Automotive Electronic Systems
TREVOR MELLARD
NEWNES
Newnes
An imprint of Butterworth-Heinemann Ltd
Linacre House, Jordan Hill, Oxford 0 X 2 8DP
φ . PART OF REED INTERNATIONAL P.L.C.
OXFORD LONDON BOSTON SYDNEY
MUNICH NEW DELHI SINGAPORE
TOKYO TORONTO WELLINGTON
First published 1987
Reprinted 1987,1988,1991
© Butterworth-Heinemann Ltd 1987
All rights reserved. No part of this publication
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Designs and Patents Act 1988 or under the terms of a
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90 Tottenham Court Road, London, England WIP 9HE.
Applications for the copyright holder's written
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British Library Cataloguing in Publication Data
Mellard, Trevor
Automotive electronic systems
1. Motor vehicles — Electronic equipment
I. Title
629.27 TL272.5
ISBN 0 7506 0436 0
Printed and bound in Great Britain by
Hartnolls Limited, Bodmin, Cornwall
Preface
Progress in the application of electronics to the auto The diagrams, drawings and illustrations are used
mobile has been rapid, and the next era will bring many to help explain theoretical principles and are in no way
more developments. To rise to the challenge offered, intended to be a design base for practical working
the service technician needs an understanding of elec circuits.
tronic principles, technology and practices. The book
is designed to give an outline of the technological The design base for the book assumes that the reader
principles and practices used in modern electronic will have an understanding ofbasic electrical, electronic
automotive systems. The inner workings of the elec and microelectronic principles, or will have accesss to
tronic control units are included to enhance the reader's the many textbooks available covering this level of the
comprehension of electronic systems, but at no point subject.
should readers feel that they are getting out of their
depth. The book seeks to build upon this and provide up
dating information and knowledge for experienced
The descriptions and explanations are clear and pre electronics people as well as the automobile technician
cise and are further consolidated by the extensive use and student.
of illustrations alongside the written material.
Acknowledgements
The author is grateful and wishes to acknowledge the Society of Automotive Engineers
valuable assistance received in the preparation of this Association of Motor Vehicle Teachers
book to the following; for their permission to reproduce Society of Auto-Electrical Technicians
illustrations, and for help in providing material: The Institute of Electrical and Electronic Engineers
AB Electronics Products Group Prof. D.M. Embry Lotus Car Company Limited
C. ENG. F.J. OP Der Winkel VAG (UK) Limited
Austin Rover Group Limited
Armstrong Patents Co. Limited Every effort has been made to identify and reach copy
Robert Bosch Limited right holders; much of the material is drawn from
Ford Motor Company past experiences and the reading of vast amounts of
Vauxhall Motors Limited information for the preparation of education and train
Lucas Electrical Limited ing programmes. If I have unwittingly infringed any
Nissan Motor Company Limited copyright, I apologise and would be grateful to hear
Toyota Motor Company Limited from any such source.
Introduction
Although automotive electronics has only recently required to fault diagnose and repair automotive elec
come into the limelight, the relationship between motor tronic systems - thus providing the car-owning public
vehicles and electronic technology dates back more with well trained, educated and quick-thinking service
than half a century. technicians, capable of providing good service and
quality repairs.
Today, electronic devices are controlled either by
analogue or by digital systems, but the trend is towards There is little available in the way of educational
the exclusive use of digital. Eventually every computer material at the moment. This book seeks to rectify this
system in the car may well be linked together, providing and meet the objectives outlined above by providing
fully integrated control of the entire vehicle and out the much needed information in a clear precise manner.
standing improvement in function and reliability.
The book will be of benefit to students on a variety
Automotive electronic equipment comes in various of City and Guilds and BTEC technician courses. It
shapes and forms ranging from small signalling devices will serve also as an ideal reference text for qualified
housing transistors and diodes, to fully fledged com craftsmen, technicians and engineers who wish to keep
puters with microprocessors, IC memories and input/ their skills and knowledge up to date, or for whom it is
output interfaces. To date, electronics has been used mandatory to have a working knowledge of automotive
in a wide variety of applications including the engine, electronics.
transmission, brakes, driving operators, power sources,
instrument panel displays, air conditioning and radios. Automotive electronics is here to stay, it is the only
way to control today's car and the car of the future. It
This integration of electronics and the automobile is essential to all concerned with motor vehicle engin
makes possible control capabilities far beyond those eering to keep up with the latest - and future - devel
realised through mechanical technology: the result opments. This book provides that opportunity and
being dramatic improvements in all phases of auto much more.
motive function, from driving performance to fuel
efficiency, exhaust purification, safety and comfort. No About the author
doubt, too, that electronics'will find many more appli Trevor Mellard teaches automotive electronics at West
cations in an ever wider variety of automotive fields. Glamorgan Institute of Higher Education, Swansea,
where he is Head of the School of Automobile Engin
The rapid application of electronics to automotive eering. Mr Mellard has taught courses in automotive
technology has also created problems for the industry's electrical and electronics at both FE and advanced FE
service personnel. Students, technicians, managers, levels. As a member of both City and Guilds and
teachers and car owners are often bewildered by the BTEC working parties and committees he has written
sight and operation of automotive electronic systems, education and training material, designed electrical and
and many do not feel confident in servicing, repairing electronic curricula and learning modules. At present
and fault diagnosing them. Therefore the need exists he is a City and Guilds examiner in electrical and
for a wide variety of persons, in particular service
technicians and engineers, to be fully versed in the
operating principles, skills, knowledge and abilities
xii Automotive Electronic Systems
electronic work, and an advisor to BTEC in the same the Institute of Road Transport Engineers and a Mem
subject areas. Professionally he is a Member of the ber of the Society of Auto-Electrical Technicians. He
Chartered Institute of Transport, Member of the Insti is Honorary Secretary of a learned Society, the Associ
tute of the Motor Industry, an Associate Member of ation of Motor Vehicle Teachers.
1
Fundamentals of automotive technology
Electronic technology is now applied to a diverse 3 suspension
spectrum of automobile operations; helping to 4 steering
improve driving performance, fuel efficiency, 5 brakes
emissions purification, comfort and pleasure. In 6 instrumentation.
this chapter we consider the relevant systems of an
automobile which can be controlled electronically: Fig 1.1 shows the layout of these systems. Fol-
lowing chapters discuss how electronic control of
1 engine these systems can take place.
2 transmission
6 Instrumentation
1 Engine and
2 Transmission unit
A Rear suspension 3 Steering system
5 Brake units
4 Front suspension
Fig 1.1 Basic automobile systems
2 Automotive Electronic Systems
Internal combustion engine where it is compressed and then ignited. The
burning of the fuel (combustion) causes a rapid
The engine is a self-contained power unit which rise in cylinder pressure which is converted to
converts the heat energy of fuel into mechanical useful mechanical energy by the piston and crank
energy for moving the vehicle. Because fuel is shaft.
burned within, the engine is known as an internal
combustion (IC) engine. In the IC engine, an air- The fuel may be ignited either by a spark or by
fuel mixture is introduced into a closed cylinder compression giving rise to classifications of spark-
Gasket (τς±
Rocker shaft
Rocker cover
Crankshaft
bearing cap
Engine support^ ifej^A
Fig 1.2 An in-line overhead valve engine -fixed parts and valve mechanism
Fundamentals ofautomotive technology 3
ignition (SI) and compression-ignition (CI) causing a rapid and extreme rise in cylinder press
engines. An exploded view of a typical spark ure, to such an extent that the piston is forced
ignition petrol engine is shown in Fig 1.2, detailing down the cylinder and the connecting rod gives
the major components. the crankshaft a powerful turning effort. This is
The four strokes of such an engine are shown the combustion stroke, also called the power
in Fig 1.3. At the beginning of the induction stroke stroke, shown in Fig 1.3(c).
Fig 1.3(a) the inlet valve opens and the piston Once the mixture has been burned it must be
travels down the cylinder from top dead centre removed from the cylinder as quickly as possible.
(TDC) to bottom dead centre (BDC). The partial In the exhaust stroke (Fig 1.3(d)) the rising piston
vacuum created by the moving piston causes the pushes the hot gases and combustion products out
air-fuel mixture to rush in from the inlet manifold of the cylinder through the open exhaust valve
and through the open valve, into the cylinder. and exhaust system into the earth's atmosphere.
Exhaust Exhaust l ^ ^ i j Inlet
(a) Induction stroke (b) Compression stroke (c) Combustion stroke (d) Exhaust stroke
Fig 1.3 Four stroke cycle principle of operation
The correct air-fuel mixture is provided by the This sequence of events is repeated continually,
carburettor. When the piston reaches the end of with power delivered to the crankshaft on only
its stroke the inlet valve closes, sealing the top end one of the four strokes - the combustion stroke.
of the cylinder as both valves are closed. Crankshaft rotation continues through the other
In Fig 1.3(b) the piston is moving up the cylin strokes due to the kinetic energy of the heavy
der, compressing the air-fuel mixture between the flywheel which is connected to the crankshaft.
piston and cylinder head to a very small volume - Note that the crankshaft rotates through two full
the compression stroke. Just before TDC an elec revolutions for each four-stroke cycle and a spark
trical spark, generated across the electrodes of the occurs only once in the cylinder. In a multi-
spark plug, ignites the air-fuel mixture. For good cylinder engine, power strokes of each cylinder
performance the timing of the spark must be are staggered so that power is delivered almost
closely controlled. continuously to the crankshaft for a smooth oper
As the mixture burns, the hot gas expands ation.
4 Automotive Electronic Systems
Mixture supply system moves towards the horizontal position the airflow
is restricted (throttled) and engine power and
Fuel stored in a large tank, is fed via a pump to speed is reduced accordingly. In normal operation
the carburettor. The carburettor (Fig 1.4) mixes the air-fuel ratio (by mass) varies, typically, in the
the liquid petrol with filtered air on its way to the range 12:1 to 17:1.
Air filter
Carburettor Air in
(fuel-in) Valve mechanism
Inlet manifold Piston
Fig 1.4 Basic fuel supply system
cylinders and in the process turns it into a vapour. Choke valve Float needle
The inlet manifold (Fig 1.5) directs the mixture
to the cylinders. The ratio of air to fuel in the Venturi Mixture supply principles
mixture delivered to the cylinder is controlled by
the size and shape of the carburettor bore and Fig 1.5
venturi, and the size of the fuel metering jets. The
standard manual control for the amount of air and
fuel mixture delivered to the engine is the throttle
valve, which is controlled by the driver's
depression of the accelerator pedal. The throttle
valve is simply a round disc, mounted on a thin
pivot shaft so that it can be tilted at different
angles under the control of the accelerator pedal.
In the vertical position the throttle valve offers
virtually no restriction and the full volume of
air and fuel passes to the cylinders to produce
maximum engine power. As the throttle valve
Fundamentals of automotive technology 5
Ignition systems ignition coil. This current flow creates a magnetic
field in the primary coil. When the contact breaker
The basic ignition system of the SI petrol engine points open, interrupting the flow of current (Fig
is the Kettering system (Fig 1.6). The battery 1.7(b)) a rapid voltage change is produced across
provides a low voltage (12 V) source of direct cur the primary winding and a high voltage (15,000-
rent. When the ignition switch is turned on and 20,000 V) is induced across the secondary winding.
the contact breaker points are closed (Fig 1.7(a)) This secondary voltage is high enough to jump
current flows through the primary winding of the the gap of a spark plug, creating a spark between
Ignition switch
σ
Distributor
Fig 1.6 Basic Kettering ignition system used in SI petrol engines
Ignition coil - N- ' eFld
Field >/^\ " / / - ^ Γ * " " collapsing
expanding 3M> ΤΤΓ-
/ / / SUTTEE Λ\ '/ ' t \\
Ignition ■IIIi ΓΪι'- Hi II !'' I'
switch ' II IΊ
—o-^— IIi
_LLL
Battery
"Capacitor M L , $ΏΙ3> ^i^JMiV/stcondafy
IAP-^ J iJ/ Secondary V^TSNÄ^V voltage
>-yy/ voltage
wv*,-Primary f—
4~ΦPrimary *-
♦voltage *?
voltage SparK
-j-J i L at I
_L_ plug
(a) (b)
Fig 1.7 Basic Kettering ignition system principles (b) with contact breaker points open
(a) with contact breaker points closed
6 Automotive Electronic Systems
the electrodes with sufficient energy and direction fuel mixture pressure - the higher the pressure,
to ignite the air-fuel mixture. To assist in the the longer the necessary burn-time. Second, the
rapid change of primary voltage a capacitor is burn-time is independent of engine speed - the
connected in parallel with the breaker points, faster the engine speed, the greater the angle
which also reduces arcing across the points - through which its crankshaft turns during the
extending useful life. burn.
The high voltage pulses generated across the To ensure the burn is always completed at the
secondary coil winding must be delivered to the correct time - whatever the load conditions and
appropriate spark plug at the correct time. The engine speed - compensation adjustments must
distributor, shown in Fig 1.6, is used for this be made to advance timing of the ignition spark
purpose. with: (1) increasing air-fuel mixture pressure, and;
(2) increasing engine speeds. These adjustments
It contains a rotary switch (rotor) and fixed cap, are automatically performed in two ways:
that connects the secondary pulse to the appro- • a device known as a vacuum advance
priate spark plug just before the corresponding
piston reaches TDC on the compression stroke. senses the pressure of the air-fuel mixture
The distributor is connected to the coil and spark within the inlet manifold and retards the
plugs by high tension plug leads and the dis- timing as the pressure decrease? (i.e. as
tributor shaft controls the opening and closing of the 'vacuum' increases). As the pressure
the contact breaker points. As the spark (ignition) is related to engine load it can be seen
timing must be related to the position of the piston that the ignition timing retards with
in the cylinder the distributor shaft rotation must increasing load.
be coupled to the crankshaft. This coupling is • a device known as a centrifugal advance
made by mechanical gearing to the camshaft, senses engine speed and advances
which is crankshaft driven. The camshaft rotates ignition timing as speed increases.
at half the speed of the crankshaft, because only Ignition timing has significant effects on engine
one spark and one valve sequence is required for performance. Correct advance of timing provides
each two revolutions of the crankshaft. complete combustion hence high power output
and low exhaust emissions. Too advanced ignition
Initial ignition timing of the Kettering system timing, however, results in the combustion press-
is set by positioning number one cylinder piston ure opposing the rising piston with a resulting loss
just before TDC, both valves closed, at the end of power, high mechanical stress conditions and
of the compression stroke. The contact breaker detonation that causes an audible pinging (knock)
points are then set to just opening with the rotor sound. Retarded ignition timing can result in
feeding the number one cylinder plug lead. Prior incomplete combustion, low power output,
to this, the gap between thefully open points must overheating and increased exhaust emission. To
be set, to give correct dwell time. balance out these conflicting features, engines are
generally tuned to a static ignition timing of about
When the engine is running, timing of the 8 to 10 degrees BTDC, with the automatic
ignition spark must be controlled so that the com- advance and retard mechanisms catering for vari-
plete charge of air-fuel mixture within a cylinder able conditions.
has fully combusted, or 'burned' through, at the
exact moment the corresponding piston starts to Diesel engine
go down the cylinder from TDC on the power
stroke; because the full combustion power is then The diesel engine, although similar to the petrol
available and maximum useful work is produced. engine in construction, is normally heavier and
In this respect the fuel mixture burn time is almost used mostly in heavy duty vehicles. Like the petrol
constant but, two factors must be taken into con-
sideration. First, the burn-time varies with air-
Fundamentals of automotive technology 7
engine it extracts its energy by burning an air-fuel Gearbox
mixture inside cylinders; the pistons, connecting
rods and crankshafts are similar and the four- The main purpose of the gearbox is to provide a
stroke cycle occurs in the same sequence. The selection of gear ratios between the engine and
main differences are in the way in which the air driving wheels, so that the vehicle can operate
and fuel are introduced into the cylinders and the satisfactorily under all driving conditions. Gear
way in which the air-fuel mixture is ignited. selection may be done manually by the driver or
automatically by a hydraulic control system.
Air alone enters the cylinder on the induction
stroke and is compressed by the upstroke of the Propellor shaft
piston in the compression stroke, during which
time it becomes extremely hot (650°C) and press- The function of the propellor (drive) shaft is to
urised (3.5MPa). Diesel fuel in the form of a spray transmit the drive from the gearbox to the input
is injected just before TDC. The fuel is self- shaft of the rear axle and differential assembly.
ignited by the high temperature of the compressed Flexible joints allow the rear axle and wheels to
air and the burning gases expand and force the move up and down without affecting operation.
piston down.
Rear axle and differential
The pressure developed by the piston's com-
pression stroke is greater than in a petrol engine - The rear axle and differential unit transmits the
normally two to three times higher. The com- engine's rotational power through 90° from prop-
bustion process lasts for most of the power stroke shaft to axle shaft to road wheels. A further
and the resulting pressure remains approximately function is to allow each driven wheel to turn at
the same throughout the stroke. a different speed; essential when cornering because
the outer wheel must turn further than the inside
Power and speed output of the diesel engine are wheel. A third function is to introduce another
controlled by the quantity of fuel injected into the gear ratio for torque multiplication.
cylinders, as the amount of air that enters the
cylinder on each induction stroke is almost Suspension
constant. The fuel is injected through an injector
spray nozzle at each cylinder, supplied by a fuel The axles and wheels are isolated from the chassis
injection pump. The amount of fuel injected into by a suspension system. The basic job of the
each charge of air is extremely small and each shot suspension system is to absorb the shocks caused
of fuel is measured with great accuracy. by irregular road surfaces that would otherwise be
transmitted to the vehicle and its occupants, thus
Transmission helping to keep the vehicle on a controlled and
level course, regardless of road conditions.
The transmission system comprises clutch, gear-
box, propellor shaft, rear axle and differential and Steering
the driven road wheels.
Clutch The steering system, under the control of the
driver at the steering wheel, provides the means
The clutch or torque converter has the task of by which the front wheels are directionally turned.
disconnecting and connecting the engine's power The steering system may be power assisted to
from and to the driving wheels of the vehicle. This reduce the effort required to turn the steering
action may be manual or automatic. wheel and make the vehicle easier to manoeuvre.
8 Automotive Electronic Systems
Brakes Engine - starting and ignition.
purposes
The braking system on a vehicle has three main Safety and - lighting, horn, wipers, washers
functions. It must be able to reduce the speed of convenience etc.
the vehicle, when necessary; it must be able to Driver
- instrumentation and warning
stop the car in as short a distance as possible; it information lamps.
must be able to hold the vehicle stationary. The Of these devices instrumentation is, perhaps,
braking action is achieved as a result of the friction most influenced by the advance of micro
developed by forcing a stationary surface (the electronics. The basic electromechanical systems
brake lining) into contact with a rotating surface of:
(the drum or disc).
Speedometer - for indicating vehicle speed.
Each wheel has a brake assembly, of either the
drum type or the disc type, hydraulically operated Engine oil - warning lamp or gauge to show
when the driver applies the foot brake pedal.
pressure operating limits.
Engine coolant - warning lamp or gauge to show
temperature operating limits.
Battery - warning lamp or gauge to
Instrumentation charging indicate satisfactory/
unsatisfactory action.
The motor vehicle incorporates a number of elec Fuel tank - gauge to show amount of fuel
trical devices which are used for: content in the fuel tank.
Battery - alternator and regulator. are giving way to computerised vehicle
charging management information centres.
Automotive electronic and microprocessor systems
A system is defined as an orderly arrangement of ing a decision a system may store the information
physical objects. It can be organised into three for a time, or process it as the result of other
basic elements: input, processor and output, as information stored permanently in memory.
shown in Fig 2.1. The input signal is usually the Finally, as the result of the decision, an action
outside the system takes place.
This three-element system configuration is very
Input Processor Output simple. Typically, each element consists of more
than one process. For example, Fig 2.2 shows a
Cause Process Effect more elaborate control system, with all the parts
likely to be found in a general system of this type.
Fig 2.1 Basic idea ofa system having organized inpSuetsnsors convert physical
and outputs and) into electronic signals quantities (measur-
and apply them to
the input circuits. In turn, the input circuits con
cause of a change in the system and the output vert and amplify the signals if necessary and pass
action which occurs as a result of the input is them into the system's control circuits. Here the
called the effect, while the response of the system control circuits process the input information and
to an input signal is called the process where the decide on the appropriate action to be taken. The
input is processed to effect the desired output. A signals required to produce these actions are sent
system can be purely analogue in nature, purely to the output circuits where the signals are recon
digital, or a mixture of both. verted again and amplified to operate the actuators
Initially, a system senses external information, and/or display devices. Actuators are devices such
converting it to a form that can be handled intern as solenoid valves, relays and motors that perform
ally. Then decisions are made, based on the input an action. Display devices provide visible and/or
information, by process or manipulation. In mak audible information.
Measured Analogue Digital Digital Analogue
physical signal signal signal signal
quantity USensor Observed
Converter Digital Converter Actuator action
and signal and or
amplifier processor ►
amplifier display
Fig 2.2 Generalized control system
10 Automotive Electronic Systems
Control loop systems Microcomputer-based control system
Any control system can be classified as open- The microprocessor is a very large scale inte
loop or closed-loop. In an open-loop system, the gration (VLSI) circuit whose final function is
control circuits do not monitor the system's output determined by the sequence of instructions,
to determine if the desired control action was known as the program, given to it. Individual
achieved (Fig 2.3(a)). instructions enable the microprocessor to carry
out each step towards completion of a complex
Input Sensor Signal Output^ circuit function. The basic microprocessor is
processor therefore not a dedicated device confined to one
Actuator particular application, although the majority of
microprocessors used for control purposes do have
(a) Open loop built-in mask-programmable storage circuits
which allow them to be dedicated to specific con
Input Sensor uSignal Output ^ trol functions.
processor Actuator iFeedback The microprocessor can do nothing on its own,
sensor requiring a certain amount of supporting
Ak hardware, memory and input/out circuits. Fig 2.4
1 shows the relationship between the four basic
Correction^ Feedback functions of microprocessor, memory, input and
signal ^^ signal output. The microprocessor is the central control
ι processor function for the system, sometimes called the cen
tral processing unit (CPU). It performs this pro
(b) Closed I0 0 P cessing control function under the direction of
instructions stored in the system memory. These
Fig 2.3 Block diagrams of instructions make up the system program.
(a) open loop and
(b) closed loop control systems The memory also provides storage of data,
function tables, and decision tables. When used
In a closed-loop system, on the other hand, a for engine management systems it would be pro
feedback sensor and circuit continually monitor grammed with all the information necessary to
the system's output; developing a correction control functions such as fuel injection quantities
signal, applied to the control circuit, which adjusts and ignition advance characteristics. For example,
the output towards the desired value (Fig 2.3(b)). the memory might contain 16 load and 16 speed
Generally, control systems comprise only closed-
loop designs, as open-loop systems are not accur
ate due to their very nature.
Sensor j£ Address bus n i Outputs
vinputs -NJ Input Micro (actuators)
-^ port(s) processor Memory Output
port(s)
Data bus 7T is.
7T7T
\L
Control bus
Fig 2.4 A microprocessor-based system
Automotive electronic and microprocessor systems 11
parameters which would permit the use of 256 a sequence of instructions which carry out this
ignition advance positions based on a combination fetch-execute cycle. Use is made of specific seg-
of these stored values. But the microprocessor has ments of memory, known as registers, which store
also to consider other parameters when deciding single words of data or instructions. These regis-
the actual ignition timing point - starting period, ters can be part of the microprocessor itself, or in
coolant temperatures, dwell time, combustion some instances, part of the general system
knock, etc. memory. One register, known as the program
counter, is always set to the address of the next
Input ports allow inputs of system data from location of memory which is to be read. Thus, it's
sensors and manual controls into the system. the program counter which lets the micro-
The microprocessor interprets the data and processor know which memory address contains
implements output control decisions under the the next instruction. A second register, the
guidance of the program stored in memory. Out- instruction register, is used to temporarily store
put ports provide the means for the micro- an instruction read from memory, until such time
processor to send the output control signal to the as the microprocessor can decode the instruction.
device that carries out the desired action. Other registers, without specific names, are also
used to temporarily store data words.
The microprocessor communicates with the
other elements of the internal system by sending The fetch operation involves getting an instruc-
digital binary codes along conductors called buses. tion out of a specific memory address, and passing
Initially, the microprocessor may send a binary it along the data bus to the microprocessor. In the
address code (on the address bus) which deter- first part of the fetch cycle the address held in the
mines which input/output port or memory program counter i.e. the address of the instruction,
location is to be brought into action. Control sig- is transmitted to the memory via the address bus.
nals (on the control bus) are then sent, which The memory is set to 'read' by the microprocessor
determine the direction of data flow (i.e. to or control section, causing the data at the addressed
from the microprocessor) and when the transfer memory location to be put on to the data bus and
of data is to take place. Finally data codes are sent be transmitted to the microprocessor's instruction
to or from the microprocessor along the data bus. register.
Each action the microprocessor takes is under After the instruction is decoded the micro-
the direction of an instruction from memory. processor increments by 1 the program counter
Thus, the microprocessor must know where the to locate the next portion of the instruction, reads
instruction is located in memory (i.e. its address) the memory content of that address and stores it
and it must fetch the instruction code from mem- in another temporary register. This may continue
ory before the instruction can be carried out. Once for a third cycle until the microprocessor has all
inside the microprocessor an instruction decoder the information necessary to execute the instruc-
circuit interprets the instruction, then a controller tion.
generates the required control signals to carry out
(execute) the instruction. This sequence of (1) The execute operation involves carrying out
addressing memory and fetching instructions, (2) the process defined by an instruction. Storage,
decoding and executing instructions, is repeated arithmetic, logical or shift functions are performed
for all instructions in the program. on the data during this stage.
Basic principles of operation After execution of the instruction, the micro-
processor fetches the next instruction to be
The operation of a microprocessor is based upon executed, and so on until a halt instruction is
executed. The fetch-execute cycle for a typical 8-
bit microprocessor takes about 3 microseconds.
12 Automotive Electronic Systems
Subroutines teristic 'diagrams' or 'maps' can be stored in the
form of tables in memory. Fig 2.5 shows an exam
A well-structured program contains a number of ple of a possible ignition advance map, illustrating
'mini' programs or subroutines. Each subroutine how the ignition advance ideally varies according
is dedicated to a specific task; handling inputs, to vacuum and speed. The map is stored as tables
performing computation, logical data manipu in the system memory. As the microprocessor
lation, or some output function. The instructions receives input information regarding vacuum and
within a subroutine are stored and located sequen speed of the engine it can then look up the cor
tially in the memory. When the main program responding ignition advance setting in the table,
requires that a subroutine be performed, an adjusting the timing to suit.
instruction is executed which tells the micro Memory tables may represent several functions
processor at what address the subroutine is that have to be implemented for control purposes.
located i.e. the program counter is set to the first For example, engine temperature and warm-up
instruction location address of the subroutine. times, sensor linearisation, fuel quantity and injec
The last instruction in the subroutine tells the tion times etc.
microprocessor to return to the part of the main
program it previously had left. Interrupts
Memory tables Another important function that the micro
processor system can do is respond to high
Automotive electronic control systems have been priority signals from the input, output or feedback
developed to the stage where complicated charac- circuits. These interrupt signals get the immediate
attention of the microprocessor, which stops what
it was doing and jumps to a subroutine designed
to handle the condition that caused the interrupt
signal. The importance of the interrupt signalling
function is that it allows the microprocessor to
handle other jobs without the time consuming job
of continually monitoring all circuits. When a
circuit needs immediate service it can get the
microprocessor's attention by using an interrupt
signal. It could be that the ignition is too far
advanced and combustion knock is occurring. The
knock interrupt signal is fed to the microprocessor,
interrupting its normal operation, causing the
Fig 2.5 Pictorial representation sotofirgendiatisontatbilmesinigntmhsiayctsrtoewpmirlol creesstaorrdtothfeetcihgnaintidonexteicmuitnega subroutine
details. Such a representation is to prevent
memory further knocking.
Sensors
Performance of any control system is, first of all, material such as nickel or cobalt oxides, which has
related to the accuracy with which information a predictable change in resistance as the tem
about the operating variables or parameters is perature is changed. On heating the thermistor,
relayed to the controlling process. The sensors or electrons break free from the semiconductor's
transducers used to monitor the variables do so covalent bonds thus reducing the thermistor's
by converting (transducing) the variables' physical resistance (Fig 3.1(a)). The change in resistance
quantities (measurands) into related electrical sig with change in temperature is shown in graphical
nals. Common measurands in automobiles are form in Fig 3.1(b). Because the resistance
temperature, pressure, speed, position, flow and
oxygen concentration. Semiconductor
Sensors operate in many ways to transduce a material p (resistivity)
measurand into an electrical signal. A knowledge
of the methods, their advantages and disad I Low temperature Temperature
vantages, is essential to the service engineer, who High resistance (b)
has to check their operating performances.
ΘΘΘΘΘΘΘ
Temperature sensors ΘΘΘΘΘΘΘ
Temperature is an important input variable in High temperature
engine control systems; in particular with regard Low resistance
to fuel metering and ignition timing, where knowl
edge of the changing coolant temperature and air (a)
temperature is essential. Of the three temperature
sensing technologies available; thermocouple, Fixed control Γ~\
resistive temperature device and thermistor, the resistor
thermistor is the one in common usage. A typical
temperature sensor consists of a thermistor pellet, Vin tThermistor V o u t =(-§Jp-)v
mounted in a housing which may be inserted in a (Reference voltage)
fluid or in an air stream. RT R + R T '
I
Thermistors
(O
The thermistor is made of a semiconductor
Fig 3.1 (a) Semiconductor resistance change with
temperature
(b) Thermistor temperature sensor characteristic
(c) Temperature sensor circuit
14 Automotive Electronic Systems
decreases with an increase in temperature the ther- Air temperature
mistor is said to have a negative temperature
coefficient (NTC). The resistance change can The design principle of the nickel thick-film ther-
amount to between 5 and 10% per degrees centi- mistor (Fig 3.2) makes the device most suitable
grade, which for a 10 kilohm thermistor element for sensing intake air temperature. Intake air tem-
is between about 500 and 10,000 ohm over the perature can change rapidly but the thick-film
operating temperature range of an engine. thermistor's small thermal time constant means
that it can still monitor these changes successfully.
Due to this sensitivity, it is possible to make In addition, it exhibits an almost linear tem-
measurements to within 0.05°C, although the rela- perature characteristic curve (Fig 3.2(c)).
tive sensitivity decreases at high temperature
changes as shown by the flattening out of the curve In some electronic control systems a negative
in Fig 3.1(b). As the output is only a physical temperature coefficient thermocouple is used to
change in resistance it usually has to be changed measure the inside wall temperature of the inlet
into a voltage or current. To achieve this the manifold. The inside wall temperature of the inlet
sensor is connected in a circuit to act as part of a manifold is really a mixed value comprising mani-
potential divider, as shown in Fig 3.1(c), where fold wall and fuel-air mixture temperatures - the
the voltage drop across the thermistor varies with value being used to influence the volume of fuel
the change in resistance. Service checks should to be supplied by an electronic carburettor.
ensure that the current flow through the ther-
mistor is not high enough to cause self-heating, Thermocouples
due to the power dissipated within the thermistor.
Protection Paladium Control circuit The thermocouple works on the principle that a
silver . voltage is generated by a circuit comprising two
connection Thick-film1 junctions of two dissimilar metals (Fig 3.3). When
sensor paste one of the junctions is kept at a constant tem-
r perature the voltage change of the circuit is a
(b) function of the temperature change at the other
Nickel-based Ceramic junction, and is linearly related. With an operating
paste thick-film aluminium range of 250°C to 2000°C they are most useful for
measuring exhaust gas and turbocharger tem-
substrate peratures.
(a) n Thermally
generated
400 H voltage
Metal A
(platinum)
Variable
ttoembpeeratu.vr%"e" oodz^_^=|xxxx
measured' Constant
\ temperature
Metal B
0 +40 +80 (rhodium)
Temperature °C
Fig 3.3 Thermocouple temperature sensing principle
(c)
Intake air mass sensors
Fig 3.2 Structure and characteristics of a thick-film or
temperature sensor To accurately control the air-fuel ratio it is necess-
(a) Cross-section ary to measure the intake air mass exactly. The
(b) Plan view
(c) Resistance-temperature characteristic
Sensors 15
mass of air per kilogram of fuel in a mixture of air Full-load Secondary Primary
and fuel gives the air-fuel ratio of the mixture. stop winding winding
For the complete combustion of an air-petrol mix
ture the ratio is approximately 15 parts of air to 1 Diaphragm Plunger
part of fuel (15:1) by mass. By measuring the mass suspension
of air drawn into a cylinder, the correct mass of ^ spring
fuel for complete burning can be determined and
injected. ito intake
manifold
Currently, two methods are used to measure
intake air mass. One of these measures the air Plunger
mass indirectly by measuring the inlet manifold Manifold pressure
pressure; the other measures the air mass directly.
Both methods must compensate for changes in (a)
air density caused by ambient temperature and
altitude variations.
Absolute pressure sensors •J 1 — c
When the engine is running, the inlet manifold ί Secondary 1 ,
pressure varies from nearly zero with the throttle t1
valve closed to almost atmospheric pressure when I
the throttle valve is fully open. Therefore manifold
absolute pressure can be used as an indication of AC excitation ▼ •J »II
the load operating conditions of the engine, and 10kHz oscillator
hence the air-fuel ratio to satisfy the operating £ Secondary 2 [
condition. Primary
Movable magnetic Typical secondary
core connection
(b)
Aneroid absolution sensor Constant Direction of
supply
The basic aneroid is a sealed, evacuated chamber, current magnetic field
rigid except for one end which is a thin plate (flux density)
that can flex easily. The thin plate, acting as a Lorentz
diaphragm, deflects according to the pressure force Direction of
exerted on it, in this case manifold pressure. Fig electron flow
3.4(a) shows a cross-section of a typical pressure Ve
sensor formed by two evacuated aneroids and
suitable mechanisms for converting diaphragm Hall voltage
deflection into electrical signals. In practice two
methods are used for this conversion:
(1) Linear variable differential transformer (O
(LVDT) - as shown in Fig 3.4(a)
The LVDT has a moveable core attached to the Fig 3A
diaphragm. When the diaphragm is deflected by (a) Aneroid chamber-type absolute pressure sensor using
manifold pressure change it varies the output of an LVDT transducer
the transformer. The primary section of the trans (b) LVDT principle
former comprises an oscillator and primary wind- (c) Hall effect principle
16 Automotive Electronic Systems
ing to create an alternating magnetic flux (Fig Flexible air dielectric Diaphragm plates
3.4(b)). The output (secondary) windings are cen diaphragm (hollow)
tre tapped to give two balanced coils with equal
output when the transformer core is in the central Manifold Γ
position. The secondary coils are not only equal vacuum —J
but are of opposing polarities, so they cancel each
other out, and the output voltage is zero. As the Sealed ^~ (a)
core is displaced from the central position by housing
manifold pressure, the output voltage of one wind (evacuated)
ing is more than the other, so that the output
voltage varies in direct proportion to the position ΘOscillator Phase
of the core. A separate signal processing circuit detector
produces an analogue DC voltage proportional to
the manifold pressure. Vout
(2) Hall effect detector (b)
When electrons move through a semiconductor
layer (Fig 3.4(c)), perpendicular to the lines of Fig 3.5 Capacitive absolute pressure sensor
flux of a magnetic field, an electromagnetic force (a) Construction
(the Lorentz force) acts on each electron per (b) Detecting circuit
pendicular to the direction of current flow. As the
result of this force, electrons drift to one side of As the plates deflect with manifold pressure
the semiconductor causing a surplus of electrons variation, the capacitance of the capsule changes
and a negative polarity at this side, and a depletion because the distance between the plates varies.
of electrons and a positive polarity at the other Capacitance increases (distance decreases) as
side. The voltage generated across the two sides manifold pressure increases. The capacitance is
due to this Hall effect is known as the Hall voltage, measured by connecting the capsule into a series
and the stronger the magnetic field the higher the resonant circuit and applying an AC signal of
Hall voltage. known frequency and amplitude from an oscil
lator, as shown in Fig 3.5(b). The oscillator fre
In pressure sensors using a Hall effect detector, quency is the same as the circuit's resonant fre
the deflection of the diaphragm moves the position quency of atmospheric (absolute manifold)
of a permanent magnet causing a change of mag pressure. At this balanced frequency the voltage
neticfieldstrength acting on a Hall semiconductor across the inductor and capacitor sensor are equal,
element (Fig 3.4(c)). The Hall voltage generated but of opposite phase and so cancel each other
by the semiconductor element is directly pro out. At a manifold pressure equal to atmospheric
portional to the magnetic field strength and thus pressure, therefore, the full supply voltage is
the inlet manifold pressure. applied across the resistor and a zero output is
given from the phase detector. As the manifold
Capacitive absolute pressure sensor pressure decreases, the capacitance falls, the cir
This is another sensor that uses an evacuated cuit's resonant frequency changes and a voltage is
chamber, but has two thin metal diaphragms created across the resistor. The phase of the
which act as the plates of a capacitor, as shown resistor voltage varies sharply relative to the ref
in Fig 3.5(a). The plates are insulated from the erence phase. The phase detector detects the
chamber by a dielectric material and the capacitor change in phase and supplies an output voltage
capsule is inside a sealed housing which is con
nected to manifold pressure.
Sensors 17
proportional to the change in phase and hence fv+
proportional to manifold pressure.
Λ
Semiconductor absolute pressure sensor
The semiconductor absolute pressure sensor com • V out -
prises a diaphragm of n-type silicon, with four R2
piezoresistors diffused in it, connected to form a
Wheatstone bridge (Fig 3.6). The resistors are
Diffused resistors Silicon
I db u LJ η ^ (a) (b)
Base plate
Fig 3.7 Piezoresistor Wheatstone bridge arrangement
1 P, manifold vacuum for the semiconductor absolute pressure sensor
Epoxy ^ (a) Piezoresistor bridge diffused in silicon diaphragm
mounting (b) Piezoresistors in Wheatstone bridge form
supports a
fail Ί -pressure
i(Pi ) seal
Fig 3.6 Basic construction ofa semiconductor absolute
pressure sensor
same time R2 and R3 decrease in value by the
same amount as these resistors (radial resistors) are
formed by diffusing an impurity element into the being elongated. This unbalances the bridge and
n-silicon diaphragm, thereby forming a diffused a voltage difference is created at the bridge ter
strain gauge. The piezoresistors are formed minals, which in turn lowers the mid-point voltage
around the edge of the diaphragm, two radially of the Rx R2 divider (point A) and raises the
and two tangentially. The set-up is sealed and an midpoint voltage of the R3 and R4 divider (point
evacuated chamber is formed between the glass B), resulting in a change in voltage between A and
plate and the central area of the silicon diaphragm. B which is proportional to the pressure change.
External connections to the resistors are made Since this output is about 50mV/80kPa, and
with fine gold wires connected to the metal bond rather small for control systems, it is amplified to
ing pads. This complete assembly is placed in a several volts at 80 kPa by an operational amplifier.
sealed housing which is connected to the manifold
by a small bore tube. Potentiometer airflowsensor
A piezoresistor is made from a material in which A sensor plate, mounted on a pivot shaft in the
the resistance changes, with a change in twisting sensor housing and loaded by a calibrated coil
or bending force, due to stress in the material, spring, protrudes into the air inlet recess (Fig 3.8).
thus providing a variable resistance output. Fig The higher the rate of airflowthe more the sensor
3.7 shows the circuit diagram used with this type plate is rotated. To steady the sensor plate, a
of sensor. The semiconductor bridge is supplied compensation plate swings simultaneously in a
with a constant voltage, and a current of approxi damping housing to provide a smooth action. The
mately 1 mA. When there is no strain on the rotating sensor plate is connected to a poten
diaphragm all four resistances are equal, the tiometer (not shown), the output of which is a
bridge is balanced and the output voltage from voltage signal corresponding to the angle of
the bridge is zero. As the stress of applied pressure deflection. The potentiometer in such a sensor is
causes the diaphragm to deflect, the resulting made using thick-film techniques, with a ceramic
strain on the piezoresistors causes them to change substrate onto which a potentiometer track of
in value; in such a way that RY and R4 (tangential wear-resistant material is overlaid. To obtain the
resistors) increase in value proportional to press required output characteristic, the potentiometer
ure because they are being compressed and at the is 'fine tuned' by baking onto the substrate several
18 Automotive Electronic Systems
Bypass screw (CO adjustment) Bypass and detecting changes in the electric current (500-
1200 mA) due to heat transfer from the hot-wire
^ S passage to the cold-air stream. A typical sensor con-
struction is shown in Fig 3.9(a).
The small wire-wound sensing probe is placed
in a by-pass channel of the inlet air path, which
has its outlet at the venturi port. The pressure
Compensation^ Hot-wire probe
plate Electronic
circuit
Damping
volume
Fig 3.8 Potentiometer airflow sensor principle
film resistors of ceramic metal (cermet) which are Air temperature
connected to sections of the track by very narrow probe
conductive straps.
(a)
When measuring the quantity of air drawn into
the cylinder of an engine using an air flow sensor Intake air" F^ «r V+
of this type an error is introduced, depending on mass per - RcJ
the altitude, which must be compensated for. This unit of
sensor is also prone to pulsation errors and the time 0^
moving parts are subject to wear. \I
Vout
Measuring intake air mass flow indirectly by
the means we have seen here is quite inaccurate, (b)
because the air volume is determined by the air
pressure and temperature at a point in the Fig 3.9 Hot-wire air massflowsensor
manifold, so the measured air mass differs from (a) Typical configuration
the air mass actually charged into the cylinder. (b) Control circuit
This can be minimised by using a more direct
measuring method. difference between the inlet and outlet of the
by-pass channel achieves an air flow which is
Direct air flow sensors proportional to that of the main inlet airflowpath.
Another sensing probe with the same construction
Compared with indirect pressure sensing methods as the hot-wire probe is also installed in the by-
direct measurement of air mass flow detects air pass measuring channel, and is used to measure
flow quickly and accurately. This allows highly the inlet air temperature to compensate for tern-
accurate control of the air-fuel ratio, which is a
most important factor in improving combustion.
Hot-wire airflow sensor
Hot-wire anemometry is a well established
method for measuring mass flow. It involves plac-
ing a very thin resistance wire, of 50-100//m
diameter, in the air stream, heating it to a very
high temperature by means of an electric current
Sensors 19
perature variations. The compensating wire is the hot-wire sensor has to meet two basic require
called a 'cold-wire' and has negligible current ments:
passing through it. The air-flow sensor also deter 1 the temperature coefficient of resistance
mines the temperature and density of the air by
measuring the resistance of the hot-wire probe. must be constant over a wide temperature
range, to facilitate temperature com
Initially, the sensing wire has a controlled cur pensation
rentflowingthrough it to raise its temperature to 2 the surface condition must be stable so
a known and constant difference in temperature that little change is brought about by oxi
(100 °C) above the cold-wire temperature. When dation and contamination.
the engine is running the air flow velocity has a Platinum is one of very few resistor materials
cooling effect on the sensing wire, due to the that satisfies both these conditions. But even so,
heat transfer from resistor to air, reducing its contamination of the wire surface still sometimes
resistance. The current is increased to keep the occurs and some control circuits counteract this
resistance and temperature at the constant differe by heating the wire to a much higher temperature
nce. Should there be a change in resistance of the (1000 °C) for, say, 1 second every time the engine
cold-wire due to air temperature, this will be is switched off. This action burns off any dirt and
compensated for by further regulating the hot contaminants on the hot wire.
wire current. The changing current required to When the mass of air drawn by the engine is
maintain the constant temperature difference is measured directly by the hot wire method the
proportional to the air flow velocity, temperature measurements are independent of changes in the
and density of the incoming air. density of the air.
The two wire probes usually form part of a Vortex airflow sensor
conventional bridge configuration as shown in Fig This sensor uses the phenomenon of Karmen
3.9(b). In this circuit the hot-wire sensing probe Vortex Street and comprises an air channel with
is used as one of the resistors (RHw)on o n e branch a triangular strut, ultrasonic transmitter and
of the bridge, while the cold-wire resistor (RCw) receiver to detect the vortexes and an electronic
is used on the other branch: when balanced the control and signal processing circuit. The basic
bridge voltage is zero. The unbalanced voltage of structure of the sensor is shown in Fig 3.10.
the bridge is amplified and fed, via a transistor,
back into the bridge circuit, so that when the When the inlet air passes through the air chan
temperature of the hot-wire falls and its resistance nel vortexes are generated in the wake of the
decreases, the circuit acts so as to increase the triangular strut, the frequency of which is nearly
voltage applied to the bridge and so increase the proportional to the airflow velocity in the air chan
current in the hot-wire resistor, thereby raising nel, therefore air velocity (volumetric airflow
its temperature to the correct level. The control quantity) can be measured by measuring the vor
circuit is constantly balanced and maintains the tex frequency.
relationship:
In this type of sensor bymorph-type ceramic
R2 RHW = RI R-CW ultrasonic transducers, composed of titanate-zir-
conia-lead (PZT), are used for both the ultrasonic
It is important that the controlled current flow transmitter and receiver. The control circuit uses
is not affected by air temperature, but depends a feedback oscillator operating at about 40 kHz to
solely on the airflowvelocity. Therefore the resist energise the ultrasonic transmitter. The ultrasonic
ance of the hot-wire probe is proportional to the wave from the transmitter passes through the air
difference between the hot-wire velocity tem flow containing the vortexes before it reaches the
perature and the cold-wire temperature, giving receiver. Thus the frequency and amplitude of the
full temperature compensation. The resistor for ultrasonic waves are periodically varied (modu-
20 Automotive Electronic Systems
To engine
Bypass channel
for regulating
air flow
, Filter
element
Air
cleaner
(a) Temperature
sensor
Fig 3.10 Vortex airflowsensor
(a) Cutaway (b)
(b) Cross-section
lated) by the strength of the vortexes. The received Air pressure sensing
wave is demodulated and converted into an alter
nating signal or rectangular waves (pulses), the Measurement of this parameter permits detection
amplitude of which corresponds to the circulating of either a moderate air leak or a sudden failure
strength of the vortexes and the frequency of in the air supply of an air braking system, as well
which corresponds to the number of vortexes gen as monitoring the working pressure.
erated. The output frequency is around 30 to
50 Hz at idling condition, and reaches 1.0 to Aneroid LVDTpressure sensor
1.5 kHz at full load. The metal diaphragms provide an almost linear
displacement with gauge pressure. The aneroid
This airflow sensor as described above, meas capsule vents to atmospheric pressure and drives a
ures volumetric airflow rate, so that output data linear variable transformer, or other device, which
have to be converted to the mass airflow rate for outputs a voltage proportion to gauge pressure.
application to a fuel metering system. An inlet
air temperature sensor is incorporated for this Potentiometric sensor
purpose. Also a barometric pressure sensor would This is widely used since it is simply a diaphragm
be used for the correction if operated at high driving a potentiometer. Although it is of low cost
altitudes. and has a high level output, it has moving parts
that wear out, change its calibration and cause
Air and oil pressure sensors failure. The above two sensors are now being
replaced by more reliable sensors that have fewer
Many of the sensing methods described earlier moving parts and no frictional contact.
can be used for air and oil pressure sensing.
Sensors 21
Capacitive pressure sensor Fuel flow rate sensors
In the common approach, the capacitive pressure
sensor is circular and composed of an aluminium Accurate measurement of fuelflowrate is essential
diaphragm bonded to a thicker aluminium sub- to a real time computation of economy, and par-
strate to form the capacitive plates. A conductive ameters of driver information systems. However
electrode is screened on each plate. When pressure a measurement of fuel flow rate is not really poss-
is applied to the diaphragm it deflects towards the ible in the diesel engine, where the fuel flow rate
substrate, causing decrease in capacitance. does not correspond to the actual amount of fuel
used - a substantial amount of the fuel supplied
Semiconductor pressure sensors to the injectors is for cooling purposes and a
This sensor uses a silicon diaphragm and diffused return fuelflowsystem is employed. Therefore the
piezoresistors, instead of a discrete potentiometer. methods described here are only suitable for petrol
When pressure is applied to the diaphragm the engine vehicles.
resistance changes in proportion to the twisting
or bending force on the material. See absolute Turbine flowmeter
pressure sensors for full details.
The turbine flowmeter has a long history of use
Thick-film pressure sensors for accurate measurement of liquid flow and, only
Similar in design and construction to the semicon- recently, has been adapted for automotive use.
ductor diaphragm type, but instead of using a This type of sensor uses a turbine whose impeller
silicon diaphragm and diffused piezoresistor, its (rotor) is activated by fuel flowing tangentially
diaphragm is aluminium and the resistors are (Fig 3.11(a)) or axially (Fig 3.11(b)) through it.
thick-film piezoresistor (mainly glass and metal
oxides). The thick-film material's resistance The sensor is fitted into the fuel feed line to the
decreases in compression and increases in tension carburettor and produces afixednumber of pulses
as the diaphragm deflects under applied pressure. for every litre of fuel passing through. Generally,
The relative change in resistance is linear in the flowmeters work on the light interruptor principle,
whole range of the applied strain. such that as the fuel flows through the turbine
Photo-source ^—^ Photo-detector
Fuel in ► ** Fuel out
Oil pressure sensing Tangential Turbine
fuel flow
Oil pressure is one of the most important variables
to be measured on an engine, ideally requiring the (a)
use of a continuous oil pressure sensor rather than
a pressure switch. The sensing methods used in 4Turbine A Fuel out
oil pressure sensors are similar to those used in air
pressure sensing, although oil pressure sensors .4 >>'>)>>> k ^ °
require a higher temperture range (130°C) because Photo-source
the sensor is mounted directly on the engine block. Photo-detector
Also the pressure required to give full scale
deflections are different, typically lower in the oil Fuel in
pressure sensor. The various pressure ranges are
obtained by changing the geometrical dimension (b)
of the diaphragm. The output from either the oil
or air sensor can be a voltage or resistance change. Fig 3.11 Turbine flowmeter
(a) Tangential
(b) Axial
22 Automotive Electronic Systems
unit it causes the turbine impeller to rotate and simplest concepts for this sensor are a rotary type
interrupt a beam of light transmitted by a LED hydraulic motor gearpump or a piston moving in
and received by a phototransistor. The number of a chamber.
pulses produced is proportional to the amount of
fuel flowing past the turbine. The voltage pulse Piston flowmeter
output is a square wave switching between earth
(OV) and vehicle voltage (12V). Consists of four radial pistons connected to the
same crankshaft, each piston acting as a valve
The axial turbine attempts to solve the problem mechanism, directing fuel flow to and from the
of bubbles being recognised as fuel by using the adjacent pistons, with the magnitude and direction
fact that light may be refracted through the fuel. of rotation of the crankshaft being sensed by a
When the turbine is full of fuel the sensor is photoelectric transmitter or magnetic tachometer,
aligned so that the light beam is refracted through Fig 3.13.
the fuel from LED to phototransistor. When a
bubble occupies the space between consecutive
impeller blades the light is not refracted enough
to reach the phototransistor, so to the sensor it
appears that the turbine has not turned. Large
bubbles are therefore not counted even though
the rotor turns when they and fuel pass through.
Ball-in-race flowmeter Fig 3.13 Four piston positiveflowrate sensor
The ball-in-race flowmeter uses an opaque ball
which is propelled around a toroidal channel by Gear flowmeter
the fuel being measured. Each time the ball passes
around the toroid it interrupts a light beam, gen- In this concept a gear pump converts the flow
erating a pulsed output signal as in Fig 3.12. measurement into a differential pressure measure-
ment. The gear pump is in line with the fuel flow
Interrupter and is driven by the fuel at such a speed that the
ball —— average differential pressure across the pump is
zero. Under this condition the pump becomes a
Photo-source positive displacement hydraulic rotor. To obtain
Toroidal the output signal the rotational speed of the shafts
passage is sensed by a magnetic pick up and is a measure
of the fuel flow rate.
uel|t| III Fuel
in T out
Fig 3.12 Ball-in-raceflowrate sensor
Displacement flowmeter Motion sensors
The general operating principle of this type of Many automotive functions to be controlled are
flowmeter is the charging and discharging of a concerned with motions:
fixed volume of liquid in a chamber. The meas- 1 Crankshaft rotation, where it is necessary
ured frequency of the cycle of operation is mul-
tiplied by the volume to give the flow rate. The to know both rotational speed and angular
position
Sensors 23
2 Road wheel speed, where both absolute Optical sensors
and differential values are needed if wheel
lock is to be detected Component speed and position can also be sensed
3 Transmission shaft speed, where the using optical methods. If an aperture disc is
information is used to select transmission positioned between a light source and a photo-
ratios or maintain a constant cruising electric detector then a signal will be generated as
speed or provide trip information for the the disc is rotated and the detector is alternatively
driver. illuminated and blocked. The light need not be
A number of attractive technologies have been visible and can range from infra-red to ultra violet.
used which meet the stringent automotive require- Generally, a LED and phototransistor are used as
ments. All have some advantage and disadvantage light source and detector. The resultant signals
which affect their quality of performance and ser- are timed reference and may be used to sense
viceability. position, flow and speed of rotation. There are
two limitations, however, that must be taken into
account when testing such sensors for faults or
when checking their calibration co-efficients.
Hall effect sensors First, as both light source and detector are
semiconductors there is a temperature limitation.
When used to sense speed or position a Hall Second, there may also be the problem of obscur-
semiconductor element may be placed between ation due to accumulated dirt.
the poles of a permanent magnet system. When a
current flows through the semiconductor element Wiegand sensors
a Hall voltage is generated at right angles to the
current flow and perpendicular to the direction of This is an interesting type of sensor which is
the magnetic flux. The value of the Hall voltage now finding wide application in vehicle electronic
is proportional to the product of the magnetic field systems. Named after the person who discovered
strength and the current. If one of these quantities and announced it (John R. Wiegand), the Wiegand
is held constant then the Hall voltage generated Effect is the name given to a magnetic phenom-
will be directly proportional to the other variable enon which occurs in a specially work-hardened
quantity. In motion sensing it is the strength of small diameter ferro-magnetic wire. The wire is
the magnetic field that is varied, and the control approximately 0.3 mm diameter, drawn in such a
current is held constant, hence the Hall voltage is manner as to give it a soft magnetic core and a
proportional to the magnetic field strength.
hard magnetic outer sheath. The whole structure
The Hall sensor is wear free and produces a is then subjected to a longitudinal twisting force,
constant output voltage throughout the system's to make it a bistable magnetic device; that is, it
life. Also, the output voltage remains predictable can exist stably in either of two magnetic states due
over a wide frequency range although the accuracy to the properties locked in by the stress patterns
of the voltage is dependent upon the constancy of created during the twisting process. The direction
the magnetic field and the exciting current. The of magnetisation changes quite abruptly when the
Hall sensor can detect zero motion and is ideal for wire is exposed to a sufficiently strong external
application to odometer systems, driver infor- magnetic field.
mation systems, and ignition timing position sys- In its original state both core and sheath are
tems. The generated Hall voltage is relatively weak polarised in the same direction and this is main-
so it is normally amplified and processed by elec- tained in a field free condition due to the per-
tronic devices built into the Hall semiconductor mantly magnetised sheath (Fig 3.14(a)). When the
chip. wire core is exposed to an opposing magnetic field
24 Automotive Electronic Systems
the soft magnetic centre magnetically switches When testing it must be remembered that a
(Fig 3.14(b)), but returns to its original direction change of flux polarity is required. A change of
when the magnetic field is removed. In a typical field strength of the same polarity will not produce
Wiegand sensor arrangement (Fig 3.14(c)) a sens a signal.
ing coil is wound around the Wiegand wire and
allows the switching field to be detected. The The Wiegand Sensor is an effective transducer
multi-fingered vane rotates in a horizontal plane for providing digital pulses in response to motion
to interrupt the magnetic field and cause the Wie or displacement, typical automotive applications
gand wire to switch. The lower permanent magnet include: ignition triggers; speed signals for speed
provides the positive saturating flux to the Wie ometer and tachometer drive; crankshaft speed
gand wire. The upper magnet, whose field the and position; flowmeter; wheel sensor for anti
skid systems; and transmission control signals.
(a) (b) Inductive sensors
Multi-fingered Sensing coil An inductive sensor consists of a sensing coil and
vane «^* Wiegand wire a permanent magnet, forming a closed magnetic
circuit (Fig 3.15(a)). The permanent magnet cre
ates a magnetic flux in the magnetic circuit, the
value of which depends on the field strength of
the magnet and on the reluctance of the magnetic
circuit. When the air gap of the magnet is bridged
by the finger of a vaned rotor or similar impulse,
the reluctance is reduced and the magnetic flux
increases (Fig 3.15(b)).
This induces a voltage across the sensing coil,
the polarity of which depends on the direction of
the change of magnetic field, controlled by the
(O Vo mVo
mSensing
Fig 3.14 Wiegand effect and sensor
(a) Wiegand wire in unswitched state coil >h^_
(b) Switched Wiegand wire
(c) Typical Wiegand sensor arrangement
fingers of the vane interrupt, provides the negative Έ s\Steel' X
saturating flux. This symmetrical magnet field
arrangement generates an output signal with equal Permanent disc Magnetic
positive and negative pulses. The positive pulse magnet flux flow
occurs when the leading edge of a vane finger
enters the slot of the Wiegand module and the (a) Air gap, high reluctance to (b) Metal disc between poles
negative pulse occurs when the trailing edge leaves magnetic flux flow, no of magnet, low reluctance
the module. A typical sensing coil has around 1300 induction in sensing coil and magnetic flux flows
turns and the induced potential, on open circuit, round the circuit inducing
is between 2.0 and 2.5 V, with a pulse width of a voltage into the sensing
twenty microseconds at half amplitude. The value coil
of the induced voltage is almost independent of
the rate of change of flux reversal. Fig 3.15 Basic structure and principle of an inductive
sensor
(a) High reluctance to magneticflux,no induction in
sensing coil
(b) Impulsefingercuts air gap, low reluctance and hence
an induced voltage in sensing coil
Sensors 25
polarity of the permanent magnet, the winding
direction of the coil, and whether the reluctance
is increasing or decreasing. Note the rotational
direction of the impulse wheel does not influence ->I-H=>T-P>~^
the polarity of the output voltage.
Current
Since the value of output voltage is proportional -L. amplifier
to the number of winding turns and the rate of
change of the magnetic flux. This means that the Output
output voltage of the inductive sensor is pro voltage
portional to the rate of transition of reluctance Fig 3.17 DC excited inductive sensor
and, hence, the speed with which the impulse
finger cuts, or even passes close by, the air gap of speeds or fast changes of rotational angles have to
the magnet. be measured. Currently they are used for diag
Permanently magnetised inductive sensors such nostic and service adjustments of ignition angles
as described here are common, but other versions and injection timing.
of inductive sensors are also used.
Quenched oscillator speed sensor
AC excited inductive sensor This sensor is basically a pulse generator, con
If a toothed impulse wheel is rotated close to sisting of an inductive sensor, control circuit and
an inductive sensor, whose coil is excited by an a four pole rotor which is connected to a vehicle's
alternating current, there will be a periodic change drive shaft (Fig 3.18(a)).
in the coil's impedance and the AC signal through The oscillator produces a high frequency (3 to
the coil will be modulated. This modulation may 4 MHz) signal whenever a voltage is produced
be used to determine rotational angle and speed. across the sensing coil of the inductive sensor.
Fig 3.16 shows a typical AC excited inductive Thus, as the rotor blades pass the sensor the
sensor arrangement. This type of sensor has the
advantage of being able to detect zero motion. Four pole
rotor
Inductive sensor
Multivibrator
-W- ! i-t>M>i—■
Output
_j Output
^ voltage voltage
(a)
Fig 3.16 AC excited inductive sensor High frequency signal 3to4MHz Square wave
output ^
DC excited inductive sensors Filter
Magneticfieldstrength in this sensor is controlled Quenched signal
by passing direct current through the coil, allow
ing control of the output voltage to a set maximum (b)
value, irrespective of impulse speed (Fig 3.17).
Fig 3.18 Quenched oscillator speed sensor
These sensors are used where high rotational (a) Typical arrangement
(b) Resultant output signal, before and after filtering
26 Automotive Electronic Systems
oscillator is turned on and off i.e. the high fre Rich mixture Lean mixture
quency signal is quenched as each rotor blade (lack of air) (excess air)
passes the sensor (Fig 3.18(b)). A filter filters out
the high frequency signal to produce a regular ^2300 \\ \ C H / . N\
square wave pulse.
CL co\\\···'
Feedback sensors
/ ' y
Closed-loop control using feedback makes it poss
ible to maintain engine variables very precisely: ODc V· V■ CH Hydrocarbons
in particular the point of ignition and air-fuel σ CO Carbon monoxide
ratio - both of which have a critical effect on fuel A NOx Oxides of nitrogen
economy and exhaust gas emission levels. Two υ NO/*/ \\ ·. / / ..N·>·;' O2 Oxygen
main types of feedback sensors are used: lambda \\ • • A . . · '
oxygen sensors - to create feedback signals regard > V/
ing instantaneous mixture composition; knock υo t / / \
sensors - providing feedback signals when det '— - I— I ^* I x
onation occurs.
0-8 0 9 10 1-1 1.
Excess-air factor λ
(a)
Rich mixture Lean mixture
mV (lack of air) (excess air)
1000-
> 800-I
en ,
3 600-j
Lambda oxygen sensor σ^) 200-j
With this type of sensor signal generation is based 0-8 " ^ 10 1M1 1-2
on measuring the residual oxygen in the exhaust Excess-air factor λ
gas. The name originates from the Greek letter
(λ) which is used to denote the equivalence ratio (b)
or excess air factor.
Fig 3.19 Excess airfactor and
Power, fuel and exhaust gas composition of the (a) Effect on exhaust gas composition
SI engine are all dependent on the composition of (b) Typical lambda oxygen sensor output voltages
the air-fuel mixture. Complete combustion occurs
with an air-fuel ratio of approximately 15:1, or its cylinder gases and λ. If an engine is supplied with
equivalent ratio (λ= 1), where: the correct mixture of petrol and air (CH + 02N2),
the exhaust gases should consist of a mixture of
actual quantity of air carbon dioxide, water vapour and nitrogen
λ = -theoretical air requirement (CO2 + H2O + N2). Due to various factors com
bustion may not be 100% efficient and oxygen
or will be present in the exhaust gases.
λ = - actual air-fuel ratio
theoretically correct air-fuel ratio (14.7:1) Sensors which measure the amount of oxygen
present in exhaust gases, and hence determine the
so that when the actual air quantity or air-fuel air-fuel mixture, are known as exhaust gas oxygen
ratio supplied to the cylinder equals the theoretical (EGO) sensors and generate a voltage (Fig
requirement λ will equal 1 (100% combustion 3.19(b)), dependent on the air-fuel ratio which
efficiency). It then follows when insufficient air (a can be used to feed back information to enable
rich mixture) is supplied λ will be less than 1, and virtually complete combustion to take place. Two
with excess air (a lean mixture) λ will be greater types of material are used for EGO sensors; zir-
than 1. conia oxide (Zr02) and titanium oxide (Ti02).
Fig 3.19(a) shows graphically the relationship The operation of EGO sensors depends on the
between levels of various compounds within the
Sensors 27
porosity of the material which allows diffusion of by the oxygen content. The exhaust gas oxygen
oxygen ions in the exhaust gas. When a difference partial pressure for a rich mixture varies over the
occurs between two sensing plates a voltage is range 10"16 to 10 ~32 of atmospheric pressure. For
generated. This voltage changes sharply at λ = 1 a lean mixture this is around 10"2 atmospheric
and is the basis of the sensor's feedback signal. pressure. If the oxygen proportions at the two
sensing probes are equal the output voltage will
Zirconia oxide be zero. But, when the oxygen content at each
Zr02 probe differs, a voltage is generated across
The principal operation is based on the ceramic the two. The electrolytic action which generates
material's ability to conduct oxygen ions which this voltage is shown in Fig 3.20(a), where the
accumulate on the Zr02 surface. Two sensing negative oxygen ions under atmospheric pressure
probes are used, one is reference to the oxygen permeate the porous electrode and accumulate on
content of ambient air at normal atmospheric its surface, giving it a negative charge the size of
pressure; the other is exposed to the exhaust gas which is dependent upon the amount of oxygen
and senses that part of the gas pressure caused in the exhaust gas.
Exhaust gas Ambient air Titanium oxide
Titanium oxide exhaust gas sensors need no ref
Electrodes (a) erence to ambient air. Titanium oxide is a
semiconductor with a very high resistivity at room
v .Exhaust gas. ,i temperature when it is pure and in air. Loss of
L^Ö2Exhaust pipe oxygen results in molecular vacancies which act as
electron donors. As the oxygen content decreases,
: Porous more electrons become 'free' for electrical con
protective coating duction, and the material's resistivity decreases.
At low oxygen concentrations the Ti02 material
-Electrons free is an n-type semiconductor. At ambient air oxygen
for conduction concentrations the material has maximum resist
ance. Beyond this point the material becomes a p-
Tsei0m2.iconductor type semiconductor. Thus, the resistance of the
TiOz oxygen sensor changes very rapidly when
the air-fuel ratio changes from rich to weak and
vice versa.
The Ti02 sensor (Fig 3.20(b)) requires some
form of temperature compensation. In practice
two series elements are used
1 the oxygen-sensing porous Ti02 ceramic
semiconductor
2 a densified Ti02 ceramic used as a
matched thermistor for temperature com
pensation.
(b)
Fig 3.20 Principles oflambda oxygen sensor usedEaGsO sensor construction
exhaust gas oxygen (EGO sensors) Both types of sensors can be positioned into
(a) Zirconia oxide EGO sensor exhaust manifold or tailpipe, and are generally
(b) Titanium oxide EGO sensor
heated to normal operating temperature by the
28 Automotive Electronic Systems
exhaust gas. The voltage and internal resistance into the inside of the Zirconia element, make it
of the sensors are temperature dependent and possible for the EGO sensor to be installed in
reliable operation typically occurs at temperatures any position of the exhaust system. The heating
over 300°C. The construction of both types of element is designed to operate at a nominal 13
sensors are shown in Fig. 3.21. volts and provide rapid heat-up of the sensor
from cold. Power consumption is about 9 watts at
Sensor temperature is a decisive factor with 850°C.
respect to the quality of emission control. The
sensor must be positioned in the exhaust system
Knock sensors
The charge of air-fuel mixture in the engine
cylinder should burn quickly to permit optimum
performance and high speed but, equally so, it
should burn progressively, smoothly and not too
quickly to create early detonation i.e. knocking of
the engine. There are many factors which may lead
to knocking combustion; different fuel qualities;
advanced ignition timing; aging and environ
mental effect. A knocking combustion process
leads to an extremely sharp rise in cylinder press
ure and produces pressure oscillation. Depending
on its intensity the piston and crank components
may be overloaded, or the engine may overheat
and lead to serious mechanical damage. The use
of knock sensors enables an engine to run on the
threshold of 'knock' and should knock occur the
sensor feeds the information back to an electronic
/ 1 Ceramic control unit which retards the ignition timing to
Steel protection tube Lead wires cement
reduce the knock detected.
Various technologies have been used to sense
(b) the vibrations associated with knocking pressure
Fig 3.21 Construction ofexhaust gas oxygen sensforresquencies and convert them into electrical volt
(a) Zirconia oxide age signals. For automotive use accelerometer sen
(b) Titanium oxide
sors appear to be the most suitable: where piezo-
ceramic materials are used as the active elements
so as to ensure that the sensor's active element to measure the structure-borne vibration of the
reaches 280-300°C within 30 seconds of engine engine. Their high natural frequencies and almost
start-up. To achieve this the sensor is usually linear sensing of the vibration frequencies up to
very close to the engine exhaust manifold but the more than 15 kHz provide ideal information that
position must also ensure that element does not can be analysed for knocking.
exceed 850°C or severe aging of the electrode will
occur. In many engines it is difficult to find the The piezo-ceramic active element usually con
optimum installation position to give a tem sists of lead, zirconia and titanium which gives a
perature range between 500 and 800°C over the piezo-electrical activity up to 360°C. Typical
entire operating range from idle to full load. The sensor construction is shown in Fig 3.22. This
introduction of a heating element which projects style of construction enables the sensor to be easily
attached to the most favourable point (a knock
Sensors 29
Connector to the electronic control unit, which decides
Spring/preload whether or not to retard the ignition point of next
combustion in a particular cylinder.
seismic mass Fluid level sensors
Piezo electric Fluid level sensors are used to sense information
crystal about all fluid containers on the vehicle.
Resistive types
Fig 3.22 Construction of piezoelectric accelerometer, A common application is thefloat-typelevel trans
mitter using a thick film resistive track and wiper.
knock sensor Fig 3.23(a) shows a typical sensor. Fig 3.23(b)
shows circuits for low fuel warning, incorporated
resonant site) on the engine for measuring knock. within the sensor itself. When there is sufficient
Where one sensor is used it is positioned between
the centre two cylinders. More than one sensor Outlet Palladium silver
would be placed between two cylinder groupings. conductor and resistive link
Piezo-electric devices produce a voltage when Pick up
the ceramic crystals are subjected to mechanical pipe
stress variations. This coupling between mech
anical and electrical energy generates voltage Thick film Filter
across opposite faces, proportional to the cause of ceramic tile
the stress variations. The seismic mass of the
sensor is initially torqued down to pre-stress the (a)
piezo-electric sensing element. This allows the
piezo-electric crystal to also act as a spring. As a Track«
knock occurs in the engine cylinder, it is trans
mitted via the engine block to the sensor, causing Wiper-* ϋ — \ ' 1200n | J 1200n
the seismic mass-piezo-electric crystal spring
arrangement to vibrate in sympathy. Resultant Low Adequate
deformation of the piezo-electric crystals gen (b)
erates an output voltage proportional to the orig
inal knock. Fig 3.23 Fuel volume sensor
(a) Construction
Output voltage is typically 10-25 mV/g giving (b) Arrangement of track resistances to activate low fuel
a voltage sensitivity of 25 mV for every 9.81 m/s
of acceleration (g). The sensor is mounted by the warning
integral stud and to maintain sensitivity maximum
torque values must not be exceeded. fuel the resistance between ground and the ter
minal to the control circuit is 180 ohms. This value
Pressure vibration frequencies other than the changes to 1380 ohms, however, when the fuel
typical knock frequency are also contained in the level is low - the control circuit detects this large
structure-borne vibration signal. The charac change and issues a low fuel warning to the driver,
teristic knocking signal must be electronically when about 5 to 8 litres of fuel are left.
extracted from other signals, then analysed by an
evaluation circuit, the result of which is supplied
30 Automotive Electronic Systems the dielectric material and the sensor will give a
'full' capacitive reading.
Fixed ^ ^ ^
housing^^*" When the fluid is below this level, the capaci-
tance element will be partially or totally uncovered
Potentiometer Actuator Moving and surrounded in part or total by air, so pro-
wiper piston piston and ducing a change in capacitance. Fig 3.26 shows
slider ^ the construction of such a sensor.
Air exits
(a) (b)
Fig 3.24 Linear deflection sensor
(a) Principle (b) Construction
Fig 3.24 shows the principle and typical con- Measuring
struction of a resistive sensor for measuring the capacitor
linear deflection of a vehicle, which may be used
as input data for a headlamp levelling system - Liquid entry
useful if the vehicle is loaded. Fig 3.25 shows the
principle and construction of a resistive sensor Compensation
designed to measure angular position. capacitor
Common connection
Wiper arm Thick-film
resistor track Fig 3.26 Capacitive level sensor. Capacitance is
proportional to the level of the fluid
Angle of AC impedance type
deflection
This type of sensor relies on the conductivity of
(a) (b) the fluid whose level is being measured. An AC
signal is transmitted between a transmitter and
Fig 3.25 Angular position sensor receiver in the sensor, so that the received signal
amplitude is a measure of fluid level. Fig 3.27
(a) Principle (b) Construction shows construction. Service checks should ensure
that the resistance of the fluid does not change
Capacitive types substantially due to additives or contaminants and
so give false signals.
The capacitive fluid level system has many advan-
tages, not the least it is totally solid state, having Hot wire type
no moving parts. Two electrically insulated tubes
or probes act as the plates of the capacitor, and The application of an electric current to a hot
the operating principle is based on the difference wire heating element produces a temperature rise
between the dielectric constants of the fluid and dependent upon the heat dissipation into the sur-
of air. When the liquid is at the full level the rounding medium, fluid or air. When covered by
capacitance element is covered and the fluid forms fluid, maximum heat dissipation occurs so that the
Sensors 31
Connector
7 ohm
resistance wire
Transmitting
probe
Fig 3.28 Typical oil level (dipstick) sensor
Sensing probe contacts are embodied in the fixed part of the
sensor body, so that a variable reluctance air gap
Fig 3.27 AC impedance level sensor is created and controlled by the level of the fluid.
When the system is full the magnet will be in close
wire is at its lowest temperature and resistance. proximity to the reed switch applying a strong
When not covered by fluid, heat dissipation is magnetic field to close and hold the contacts
reduced and the wire is at a higher temperature closed. As the fluid level falls the air gap increases
and higher resistance value. Sensor details are reducing the strength of the magnetic field on the
shown in Fig 3.28. Hot wire sensors are most reed, until it switches to the open position and
commonly used for engine oil monitoring, where provides the low level output signal. Fig 3.29
a short application of current, immediately upon illustrates a typical sensor used for fluid level
turning the engine on, indicates whether sufficient sensing in windscreen washer tanks.
oil is present. Service checks should take into
account the following factors: In the body of the sensor are two resistors,
1 the switching point from high to low connected in such a way that when the switch is
closed (float up) the resistance between the pins
levels is dependent upon temperature of in the connector is 180 ohms and when the switch
the wire
2 the switching point may also be affected Reed switch (closed)
by the moisture content of the oil and
deposits on the sensor Float up
3 accurate monitoring also depends upon (fluid level high)
low resistance cables and terminal con-
nections. Magnet
Reed switch (open)
2A
Reed switched type Float down
(fluid level low)
In this type of sensor a permanent magnet is Fig 3.29 Construction of typical low wash/coolant fluid
fixed to a fluid operated float. The reed switching level sensor using float and reed switch principle
32 Automotive Electronic Systems
is open (float down) the resistance is 1380 ohms. For low screenwash, the switch operates with
One pin in the connector is earthed to the vehicle about one quarter of thefluidleft in the container.
body locally via the wiring harness, the other is For low coolant, the switch operates when the level
taken to the control assembly input. in the overflow container is just below minimum.
Actuators
Actuators are the devices in a control system which plunger-type of solenoid is illustrated in Fig
convert an electrical input to a mechanical action. 4.1(b). Another type of solenoid, theflat-facetype,
They provide a controllable force under command is illustrated in Fig 4.1(c). Typical applications of
of the control unit. There are two basic types - solenoids in automotive systems are as fuel cut
solenoids and motors. off valves (Fig 4.1(d)) or fuel injector valves (Fig
4.1(e)).
Solenoid The main disadvantage of the solenoid is due
to the fact that the force produced by the linear
The solenoid is an electromagnetic device that movement is proportional to the square of the
produces a linear (straight line) mechanical force.
The input signal produces a current flow in the Solenoid
solenoid coil, which in turn creates a strong mag winding
netic field within and around the coil. This mag
netic field applies an attractive force to the metal
armature, pulling the armature into the centre of Force
the coil. A spring forces the armature back out
when the input signal is removed.
Fig 4.1(a) illustrates the operating principle of (b)
such a basic solenoid. Construction of a typical
Solenoid magnetic field Flux flow Control spring
polarizes the armature
Soft iron
' armature
' I iχ
V in s ::z:g Axis of force
+ ve o —
I ■'ΖΐχόάfcοOόOοOάOόOοO' C
Attractive force Solenoid 5*
winding
Magnetic
attractive force
(a) (O
Fig 4.1 Solenoid actuator
(a) Principle (b) Plunger-type solenoid cross-sectional construction
(c) Flat face-type cross-sectional construction
34 Automotive Electronic Systems Current
Engine Voltage > V ^ 1
side
Fuel cut-off
solenoid
valve assembly
~CZZ7 Activation period Holding period
(f)
Solenoid (d) Filter
plunger Solenoid
Fuel
injector
(d) Fuel cut-off solenoid valve
(e) Cross-section offuel injector valve
(e) (f) Solenoid characteristics
distance between the core and the magnetic pole Moving armature
piece. This limits solenoid use to applications
requiring stroke distances of not more than 8 mm Alternative contact Rexible spring strap
or so. Another disadvantage is the time taken for position
the solenoid to operate - the activation period.
Both disadvantages are illustrated in Fig 4.1(f). Battery Ct o-
Circuit C2 o-
Service checks should ensure that all operating
lengths and gaps are within the specifications, as Solenoid Iron core
well as the electrical factors. winding Frame and iron path
Magnetic flux for flux
Relays when current is
switched on
A relay is an important part of many electrical
control systems because it is an indirectly operated (a)
electrical switch that is useful for remote control
and to control high current devices with a low Glass e -oC2
current control signal. envelope Circuit
Precious metal
The relay is basically a solenoid with a fixed Battery coated contacts Nickel iron
core, in which the magnetic force is used to pull reeds
down an armature towards the core, and through w,
the use of mechanical levers force electrical con
tacts to close. When the coil is demagnetised a (b)
return spring opens the contacts (Fig 4.2).
Fig 4.2 Two types of magnetic relays, using the solenoid
principle
(a) Basic relay in cross-section
(b) Cross-section of reed relay
Actuators 35
Some relays have normally closed contacts - range of powers. For this discussion, however, we
the magnetic force in such devices opens them. will look only at motors of about 100-150 watts,
which include the majority of instrumentation and
Electric motors servo applications.
There are three main types of DC motors:
The electric motor is probably the most versatile 1 Iron-cored
of all actuators used in automotive control 2 Ironless rotor
systems. Like solenoids, the basic operating prin 3 Printed circuit.
ciple is of an armature moving, owing to a mag
netic field. Unlike solenoids, whose armatures Iron-cored motor
move linearly, motor armatures generally rotate. This is the traditional type of motor construction
When coupled with low cost solid state control in which copper wire is wound onto an iron
electronics a wide variety of motor-controlled pro armature. The iron armature both supports the
portional actuators are possible. In practice there armature coil and concentrates the magnetic field
are a number of different motor types that are created by passing current through the coil. The
commonly used in actuating systems. permanent field magnets are located around the
DC motors circumference of the motor, and commutation is
by carbon brushes onto a copper commutator.
The basic DC motor construction is shown in Fig These motors are most commonly used in drive
4.3. All motors work on the basic magnet principle rather than servo applications. Fig 4.4 shows an
that like poles repel and opposites attract. In the iron-cored DC motor whose field magnet is, in
case of the DC motor a main stationary magnetic fact, created by a coil.
field is produced by two permanent magnetic pole
pieces. Another magneticfieldis produced around Ironless rotor (coreless) motor
a current-carrying armature winding. The two In this type of motor a central cylindrical per
manent magnet is used around which a cylindrical
coil of wire rotates. High grade precious metals are
used in the brushes and commutators to mimimise
contact resistance and friction. With no iron in
the rotor there is low inertia, high efficiency, good
performance-to-weight ratio and excellent servo
characteristics.
Fig 4.3 Basic structure ofa DC motor. Armature Pshrianjtted circuit motor
used to activate control mechanisms These motors comprise a disc of insulating
material onto which a circuit of resistive con
magneticfieldsystems oppose each other resulting ductors is printed. Carbon brushes contact with
in a force being exerted on the armature, causing these conductors, and pass the current to create a
the armature to rotate. In order that the armature's magneticfieldwhich interacts with radially placed
magnetic field is always in the correct direction to permanent magnets. The printed disc has as many
maintain armature rotation, a commutator and commutator bars as conductors to give a very
brushes arrangement, illustrated in Fig 4.3, is used smooth rotation, and no torque variation during
to switch the direction of armature current during its revolution. These motors create very good
rotation. DC motors can be made in a very wide medium- to high-power drive and servo motors.
36 Automotive Electronic Systems
Direction of Linear DC motors
rotation The main disadvantage of conventional DC
Armature Commutator motors is that the speed of rotation has to be
reduced and converted by a gearbox mechanism
Armature (Fig 4.5(a)) to provide a linear output. Fig 4.5(b)
winding
Motor pinion
Field
Brushes
(a)
Rotating armature Commutator
winding χ / and brushes
oinRotation E l f f l ä ö 3 8 -
Permanenter
magnet * L
(b)
Threaded shaft Feedback
closed-loop
♦ ■+ H\\iW\\\| Lead screw |\\\\\\\\\\\\\>\\\\\v Potentiometer —/. Fig 4.5 Linear DC motor actuators
(a) Greer mechanism
0—<l· Controller Trigger (b) Typical actuator
signal
DC motor shows a small electric linear actuator which com
driving lead screw prises a low voltage DC motor and short gear train
driving a lead screw to provide the linear output,
(0 primarily designed for operating flaps in a heater
or air conditioning unit.
Fig 4.4 Typical types of DC motors
(a) Iron-cored motor One interesting development is the adaptation
(b) Ironless motor of a DC motor to provide linear motion directly,
(c) DC servo motor system over a long stroke with a constant mechanical
force. Fig 4.6 shows the operating principle of
DC servo motors such a moving winding arrangement; in which the
The servo motor makes use of a variable DC stationary centre core is a high flux permanent
voltage for speed control. A potentiometer driven magnet and the moving winding is wound around
by the motor provides speed and position feedback the actuator piston assembly. When current is
to form a self-contained positioning system. An applied to the coil it creates a magneticfieldsystem
example is shown in Fig 4.4(c). These motors are of the same polarity as the magnetic core. The two
used where precise position control is necessary magneticfieldsystems interact, resulting in a force
along with high-speed operation.
Actuators 37
good torque when no power is applied, though the
torque characteristic may be a limitation because it
Force is inversely proportional to speed. If the motor is
switched instantly from stop to full speed it will
stall, therefore speed must be progressively
increased. One other characteristic to bear in mind
Fig 4.6 Moving winding linear DC motor is their resonance point - at a low speed when the
torque is drastically reduced it may stall. Also, a
special driver unit is required with two input
on the winding causing it to be expelled away terminals; one to receive the driver pulse train and
from the stationary magnet in a linear direction. the other to designate the direction of rotation.
Stepper motors The most common type of stepper motors in
automotive use have four phases giving five con
In a stepper motor many field windings are nections, one common positive supply and four
switched in rotation around a permanent negative returns. The electronic control unit
magnet armature. The armature follows in small earths combination ofthe returns causing the rotor
incremental steps. Fig 4.7 shows the basic oper to rotate in 7.5° (48 steps/rev) steps with up to
ating principle of stepper motors, illustrated with some 150 combinations depending on the number
mechanical switches to switch current to the field of revolutions made by the rotor. A motor working
coils. In a practical stepper motor switching is on 48 pulses to rotate the spindle through one
electronic and digitally controlled. Rotational revolution requires 96 pulses per second to operate
speed depends on how fast the magnetic field is at a speed of 120 rev/min.
incremented or rotated round the field system.
Output movement from a stepper motor can be
Stepper motors have a good stability and also rotary or linear, and it is often the case that the
motor is not required to run at the optimum speed
ICode SI 52 S3 S 4 3®i and torque. In which case a gearbox is used to
1 00 1 1 modify the motor performance to match the load
Mm requirements, and/or convert the rotary action to
2 100 1 a linear one.
3 1100
Rotary solenoids (torque motor)
4 0 1 1 oI This type of solenoid actuator is used to provide
1 indicates switch closed
0 indicates switch opened
rotational motion instead of linear motion.
LJ U Fig 4.8 shows the basic structure of torque
motor which has two windings, which during an
Codel nCode 2 N ( V O actuation period are loaded alternatively with volt
ages to exert opposing forces on the free rotating
n armature. By virtue of the armature's inertia the
Code U Ul. Code 3 armature rotates to an angle directly cor
responding to the ratio of the voltages applied to
s N//t N Π the two windings. It is the ability of this type of
solenoid to combine large torque capability with
'PI very fast response times that make it suitable in
Fig 4.7 DC stepper motor principle many automotive control applications.
38 Automotive Electronic Systems
Electrical connection I Winding 1 Vacuum Solenoids
Housing Winding 2 valve
^^.Air
Permanent b X ] = j = valve
magnet
Armature Diaphragm
Return spring
Rotation Fig 4.9 Typical electro-pneumatic actuator
Fig 4.8 Rotary solenoid actuator (torque motor) and/or partial vacuum in the inlet manifold) is
controlled by solenoid operated valve(s) which
Electro-pneumatic actuator regulate the pressure difference either side of the
This type of actuator is a vacuum operated dia diaphragm, so producing a linear movement. Fig
phragm device, which can move against a return 4.9 outlines the operating principles of a typical
spring. The operating pressure (atmospheric electro-pneumatic actuator.
Generator electronic systems
The satisfactory performance of a vehicle's elec- r
trical and electronic components depends to a
certain extent on a reliable and constantly available Fig 5.2(a) illustrates the principle of AC single
supply of power. It is the task of the generator phase generation. Typically, vehicle alternators
and its associated control electronics to provide use three sets of output windings, spaced at 120°
the on-board electrical power to supply loads and intervals around the armature (Fig 5.2(b)) so that
for storage in the battery. Fig 5.1 shows in block the rotating magnetic field cuts through three
diagram form the generator system. fields and each output winding produces an AC
Trcmsduction Uncontrolled Controlled DC
Mechanical V.. DC t., DC Regulated
input
Voltage DC output
regulation —►
Fig 5.1 Block diagram of a generator system
Stationary Rotating R Y / /* B
magnetic field \
Voutput winding \/ // /\
U;
m .—r_l n V \ *>f/// A \ <\ \ \\\ ///
+ L_J/ i i iN , ♦ i—~H fo \h \
f \ /
\ /
/\
■ ^ $ ;|s /
1•
a (a) 0° 90° 180° 270° 360°
s'$\s> Degrees of rotor rotation
s (O
\?'· Fig 5.2 Alternator voltage generation principles
(a) Simplified single phase alternator. In practice an
(b)
electromagnetic field system is used rather than a
permanent magnet
(b) Arrangement of output windings in a three phase
alternator
(c) Voltage curves over one revolution of rotating
magnet. Connection of the windings results in a three
phase alternating voltage
40 Automotive Electronic Systems
output. The voltage waveforms of such a three- giving rise to this configuration's name: half-wave
phase alternator are illustrated in Fig 5.2(c). To rectifier.
minimise the number of output terminals the three
windings are connected in either star or delta A more efficient method uses two diodes to give
formations (Fig 5.3). full wave rectification, in which both half cycles
are rectified. Use is made of an output winding
In the star connection method the amplitudes with a centre tap to achieve this. During positive
of the total generated output voltage and any half cycles (Fig 5.5(a)) diode Όλ conducts; during
one phase voltage differ by the factor y/3:l (i.e. negative half cycles (Fig 5.5(b)) diode D2
1.73:1). The output current amplitude however, conducts. The resultant waveform is shown in Fig
equals the amplitude of any one phase current. 5.5(c).
Conversely, in the delta connection method the
amplitude of the output voltage equals the ampli W, +r ^->D.li·-
tude of any phase voltage but the output current
is y/3 times one phase current. -Of- - i i Vehicle
Dl( DC? ' bctftery
B! (a)
T-
(a) 3 phase with (b) Star connection (c) Delta connection Stator LA -C*
six line conductors of 3 phase windings of 3 phase wind winding 3|AC
ings
Fig 5.3 Alternator starter winding connections * ►! - *
DJ(ON) I T 1 " Vehide
' D C U - battery
Rectification
(b)
Conversion from AC to DC is known as rec
tification and is done with semiconductor diodes. 'Max Average DC value
Fig 5.4 shows how a single diode allows the posi 63% of maximum
tive half cycle of an alternating current wave to
pass through but blocks the negative half cycle,
■^- Rectifying (c)
diode
Fig 5.5 Full wave rectification ofa single phase
r1 alternating current
(a) Positive halfcycle from Wl conducted by diode Dl:
! Single phase j
! stator winding ; Diode D2 not conducting due to negative bias at W2.
First halfcycle passed through to the battery
Alternator VDC Stator output current (b) Second halfcyclefindsW2 positive and diode D2
before diode conducts current to battery while diode Dl is
negatively biased and is off
i Rotary K-A-j- (c) Single phasefull wave rectification output waveform
! electromagnetic! Average DC value
j field system ! 37*/· of maximum
Vehicle
battery I- Half wave rectified DC
after diode
Fig 5.4 Single phase half wave rectification
Generator electronic systems 41
Another method of full-wave rectification uses 3 phase
a bridge rectifier, without a tapped output winding stator output
(Fig 5.6). During positive half cycles diodes D , windings
and D3 conduct, while during negative half cycles
diodes D 2 and D 4 conduct.
1 * 4AC D| ONIC DC
"^ Vehicle
w, ΐ 4-D» -^r battery (a)
L ON ~- v ,* AveragvealDuCe
Stator
winding
W,
(a)
w, u DC
» ~zp" Vehicle
Stator tj L_t-*— battery
winding \OFF 1 (b)
AC D«i
W, Fig 5.7 Three phase alternator andfull wave bridge
rectifier
(b) (a) Positive halfcycle offirstphase
Fig 5.6 Single phase full wave bridge rectification (b) Output waveform comprising six halfcycles per
revolution
(a) During the positive halfcycle point Wl is positive
and diodes Dl and D4 conduct
(b) During the negative halfcycle point W2 is positive rectify the excitation current and the generator
so diodes D2 and D4 conduct
current, but they also prevent the battery from
discharging through the three-phase stator wind
Bridge rectification can be used with a three- ings when the engine is stopped or the operating
phase alternator, as shown in Fig 5.7(a), to give
DC output. During each half cycle of each phase Main rectifying D+
three diodes conduct, so the output waveform of diodes B+
a three-phase full wave rectifier comprises six
positive half cycles per revolution, as shown in Auxiliary w Main
Fig 5.7(b). As the waveform is much smoother field ^ rectifying
than the waveform of a single phase full wave diodes diodes
rectifier (Fig. 5.6(c)) it can be appreciated that the 3 phase (also used with
average DC value is correspondingly greater. stator out-l |* auxiliary field
put winding diodes for full
An electromagnetic field system is used in auto ■^- wave rectification
motive alternators and the excitation current is of auxiliary field
tapped from the three-phase alternator output in current)
a process known as self-excitation. A separate
DF
-o J
ΠΤΠ D -
diode circuit (auxiliary field diodes) is used to V Electromagnetic
rectify the excitation field current. Fig 5.8 shows ' field winding
a typical circuit. Fig 5.8 Rectification ofself-excited electromagnetic
The rectifier diodes in the alternator not only rotorfieldcurrent
42 Automotive Electronic Systems
speed is so low that the output voltage is less than During times when the rectifier output is above
battery voltage. Current can only flow from the battery voltage the battery is recharged. A possible
alternator to the battery and not vice versa. The smoothed DC voltage is shown in Fig 5.9. The
silicon diodes block the current against a voltage amount with which the output varies above and
of up to approximately 350-400 volts. If this below the optimum is called the ripple voltage.
breakdown voltage were exceeded the diodes
would be destroyed by the high breakdown Voltage regulation
current.
Voltage drop across the main diodes in oper The voltage generated by an alternator increases
ation causes a power loss which is dissipated in as its rotational speed increases, and also as the
the form of heat. Power diodes (25 W) are used strength of the excitation field current increases.
for this reason and they are mounted on heat sinks At about 10,000 rev/min the open circuit voltage
to aid cooling. Care must be taken not to reverse on full excitation current (2A) could be as high as
battery polarity otherwise a short circuit is created 140 volts, which is far too high for an automotive
through the power diodes and the excessive cur system. The principle of voltage regulation con
rent flow could destroy them. The auxiliary field sists of varying the value of the auxiliary field
exciter diodes carry a relatively small current excitation current in a way which maintains a
hence the exciter diodes are smaller and need working voltage of approximately 14 volts, i.e.
sufficiently high to provide power for all the
dissipate only about 1W.
vehicle's electrical systems and recharge the
Filtering battery, but not too high to cause damage. The
voltage regulator is designed to reduce the field
The output waveform of the three-phase full wave current as the stator voltage output increases thus
rectifier is fairly smooth in appearance and the preventing the output voltage exceeding the safe
average DC value closely approaches the working limit of around 14 volts. As long as the
optimum. Further smoothing can be effected by alternator and rectifier's output voltage remains
filtering, and capacitive and/or inductive circuits below the working limit, the electronic regulator
may be used for this purpose. In automotive sys is not in operation. When the voltage exceeds
tems the battery creates an inherent smoothing 14 volts the regulator automatically reduces the
effect; as current is supplied from the battery into output voltage, depending on the load on the
the system during times when the rectifier output alternator, by interrupting the excitation current.
would otherwise be below the battery voltage. The magnetic field strength of the rotor decreases
Optimum_
value
Ripple voltage,
measured in volts
or as a percentage
of the optimum
value
Time(s) Fig 5.10 1500
Rotor speed (rev/min)
Fig 5.9 A smoothed DC voltage, such as may be derived
from an automotive generating system Alternator and control characteristics
Generator electronic systems 43
and so does the output voltage. Fig 5.10 shows Electronic regulators
typical alternator and control characteristics.
The advantages of the electronic regulator are so
When the voltage drops below a pre-set mini significant that it has become an item of standard
mum the excitation current is increased again, and equipment. The electronic regulator is usually
the output voltage rises until the maximum voltage small enough to be alternator-mounted: Body-
is exceeded again. This switching cycle takes place mounted regulators are now only used with alter
automatically and at a frequency that ensures the nators having high excitation currents or where
output voltage is regulated to the desired mean special requirements are demanded.
value. At low speeds the excitation current flows
for a relatively long period of time, so that its The most important electronic components in
mean value is high (1.5A). At high speeds the the regulator are transistors and a Zener diode.
excitation current flows only briefly, hence the The transistor has the function of switching the
mean value is low (0.25 A). Fig 5.11 shows possible
A control B power switching
(B+ve)
Condition at speed n, Condition at speed
| Regulator
ifflffl ilidll ifcliPOff ! IIIMIII Offwin |Offon Off on
lllliniillll
I max h-~"-—''\ Imax
l m , ^ sü ΤθΠ2 ♦Joffl*
//// Toff, | Ton, τ\γ
1 ***«».. (from auxiliar)y
Time t —
Fig 5.11 Excitation current Iexc as a function of on time I i i (B-ve)
Ton and off-time ToS. The relationship between on-time
and off-time is decisive with regard to the magnitude of Fig 5.12 Electronic voltage regulator. Circuit diagram
the resulting mean excitation current Im. The excitation field excitation current on and off while the Zener
current rises along curve a and decays (decay current iA) diode is used for controlling the transistors by
along curve b. The graph is intended to convey a general acting as a voltage sensitive switch. Fig 5.12 shows
a typical electronic regulator circuit.
impression only. The values shown are not in accordance
with actual values.
The regulator can be considered to be made up
switching characteristics of the excitation current of two parts (1) part A: transistor T b potential
and its mean values. divider Rb R2, and R3, and the Zener Diode ZD
(the control stage) and (2) Part B: transistors T2
Throughout this high speed switching process and T3, resistor R5 (the power stage). The prin
thefieldwinding represents a high inductive load, ciple of operation can be best understood by con
and so the excitation current does not undergo
abrupt changes - due to self-induction. Output sidering what happens as the generation voltage
current may be self-limiting as the output winding rises and falls due to variation in alternator speed.
reaction limits the maximum current flow at full
load, so at high speeds a weak magnetic flux is Low voltage operation
sufficient for generating the necessary output volt Whenever the actual output voltage is below the
age. maximum preset generator voltage, no current
44 Automotive Electronic Systems
flows to the Zener diode and so no base current is tection diode Dp, is connected in parallel with the
supplied to transistor Tj which is therefore in rotor field excitation winding, allowing decay of
the non-conducting state (switched off). With tran residual current whenever a voltage spike occurs.
sistor Tj off, control current flows from the posi The self-induced voltage spikes are of a higher
potential than the generated output voltage and
tive ( + ve) terminal via resistor R6 to the base of so diode Dp conducts as they occur, dissipating
transistor T2, switching the transistor on. This in their energy as heat.
turn, supplies base current to power transistor T3,
switching it, too, on. Transistors T2 and T3 form Additional protection is also usually provided
a circuit known as a Darlington pair - a high against overvoltage which may occur as a result of:
power gain amplifying circuit. With transistor T3 regulator failure; switching off of high inductive
on, field excitation current flows through the field loads; loose contacts; or breaks in conductor
windings and the transistor, causing the generated cables. A Zener diode ZDp, is often used as a
output voltage to rise. protective device, reverse biased between the bat
tery positive (B + ) terminal and earth. The break
High voltage operation down voltage of this Zener diode is typically
When the generated output voltage exceeds the between 20 and 30 volts and provides adequate
working limit (14V), the breakdown voltage of protection for rated outputs up to 35 A. Whenever
Zener diode ZD is also exceeded, and so the Zener an overvoltage condition exists the Zener diode
diode conducts. Control current now flows via conducts and current flows through to earth, pro
tecting the regulator.
resistors Rj and R2, and the diodes D, and D2, to
the base of transistor T b switching Tj on. The This operating principle is basically the same
base voltage of transistor T2 drops below 0.6V for all types of alternator electronic regulators,
and so base current stopsflowing,resulting in the though they may look different in appearance and
transistor pair being switched off. With transistor circuit layout.
T3 off no field excitation current flows and so the
generated output voltage falls again.
These cycles are repeated continuously swit Hybrid voltage regulator (Fig 5.1 3)
ching the field excitation current on and off to
regulate the generated output voltage to around The current trend is towards the use of integrated
14 V. The ratio of on to off times of the excitation circuit regulators, comprising a hybrid IC with
current depends upon the rotational speed of the built in monolithic integrated circuits (MIC). The
engine and the electrical load requirements.
basic principle ofoperation is still the same, where
To aid stability, resistor R{ and capacitor Q by the MIC detects the alternator output voltage
form afilterwhich smooths the generated voltage. and, by switching transistors on and off, controls
Diodes Dj and D2 act as temperature com the current in the field windings, maintaining the
pensating elements to ensure that the output volt alternator output voltage at a constant level.
age is not only temperature stable, but is slightly
reduced during the hot summer periods and
increased in winter. This is useful to counteract Operation
the higher operating current drain on the battery
in the cold, dark months. At the instant power When the ignition is switched on, without the
transistor T3 switches off, a large voltage spike is engine running, battery voltage is applied to the
generated in the rotor field winding due to self- IC, and transistor Tl is on. Currentflowsthrough
induction as the field current is interrupted. This the field winding circuit via Tl to earth, exciting
self-induced voltage spike might damage the tran thefieldwindings. In this condition (i.e. the alter
sistor, so to provide effective protection a pro nator is not turning) the phase voltage at terminal
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Automotive Electricity and Electronics 5th Edition by James D. Halderman (z-lib.org)
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