Motion Control Fundamentals

MOTION CONTROL FUNDAMENTALS

ANSWERS TO COMMON QUESTIONS AND TIPS TO PERFECT YOUR SYSTEM

MOTION CONTROL FUNDAMENTALS
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Motion Control Fundamentals REXELUSA.COM

Understanding Motion System Components Page 4
Page 8
Sizing a Motion System Page 18
Page 20
Finding the Right Inertia Ratio Page 22
Designing Motion Safety
Tuning a Servo System

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MOTION CONTROL FUNDAMENTALS

Understanding Motion System Components

Servo Motors

A servo motor is a rotary or linear motor that allows for precise control of position, velocity, and acceleration.
A servo motor must be paired with a feedback device to allow for the tight, closed-loop control. A closed loop
control system is when a feedback device (such as an encoder) is used to automatically achieve and maintain
the desired output condition by comparing the desired condition with the actual condition. An open loop
system does not have a feedback device and because of this the system does not know if it is running at the
desired state. Think of this as the difference between trying to maintain a speed in your car with your eyes
closed (open loop) vs watching the speedometer (closed loop).
There are three main types of electric motors in the industrial world: brushed motors, brushless motors, and
induction motors.

BRUSHED MOTORS

Brushed DC motors use a mechanical system (brushes) to deliver the electrical power to the motor while
induction and brushless DC motors use an electronic system. A brushed motor has an armature with the motor
windings in the center of the motor with magnets bonded to a ring that surrounds the rotor. As the brushes
come into contact with sections of the rotating shaft, power is transferred to the windings. Over time however,
the brushes will wear out.

BRUSHLESS MOTORS

Brushless and AC Induction motors pass their current through the stator in the outside of the motor, which will
then turn the magnets in the center of the motor. AC Induction motors are spun due to the induction of the
rotating magnetic field within the stator due to the changing AC current. In a brushless DC motor, permeant

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magnets are attached to the rotor and as current passes through the stator the rotor will rotate to align the
electromagnetic poles in the stator.

INDUCTION MOTORS

The most common types of motors used in industrial applications are brushless and induction motors. Servo
motors are most commonly brushless motors, though you can run induction motors in a closed loop as well.
Depending on the application, the motor will either be a DC motor or an AC motor. DC motors are typically
used in smaller applications, and AC motors give the best performance.

For servo motors, the feedback device is the most important part of the system. Without a feedback device, the
motor runs in an open loop and cannot reliably position the load, or even move at all. The most common types
of feedback devices are encoders and resolvers. Resolvers are a more robust device, handling high temperatures
and vibrations, but encoders allow for more precise control.

Feedback Devices: What are they for?

INCREMENTAL ENCODERS

Incremental encoders rely on the drive to interpret the position based on the output pulses that the encoder
sends to the drive. The outputs for an incremental encoder are typically two square waves (A and B), used to
determine the rotation direction. Sometimes this signal is combined with an index pulse, which occurs once per
revolution (Z). Combining the two square waves will then give you a “quadrature” signal.

The other common type of incremental encoder is called sine-cosine (Sin/Cos). The main difference between
the two is that sine-cosine encoders send their signal as 1-volt peak-to-peak (1 Vpp) analog sine waves on the
A and B channels. Because of the Sin/Cos encoder’s continuous wave signal, they can achieve much higher
resolutions.

Both types of incremental encoders can use X4 encoding to quadruple the resolution of the feedback device.
The Sin/Cos encoders count the number of times the signal crosses 0V for a given period. And square wave
encoders count both the rising and falling edges of both square waves for each period.

The main downside to these motor encoders is that the servo system loses its position information when the
power is turned off. This means the system needs to find its home again at power-up. At the least, this will lead
to longer startup times for machines, but at the worst, homing can be impossible due to tooling designs. To
solve this issue, many engineers started to use absolute encoders.

ABSOLUTE ENCODERS

An absolute encoder maintains position information when power is removed from the system. This allows the
system to know its position immediately and start working on power-up. An absolute encoder will use a coded
wheel that allows each encoder position to read a unique value. Because each position has a unique value, if the
motor is turned while power is off, the motor will know its new position when power is restored.

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MOTION CONTROL FUNDAMENTALS

One issue with this setup is if the motor is turned more than one revolution while power is off, the system will no
longer reliably know the position of the output mechanisms. To solve this issue, some motors come with multi-
turn encoders that use multiple discs or gearing to allow the motor to turn more than once while the power is off
and still retain its position information. Rockwell Automation® servo motors typically come with 12-bit, multi-turn
encoders, which means they can be turned 4,096 revolutions with the power off before they lose track of their
position. The downside of multi-turn encoders is that they often do not have as high a resolution as single-turn
encoders. To solve this issue, Rockwell Automation pairs a single-turn (18-23 bit, 262,144-8,388,608 counts!)
with a multi-turn encoder. This system gives very high motor encoder resolutions with the benefits of multi-turn
encoders.

Absolute encoders will either send the raw encoder pulses or use a communication interface to communicate
back to the drive. These interfaces send the absolute position in sync with a clock signal. To improve these
designs, some motors now come with a DSL encoder that communicates with the servo drive using only two
wires. This digital communication allows feedback signals to run in the same cable as the motor power and
allows more two-way communication with motor encoders and drives.

Servo Drives

Servo drives interpret the motion controller command signal and control the amount of speed and torque
delivered by the motor. Drives accomplish this task by converting plant power to the voltage and current levels
required by the motor to control the application.

Motor Cables

Motor cables are important to any motion systems. They are responsible for handling the power and
communication between the motor and the drive. When low quality cables are used the risk of issues from
electrical noise and poor connections increases. The benefits of high-quality cables include: longer life,
eliminated noise issues, and better system performance.

When selecting motor cables, it is important to know if high flex cables are required for the application. If the
cable will be bending back and forth during machine operation, a standard cable will break conductors in a
relatively short time because it is not designed for continuous flexing. Flex rated cables are designed for this
type of application and will end up with a much longer life and better performance.

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Sizing a Motion System

Sizing Servo Systems

For most applications, sizing a servo system is a pretty
straightforward task but takes a good amount of
preparation and understanding to make it that way.
Before any engineer can start to size a servo axis, they
must gather all the necessary information. The easiest
way of going about this is to start from the load and
work back to the actuator. This is especially important
when sizing multiaxis gantries, as the axes closer to
the load need to be sized first so the load for the later
axes can be accurate.

Using Modeling
Software

When sizing a linear axis, an engineer needs to know
the mass and the center of mass (COM) of the load
relative to the actuator’s carriage. You can usually find
the value in the solid modeling software used to build
the system. An example of where you would find this
information in SolidWorks® is shown below. In addition
to this value, we need to know the move distances,
speeds, dwells, and cycle times to properly size
the motor and actuator. These items can be nicely
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summed up in a system timing chart. This is a good visual way to layout the process of the system that translates
well into the sizing software when building the motion profile. You may need more advanced properties, such as
jerk rates, when handling delicate materials. They can be reviewed on a per application basis.

Sizing a Rotary System

When sizing a rotary system, instead of knowing the mass and COM, we need to know the rotary inertia around
the axis of rotation; this is especially important for rotary axes to keep the inertia ratio to a proper value. You
can find this value in the mass properties of the modeling software. The engineer designing the models must
be careful to set up the proper coordinate axis to ensure the inertia value is correct. In addition to the inertia, a
timing chart with the same information as the linear axis is necessary for sizing a rotary system.

Determining Accuracy Requirements

For both rotary and linear systems, the engineer needs to determine the accuracy requirements for the system.
For linear mechanics, this will most commonly be the actuator’s repeatability as well as the accuracy, straightness,
and flatness. For rotary mechanics, the backlash of the gearbox and couplings need to be considered.

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System Integration Data

The final information needed when you size a servo axis is related to the overall integration with the larger
system. You will need information on cable lengths and voltages to integrate the electrical system. You need the
controller model to determine what type of drive can be used and what safety system. Typically, the drive and
motion control requirement dictates the required controller be a motion controller (ex. -ERM CompactLogix™
controllers).
This is not a complete list of parameters that need to be considered; different factors can arise in different
applications such as temperature, vacuum, hazardous environments, and more.

Building the Motion Profile

Once all the information on your system has been gathered, you need to put the movements into a motion
profile. We will use a simple application for our example. Let’s say we have a 50 kg load that we want to move
500mm in one second, and we will do this every two seconds. The easiest way to calculate all the information for
our profile is to enter it into a program like Rockwell Automation’s Motion Analyzer.

This can be calculated by hand with a few equations if you so choose or if you want to understand the math
behind the program. To size the rest of the system by hand, we would need the maximum velocity, acceleration,
and force/torque. For this we will also assume a standard trapezoidal move profile.
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For a linear application:

For a rotary application:

Sizing/Selecting Mechanics

OVERVIEW

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SPEED

For our example application, we have a peak speed of 750 mm/sec. This means that we can use any of the
options shown in the overview table. Typically, screws are the most limited with peak speeds being around 1200
mm/sec but some can go faster than that. Linear Servos, Belt Drives, and Rack & Pinion systems can typically all
achieve multiple meter/sec speeds without issue.
Usually belt and screw actuators will be the best starting point with screws being the most common option
unless longer strokes or higher speeds are required. The reason for this is belt actuators typically need a gearbox
to compensate for the system inertia while the screw actuators do not. We will touch on this more later.

THRUST FORCE

For our application, we do not have any applied force, so the only force comes from the acceleration. Using the
earlier equation, the force required works out to 112.5 N.

MOMENT LOADING

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For this application, we will say the center of mass of the load is located off the carriage by 100mm in the Y
direction and 50mm in the Z direction. To quickly calculate the moment loading on the actuator, we can use the
following:

Note that most actuators will be the weakest in the Mx rating. This loading will give the bearing system the
least amount of mechanical advantage to support the load. To help compensate for this, we need to select and
actuator with a wide base to support the overhung load better.

ACTUATOR SELECTION

If we use a sizing tool like that provided by Tolomatic™, we will get the values for the speed, forces, and moments
based on the motion profile we enter. Tolomatic’s sizing tool will then use this information to provide a list of
recommended actuators. Once an actuator is selected you can see how much of the actuator’s capacity is used
by the current application.

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MOTION CONTROL FUNDAMENTALS

Sizing/Selecting Motors

SPEEDS

If we are working on a rotary application, then we already know the speed that the motor will need to be capable
of. If we are working on a linear application, then we will need to make some more calculations to convert our
data to rotary values to select the motor. Luckily, it is simple to convert from a linear to rotary speed.
For a screw system:

For a belt system:

Both equations will give you the required peak motor speed in revolutions per minute (RPMs). For our example
application, we can also get this information from our Motion Analyzer software. If we choose an actuator with a
10mm screw, we will need a motor capable of at least 4500 RPMs.

TORQUE

When calculating the required toque, we will have to focus on both the peak torque as well and the continuous
(RMS) torque. The continuous torque is calculated based on he duty cycle of the system as well as the required
torque at various stages of travel.
Continuous torque:

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For the peak torque of a screw system:
For the peak torque of a belt system:

For our example system, a VPL-B0633T-PJ12AA will work just fine and give us a comfortable safety factor.

INERTIA

Ensuring the correct inertia ratio is one of the most important parts of sizing a servo system. Even if the
mechanics and motor can handle all the required speeds, forces, and torques, you will still have a hard time
properly controlling the system if the inertia ratio is too high. Most of the time when working with a screw-
based system there is enough mechanical advantage that the inertia ratio will not be an issue. With a belt-based
system you will almost always have to use a gearbox to compensate for the system inertia.
For our example application, the 10mm screw would give an inertia ratio of 4.67:1 and a 40mm pulley belt
actuator would give a ratio of 673:1!

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MOTION CONTROL FUNDAMENTALS

Sizing/Selecting Drives

CURRENT/ POWER

Once you have selected the proper motor for your application, there are a couple ways to select the proper
drive. The simplest approach is to use something like a Design Guide to select the recommended drive for the
given motor. This will work out just fine but depending on the application the drive may end up oversized.

The other option is to take the data from Motion Analyzer and use that to select the drive. Based on the info
below we see that we can use the 2198-H003-ERSx servo drive. If we had used the design guide, we would be
using a drive 1 or 2 sizes larger than necessary.

We also need to check to make sure the internal shunt will be capable of handling any regen in the system. This
is another task that can be automatically handled in Motion Analyzer but if you need to calculate it manually, we
can see that the average regen comes to about 7.5W. The drive we selected is capable of handling up to 30W.
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Finding the Right Inertia Ratio

There is no formula for determining the ideal inertia ratio. However, most servo sizing guidelines aim for the
inertia ratio 10:1 or lower but having mechanically stiff systems over highly compliant ones is also important
and allows for some more flexibility in having a proper ratio. Higher ratios cause the motor to work harder than
necessary and can lead to increased settling time during aggressive moves, which decreases efficiency and
increases operating costs and cycle times. With a proper inertia ratio, you can use the tuningless features in
Kinetix® servo drives to eliminate any need for an engineer to tune the system, resulting in low maintenance and
quicker startups. If the inertia ratio is very high (over 10:1), then the motor may not be able to control the system
at all, even at a standstill. This will lead to the motor using an excessive amount of power just to keep the load
stationary or require a lot of engineering time to tune the system into a stable state.

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If the inertia ratio is too high, there are two ways to reduce it: increase the drive ratio (output distance per
motor rev) or use a larger motor. Gearboxes are frequently required in belt-driven systems because of the low
mechanical advantage of the pulleys. They can also significantly reduce the system’s inertia ratio since the gear
ratio has an inverse square effect on the inertia of the load.
The second method for reducing the inertia ratio is to use a larger motor with higher inertia (sometimes moving
to a medium inertia motor). However, because of the space and cost required, a gearbox is the preferred
solution.

Why is it Important?

For the servo system to control the load effectively and efficiently during acceleration and deceleration, the
motor and load inertias should be as close to equal as possible, but a 1:1 inertia match is rarely practical or
achievable. Many factors affect what is considered an acceptable inertia ratio, but one of the most important
is the system compliance or wind-up. Mechanical components are not perfectly rigid, and as more compliant
components are added, the more compliance the system will have. System compliance is largely driven by more
flexible components such as belts and couplings. In general, the higher the compliance, the lower the inertia ratio
should be.
Note: having an inertia ratio <10:1 is ideal but with Rockwell Automation’s Load Observer (available on Kinetix 5300,
5500, and 5700) and mechanically stiff systems there is some flexibility.

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Designing Motion Safety

The safe torque off (STO) function is used to prevent unexpected motor rotation in the event of an emergency
while the drive remains connected to the power supply. This is a very common, industry-standard way of
keeping an operator safe while around moving equipment. While STO is great for ensuring a motor does not
become energized, it can cause issues when stopping a loaded machine. To bring high-inertia loads to a more
controlled stop, we can use a newer function called a safe stop.

Safe Stopping

To safely bring a load to a stop, we have two options: safe stop 1 (SS1) and safe stop 2 (SS2). Both work in the
same manner to bring the load to a stop but differ once the motor is at a standstill.

SAFE STOP 1

Using SS1 reduces the risk of danger and increases the productivity of a system by bringing the load to
a controlled stop using the motor’s active torque. SS1 allows for reduced safety clearances in a machine,
potentially reducing a machine or system’s overall footprint. Using SS1 also eliminates the need for wear-prone
mechanical brakes for stopping the motor. Once the motor is verified to be at a standstill, the drive enters an
STO state.

SAFE STOP 2

SS2 operates like SS1 in how it brings the load to a rapid, controlled stop, but once the motor is verified to be at
a standstill, the safety controller monitors the zero-speed condition while the motor torque is used to hold the
load in place. If the load begins to move for any reason, the drive will immediately enter STO and cut power
to the motor. Using SS2 can eliminate the need to rehome a machine after entering the safe stop because the
motor remains energized and does not lose its position. Rotating blade mechanisms are another example where
SS2 is helpful. If an operator accessing a machine needs to put it into an STO state to work on it safely, without

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the addition of interlocks, the rotating blades may be able to move and cause harm. With SS2, the motor will
keep the blades stationary, reducing the risk of operator injury.

SAFE STOP 1 vs SAFE STOP 2

Both SS1 and SS2 work with the flexibility of networked safety functions. Because they run through a safety
PLC, you can adjust all the parameters on the fly based on machine conditions and user inputs, if necessary.
Compared to other safety functions, the deceleration rate of both commands is adjustable and can be
programmed to protect both personnel and equipment more efficiently.
The most significant difference between the two stops is that only SS1 can be used in emergency stopping
situations. Another difference between these two types of stops has to do with the end state. In an SS1 function,
the motor enters STO once it is verified to be at a standstill. In SS2, the motor remains energized and enters
zero-speed monitoring or a safe operating stop.
The differences between the way these two functions operate should make it clear that SS2 is not acceptable
for emergency stopping situations. If an operator is in the way of moving tooling and SS2 is commanded by the
e-stop, they may be trapped by the equipment or in further danger because the motor remains energized and
can still output torque at the end of the stop.
For information on safety functions, visit the Rockwell Automation Safety Function Library.

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MOTION CONTROL FUNDAMENTALS

Tuning a Servo System

Now that you are ready to get your motion system running, you are faced with one of the more challenging
tasks: tuning the system. This once frequent, time-consuming task has been the bane of controls engineers
across the automation industry. Thankfully, tuning a servo motor has become less common thanks to tools such
as load observer and other tuningless features. However, on servo systems where these tools are not present,
manual tuning is still the go-to method for attaining optimal performance. So, let us walk through tuning a servo
motor.

Trial and Error

Because servo tuning is not an exact science, the best tool an engineer can have is their experience in the
art of tuning a system. There is no one “right” set of tuning parameters for any given application. With close
observation of the system as you adjust parameters through a trial-and-error approach, an initially rough system
can be made to run much more efficiently.

Design for Tuning

Before a controls engineer can begin to tune a servo system, the mechanical engineers need to design a system
that can be handled and tuned in the first place. If the servo system has an inertia ratio that is too large (>10:1),
no amount of skill and time will be able to get the system to run properly. But if the mechanics are designed well,
the controls engineer can start to tune a servo motor.

Auto-Tune

Some servo systems offer an “auto-tune” option in their software, which can help the engineer get to a much
closer starting point and shorten the tuning process. The auto-tuning process will test the motor and drive
combination in a short test move and set and identify more parameters than can be handled in a manual
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process. However, in certain systems, especially those that are highly dynamic, the auto-tuning process will not
deliver the best results. And if there is too much compliance in the system, it will make the entire process more
difficult.

Manual Adjustment Terms

Whether or not you perform an auto-tune, the process of tuning a servo motor involves manually changing
various gains in the servo loop (typically the Loop and Integrator Bandwidths). This is an iterative, trial-and-error
process. So, you need to make small adjustments until the motion is smooth, there is little or no audible noise in
the system, there is little or no position error after the move, and the velocity error and overshoot is minimized
during motion.

Tuning a Servo System

The main gains that are adjusted in a tuning system will be the proportional (loop bandwidth), integral
(integrator bandwidth), and derivative gain (system damping).
The proportional gain is related to the stiffness of a system and is typically adjusted first. This gain determines
how much voltage is applied to decrease the position/velocity error in the system. The amount of force applied
to the system is proportional to the error in the system.
The integral gain provides a force at the end of the move to get the axis to a point where there is zero error
in both the velocity and position. The term integral comes from the fact that the system will accumulate
(integrate) the error, and the greater the cumulative error, the larger the integral and force become. Both the
proportional gain and integral gain for the position and velocity loops can be easily adjusted using just the
system bandwidth slider in the Rockwell Automation software.

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The derivative gain acts as a damper on the system to reduce overshoot and oscillations. The derivative gain
looks at the rate of change of the system error and applied a force relative to that.

Find the Balance

There are different schools of thought on how to best go about tuning a system, but the most common involves
increasing the proportional gain (or system bandwidth) until the system becomes overtuned. The system will
become overtuned when it overshoots the target by a large amount or begins to oscillate and never reaches
a point of zero/minimal error. You can then slightly back off the proportional gain and increase the damping
until you find a good balance between fast response and low overshoot. The gains are then slightly increased in
achieving optimal performance without overtuning the system. The integral gain can then be used to remove
the last of the error but should only be used sparingly to keep the system stable.
Remember, tuning is not an exact science, and the best tool an engineer can have is their experience in the art
of tuning a system.

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MOTION CONTROL FUNDAMENTALS

MMOOTTIIOONN CCOONNTTRROOLL SSOOLLUUTTIIOONNSS

QQuuaalliittyyssoolluuttiioonnssffoorrssaaffeellyyccoonnttrroolliinnggyyoouurrmmaacchhiinneess..

High-performance products you can trust to increase efficiency,
uptime, and throughput while reducing operating costs.

Horizon Solutions, a Rexel banner, partners with leading manufacturers of servo motors and drives, gearboxes,
actuators, and bearings.
Find the products you need for everyday and high-performance automation. We offer products for just about
every motion control application from PLCs to linear mechanics to keep your plant safe and efficient. We can
deliver complete solutions and turnkey projects to keep your systems running.

Motion Control Manufacturer Listing

GEARBOXES

� Apex Dynamics, USA
� Stober
� WITTENSTEIN


LINEAR BEARING RAILS AND BALL SCREWS

� LinTech®

LINEAR STAGES (BALL SCREW, ROLLER SCREW, AND BELT)

� Exlar®
� LinTech®
� LinMot®
� Rockwell Automation
� Tolomatic

LINEAR SERVO MOTORS

� LinMot®
� Rockwell Automation®

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MOTORS AND DRIVES

� Advance Micro Controls (AMCI)
� Elwood®
� Exlar®
� Rockwell Automation
� Tolomatic

ROBOTICS

� Codian

APEX
DYNAMICS

Contact an Automation Specialist today to learn more about our motion control offerings.
Email [email protected]

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