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Computer Aided Drafting, Design and Manufacturing CADDM
Volume 24, Number 2, June 2014, Page 43

The Design of Wheelchair Lifting Mechanism and Control System

ZHAO Cong1, WANG Zheng-xing2, JIANG Shi-hong1, ZHANG Li1,3, LIU Zheng-yu1,3

1. School of Machinery and Automobile Engineering, HFUT, Hefei 230009, China;
2. Zhejiang Henglin Chair Industry Co., Ltd. Anji 313300, China;
3. Engineering Research Center of Safety-critical Industry Measure and Control Technology of Ministry of Education, Hefei 230009, China.

Abstract: In order to achieve a wheelchair lift function, this paper designs a tri-scissors mechanism. Through the so-called H-type
transmission and L-type swing rod, the three scissors mechanisms lift in the same rate with only one liner motor while ensuring the
stability of the lift. Finite element analysis in ANSYS is performed to verify the material strength. The control system with Sunplus
SCM achieves the voice control of wheelchair walking and lifting.
Key words: wheelchair lift; scissors mechanism; ANSYS analysis; Sunplus SCM

1 Introduction small as possible; in the other hand, to make the lift
process smooth and rate stable, wheelchair anchor
As one feature of intelligent wheelchair, lift scissor mechanism moving range at the highest
function can solve many problems for disabled people. position and the lowest position must be as large as
In real life, different figure and different condition possible. These two requirements are mutually
require different height of wheelchair’s sitting board. contradictory and difficult to balance.
Therefore, the design of an automatic lifting
mechanism which can meet different applying This paper designs a tri-scissors mechanism while
condition with simple structure and stability is using only one liner motor driving three scissor lift
becoming more and more important. mechanism. In order to achieve the same rate of three
scissor lift mechanism, the paper also design a
Currently, the realization of wheelchair lift mainly so-called H-type transmission and L-type swing rod.
contains two methods. One of them puts the liner This method solve the problem of wheelchair lift
motor vertically which can apply direct push force on while not increasing the number of liner motor which
the bottom of the sitting board and then the wheelchair means not increasing the cost of wheelchair.
rise and fall through the length change of liner motor.
With this method push force is focus on only one 2 Design of Wheelchair Lifting Mechanism
application point which will result in poor stability
when wheelchair lifts. Limited by the lift distance, this 2.1 The Basic Structure of Scissor Mechanism
method can’t use all the length of liner motor. If we
increase the stability by increasing the number of liner The basic structure of scissor mechanism is shown
motor it will also increase the cost which is not in Fig.1 and the number of moving parts of the
allowable. In the other method, there are two scissor mechanism is 5 with 7 low pairs and 0 high pairs, by
mechanism mounted on the left and right sides of the the calculation formula of degree of freedom:
wheelchair. This will increase the stability in some
way while it also has some fatal disadvantages. In one F=3n-2p-q
hand, in order to stabilize the wheelchair, wheelchair
anchor scissors mechanism moving range at the where n, p and q are respectively the number of
highest position and the lowest position must be as moving parts, low pairs and high pairs.

Let n=5, p=7, q=0, then F=3×3-2×4-0=1, i.e. the
mechanism has only one degree of freedom.

Project Item: Supported by Special Topic of the Ministry of Education about Humanities and Social Sciences (12JDGC007); ZheJiang Henglin Chair Industry
Co., Ltd.-Hefei University of Technology Green Seat and Intelligent House Innovate Team.

Corresponding author: ZHANG Li, Female, Master, Professor, E-mail: 77zhangli@ hfut.edu.cn.

44 Computer Aided Drafting, Design and Manufacturing (CADDM) , Vol.24, No.2, Jun. 2014

Obviously when thrust or tension is applied to the can be adjusted by the distance to drive the third
sliding side, the scissor platform will rise and fall scissors.
accordingly. The mechanism can achieve the direction
conversion of two perpendicular forces within the
same plane.

Fig.1. Scissors mechanism schematic diagram. Fig.2. H-type transmission and L-type swing rod
mechanism schematic diagram.
We apply three sets of scissor lifting mechanism,
two of them placed on both sides of the wheelchair 2.3 Wheelchair Lifting Mechanism Design
symmetrically and the other one placed on rear side
laterally. The three scissor lifting mechanism must lift Intelligent wheelchair lift mechanism includes
at the same speed in order to achieve a stable H-type transmission mechanism, L-type swing rod
three-point structure. The left and right sides of the mechanism, scissors mechanism and liner motor. The
scissor are designed symmetrically which front of top of the scissors mechanism drive the wheelchair lift
them fixed articulated chassis and rear end sliding and the bottom of the scissors mechanism fit three
along the chute. They are driven by the same liner sliders on the wheelchair frame. Two sliders are
motor and easy to implement while lifting. The third longitudinally placed on both sides of the frame whose
scissor lifting mechanism is placed on rear side front part hinged and rear part sliding. The third slider
laterally. To ensure the stability of the wheelchair, we are laterally placed on the rear frame whose both sides
need to let the scissors mechanism support center is sliding toward the middle of the two ends with
always in a wheelchair longitudinal center plane. scissors. When the liner motor rotates, its putting end
Different from both sides, the third scissor need to will drive the H-type transmission move forward. The
move by opposite direction and the movement must be front part of the H-type transmission drive the both
half the rate of the both sides. sliders move along the link at the bottom and the rear
part of it drive the L-typed swing rod rotate along the
2.2 Transmission Mechanism and Swing Rod kingpin by a pair of initiative sliders when moving
Design forward. The other part of the L-typed swing rod drive
the rear link slide inwards symmetrically along the
As can be seen in Fig.2, we design the so-called chute by the slider in order to stimulate wheelchair
H-type transmission and L-type swing rod to achieve lift.
the previously described function. H-type
transmission’s central part is directly connected to the In this configuration, there are two special designs.
liner motor with one side connected to the end of the First, the distance of travel of the both sides’ bottom is
scissors and the other side connected to the L-type twice the distance of the rear link. Second the distance
swing rod by a slider. The liner motor’s elongation between the slider of the transmission mechanism to
and contraction will drive the H-type transmission kingpin is twice the distance between the slider of the
which can make the left and right sides scissors slide link to kingpin. The former is designed to guarantee
and drive the L-type swing rod rotate around the the equal scissors lift distance and the second is
kingpin. Since any point on the L-type swing rod designed to guarantee the equal scissors lift speed.
shares the same angular velocity which means the These two are designed together to guarantee the
speed of any point on the L-type swing rod is scissors lift stable during all the progress.
proportional to the distance between this point and
kingpin. Because of the special L-typed swing rod, the Based on the above design ideas, the author
direction of velocity changes perpendicularly in the establishes the digital model of the wheelchair lift
same plane. Use the slider as a guiding mechanism so mechanism in SolidWorks 2012. Upon examination,
we can transmit the vertical speed to lateral speed that no interference occurs when the lift mechanism moves
and the wheelchair board is always in the same plane
and keep its speed stable when it lifts which means it

ZHAO Cong et al., The Design of Wheelchair Lifting Mechanism and Control System 45

has already achieve the design requirement.

3 Stress and Strain Analysis

In order to verify the strength of the designed Fig.7. Brace displacement result.
agency and build a foundation for the subsequent
optimization of the design, stress and strain analysis
are performed in ANSYS. This paper performs stress
and strain analysis on some typical parts and shows
the results in Fig.3~Fig.8.

Fig.3. Strut node elastic strain. Fig.8. Wheel elastic strain.
Fig.4. Strut node displacement result.
Through the results file, we can learn the maximum
Fig.5. Joint elastic strain. displacement and maximum stress of the key
Fig.6. Joint displacement result. components. Through the results of the key
components we can know that the maximum stress
occurs in the scissors lift mechanism where two shaft
cross shown in Fig.3. The maximum strain is located
in the squeeze part of the shaft with value of 270MPa.
Through the material table we can acknowledge that
the maximum yield strength of the selected 45# steel
under normal temperature is no less than 355MPa.
The actual stress is less than the maximum allowable
stress. The high safety factor means that the design
requirement has been satisfied.

4 Control System Design

This paper chooses voice as intelligent wheelchair’s
control method. In all voice control chips, Sunplus
SPEC061A voice control module is particularly strong
with the characteristics of high reliability, small size,
high integration and high expansibility.

4.1 Module Control Circuit

Fig.9 is an intelligent wheelchair drive circuit using
differential method to achieve steering. In this paper, a
full-bridge driver circuit is used to control the rear
wheel’s direction and speed. By adjusting the
rotational speed difference we can control the size of
steering radius. The smaller the rotation speed
difference, the larger the steering radius, whereas the
larger the difference, the smaller the steering radius. In
Fig.9, the four transistors Q1, Q2, Q3 and Q4

46 Computer Aided Drafting, Design and Manufacturing (CADDM) , Vol.24, No.2, Jun. 2014

constitute the full bridge circuit as four bridge arms. type photoelectric sensor. It is a set of photoelectric
Q2 and Q3 are controlled by Q5. Q1 and Q4 are sensor composed by transmitter and receiver. Output
controlled by Q6. signal is issued after modulated and receiver
demodulates the reflected output which can avoid the
interference of visible light effectively.

Fig.9. Drive circuit. Fig.12. Photoelectric detection circuit.

The voice input circuit is shown in Fig.10. VMIC is 4.2 Control System Design
the power cable for microphone. VCM is reference
voltage. VSS is analog ground of the system. Foot 1 The overall control system circuit is shown in
and foot 2 are the microphone’s active and negative Fig.13. We can achieve coordinated operation of
input pin. various functions by connecting each module to the
corresponding interfaces of Sunplus SPCE061A. The
port 1 and port 2 of rear wheel driven circuit are
connected to foot IOB8 and IOB9 of SPCE061A. The
port 1 and port 2 of servo motor driven circuit are
connected to foot IOB10 and IOB11 of SPCE061A.
The port 1 and port 2 of voice input circuit are
connected to foot MICP and MICN of SPCE061A.
The port 9 of voice output circuit is connected to the
foot DAC1 of SPCE061A. The port OUT of
photoelectric detection circuit is connected to the foot
IOB12 of SPCE061A.

Fig.10. Voice input circuit.

The voice output circuit is shown in Fig.11. VSS is Fig.13. Control system design.
the analog ground. VDDH is reference voltage. The
audio signal is generated by SPCE061A and input to
voice output circuit J9 through foot DAC. R9 is the
volume potentiometer and it can integrate the audio
signal to amplifier SPY0030 after adjusting. The
signal is amplified by the audio and output from
SPY0030 to external speakers which can play tips.

Fig.11. Voice output circuit. 5 Conclusions

The photoelectric detection circuit is shown in In this paper we analyzes the common features of a
Fig.12. The photoelectric circuit uses E18-D80NK-N wheelchair lift mechanism and designs a tri-scissors
mechanism. Through the so-called H-type
transmission and L-type swing rod, the three scissors
mechanisms lift in the same rate and is stable due to
the reasonable installation position. The advantage of

ZHAO Cong et al., The Design of Wheelchair Lifting Mechanism and Control System 47

this design is that it does not increase the costs and 2012. He is currently a graduate in the Department of
improve the mechanical stability at the same time. We Mechanical at Hefei University of Technology. His
perform finite element analysis in ANSYS and build a research interests include vehicle engineering,
foundation for the subsequent optimization of the wheelchair control and vehicle dynamic.
design and applied Sunplus SCM to achieve voice
control of walking and lifting. WANG Zheng-xing was born in
1975, and graduated in Zhejiang
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ZHAO Cong received his B.Eng
degree in Mechanical from Hefei
University of Technology, China, in