Lecture 20
Dimitar Stefanov
Microprocessor control of Powered
Wheelchairs
•Flexible control; speed synchronization of both driving
wheels, flexible control algorithm; algorithm for power
limitation, diagnostic of the current wheelchair condition,
information about the current battery charge, etc.
•The control of the powered wheelchair can be tuned easily
to the individual user’s needs. Easy adjustment of the
maximal speed in different regimes (forward, backward,
turning)
•Acceleration and deceleration rate, input filter parameters
(tremor suppression), input device dead zone.
•Tuning –the controller is set into the “program
mode” regime; tuning via special keys or tuning by
using the wheelchair joystick; tuning via special
tuning device, which is connected to the
microprocessor controller; series of menus are used to
tune the parameter of interest.
•Contour velocity control – velocity feedback; the
desired speed remains constant despite of the terrain
characteristics (rough and smooth surface) and the
terrain inclination; speed feedback – by optical
encoders, tachogenerators, or by using the back
electromotive force (EMF) of the motor.
Wheelchair model
1. Model of DC motor with permanent magnets
Va – armature voltage;
ia – armature current;
Ra – armature resistance;
Kv – motor voltage constant;
wm – motor angular velocity;
La – armature inductance;
Jm – motor inertia;
Kt – motor torque constant;
Tm – motor torque.
can be neglected because the time constant of the
chair is typically on the order of 1/100 times larger.
Torque loss – result from the friction if wheels, bearing and rolling.
Free-body diagram of a powered wheelchair and rider on an inclined surface
Q – pitch (slope) angle;
F – incline angle
M- mass of the wheelchair/rider system
Acceleration of the wheelchair/rider system along the x
and y axes:
vr – linear velocity of the right wheel
vl - linear velocity of the left wheel
l – distance between the the center of mass and the rear axles
W – width of wheelchair between the rear wheels.
Forces, acting at the center of the mass (M) of the
wheelchair/rider system:
g – acceleration due to gravity.
Linear acceleration of the left and the right wheels
and chair angular acceleration about the “z” axis:
where
Microprocessor-based feedback wheelchair
controller
•Differential torque control of the powered rear
wheels; free motion of the front castor wheels.
•Feedback speed control maintains the desired
wheelchair speed in presence of disturbances, e.g.
side slopes, changes of the terrain, changes in the
texture of the rolling surface.
•Easy user’s control.
Problem:
•User should feel the resistance of the wheelchair
in order to be able to sense bearing failure of low
tire pressure.
Solutions:
• Sensors which give information about the
power consumption and interface, which transmit
the information to the user
•P – controller.
Typical interface – two axis joystick – set the
direction and speed of the wheelchair motion.
•In case of PI feedback control, user can easily
keep the desired direction (no correcting
commands are required).
•The difference between the joystick signal
and the velocity signal of the velocity sensor
becomes an error signal for the controller.
•In case of open loop, user continuously adjust
the speed proportion of the wheels in order to
maintain the desired direction.
Speed control of the rear wheels:
1. Tachogenerators
2. Optical sensors
3. Hall – sensors
4. In case of armature controlled permanent magnet D.C. motors,
the back electromotive force (EMF) is proportional to the motor
speed
Angular and linear velocity of the wheelchair, expressed by the
velocity of the left and the right of the rear wheels:
Val – linear velocity of the wheelchair
Vwl – angular velocity of the wheelchair
vr – linear velocity of the right wheel
vl - linear velocity of the left wheel.
The input to the left motor servo amplifier (VsL) :
Where:
Vcv, Vcw - output signals of the wheelchair
controller
Vcv –signal, proportional to the linear velocity
Vcw – signal, proportional to the angular
velocity.
The input to the right motor servo amplifier (VsR)
:
Feedback controller – allows independent limitation of
the linear and the angular speed of the wheelchair:
independent limitation of acceleration and deceleration
during forward and backward movement.
Choice of the parameters of the microprocessor
wheelchair controller:
•Gain of the system
•Sampling rate – determined the closed-loop gain
•The longer the sampling time, the lower gain of the the
PI controller; high gain and high accuracy in case of
faster sampling time.
•Average sampling rate - 0.01s.
Shared control
•Called sensor-based control
•Array of external and internal sensors is used by the
control system
The selection of appropriate interface is from
great importance for the wheelchair
performance.
Input signals from the user – set the desired
speed and turning radius:
1. Continuous variable input signals
2. Discrete-steps variable input signals
•Proportional input devices – joystick, head
control
•Latched inputs – switches, sip-and-puff
devices, voice recognition devices
Signal pre-conditioning:
•Simple low-pass filter (suppresses fast user’s
commands)
•Sliding window signal averager
•Neural network or fuzzy logic based signal
preconditioning – tremor suppression.
Two basic approaches for tremor suppression:
1. Driving behavior is recorded on an actual or
virtual driving course.
1. Driving data are used to train the neural
network or to adjust the parameters of
fuzzy controller.
2. Tremor data are measured while the joystick is
in the neutral position.
1. Neutral position tremor is measured and
used as a background noise to develop an
adaptive noise cancellation.
Approach 1. An example.
Approach 2. An example.
Pre-conditioning – removes input
device drift and sensor imbalance.
Schemes for failure detection:
•The output signals to the motors become zero
when signal failure is detected
•Redundant circuitry schematics
•Memorizing the “history” of the wheelchair
behavior; comparison the wheelchair response to
one and the same input commands at the current
and the past moments.
Failure detection of the input device. An example.