Dimitar Stefanov

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Lecture 20

• Dimitar Stefanov

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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 users 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.

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• 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.

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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.
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Torque loss result from the friction if wheels,
bearing and rolling.
Free-body diagram of a powered wheelchair and
rider on an inclined surface
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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.
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Forces, acting at the center of the mass (M) of
the wheelchair/rider system
g acceleration due to gravity.
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Linear acceleration of the left and the right
wheels and chair angular acceleration about the
z axis
where
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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 users control.

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• 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.

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• 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.

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• Speed control of the rear wheels
• Tachogenerators
• Optical sensors
• Hall sensors
• 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.
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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)
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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.

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Shared control

• Called sensor-based control
• Array of external and internal sensors is used by
  the control system

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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
• Continuous variable input signals
• Discrete-steps variable input signals

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• 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 users
  commands)
• Sliding window signal averager
• Neural network or fuzzy logic based signal
  preconditioning tremor suppression.

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• Two basic approaches for tremor suppression
• Driving behavior is recorded on an actual or
  virtual driving course.
• Driving data are used to train the neural network
  or to adjust the parameters of fuzzy controller.
• Tremor data are measured while the joystick is in
  the neutral position.
• Neutral position tremor is measured and used as a
  background noise to develop an adaptive noise
  cancellation.

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Approach 1. An example.
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Approach 2. An example.
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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.

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Failure detection of the input device. An
example.