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Lecture 20. Dimitar Stefanov. Microprocessor control of Powered Wheelchairs.
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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: • 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.
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: • Continuous variable input signals • 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: • 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.
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.
