Med. & Biol. Eng. & Comput., 1979, 17, 110-114
Versatile wheelchair control system J, A y l o r
R. R a m e y
J. S c h a d d e g g
S. R e g e r
Universityof Virginia,RehabilitationEngineeringCentreand Schoolof Engineering& AppliedScience,Charlottesville,Virginia22901, USA A b s t r a c t - - A n electric wheelchair control system is described that offers an increased ease of operation to conventional wheelchair users a n d makes it possible for persons with severe pathologic h a n d movements to control wheelchairs in a safe and satisfactory manner. The control system is designed so that its characteristics are easily adjusted by the therapist to match the patient's control abilities. Chair acceleration, maximum speed and joystick-position averaging are adjusted by means of p l u g - i n modules. There is only a disconnect relay that is activated when the joystick is grasped or addressed with a mouth stick. This relay functions as a "deadman control" by applying dynamic braking when the joystick is released. The absence o f other relays results in noise-free operation. Cruise control is i n c l u d e d to permit the drive motors to develop f u l l torque automatically when required, regardless of the joystick position. This contro/ system can be applied to any existing wheelchair that uses two motors for steering/propulsion, provided two batteries are used to provide a + 6 , O, --6. or + 12, O, -- 12V supply. The design focused on portability so that the control system does not interfere with folding the wheelchair.
clock provides 400 Hz power for the excitation of the induction potentiometers. There are two potentioWHEELCHAIR users who possess motion disorders, meters, one for each motor channel. Each motor can such as hyperkinesia or otherwise, lack active control be independently driven forward-neutral-reverse at because of injury or advanced age and often find it any desired speed. This is a proportional control difficult or impossible to control a modern electric system where the chair speed is approximately wheelchair. Many of these patients are not able to proportional to the joystick angle off of the vertical. steer the conventional joystick-controlled wheelFig. 1 shows only one pair of the two identical chair without exhibiting large-position amplitude deviations from a desired straight-line course. A channels. The signal from the induction potentiocontrol system is needed that overcomes the driving meter is amplified, rectified and time averaged. The averaging can be adjusted by the therapist to match obstacles presented by hand instability. The control system presented here will time- the driving characteristics of the wheelchair to those average out hand tremors and is unresponsive to of the patient's ability. A person with little tremour rapid and erratic movements. This feature was movement would require very little time averaging, particularly helpful for a test driver with traumatic whereas one with severe spastic movement would mid-brain injury, complicated by ataxia, dysmetria require extensive time averaging and a corresand extension tremour*. By using this control ponding reduction in wheelchair speed. As long as system, this patient was able to steer straight down the tremour movement contains directional inforhallways, easily negotiate an obstacle course that mation (mean net displacement to one side), the required sharp turns (turning radius of 3 feet) and control system can extract this information and use it to control the wheelchair. pass readily through ordinary doorways. Following the signal processor is a summing amplifier, the output of which is further power 2 Control system amplified to drive the wheelchair motors. The gain K s is adjustable and controls the upper speed limit With reference to Fig. 1, a joystick controller is of the chair. It must be decreased when extensive used except that the usual wirewound potentiotime averaging is required, because the decision meters have been replaced with induction potentioresponse rate is delayed by the time averaging and, meters, which should be wear free. A solid-state therefore, the rate (chair speed) at which events occur must be slowed down. Received 1lth April 1978 *Ataxia is theirregular hunting or oscillation of the limb on voluntary Feedback from the motor drive to the summing motion, dysmetria is the missing of the mark when the searching amplifier is employed to provide maximum motor finger nears its target and intension tremouf is the regular of relaythtorque, when required, without the patient taking microscillation of thelimb during voluntary motion. 1 Introduction
0140-0118/79/010110 -'F 05 $01 950/0
9 IFMBE: 1979 110
Medical & Biological Engineering & C o m p u t i n g
any personal action. F o r instance, when starting a chair whose caster wheels are not pointing in the direction of desired travel, it is often necessary to momentarily advance the joystick of a conventional chair to the maximum position and then quickly return it to the desired speed position as soon as the chair starts to move. With this control system, it is necessary to advance the joystick to the speed position desired, and, because of the feedback loop, whatever motor torque is necessary to start the chair will be applied, even though the speed is set at slow. This greatly simplifies the driving requirements, and, in effect, the patient has a 'cruise control' similar to that used on automobiles. As no relays are used in the operation of the chair, the only sound is that of the motors. As a result, the chair is extremely quiet, compared with many commercially available chairs. A power disconnect relay engages when the user rests his hand on the joystick. The purpose of this relay is to function as a 'deadman switch' and halt the chair should anything go wrong. F o r many injuries, the patient's first reaction is to throw up his hands when excited, and this would immediately stop the chair. Other switch locations can be determined for users who might show a tendency to clutch the joystick at the onset of trouble. The power disconnect relay is also used as a 'brake' for the wheelchair. Several contacts of this relay are used to short the respective motors so that dynamic braking is employed. 3 Joystick controller Numerous types of electrical coupling to the joystick are possible and could be classified as
(a) resistive, such as wirewound potentiometers (b) inductive, such as the induction potentiometers used in the controller described here (c) capacitive, where the joystick moves a common plate relative to a set of quadrant plates (d) photoelectric, where a light beam is modulated by a moving screen of graded transmission density or a coded disc (Gray code for digital control systems). With the exception of the wirewound potentiometer, all of these transducer systems can be constructed to be wear free. The induction potentiometer was selected for this application as it was readily available. On a production basis, any o f the above would be possible, and production cost anyalsis of the system would be the best method of selecting the joystick transducer to use.
4 Signal processor The signal-processor electronics (Fig. 2) is fabricated on a plug-in circuit board for ease of access. Part of the circuit is dependent on the particular type of joystick transducer selected. Certain transducers, such as the wirewound potentiometers and some of the photoelectric devices, produce an analogue voltage proportional to the joystick position and require minimal interfacing to the other signal-processing electronics. Other transducers, such as induction potentiometers and coded discs, require additional circuitry to obtain a signal that is proportional to position. With the selection
chair acceleration adjuslment (provides maximum starting torque independent of
i f I
adjustable time averaging to compensate for erratic hQnd movement
maximum chair speed adjustment (must be
decreased as averaging increased)
Fig. I Wheelchair control system M e d i c a l & B i o l o g i c a l Engineering & C o m p u t i n g
of the induction potentiometer as the transducer, two additional circuits are necessary. The basic operation of the induction potentiometer is similar to that of a transformer. An excitation is applied to a primary winding and a voltage is induced in its secondary winding. This induced secondary voltage in the case of the induction potentiometer varies in both phase and amplitude with the position of the secondary winding. The two additional circuits required are the 400 Hz excitation circuitry and the phase and amplitude detection circuits. The 400 Hz excitation signal is generated using a Fairchild NE555 timer chip. Although the induction potentiometer can operate successfully using the square wave output of the timer, a ]owpass filter is included in its output resulting in a 400 Hz sinewave in order to reduce the propagation of high-frequency signals throughout the system. The phase and amplitude detection of the output of the potentiometer is performed by operational amplifiers OAt and OA2 shown in Fig. 2. The actual output of the potentiometer is a 400 Hz sinewave whose amplitude increases from zero as the joystick is displaced from its normal upright position. The phase of the signal
with respect to the excitation is a function of the direction of that displacement; i.e. a forward movement of the joystick produces an inphase signal and a backward displacement results in a signal phase shift of 180 ~. Phase detection is accomplished by the summing of the excitation signal with the signal out of the potentiometer. An inphase condition at the potentiometer results in a signal that increases in amplitude above that of the excitation signal, while a 180 ~ phase shift results in a reduction in the summed amplitude. Operational amplifier OA1 functions as a zero-crossing detector with an output that is phase shifted by 180 ~ OA2 sums twice this signal with the original signal, producing a full-wave rectified signal. Filtering by integration is also performed by OA2. Amplifiers OAa and OA5 process the signals of the actual feedback control system. This circuitry would be necessary for any transducer chosen. The major role of OA3 is the summation of the desired speed signal from the transducer and the feedback signal which is proportional to actual speed. This resulting signal is used to control the motor-drive system. Because a bipolar signal is required by the power circuits, a bias is summed with the two
R9 39kG f 1 R7 ~---- 33 k.O. I
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drive A ...__ --"
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R 23...~8.2 k.O. tachometer feedback Fig. 2 Signal proeessor C7, R1 s and R19 are individually adjusted for each patient and the make andmodel of wheelchair and included in a plug-in module
Medical & Biological Engineering & Computing
signals in order to remove the offset voltage introduced by the phase-amplitude detector circuitry. The gain of the forward control loop Ke is also determined by OAa. Amplifier OA4 is included to produce a control signal of equal amplitude with a 180 ~ shift in phase to that of the output of OA3. This signal is used to drive the second half of the motor control bridge. Although accurate measurements of motor speed can only be obtained through use of a tachometer, this technique would be too expensive for wheelchair use. An alternative method of speed measurement is the measurement of the back electromagnetomotive force (e.m.f.) produced by the motor. This e.m.f, increases with motor speed and can be measured as a voltage on the motor terminals if the motor is driven by a current source. Although the method is not as accurate as a tachometer, speed regulation is possible using this signal. The back e.m.f. voltage is measured by OAs. As the power electronics actually reverse the voltage to the motor, a differential configuration for OA5 is necessary. Diodes are placed on the amplifier inputs to protect it against motor voltage spikes. The gain of the feedback loop is determined by the feedback resistor of OAs. providing an adjustment on the chair acceleration. The feature of automatically compensating for spastic- or tremour-type motion in a user is Controlled in the signal processor. This feature can be included by the addition of more capacitance in the
T5 i~.>~o drive A
[ variable resistor d.c. drive motor
feedback around OA2. When this is done, OA2 operates as an integrator, permitting motion information to be extracted from spastic motion of the joystick. Such a feature, hox~ever, creates a sluggish response to joystick commands, and a reduction in the forward loop gain or the gain of OA~ may be necessary.
5 Motor-drive system Each of the two wheelchair drive motors are controlled by the bridge arrangement shown in Fig. 3. One induction potentiometer and its associated signal-processing channel control one of the drive motors. Output from the control processor consists of two phase-inverted signals (drive A and drive B). These signals are power amplified by power operational amplifiers T5 and T6. In turn, these signals drive opposite arms of the power transistor bridge T3A-TaB and T4B-T4A through the driver transistor T1A-T1B and T2B-T2A. If drive A is positive and drive B is negative, then base current is driven into T1A which drives T4A on. Simultaneously, base current is driven out of T2B and hence drives TaB on. Motor armature current is established from node 2 to node 1. If drive A had been negative and drive B positive, the motor armature current would have been from node 1 to node 2. In this manner the motor may be driven forward-neutral-reverse in a proportional control mode. The only power source requirement is a dual battery system with centre tap grounded. Voltagesof +6, 0, - 6 , or +12, 0, - 1 2 V are usable, but, because of the collector-emitter voltage drop of approximately 0- 5 to 0.7 V for the power transistor, the system efficiency will be better at the higher operating voltages. To prevent potentially destructive motor transient voltages from damaging the power transistors, varistors are placed across the motor and each power transistor.
feedback to signal processor Fig. 3 Motor-drive system T 2 2N3740
T~ 2N3766 NPN PNP
} . power drivers
T3 2N5302 NPN T4 2N4399 PNP
} . power transistors
Ts, T6 MC 1438R power operational amplifiers VR GE overvoltage protection varistor V33ZA5
Medical & Biological Engineering & Computing
The wheelchair control system has been installed on both 12 V and 24 V Everest & Jennings standard electric wheelchairs. The initial design was done on the 12 V version, and the replacement of the 12 V battery with two 6 V batteries was necessary. This controller was installed on another wheelchair of a patient at the Woodrow Wilson Rehabilitation Centre, Fishersville, Virginia, for evaluation. The new controller enabled the patient, previously mentioned in Section 1, to easily negotiate an obstacle course that he was unable to handle with a standard electric wheelchair. With only a few small changes in component values, the controller was installed on a 2 4 V Everest & Jennings wheelchair. Since the wheelchair
used two 12 V batteries, only the electronics of the Everest & Jennings chair were removed. The 24 V modified chair is presently being evaluated as to reliability at the Rehabilitation Engineering Centre at the University of Virginia. Acknowledgments--The authors are indebted to the physical therapists of the Woodrow Wilson Rehabilitation Centre for the field tests and to the Rehabilitation Engineering Centre at the University of Virginia for its support.
BRUMLIK, J. (1967) Disorders of motion. Am. J. Phys. Med. 46, 536-543. L1PSKIN, R. (1970) An evaluation program for powered wheelchair control systems. Bull. Prosthetics Res. 8PR 10-14, 121-129. LOZACH,Y. et aL (1975) New head control for quadriplegic patients. Ibid. BPR 10-23, 151-157. L~PSKIN, R. (1974) VA prosthetics center program for electric wheelchairs and other nonlicensed mobility aides. Ibid., BPR 10-22.
Medical 8= Biological Engineering & Computing