Patent GB-1514482-A (GB)
Title: Wheelchair control circuit
IPC: B60L15/20
Filed: 1974-05-23
Abstract: 1514482 Control of D.C. motors POSSUM CONTROLS Ltd 12 May 1975 [23 May 1974] 23004/74 Heading H2J A control circuit for controlling the two electric motors of a wheelchair comprises two control potentiometers having wipers connected to a common control lever so as to be movable both independently and simultaneously whereby, in operation, voltages at the wipers have magnitudes dependent upon the positioning of the control lever, deviations on opposite sides of a predetermined magnitude voltage corresponding to a datum position of the lever constituting control signals for forward and reverse operation of the motors, a respective reversing relay for each motor, a respective detector circuit for each potentiometer and associated relay, each including means for detecting an output voltage from its associated potentiometer representative of a control signal for reverse operation of the associated motor to energize the associated reversing relay, and means for driving each motor in dependence upon the deviation of the output voltage at the wiper of the associated potentiometer from the datum. The circuit shown in Fig. 1 is for controlling one motor 21 only, the control circuit for the other motor being identical. The output of the control potentiometer 10 is applied to a pulse width modulator 13, via an amplifier 11 and circuit 12, the modulator being supplied with a constant frequency sawtooth waveform signal from an oscillator 14. The pulse output of the modulator is then applied to the motor via AND gate 15, power amplifier 16, resistor 17, power/ brake relay switch 18 (which is shown in the brake position), and reversing relay switches 19, 20. When the control lever is moved to put the motor in reverse, a detector 34 detects the change in output of the potentiometer, and energizes the reversing relay (having switches 19, 20) via gates 33, 50 and a bistable circuit 31, 32. When the control lever is returned to its datum position, a detector 27 detects the absence of pulses at the output of modulator 13 and thereby causes a power/brake relay 25 (having switch 18) to be energized to move switch 18 to the position shown so that a resistor 26 is connected across the armature of the motor 21. When the control lever is moved to the full-speed position, a detector 70 senses the high level d.c. output of modulator 13 and, after a delay of one second, energizes a relay 71 having contacts 72 which close to switch the battery supply directly to the contacts 18. At the same time the power amplifier is switched off automatically. Fig 2 (not shown) shows details of the basic circuitry just described. However, the system shown in Fig. 1 includes means for protecting the circuit against damage or abuse. To protect the motor against overload caused, e.g., by stall conditions, a current detector 35 detects excessive current through resistor 17 and a cut out circuit 36 is activated for a specific time, e.g. seven seconds, which causes the output of detector 27 to be held low via OR gate 68. AND gate 15 then ensures that no pulses reach the power amplifier 16 and the relay 25 is held in the brake position. The cutout also de-energizes a parking brake relay 66 (having contacts 67) to apply a heavy brake to the motor and to switch off an indicator lamp 69. To ensure that the reversing contacts change on "no-load" condition, a detector 54 senses the armature voltage and supplies a signal to gates 33, 50 via an OR gate 51. If the control lever is moved from a "forward" position to a "reverse" position, before the relay 30 has time to operate power drive is removed by the changed output of a coincidence detector 53 acting through the OR gate 68, detector 27 and gate 15. The coincidence detector 53 receives its inputs from detector 34 and from a delay circuit 52 connected to one output of the bistable circuit. The output of detector 53 remains changed for the period of the delay of device 52 and during this time the contacts 19 and 20 and contacts 18 change to apply the brake. To further protect against large control lever movements, e.g. "full ahead" to "full reverse", a detector 60 is provided to detect a decreasing output of circuit 12 crossing a preset threshold and triggers a retriggerable delay circuit 61 which provides a 0À8 second pulse to gate 51. The presence of this pulse in a reversal situation inhibits change of the bistable circuit 31, 32 and thus prevents reversal of contacts 19, 20 while not interfering with braking and power removal.