GB2107487A - Electric motor control systems - Google Patents

Abstract

In an electric motor control system motor M is controlled from an impedance bridge circuit. An adjustable control impedance TH1 is set by manual or automatic control and an adjustable feedback impedance FB1 is adjusted mechanically by rotation of the motor M. The control system has means R1, TR.1 for monitoring a stall-indicating characteristic of the motor and means Ic.2d, TR.2 to cut off the power supply to the motor when the characteristic indicates that the motor is close to or has reached a stall condition. It also has comparator means Ic.2a for comparing the potential inputs such that when the power supply to the motor has been cut-off it can only be restored in response to the output of the comparator if there is imbalance between the potential inputs in the opposite sense to that existing when the power supply was cut-off. <IMAGE>

Description

SPECIFICATION Electric motor control systems This invention relates to control systems for electric motors, and more particularly for electric motors used in the heating, ventilating and air conditioning equipment of motor vehicles for operating flap valves, water valves or other controls of such equipment.

Control systems for electric motors used to operate flap valves or water valves are designed to effect rotation of the motor to position the valve proportionately throughout its range of travel under manual or automatic control. When operated under these controls a condition can occur such that the valve is fully over to one extreme of travel or the other. Desirably therefore the control system must discriminate when such conditions occur and remove the power from the motor and then recognise when the automatic or manual control has returned to a condition where proportional control of the valve is again possible.

The object of the invention is to provide a motor control system which achieves the above desirable objectives.

According to the invention there is provided an electric motor control system in which the power supply to the motor is controlled from an impedance bridge circuit, one arm on one side of the bridge comprising an adjustable control impedance whose setting is determined by manual or automatic control and one arm on the other side of the bridge comprising an adjustable feedback impedance whose setting is adjusted mechanically by rotation of the motor, the respective outputs from said one side and said other side of the bridge providing potential inputs for controlling the power supply to the motor, said control system having (i) means for monitoring a stall-indicating characteristic of the motor and to cut off the power supply to the motor when said characteristic indicates that the motor is close to or has reached a stall condition, and (ii) comparator means for comparing said potential inputs such that when the power supply to said motor has been cut-off it can only be restored in response to the output of said comparator if there is imbalance between said potential inputs in the opposite sense to that existing when said power supply was cut-off.

The invention will now be further described by way of example with reference to the accompanying circuit diagram of one motor control system in accordance with the invention for controlling a flap valve in the ventilating equipment of a motor vehicle.

Referring now to the circuit diagram, series connected resistor R4 and Zenar diode Z4 create a stable voltage source from the supplied voltage source of the vehicle battery/alternator. The stable voltage source drives a bridge circuit, one arm of which on one side of the bridge comprises adjustable control resistor TH. 1 and another arm on the other side of the bride comprises adjustable feed back resistor FB.1. Adjustment of resistor TH.1 is by automatic or manual control to set the flap valve to a desired position. Adjustment of resistor FB.1 is effected through a mechanical connection to the motor so that its setting is determined by the angular position of the motor rotor.

Points A and B are the output points of the bridge circuit and provide respective input potential signals to the rest of the circuit. The difference between the output potentials at A and B is therefore determined by the relative settings of TH.1 and FB.1.

lc.1 is a dual power amplifier comprising amplifiers a and b, IC.2 is Quad Norton operational amplifier comprising amplifiers ato d, Tr.1 is an NPN transistor which acts as an amplifier switch and TR.2 isan N-channel Mosfettransistor.

lc.1 is used as a power source for the D.C. motor M which is connected across the respective outputs of the amplifiers a and b of It.1. The inputs of the amplifiers a and b of lc.1 are connected in bridge form, such that any potential differences between the respective inputs to the amplifiers a and bare amplified positively by one amplifier, and negativeiy by the other. Hence by changing the polarity of the inputs the motor M can be driven in a clockwise or counterclockwise direction.

Resistor R1 acts as a sensing element for the current amplifierTR.1. voltage is generated across R1 by the current passing through it, which is proportional to the current flowing through the motor M. The voltage generated is filtered and smoothed by resistor/capacitor circuit R2/C2 and when enough currentflowsthrough R1 indicating that the motor M has stalled and proportional control from the bridge circuit is no longer possible, TR.1 will be turned on, and its collector will go 'low'.

Amplifier Ic.2d works as a NAND flip-flop, e.g.

when one input goes low the output is set high, and when the other input goes low the output is set low.

In this case when the collector of TR.1 goes low, the output of lc.2d goes high. This activates TR.2 which is a Mos transistor working in switch mode. When TR.2 goes high, TR.2 source/drain resistance is low, the inputs to amplifier IC.1 are shorted-out, and hence go to the same potential and the motor M has no power applied to it.

The purpose of amplifiers lc.2a., and lc.2b/lc.2c is to determine when the bridge has returned to a state where proportional control is possible. This is achieved in the following manner. Amplifier lc.2a amplifies the difference between the outputs of the bridge circuit. Ic.2b is a comparator which can be adjusted by RV.1. The output of amplifier lc.2b is ac coupled to amplifier lc.2c, which is a dual - edge triggered monostable, with negative - pulse output.

If it is assumed that the flap valve has reached the end of its travel, which has stalled the motor, this has triggered TR.1 which has latched lc.2d and turned on TR.2, which has shorted the inputs to the motor M and the motor M now cannot move. This state will remain unchanged until the flip-flop ic.2d is reset by a negative-going pulse at input 2 of amplifier. lc.2d.

As the feedback potentiometer FB.1 is mechanical ly connected for adjustment by the motor M, when the flap valve is at the end of its travel FB.1 cannot changevalue andthe outputfromithisside of the bridge will remain static at a particular potential. The output from the other side withe bridgeTH.1 can now fluctuate, without affecting thopower supply output to the motor M, as long as itsivalue is greater than that from the side of the bridge containing FB.1.

As soon as the output from the side of the bridge containing resistorTH.1 falls below that:ofthe other side of the bridge containing FB.1 the comparator lc.2b change state. This triggers the monostable IcSc which gives a negative going pulse, resettingth flip-flop lc.2d and turning-off (high impedance; ra which unclamps the short-circuit across ts to the amplifier Ic.1, allowing the motor M to rrsoveato the new position prescribed by the bridge circl The motor M can now move freely until balance of the bridge is achieved.

Thus the circuit will operate in a prescribed range as a normal proportional controlling servo ampsfer circuit. However, should the flap valve reach thefuil extent of its travel,the cirucit will turn itself off, cutting powerto the motor M, until such time as the bridge circuit returnsto within the prescribed limits Then the servo-amplifierwill be unclamped and the motor M is free to operate again as normal.

In the above described example the characteristic of the motor used to indicate a stall or imminent stali is the current drawn by the motor. As an alternative the stall-indicating characteristic could be motor speed. For example a pulse could be derived from the rotation of the motor and whose frequency is proportional to motor speed. Hence if the motor is running very slowly, as it does for example when a flap valve engages a resilient seating in the closed position, or the motor stalls, the decrease in. frequen cy is used to trigger the cut-off of the powersupply to the motor.

Claims (7)

1. An electric motor control system in which the power supply to the motor is controlled from an impedance bridge circuit, one arm on one side ofthe bridge comprising an adjustable control. impedance whose setting is determined by manual or automatic control and one arm on the other side of the bridge comprising an adjustable feedback impedance whose setting is adjusted mechanically by rotation of the motor, the respective outputs from said one side and said other side of the bridge providing potential inputs for controlling the power supply to the motor, said control system having (i) means for monitoring a stall-indicating characteristic of the motor and to cut off the power supply to the motor when said characteristic indicates that the motor is close to or has reached a stall condition, and (ii) comparator means for comparing said potential inputs such that when the power supply to said motor has been cut-off it can only be restored in response to the output of said comparator if there is imbalance between said potential inputs in the opposite sense to that existing when said power supply was cut-off.

2. An electric motor control system according to Claim 1, wherein said potential inputs are fed to respective amplifiers,the difference between the outputs of the amplifiers constituting the potential difference between the motor terminals.

3. An electric motor control system according to Claim 1 or 2, wherein said monitoring means is arranged to control switching means which latches a flip41p to cause the power supply to the motor terminals to be cut off when the motor is close to or twas reached a stall condition, said flip-flop being arranged to be reset in response to the putput from said comparator means indicating that the inbalance between said potential inputs has changed sense following the latching of said fli-p-flop.

4. An electric motor control system according to Claim 3 wherein said bridge circuit is a D.C. bridge circuit and said comparator means isA.C. coupled to a monostablewhich produces a pulsed output to reset said flip-flop.

5. An electricmotorcontroisystem according to Claim 3 or 4 wherein the output from said flip-flop operates second switching means to effect short circuiting of said motor terminals in order to cut off the power supply to the motor.

6. An electric motor control system according to Claim 5, wherein said second switching means comprises a Mosfettransistor operating in switch mode.

7. An electric motor control system substantially as hereinbefore described with reference to the accompanying drawing.