GB2078367A - Control device - Google Patents

Abstract

A control device for a motor 24 has a pulse width modulation control circuit 22. In place of the usual potentiometer, this is controlled by a device comprising a light dependent resistor 18, two polarising filters 14, 16, and a light emitting diode 12. On rotation of a shaft 26, the relative planes of polarisation of the filters are changed, varying the transmission of light from the LED 12 to the LDR 18, and changing the latter's resistance to alter the motor speed. The invention is not limited to the control of motor speeds. <IMAGE>

Description

SPECIFICATION Control device This invention relates to control devices.

The potentiometer or variable resistor is a well known device for allowing manual control of an electrical or electronic circuit in many types of equipment. It has the advantage of being cheap, but it is well known that it is very inaccurate. In particular its response is far from being linear, and in wirewound potentiometers tends to vary stepwise.

Potentiometers are used, for example, in the pulse width modulation type of motor controller for motors such as used on forklift trucks and electric cars.

The present invention provides a control device comprising a light sensitive electrical element arranged to receive light from a source thereof through at least a pair of polarising filters, one filter being movable with respect to the other so as to vary the amount of light impinging on the light sensitive element from the source.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawing, which is a schematic diagram of apparatus for controlling the speed of a motor.

Referring to the drawing, the speed of a d.c. motor 24 for a forklift truck is controlled by a conventional pulse width modulation control circuit 22, which acts by supplying the motor 24 with pulses of current.

The width of these pulses governs the amount of power supplied to the motor and hence its speed.

In place of the conventional potentiometer control of the controller 22, a light dependent resistor 18, type ORP12, is connected to the controller 22 by wires 20. A disc 16 of a sheet of polarising filter material is secured over the light sensitive face of the light dependent resistor (LDR) 18. The LDR 18 is illuminated through the filter 16 by a light emitting diode (LED) 12, which is supplied from a suitable power supply 10. The power supply 10 may be stabilised if desired to ensure a constant light output from the LED 12.

Between the filter disc 16 secured to the LDR 18 and the LED 12, there is a further disc of polarising filter material 14. This disc 14 is mounted on a shaft 26 for rotation therewith. The shaft 26 may be for example rotated by a twist-grip device or a knob (not shown), so that an operator can adjust the orientation of the filter disc 14 with respect to the filter disc 16. By rotating the shaft 26, therefore, the direction of polarisation of the disc 14 can be varied from a position in which it is parallel to that of the disc 16 to a position in which it is perpendicular thereto. The transmission of light from the LED 12 to the LDR 18 varies correspondingly between a maximum and a minimum. This variation is to a high degree of accuracy linearly dependent on the rotation of the shaft 26.The shaft 26, or the knob or twist-grip to which is is attached, may be provided with end stops so as to limit its travel to the 90C of rotation between these maximum and minimum positions.

The variation of the amount of light transmitted to the LDR 18 varies its resistance, and this varying resistance acts on the controller 22 in exactly the same way as would a potentiometer. However, the variation in resistance, and hence the control, can be much more accurately linear, depending only on the linearity of the response of the LDR 18. Also, the variation does not suffer from the stepwise variations often inherent in potentiometer control, and the noise problems often eventually caused by dust and by the continual wiping of the wiper of the potentiometer along its track are also avoided.

It will be appreciated that the above description is merely illustrative, and many variations can be made. For example a different kind of light source can be used rather than the LED 12, such as an incandescent light bulb, and different mechanical arrangements can be used to rotate the disc 14 rather than the shaft 26. Rather than having manual rotation of the shaft 26, the arrangement described might be used in place of a potentiometer acted on by a motor in a servo controlled system. The invention is not limited to the control of motor speeds; it could of course easily be used in any other situation in which a potentiometer or variable resistor might be used.Instead of having one filter rotatable with respect to the other, a similar effect can be achieved by moving one filter across another into and out of the path of the light with the directions of polarisation of the two filters being generally at right angles to each other.

1. A control device comprising a light sensitive electrical element arranged to receive light from a source thereof through at least a pair of polarising filters, one filter being movable with respect to the other so as to vary the amount of light impinging on the light sensitive element from the source.

2. A control device according to claim 1 wherein the filters are relatively rotatable between a first position in which their planes of polarisation are parallel, and a second position in which their planes of polarisation are crossed.

3. A control device according to claim 2 wherein one filter is mounted on a shaft which is arranged for manual rotation.

4. A control device according to any one of the preceding claims wherein the light sensitive electrical element is a light dependent resistor.

5. A control device according to any one of the preceding claims wherein one of the filters is secured over a light sensitive face of the light sensitive element.

6. A control device according to any one of the preceding claims provided with a light emitting diode as said light source.

7. A control device substantially as described herein with reference to the accompanying drawings.

8. A motor having a control circuit including a control device according to any one of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Control device This invention relates to control devices. The potentiometer or variable resistor is a well known device for allowing manual control of an electrical or electronic circuit in many types of equipment. It has the advantage of being cheap, but it is well known that it is very inaccurate. In particular its response is far from being linear, and in wirewound potentiometers tends to vary stepwise. Potentiometers are used, for example, in the pulse width modulation type of motor controller for motors such as used on forklift trucks and electric cars. The present invention provides a control device comprising a light sensitive electrical element arranged to receive light from a source thereof through at least a pair of polarising filters, one filter being movable with respect to the other so as to vary the amount of light impinging on the light sensitive element from the source. An embodiment of the invention will now be described by way of example, with reference to the accompanying drawing, which is a schematic diagram of apparatus for controlling the speed of a motor. Referring to the drawing, the speed of a d.c. motor 24 for a forklift truck is controlled by a conventional pulse width modulation control circuit 22, which acts by supplying the motor 24 with pulses of current. The width of these pulses governs the amount of power supplied to the motor and hence its speed. In place of the conventional potentiometer control of the controller 22, a light dependent resistor 18, type ORP12, is connected to the controller 22 by wires 20. A disc 16 of a sheet of polarising filter material is secured over the light sensitive face of the light dependent resistor (LDR) 18. The LDR 18 is illuminated through the filter 16 by a light emitting diode (LED) 12, which is supplied from a suitable power supply 10. The power supply 10 may be stabilised if desired to ensure a constant light output from the LED 12. Between the filter disc 16 secured to the LDR 18 and the LED 12, there is a further disc of polarising filter material 14. This disc 14 is mounted on a shaft 26 for rotation therewith. The shaft 26 may be for example rotated by a twist-grip device or a knob (not shown), so that an operator can adjust the orientation of the filter disc 14 with respect to the filter disc 16. By rotating the shaft 26, therefore, the direction of polarisation of the disc 14 can be varied from a position in which it is parallel to that of the disc 16 to a position in which it is perpendicular thereto. The transmission of light from the LED 12 to the LDR 18 varies correspondingly between a maximum and a minimum. This variation is to a high degree of accuracy linearly dependent on the rotation of the shaft 26.The shaft 26, or the knob or twist-grip to which is is attached, may be provided with end stops so as to limit its travel to the 90C of rotation between these maximum and minimum positions. The variation of the amount of light transmitted to the LDR 18 varies its resistance, and this varying resistance acts on the controller 22 in exactly the same way as would a potentiometer. However, the variation in resistance, and hence the control, can be much more accurately linear, depending only on the linearity of the response of the LDR 18. Also, the variation does not suffer from the stepwise variations often inherent in potentiometer control, and the noise problems often eventually caused by dust and by the continual wiping of the wiper of the potentiometer along its track are also avoided. It will be appreciated that the above description is merely illustrative, and many variations can be made. For example a different kind of light source can be used rather than the LED 12, such as an incandescent light bulb, and different mechanical arrangements can be used to rotate the disc 14 rather than the shaft 26. Rather than having manual rotation of the shaft 26, the arrangement described might be used in place of a potentiometer acted on by a motor in a servo controlled system. The invention is not limited to the control of motor speeds; it could of course easily be used in any other situation in which a potentiometer or variable resistor might be used.Instead of having one filter rotatable with respect to the other, a similar effect can be achieved by moving one filter across another into and out of the path of the light with the directions of polarisation of the two filters being generally at right angles to each other. CLAIMS

1. A control device comprising a light sensitive electrical element arranged to receive light from a source thereof through at least a pair of polarising filters, one filter being movable with respect to the other so as to vary the amount of light impinging on the light sensitive element from the source.

2. A control device according to claim 1 wherein the filters are relatively rotatable between a first position in which their planes of polarisation are parallel, and a second position in which their planes of polarisation are crossed.

3. A control device according to claim 2 wherein one filter is mounted on a shaft which is arranged for manual rotation.

4. A control device according to any one of the preceding claims wherein the light sensitive electrical element is a light dependent resistor.

5. A control device according to any one of the preceding claims wherein one of the filters is secured over a light sensitive face of the light sensitive element.

6. A control device according to any one of the preceding claims provided with a light emitting diode as said light source.

7. A control device substantially as described herein with reference to the accompanying drawings.

8. A motor having a control circuit including a control device according to any one of the preceding claims.

9. A motor according to claim 9 wherein the control circuit is of the pulse width modulation type.