GB2126347A

GB2126347A - Inductive proximity sensors

Info

Publication number: GB2126347A
Application number: GB08223469A
Authority: GB
Inventors: Albert Everett Sloan
Original assignee: SLOAN POWER ELECTRONICS LIMITE
Current assignee: SLOAN POWER ELECTRONICS LIMITE
Priority date: 1982-08-16
Filing date: 1982-08-16
Publication date: 1984-03-21
Also published as: GB2126347B

Abstract

A device provides an electrical output dependent on the position of a hand operated lever or an accelerator pedal, includes a square wave oscillator (9) which switches on and off a transistor (7) at a fixed frequency. A coil (1) is in series with the transistor (7) together with a resistor (10) for measuring the current flow through the coil (1) whose impedance varies in accordance with the proximity of a non-magnetic conductive element (5) connected to the lever or pedal. A differential amplifier (16) provides a voltage output which is proportional to the distance between coil (1) and element (5), one input thereof being taken from the resistor (10) via diode (13) and the other input from a zero-setting potentiometer (17). <IMAGE>

Description

SPECIFICATION Solid state analogue proximity device Many types of equipment used in the mechanical handling industry such as cranes, Battery electric fork lift trucks and Battery electric road vehicles are controlled by the operator by means of a manually operated control lever or accelerator pedal. Such control mechanisms normally rely upon the rotation of a potentiometer to provide an output which varies as the control lever is moved. Other methods are used in so called 'solid state accelerators' such as varying the mutual coupling between two coils, which are mounted in close proximity, by moving a steel vane between the coils. Other means are known but in common with the methods described they all rely on some direct mechanical coupling between the device and the control lever or pedal.In addition, the electrical parts of such devices are usually mounted inside an enclosure and the movement of the control lever must be transmitted to the active parts of the device via a shaft rotating in a bush or via a plunger passing through the wall of the enclosure. Such systems are obviously prone to wear or the ingress of moisture, both of which may impair the operation of the device. Although these defects may be limited by good design and the use of high quality materials, the cost of the components and the cost of assembly, reduces the acceptability of these devices.

A preferred device would be totally encapsulated and require no moving parts or direct mechanical coupling to the control lever or pedal. Such a device is herein described. The device is designed to produce an output voltage depending upon the proximity of an electrically conductive material other than magnetic materials such as steel or ferrite. It is preferable that the device does not respond to steel as in most cases it is likely to be mouned onto a steel bulkhead or footplate, which may of course interfere with the proper operation of the detector. The principle of operation is as follows: A specially designed coil is supplied with a continuous chain of voltage pulses of constant amplitude and duration. During each pulse the current in the coil rises exponentially depending upon the circuit impedance.If a piece of conductive material is placed close to the centre of the coil, a current is induced in the material by transformer action. The low impedance of the material is reflected to the primary of the 'transformer" ie. the coil and the impedance of the coil circuit is consequently reduced. The change in impedance may be sensed in a number of ways and a signal may therefore be produced which depends upon the distance between the coil and the conductive material. Such a signal may then be used to control the operation or speed of a piece of machinery. Currents would be also induced in mild steel, but a secondary effect occurs which has an opposite and approximately equal effect on the impedance of the coil circuit. A magnetic material such as steel also modifies the magnetic circuit of the coil and consequently causes an effective increase in the coil inductance.The system will therefore only repond to a material or combination of materials in which the electrostatic and electromagnetic properties are.dissimilar.

The operation of the system may be more easily understood by reference to the mechanical arrangement shown in Figure 1. and the example of an electrical circuit means shown in Figure 2. The operation of the system is as follows.

A coil 1. is encapsulated in epoxy resin together with an electronic circuit 2. The complete module so formed is shown mounted to a sheet steel plate which may for example be the footplate of a vehicle.3.An accelerator pedal 4. made of any suitable material such as steel has attatched to it a disc of non-magnetic, conductive material 5. which may, for example, be aluminium. The coil 1. is designed to have a large diameter and a very short length to enable the centre of the coil 1. to be positioned as near as possible to the upper surface of the encapsulation 6. The aluminium disc 5, is so positioned that when the pedal 4, is depressed it comes in close proximity with the coil 1, and may be allowed to come into contact with the surface of the encapsulation 6.The electronic circuit 2, is designed as, described below, to provide an output dependant upon the distance between the disc 5, and the coil 1, which is suitable for the control of the equipment eg.

fork-lift truck.

In the circuit shown in Figure 2, the coil 1, is connected to the collector of a transistor 7, the base of which is supplied from oscillator 9, which provides a continuous chain of pulses of constant width and at a constant frequency thereby turning transistor 7, on and off repeatedly. The emitter of the transistor is connected to the positive terminal of a source D.C. voltage 8. The negative end of the coil 1, is connected via resistor 10, to the negative of the voltage source 8. Diode 11 and resistor 12 are connected in series across the coil 1,to protect the transistor 7, from voltage break-down during turn off. The anode of diode 13, is connected to the negative end of coil 1, and its cathode is connected to capacitor 14, resistor 15, and the non-inverting input of amplifier 16.The other ends of capacitor 14, and resistor 15, are connected to the negative terminal of voltage source 8. The inverting input of the amplifier 16, is connected to the slider of potentiometer 17, the ends of which are connected across the voltage source 8. A resistor 18, is connected from the inverting input and the output 19, of amplifier 16.

When transistor 7, turns on, a pulse appears at its collector as shown in Figure 3(a). During time t., if the disc 5, is remote from the coil 1, a current pulse as shown in Figure 3(b). will flow through the coil 1, and resistor 10. The current pulse produces a voltage across resistor 10, and a voltage dependant upon the peak voltage will be stored on capacitor 14, via the diode 13. Under these conditions the slider of potentiometer 17, is adjusted to cause the the amplifier output 19, to equal zero. The minimum output therefore coincides with the lowest value of peak current passing through coil 1, and the resistor 10.As the aluminium disc 5, is moved towards the coil 1, the effective impedance of the coil 1, assumes a lower value and the current in the coil 1, during the time twill increase at a greater rate and the peak current will be correspondingly greater. In the limit, when the disc 5, is in close proximity with the coil 1, the voltage appearing across resistor 10 will be as shown in Figure 3(c). The peak voltage stored on capacitor 14, will also be greater and the output of the amplifier 16, will increase to a value determined by its gain which may be fixed by the value of resistor 18.

This description refers to a disc of material which is moved in relation to the coil along its axis, however the system would work equally well with material of a different shape and where it is moved perpendicular to the axis of the coil along the face of the coil and towards or away from the centre of the coil.

One embodiment of the circuit is described but there are many other ways of detecting the variation in impedance of the circuit dependant on the position of the conducting material. Similarly, the device would work equally well if it were mounted onto a moving vehicle or member when it would operate as the vehicle approached conductive material which could be operative in stopping the vehicle, or producing a warning signal thereby operating as an anti-collision device.

Claims

1. An electronic device for providing an electrical output which is dependent on the proximity of a non-ferrous element connected to a device to be controlled: said electronic device including: means for generating a series of pulses: means for supplying these pulses to an electrical element of an electro-magnetic field generating device; means for detecting the change in impedance across said electrical element as a result of a variation in the distance between said electro-magnetic field generating device and said element of the device to be controlled; and means for providing an electrical output from the electronic device, which is a function of the impedance of the electrical element of the electro-magneticfield generating device.

2. An electronic device according to Claim 1, wherein said electro-magnetic field generating device is a coil and said element is made of a non-magnetic, electrically conductive material.

3. An electronic device according to Claim 2, wherein the coil has a relatively large diameter in comparison with its length which is relatively short.

4. An electronic device according to Claim 2 or 3, wherein said coil is encapsulted in epoxy resin.

5. An electronic device according to any one of the preceding claims, wherein said means for generating a series of current pulses includes an oscillator which provides a continuous chain of pulses of constant width and constant frequency.

6. An electronic device according to Claim 5, wherein said means for supplying the pulses to the electrical element of the electro-magneticfield generating device is a transistor which is switched on and off by said chain of pulses output from the oscillator.

7. An electronic device according to Claims 2 and 6 wherein said transistor and coil are connected in series across a D.C. voltage supply and a series combination of a resistor and diode are connected in parallel with the coil to protect the transistor against voltage surges on switching.

8. An electronic device according to any one of the preceding claims, wherein said means for detecting a change of impedance across the electrical element comprises a resistor in series therewith.

9. An electronic device according to Claim 8, wherein the means for providing an electrical output which is a function of the impedance of the electrical element includes a differential amplifier, one input of which is connected to the junction between said resistor and said electrical element, the other input of which is connected two a potentiometer which is used to calibrate the output of the differential amplifier with respect to the distance of said element connected to the device to be controlled and the electro-magneticfield generating device.

10. An electronic device according to any one of the preceding claims, wherein said device to be controlled is a vehicle or mechanical handling device, the element being an accelerator pedal or manually operated control lever.

11. An electrical device for providing an electrical output which is dependent on the proximity of an element connected to a device to be controlled, substantially as herein described with reference to and as illustrated in the accompanying drawings.