GB2128750A

GB2128750A - Displacement transducer system

Info

Publication number: GB2128750A
Application number: GB08326409A
Authority: GB
Inventors: John Harrison Asbery
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-10-15
Filing date: 1983-10-03
Publication date: 1984-05-02
Also published as: GB2128750B; GB8326409D0

Abstract

A transducer system includes a variable capacitor 13 formed by a pair of relatively movable elements, with a dielectric medium between. The capacitor 13 is connected in series with a fixed capacitor 14 across the output of an A.C. generator 15. The voltage across the capacitor 14 is sensed to provide a D.C. output signal, which is compared with a reference voltage in a comparator 19 and the difference is supplied to a memory circuit 21 which controls the output voltage of the generator 15. The reference voltage is chosen to be equal to the desired value of the output signal at either the maximum or minimum value of the capacitor 13. In the embodiment the movable one of the pair of elements spends a substantial portion of its time at a stop, such that the capacitance is a maximum. <IMAGE>

Description

SPECIFICATION Transducer system This invention reiates to a transducer system of the kind comprising a pair of elements which form the plates of a capacitor, said elements being relatively movable thereby resulting in a change in the capacitance value of the capacitor, a dielectric medium disposed between said elements, and electrical circuit means responsive to the electrical capacitance between said elements for providing an output signal representative of the relative position of said elements.

Such transducers can be simple in form but in common with many transducers they have the disadvantage that the output can vary as the temperature varies. In the present case the dielectric constant of the dielectric material varies with temperature.

The object of the invention is to provide a transducer system of the kind specified in a simple and convenient form.

According to the invention a transducer system of the kind specified comprises means for comparing said output signal with a reference signal, and means responsive to the difference signal obtained as a result of comparing said signals, for adjusting the output signal to the desired value when the capacitance value is at its maximum or minimum value.

An example of a transducer system in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of the transducer and Figure 2 shows the electrical circuit of the transducer system.

Referring to Fig. 1 a linear transducer is shown and this comprises a pair of elements 10, 11 which are movable relative to each other. A dielectric medium 1 2 is disposed between the elements. In the particular example, the element 10 is a hollow cylindrical member and the element 11 is a cylindrical member slidable within the element 10.

The member 11 may be a piston of a fuel pumping apparatus. It is desired to provide an indication of the relative axial positions of the elements. The elements as will be appreciated form a capacitor and referring now to Fig. 2, the capacitor formed by the elements is indicated at 1 3.

Connected in series with the capacitor 1 3 is a further capacitor 14 and an alternating voltage is applied across the series combination of the capacitors by means of an AC generator 1 5. The capacitors 1 3 and 14 constitute a capacitive voltage divider and the value of the capacitor 14 is chosen so that the AC voltage across the capacitor 1 3 remains substantially constant.

The voltage developed across the capacitor 14 is applied to a difference amplifier 1 6 the output of which is applied to a peak to DC voltage converter 17. The DC signal obtained at the output of the converter constitutes the output signal and this will vary depending upon the relative position of the elements 10 and 11. With the arrangement as described, the output signal will also vary as the temperature of the dielectric material 1 2 varies.

In order to overcome this problem the output signal is applied to one input of a comparator 1 9 the other input of which is supplied with a reference voltage from a potentiometer chain 20. The output of the comparator is applied to a memory circuit 21 which includes a capacitor 22 having a resistor 23 connected in parallel therewith. The capacitor 22 can be charged by way of a diode 24 and a series resistor 25 and the voltage across the capacitor 22 is applied to the AC generator 1 5 in order to control the output voltage thereof.

In the particular example the element 11 spends a substantial portion of its time in contact with a stop 26 and in this position the capacitance value between the elements 10 and 11 is at its maximum. The output signal produced by the converter 1 7 will also be at its maximum value and it is arranged that this value corresponds to the reference voltage applied to the comparator 1 9. If for example the output signal when the element 11 is in contact with the stop, exceeds the reference signal then the output provided by the comparator 1 9 will charge the capacitor 22 so that the control voltage applied to the generator 1 5 will be increased. This will effect a reduction in the output voltage of the generator so that the output signal is restored to its correct value.If the voltage at the output is less than the reference voltage then the voltage across the capacitor 22 will gradually fall due to the parallel resistor 23 and the output voltage of the generator will be increased until the output signal once again equals the reference voltage. The time constant of the capacitor 22 and the resistor 23 has to be chosen with care and it must be such that the change in the capacitor voltage which occurs due to leakage, is small as compared with the time taken for the element 11 to move away from its stop and to return to its stop. Each time the element returns to the stop the small quantity of charge which has been lost from the capacitor 22 during the travel of the element 11 will be made up by the action of the comparator 1 9 and the diode 24 and resistor 25.

The time constant of the capacitor 22 and the resistor 23 will however not be large as compared with the time taken for the temperature of the dielectric to vary. If for example the dielectric constant increases such as to increase the capacitance value of the capacitor 13, the output signal will increase and correc tion as described will take place to reduce the output voltage of the generator 1 5 and vice versa.

The circuit as described effects correction each time the moving element 11 comes into contact with the stop 26 and providing the time constant of the capacitor 22 and resistor 23 are as described, the aforesaid output signal will reflect the position of the element 11 relative to the element 10.

In the example described the correction is carried out when the capacitance value between the elements 10 and 11 is at its maximum value. The circuit can be modified so that correction occurs when the capacitance value between the elements is at a minimum.

Claims

1. A transducer system comprising a pair of elements which form the plates of a capacitor, said elements being relatively movable thereby resulting in a change in the capacitance value of the capacitor, a dielectric medium disposed between said elements, electrical circuit means responsive to the electrical capacitance between said elements for providing an output signal representative of the relative position of said elements, means for comparing the output signal with a reference signal, and means responsive to the difference signal obtained as a result of comparing said signals for adjusting the output signal to the desired value when the capacitance value is at its maximum or minimum value.

2. A system according to Claim 1 including an AC generator for providing an AC voltage which is applied across a series circuit including a fixed capacitor and the capacitor formed by said elements, the means responsive to the difference signal effecting adjustment of the AC voltage produced by said generator.

3. A system according to Claim 2 in which said means for comparing the output signal and the reference signal comprises a comparator and the means responsive to the difference signal comprises a memory circuit including a memory capacitor having a discharge resistor in parallel therewith and a diode through which the memory capacitor is charged by the difference signal.

4. A system according to Claim 3 in which said electrical circuit means comprises a difference amplifier responsive to the AC voltage developed across said fixed capacitor and a peak to DC voltage converter the input of which is connected to the output of the difference amplifier and the output of which provides said output signal.

5. A system according to Claims 3 or 4 in which the value of the reference voltage corresponds to the desired value of the output signal at the minimum or maximum capacitance value of the capacitor formed by said elements.

6. A transducer system comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.