GB2133890A

GB2133890A - Displacement transducer

Info

Publication number: GB2133890A
Application number: GB08400491A
Authority: GB
Inventors: Keith Heppenstall
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1983-01-19
Filing date: 1984-01-10
Publication date: 1984-08-01
Also published as: GB8400491D0

Abstract

A displacement transducer comprises a first and a second series of conductive elements formed on a pair of relatively movable members 11, 13, the elements being spaced apart in the direction of relative movement of the members. A third and fourth series of conductive elements are formed on a further pair of members 14, 12 relatively movable in synchronism with the members 11, 13. The number of elements in the first and second series is different to the number of elements in the third and fourth series whereby the variation of capacitance between the third and fourth series of elements will move into and out of phase with the variation of the capacitance between the first and second series of elements, means being provided to determine the phase difference. In Fig. 1 discs 13 and 14 are mounted on a shaft and discs 11 and 12 are fixed. <IMAGE>

Description

SPECIFICATION Displacement transducers This invention relates to displacement transducers of the capacitive type and has for its object to provide such a transducer in a simple and convenient form.

According to the invention a displacement transducer of the capacitive type comprises first and second series of conductive elements mounted on a first pair of relatively movable surfaces, the capacitance between said first and second series varying as relative movement takes place between the surfaces, third and fourth series of conductive elements mounted on a second pair of relatively movable surfaces which are movable in synchronism with the first mentioned pair of surfaces, the capacitance between said third and fourth series of elements varying as relative movement takes place between the surfaces, the elements of each series being spaced apart in the direction of relative movement of the surfaces, characterised in that the number of conductive elements in the first and second series and the third and fourth series is different and means for comparing the phase relationship of the capacitance signals produced by the first and second series and the third and fourth series of conductive elements.

An example of a displacement transducer in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side elevation of the transducer, Figure 2 shows the conductive patterns on disc forming part of the transducer.

Figure 3 is a circuit diagram of one part of the circuit associated with the transducer, and Figure 4 is a pulse diagram.

Referring to the drawings the displacement transducer is intended to be used for providing an indication of the angular movement of a shaft 10 and includes an outer pair of fixed discs 11, 12 and an inner pair of discs 13, 14 which are mounted on the shaft 10 so as to be angular movable therewith. The disc 11 and 13 are secured to some convenient support not shown.

The outer disc 11, 12 have a conductive pattern on their inner surfaces part of which is indicated in the upper portion of Figure 2 and the discs 13, 14 have on their surfaces presented to the discs 11 and 12 respectively a conductive pattern generally as shown in the lower portion of Figure 2. The pattern on the disc 11 is described as a first series of conductive elements, that on the disc 1 3 as a second series of conductive elements, that on the disc 12 as a third series of conductive elements and that on the disc 14 as a fourth series of conductive elements.The first and third series of conductive elements are substantially identical the difference being in the number of elements and as will be seen from the upper portion of Figure 2, each series comprises a plurality of radially disposed and angularly spaced conductive elements 1 5 alternate elements being connected to inner and outer annular connection strips 16, 1 7 respectively. The connection strips are connected to terminals 18, 1 9 respectively which conveniently are formed on a lug of the disc. The disc is formed from electrically insulating material and conveniently the elements are in the form of copper applied to the surface of the disc by printed circuit techniques.

The second and fourth series of elements are formed by a plurality of radially disposed conductive elements 20 which are each connected to an annular connector member 21 which is itself connected to a terminal 22 the latter being electrically connected to the shaft 10 so that in use it is earthed. The angular spacing of the elements 20 is the same as the angular spacing of the elements 1 5 and the only difference between the first and second series and the third and fourth series, is the number of elements. For example, the number of elements in the third and fourth series may be one less than the number of elements in the first and second series.

The terminals 18, 19 of the discs 11 and 12 are connected to an electric supply such that they have opposite electrical polarity, the midpoint of the supply preferably being earthed. The terminal member 22 is also earthed and as a result, as the shaft is rotated, the capacitance values of the elements 15 will vary as the elements 20 move into and out of register therewith. Since the elements 1 5 are divided into two groups, two signals can be obtained making from the two fixed discs, four signals in all. The signals from the first and third series of elements will have a varying phase relationship because of the different number of conductive elements and the phase relationship can be measured to provide an indication of the absolute position of the shaft.

If the difference in the number of elements is one then at one position only of the shaft will the signals show no phase difference. If the difference in the number of elements is two then at two positions of the shaft will the signal show no phase difference.

The circuit shown in Figure 4 is for connection to the terminals 18, 1 9. In the particular example therefore there are four such circuits. The purpose of the circuit is to supply the polarizing voltage and to provide an output voltage which reflects the change in capacitance as the discs rotate. The circuit includes a high gain amplifier 23 having a feedback impedance 24 connected between its output and one input the latter being connected to the appropriate terminal 18, 1 9. The other input of the amplifier is connected to the positive terminal of a DC supply 25 having its other terminal connected to earth. The particular terminal 1 8, 1 9, to which the circuit shown in Figure 3 is connected will be positive and as the capacitance to earth of the terminal varies upon rotation of the discs so the output voltage will vary about the level determined by the voltage of the supply 25.

The outputs of the two circuits associated with the same fixed disc are combined and shaped to produce an output waveform such as is shown at 26 in Figure 4, the other disc producing the waveform shown at 27. These two waveforms are supplied to a "D" type bistable one to the clock input and the other to the main input, the Q output being indicated at 28.

If desired the disc 13 and 14 can be combined but in this case it will be advantageous to provide an electrostatic screen between the two series of elements 20 on the disc.

Claims

1. A displacement transducer of the capacitive type comprising first and second series of conductive elements mounted on a first pair of relatively movable surfaces, the capacitance between said first and second series varying as relative movement takes place between the surfaces, third and fourth series of conductive elements mounted on a second pair of relatively movable surfaces which are movable in synchronism with the first mentioned pair of surfaces, the capacitance between said third and fourth series of elements varying as relative movement takes place between the surfaces, the elements of each series being spaced apart in the direction of relative movement of the surfaces, characterised in that the number of conductive elements in the first and second series and the third and fourth series is different and means for comparing the phase relationship of the capacitance signals produced by the first and second series and the third and fourth series of conductive elements.

2. A transducer according to Claim 1 in which adjacent ones of the elements of the first and third series of conductive elements are connected to opposite terminals of a source of electric supply.

3. A transducer according to Claim 1 or Claim 2 in which the surface carrying the second and fourth series of elements are rotatable with an angularly movable shaft.

4. A transducer according to Claim 3 in which the surfaces carrying the second and fourth series of elements are defined by surfaces on a pair of discs carried by the shaft.

5. A transducer according to Claim 2 in which the surfaces carrying the second and fourth series of elements are defined by the surfaces of a disc carried by the shaft, the disc incorporating an electrostatic shield between the second and fourth series of elements.

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