GB2133889A

GB2133889A - Capacitance displacement transducers

Info

Publication number: GB2133889A
Application number: GB08400007A
Authority: GB
Inventors: John Harrison Asbery
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1983-01-19
Filing date: 1984-01-03
Publication date: 1984-08-01
Also published as: GB8400007D0

Abstract

A capacitance displacement transducer, exemplified by a rotary type in Fig. 3, comprises a pair of members in the form of discs one of which 12, is fixed and the other of which is rotated by a shaft. The disc 12 has two series of conductive strips 16,17 the strips being radially disposed and alternately arranged. The strips of each series are connected to respective connection points 18, 19 which in use are of opposite electrical polarity. The rotary disc has a further series of strips arranged in like manner to the series of strips 16,17, but the number of strips on the rotary disc being half the number on the disc 12. The discs are in close proximity so that as the rotary disc rotates the polarity of the strips thereon will vary. <IMAGE>

Description

SPECIFICATION Capacitance transducers This invention relates to capacitance type transducers of the kind comprising a pair of relatively movable members, first and second series of conductive elements mounted on the members respectively, the elements of each series being spaced apart by the same amount in the direction of relative movement of the members and means for providing an indication of the variation in capacitance between the two series of elements as the members are moved relative to each other.

The object of the present invention is to provide such a transducer in a simple and convenient form.

According to the invention a third series of conductive elements is provided, the third series of elements being fixed and alternately arranged with the elements of the first series, the third series of elements being positioned so that when the second series of elements is in register therewith, the resulting capacitance value will be substantially the same as obtained when the second series of elements is in register with the first series of elements, said first and third series of elements being connected to supply means whereby the first and third series of elements have equal but opposite polarity.

An example of a capacitance transducer in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows the general arrangement of the transducer which in this example is of the rotary type, Figure 2 is an end view of one face of a rotary disc forming part of the transducer, Figure 3 is a view similar to Figure 2 showing an end view of a fixed disc forming part of the transducer.

Figure 4 is an equivalent circuit of part of the transducer seen in Figures 1-3.

Figure 5 is one example of a supply and detection circuit, Figure 6 is a view similar to Figures 2 and 3 showing the outer surfaces of the outer pair of the discs of Figure 1, and Figure 7 is a view similar to Figure 1 showing a modification.

Referring to Figure 1 of the drawings, the transducer comprises a central rotary disc 10 which is carried by a shaft 11, the shaft 11 either being the shaft the angular movement of which is to be observed or a shaft which is connected to the aforesaid shaft. The disc 10 is located in close spaced relationship between a pair of fixed discs 12, 13 and the discs are formed from electrically insulating material conveniently of the type used in the manufacture of printed circuit boards.

The discs 12 and 1 3 have on their faces presented to the disc 10, an electrical pattern as indicated in Figure 3 and the disc 10 has on each face, a pattern as shown in Figure 2.

Considering first the discs 1 2 and 1 3 and the pattern on the faces thereof presented to the disc 10. As shown in Figure 3 the discs have a central aperture 1 4 through which the shaft 11 can pass.

Each disc is also provided with a connection lug 1 5. The pattern is conveniently formed by printed circuit techniques and comprises first and third series of conductive elements 1 6, 1 7. The elements 1 6 extend inwardly from an outer annular connection strip 1 8 and the conductive elements 1 7 extend outwardly from an inner annular connection strip 1 9. Each connection strip is divided into two parts, each part having a connection point on the lug 1 5. The conductive elements 1 6, 1 7 extend in the radial direction and the elements of each series are alternately arranged with equal angular spacing between adjacent elements.

The conductive patterns on the discs 1 2 and 1 3 also include an annulus 20 which has a connection extending to the lug 1 5. In addition the connection to the annulus 20 and also the annulus itself, are surrounded by a guard strip 21 which also has a connection on the lug 1 5.

Turning now to Figure 2 and the conductive pattern thereon. The pattern includes a second series of conductive elements 22 which are interconnected by an intermediate annular connection strip 23 the latter being connected to an annulus 24 of the same size as the annuli 20 on the discs 12 and 13. As previously mentioned, the disc 10 has the conductive pattern shown in Figure 2 on both sides and the conductive elements 22 which extend in a radial direction and are of a length substantially equal to the overlap length of the elements 1 6, 17, are aligned on both sides of the disc. Moreover, the spacing between adjacent conductive elements 22 is the same as the spacing between the elements 1 6 of the first series.When the discs are in assembled relationship the running clearance between the discs is kept to a minimum and the annuli 20 and 24 on the presented sides of the discs 10 and 12 and 10 and 13, provide a capacitive connection from the second series of conductive elements 22 to the terminal on the lug 1 5 which is connected to the annulus 20.The equivalent circuit of the presented faces of the disc and the disc 12 or 13 is seen in Figure 4 and it will be seen that in use, the two portions of the annular connection strips 18 and 19 are interconnected at the lug 1 5. The capacitor 25 in Figure 4, is constituted by the annuli 20, 24. It will be appreciated that the equivalent circuit of the other side of the disc 10 together with the adjacent surface of the outer disc 12 or 13 is the same and it is arranged that the two circuits are connected in parallel. As indicated in Figure 4, the connection to the first series of conductive elements is assigned the reference numeral 26 that to the third series of elements the reference numeral 27 and that to the second series of elements by way of the capacitor 25, the reference numeral 28.

In Figure 5 there is shown a supply circuit to the various terminals. The circuit inciudes an oscillator 31 the output of which is connected to the connection 26 and also to an inverter 32 the output of which is connected to the connection 27. The output of the oscillator is also supplied to one terminal of a phase sensitive rectifier 33 the other terminal of which is connected to the connection 28 and the output 30 of the phase sensitive rectifier 33 constitutes the output from the transducer. The oscillator produces an alternating voltage at its output and this is applied to the first series of connection members 16.The inverter produces a voltage in phase but of opposite polarity and this voltage is applied to the third series of conductive members 1 7. During relative angular movement of the discs the second series of conductive elements 22 will move from the position shown in which the polarity of the connection 28 will correspond to the polarity of the connection 26, to a position in which the conductive elements 22 are opposite the conductive elements 1 7. The polarity of the connection 28 will therefore be reversed. In practice however this may not be the case bearing in mind that the oscillator provides an alternating current output and it is just possible that by the time the conductive elements 22 have moved into line with the conductive elements 17, that the polarity of the latter may be the same.The fact is however that an alternating voltage appears at the connection 28 and its phase is detected in the rectifier 33 relative to the phase of the output voltage of the oscillator. The output signal obtained at the output 30 will therefore be a series of pulses the peaks of which correspond to the passage by the conductive elements 22, past the conductive elements 16 and 17.

The guard strip 21 will normally be connected to earth and acts to minimise the capacitance between the annulus 20 and the remaining portion of the conductive pattern.

Figure 6 shows the outer side of the discs 12 and 13 and it will be seen that an inner annulus 34 is provided which is of approximately the same size as the annulus 20. it provides a screen for the annulus 20 and it may be connected to earth but optionally it may be connected by way of a "boot strap" circuit to the connection 28 so that its voltage rises and falls with that of the annulus 20.

Using this technique the effective capacitance between the annulus 20 and the annulus 34 is minimised. The outer annulus 35 is provided to screen the first and third series of conductive elements together with the associated annular connection strips. The annulus 35 may be connected to earth.

In the example described the first and third series of conductive elements are formed on the same surface. An alternative arrangement would be to have the disc 10 with the second series of conductive elements on both sides and have the disc 1 2 with the first series of elements and the disc 1 3 with the third series of elements. It is also possible to utilise a single fixed disc with a conductive pattern as shown in Figure 3 and a rotating disc with a single pattern as shown in Figure 2, on the side presented to the fixed disc.

It will also be appreciated that if the annulus 34 shown in Figure 6 is to be connected to earth, then the whole of the surface of the disc may be provided with a conductive coating and earthed.

The screening provided by this complete conductive coating may in practice be provided by some component external of the transducer for example a metal gear wheel or the like forming part of the apparatus with which the transducer is associated.

A further modification would be to provide the annulus 20 on a separate disc.

In the arrangement shown in Figure 7 three fixed discs 36, 37 and 38 are provided together with two discs 39, 40 which are mounted on the shaft 11, the disc 39 being positioned between the discs 36, 37 and the disc 40 between the discs 37, 38.

The disc 36 has on its face presented to the disc 39, the first and third series of elements 16, 1 7 as shown in Figure 3, the elements being connected to external terminals. The disc 39 has on its face presented to the disc 36, the second series of elements 22 which are connected by a connection wire 41 to an annular conductive coating on the face of the disc 40 which is presented to the adjacent face of the disc 38 and this face of the disc 38 has a complementary coating which is connected to an external terminal. The disc 37 is provided with a conductive coating which is earthed.

Claims

1. A capacitance type transducer comprising a pair of relatively movable members, first and second series of conductive elements mounted on the members respectively, the elements of each series being spaced apart by the same amount in the direction of relative movement of the members, means for providing an indication of the variation in capacitance between the two series of elements as the members are moved relative to each other, a third series of conductive elements, said third series of elements being fixed and alternately arranged with the elements of the first series, the third series of elements being positioned so that when the second series of elements is in register therewith, the resulting capacitance value will be substantially the same as obtained when the second series of elements is in register with the first series of elements, said first and third series of elements being connected to supply means whereby the first and third series of elements have equal but opposite polarity.

2. A tranducer according to Claim 1 in which said supply means comprises an alternating current supply and said means for providing an indication of the variation in capacitance comprises a phase detector which is supplied with the voltage applied to the first series of conductive elements and the voltage developed on the third series of elements.

3. A transducer according to Claim 1 in which said members are of disc form, one of the discs being supported upon a rotary shaft, the other disc being fixed, the second series of conductive elements being formed on the face of said one disc which is presented to the other disc, the second series of conductive elements comprising radially disposed conductive strips which are equiangularly spaced about the axis of the shaft, the first and third series of conductive elements being arranged in like manner on the face of the other disc which is presented to the one disc, the number of conductive elements in each series being the same.

4. A transducer according to Claim 3 in which said first and third series of elements are connected to connection points on a terminal lug on said other disc.

5. A transducer according to Claim 4 in which said second series of elements are connected to an annulus of conductive material on said one disc, said annulus being capacitatively coupled to a further annulus on the adjacent face of said other disc, said further annulus being connected to a connection point on said terminal lug.

6. A transducer according to Claim 5 in which said further annulus, its connection point and the connection therebetween are surrounded on the face of the other disc by a guard strip.

7. A transducer according to Claim 5 including a shielding annulus disposed on the outer face of said other disc, said shielding annulus corresponding with said further annulus and being connected in use to a bootstrap or like circuit whereby its potential varies in the same way as that of the further annulus.

8. A transducer according to any one of Claims 3-7 including a further fixed disc mounted on the opposite side of said one disc from said other disc, said further disc being provided with conductive elements of said first and third series, said one disc being provided with conductive elements of said second series on its face presented to the further disc.

9. A transducer according to Claim 4 in which said second series of elements is connected to an annulus of conductive material formed on one face of a further rotary disc, said annulus being capacitatively coupled to an annulus formed on a fixed disc disposed on the opposite side of said further rotary disc to said one disc.

10. A transducer according to Claim 9 including a fixed shield disc disposed between said rotary discs.

11. A capacitance type transducer comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figures 1-6 of the accompanying drawings.

12. A capacitance type transducer comprising the combination and arrangement of parts substantially as hereinbefore described with reference to Figures 1-6 as modified by Figure 7 of the accompanying drawings.