GB2133891A

GB2133891A - Capacitance transducer

Info

Publication number: GB2133891A
Application number: GB08400629A
Authority: GB
Inventors: John Harrison Asbery
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1983-01-19
Filing date: 1984-01-11
Publication date: 1984-08-01
Also published as: GB8400629D0

Abstract

A capacitance transducer comprises a pair of relatively movable electrically insulating members 10, 11 upon each of which are mounted a main series of conductive elements 10A, 11A respectively. The elements of each series are equally spaced in the direction of relative movement of the members. Each member also has at least one further series of elements 10B, 11B. The main and further series of elements register with each other at predetermined relative positions of the members and at other positions only one series of elements on one member can register with another series of elements on the other member. The elements on each member are connected together and the capacitance between the elements on the two members can be observed to determine the positions of the members. <IMAGE>

Description

SPECIFICATION Capacitance transducer This invention relates to a capacitance transducer of the kind comprising a pair of electrically insulating support members which are movable relative to each other during the use of the transducer, conductive elements on the support members respectively and connection means whereby the variation of capacitance value between the elements on one member and the elements on the other member can be observed as the members are moved relative to each other.

A known form of such a transducer has the conductive elements on each member spaced an equal distance apart in the direction of relative movement of the members. The spacing of the elements on each member is the same. As a result during relative movement of the members the capacitance value between the elements on the two members decreases as the elements on the two members move out of register and increases as the elements move into register. The maximum value of capacitance occurs when the elements are in register and the maximum value occurs when the elements on one member are halfway between the elements on the other member.

If a high resolution is required of the transducer, the elements must of necessity be narrow in the direction of relative movement and the capacitance value will show sharp pulses of low amplitude. If a low resolution is required and the elements are narrow, sharp pulses again of low amplitude will be obtained. One of the problems associated with capacitance transducers is the noise level in the measuring and/or indicating circuits and if an attempt is made to increase the capacitance value obtained when the elements are in register, by increasing the areas of the conductive elements by making them wider in the direction of movement of the members, the resulting pulses are less sharp although the amplitude will be increased.The elements may be extended in the direction at right angles to the direction of movement but there may be physical constraints on the amount of extension which can take place. The object of the invention is to provide a capacitance transducer of the kind specified in an improved form.

According to the invention in a capacitance transducer of the kind specified each member is provided with a main series of conductive elements, adjacent elements of the main series being equally spaced apart in the direction of relative movement of the members, the spacing of the elements on the two members being the same, each member being provided with at least one further series of elements positioned so that the registration of the further series of elements on the two members takes place only when the main series of elements on the two members are in register.

An example of a capacitance transducer in accordance with the invention will now be described with reference to the accompanying diagrams in which: Figure 1 shows the physical arrangement of two support members forming the rotary transducer, and Figure 2 is a view to an enlarged scale showing elements on the members.

The transducer shown in the diagram is a rotary transducer and comprises a pair of electrically insulating discs 10, 11. For convenience the disc 10 is shown to be of larger diameter than the disc 11 but in practice the two discs will be of the same diameter. The disc 10 is fixed to a support 9 and the disc 11 is mounted upon a shaft 1 3 the rotary axis of which is indicated at 1 2 and the angular displacement of which it is required to measure. In the particular example it is required to provide an indication at intervals of 900 of angular movement of the shaft.

The discs 10 and 11 are provided on their presented faces, with a main series of conductive elements 1 or, 1 A respectively. The elements of the series of elements 1 OA are spaced by 900 as also are the elements 1 A. Each element is formed from conductive material and conveniently the elements are printed upon the discs using printed circuit techniques. The elements 1 OA are connected together as also are the elements 1 A the latter being connected to a slip ring 14 with which is associated a brush 1 5 whereby the elements on the disc 11 can be connected to a sensing device the other terminal of which is connected to the elements on the disc 10.The capacitance value between the elements can therefore be measured and if the elements 1 OA and 1 A only are provided, as the disc 11 rotates, the capacitance value will have four peaks per revolution of the shaft, the peaks occurring at 900 intervals.

In order to increase the amplitudes of the peaks each member is provided with, in the particular example, four further series of elements.

Considering the first of the further series of elements, those on the disc 10 are referenced 10B and those on the disc 11, 11 B. It will be noted that the elements 10B and 1 1 B are spaced in the direction of rotation of the shaft from the elements 1 OA and 1 A by the same amount. it will be noted also that the spacing of the elements of the main and further series is such that during rotation of the disc while the elements 1 1A may move into register with the elements 1 OB, at that instant of registration, the elements 11 B are out of register with the elements 1 or. Similarly the elements 1 B may register with the elements 1 OA but at that time the elements 1 A are out of register with the elements 1 OB. As the shaft rotates therefore a high amplitude pulse will be obtained four times per revolution of the shaft, the high amplitude pulse corresponding to the simultaneous registration of the elements 1 OA and 10B with the elements 1 A and 11 B respectively.There will however be a further peaks of reduced amplitude as the shaft moves to bring the elements 11 A into register with the elements 1 OB and the elements 1 B into register with the elements 1 0A.

The example shown in the diagram has in total, four further series of elements, the additional further elements on the disc 10 being referenced 1 or, 1 OD and 1 0E and those on the disc 11 being referenced 1 C, 11 D and 11 E. The patterns of the elements on the two discs are identical but it will be noted that on each disc the angular spacing separating one element from the adjacent elements is different.

In the particular example there are four groups of elements on each disc with the last element of one group constituting the first element i.e. the main element 1 OA of the next group. The spacing in a group of elements follows the series 1, 2, 4, 5, 13 giving in total 25. 1 unit of space therefore corresponds since each group extends over 90 , to 3.60. The spacing between the elements 1 0A and 1 OB is 3.60, that between the elements 1 OB and 1 OC is 7.20 and so on. In practice because the elements have a finite width the spaces between the elements are slightly smaller than quoted above.As the disc 11 is rotated the elements on the disc 11 will move into register with the respective elements on the disc 10 four times per revolution of the shaft and the capacitance value will therefore be high. As the shaft continues to rotate elements on the disc 11 will move into register with elements on the disc 10 at partial registration positions but only four elements on each disc will be brought into register at the same time. In the example at the point of complete registration, as indicated in the diagram, twenty pairs of elements are in register and at any other position only four pairs of elements can be in register.

In the diagram the elements of each series are shown to be of the same width. It is also possible to vary the widths of the elements in the series of elements. It will be appreciated that the mathematical series quoted above is by way of example only and other mathematical series may be employed. It will further be understood that the concept above may be applied to linear transducers although it will be appreciated that the fixed member will have to be longer than the moving member if it is required that the amplitude of the pulses obtained upon movement of the moving member should remain constant throughout the design range of movement of the transducer.

In the example shown in the diagram the capacitance value between the elements on the two discs is measured. The capacitance value will peak four times per revolution of the shaft 1 3 and then fall to a low value with small peak at each partial registration position. The effect can be enhanced by positioning conductive strips 1 6 in the spaces between the elements on for example the disc 10, and connecting the strips together and to a negative supply terminal with the elements on the same disc being connected to a positive supply terminal. The strips must be electrically insulated from the elements and the effect will be that at the 900 positions of full registration the slip ring 14 will assume maximum positive polarity and will then move negatively to a maximum negative value as the elements on the disc 11 register with the strips. When the aforesaid partial registration occurs the elements on the disc 11 will move slightly positively. The small ripple produced by the partial registrations can be easily filtered.

Claims

1. A capacitance transducer comprising a pair of electrically insulating support members which are movable relative to each other during the use of the transducer, conductive elements on the support members respectively and connection means whereby the variation of capacitance value between the elements on one member and the elements on the other member can be observed as the members are moved relative to each other, each member having a main series of conductive elements, adjacent elements of the main series being equally spaced apart in the direction of relative movement of the members, the spacing of the elements on the two members being the same, each member being provided with at least one further series of elements positioned so that the registration of the further series of elements on the two members takes place only when the main series of elements on the two members are in register.

2. A capacitance transducer according to Claim 1 in which when the members have more than one further series of conductive elements, the spacing between the elements of the main and further series of elements is such that at any partial registration position of the members only one series of elements on one member can be in register with one series of elements on the other member.

3. A capacitance transducer according to Claim 1 or Claim 2 in which the transducer is a rotary transducer and said members are of disc like form, one of said discs being fixed and the other being mounted upon a rotary shaft.

4. A capacitance transducer according to Claim 3 in which said other disc carries a slip ring to which the elements on the disc are connected, the transducer including a brush whereby electrical connection can be made to the slip ring.

5. A capacitance transducer according to any one of Claims 1-4 in which the widths in the direction of movement of the members of the elements in the different series varies.

6. A capacitance transducer according to Claim 1 in which the spaces between adjacent elements on one member are occupied by conductive strips, said strips in use being connected together and to one terminal of a source of supply, the elements on the disc being connected to the other terminal of the source of supply.

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