GB2035569A - Capacitive transducers - Google Patents

Abstract

A capacitive position transducer comprises a pair of electrodes having opposed first surfaces between which a member moves to vary the capacitance therebetween and thus indicate the position of the member. A further electrode forms with a second surface of one of the pair of electrodes a reference capacitor, the second surface of the one of the pair of electrodes being opposed to its first surface.

Description

SPECIFICATION Improvements in or relating to capacitive variable transducers (C.V.D.T.) A capacitive (variable) displacement transducer (CVDT) in which greater convenience in and accu racy of mechanical measurements is made possible.

The measurements may be made only one stage removed from national primary standards. The benefits come from the novel arrangements of the electrodes and the proper intercomparison of the principal coefficients of capacitance and having due regard to the cross coefficients. The construction is relatively tolerant to the irregularities of manufacture and is conceived so as to be rigid and stable.

Moreover despite the low structural compliance the elements are substantially uncoupled to their surroundings. The structure is almost isoexpansive and isoelastic, has inherently low quadrate field perturbations, minimal guard, uses space-multiplexed field distributions and is almost co-extensive with the measuring elements. It is of the essence that principal and cross capacitance coefficients are associated with the digital and analogue conversion, the proof of fidelity and the harmonisation of the electrical and mechanical zero positions.

The sketches show several partial and some fuller diagrammatic arrangements to help the description of particular or more general workings.

Fig. 1 shows how the principal coefficient of capacitance is related to the displacement of one of the elements.

Fig. 2 shows a form of electrical apparatus used to measure the change in capacitance by analogue and digital methods.

Fig. 3 shows a refinement of fig. 1.

Fig. 4 shows the scheme of construction to give low compliance and be uncoupled from its surroundings.

Fig. 5 shows additional electrodes for the introduction of other control and measuring voltages.

Fig. 6 shows an example of fixed construction which may be used for its advantageous stability in both electrical and mechanical measurements.

In fig 1 the capacitance between electrode (2) and electrode (3) is, ideally, altered in direct proportion to the movement of the measuring shaft (1 ) by such a displacement as (6). The condition for this proportionality is that the spacing be uniform, i.e. t (5), be uniform and all cross coefficients be constant. A measurement of capacitance at the terminals (4) then sofficesto determine the displacement of the measuring shaft.

In fig 2 one possible scheme to measure the changes in capacitance, Cx, now at (5) is to use the capacitor (2) as a standard of comparison (Ref.A) and choose the voltages (1 ) & (6) to suit the relative magnitudes of (2) and (5) (Ref. B1). The junction of the capacitors is monitored by the electrical circuits attached to it, often in the form of the analogue to digital convertor (3) and presented in the display (4) (Ref B2). The mechanical arrangement is shown diagrammatically in Fig 3 where it can be seen that a single electrode (3) is shared and the element (2) of Fig 2 is embodied as (1) in Fig 3.

So much is known as the basis of this kind of measurement.

The essence of the improvements disclosed lies in: i) greater mechanical rigidity by a new arrangement of electrodes and redefinition of the principal coefficients of capacitance.

ii) providing a means of proving the fidelity of the analogue and digital processes.

iii) a relaxation of the requirements of uniformity of structural dimensions.

iv) all of these without the obtrusive increase in the size of all or even one of the dimensions.

In Fig 4 is shown the scheme of assembly to give a rigid structure in close reference to the fiduciary plane (1) by the massive bearing (2) carrying a solid shaft (4). The terminal (now more conveniently a shielded cable) is insulated by (5) and connected to electrode (3b). Such other cables as are required can be mounted and connected in the same sort of way.

In Fig 5 are shown in outline diagrammatic form the ideas for which a claim is made to novelty, and it can be seen that it is now possible to introduce other signals for control and monitoring in a form and of magnitude not hitherto available, without influence on the principal displacement signal. The corresponding mechanical layout in Fig 5b shows how these may be incorporated in the desired rigid form and, without excessive increase in size, the desired principal and cross coefficients of capacitance can be defined.

In Fig Sc a form of interconnection is given by way of example to show how the electrode structure can be used to accept and provide signals to suit a variety of measuring requirements.

In Fig 6 it is shown that if the mechanical parameters are fixed then the electrical parameters are as well and the device now could find use in precise feed-back circuits and such like.

Because of its rigid structure and well defined electrical performance it is especially suited to applications in metrology, automatic inspection, machine tool control and servo-systems.

Some references are given to avoid over-lengthy descriptions in the text: Al Wolfendale BP 1 347 235 B1 Blumlein BP323 037 Sept1928 B2 Gibert US 3 074 057 Clearly the list is not exhaustive but it is believed that by the present disclosure shows a new and advantageous form of construction, useful in itself, which can now make more use ofthe combined properties of the Blumlein bridge and modern charge handling digital conversion systems. The form of the structure is not limited to a coaxial system and may be combined with concentric elements or may be double coaxial or mixed according to the needs of a particular application.

1. A capacitance transducer comprising not less than four and usually more (the additional elements) elements for at least one of which both the obverse and the reverse sides are active, the obverse side faced by a counter electrode or element with a mov

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to capacitive variable transducers (C.V.D.T.) A capacitive (variable) displacement transducer (CVDT) in which greater convenience in and accu racy of mechanical measurements is made possible. The measurements may be made only one stage removed from national primary standards. The benefits come from the novel arrangements of the electrodes and the proper intercomparison of the principal coefficients of capacitance and having due regard to the cross coefficients. The construction is relatively tolerant to the irregularities of manufacture and is conceived so as to be rigid and stable. Moreover despite the low structural compliance the elements are substantially uncoupled to their surroundings. The structure is almost isoexpansive and isoelastic, has inherently low quadrate field perturbations, minimal guard, uses space-multiplexed field distributions and is almost co-extensive with the measuring elements. It is of the essence that principal and cross capacitance coefficients are associated with the digital and analogue conversion, the proof of fidelity and the harmonisation of the electrical and mechanical zero positions. The sketches show several partial and some fuller diagrammatic arrangements to help the description of particular or more general workings. Fig. 1 shows how the principal coefficient of capacitance is related to the displacement of one of the elements. Fig. 2 shows a form of electrical apparatus used to measure the change in capacitance by analogue and digital methods. Fig. 3 shows a refinement of fig. 1. Fig. 4 shows the scheme of construction to give low compliance and be uncoupled from its surroundings. Fig. 5 shows additional electrodes for the introduction of other control and measuring voltages. Fig. 6 shows an example of fixed construction which may be used for its advantageous stability in both electrical and mechanical measurements. In fig 1 the capacitance between electrode (2) and electrode (3) is, ideally, altered in direct proportion to the movement of the measuring shaft (1 ) by such a displacement as (6). The condition for this proportionality is that the spacing be uniform, i.e. t (5), be uniform and all cross coefficients be constant. A measurement of capacitance at the terminals (4) then sofficesto determine the displacement of the measuring shaft. In fig 2 one possible scheme to measure the changes in capacitance, Cx, now at (5) is to use the capacitor (2) as a standard of comparison (Ref.A) and choose the voltages (1 ) & (6) to suit the relative magnitudes of (2) and (5) (Ref. B1). The junction of the capacitors is monitored by the electrical circuits attached to it, often in the form of the analogue to digital convertor (3) and presented in the display (4) (Ref B2). The mechanical arrangement is shown diagrammatically in Fig 3 where it can be seen that a single electrode (3) is shared and the element (2) of Fig 2 is embodied as (1) in Fig 3. So much is known as the basis of this kind of measurement. The essence of the improvements disclosed lies in: i) greater mechanical rigidity by a new arrangement of electrodes and redefinition of the principal coefficients of capacitance. ii) providing a means of proving the fidelity of the analogue and digital processes. iii) a relaxation of the requirements of uniformity of structural dimensions. iv) all of these without the obtrusive increase in the size of all or even one of the dimensions. In Fig 4 is shown the scheme of assembly to give a rigid structure in close reference to the fiduciary plane (1) by the massive bearing (2) carrying a solid shaft (4). The terminal (now more conveniently a shielded cable) is insulated by (5) and connected to electrode (3b). Such other cables as are required can be mounted and connected in the same sort of way. In Fig 5 are shown in outline diagrammatic form the ideas for which a claim is made to novelty, and it can be seen that it is now possible to introduce other signals for control and monitoring in a form and of magnitude not hitherto available, without influence on the principal displacement signal. The corresponding mechanical layout in Fig 5b shows how these may be incorporated in the desired rigid form and, without excessive increase in size, the desired principal and cross coefficients of capacitance can be defined. In Fig Sc a form of interconnection is given by way of example to show how the electrode structure can be used to accept and provide signals to suit a variety of measuring requirements. In Fig 6 it is shown that if the mechanical parameters are fixed then the electrical parameters are as well and the device now could find use in precise feed-back circuits and such like. Because of its rigid structure and well defined electrical performance it is especially suited to applications in metrology, automatic inspection, machine tool control and servo-systems. Some references are given to avoid over-lengthy descriptions in the text: Al Wolfendale BP 1 347 235 B1 Blumlein BP323 037 Sept1928 B2 Gibert US 3 074 057 Clearly the list is not exhaustive but it is believed that by the present disclosure shows a new and advantageous form of construction, useful in itself, which can now make more use ofthe combined properties of the Blumlein bridge and modern charge handling digital conversion systems. The form of the structure is not limited to a coaxial system and may be combined with concentric elements or may be double coaxial or mixed according to the needs of a particular application. CLAIMS

1. A capacitance transducer comprising not less than four and usually more (the additional elements) elements for at least one of which both the obverse and the reverse sides are active, the obverse side faced by a counter electrode or element with a mov able element between them, the reverse side also faced with a counter electrode or element such that the capacitance of the obverse side to its counter electrode as modified by the movable element which is not necessarily homogeneous is measurable in terms of the capacitance of the reverse side and its counter electrodes wherein the dielectric is not restricted and may be chosen by design.

2. Atransducer according to the first claim in which the movable element is conductive and connected to common.

3. Atransducer according to the first claim in which the movable element is a dielectric.

4. Atransducer according to claim 1 in which the movable member may be affected by extraneous fields and the effects of these are cancelled by the action of the designated additional element.

5. Atransducer according to the first claim in which the elements are of substantially cylindrical form and the movable element is hollow.

6. Atransducer in which the elements are parts of cylinders and the movable element is a solid cytinder.

7. Atransducer according to claim 1 in which some of the elements are cylindrical and some are planar and the moving element may take-transla- tional or rotational motion.

8. Atransducer accordingtothe-first claim in which some of the elements are cylindrical and some are planar and the movable element is constructed respond to very small movements.

9. Atransducer according to the first claim in which one of the additional electrodes is temperature sensitive and is used to apply correction possibly through another of the additional elements.

10. Atransducer according to the first claim in which the movable element is fixed and the capacitive properties assume the first importance.

11. Atransducer according to the first claim in which the movable electrode is absent and one of the elements or one of the additional elements is temperature sensitive and the out-put readings are particularly adapted to show temperature.

12. Atransducer according to the first claim in which one of the elements including the movable element is sensitive to pressure and the out-put readings are particularly adapted to read this.

13. Atransducer according to the first claim in which the moving member is a spring-like object and the out-put readings are particularly adapted to read the forces acting on said movable member.

14 Atransducer accordingto the first claim in which the movable element is a liquid.

15. Atransducer according to claim 1 in which the movable element is a particulate solid.

16. A transelucer according to the first claim in which the movableelement is a gas.

17. Atransducer substantially as herein described with reference to the figures 7, 8 and 10.