GB2221035A

GB2221035A - Capacitive displacement sensor

Info

Publication number: GB2221035A
Application number: GB8817152A
Authority: GB
Inventors: Raymond John Whorlow
Original assignee: Glacier Metal Co Ltd
Current assignee: Federal Mogul Engineering Ltd
Priority date: 1988-07-19
Filing date: 1988-07-19
Publication date: 1990-01-24
Also published as: GB2221035B; GB8817152D0

Abstract

A displacement sensor includes a capacitor element (8) forming part of an oscillator (15). The capacitor (8) is installed close to a shaft (12) whose position is to be measured. The capacitor preferably comprises a three layer sandwich, the middle layer (6) of which is insulated from the other two layers and acts as a guard layer. One of the other layers is probe layer (8) and the other is a support or, as shown, the surface of a recess in pole piece (1) of an electromagnet. <IMAGE>

Description

Improvements in and relating to displacement sensors This invention relates to displacement sensors particularly, but not exclusively useful for magnetic bearings. It is known to use electromagnets to support a rotatable shaft without physical contact. It is also known to determine the position of the shaft relative to the supporting magnets and to use this determination to apply a correction for any unwanted displacement from a selected position.

Determination of the position of a shaft in a system where only relatively small displacements are significant is not easy. Magnetic sensors can be used, provided care is taken to ensure that there is no significant interaction with the magnetic fields created by the magnets of the bearing itself. Sensor size and position are also to be considered.

It is an object of the present invention to provide an improved displacement sensor.

According to this invention, a displacement sensor comprises an oscillator, the frequency of which is at least in part determined by a capacitor element which in use is installed close to a shaft whose position is to be measured. The capacitor element is preferably constituted by a three-layer sandwich, the centre or middle layer being insulated from the other two. One outer layer constitutes a probe; the other outer layer may be a support element, or it may be constituted by the inner surface of a recess in the working face of the pole piece of an electromagnet. In use, the middle layer is preferably constituted as a guard ring to minimise the effect of capacitance between the probe and the other outer layer. This can be accomplished in known manner by sampling the probe signal and feeding it onto the middle layer, for example, by means of a voltage follower.

The oscillator frequency is selected to be at least high enough to avoid interaction with the anticipated frequency of events to be sampled. A convenient frequency for manv purposes would be of the order of 100 to 500 kHz. The oscillator may be attached directly to the capacitor element. Such an implementation can be embedded into the pole piece of an electromagnet. For example, it is possible to use as the probe a relatively small brass (or other conductive metal) plate, whose effective capacitance to the shaft to be supported is as little as 4 pf. Alternatively the capacitor element may be coupled to the oscillator by a wire extending through the pole piece. In such a case the 3-element construction is particularly preferred, with the middle or inner layer constituted as a guard ring The coupling to the latter preferably also extends through the pole piece.One way of accomplishing this is to use a coaxial feeder cable through a hole in the pole piece, the outer of the cable being attached to the guard ring and the inner being attached to the probe.

This type of construction has the advantage that the oscillator and related electronic circuitry may be located remote from the probe, for example behind the electromagnet, or even further away.

In order that the invention be better understood one embodiment of it will now be described by way of example with reference to the accompanying drawing in which the sole Figure is a schematic side view of a capacitor element according to the invention.

In the Figure an electromagnet pole piece 1 has an exposed face adjacent a shaft 12 whose displacement relative to the pole piece is to be determined. In the interests of simplicity, unnecessary details of the electromagnet itself are not shown and need not be further discussed.

The pole piece is provided with a capacitor element comprising an insulation layer 5, a brass foil layer 6, a further insulation layer 7 and a brass foil layer 8. The latter constitutes the working part of the element whilst the other foil layer is used as a guard ring, as will be further discussed shortly. A hole 9 drilled through the pole piece provides for electrical connections to the element. A coaxial feed line 10 is employed, the outer (or shielding) of the cable is connected to the guard ring 6 and the inner of the cable is connected to the working part 8.

At the opposite end, behind the magnet, the cable connects the capacitor defined between the working part 8 and the shaft 12 to an oscillator 15, the capacitor forming part of an oscillatory circuit operating at 150 kHz. Also a voltage follower 16 is connected between the element and the guard ring 8 to eliminate or at least minimise the effect of the body of the pole piece on the perceived capacitance variations due to relative displacement of the shaft.

Conventional electronics are used to develop a control signal from changes in the oscillator frequency caused by capacitance changes due to relative displacement of the shaft. The resultant control signal may then be used in the development of a correction/control signal for the electromagnet system, in known manner.

Claims

1. A displacement sensor comprising an oscillator, the frequency of which is at least in part determined by a capacitor element which in use is installed close to a shaft whose position is to be measured.

2. A sensor according to Claim 1 wherein the capacitor element is constituted by a three layer sandwich, the middle layer of which is insulated from the other two layers, one of which is a probe layer.

3. A sensor according to Claim 2 wherein said middle layer is constituted as a guard ring to minimise the effect of capacitance between what is, in use, the probe layer and the other layer.

4. A sensor according to Claim 3 including means for sampling a signal on the probe layer and feeding it onto the middle layer.

5. A sensor according to any preceding claim where in the oscillator frequency is in the range 100 to 5 00 KHz.

6. A sensor according to any preceding claim wherein the oscillator is directly attached to the capacitor element.

7. A sensor according to Claim 6 adapted to be embedded into the pole piece of an electromagnet.

8. A sensor according to any of Claims 1 - 5 wherein the capacitor element is embedded into the pole piece of an electromagnet and is connected to the oscillator by at least one wire extending through said pole piece.

9. A sensor substantially as herein described with reference to and as illustrated by the accompanying drawing.