GB2080542A

GB2080542A - Measurement of shaft angle displacement

Info

Publication number: GB2080542A
Application number: GB8121216A
Authority: GB
Original assignee: Short Brothers PLC
Current assignee: Short Brothers PLC
Priority date: 1980-07-16
Filing date: 1981-07-09
Publication date: 1982-02-03
Also published as: GB2080542B

Abstract

Angular displacement of a shaft 11 relative to a fixture 17 from a rest position of the shaft is detected by a strain gauge 18 mounted to a beam 14. One end of the beam is fastened 13 to the shaft 11 and the other end is fastened 15 to a mount 16 on the fixtures 17. Changes of the electrical resistance of the strain gauge 18 with fixing of the beam 14 upon angular displacement of the shaft 11 from its rest position are measured by a bridge circuit end the device is calibrated to provide an output representative of the magnitude of the angular displacement. For large angular displacements, the beam may be extended so as to circle the shaft several times. <IMAGE>

Description

SPECIFICATION Measurement of shaft angle displacement The present invention relates to a method of, and apparatus for, measuring angular displacement and, more particularly but not exclusively, is concerned with the use of a sensing device which provides an electrical output to determine with high accuracy the angular displacement of a shaft from a rest position thereof, Transducers for obtaining an electrical signal from a rotary movement of a device have previously been proposed, but in general they require high precision assembly techniques and extensive electronic circuitry if they are to provide accurate information.It is an object of the present invention to overcome these disadvantages, providing a way of measuring a shaft angle which, while based on simple principles, will produce in the first instance an output which is of such a form that it can be used with a minimum of electronic circuitry to provide accurate information on the position of the shaft.

According to a first aspect of the invention there is provided a method of measuring angular displacement of a member relative to a fixture, which method comprises the steps of: i) fastening one end of a flexible beam of a sensing device to the member and the other end to the fixture such that the beam flexes in proportion to said angular displacement; ii) obtaining from the device a signal indicative of the amount by which the beam flexes; and iii) calibrating the sensing device to provide an output which is representative of the said angular displacement.

According to a second aspect of the present invention there is provided apparatus including a member mounted for rotation relative to a fixture and a sensing device to measure the angular displacement of the member relative to the fixture, the sensing device comprising a flexible beam so secured as to co-operate at one of its two ends with the fixture and at the other of its ends with the member, and to flex in proportion to angular displacement of the member in at least one direction from a rest position of the member, and a sensing means located intermediate the beam ends which respond to flexing of the beam to generate a signal which varies with the amount of angular displacement of said member from said rest position, the sensing device being calibrated to provide as output a signal representative of the amount of said angular displacement.

According to a third aspect of the present invention there is provided a device for measuring angular displacement of a member relative to a fixture, which device comprises a flexible beam which carries intermediate its ends a sensing means which responds to flexing of the beam, a mounting to which one end of the beam is connected, and a clamp to which the other end of the beam is connected and by which the beam can be secured to said member whereby, in use, the said mounting is secured to the fixture and the beam flexes in proportion to angular displacement of said member in at least one direction from a rest position thereof relative to said fixture, the measuring device being so constructed and arranged that it provides an output signal representative of said angular displacement.

While the said member will usually be a shaft, it need not be.

Preferably the beam comprises a strip of beryllium copper alloy secured at one end to the fixture and at the other end to the shaft or a component which moves with the shaft.

Advantageously the sensing means comprises one or more strain gauges mounted to the beam.

Such strain gauges may be of the bonded foil type or of the thin film type.

The end of the beam coupled to the shaft may be so coupled through a crank arm which is mounted to the shaft for rotation therewith.

Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, wherein: Figure lisa diagrammatic plan view of a first embodiment of apparatus according to the invention; Figure 2 is a diagrammatic side elevation of the apparatus shown in Figure 1; Figure 3 is a diagrammatic plan view of a second embodiment of apparatus according to the invention; Figure 4 is a diagrammatic side elevation of the apparatus shown in Figure 3; Figure 5 is a diagrammatic plan view of a third embodiment; and Figure 6 is a diagrammatic plan view of a fourth embodiment of apparatus according to the invention.

Referring now to the drawings, and more particu marly to Figures 1 and 2, a device is shown for measuring the amount of angular displacement of a shaft 11. The device comprises a crank arm 12 clamped to the shaft for rotation therewith and extending radially from the shaft to a free end of the crank arm to which is connected by means of a screw 13 a flexible beam 14 which is composed of a thin, flat, elongate strip of beryllium copper alloy.

The other end of the beam 14 is connected by means of a screw 15 to a mount 16, itself fixedly mounted to the housing 17 of the shaft 11 relative to which displacement of the shaft is to be measured.

Upon rotation of the shaft 11, and therefore also movement of the crank arm 12, relative to the housing 17 the beam 14 flexes and strain gauges 18, one mounted to each opposed flat face of the beam at a point intermediate its ends respond to this flexing by changes in the electrical resistance of each strain gauge. The strain gauges are connected in a full bridge gauge circuit (known per se) which is able to compensate for such changes in their resistance as resu It from changes in temperature and which is calibrated to provide a linear output representative of the angular displacement of the shaft 11 from its rest position. The connections and bridge circuit are not shown in the drawings for reasons of clarity but their provision will be obvious to those skilled in the art.

The embodiment shown in Figures 1 and 2 is suitable for measuring small rotational displacements of the order of 5 to 100 from a rest position of the shaft.

A second embodiment is shown in Figures 3 and 4 and comprises a substantially rectilinear beam 14 held at one end to the fixed mounting 16 by a beam clamp 19. The other end is coupled to the crank arm 12 by means of a pin 20 fixedly mounted to the crank arm at the free end thereof. The pin 20 just touches the beam 14 at the rest position of the shaft and, with anti-clockwise displacement of the shaft and crank arm 12, the pin 20 bears on the beam 14 and causes it to flex (see arrow in Figures 4), This embodiment is suitable for very low angular displacements in one direction only. It is inaccurate for higher angular displacements due to friction effects in the rubbing contact between the pin 20 and the face of the beam 14.

Athird embodiment, shown in FigureS, differs only from the first in that the crank arm 12 is of different length and the beam 14 is irregularly curved so that one end thereof is connected to the fixed mounting 16 closer to the shaft 11. The embodiment of Figure 6 is similar to that of Figure 5 but is capable of providing a larger output signal from the strain gauge 18. Modifications such as these may be employed to render the transducer more suitable for measuring particular ranges of angles of rotational displacement different from that of the Figure 1 embodiment.

To measure greater magnitudes of angular displacement, it would of course be possible to extend the length of the beam, even so as to encircle the shaft several times, or to use alternative materials for the beam. The strain gauges 18 may be bonded foil strain gauges although, in general, thin film strain gauges are preferred since they can give greater accuracy and repeatability.

Claims

1. A method of measuring angular displacement of a member relative to a fixture, which method comprises the steps of:

i) fastening one end of a flexible beam of a sensing device to the member and the other end to the fixture, such that the beam flexes in proportion to said angular displacement;

ii) obtaining from the device a signal indicative of the amount by which the beam flexes; and

iii) calibrating the sensing device to provide an output which is representative of the said angular displacement.

2. Apparatus including a member mounted for rotation relative to a fixture and a sensing device to measure the angular displacement of the member relative to the fixture, the sensing device comprising a flexible beam so secured as to co-operate at one of its two ends with the fixture and at the other of its ends with the member, and to flex in proportion to angular displacement of the member in at least one direction from a rest position of the member, and a sensing means located intermediate the beam ends which respond to flexing of the beam to generate a signal which varies with the amount of angular displacement of said member from said rest position, the sensing device being calibrated to provide as output a signal representative of the amount of said angular displacement.

3. Apparatus as claimed in claim 2 wherein said member is a shaft.

4. Apparatus as claimed in claim 3 wherein said other end of the beam is coupled to the shaft by a crank arm connected to the sbaftfor rotation therewith.

5. A device for measuring angular displacement of a member relative to a fixture, which device comprises a flexible beam which carries intermediate its ends a sensing means which responds to flexing of the beam, a mounting towhich one end of the beam is connected, and a clamp to which the other end of the beam is connected and by which the beam can be secured to said member whereby, in use, the said mounting is secured to the fixture and the beam flexes in proportion to angular displacement of said member in at least one direction from a rest position thereof relative to said fixture, the measuring device being so constructed and arranged that it provided an output signal representative of said angular displacemeat;

6. Apparatus as claimed in claim 2,3 or 4, or a device, as claimed in claim 5 wherein said sensing means is a strain gauge mounted to the beam.

7. Apparatus or a device as claimed in claim 6 wherein the strain gauge is a thin film strain gauge.

8. Apparatus or a device as claimed in claim 6 or 7 wherein the beam is of a beryllium copper alloy suitable for use with a strain gauge.

9. Apparatus or a device as claimed in claim 6,7 or 8 including a full bridge gauge circuit.

10. Apparatus or a device as claimed in any one of claims 6 to 9 including a source of electrical power.

11. A device for measuring angular displacement substantially as hereinbefore described with reference to, and as shown in, any one or more of the accompanying drawings.