GB2200457A

GB2200457A - Strain measurement transducer

Info

Publication number: GB2200457A
Application number: GB08701212A
Authority: GB
Inventors: James Cahill; Malcolm Victor Wilson
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-01-21
Filing date: 1987-01-21
Publication date: 1988-08-03
Anticipated expiration: 2007-01-21
Also published as: GB8701212D0; GB2200457B

Abstract

A transducer which measures engineering strain in the surface of a material under stress is frictionally coupled to the measurement surface via two sharp points (1), such that a change in strain at the surface raises stresses in two cantilever plates (2) mounted on a rigid backplate (4). The cantilever plates have commercial straingauges (3) bonded one on each of the four faces, and connected electrically in a Wheatstone bridge network so as to produce an electrical analogue of the frictionally induced surface strain for measurement via conventional electronic instruments. The transducer is compensated electrically and mechanically for inputs other than frictionally induced linear deflection between its points arising from strain in the measurement surface, so that it may be hand held, or held in a simple clamp, against the measurement surface. Mounting portion (5) is connected to the backplate (4) by pivots (6) so that displacement of the mount in plane A-A does not affect the output, while displacement in plane B-B merely produces rocking about the points (1). The bridge connection is effective to nullify the effects of any backplate displacement parallel to the surface under test. <IMAGE>

Description

STRAIN MEASUREMENT TRANSDUCER This invention relates to a transducer for the measurement of engineering strain in engineering components under stress.

The process of the design and development of engineering components in a wide range of industries often requires the measurement of engineering strain.This can be accomplished by a variety of commercially available methods, most of which require the physical attachment of a measurement system to the component under study or the attachment of a transducer by mechanical means. The component is then put through a loading cycle and the measurement carried out.

The invention described here can carry out measurements while being held in the hand, frictionally attached to the component by means of hand pressure only. This allows the investigator to move the transducer over the surface of the component being studied at will, so that many measurements may be rapidly and easily taken to facilitate analysis and understanding of stress patterns and load flows.

In the present invention, strain at the surface of a component under study is frictionally coupled into a transducer section which converts the measured strain into an electrical signal, the device being able to be hand held during the measurement process.

A specific example of the construction of the transducer will now be described, with reference to the accompanying drawings, in which: Fig 1 shows in perpective the general construction; Fig 2 shows the electrical connection arrangement; Fig 3 shows the various compensation modes, viz: Fig 3a shows the transducer picking up engineering strain for measurement Figs 3b, 3c show the force inputs which are mechanically/electrically compensated.

The transducer has three main features: (a)A pair of sharp points (1) which, when loaded by hand pressure against the surface of a component under study,frictionally couple to the surface so that strain in the surface is transferred to the transducer.

(b)A pair of cantilever plates (2) which are stressed by the frictionally coupled strain, and which have bonded to them, one to each face, commercially available straingauges (shown diagrammatically 3) which convert the strain in them into an electrical analogue signal able to be conditioned by conventional analogue electronic instrumentation.These measurement areas are shaped such that the strain in them is uniform, by being triangular in form with the pickup point forming the apex of the triangle.The arrangement of the electrical connections to the straingauge transducers is by a Wheatstone bridge network arranged such that the measurement sections are sensitive only to direct strain between the pickup points, with strains due to hand induced load inputs from the operator being negated by electrical compensation.( figs 2 and 3) (c) A body (4),either integral with the measurement sections, or alternatively bolted or bonded to them, connecting the two measurement sections together so as to react the forces consequent upon strain input via the pickup points,and which has a mounting (5) and pivot(6) to allow the operator to hold the transducer and apply it to the surface of the component under test, the whole assembly being also arranged so that the effects of hand holding of the device on the strains measured are minimised.

Referring to Figs 1 and 3, manual inputs in the plane A-A are negated by the pivot mounting (5), and inputs in the plane B-B are negated by the device pivotting about the points (1). The pivotted mounting is arranged to adapt into a hand holder for manual operation, or to a mechanical clamp providing the same functions as a human hand.

Referring to Figs 2 and 3, input other than direct strain between the pickup points is electrically negated by the connection arrangement into the conventional Wheatstone bridge network.Thus input as at Fig 3b gives equal inputs on gauges 1 and 3, and on gauges 2 and 4, giving no change in output.Similarly, direct force onto the transducer from the operator to allow frictional pickup of strain, as in Fig 3a gives an equal change on all four gauges, giving no change in output.

The reference gauge length of the transducer is set by the distance between the pickup points, L.In a typical transducer this would be in the order of 5 to 50 millimetres.

Claims

1 A strain transducer comprising two cantilever plates with sharp points, which are connected by a rigid backplate or body incorporating a mounting pivot, and which have attached to their four faces commercial resistance straingauges arranged in a Wheatstone bridge electrical network.The transducer, by frictional coupling via the sharp points to a stressed material surface, produces an electrical analogue signal for strain measurement via conventional electronic instruments.

2 A transducer as Claim 1 in which the cantilever plates and the backing plate form one integral component.

3 A transducer as Claim 1 in which the cantilever plates are mechanically bolted or rivetted to the backing plate

4 - A transducer as Claim 1 in which the cantilever plates are bonded or welded to the backing plate.

5 A transducer as Claim 1 or Claims 2 to 4 in which -the sharp points are integral with the cantilever plate material.

6 A transducer as claim 1 or Claims 2 to 4 in which the sharp points are separate and of a different material to the cantilever plates, but rigidly attached to them.

7 A transducer as Claims 1 to 6 in which the commercial straingauges are of the foil or semiconductor type bonded to the cantilevers by commercial straingauge adhesive.

8 A transducer as Claims 1 to 6 in which the commercial straingauges are of the high temperature welded type, welded to the cantilevers.

9 A transducer as Claims 1 to 8 in which the frictional force is generated by the device being hand held against the measurement surface.

10 A transducer as Claims 1 to 8 in which the frictional force is generated by means of a simple clamping device.

11 A transducer substantially as described herein with reference to figs 1 to 3 of the accompanying drawings.