GB2141548A

GB2141548A - Strain-gauge transducer

Info

Publication number: GB2141548A
Application number: GB08316385A
Authority: GB
Inventors: Arnold Pitcher
Original assignee: Welwyn Electronics Ltd
Current assignee: Welwyn Electronics Ltd
Priority date: 1983-06-16
Filing date: 1983-06-16
Publication date: 1984-12-19
Also published as: GB8316385D0

Abstract

A transducer for measurement of a physical effect such as a force or load(L) comprises a primary member (1), e.g. of EN24 steel, adapted to be deflected when subjected to the effect and having elastic properties such that the effect can be measured with repeatability and reliability to a required degree of accuracy. A secondary member (2), preferably of aluminium or aluminium alloy having an electrically insulating surface film comprising anodised aluminium, is secured in association with the primary member (1) and has adhered thereto film electrical resistance elements (4, 5) whose electrical resistance varies as a function of mechanical strain induced in the secondary member (2) as a result of deflection thereof by the primary member (1), terminals (6, 7 and 8, 9) being provided for the elements (4, 5). The elements (4, 5) comprise electrically conductive or resistive particles dispersed in an organic polymer. The members (1, 2) are clamped at one end between supports (10, 11) and may also be secured together at their free ends. <IMAGE>

Description

SPECIFICATION Transducer This invention relates to electromechanical transducers incorporating electrical resistance strain gauges. More particularly, the invention relates to a transducer incorporating an electrical resistance strain gauge element in the form of a conductive polymer comprising a dispersion of electrically conductive or resistive particles in an organic polymer.

In UK Patent Application No 8115089 (publication No GB 2098739A), there is described a transducer for use in measuring a physical effect such as force, weight or load and which in an exemplified embodiment comprises a beam, notably of aluminium or aluminium alloy, arranged to be firmly clamped at one end in a support in a cantilever configuration.

One or more electrical resistance elements whose electrical resistance varies as a function of applied mechanical strain is or are deposited on the surface of the beam, the surface being anodised to render it electrically insulating when the beam comprises aluminium or aluminium alloy.

The or each resistance element comprises a dispersion of electrically conductive or resistive particles, such as carbon, in an organic polymer suitably selected from epoxy, alkyd, polyester, acrylic or silicone materials, or copolymers thereof.

Electrically conductive terminations of film form are provided for the or each resistance element. When the beam is deflected, eg by application of a force or load at the unsupported end thereof, the electrical resistance of the element changes. The change in resistance is used to provide a measure of the applied force or load.

The strain sensitive resistance element is most conveniently deposited on the surface of the beam by screen printing of a suitable composition onto the beam. A heat treatment is then carried out to effect necessary stoving and polymerisation of the conductive polymer element. This heat treatment, which may involve temperatures of between 1 00 C and 250"C is also used to optimise the electrical properties of the element.

It has been found that although a beam of material such as aluminium or aluminium alloy having an anodised surface is advantageous for use as a support for the deposited conductive polymer resistance element it is disadvantageous in an important respect. This relates to the elastic properties of the beam, which are very important in a transducer, and transducer beams are usually constructed from materials which are chosen and specially heat treated to provide elastic properties suitable for the particular application for which the transducer is intended.

However, the heat treatment which it is necessary to give to the conductive polymer resistance element or elements following deposition degrades the elastic properties of certain materials such as aluminium, making them unsuitable for use by themselves as the principle member in a transducer.

The present invention provides a transducer for use in the measurement of a physical effect, said transducer comprising a primary member adapted to be deflected when subjected to said physical effect and having elastic properties such that said effect can be measured with repeatability and reliability to a required degree of accuracy; a secondary member adapted and arranged to be secured in association with said primary member, said secondary member having adhered thereto a film electrical resistance element whose electrical resistence varies as a function of mechanical strain induced in said secondary member as a result of deflection thereof by said primary member said resistance element comprising a dispersion of electrically conductive or resistive particles in an organic polymer, electrically conductive terminals being provided for said element.

The said secondary member suitably comprises aluminium or aluminium alloy and is preferably provided with an electrically isulating surface film comprising anodised aluminium.

The said primary and secondary members may be secured in intimate contact.

Alternatively, the said primary and secondary members may be arranged such that certain regions thereof are secured together and other regions thereof are spaced apart.

In one embodiment said primary and secondary members comprise elongate beams overlying one another and clamped at one end in a support in a cantilever configuration. The other end of the primary member, which may be clamped to the corresponding end of the secondary member, is arranged to be deflected by subjection to said physical effect, the secondary member following the deflection of the primary member. Alternatively the said beams could be clamped at a central region thereof in a support in a double cantilever configuration and having two free ends of said primary member arranged to be deflected by subjection to said physical effect, corresponding regions of said secondary member following the deflections of said primary member. Adjacent free ends of the primary and secondary members may be clamped together.

In a further embodiment, the said primary and secondary members comprise elongate beams overlying one another and secured together at opposite ends thereof and arranged such that with one end of said beams supported and the other end subjected to said physical effect said beams undergo double bending of a reversed curvature nature.

The overlying beams may be slightly spaced apart by means of shims or spacer members interposed between the beams at opposite ends thereof.

The invention is now described by way of example with reference to the accompanying drawings in which Fig. 1 illustrates an exploded perspective view of a transducer according to the invention; Fig. 2 illustrates a perspective view of the transducer of Fig. 1 clamped in a support and with a physical effect applied.

An embodiment of a transducer according to the invention for measuring a physical effect such as, for example, force, load, weight, displacement, pressure or acceleration, comprises a primary member in the form of an elongate beam 1 having elastic properties suitable for accommodating the desired physical effect when applied. An example of a suitable material for the beam 1 is a steel, eg of type EN 24. A secondary member, in the form of an elongate beam 2 is constructed from aluminium or aluminium alloy. An electrically insulating film 3 is provided on the upper major surface of the beam 2. Film 3 is preferably provided by anodising the surface of the aluminium or aluminium alloy material of the beam 2. A pair of film electrical resistance elements 4 and 5, are deposited on the anodised insulating film 3 on the beam 2.The resistance elements 4 and 5 comprise a dispersion of electrically conductive or resistive particles in an organic polymer as described in published UK Patent Application No GB2098739A, and are suitably applied by a screen printing process. An appropriate electrical resistance composition is available from Electro-Science Laboratories of Pennsauken, USA and having series reference RS-150.

Film electrically conductive terminals 6, 7 are provided for resistance element 4 and similar terminals 8, 9 are provided for resistance element 5. The terminals are conveniently deposited by screen printing and suitably comprise a mixture of a powdered electrically conducting material, such as silver, and an organic resin. An example of such a material is type T2 100 manufactured by EMD-Cermalloy Inc. of Conshohocken, USA.

The resistance elements 4 and 5 and the terminals 6, 7, 8, 9 are heated to achieve the necessary polymerisation of the organic resin and to optimise the electrical properties of the resistance elements 4 and 5. The heating process may involve periods from a few hours to about twenty-four hours at temperatures of between about 100"C and 250 C. The electrical resistance of elements 4 and 5 varies as a function of mechanical strain induces in the beam 2.

The secondary member comprising the beam 2 with the resistance elements 4, 5 thereon is located in contact with the primary member comprising the beam 1. The resulting transducer assembly is clamped at one end between supports 10 and 11. The other end 1 2 (Fig. 2) of the assembly is secured to an arrangement (not shown) by means of which a physical effect, such as a load L, which it is desired to measure, can be applied to the transducer assembly. The arrangement secured to the end 1 2 of the assembly may, for example, be a platform of weighing apparatus. Preferably the two beams 1 and 2 are secured together at the end 1 2.

When a load L is applied, the primary member comprising beam 1, which serves as the supporting member for the load, is deflected in such a way that the beam 1 undergoes double bending of a reversed curvature nature (sometimes known as contraflexure) and takes up a shape which in some respects resembles a flattened "S". The secondary member, comprising the beam 2, operates to follow the deflection of the beam 1.

Mechanical strain is induced in the beam 2 as a result of the deflection of the beam 2 by the beam 1 and this strain is monitored by the resistance elements 4 and 5. Tensile strain is induced in the region of beam 2 on which resistance element 4 is deposited and compressive strain is induced in the region of beam 2 on the resistance element 5 is deposited. By appropriately connecting elements 4 and 5 in well-known manner in a Wheatstone Bridge network, the load L can be monitored.

If desired, the beams 1 and 2 could be arranged to be slightly spaced apart by interposing suitable shims or spacer members at each end.

Claims

1. A transducer for use in the measurement of a physical effect, said transducer comprising a primary member adapted to be deflected when subjected to said physical effect and having elastic properties such that said effect can be measured with repeatability and reliability to a required degree of accuracy; a secondary member adapted and arranged to be secured in association with said primary member, said secondary member having adhered thereto a film electrical resistance element whose electrical resistance varies as a function of mechanical strain induced in said secondary member as a result of deflection thereof by said primary member said resistance element comprising a dispersion of electrically conductive or resistive particles in an organic polymer, electrically conductive terminals being provided for said element.

2. A transducer according to Claim 1 in which the said secondary member comprises aluminium or aluminium alloy.

3. A transducer according to Claim 2 in which the secondary member of aluminium or aluminium alloy is provided with an electrically insulating surface film comprising ano dised aluminium.

4. A transducer according to Claim 1, 2 or 3, in which the said primary and secondary members are secured in intimate contact.

5. A transducer according to Claim 1, 2 or 3 in which the said primary and secondary members are arranged such that certain regions thereof are secured together and other regions thereof are spaced apart.

6. A transducer according to any preceding Claim in which said primary and secondary members comprise elongate beams overlying one another and clamped at one end in a support in a cantilever configuration, the other end of the primary member being arranged to be deflected by subjection to said physical effect, the secondary member following the deflection of the primary member.

7. A transducer according to Claim 6 in which the said other end of the primary member is clamped to the corresponding end of the secondary member.

8. A transducer according to any of Claims 1 to 5 in which the said beams are clamped at a central region thereof in a support in a double cantilever configuration and having two free ends of said primary member arranged to be deflected by subjection to said physical effect, corresponding regions of said secondary member following the deflections of said primary member.

9. A transducer according to Claim 8 in which adjacent free ends of the primary and secondary members are clamped together.

10. A transducer according to any of claims 1 to 5 in which the said primary and secondary members comprise elongate beams overlying one another and secured together at opposite ends thereof and arranged such that with one end of said beams supported and the other end subjected to said physical effect said beams undergo double bending of a reversed curvature nature.

11. A transducer according to Claim 10 in which the overlying beams are slightly spaced apart by means of shims or spacer members interposed between the beams at opposite ends thereof.

1 2. A transducer constructed and arranged substantially as herein described with reference to the accompanying drawings.