GB2360361A

GB2360361A - Strain gauge with matching resistors on both surfaces of a substrate

Info

Publication number: GB2360361A
Application number: GB0024368A
Authority: GB
Inventors: Robin T A Stevens
Original assignee: IND DATALOGGERS Ltd
Current assignee: IND DATALOGGERS Ltd
Priority date: 2000-03-17
Filing date: 2000-10-05
Publication date: 2001-09-19
Anticipated expiration: 2020-10-05
Also published as: GB0006551D0; GB2360361B; GB0024368D0

Abstract

The strain gauges comprise a flat insulating substrate carrying a metallic layer formed as a labyrinthine pattern. The substrate carries the metallic layer formed as a labyrinthine pattern on both of its surfaces and in a substantially matching relationship. The metal patterns are connected so that changes is resistance balance each other. The insulating substrate is a synthetic resin composition reinforced to provide strength but sufficiently resilient to avoid cracking when subjected to moderate stresses or strains such as glass fibre reinforced epoxy resin. Alternatively the substrate may be a thin insulating film. The metal pattern may be formed from a high resistance alloy, such as a copper/nickel or nickel/chromium alloy, or a pure metal such as copper. The substrate may also carry amplifier whereby, if power is provided to the amplifier, an electrical output signal is obtained which relates to the strain exerted on the gauge. The resistance change from the metal pattern or the output signal from its associated amplifier can be arranged to be proportional to the bending moment applied to the substrate.

Description

2360361 IMPROVED STRAIN GAUGE DEVICES This invention relates to improved

strain gauge devices and their application for measurement and 5 control purposes.

Strain gauges comprise one or more electrical conductors whose resistance changes when deformation occurs. Such gauges may consist of wire or f oil conductors bonded or otherwise f irmly attached to a specimen whose deformation is to be measured. Wire or foil gauges may be directly bonded. to the specimen whose deformation is to be measured or,"they may be deposited on a substrate which is, subsequently bo nded to the specimen under measurement,. The construction and use of strain gauges is well known and described in the text books, Strain Gauge Technology edited by A.L. Window (Elsevier Applied Science) and The Bonded Electrical Resistance Strain Gage by W. Murray and W.R. Miller (Oxford University Press). Foil strain gauges and known in which a layer of a resistive alloy is deposited on a substrate, a conductor pattern is printed on the alloy layer using a photoresist, straight printing or other technique and thereafter the unwanted portions of the alloy layer are removed by etching. Alternatively the unwanted portions of the alloy layer may be removed by etching using a high intensity energy source such as a laser.

The present invention provides an improved strain gauge together with associated devices and apparatus incorporating such gauges.

According to the present invention there is provided a strain gauge comprising a flat insulating substrate carrying a metallic layer formed as a labyrinthine pattern, 2 characterised in that the substrate carries the metallic layer formed as a labyrinthine pattern on both of its surfaces and in a substantially matching relationship.

The insulating substrate is preferably a synthetic resin composition reinforced to provide strength but sufficiently resilient to avoid cracking when subjected to moderate stresses or strains. Glass fibre reinforced epoxy resin is a preferred substrate. In an alternative embodiment the insulating substrate is a thin insulating film which carries the metal pattern. The film may carry adhesive on one side so that it can be attached to a specimen directly and a second identical layer is placed on top of it. The.film carrying the second pattern becomes the insulating substrate. The metal forming the labyrinthine pattern is preferably bonded to the opposite surfaces of the substrate before the formation of the pattern. The metal layer may be applied as a thin foil bonded by an adhesive film or formed by vacuum deposition directly on to each surface of the substrate. The conducting foil pattern may be created by conventional means such as coating with a photoresist composition followed by actinic light exposure to form a resist pattern and etching away the areas unprotected by the resist. The metal may be a high resistance alloy, such as a copper/nickel or nickel/chromium alloy, or a pure metal such as copper. The use of substantially matching metal patterns on each side of the substrate enables considerable compensation for resistance changes due to variations in temperature to be obtained. The metal patterns are connected so that changes is resistance balance each other. A particular advantage of this arrangement is that inexpensive and easily managed metals, such as copper, may be used for the pattern in spite of their comparatively high temperature coefficient of resistance.

3 The labyrinthine metal pattern on the substrate may take any conventional form. The pattern is duplicated on each side of the substrate so that the conductors forming the gauge may be connected in a bridge configuration in a manner that compensates for resistance changes caused by variations in temperature. Such a bridge configuration balances out the effects of axial loads upon the specimen to which the strain gauge is attached. It also balances out the effects of dimensional changes in the specimen due to its linear coefficient of expansion with changes in temperature. This avoids the need to match the linear coefficient of expansion of the metal used to fabricate the pattern with that of the specimen. to which it is attached. Bending moments on, the gauge are detected efficiently without interference from temperature effects or the effects of axial loading. Matching pairs of rosette shaped patterns may be added to the substrate to cancel out unwanted motions and temperature effects. The leads f rom the rosettes are connected into the resistance measuring circuit in conventional manner to achieve the desired cancellation effect.

In a preferred version the substrate of the strain gauge according to the invention includes a conductor pattern and associated amplifier whereby if power is provided to the amplifier an electrical output signal is obtained which relates to the strain exerted on the gauge. While the amplifier may be formed from discrete components connected to a conducting circuit pattern on the substrate it is preferred that the amplifier.is formed as an integrated circuit. The latter arrangement minimises the number of connections required to the assoc iated gauge pattern. The preferred configuration of the integrated circuit amplifier is the so-called "surface mount" form.

Such a configuration permits the conductor pattern forming 4 the strain gauge, the connecting conductors and the amplifier to all be located on one side of the substrate. The other side of the substrate is bonded, bolted or otherwise attached to the specimen from which strain 5 information is required to be measured.

The gauge/amplifier assembly enables a bridge circuit comprising the conductor pattern forming the gauge to be supplied with power and to provide a signal to the amplifier. This signal is then amplified to a level at which it may be transmitted to associated equipment such as a measurement display, a recordin g.system or a servo system in which it is used as the error.- signal. In some cases it may be convenient to include an analogue to digital conversion circuit within the amplifier integrated circuit package or in a separate package so as to send a digital signal from the assembly. The output signal from the amplifier can be arranged to be proportional to the change in resistance of the gauge caused by some change in the specimen to which the gauge is attached. By locating the amplifier and associated circuitry on the substrate close to the connections to the gauge or gauges the effect of rogue signals picked up from outside sources is reduced. The output from the amplifier fed to the control or measurement equipment is at a high level compared with interfering signals and their effect is negligible.

A strain gauge according to the invention having dimensions in the region of 600 mm by 300 mm may be formed as a platform which can take the weight of a human being by using a substrate thick enough to take this weight or by bonding to a separate support substrate., The platform must be spaced f rom the surface on which it is placed so that it can flex to some extent. Three or more rubber feet can provide this suspension.

Changes in the resistivity of the gauge will occur as when a person standing on it moves. These changes can be amplified and fed to a microprocessor where they can be used to provide signals which interact with computer programs, particularly those involving viewing screens, so that the person on the platform can interact and control features on the screen by body movements instead of using manual controls as in most interactive computer games.

In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which:

Figure la is a plan view of the surf ace of a strain gauge carrying two metal labyrinthine patterns according to the invention for forming half a bridge, Figure 1b is a side view of the a strain gauge shown in Figure la showing the substantially matching metal patterns, Figure 2a is a plan view of the surface of a strain gauge carrying two twin metal labyrinthine patterns according to the invention forming a full bridge, Figure 2b is a side view of the a strain gauge shown in Figure 2a showing the substantially matching metal patterns.

A strain gauge according to the invention, see Figure la, consists of a rectangular insulating substrate 1 carrying a labyrinthine metal foil patterns 2 and 3. Each foil pattern has enlarged portions 4 to facilitate electrical connections to the patterns. The substrate 1 is attached to a f lat specimen 5 which is capable of bending as shown schematically by the arrow 6. The attachment may be by adhesive or fixings such as screws, bolts, etc. The f oil patterns 2 and 3 have the same patterns which are substantially matched on each side of the substrate 1. As a consequence temperature variations 6 will affect both patterns so that if they are connected as two arms of a bridge the changes in resistance caused by these variations will cancel each other. Resistance changes resulting from the bending of the specimen 5 can be sensed without temperature variations distorting the results.

A strain gauge having twin patterns, see Fig-ure 2a, consists of a rectangular insulating substrate 10 carrying a labyrinthine metal foil patterns 12 and 13. Each foil pattern is has twin conductors so that the pattern can be connected to form a full bridge. The ends of the conductors forming the patterns 14.are enlarged to facilitate electrical connections to the patterns. The substrate 11 is attached to a flat specimen 15 which is capable of bending as shown schematically by the arrow 6. Attachment to the specimen 15 is carried out using the same methods described with respect to Figure 1. The foil patterns 12 and 13 have the same patterns which are substantially matched on each side of the substrate 11 so as to provide temperature compensation as described previously.

In a preferred embodiment the ends of the conductors of the labyrinthine patterns are connected directly to an integrated circuit amplifier or similar circuit fitted on the substrate. The amplifier is supplied by power externally and provides either a balanced signal or unbalanced signal relating to the amount of bending moment applied to the specimen 5 or 15. The amplifier may be connected to a substrate mounted analogue to digital converter circuit so that the output signal is in digital form.

The gauge may take the form of a board which can take the weight of a standing human being, e.g. more than 80 kg, and the bending moment will vary as the being moves its centre of gravity. The output signal obtained may be used as the input to a computer system having a visual display.

Movement of the being on the board can be used to control moments of figures or other items shown by the visual display. Such an arrangement avoids the mechanical coupling and short life of potentiometers previously used for such applications. 10 8

Claims

1. A strain qauqe comprisincr a f lat insulatinq substrate carrying a metallic layer formed as a labyrinthine pattern, characterised in that the substrate carries the metallic layer formed as a labyrinthine pattern on both of its surfaces and in a substantially matching relationship.

2. The strain gauge as claimed in claim 1, characterised in that the insulating substrate is a synthetic resin composition reinforced to provide strength but sufficiently resilient to avoid cracking when subjected to moderate stresses or strains.

3. The strain gauge as claimed in claim 2, characterised in that the insulating substrate is a glass fibre reinforced epoxy resin.

4. The strain gauge as claimed in claim 2, characterised in that the insulating substrate is a thin insulating film.

5. The strain gauge as claimed in any of the preceding claims, characterised in that the metal forming the labyrinthine pattern is bonded to the opposite surfaces of the substrate before the formation of the pattern.

6. The strain gauge as claimed in claim 5, characterised in that the metal layer is applied as a thin foil bonded by an adhesive film or formed by vacuum deposition directly on to each surface of the substrate.

7. The strain gauge as claimed in any of.the preceding claims, characterised in that the conducting foil pattern is created by coating its surface with a photoresist composition followed by actinic light exposure to form a resist pattern and etching away the areas unprotected by the resist.

8. The strain gauge as claimed in any of the preceding claims, characterised in that the metal is a high resistance alloy, such as a copper/nickel or nickel/chromium alloy.

9. The strain gauge as claimed in any of the preceding claims 1 to 7, characterised in that the metal is a pure metal such as copper.

10. The strain gauge as claimed in any of the preceding claims, characterised in that the metal patterns are connected so that changes is resistance balance each other.

11. The strain gauge as claimed in any of the preceding claims, characterised in that the substrate carries a metal pattern and an associated amplifier whereby, if power is provided to the amplifier, an electrical output signal is obtained which relates to the strain exerted on the gauge.

12. The strain gauge as claimed in claim 11, characterised in that the amplifier is formed as an integrated circuit.

13. The strain gauge as claimed in claim 12, characterised in that the amplifier is formed as an integrated circuit in surface mount form.

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14. The strain gauge as claimed inany of the preceding claims 11 to 13, characterised in that an analogue to digital conversion circuit is included within the amplifier integrated circuit package or in a separate package so as to provide a digital output signal.

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15. The strain gauge as claimed in any of the preceding claims, characterised in that one side is bonded, bolted or adhesively attached to the specimen from which bending stress is required to be measured. 5

16. The strain gauge as claimed in any of the preceding claims, characterised in that the resistance change from the metal pattern or the output signal from an associated amplifier is arranged to be proportional to the bending moment applied to the substrate.

17. The strain gauge as claimed in zany of the preceding claims, characterised in that the.-resistance change from the metal pattern or the output signal from an associated amplifier is arranged to be proportional to the bending moment applied to the substrate.

18. Strain gauges as claimed in claim 1 and as herein described.

19. Strain gauges as herein described and illustrated in the accompanying drawings.