WO2006037969A1

WO2006037969A1 - Strain gauge

Info

Publication number: WO2006037969A1
Application number: PCT/GB2005/003777
Authority: WO
Inventors: Ken Hall
Original assignee: Zi Medical Plc
Priority date: 2004-10-01
Filing date: 2005-09-30
Publication date: 2006-04-13
Also published as: GB0421858D0

Abstract

A strain gauge assembly (10) suitable for mounting on a circuit board (14) comprising; a substrate (28) having a first end (30) and a second end (32); one or more mounting pads (34); and a connector (38) located near to an edge of the first end (30) of the substrate; and one or more electrical pathways (36) extending from the connector (38) to a strain gauge (40) located towards the second end (32) of the substrate; wherein the strain gauge assembly (10) is adapted for cantilevered attachment by the mounting pads (34) over an edge (22) of the circuit board (14); and resilient deformation relative to the circuit board (14). A method of manufacturing the strain gauge assembly (10) and an application therefor is also disclosed.

Description

DESCRIPTION

Title: Strain Gauge

The present invention relates to strain gauges and in particular to surface mounted strain gauges for circuit boards.

Strain gauges are well known and sometimes comprise a thin strip of polymeric material, into or onto which an electrically resistive track is laid. A voltage is then applied to the track and the current monitored. When the gauge is strained, the track either lengthens and thins or shortens and thickens depending on the nature of the strain. This dimensional change causes the electrical resistance of the track to either increase or decrease, which causes the measured current to decrease or increase, respectively.

Multiple tracks may be provided for measuring strains in various directions.

Strain gauges are expensive because of their relative complexity and small size. Furthermore, using strain gauges tends to be expensive, as they must usually be applied, i.e. stuck to a surface using an adhesive, by a skilled technician. A further problem with known strain gauges is that they cannot be easily wired up due to the small size of the contacts and the fragility of the devices themselves.

Applications of strain gauges are varied, and include, materials testing applications, avionics for continuous monitoring of aircraft components in flight and weighing / force measurement devices. Another application of strain gauges is measuring the torque transmitted by a motor.

In the latter case, a motor can be mounted such that it can rotate to a certain extent about its axle. The motor is secured using resilient legs that extend tangentially from its housing. One or more of the legs have a strain gauge associated therewith that can detect deformations in that leg. Thus, in use and with a load applied, the motor will try to twist about its axle with a torque equal to the torque in the motor system. However, the legs will restrain that torque, and the strain gauge or gauges will detect forces transmitted through the legs to counter the torque.

Where the motor is very small, or mounted directly on a circuit board, using a strain gauge is generally impractical and/or expensive. One application where measuring the torque in a circuit board-mounted motor is in the art of syringe driver technology.

A syringe driver generally comprises a mechanical assembly adapted to engage with a syringe. The mechanical assembly is used to decant a specified amount of liquid, e.g. a drug, from the syringe into a user at a desired rate. As syringe driver technology has developed, certain advantages have been discovered in varying the speed of drug delivery. For example, it is desirable to monitor the backpressure in a syringe, i.e. the pressure of the user's blood against which the drug is being forced, to ensure that the drug delivery rate is compliant with one or more physiological parameters.

In a syringe driver system utilising a motor to drive the syringe, it has been found that the backpressure is a function of the motor torque required to drive the syringe. A need has therefore arisen to monitor the motor torque. The disadvantages of using a strain gauge as discussed above, i.e. complexity, small size, cost etc., are prohibitive of using onboard, circuit board-mounted strain gauges.

The present invention aims to overcome one or more of the above problems. It is also an object of the present invention to provide an improved strain gauge.

Accordingly, a first aspect of the invention provides a strain gauge assembly suitable for mounting on a circuit board comprising; a substrate having a first end and a second end; one or more mounting pads; and a connector located near to an edge of the first end of the substrate; and one or more electrical pathways extending from the connector to a strain gauge located towards the second end of the substrate; wherein the strain gauge assembly is adapted for cantilevered attachment by the mounting pads over an edge of the circuit board; and resilient deformation relative to the circuit board.

The circuit board can be of any type, used in any application. Likewise, the strain gauge can be of any suitable type.

The substrate is preferably tapered. Where the substrate is tapered, the first end is preferably wider than the second end.

The substrate is preferably manufactured of a polymeric material, although any resilient, electrically insulative material would suffice. The substrate is preferably tapered and preferably substantially trapezoidal in shape, having one pair of opposite sides parallel. The substrate may have a constant thickness or may thin towards one end.

The one or more mounting pads are preferably manufactured of a solderable material, such as copper or tin. The mounting pads may optionally serve as electrical connections between the substrate and circuit board, hence good electrical conductivity is a possible desirable feature.

The connector can be of any suitable type. A multiple-pin, push fit connector is preferred for ease of use. Additional securing means may be associated with the connector, such as a securing clip. The connector is preferably adapted for engagement with a complementary connector located on the circuit board.

The electrical pathways of the invention may be wires or other suitable connectors. Tracks formed from deposited or etched-away copper, aluminium or tin are possible

alternatives to wires. The strain gauge is located towards the second end of the substrate, preferably substantially centrally.

A second aspect of the invention provides a method of manufacturing a strain gauge assembly comprising the steps of; depositing one or more mounting pads and one or more conducting tracks on a substrate; affixing a strain gauge and a connector to the substrate, the conducting tracks forming at least one electrical connection between the strain gauge and connector; and cutting the substrate to form a strain gauge assembly; the method being characterised in that the mounting pads are located near to an edge of the end of the substrate.

The order of the method steps is not important, e.g. the order in which the tracks/mounting pads are deposited or the order in which the strain gauge/connector are affixed.

The substrate is preferably tapered, e.g. trapezoidal in shape. Where the substrate is tapered, the mounting pads are preferably located at the wider end thereof. The step of depositing one or more mounting pads and one or more conducting tracks on a substrate can be achieved by any suitable means. Laying-down of electrical wires or forming electrical tracks by Printed Circuit Board (PCB) techniques are possible means.

The strain gauge and connector can be affixed to the substrate by any suitable means, e.g. gluing or by clipping thereto.

The substrate can be cut to shape at any stage of the manufacturing process, although it is envisaged that doing so as a last step would be most convenient.

A third aspect of the invention provides a torque monitor for a motor comprising a motor resiliently mounted via a mounting means to a circuit board, the mounting means being arranged to interact with a strain gauge assembly adapted for cantilevered attachment by mounting pads over an edge of the circuit board and resilient deformation relative to the circuit board; wherein the strain gauge assembly comprises; a substrate having a first end and a second end; one or more mounting pads and a connector located near to an edge of the first end of the substrate; and one or more electrical pathways extending from the connector to a strain gauge located towards the second end of the substrate.

The torque monitor may be implemented by any suitable means. In a possible embodiment, the strain gauge assembly is arranged to overhanging a cut out, edge or aperture in the circuit board. A motor may then be resiliency fastened to the circuit board using a mounting means. The mounting means is preferably adapted to interact with the strain gauge assembly, i.e. by causing it to flex under the influence of a compressive force generated by the torque in the motor during use. Flexure of the strain gauge in use preferably causes a detectable signal from the strain gauge to be generated.

A preferred embodiment of the invention shall now be described, by way of example only, with reference to the accompanying drawings, in which;

Figure 1 shows a schematic end view of the strain gauge assembly used in conjunction with a motor;

Figure 2 shows how the strain gauge assembly is affixed to a circuit board;

Figure 3 is a schematic manufacturing step diagram for the invention; and

Figure 4 shows a circuit board lay-up prior to cutting.

Referring now to Figure 1, a strain gauge assembly 10 according to the invention is shown overhanging a cut out 12 in a circuit board 14. A motor 16 is resiliency fastened to the circuit board using legs 18. The intended direction of rotation of the motor 16 is denoted by arrow A. When the motor 16 is in use, a compressive force is thereby transmitted through leg 18'. The force causes the strain gauge assembly 10 to flex, which causes a detectable signal from the strain gauge to be generated. Of course, forces in the opposite sense can be measured so long as a tensional force can be adequately transmitted to the strain gauge assembly 10.

Figures 2 and 3 show the strain gauge assembly 10 of the invention in greater detail from above. The circuit board 14 has a square-shaped cut out 12 therein. Copper locating pads 20, formed by conventional PCB manufacturing techniques, are located on the circuit board near to the inner edge 22 of the cut out 12. A female connector 24 is also provided, which is addressed by a series of copper tracks 26, also manufactured via conventional PCB techniques.

The strain gauge assembly 10 comprises a tapered substrate 28 having a wide end 30 and a narrow end 32. Mounting pads 34 are provided on the corners of the substrate 28 at the wide end 30 thereof. Tracks 36 are provided to connect a male connector 38 to a strain gauge 40. The substrate 28 is manufactured of a resilient polymeric material and the mounting pads 34 and tracks 36 are disposed thereon using conventional manufacturing techniques.

As can be seen, the strain gauge assembly 10 is adapted for cantilevered location over an edge 22 of the circuit board 14 by soldering respective mounting pads 20 & 34 to one another and by engaging the male connector 38 with the female connector 24. Thus, electrical signals generated by the strain gauge 40 during flexure can be detected via the copper tracks 26 on the circuit board 14.

A force can be applied to any part of the strain gauge assembly 10, although the nearer to the narrow end 32 and centrally that force is applied, the greater the sensitivity that is to be

expected.

Location of the strain gauge assembly 10 is relatively straightforward, as it held in-situ by the connectors 24 & 38 whilst a technician or robot solders the mounting pads 20 & 34 in a conventional manner. Removal and replacement of the strain gauge assembly 10 is, likewise, relatively straightforward and is accomplished by the reverse of the above.

In an alternate arrangement, (not shown) the mounting pads are used to provide an electrical connection between the copper tracks of the circuit board and the strain gauge, hi this embodiment just the arrangement of tracks on the strain gauge assembly and the type of connectors has been changed. Furthermore, it will be appreciated that a cut out is not required, as the strain gauge assembly can be mounted overhanging the edge of the circuit board itself. hi a further alternate embodiment of the invention (not shown) the strain gauge assembly is located overhanging an aperture in the substrate. The present invention thereby enables convenient location of the strain gauge assembly at any desired location on a circuit board.

Figure 3 shows a possible manufacturing route for the invention. In Figure 3a, a bare substrate 28 is shown; in Figure 3b, copper mounting pads 34 and tracks 36 have been formed by conventional PCB manufacturing techniques; in Figure 3c the strain gauge 40 is stuck to the substrate using a suitable proprietary adhesive; and in Figure 3d, the connector 38 is affixed using conventional techniques.

Finally, Figure 4 shows how it is possible to scale-up production by manufacturing a plurality of strain gauge assemblies 10 on a single substrate. It is envisaged that the various manufacturing steps by carried out using automated techniques, e.g. using robots or manipulators to position the various components and to make the necessary electrical connections etc. The tapered shape of individual strain gauge assemblies 10 is formed by slicing the substrate 28, post-manufacture along the cut-lines denoted C.

As can be appreciated by the above description of the invention, considerable cost savings can be accomplished by mass-manufacturing the strain gauge assemblies in accordance with a regime such as that depicted in Figures 3 and 4. Furthermore, providing an easy to install and replace strain gauge assembly, which does not require a skilled or specially trained technician to install, can lead to considerable labour and cost savings.

Claims

1. A strain gauge assembly suitable for mounting on a circuit board comprising; a substrate having a first end and a second end; one or more mounting pads; and a connector located near to an edge of the first end of the substrate; and one or more electrical pathways extending from the connector to a strain gauge located towards the second end of the substrate; wherein the strain gauge assembly is adapted for cantilevered attachment by the mounting pads over an edge of the circuit board; and resilient deformation relative to the circuit board.

2. A strain gauge assembly as claimed in claim 1, wherein the substrate is tapered.

3. A strain gauge assembly as claimed in claim 2, wherein the first end is wider than the second end.

4. A strain gauge assembly as claimed in any of claims 1 to 3, wherein the substrate is manufactured of resilient, electrically insulative material

5. A strain gauge assembly as claimed in any of claims 1 to 4, wherein the one or more mounting pads are manufactured of a solderable material.

6. A strain gauge assembly as claimed in any of claims 1 to 5, wherein the mounting pads serve as electrical connections between the substrate and circuit board.

7. A strain gauge assembly as claimed in any of claims 1 to 6, wherein the connector is a push-fit connector.

8. A strain gauge assembly as claimed in any of claims 1 to 7, wherein the electrical pathways comprise one or more tracks formed from deposited or etched-away copper, aluminium or tin.

9. A strain gauge assembly as claimed in any of claims 1 to 8, wherein the strain gauge is located towards the second end of the substrate, substantially centrally thereof.

10. A method of manufacturing a strain gauge assembly comprising the steps of; depositing one or more mounting pads and one or more conducting tracks on a substrate; affixing a strain gauge and a connector to the substrate, the conducting tracks forming at least one electrical connection between the strain gauge and connector; and cutting the substrate to form a strain gauge assembly; the method being characterised in that the mounting pads are located near to an edge of the end of the substrate.

11. A method of manufacturing a strain gauge assembly as claimed in claim 10, wherein the substrate is tapered.

12. A method of manufacturing a strain gauge assembly as claimed in claim 11, wherein the mounting pads are located at the wider end thereof.

13. A method of manufacturing a strain gauge assembly as claimed in claim 10, 11 or claim 12, wherein the conducting tracks comprise one or more tracks formed from deposited or etched-away copper, aluminium or tin.

14. A method of manufacturing a strain gauge assembly as claimed in claim 13, wherein the tracks are formed by Printed Circuit Board (PCB) techniques.

15. A method of manufacturing a strain gauge assembly as claimed in any of claims 10 to 14, wherein the strain gauge and/or connector are affixed to the substrate using glue or one or more clips.

16. A method of manufacturing a strain gauge assembly as claimed in any of claims 10 to 15 wherein the substrate cutting cut step comes last.

17. A torque monitor for a motor comprising a motor resiliently mounted via a mounting means to a circuit board, the mounting means being arranged to interact with a strain gauge assembly adapted for cantilevered attachment by mounting pads over an edge of the circuit board and resilient deformation relative to the circuit board; wherein the strain gauge assembly comprises; a substrate having a first end and a second end; one or more mounting pads and a connector located near to an edge of the first end of the substrate; and one or more electrical pathways extending from the connector to a strain gauge located towards the second end of the substrate.

18. A torque monitor as claimed in claim 17, wherein the strain gauge assembly is arranged to overhang a cut out, edge or aperture in the circuit board.

19. A torque monitor as claimed in claim 17, wherein a motor is resiliently fastened to the circuit board using a mounting means, the mounting means being adapted to interact with the strain gauge assembly.

20. A torque monitor as claimed in claim 19, wherein the mounting means is adapted to flex under the influence of a compressive force generated by the torque in the motor during use.

21. A torque monitor as claimed in any of claims 17 to 21, wherein flexure of the strain gauge in use causes a detectable signal from the strain gauge to be generated.