GB2303454A

GB2303454A - Strain gauge beam assembly

Info

Publication number: GB2303454A
Application number: GB9614226A
Authority: GB
Inventors: Michael Andrew Kellett
Original assignee: LAMERHOLM FLEMING Ltd
Current assignee: LAMERHOLM FLEMING Ltd
Priority date: 1995-07-14
Filing date: 1996-07-05
Publication date: 1997-02-19
Anticipated expiration: 2016-07-05
Also published as: GB2303454B; GB9614226D0

Description

STRAIN GAUGE ARRANGEMENTS This invention is concerned with strain gauge arrangements.

In particular, but not exclusively, the invention is concerned with strain gauge arrangements suitable for use in detecting movement/displacements of moving bodies.

Summarv of the Invention According to a first aspect of the present invention, there is provided a strain gauge assembly comprising two beams, superimposed or stacked in spaced relation, and inter-joined to one another and to a beam support so as to permit movement (of the beams) in one degree of freedom whilst substantially reducing movement (of the beams) in the other degrees of freedom.

Preferably, said assembly further comprises an instantaneous position follower positioned so as to be able to co-operate with the surface whose positional displacement it is desired to measure.

Ideally, said instantaneous position follower is a rotatable follower such as a ball race, rotatably mounted at the end of said strain gauge assembly distant from said beam support.

In a preferred form, each of said beams has one or more apertures therethrough, the aperture(s) preferably being so sized and shaped as to control the location of the point of maximum stress development within the beam as a consequence of flexure of the beam.

Advantageously, said strain gauge assembly further comprises a dual strain gauge device located on one surface of one of said beams, preferably at a position of maximum stress development.

Ideally, each of said beams is symmetrical about the axis perpendicular to said beam support.

Advantageously, each of said beams is formed (for example by stamping or double-sided etching) from sheet steel, where the grain of the steel is orientated perpendicular to the direction of maximum stress development.

In a preferred form, said strain gauge assembly is associated with electronic circuitry which monitors the output of said strain gauges resulting from flexure of said beams.

Ideally, said circuitry incorporates means for removing any signal offset output by said strain gauges when the beams are in their nominal position.

Preferably, this offset removal (or "zeroising") is actuable on demand and occurs electronically i.e without the need for mechanical calibration.

It will be appreciated that the invention includes specifically within its scope a strain gauge assembly substantially as described herein with reference to and as illustrated by any appropriate combination of the accompanying drawings.

According to a second aspect of the invention, there is provided a strain gauge arrangement comprising two or more strain gauge assemblies substantially as described above mounted adjacent one another on said beam support. Preferably, the strain gauge arrangement comprises two strain gauge assemblies.

According to a third aspect of the invention, there is provided an automated teller machine (ATM) or the like including one or more strain gauge arrangements substantially as described above.

Brief Description of the Drawings In the accompanying drawings: Figure 1 is a schematic side view of a strain gauge assembly embodying one aspect of the invention; Figure 2 is a schematic view of a beam; Figure 3 is a schematic end view of a strain gauge assembly; Figure 4 is a schematic front view of a strain gauge arrangement, incorporating two strain gauge assemblies; Figure 5 shows a second embodiment of the strain gauge arrangement; Figure 6 is a general circuit diagram for the strain gauge arrangement; and Figure 7 is a schematic circuit diagram of the electronic zeroising circuitry.

Description of the Preferred Embodiments Referring to Figure 1, strain gauge assembly 1 including two superimposed parallel, spaced-apart beams 2 (see description below) has at one end a ball race 3 which, in use, operationally acts as a position follower. In its nominal (i.e rest) position ball race 3 of strain gauge assembly 1 bears against surface 4 of a roller 5, causing a nominal degree of flexure of beams 2.

Ball race 3 in use, will be displaced in the direction indicated by arrow Y in response to the passage of, for example a banknote, in the direction indicated by arrow X between ball race 3 and surface 4 of roller 5. The banknote could alternatively approach ball race 3 and surface 4 from the opposing direction to arrow X.

Turning now to Figure 2, which shows a single beam 2 which can be either stamped or double-sided etched from sheet metal, for example steel. The beam 2 is symmetrical about axis Z and, preferably, the grain of the steel is parallel to axis Z.

Beam 2 is of a generally rectangular overall shape and includes a symmetrically positioned cut-away region 7 at one end thereof, this forming two symmetrically positioned strips 8 and 9. In use, in the strain gauge assembly, these strips 8 and 9 are located at the end of beam 2 which contacts ball race 3.

Two apertures 10 and 11 are provided in beam, these apertures being symmetrical about axis Z.

Three holes 12 are provided in beam 2 as shown in Figure 2. At the opposite end of beam 2, each of strips 8 and 9 is provided with a hole 13 for fixing to ball race 3 (not shown).

The overall shaping of beam 2 creates flexibility conditions such that upon relative displacement between the ends of beam 2, stress is concentrated at certain regions as the beam flexes, for example, the region generally shown at 14.

Figure 3 is an end view of a strain gauge assembly. Two of the beams 2 are positioned parallel to one another, in spaced relation, and fixed via screws through holes 13 to ball race 3. The opposite ends of beams 2 are fixed, via screws 15 through holes 12, to a metal beam support 16. PCB 17 containing the electronic circuitry (described below) is mounted to beam support 16 by fixings 18 and is electrically connected by connection 19 to a dual strain gauge mounted at region 14 of one of the beams 2. The dual strain gauge (not shown) is ideally situated on beam 2 nearest beam support 16 (the uppermost beam with the apparatus in its normal attitude of use).

As shown in Figure 4, two of the strain gauge assemblies 1 can be mounted side by side on beam support 16 to form a strain gauge arrangement 21.

Output results from one of the strain gauge assemblies can then be compared to the results from the other so as to reduce the likelihood of error.

Figure 5 shows a second embodiment of the strain gauge arrangement 21, whereby the mounting of strain gauge assemblies 1 to beam support 16 differs from that shown in Figure 4. The embodiment shown in Figure 5 is also shown in Figure 1. Beams 2 are connected at the end containing holes 12 to a support bracket or the like 22 which is itself mounted from two parallel support bars contained within beam support 16. Figure 5 also shows how rollers 5 interact with ball races 3, a side view of which is shown in Figure 1.

The present invention is particularly suitable for use inside an automated teller machine (commonly known as an ATM) to monitor the number of banknotes being dispensed by the machine. In use, the banknotes are driven by roller 5 in the direction indicated by arrow X in Figure 1 so that they pass between roller 5 and ball race 3. The finite thickness of the banknote(s) cases ball race 3 to be displaced in the direction indicated by arrow Y in Figure 1. This displacement causes beams 2 to flex and the stress pattern caused by such flexure is concentrated at region 14 where the dual strain gauge (not shown) is located. Information from the dual strain gauge is output to PCB 17 (via connection 19) where it is processed.

Although this could be carried out by a single strain gauge assembly, it is preferred to use the strain gauge arrangement (comprising two strain gauge assemblies) in order to reduce errors introduced, for example, by inadvertently folded or damaged banknotes.

Since the degree of displacement of ball race 3 is relatively small and many potentially error-introducing conditions may prevail (such as variations in roller speed, bearing friction etc) it is essential to reduce potential errors to a minimum for accurate results. The arrangement of two parallel beams 2, one on top of the other, in each strain gauge assembly provides significant rigidity in every degree of freedom other than the degree of freedom in which it is desired to measure i.e flexure in direction Y. The methods of mounting beams 2 to beam support 16 also increase this rigidity. Thus the two parallel beams 2 are able to move much more freely in the Y direction than any other.

When each strain gauge assembly 1 is in its nominal (i.e rest) position, beams 2 should have a nominal deflection of 2mm in order to apply the correct force to roller 5. However, manufacturing and installation tolerances mean that this nominal deflection ise 0.5mm. Since a typical banknote has a thickness of 0.1mm, the strain gauge output needs to be "zeroised" by removing the nominal offset (at least to an accuracy of + 0.03mm.

Conventionally, this offset would be reduced or eliminated by mechanically calibrating the device but the present invention uses an electronic means to automatically remove this undesired offset. A schematic circuit diagram of the offset-removing circuitry ("the electronic zeroising circuitry") is shown in Figure 7.

With the system in its nominal position, the SET ZERO line can be activated on demand. This causes the output 25 from the dual strain gauge to be compared with a reference voltage (in this case 1.05V). Microprocessor 26 then uses successive approximation on the output of comparator 24 to adjust the signals from the digital to analogue convertor 27 to achieve an output 25 as close to the reference voltage (1.05V) as possible thus electronically eliminating the offset.

Figure 6 shows the general circuit diagram for the strain gauge arrangement (i.e two strain gauge assemblies). The connections to the dual strain gauge on one assembly appear at P1, P2, P3 and the connection to the second dual strain gauge (on the other strain gauge assembly) appear at P4, P5, P6.

Although the circuitry for both strain gauge assemblies shares some components, the two circuits are not operationally inter-connected.

Throughout this specification the term "ball race" is meant to cover any suitable freely rotatable bearing having balls or rollers as its rolling elements.

Claims

1. A strain gauge assembly comprising two beams, superimposed or stacked in spaced relation, and inter-joined to one another and to a beam support so as to permit movement (of the beams) in one degree of freedom whilst substantially reducing movement (of the beams) in the other degrees of freedom.

2. A strain gauge assembly as claimed in Claim 1 which further comprises an instantaneous position follower positioned so as to be able to co-operate with the surface whose positional displacement it is desired to measure.

3. A strain gauge assembly as claimed in Claim 2 wherein said instantaneous position follower is a rotatable follower such as a ball race, rotatably mounted at the end of said strain gauge assembly distant from said beam support.

4. A strain gauge assembly as claimed in any of the preceding Claims wherein each of said beams has one or more apertures therethrough, the aperture(s) preferably being so sized and shaped as to control the location of the point of maximum stress development within the beam as a consequence of flexure of the beam.

5. A strain gauge assembly as claimed in any of the preceding Claims which further comprises a dual strain gauge device located on one surface of one of said beams.

6. A strain gauge assembly as claimed in Claim 5 wherein said dual strain gauge device is located at a position of maximum stress development.

7. A strain gauge assembly as claimed in any of the preceding Claims wherein each of said beams is symmetrical about the axis perpendicular to said beam support.

8. A strain gauge assembly as claimed in any of the preceding Claims wherein each of said beams is formed (for example by stamping or doublesided etching) from sheet steel, where the grain of the steel is orientated perpendicular to the direction of maximum stress development.

9. A strain gauge assembly as claimed in any of Claims 5 to 8 wherein said strain gauge assembly is associated with electronic circuitry which monitors the output of said strain gauges resulting from flexure of said beams.

10. A strain gauge assembly as claimed in Claim 9 wherein said circuitry incorporates means for removing any signal offset output by said strain gauges when the beams are in their nominal position.

11. A strain gauge assembly as claimed in Claim 10 wherein the removal of said offset (or "zeroising") is actuable on demand and occurs electronically ie without the need for mechanical calibration.

12. A strain gauge assembly substantially as described herein with reference to and as illustrated by any appropriate combination of the accompanying drawings.

13. A strain gauge arrangement comprising two or more strain gauge assemblies substantially as described in any of the preceding Claims wherein said strain gauge assemblies are mounted adjacent one another on said beam support.

14. A strain gauge arrangement as claimed in Claim 13 wherein said strain gauge arrangement comprises two strain gauge assemblies.

15. An automated teller machine (ATM) or the like including one or more strain gauge arrangements substantially as described in Claim 13 or Claim 14.