GB2245978A

GB2245978A - Force measuring apparatus

Info

Publication number: GB2245978A
Application number: GB9010969A
Authority: GB
Inventors: James Edward Oakley
Original assignee: MECMESIN Ltd
Current assignee: MECMESIN Ltd
Priority date: 1990-05-16
Filing date: 1990-05-16
Publication date: 1992-01-15
Also published as: GB9010969D0

Abstract

An apparatus for measuring an applied force includes a support case (12) and an arm (16) to which force is applied, movably mounted on the support. A cantilever spring biassing means (26) urges the arm towards a predetermined normal position relative to the support, and an electronic displacement sensor measures displacement of the arm relative to the support. The displacement sensor comprises a ladder of capacitive strips (48) on a carrier 38 mounted on springs 40a, 40b fixed to the support at 44a, 44b and coupled to the arm by roller 46 and a capacitance detector mounted on the support. A calibration member 32 having slots 30a, 30b to receive the forked ends of cantilever spring 26 is adjustable in direction of arrow 34 to alter the degree of coupling between arm 16 and carrier 38. In use a force is applied to the arm causing lateral displacement, and the degree of displacement indicative of the applied force is measured by the displacement sensor. In an alternative embodiment the arm 16 is movable axially by applied force against spring bias. <IMAGE>

Description

FORCE MEASURING APPARATUS The present invention relates to apparatus for measuring an applied force.

A conventional gauge for measuring an applied force employs an arm to which the force is applied, a spring and a mechanical dial pointer to indicate the force. With such an analog display, measurements may typically be taken to an accuracy of only around 1 or 2 per cent of the full scale reading.

Gauges of this type are suitable for use in the laboratory, and may be used for measuring expansion and compression forces, as well as lateral forces in, for example, electrical relays or the starting torque of an electric motor.

If a more precise gauge is required, a more sensitive and hence more expensive mechanism must be used.

According to the present invention, there is provided apparatus for measuring an applied force, comprising a support, an arm to which the force to be measured is applied, the arm being movably mounted on the support, biasing means for urging the arm towards a predetermined normal position relative to the support, and an electronic displacement sensor for sensing displacement of the arm relative to the support, which displacement is indicative of the applied force.

With such an apparatus, the precision depends to a large extent on the resolution of the displacement sensor. By using a sensitive displacement sensor, highly accurate measurements can be taken. The output from the sensor is in the form of an electrical signal, which can be interpreted and displayed as a precise digital reading.

Preferably the arm is mounted for lateral deflection, or alternatively for longitudinal deflection.

Preferably the displacement sensor includes a track or ladder arrangement of capacitive strips and a detector for sensing movement of the strips relative to the detector. One part of the displacement sensor can be coupled to the arm, and the other part fixed relative to the support. Displacement sensors of this type can resolve to an accuracy of hundredths of a millimetre, enabling a more precise measurement to be taken than with the mechanical type gauge discussed hereinbefore.

Capacitance-ladder displacement sensors are known from their use in digital-style calipers. In these calipers, the ladder of capacitance strips is attached to a fixed caliper jaw, and the capacitance detector is attached to a slidable jaw. This direct coupling achieves a resolutioon to 0. O1Itrn. However, such accuracy using a relatively cheap sensor has not been achieved hitherto in force gauges. The present invention provides an apparatus that takes advantage of the accuracy of the type of displacement sensors commonly used in digital calipers.

Another desirable feature is a calibrator for enabling the amount of coupling between the arm and the displacement sensor to be adjusted.

This enables a greater variance in the characteristics of springs used as the biasing means to be tolerated without compromising the precision of the instrument.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a force measuring instrument with its top cover removed; Figure 2 is a transverse section taken along the line II-II of figure 1; and Figure 3 is schematic diagram illustrating a modified form of the instrument.

Referring to figure 1, a force measuring instrument 10 includes a case 12 with a side wall 14. The case 12 acts as a support for an arm 16 movably mounted within the case 12, the arm extending out through a slot 18 in the wall 14 at one end of the case 12. The slot 18 is wider than the arm 16 to permit lateral deflection of the arm 16 relative to the case 12. The sides of the arm 16 taper inwardly away from the case 12, the free end 20 of the arm being in the shape of a rounded point. In use, a lateral force is applied to the arm in a direction parallel to the arrows 22, and the instrument 10 measures the force applied relative to the supporting case 12.

A biasing spring 26 is attached to the end of the arm 16 inside the case 12. The spring 26 is in the form of a double-limbed cantilever spring element, having a bridge portion 24 that is attached to the arm 16, and two limbs 28a and 28b extending away from the arm 16.

The ends of the limbs 28a and 28b are held captive in grooves 30a and 30b, respectively, of an adjustable calibration member 32. The calibration member is movable longitudinally in the direction of the arrows 34, but it is securable to the case 12 by means of two locking bolts 36. As explained hereinafter, the calibration member 32 enables the position of the spring 26 to be adjusted to calibrate the instrument.

The cantilever spring 26 acts as a biasing means to urge the arm towards a predetermined normal position in which the arm is centred relative to the case 12, as illustrated in figure 1.

Referring to figures 1 and 2, a scale carrier 38 is supported on one side of the limbs 28a and 28b of the cantilever spring 26, by means of two support springs 40a and 40b. One end of each support spring 40a, 40b, engages in a respective slot 42a, 42b, in the scale carrier 38, and the other end is held captive in a respective projection 44a, 44b fixed relative to the case 12. The carrier 38 is coupled to the arm 16 by means of a guide in the form of a roller 46 attached to the carrier 38. The diameter of the roller is substantially equal to the distance between the limbs 28a, 28b, of the cantilever spring 26, and the the roller 46 engages snugly between the limbs 28a and 28b.

A ladder of capacitive strips in the form of a linear track 48, shown schematically in figure 1 as a dotted line, is mounted on the underside of the carrier 38. A capacitance sensitive detector 50 is mounted on the case 12 closely adjacent the underside of the carrier 38 to detect movement of the capacitive ladder track 48, and hence movement of the carrier 38. The ladder of capacitive elements and the capacitance sensor 50 together form a displacement sensor for measuring displacement of the arm 16 relative to the case 12.

Lateral movement of the arm 16 causes corresponding lateral movement of the limbs 28a and 28b of the cantilever spring 26, and this lateral movement is transmitted to the carrier 38. The support springs 40a and 40b act as suspension for the carrier 38, to keep the capacitive ladder track 48 positioned over the detector 50.

The normal position of the arm 16 corresponds to approximately the mid-point of the capacitive ladder track 48. The arm can be deflected from the normal position in either of the two opposite directions indicated by the arrows 22. The displacement sensor measures over a range of +/- 4mm relative to the normal position, and is accurate to +/,- 0. OllErn, ie. it can resolve to an accuracy of 0.25% of the full scale reading.

The output from the detector 50 is fed to an electronic circuit (not shown) which interprets the output as the applied force. The circuit can be of a conventional design intended to match the capacitive displacement sensor, and the measured force displayed using a digital display. The electronic circuit and display can be incorporated within the case 12, or alternatively it be remote from the case 12 and connected by suitable leads.

In use, a force to be measured is applied laterally to the end 20 of the arm 16. The force causes deflection of the arm 16 against the bias of the cantilever spring 26, the amount of deflection depending on the force applied. The deflection of the arm causes a corresponding displacement of the carrier 38, which displacement is measured by the detector 50, and interpreted by the electronic circuit as the measured force.

The calibration member 32 can be used to calibrate the instrument 10. By moving the calibration member 32 in the direction of the arrows 34, the degree of leverage, or coupling, between the arm 16 and the carrier 38 can be adjusted. The calibration member 32 is positioned so that the movement of the arm is matched to the +/- 4mm linear movement of the carrier 38. Once the calibration member 32 has been correctly positioned, it is secured relative to the case 12 by tightening the locking bolts 36.

It will be appreciated that the embodiment described above is simple and fairly compact. The capacitive displacement sensor is relatively inexpensive, and so the instrument is not prohibitively expensive to build. The use of an electronic sensor and a digital display enables very accurate force measurements to be made. The accuracy is roughly 10 times better than with the analog gauge described hereinbefore. The instrument is suitable for use in a laboratory as well as in industrial applications, where the remote display for the apparatus might be used.

Capacitance-ladder displacement sensors are known from their use in digital-style calipers. In these calipers, the ladder of capacitance strips is attached to a fixed caliper jaw, and the capacitance detector is attached to a slidable jaw. This direct coupling achieves a resolutioon to 0.01mEL However, such accuracy using a relatively cheap sensor has not been achieved hitherto in force gauges. The present invention provides an apparatus that takes advantage of the accuracy of the type of displacement sensors commonly used in digital calipers.

A modified form of the instrument is shown schematical ly in figure 3. The instrument is intended to measure a longitudinal force 5, and it can measure both expansive and compressive forces. The design of the instrument is very similar to that described above, however, a coil spring 26' applies a longitudinal biasing force rather than the lateral force applied in the previous emebodiment.

Similarly, the carrier 38 lies parallel with the longitudinal direction of the arm 16' so that the displacement sensor measures longitudinal movement of the arm 16'. The operating principle of the modified instrument is, however, the same as before, and enables measurements to be taken with equal precision.

It will be appreciated that although in the embodiments described above, the displacement sensor comprises a relatively movable capacitance-ladder and detector, other forms of displacement sensors could be used in alternative embodiments. For example, an optical detector and an optical track could replace the capacitance detector and the capacitance track, respectively. The apparatus is particularly suitable for use with the types of displacement sensors used in conventional digital calipers.

Claims

1. Apparatus for measuring an applied force, comprising a support, an arm to which the force to be measured is applied, the arm being movably mounted on the support, biasing means for urging the arm towards a predetermined normal position relative to the support, and an electronic displacement sensor for sensing displacement of the arm relative to the support, which displacement is indicative of the applied force.

2. Apparatus according to claim 1, wherein the arm is mounted for lateral movement.

3. Apparatus according to claim 1, wherein the arm is mounted for longitudinal movement.

4. Apparatus according to claim 1, 2 or 3, wherein the displacement sensor comprises a ladder of capacitive strips coupled to one of the arm and the support, and a capacitance sensitive detector coupled to the other of the arm and the support.

5. Apparatus according to claim 4, wherein the ladder of capacitive strips is coupled to the arm, the ladder extending linearly substantially parallel to the direction of deflection of the arm from the normal position.

6. Apparatus according to claim 5, wherein the ladder is mounted on a movable carrier, the carrier being supported at its ends by means of support springs, the carrier being coupled to the arm.

7. Apparatus according to claim 6, wherein the arm is mounted for lateral movement, the biasing means comprising a multi-limbed cantilever spring coupled at one end to the arm, and at its other end to the support, the carrier being coupled to the arm by means of a guide attached to the carrier, the guide engaging between two of the limbs of the cantilever spring.

8. Apparatus according to claim 7, wherein the cantilever spring is coupled to the support by means of an adjustable calibration member, which calibration member is selectively movable in a direction parallel to the longitudinal direction of the arm when in the normal position, whereby the relative movement of the carrier with the arm can be adjusted.

9. Apparatus for measuring an applied force substantially as hereinbefore described with reference to figures 1 and 2, or to figure 3, of the accompanying drawings.

9. Apparatus according to any of the preceding claims, wherein the arm is movable from the normal position in two opposite directions, the normal position corresponding to approximately the mid-position of the displacement sensor.

10. Apparatus for measuring an applied force substantially as hereinbefore described with reference to figures 1 and 2, or to figure 3, of the accompanying drawings.

AMENDMENTS TO THE CLAIMS H AVE BEEN FLED AS FOLLOWS.

1. Apparatus for measuring an applied force, comprising a support, an arm to which the force to be measured is applied, the arm being movably mounted on the support, biasing means for urging the arm towards a predetermined normal position relative to the support, and an electronic displacement sensor for sensing displacement of the arm relative to the support which displacement is indicative of the applied force, the displacement sensor comprising a ladder of capacitive strips coupled to one of the arm and the support, and a capacitance sensitive detector coupled to the other of the arm and the support.

4. Apparatus according to claim 1, 2 or 3, wherein the ladder of capacitive strips is coupled to the arm, the ladder extending linearly substantially parallel to the direction of deflection of the arm from the normal position.

5. Apparatus according to claim 4, wherein the ladder is mounted on a movable carrier, the carrier being supported at its ends by means of support springs, the carrier being coupled to the arm.

6. Apparatus according to claim 5, wherein the arm is mounted for lateral movement, the biasing means comprising a multi-limbed cantilever spring coupled at one end to the arm, and at its other end to the support, the carrier being coupled to the arm by means of a guide attached to the carrier, the guide engaging between two of the limbs of the cantilever spring.

7. Apparatus according to claim 6, wherein the cantilever spring is coupled to the support by means of an adjustable calibration member, which calibration member is selectively movable in a direction parallel to the longitudinal direction of the arm when in the normal position, whereby the relative movement of the carrier with the arm can be adjusted.

8. Apparatus according to any of the preceding claims, wherein the arm is moveable from the normal position in two opposite directions, the normal position corresponding to approximately the mid-position of the displacement sensor.