AU731312B2 - Load cell - Google Patents

Description

WO 98/48250 PCT/AU98/00290 LOAD CELL FIELD OF THE INVENTION This invention relates to devices for force measurement in which the bending moment produced in a transducer body is the basis of measurement, and in which the bending mode is single bending. "Single bending" in this specification refers to devices in which the neutral line of the transducer body bends in one direction only from its rest or zero load position.

BACKGROUND ART Known devices of this kind typically include a cantilever beam with a load applied near its free end and a strain gauge located between the point of load application and the support, with the plane of the gauge being essentially at right angles to the load line and lines of applied forces. Such devices suffer from several disadvantages.

Because the bending moment at the gauge location is a function of distance from the point of load application, the bending moment varies over the length of the gauge, so that for a beam of uniform cross-section, the strain field at the gauge location will be non-uniform. This can cause gauge creep and apparent nonlinearity.

This effect can be reduced by increasing the distance between the load line and the gauge, but at the expense of compactness. An alternative is to vary the cross-section of the beam at the gauge location to compensate for the otherwise uneven strain field, but this is an expensive option.

Another disadvantage of such known devices arises from the strain field generated by the two reaction forces at the mounting end of the beam. If the distance between the gauge and the mounting is short, a variation in strain distribution across the cross-section of the beam will occur with changes in applied load.

This effect can be reduced by increasing the length of the beam between the support and the gauge, again at the expense of the load cell dimensions.

A further disadvantage of known single bending systems, particularly those in which the beam is cantilevered, arises from the fact that as the beam bends under increasing load, the angle between the beam and the load line changes, producing non-linearity.

These problems have until now been overcome either by the use of a long bending beam, which is often unsuitable, or by the use of shapes requiring intricate machining which enable the dimensions of the device to be contained, but at considerable cost, and in no case has it been possible to configure such a device as a simple tension load cell.

SUMMARY OF THE INVENTION The present invention provides a device which addresses these problems and provides a simple bending beam load cell which can be manufactured relatively economically. Cells of the present invention are characterised by the use of 0 *00 single bending for the measurement of strain in a plane which is not at right angles to the load line, and which in preferred forms of the invention is in fact substantially parallel to the load line.

i: In one preferred form the invention broadly resides in a load cell including a beam configured for single bending in response to a load force, the beam having a principal axis and a measuring portion displaced from the line of action of the load force, the principal axis in said portion in the unstressed condition of the beam not being at right angles to said line of action, and strain gauge means on said measuring portion responsive to strain arising from said bending.

WO 98/48250 PCT/AU98/00290 3 For use as a tension load cell, such a device may be constructed so that the force to be measured and the reaction force are applied to the internal, opposed, sides of arms which couple the forces to the measuring portion. In this way, the attachment and disconnection of the load can be very simple, and the load line will be maintained without the need for complicated or expensive measures. The strain gauge set can be placed on an accessible and flat surface, and the entire device can be made without machining.

Preferably, the measuring portion is substantially linear and is disposed substantially parallel to the line of action of the force to be measured. In this arrangement, the bending moment in the measuring portion arising from the force to be measured will be substantially independent of the position along the measuring portion at which it is measured. The bending moment gives rise to simple bending of the measurement portion, and is readily measured by means of a strain gauge set mounted on one side of the measurement portion.

Preferably such a tension load cell is formed, for example by means of bending or otherwise from a single piece of steel, with a pair of legs preferably extending substantially at right angles to the line of force and preferably also at substantially right angles to the measurement portion, the legs being provided on their inside surfaces with respective force-engaging means such as knife edges.

Unusually in the field of load measurement, the use of knife edges does not, in this device, lead to problems with the repeatability of measurements, but it is to be observed that the manner by which load forces are applied to the device should be chosen to suit the purpose for which the device is to be used. It is to be observed that while in the examples which follow, load forces are applied to the device from within the legs, in many applications it will be preferable to apply loads externally, or by means of devices which pass through the legs.

Of particular interest is a form of the invention which enables a multiple capacity instrument to be produced, in which the bending moment arm is used WO 98/48250 PCT/AU98/00290 4 as the variable parameter. This can be achieved in devices according to the invention, by providing two (or more, if required) sets of force-engaging means, typically knife edges, at different locations on the arms of the device.

In other applications of the invention other configurations of the device can be used, as will appear below in the form of exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example only, in relation to several embodiments, illustrated in the accompanying drawings.

In the drawings: Fig. 1 shows in side elevation a first embodiment of the invention; Fig. 2 shows in side elevation a second embodiment of the invention; Fig. 3 shows in side elevation a third embodiment of the invention; Fig. 4 shows in schematic side elevation a fourth embodiment of the invention, and Fig. 5 shows in side elevation a fifth embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The embodiment illustrated in Fig. 1 consists of a strip of spring steel 10 bent at 11 and 12 to form a bight portion 13, an upper leg 14, and a lower leg 15. Each of the legs has on its inside surface, at points equally spaced from the bight portion 13, a knife edge 16 and 17.

Mounted on the inner surface of the bight portion is a strain gauge set 18 designed to respond to strain arising from simple bending of the bight portion.

The strain gauge is connected by means of cabling to conventional processing circuitry and a visual display, not shown here.

If such a device is suspended by means of the upper knife edge 16, and a load WO 98/48250 PCT/AU98/00290 applied to the lower knife edge 17, the device will, after calibration, indicate the applied load with a great degree of linearity and repeatability. The device is self-aligning and exhibits dimensional stability. It will be seen that the device may be readily installed with and removed from the weighing apparatus, and it has been found that the weight indication is substantially errorless with repeated removal and reinstallation. Using standard spring steel of Rockwell 32 hardness, a prototype device exhibited linearity of better than 1 in 1000.

The load and reaction forces may be applied to the device by means other than knife-edges. For example, where the device is to be built into a structure such as a silo or for supporting a truck load carrier, pins or bushes may be used.

If desired, the legs of the device may be pre-set to taper slightly towards each other so that they become parallel approximately in the middle of the designed weight range. By disposing the legs substantially at right angles to the line of action of the load and reaction forces the effect of dimensional changes with the application of load is minimised.

The shape of the bight portion may be varied to suit the application for which the device is intended. For example, the central portion on which the gauge is located may be of thinner cross-section than the outer regions and the legs, to provide greater sensitivity.

The structure of the device, involving loading from inside, is inherently safe, and because the device is open on the side opposite the measuring portion, the load and support may be introduced with great simplicity.

The configuration of this embodiment of the invention also lends itself to a simple arrangement to prevent overload. By extending the legs beyond the force application points, motion of the legs under load will be amplified, and may be restricted in this extended region, to prevent overloading.

In the embodiment illustrated in Fig. 2 an additional pair of knife edges 19 and WO 98/48250 PCT/AU98/00290 6 have been added, located closer to the bight portion 13. If these knife edges are engaged with the load and support a greater load will be supported for a given output from the strain gauges 18.

The device can therefore operate in two load ranges without a sacrifice of sensitivity in the lower range, a characteristic which cannot normally be obtained in strain gauge-based load cells.

Devices according to the above embodiments of the invention are suitable for retrofitting in weighing systems designed with mechanical scales. For example, where previously mechanical scales have been used in conjunction with a pull rod tensioned by the load force, electronic weighing can be retrofitted with minimum change to the mechanism or structure by suspending the pull rod from a device of the kind illustrated and substituting the scale mechanism by processing circuitry for the strain gauge, and a visual display.

Such mechanical scales are found with markedly different load capacities and may present widely differing pull rod forces for a given load. For example, the pull rod forces at full capacity for different scales may vary from 20 kg to 120 kg. By using the multiple capacity form of the invention illustrated in Fig. 2, such a range of full capacity loading may be accommodated at acceptable resolution by means of a single device, enabling those in the field to carry only one cell type.

While it is preferred that the measuring portion is linear and parallel to the line of action of the force, this is not essential, and devices may be constructed in which the measuring portion lies at an angle to the line of action of the force, although still displaced from it. Such an arrangement will lack the advantage of having a region over which the bending moment is constant, but will otherwise provide satisfactory results.

WO 98/48250 PCT/AU98/00290 7 Fig. 3 illustrates such a device, where the bight portion 13 lies in an oblique position relative to the line of action of the load between the knife edges 16 and 17.

Another configuration of force measuring device according to the present invention, and indeed one which perhaps shows the principles of the invention applied in their simplest form, is shown schematically in Fig. 4. Here a short stiff arm 14' from which a load is to be suspended extends from a measuring portion 13' which is mounted vertically in a support 21.

While devices of the invention can be manufactured with great economy by bending from a single bar of steel, they may also be manufactured from separate components, as illustrated for example in Fig. 5, where the bight portion 13" comprises a steel bar and the upper and lower legs 14" and 15" comprise short bars inserted into apertures adjacent each end of the bar 13". Where knife edges are used as the means of connection of the load forces to the device, these are mounted on the legs 14" and 15" as shown.

It will be clear that many alternatives will exist for the construction of devices embodying the invention, as dictated by particular weighing or force measuring applications.

Claims (8)

1. A load cell including a beam configured for single bending in response to a load force, the beam having a principal axis and a measuring portion displaced from the line of action of the load force, the principal axis in said portion not being at right angles to said line of action, and strain gauge means on said measuring portion responsive to strain arising from said bending.

2. A load cell according to claim 1 in which the measuring portion is substantially linear and is disposed substantially parallel to the line of action of said load.

3. A load cell according to claim 1 in which the force to be measured and the reaction force are applied to the internal, opposed, sides of arms which couple the forces to the measuring portion.

4. A load cell according to claim 1 formed with a pair of legs extending substantially at right angles to the line of force and also substantially at right angles to the measurement portion, the legs being provided on their inside 0 surfaces with respective force-engaging means.

5. A load cell according to claim 4 formed by means of bending or otherwise from a single piece of steel.

6. A load cell according to claim 1 in which the measuring portion is substantially linear and is disposed obliquely to the line of action of the force to 25 be measured.

7. A load cell according to claim 1 in which the force to be measured is applied to an arm which couples the force to the measuring portion. WO 98/48250 PCT/AU98/00290 9

8. A load cell according to claim 7 in which said measuring portion is substantially linear and said arm is substantially normal thereto.