GB2128761A

GB2128761A - Force-measuring transducer

Info

Publication number: GB2128761A
Application number: GB08326834A
Authority: GB
Inventors: Kaj Erik Hilding Blomster
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-10-11
Filing date: 1983-10-07
Publication date: 1984-05-02
Also published as: JPS59132323A; FI823447A0; DE3336069A1; SE445858B; GB8326834D0; FI64719C; FI64719B; SE8305108D0; SE8305108L

Abstract

In a force-measuring transducer for electrical balances and the like, the strain in a loaded measuring body is measured by resistance strain gauges (8). The measuring body comprises a diagonally loaded, oblique parallelogram- shaped body with its two opposite sides (3) thick and comparatively inflexible, whereas the other sides, the so-called measuring sides (2) are thin and are perpendicular to or almost perpendicular to the loading line. In addition, the measuring body may at need be shaped so as to form an overload protection (1) or be provided with such protection by a distance piece (4). <IMAGE>

Description

SPECIFICATION Force-measuring transducer The invention relates to a force-measuring transducer for electronic balances and the like, based on the strain and compression in the surface layer of a loaded measuring body which is measured in a known manner using resistive strain gauge techniques where the resistance of the strain gauges is measured by a resistance bridge.

There are a large number of measuring bodies or load cells of different shapes known for this type of force-measuring transducer.

But, in addition to being quite expensive, they have certain other drawbacks. For instance, in longitudinal loading of cylindrical measuring bodies, surface tensions in different directions are formed requiring the use of differently directed resistive strain gauges, and it is not always possible to place these in the best possible position. Diagonally loaded square strain gauge cells also exist, such as the device according to U.S. Patent Specification No. 2,986,931. However this latter device, apart from being relatively expensive, has the drawback that the sides of the square are subjected to tension and compression forces, in addition to bending. The main object of the present invention is to provide a more competitive transducer which uses a load cell or measuring body which is less expensive and more accurate than the known load cells.The characteristic feature of the present invention is that it comprises a force-measuring transducer made of a diagonally loaded, essentially oblique, parailelogram-shaped body with one pair of opposite sides thick and comparatively inflexible, whereas the other pair of opposite sides, which are the measuring sides where the strain gauges are located, are thin and make an angle of approximately 90 . In the preferred embodiments of the invention the measuring sides form an angle of 90 with the loading line and this difference from the device of U.S. Patent No. 2,986,931 means that the thin sides, i.e. the measuring sides of the oblique parallelogram, are subjected to a rather pure bending.

A cheap way of producing a transducer according to the present invention is by making the cell or the measuring body from a drawn profiled metal bar by cutting it to suitable pieces according to the desired measuring range, the different measuring ranges therefore requiring no separate profile drawing tool.

The measuring body or load cell of the present invention can at need be shaped so as to form two surfaces serving as an overload protection, these surfaces coming into mutual contact to restrict the deformation.

The invention is described below with reference to the enclosed drawings, which present the invention in principle and illustrate a few working examples, and in which: Figure 1 presents the invention in principle by showing an unloaded cell and the deforma tions when the cell is compression loaded, and drawing or tension loaded, Figure 2 is an axonometric projection of one working example of the cell, Figures 3, 4 and 5 present three different working examples of the cell, Figures 6, 7 and 8 present the upper part of a cell where 0, one and two narrowings are made in the thin side, Figures 9, 10 and 11 present the upper part of a cell where the thin sides form different angles with the loading line, Figures 12, 13 and 14 present different arrangements for overload protection of a cell when compression loaded.

The transducer according to the invention can be used under both compression loading and drawing loading. Fig. 1 shows in a highly exaggerated manner how the unloaded trans ducer cell on the left is deformed by compressimon and drawing loading. In order to give overload protection when the cell is under compression, the cell is provided with parallel planes 1 perpendicular to the loading line, which in the case of overloading under com pression, as shown in the centre of Fig. 1, come into mutual contact to prevent the thin sides 2 from further bending and breaking.

The overload limit can and naturally must be set so that no permanent deformations occur in the thin sides. Under drawing loading, of course, such single overload protection cannot be used and overloading has to be prevented by other means.

The construction of the transducer measur ing body or cell according to the invention is further illustrated in Fig. 2. In addition to the central planes 1 and the thin sides 2 of the oblique parallelograms, there are the thick, practically inflexible sides of the parallelo grams. A distance piece 4 can be used be tween the planes 1 as overload protection. At both ends of the cell (e.g. above and below) there are planes 5 at right angles to the loading line and thus parallel to the planes 1.

The upper and lower planes 5 are provided at their centres with threaded fastening holes 6 whose axes are parallel in direction to the loading line. As already mentioned the cells are made by cutting pieces at right angles, from a drawn profiled metal bar, the pieces being of suitable thickness for the desired measuring range. The thickness of the cell can also be changed by making cuts along the broken lines 7 in Fig. 2. The resistive strain guages 8 may be placed advantageously at the points shown in Fig. 2, but they can also be placed elsewhere on the thin side, or on both sides of it.

Fig. 3 shows schematically a cell with the said central planes 1, and without distance pieces 4. Fig. 4 shows a cell without any central planes 1, which is suitable for use with drawing loads, when the central planes 1 are not used. Fig. 5 shows a cell where the thick sides of the oblique parallelograms are curved, and which is provided with planes 1 for a distance piece. Fig. 6 shows the upper part of a cell whose thin side has no narrowings, Fig. 7 a thin side with one narrowing and Fig. 8 a thin side with two narrowings.

These narrowings are mostly used at small loads, when greater sensitivity is required from the cell. Fig. 9 shows a cell where the thin sides are at a 90 angle with the loading line. In Fig. 10 the corresponding angle is 120 , and in Fig. 11 60 . The two latter cases should be avoided, however, because the more deviation from 90 , the more inaccurate the transducer. Fig. 1 2 shows how the distance piece 4 of the overload protection can be placed perhaps preferably in the cell according to Fig. 3. The distance piece 4 can be glued to the lower end and exchanged at need. In the case according to Fig. 13 no distance piece is required, but the central planes of the cell are so close to each other that they function already as such as overload protection.Fig. 14 shows how the distance piece 4 can be placed in the cell according to Fig. 5.

When the cell is loaded, deformations occur in both thin sides in exactly the same way.

When both ends of the thin sides are fixed to the thick sides (see Fig. 1), the thin sides bend to S-shape on loading the cell, as shown in Fig. 1, where it is easy to measure the deformations. The Figs. 1, 2, 6, 7 and 8 give examples of placing of the resistive strain gauges 8.

Compared to previous solutions a transducer according to the invention offers the following advantages: -The oblique parallelogram shape, where the thin sides are situated at right angles to the loading line, gives highly exact values.

-In the cell according to the invention it is easy to obtain overload protection.

-The manufacture of the cell is particularly economical when the cell is made from a drawn profiled metal bar by cutting to suitable pieces.

The present invention herein described also comprehends a strain gauge load cell which comprises a first pair of opposed arms which are deformable measuring arms and a second pair of opposed arms which are relatively thick and inflexible in comparison with the deformable measuring arms, the body being deformable by application of a load to diagonally opposed corners thereof which corners are defined by one deformable measuring arm and one relatively thick inflexible arm, the deformable measuring arm making an angle between 60 and 120 with the loading line at each of the diagonally opposed corners, and at least one strain gauge being located on each deformable measuring arm, the strain gauge being similarly placed on each of the deformable measuring arms in relation to the loading line.

Claims

1. A force-measuring transducer for measuring the strain and compression in the surface layer of a loaded measuring body in a manner using resistive strain gauge techniques where the resistance of the strain gauge is measured by a resistance bridge, the loaded measuring body being a strain gauge load cell comprising a diagonally loaded, essentially oblique, parallelogram-shaped body with two opposite sides thick and comparatively inflexible, and the other two opposite sides, which are the measuring sides, where strain gauges are located, thin and making an angle of not more than 120 and not less than 60 with the loading line.

2. A transducer according to Claim 1 wherein the body is shaped so as to form two surfaces serving as an overload protection, which surfaces can come in mutual contact to restrict the deformation.

3. A transducer according to Claim 1 or Claim 2, wherein the thin measuring sides make an angle of 90 to the loading line.

4. A transducer according to any one of the preceding claims wherein the thick inflexible sides are curved.

5. A transducer according to any one of the preceding claims, wherein the thin measuring sides are provided with one or two narrowings.

6. A transducer according to any one of the preceding claims, wherein the centre of the body is provided with two planes at right angles to the loading line.

7. A transducer according to Claim 6 further including a separate distance piece placed between the two planes to provide overload protection.

8. A method of producing a transducer according to any one of the preceding claims, wherein the measuring body is shaped so that it can be easily made from a drawn profiled metal bar by cutting into suitable pieces according to the desired measuring range.

9. A force-measuring transducer including a strain gauge load cell substantially as hereinbefore described with reference to any one of the figures of the accompanying drawings.