GB2151799A - Measuring device - Google Patents

Abstract

A measuring device comprises a measuring spring 1 of ferromagnetic material which is subjected to deformation stress by the quantity F to be measured and having strain-measuring elements 3,4,5,6 cooperating with the measuring spring 1, the measuring spring 1 being subject to a magnetic field 8 in the region of the strain-measuring elements 3,4,5,6 during the measuring operation. The strength of the magnetic field may be adjusted to reduce the hysteresis error in the material of the measuring spring. <IMAGE>

Description

SPECIFICATION Measuring device The invention relates to measuring devices having measuring springs of ferromagnetic material which are subjected to deformation strain by the quantity to be measured and having strain-measuring elements co-operating therewith.

The resolution of the measuring range of such a measuring device, that is to say the fineness of the division of the measuring range, depends on the hysteresis of the material of the measuring spring.

The greater is the relative reversal error of the hysteresis curve of the measuring-spring material, the less is the resolution of the measuring device. It is true that the hysteresis of some of the usual measuring-spring materials can be improved by various measures, for example by exterme cold-working.

This is only possible to a limited extent, however, so that limits are imposed on an improvement in resolution which can be achieved thereby.

The invention seeks to improve the resolution of measuring devices of the type mentioned at the beginning beyond the extent which could be achieved by the measures hitherto known.

According to the invention, there is provided a measuring device having a measuring spring of ferromagnetic material which is subjected to deformation strain by the quantity to be measured and having strain-measuring elements co-operating therewith, wherein in the region of action of the strain-measuring elements the measuring spring lies in a magnetic field at least during the measuring operation.

Various measuring-spring materials have different hysteresis characteristics I and also other different characteristics. For example, the temperature coefficient of Young's modulus can be influenced in many materials I for example in ironnickel alloys I while this is not possible with other materials I for example copper-beryllium alloys.

Now if the measuring spring material has to be selected according to another such characteristic, then the limitation of the resolution caused by the hysteresis, that is to say the reversal error, of this material must necessarily be accepted into the bargain. Now it is an advantage of the invention that, with it, the resolution can be improved even in such cases.

The invention will now be described in greater detail, by way of example, with reference to the diagrammatic drawings in which all parts which are not necessary for an understanding are omitted and in which: Figure 1 is a perspective illustration of a measuring device with a measuring spring subjected to bending stress by a force to be measured; Figure 2 is a side view of the device shown in Figure 1 with a magnet disposed above the measuring spring; Figure 3 is a side view of a measuring device with a measuring spring subjected to bending stress by a force to be measured, wherein a magnet is disposed in a window-like recess in the measuring spring; and Figure 4 shows a view in the direction of the arrow A of a device as shown in Figure 3 but in which a magnet is disposed at each side of the measuring spring instead of the magnet disposed in the recess in the measuring spring.

In Figure 1, a measuring spring 1 of ferromagnetic material is secured at one side to a stationary base 2. At its part projecting freely beyond the base 2, the measuring spring 1 is made considerably thinner and so forms a measuring section on the upper surface 4 of which, electrical strain gauges 3 to 6, indicated only diagrammatically, are disposed. On the other side of the measuring section, the measuring spring 1 has an L-shaped extension 7, the longer arm of which is taken back under the measuring section, extending parallel to the measuring spring.

The force F to be measured acts at the end of this arm along a line which extends perpendicular to the longitudinal extent of the measuring spring 1, substantially through the middle of the measuring section. Thus the measuring spring represents a bending test beam which is constructed in known manner and the measuring section of which is bent in s-shape under the action of the force F.

In the course of this, the strain gauges 3 and 5 are extended and the strain gauges 4 and 6 are compressed by the measuring spring material and convert these into corresponding electrical signals.

The strain gauges 3 to 6 are connected, in a known manner not illustrated, into an electrical measuring bridge which delivers a signal corresponding to the action of the force F.

Naturally, the strain gauges may be all or partially disposed at the under surface of the measuring section. Also, a different number of strain gauges may be provided in an appropriate arrangement.

Such variations, with corresponding connection of the measuring bridge, are known.

A magnetic field, the lines of flux 8 of which penetrate through the measuring section, is caused to act on the measuring section. As a result of the action of the magnetic field, the reversal error of the material of the measuring spring is reduced.

The extent of the reduction can be adjusted by selection of the magnetic field strength. An arrangement of the magnetic field, wherein the magnetic lines of flux extend substantially perpendicular to the longitudinal direction of the measuring spring, is particularly advantageous.

In the embodiment shown in Figure 2, a magnet 9 is disposed above the measuring section by means of a holder 10 so that the magnetic lines of flux penetrate through the measuring section. The position of the magnet is selected above the measuring section in order to avoid hampering the pulling action of the force F. The magnet could, also be disposed elsewhere, for example below the measuring section, if the adverse effect on the pulling action of the force F is avoided by other means.

In the embodiment illustrated in Figure 3, the measuring spring has the form of a bending test beam 11 which is secured at one side to a base 12 and on the free end of which, a tensile force F acts.

Disposed in a window-like recess in the bending test beam, the marginal zones 13, 14 of which form two measuring sections lying parallel to one another at the same time, is a magnet 15 such that its lines of flux penetrate through both measuring sections. The arrangement and bridge connection of the strain gauges as well as the mode of operation of the apparatus can be described in connection with Figure 1.

The embodiment shown in Figure 4 corresponds to the embodiment illustrated in Figure 3 with the exception of the magnet arrangement. To this extent, the same reference symbols are used. Here, however, the magnetic field, the lines of flux of which penetrate through the measuring sections 13 and 14, is produced by two magnets 16 and 17 dis posed on either side of the two measuring sections. The holding means 18, with which the two magnets are secured to the base 12, can serve at the same time, as a yoke for the return of the magnetic lines of flux.

As the various embodiments show, the means for the production of the magnetic field can be dis posed in any position in relation to the measuring section(s) if care is taken to ensure that the magnetic lines of flux penetrate through the measuring section. One or more magnets I permanent magnets or electromagnets I may be used to produce the magnetic field. The use of electromagnets is advantageous because the magnetic field is then adjustable in its strength and so can easily be adapted to different requirements with regard to resolution. Also, electromagnets can be switched off when no measuring is being carried out with the measuring device.

Claims (12)

1. A measuring device having a measuring spring of ferromagnetic material which is subjected to deformation strain by the quantity to be measured and having strain-measuring elements co-operating therewith, wherein in the region of action of the strain-measuring elements, the measuring spring lies in a magnetic field at least during the measuring operation.

2. A measuring device as claimed in claim 1, wherein the magnitude of the magnetic field is adjustable.

3. A measuring device as claimed in claim 1 or 2, wherein the magnetic lines of flux extend substantially perpendicular to the longitudinal direction of the measuring spring.

4. A measuring device as claimed in claim 1, 2 or 3, wherein a reversal error of the hysteresis curve of the measuring spring material is at least partially reduced by suitable dimensioning of the magnetic field strength.

5. A measuring device as claimed in claim 4, wherein the spring material is an Fe Ni alloy.

6. A measuring device as claimed in claim 4 or 5, wherein the magnetic field is in the range between 40 and 200 mT.

7. A measuring device as claimed in any one of claims 1 to 6, wherein one or more magnets is (are) disposed above and/or below the measuring spring so that the magnetic lines of flux penetrate through the measuring spring in the region of action of the strain-measuring elements.

8. A measuring device as claimed in any one of claims 1 to 6, wherein a magnet is disposed in a window-like recess in the measuring spring such that the magnetic lines of flux penetrate through the measuring spring in the region of action of the strain-measuring elements.

9. A measuring device as claimed in any one of claims 1 to 6, wherein one or more magnets is (are) disposed at one or both sides of the measuring spring such that magnetic lines of flux penetrate through the measuring spring in the region of action of the strain-measuring elements.

10. A measuring device as claimed in any one of claims 7 to 9, wherein a permanent magnet or magnets is (are) used as the one or more magnets.

11. A measuring device as claimed in any one of claims 7 to 9, wherein an electromagnet or magnets is (are) used as the one or more magnets.

12. A measuring device substantially as described herein with reference to the drawings.