GB2292811A

GB2292811A - Torque sensor with a strain gauge

Info

Publication number: GB2292811A
Application number: GB9516607A
Authority: GB
Inventors: Dieter Dorrie; Paul Schwerdt
Original assignee: Daimler Benz AG; Mercedes Benz AG
Current assignee: Daimler Benz AG
Priority date: 1994-08-27
Filing date: 1995-08-14
Publication date: 1996-03-06
Anticipated expiration: 2015-08-14
Also published as: FR2724018B1; DE4430503C1; GB9516607D0; ITRM950576A0; FR2724018A1; IT1277917B1; ITRM950576A1; GB2292811B

Description

1 Toraue sensor with a str _,. arrangement 2292811 The invention relates

to a torque sensor having a strain gauge arrangement adapted to be fitted to a measuring element which exhibits mechanical stresses under the action of torque, the measuring element being f ormed from a region, lying in an axial plane, of a disc element and the disc element furthermore containing a first torque transmitting part adjoining the measuring element region radially on the inside and a second torque transmitting part adjoining the measuring element region radially on the outside, of which one forms the torque-introducing and the other the torquedelivering side for the measuring element region. The torque sensor is introduced mechanically into the transmission path of the torque to be measured via the torque-introducing and a torque- delivering sides.

Sensors for measuring a torque are known, in which strain gauges, which are connected as measuring bridges, are arranged on a cylinder outer surface, via which the torque to be measured is transmitted, at an angle of 450 with respect to the generating line of the cylinder. Consequently, the strain gauges extend over a certain axial region and convert the mechanical stresses, induced by a torque, of the cylinder surface serving as measuring element into corresponding electric measured signals. Apart from such sensors with a strain gauge arrangement, such sensors with spring element geometries and such sensors with a magnetostrictive measuring layer are also known, see for example Patent Specification EP 0 285 259 B1.

It is often desirable to have available sensors with the smallest possible axial dimensions, in order to fulfil the increasing requirement for compact, costeffective construction of torque-generating machines and the associated torque-transmitting parts, and in order to be able to fit such sensors even under restricted incorporation conditions. For torque sensors which are based on the functional principle with a magnetostrictive measuring 2 layer, implementations with a comparatively short axial constructional length have already been proposed. Thus, in the case of a sensor disclosed in the US Patent specification 4,697,460, a magnetic disc is arranged transversely with respect to the axis of rotation, opposite which axially a magnetic sensor arrangement with an excitor and sensor coil is placed, which places limits on the reduction of the axial constructional length. In the unprepublished German Patent Application P 4333199.8, a sensor is specified in which the axial expansion of a cylindrical outer surface carrying the magnetic measuring layer determines the axial constructional length of the sensor.

For torque sensors based on the strain gauge principle, various implementations with relatively short axial constructional lengths have also been proposed. Thus, a torque sensor is disclosed in the Laid-Open Specification WO 92/18840. In the case of this sensor, the measuring element region comprises four radial webs which extend, flush on one axial side, to the radially inner and outer adjoining torque transmitting parts, have an axial thickness which is uniform in the radial direction and are provided with the respective strain gauges on their outer side. Accordingly, the thickness of the strain gauge arrangement contributes to the axial constructional length of this sensor. Further torque sensors with strain gauge arrangement and relatively small axial constructional lengths are described in the specifications DE 27 08 484 C2 and DE 37 15 472 C2 and the Laid-Open Specifications DE 32 13 319 Al and DE 42 08 522 A2. In this case, DE 32 13 319 Al discloses a sensor in which the measuring element comprises four radial webs whose axial thickness is constant along the radial extent, while their thickness in the circumferential direction of the associated sensor element decreases from the inside to the outside along the radial direction. In the case of the torque sensor which is shown in DE 42 08 522 A2 and picks up shear forces, the measuring element comprises

3 f our radial webs with an axial thickness which is reduced with respect to the radially inner and outer adjoining torque transmitting parts and is constant along the radial direction, which is achieved by the introduction on both sides of cutouts into an associated sensor disc element.

It is further known to transmit the electric sensor signals and the power supply without contact f rom and to the sensor via telemetric arrangements, see, f or example, Laid-open Specif ications DE 38 25 706 A1 and DE 40 25 279 Al.

The invention is based on the technical problem of providing a torque sensor with a strain gauge arrangement which has a comparatively small axial constructional length and advantageous measuring properties.

According to the present invention there is provided a torque sensor having a strain gauge arrangement adapted to be fitted to a measuring element which exhibits mechanical stresses under the action of torque, the measuring element being formed f rom a region, lying in an axial plane, of a disc element and the disc element furthermore containing a first torque transmitting part adjoining the measuring element region radially on the inside and a second torque transmitting part adjoining the measuring element region radially on the outside, of which one forms the torque-introducing and the other the torquedelivering side for the measuring element region, the latter having an axial thickness which is smaller than the torque transmitting parts, wherein the measuring element is formed as an annular region whose axial thickness decreases in the radial direction from the inside to the outside.

The strain gauge arrangement is located on an annular region of a disc element, surrounding the axis of rotation in a transverse plane, the annular region being of sufficiently low thickness to generate mechanical stresses, f or example shear stresses, when a torque acts via the torque transmitting parts of the disc element which connect radially on the inside and the outside, said stresses being 4 sensed by the strain gauge arrangement. A sensor constructed in such a way can be implemented in an extremely axially short constructional form, since the axial expansion of the disc element on the basis of the strain gauges lying in an axial plane with the measuring element is not determined by the sensing elements, but rather the disc element can be axially reduced to a thickness which still ensures the mechanical stability necessary for transmitting the torque. Since the electric energy which is necessary for the strain gauge arrangement and the electric signals generated by the strain gauges can readily be transmitted using wires or in a wire-free manner by means of electric or electronic components having relatively small dimensions, the reduction of the axial sensor constructional length is also not limited by these energy and signal transmission components. Metrologically advantageous, very homogeneous shear stresses for detection by the strain gauges are achieved by the axial thickness of the annular measuring element region decreasing radially from the inside to the outside.

A further development of the invention enables a constructively simple and thus cost-effective production of the torque sensor, in that the disc elements provided, from which the sensor is constructed, are configured such that they can be produced largely as rotating part.s. The second disc element provided in the case of this refinement is especially expedient for an implementation of a further development, in which the torque introduction and the torque delivery for the sensor lie approximately on the same radius, which is advantageous both from the point of view of torque transmission and also from the point of view of assembly, since the sensor can be fastened on the torqueintroducing and on the torque-delivering side! in each case at the same radial height to the associated, adjacent sections in the torque transmission path.

In a further embodiment, a sensor implementation is specif ied in which the electric energy for the strain gauge arrangement and the measured signals generated by the latter are transmitted in a wire-free manner. To construct such an electric device, those skilled in the art have access to a multiplicity of electric and/or electronic components which have sufficiently small dimensions which do not extend beyond the mechanical dimensions of the sensor disc element which in any case are necessary to ensure the necessary mechanical stability, so that the reduction of the axial constructional length is not limited by this type of electric signal transmission. This is additionally ensured by means of the measure of carrying out the electric signal transmission to and from the sensor via its circumference. In a particularly advantageous refinement of such a device, electric signal transmission is carried out in a wire-free manner to and f rom the sensor radially f rom and to the outside, the voltage converter module being accommodated within the sensor disc element or elements, so that even here no increase in the axial sensor constructional length is necessary because of its electric signal processing part. Any connecting leads which may be necessary f rom the voltage converter module to the radial sensor outside and/or to the strain gauge arrangement can thus be led in cutouts or holes in the disc element or elements of the sensor, such that they do not project axially beyond the sensor element, in spite of a low axial constructional length of the sensor.

Preferred embodiments of the invention are shown in the drawings and are described below. In the drawings:

Fig. 1 shows a longitudinal section through a torque sensor with a strain gauge arrangement and wirefree electric signal transmission, having a short axial constructional length and Fig. 2 shows a perspective view of a torque sensor which is slightly modified in terms of construction with respect to that of Fig. 1 but is functionally equivalent, one quarter of the sensor being broken away to illustrate the internal construction.

Apart from slight modifications in terms of mechanical construction, in particular With respect to the 6 exact cross-sectional f orm. of the disc elements used and the course of lead-guiding holes in these disc elements, the torque sensor shown in Figure 2 corresponds to that of Fig. 1, for which reason all the corresponding parts of the sensor of Fig. 2 are provided with the same reference symbols as are used f or the sensor of Fig. 1, so that the following description jointly relates to both sensors, providing no differentiation is specifically made.

The torque sensors shown comprise a f irst disc element I and a second disc element 9. Both disc elements 1, 9 are produced at low cost as rotating parts which are rotationally symmetric about their central axis 2, and into which holes only mentioned further below are subsequently introduced. The first disc element 1 comprises a radially inner torque transmitting part 3, a radially outer torque transmitting part 4 and an annular region 5, lying between these torque transmitting parts 3, 4 and having a comparatively small axial thickness d2l the latter extending more precisely between the inner radius r2 of the radially outer 4 and the outer radius ri of the radially inner torque transmitting part 3. This reduced thickness d2 leads to notable shear stresses occurring in the annular region 5 when a torque passes through the first disc element 1, while the torque transmitting parts 3, 4, which are by comparison configured with a distinctly greater axial extent d, dl, remain mechanically more resistant to stress. These shear stresses can be registered by a strain gauge bridge 6 which for this purpose is arranged on one side of" the annular region 5 and converted into electric signals. The strain gauge bridge 6 here is located within one of the annular cutouts 22, introduced on both sides of the annular region 5 to form the same in the first disc element 1, within a section of the annular measuring element 5 in which the shear stresses occur. In order to generate shear stresses which are as homogeneous as possible in the annular measuring element 5, in the case of the sensor of Fig. 1 the measuring element is designed with a trapezoidal cross- 7 section such that its axial thickness d2 decreases in the radial direction from the inner bounding radius r, to the outer bounding radius r 2' The radially outer torque transmitting part 4 has a hole circle 7, which contains a predetermined number of screw holes arranged at equidistant angular spacing, which are used for the rotational ly-f ixed fitting of the first disc element 1 to one side of a component whose torque is intended to be monitored, by means of screw connections. While the radially inner torque transmitting part 3, on the side on which the strain gauge bridge 6 is f itted in the annular region 5, ends axially at the outer torque transmitting part 4, it 3 extends on the opposite side, with a stepped radial taper, axially beyond the dimension d, of the radially outer torque transmitting part 4 with an axial length d which is greater by comparison. The second disc element 9 is placed on the projecting axial end region of the radially inner torque transmitting part 3, extending parallel to the first disc element 1, and is electron- beam welded to the latter along the contact surf ace 10. The second disc element 9 in this arrangement ends on the outer side f lush with the associated end of the radially inner torque transmitting part 3 of the first disc element 1, so that the axial length d of this radially inner torque transmitting part 3 and hence of the disc element 1 simultaneously represents the entire axial constructional length of the torque sensor.

The second disc element 9 is likewise provided, on the same radius R as the first disc element 1, with a hole circle 8 which contains a predetermined number of screw thread holes arranged at equidistant angular spacing, with which holes the torque sensor can be fastened to a further connecting part, provided f or this purpose, of a torquemonitored component, on that side lying opposite the hole circle 7 of the first disc element 1. In this arrangement, the screw connections of the f irst hole circle form the torque- introducing elements and the screw connections of the 8 other hole circle form the torque-delivering elements, the selection of which hole circle 7, 8 assumes which function being arbitrary, from the point of view of the construction of the sensor, and can be directed towards the respective drive and loading machine whose torque is intended to be measured. The torque is consequently led within the sensor from the hole circle 8 of the second disc element 9 via the latter, via the radially inner torque transmitting part 3 of the first disc element 1 and via the annular region 5, where it induces the shear stresses sensed by the strain gauge bridge 6, to the radially outer torque transmitting part 4 of the first disc element 1, and is delivered from there via the hole circle 7, or else the torque transmission course is reversed, if the torque is introduced at the hole circle 7 of the first disc element 1.

It is evident that the torque sensors shown in Figs. 1 and 2 have the advantage from the point of view of assembly and torque transmission that the two hole circles 7, 8 forming the torque connection lie on the same radius R and, nevertheless, the sensor is produced in a simple manner from the two rotating part disc elements 1, 9. As an alternative it is of course also possible to machine the sensor shown or one of similar construction from a workpiece, with a somewhat increased constructional cost. If necessary, this can be linked with providing the second hole circle 8 directly on the radially inner torque transmitting part 3 of the f irst disc element 1, which can then have a suitably modified configuration, in that, for example, it does not taper radially in the axially projecting region, but rather extends outwards with a constant outer radius ri or even with an increasing radius. In this case, the second disc element 9 can be dispensed with, if appropriate.

The entire electrical part of the torque sensors of Figs. 1 and 2 is accommodated within the axial region specified by the axial length d of the inner torque transmitting part 3 of the f irst disc element 1 and thus does not increase the axial constructional length of the 9 sensors. In particular, the coupling in of the electric energy f or the strain gauge bridge 6 is carried out via a commonly available telemetry system having a generator 12 arranged in a stationary position radially outside the sensor element 1, 9 and which, in this example, is designed with a head-shaped coupling part, this generator head 12 lying opposite an electric winding 13 [ in the case of the sensor of Figure 2, an electrically conductive separate band between two adjacent connecting points] arranged on the circumference of the second disc element 9, leaving an air gap 21. The winding 13 in this arrangement is seated in an annular part 20 of U-shaped cross-section, which is introduced into a cutout matched thereto on the circumference of the second disc element 9 such that it ends flush in the radial direction with the circumference of the second disc element 9. Instead of the head-shaped coupling part, the generator can also have an annular primary winding which surrounds the winding 13 on the disc element 9 in an electromagnetically coupling manner, leaving an annular gap.

As can be seen f rom Figure 2 - in Figure 1 the electrical leads are left out on grounds of clarity - a set of electrical leads 19 leads f rom the outside winding or from the band 13 to an electronics module 17 arranged in the interior of the radially inner torque transmitting part 3 of the first disc element 1, the electronics module rotating concomitantly when the sensor is mounted. The electronics module 17 here is seated in a central, rotationally symmetrical, stepped cutout 18 in the radially inner torque transmitting part 3. In this arrangement, the latter is of can-shaped configuration, in that the cutout 18 is delimited on one axial side by a bottom region 23 of this transmitting part 3. The connecting essentially radially, through leads 19 are led, extending a cutout 16 which is introduced in the second disc element 9 on that region facing the radially outer torque transmitting part 4, and through a hole 15 in the radially inner torque transmitting part 3, which hole leads from that side of the f irst disc element I f acing the second disc element 9 to that side facing away from the latter into the region of the cutout 18 in the radially inner torque transmitting part 3 [the precise course of the lead-throughs 15, 16 f or both the sensors according to Figs. 1 and 2 varying somewhat]. As can also once more be seen from Fig. 2, a further set of connecting leads 11 leads from the radially centrally arranged electronics module 17 through a radial hole 14 in the radially inner torque transmitting part 3 to the strain gauge bridge 6. It is evident that all the electrical leads are laid in such a way that they do not project beyond the outer dimensions of the sensor given by the two disc elements 1, 9.

The sensor of Figure 1 additionally offers protection against unauthorized manipulations. During the assembly of the sensor shown there, as soon as the strain gauge bridge 6 is fitted, the electronics module 17 introduced from the open pot side of the inner torque transmitting part 3 into the cutout 18 and the electrical connecting leads have been laid, the relevant access side for these elements is closed with a sealing lid 24. The latter is fastened to the first disc element 1, from the axial outer side of the latter, in such a manner that it can only be released again without destroying it by a person authorized to do this. The lid 24 extends radially as far as the inner radius r2 of the outer torque tr ansmitting part 4. Together with the design of the sensor, which is closed on the opposite side, this has the consequence that the electronics module, the strain gauge bridge 6 and the connecting leads are secured against unauthorized interventions.

Figure 3 shows, for a further torque sensor version, the associated electrical equivalent circuit diagram. This sensor essentially corresponds in its construction to those described above, to which extent the same reference symbols for functionally similar, elements are selected. This sensor differs in that the generator-side part of the electromagnetic coupling path between the 11 rotating sensor part 25 and a stationary component 26 surrounding the sensor comprises a primary winding band 12b instead of the head-shaped generator coupling part 12. Mounted on the stationary component 26 is an RF-generator unit 12a which is electrically connected, on the one hand, to the primary winding band 12b and, on the other hand, via a cable connection 27 to an associated evaluation unit 28. As in the case of the above sensors, the winding 13 on the radial outer side of the sensor lying opposite the generator winding 12b is connected to the electronics module 17 which is accommodated in the interior of the sensor 25 via a harness 19 which is led through, said electronics module, apart from its function as a voltage converter unit, also having the function of a sensor signal amplifier. A calibration resistor 29 on the electronics module 17 can also be seen. The strain gauge bridge 6 is connected to the electronics module 17 by means of the harness 11 and, at selected points, has measuring locations 6a. Of course, the strain gauge bridge 6 can also be subdivided into two diametrically opposite half-bridges having two measuring locations 6a in each case and connected to each other.

When operating the respective sensors, an electric voltage is induced in the circumferential winding 13 with the aid of the generator 12; 12a, 12b, is led via the electrical connecting leads 19 to the rotating electronics module 17, is converted there and is fed as a DC voltage via the second set of connecting leads 11 to the strain gauge bridge 6 for the electrical supply of the latter. The measured signal output on the bridge diagonal of this strain gauge arrangement 6 is led via the same electric connecting path radially outwards in the opposite direction to the electrical voltage supply via the generator 12; 12a, 12b and the cables 27 to the evaluation unit 28.

It goes without saying that, for those skilled in the art, apart from those already mentioned, further versions of the torque sensors shown can be implemented within the framework of the invention. Thus, for example, 12 another conventional type of electric signal transmission between the rotating sensor and the outer, stationary environment can be provided. Also, the precise configuration of the disc element containing the annular, mechanically stressable region, and of a further disc element provided, if appropriate, can be adapted to the respective application. It is common to all sensors of this type that they use a strain gauge arrangement for torque sensing and have an extremely short axial constructional shape, since the mechanically stressing region on which the strain gauge arrangement is seated extends in an axial plane. This very short axial constructional length makes it possible, for example as a particular advantage, to introduce such a sensor for torque registration into a propeller-shaft train with universal joint sliding pieces in a simple manner by pushing the universal joint sliding pieces axially away from each other to a small degree, which is possible in most cases, in the case of such arrangements, following which the sensor can be introduced there.

13 claims A torque sensor having a strain gauge arrangement adapted to be f itted to a measuring element which exhibits mechanical stresses under the action of torque, the measuring element being f ormed f rom a region, lying in an axial plane, of a disc element and the disc element furthermore containing a first torque transmitting part adjoining the measuring element region radially on the inside and a second torque transmitting part adjoining the measuring element region radially on the outside, of which one forms the torque-introducing and the other the torque-delivering side for the measuring element region, the latter having an axial thickness which is smaller than the torque transmitting parts, wherein the measuring element is f ormed as an annular region whose axial thickness decreases in the radial direction from the inside to the outside.

Claims

2. A torque sensor according to Claim 1, f urther comprising a second disc

element which is arranged parallel to the f irst disc element and is rotationally fast to the first torque transmitting part, both disc elements being of a form which can largely be implemented as a rotating part and one of the second disc element and the second torque transmitting part of the first disc element, serves as the torque-introducing connecting part and the other serves as the torque-delivering connecting part of the sensor.

3. A torque sensor according to Claim 2, wherein, on the second disc element and on the second, radially outer torque transmitting part of the f irst disc element, elements are provided in each case for introducing torque into and for delivering torque out of the sensor and lie at the same radial height.

14 4. A torque sensor according to any one of Claims 1 to 3, wherein, on the circumference of at least one disc element, a device is provided f or the wire-f ree and contact free coupling in of electric energy for the strain gauge arrangement and for coupling out the measured signal generated by the latter to the stationary outer environment.

A torque sensor according to Claim 4, wherein the device for wire-free and contact-free transmission of electric energy and electric signals contains a telemetry system having a winding arranged on the circumference of at least one disc element and a stationary generator part lying radially opposite the winding, leaving an air gap, and a voltage converter module which is electrically connected, on the one hand, to the winding and, on the other hand, to the strain gauge arrangement, is accommodated by the disc element or elements.

6. A torque sensor having a strain gauge arrangement substantially as described herein with reference to and as illustrated in the accompanying drawings.