GB2202997A

GB2202997A - Dynamometer

Info

Publication number: GB2202997A
Application number: GB08705723A
Authority: GB
Inventors: David John Runchman; Roger Herbert Petheram
Original assignee: MECHTRIC ENGINEERING Ltd
Current assignee: MECHTRIC ENGINEERING Ltd
Priority date: 1987-03-11
Filing date: 1987-03-11
Publication date: 1988-10-05
Anticipated expiration: 2007-03-11
Also published as: GB2202997B; GB8705723D0

Abstract

A dynamometer includes a torque-sensing element 15, including a pair of discs 16 interconnected by rods 17, the element 15 being mounted in bearings 20 on the input shaft 23. Further bearings 18 are carried in frame supports 12 and journal the element 15. The input shaft 23 carries a pair of eddy current discs 22 respectively disposed in two gaps defined by the discs 16 and a pair of static discs 11 mounted on the frame. Electromagnetic coils 14 generate magnetic fields across those two gaps, the field strength being controlled dependent upon the rotational rate of the shaft 23, sensed by an optical detector 28, 29 and dependent upon a load cell 31 output reacting to the torque of the sensing element 15. Frictional losses in the bearings 20 are added to the torque sensed by element 15, so eliminating errors in sensed torque resulting from bearing losses. <IMAGE>

Description

DYNAMOMETERS This invention relates to dynamometers, and in particular to so-called eddy current dynamometers which are adapted to permit the assessment of the power output and torque characteristics of a motor.

A known form of dynamometer for measuring the power and torque characteristics of a motor includes an input shaft carrying a disc lying within a magnetic field and arranged so that rotation of the shaft causes eddy currents to be induced which generate a field resisting rotation of the shaft. By having the disc lying within a magnetic field between a fixed stator and a sensor element rotatable co-axially with the shaft, an assessment of the power input and torque characteristics of a motor driving the shaft may be made by monitoring the rotational movement of the sensor element. This may be achieved by having the sensor element operating against a known load (for example a spring) or by using the movement of the sensor element indirectly to control the magnetic field strength within which the disc is rotating.

A disadvantage of the above-described known form of eddy current dynamometer is that when the dynamometer is to be used for assessing the power output or torque of a relatively small motor, the mechanical losses in the dynamometer may become significant, especially if the input shaft is to be rotated by the motor at a relatively high rate. Typically, the input shaft of such a dynamometer would be mounted on ball bearings, and though the frictional losses in such bearings may be relatively small - especially when precision bearings are employed - nevertheless the frictional losses may still be most significant when testing small high speed electric motors.

It is a principal aim of the present invention to provide an eddy current dynamometer where the losses resulting from friction in the bearings supporting the driven input shaft are reduced to a minimum, so as to permit more accurate measurements to be made, especially when testing motors having relatively small output powers and torques, such as may be encountered in the case of small high-speed electric motors.

Accordingly, this invention provides an eddy current dynamometer comprising a frame supporting a ferromagnetic static plate, a torque sensing element mounted on the frame for turning movement about an axis extending normally with respect to the static plate, the torque sensing element including a sensing plate disposed parallel to but spaced from the static plate, electromagnetic means to create a magnetic field in the gap between the static and sensing plates, an input shaft carried in bearings provided in the torque sensing element and having an eddy current disc located in said gap between the static and sensing plates, first transducer means operatively connected to the torque sensing element to provide a first electrical signal representative of the torque applied to the torque sensing element resulting from rotation of the input shaft, second transducer means to provide a second electrical signal representative of the rotational speed of the input shaft, and control means operating on the first and second electrical signals and controlling supply of drive current to the electromagnetic means, the control means also providing an output indicative of at least one of the shaft rotational speed, the torque and the power applied thereto.

It will be appreciated that in the eddy current dynamometer of this invention, the input shaft which is driven by a motor under test is journalled in bearings provided in the torque sensing element itself - that is to say, that element of the dynamometer to which torque is imparted by virtue of the rotation of the input shaft and the resultant induced eddy currents. As a consequence, apart from the torque imparted to the torque sensing element as a consequence of the induced eddy currents, the torque sensing element also will be subjected to a torque directly resulting from the friction of the bearings which carry the input shaft.

That torque resulting from the bearing friction will be in the same sense as the torque resulting from the induced eddy currents, and consequently the sensed torque as determined by the first transducer means will be the sum of those two torques. Thus, when determining the power and torque characteristics of a relatively small high speed motor, any losses in the bearings carrying the input shaft of the dynamometer no longer will be significant, in that the losses in those bearings are added in to the measuring system determining the torque imparted to the torque sensing element.

The first transducer means may sense the angular movement of the torque sensing element away from a datum position against a known bias, provided for example by a spring. In this case, the first transducer means may comprise an angular sensor or a linear transducer appropriately coupled to the torque sensing element.

Most preferably, however, the first transducer means takes the form of a load cell which is arranged to resist the turning movement of the torque sensing element. Conveniently, this is achieved by mounting the load cell on the frame and having an abutment on the torque sensing element which abutment engages the load cell, so that the cell may directly sense the torque imparted to the torque sensing element. In this way, the angular movement of the torque sensing element may be very small, and so the effects of any friction in the bearings mounting of the torque sensing element on the frame may be ignored.

The electromagnetic means should be driven to provide an appropriate magnetic field strength in the gap between the static and sensing plates, having regard to the power output and rotational speed of the motor under test, but in order to allow the testing over a wide range of variables, it is greatly preferred for the control means to be arranged to control the supply of drive current to the electromagnetic means dependent upon the first electrical signal provided by the first transducer means. In this way, the magnetic field across said gap between the static and sensing plates may be adjusted to such a level that the torque sensing element has a torque imparted thereto lying within a defined range - and this is particularly advantageous when it is desired to limit the angular movement of the torque sensing means and the first transducer means takes the form of a load cell.

In one preferred embodiment of this invention, the frame supports a pair of static plates in a parallel, spaced-apart disposition, the torque sensing element being disposed between that pair of static plates and having a pair of sensing plates disposed one adjacent each static plate respectively, so that a gap is defined between each pair of static and sensing plates. The input shaft may then extend with clearance through apertures in both static plates the shaft being carried by bearings provided in the torque sensing element between the static plates. Advantageously, such bearings may be provided in bores in the sensing plates themselves, further bearings being provided as appropriate between the torque sensing means and the frame to permit the torque sensing element to turn about the axis of the input shaft.For this arrangement the input shaft should be provided with two eddy current discs, one located in each gap respectively between adjacent pairs of static and sensing plates, so that rotation of the input shaft imparts torque to the torque sensing element by virtue of the reaction from the induced eddy currents resulting from the rotation of the discs in the magnetic field between the static and sensing plates.

For the embodiment as described above, the electromagnetic means conveniently comprises at least one electromagnetic coil provided on a ferromagnetic core extending between the pair of static plates.

Preferably, a plurality of such ferromagnetic cores provided with coils should be provided, equi-spaced about the axis of the dynamometer - and typically four such cores and coils may be provided. In order to complete the magnetic circuit in the optimum manner, it is preferred for there to be a like plurality of ferromagnetic bars extending between the sensing plates of the torque sensing element.

The second transducer means may take any appropriate form to sense the rotational speed of the input shaft, but it is preferred for that second transducer means not to impart a drag on the rotation of the shaft. For example, the second transducer means may comprise a radiation transmitter and receiver pair, operating for instance in the infra-red region, an optical chopper being mounted on the input shaft and being arranged to pass between the transmitter and receiver.

By way of example only, one specific embodiment of dynamometer constructed and arranged in accordance with the present invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical section through the dynamometer of this invention; Figure 2 is a diagrammatic transverse sectional view through the dynamometer of Figure 1; and Figure 3 is a block diagram of the control arrangement for the dynamometer of Figures 1 and 2.

The embodiment of eddy current dynamometer of this invention and shown in the drawings comprises a base plate 10 on which is mounted a pair of soft iron static discs 11, by means of supports 12. The discs 11 lie parallel to one another and four iron core rods 13 extend between, to form an external magnetic path for the dynamometer. On each core rod 13 is mounted an electromagnetic coil 14, for a purpose to be described below.

Disposed between the static discs 11 is a torque sensing element 15, comprising a pair of torque sensing discs 16 each made of a non-magnetic material such as aluminium, the discs 16 being interconnected by means of four steel rods 17. The supports 12 are arranged so as to extend centrally into the torque sensing element 15, suitable apertures being provided in those supports for the core rods 13 and steel rods 17 to pass therethrough.

Within the torque sensing element 15, each support 12 carries a ball bearing 18, a hub piece 19 being fitted into the inner race of each bearing and carrying one of the torque sensing discs 16 respectively. In this way, the torque sensing element is mounted on the base plate 10 for limited angular movement.

Press fitted into a bore in each torque sensing disc 16 is a further ball bearing 20, an input shaft 21 extending through the inner races of those bearings 20.

The shaft 21 projects beyond the discs 16 so as to pass through apertures provided centrally in the static discs 11. Disposed in each gap between a static disc 11 and the adjacent torque sensing disc 16 is an eddy current disc 22, pinned to the shaft 21 for rotation therewith.

One end portion 23 of the shaft 21 projects through a casing 24 for the apparatus, which casing 24 is mounted on the base plate 10, the projecting end portion 23 of the shaft 21 being provided with a drive peg 25, to permit the shaft to be driven by a motor under test.

The other end portion 26 of the shaft projects through the adjacent static disc 11 and is provided with a perforated wheel 27, passing through the slot 28 of an infra-red transmitter and receiver pair 29, intermittently to interrupt the infra-red beam falling on the receiver upon rotation of the shaft 21.

An arm 30 is rigidly connected to the right hand (in Figure 1) torque sensing disc 16 so as to extend parallel to the axis of the shaft 21, the arm passing through a suitable opening in the adjacent static disc 11. A load cell 31 is mounted on the base plate 10, the arm 30 bearing on the load detecting element of that load cell, whereby the output from the load cell may indicate the torque imparted to the torque sensing element in an anti-clockwise sense, when viewed endwise from the load cell end of the dynamometer.

As will be appreciated from Figure 3, the outputs from the infra-red transmitter and receiver pair 29 and the load cell 31 are led to an electronic control circuit 35, which circuit operates on those outputs and selects an appropriate drive current for the coils 14, dependent thereon. The control circuit 35 also activates a display device 36, a manual control 37 selecting the required parameter to be displayed.

It will be appreciated that with the construction described above, there is a magnetic circuit extending from the coils 14 along the core rods 13, through the static discs 11 and across the gap between those discs and the adjacent torque sensing discs 16, and then along the steel rods 17. Magnetic fields will thus be created within the gaps between the static discs 11 and the torque sensing disc 16, the strength of which field depends upon the current flow through the coils 14.

In use, a motor to be tested is appropriately coupled to the shaft 21 so as to effect the rotation thereof in an anti-clockwise sense, as viewed endwise from the direction of the perforated wheel 27. The control circuit 35 is operated to energise the coils 14 and then, upon rotation of the shaft 21, the induced eddy currents resulting from the rotation of the eddy current discs 22 within the magnetic fields existing in the gaps in the magnetic circuit between the static discs 11 and the respective adjacent torque sensing discs 16 of the torque sensing element 15, will impart a torque to the torque sensing element 15. However, the friction in the bearings 20 carrying the shaft 21 also will impart a torque to the torque sensing element 15, which torque resulting from that friction will be in the same sense as the torque resulting from the induced eddy currents. Consequently, the load (and so the torque) detected by the load cell 31 will be the sum of the induced electromagnetic torque imparted to the load sensing element and the frictional torque imparted thereto. That torque, as determined by the control circuit 35, may be displayed on display device 36, after appropriate operation of the selection control 37.

Equally, the control circuit operating on the output of the transmitter and receiver pair 29 determines the rotational speed of the shaft 21 and again this may be displayed on the display device 36, if required. The control circuit further may assess the effective power output of the motor driving the shaft 21, and again this may be displayed in any suitable manner.

The control circuit 35 should operate so as to adjust the drive current to the coils 14, to ensure that the load being sensed by the load cell 31 lies within the design range for the load cell 31. By controlling the current in this way, the dynamometer may operate over a very wide range of power inputs and rotational speeds, and the friction in the bearings mounting the shaft 21 will not have a significant influence on the operation of the dynamometer, since that friction will be taken into account in determining the torque imparted to the torque sensing element 15.

Claims

1. An eddy current dynamometer comprising -a frame supporting a ferromagnetic static plate, a torque sensing element mounted on the frame for turning movement about an axis extending normally with respect to the static plate, the torque sensing element including a sensing plate disposed parallel to but spaced from the static plate, electromagnetic means to create a magnetic field in the gap between the static and sensing plates, an input shaft carried in bearings provided in the torque sensing element and having an eddy current disc located in said gap between the static and sensing plates, first transducer means operatively connected to the torque sensing element to provide a first electrical signal representative of the torque applied to the torque sensing element resulting from rotation of the input shaft, second transducer means to provide a second electrical signal representative of the rotational speed of the input shaft, and control means operating on the first and second electrical signals and controlling the supply of drive current to the electromagnetic means, the control means also providing an output indicative of at least one of the shaft rotational speed, the torque and the power applied thereto.

2. An eddy current dynamometer according to claim 1, wherein the first transducer means senses the angular movement of the torque sensing element away from a datum position against a known bias.

3. An eddy current dynamometer according to claim 2, wherein the known bias is provided by a spring.

4. An eddy current dynamometer according to claim 2 or claim 3, wherein the first transducer means comprises an angular sensor or a linear transducer appropriately coupled to the torque sensing element.

5. An eddy current dynamometer according to claim 1, wherein the first transducer means comprises a load cell which is arranged to resist the turning moving of the torque sensing element.

6. An eddy current dynamometer according to any of the preceding claims, wherein the electromagnetic means is driven to provide a magnetic field strength of a suitable intensity in the gap between the static and sensing plates, having regard to the power output and rotational speed of the motor under test.

7. An eddy current dynamometer according to claim 6, wherein the control means is arranged to control the supply of drive current to the electromagnetic means dependent upon the first electrical signal provided by the first transducer means.

8. An eddy current dynamometer according to claim 7, wherein the frame supports a pair of static plates in a parallel spaced-apart disposition, the torque sensing element being disposed between that pair of static plates and having a pair of sensing plates disposed one adjacent each static plate respectively, whereby a gap is defined between each pair of static and sensing plates.

9. An eddy current dynamometer according to claim 8, wherein the input shaft extends with clearance through apertures in both static plates, the shaft being carried by bearings provided in the torque sensing element between the static plates.

10. An eddy current dynamometer according to claim 9, wherein said bearings are provided in bores in the sensing plates, further bearings being provided between the torque sensing means and the frame to permit the torque sensing element to turn about the axis of the input shaft.

11. An eddy current dynamometer according to any of the preceding claims, wherein the electromagnetic meant comprises at least one electromagnetic coil provided on a ferromagnetic core extending between a pair of static plates.

12. An eddy current dynamometer according to claim 11, wherein a plurality of ferromagnetic cores are provided with coils, equi-spaced about the axis of the dynamometer.

13. An eddy current dynamometer according to claim 12, wherein there are a like plurality of ferromagnetic bars extending between the sensing plates or the torque sensing element to complete the magnetic circuit.

14. An eddy current dynamometer according to any of the preceding claims, wherein the second transducer means is adapted not to impart a drag on the rotation of the shaft.

15. An eddy current dynamometer according to claim 14, wherein the second transducer means comprises a radiation transmitter and receiver pair, in combination with an optical chopper mounted on the input shaft and being arranged to pass between the transmitter and receiver.

16. An eddy current dynamometer substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.