GB2225854A

GB2225854A - Resonantly vibrating torque sensor

Info

Publication number: GB2225854A
Application number: GB8828742A
Authority: GB
Inventors: James Christopher Reynolds
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-12-08
Filing date: 1988-12-08
Publication date: 1990-06-13
Anticipated expiration: 2008-12-08
Also published as: GB8828742D0; GB2225854B

Abstract

A torque sensor is formed by a resonant bridge 6 linking points on the surface of a shaft 2. The bridge 6 is formed by removing two segments 4 of the material of the shaft e.g. by spark erosion and another such bridge is formed on the opposite side of the shaft. The bridges are resonantly driven and sensed by piezoelectric elements 18, 19 connected with conductive layers 11, 13, 15, 17 on flanges 8, 9 rotating with the shaft and connected with an alternating source 31 and a signal processor 34 via capacitative plates 28, 32 and 30, 33. The processor 34 calculates the torque from the two measured resonant frequencies of the bridges. <IMAGE>

Description

Torque Sensor This invention relates to torque sensors, specifically those for sensing the torque on a rotating shaft.

Referring to Figure 1, the torque on a shaft 1 can be calculated by measuring the relative angular positions of the shaft 1 at two points P1 and P2 axially spaced apart along the shaft 1 firstly when there is no torque acting on the shaft and secondly when there is torque acting on the shaft.

For a shaft 1 of radius r and two points P1 and P2 separated by a distance L, the torque on the shaft is proportional to the change in the relative angular positions of the shaft 1 at the points P1 and P2 when the torque is acting compared to the relative angular positions when no torque is acting.

One known method of carrying out this procedure is to measure the angular positions of a shaft at two points using optical or electrical angle encoders and measure changes in the relative angular position by substracting one angular position from the other. A problem with this method is that in real systems the changes in relative angular position are very small compared to the full circle angular movement of the rotating shaft. As a result relatively small errors in measuring the absolute angular positions of the shaft at the two points result in proportionaly large errors in the calculation of the relative angular positions of the two points.

It was an object of this invention to produce a torque sensor overcoming this problem at least in part.

This invention provides a system for sensing the torque on an object including - a mechanically resonant element linking a first and a second point on the object such that the resonant frequency of the element is dependent on the torque acting on the body between the first and second points.

This allows the relative angular position of the body at two points to be measured directly and so overcomes the problems of inaccuracy encountered in known methods as described above.

Advantageously the mechanically resonant element is at an angle to the torque axis because if the element is parallel to the torque axis changes in torque cause very little elongation of the element, and so the sensitivity of the system is very low.

Preferably two mechanically resonant elements linking points in two planes perpendicular to the torque axis are used, the two elements being at equal and opposite angles to the torque axis. This construction means that changes in the strain on the resonant elements due to temperature changes or linear strain on the body will be equal but changes in the strain on the resonant elements due to torque on the body will be equal and opposite. This will make it relatively easy to separate strain changes due to torque from those cause by other factors.

One way of performing the invention will now be described by way of example - with reference to the accompanying drawings in which, Figure 1 is a schematic diagram used to explain the theoretical basis of the invention, Figure 2 is a plan view of a torque sensor constructed in accordance with the invention; and Figure 3 is a side elevation of the torque sensor of Figure 2.

Referring to Figure 1, a shaft 1 has a radius r and a torque is applied to the shaft about the axis 36. The axis about which torque is applied, in this case the axis 36, will be called the torque axis in this specification.

It is known that the torque T acting on a shaft of radius r between two planes seperated by a distance L along the torque axis

Where G is the shear modulus of the material of the shaft, and is the change in relative angular position of the two points when the torque is applied.

If strain is applied to a bridge, that is a selfsupporting structure having a resonant frequency, the resonant frequency of the bridge will alter, the change in frequency f will be;

where vC is the applied strain, t the thickness of the bridge, E the Youngs modulus of the material of the bridge and P its density.

If a bridge links two planes separated by a distance L along a shaft and the bridge is at an angle t to the shaft axis, the strain in the bridge will be,

Substituting from equation (1) for 0

Substituting equation 4 into equation 2 the change in the resonant frequency of the bridge is,

This change in the resonant frequency is only correct if the change in the moment of inertia of the shaft between the two planes produced by the bridge is insignificant. If the change in moment of inertia is taken into account, the change in the resonant frequency of the bridge is,

Where I is the moment of inertia per unit length of the shaft between the two planes.

Referring to Figures 2 and 3 a steel shaft 2 has a circular cross section and rotates and bears a torque about an axis 3.

Two segments 4 and 5 of the shaft 2 situated at the same position along the shaft 2 but on opposite sides of it are removed by spark erosion, leaving first and second bridges 6 and 7 which are linked to the main body of the shaft 2 at each end of the segments 4 and 5 only.

A pair of circular flanges, 8 and 9 respectively, are formed from steel integrally with the shaft 2 are situated on either side of the segments 4 and 5.

The first flange 8 carries a first electrically insulating layer 10 on one of its faces, this first insulating layer is in the shape of a circular annulus coaxial with the shaft 2. The first insulating layer 10 bears a first conductive layer 11 in the shape of a circular annulus co-axial with the shaft 2. The effect of the first insulating layer 11 is to electrically insulate the first conductive layer 11 from the flange 8.

Similarly the other face of the first flange 8 carries a second electrically insulating layer 12 and a second conductive layer 13 and the two faces of the second flange 9 carry a third insulating layer 14 and third conductive layer 15 and a fourth insulating layer 16 and a fourth conductive layer 17 respectively.

The first bridge 6 has a first piezo-electric element 18 and a second piezo-electric 19 secured to it.

Similarly the second bridge 7 has third and fourth piezoelectric elements 20 and 21 secured to it.

The first conductive layer 11 is connected to the first piezo-electric elements 18 by a wire 22, which passes through a bore 23 through the first flange 8 and its associated conductive and insulating layers 10 to 13.

The second conductive layer 12 is connected to the fhird piezo-electric element 20 by a wire 24. The third conductive layer 15 is connected to the second piezoelectric element 19 by a wire 25. The fourth conductive layer 17 is connected to the fourth piezo-electric element by a wire 26, which passes through a bore 27 through the second flange 9 and its associated layers 14 to 17.

A conductive plate 28 is held close to the first conductive layer 11. A signal generator 29 applies an alternating voltage to the plate 28, this induces an alternating voltage at the same frequency in the first conductive layer 21. Thus the first conductive layer 11 and the first conductive plate 28 act as the two plates of a capacitor, because the first conductive layer 11 is formed as an annulus about the axis of rotation of the shaft 1 this capacitor action will occur throughout the rotation of the shaft 1. The wire 22 then applies this voltage to the first piezo-electric element 18, as a result the first piezo-electric element 18 applies mechanical impulses to the first bridge 6 at the frequency of the alternating voltage from the signal generator 29.

These mechanical impulses cause the first bridge 6 to vibrate at its resonant frequency.

These vibrations cause the second piezo-electric element 19 to generate alternating voltage at the frequency of vibration of the first bridge 6. This voltage is applied to the third conductive layer 15 by the wire 25. The alternating voltage on the third conductive layer 15 induces an alternating voltage at the same frequency in a second, conductive plate 30, held close to the third conductive layer 15.

Similarly, an alternating voltage from a signal generator 31 is applied by way of a second conductive plate 32, the second conductive layer 13 and the wire 24 to the third piezo-electric element 20. This causes the second bridge 7 to vibrate at its resonant frequency. An alternating voltage at the resonant frequency of the second bridge 7 is produced in the fourth conductive plate 33 by the fourth piezo-elelctric element 21, the wire 26 and the fourth conductive layer 17.

Thus alternating voltages are induced in the third plate 30 and fourth plate 33 at the resonant frequencies of the first and second bridges 6 and 7 respectively.

These voltages are supplied to a signal processing system 34 which calculates the torque in the shaft 2 from these two frequencies and displays this value on the display 35.

Although the bridges 6 and 7 have been described as being cut from the material of the shaft they could of course be formed seperately-and secured to the shaft by, for example gluing, The bridges 6 and 7 have also been described as being cut from the material of the shaft 2 by spark erosion, they could of course be formed by any other material shaping process such as chemical etching.

Although it is advantageous for the bridges 6 and 7 to be on opposite sides of the shaft 2 because this reduces interference between them they could be placed anywhere around the circumference of the shaft 2.

Claims

CLAIMS 1. A system for sensing torque on an object including a mechanically resonant element linking a first and a second point on the object such that the resonant frequency of the element is dependent on the torque acting on the object between the first and second points. 2. A system as claimed in claim 1 in which the mechanically resonant element is at an angle to the torque axis. 3. A system as claimed in claim 2 in which two mechanically resonant elements are used, each of the resonant elements linking a first and a second point, both of the first points being in a first plane perpendicular to the torque axis, both of the second points being in a second plane perpendicular to the torque axis, and the two resonant elements being at an equal but opposite angle to the torque axis. 4. A system as claimed in any preceding claim in which the body rotates about the torque axis. 5. A system as claimed in any preceding claim in which the resonant element or elements can be driven into resonance by a first piezo-electric means. 6. A system as claimed in any preceding claim in which vibration of the resonant element or elements can be converted into an electric signal by a second piezoelectric means. 7. A system as claimed in claim 5 in which the first piezo-electric means are driven from a power source sited off the body by capacitive means. 8. A system as claimed in claim 6 in which the electric signal is sent to a data-processing system sited off the body by capacitive means. 9. A system as claimed in claim 7 or claim 8 in which the capacitive means comprises a first annular plate on the body and a second plate off the body. 10. A system as claimed in claim 9 in which the annular plate is supported by a flange on the body. 11. A torque sensor substantially as shown in and substantially as described with reference to Figures 2 and 3 of the accompanying drawings. Amendments to the claims have been filed as follows

1. A system for sensing torque on a body including a mechnically resonant element, extending in a direction having a component parallel to the torque axis and linking a first and a second point on the body such that the resonant frequency of the element is dependent on the torque acting on the body between the first and second points.

2. A system as claimed in claim 1 in which the resonant element extends in a direction having a component perpendicular to the torque axis.

3. A system as claimed in claim 2 in which two mechanically resonant elements are used, each of the resonant elements linking a first and a second point, both of the first points being in a first plane perpendicular to the torque axis, both of the second points being in a second plane perpendicular to the torque axis, and the two resonant elements being at an equal but opposite angle to the torque axis.

4. A system as claimed in any preceding claim in which the body rotates about the torque axis.

5. A system as claimed in any preceding claim in which the resonant element or elements can be driven into .resonance by a first piezo-electric means.

6. A system as claimed in any preceding claim in which vibration of the resonant element or elements can be converted into an electric signal by a second piezo electric means.

7. A system as claimed in claim 5 in which the first piezo-electric means are driven from a power source sited off the body by capacitive means.

8. A system as claimed in claim 6 in which the electric signal is sent to a data-processing system sited off the body by capacitive means.

9. A system as claimed in claim 7 or claim 8 in which the capacitive means comprises a first annular plate on the body and a second plate off the body.

10. A system as claimed in claim 9 in which the annular plate is supported by a flange on the body.

11. A system as claimed in any preceding claim wherein the resonant member extends across a recess in the body.

12. A system as claimed in any preceding claim wherein a portion of the body is adapted to form the resonant member.

13. A torque sensor substantially as shown in and substantially as descirbed with reference to Figures 2 and 3 of the accompanying drawings.