GB2255183A

GB2255183A - Torque sensor

Info

Publication number: GB2255183A
Application number: GB9107915A
Authority: GB
Inventors: David Alick Burgoyne
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-04-15
Filing date: 1991-04-15
Publication date: 1992-10-28
Also published as: GB9107915D0

Abstract

A sensor for detecting torque imposed between a shaft (not shown) and an element (11) mounted on the shaft comprises a magnet (12) and a linear hall effect sensor (14). The element (11) is mounted on the shaft by a ball screw housing (16) and a lead screw shaft (22) so that torque applied to the ball screw housing is carried by bearing balls to the shaft to generate an axial movement of the housing on the lead screw shaft. The axial movement of the housing is so resisted by upper and lower disc springs (24, 26). Attached to a housing (36) which is bolted to the element (11) and ball screw housing (16) is a ring magnet (12). Mounted outside the magnet on a mounting ring (73) is at least one linear hall effect sensor which detects translational movement of the magnet. The polarity and magnitude of the sensor output indicate the direction and magnitude of the torque input. <IMAGE>

Description

TORQUE SENSING The present invention concerns a device for sensing and measuring torque applied to a shaft and more particularly but not exclusively, a device for sensing and measuring torque in a power steering system.

Conventional power steering systems are hydraulic and so have certain disadvantages. These could be avoided by the use of an electrical servo system but this would be reliant on accurate measurement of the torque applied to the steering wheel shaft. Such measurements are usually made using a strain gauge on the shaft together with slip rings. These are bulky additions to the steering system and are expensive. These problems are compounded when, in order to ensure functional reliability, a plurality of strain gauges are fitted.

It is an object of the present invention to measure electrically the torque applied to the wheel shaft obviating the need to use strain gauges or slip rings.

According to the present invention there is provided a device for detecting a torque imposed between a shaft and an element mounted on the shaft, the device comprising a magnet for mounting on one of the shaft and the element, and a magnetic field sensor for mounting on the other of the shaft and the element, the mounting of the element to the shaft being such that the element moves longitudinally on the shaft in proportion to the magnitude of the torque imposed between the shaft and the element, and the magnetic field sensor being capable of generating a signal indicative of the amount of such longitudinal movement whereby processing of said signal will yield as output an indication of the magnitude of the torque.

Preferably the magnetic field strength sensing component is a linear Hall effect sensor.

Preferably the means for mounting the element to the shaft is a ball screw assembly.

Preferably the magnet is annular and co-axial with the shaft.

Preferably the magnetic field sensor is on a ring outside the annular magnet.

Preferably a plurality of magnetic field sensors are sited around the ring outside the annular magnet.

Preferably the disc springs are Belleville washers.

Preferably the device includes a mu-metal shield.

For a better understanding of the present invention and to show more clearly how it may be put into effect, reference will now be made to the accompanying drawings in which: Figure 1 is an expanded perspective view of a preferred embodiment of the invention.

Figure 2 is a view from above of the Figure 1 embodiment.

Figure 3 is a longitudinal diametral section of the Figure 1 embodiment.

Figure 4 is a view from below of the Figure 1 embodiment.

Figure 5 is an expanded longitudinal diametral section of the Figure 1 embodiment.

Referring to Figure 1, a device 10 for detecting a torque imposed between a shaft (not shown) and an element 11 mounted on the shaft comprises a magnet 12 and a magnetic field sensor 14. The element 11 is mounted on the shaft by a ball screw housing 16 containing a plurality of bearing balls 18 which run in helices 20 of a lead screw shaft 22 so that torque applied to the ball screw housing 16 is carried by the bearing balls to the shaft to generate an axial movement of the housing 16 on the lead screw shaft 22.

The axial movement of the housing 16 is resisted by an upper disc spring 24 and a lower disc spring 26. More particularly, an upwardly directed axial thrust is transferred from the element 11 (which in the illustrated embodiment is a boss for a vehicle steering wheel) through a thrust washer 28 to the disc spring 24 (which is a Belleville washer) to a location washer 30 for the disc spring, a further washer 32 and a lock nut 34 on the not shown shaft.

Force downwardly directed from the ball screw housing 16 is conveyed to a mounting boss 36, itself secured to the steering wheel boss 11 and ball screw housing 16 by bolts 38. The boss 36 has a downwardly depending externally threaded portion 40 below a shoulder 42.

Against the shoulder is located a first PTFE bearing disc 44 and second PTFE bearing disc 46, and sandwiched between them is the magnet 12. Then, on the threaded portion 40 is secured a pair of internally threaded locking rings 48 and 50, which serve to give some freedom of rotational movement for the magnet 12 on the shaft, between the low-friction discs 44 and 46.

Downward thrust from the locking rings continues through a thrust washer 52, the disc spring 26 and a lower boss 54 which has on its top surface a seating 56 for the periphery of the disc spring 26. Below the boss 54 is a washer 58 and lower lock nut 60.

On a flange 62 of the lower boss 54 is carried a ball bearing 64, which itself carries an annular support base 66. A circlip 68 retains the base 66 to the bearing 64.

Three axially arranged screws 70 serve the purpose of retaining a mu-metal shield 72 around a cylindrical space above the base 66, and also maintain, spaced above the base 66 a mounting ring 73 level with the magnet 12 and circumferentially outside it. The screws 70 are fixed to the structure surrounding the device by brackets (not shown). The brackets prevent rotational movement of the base 66, the mu-metal shield 72, the mounting ring 73, and the Hall effect sensor 14, relative to the surrounding structure. The radially outward facing edge of the magnet 12 carries a pair of slots 74 which cooperate with a pair of spigots 76 on the inside edge of the mounting ring 73. The sensor 14 is a Hall effect semi conductor device which is carried in the inner face of the mounting ring 73.

In production embodiments of the device there are likely to be a number of sensors arranged at regular intervals around the mounting ring 73, the outputs of which being then available for multiplexing.

In use of the device, a torque is applied to the steering wheel boss 11, causing the ball screw housing 16 to move on the helix 20 and generate a thrust either upward or downward depending upon the direction of the imposed torque. Either way, the mounting boss 36 moves axially on the shaft, causing the magnet also to move axially with respect to the shaft. The mu-metal shield 72 and sensor 14, however, do not move axially relative to the lower lock nut 60, and so there is relative axial movement between the magnet 12 and the Hall effect sensor 14 on its mounting ring 73. This movement is picked up by the sensor and generates signals (as is known in itself) indicative of the extent of relative movement between the magnetic field and the sensor.

When torque is imposed on the boss 11 and hence the ballscrew housing 16, relative rotation of the magnet 12 and the mounting 73 is constrained by cooperation between the slots 74 and the spigots 76.

The device can be calibrated so that the signal is then indicative of the amount of torque imposed on the shaft which carries the device.

Claims

1. A device for detecting a torque imposed between a shaft and an element mounted on the shaft, the device comprising a magnet for mounting on one of the shaft and the element, and a magnetic field sensor for mounting on the other of the shaft and the element, the mounting of the element to the shaft being such that the element moves longitudinally on the shaft in proportion to the magnitude of the torque imposed between the shaft and the element, and the magnetic field sensor being capable of generating a signal indicative of the amount of such longitudinal movement whereby processing of said signal will yield as output an indication of the magnitude of the torque.

2. A device as claimed in Claim 1 wherein the magnetic field strength sensing component is a linear Hall effect sensor.

3. A device as claimed in Claim 1 or 2 wherein the means for mounting the element to the shaft is a ballscrew assembly.

4. A device as claimed in Claim 1, 2 or 3 wherein the magnet is annular and co-axial with the shaft.

5. A device as claimed in Claim 4 wherein the magnetic field sensor is on a ring outside the annular magnet.

6. A device as claimed in Claim 5 wherein a plurality of magnetic field sensors are sited around the ring outside the annular magnet.

7. A device as claimed in any one of the preceding claims wherein the disc springs are Belleville washers.

8 A device as claimed in any one of the preceding claims wherein the device includes a mu-metal shield for the magnetic field sensor.

9 A device for detecting a torque substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Amendments to the ciakns have been fled as folows 1. A device for detecting a torque imposed between a shaft and an element mounted on the shaft, the device comprising a permanent magnet for mounting on one of the shaft and the the element such that the North and or South pole end of the magnet are axially mounted such that the lines of magnetic flux are parallel to the longitudinal movement of the shaft in proportion to the magnitude of the torque imposed between the shaft and the element, and at least one linear hall effect sensor positioned so that it is capable of generating a signal indicative of the amount of and direction of such longitudinal movement whereby processing of said signal will yield as output an indication of the magnitude and direction of the torque.

2. A device as claimed in Claim 1 wherein the means for mounting the element to the shaft is a low friction rotational assembly.

3. A device as claimed in Claim 1 or 2 wherein the magnet is annular and co-axial with the shaft.

4. A device as claimed in Claim 3 wherein the magnetic field sensor is on a ring outside the annular magnet.

5. A device as claimed in Claim 4 wherein a plurality of magnetic field sensors are sited around the ring outside the annular magnet.

6. A device as claimed in any of the preceding claims wherein the axial movement of the element mounted to the shaft is resisted by at least one upper and lower spring.

7. A device as claimed in any one of the preceding claims wherein the device includes a mu-metal shield for the magnetic field sensor.

8. A device as claimed in any one of the preceding claims wherein at least one optional upper and lower thrust ball bearing can be used to reduce residual friction.

9. A device as claimed in any one of the preceding claims wherein the sensor mounting ring assembly is located in position either by a ball bearing as described or a taper roller bearing.

10. A device for detecting a torque substantially as hereinbefore described with reference to and as shown in the accompanying drawings.