GB2306641A - Optical torque sensors for vehicle steering systems - Google Patents

Abstract

An optical torque sensor has an input shaft (1) and an output shaft (5) and a pair of spaced apart metal discs (26, 27) whose relative angle of rotation, whilst spinning as a pair, is indicative of the torque applied between the input and output shafts (1, 5), and in which the discs are mounted onto a hub assembly (20) which comprises inner and outer rigid rings (21, 22) the rings being connected by flexible vanes (23) which allow relative rotation between the two discs whilst ensuring that they are stiffly constrained to remain concentric and parallel. The torque sensor is suitable for incorporation in a power-assisted steering system for vehicles.

Description

OPTICAL TORQUE SENSORS AND STEERING SYSTEMS FOR VEHICLES INCORPORATING THEM This invention relates to optical torque sensors especially, but not exclusively, for incorporation in power-assisted steering systems for vehicles.

One known construction of optical torque sensor comprises a pair of closely spaced high precision thin metal discs whose relative rotation, whilst spinning as a pair, is detected by optical means. This relative rotation, which is approximately + 1.5 full scale, with up to + 0.3 over travel, is a function of the twist, and hence the torque applied, in a torsion bar which is attached at its ends to input and output shafts of the sensor. It is important to maintain the relative axial spacing of the discs, their concentricity and their parallelism within close limits, typically i 0.050mm, whilst allowing them to rotate.

Problems arise when manufacturing such a torque sensor as a mass produced automotive product since it is necessary to ensure that the thin metal discs are mounted in the torque sensor in such a manner as substantially to avoid friction, wear, hysteresis, backlash and contamination, during the operation of the sensor.

According to our invention we provide an optical torque sensor comprising an input shaft and an output shaft and having a hub component comprising an inner and an outer rigid ring, and flexible vanes joining the inner and outer rings together, the inner and outer rigid rings comprising mountings for a pair of closely spaced discs whose relative rotation is a function of the angle of twist, and hence torque applied, between the input and output shafts.

Preferably, the discs comprise high precision metal discs, and the hub component may preferably comprise a one-piece plastics moulding.

In operation the hub component acts like a bearing with limited angular travel and the rings are angularly moveable relative to each other through a small angular distance about a central axis but are stiffly constrained to remain concentric and parallel.

The rings ensure that the discs are very accurately positioned in terms of their relative angle at a zero torque condition.

Preferably very flat and parallel metal support plates are joined to the rings, suitably by means of heat staking to pillars which are compliant to allow for differential thermal expansion between the hub and the plates, the plates forming connections between the discs and the hub component.

The heat-staking process, for example by the use of ultrasonics, is used to set the axial spacing of the support plates, independently of the accuracy of the hub moulding, as originally manufactured.

During the assembly of the sensor adhesive may be used to set the concentricity and the rotational alignment of the discs, independently of the accuracy of the hub component and the plates.

Conveniently dogs are provided on the moulding to give protection, in terms of errant torque readings, against failure of the adhesive and/or the heat staking.

Limit stops may be extended axially and radially inwardly, respectively, to provide engagement between the input and output shafts and the inner and outer rings, respectively. The rotational alignment of the optical disc is therefore maintained to high accuracy and with respect to that of torsion bar limit stops. This ensures that the optical discs do not over/under travel and it also aids the assembly of a pre-assembled opto-electronic sensor unit to the mechanical shafts.

The final assembly is sealed on opposite sides of the optical discs by first and second seals each located on the inboard side of a respective one of a pair of bearings in which the input shaft is rotatably mounted.

The relative rotational movement can be imparted by slots on the input shaft and dogs on the output shaft which engage with the rings on the component.

One embodiment of our invention is detailed in the accompanying drawings in which: Figure 1 is a longitudinal section through a power-assisted steering system for a vehicle; Figure 2 is a portion of the system of Figure 1 on an enlarged scale; Figure 3 is an end view of the hub component with the support plates assembly for clarity; Figure 4 is a section on the line B-B of Figure 3; Figure 5 is a section on the line C-C of Figure 3; Figure 6 shows the support plates staked to the hub component; Figure 7 shows one optical disc; and Figure 8 shows the other optical disc The power-assisted steering system illustrated in the drawings comprises a steering input shaft 1 coupled at one end through a splined portion 2 to a steering wheel not shown and at the other end to a quill or torsion bar 4 which transmits steering torque to a steering output shaft 5.

The steering output shaft 5 is coupled to a steering transmission 6 through a universal joint 7.

The input shaft 1 is journaled for rotation in an outer casing 8 between axially spaced bearings 9 and 10. Similarly the output shaft 5 is journaled for rotation between inner and outer axially spaced bearings 11 and 12.

An electric motor 14 is housed in the casing 8. The motor comprises a stator 13 and a rotor in the form of a one-piece flexible hub assembly 20. The hub assembly 20 comprises a plastics moulding mounted on a journal 21 in which the inner end of the input shaft 2 is angularly movable. The use of the inherent flexibility of plastics avoids the need for lubrication, and hence seals, and so minimises friction, wear, hysteresis as well as free play. The moulded hub 20 consists of relatively stiff inner and outer relatively stiff rings 21 and 22 which are spaced apart by four angularly spaced flexible vanes 23. These vanes are arranged to be resistant to deformation in their own planes but flexible in a direction normal to such planes.This enables the rings 21 and 22 to rotate by small angular distances relative to each other about a central axis, but to be stiffly constrained to remain concentric and parallel the chosen thickness of the vanes will be a compromise between acceptable strain level at maximum angular travel and the ease of moulding.

In one example the vanes 23 are substantially 0.6mm thick and the calculated strain at 1.80 of movement for this thickness is 2.6%. The maximum occurs at the inner end, namely the root, of each vane 23. The strain for the given angle is proportional to the thickness and, for the purpose of evaluation it can be assumed that % strain = 0.24 x vane thickness (mm) x movement (dog) A second compromise will concern the use of a filled or an unfilled polymer to construct the hub component 20, and either technique may be used depending upon the flexibility of the vanes 23 and the ease of moulding.

Individual inner and outer support plates 24 and 25 are heat staked to the inner and outer rings of the hub component at the input side facing the bearing 21. Optical discs 26 and 27 are bonded to the inner and outer support plates 24 and 25 as clearly shown in Figures 4 and 5. The concentricity and angular positioning of the optical discs, both relative to each other and relative to features on the hub component 20 which will engage with the shafts 2 and 5 and which are to be described, are determined by accurate fixtures during the bonding operation. Hence these critical alignments do not directly depend on the accuracy with which the hub component 20 and the support plates 24 and 25 are manufactured. However there is a requirement to achieve an axial spacing between the optical discs 26 and 27 of 0.050 to 0.150mm.This will require either: 1. that the combined tolerance of (a) the axial offset of the seating registers for the two plates 24 and 25 (b) the thickness tolerance for the plates fall within this variation, or 2. a method is developed for "bedding" the plates onto the seating registers, with their appropriate axial offset, at the time of their assembly to the moulded hub component 20. Conveniently this can be achieved by the use of ultrasonics.

Angularly spaced pillars 30 and 31 are provided on hub moulding 20. These pillars act in securing the support plates to the hub component and are received in complementary angularly spaced openings 32 and 33 in the inner and outer support plates 24 and 25 as illustrated there are six pillars for the outer plate 25, and four pillars for the inner plate 24. The number and positioning of the pillars is dictated by packaging constraints. The outer ring 22 is also provided with six dogs 35 which accurately engage with complementary slots 36 in the periphery of the outer plate 25. These ensure accurate radial and rotational location of the plate with respect to the hub component 20 whilst allowing for differential thermal expansion between the plastics material of the hub component 20 and the steel of the steel support plate.

Conveniently a root-end-tip clearance is provided between the dogs 35 and the slots 36 so that a small amount of radial growth can occur. The staking pillars 30 and 31 are slightly flexible in a radial direction in order to accommodate differential expansion. Two of the dogs 35 are sufficiently long as to project through complementary openings 40 in the optical disc 27 and openings 41 in the optical disc 26. This limits rotation of the optical disc that would occur as a result in failure of its bonding to its respective support plate.

The accuracy of the moulding is significant in that this regard since the greater the clearance needed between this pair of dogs and the openings in the optical discs 26 and 27 to allow for moulding variations, then the greater will be the torque signal deviation resulting from a bonding failure. The arrangement for attaching the inner plate 24 is similar except that dogs 45 on the inner plate 24 engage in slots 46 in the hub in the ring. To protect against failure of the bonding for the optical disc 26 to the inner plate 24 the disc is provided with diametrically opposed dogs 47 which engage, with small clearance, in the slots 46.

The heat-staking process for attaching the support plates 24 and 25 to the hub component 20 consists of operations to flatten the end portion of each pillar into a countersunk recess surrounding each of the attachment openings 32 and 33 in the plates 24,25. This is accomplished by melting the plastics material.

When the hub component 20 together with the support plates 24 and 25 and the optical discs 26 and 27 are incorporated in the torque sensor relative rotational movement is provided by slots 50 in the input shaft 1 and dogs 51 on the output shaft 5 which engage with the inner and outer rings 21 and 22, respectively.

Axially spaced seals 60 and 61 are disposed on opposite sides of the hub component 20 in order to seal the torque sensor against the ingress of dirt. The seal 60 is located on the input side of the bearing 10 and seals 61, which is of L shaped outline, is accommodated against the output side of the component 20, adjacent to the bearing 11, and seals against two faces, mutually at right angles to each other.

LED's 62 and 63 illuminate the optical discs 26 and 27 through axial passages 64 and 65 in the stator 13, and optical signals are received by complementary detectors 66 and 67.

Claims (13)

1. An optical torque sensor comprising an input shaft and an output shaft and having a hub component comprising an inner and an outer rigid ring and flexible vanes joining the inner and outer rings together, the inner and outer rigid rings comprising mountings for a pair of closely spaced discs, whose relative rotation is a function of the angle of twist, and hence torque applied, between the input and output shafts.

2. An optical torque sensor according to Claim 1 in which the discs comprise a pair of high precision metal discs.

3. An optical torque sensor according to Claim 1 or Claim 2 in which the hub component is a one-piece plastics moulding.

4. An optical torque sensor according to any preceding claim in which the flexible vanes are adapted so that the inner and outer rigid rings are angularly moveable relative to each other through a small angular distance about a central axis but are stiffly constrained to remain concentric and parallel.

5. An optical torque sensor according to any preceding claim in which a plurality of flat and parallel metal support plates are joined to the rings through pillars which are compliant to allow for differential thermal expansion between the hub and the support plates, the support plates forming connections between the discs and the hub component.

6. An optical torque sensor according to any preceding claim in which the support plates are joined to the pillars using a heat staking process.

7. An optical torque sensor according to Claim 6 in which the heat staking process comprises an ultrasonic process.

8. An optical torque sensor according to any one of Claims 5 to 7 in which the discs are bonded to the support plates using an adhesive.

9. An optical torque sensor according to any preceding claim in which a plurality of dogs are provided on the hub component.

10. An optical torque sensor according to any preceding claim in which limit stops are provided which extend axially and radially inwardly, respectively, to provide engagement between the input and output shafts and the inner and outer rings, respectively.

11. An optical torque sensor according to any preceding claim which is sealed on opposite sides of the optical discs by first and second seals each located on the inboard side of a respective one of a pair of bearings in which the input shaft is rotatably mounted.

12. An optical torque sensor according to any preceding claim in which relative rotational movement between the discs can be imparted by slots provided on the input shaft and dogs provided on the output shaft which engage with the rigid rings of the hub component.

13. An optical torque sensor having a hub component subtantially as described herein with reference to and as illustrated in the accompanying drawings.