A steering column assembly for a vehicle

Abstract

A steering column assembly (10) for a vehicle comprises a steering column (14) with a steering wheel (18) at one end and a gear (24) connected to the opposite end. A motor (28), controlled by control

Description

The present invention relates to steering column assemblies for vehicles and in particular to such assemblies for use with a steer-by-wire hand wheel actuator. In steer-by-wire arrangements, a handwheel (steering wheel) is connected to one end of a rotatably mounted shaft whose angular displacement is measured to generate a signal which is used to control the orientation of the steered wheels of the vehicle. The arrangement is commonly also provided with an electric motor connected to the shaft to provide a controlled amount of torque in the opposite direction to the torque applied by the driver, in order to provide a sensation of road feel to the driver. A typical steer-by-wire arrangement is shown in Figure 1, which illustrates a vehicle steering system in which a hand wheel actuator supports a steering wheel operated by a driver and measures the driver demand, usually the steering angle. A steering controller converts the driver demand as measured by the hand wheel actuator into a position demand that is sent to a front axle actuator. The front axle actuator controls the steering angle of the road wheels to achieve the position demand. The front axle actuator can feedback operating states and measurements to the steering controller. The steering controller combines the feedback from the front axle actuator with other information measured in the vehicle, such as lateral acceleration, to determine a target feedback torque that should be sensed by the driver. This feedback demand is then sent to the hand wheel actuator and is provided by controlling motors in the hand wheel actuator. In such arrangements, as shown schematically in Figure 2, typically two electric motors M1, M2 under the control of respective motor controls C1, C2 under the overall control of a hand wheel actuator control HWA C1 drive a worm screw engaged with a worm gear which rotates with the shaft to which the steering wheel is connected. It is desirable to take steps to bias the worm screw into engagement with the worm gear in order to reduce gear rattle which can occur when the torque and direction of the motor are reversed. In certain torque ranges, one of the motors supplies a torque to provide feedback to the hand wheel actuator while the other motor applies a smaller torque (an “offset torque”) in the opposite direction to provide an “active” lock to eliminate or reduce transmission rattle. The roles of the motors can change, depending on which direction the vehicle is being steered. A disadvantage of the worm and wheel type of gearbox is that it has a relatively high friction due to the sliding nature of the worm mechanism. The friction varies with the applied torque. At higher levels of driving torque the friction torque from the gearbox presents a large difference between the motor torque and the torque felt by the driver at the handwheel. It is an aim of the present invention to provide a steering column assembly that can compensate for the gearbox friction. In accordance with a first aspect of the present invention, a steering column assembly for a vehicle comprises: an elongate steering column mounted for rotation about its longitudinal axis and configured for attachment of a steering wheel at one end; a first gear connected to the steering column at a location spaced from the steering wheel attachment location and configured to rotate with the steering column; a first motor having an output shaft which is also displaceable axially, along its rotational axis, through a predetermined range; a first worm screw directly connected to, rotatable with and axially displaceable with the output shaft of the first motor and engaged with the first gear; biasing means configured to resist axial displacement of the output shaft in both axial directions; control means configured to operate the first motor; and motor position sensor means for sensing the rotational and axial positions of the motor output shaft, the sensor means comprising a target member fixedly mounted to the motor output shaft and a sensor for detecting the target member. The present invention provides an alternative way of estimating the torque provided by the motor which potentially allows the conventional torque sensor to be omitted, with a consequent cost benefit. This estimation of torque can be used to compensate for the friction in the mechanical components of the assembly, in order to provide a more accurate value of feedback torque to the handwheel of the vehicle. In accordance with a second aspect of the present invention, a steering column assembly for a vehicle comprises: an elongate steering column mounted for rotation about its longitudinal axis and configured for attachment of a steering wheel at one end; a first gear connected to the steering column at a location spaced from the steering wheel attachment location and configured to rotate with the steering column; first and second motors, each having an output shaft, each output shaft also being displaceable axially, along its rotational axis, through a predetermined range; biasing means configured to resist axial displacement of each output shaft in both axial directions; first and second worm screws directly connected to and rotatable with the output shaft of the first and second motors respectively and engaged with the first gear; control means configured to operate the first and second motors; and motor position sensor means for sensing the rotational and axial positions of the motor output shafts, each sensor means comprising a target member fixedly mounted to a respective motor output shaft and a sensor for detecting the target member. The present invention will typically be used with two motors for applying torque to the steering column via the first gear. Preferably, the or each worm screw forms part of the output shaft of its associated motor. The assembly may comprise a housing, and the output shaft of the or each motor may be slidably mounted with respect to the housing. For example, the output shaft of the or each motor may be rotatably mounted in first and second spaced-apart bearings, and the bearings may be slidably mounted in the housing. One or both of the bearings of the output shaft of the or each motor may be located in a bush mounted in the housing. Alternatively, the steering column assembly may further comprise a housing, and the output shaft of the or each motor may be constrained radially by the biasing means. In that arrangement, one or both of the bearings on the output shafts would be incorporated into the associated biasing means, which forms an axially compliant member, and there would be no requirement for axial sliding of the bearing / s, nor for an associated bush / es to facilitate such sliding. The steering column assembly may comprise first and second spaced-apart biasing means associated with the output shaft of the or each motor and configured to urge the associated output shaft in opposite directions. The output shaft of the or each motor may comprise first and second spaced-apart radially-extending projections configured to receive the force applied by the first and second spaced-apart biasing means respectively. The first and second spaced-apart radially-extending projections may be configured to engage the first and second spaced-apart bearings respectively. The first and second spaced-apart biasing means may be substantially identical. The first and second spaced-apart biasing means may comprise axial springs. In one embodiment, one or each of the first and second spaced-apart biasing means comprises a resiliently deformable material. The first and second spaced-apart biasing means may be configured to bias the associated motor output shaft into a position at the centre of its travel under quiescent conditions. The steering column assembly may comprise a reaction member, e.g. a reaction plate, engaged with one of the first and second spaced-apart biasing means. The or each reaction member may be configured to abut one of the bearings of the output shaft. The steering column assembly may further comprise a retaining member configured to retain one of the first and second spaced-apart biasing means. The reaction plate (e.g. a reaction plate) may be configured to abut the retaining member to limit the longitudinal displacement of the output shaft in a first direction. The target member may be located at one end of the output shaft of the or each motor. In one embodiment, the target member comprises a magnetic member. Preferably, the sensor is configured to detect the angular and longitudinal position of the target member. There may be a plurality of sensors for sensing the rotational and axial positions of the target member. As the target member rotates, each sensor measures a sinusoidal signal with each cycle / revolution of the associated motor output shaft. The two or more sinusoids can be mathematically combined and can be used to calculate the angle of the target member, e.g.) by an ATAN algorithm. For example, there may be a pair of sensors for sensing the rotational and axial positions of the target member. In one embodiment, the sensors are angularly spaced by 90° with respect to the rotational axis of the associated motor output shaft. There may be a plurality of pairs of sensors for sensing the rotational and axial positions of the target member. For example, there may be two pairs or three pairs of sensors. The sensors may be equally angularly spaced with respect to the rotational axis of the associated motor output shaft. The target member may be aligned coaxially with the rotational axis of the associated motor output shaft. The target member may be offset with respect to the rotational axis of the associated motor output shaft. The plurality of sensors may incorporated into a common integrated circuit, or may comprise discrete sensors. The rotational axes of the output shafts of the first and second motors may be substantially parallel. The first gear may be connected to the steering column at the opposite end of the steering column from the steering wheel. The worm screws may be positioned on opposite sides of the rotational axis of the first gear. The rotational axis of the output shafts of the or each motor is preferably oriented substantially perpendicularly to the rotational axis of the steering column. The present invention also includes a vehicle comprising a steering column assembly in accordance with the present invention. In accordance with a further aspect of the present invention, there is provided a method of operating a steering column assembly for a vehicle in accordance with the present invention, comprising determining a value for the angular displacement of the or each motor shaft from the motor position sensor means to determine direction of friction torque for compensation. In accordance with a further aspect of the present invention, there is provided a method of operating a steering column assembly for a vehicle in accordance with the present invention, comprising determining a value for the axial displacement of the or each motor shaft from the motor position sensor means to determine the direction of the friction torque for compensation. In accordance with a further aspect of the present invention, there is provided a method of operating a steering column assembly for a vehicle in accordance with the present invention, comprising determining a value for the axial displacement of the or each motor shaft from the motor position sensor means to control the worm thrust force in a closed-loop feedback. By way of example only, a specific embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a typical steer-by-wire steering column assembly for a road vehicle; Figure 2 is a schematic illustration of the general arrangement of a control system for a typical steer by wire steering column assembly for a road vehicle; Figure 3 is a perspective view, of an embodiment of steering column assembly in accordance with the present invention; Figure 4 is a vertical cross-section through the steering column assembly of Figure 3, looking in the direction of arrows IV - IV of Figure 3; Figure 5 is a horizontal cross-section through the steering column assembly of Figure 3, looking in the direction of arrows V - V of Figure 3; Figure 6 is a view of the portion of the steering column assembly indicated at A in Figure 4, to an enlarged scale; Figure 7 is a perspective view of a first example of target magnet assembly which can be used with the present invention; Figure 8 is a side view of the target magnet of Figure 7, also showing a target magnet sensor and magnetic field lines; Figure 9 is a schematic side view showing the forces on a gear and a worm screw during operation of the present invention; Figure 10 is a graph showing an example of the typical variation of the signals of two target magnets with angular displacement; Figure 11 is a graph illustrating the typical variation of field strength with distance between a target magnet and its associated sensor; Figure 12 is a plan view of a second example of target magnet assembly which can be used with the present invention; Figure 13 is a plan view of the target magnet assembly of Figure 12; Figure 14 is a plan view of a third example of target magnet assembly which can be used with the present invention; Figure 15 is a plan view of the target magnet assembly of Figure 14; Figure 16 is a plan view of a fourth example of target magnet assembly which can be used with the present invention, which is a variant of the target magnet assembly of Figure 12; Figure 17 is a plan view of a fifth example of target magnet assembly which can be used with the present invention, which is a variant of the target magnet assembly of Figure 14; Figure 18 is a block diagram illustrating a first control method of the steering column assembly in accordance with the present invention, which utilises an angle-based friction compensation scheme; Figure 19 is a block diagram illustrating a second control method of the steering column assembly in accordance with the present invention, which utilises a force-based detection scheme; Figure 20 is a flow diagram illustrating the friction compensation logic of the control method of Figure 19; and Figure 21 is a block diagram illustrating a third control method of the steering column assembly in accordance with the present invention, which utilises a closed-loop worm shaft torque control. A steer-by-wire steering column assembly 10 comprises a straight elongate steer-by-wire steering column 14 (only the lower end of which is illustrated) which is rotatably mounted about its longitudinal axis 16. A steering wheel 18 illustrated schematically in Figure 1 is secured to a splined portion 15 of the upper end of the steering column 14, by means of which the steering column 14 can be rotated by a driver. In the drawings, the steering column 14 is shown as a single shaft, but in practice it is likely to be formed from a number of components. For example, the steering column may be formed from several telescopic parts and may have a torque sensor (e.g. a torsion bar assembly) between the steering wheel end and the opposite end, although the present invention may remove the need for such a torque sensor. The opposite, lower end of the steering column 14 is received in a housing 20, in which it is rotatably mounted by means of two spaced-apart bearings 22a, 22b within the housing 20, as shown in Figure 5. As best seen in Figures 4 to 6, a spur gear (pinion gear) 24 is fixedly mounted on the lower end of the steering column 14 between the two bearings 22a, 22b, and is thereby constrained to rotate with the steering column 14 and steering wheel 18. The housing 20 also encloses two identical electric motors 28, 30, each having a respective identical motor output shaft 32, 34 driven directly by the respective motor 28, 30. The output shafts 32, 34 extend with their longitudinal axes 36, 38 parallel to each other and perpendicular to the rotational axis 16 of the steering column 14 and each is formed into a respective worm screw portion 40, 42 towards its upper end. The worm screw portions 40, 42 are identical, and are meshed with the spur gear (pinion gear) 24 at diametrically opposed positions on opposite sides of the gear. Each shaft 32, 34 is rotatably mounted within the housing 20 by means of upper and lower bearings 46, 48 located just beyond the upper and lower ends respectively of the worm screws portions 40, 42, the axially inner face of each bearing 46, 48 being seated on a radially extending shoulder formed by a respective peripheral radial flange portion 54, 56 of the output shaft 32, 34. Figure 6 shows the portion of the steering column assembly 10 indicated at A in Figure 4 to a larger scale and shows the mounting of one motor output shaft 32, but the mounting of the other motor output shaft 34 is identical. The bearings 46, 48 support their respective shafts 32, 34 both radially and axially and are mounted in the housing via a low-friction bush 60 which allows the bearings to be displaced in a direction along the axes 36, 38 of the respective motor output shaft 32, 34. Each bearing 46, 48 is associated with an annular axial spring 62 located around the respective shaft 32, 34, which exerts a thrust on the bearing via a spring reaction plate 64. The spring reaction plate 64 is formed from an annular plate portion 65 having planar inner and outer surfaces 66a, 66b extending perpendicularly to the rotational axis 36, 38 of the respective motor output shaft 32, 24, and a circumferentially extending peripheral tubular rim 67 extending above the level of the inner and outer surfaces 66a, 66b of the annular plate portion 65. As best seen in Figure 6, the longitudinally inner end of the peripheral tubular rim 67 of the reaction plate 64 is urged towards engagement with the associated bearing 46, 48 by means the associated axial spring 62 which is retained in contact with the longitudinally outer face of the spring reaction plate 64 by means of a respective axial spring retainer plate 68 which is releasably secured to the housing by bolts 69. The spring retainer plate 68 also forms a travel limiter which is engaged by the longitudinally outer edge of the peripheral annular rim 66 of the reaction plate 64 when the associated motor output shaft 32, 34 has been displaced by a predetermined distance along its longitudinal axis 36, 38, which constrains the movement of the motor output shaft and limits the stress on the axial spring 62. In the embodiment shown, the axial springs 62 are each in the form of a deformable annular rubber washer, but any arrangement which exerts an increase in force with axial displacement can be used, for example a disc spring or a compression spring. As shown in the figures, the axial springs 62 associated with the upper and lower bearings 46, 48 act in opposite directions on the respective motor output shaft 32, 34. Preferably, the two axial springs 62 acting on each output shaft 32, 34 are preloaded together and the force / displacement characteristics of the axial springs 62 and the preload are preferably selected so that each motor output shaft 32, 34 is biased into a position at the centre of its travel under quiescent conditions. Preferably, the force / displacement of the axial springs 62 is substantially linear so that the force is approximately proportional to the displacement. Preferably, the motors 28, 30 are designed so that their torque output is not affected by the axial displacement of the shafts 32, 34. A disc-shaped motor position sensor (MPS) target magnet 70, 72 is fixedly secured to the end of each motor output shaft 32, 34 adjacent to the respective motor 28, 30, i.e. on the same side of the worm screw 40, 42 as the motor, by means of a respective mounting cup 74, 76 fixedly mounted on the end of each motor output shaft 32, 34 and rotatably mounted in respective apertures 78, 80 in a lower wall 82 of the housing 20. The MPS target magnets 70, 72 thereby move with their respective motor output shafts 32, 34 and both their rotation and their axial position (and thereby the rotation and axial position of the shafts 32, 34) are detected by a respective MPS sensor 84, 85 mounted on the inner face of a planar MPS sensor circuit board 86 mounted at the base of the housing 20 and extending parallel to the longitudinal axis 16 of the steering column 14 and perpendicular to the longitudinal / rotational axes 36, 38 of the output shafts 32, 34. The MPS target magnets 70, 72 and the MPS sensors 84, 85 are known, and different types of magnets and sensors may be used depending on the circumstances. The MPS target magnets 70, 72 and sensors 84, 85 may alternatively be mounted at the opposite end of the shafts 32 and 34 compared with that shown in the figures, i.e. on the opposite side of the worm screws 40, 42 from the associated motors 28, 30. One example of target magnet 70, 72 is shown in Figures 7 and 8. In this example, the target magnet comprises a cylindrical magnet assembly formed from two semi-cylindrical halves 90, 92 which are of the same dimension and each of which has a planar face 94 which abuts the planar face 94 of the other half 90, 92 to form a cylindrical magnet assembly. The magnet halves 90, 92 are magnetised with opposite polarity to produce a field F (shown schematically in Figure 8) that rotates with the associated motor output shaft 32, 34. The MPS sensors 84, 85 are connected to monitoring and control circuitry on the MPS sensor circuit board 86 which is configured to determine both (a) the rotational position of the target magnet 70, 72 (and therefore the rotational position of the associated motor output shaft 32, 34) and (b) the axial position of the target magnet 70, 72 (and therefore the linear displacement / axial position of the associated motor output shaft 32, 34 along its respective rotational axis 36, 38). Signals from the MPS sensors 84, 85 are fed an electronic control unit (ECU, not shown) which controls the operation of the two motors 28, 30 in order to apply a desired torque from each motor 32, 34 to the steering column 14 and thereby to the steering wheel 18 in a known manner, as will be described. The MPS sensing element can be implemented using several different technologies including Hall effect, AMR, GMR, and TMR. In general several sensing elements are implemented on a single semiconductor die. These elements may be combined in a bridge arrangement to align with the field of the target magnet at different angles. As the magnet rotates, these elements provide sinusoidal signals with a known phase offset. Figure 10 shows a pair of sinusoids in quadrature but other sensor may have more elements to give 3-phase or 4-phases. As the distance between the target magnet and the sensor element changes, the overall field strength detected by the sensing elements varies. Figure 11 is a graph showing the typical variation of field strength (y-axis) against the distance between the target magnet and the sensor element. Typically there is a portion of the operating condition in which the field strength variation is substantially linear, and it can be made more accurate by compensating for inherent nonlinearities, the operating temperature and any errors that can be determined a priori, such as mechanical runout. Therefore the angle (angular displacement or orientation) of the magnets can be determined from the phase of the quadrature signals and their axial position can be determined from the overall amplitude of the signals. It is sometimes desirable to have multiple sensing elements to ensure continued operation in the event of a fault. This can be achieved by adding extra sensing elements (e.g. additional semiconductor dies) that measure the same target magnet. In use, the steering assembly 10 is installed in a vehicle, and typically the steering column 14 is inclined to the horizontal by around 20° to 25°. Rotation of the steering wheel 18 by a driver results in rotation of the steering column 14 which is measured in a known manner and used to control the orientation of the steered wheels of the vehicle by means of an electronic control unit in a steer-by-wire manner, i.e. without any direct mechanical connection between the steering wheel and the steered wheels. Rotation of the worm screw portions 40, 42 by their respective motors 28, 30 is controlled by the ECU to apply feedback torque to the steering column 14 and the steering wheel 18, in order to provide a sensation of road feel to the driver. By using two worm screws 40, 42, the torque applied to each of them can be controlled in order to reduce backlash and gear rattle. Rotation of the motor output shafts 32, 34, and thereby of the worm screws 46, 48, results in rotation of the annular MPS magnets 70, 72 mounted on the motor output shafts 32, 34 on the opposite side of the worm screws 46, 48 from the respective motors 28, 30. Rotation of the MPS magnets 50, 52 is detected by the respective motion position sensors 58, 60 on the MPS sensor board 62 and is used by the ECU formed by the PCBs 66, 68 to control the motors 28, 30 in order to apply an appropriate amount of torque to the worm screws 46, 48. When torque is applied to the motor output shaft, the worm will apply a force to the gearwheel. The force on the motor output shaft 32 / 34 will move the motor output shaft 32 / 34 away from its central position until it is reacted by the axial spring. The thrust that the motor output shaft 32 / 34 applies to the gearwheel is reacted by the compliant elements and consequently the axial movement of the motor output shaft 32 / 34 will indicate the direction of the force the worm is applying to the gearwheel. If the friction in the axial movement is low and the compliant elements act in a proportional manner, it is possible to estimate the force applied to the gearwheel by the worm and hence the torque applied to the gearwheel by each motor output shaft 32 / 34. As shown in Figure 9, when a torque is applied to the motor output shaft 32 / 34, the worm 40 / 42 will apply a force F to the gearwheel 24. The force on the motor output shaft 32 / 34 will move the it away from its central position until it is reacted by one of the axial springs 62. The thrust that the motor output shaft 32 / 34 applies to the gearwheel 24 is reacted by one of the axial springs 62 (depending on the direction of the thrust applied to the gearwheel 24) and consequently the axial movement of the motor output shaft 32 / 34 will indicate the direction of the force F which the worm 40 / 42 is applying to the gearwheel 24. If the friction in the axial movement is low and the axial springs 62 are configured to act in a proportional manner, it is possible to estimate the force applied to the gearwheel by the worm 40 / 42 and hence the torque applied to the gearwheel 24 by each motor output shaft 32 / 34. A second example of magnet assembly, is illustrated in Figures 12 and 13. The magnet assembly is formed from two semi-annular magnetic discs 95, 96 mounted on the outer face of a circular disc-shaped magnet carrier 97 which is mounted coaxially to one end of one of the motor output shaft 32 / 34 (the lower end in this example). The semi-annular magnetic discs 95, 96 are of the same dimension and are magnetised with opposite polarity to produce a field F (similar to that shown schematically in Figure 8) that rotates with the associated motor output shaft 32 / 34. In addition, two identical, individual magnetic field sensors 98a, 98b (which in this embodiment are linear Hall effect sensors) are mounted on the MPS sensor circuit board 86, equidistant from the rotational axis 36 / 38 of the associated motor output shaft 32 / 34 (and of the disc-shaped magnet carrier 97) and angularly spaced apart by an angle a = 90°. As the magnetic semi-annular magnetic discs 95, 96 rotate, each magnetic field sensor 98a, 98b measures a sinusoidal signal with each cycle / revolution of the motor output shaft 32 / 34. The two or more sinusoidal signals can be mathematically combined and can be used to calculate the angle of the magnet, for example by using an ATAN algorithm. A third example of magnet assembly, which is a modification of the arrangement of Figures 12 and 13, is illustrated in Figures 14 and 15. Most of the features of the third example are the same as the second example, and corresponding features are identified by the same reference numerals. The only difference is that four identical, individual magnetic field sensors 98a, 98b, 98c, 98d (which in this embodiment are linear Hall effect sensors) are mounted on the MPS sensor circuit board 86, equidistant from the rotational axis 36 / 38 of the associated motor output shaft 32 / 34 and angularly spaced apart from the adjacent sensors by an angle a = 90°, each of the sensors therefore being diametrically opposed to another one of the sensors with respect to the rotational axis 36 / 38. The use of four magnetic field sensors 98a, 98b, 98c, 98d allows compensation for mechanical runout. Additional pairs of magnetic field sensors can be added (e.g. six devices spaced 60° apart) to improve runout and stray field compensation. In the arrangements illustrated in Figures 12 to 15, the centre of the circular disc-shaped magnet carrier 97 is aligned coaxially with the rotational axis 36 / 38 of the motor output shaft 32 / 34, and consequently the magnetic field sensors 98a, 98b, 98c, 98d are always at the same radial position of each of the two semi-annular magnetic discs 95, 96. In the modified arrangements illustrated in Figures 16 and 17 (in which features corresponding to the arrangements shown in Figures 12 to 15 are identified by the same reference numerals), the centre 99 of the circular disc-shaped magnet carrier 97 is offset by a distance d from the rotational axis 36 / 38 of the motor output shaft 32 / 34. This results in a continuous variation of the radial position of the two semi-annular magnetic discs 95, 96 with respect to the two magnetic field sensors 98a, 98b (Figure 16) and 98a, 98b, 98c, 98d (Figure 17). For the arrangement of Figure 16, with two magnetic field sensors 98a, 98b angularly spaced apart by 90° with respect to the rotational axis 36 / 38 of the motor output shaft 32 / 34, the sensors 98a, 98b will produce two sine waves, with a relative phase shift of 90°. If the signals from the sensors 98a, 98b are identified as A and B respectively, the angle is given by arctan2(A,B) and the amplitude by sqrt(A2+B2). For the arrangement of Figure 17, with four magnetic field sensors 98a, 98b, 98c, 98d angularly spaced apart by 90° with respect to the rotational axis 36 / 38 of the motor output shaft 32 / 34, a sine and cosine can be generated by combining the signals from the sensors. As one example, if the signals from the sensors 98a, 98b, 98c, 98d are identified as A, B, C and D respectively, X = A+ B + C- D and Y = A- B- C + D. The angle and the amplitude can then be found in the same manner as for the 2-sensor case using X and Y, i.e. arctan2(X,Y) and sqrt(X2+Y2). Runout of the motor output shaft 32 / 34 will increase errors in these values but there are well-known processing techniques than can partially compensate for the runout errors. Without compensation, the amplitude measurement can be more sensitive to runout than the angle measurement. In all of the arrangements described above, the sensors can be implemented either using discrete sensor devices (e.g. spaced on a PCB) or within a single integrated circuit. For the purpose of measuring the airgap between the magnets and the sensors as well as the angle, there may some advantage in using discrete sensors that can be spaced further apart on a PCB than in an integrated circuit, although either arrangement may be used. The greater distance between devices will tend to reduce the sensor sensitivity to runout errors of a given magnitude. Three different control methods are disclosed as follows: • Use of MPS angular measurement to determine direction of friction torque for compensation • Use of MPS axial displacement measurement to determine the direction of the friction torque for compensation • Use of the MPS axial displacement to control the worm thrust force in a closed-loop. These control methods operate in the region in which the motor output shaft 32 / 34 is free to move. At higher loads, the motor output shaft 32 / 34 will reach its travel limit. Additional features in the controller will identify and react to this condition and modify the control strategy, but this is not described here. Another complexity that is omitted is the behaviour of the controller at different limiting conditions (such as maximum motor current and minimum supply voltage) and behaviour in a faulty state. In this description and corresponding drawings, the terms “motor output shaft” and “wormshaft” are used interchangeably. It should also be noted that in Figures 18, 19 and 21, the thick lines denote a vector of multiple signals. Figure 18 shows a source for the overall HWA feedback torque demand which is determined according to the need of the SbW control system (see Figure 1) and which utilises an anglebased friction compensation scheme. A closed-loop torque regulator 100 sets the target gearbox torque demand using an observer 102 and a post-filter 104 to estimate the current gearbox torque at block 106. The regulator 100 is responsible for stabilising the system and achieving an adequate time response. This feedback torque demand is then split into the two motor torque demands by the allocation block 110. Each motor 28, 30 is then controlled by a respective motor control block 112, 114 to achieve the demanded torque. This includes parameters such as the calculation of motor current demand according to the torque demand, control of the motor phase currents using an inverter in the ECU and angular position information from the MPS. Both of the motors are coupled into the mechanical system 116 and the resulting movement (angular and longitudinal) of the motor output shafts 32 / 34 is measured by the MPS 84, 85 at the end of each motor output shaft 32 / 34. The MPS measurements are used at block 118 to determine the system state that includes the angle and angular velocity of both of the motor output shafts 32 / 34. The system state is fed back to the observer 102 and the closed loop regulator 100. When the motor output shaft 32 / 34 is stationary, the gearwheel is “locked” to the motor output shaft 32 / 34 by the friction between the worm and wheel. Due to the allocation strategy, at low to medium loads, one motor 28, 30 will be lightly loaded and thus have a low friction. When the driver starts to move the handwheel away from a stationary condition, the high-torque motor output shaft 32 / 34 will remain locked and the motor output shaft 32 / 34 will move axially until the torque is high enough to break away from the friction. In contrast, the lightly loaded motor output shaft 32 / 34 will be free to rotate. The friction compensation block 108 has to determine level and direction of the compensation torque. The level of friction compensation torque is determined from the overall level of torque to be applied and the system state. When the actuator is moving, the friction direction can be determined. When the actuator is stationary, the direction of the friction torque can be difficult to determine. The friction compensation block 108 shown in Figure 12 identifies this condition by monitoring the difference between the MPS angular positions in the stuck / stationary state. When one motor output shaft 32 / 34 starts to move, the direction of the friction can be determined and the friction compensation can be applied in the correct direction to overcome the load-dependent friction. Thus the additional compliance of the motor output shaft 32 / 34 allows the driver to move the wheel by a small amount in the stationary / stuck mode and provides time to identify which direction the friction compensation torque shall be applied. A second control method which utilises the axial measurement of the motor output shaft 32 / 34 deflection that can be obtained from the MPS 84, 85 is shown in Figure19. Many of the features of this control methods are similar to the first control method shown in Figure 18, and the same reference numerals are used to identify the same features. The block diagram is similar to Figure 12. The main differences are that the calculated system state is augmented by blocks 120 which each estimate the axial forces on each worm shaft calculated from the measured motor output shaft 32 / 34 displacements in the axial direction. The calculate system state block 118 combines the motor angle and velocity with the estimated motor output shaft 32 / 34 forces. The friction compensation block 108 uses the change in estimated motor output shaft 32 / 34 force to determine the direction of the friction compensation in the stuck / stationary state. The friction compensation demand is calculated according to the direction of the friction and the estimated magnitude. The estimated magnitude is proportional to the applied torque and is therefore calculated from the overall torque demand. The friction compensation direction can be calculated from the sign of the velocity of the motor output shaft 32 / 34 when the motor output shaft 32 / 34 is moving. When the motor output shaft 32 / 34 is stationary, the gearwheel is “locked” to the motor output shaft 32 / 34. Hence movement of the motor output shaft 32 / 34 can be used to determine the direction the driver is applying a net torque to the handwheel. The flowchart shows that an initial estimate of the motor output shaft 32 / 34 force is recorded close to the instance when the velocity becomes zero. This is done for both motors. If either estimated motor output shaft 32 / 34 force changes from the recorded value, the change in estimated force is used to estimate the direction of movement of the gearwheel and hence the direction that the friction compensation shall act. The principle of the second control method is illustrated further in Figure 20, in which “step” is abbreviated to “S”. The friction compensation direction can be calculated from the sign of the velocity of the wormshaft when the wormshaft is moving. When the wormshaft is stationary, the gearwheel is “locked” to the wormshaft. Hence movement of the wormshaft can be used to determine the direction the driver is applying a net torque to the handwheel. The flowchart shows that an initial estimate of the wormshaft force is recorded close to the instance when the velocity becomes zero. This is done for both motors. If either estimated wormshaft force changes from the recorded value, the change in estimated force is used to estimate the direction of movement of the gearwheel and hence the direction that the friction compensation shall act. Specifically, at S200 it is determined whether the motor velocity is at or near zero. If no, then at S202 it is determined to use velocity to determine the friction direction and at S204 the friction compensation torque demand is calculated. On the other hand, if it is determined at S200 that the motor velocity is not at or near zero, then at S206 the estimated worm shaft force is recorded and subsequently at S208 the system then waits for either forced to change. When either force has changed, then at S210 the direction of force is used to determine the friction direction and at S204 the friction compensation torque demand is calculated. A third control method, shown in Figure 21, uses the force estimated from the axial measurement of the position of the motor output shafts 32 / 34 directly in a closed-loop controller. Many of the features of this control methods are similar to the first and second control methods shown in Figures 18 and 19, and the same reference numerals are used to identify the same features. In this arrangement, the overall HWA feedback torque demand is determined according to the need of the SbW control system (see Figure 1). The total gearwheel torque is estimated in block 122 from the measured system state (that includes the estimated worm axial force) calculated at block 118 and post-filtered. The closed-loop torque regulator block 100 then determines an overall gearwheel torque demand which is allocated via the torque torque demand allocation block 110 and to the two motor torque demands. Each motor then has a closed-loop controller block 124 that uses the estimate of the motor output shaft torque (obtained from the axial movement of the motor output shaft 32 / 34) as the principal feedback component. The closed-loop wormshaft torque controller blocks 124 determine the target motor torque for each motor 28, 30. The controller blocks 124 must be stable in the case of the wormshaft axial position oscillating and will substantially compensate for the friction between the worm and gear wheel 24 since it is controlling the net force that is applied to the gearwheel in all conditions. Figure 21 shows that the MPS displacement measurement is used in blocks 126 to calculate an estimate of torque applied to the gearwheel by each motor 28, 30. This is compared to the scaled torque demand for each motor and the error is used to update the demand to the motor current control (which uses the angle form the MPS). Finally the ECU drivestage is controlled to achieve the required level of current in the motor windings. At higher levels of torque, the wormshaft torque estimate will be saturated and the controller will have to switch to an open-loop control method (switching and open-loop controller not shown). The invention is not restricted to the details of the foregoing embodiment. For example, the magnetic target members 50, 52 and their associated sensors 58, 60 may 5 be replaced with other means of detecting the angular and longitudinal positions of the output shafts of the first and second motors. For example, the detecting means may comprise optoelectronic sensing means, a variable reluctance resolver (in which case the target member would be the resolver rotor) or some other means for detecting the angular and longitudinal positions of the motor output shafts.

Claims

1. A steering column assembly for a vehicle, comprising:an elongate steering column mounted for rotation about its longitudinal axis and configured for attachment of a steering wheel at one end;a first gear connected to the steering column at a location spaced from the steering wheel attachment location and configured to rotate with the steering column;a first motor having an output shaft which is also displaceable axially, along its rotational axis, through a predetermined range;a first worm screw directly connected to, rotatable with and axially displaceable with the output shaft of the first motor and engaged with the first gear;biasing means configured to resist axial displacement of the output shaft in both axial directions;control means configured to operate the first motor; andmotor position sensor means for sensing the rotational and axial positions of the motor output shaft, the sensor means comprising a target member fixedly mounted to the motor output shaft and a sensor for detecting the target member.

2. A steering column assembly for a vehicle, comprising:an elongate steering column mounted for rotation about its longitudinal axis and configured for attachment of a steering wheel at one end;a first gear connected to the steering column at a location spaced from the steering wheel attachment location and configured to rotate with the steering column;first and second motors, each having an output shaft, each output shaft also being displaceable axially, along its rotational axis, through a predetermined range;biasing means configured to resist axial displacement of each output shaft in both axial directions;first and second worm screws directly connected to and rotatable with the output shaft of the first and second motors respectively and engaged with the first gear;control means configured to operate the first and second motors; andmotor position sensor means for sensing the rotational and axial positions of the motor output shafts, each sensor means comprising a target member fixedly mounted to a respective motor output shaft and a sensor for detecting the target member.

3. A steering column assembly as claimed in claim 1 or claim 2, wherein the or each worm screw forms part of the output shaft of its associated motor.

4. A steering column assembly as claimed in any of the preceding claims, further comprising a housing, and wherein the output shaft of the or each motor is slidably mounted with respect to the housing.

5. A steering column assembly as claimed in claim 4, wherein the output shaft of the or each motor is rotatably mounted in first and second spaced-apart bearings, and wherein the bearings are slidably mounted in the housing.

6. A steering column assembly as claimed in claim 5, wherein one or both of the bearings of the output shaft of the or each motor is located in a bush mounted in the housing.

7. A steering column assembly as claimed in any of claims 1 to 4, wherein the steering column assembly further comprises a housing, and the output shaft of the or each motor is constrained radially by the biasing means.

8. A steering column assembly as claimed in any of the preceding claims, comprising first and second spaced-apart biasing means associated with the output shaft of the or each motor and configured to urge the associated output shaft in opposite directions.

9. A steering column assembly as claimed in claim 8, wherein the output shaft of the or each motor comprises first and second spaced-apart radially-extending projections configured to receive the force applied by the first and second spaced-apart biasing means respectively.

10. A steering column assembly as claimed in claim 8 or claim 9 when appendant to claim 5 or claim 6, wherein the first and second spaced-apart radially-extending projections are configured to engage the first and second spaced-apart bearings respectively.

11. A steering column assembly as claimed in claim 10, wherein the first and second spaced-apart biasing means are substantially identical.

12. A steering column assembly as claimed in any of claims 8 to 11, wherein the first and second spaced-apart biasing means comprise axial springs.

13. A steering column assembly as claimed in claim 12, wherein one or each of the first and second spaced-apart biasing means comprises a resiliently deformable material.

14. A steering column assembly as claimed in any of claims 8 to 13, wherein the first and second spaced-apart biasing means are configured to bias the associated motor output shaft into a position at the centre of its travel under quiescent conditions.

15. A steering column assembly as claimed in any of claims 8 to 14, comprising a reaction member engaged with one of the first and second spaced-apart biasing means.

16. A steering column assembly as claimed in claim 14, wherein the or each reaction member is configured to abut one of the bearings of the output shaft.

17. A steering column assembly as claimed in claim 16, further comprising a retaining member configured to retain one of the first and second spaced-apart biasing means.

18. A steering column assembly as claimed in claim 17, wherein the reaction plate is configured to abut the retaining member to limit the longitudinal displacement of the output shaft in a first direction.

19. A steering column assembly as claimed in any of the preceding claims, wherein the target member is located at one end of the output shaft of the or each motor.

20. A steering column assembly as claimed in claim 19, wherein the target member comprises a magnetic member.

21. A steering column assembly as claimed in any of the preceding claims, wherein the sensor is configured to detect the angular and longitudinal position of the target member.

22. A steering column assembly as claimed in any of the preceding claims, comprising a plurality of sensors for sensing the rotational and axial positions of the target member.

23. A steering column assembly as claimed in claim 22, comprising a pair of sensors for sensing the rotational and axial positions of the target member.

24. A steering column assembly as claimed in claim 23, comprising a plurality of pairs of sensors for sensing the rotational and axial positions of the target member.

25. A steering column assembly as claimed in any of claims 22 to 24, wherein the target member is aligned coaxially with the rotational axis of the associated motor output shaft.

26. A steering column assembly as claimed in any of claims 22 to 24, wherein the target member is offset with respect to the rotational axis of the associated motor output shaft.

27. A steering column assembly as claimed in any of claims 22 to 26, wherein the plurality of sensors are incorporated into a common integrated circuit.

28. A steering column assembly as claimed in claim 2 or any of claims 3 to 27 when appendant to claim 2, wherein the rotational axes of the output shafts of the first and second motors are substantially parallel.

29. A steering column assembly as claimed in any of the preceding claims, wherein the first gear is connected to the steering column at the opposite end of the steering column from the steering wheel.

30. A steering column assembly as claimed in claim 2 or any of claims 3 to 29 when appendant to claim 2, wherein the worm screws are positioned on opposite sides of the rotational axis of the first gear.

31. A steering column assembly as claimed in any of the preceding claims, wherein the rotational axis of the output shafts of the or each motor is oriented substantially perpendicularly to the rotational axis of the steering column.

32. A vehicle comprising a steering column assembly as claimed in any of the preceding claims.

33. A method of operating a steering column assembly for a vehicle as claimed in any of the preceding claims, comprising determining a value for the angular displacement of the or each motor shaft from the motor position sensor means to determine direction of friction torque for compensation.

34. A method of operating a steering column assembly for a vehicle as claimed in any of the preceding claims, comprising determining a value for the axial displacement of the or each motor shaft from the motor position sensor means to determine the direction of the friction torque for compensation.

35. A method of operating a steering column assembly for a vehicle as claimed in any of the preceding claims, comprising determining a value for the axial displacement of the or each motor shaft from the motor position sensor means to control the worm thrust force in a closed-loop feedback.

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