ELECTROMAGNETIC CONTROL APPARATUS FOR VARYING THE DRIVER STEERING EFFORT OF A HYDRAULIC POWER STEERING APPARATUS

Abstract: A hydraulic power steering apparatus having conventional relatively rotatable valve spool (54) and valve body (64) for flow regulation, a mechanically resilient element (90) for generating a mechanical centring force in relation to the relative rotation of the valve spool and valve body which tends to restore the valve spool and valve body to a centred condition, and an integral electromagnetic mechanism (92) which variably opposes the mechanical centring force to adjust the level of operator steering input required to produce a given level of power assist. The electromagnetic mechanism comprises an exciting coil (130) and a pair of flux conducting elements (106,108) supported for rotation with the valve spool and valve body, respectively. The flux conducting elements have interleaved teeth (112,114) which are magnetically coupled to the exciting coil to define a magnetic circuit having an inter-tooth working air gap which varies with the relative rotation of the valve spool and valve body.

Description: This invention relates to an automotive hydraulic power assist steering apparatus, and more particularly to an electromagnetic control apparatus for varying the driver steering effort required to produce a given level of power assist. The conventional hydraulic power assist steering apparatus comprises a hydraulic actuator for moving the steering linkage in relation to the fluid flow supplied thereto, and a rotary hydraulic control valve assembly for controlling fluid flow to the actuator in relation to the operator exerted steering torque. The control valve generally includes a cylindrical valve body rotatable within the valve housing and a valve spool rotatably disposed within the valve body. Hydraulic fluid is supplied to a cavity formed in the valve spool, and the valve body is grooved to receive fluid flow in relation to the amount of relative rotation between valve spool and valve body. The fluid so received is then directed to the actuator so that steering assist is developed in relation to the relative rotation of the valve body and valve spool. The valve spool is manually rotated by the operator of the vehicle and is connected to mechanically drive the steering linkage through a lost motion coupling. A resilient element, such as a torsion bar, couples the valve spool and valve body to provide a centring force for aligning the valve spool and valve body, and to permit relative rotation therebetween in relation to operator exerted steering torque, at least within the limitations of the lost motion coupling. In apparatus of the type described above, the level of driver steering effort assist required to produce a given level of power assist depends primarily on the compliance of the torsion bar. If the torsion bar has relatively high compliance, a relatively low level of driver steering effort is required. This is generally desirable in low speed operation of a vehicle where relatively high steering forces are required. If the torsion bar has relatively low compliance, a relatively high level of driver steering effort is required. This is generally desirable in high speed operation of a vehicle where relatively low steering forces are required. To overcome the engineering trade-off described above, various arrangements have been proposed for varying the driver steering effort for a given level of power assist as a function of vehicle speed. An example of one such arrangement is given in US-A-4,629,025. In that arrangement, a controlled portion of the hydraulic fluid pump output is returned to the reservoir of the fluid pump to reduce fluid flow to the steering actuator with increasing vehicle speed. JP-A-61-241271 and JP-A-62-43364 describe electromagnetic control apparatus for varying the driver steering effort in accordance with the preamble of claim 1. Hydraulic power assist steering apparatus in accordance with the present invention comprises first and second hydraulic elements which are relatively rotatable and which are connected between a steering shaft and a steering gear for generating hydraulic flow in relation to their relative rotation and apparatus for variably coupling the first and second hydraulic elements to vary the level of steering assist produced for a given driver manipulation of the steering shaft, the apparatus comprising mechanical resilient means connected between the first and second hydraulic elements for developing a mechanical centring force in relation to relative rotation thereof which tends to restore the first and second hydraulic elements to a centred condition of minimum hydraulic flow; and electromagnetic circuit means including an exciting coil and first and second flux conducting elements magnetically coupled to the exciting coil and supported for rotation with the first and second hydraulic elements, respectively; said hydraulic power assist steering apparatus being characterised in that the first and second flux conducting elements have teeth which define a magnetic circuit having a working air gap which varies with the relative rotation of the first and second hydraulic elements, the flux conducting elements being angularly oriented such that the teeth of one flux conducting element are equally attracted to juxtaposed teeth of the other flux conducting element when the first and second hydraulic elements are in the centred condition, whereby when the first and second hydraulic elements are relatively displaced from the centred condition, the exciting coil and flux conducting elements develop a net magnetic attractive force which opposes the mechanical centring torque in relation to the energization of the exciting coil. The present invention is directed to a hydraulic power assist steering apparatus having conventional relatively rotatable valve spool and valve body elements for flow regulation, a mechanically resilient element (such as a torsion bar) for generating a mechanical centring force/torque in relation to the relative rotation of the valve spool and valve body which tends to restore the valve spool and valve body to a centred condition of low hydraulic flow, and an integral electromagnetic mechanism which variably opposes the mechanical centring force/torque for adjusting the driver steering effort required to produce a given relative rotation of the valve spool and valve body, and hence, a given level of power assist. The integral electromagnetic mechanism of this invention comprises an exciting coil and first and second flux conducting elements supported for rotation with the valve spool and valve body, respectively. The flux conducting elements have interleaved teeth which are magnetically coupled to the exciting coil, thereby to define a magnetic circuit having an inter-tooth working air gap which varies with the relative rotation of the valve spool and valve body. The flux conducting elements are relatively oriented such that the teeth of one element are equally displaced from juxtaposed teeth of the other element when the valve spool and valve body are in a centred relation for minimum hydraulic flow. When driver exerted steering force displaces the valve spool and valve body from the centred relation, the coil and flux conducting elements cooperate to produce a net magnetic attractive force which opposes the mechanical centring force of the resilient element, thereby defining an overall centring force which is less than the mechanical centring force. The resilient element is designed to be relatively stiff (low compliance), as is generally desired at relatively high vehicle speeds. The coil current is scheduled in relation to vehicle speed and the relative displacement of the valve spool and valve body so that the effective compliance of the coupling is effectively increased with decreasing vehicle speed, thereby reducing the driver steering effort required to produce a given level of steering assist with decreasing vehicle speed. The relative displacement dependence is employed to linearize the magnetic attractive force. A driver preference input may also be used to tailor the response for individual tastes. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1a is a sectional view of a hydraulic control valve assembly incorporating the integral electromagnetic mechanism of this invention and a block diagram of a computer-based control unit therefor; Figure 1b is a section of the valve assembly of Figure 1a along the line 1b - 1b; Figures 2a and 2b schematically depict linearized views of the magnetic flux conducting elements of the mechanism depicted in Figures 1a - 1b; Figure 3a graphically depicts the nonlinear attractive force generated by the electromagnetic mechanism of Figures 1a - 1b as a function of relative displacement for various coil current magnitudes; Figure 3b graphically depicts the linearized attractive force generated by the electromagnetic mechanism of Figures 1a - 1b as a function of relative displacement for various coil current magnitudes; Figure 3c graphically depicts the scheduled relationship between coil current and relative displacement for various vehicle speeds; Figure 4 graphically depicts the overall variation in driver steering effort for various vehicle speeds; and Figure 5 is a flow diagram representative of computer program instructions executed by the computer based control unit of Figure 1a in controlling the excitation of the electromagnetic mechanism of this invention. Referring to Figures 1a and 1b, the reference numeral 10 generally designates a portion of a (motor vehicle hydraulic power assist rack and pinion) steering gear. The steering gear 10 is disposed within a two-piece housing 12 having a (cylindrical smooth wall) bore 14 formed therein. A (cylindrical rotary power steering gear) valve assembly 16 disposed within the bore 14 includes an (elongated cylindrical) spool shaft 18 mounted for turning movement in the two-piece housing 12 by a bearing assembly 20. The inboard end of spool shaft 18 projects through an annular fluid seal 22 for connection to a conventional steering shaft and operator manipulated handwheel, not shown. The outboard end of the spool shaft 18 is splined as indicated by the reference numeral 24 with an (elongated) pinion gear 28 to define a lost motion mechanical coupling therebetween. The pinion gear 28 is rotatably mounted in the two-piece housing 12 by a sleeve bearing 30 and a ball bearing assembly 32 which receives a shank portion 34 of pinion gear 28. A nut 36 threaded onto the outboard end of the shank portion 34 secures the pinion gear 28 with the housing 12. A cup-like cover 38 frictionally fits into the end of the two-piece housing 12 to provide service access. The teeth of pinion gear 28 mesh with a toothed portion 40 of an (elongated) rack 42 mounted for linear sliding movement within the two-piece housing 12. The rack 42 is operatively connected to the steerable wheels of the vehicle by suitable ball joints and tie rods, not shown. In such an arrangement, the linear movement of the rack 42 turns the steerable wheels of the vehicle for vehicle steering purposes. The rack 42 is also coupled to a fluid operated power cylinder mechanism or actuator, not shown, for applying a steering assist force to rack 42. As described below, the (power steering) valve assembly 16 is adapted to direct hydraulic fluid to right or left turn chambers of the power cylinder to apply right-hand or left-hand steering assist force to rack 42. A power cylinder meeting the above description is described in detail in US Patent No 4,454,801, such patent being incorporated herein by reference. Close meshing engagement between the teeth of the pinion gear 28 and the rack 42 is achieved by a rack contact shoe 46 which is slidably mounted in a housing bore 47. A helical spring 48, seated between the rack contact shoe 46 and an adjusting plug 50, tensions the rack contact shoe 46. Adjusting plug 50 is threaded into the end of housing bore 47 and can be axially adjusted therein to vary the spring force. An adjuster plug nut 52 maintains the adjusting plug 50 in a selected position. The spool shaft 18 of the (power steering) valve assembly 16 has a (cylindrical) valve spool 54 formed thereon. The valve spool 54 has a plurality of (arcuate, axially extending) oil transfer slots 56 formed in the periphery thereof. The valve assembly 16 also includes a (cylindrical) valve body 64 rotatably mounted within bore 14 on the valve spool 54. The outboard end of the valve body 64 extends over the end of the pinion gear 28 and is drivingly connected thereto by a radial pin 66. Valve spool 54 and valve body 64 define first and second hydraulic elements which are relatively rotatable. The valve body 64 defines right turn, supply and left turn chambers 76, 78, 80 respectively between it and the bore 14. A hydraulic pump 82 supplies fluid to the supply chamber 78, and such fluid is directed to the right turn and left turn chambers 76, 80 via the oil transfer slots 56 of valve spool 54 and drilled passages 84 and 86, depending on the direction and degree of relative rotation between valve spool 54 and valve body 64. The right turn and left turn chambers 76, 80 are connected to the right (RT) and left (LT) chambers of the power cylinder (actuator) as indicated for generating a corresponding level of steering assist force in rack 42 as described above. An exhaust passage chamber 88 returns hydraulic fluid to a fluid reservoir 86 of hydraulic pump 82. A detailed description of the valve assembly 16 and the hydraulic system thereof is set forth in the above-referenced US Patent No 4,454,801. A resilient centring coupling between the valve spool 54 and valve body 64 is provided by the combined operation of a torsion bar 90 (mechanical resilient means) and the electromagnetic mechanism (electromagnetic circuit means) of this invention, generally designated by the reference numeral 92. Together, the torsion bar 90 and electromagnetic mechanism 92 permit the valve spool 54 to be rotated relative to the valve body 64 in relation to the operator exerted steering torque so that the valve assembly 16 directs fluid to the power cylinder (not shown) for producing the desired level of steering assist force. On termination of the operator exerted steering torque, the torsion bar 90 centres the valve body 64 and valve spool 54 to terminate steering assist force. The torsion bar 90 extends concentrically through an axial opening 69 in the spool shaft 18. A cross pin 70 connects the input end of torsion bar 90 to spool shaft 18. The output end of torsion bar 90 is splined and staked at 72 to the pinion gear 28. Bearing sleeve 74 supports the inner end of the spool shaft 18 on a cylindrical portion of the torsion bar 90. The electromagnetic mechanism 92 comprises a pair of (relatively rotatable magnetic flux conducting) elements 106, 108 magnetically coupled to a (stationary) exciting coil 130. The element 106 is secured to the pinion gear 28 via a non-magnetic core 107 for rotation with the valve body 64, and is magnetically coupled to the exciting coil 130 via pole pieces 136 and 137. The element 108 is secured to the valve spool 54 via a non-magnetic core 110 for rotation therewith and is magnetically coupled to the exciting coil 130 via pole pieces 138 and 139. The exciting coil 130 is wound in an insulating bobbin 132 and has lead ends 170, 172 passing through a suitable opening 174 in the two-piece housing 12 for connection to a computer-based control unit 178 (control means), described below. The (flux conducting) elements 106, 108 have (axially extending and interleaved) teeth 112, 114 which permit a limited relative rotation without mechanical interference, the limit of such rotation being determined by the lost motion coupling described above. When the exciting coil 130 is energized with direct current, juxtaposed teeth 112, 114 assume alternate magnetic polarities as indicated in the linearized representations of Figures 2a - 2b and magnetic flux encircles the exciting coil 130, crossing the inter-tooth working air gaps. The elements 106, 108 are angularly oriented at the installation thereof such that each tooth of one element is centred between a pair of teeth of the other element when the valve spool and valve body 54, 64 are mutually centred to produce minimum hydraulic flow. This condition is depicted by the linearized diagram of Figure 2a. In this condition, the torsion bar 90 is fully relaxed and the integral electromagnetic mechanism 92 of this invention is magnetically balanced, regardless of the coil energization. As a result, neither produces a net torque on the valve spool 54 or valve body 64. However, when there is a relative rotation of the valve spool and valve body 54, 64, mechanical energy is stored in the torsion bar 90 and the working air gaps between opposite polarity teeth 112, 114 of the electromagnetic mechanism 92 decrease as shown in Figure 2b. As in a conventional steering apparatus, the torsion bar 90 responds by exerting a mechanical centring torque which tends to restore the valve spool and valve body 54, 64 to their mutually centred condition. However, the reduced air gaps unbalance the electromagnetic mechanism 92, producing a net attractive force (torque) in relation to the coil current which aids the driver and opposes the mechanical centring torque. Thus, the net centring torque is determined by the difference between the mechanical centring torque and the magnetic attractive torque. The magnetic attractive force varies as a function of the relative displacement and coil current as illustrated in the traces 120 - 124 of Figure 3a. Trace 120 represents the force for relatively low coil current, trace 124 represents the force for relatively high coil current and trace 122 represents the force for an intermediate level of coil current. The traces 120′ - 124′ of Figure 3b illustrate a preferred, linear relationship between magnetic attractive force and relative displacement, corresponding to the nonlinear traces 120 - 124. This linearization may be achieved according to this invention either by suitably shaping the teeth 112, 114, or by scheduling the coil current as a function of the relative displacement as illustrated by the traces 125 - 127 of Figure 3c. In the latter approach, the trace 125 represents the coil current for relatively high vehicle speeds, trace 127 represents the coil current for relatively low vehicle speeds and trace 126 represents the current for an intermediate vehicle speed. Such latter approach is employed in the illustrated embodiment and requires either a sensor for measuring the relative displacement or an estimation of the relative displacement based on readily available information characterizing the coil geometry and measured electrical parameters including the coil voltage and rate of change in coil current. As indicated above, the effective resiliency of the coupling between valve spool 54 and valve body 64 is determined by the difference of the relatively large mechanical centring force of the torsion bar 90 and the relatively small electromagnetic attractive force of the electromagnetic mechanism 92 of this invention. The overall or net centring force is depicted as a function of relative displacement in Figure 4. The torsion bar centring force is fixed for a given installation, but electromagnetic attractive force is variable with the exciting coil energization current, resulting in an overall centring force as represented by the family of curves shown in Figure 4. In a steering apparatus incorporating the electromagnetic mechanism 92 of this invention, the torsion bar 90 is designed to be relatively stiff (low compliance) so that a relatively high level of driver steering effort is required to produce a given level of steering assist. This characteristic is generally desired at relatively high vehicle speeds, and is illustrated in Figure 4 by the trace 146. The coil current is increased with decreasing vehicle speed as illustrated in Figure 3c to reduce the overall or net centring torque, thereby reducing the level of driver steering effort required to produce a given level of steering assist. Successively increasing coil current levels produce assist characteristics represented by the traces 148 in Figure 4. A driver preference input may also be used to tailor the response for individual tastes. As indicated above, the maximum relative displacement limits (MAX) are defined by the splined lost motion coupling between spool shaft 18 and pinion gear 28. Once the maximum displacement has occurred, further rotation of the steering wheel is mechanically transmitted to the pinion gear 28 via the coupling. In the illustrated embodiment, the coupling permits relative displacements of approximately ± 7 mechanical degrees. The computer-based control unit 178 of Figure 1 is supplied with operating power from the vehicle storage battery (not shown) and comprises a microcomputer (uC) 180, an input/output (I/O) device 182, an input counter (INP CTR) 184, and a pulse-width-modulation driver (PWM) 186, all of which may be conventional devices. The microcomputer 180 communicates with the remainder of the apparatus via I/O device 182; in response to various input information, microcomputer 180 executes a series of predetermined program instructions for developing an output command pertaining to the required energization of the exciting coil 130. Program instructions are described below in reference to the flow diagram of Figure 5. The computer based control unit 178 inputs include an oscillatory vehicle speed signal on line 188, which may be obtained from a conventional speed pickup, not shown, and a relative displacement signal on line 198, which may be obtained from a conventional rotary potentiometer, schematically designated by the reference numeral 190. The speed signal is applied to the I/O device 182 through the input counter 184, which divides the frequency of the speed signal by a predetermined factor. The relative displacement signal is applied to an analogue input port of the I/O device 182. The PWM command for exciting coil 130 is applied to the PWM driver 186 for correspondingly modulating the exciting coil 130 with current from the vehicle storage battery, not shown. A signal indicative of the coil current is developed by the PWM driver 186 on line 176 with a suitable current shunt, such signal being applied as an input to an analogue port of the I/O device 182 for use in a closed-loop control of the coil current. Open-loop voltage control may alternately be employed, if desired. Figure 5 depicts a simplified flow diagram representative of computer program instructions which would be executed by the computer-based control unit 178 of Figure 1 in carrying out the control. The block 150 designates a series of program instructions executed at the initiation of each period of vehicle operation for initializing the various registers and program variable to predetermined values. Thereafter, the blocks 152 - 162 are executed as explained below. The decision blocks 152 and 162 detect a low to high transition of the vehicle speed signal output of input counter 184. When the transition is detected, the instruction blocks 154, 156, 158 and 160 are sequentially executed to calculate the vehicle speed N v , to read the relative displacement RD and coil current I c , and to calculate and output a PWM pulse width to the PWM driver 186. Calculation of the vehicle speed at block 154 is based on the elapsed time between low-to-high transitions of the input counter carry bit, such time being inversely proportional to vehicle speed N v . Calculation of the PWM pulse width command is based on the deviation of the desired coil current from the measured coil current I c , the desired current being determined in accordance with vehicle speed and relative displacement, as indicated in Figure 3c. While this invention has been described in reference to the illustrated embodiment, various modifications will occur to those skilled in the art. In essential form, the control apparatus of this invention comprises a stationary coil and a pair of relatively rotatable flux conducting elements. The function of the magnetic circuits can be achieved with alternate configurations, and it should be understood that the scope of this invention is defined solely by the appended claims. Moreover, various control parameters, such as driver preference (light, medium, or heavy effort) or pressure feedback may be used, either separately or in combination with the above-described vehicle speed parameter.

Claims: Hydraulic power assist steering apparatus comprising first and second hydraulic elements (54,64) which are relatively rotatable and which are connected between a steering shaft (18) and a steering gear (28) for generating hydraulic flow in relation to their relative rotation; apparatus for variably coupling the first and second hydraulic elements (54,64) to vary the level of steering assist produced for a given driver manipulation of the steering shaft (18), the apparatus comprising mechanical resilient means (90) connected between the first and second hydraulic elements (54,64) for developing a mechanical centring force in relation to relative rotation thereof which tends to restore the first and second hydraulic elements (54,64) to a centred condition of minimum hydraulic flow; and electromagnetic circuit means (92) including an exciting coil (130) and first and second flux conducting elements (106,108) magnetically coupled to the exciting coil (130) and supported for rotation with the first and second hydraulic elements (54,64), respectively; characterised in that the first and second flux conducting elements (106,108) have teeth (112,114) which define a magnetic circuit having a working air gap which varies with the relative rotation of the first and second hydraulic elements (54,64), the flux conducting elements (106,108) being angularly oriented such that the teeth (112) of one flux conducting element are equally attracted to juxtaposed teeth (114) of the other flux conducting element when the first and second hydraulic elements (54,64) are in the centred condition, whereby when the first and second hydraulic elements (54,64) are relatively displaced from the centred condition, the exciting coil (130) and flux conducting elements (112,114) develop a net magnetic attractive force which opposes the mechanical centring torque in relation to the energization of the exciting coil (130). Hydraulic power assist steering apparatus as claimed in claim 1, wherein the mechanical resilient means is a torsion bar (90). Hydraulic power assist steering apparatus as claimed in claim 1 or claim 2, comprising control means (178) for varying the energization of the exciting coil (130), thereby to correspondingly vary the magnitude of the magnetic attractive force for a given relative displacement of the first and second hydraulic elements (54,64). Hydraulic power assist steering apparatus as claimed in claim 3, wherein the control means (178) varies the coil energization in relation to the speed of the vehicle, whereby the driver manipulation of the steering shaft (18) required to produce a given relative rotation of the first and second hydraulic elements (54,64), and hence the hydraulic flow, varies with the speed of the vehicle. Hydraulic power assist steering apparatus as claimed in claim 4, wherein the mechanical centring force of the mechanical resilient means (90) is relatively high so that a relatively high level of driver steering effort is required to produce a given level of assist; and the control means (178) increasingly energizes the exciting coil (130) in inverse relation to the vehicle speed so that the driver steering effort required to produce a given level of assist increases in direct relation to the vehicle speed.

Inventors: GRABER, DAVID WAYNE, PAWLAK, ANDRZEJ MARIAN, ZURASKI, JEFFREY ALAN
Applicants: GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: B62D6
IPC codes: B62D6