NONCONTACTING POSITION DETECTING APPARATUS FOR AN AUTOMOTIVE STEERING GEAR

Abstract: A noncontacting automotive steering angle sensor involving the application of a two element (136,138) magnetoresistive sensor to an otherwise conventional rack and pinion steering gear (10). The rack (42) is directly coupled to the steerable wheels, and the sensor is integrally mounted in a spring-loaded rack contacting mechanism (contact shoe) (92) used to maintain a meshing engagement between the pinion gear (28) and the rack. A variable permeability surface (100) is formed on the surface of the rack along the travel path of the sensor such that axial movement of the rack differentially varies the magnetic flux density detected by each element of the sensor. A detector circuit (122) comprising a bridge or simple voltage divider responsive to the differential resistances of the sensor elements develops an electrical signal indicative of the rack position.

Description: This invention pertains to rack driven automotive steering gear, and more particularly to a noncontacting apparatus for detecting the axial position of the rack, as disclosed in EP-A-90697. The steering angle of an automobile is often used as an input parameter for sophisticated vehicle control systems such as four-wheel steering, active suspension, anti-lock braking and traction control. When an electrical signal is required, rotary potentiometer devices are typically employed to sense the handwheel or pinion position. This approach is straightforward but involves the usual drawbacks associated with contacting sensors and fails to account for any discrepancies (due to compliance and hysteresis, for example) between the position of the handwheel or pinion and the actual steering angle. Apparatus for detecting the position of the rack in accordance with the present invention is characterised over EP-A-90697 by the features specified in the characterising portion of Claim 1. The present invention is directed to an improved noncontacting automotive steering angle sensor involving a novel application of a two element magnetoresistive sensor to an otherwise conventional rack and pinion steering gear. The rack is directly coupled to the steerable wheels, and the magnetoresistive sensor is integrally mounted in a spring-loaded rack contacting mechanism (contact shoe) used to maintain a meshing engagement between the pinion gear and the rack. The two magnetoresistor elements of the magnetoresistive sensor are positioned on an axis transverse to the axis of rack displacement, and a surface feature is formed on the rack along a axis diagonal to the axis of rack displacement such that axial movement of the rack differentially varies the magnetic coupling between the rack and the magnetoresistive sensor. This produces a differential change in the electrical resistances of the magnetoresistor elements, and a detector circuit comprising a bridge or simple voltage divider responsive to the differential resistances of the magnetoresistor elements develops an electrical signal indicative of the rack position. In the illustrated embodiment, the diagonal surface feature is defined by a rack mounted slide bar having a diagonally extending groove formed therein. The slide bar is received within a channel formed in the rack contacting mechanism so that the surface feature defined by the slide bar remains properly oriented with respect to the magnetoresistive sensor, even though the rack experiences other than axial displacement. In this form, the modifications to a conventional rack and pinion gear are relatively minimal, and the output signal is substantially insensitive to mechanical nonlinearities of the steering gear. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a cross section of a rack and pinion steering gear including a magnetoresistive sensor and control module in accordance with this invention; Figure 2 is a section of the rack and pinion steering gear of Figure 1 taken along lines 2--2; Figures 3a - 3d are schematic representations of the magnetoresistive sensor and slide bar of Figure 1 for various axial positions of the rack; Figure 4 is a graph depicting the output voltage of the control module of Figure 1 as a function of rack position; Figure 5 is a circuit diagram of the control module of Figure 1; and Figure 6 is an exploded view of the magnetoresistive sensor of Figure 1. Referring to Figures 1 - 2, the reference numeral 10 generally designates a portion of a motor vehicle hydraulic power assist rack and pinion steering gear, hereinafter, for convenience, referred to as steering gear. The steering gear 10 is disposed within a housing 12 having a (smooth wall) bore 14. A (cylindrical rotary power steering gear) valve assembly 16, disposed within the bore 14, includes an (elongated cylindrical) spool shaft 18 and valve body 64 supported for independent turning movement therein. The inboard end of spool shaft 18 is supported on a roller bearing 22 and projects through an (annular) fluid seal 23 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 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 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 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, axial displacement of the rack 42 turns the steerable wheels 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 valve assembly 16 is adapted to direct hydraulic fluid to right or left turn chambers (RT, LT) 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. The spool shaft 18 of the 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 body 64 is rotatably mounted on the valve spool 54 via sleeve bearings 20 and 21. The outboard end of the valve body 64 extends over the end of the pinion gear 28 and is drivingly connected thereto by radial pin 66. The valve body 64 defines supply chamber 78, and right and left turn chambers 76, 80 between it and the valve 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 68 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 a torsion bar 90, which permits 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 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 72 of torsion bar 90 is splined and staked 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. Close meshing engagement between the teeth of the pinion gear 28 and the rack 42 is achieved by a rack contact shoe 92 having a (machined) arcuate surface 94 which is resiliently urged into contact with the rack 42 opposite the pinion gear 28. The rack contact shoe 92 includes a slot 96 and two concentric openings 97 and 98 radially aligned with the centre of the rack 42. A slide bar 100 axially secured to the rack 42 by a pin 102 is received within the (contact shoe) slot 96; a tip 104 of a magnetoresistive sensor housing 106 is received within the opening 97; and a flange 108 of the magnetoresistive sensor housing 106 is received within the opening 98. The openings 97 - 98 are axially offset such that the tip 104 of magnetoresistive sensor housing 106 is slightly displaced from the outboard surface of slide bar 100. A locating pin 110 on the outboard end of the rack contact shoe 92 mates with a corresponding opening 112 in the flange 108 to angularly orient the magnetoresistive sensor housing 106 with respect to the slide bar 100. A helical spring 114 is seated between the flange 108 and an adjusting plug 116 to seat the magnetoresistive sensor housing 106 against rack contact shoe 92 and to tension the rack contact shoe 92 against rack 42. Adjusting plug 116 is threaded into the housing 12 and can be axially adjusted therein to vary the spring force. An adjuster plug nut 118 maintains the adjusting plug 116 in a selected position, and a sensor lead-in cable 120 passes through suitable openings in the magnetoresistive sensor housing 106 and adjusting plug 116 for connection to a control module 122 (developing means). As seen most clearly in reference to Figures 1 and 3a - 3d, the (ferromagnetic) slide bar 100 has an end-to-end groove 130 (defining means defining a surface feature) extending diagonally across the width of the slide bar, defining a pair of tapered ridges 132, 134. As described below in reference to Figure 6, the magnetoresistive sensor of Figures 1 - 2 houses a pair of juxtaposed magnetoresistor elements 136, 138 which correspond in overall width to the slide bar 100. When assembled, as described above in reference to Figures 1 - 2, the magnetoresistor elements 136, 138 are oriented with respect to the tapered ridges 132, 134 substantially as shown in Figures 3a - 3d. Thus, the magnetoresistor element 136 is positioned directly over and magnetically interacts with the tapered ridge 132 and the magnetoresistor element 138 is positioned directly over and magnetically interacts with the tapered ridge 134. Figures 3a - 3d depict the slide bar 100 and magnetoresistor elements 136, 138 for four different positions of the rack 42. For ease of illustration, the magnetoresistor elements 136, 138 are depicted as moving with respect to the rack 42. In Figure 3a, the rack 42 is centred for straight ahead vehicle operation, and the active ridge area seen by magnetoresistor element 136, is substantially equal to that seen by the magnetoresistor element 138. In this case, the flux densities of the magnetoresistor elements 136 and 138, and therefore their characteristic resistances, are substantially equal. In Figures 3b and 3c, the rack 42 is fully displaced to the right and left, respectively, maximizing the active ridge area seen by one magnetoresistor element 136, 138 while minimizing the active ridge area seen by the other magnetoresistor element 138, 136. In this case, the flux density around one magnetoresistor element greatly exceeds that around the other magnetoresistor element, resulting in a relatively large difference in the magnetoresistor element resistances. In Figure 3d, the rack 42 is partially displaced to the left so that the active ridge area seen by magnetoresistor element 136 is somewhat greater than that seen by the magnetoresistor element 138, giving rise to an intermediate difference in the magnetoresistor element resistances. In any case, the resistance differential as between the magnetoresistor elements 136 and 138 is detected by the control module 122 of Figure 5, described below to provide a measure of the rack position. Figure 6 is an exploded view of the magnetoresistive sensor depicted in Figures 1 - 2. The magnetoresistor elements 136, 138 (available from Siemens in a two-pack assembly as shown) are mounted on a permanent magnet 140 and mica insulator 142. The assembly is then potted inside the tip 104 of the magnetoresistive sensor housing 106, as indicated by a potting material block 144. A bundle 146 of three lead-in wires 160, 162, 164 passes through the potting material block 144 and an opening 148 in the housing 106 for attachment to the (external) sensor lead-in cable 120 via circuit board 150. The housing section 152 is press fit into a stem 154 of magnetoresistive sensor housing 106, and the circuit board 150 is potted therein as indicated by the potting material block 156. A retaining ring 158 is secured to the housing section 152, completing the magnetoresistive sensor. When assembled in relation to the steering gear 10, as shown in Figures 1 - 2, the magnetoresistor elements 136, 138 are separated from the slide bar 100 by a slight air gap, and the permanent magnet 140 establishes a pair of magnetic flux paths through the magnetoresistor elements 136, 138. The flux passing through magnetoresistor element 136 passes through the tapered ridge 132 and the tip 104; the flux passing through magnetoresistor element 138 passes through the tapered ridge 134 and the tip 104. In each case, the effective area of the tapered ridge 132, 143 determines the flux density, and therefore, the electrical resistance of the magnetoresistor elements 136, 138. Figures 4 and 5 refer to the operation of the control module 122. Figure 5 depicts the control module circuit and Figure 4 depicts the circuit output voltage V o as a function of rack position. Referring to Figure 5, the magnetoresistor elements 136 and 138 are illustrated as a serially connected variable resistor divider. The sensor lead-in cable 120 of Figures 1 - 2 comprises the lead-in wires 160, 162 and 164, the lead-in wire 162 being common to both magnetoresistor elements 136,138. Also, as seen in Figure 1, the control module 122 is supplied with power and ground connections to a storage battery 165. The reference numeral 166 designates a voltage regulator (VR) for supplying a regulated voltage via line 168 to the magnetoresistor divider, and to a separate resistor divider comprising the resistors 170, 172. The junction between magnetoresistor elements 136 and 138 is connected via resistor 174 to the inverting input of operational amplifier 176, and the junction between resistors 170 and 172 is connected via line 178 to the noninverting input of operational amplifier 176. The feedback resistor 180 determines the amplifier gain, and the output voltage V o of the circuit appears on line 182. Those skilled in the art will understand that the circuit of Figure 5 will provide an analogue output voltage V o which has an offset determined by the ratio of resistors 170, 172. Together the resistor dividers define a bridge circuit, and the output voltage of the operational amplifier 176 varies in relation to the differential resistance of the magnetoresistor elements 136 and 138. In the illustrated embodiment, the regulated voltage is approximately 2.50 volts, and the circuit elements are chosen to provide an offset voltage of approximately 1.25 volts, as seen in Figure 4. For right-hand displacement of the rack 42, the output voltage varies over the approximate range of 1.25 volts to 2.50 volts, as indicated by the broken trace 190. For left-hand displacement of the rack 42, the output voltage varies over the approximate range of 0.00 volts to 1.25 volts as indicated by the solid trace 192. In the above manner, the sensor arrangement of this invention provides a simple, reliable and accurate indication of the steering rack position without any of the disadvantages of contacting sensor arrangements. The arrangement is inexpensive to produce and requires only minimal modification of a conventional rack and pinion steering gear. While this invention has been described in reference to the illustrated embodiment, it is anticipated that various modifications will occur to those skilled in the art. For example, the function of slide bar 100 could be achieved by forming a diagonal groove in the rack 42 itself and limiting the rotary displacement of the rack. Similarly, the shape of the groove itself could be modified to produce an output voltage characteristic different from that shown in Figure 4.

Claims: Apparatus for detecting the position of a rack (42) as a measure of the steering angle in a steering gear (10) of a motor vehicle in which steering is achieved via linear displacement of the rack (42) with respect to a housing (12) along a longitudinal axis, comprising magnetic means (136, 138) secured to the housing (12) in proximity to the rack (42) on an axis transverse to the longitudinal axis; and defining means (130) defining a surface feature on the rack (42) adjacent the magnetic means (136,138); characterised in that the magnetic means comprises a magnetoresistive sensor including first and second magnetoresistor elements (136,138); in that the surface feature is on an axis diagonal to the longitudinal axis for differentially altering the magnetic coupling between the rack (42) and the magnetoresistive sensor in relation to the displacement of the rack; and by developing means (122) for developing an indication of rack position in relation to the consequent differential resistances of the magnetoresistor elements (136,138). Apparatus as claimed in Claim 1, wherein the defining means is defined by a slide bar (100) of ferromagnetic material secured to the periphery of the rack (42) along the longitudinal axis thereof, the slide bar having a groove (130) formed therein substantially along the diagonal axis to define first and second tapered ferromagnetic surfaces (132,133) magnetically coupled to the magnetoresistive sensor through the first and second magnetoresistor elements (136,138). Apparatus as claimed in Claim 2, wherein the magnetoresistor elements (136,138) are juxtaposed and mounted on a permanent magnet (140) within a magnetoresistive sensor housing (106) so that magnetic flux entering the first and second tapered surfaces (132,133) from the permanent magnet is returned to the permanent magnet through the magnetoresistive sensor housing. Apparatus as claimed in Claim 1, wherein the steering gear (10) includes a pinion gear (28) drivingly engaged with the rack (42), and a contact member (92) secured to the housing (12) opposite the pinion gear and resiliently contacting the rack for maintaining the engagement between the pinion gear and the rack; and the surface feature is defined by a slide bar (100) of ferromagnetic material secured to the rack for linear displacement therewith and received within a slot (96) formed in a rack contacting surface (94) of the contact member; and the magnetoresistive sensor is mounted in a central opening (97,98) of the contact member so as to position the magnetoresistor elements (136,138) in proximity to the surface feature of the slide bar. Apparatus as claimed in Claim 4, wherein the surface feature of the slide bar (100) is defined by a groove (130) formed across its width to define first and sevariable ferromagnetic surfaces (132,134) on either side of the groove which magnetically interact with the magnetoresistive sensor through the first and second magnetoresistor elements (136,138), respectively.

Inventors: GOINGS, JANET S., GOLEBIEWSKI, RONALD T., KARADSHEH, SAM M., SCHROEDER, ALLEN G., SCHROEDER, THADDEUS
Applicants: GENERAL MOTORS CORPORATION
Application year: 1990
Main classification: B62D15
IPC codes: B62D5, G01B7, B62D15, B62D3