Steer-by wire handwheel actuator

[0002] This application claims the benefit of U.S. provisional application No. 60/299,342, filed Jun. 19, 2001 the contents of which are incorporated by reference herein in their entirety.

[0003] This invention relates to a steer-by-wire system, and more particularly to a steer-by-wire handwheel actuator.

[0004] It is known in the art to have a steering system with mechanical linkage from the steering wheel to the steerable road wheels. Even with power assist, the driver of a mechanically linked vehicle can feel the forces of the road against the steerable wheels through the steering wheel. Indeed, this is a desired feature of direct linkage and is sought out by purchasers of high performance vehicles in the form of so-called “rack-and-pinion” steering. The road forces felt in the steering wheel give the driver feedback he can use to anticipate and control the vehicle, or at least create the comfortable feeling that he is in control of the vehicle. Remove this feedback, such as in the case of “mushy” power steering, and the driver will have the uncomfortable feeling of being separated from the road wheels, not quite in control, and will tend to oversteer his vehicle, particularly in demanding situations such as sharp or sudden turns.

[0005] By definition, a steer-by-wire system has no mechanical link connecting the steering wheel from the road wheels. In effect, the steering wheel is little more than a joystick, albeit an extremely sophisticated joystick. Exemplary of such known steer-by-wire systems is commonly-assigned U.S. Pat. No. 6,176,341, issued Jan. 23, 2001 to Ansari, which is wholly incorporated herein by reference. What is needed is to provide the steer-by-wire driver with the same “road feel” that a driver receives with a direct mechanical link. Furthermore, it is desirable to have a device that provides a mechanical back up “road feel” in the event of multiple electronic failures in the steer-by-wire system. In addition, a device that provides positive on-center feel and accurate torque variation as the handwheel is rotated is also desirable. Existing steer-by-wire devices produce excessive lash, excessive noise and insufficient over-load torque capability as the handwheel is rotated to its end of travel in either direction.

[0006] A steer-by-wire steering system is defined as a steering system with no mechanical connection between a steering wheel and a set of steering gears or actuators. Such systems are advantageous because they permit auto and other vehicle designers great latitude in use of space that would normally be taken up by mechanical linkages, among other reasons.

[0007] In an exemplary embodiment of the invention, a steer-by-wire handwheel actuator is described, which provides feedback of road forces to the operator of a steer-by-wire vehicle. A handwheel actuator comprises a driver input shaft; a gear train connected to the driver input shaft; a motor responsive to control signals from a controller and connected to the gear train; an electro-mechanical brake responsive to the control signals from the controller and connected to one of the driver input shaft and the gear train; and a stop mechanism attached to a housing and coupled to one of the electro-mechanical brake and the gear train.

[0008] FIG. 1 is a perspective view of a prior art hard contact driving system;

[0009] FIG. 2 is a schematic representation of the steer-by-wire handwheel actuator in signal communication with a steer-by-wire steering system;

[0010] FIG. 3 is a perspective view of an exemplary embodiment of a steer-by-wire handwheel actuator;

[0011] FIG. 4 is a rear/plan view of a an exemplary embodiment of a steer-by-wire handwheel actuator of FIG. 3;

[0012] FIG. 5 is a front/plan view of a an exemplary embodiment of a steer-by-wire handwheel actuator shown in FIG. 4 detailing cross sectional divisions that follow;

[0013] FIG. 6 is a cross section view of an exemplary embodiment of a steer-by-wire handwheel actuator depicted in FIG. 5, Section 6—6;

[0014] FIG. 7 is a cross section view of an exemplary embodiment of a steer-by-wire handwheel actuator depicted in FIG. 5, Section 7—7;

[0015] FIG. 8 is a cross section view of an exemplary embodiment of a steer-by-wire handwheel actuator depicted in FIG. 5, Section 8—8;

[0016] FIG. 9 is a cross section view of a modular unit attached to a steering shaft to provide auxiliary steering resistance;

[0017] FIG. 10 is a cross section view of an alternative embodiment of the steer-by-wire handwheel actuator of FIG. 3 with a combination worm gear and direct drive power transmission; and

[0018] FIG. 11 is another cross section view of the alternative embodiment of the steer-by-wire handwheel actuator of FIG. 10 with worm gear power transmission taken perpendicular to the view of FIG. 10.

[0019] Referring to FIG. 1, a typical prior art steering system is depicted. A steering wheel 10 is connected to a steering column 11 which in turn is connected to a steering intermediate shaft 12 via universal joint 14. Another universal joint 16 couples intermediate steering shaft 12 to an electric power steering assist assembly (rack assist) 18. It is evident that mechanical direct connection exists throughout the prior art driving system.

[0020] FIG. 2 is a schematic representation of a steer-by-wire steering system 600 as it is comprises a controller 400, a first electro-mechanical actuator 202 and a second electro-mechanical actuator 302, each actuator 202, 302 in signal communication 400a, 400bwith the controller 400. In one embodiment, the first and second electro-mechanical actuators 202, 302 comprising a motor, crank arm, steering arm and tie rod, are in turn connected respectively to a first wheel 200 and a second wheel 300 and are operative thereby to steer the wheels 200, 300 under the command of the controller 400. In another embodiment, it will be appreciated by one skilled in the pertinent art that one actuator is optionally linked to both road wheels 200, 300 to operably steer wheels 200, 300 using one motor to actuate the sole actuator.

[0021] Still referring to FIG. 2, a handwheel actuator 100 of the present disclosure is in signal communication 100a, 100bwith the controller 400 of the steer-by-wire steering system 600. Handwheel actuator 100 is in further communication with an external motive source 500, such as a driver from whom the handwheel actuator 100 receives steering commands by way of a driver input shaft 102. The controller 400 is also operative to receive as input thereto a signal 700 indicative of vehicle velocity, as well as a signal 500aindicative of the position of the driver input shaft 102 and a signal 500bindicative of the torque on the driver input shaft 102. Signals 500aand 500bare generated from sensors disposed in handwheel actuator 100 proximate shaft 102.

[0022] With reference to FIG. 3, therein depicted is a representation of the handwheel actuator 100 of a preferred embodiment. The handwheel actuator 100 includes a housing 130, a driver input shaft 102 and a gear train 104 coupled to the driver input shaft 102. In the interest of clarity, the gear train 104 comprises a driver feedback pulley 106, a speed reducer pulley 108, and a spur gear 110 (See also FIG. 7, as gear train 104 is shaded completely). The driver input shaft 102 is rotatably positioned between an upper bearing 54 and a lower bearing 56, and position sensors 20 and torque sensor 22 operably connected to driver input shaft 102. Position sensors 20 electronically detect the angular position of the driver input shaft 102, while the torque sensor 22 electronically detects and evaluates the torsional force acting on the driver input shaft 102. The angular displacement of the hand steering wheel 10 is detected by sensors 20, 22, processed, and applied to a servo motor (not shown) to move steerable wheels (not shown). The handwheel actuator 100 of FIGS. 3 and 6 includes an electric motor 114 having a motor shaft 116 rotatively driven by the motor 114. Attached or optionally formed into the motor shaft 116 is a motor pulley 118. Attached to the motor pulley 118 is a belt 128 positively driving pulley 108 of the gear train 104. Continuing in FIG. 3 in conjunction with FIG. 7, the handwheel actuator 100 further includes an electro-mechanical brake 120 having an electro-mechanical brake shaft 122 rotatively controlled by the electro-mechanical brake 120. Attached or optionally formed into the electro-mechanical brake shaft 122 is a pulley 124 driving a belt 136 operably connected to the gear train 104. More specifically, the belt 136 is connected to the driver feedback pulley 106 in an exemplary embodiment.

[0023] It should be noted that a preferred embodiment of the handwheel actuator utilizes an electric sine commutated brushless motor 114 for its primary power transmission because the sine wave commutation provides for a low torque ripple. Furthermore, it is preferred that the belt 128 used to transmit the power from the motor 114 is a small-cogged belt to provide positive drive, high efficiency, low noise, and nearly zero lash. In using a cogged belt, it has been found to yield approximately 98% efficiency. In addition, a preferred embodiment uses a magnetic particle brake for the electro-mechanical brake 120, however alternative embodiments also include electro-rheological fluid devices.

[0024] Magnetorheological fluids suitable for use in the handwheel actuator 100 are disclosed in U.S. Pat. Nos. 5,896,965, issued Apr. 27, 1999, to Gopalswamy et al. for a Magnetorheological Fluid Fan Clutch; 5,848,678, issued Dec. 15, 1998, to Johnston et al. for a Passive Magnetorheological Clutch; 5,845,752, issued Dec 8, 1998, to Gopalswamy et al. for a Magnetorheological Fluid Clutch with Minimized Resistance; 5,823,309, issued Oct. 20, 1998, to Gopalswamy et al. for a Magnetorheological Transmission Clutch; and 5,667,715, issued Sep. 16, 1997, to Foister, R. T. for Magnetorheological Fluids; the disclosures of all of which are incorporated herein by reference in their entirety. An alternative embodiment utilizing an electro-rheological fluid device having magnetorheological fluid for obtaining a variable resistance to the driver input shaft is disclosed in Patent Application number DP-300272, entitled “Variable Road Feedback Device For Steer-By-Wire System.”

[0025] It is to be noted that utilizing a magnetic particle brake or a magnetorheological fluid device provides virtually no resistance to a driver input shaft when there is no magnetic force induced by a control module. However, when it becomes desirable to give the vehicle operator a feel of the road, a control module energizes a magnetic field in the magnetic particle brake or the magnetorheological fluid device causing the magnetic particle brake or the magnetorheological fluid device in turn to provide variable passive resistance. The variable passive resistance along with active resistance provided by the electric motor gives the vehicle operator a feel of the road by transferring the resistance upon the steering wheel. Thus, causing the vehicle operator to “feel” or sense the road.

[0026] Returning to FIG. 3 and incorporating FIG. 8, the handwheel actuator 100 illustrates generally a stop mechanism 126 enclosed by housing 130 and comprising the spur gear 110 of the gear train 104 geared to a reducing spur gear 132. Reducing gear 132 further comprises interior arcuate stop guides 126a, 126b, one on either side of the gear 132 having a first stop pin 126cdisposed within said stop guide 126aand having a having a second stop pin 126ddisposed within stop guide 126b, such that as spur gear 110 rotates, reducing gear 132, and thus spur gear 110, is mechanically restrained as the first stop pin and second stop pin make simultaneous contact with one pair of two ends 126e, 126fof one end of the interior arcuate stop guides 126a, 126b(See FIG. 8). The other end of the internal arcuate stop guides 126a, 126bis not shown. The first stop pin and second stop pin are attached to the housing 130. In an exemplary embodiment, the stop pins 126c, 126dare rubber coated and provide over 100 Nm of overload torque capability.

[0027] Still referring to FIG. 3, a modular unit 140 is attached to the shaft 102 that acts as a mechanical back-up device to provide auxiliary steering resistance in the steer-by-wire system 600. Modular unit 140 allows full lock-to-lock travel of the steering handwheel 10 while providing torsional resistance to handwheel rotation up to a specified saturation torque.

[0028] Referring to FIG. 9, modular unit 140 is shown in more detail and described below. Modular unit 140 provides an adjustable on-center feel and return-to-center mechanism for the steer-by-wire handwheel actuator 100 to provide a passive steering system feel similar to current production hydraulic assisted rack and pinion systems. Modular unit 140 provides the driver with force feedback throughout the range of travel of the handwheel 10. Modular unit 140 comprises a ball screw assembly 142 including a hollow sleeve 144 that slip fits over the ball screw assembly 142 and is rotationally fixed using an anti-rotation pin assembly 148 and a key 149 opposite thereto. Rotation pin assembly 148 includes a pin 150 slidably engaged against an exterior sleeve 144. Anti-rotation pin assembly 148 further includes a pin retainer 152 and screw 154 that is disposed in an aperture 156 of retainer 152 and received in a threaded aperture 158 of housing 162 defining modular unit 140. Pin 150 is configured with a slot 163 positioned thereon to allow a portion of retainer 152 to be received therein while maintaining pin 150 slidably disposed in an aperture formed in housing 162 to engage an outside surface of a sleeve 144 when screw 154 is fixed against retainer 152 by tightening screw 154 in threaded aperture 158. Sleeve 144 has a channel 164 configured on a periphery thereof to allow axial translation of sleeve 144 while limiting rotation thereof. Sleeve 144 engages a threaded portion 166 of shaft 102 via a ball nut 168 disposed in sleeve 144. Ball nut 168 is retained in sleeve 144 with a ball nut retainer nut 169. Ball nut 168 has complementary threads to engage threaded portion 166. Screw shaft 102 is supported in a ball bearing assembly 170 that is disposed at one end in housing 162.

[0029] Bearing assembly 170 is retained in housing 162 with a bearing cap 172 that is mechanically fastened to housing 162 with mechanical fasteners 174. A bearing nut 176 is engageable with another threaded portion of shaft 102 to fix shaft 102 relative to bearing assembly 170 which is fixed relative to housing 162. At an opposite end 178 of housing 162 an end cap 180 encloses a cavity formed in housing 162. At the same end 178 a retaining nut 182 is threaded onto the end of sleeve 144 and an external spring return nut 196 is threaded into housing 162. Disposed against retaining nut 196 and retaining nut 182 is a first spring retaining washer 188 having an aperture allowing ball screw 144 to slide therethrough. A second spring retaining washer 190 is disposed against a shoulder 192 of sleeve 144 and shoulder 198 of housing 162. Like washer 188, washer 190 includes an aperture that allows sleeve 144 to slide therethrough. A plurality of biasing members 194 is disposed intermediate washers 188, 190. Each biasing member is preferably a disc spring or Belleville washer. The plurality of biasing members 194 is more preferably a stack of disc springs circumferentially disposed about sleeve 144 while allowing translation of sleeve 144 therethrough. The stack of disc springs are preferably formed by alternating the orientation of contiguous disc springs to provide a compression type biasing member 194. The spring pack or plurality of biasing members 194 is stacked in series to provide desired spring load and maximum travel. The springs are designed and preferably preloaded to a stack height that gives a nonlinear spring rate with a very gradual slope.

[0030] Spring retaining nut 182 is preferably configured as an adjustment preload nut that is threaded onto the sleeve 144 and tightened to a specified position to set the appropriate spring preload on the biasing members 194. An external spring return nut 196 has exterior threads threadably engaged with complementary threads in housing 162 at end 178 for engaging washer 188 when sleeve 144 translates toward end 178 pushing washer 192 to compress biasing members 194 which push against washer 188 which is prevented from translating toward end 178 by fixed nut 196. When sleeve 144 translates away from end 178, nut 182 pushes against washer 188 to compress biasing members 194 against washer 192 that is prevented from translating away from end 178 by a shoulder 198 formed in housing 162. The preload on the biasing members 194 is configured to provide an identical bias when shaft 102 is rotated in either direction since the spring pack or plurality of biasing members 194 is configured in a parallel arrangement to bias the shaft indicative of a return-to-center position. It will be recognized that although the plurality of biasing members has been described as a single series stack of disc washers, multiple stacks are also contemplated. More specifically, it is contemplated that a first stack may be disposed on one side of ball nut 168 while a second stack is disposed on the other side of ball nut 168. In this manner, when shaft is rotated in one direction, the first stack is compressed and when shaft 102 is rotated in an opposite direction, the second stack is compressed.

[0031] In operation, as the hand wheel 10 is rotated from the center position, the steering shaft 102 rotates at the same speed. As the steering shaft 102 rotates, the ball nut 168 and hence sleeve 144 translates left or right in an axial direction relative to shaft 102 shown in FIG. 9 depending on direction of rotation. When the sleeve 144 translates it compresses the spring stack giving a gradual load increase on the ball nut 168. The axial load increase reflects through the ball nut 168 giving an increasing torque feed back to the driver as the hand wheel 10 is turned further from center. The mechanism will not run out of travel since the hand wheel actuator will reach its end of travel stop before maximum travel of the ball nut 168 is reached. When the driver releases the hand wheel 10 the ball nut 168 will backdrive on the screw due to the axial load imbalance until it reaches the equilibrium load at the center position.

[0032] Turning to FIG. 4, a preferred embodiment depicted in FIG. 3 is shown from the rear, and FIG. 5 illustrates the front/plan view of FIG. 4. FIG. 5 also illustrates three cross sections taken for the Figures referenced above that follow. FIGS. 4 and 5 illustrate the housing 130 coupled to a bracket 160 for attaching a handwheel actuator to a vehicle mounting interface.

[0033] In FIG. 2, in conjunction with FIGS. 6 and 7, the handwheel actuator 100 is operative to accept as input thereto, firstly, steering commands by way of the driver input shaft 102 from an outside motive source such as a driver 500, secondly, the first control signal 100afrom the controller 400 to the motor 114 and thirdly, a second control signal 100bfrom the controller 400 to the electro-mechanical brake 120. The steering commands are typically the clockwise or counterclockwise rotation of the driver input shaft 102. The first control signal 100aoriginating from the controller 400 controls the input to the speed reducer pulley 124 by the motor 114. The second control signal 100bcontrols a feedback torque directed to the driver input shaft 102 by the electro-mechanical brake 120.

[0034] In FIGS. 6 and 7, the clockwise or counterclockwise rotation of the driver input shaft 102 instigates the action of the speed reducer pulley 124. By way of pulley 108 the motor shaft 116 is positively driven by belt 128. Such engagement results in a gear ratio, R₁, of the speed reducer pulley 124. As vehicular operating conditions change, e.g., a change in vehicular speed or a change in the position of the driver input shaft 102, plus the application of road forces acting upon the steer-by-wire system 600 (See FIG. 2), the first control signal 100a, based upon the changing operating conditions, activates the motor 114 so as to control speed reducer pulley 124, namely resisting or assisting to the motion of reducing pulley 108 and allowing controlled rotation thereof. Based upon the aforesaid changing operating conditions, the second control signal 100bfrom the controller 400 activates the electro-mechanical brake 120 so as to provide a resistive torque to the speed reducer 104 by way of the engagement of the electro-mechanical brake 120 to the driver input shaft 102 through the electro-mechanical brake shaft 122, the pulley 124, belt 136, and the driver feedback pulley 106. The resistive torque results in a feedback torque that provides the driver 500 with additional tactile response to steering commands.

[0035] Referring now to FIGS. 10 and 11, an alternative embodiment for eliminating pulleys 118,108 and 124,106 and corresponding belts 128 and 136, respectively, are illustrated. Motor 114 is in direct drive relationship with motor shaft 116 operably connected to shaft 102 absent a connection via a speed reducer. Motor 114 includes a motor stator 206 surrounding a motor rotor 208 operably connected to shaft 116 for rotation thereof when rotor 206 is energized. Motor shaft 116 is connected to driver input shaft 102 via a sleeve assembly 212 having a torsion bar 214 disposed therein for torque sensing by torque sensor 22. Sleeve assembly 212 is connected to a worm gear 216 that is engaged to a worm 218. Worm 218 is rotatably supported between two bearings 222 and is operably connected to brake 120 via shaft 122 at one end thereof. In an exemplary embodiment, brake 120 provides up to about 6 Nm of torque feedback by braking that is transmitted to worm 218, worm gear 216, to sleeve assembly 212, and then to driver input shaft 102, while motor 114 provides up to about 4 Nm of torque feedback in conjunction with the 6 Nm from brake 120 to driver input shaft for a total of up to about 10 Nm. Motor 114 generates the torque through shaft 116, to sleeve assembly 212, and then to the driver input shaft 102. Using the worm and worm gear drive with the direct drive motor connection, zero lash and comparable noise are experienced compared with the belt drive previously described above.

[0036] It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but intended to cover the invention broadly within the scope and spirit of the claims.