Abstract:
A hand wheel actuator for a steer-by-wire steering system having a steering feel that meets drivers&#39; expectations is provided with a first housing having an aperture, a steering shaft extending through the aperture and rotatably supported by the housing, an electric motor, a position sensor and torque sensor, and a mechanical steering feel enhancement device. The position and torque sensors, are positioned within said first housing and configured to detect the angular displacement of the steering shaft and torque applied against the steering shaft respectively. The mechanical steering feel enhancement device is attached to the first housing and provides at least one of a positive stop function, a return-to-center function, or a friction/weight function by mechanical means. The mechanical steering feel enhancement device is in mechanical communication with said steering shaft and is disposed in a second housing and existing as a separate unitary subassembly prior to being installed on said first housing. The electric motor is mounted to one of the first housing and second housing and has an output shaft that is in mechanical communication with the steering shaft for imparting a torque to the steering shaft.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a hand wheel actuator for a steer-by-wire system of a vehicle.  
         BACKGROUND  
         [0002]    Automobiles are conventionally equipped with a pair of front road wheels that are steered to enable the vehicle to turn left and right as it maneuvers on land. It is also known to provide for steering rear wheels in automobiles. In the past, vehicle steering systems commonly employed a mechanical linkage between the driver-operated hand wheel and the front road wheels of an automotive vehicle. As the driver rotated the hand wheel, a mechanical linkage through the vehicle&#39;s tie-rods actuated the road wheels sometimes with the assistance of a power assist steering motor or hydraulic piston.  
           [0003]    Recently, steer-by-wire steering systems have been introduced into automotive vehicles to provide road wheel steering function. Included in a typical steer-by-wire steering system is a hand wheel actuator for monitoring the angular position of the steering wheel, and road wheel motor actuators which are controlled by controllers in response to tracking the sensed angular displacement of the hand wheel from a central position. In contrast to prior steering systems, the steer-by-wire steering system does not employ a mechanical linkage between the steering wheel and the individual road wheels. 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.  
           [0004]    Prior art hand wheel actuators typically include a number of components including a hand wheel actuator, an electronic control unit, and a road wheel actuator. The hand wheel actuator includes sensors and motors for sensing the position of the hand wheel and/or torque transmitted through the steering shaft. The hand wheel actuator also includes an electric motor, which provides feedback to the driver and a agreeable steering feel. Thus, the hand wheel actuator and the electronic controller are in constant communication.  
           [0005]    Previously, the electronic controller was housed in any convenient location behind the dashboard of the vehicle or in the engine compartment. However, this requires additional assembly steps.  
           [0006]    Furthermore, as mentioned previously, hand wheel actuators typically include an electric motor to provide feedback to the driver and improve the feel of the hand wheel. Drivers expect hand wheels to have a certain feel that is consistent with a mechanical connection with the road wheels. However, absent such a mechanical connection, aspects of this desired feel must be synthesized. The required feel typically includes some inertia, some friction-related drag, and some centering force that biases the hand wheel toward a center position and a positive stop to limit rotation of the hand wheel to correspond to the limits of rotation of the road wheels. It would be desirable to provide such a mechanical steering feel enhancement device to provide as many aspects of the desired steering feel as possible in order to relieve the electric motor of these tasks. However, prior art mechanical steering feel enhancement devices required extra assembly steps and were therefore cumbersome to install. In addition, prior art return-to-center springs have had the draw back that the force is linearly proportional to displacement. It would be desirable to provide a mechanical steering feel enhancement device that is simple to install or assemble, and provides a satisfactory steering feel.  
         SUMMARY  
         [0007]    The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a hand wheel actuator for a steer-by-wire steering system having a steering feel that meets drivers&#39; expectations is provided with a first housing having an aperture, a steering shaft extending through the aperture and rotatably supported by the housing, an electric motor, a position sensor and torque sensor, and a mechanical steering feel enhancement device. The position and torque sensors are positioned within said first housing and configured to detect the angular displacement of the steering shaft and torque applied against the steering shaft respectively. The mechanical steering feel enhancement device is attached to the first housing and provides at least one of a positive stop function, a return-to-center function, or a friction/weight function by mechanical means. The mechanical steering feel enhancement device is in mechanical communication with said steering shaft and is disposed in a second housing and existing as a separate unitary subassembly prior to being installed on said first housing. The electric motor is mounted to one of the first housing and second housing and has an output shaft that is in mechanical communication with the steering shaft for imparting a torque to the steering shaft. 
       
    
    
       [0008]    The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings, in which:  
         [0009]    [0009]FIG. 1 shows a first schematic diagram of a steer-by-wire system;  
         [0010]    [0010]FIG. 2 shows a second schematic diagram of a steer-by-wire system;  
         [0011]    [0011]FIG. 3 shows a perspective view of a driver interface electronic module;  
         [0012]    [0012]FIG. 4 shows a plan view of a driver interface electronic module;  
         [0013]    [0013]FIG. 5 shows a cross-section view of a first embodiment of a hand wheel actuator incorporating the driver interface electronic module of FIGS. 3 and 4;  
         [0014]    [0014]FIG. 6 shows a front view of a stop gear of the hand wheel actuator of FIG. 5;  
         [0015]    [0015]FIG. 7 shows a second embodiment of a hand wheel actuator;  
         [0016]    [0016]FIG. 8 shows a front view of a return-to-center and steering feel enhancement device of the hand wheel actuator of FIG. 7;  
         [0017]    [0017]FIG. 9 shows a cross-section view of a hand wheel actuator having a constant-force spring;  
         [0018]    [0018]FIG. 10 shows a diagram depicting the operation of the constant-force spring of the actuator shown in FIG. 9;  
         [0019]    [0019]FIG. 11 shows a cross-section view of another embodiment of a hand wheel actuator having a constant-force spring; and  
         [0020]    [0020]FIG. 12 shows yet another embodiment of a hand wheel actuator having a constant-force spring. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 shows a schematic representation of a steer-by-wire system comprising a hand wheel actuator  10 , road wheel actuator  8 , and data bus  2 . Hand wheel actuator  10  comprises a hand wheel  12  connected via a steering shaft to a feedback device  14  that provides driver feedback. Torque and position sensors are mounted to driver interface electronic module (DIEM)  3 , which also includes the electronic control unit for controlling road wheel actuator  8  via data bus  2  and feedback device  14  via wiring  11 . Signals to and from road wheel actuator  8  are multiplexed on data bus  2  as is known in the field of distributed (networked) control. By utilizing a network rather than direct signal lines as known in the prior art, the steer-by-wire system may be integrated into an overall chassis control system including an integrated chassis control unit (ICCU)  7 , which may include, active braking control, active suspension, active roll, and vehicle state sensors including, but not limited to, yaw rate, lateral acceleration, and roll rate. Plural data busses  2  may be provided for redundancy while still reducing the overall number of wiring connections required.  
         [0022]    The second communication bus  4  provides interface and data exchange with non-chassis systems such as the main electrical system, vehicle diagnostics, and multi-media systems. Thus displays and controls incorporated into DIEM  3  can communicate with these auxiliary systems via second communications bus  4 .  
         [0023]    [0023]FIG. 2 shows an alternative embodiment of a steer-by-wire system in which data bus  2  is replaced with signal lines  15 . Because the electronic control unit for the steer-by-wire system is incorporated into DIEM  3 , extra steps required for installing the electronic control unit separately are not required.  
         [0024]    DIEM  3  will now be described in more detail with reference to FIGS. 3 and 4. DIEM  3  is an electronic control center that includes a control module  22  that houses various control electronics including an electronic control unit  5  for hand the steer-by-wire system as described above. DIEM  3  is designed to be positioned just behind hand wheel  12  (FIGS. 1, 2) and is mounted within a steering shaft housing (not shown) and includes a hole  24  through which a steering shaft  13  passes, permitting position and torque sensors (further described below) to be mounted to DIEM  3 , thereby further reducing complexity of assembly. DIEM  3  is modular, in that it exists as a subassembly prior to being mounted on steering shaft housing  30 , thereby providing a simpler assembly than mounting each component included in DIEM  3  indificually. Further information regarding DIEM  3  is available in commonly-assigned U.S. patent application Ser. No. 09/858,236 entitled, “Distributed Control Architecture For Mechatronic Automotive Systems” and filed May 15, 2001 by Scott A. Millsap, et al., and which is wholly incorporated herein by reference.  
         [0025]    One or more multi-function stalk switches  28  are connected to DIEM  3  to allow the driver to control various aspects of the vehicle, such as, for example, turn signals, headlights, windshield wipers, cruise control, entertainment system, etc., as is generally known in the art. An electrical connector  21  may be provided for connection to the vehicle horn and airbag actuator. Actuation of the various controls generates output signals via connector  23  to signal line  16  and multiplexed data bus  4  (FIG. 1) so that the proper vehicle component will receive the driver&#39;s instructions and be able to respond appropriately. Alternatively, signal lines to and from driver interface controls may be passed directly to the associated component via a wiring harness (not shown) as is generally known. As will be further described below, DIEM  3  also includes at least torque and at least one position sensor to detect the position of hand wheel  12  and torque on steering shaft  13 . This information is processed within control module  22  which includes an electronic control unit  5  for the steer-by-wire system as described above. Signals to and from road wheel actuator  8  are transmitted via connector  25 , signal line  15 , and data bus  2  as described above with respect to FIG. 1. Alternatively, such signals are passed directly to and from road wheel actuator  
         [0026]    Electronic control unit  5  may be integrated with the various other functions of control module  22  discussed above or it may include distinct electronics. Furthermore, multiple electronic control units  5  (only one shown) may be provided for redundancy.  
         [0027]    [0027]FIG. 5 shows a first embodiment of a hand wheel actuator  10  for a steer-by-wire system. A steering shaft is mounted for rotation on bearings  32  within steering shaft housing  30 . Top end  33  of steering shaft  13  is adapted to be attached to a hand wheel  12  (see FIGS. 1, 2). DIEM  3  is attached to steering shaft housing  30  adjacent to a top end  33  of steering shaft  13  so that it is positioned just behind hand wheel  12  in use (as shown in FIGS. 1, 2). DIEM  3  includes a position sensor  36 , a torque sensor  34  accurately supported over shaft  13  by bearings  31 , and control module  22 . Plural sensors may be provided for redundancy (not shown). Torque sensor  34  is a non-compliant type torque sensor that does not require a torsion bar. Steering shaft  13  includes section  17  connected via pins  39  into steering shaft  13 . Section  17  provides a predictable response to torque, which is measured by non-compliant torque sensor  34  in the known manner. For more information on this technology, please refer to U.S. patent application Ser. No. 09/825,794 entitled, “Steering Column With Non-Compliant Torque Sensor” filed Apr. 4, 2001 by Ratko Menjak, and is wholly incorporated herein by reference.  
         [0028]    Motor  40  provides the motive force of feedback to the driver operating hand wheel  12 . Motor  40  is connected via transmission  45  to steering shaft  13 . Transmission  45  includes a motor pulley mounted onto or formed into the output shaft of motor  40 , a pulley  46  fixed to shaft  13 , and a belt  44  extending around motor pulley  42  and pulley  46 , thereby placing motor  40  into mechanical communication with steering shaft  13 .  
         [0029]    Steering shaft housing  30  also includes a steering enhancement device in the form of positive stop mechanism  50 . Positive stop mechanism  50  engages a pinion  151  attached to or formed into steering shaft  13 . Pinion  151  engages stop gear  153  mounted onto pin  155  for rotation about its axis. Pin  155  is supported by housing portion  152 . Stop gear  153  includes teeth for engaging pinion  151  and stop tooth  154  (see FIG. 6) for limiting rotation of stop gear  153  and pinion  151 , and therefore steering shaft  13 . Preferably, though not necessarily, pinion  151  and stop gear  153  are sized to permit steering shaft  13  to rotate 540° (1½ revolutions) in either direction from a central position. By incorporating positive stop mechanism into steering shaft housing  30 , fewer assembly steps are required during manufacture of hand wheel actuator  10 .  
         [0030]    Return-to-center device  90  comprises a spring-loaded ball-screw mechanism connected to a lower end of steering shaft  13  for providing a biasing torque against steering shaft  13  that biases steering shaft  13  towards the center position. Details of the operation of this type of return-to-center device is provided in commonly-assigned U.S. patent application Ser. 10/068,703 filed Feb. 4, 2002 by Ratko Menjak et al., which is wholly incorporated herein by reference.  
         [0031]    Each of motor  40 , positive stop mechanism  50 , and return-to-center device  90  contribute to driver feed-back sensations. Mechanical feedback devices such as positive stop mechanism  50  and return-to-center device  90  reduce load and power requirements of motor  40 . For example, a return-to-center device  90  provides a biasing force to return the steering wheel to a central position while electric motor and transmission  45 , which may include gear reduction means, provides feedback from the road wheels to the hand wheel. A stronger electric motor  40  may eliminate the need for gear reduction in transmission  45 . For closed-loop control of motor  40 , it is necessary that motor  40  be in mechanical communication with steering shaft  13  on a side of torque sensor  34  opposite of hand wheel  12 . However, positive stop mechanism and return-to-center device may be on either side of torque sensor  34 .  
         [0032]    [0032]FIGS. 7 and 8, e.g., show a second embodiment of a hand wheel actuator  10 . In this device, DIEM  3  (not shown) may be attached at top end  33  of steering shaft  13 . In this embodiment, however, redundant position sensors  36 ,  38  and non-compliant torque sensor  34  are integrated in a single housing  30  with positive stop mechanism  50 , which is positioned between top end  33  and section  17 , the latter being coupled to non-compliant torque sensor  34  fir sensing applied torque.  
         [0033]    Positive stop mechanism  50  includes housing  62  that is attached to steering shaft housing  30  in a conventional manner. Positive stop mechanism  50  comprises a pinion  51  formed or attached to steering shaft  13 , the pinion  51  engaging an inside gear  52  as seen in FIG. 8. Inside gear  52  rotates on shaft  53  supported by housing  62 , shaft  53  being positioned parallel and adjacent to steering shaft  13 . Inside gear  52  includes a sloped outer surface  54  that interacts with roller cam  55  to provide a return-to-center function and improved steering feel. Roller  55  supported by roller support  56 , which includes a housing and compression spring  57  for biasing roller cam  55  against sloped outer surface  54 . As steering shaft  13  rotates, pinion  51  causes inside gear  52  to rotate on shaft  53 . Sloped surface  54  forces roller cam  55  out, causing spring  57  to compress. When inside gear  52  rotates close to 180°, pinion  51  engages stop surface  59 , thereby preventing further rotation of either the inside gear  52  or pinion  51  and thereby providing a positive stop function.  
         [0034]    Pinion  51  and inside gear  52  are preferably sized to permit the steering shaft  13  to rotate one and one-half turns in either direction from a centered direction, though other amounts are possible. The force of roller cam  55  against sloped outer surface  54  caused by spring  57  generates a torque against inside gear  52 , which provides a return-to-center function against steering shaft  13 . Use of a roller cam  55  improves steering feel, and by shaping the sloped outer surface  54 , the torque profile of the return-to-center function can be customized. For example, a uniform torque can be provided, or the torque can be increased at the extremes of rotation by increasing the slope with respect to the tangent at either end of the rotation. Furthermore, it should be noted that the position of roller cam  55  with respect to pinion  51  need not be aligned; the roller cam may be positioned at any convenient location along the circumference of the inside gear  52 , with the understanding that the sloped outer surface must also be adjusted to accommodate the location of roller cam  55  for returning the steering shaft to a center location.  
         [0035]    Assembly of positive stop mechanism simply requires fitting housing  62  having inside gear  52  and roller cam and roller support  56  preassembled thereto over steering shaft top end  33  and attaching housing  62  to steering shaft housing  30  so that inside gear  52  engages pinion  51 .  
         [0036]    Motor  40  is coupled via transmission  45 , which may include a planetary gear set to reduce the turning ratio, to a lower end of steering shaft  13 . Note that torque sensor  34  will only detect torque exerted against motor  40 . Redundant position sensors  36 ,  38  are positioned on steering shaft  13  between non-compliant torque sensor  34  and transmission  45 .  
         [0037]    [0037]FIGS. 9 and 10 show an embodiment of a hand wheel actuator  10  incorporating a constant-force spring  80 . Shown here is a hand wheel actuator similar to previous embodiments having steering shaft  13  with a DIEM  3  positioned at top end  33  of shaft  13  and motor  40  engaging steering shaft  13  via transmission  45  at a bottom end thereof. DIEM  3  includes position sensors and a non-compliant torque sensor as discussed above with respect to the first embodiment shown in FIG. 5. Positioned on a lower portion  48  of steering shaft  13  intermediate of DIEM  3  and transmission  45  is steering return/resistance device  70 .  
         [0038]    Steering return/resistance device  70  is incorporated into steering shaft housing  30  and includes an auxiliary shaft  78  in mechanical communication with steering shaft  13  via a first pulley  71  attached to or formed into shaft  13 , a second pulley  75  attached to or formed into auxiliary shaft  78 , and a belt  73  engaging both the first pulley  71  and second pulley  75 . Belt  73  is preferably a timing belt, i.e., has teeth or cogs that mate with mating formations on first and second pulleys  71 ,  75 , for maintaining a constant turn ratio between first and second pulleys  71 ,  75 . Auxiliary shaft  78  is supported on ball bearings positioned at either end of auxiliary shaft  78 . Fixed or otherwise formed on shaft  78  is a first spool  82  upon which a length of constant force spring  80  is wound. An additional length of constant force spring  80  is wound on second spool  84 , which is fixed or otherwise formed into steering shaft  13 .  
         [0039]    Constant force spring  80 , when not stressed, appears as shown in profile in FIG. 10 in a central position in which a first segment is spiral shaped, a second central segment is relatively straight, and a third segment is spiral shaped. As the relatively straight central segment is wound on either first spool  82  or second spool  84 , it exerts a torque against the pulley tending the pulley back to the unstressed central position, thereby providing an effective return-to-center function. Each end of constant force spring  80  is fixed to a respective one of the spools. When steering shaft  13  reaches a limit of rotation, e.g., 540° or one and one-half revolution from the central position, the constant force spring is completely unwound from one of the spools, thereby preventing further rotation. In addition to providing a return-to-center function, constant force spring  80  enhances the overall steering feel felt by the driver, and approximates a driver&#39;s expectations.  
         [0040]    Other variations on the use of the constant force spring can be envisioned. For example, belt  73  may be replaced with a chain. Alternatively a pair of mating gears can be used to provide mechanical communication between steering shaft  13  and auxiliary shaft  78 . In this case, the constant force spring would be wound in opposite directions around the spools because the relative motion of the gears would require it.  
         [0041]    [0041]FIG. 11 shows another embodiment of a hand wheel actuator  10  having a steering return/resistance device  70 . In this case, as with the second embodiment discussed above with respect to FIGS. 7 and 8, DIEM  3  does not include torque or position sensors, or includes only position sensors. Instead, redundant position sensors  36 ,  38  and non-compliant torque sensor  34  are integrated in a single housing  30  with steering return/resistance device  70 . Steering return/resistance device  70  is essentially the same as previously discussed with respect to FIGS. 9 and 10.  
         [0042]    [0042]FIG. 12 shows yet another embodiment of a hand wheel actuator  10 . This embodiment is similar to the previous embodiment shown in FIG. 11 with the only difference being a relocation of motor  40  from the lower end of shaft  13 . In this embodiment, shaft  13  includes a pulley  46  positioned on steering shaft  13  intermediate of position sensors  36 ,  38  and steering return/resistance device  70 . A belt  44  extends between pulley  46  and a small pulley  42  positioned on an output shaft of motor  40 . Transmission  45  therefore comprises a belt transmission assembly for conveying torque from motor  40  to steering shaft  13 .  
         [0043]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Terms such as “first” and “second” are used herein merely to distinguish between two like elements, and are not intended to imply an order such as of importance or location. It is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.