Drive-by-wire steering systems having steering wheel return mechanism

[0002] This disclosure relates generally to drive-by-wire steering systems. More specifically, this disclosure relates to drive-by-wire steering systems having a steering wheel return mechanism.

[0003] Vehicles require a steering system to control the direction of travel. Previously, mechanical steering systems have been used. Mechanical steering systems typically include a mechanical linkage or a mechanical connection between the steering wheel and the vehicle's road wheels. Thus, movement of the steering wheel causes a corresponding movement of the road wheels. Movement of such mechanical systems is often power assisted through the use of hydraulic assists or electric motors.

[0004] Mechanical steering systems are being replaced and/or supplemented by electrically driven steering systems, commonly known as “steer-by-wire” systems. Such steer-by-wire systems to varying extents replace, for example, the mechanical linkage between the steering wheel and the vehicle wheels with an electrically assisted system.

[0005] This migration to steer-by-wire systems is being made to improve fuel economy, increase vehicle modularity, reduce load on the engine of the vehicle, reduce vehicle weight, and provide four-wheel-steering. For example, the use of steer-by-wire systems eliminates the need for hydraulic fluids, provides a tighter turning radius, and reduces the weight of the vehicle.

[0006] Additionally, steer-by-wire systems eliminate various undesirable problems present in mechanical systems. For example in steer-by-wire systems, the steering wheel is mechanically isolated from the road wheels. Thus, excessive deleterious feed back to the steering wheel in the form of shudders, and steering wheel kickback from the road wheels is eliminated.

[0007] Unfortunately, mechanically isolating the steering wheel from the road wheel also eliminates desired feed back. For example, during the use of mechanical steering systems, an operator applies a force to the steering wheel to turn the road wheels of the vehicle. After releasing the turning force on the steering wheel, the gyroscopic and other forces on the road wheels tend to act on the mechanical steering system to return the steering wheel to its normal or center position. Unfortunately, the mechanical isolation provided by drive-by-wire steering systems eliminates this desired feedback. Namely, during the use of drive-by-wire steering systems, the steering wheel maintains its turned position after being released instead or returning to its center position.

[0008] In vehicles having mechanical steering systems, the force applied by the operator to the steering wheel to turn the road wheels of the vehicle is typically proportion to the amount or degree of vehicle turn desired. Namely, in order to turn the vehicle slightly, only a slight force must be applied to the steering wheel. Conversely, in order to turn the vehicle sharply, a large force must be applied. It is known to provide mechanical steering systems with power assistance through the use of hydraulic assists or electric motors to reduce the amount of force applied to the steering wheel necessary to turn the road wheels. Thus, it has been seen that “over assisting” by removing all of the force associated with turning the vehicle, or even making the force required to turn the vehicle constant regardless of the degree of turn changes the “feel” of the steering system.

[0009] In mechanical systems, the amount of assistance applied by the hydraulic assists or electric motors has been regulated so as to avoid these “over assist” problems. However, during the use of drive-by-wire steering systems the force applied to the steering wheel necessary to turn the wheels is both minimal and constant due to the mechanical isolation of the steering wheel from the road wheels. Thus, prior drive-by-wire steering systems often suffer from the same problems experienced in overly assisted mechanical steering systems.

[0010] A drive-by wire steering system comprising a steering shaft and a steering wheel return mechanism is provided. The steering shaft is configured for a first angular displacement about a first axis. The steering shaft comprises a first geared portion and a first end, where the first end is connectable to a vehicle's steering wheel. The steering wheel return mechanism comprises a torsion member and a second geared portion. The first geared portion and the second geared portion are operatively engaged such that the first angular displacement of the steering shaft imparts a second angular displacement about a second axis to the steering wheel return mechanism. The torsion member is configured to generate a returning torque on the steering wheel return mechanism upon the second angular displacement of the steering wheel return mechanism. The returning torque has a direction opposite the first angular displacement, and is proportional to the second angular displacement.

[0011] A method of returning a steering shaft in a drive-by wire steering system to a center position is provided. The method comprises engaging a steering wheel return mechanism to the steering shaft and connecting a torsion member to the steering wheel return mechanism. The steering wheel return mechanism is engaged to a steering shaft such that an angular displacement of the steering shaft about a first axis imparts a second angular displacement about a second axis to the steering wheel return mechanism. The steering shaft is connectable to the vehicle's steering wheel. The torsion member is connected to the steering wheel return mechanism such that the torsion member generates a returning torque on the steering wheel return mechanism when subjected to the second angular displacement. The returning torque acts on the steering wheel return mechanism to return and maintain the steering wheel at the center position.

[0012] A method of improving the stability of a vehicle having a drive-by wire steering system is provided. The method includes calculating a maximum desirable angular displacement of a steering wheel from vehicle conditions, comparing an actual angular displacement of the steering wheel to the maximum desirable angular displacement, and activating an over-rotation device if the actual angular displacement is greater than or equal to the maximum desirable angular displacement. The vehicle conditions include sensed inputs and constant inputs. Upon activation of the over-rotation, the steering wheel is not displaceable past the maximum desirable angular displacement.

[0013] The above-described and other features are appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.

[0014] FIG. 1 is a schematic view of a steer-by wire system for a vehicle;

[0015] FIG. 2 is a sectional view of an exemplary embodiment of steer-by wire system;

[0016] FIG. 3 is an exemplary embodiment of a return to center mechanism of the steer-by wire system of FIG. 2, taken along circle 3—3;

[0017] FIG. 4 is a sectional view of the center feel mechanism of FIG. 3, taken along lines 4—4;

[0018] FIG. 5 is a view of the center feel mechanism of FIG. 4 in a first position;

[0019] FIG. 6 is a view of the center feel mechanism of FIG. 4 in a second position;

[0020] FIG. 7 is a view of the center feel mechanism of FIG. 4 in a third position;

[0021] FIG. 8 is a sectional view of an alternated exemplary embodiment of steer-by wire system;

[0022] FIG. 9 is an alternate exemplary embodiment of a return to center mechanism of the steer-by wire system of FIG. 8, taken along circle 8—8;

[0023] FIG. 10 is a sectional view of the center feel mechanism of FIG. 9, taken along lines 10—10;

[0024] FIG. 11 is an exemplary embodiment of a steering wheel return mechanism;

[0025] FIG. 12 is a sectional view of the center feel mechanism of FIG. 11, taken along lines 12—12;

[0026] FIG. 13 is a sectional view of the center feel mechanism of FIG. 11, taken along lines 13—13; and

[0027] FIG. 14 is an exemplary embodiment of an algorithm for a steering wheel over rotation device.

[0028] Referring now to FIG. 1, a drive-by-wire steering system 10 for use in a vehicle 11 is illustrated. The steering system 10 allows the operator of the vehicle 11 to control the direction of the road wheels 12 through the manipulation of a steering wheel 14. The steering wheel 14 is operatively coupled to a steering column or shaft 16. The steering column 16 is installed in a main housing 18 such that the column is rotatable within the housing.

[0029] The road wheels 12 are connected to knuckles 20, which are in turn connected to tie rods 22. The tie rods 22 are connected to a steering assembly 24. The steering assembly 24 comprises an electric motor 26 and a steering rod 28. The steering rod 28 is operatively coupled to the electric motor 26 such that the motor is adapted to move steering rod 28. The lateral movement of the steering rod 28 controls the direction of the road wheels 12 through the knuckles 20 and tie rods 22 in a known manner.

[0030] One or more sensors 32 detect angular displacement or travel 30 of the steering column 16, as well as detecting the torque of the angular displacement. The sensors 32 provide electric signals 34 to a controller 36 indicative of the angular displacement 30 and torque. The controller 36 sends and receives signals 40 to/from the electric motor 26 to actuate the electric motor in response to the angular displacement 30 of the steering wheel 14.

[0031] In use, the steering wheel 14 is angularly displaced 30 such that the steering column 16 is also angularly displaced. The sensors 32 detect the angular displacement 30 of the column 16, and the sensors send signals 34 to the controller 36 indicative of the relative amount of the angular displacement of the column. The controller 36 sends signals 40 to the motor 26 indicative of the relative amount of the angular displacement 30. In turn, the motor 26 laterally moves the steering rod 28. Thus, the controller 36 controls the distance that the steering rod 28 is moved based on the amount of the angular displacement 30 of the column 16. The movement of the steering rod 28 manipulates the tie rods 22 and knuckles 20 to reposition the road wheels 12 of the vehicle 11. Accordingly, when the steering wheel 14 is turned, the road wheels 12 are turned.

[0032] In mechanical steering systems, the rotation of the steering wheel 14 is limited by the travel of the road wheels 12. This is usually equal to a rotation of the steering wheel 14 of about 1.5 times in either direction. However, in drive-by-wire steering system 10 the steering wheel 14 is mechanically isolated from the road wheels 12. Thus, the rotation of the steering wheel 14 is not limited. Now, it has been determined that limiting the rotation of the steering wheel 14 to about 1.5 times in either direction (e.g. about ±540°) is desirable. Also, limiting movement of the steering wheel 14 to about ±540° protects the sensors from over rotation.

[0033] Referring now to FIG. 2, an exemplary embodiment of a drive-by-wire system 10 is shown. Here, the steering column 16 is rotatably mounted in main housing 18 and a lower housing 42 by way of bearings 44. Lower housing 42 has an upper portion 46 that is connected to a lower portion 48 of main housing 18. For example, a bolt 50 secures lower housing 42 and main housing 18.

[0034] The steering column 16 includes a geared portion 52 defined at its lower end 54, namely at the end opposite steering wheel 14. The geared portion 52 is in operative contact with a center feel mechanism 56. The center feel mechanism 56, as well as the geared portion 52 of the steering column 16, is housed within the lower housing 42.

[0035] The lower end 54 of the steering column 16 is operatively coupled to a secondary shaft 58 by way of a torque sensor 60 having a torsion bar 62. Additionally, position sensors 64 are operatively positioned proximate the steering column 16 and/or the secondary shaft 58 to detect the angular displacement 30 of the steering column and/or the secondary shaft, respectively. The sensors 60 and 64 provide signals 34 to the controller 36.

[0036] For example, the sensor 60 detects characteristics of the movement or angular rotation 30 of the steering column 16 by detecting the torque and speed of the angular displacement of the steering column. However, the sensor 60 operates within a predetermined range of motion. Namely, the sensor 60 typically has a range of motion of about ±540°.

[0037] The secondary shaft 58 is connected to an electric servomotor 66 through a planetary gear reducer 67. The motor 66 is operatively connected to the controller 36. The motor 66, as controlled by controller 36, is configured to angularly displace 30 the secondary shaft 58, which in turn angularly displaces the steering column 16. Accordingly, the steer-by-wire system 10 is configured to control the direction of road wheels 12 without the manipulation of steering wheel 14 by a driver, and is configured to communicate road feel to the-steering wheel 14.

[0038] For example in an exemplary embodiment, the road wheels 12 include a sensor (not shown) configured to detect forces on the road wheels. The sensor provides signals to the controller 36 indicative of such forces on the road wheels 12. The controller 36 actuates the motor 66 in response to such road forces to simulate road feeling on the steering wheel 14.

[0039] Also, the motor 66 is used to return or help return the steering wheel 14 to its center position. For example, after turning the vehicle 11, the operator typically releases the steering wheel 14, expecting the steering wheel to return to its center position as in mechanical steering systems. Once the controller 36 detects via the sensors 60 and 64 that the operator has released the steering wheel 14, the controller activates the servomotor 66 to return the steering wheel to its center position as expected.

[0040] However, the activation of the motor 66 consumes energy from the battery (not shown) of the vehicle 11, and thus reduces the overall efficiency of the vehicle. Additionally, the motor 66 and the planetary gear reducer 67 add weight and expense to the vehicle 11, and take up valuable space within the vehicle.

[0041] Now, it has been determined that the planetary gear reducer 67 can be eliminated or reduced in size and/or weight by incorporating the center feel mechanism 56 having a steering wheel return mechanism 59 into the drive-by-wire system 10. Thus, the drive-by-wire steering system 10 having the center feel mechanism 56 and the return mechanism 59 eliminates the cost and weight of the planetary gear reducer 67, and reduces the size of the steering system 10, while eliminating many of the deleterious effects of mechanically isolating the steering wheel 14 from the road wheels 12. Further, it has been determined that the motor 66 can be reduced in size and/or usage by incorporating the center feel mechanism 56 having the steering wheel return mechanism 59 into the drive-by-wire system 10.

[0042] A first exemplary embodiment of the center feel mechanism 56 is illustrated in FIGS. 2-7. The center feel mechanism 56 is configured to limit rotation of the steering wheel 14 and the steering column 16 to about 1.5 rotations (e.g., ±540°). The center feel mechanism 56 provides the steering column 16 with about 1080° of angular displacement 30. Accordingly, the center feel mechanism 56 improves the feel of the steering system 10 by more closely mimicking the feel of mechanical steering systems, and is configured to protect the sensors 60 and 64.

[0043] The center feel mechanism 56 includes a geared portion 68, a cam face 70, and a stop portion 72. The center feel mechanism 56 defines a home or center position 74 (FIG. 5), a positive or rightmost position 76 (FIG. 7) and a negative or leftmost position 78 (FIG. 6). The stop portion 72 is defined within the geared portion 68, and is located diametrically opposed from the center position 74. Thus, the center feel mechanism 56 defines the positive position 76 and the negative position 78 on either side of the stop portion 72, respectively.

[0044] The center feel mechanism 56 is rotatably mounted on a stop shaft 80 such that the geared portion 68 is engaged with the geared portion 52 of the column 16. Accordingly, the angular displacement 30 of the steering wheel 14 is translated to the center feel mechanism 56 by the geared portions 52 and 68. The rotation of the column 16 about its axis or centerline 82 causes the geared portion 52 to drive the geared portion 68 such that the center feel mechanism 56 rotates about the center feel mechanism axis or centerline 84. In sum, the rotation of the column 16 about the centerline 82 causes the geared portion 52 to impart rotation to drive the geared portion 68 such that the center feel mechanism 56 rotates about the centerline 84. The rotation of the center feel mechanism 56 is limited when the center feel mechanism has rotated to the point where the column 16 abuts or interferes with the stop portion 72 at either the positive position 76 (FIG. 7) or the negative position 78 (FIG. 6).

[0045] Moreover, the geared portions 52 and 68 are configured such that the rotation of the column 16 of about ±540° rotates the center feel mechanism 56 an angle 57 prior to the steering column abutting or interfering with the stop portion 72. In an exemplary embodiment, the angle 57 is about ±140°. Thus, the angular displacement 30 of the steering wheel 14 of +540° rotates the center feel mechanism 56 by angle 57 of about +140° from the center position 74 to the positive position 76, while the angular displacement of the steering wheel of −540° rotates the center feel mechanism by angle about −140° from the center position 74 to the negative position 78. In sum, the steering column 16 has a total range of motion of about 1080° and the center feel mechanism 56 has a total range of motion of about 280°.

[0046] When the center feel mechanism 56 is in the center position 74, the steering wheel 14 is in its center or normal position. In this position, the road wheels 12 are pointed parallel to the vehicle (e.g., line 86 in FIG. 1). However, angular displacement 30 of the steering wheel 14 to its rightmost or positive position causes the center feel mechanism 56 to rotate to the positive position 76. Here, the motor 26 moves the road wheels 12 via the steering assembly 24 such that the road wheels are pointed to the right (e.g., line 88 in FIG. 1). Similarly, angular displacement 30 of the steering wheel 14 to its negative or leftmost position causes the center feel mechanism 56 to rotate to the negative position 78. Again, at this point the motor 26 moves the road wheels 12 via the assembly 24 such that the road wheels are pointed to the left (e.g., line 90 in FIG. 1).

[0047] Of course, it should be recognized that the center feel mechanism 56 is described above by way of example as being configured for the angular displacement 30 of the column 16 of about ±540° translating into rotation of the stop portion 72 of about ±140°. The center feel mechanism 56 configured to provide alternate amounts of angular displacement 30 for the steering wheel 14 and/or the center feel mechanism 56 are contemplated.

[0048] The steering system 10 having the center feel mechanism 56 provides the drive-by wire steering system with the “feel” of a mechanical steering system. Thus, the steering system 10 improves the “drivability” or “feel” of vehicles 11 having such drive-by-wire steering systems. Additionally, the steering system 10 having the center feel mechanism 56 protects the sensors 60 and 64 from over rotation.

[0049] As illustrated, the drive-by-wire steering system 10 controls the direction of both the front and rear sets of road wheels 12 of the vehicle 11. However, control of only the front or rear set of the road wheels 12 is contemplated. Additionally, the steering system 10 is illustrated controlling the front and rear set of road wheels in a similar direction. Of course, the steering system 10 controlling the front and rear set of road wheels 12 in a different direction, and/or controlling the front and rear set of road wheels in a similar different at some speeds, and a different direction at other speeds are contemplated.

[0050] Referring again to FIG. 5, an alternate embodiment of the center feel mechanism 56 is illustrated. Here, the stop portion 72 further includes adjustment screws 92 and 94 shown in phantom. The adjustment screws 92 and 94 are configured to provide the center feel mechanism 56 with the ability to adjust or calibrate the positive position 76 and the negative position 78, respectively. The adjustment screws 92 and 94 are tightenable to the point where the screw(s) protrudes from the stop portion 72 (e.g., past geared portion 68). Conversely, the adjustment screws 92 and 94 are retractable to the point where the screw(s) does not protrude through the stop portion 72.

[0051] Thus, with the adjustment screws 92 and 94 protruding past the geared portion 68 the rotation of the center feel mechanism 56 is limited when the center feel mechanism has rotated to the point where the column 16 abuts the adjustment screw 92 at the positive position 76 or the adjustment screw 94 at the negative position 78. Alternately, with the adjustment screws 92 and 94 retracted to the point where the screw(s) do not protrude past the geared portion 68, the rotation of the center feel mechanism 56 is limited when the center feel mechanism has rotated to the point where the column 16 abuts the stop portion 72 at the positive position 76 or the negative position 78. In this manner, the adjustment screws 92 and 94 are configured to make fine tune or calibration type adjustments to the angle 57. Thus, in the example where the steering column 16 has a total range of motion of about 1080° and the angle 57 of the center feel mechanism 56 is about 280°, the adjustment screws 92 and 94 are configured to make fine tune or calibration type adjustments to the angle 57 of the center feel mechanism 56 by about ±5°.

[0052] In sum, one aspect of the center feel mechanism 56 is that it provides the steering system 10 with stop portion 72, which prevents the over rotation of the sensors 60 and 64, and more closely mimics a mechanical steering system by preventing infinite rotation of the steering wheel 14.

[0053] Referring again to FIGS. 2-3, the center feel mechanism 56 also provides the steering system 10 with the ability to mimic or simulate a mechanical steering system by providing desired feedback to the steering wheel 14. For example, the center feel mechanism 56 is configured to apply a returning torque (Tr) to the steering wheel 14. The returning torque (Tr) has a direction opposite that of the angular displacement 30. During a turn of the vehicle 11, the returning torque (Tr) provides a resistance to the angular displacement 30 to mimic mechanical steering systems. The center feel mechanism 56 is configured such that the returning torque (Tr) applied to the steering wheel 14 by the center feel mechanism 56 is proportional to the degree of turn of the steering wheel to more closely mimic or simulate the “feel” of a mechanical steering system. Additionally, after the completion of a turn of the vehicle 11, the returning torque (Tr) acts to return the center feel mechanism 56 to the center position 74.

[0054] The center feel mechanism 56 includes a cam face 70 and an urging member 96. The urging member 96 comprises a cam follower 98 biased into operative engagement with the cam face 70. The housing 42 includes an extension portion 100 extending radially outward therefrom. The cam follower 98 is rotatably mounted on a riser 102 by way of a bolt 104. Thus, the cam follower 98 is rotatable about an axis 106 that is parallel to the centerline 84 of rotation of the center feel mechanism 56.

[0055] A compression member 108 is retained in the extension portion 100 by way of a cap 110. The riser 102 is slidably retained in the extension portion 100 such that the cam follower 98 moves radially toward and away from the cam face 70. The compression member 108 is configured to exert a spring force (Fo) on the riser 102 to bias the riser toward the cam face 70. In this way, the cam follower 98 is in operative engagement with the cam face 70.

[0056] The cam face 70 has a detent 112 defined at center position 74 and a cam profile 114. The cam profile 114 is defined on both sides of the detent 112, and each side is preferably symmetrical to the other. The cam profile 114 is configured to translate the spring force (Fo) of the compression member 108 into the returning torque (Tr) on the mechanism 56.

[0057] In use, the angular displacement 30 of the steering wheel 14 rotates the column 16 about the centerline 82 to cause the geared portion 52 to drive the geared portion 68 to rotate that the center feel mechanism 56 about the centerline 84. The rotation of the center feel mechanism 56 about centerline 84 causes the cam face 70 to act upon the cam follower 98. The cam follower 98 rides along the cam profile 114 of the cam face 70 by overcoming the spring force (Fo) exerted by the compression member 108. By overcoming the spring force (Fo) exerted by the compression member 108, the riser 102 is slid radially away from the cam face 70, which further compresses the compression member and further increases the spring force (Fo). Thus, the spring force (Fo) of the compression member 108 is translated into the returning torque (Tr) by the contact of the cam follower 98 and the cam profile 114.

[0058] The action of the cam profile 114, the detent 112, and the compression member 108 create a returning torque (Tr) on the center feel mechanism 56. Thus, the center feel mechanism 56 transmits the returning torque (Tr) to the column 16. In this manner, the center feel mechanism 56 has the tendency to return and maintain the steering wheel 14 in position 74, which provides the steering system 10 with a center feeling.

[0059] The cam profile 114 is configured to provide the returning torque (Tr) with a variable resistance as a function of the amount of the angular displacement 30 of the steering wheel 14. Thus, the center feel mechanism 56 prevents and/or mitigates the feeling that the drive-by-wire system 10 “over assists” by removing all of the force associated with turning the vehicle 11, or even making the force required to turn the vehicle constant regardless of the degree of turn. Thus, incorporating the center feel mechanism 56 into the drive-by-wire system 10 enables the elimination of and/or reduction in size of the planetary gear reducer 67. Further, incorporating the center feel mechanism 56 into the drive-by-wire system 10 enables the reduction in size and/or usage of the motor 66.

[0060] In an alternate embodiment, the cap 110 is secured to the extension portion 100 by way of a thread 116. The thread 116 is configured such that the cap 110 is adjustable to increase and/or decrease the force with which the compression member 108 biases the cam follower 98 into the cam face 70.

[0061] A second exemplary embodiment of the center feel mechanism 56 is illustrated in FIGS. 8-10.

[0062] Again, the center feel mechanism 56 includes the cam face 70 and urging member 96. In this embodiment, the urging member 96 comprises two cam followers 98 biased toward the cam face 70. The housing 42 includes the extension portion 100 extending radially outward therefrom. The cam followers 98 are rotatably mounted on the riser 102 by way of the bolts 104. Thus, the cam followers 98 are rotatable about the axes 106 that are parallel to the centerline 84 of rotation of the center feel mechanism 56.

[0063] The compression member 108 is retained in the extension portion 100 by way of the cap 110. The riser 102 is slidably retained in the extension portion 100 such that the cam followers 98 are adapted to move radially toward and away from the cam face 70. The compression member 108 acts on the riser 102 to slide the riser toward the cam face 70 such that only one of the cam followers 98 is in operative engagement with the cam face.

[0064] The cam face 70 has a cam profile 114 comprising three zones, namely first zone 118, second zone 120, and third zone 122. The first zone 118 runs along the cam face 70 between points 124. The second zone 120 runs along the cam face 70 between point 124 and point 126. The third zone 122 runs along the cam face 70 between point 126 and point 128.

[0065] The first zone 118 has a constant radius from the centerline 84 of the center feel mechanism 56. The center feel mechanism 56 is at the starting position 74 when the cam followers 98 are positioned at points 124, namely when the center feel mechanism is in the first zone 118. The second zones 120 have a radius from the centerline 84 of the center feel mechanism 56 that increases sharply from point 124 to point 126. However, the third zones 122 have a radius from the centerline 84 of the center feel mechanism 56 that increases mildly from point 126 to point 128.

[0066] The zones 118, 120, and 122 result in a cam profile 114 that provides variable return torque (Tr) to the steering wheel 14 depending upon the degree of angular displacement 30. The angular displacement 30 of the steering wheel 14 rotates the column 16 about centerline 82 such that the center feel mechanism 56 rotates about its centerline 84. The rotation of the center feel mechanism 56 about its centerline 84 causes the cam face 70 to act upon the cam followers 98. The leading cam follower 98 (i.e., the cam follower in the direction of rotation) rides along the cam profile 114 of the cam face 70. This overcomes the spring force (Fo) exerted by the compression member 108 to slide the riser 102 radially away from the cam face 70. At this point, the trailing cam follower 98 (i.e., the cam follower in opposite the direction of rotation) is no longer in contact with the cam face 70 such that the spring forces (Fo) of the compression member 108 are transferred only at the cam follower in contact with the cam face.

[0067] The returning torque (Tr) is equal to the spring force (Fo) of the compression member 108 multiplied by the distance (X) the spring force is applied from the centerline 84 (e.g., Tr=Fo*X). As provided above, the zones 118, 120, and 122 of the cam profile 114 have differing radii, which increase from point 124 to point 128. In use, the angular displacement 30 of the mechanism 56 causes the compression member 108 to be compressed an amount corresponding to the increase in the radii of the cam profile 114. Thus, the spring force (Fo) of the compression member 108 increases as the cam followers 98 are rotated along the cam profile 114. This causes a corresponding increase in the returning torque (Tr) as the cam followers 98 are rotated along the cam profile 114.

[0068] Accordingly, the action of the cam profile 114 and the compression member 108 create the returning torque (Tr) on the center feel mechanism 56. The returning torque (Tr) on the center feel mechanism 56 provides the steering system 10 with the tendency to return and maintain the steering wheel 14 in the center position 74. Additionally, the returning torque (Tr) on the center feel mechanism 56 increases as the mechanism is angularly rotated 30 from the center position 74 towards either the rightmost position 76 or the leftmost position 78.

[0069] For example, a return torque (Tr) of about 1 Newton-meter (nm) is required to move the center feel mechanism 56 from first zone 118. The angular rotation 30 of the steering wheel 14 from the first zone 118 to the end of second zone 120 (e.g. about ±300° of steering wheel 14 rotation) causes a subsequent increase the torque from 1 nm to about 3 nm. Finally, the returning torque (Tr) increase from about 3 nm to about 4.5 nm from the end of the second zone 120 to the end of third zone 122 (e.g., about ±300° to about ±540° of steering wheel 14 rotation). It should be recognized that the returning torques (Tr) provided above for zones 118, 120, and 122 are provided by way of example only. Of course larger or smaller returning torques, more or less cam zones, and the like are contemplated.

[0070] It is seen that the center feel mechanism 56 provides the returning torque (Tr) with a variable resistance as a function of the amount of angular displacement 30 of the steering wheel 14. Thus, the center feel mechanism 56 prevents and/or mitigates the feeling that the drive-by-wire system 10 “over assists” the driver by removing all of the force associated with turning the vehicle 11. Further, the center feel mechanism 56 prevents and/or mitigates the feeling that the drive-by-wire system 10 “over assists” the driver by making the force required to turn the vehicle constant regardless of the degree of turn. Accordingly, incorporating the center feel mechanism 56 into the drive-by-wire system 10 enables the elimination of and/or reduction in size of the planetary gear reducer 67. Further, incorporating the center feel mechanism 56 into the drive-by-wire system 10 enables the reduction in size and/or usage of the motor 66.

[0071] In an alternate embodiment, the cap 110 is secured to the extension portion 100 by way of a thread 116. The thread 116 is configured such that the cap 110 is adjustable to increase and/or decrease the spring force (Fo) with which the compression member 108 biases the follower 98 into the cam face 70. The compression member 108 is preloaded to a higher spring force (Fo) by tightening the cap 110, and the compression member is relaxed to a lower spring force (Fo) by loosening the cap.

[0072] Referring now to FIGS. 11-13, an exemplary embodiment of the steering wheel return mechanism 59 is illustrated. The steering wheel return mechanism 59 is configured to supplement and/or replace the returning torque (Tr) generated by the center feel mechanism 56 with a second returning torque (STr). In this embodiment, second returning torque (STr) eliminates and/or mitigates the need for the planetary gear reducer 67 and or the electric motor 66.

[0073] The return mechanism 59 comprises a torsion member 130 configured to generate the second returning torque (STr) on the center feel mechanism 56. The housing 42 includes a finger 132. Then center feel mechanism 56 includes a post 134. The torsion member 130 is connected to the housing 42 at finger 132. Additionally, the torsion member 130 is disposed about the column 16 such that the post 134 acts as a fulcrum about the centerline 136 of the post. Thus, the angular displacement 30 of the center feel mechanism 56 about its centerline 84 causes the finger 132 and the post 134 to load the torsion member 130. In this manner, the torsion member 130 generates a torsion spring force (TFo) on the center feel mechanism 56. The degree of the angular displacement 30 of the center feel mechanism 56 about its centerline 84 is proportional to magnitude of the torsion spring force (TFo) that is generated.

[0074] The second returning torque (STr) is equal to the torsion spring force (TFo) of the torsion member 130 multiplied by the distance (X) the torsion spring force is applied from the centerline 84 (e.g., STr=TFo*X). In this example, the distance (X) is the distance between the centerline 84 of the column 16 and the centerline 136 of the post 134.

[0075] It is seen that the return mechanism 59 is configured to provide the second returning torque (STr) with a variable resistance as a function of the amount of angular displacement 30 of the steering wheel 14. Thus, the return mechanism 59 prevents and/or mitigates the feeling that the drive-by-wire system 10 “over assists” the driver by removing all of the force associated with turning the vehicle 11. Further, the return mechanism 59 prevents and/or mitigates the feeling that the drive-by-wire system 10 “over assists” the driver by making the force required to turn the vehicle constant regardless of the degree of turn. Finally, the return mechanism 59 is configured to eliminate and or mitigate the need for the motor 66 and the planetary gear reducer 67 through the generation of the second returning torque (STr).

[0076] It is contemplated to use the return mechanism 59 to generate the second returning torque (STr) either alone, or in conjunction with the center feel mechanism 56 generating the returning torque (Tr). Thus, the return mechanism 59 is adapted to supplement and/or replace the center feel mechanism 56 that generates the returning torque (Tr) (e.g., a center feel mechanism comprising cam face 70 and cam follower(s) 98).

[0077] Also illustrated in FIG. 11 is an over-rotation device 138. The vehicle 11 is subject to instability based on the degree of turn of the road wheels 12 in relation to, for example, the speed of the vehicle. Thus, the drive-by-wire system 10 includes the over-rotation device 138, which is configured to prevent the over-rotation of the road wheels 12 in relation to the conditions of vehicle 11.

[0078] In a first exemplary embodiment, the over-rotation device 138 is configured to prevent over-rotation of the road wheels 12 by limiting, for example, the amount of angular displacement 30 of the steering wheel 14. In this embodiment, the over-rotation device 138 comprises a solenoid 140 adapted to move a pawl 142 into teeth 144. Here, the solenoid 140 and pawl 142 are connected to the housing 42, while the teeth 144 are disposed on the center feel mechanism 56. Engaging the pawl 142 with the teeth 144 prevents rotation of the center feel mechanism 56, which prevents the rotation of the column 16 and the steering wheel 14 due to the engagement of the geared portions 52 and 68. Thus, preventing the rotation of the center feel mechanism 56 prevents the movement of the road wheels 12 to ensure the stability of the vehicle 11.

[0079] In this embodiment, the controller 36 determines a maximum desirable angular displacement for the steering wheel 14 based on inputs such as, but not limited to, the vehicle's speed, weight, center of gravity, road conditions, suspension conditions, brake conditions, and the like. The sensors 60 and 64 provide the signals 34 to the controller 36 indicative of the angular displacement 30 of the steering wheel 14. The controller 36 constantly compares the angular displacement 30 of the steering wheel 14 to the maximum desirable angular displacement for the steering wheel.

[0080] The controller 36 provides a control signal 146 to activate the solenoid 140 to engage the pawl 142 with the teeth 144. This prevents the angular displacement 30 of the steering wheel 14 beyond the determined maximum desirable angular displacement. Next, the controller 36 provides the control signal 146 to deactivate the solenoid 140, thus disengaging the pawl 142 from the teeth 144 as the controller updates and recalculates the maximum desirable degree of angular displacement.

[0081] Referring now to FIG. 14, an exemplary embodiment of a control algorithm 145 of the over rotation device 138 is illustrated. Here, the vehicle inputs 148 are provided to the controller 36 at step 150. The vehicle inputs 148 include sensed inputs such as, but not limited to, the vehicle's speed, yaw rate, road conditions, suspension conditions, brake conditions, and the like. The vehicle inputs 148 also include constant inputs such as, but not limited to, the vehicle's weight, center of gravity, and the like.

[0082] Next, the controller 36 calculates a maximum desirable angular displacement (MAX_sad) at step 152, and compares the angular displacement 30 from sensors 60 and 64 to the calculated MAX_sadat step 154. If the angular displacement 30 is equal to or larger than the MAX_sad, then the controller 36 activates the over rotation device 138 via the control signal 146 at step 156. However, if the current angular displacement 30 is less than the MAX_sad, then the controller 36 deactivates the over rotation device 138 via the control signal 146 at step 158. Finally, step 160 returns to step 148 to repeat the algorithm 145. In this manner, the algorithm 145 continuously controls the over rotation device 138 to prevent the over-rotation of the road wheels 12 by limiting the angular displacement 30 of the steering wheel 14.

[0083] It should be recognized that the control signal 146 is described above by way of example only as activating the solenoid 140 for mechanically restricting the movement of the road wheels 12 and or the steering wheel 14. Other configurations of the over rotation device 138 are contemplated. For example in an alternate embodiment, the over rotation device 138 is a logic switch within the controller 36. Here, the control signal 146 activates and deactivates the over rotation device 138 by controlling the signal 40 from the controller 36 to enable/disable the electric motor 26. Thus, in this embodiment the signal 40 only enables actuation of the electric motor 26 to turn the road wheels 12 up to MAX_sadregardless of how much angular displacement 30 is applied to the steering wheel 14. In this manner, the algorithm 145 electrically controls the over rotation device 138 to prevent over-rotation of the road wheels 12.

[0084] It should also be recognized that the over rotation device 138 is described above by way of example only being included in the drive-by-wire steering system 10. Of course, the over rotation device 138 is useful in other steering systems, such as, but not limited to, mechanical steering systems, and electrically or hydraulically assisted steering systems.

[0085] While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.