Quadrant dependent active damping for electric power steering

An electric power steering system includes a steering wheel, an electric assist motor operatively coupled to the steering wheel, and an electronic controller operatively coupled to the motor for receiving a first signal representative of torque applied to the steering wheel, and a second signal representative of angular velocity of the motor. The electronic controller produces an active damping motor command signal according to a function of the first and second signals. The active damping motor command signal is scaled by a first factor if the first and second signals are both positive, or if the first and second signals are both negative, to provide a scaled motor command signal. The active damping motor command signal is scaled by a second factor if the first signal is positive and the second signal is negative, or if the first signal is negative and the second signal is positive, to provide the scaled motor command signal. The scaled motor command signal is applied to the motor.

BACKGROUND

In a vehicle equipped with electric power steering (EPS), a steering assist force is provided by an electric motor operatively coupled to a steering wheel through a steering column or shaft. Typically, such systems include a controller programmed to derive an assist torque signal along with a “return to center” (i.e., neutral position) torque signal, thereafter summing these torque signals to generate a motor command signal. The assist torque signal is derived using an applied torque signal indicative of torque applied to the steering wheel by a vehicle operator. The assist torque signal provides a power steering boost torque, and the neutral position torque signal provides a return-to-center bias torque.

During EPS operation, the motor command signal is applied to the electric motor, and a sensing device is utilized to sense the angular velocity of the electric motor. Based upon the angular velocity of the electric motor and the applied torque signal, four operational quadrants may be defined. Quadrant I is defined as a motor operating condition wherein the applied torque signal and the angular velocity are both positive. Quadrant II is defined as a motor operating condition wherein the applied torque signal is negative but the angular velocity is positive. In practice, Quadrant II situations arise when the applied torque signal specifies a reversal in motor rotation. In Quadrant III, the applied torque signal and the angular velocity are both negative, whereas in Quadrant IV, the applied torque signal is positive, but the angular velocity is negative. Quadrant III mirrors Quadrant I, but with opposite signs for angular velocity and applied torque signals. Similarly, Quadrant IV mirrors Quadrant II, but with opposite signs for angular velocity and applied torque signals. Although Quadrants I and II are discussed herein, it should be noted that any discussion of Quadrant I also applies to Quadrant III, and also that any discussion of Quadrant II also applies to Quadrant IV.

As a practical matter, EPS systems may exhibit free rotational oscillation resonances that, if left undamped, may not result in a crisp, controlled feeling to the steering. EPS systems are non-linear, providing a steering assist force which varies as a function of vehicle speed, road conditions, and the driving maneuvers being performed. In spite of these variations, good EPS system performance is characterized by linear behavior as perceived from the driver's point of view. Drivers expect steering behavior to remain consistent throughout a wide variety of operating conditions.

In order to impart a stable and precise feel to an EPS system, input-dependent active damping may be utilized. An example of such a technique is presented in U.S. Pat. No. 5,919,241 (“the '241 patent”), Vehicle Having Electric Power Steering With Active Damping, filed Dec. 13, 1996, incorporated by reference herein in its entirety and assigned to the assignee of the present application. In the '241 patent, active damping provides an active damping torque signal that is further summed along with the assist torque signal and the return to center torque signal to produce the torque command signal. This active damping torque signal is derived as a function of a filtered steering shaft position and a sensed vehicle velocity. A filtering mechanism for generating the steering shaft angular velocity applies the amplitude and phase characteristics of a differentiator from 0 Hz through the resonant frequency of free rotational oscillation of the EPS system to the steering shaft position.

A continuing source of difficulty in tuning and controlling EPS systems is a perception by the driver of a difference in damping between operation in Quadrant I as opposed to Quadrant II. However, existing active damping techniques do not modify damping behavior in response to a quadrant transition. As a result, these techniques provide undesirable, inappropriate, and oftentimes annoying tactile feedback to the steering wheel throughout one or more quadrants. Accordingly, what is needed is an improved technique for applying active damping to an EPS system that may operate in any of a plurality of quadrants.

SUMMARY

The above described and other features are exemplified by the following Figures and Description in which an electric power steering system is disclosed that includes a steering wheel, an electric assist motor operatively coupled to the steering wheel, and an electronic controller operatively coupled to the assist motor for receiving a first signal representative of torque applied to the steering wheel and a second signal representative of angular velocity of the assist motor. In response to a sensed position of the steering wheel, the electronic controller produces an assist torque command and a “return to center” torque command. The electronic controller produces an active damping motor command signal according to a function of the first and second signals. The active damping motor command signal is scaled by a first factor if the first and second signals are both positive, or if the first and second signals are both negative, to provide a scaled motor command signal. The active damping motor command signal is scaled by a second factor if the first signal is positive and the second signal is negative, or if the first signal is negative and the second signal is positive, to provide the scaled motor command signal. The scaled motor command signal is summed with the assist torque command and the “return to center” torque command and then applied to the electric assist motor.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is a block diagram of a motor vehicle10provided with an exemplary electric power steering (EPS) system12. The EPS system12may include a conventional rack and pinion steering mechanism14, which includes a toothed rack16and a pinion gear (not shown) under a gear housing18. As a steering input member (e.g., a steering wheel20) is turned, a steered member or upper steering shaft22turns a lower steering shaft24through a universal joint26. In turn, the lower steering shaft24turns the pinion gear. The rotation of the pinion gear moves the rack16, which then moves a pair of tie-rods28(only one shown) coupled to a pair of steering knuckles30(only one shown) to turn a pair of road wheels32(only one shown).

Electric power assist is provided through a controller34in conjunction with a power assist actuator, such as an electric motor36. The controller34receives electric power from a vehicle electric power source38through a line40. Inputs to the controller34include a signal42representative of the vehicle velocity, as well as a signal44representative of steering pinion gear angle from a column or shaft rotational position sensor46. A motor velocity signal, ωM, may be determined by differentiating an output of rotational position sensor46. However, it is contemplated that alternate embodiments may obtain motor velocity signal ωMfrom a velocity sensor such as, for example, a tachometer or a resolver. As steering wheel20is turned, a torque sensor48senses the torque applied to steering wheel20by the vehicle operator and provides an input steering torque signal50to controller34. In addition, as the rotor of motor36turns, motor position signals52for each phase are generated within motor36and are provided to the controller34.

In response to received signals representative of vehicle velocity, operator-applied torque, steering pinion gear angle and rotor position, controller34derives desired motor voltages or currents and provides such voltages or currents through a motor command signal54to motor36. Thereby, motor36supplies a torque assist to upper and lower steering shafts22,24through a worm56and associated worm gear58. If torque sensor48is of the type that requires upper steering shaft22to be separated at the sensor between upper and lower sections (allowing some range of rotational independence), both rotational position sensor46and worm gear58are associated with the lower section of the steering shaft below torque sensor48, as shown.

Referring now toFIG. 2, there is shown a block diagram of an active damping system associated with the EPS controller34shown inFIG. 1. As described earlier, a desired assist torque is derived at block60. The desired assist torque, in turn, determines a desired assist torque current or voltage that represents the amount of motor current or voltage, respectively, to be commanded, specifically responsive to the inputs of input steering torque signal50and vehicle velocity signal42. The desired assist torque current or voltage is outputted by block60, illustratively in the form of an assist torque command signal64.

An active damping block70is also included in controller34, in order to determine an active damping torque command signal72representing an active damping voltage or current. Preferably, the active damping torque command signal72is derived from motor position signals52and vehicle velocity signal42, and is outputted to summation block68. The magnitude of the active damping torque command signal72is subtracted from the difference between the assist torque command signal64and the return to center torque command signal66. Alternatively, however, active damping block70may receive pinion gear angle signal44rather than motor position signal52. In an alternate embodiment, the return to center torque block62may be omitted altogether, since vehicle chassis characteristics may themselves provide a return to center torque.

At block88, a quadrant detector uses input steering torque signal50and motor velocity signal, ωMderived from rotational position sensor46(FIG. 1) to identify a quadrant in which motor36(FIG. 1) is operating. Recall that input steering torque signal50represents an applied torque signal indicative of torque applied to the steering wheel by a vehicle operator. Based upon motor velocity signal ωMrepresentative of the angular velocity of motor36(FIG. 1), as well as input steering torque signal50(FIG. 2) representative of an applied torque signal, motor operation in one of four quadrants may be determined. Quadrant I is defined as a motor operating condition wherein the applied torque signal and the angular velocity are both positive. Quadrant II is defined as a motor operating condition wherein the applied torque signal is negative but the angular velocity is positive. In practice, Quadrant II situations arise when the applied torque signal specifies a reversal in motor rotation. In Quadrant III, the applied torque signal and the angular velocity are both negative, whereas in Quadrant IV, the applied torque signal is positive, but the angular velocity is negative. Quadrant III mirrors Quadrant I, but with opposite signs for angular velocity and applied torque signals. Similarly, Quadrant IV mirrors Quadrant II, but with opposite signs for angular velocity and applied torque signals.

Quadrant I and Quadrant III scaling factors82represent a set of scaling factors that are applicable to motor operation in Quadrant I or Quadrant III. These scaling factors82are stored in an electronic memory readable by controller34(FIG. 1), illustratively as one or more look-up tables. Quadrant II and Quadrant IV scaling factors84(FIG. 2) represent a set of scaling factors that are applicable to motor operation in Quadrant II or Quadrant IV. These scaling factors84are also stored in an electronic memory readable by controller34(FIG. 1), illustratively as one or more look-up tables. Optionally, scaling factors82and84(FIG. 2) may be stored in the same look-up table or in different look-up tables. Pursuant to a further embodiment, scaling factors82and84are selected to be within the range of approximately 0% to 500%.

At block86, a scaling selection function selects the appropriate scaling factors based upon the quadrant detected by quadrant detector in block88. If the detected quadrant is Quadrant I or Quadrant III, then Quadrant I and III scaling factors are applied to the active damping signal generated at block70, illustratively using a multiplier73. On the other hand, if the detected quadrant is Quadrant II or Quadrant IV, then Quadrant II and IV scaling factors are applied to the active damping signal generated at block70, illustratively using multiplier73. The output of multiplier73represents a scaled active damping signal that has been scaled based upon the quadrant in which motor36(FIG. 1) is operating.

As described earlier, a desired return to center torque is derived at block62(FIG. 2). The desired return to center torque thus determines the magnitude of a return to center torque current or voltage, and is responsive to vehicle velocity signal42and pinion gear angle signal44. The desired return to center torque current or voltage is outputted by block62through a return to center torque command signal66. Signals64and66are inputted to summation block68, wherein the magnitude of the return to center torque command signal66is subtracted from the magnitude of the assist torque command signal64.

FIG. 3is a block diagram of a second active damping system associated with the controller ofFIG. 1. In this embodiment, torque command or assist dependent damping, as well motor velocity frequency dependent damping, is further provided in the determination of a total quadrant-dependent damping torque. An assist dependent damping block74was incorporated to aid in stability of the system without compromising on-center feel. By changing the amount of damping as a function of quadrant operation and assist level, larger values of damping may be provided at higher assist torques, while lesser or no extra damping may be provided at low assist torques (such as encountered on-center). Thereby, the return ability and on-center feel of the system is prevented from being adversely affected. Additional details regarding assist dependent damping may be found in U.S. application Ser. No. 09/829,311, filed Apr. 9, 2001, assigned to the assignee of the present application, the contents of which are incorporated herein by reference.

At block88, a quadrant detector uses input steering torque signal50and motor velocity signal, ωMderived from rotational position sensor46to identify a quadrant in which motor36(FIG. 1) is presently operating. Based upon motor velocity signal ωMrepresentative of the angular velocity of motor36(FIG. 1), as well as input steering torque signal50representative of an applied torque signal, motor operation in one of four quadrants may be determined. Quadrants I, II, III, and IV were defined previously in conjunction withFIG. 2, and these definitions also apply to the system ofFIG. 3.

Quadrant I and Quadrant III scaling factors82(FIG. 3) represent a set of scaling factors that are applicable to motor operation in Quadrant I or Quadrant III. These scaling factors82are stored in an electronic memory readable by controller34(FIG. 1), illustratively as one or more look-up tables. Quadrant II and Quadrant IV scaling factors84(FIG. 3) represent a set of scaling factors that are applicable to motor operation in Quadrant II or Quadrant IV. These scaling factors84are also stored in an electronic memory readable by controller34(FIG. 1), illustratively as one or more look-up tables. Optionally, scaling factors82and84(FIG. 3) may be stored in the same look-up table or in different look-up tables. Pursuant to a further embodiment, scaling factors82and84are selected to be within the range of approximately 0% to 500%.

At block86, a scaling selection function selects the appropriate scaling factors based upon the quadrant detected by quadrant detector in block88. If the detected quadrant is Quadrant I or Quadrant III, then Quadrant I and III scaling factors are applied to the active damping signal generated at block70, illustratively using a multiplier73. On the other hand, if the detected quadrant is Quadrant II or Quadrant IV, then Quadrant II and IV scaling factors are applied to the active damping signal generated at block70, illustratively using multiplier73.

The output of multiplier73represents a scaled active damping signal that has been scaled based upon the quadrant in which motor36(FIG. 1) is operating. This scaled damping signal is combined with an assist torque signal produced at block60(FIG. 2) and a return to center torque signal produced at block62(FIG. 2), illustratively using a summer68, to provide a scaled motor command signal54.

A velocity compensation filter76was added to the motor velocity path and used in conjunction with the assist dependent damping block74in order to aid in stabilizing systems with analog velocity sensors. The velocity compensation filter76further improves stability, disturbance rejection, and on-center feel properties of the system. Filter76may include any general first, second, or higher order filter with appropriate characteristics. Additional details regarding the unity gain frequency dependent damping filter76may be found in U.S. Provisional Application Ser. No. 60/297,066, filed Jun. 8, 2001, assigned to the assignee of the present application, the contents of which are incorporated herein by reference.

While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.