System and method for reducing steering wheel vibration in electronic power steering systems

Methods and apparatus are provided for reducing steering wheel vibrations in electronic power steering systems. The apparatus includes a processor and performs a method that estimates an average angular wheel speed of at least one wheel of the vehicle; determines an average wheel angle from the average angular wheel speed; generates sine and cosine functions of the average angular wheel angle; processes the sine and cosine functions with a steering wheel torque signal received from a torque sensor to extract amplitude components of at least one of the sine and cosine functions; reconstructs the sine and cosine functions using the steering wheel torque signal; combines the sine and cosine functions to provide a control signal, and controls the EPS system via the control signal to attenuate smooth road shake vibrations in a steering wheel of the vehicle.

TECHNICAL FIELD

The technical field generally relates to Electric Power Steering (EPS) systems, and more particularly relates to techniques for reducing periodic steering wheel vibrations that occur within an EPS system.

BACKGROUND

Electric Power Steering (EPS) systems use an electric motor that can be coupled directly to either the steering gear or steering column to reduce a driver's effort in steering the vehicle. During operation of the vehicle, the driver will apply a force to the steering wheel in an effort to steer the vehicle. This results in a “driver torque” being applied to a shaft that is coupled to the steering wheel. Torque sensors detect torque being applied to the steering column by the driver, and communicate this information to an electronic control unit. The electronic control unit generates a motor control signal that is applied to the electric motor causing it to generate a “motor assist torque” that is combined with the driver torque. This combined torque is then used to steer the vehicle. This allows varying amounts of assistance to be applied depending on driving conditions.

There are numerous types of unwanted vibrations, noises, pulsations, disturbances, and other forms of fluctuating vibratory energy that can exist in a vehicle; these phenomena are hereafter collectively and broadly referred to as “vibrations.” Vibrations can have many sources, including external sources such as irregular road surfaces, as well as internal sources.

Periodic vibrations caused by internal sources can propagate throughout the vehicle and can cause an undesirable shake or movement of certain vehicle components that is noticeable to the driver. For instance, periodic vibrations generated at the wheel assemblies can combine to create a dynamic torque on a steering wheel column that causes the steering wheel to cyclically turn at small amplitudes in either direction. When this type of event occurs on a flat or smooth road surface, it is all the more apparent to the driver and is sometimes referred to as “smooth road shake” (SRS). SRS generally is most noticeable between approximately 50 to 100 mph, and exhibits a frequency of about 10 to 20 Hz. At 50 mph, smooth road shake occurs at approximately 10 Hz. SRS tends to increase as an approximately linear function of speed, such that, at 100 mph, SRS occurs at approximately 20 Hz. These vibrations can be sensed by the driver of the vehicle and such steering wheel vibrations may be distracting or annoying to the driver.

Accordingly, it is desirable to provide improved methods, systems and apparatus for suppressing steering wheel vibrations in vehicles having an EPS system. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

SUMMARY

An apparatus is provided for reducing steering wheel vibrations in electronic power steering systems. In one embodiment, the apparatus includes a sensor providing an average angular wheel speed of at least one wheel of a vehicle and a processor configured to: determine an average wheel angle from the average angular wheel speed; generate sine and cosine functions of the average angular wheel angle; process the sine and cosine functions with a steering wheel torque signal received from a torque sensor to extract amplitude components of at least one of the sine and cosine functions; reconstruct the sine and cosine functions using the steering wheel torque signal; combine the sine and cosine functions to provide a control signal and control the EPS system via the control signal to attenuate smooth road shake vibrations in a steering wheel of the vehicle.

A method is provided for reducing steering wheel vibrations in electronic power steering systems. In one embodiment, the method estimates an average angular wheel speed of at least one wheel of the vehicle; determines an average wheel angle from the average angular wheel speed; generates sine and cosine functions of the average angular wheel angle; processes the sine and cosine functions with a steering wheel torque signal received from a torque sensor to extract amplitude components of at least one of the sine and cosine functions; reconstructs the sine and cosine functions using the steering wheel torque signal; combines the sine and cosine functions to provide a control signal, and controls the EPS system via the control signal to attenuate smooth road shake vibrations in a steering wheel of the vehicle.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

In this document, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Numerical ordinals such as “first,” “second,” “third,” etc. simply denote different singles of a plurality and do not imply any order or sequence unless specifically defined by the claim language.

Additionally, the following description refers to elements or features being “connected” or “coupled” together. As used herein, “connected” may refer to one element/feature being directly joined to (or directly communicating with) another element/feature, and not necessarily mechanically. Likewise, “coupled” may refer to one element/feature being directly or indirectly joined to (or directly or indirectly communicating with) another element/feature, and not necessarily mechanically. However, it should be understood that, although two elements may be described below, in one embodiment, as being “connected,” in alternative embodiments similar elements may be “coupled,” and vice versa. Thus, although the schematic diagrams shown herein depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment.

Finally, for the sake of brevity, conventional techniques and components related to vehicle electrical and mechanical parts and other functional aspects of the system (and the individual operating components of the system) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the invention. It should also be understood thatFIGS. 1-3are merely illustrative and may not be drawn to scale.

Referring toFIG. 1, an exemplary embodiment of an electric power steering (EPS) system100for a vehicle102is shown. The embodiment ofFIG. 1illustrates a column EPS system (or CEPS system). The electric power steering system100may comprise a conventional rack and pinion steering mechanism104, which includes a toothed rack (not shown) and a column pinion gear (not shown) under gear housing106. As the steering wheel108is turned, an upper steering shaft110turns a lower shaft112through a rotary joint114; and the lower steering shaft112turns the column pinion gear. Rotation of the column pinion gear moves the rack, which moves tie rods116(only one shown), which move steering knuckles118(only one shown) to turn tires120(only one shown).

The electric power assist is provided through a controller122and a power assist actuator comprising an electric drive motor124. The controller122receives electric power from a vehicle electric power source126through a line128, a signal representative of the vehicle velocity on line130and column pinion gear angle from a column rotational position sensor132on line134. As the steering wheel108is turned, a torque sensor136senses the torque applied to steering wheel108by the vehicle operator and provides an operator torque signal to controller122on line138. In response to the vehicle velocity, operator torque, and in some cases, column pinion gear angle signals received, the controller122derives desired electric motor currents and provides values for such currents through a bus140to the electric drive motor124, which supplies torque assist to steering shaft110through worm gear142and motor pinion gear144.

Accordingly to exemplary embodiments, the controller122includes a module146that functions to attenuate or reduce smooth road shake (SRS) and other steering wheel vibrations. The module146applies adaptive logic to process the vehicle speed (from line130) and steering wheel torque (from line138) that may be provided via conventional sensors for these functions. As will be discussed in detail in connection withFIGS. 3-4below, the module processes these signals to produce a corrective signal used by the controller122to reduce or attenuate SRS or vibratory effects that may be noticed and annoying to the driver of the vehicle102.

Exemplary embodiments of the present disclosure are also applicable to rack mounted electric power steering system.FIG. 2depicts an example of an EPS system where the electric motor is mounted on the rack of the steering system (an REPS system). The electric power steering system200comprises a conventional rack and pinion steering mechanism202, which includes a toothed rack204which is connected to the tie rods (not shown inFIG. 2) for directing the turning of the tires (not shown inFIG. 2). The steering column has an assembly206having a column pinion gear208which is meshed with the teeth210of the toothed rack204so that turning of the steering column applies a torque at the toothed rack that results in the toothed rack translating left or right, depending on the direction of the turning of the steering column. The electric drive motor212of the electric power steering system is connected to the toothed rack by a motor pinion gear214, wherein the motor pinion gear may be mechanically connected, for example, by a belt or gear interface via, for example, a ballscrew mechanism216. The electrical operation is as generally described with respect to the CEPS configuration ofFIG. 1, as it is adapted to the REPS configuration ofFIG. 2, where the module146of the controller122processes the vehicle speed (from line130) and the steering wheel torque (138) to provide the controller122with a corrective signal that the controller can use to attenuate or reduce SRS and other vibratory effects that may be noticed by the driver of the vehicle.

Referring now toFIG. 3, a block diagram of a controller for an electronic power steering (EPS) system is illustrated. Typically, EPS systems that provide compensation for smooth road shake (SRS) employ a highly accurate measurement or approximation of angular wheel velocity (speed). This measurement may be provided by dedicated sensors positioned in or near wheel assemblies or may be estimated by use of anti-lock braking systems (ABS) in those vehicles employing ABS. However, the cost of dedicated sensors may be prohibitively high for some vehicles, and not all vehicles employ an ABS. Accordingly, the present disclosure provides compensation for SRS without the requirement of high accuracy angular wheel velocity data. Simple estimates of angular wheel velocity such as, for example, by a speedometer reading or by sensing a rotating gear in a transmission is sufficient for the SRS attenuation provided by the present disclosure.

As illustrated inFIG. 3, the EPS controller122includes in fundamental embodiments a module146and a processor300which may be coupled to a memory302(optionally, the processor300may have an internal memory). The module146provides a heterodyning function employing the speed data130and the steering wheel torque signal138which is used to provide correction for (i.e., attenuate) SRS effects such as those described in U.S. Patent Publication No. 2012/0061169 filed on Sep. 15, 2010 (which is hereby incorporated by reference). The speed data130is processed via an integrator304to provide an average wheel angle. This value may be stored (and updated) in block306which feeds a sine function generator308and a cosine function generator310which provide sine and cosine functions (308′ and310′ respectively) for the average wheel angle. The sine and cosine signals are individually mixed with the torque signal138via mixers312and314. This extracts the sine and cosine component of the torque signal138, which is integrated via integrators316and318. The output (320and322) of these integrators provide data that will be processed via a Fast Fourier Transform (FFT) analysis to determine offset values (periodically per sample) that will be used to compensate (i.e., attenuate) SRS effects as described more fully below. After extracting this data, sine and cosine signals are reconstructed by mixing the data (320and322) with the sine and cosine functions (308′ and310′ respectively) in mixers323and325. The reconstructed signals may be scaled (via amplifiers324and326respectively) before being combined (via summer328). The output of summer328is a control signal for the EPS system that may be scaled (via amplifier330) and transmitted to the processor300via line332.

The control signal332is received by the processor300together with other inputs334as may be desired for any particular implementation. The processor300may combine, merge or supplement the control signal332with a number of other signals as is known in EPS systems. Non-limiting examples of such other signals include steering wheel angle, steering wheel torque, steering wheel velocity, steering wheel acceleration, steering wheel torque gradient, vehicle speed, vehicle fore-aft acceleration, and vehicle lateral acceleration. The processor then provides a control signal336to the EPS actuator (motor124or212) which provides compensation for SRS due to the component provided by control signal332. The processor300also receives the data320and322for performing the FFT analysis on the data and storing the results in the memory302. Generally, only one of the output data (320or322) needs to be processed via the FFT function. Optionally, the controller300may distribute the FFT computations to other controllers in the vehicle via a bus338. Distributing the FFT analysis provides an advantage in the event that the processor300is busy and other processors are available if not presently occupied by their intended functions.

FIG. 4illustrates flow diagrams useful for understanding the method and modes of operation for attenuating SRS in an EPS system. The various tasks performed in connection with the method ofFIG. 4may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description of the method ofFIG. 4may refer to elements mentioned above in connection withFIGS. 1-3. In practice, portions of the method ofFIG. 4may be performed by different elements of the described system. It should also be appreciated that the method ofFIG. 4may include any number of additional or alternative tasks and that the method ofFIG. 4may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. Moreover, one or more of the tasks shown inFIG. 4could be omitted from an embodiment of the method ofFIG. 4as long as the intended overall functionality remains intact.

As illustrated inFIG. 4, the method400operates in three modes402,404and406. Mode 1 (402) is a learning mode, Mode 2 (404) is a verification mode and Mode 3 (406) is a monitoring mode of operation. In each mode, the FFT data (320or322) is employed to aid in providing the control signal (332inFIG. 3) to attenuate SRS. The method400begins in Mode 1 (402) where the FFT data (320or322) is stored (for example in memory302ofFIG. 3) in step408. In some embodiments, the FFT data is only stored at such time when vehicle conditions are valid for collecting and storing the FFT data. Non-limiting examples of such conditions are when the vehicle is traveling at a substantially constant speed and particularly at speeds exceeding 45 miles per hour (72.42 kilometers per hour). Next, block410performs the FFT analysis such as via processor300(ofFIG. 3) or via other processors via bus338. In some embodiments, the FFT data is only processed at such time when conditions are valid for processing the FFT data. Non-limiting examples of such conditions are when a processor within the vehicle is available, such as, when the vehicle has stopped (e.g., stop sign, other traffic control signal or after key off is detected) during which time the processor300does not need to control the EPS system or with some other processor is not then required to be performing its intended function. After performing the FFT analysis, block412stores the results which include the average speed, an offset value which in some embodiments is the frequency difference between a stored based value and a calculated value (sometimes expressed as a percentage) and an offset polarity, which may be assumed to be a positive polarity during the initial execution of Mode 1 (402). At the completion of Mode 1 (402) step414triggers Mode 2 (406) to commence.

During Mode 2 (404), a percent of the offset value (derived in Mode 1) is applied to the average wheel velocity together with the offset polarity in step416. In some embodiments, fifty percent (50%) of the offset value is used. As noted above, during the initial execution of the Mode 1, the offset polarity was assumed to be positive. Mode 2 verifies (or corrects) the offset polarity for further processing during Mode 3. To perform the verification process, the FFT data is again stored (block418) and processed (block420) as previously described in connection with blocks408and410. Thus, block420provides a current offset value that can be compared in block422with the first offset value generated during Mode 1. If the current offset value exceeds the first offset value, then the offset polarity is set as negative. Otherwise, the offset polarity remains at the originally assumed positive polarity. The offset polarity having been verified (or corrected), step424triggers commencement of Mode 3 (406).

During Mode 3 (406) the method400operates to monitor the continually derived offset value to attenuate SRS. To do this, the learned offset value (from Mode 1) and the verified offset polarity (from Mode 2) is applied to the average wheel velocity for the next computation by the heterodyning module (146inFIG. 3) in step426. Blocks428and430again store and process the FFT data (320or322inFIG. 3) when conditions are valid to provide a current offset value. Decision432compares the current offset (computed in Mode 3) with an offset threshold. In some embodiments, the offset threshold is approximately one-half of one percent (0.5%). If the current offset is less than the threshold, then the method400remains in Mode 3 and loops back to step426. Conversely, if the threshold has been exceeded, step434triggers Mode 1 (402) to begin again and the method repeats. By applying the learned offset value (from Mode 1) and learned polarity (from Mode 2), any error due to the simple estimation of the average angular wheel velocity is quickly corrected and the control signal323to compensate for the effects of SRS is provided. In this way, the present disclosure provides compensation (attenuation) of the effects of SRS without a highly accurate measurement or estimation of average angular wheel velocity making the present disclosure an affordable and readily implemented SRS compensation system.