Abstract:
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.

Description:
TECHNICAL FIELD 
       [0001]    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 
       [0002]    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&#39;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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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 
       [0006]    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. 
         [0007]    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. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    The exemplary embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0009]      FIG. 1  is a schematic representation of an column electric power steering (CEPS) system in accordance with an embodiment; 
           [0010]      FIG. 2  is a partly sectional view of a rack electric power steering (REPS) system in accordance with an embodiment; 
           [0011]      FIG. 3  is block diagram of a controller for an electronic power steering (EPS) system in accordance with an embodiment; and 
           [0012]      FIG. 4  is a flow diagram illustrating the modes of operation of the controller of  FIG. 3  in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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 that  FIGS. 1-3  are merely illustrative and may not be drawn to scale. 
         [0017]    Referring to  FIG. 1 , an exemplary embodiment of an electric power steering (EPS) system  100  for a vehicle  102  is shown. The embodiment of  FIG. 1  illustrates a column EPS system (or CEPS system). The electric power steering system  100  may comprise a conventional rack and pinion steering mechanism  104 , which includes a toothed rack (not shown) and a column pinion gear (not shown) under gear housing  106 . As the steering wheel  108  is turned, an upper steering shaft  110  turns a lower shaft  112  through a rotary joint  114 ; and the lower steering shaft  112  turns the column pinion gear. Rotation of the column pinion gear moves the rack, which moves tie rods  116  (only one shown), which move steering knuckles  118  (only one shown) to turn tires  120  (only one shown). 
         [0018]    The electric power assist is provided through a controller  122  and a power assist actuator comprising an electric drive motor  124 . The controller  122  receives electric power from a vehicle electric power source  126  through a line  128 , a signal representative of the vehicle velocity on line  130  and column pinion gear angle from a column rotational position sensor  132  on line  134 . As the steering wheel  108  is turned, a torque sensor  136  senses the torque applied to steering wheel  108  by the vehicle operator and provides an operator torque signal to controller  122  on line  138 . In response to the vehicle velocity, operator torque, and in some cases, column pinion gear angle signals received, the controller  122  derives desired electric motor currents and provides values for such currents through a bus  140  to the electric drive motor  124 , which supplies torque assist to steering shaft  110  through worm gear  142  and motor pinion gear  144 . 
         [0019]    Accordingly to exemplary embodiments, the controller  122  includes a module  146  that functions to attenuate or reduce smooth road shake (SRS) and other steering wheel vibrations. The module  146  applies adaptive logic to process the vehicle speed (from line  130 ) and steering wheel torque (from line  138 ) that may be provided via conventional sensors for these functions. As will be discussed in detail in connection with  FIGS. 3-4  below, the module processes these signals to produce a corrective signal used by the controller  122  to reduce or attenuate SRS or vibratory effects that may be noticed and annoying to the driver of the vehicle  102 . 
         [0020]    Exemplary embodiments of the present disclosure are also applicable to rack mounted electric power steering system.  FIG. 2  depicts 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 system  200  comprises a conventional rack and pinion steering mechanism  202 , which includes a toothed rack  204  which is connected to the tie rods (not shown in  FIG. 2 ) for directing the turning of the tires (not shown in  FIG. 2 ). The steering column has an assembly  206  having a column pinion gear  208  which is meshed with the teeth  210  of the toothed rack  204  so 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 motor  212  of the electric power steering system is connected to the toothed rack by a motor pinion gear  214 , wherein the motor pinion gear may be mechanically connected, for example, by a belt or gear interface via, for example, a ballscrew mechanism  216 . The electrical operation is as generally described with respect to the CEPS configuration of  FIG. 1 , as it is adapted to the REPS configuration of  FIG. 2 , where the module  146  of the controller  122  processes the vehicle speed (from line  130 ) and the steering wheel torque ( 138 ) to provide the controller  122  with 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. 
         [0021]    Referring now to  FIG. 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. 
         [0022]    As illustrated in  FIG. 3 , the EPS controller  122  includes in fundamental embodiments a module  146  and a processor  300  which may be coupled to a memory  302  (optionally, the processor  300  may have an internal memory). The module  146  provides a heterodyning function employing the speed data  130  and the steering wheel torque signal  138  which 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 data  130  is processed via an integrator  304  to provide an average wheel angle. This value may be stored (and updated) in block  306  which feeds a sine function generator  308  and a cosine function generator  310  which provide sine and cosine functions ( 308 ′ and  310 ′ respectively) for the average wheel angle. The sine and cosine signals are individually mixed with the torque signal  138  via mixers  312  and  314 . This extracts the sine and cosine component of the torque signal  138 , which is integrated via integrators  316  and  318 . The output ( 320  and  322 ) 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 ( 320  and  322 ) with the sine and cosine functions ( 308 ′ and  310 ′ respectively) in mixers  323  and  325 . The reconstructed signals may be scaled (via amplifiers  324  and  326  respectively) before being combined (via summer  328 ). The output of summer  328  is a control signal for the EPS system that may be scaled (via amplifier  330 ) and transmitted to the processor  300  via line  332 . 
         [0023]    The control signal  332  is received by the processor  300  together with other inputs  334  as may be desired for any particular implementation. The processor  300  may combine, merge or supplement the control signal  332  with 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 signal  336  to the EPS actuator (motor  124  or  212 ) which provides compensation for SRS due to the component provided by control signal  332 . The processor  300  also receives the data  320  and  322  for performing the FFT analysis on the data and storing the results in the memory  302 . Generally, only one of the output data ( 320  or  322 ) needs to be processed via the FFT function. Optionally, the controller  300  may distribute the FFT computations to other controllers in the vehicle via a bus  338 . Distributing the FFT analysis provides an advantage in the event that the processor  300  is busy and other processors are available if not presently occupied by their intended functions. 
         [0024]      FIG. 4  illustrates 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 of  FIG. 4  may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description of the method of  FIG. 4  may refer to elements mentioned above in connection with  FIGS. 1-3 . In practice, portions of the method of  FIG. 4  may be performed by different elements of the described system. It should also be appreciated that the method of  FIG. 4  may include any number of additional or alternative tasks and that the method of  FIG. 4  may 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 in  FIG. 4  could be omitted from an embodiment of the method of  FIG. 4  as long as the intended overall functionality remains intact. 
         [0025]    As illustrated in  FIG. 4 , the method  400  operates in three modes  402 ,  404  and  406 . 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 ( 320  or  322 ) is employed to aid in providing the control signal ( 332  in  FIG. 3 ) to attenuate SRS. The method  400  begins in Mode 1 ( 402 ) where the FFT data ( 320  or  322 ) is stored (for example in memory  302  of  FIG. 3 ) in step  408 . 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, block  410  performs the FFT analysis such as via processor  300  (of  FIG. 3 ) or via other processors via bus  338 . 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 processor  300  does 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, block  412  stores 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 ) step  414  triggers Mode 2 ( 406 ) to commence. 
         [0026]    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 step  416 . 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 (block  418 ) and processed (block  420 ) as previously described in connection with blocks  408  and  410 . Thus, block  420  provides a current offset value that can be compared in block  422  with 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), step  424  triggers commencement of Mode 3 ( 406 ). 
         [0027]    During Mode 3 ( 406 ) the method  400  operates 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 ( 146  in  FIG. 3 ) in step  426 . Blocks  428  and  430  again store and process the FFT data ( 320  or  322  in  FIG. 3 ) when conditions are valid to provide a current offset value. Decision  432  compares 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 method  400  remains in Mode 3 and loops back to step  426 . Conversely, if the threshold has been exceeded, step  434  triggers 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 signal  323  to 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. 
         [0028]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the appended claims and the legal equivalents thereof.