Abstract:
Provided is a method for operating a power steering system and to such a power steering system. A motor torque is computed by an electronic processor and is established by suitable motor actuation, wherein a plausibility check of the computed motor torque is carried out, taking a measured manual torque into consideration. A value of the additional torques of at least one special function is further considered in the plausibility check.

Description:
[0001]    This is a continuation of PCT/EP2011/070087 filed Nov. 15, 2011. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for operating a power steering system, to such a power steering system, to a control unit, and to a computer program and a computer program product for carrying out the method. 
         [0003]    Power steering systems, which are also referred to as power-assisted steering systems, are used to achieve a reduction in the force that the driver must apply for steering. When using electric power steering (EPS) as the power-assisted steering system, a motor torque is computed depending on various input variables, the motor torque being superimposed, as added torque, on the manual torque that is applied by the driver. 
         [0004]    So as to limit potential impact from faults in EPS systems, additional measures are known, which hereafter are referred to as safety measures. To this end, monitoring of the combination of manual and torque motor torque can be used to prevent safety-relevant operating situations by providing for a limitation to allowed combinations in the function path. 
         [0005]    The published prior art DE 10 2009 000 165 A1 describes a method for operating a power steering system, in which a motor torque is computed by an electronic processor and established by suitable motor actuation. A plausibility check of the computed motor torque is carried out within the framework of the safety design, which is represented by a three-level design, for example. Contributions above the motor torque limit curve are integrated, and an integrator is decremented by contributions below the motor torque limit curve. The target motor torque is limited in level 1 to a first integration threshold. The limitation is monitored in level 2. Additionally, an intrinsically safe motor torque limit curve is used, as a function of an actual torque. 
         [0006]    Over the past few years, special functions have been further developed, which utilize the EPS system by way of torque superposition, so as to achieve added value in terms of driver assistance, Examples include a parking steering assist function or a lane-tracking assist function (lane departure warning and lane maintenance support). As the degree of assistance rises, increasingly greater superimposed steering torques are required. These functions alter the manual torque depending on particular situations. Consequently, the combination of manual torque and motor torque is also altered, which has not been taken into account in the existing design, 
       SUMMARY OF THE INVENTION 
       [0007]    The method that is described is used to operate a power steering system, wherein a motor torque is computed by an electronic processor and established by suitable motor actuation, wherein a plausibility check of the computed motor torque is carried out, taking a measured manual torque into consideration. In addition, a value of the added torques of at least one special function is considered. 
         [0008]    In the method, the measured manual torque can be corrected by the typically appropriately signed value of the customarily additive added torques of the special function. 
         [0009]    In one embodiment of the described method, the measured manual torque is increased. As an alternative, the manual torque is reduced. 
         [0010]    In one embodiment of the described method, the plausibility check is carried out within the scope of a 3-level design, for example. 
         [0011]    In a particular embodiment, the manual torque, which is customarily detected by way of a manual torque sensor, is controlled by specifying a target manual torque. 
         [0012]    In addition, a power steering system for a motor vehicle is described, which is particularly suitable for carrying out a method of the type described above, This power steering system comprises an electronic processor for computing a motor torque, an electric motor for introducing a customarily specified added torque or motor torque, and a device for detecting a current manual torque. Customarily, a manual torque sensor, for example a torsion bar torque sensor, is used for this purpose. The power steering system is characterized in that a plausibility cheek of the computed motor torque is carried out, taking the measured manual torque into consideration, and that a value of the added torques of at least one special function is further considered in the plausibility check. 
         [0013]    Moreover, a control unit is described, which is notably provided for use in a power steering system of the type described. This control unit comprises an electronic processor, which is used to compute a motor torque that is subjected to a plausibility check. A value of the added torques of a special function is further considered. 
         [0014]    In the described method, a basic distinction can be made, concerning the behavior of the safety function, between cases in which the special function increases the manual torque and cases in which the special functions reduce the manual torque. 
         [0015]    An increase of the manual torque means a reduction in the assistive power, which is to say the EPS motor must contribute less torque to the system. The motor torque thus always remains within the limits, since a reduction of the assistive power constitutes the transition to a safe state (no assistance) from a safety perspective. Thus, no functional limitation lakes place. 
         [0016]    However, increasing the manual torque while affecting the special function allows a considerably higher EPS motor torque, even though this is not functionally necessary and should definitely be avoided from a safety perspective, because in this situation an unchecked system fault that produces motor torque in the corresponding direction could develop, resulting in uncontrollable effects. 
         [0017]    It is also possible to consider the behavior of the current safety function in the special functions that lower the manual torque. 
         [0018]    The superimposition of torques is intended to make it easy for the driver to steer in the right direction. “Making it easy” in this case shall be understood to mean that the manual torque is reduced, which means that the EPS motor torque is increased. 
         [0019]    In terms of safety, this process constitutes over-assistance, which is to say higher EPS motor torque with a lower manual torque, Meeting with only little resistance, the driver can thus steer in the direction of the steering stop and would be limited by the described plausibility check of the computed motor torque (motor torque limiter), which is to say the motor torque limiter would not allow these additional torques and limit the motor torque. However, these cases do not involve a fault that is to be prevented, but a functional property of the special function in question, which is to say the motor torque limiter must be developed so as to allow functionality of this type. 
         [0020]    Some of the advantages of the invention include allowing a torque-based EPS special function to be performed without functional limitation, without necessitating deactivation of the safety function and modification of the accepted, approved parameters of the safety function. Possible effects of a fault are thereby mitigated. The procedure can be established as part of the safety validation. 
         [0021]    The described method is particularly suitable for use in EPS systems comprising means for introducing added torque so as to achieve free steering torque influence, means for detecting the current manual torque, means for detecting electrical variables of the electric motor, and means for detecting the rotor position or rotor speed of the electric motor. 
         [0022]    The described control is typically carried out by way of a control system, which is suitable for controlling a specified added torque. During the control process, the following method steps are typically carried out: 
         [0023]    1. computing the target manual torque; 
         [0024]    2. determining the actual manual torque; 
         [0025]    3. minimizing the control deviation between the target torque and actual torque by computing a controlled target motor torque; and 
         [0026]    4. controlling the current so as to establish the target motor torque. 
         [0027]    The control process is thus carried out by way of a control system tor generating added torques, wherein advantageously a microprocessor is provided, which communicates with an intelligent safety processor (watchdog). The 3-level design is typically employed to protect the computer core. 
         [0028]    In at least some of the embodiments, the described method is based on the following requirements in regard to the BPS special functions: the superposed torques must either be limited to intrinsically safe numbers with respect to the value and gradient, or must assure ASIL D compliance by way of plausibility check measures. Additionally, the superposed torques must only be requested in appropriate special situations and must be monitored and limited in terms of time. The superposed torques must also be subject to a plausibility check of the application direction and exclude undesirable, potentially oscillating changes in signs. 
         [0029]    The described computer program comprises program code means for carrying out all the steps of a method, as described above, if the computer program is executed on a computer or a processor. 
         [0030]    In addition, a computer program is described, comprising program code means for carrying out all the steps of a method described above if the computer program is executed on a computer or a corresponding processor. 
         [0031]    The computer program product comprises these program code means, which are stored on a computer-readable data medium. 
         [0032]    This computer program can be stored on a computer-readable data medium, such as a diskette, CD, DVD, hard drive, USB memory stick or the like, or on an Internet server as a computer program product. From there, the computer program can be transferred into a storage element of the control unit. 
         [0033]    Further advantages and embodiments of the invention will be apparent from the description and the accompanying drawings. 
         [0034]    The above characteristics and those explained below can, of course, be used not only in the described combination, but also in other combinations, or alone, without departing from the scope of the present invention. 
         [0035]    The invention is schematically illustrated in the drawings based on an exemplary embodiment and will be described in detail hereafter with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIGS. 1   a  and  1   b  show an embodiment of the power steering system according to the invention; 
           [0037]      FIG. 2  shows a graph of the progression of the motor torque limit curve, illustrating the safety function for special functions that increase the manual torque; and 
           [0038]      FIG. 3  shows a graph of the progression of the motor torque limit curve, illustrating the safety function for special functions that reduce the manual torque. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  shows, in sub- FIGS. 1   a  and  1   b,  an embodiment of the power steering system, which, in the overall, is denoted by reference numeral  10 .  FIG. 1   a  also shows a steering wheel  12 , a front axle  14  comprising two linked wheels  16 , an electric motor or EPS motor  18 , a control unit  20  comprising an electronic processor  21 , and a manual torque sensor  22 . The control unit  20  comprising the processor  21  implements a motor actuation function for actuating the motor  18 . 
         [0040]    According to  FIG. 1   b,  which describes software that is executed in the processor  21  of  FIG. 1   a,  a feedback control unit  23  and a block  24  are provided, which specifies predetermined values for the feedback control unit  23 . In this block  24 , a driver assistance interface  25  and a region  26  for specifying steering sensation are provided. 
         [0041]    The steering sensation is introduced into the system by way of the path M steering sensation  (arrow  27 ), and the superposed torques are introduced by way of the path M driver assistance  (arrow  28 ). The manual torque sensor signal is input by the manual torque sensor  22  in  FIG. 1   a  at point  29 . The motor torque for the motor  18  in  FIG. 1   a  is specified at point  30 . 
         [0042]    The approach in this solution is based on the correction of the manual torque so as to compensate for the change caused by the added torque by way of the special function, and thus leaves the coordinated and approved parameters unchanged. A manual torque is thus computed, which would be required for the identical steering motion without a special function. This corrected signal is supplied to the safety function. As a result, no functional limitations occur in connection with the special functions. 
         [0043]      FIG. 2  shows a graph of a progression  30  of the motor torque limit curve. The actual manual torque is plotted on the abscissa  32  and the motor torque is plotted on the ordinate  34 . The illustration is intended to clarify the correction of the safety function for special functions that increase the manual torque. 
         [0044]    In the example shown, for example, a right-hand curve is driven. An added torque of, for example, M Offset =−5 Nm is applied by the action of a special function. The measured manual torque is thereby increased in terms of the value and is approximately M Manual =−8 Nm. The contribution of the EPS motor decreases because the motor has to provide less assistance, which is to say no restrictions exist functionally. However, at 8 Nm manual torque, the safety function allows maximum motor torque (first range  50 ), although functionally this is not required and should be avoided from a safety perspective. A substitute manual torque M Manual*  is calculated by way of the described correction: 
         [0000]        M   Manual*   =M   Manual   −M   Offset =−8 Nm−(−5 Nm)=−3 Nm
 
         [0045]    The substitute manual torque corresponds to the manual torque that is required to travel the curve without the special function. The allowed motor torque is shifted from the first range  50  to a second range  52  (arrow  54 ), as a result of which the possible impact of faults is considerably less. 
         [0046]    In  FIG. 3 , the motor torque limit curve  30  has been plotted in a manner corresponding to  FIG. 2 . The illustration is intended to clarify the correction of the safety function for special functions that reduce the manual torque. 
         [0047]    Here, for example, a right-hand curve is driven, and an added torque of M Offset =+5 Nm, for example, is applied by the action of a special function. The measured manual torque is thereby reduced in terms of the value and is approximately M Manual =0 Nm. The contribution of the EPS motor increases because the motor has to provide greater assistance. However, due to the plausibility check measure, only low EPS motor torque is allowed at low manual torque, which is to say the request for the special function is possible only to a very limited extent and is limited (first range  60 ). 
         [0048]    However, the curve cannot be traveled with a manual torque of approximately 0 Nm within the range of normal steering sensation coordination. Rather, higher manual torque is required for this purpose, without the special function. At higher manual torque, in turn, higher EPS motor torque is allowed. For this reason, it is functionally appropriate and necessary to correct the manual torque for the safety function, and a substitute manual torque M Manual*  is computed as follows: 
         [0000]        M   Manual*   =M   Manual   −M   Offset =0 Nm−(−5 Nm)=−5 Nm
 
         [0049]    The substitute manual torque corresponds to the manual torque that is required to travel the curve without the special function. The allowed motor torque is shifted from the first range  60  to a second range  62  (arrow  64 ), and the request for torque for the special function is allowed. 
         [0050]    This method can be established as part of a comparison of the vehicle response. The decisive factor for the vehicle response, which must be kept within the controllable range, is the toothed rack motion, because this steers the wheels. The motion is obtained from the sum of the manual torque and EPS servo torque. If the driver is required to provide a lower manual torque contribution due to special functions, die contribution of the servo-motor increases accordingly. However, the sum of manual torque and EPS motor torque on the toothed rack remains the same, which is to say only the amplification factor between the manual torque and the motor torque is altered by torque-based added functions. In this case, however, the appropriately signed sum of manual torque and added torque or motor torque is relevant for limiting the motor torque. 
         [0051]    Consequently, from a safety perspective, in observing the vehicle behavior, there is no difference between a driver with special function and a corrected safety function, and an experienced driver who is able to control the situation within the limit range without the special function, and does so by drastically steering the steering wheel with a high manual torque, and thus high allowable motor torque.