Abstract:
A method is for compensating for a rotor angle deviation of a motor, which may be used as an actuator, e.g., as a servo actuator, in a steering system. The rotor angle deviation is compensated for by a piecewise linearized control of the difference between a desired rotor angle, which is based on a manual torque and a stored rotor angle, and a measured rotor angle, a control factor of the control being variable as a function of the control range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to Application No. 10 2004 048 107.5, filed in the Federal Republic of Germany on Oct. 2, 2004, which is expressly incorporated herein in its entirety by reference thereto. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for compensating for a rotor angle deviation of a motor and to a steering system. 
   BACKGROUND INFORMATION 
   Steering systems such as electronic power steering systems, for example, exhibit deviations between the specified torque determined by the driver of the vehicle and the rotor angle actually set by the steering system. The torque, also called manual torque, is introduced into the steering system via a steering handle such as a steering wheel or a joystick and the like. A desired rotor angle (desired RW) is derivable from the manual torque. In an idealized manner, a linear relationship is assumed between the manual torque and the desired rotor angle. In conventional steering systems, the manual torque is transmitted via a torsion element known as a torsion bar (TB). The driver of the vehicle is supported by an electrical or hydraulic motor or actuator such that the manual torque to be introduced may be lower. The actuator transmits a rotor angle (RW) onto an actuator shaft. In the process, however, the steering system does not follow the specified manual torque completely. The deviations are attributed to numerous causes. Generally, the deviations are attributed to static deviations and dynamic deviations, which, being system-related, are sought in static frictions, in a hysteresis behavior of the steering system, in liquid friction losses, in velocity losses and many other causes. 
   German Published Patent Application No. 199 56 713 describes an electric power steering device, the control unit of which calculates a current control value for the motor of the power assisted steering device. Deviations of the specified current control value from the actual power steering device control are attributed to additional loads, which are caused by feedback effects of the roadway surface on the rack-and-pinion steering gear. The feedback effects are to be absorbed with the aid of an additional element, an elastic body. 
   U.S. Patent Application Publication No. 2003/006088 describes a compensation table for the kinetic friction being stored in the engine control unit. 
   A similar approach is described in German Published Patent Application No. 102 21 678, which attributes the friction in the steering system to a hysteresis torque, which is to be taken into account in the desired torque input. The hysteresis characteristic curve is ascertained as a function of the non-compensated desired torque. 
   German Published Patent Application No. 199 20 975 subdivides the causes for the friction losses in a more differentiated manner. Five different kinds of friction are distinguished and calculated in terms of control engineering. For this purpose an estimated value is assumed for the static friction. The control provided thus estimates the system deviation of the steering, which has a separately energized DC motor as an actuator. 
   On the whole, the foregoing steering systems represent an attempt to improve the feel of the steering for a driver of the vehicle. Conventional friction compensations partly have the tendency to overcompensate in the case of steering systems that have little friction. 
   SUMMARY 
   An example embodiment of the present invention may provide optimized friction compensation for steering systems. 
   Since the rotor angle deviation of the motor used in a steering system is controlled in a compensating manner via a piecewise linearized control on the basis of a desired rotor angle and the measured rotor angle, whereby at least of the control factors of the controls is changed as a function a the control range, the control of the steering system may be optimized in the operation of a steering system according to the method of an example embodiment of the present invention. The term “control range” refers to the deviation to be controlled. The control, which operates with actual measured values of the steering system, determines the required compensation. An overcompensation may be clearly reduced if not avoided. 
   One of the control factors of the control may be changed by a gradient change as a function of the control range. The gradient change may always provide an optimum control factor for the current rotor angle position. It changes continuously within an admissible range of values. If only slight adjustments remain to be made, then the control factor is changed such that a manipulated variable of the control is adjusted only minimally. 
   According to a design of the control loop of the steering system, the control may be designed such that, when the compensation exceeds the desired rotor angle, the control breaks off the compensation for the rotor angle deviation. This may prevent overcompensation, the so-called overswinging of the control. Steering systems that are only slightly affected by friction are therefore not overcompensated. 
   In contrast to conventional, very complex control systems, which may make all sorts of estimates using many input variables, the steering system makes do with one single control, which compensates for all friction-related deviations of the rotor angle when a manual torque is applied. The degree of complexity may be reduced, which may have advantageous effects on the stability and the mutual, reinforcing influence. 
   The controller deliberately has ranges of values in which it no longer operates in a linear manner. It is linearized in a piecewise manner. The linear share of the control may include a controller, which includes at least one controller either of a P-type or of an I-type, e.g., a PI-controller. Using a P-controller, the rotor angle deviation may be quickly eliminated. A PI-controller is used to reduce the system deviation. If the control factor of the I-controller is adjustable, then this may reduce the variability, the overswinging or overcompensating tendency. The variability of the I-controller may be designed such that the control factor of the I-controller in quantitative terms is provided with a maximum control factor in the case of input variables that are in quantitative terms far from zero, and that the controlled system may be provided with variable, that is to say, quantitatively decreasing control factors in an input variable range that is quantitatively near zero. With the quantitative delimitation of the control factor, the control fundamentally may move within a technically meaningful range of values and may not drift into an extreme position. 
   As a further measure for maintaining the controller within a range of values utilizable by the actuator, the values of the manipulated variable of the control may be limited. The compensation for the rotor angle deviation is limited within an admissible maximum range of values by influencing the manipulated variable via a value limiter. The value limiter has at least three sections or ranges. Within a first range, it operates in a linear manner, and within a second and a third range, the manipulated variable is quantitatively limited to the maximum manipulated variable. 
   As particular stabilizing measures may be taken on the output side of the controller, the method may also be stabilized by measures for the input variables. Thus in an exemplary embodiment, the manual torque may be quantitatively limited to a maximum value. The control may set in with any change of the manual torque. It may be provided, however, for the control to operate only once a limiting value of the manual torque has been exceeded and to set in only afterwards. Minimal changes may thus be absorbed and the control may not correct itself permanently. 
   The method may be based on the assumption that a desired rotor angle deviation is determined from the difference of the manual torque with respect to a previously stored manual torque, multiplied by a gear ratio and divided by a factor for the system stiffness. 
   A first control, the positive compensation control, may be selected if the manual torque is above a threshold value, and a second control, the negative compensation control, may be selected if the magnitude of the manual torque having a negative sign is above a threshold value. For this purpose, the threshold values of the manual torque may be quantitatively identical for the positive and negative compensation. 
   The foregoing may be used in a steering system, e.g., for motor vehicles such as passenger cars. Their friction-dependent rotor angle deviation may operate according to a method according to an example embodiment of the present invention. 
   According to an example embodiment of the present invention, a method for compensating for a rotor angle deviation of a motor includes: piecewise linearized controlling of a difference between a desired rotor angle and a measured rotor angle, the desired rotor angle being based on a manual torque on a steering handle and a stored rotor angle, a control factor of the controlling being variable as a function of a control range. 
   The motor may include an actuator in a steering system. 
   The control factor may be variable by a gradient change as a function of the control range. 
   The controlling may include breaking off the compensation for the rotor angle deviation in the event that the compensation exceeds the desired rotor angle. 
   The controlling may include compensating for all friction-related deviations of the rotor angle under influences of the manual torque. 
   The controlling may be performed by a controller that includes at least one of (a) a P-controller, (b) an I-controller and (c) a PI-controller. 
   A controlled system of the I-controller may be provided with a maximum control factor for input variables that are quantitatively far from zero and may be provided with variable, quantitatively decreasing control factors in an input variable range that is quantitatively near zero. 
   The method may include influencing a manipulated variable by a value limiter within an admissible maximum range of values. 
   The value limiter may be linear within a first range and limited to the maximum manipulated variable within a second range and a third range. 
   The method may include quantitatively limiting the manual torque to a maximum value. 
   The controlling may set in in accordance with a change in the manual torque. 
   The controlling may set in only when the manual torque exceeds a limiting value. 
   The method may include determining a desired rotor angle deviation from a difference of the manual torque with respect to a previously stored manual torque, multiplied by a gear ratio and divided by a factor for a system stiffness. 
   The method may include selecting a first control if the manual torque is above a first threshold value and selecting a second control if a magnitude of the manual torque having a negative sign is above a second threshold value. 
   The first control may include a positive compensation control. 
   The second control may include a negative compensation control. 
   The first threshold value and the second threshold value may be quantitatively identical. 
   According to an example embodiment of the present invention, a steering system includes: a device adapted to perform piecewise linearized controlling of a difference between a desired rotor angle and a measured rotor angle, the desired rotor angle being based on a manual torque on a steering handle and a stored rotor angle, a control factor of the controlling being variable as a function of a control range. 
   The steering system may be arranged as a steering system for a motor vehicle. 
   Example embodiments of the present invention are described in more detail with reference to the appended Figures 

   
     BRIEF DESCRIPTION OF THE DRAWINGS. 
       FIG. 1  is a state diagram of a control according to an example embodiment of the present invention. 
       FIG. 2  illustrates a method for compensating for a rotor angle deviation for positive rotor angle deviations. 
       FIG. 3  illustrates a method for compensating for a rotor angle deviation for negative rotor angle deviations. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates the sequence of a control for steering systems according to an example embodiment of the present invention in the form of a state diagram. In state  1 , the current variables are first saved by a recopying process  10  in a recopying and initialization step. Afterwards, a check is performed by a comparison of values as to whether the manual torque introduced, that is, the torque that a driver of the vehicle intends to transmit to the steering system as the selected torque, is to initiate a further processing step in a new state, the control state  3  and the control state  5 , so as to result in a control  38 ,  58 , the actual compensation control. Within the scope of the recopying process  10 , the values of the rotor angle (RW), of the manual torque (TBT) and of the desired rotor angle (Desired RW or DeRW) are described. The rotor angle (RW) and the manual torque (TBT) are described using the current measured values. The desired rotor angle (DeRW) is set to  0  so that within the context of control  38 ,  58  it is then able to receive the calculated value. If the manual torque is greater than a limiting manual torque value  12  (ZeroTBT), the system branches into the positive control state  3 . If the absolute value of the manual torque is greater than a limiting manual torque value  14 , the system branches into the negative control state  5 . If the limiting manual torque values (ZeroTBT) are undershot, then the control system remains in state  1 , which may also be referred to as an archiving and initialization state for the control. 
   The continuing specification considers a controller according to  FIG. 2  and  FIG. 3 , which includes an integrator in the form of an I-controller  124 ,  224  and a proportional controller (P-controller)  136 ,  236 , and which by an addition in adder  144 ,  244  forms a PI-controller. It should be understood that example embodiments of the present invention are not limited only to PI-controllers, but that in their place PID-controllers or any other kind of control factor-adaptive controller type such as a pure P-controller or a controller of a higher order may be provided. For simplicity, the further remarks are presented for a PI-controller, including an I-controller  124 , a P-controller  136  and an adder  144 , having positive integration and compensation, or for a PI-controller, including an I-controller  224 , a P-controller  236  and an adder  244 , having negative integration and compensation. 
   The two states  3 ,  5  illustrated in  FIG. 1 , which yield the positive control state for compensating for a positive friction value, the friction compensation manipulated variable (FrictionComp)  150 , and the negative control state for compensating for a negative friction value, the friction compensation manipulated value (FrictionComp)  250 , are similar in structure. The different sign in the manual torque (TBT), however, is taken into account by sign multiplication by (−1) or by a reversal of the comparison operators. In states  3 ,  5 , an ascertainment is made in a first comparison  30 ,  50  as to whether the I-share of the position controller (IPart) is quantitatively higher than a specified I-share for a zero limit (ZeroPart). If the I-share (IPart) is quantitatively above an I-share limit value (ZeroIPart), then the respective compensation control  38 ,  58  is activated. If the compensation limit for the compensation (TBTReady) exceeds  32  or undershoots  52  the manual torque (TBT) and the limit for the lower manual torque  34 ,  54  is undershot, then the positive  38  or negative compensation control  58  may be started in states  3 ,  5 . Otherwise a limiting value persistence  70  is checked in safety state  7  or a transition is made from control state  3  to control state  5  or vice versa following a storage step of variables  36 ,  56 . 
   Favorable limits for a stable state change are limits of approximately 0.1 Nm for example. Below a manual torque (TBT) of approximately 0.1 Nm, the control assumes that the driver of the vehicle did not intend to perform a driving maneuver. As a result, the vehicle becomes more stable overall when there are smaller manual torque fluctuations in straightforward driving. The two control according to  FIG. 2  and  FIG. 3  are similar. Due to the different signs of the variable of the measured rotor angle (wrsRotrangle)  110 ,  210 , of the manual torque (mstTorsionBarTorque)  102 ,  202  and of the stored rotor angle (StoredRW)  100 ,  200 , the I-controller  124 ,  126  illustrated in  FIG. 2  or the I-controller  224 ,  226  illustrated in  FIG. 3  and the P-controller  136  illustrated in  FIG. 2  or the P-controller  236  illustrated in  FIG. 3  are established using positive and negative values. The value of I-controllers  124 ,  126 ,  224 ,  226  is limited in its maximum I-value (IPart) to a maximum value by the limiting value  128 ,  228  in the limiter of I-controller  126 ,  226 . From the difference between the subtracter  112 ,  212  of the manual torque (TorsionBarTorque)  102 ,  202  and the stored manual torque (StoredTBT)  104 ,  204 , a rotor angle, which may be added to the stored rotor angle (StoredRW)  100 ,  200  by an adder  118 ,  218 , is determined using a position factor (TBTToPos)  106 ,  206  and an optional value limiter  116 ,  216  having a limiting value (maxOffsetangle)  100 ,  200 . 
   Following the addition, controller  38 ,  58  has the desired rotor angle (DesiredRW)  120 ,  210  available, from which the measured rotor angle (Rotorangle)  110 ,  210  is subtracted by a subtracter  122 ,  222 . The signal of subtracter  122 ,  222  is applied parallel to an I-controller including the elements  124 ,  126 ,  224 ,  226  and to a P-controller  136 ,  236 . The individual signals after the I-controller, including the elements  124 ,  126 ,  224 ,  226 , and after the P-controller  136 ,  236  are added to form a signal of a PI-controller via an adder  144 ,  244 . Multipliers  130 ,  140 ,  230 ,  240 ,  132 ,  134 ,  232 ,  234  are provided for value adjustment. At the same time, value limiters  146 ,  148 ,  246 ,  248  may stabilize the control and may eliminate its tendency to oscillate. 
   The measures of stabilization by splitting signals and limiting signals may be optional and may not need to be present for a simple implementation of a control according to an example embodiment of the present invention. Behind adder  144 ,  244 , the friction compensation manipulated value  150 ,  250  may be read off, which is then applied to an actuator of the steering system. 
   REFERENCE NUMERAL LISTING STATE 
   
       
         3  control state 
         5  control state 
         7  safety state 
         10  recopying and initialization step 
         12  limiting manual torque value comparison 
         14  limiting manual torque value comparison 
         30  first comparison 
         32  compensation limit comparison 
         34  lower manual torque comparison 
         36  variables storage step 
         38  compensation control 
         50  first comparison 
         52  compensation limit comparison 
         54  lower manual torque 
         56  variables storage step 
         58  compensation control 
         70  limiting value persistence 
         100  stored rotor angle 
         102  manual torque 
         104  stored manual torque 
         106  position factor 
         108  limiting value 
         110  rotor angle 
         112  substracter 
         114  multiplier 
         116  value limiter 
         118  adder 
         120  rotor angle 
         122  substracter 
         124  I-controller 
         126  I-controller 
         128  limiting value 
         130  multiplier 
         132  multiplier 
         134  multiplier 
         136  proportional controller 
         140  multiplier 
         144  adder 
         146  value limiter 
         148  value limiter 
         150  friction compensation manipulated value 
         200  stored rotor angle 
         202  manual torque 
         204  stored manual torque 
         206  position factor 
         208  limiting value 
         210  rotor angle 
         212  substracter 
         214  multiplier 
         216  value limiter 
         218  adder 
         220  rotor angle 
         222  substracter 
         224  I-controller 
         226  I-controller 
         228  limiting value 
         230  multiplier 
         232  multiplier 
         234  multiplier 
         236  proportional controller 
         240  multipliers 
         244  adder 
         246  value limiter 
         248  value limiter 
         250  friction compensation manipulated value