Patent Publication Number: US-2021171094-A1

Title: Method for position control for a steering system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 102019133025.4 filed Dec. 4, 2019, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for position control for an electromechanically assisted steering system of a motor vehicle. The present disclosure further relates to a control unit for an electromechanically assisted steering system, an electromechanically assisted steering system and a computer program. 
     BACKGROUND 
     Motor vehicles of the prior art are known to have a steering system with electromechanical steering assistance, wherein an electric motor of the electromechanical steering assistance can provide a torque to support the steering movement of the driver of the motor vehicle. 
     The electromechanical steering assistance assists the driver in steering and/or at least partially automatically steers the vehicle in a certain direction. In particular, the electromechanical steering assistance also steers the vehicle automatically, i.e. without a corresponding steering movement of the driver, based on a target position in a certain direction, for example using data from a camera or other sensors that capture an environment of the motor vehicle. The automatic steering of motor vehicles is used in particular in control systems of at least partially autonomous motor vehicles. 
     The steering systems of such vehicles, which are at least partially autonomous, typically have a position controller, which is designed to adjust a controlled variable to the corresponding driving situation and to transmit it to the steering system by means of the electric motor. 
     Reactions from the road to the steering system and non-linear effects can significantly affect the quality of the position control. Previously known control methods are therefore only available for certain driving situations or only with low control quality. 
     SUMMARY 
     The object of the present disclosure is to provide a method for position control for an electromechanically assisted steering system, in which the disadvantages from the prior art are at least partially no longer present. 
     The object is achieved according to the present disclosure by a method for position control for an electromechanically assisted steering system of a motor vehicle, which has electromechanical steering assistance with an electric motor and at least one controller, wherein a torque to be applied by the electric motor is the controlled variable of the at least one controller, and wherein the controlled system for the controller design of at least one controller includes at least a lower part of the steering system containing the electric motor. The method has the following steps: A frequency response of the controlled system is determined based on a mathematical model of the simplified controlled system and/or by measuring the controlled system, in particular the entire controlled system. The controller parameters of the at least one controller are adjusted based on the frequency response of the controlled system. At least one target position is generated and/or received. At least one actual position is recorded and returned, and the controlled variable is adjusted by means of the at least one controller based on a comparison of the at least one target position with the at least one actual position. 
     The controlled system for the design of the controller is therefore at least the lower part of the steering system, which contains the components below a torque sensor of the steering system. More specifically, the lower part of the steering system thus comprises the electric motor and other components arranged between the torque sensor and the wheels of the motor vehicle. In particular, the lower part of the steering system comprises a gearbox coupled to the electric motor. In addition, the lower part of the steering system may include a rack coupled to the wheels of the motor vehicle, more specifically to the front wheels and the steering column. 
     The at least one controller is part of a closed control circuit. The target position represents the control variable of the controller, while the actual position is the controlled variable of the controller. 
     The frequency response for the controller design can be determined by analytically calculating it based on the mathematical model of the lower part of the steering system, wherein the frequency response then depends on model parameters of the mathematical model. Alternatively or additionally, the frequency response can be determined by measuring the steering system, more precisely the entire controlled system. 
     In particular, the transmission function of the controlled system is determined only once and then stored for further use in a control unit of the steering system. 
     Furthermore, the controller parameters of the at least one controller can be determined once based on the frequency response of the controlled system. The controller parameters can then be stored for further use in a control unit of the steering system. 
     In other words, the controller parameters are thus determined, in particular optimized, at least once based on the frequency response of the controlled system. 
     The position control method described above can be applied to any type of steering system. In particular, the method according to the present disclosure is suitable for an electromechanically assisted steering system with steering column assistance as well as for steering systems in which a torque to be applied is transmitted to the rack via a pinion, a dual pinion, or a belt drive. Furthermore, the method according to the present disclosure is also suitable for steer-by-wire steering systems, in which there is no active mechanical connection between the steering wheel and the wheels. 
     The mathematical model can be based on a physical substitution model of the controlled system, in particular a simplified physical substitution model. In the physical substitution model, individual sections of the controlled system can be modeled as a mass with a spring and/or at least one attenuation, wherein the mass has a predefined moment of inertia in each case. 
     One aspect is that the controller parameters of the at least one controller are determined based on the symmetric optimum method. This method is particularly advantageous when disturbance affects the controlled system, for example in the form of friction and/or in the form of reactions from the road. In other words, the controller design, i.e. the determination of the controller parameters according to the method of the symmetrical optimum, allows that disturbance torques and/or disturbance forces can be well controlled and good control behavior is ensured. 
     According to a further aspect, the at least one actual position is a rotation angle of the electric motor and/or a rotation angle below a torque sensor of the steering system. In particular, the rotation angle of the electric motor is used when the angle of rotation below the torque sensor is not available by measurement. 
     In one embodiment of the present disclosure, an observer is designed, which determines the disturbance torques acting on the controlled system and/or frictional torques occurring in the controlled system. In other words, the disturbance torques and/or frictional torques are estimated, for example by means of a Kalman filter, wherein a state space model can be used for the estimation. In particular, the state space model is based on the above-mentioned simplified physical substitution model of the lower part of the steering system. The disturbance torques are, for example, reactions from the road that affect the steering system. The observer thus provides estimates of the disturbance torques and/or the frictional torques occurring in the controlled system. These estimates can be used to compensate for the disturbance torques and/or frictional torques by means of the controller. 
     Preferably, based on the determined disturbance torques and/or frictional torques, the controlled variable of the at least one controller is adjusted so that the disturbance torques and/or frictional torques are at least partially compensated, in particular fully compensated. The influence of the disturbance torques, for example the reactions of the road on the steering system, and/or the frictional torques in the steering system is thus at least partially eliminated, in particular completely eliminated. Accordingly, the method according to the present disclosure is also suitable for types of steering systems in which greater frictional torques occur. 
     Further preferably, the determined disturbance torques and/or frictional torques are introduced into the control circuit downstream of the at least one controller by a first insertion transfer function. In this way, the influence of the disturbance torques and/or the frictional torques is taken into account in the control circuit. In particular, the torque applied by the electric motor is adjusted based on the determined disturbance torques and/or frictional torques. In other words, the disturbance torques and/or frictional torques are at least partially compensated in this way, in particular completely compensated. 
     According to one aspect of the present disclosure, a torque resulting from a position of the steering wheel relative to the lower part of the steering system is determined and based thereon the controlled variable of the at least one controller is adjusted, in particular wherein the resulting torque is determined by means of a torque sensor. In particular, this is a torque applied by the driver. The torque applied by the electric motor is then adjusted accordingly based on the determined resulting torque. 
     A further aspect provides that the determined torque is introduced into the control circuit downstream of the at least one controller by means of a second insertion transfer function. In this way, the influence of the torques acting on the upper part of the steering column is taken into account in the control circuit. In particular, in this way the torque to be applied by the electric motor is adjusted based on the determined resulting torque. 
     The at least one controller can be a PID controller. In particular, parameters of the PID controller are determined based on the frequency response of the controlled system. For example, the parameters of the PID controller are determined according to the symmetric optimum method. 
     Another aspect provides that a PI controller and a P controller are provided in a cascade circuit, in particular wherein the PI controller is the lead controller and the P controller is the follower controller. In particular, parameters of the PI controller and/or the P controller are determined based on the frequency response of the controlled system. For example, the parameters of the PI controller are determined using the symmetric optimum method. 
     In this case, the controlled variable of the inner control circuit can be a velocity, in particular an angular velocity, while the controlled variable of the outer control circuit is the position. For example, thus the angular velocity is controlled in the lower control circuit and the rotation angle is controlled in the upper control circuit. 
     The object is further achieved according to the present disclosure by a control unit for an electromechanically assisted steering system of a motor vehicle, wherein the electromechanically assisted steering system has electromechanical steering assistance with an electric motor and at least one controller, wherein a torque to be applied by the electric motor is the controlled variable of the at least one controller, and wherein the controlled system for the design of at least one controller comprises at least a lower part of the steering system containing the electric motor, wherein the control unit is designed to carry out a method described above. As regards the advantages, reference is made to the above explanations relating to the method. 
     The at least one controller can be integrated in the control unit. 
     The control unit may be designed to determine at least a required torque based on measurement data from the steering system and to transmit corresponding control commands to the electric motor or to a current controller of the electric motor, so that the electric motor provides at least the required torque. 
     In particular, the control unit is designed to control, in particular to steer, the motor vehicle at least partially automatically, in particular fully automatically. In this case, therefore, the electric motor does not merely provide an auxiliary torque, but rather a full torque necessary for the control of the motor vehicle. 
     The object is also achieved according to the present disclosure by an electromechanically assisted steering system with a control unit described above and electromechanical steering assistance which has an electric motor and a controller, wherein a torque to be applied by the electric motor is the controlled variable of the at least one controller, and wherein the controlled system for the design of the at least one controller comprises at least a lower part of the steering system containing the electric motor. As regards the advantages, reference is made to the above explanations relating to the method. 
     In particular, it is an electromechanically assisted steering system with steering column assistance or an electromechanically assisted steering system in which an additional torque is transmitted to the rack via a pinion, a dual pinion, or a belt drive. The steering system can also be a steer-by-wire steering system with no active mechanical connection between the steering wheel and the wheels. 
     The object is further achieved according to the present disclosure by a computer program with program code means, which are designed to cause the above-described steering system to carry out a method described above when the computer program is executed on a computing unit of the control unit of the steering system. As regards the advantages, reference is made to the above explanations regarding the method. 
     “Program Code Means” here and below means computer-executable instructions in the form of program code and/or program code modules in compiled and/or uncompiled form, which may be available in any programming language and/or in machine language. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and properties of the present disclosure arise from the following description and the attached drawings to which reference is made. In the figures: 
         FIG. 1( a )  shows a first oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 1( b )  shows a second oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 1( c )  shows a third oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 1( d )  shows a fourth oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 1( e )  shows a fifth oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 1( f )  shows a sixth oblique schematic view of an electromechanically assisted steering system according to the present disclosure; 
         FIG. 2  shows in an oblique schematic view a steer-by-wire variant of the electromechanically assisted steering system according to the present disclosure; 
         FIG. 3( a )  shows a model of a lower part of the electromechanically assisted steering system of  FIG. 1 ; 
         FIG. 3( b )  shows a model of a lower part of the electromechanically assisted steering system of  FIG. 2 ; 
         FIG. 4  shows a block diagram of a controller of the electromechanically assisted steering system of  FIG. 1 or 2  according to a first embodiment; and 
         FIG. 5  shows a block diagram of a controller of the electromechanically assisted steering system of  FIG. 1 or 2  according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1 ( a )  schematically shows a steering system  10  for a motor vehicle, wherein the steering system  10  has a gearbox  12  and is designed as an electromechanically assisted steering system with steering column assistance (“column drive EPS”). 
     The steering system  10  has a steering wheel  14 , which is connected to a first pinion  18  via an upper part of a steering column  15  and via an intermediate steering shaft  16 . The first pinion  18  meshes with a rack  20 , so that it is subjected to a torque. 
     On the steering column  15 , a torque and/or a steering angle sensor  22  is arranged, which is designed to measure a steering torques and/or a steering angle. In particular, it is thus a steering torque and steering angle sensor, which is also referred to as a “torque and angle sensor (TAS)” and can provide a steering angle in addition to the steering torque. 
     Furthermore, an electric motor  24  is provided, which has a torque transmission connection to the gearbox  12 . 
     As indicated in  FIG. 1( a ) , the gearbox  12  may be designed in different ways, for example as a worm gear, as a spur gear or as a bevel gear. 
     In any case at least one torque provided by the electric motor  24  is transmitted to the intermediate steering shaft  16  via the gearbox  12  to carry out a steering movement. 
     The electric motor  24  has a signal transmission connection to a control unit  26  of the steering system  10 , which is only schematically indicated in  FIGS. 1( a ) to ( f ) . 
     The control unit  26  is designed to determine at least a torque to be applied and to transmit corresponding control commands to the electric motor  24  based on measurement data from the steering system  10  so that the electric motor  24  provides at least the torque to be applied. 
     The control unit  26  is further designed to control, in particular to steer, the motor vehicle at least partially automatically, in particular fully automatically. In this case, therefore, the electric motor  24  provides not only a supporting torque, but rather the entire torque necessary for the control or steering of the motor vehicle. 
     The steering system  10  shown in  FIG. 1( b )  differs from that shown in  FIG. 1( a )  in that the electric motor  24  does not have a torque transmission connection via the gearbox  12  to the intermediate steering shaft  16 , but to the first pinion  18 . The steering system  10  therefore has a single pinion drive, which is also referred to as a “single pinion EPS”. 
     The steering system  10  shown in  FIG. 1( c )  has a second pinion  18 ′, which is meshed with the rack  20 . The electric motor  24  has a torque transmission connection to the second pinion  18 ′ via the gearbox  12 . In this case, the steering system  10  is a steering system with dual pinions, which is also referred to as a “dual pinion EPS”. 
     In  FIGS. 1( d ) through 1( f ) , further possible designs of the electromechanically assisted steering system  10  are shown. 
     More specifically,  FIG. 1( d )  shows a steering system  10  with a concentric rack-and-pinion drive by means of a recirculating ball nut  27 . The electric motor  24  is arranged directly on the rack  20  here and subjects the rack  20  to the supporting torque via the recirculating ball nut  27 . 
       FIG. 1( e )  shows a drive in which the gearbox  12  is in the form of a bevel gear, and in which a recirculating ball nut  27  is attached to the rack  20 . The electric motor  24  subjects the rack  20  to the supporting torque via the gearbox  12  and the recirculating ball nut  27 . 
       FIG. 1( f )  shows a belt drive  12 ′ with a recirculating ball nut  27  attached to the rack  20 . A supporting torque applied by the electric motor  24  is transmitted via a belt of the belt drive  12 ′ to the recirculating ball nut  27  and via this to the rack  20 . 
       FIG. 2  shows another embodiment of the steering system  10 . The steering system is designed here as a steer-by-wire steering system, i.e. there is no mechanical working connection between the steering wheel  14  and the rack  20 . Rather, the steering angle sensor  22  determines a steering angle and transmits it to the control unit  26 . The control unit  26  controls the electric motor  24 , which is connected to the rack  20  by a belt drive, for the generation of a torque necessary for the control or steering of the motor vehicle. In addition, the steering system  10  here has a steering wheel actuator  24 ′, which can subject the steering wheel  14  to a torque, for example to generate road feedback. 
     The electromechanically assisted steering system  10  is designed to carry out a method for position control described below on the basis of  FIGS. 3 to 5 . 
     More specifically, the control unit  26  comprises a computer program with program code means, which are designed to cause the steering system  10  to carry out the position control method described below when the computer program is executed on a computing unit or a processor of the control unit  26  of the steering system  10 . 
     “Program Code Means” means here and the below computer-executable instructions in the form of program code and/or program code modules in compiled and/or uncompiled form, which may be available in any programming language and/or in machine language. 
     The method for position control is described below by way of example on the basis of  FIG. 1( a ) . It should be noted, however, that the method can also be carried out with the steering systems  10  according to  FIGS. 1( b ) through ( f )  and  FIG. 2 . 
     First, a physical substitution model of the simplified controlled system in the form of a lower part of the steering system  10  is created, wherein the lower part of the steering system  10  contains the electric motor  24 , the gearbox  12 , at least part of the intermediate steering shaft  16 , the rack  20  and the first pinion  18 . 
       FIGS. 3( a ) and ( b )  show two variants of such a physical substitution model. 
     In the model of  FIG. 3( a )  the coupling of the electric motor  24  to the intermediate steering shaft  16  via the gearbox  12  is modeled by a first mass  28  with a moment of inertia J ED , a spring  30  with a spring constant c GB  and a material attenuation  32  with a damping constant b GB . The spring  30  and the material attenuation  32  model the gearbox  12  and the mass  28  models the electric motor  24 , more precisely at least an output shaft of the electric motor  24 . The gearbox  12  also has a transmission ratio i GB . 
     In addition, by means of attenuation  34  with a damping constant b GB  it is taken into account that the torque which the electric motor  24  applies to the gearbox  12  is attenuated by viscous friction, in particular by bearing friction in the electric motor  24 . An entire counter-torque T FR,ED  caused by Coulomb friction on the electric motor  24  and in the gearbox  12  counteracts the torque T ED . 
     If the driver applies a steering torque to the steering wheel  14 , a resulting torsion rod torque T COL  also acts on the intermediate steering shaft  16 . 
     The intermediate steering shaft  16 , the pinion  18  and the rack  20 , i.e. the lower part of the electromechanically assisted steering system  10  without the electric motor  24 , are modeled with a second mass  36  and a viscous friction b DN  in the form of a second damping  38 , wherein the second mass  36  has a moment of inertia J DN . 
     An entire counter-torque T FR,DN  acting in the lower part of the steering system  10  without the electric motor  24  and caused by Coulomb friction counteracts a torque T DN  acting on the lower part of the intermediate steering shaft  16 . 
     The physical substitution model shown in  FIG. 3( a )  has two degrees of freedom, namely a rotation angle φ DN  of the lower part of the intermediate steering shaft  16  and a rotation angle φ ED  of the electric motor  24 . 
     The respective associated angular velocities of the two degrees of freedom are Ω ED ={dot over (φ)} ED  and Ω DN ={dot over (φ)} DN . 
     The physical substitution model of  FIG. 3( b )  is simplified compared to the substitution model of  FIG. 3( a ) . More specifically, the effects of the spring  30  and the material attenuation  32  are neglected. Further, the second mass  36  and the viscous friction in the form of the second damping  38  are aggregated in a modified, effective moment of inertia J lumped  of the first mass  28 ′ and an effective damping constant b lumped  of the first damping  34 ′ as well as the effective coulomb friction T FR,lumped . 
     The physical substitution model of  FIG. 3( a )  is thus simplified by reducing the number of degrees of freedom (the degree of freedom φ DN  is omitted) by introducing effective variables which take into account both the coupling through the gearbox  12  and the lower part of the steering system  10 . 
     Accordingly, the physical substitution model shown in  FIG. 3( b )  has only one degree of freedom, namely the rotation angle V E D of the electric motor  24 . 
     Based on the respective physical substitution model, a mathematical model of the lower part of the steering system  10  is created. The mathematical model is in particular differential equations, which relate an output variable of the lower part of the steering system  10 , here φ, to input variables, here, T ED , T DN  and T COL  as well as T FR,ED , T FR,DN  or T FR,lumped  and the degree of freedom of the system. 
     Based on the mathematical model, a transmission function of the lower part of the steering system  10  is then determined, which serves as the basis for the frequency response and thus the method for position control for the electromechanically assisted steering system  10 . 
     Alternatively or in addition to the procedure described above, the frequency response can also result from measuring a time response of the entire steering system  10 . 
     In  FIG. 4 , a control circuit  39  for position control according to a first variant is shown. The control circuit  39  comprises a controller  40 , which is in the form of a PID controller, as well as a controlled system  42  (G P (s)). The controlled system  42  is the entire steering system  10 . 
     The controller parameters of the controller  40  are determined based on the frequency response of the simplified controlled system and/or the measured frequency response of the entire controlled system G P (s). In other words, a transmission function G ctrl (s) of the controller  40  is determined according to predefined criteria. 
     For example, the controller parameters or the transmission function G ctrl (s) of the controller  40  are adjusted based on the symmetric optimum method. 
     Basically, the control circuit  39  works as follows: 
     The control circuit  39  receives a target position φ ref , which represents the control variable of the controller  40 . For example, the target position φ ref  is provided by the control unit  26  or by an external control unit. 
     In addition, an actual position φ, i.e. an actual rotation angle, of the lower part of the steering system  10  is detected, wherein the actual position φ represents the controlled variable of the controller  40 . The actual position φ can be the rotation angle φ ED  of the electric motor  24  or the rotation angle φ DN  of the lower part of the intermediate steering shaft  16 . 
     Based on a comparison of the target position φ ref  with the actual position φ, a control error e φ  is determined, in particular as the difference between the target position φ ref  and the actual position φ. 
     Based on the control error e φ , the controller  40  adjusts the controlled variable of the control circuit  39 , which results in the torque to be applied T DEM  by the electric motor  24 . 
     Disturbance variables acting on the controlled system, for example disturbance torques T DN  and/or the combined frictional torques T FR , can influence the quality of the position control. 
     To compensate for the influence of the frictional torques T FR  and the disturbance torques T DN , an observer  44  is provided, which can be designed based on a state space model of the controlled system  42 . 
     The observer  44  estimates the disturbance torques T DN  and/or frictional torques T FR  occurring in the controlled system  42  based on at least one vector valued input variable {right arrow over (u)} ctrl  and at least one vector valued measurement variable {right arrow over (y)} meas . 
     In particular, the at least one vector valued input variable {right arrow over (u)} ctrl  includes the controlled variable of the control circuit  39  while the at least one vector valued measurement variable {right arrow over (y)} meas  includes the determined actual position φ. 
     In other words, the disturbance torques T DN  and/or the frictional torques T FR  are estimated by means of an observer, in particular by means of a Kalman filter, wherein a state space model can be used for estimation. In  FIG. 4  y DIST  refers to the estimate of the disturbance variables provided by the observer  44 . 
     The state space model does not have to be the same as the mathematical model mentioned above. However, it is based in particular on the same physical substitution model of the lower part of the steering system  10 . 
     The determined disturbance variables y DIST  are then fed into the control circuit  39  downstream of the controller  40  by means of a first insertion transfer function G FF1 (S). 
     The determined first insertion transfer function G FF1 (S) is such that the disturbance torques T DN  and/or the frictional torques T FR  are at least partially compensated, in particular completely compensated. 
     As explained above, only a lower part of the steering system  10  was taken into account in the physical substitution model underlying this control circuit  39 . In order to also take into account the influence of the upper part of the steering column  15 , i.e. a part above the torque sensor  22 , a torque T SEN  measured by the torque sensor  22  is inserted into the control circuit by means of a torque transmission function G FF1 (s), namely downstream of the controller  40 . 
     As a result, the controlled variable of the control circuit  39  is adjusted, whereby the influence of the upper part of the steering system  10 , more precisely the influence of the measured torque T SEN , on the movement of the part below the torque sensor  22  is taken into account. The measured torque T SEN  is in particular the torsion rod torque T COL  resulting from a position of the steering wheel  14  relative to the lower part of the steering system  10 . 
     Consequently, in the position control method described above, on the one hand disturbances in the lower part of the steering system and on the other hand the torque acting between the upper and lower parts of the steering system  10  are compensated. 
     In  FIG. 5 , a control circuit  39  for position control according to a second variant is shown. The control circuit  39  comprises a controller  40 , which is in the form of a PI controller. 
     In contrast to the variant shown in  FIG. 4 , the control circuit  39  also comprises an inner control circuit  39 ′ with a second controller  40 ′, which is in the form of a P controller, wherein the controlled system  42  is arranged in the inner control circuit  39 ′. In other words, it is a PI-P cascade circuit, wherein the PI controller  40  is the lead controller and the P controller  40 ′ is the follower controller. 
     The controlled variable of the inner control circuit  39 ′ is the angular velocity Ω in this case. The angular velocity Ω is not normally measured directly but is determined by a time derivative based on the angle  . 
     The controlled variable of the control circuit  39  is the rotation angle φ and the output of the controller  40  provides the target angular velocity for the inner control circuit  39 ′. 
     The controlled variable of the control circuit  39  may be the rotation angle φ ED  of the electric motor  24  or the rotation angle φ DN  of the lower part of the intermediate steering shaft  16 . 
     Accordingly, the controlled variable of the inner control circuit  39 ′ may be the angular velocity Ω ED  of the electric motor  24  or the angular velocity Ω DN  of the lower part of the intermediate steering shaft  16 . 
     The influence of disturbance variables T FR  and T DN  as well as the influence of the torque T SEN  resulting from the position of the steering wheel  14  relative to the lower part of the steering system  10  as well as the influence of disturbance variables T FR  and T DN , analogous to the case described above with respect to  FIG. 4 , is taken into account or compensated by introducing the estimated disturbance variables y DIST  and the measured torque T SEN  into the control circuit ( 39 ) by means of a first insertion transfer function G FF1 (s) or by means of a second insertion transfer function G FF2 (s). 
     The insertion into the control circuit  39  is carried out here downstream of the controller  40 , but upstream of the inner control circuit  39 ′.