Patent Publication Number: US-11034382-B2

Title: Steering apparatus for vehicles

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2018/039148 filed Oct. 22, 2018, claiming priority based on Japanese Patent Application No. 2017-204910 filed Oct. 24, 2017. 
     TECHNICAL FIELD 
     The present invention relates to a high-performance steering apparatus for vehicles that obtains a desired steering torque by performing control so that a torsional angle of a torsion bar or the like follows a value corresponding to vehicle driving information, and maintains the desired steering torque without being affected by a road surface state and aging-changes of mechanism system characteristics. 
     BACKGROUND ART 
     An electric power steering apparatus (EPS) being one of steering apparatuses for vehicles provides a steering system of a vehicle with an assist torque (a steering assist torque) by means of a rotational torque of a motor, and applies a driving force of the motor which is controlled by using an electric power supplied from an inverter as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism including a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control. 
     A general configuration of the conventional electric power steering apparatus will be described with reference to  FIG. 1 . As shown in  FIG. 1 , a column shaft (a steering shaft or a handle shaft)  2  connected to a steering wheel  1  is connected to steered wheels  8 L and  8 R through a reduction mechanism  3 , universal joints  4   a  and  4   b , a rack-and-pinion mechanism  5 , and tie rods  6   a  and  6   b , further via hub units  7   a  and  7   b . In addition, a torque sensor  10  for detecting a steering torque Ts of the steering wheel  1  and a steering angle sensor  14  for detecting a steering angle θh are provided in the column shaft  2  having a torsion bar, and a motor  20  for assisting a steering force of the steering wheel  1  is connected to the column shaft  2  through the reduction mechanism  3 . The electric power is supplied to a control unit (ECU)  30  for controlling the electric power steering apparatus from a battery  13 , and an ignition key signal is inputted into the control unit  30  through an ignition key  11 . The control unit  30  calculates a current command value of an assist command (a steering assist command) based on the steering torque Ts detected by the torque sensor  10  and a vehicle speed Vs detected by a vehicle speed sensor  12 , and controls a current supplied to the motor  20  for the EPS by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value. 
     A controller area network (CAN)  40  exchanging various information of a vehicle is connected to the control unit  30 , and it is possible to receive the vehicle speed Vs from the CAN  40 . Further, it is also possible to connect a non-CAN  41  exchanging a communication, analog/digital signals, a radio wave or the like except for the CAN  40  to the control unit  30 . 
     The control unit  30  mainly comprises a central processing unit (CPU) (including a micro controller unit (MCU), a micro processor unit (MPU) and so on), and general functions performed by programs within the CPU are shown in  FIG. 2 . 
     Functions and operations of the control unit  30  will be described with reference to  FIG. 2 . As shown in  FIG. 2 , the steering torque Ts detected by the torque sensor  10  and the vehicle speed Vs detected by the vehicle speed sensor  12  (or from the CAN  40 ) are inputted into a current command value calculating section  31 . The current command value calculating section  31  calculates a current command value Iref1 that is a control target value of a current supplied to the motor  20  based on the inputted steering torque Ts and vehicle speed Vs and by using an assist map or the like. The current command value Iref1 is inputted into a current limiting section  33  through an adding section  32 A. A current command value Irefm whose maximum current is limited is inputted into a subtracting section  32 B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current Im being fed back is calculated. The deviation I is inputted into a proportional integral (PI) control section  35  for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section  35  is inputted into a PWM-control section  36 . Furthermore, the motor  20  is PWM-driven through an inverter  37  serving as a driving section. The motor current Im of the motor  20  is detected by a motor current detector  38  and is fed back to the subtracting section  32 B. 
     A compensation signal CM from a compensation signal generating section  34  is added to the adding section  32 A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section  34  adds a self-aligning torque (SAT)  343  and an inertia  342  at an adding section  344 , further adds a convergence  341  to the added result at an adding section  345 , and then outputs the added result at the adding section  345  as the compensation signal CM. 
     Thus, in assist control by a conventional electric power steering apparatus, a steering torque applied by a manual input of a driver is detected as a torsional torque of a torsion bar by a torque sensor, and a motor current is mainly controlled as an assist current depending on the detected steering torque. However, in the case of performing control by this method, different steering torques can be generated depending on a steering angle due to a difference of a road surface state (for example, a cant of the road surface). Variations of a motor output characteristic due to long-term use can also affect the steering torque. 
     In order to solve the above problems, for example, an electric power steering apparatus shown in the publication of Japanese Patent No. 5208894 B2 (Patent Document 1) has been proposed. The electric power steering apparatus of Patent Document 1 sets a target value of the steering torque based on a relation (a steering reaction characteristic map) between the steering angle, which is determined based on a relation between the steering angle or the steering torque and an amount of response, and the steering torque in order to apply an appropriate steering torque based on a tactile characteristic of a driver. 
     THE LIST OF PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent No. 5208894 B2 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in the electric power steering apparatus of Patent Document 1, it is necessary to obtain the steering reaction characteristic map preliminarily. Further, since the apparatus performs control based on a deviation between a target value of the steering torque and a detected steering torque, an influence to the steering torque may remain. 
     The present invention has been developed in view of the above-described circumstances, and an object of the present invention is to provide a steering apparatus for vehicles that easily obtains equivalent steering torques to vehicle driving information such as a steering angle without being affected by a road surface state and aging-changes of mechanism characteristics of a steering system. 
     Means for Solving the Problems 
     The present invention relates to a steering apparatus for vehicles that comprises at least a torsion bar having an arbitrary spring constant and a sensor detecting a torsional angle, drives and controls a motor, and assists and controls a steering system, the above-described object of the present invention is achieved by that comprising: a torsional angle control section that calculates a motor current command value based on a target torsional angle corresponding to vehicle driving information and the torsional angle; wherein the torsional angle control section comprises a torsional angle feedback compensating section that calculates a target torsional angular velocity by a deviation between the target torsional angle and the torsional angle, a torsional angular velocity calculating section that calculates a torsional angular velocity by the torsional angle, a velocity control section that calculates a pre-limitation motor current command value by performing proportional compensation based on the target torsional angular velocity and the torsional angular velocity, and an output limiting section that limits upper and lower limit values of the pre-limitation motor current command value, and outputs the motor current command value; and wherein the steering apparatus for vehicles drives and controls the motor based on the motor current command value. 
     Alternatively, the above-described object of the present invention is achieved by that comprising: a torsional angle control section that calculates a motor current command value based on a target torsional angle corresponding to vehicle driving information, the torsional angle, a first rotation angle and a second rotation angle; wherein the torsional angle control section comprises a torsional angle feedback compensating section that calculates a target torsional angular velocity by a deviation between the target torsional angle and the torsional angle, a first angular velocity calculating section that calculates a first angular velocity by the first rotation angle, a second angular velocity calculating section that calculates a second angular velocity by the second rotation angle, a velocity control section that calculates a pre-limitation motor current command value by performing proportional compensation based on a target angular velocity that is obtained by the target torsional angular velocity and the first angular velocity, and the second angular velocity, and an output limiting section that limits upper and lower limit values of the pre-limitation motor current command value, and outputs the motor current command value; and wherein the steering apparatus for vehicles drives and controls the motor based on the motor current command value. 
     Alternatively, the above-described object of the present invention is achieved by that comprising: a torsional angle control section that calculates a motor current command value based on a target torsional angle corresponding to vehicle driving information, a first rotation angle and a second rotation angle; wherein the torsional angle control section comprises a torsional angle feedback compensating section that calculates a target torsional angular velocity by a deviation between a target rotation angle that is obtained by the target torsional angle and the first rotation angle and the second rotation angle, a first angular velocity calculating section that calculates a first angular velocity by the first rotation angle, a second angular velocity calculating section that calculates a second angular velocity by the second rotation angle, a velocity control section that calculates a pre-limitation motor current command value by performing proportional compensation based on a target angular velocity that is obtained by the target torsional angular velocity and the first angular velocity, and the second angular velocity, and an output limiting section that limits upper and lower limit values of the pre-limitation motor current command value, and outputs the motor current command value; and wherein the steering apparatus for vehicles drives and controls the motor based on the motor current command value. 
     Further, the above-described object of the present invention is efficiently achieved by that wherein the torsional angle control section further comprises an input limiting section that limits upper and lower limit values of the target torsional angle; or wherein the torsional angle control section further comprises a rate limiting section that limits a change amount of the target torsional angle; or further comprising: a target steering torque generating section that generates a target steering torque based on the vehicle driving information, and a converting section that converts the target steering torque into the target torsional angle used at the torsional angle control section; or wherein the target steering torque generating section comprises a basic map section that obtains a first torque signal in accordance with the vehicle driving information by using a basic map being vehicle speed sensitive, a damper calculating section that obtains a second torque signal based on angular velocity information by using a damper gain map being vehicle speed sensitive, and a hysteresis correcting section that obtains a third torque signal by performing hysteresis correction to the vehicle driving information depending on a steering state, and calculates the target steering torque by the first torque signal, the second torque signal and the third torque signal; or wherein the steering apparatus for vehicles drives and controls the motor based on a current command value that is obtained by adding an assist current command value calculated based on a steering torque to the motor current command value. 
     Effects of the Invention 
     By performing velocity control using proportional compensation to the target torsional angular velocity or the target angular velocity which is calculated based on the target torsional angle corresponding to the vehicle driving information, the steering apparatus for vehicles of the present invention operates so that the torsional angle follows the target torsional angle, obtains a desired steering torque to the vehicle driving information, and can supply an appropriate steering torque based on steering feeling of a driver. The torsional angle control section comprises the velocity control section that controls the torsional angular velocity or the angular velocity, thereby, followability to the target torsional angle can be improved, an influence to the torsional angle due to a change of a steering angle inputted from a driver can be suppressed, and followability of the torsional angle to the target torsional angle against abrupt steering can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a configuration diagram illustrating a general outline of an electric power steering apparatus; 
         FIG. 2  is a block diagram showing a configuration example of a control unit (ECU) of the electric power steering apparatus; 
         FIG. 3  is a structural diagram showing an installation example of an EPS steering system and various sensors; 
         FIG. 4  is a block diagram showing a configuration example (a first embodiment) of the present invention; 
         FIG. 5  is a block diagram showing a configuration example of a target steering torque generating section; 
         FIGS. 6A and 6B  are diagrams showing a characteristic example of a basic map; 
         FIG. 7  is a diagram showing a characteristic example of a damper gain map; 
         FIG. 8  is a diagram showing a characteristic example of a hysteresis correcting section; 
         FIG. 9  is a block diagram showing a configuration example (the first embodiment) of a torsional angle control section; 
         FIG. 10  is a flowchart showing an operating example of the present invention; 
         FIG. 11  is a flowchart showing an operating example of the target steering torque generating section; 
         FIG. 12  is a flowchart showing an operating example (the first embodiment) of the torsional angle control section; 
         FIG. 13  is a graph showing a time sequence example of a steering angle used in a simulation; 
         FIG. 14  is a simulation result in the case of not performing torsional angle control; 
         FIG. 15  is a diagram showing an output example of a target steering torque in a simulation in the case of performing the torsional angle control; 
         FIG. 16  is a simulation result in the case of performing the torsional angle control; 
         FIG. 17  is a block diagram showing a configuration example (a second embodiment) of a torsional angle control section; 
         FIG. 18  is a flowchart showing an operating example (the second embodiment) of the torsional angle control section; 
         FIG. 19  is a block diagram showing a configuration example (a third embodiment) of a torsional angle control section; 
         FIG. 20  is a flowchart showing an operating example (the third embodiment) of the torsional angle control section; 
         FIG. 21  is a block diagram showing a configuration example (a fourth embodiment) of the present invention; and 
         FIGS. 22A and 22B  are block diagrams showing an inserting example of a phase compensating section. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The present invention is a steering apparatus for vehicles to obtain an equivalent steering torque corresponding to vehicle driving information such as a steering angle, a vehicle speed and a steering state without being affecting a road surface state, and obtains a desired steering torque by performing control so that a torsional angle of a torsion bar or the like follows a value depending on the vehicle driving information. 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
     First, an installation example of various sensors that detect information related to an electric power steering apparatus being one of steering apparatuses for vehicle of the present invention, will be described.  FIG. 3  is a diagram showing an installation example of an EPS steering system and various sensors. A torsion bar  2 A is provided in a column shaft  2 . Road surface reaction force Fr and road surface information p operate on steered wheels  8 L and  8 R. An upper side angle sensor is disposed on a steering wheel side of the column shaft  2  above the torsion bar  2 A, and a lower side angle sensor is disposed on a steered wheel side of the column shaft  2  below the torsion bar  2 A. The upper side angle sensor detects a steering wheel angle θ 1 , and the lower side angle sensor detects a column angle θ 2 . A steering angle θh is detected by a steering angle sensor disposed on an upper portion of the column shaft  2 . A torsional angle Δθ of the torsion bar and a torsion bar torque Tt can be calculated by the following expressions 1 and 2 from a deviation between the steering wheel angle θ 1  and the column angle θ 2 . In the expression 2, Kt is a spring constant of the torsion bar  2 A.
 
θ 2 −Θ 1 =Δθ  [Expression 1]
 
 Kt·Δθ=Kt ·(θ 2 −θ 1 )= Tt   [Expression 2]
 
     The torsion bar torque Tt can be also detected by using, for example, a torque sensor disclosed in Japanese Unexamined Patent Publication No. 2008-216172 A. 
     Next, a configuration example of the present invention will be described. 
       FIG. 4  is a block diagram showing a configuration example (a first embodiment) of the present invention, and steering of a driver is assisted and controlled by a motor in an EPS steering system/vehicle system  100 . A vehicle speed Vs being the vehicle driving information and a steering state STs that is outputted from a right-turning/left-turning judging section  110  and indicates right-turning or left-turning, are inputted into a target steering torque generating section  120  that outputs a target steering torque Tref corresponding to the steering angle θh being the vehicle driving information, in addition to the steering angle θh. The target steering torque Tref is converted into a target torsional angle Δθref at a converting section  130 , and the target torsional angle Δθref is inputted into a torsional angle control section  140  with the torsional angle Δθ of the torsion bar  2 A. The torsional angle control section  140  calculates a motor current command value Imc so that the torsional angle Δθ becomes the target torsional angle Δθref. The motor of the EPS is driven in accordance with the motor current command value Imc. 
     The right-turning/left-turning judging section  110  judges whether steering is right-turning or left-turning based on a motor angular velocity ωm, and outputs the judgment result as the steering state STs. That is, when the motor angular velocity ωm is a positive value, the right-turning/left-turning judging section  110  judges the steering “right-turning”, and when the motor angular velocity ωm is a negative value, the right-turning/left-turning judging section  110  judges the steering “left-turning”. Instead of the motor angular velocity ωm, an angular velocity calculated by velocity calculation with respect to the steering angle θh, the steering wheel angle θ 1  or the column angle θ 2 , may be used. 
       FIG. 5  shows a configuration example of the target steering torque generating section  120 . The target steering torque generating section  120  comprises a basic map section  121 , a differential section  122 , a damper gain section  123 , a hysteresis correcting section  124 , a multiplying section  125  and adding sections  126  and  127 . The steering angle θh is inputted into the basic map section  121 , the differential section  122  and the hysteresis correcting section  124 . The steering state STs outputted from the right-turning/left-turning judging section  110  is inputted into the hysteresis correcting section  124 . 
     The basic map section  121  has a basic map, and outputs a torque signal (a first torque signal) Tref_a having the vehicle speed Vs as a parameter by using the basic map. The basic map has been adjusted by tuning. For example, as shown in  FIG. 6A , the torque signal Tref_a increases as a magnitude (an absolute value) |θh| of the steering angle θh increases, and increases also as the vehicle speed Vs increases.  FIG. 6A  shows a configuration where a sign section  121 A outputs a sign (+1, −1) of the steering angle θh to a multiplying section  121 B, a magnitude of the torque signal Tref_a is obtained from the magnitude of the steering angle θh by using a map, the magnitude of the torque signal Tref_a is multiplied by the sign of the steering angle θh, and the torque signal Tref_a is obtained. On the other hand, as shown in  FIG. 6B , the map may be configured depending on a positive and a negative steering angles θh. In this case, the mode of variation may be changed depending on whether the steering angle θh is positive or negative. 
     The differential section  122  calculates a steering angular velocity ωh by differentiating the steering angle θh, and the steering angular velocity ωh is inputted into the multiplying section  125 . 
     The damper gain section  123  outputs a damper gain D G  by which the steering angular velocity ωh is multiplied. The steering angular velocity ωh that is multiplied by the damper gain D G  at the multiplying section  125 , is inputted into the adding section  127  as a torque signal (a second torque signal) Tref_b. The damper gain D G  is obtained depending on the vehicle speed Vs by using a vehicle speed sensitive damper gain map that the damper gain section  123  has. The damper gain map, for example, as shown in  FIG. 7 , has a characteristic that the damper gain D G  increases gradually as the vehicle speed Vs increases. The damper gain map may be variable depending on the steering angle θh. The damper gain section  123  and the multiplying section  125  constitute a damper calculating section. 
     The hysteresis correcting section  124  calculates a torque signal (a third torque signal) Tref_c based on the steering angle θh and the steering state STs in accordance with the following expression 3. In the following expression 3, x and y are set to θh and Tref_c respectively (x=θh and y=Tref_c), and A hys  is a hysteresis width.
 
when right-turning  y=A   hys [1−exp{− a ( x−b )}]
 
when left-turning  y=−A   hys [1−exp{ a ( x−b )}]  [Expression 3]
 
     When switching from the right-turning steering to the left-turning steering and when switching from the left-turning steering to the right-turning steering, based on the final coordinates (x1, y1), a value “b” of the following expression 4 is substituted into the value “b” in the expression 3 after switching. Thereby, continuity when switching the steering is maintained. 
     
       
         
           
             
               
                 
                   
                     
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     Setting A hys =1 [Nm] and a=0.3 in the expressions 3 and 4, in the case that an initial angle of the steering wheel is 0 [deg] and the steering wheel is steered between +50 [deg] and −50 [deg], a changing example of the torque signal Tref_c which hysteresis correction is applied to is shown in  FIG. 8 . That is, the torque signal Tref_c from the hysteresis correcting section  124  has a hysteresis characteristic shown by “an origin point→L 1  (the thin line)→L 2  (the broken line)→L 3  (the thick line)”. 
     A hys  which is a coefficient expressing an output width of the hysteresis characteristic, and “a” which is a coefficient expressing roundness, may be variable depending on the vehicle speed Vs and/or the steering angle θh. 
     The torque signals Tref_a, Tref_b and Tref_c are added at the adding sections  126  and  127 , and the added result is outputted as the target steering torque Tref. 
     The steering angular velocity ωh is obtained by differential calculation to the steering angle θh, and low pass filter (LPF) processing is appropriately performed in order to reduce an affection of a noise in a high frequency band. The differential calculation and the LPF processing may be performed by using a high pass filter (HPF) and a gain. The steering angular velocity ωh may be calculated by the differential calculation and the LPF processing to the steering wheel angle θ 1  detected by the upper side angle sensor or the column angle θ 2  detected by the lower side angle sensor, instead of the steering angle θh. The motor angular velocity ωm may be used instead of the steering angular velocity ωh, and in this case, the differential section  122  becomes unnecessary. 
     The converting section  130  has a characteristic of −1/K t  obtained by inverting the sign of the reciprocal of the spring constant Kt of the torsion bar  2 A, and converts the target steering torque Tref into a target torsional angle Δθref. 
     The torsional angle control section  140  calculates the motor current command value Imc based on the target torsional angle  66  θref and the torsional angle Δθ.  FIG. 9  is a block diagram showing a configuration example of the torsional angle control section  140 . The torsional angle control section  140  comprises an input limiting section  141 , a rate limiting section  142 , a torsional angle feedback (FB) compensating section  143 , a torsional angular velocity calculating section  144 , a velocity control section  150 , an output limiting section  145 , and a subtracting section  146 . 
     The input limiting section  141  limits upper and lower limit values of the target torsional angle Δθref so that the torsional angle control section  140  does not output the abnormal motor current command value Imc when the target torsional angle Δθref becomes an abnormal value in a communication, a calculation of a microcomputer or an ECU, and so on. The input limiting section  141  presets the upper limit value and the lower limit value with respect to the target torsional angle Δθref. In the case that the target torsional angle Δθref is larger than or equal to the upper limit value, the input limiting section  141  outputs the upper limit value as a target torsional angle Δθref′. In the case that the target torsional angle Δθref is smaller than or equal to the lower limit value, the input limiting section  141  outputs the lower limit value as the target torsional angle Δθref′. Otherwise, the input limiting section  141  outputs the target torsional angle Δθref, as it is, as the target torsional angle Δθref′. The set upper limit value and lower limit value may be the maximum value and the minimum value of a torsional angle used in the control respectively, and may be also the maximum value and the minimum value of a detectable torsional angle respectively. This enables security of safety. 
     The rate limiting section  142  limits a change amount of the target torsional angle Δθref′ so that the value of the target torsional angle Δθref′ does not change continuously and the target torsional angle Δθref′ fluctuates discontinuously when the target torsional angle Δθref becomes an abnormal value. The target torsional angle normally changes continuously, and the rate limiting section  142  does not limit it. When the target torsional angle temporarily becomes an abnormal value by any abnormality, the discontinuous change is prevented by the limitation of the rate limiting section  142 . For example, a difference between the present and the previous target torsional angles Δθref′ is used as the change amount, when an absolute value of the change amount is larger than a predetermined value, addition or subtraction to the target torsional angle Δθref′ is performed so that the absolute value of the change amount becomes the predetermined value, and the result is outputted as a target torsional angle Δθref″. When the absolute value of the change amount is smaller than or equal to the predetermined value, the target torsional angle Δθref′ is outputted, as it is, as the target torsional angle Δθref″. The limitation may be performed such as by using a rate of the difference to the previous target torsional angle Δθref′ as the change amount. 
     Although the input limiting section  141  and the rate limiting section  142  may be arranged reversely, the arrangement as shown in  FIG. 9  is desirable. In such a case of dealing with the abnormal value and/or the discontinuous value by another method, it is possible to remove the input limiting section  141  and/or the rate limiting section  142 . 
     The torsional angle FB compensating section  143  multiplies a deviation Δθ 0 , which is calculated at the subtracting section  146 , between the target torsional angle Δθref″ and the torsional angle Δθ by a compensation value C FB  (a transfer function), and outputs a target torsional angular velocity ωref that enables followability of the torsional angle Δθ to the target torsional angle Δθref. The compensation value C FB  may be simply a gain Kpp, or may be a compensation value generally used, such as a compensation value of PI-control. The target torsional angular velocity ωref is inputted into the velocity control section  150 . By using the torsional angle FB compensating section  143  and the velocity control section  150 , the torsional angle Δθ follows the target torsional angle Δθref, and it is possible to obtain the desired steering torque. 
     The torsional angular velocity calculating section  144  calculates a torsional angular velocity ωt by differential calculation to the torsional angle Δθ, and the torsional angular velocity ωt is inputted into the velocity control section  150 . Pseudo-differential by an HPF and a gain may be performed as the differential calculation. 
     The velocity control section  150  calculates a motor current command value (a pre-limitation motor current command value) Imcb that enables followability of the torsional angular velocity ωt to the target torsional angular velocity ωref. The velocity control section  150  calculates a difference (ωref−ωt) between the target torsional angular velocity ωref and the torsional angular velocity ωt at a subtracting section  151 , multiplies the difference by a compensation value Kv ata compensating section  152 , and outputs the multiplied result as the motor current command value Imcb. Although a compensation value of proportional (P) compensation is used as the compensation value Kv, a compensation value of proportional integral (PI) compensation or the like may be used. 
     The output limiting section  145  limits upper and lower limit values of the motor current command value Imcb outputted from the velocity control section  150 , and outputs the motor current command value Imc. As with the input limiting section  141 , the output limiting section  145  performs the limitation by presetting the upper limit value and the lower limit value with respect to the motor current command value Imcb. 
     In such a configuration, an operating example of the present embodiment will be described with reference to flowcharts of  FIGS. 10 to 12 . 
     As the operation starts, the right-turning/left-turning judging section  110  inputs the motor angular velocity ωm, judges whether steering is right-turning or left-turning based on a sign of the motor angular velocity ωm, and outputs the judgment result as the steering state STs to the target steering torque generating section  120  (Step S 10 ). 
     The target steering torque generating section  120  inputs the steering angle θh and the vehicle speed Vs with the steering state STs, and generates the target steering torque Tref (Step S 20 ). An operating example of the target steering torque generating section  120  will be described with reference to a flowchart of  FIG. 11 . 
     The steering angle θh inputted into the target steering torque generating section  120  is inputted into the basic map section  121 , the differential section  122  and the hysteresis correcting section  124 , the steering state STs is inputted into the hysteresis correcting section  124 , and the vehicle speed Vs is inputted into the basic map section  121  and the damper gain section  123  (Step S 21 ). 
     The basic map section  121  generates the torque signal Tref_a corresponding to the steering angle θh and the vehicle speed Vs by using the basic map shown in  FIG. 6A or 6B , and outputs it to the adding section  126  (Step S 22 ). 
     The differential section  122  differentiates the steering angle θh, and outputs the steering angular velocity ωh (Step S 23 ). The damper gain map  123  outputs the damper gain D G  corresponding to the vehicle speed Vs by using the damper gain map shown in  FIG. 7  (Step S 24 ). The multiplying section  125  calculates the torque signal Tref_b by multiplying the steering angular velocity ωh and the damper gain D G , and outputs it to the adding section  127  (Step S 25 ). 
     The hysteresis correcting section  124  performs the hysteresis correction to the steering angle θh by switching the calculations of the expressions 3 and 4 depending on the steering state STs (Step S 26 ), generates the torque signal Tref_c, and outputs it to the adding section  127  (Step S 27 ). Although the hysteresis width A hys , “a”, x1 and y1 are preset and retained, it is possible to calculate “b” depending on steering directions (right-turning and left-turning) in advance, and retain “b” instead of x1 and y1. 
     The torque signals Tref_b and Tref_c are added at the adding section  127 , the torque signal Tref_a is added to the added result at the adding section  126 , and the target steering torque Tref is calculated (Step S 28 ). 
     The target steering torque Tref generated at the target steering torque generating section  120  is inputted into the converting section  130 , and is converted into the target torsional angle Δθref at the converting section  130  (Step S 30 ). The target torsional angle Δθref is inputted into the torsional angle control section  140 . 
     The torsional angle control section  140  inputs the torsional angle Δθ with the target torsional angle Δθref, and calculates the motor current command value Imc (Step S 40 ). An operating example of the torsional angle control section  140  will be described with reference to a flowchart of  FIG. 12 . 
     The target torsional angle Δθref inputted into the torsional angle control section  140  is inputted into the input limiting section  141 , and the torsional angle Δθ is inputted into the torsional angular velocity calculating section  144  and the subtracting section  146  (Step S 41 ). 
     The input limiting section  141  limits the upper and lower limit values of the target torsional angle Δθref by the preset upper limit value and lower limit value, and outputs the limited result as the target torsional angle Δθref′ to the rate limiting section  142  (Step S 42 ). The rate limiting section  142  limits the change amount of the target torsional angle Δθref′, and outputs the limited result as the target torsional angle Δθref″ to the subtracting section  146  (Step S 43 ). 
     The deviation Δθ 0  is calculated at the subtracting section  146  by subtracting the torsional angle Δθ from the target torsional angle Δθref″ (Step S 44 ). The deviation Δθ 0  is inputted into the torsional angle FB compensating section  143 . The torsional angle FB compensating section  143  compensates the deviation Δθ 0  by multiplying the deviation Δθ 0  by the compensation value C FB  (Step S 45 ), and outputs the target torsional angular velocity ωref to the velocity control section  150 . 
     The torsional angular velocity calculating section  144  inputting the torsional angle Δθ calculates the torsional angular velocity cot by the differential calculation to the torsional angle Δθ (Step S 46 ), and outputs it to the velocity control section  150 . 
     In the velocity control section  150 , the difference between the target torsional angular velocity ωref and the torsional angular velocity ωt is calculated at the subtracting section  151 , the proportional processing by the compensation value Kv is performed to the difference at the compensating section  152 , and the result of the proportional processing is outputted as the motor current command value Imcb to the output limiting section  145  (Step S 47 ). 
     The output limiting section  145  limits the upper and lower limit values of the motor current command value Imcb by the preset upper limit value and lower limit value (Step S 48 ), and outputs the limited result as the motor current command value Imc (Step S 49 ). 
     Current control is performed by driving the motor based on the motor current command value Imc outputted from the torsional angle control section  140  (Step S 50 ). 
     Each Order of the data inputs, the calculations, or the like in  FIGS. 10 to 12  is appropriately changeable. 
     An effect of the followability to the target steering torque by the present embodiment will be described based on a simulation result. 
     First, a simulation result in the case of performing only a conventional assist control will be shown. Assuming ordinary steering, as shown in  FIG. 13 , a simulation of responses of the steering angle θh and the steering torque (the torsion bar torque) Tt in the case of changing the steering angle θh in a sine wave shape whose amplitude is about 30 deg and whose frequency is about 1.0 Hz, is performed. In  FIG. 13 , a horizontal axis shows a time [sec], and a vertical axis shows a steering angle [deg]. 
     A time sequence waveform of the simulation result is shown in  FIG. 14 . In  FIG. 14 , a horizontal axis shows a steering angle [deg], a vertical axis shows a steering torque [N·m], and signs of them are adjusted so that when the steering angle is positive, the steering torque is also positive. Since a target steering torque does not exist in the assist control, in the case of performing only the assist control, the steering torque has a characteristic where the steering torque remains outputted continuously. 
     Next, a simulation result in the case of performing the torsional angle control will be shown. In the torsional angle control, the compensation values of the torsional angle FB compensating section  143  and the compensating section  152  in the velocity control section  150  are set to a proportional gain, and the torsional angular velocity calculating section  144  has a structure of an HPF where a cutoff frequency is 50 Hz and a transfer function is expressed by the following expression 5 (T hpf  is a time constant of a filter). 
                   s         T   hpf     ⁢   s     +   1             [     Expression   ⁢           ⁢   5     ]               
As with the case of performing only the assist control, an inputted steering angle θh is data of the sine wave shape as shown in  FIG. 13 . In this case, the target steering torque Tref outputted from the target steering torque generating section  120  becomes a waveform shown in  FIG. 15 . In  FIG. 15 , a horizontal axis shows a steering angle [deg], and a vertical axis shows a target steering torque [N·m].
 
       FIG. 16  shows a simulation result. In  FIG. 16 , a horizontal axis shows a steering angle [deg], a vertical axis shows a steering torque [N·m], and as with  FIG. 14 , signs of them are adjusted so that when the steering angle is positive, the steering torque is also positive. From  FIGS. 15 and 16 , it is found out that the steering torque follows the target steering torque comparatively well in the whole region. 
     Another configuration example of the present invention will be described. 
     Since the torsional angle Δθ of the torsion bar  2 A can be replaced with the deviation between the steering wheel angle θ 1  and the column angle θ 2  as shown by expression 1, the torsional angle control section  140  of the first embodiment shown in  FIG. 9  can be equivalently replaced with a configuration example (a second embodiment) shown in  FIG. 17  as an effect that the first embodiment has remains as it is. 
     Compared with the torsional angle control section  140  of the first embodiment, in a torsional angle control section  240  of the second embodiment, the steering wheel angle (a first rotation angle) θ 1  and the column angle (a second rotation angle) θ 2  are inputted in addition to the target torsional angle Δθref and the torsional angle Δθ, angular velocity calculating sections  244  and  247  for the steering wheel angle θ 1  and the column angle θ 2  respectively are added instead of the torsional angular velocity calculating section  144  for the torsional angle Δθ, and an adding section  248  is further added. By setting what is obtained by adding a steering wheel angular velocity (a first angular velocity) ω 1  calculated by differentiating the steering wheel angle θ 1  to the target torsional angular velocity ωref to a target angular velocity ωrefc, it is possible to regard the velocity control section  150  as a configuration of velocity control with respect to a column angular velocity (a second angular velocity). 
     Compared with the first embodiment, an operating example of the second embodiment is different in only the operation of the torsional angle control section, and other operations are the same. 
     An operating example of the torsional angle control section in the second embodiment is shown by a flowchart of  FIG. 18 . Operations from the start to step S 45  of performing the torsional angle FB compensation are the same as those of the first embodiment, and the target torsional angular velocity ωref outputted from the torsional angle FB compensating section  143  is inputted into the adding section  248 . The angular velocity calculating section  244  inputting the steering wheel angle θ 1  calculates the steering wheel angular velocity ω 1  by differential calculation (Step S 46 A). The target torsional angular velocity ωref is added to the steering wheel angular velocity ω 1  at the adding section  248 , and the added result is outputted to the velocity control section  150  as the target angular velocity ωrefc (Step S 46 B). The angular velocity calculating section  247  inputting the column angle θ 2  calculates the column angular velocity ω 2  by differential calculation (Step S 46 C), and outputs it to the velocity control section  150 . The velocity control section  150  performs the velocity control based on the target angular velocity ωrefc and the column angular velocity ω 2  (Step S 47 ). Operations after that are the same as those of the first embodiment. 
     By replacing the torsional angle Δθ with the deviation between the steering wheel angle θ 1  and the column angle θ 2 , besides the second embodiment, the torsional angle control section  140  can be equivalently replaced with a configuration example (a third embodiment) shown in  FIG. 19  as the effect that the first embodiment has remains as it is. 
     Compared with the torsional angle control section of the second embodiment, in a torsional angle control section of the third embodiment, the torsional angle Δθ is not inputted, and an adding section  349  is added. By setting what is obtained by adding the steering wheel angle θ 1  to the target torsional angle Δθref″ to a target rotation angle θrefc, multiplying the deviation Δθ 0  between the target rotation angle θrefc and the column angle θ 2  by the compensation value C FB , and setting what is obtained by adding the steering wheel angular velocity ω 1  to the multiplied result to the target angular velocity ωrefc, as with the second embodiment, it is possible to regard the velocity control section  150  as a configuration of velocity control with respect to a column angular velocity. 
     Compared with the second embodiment, an operating example of the third embodiment is different in only the operation of the torsional angle control section, and other operations are the same. 
     An operating example of the torsional angle control section in the third embodiment is shown by a flowchart of  FIG. 20 . Operations from the start to step S 43  of performing the rate limitation are the same as those of the second embodiment, and the target torsional angle Δθref″ outputted from the rate limiting section  142  is inputted into the adding section  349 . The steering wheel angle θ 1  is also inputted into the adding section  349 , and the result of adding target torsional angle Δθref″ and the steering wheel angle θ 1  is outputted as the target rotation angle θrefc (Step S 43 A). The column angle θ 2  is subtracted from the target rotation angle θrefc at the subtracting section  146 , and the deviation Δθ 0  is calculated (Step S 44 ). Operations after that are the same as those of the second embodiment. 
     In the second and the third embodiments, by simply making a reduction mechanism perform reduction ratio transformation, data obtained by dividing a motor angular velocity by a reduction ratio may be used as the column angular velocity ω 2 . In this case, the angular velocity calculating section  247  becomes unnecessary. Further, the steering angle θh may be used instead of the steering wheel angle θ 1 . 
     Even when a current command value (an assist current command value) calculated based on a steering torque in a conventional EPS, for example, a current command value Iref1 outputted from a current command value calculating section  31  or a current command value Iref2 obtained by adding a compensation signal CM to the current command value Iref1 in  FIG. 2 , is added to the motor current command value Imc outputted from the torsional angle control section in the first to the third embodiments, it is possible to obtain the desired steering torque. 
     A configuration example (a fourth embodiment) of applying the above to the first embodiment is shown in  FIG. 21 . An assist control section  400  is constituted by the current command value calculating section  31 , or the current command value calculating section  31 , a compensation signal generating section  34  and an adding section  32 A. An assist current command value Iac (corresponding to the current command value Iref1 or Iref2 in  FIG. 2 ) outputted from the assist control section  400  and the motor current command value Imc outputted from the torsional angle control section  140  are added at an adding section  460 , a current command value Ic of the added result is inputted into a current limiting section  470 , a motor is driven based on a current command value Icm whose maximum current is limited, and the current control is performed. 
     With respect to the target steering torque generating section  120  in the above embodiments (the first to the fourth embodiments), in such a case of focusing on a cost and a processing time, it is possible to omit the damper calculating section and/or the hysteresis correcting section  124 . In the case of omitting the damper calculating section, the differential section  122  and the adding section  127  can be also omitted. In the case of omitting the hysteresis correcting section  124 , the right-turning/left-turning judging section  110  and the adding section  127  can be also omitted. Further, it is possible to insert a phase compensating section  128  performing phase compensation into a front stage or a rear stage of the basic map section  121 . That is, it is possible to change a configuration of an area “R” surrounded by a broken line in  FIG. 5  to a configuration shown in  FIG. 22A or 22B . In the case of setting phase advance compensation as the phase compensation in the phase compensating section  128 , and, for example, performing the phase advance compensation by a primary filter where a cutoff frequency of a numerator is set to 1.0 Hz and a cutoff frequency of a denominator is set to 1.3 Hz, it is possible to achieve comfortable feeling. With respect to the target steering torque generating section, if it is configured based on the vehicle driving information, the configuration is not limited to the above configurations. 
     Although the present invention is applied to a column-type EPS in  FIGS. 1 and 3 , the present invention is not limited to an upstream-type EPS such as the column-type EPS, and can also be applied to a downstream-type EPS such as a rack-and-pinion type EPS. Further, in a viewpoint that the feedback control is performed based on the target torsional angle, the present invention can be applied to a steer-by-wire reaction force apparatus which comprises at least a torsion bar whose spring constant is arbitrary and a sensor for detecting the torsional angle, and so on. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           1  steering wheel 
           2  column shaft (steering shaft, handle shaft) 
           2 A torsion bar 
           3  reduction mechanism 
           10  torque sensor 
           12  vehicle speed sensor 
           14  steering angle sensor 
           20  motor 
           30  control unit (ECU) 
           31  current command value calculating section 
           33 ,  470  current limiting section 
           34  compensation signal generating section 
           100  EPS steering system/vehicle system 
           110  right-turning/left-turning judging section 
           120  target steering torque generating section 
           121  basic map section 
           123  damper gain section 
           124  hysteresis correcting section 
           128  phase compensating section 
           130  converting section 
           140  torsional angle control section 
           141  input limiting section 
           142  rate limiting section 
           143  torsional angle feedback (FB) compensating section 
           144  torsional angular velocity calculating section 
           145  output limiting section 
           150  velocity control section 
           152  compensating section 
           244 ,  247  angular velocity calculating section 
           400  assist control section