Abstract:
A steering control apparatus is provided which is capable of alleviating a decrease in the output torque of a motor during high-speed steering. The steering control apparatus includes a motor  5 , a motor current instruction value generation section  100   a   , 100   b  for generating a current instruction value for the motor  5 , a motor current detection section  102   a   , 102   b  for detecting a current flowing through the motor  5 , wherein the motor  5  is driven to rotate based on at least the current, which flows through the motor  5 , and the current instruction value. The motor current instruction value generation section includes a correction section  100   b  for correcting the current instruction value which controls the magnetic field of a field magnet of the motor  5 , the correction section being operable to correct the current instruction value for controlling the magnetic field of the field magnet of the motor  5  when a steering speed is fast.

Description:
This application is based on Application No. 2001-229685, filed in Japan on Jul. 30, 2001, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a steering control apparatus, and more specifically, it relates to an electric power steering control apparatus which controls an assisting force applied by a motor to a steering system of a vehicle based on at least the steering torque of a steering wheel and the speed of the vehicle when the steering wheel is steered to turn by a driver. 
     2. Description of the Related Art 
     There have been developed electric power steering apparatuses in which the speed of a vehicle and the steering torque applied to a steering column or shaft are detected, and a driving current determined according to the vehicle speed and the steering torque thus detected is supplied to a motor which generates an assisting force given to the steering shaft, so that the motor is thereby driven to rotate, thus assisting the force required to steer the vehicle by means of the rotating force of the motor to provide the driver with a pleasant steering sensation or feeling. 
     In the past, a DC motor with a brush has been used as a motor which gives an assisting force to a steering shaft of a vehicle, but it is impossible to perform field control with the DC motor and hence conventional electric power steering control apparatuses are not equipped with any means for generating a target current value used to carry out field control. 
     Moreover, there can be considered a steering control apparatus which employs a brushless motor in place of such a DC motor, as a motor for applying an assisting force to the steering shaft. In this case, owing to the absence of any brush within an electric motor, there will take place almost no abnormality or fault in the motor itself, and it also becomes possible to effect field control, which could not been carried out in the past as referred to above. 
     A brushless motor control apparatus performs a variety of calculations such as the calculation of a dq-axis target current based on an instruction torque, the detection of currents for respective phases (e.g., u phase and v phase) of a motor, the dq conversion of currents, the calculation of current deviations, the calculation of instruction voltage values, dq inversions, and the output of PWM control patterns. The detected respective phase currents are subjected to dq conversion in such a manner that they are controlled in a feedback manner to make their d-axis component and q-axis component equal to a d-axis target current and a q-axis target current, respectively. The d-axis component of each current means a wattless or reactive current, whereas the q-axis component is proportional to the torque of the motor when the motor is a synchronous motor and when the magnitude of the excitation magnetic field is constant. Therefore, the current feedback control for the synchronous motor is generally carried out in such a manner that the d-axis component of the detected current becomes zero and the q-axis component thereof becomes equal to a target value of the output torque. 
     In cases where a steering control apparatus is installed on a vehicle with a large weight, a large motor output torque is required, and there will be a problem that in cases where the motor characteristic is of the high-torque and low-rotation type, the output torque of the motor rapidly decreases upon rapid steering. As a result, in the case of an electric power steering control apparatus, there arises a problem that the steering operation rapidly becomes heavy upon rapid steering, whereas in the case of a steer-by-wire steering system, there is a problem in that the actual steering angle of the steered wheels does not follow the steering angle of the steering wheel upon rapid steering. 
     However, where the motor characteristic is of the low-torque and high-speed rotation type, it is necessary to increase the motor current in order to enlarge the motor output torque, and hence a large-sized motor of high power consumption is required. With a steering apparatus which is to be installed in a limited space, however, it becomes important to suppress the power consumption of a motor used therein and reduce the size thereof. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to obviate the various problems as referred to above, and has for its object to provide a steering control apparatus which is capable of alleviating a decrease in the output torque of a motor during high-speed steering without increasing the size thereof. 
     Bearing the above object in mind, according to the present invention, there is provided a steering control apparatus including a motor, a motor current instruction value generation section for generating a current instruction value for the motor, and a motor current detection section for detecting a current flowing through the motor, the motor being driven to operate based on at least the current flowing through the motor and the current instruction value, wherein the motor current instruction value generation section includes a correction section for correcting a current instruction value which controls a magnetic field of a field magnet of the motor, the correction section being operable to correct the current instruction value for controlling the magnetic field of the motor field magnet when a steering speed is high, 
     In a preferred form of the present invention, the steering control apparatus further comprises a motor control section for performing torque control on the motor in accordance with a torque instruction through vector control which is represented by a two-phase rotating magnetic flux coordinate system having a direction of a field current oriented in a d-axis direction and a direction perpendicular to the d-axis oriented in a q-axis direction, wherein the correction section corrects a d-axis current instruction value in such a manner that a d-axis current is controlled to such a predetermined value as to weaken the magnetic field of the motor field magnet when a deviation between the q-axis current instruction value and a q-axis current detection value becomes not less than a predetermined value. 
     In another preferred form of the present invention, the correction section increases a negative d-axis current instruction value for weakening the magnetic field of the motor field magnet when the q-axis current deviation is not less than a first predetermined value, and decreases the negative d-axis current instruction value for weakening the magnetic field of the motor field magnet when the q-axis current deviation is not greater than a second predetermined value. 
     In a further preferred form of the present invention, the d-axis current instruction value is limited within a preset range. 
     In a still further preferred form of the present invention, the steering control apparatus further comprises a stator phase current instruction value generation section for generating stator respective phase current instruction values from the q-axis current instruction value, wherein the current instruction value for controlling the magnetic field of the motor field magnet is corrected based on a deviation between at least one of the stator phase current instruction values and an actual corresponding stator phase current value in place of the q-axis current deviation. 
     In a yet further preferred form of the present invention, the steering control apparatus further comprises a reference steering torque generation section for generating a reference steering torque which is used to determine whether the magnetic field of the motor field magnet is to be weakened, wherein when the steering torque becomes not less than the reference steering torque, the correction section corrects the current instruction value to such a prescribed value as to weaken the magnetic field of the motor field magnet. 
     In a further preferred form of the present invention, the reference steering torque generation section generates the reference steering torque as a function of at least a vehicle speed. 
     In a further preferred form of the present invention, the steering control apparatus further comprises a steering speed detection section for detecting a steering speed of a steering wheel, wherein the correction of the current instruction value for controlling the magnetic field of the motor field magnet is effected such that when the steering speed of the steering wheel becomes not less than a predetermined value, the current instruction value is corrected to such a prescribed value as to weaken the magnetic field of the motor field magnet. 
     In a further preferred form of the present invention, the steering control apparatus further comprises: a motor control section for performing torque control on the motor in accordance with a torque instruction through vector control which is represented by a two-phase rotating magnetic flux coordinate system having a direction of a field current oriented in a d-axis direction and a direction perpendicular to the d-axis direction oriented in a q-axis direction; a speed detection section for detecting a rotational speed of the motor; and a voltage limitation value generation section for generating a voltage limitation value to a voltage applied to the motor; wherein the correction section determines a d-axis current value for setting a working point on a voltage limitation circle through calculations based on at least the rotational speed of the motor, a q-axis current instruction value, a stator winding resistance, a stator winding reactance and a motor counter electromotive voltage constant, and effects correction in such a manner that when the d-axis current value determined through calculations is a current value which weakens the magnetic field of the motor field magnet more than a d-axis current instruction value does, the d-axis current value determined through calculations becomes equal to the d-axis current instruction value. 
     In a further preferred form of the present invention, the steering control apparatus further comprises a power supply voltage detection section for detecting a power supply voltage, wherein the voltage limitation value generation section generates, as a voltage limitation value, a value obtained by multiplying the power supply voltage by a predetermined coefficient. 
     In a further preferred form of the present invention, the motor comprises a field winding type motor, and the apparatus further comprises a motor control section for performing torque control on the motor in accordance with a torque instruction in such a manner that a field winding current instruction value is corrected when a deviation between an armature current instruction value and an armature current detection value becomes not less than a predetermined value. 
     In a further preferred form of the present invention, the field winding current instruction value is corrected in such a manner that it is decreased when the armature current deviation is not less than a first predetermined value, and it is increased when the armature current deviation is not greater than a second predetermined value. 
     In a further preferred form of the present invention, the field winding current instruction value is limited to a preset minimum value. 
     In a further preferred form of the present invention, the correction section corrects the current instruction value with a correction value which is determined through calculations. 
     In a further preferred form of the present invention, the correction section corrects the current instruction value with a correction value which is obtained by referring to a table prepared in advance. 
     In a further preferred form of the present invention, the d-axis current instruction value is limited to a value which is obtained by vector subtracting a q-axis current value from a preset maximum current vector value. 
     In a further preferred form of the present invention, an integrated value of the deviation is used in place of the deviation. 
     In a further preferred form of the present invention, the correction section corrects the current instruction value for controlling the magnetic field of the motor field magnet only when a vehicle speed is not less than a predetermined value. 
     In a further preferred form of the present invention, the correction section corrects the current instruction value for controlling the magnetic field of the motor field magnet only when a steering torque is not less than a predetermined value. 
     The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a constructional view illustrating a first embodiment of the present invention. 
     FIG. 2 is a block diagram illustrating functions of the first embodiment of the present invention. 
     FIG. 3 is a flow chart illustrating d-axis target current correction processing according to the first embodiment of the present invention. 
     FIG. 4 is a view illustrating examples of waveforms before and after modulation in the first embodiment of the present invention. 
     FIG. 5 is a block diagram illustrating a second embodiment of the present invention. 
     FIG. 6 is a flow chart illustrating d-axis target current correction processing in the second embodiment of the present invention. 
     FIG. 7 is a block diagram illustrating functions of a third embodiment of the present invention. 
     FIG. 8 is a flow chart illustrating d-axis target current correction processing in the third embodiment of the present invention. 
     FIG. 9 is a block diagram illustrating functions of a fourth embodiment of the present invention. 
     FIG. 10 is a flow chart illustrating d-axis target current correction processing in the fourth embodiment of the present invention. 
     FIG. 11 is a block diagram illustrating functions of a fifth embodiment of the present invention. 
     FIG. 12 is a flow chart illustrating d-axis target current correction processing in the fifth embodiment of the present invention. 
     FIG. 13 is a block diagram illustrating functions of a sixth embodiment of the present invention. 
     FIG. 14 is a flow chart illustrating d-axis target current correction processing in the sixth embodiment of the present invention. 
     FIG. 15 is a block diagram illustrating functions of a seventh embodiment of the present invention. 
     FIGS. 16A through 16D are dq-axis vector diagrams for explaining magnetic flux weakening control according to the seventh embodiment of the present invention. 
     FIG. 17 is a flow chart illustrating d-axis target current correction processing in the seventh embodiment of the present invention. 
     FIG. 18 is a block diagram illustrating an eighth embodiment of the present invention. 
     FIG. 19 is a flow chart illustrating field current instruction value correction processing in the eighth embodiment of the present invention. 
     FIG. 20 is a steering speed vs. steering torque characteristic view illustrating effects of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings by taking, as an example, the case where the present invention is applied to an electric power steering control apparatus. 
     Embodiment 1 
     In a first embodiment of the present invention, a d-axis target current is corrected as a function of a deviation in a q-axis current thereby to decrease the magnetic field of a field magnet of a motor upon high-speed steering in which the deviation in the q-axis current increases due to voltage saturation, thus alleviate an increase in steering torque during such high-speed steering. 
     FIG. 1 is a constructional view illustrating an electric power steering control apparatus according to this first embodiment of the present invention. 
     In this figure, a motor  5 , which generates steering assisting torque, is connected through a reduction gear  4  with one end of a steering column or shaft  2 , the other end of which is connected with a steering wheel  1 . Also connected with the steering shaft  2  is a torque sensor  3  for detecting the steering torque of the steering wheel  1  to generate a corresponding torque detection value. 
     A controller  100  serves to determine a steering assisting torque based on the torque detection value of the torque sensor  3  and a vehicle speed detection value detected by a vehicle speed sensor  6 , and assist the steering operation of the steering wheel  1  by driving the motor  5  to generate the steering assisting torque thus determined. A battery  7  is connected with an ignition key  8  and the controller  100 . 
     FIG. 2 functionally illustrates an example of the electric power steering control apparatus according to the first embodiment of the present invention in which a PM brushless motor is used as a steering assisting motor. 
     In FIG. 2, a reference numeral  100  designates a microcomputer which performs steering assisting control with its software configuration being illustrated in a functional block diagram. 
     In FIG. 2, the microcomputer  100  includes a q-axis target current calculation section  100   a , a d-axis target current correction section  100   b  acting as a correcting section, a position calculation section  100   c , a uv to dq transformation section  100   d , a current control section  100   e , a dq to uvw transformation section  100   f  acting as a stator phase current instruction value generation section, a voltage utilization efficiency improvement section  100   o , a dead band (Td) correction section  100   p , an angular velocity calculation section  100   q  acting as a speed detection section, a decoupled control section  100   r , and a current detection offset correction section  100   s . Here, note that the q-axis target current calculation section  100   a  and the d-axis target current correction section  100   b  together constitute a motor current instruction value generation section. 
     The q-axis target current calculation section  100   a  performs predetermined calculations based on the torque detection signal of the torque sensor  3 , which detects the steering torque of the steering wheel  1 , and the vehicle speed detection signal of the vehicle speed sensor  6 , which detects the vehicle speed, determines a q-axis target current value (Iq*) for driving the motor  5  in the form of a PM brushless motor, and supplies the q-axis target current value thus determined to the current control section  100   e.    
     The position calculation section  100   c  determines an electrical angle θ through calculations based on the positional detection signal of a position sensor  103 , and supplies the electrical angle θ thus determined to the angular velocity calculation section  100   q , the uv to dq transformation section  100   d  and the dq to uvw transformation section  100   f.    
     The angular velocity calculation section  100   q  determines a motor rotational angular velocity ω through calculations based on the electrical angle θ, and supplies it to the decoupled control section  100   r.    
     The current detection offset correction section  100   s  calculates respective phase detection currents (Iu, Iv) by subtracting the amounts of respective phase offsets from phase current values detected by current sensors  102   a ,  102   b , respectively, and supplies them to the Td correction section  100   p  and the uv to dq transformation section  100   d.    
     The uv to dq transformation section  100   d  performs dq conversion based on the detected phase current values (Iu, Iv) and the electrical angle θ, and supplies thus converted dq-axis currents (Id, Iq) to the decoupled control section  100   r  and the current control section  100   e.    
     FIG. 3 is a flow chart for explaining the processing performed by the d-axis target current correction section  100   b.    
     In step S 1 , it is determined whether the vehicle speed detected by the vehicle speed sensor  6  is not less than a predetermined value, and when the detected vehicle speed is less than the predetermined value, the d-axis target current (Id*) is adopted as a corrected d-axis target current (Id**) in step S 5 , whereas when the detected vehicle speed is not less than the predetermined value, the control process advances to the processing of step S 2 . Then, in step S 2 , it is determined whether the steering torque detected by the torque sensor  3  is not less than a predetermined value, and when the detected steering torque is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 5 , whereas when the detected steering torque is not less than the predetermined value, the control process advances to the processing of step  3 . In step S 3 , it is determined whether a q-axis current deviation (ΔIq) is not less than a predetermined value, and when the q-axis current deviation (ΔIq) is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 5 , whereas when the q-axis current deviation (ΔIq) is not less than the predetermined value, the control process advances to the processing of step  4 . In step  4 , the d-axis target current is corrected by subtracting a corrected current value (f(ΔIq)), which is obtained as a function of the q-axis current deviation (ΔIq), from the d-axis target current (Id*), and the thus corrected d-axis target current (Id**) is supplied to the current control section  100   e.    
     The current control section  100   e  performs proportional and integral (PI) control based on deviations between the dq-axis target currents (Id**, Iq*) and the corresponding detected dq-axis currents (Id, Iq), and generates dq-axis target application voltages (Vd*, Vq*). 
     The control section  100   r  calculates non-interfering voltages based on the dq-axis detection currents (Id, Iq) and the motor angular velocity ω, and corrects the dq-axis target application voltages (Vd*, Vq*) to generate corrected dq-axis target application voltages (Vd**, Vq**), which is supplied to the dq to uvw transformation section  100   f.    
     The dq to uvw transformation section  100   f  performs dq inversion based on the corrected dq-axis target application voltages (Vd**, Vq**) and the electrical angle θ to generate three-phase target application voltages (Vu*′, Vv*′, Vw*′), which are supplied to the voltage utilization efficiency improvement section  100   o.    
     To improve the voltage efficiency, the voltage utilization efficiency improvement section  100   o  modulates the three-phase target application voltages (Vu*′, Vv*′, Vw*′) into a spatial voltage vector, and supplies the thus modulated three-phase target application voltages (Vu*″, Vv*″, VW*″) to the Td correction section  100   p . Examples of waveforms before and after such modulation are illustrated in FIG.  4 . In FIG. 4, the axis of ordinate represents the values of the three-phase target application voltages, and the axis of abscissa represents the rotational position of the motor. 
     The Td correction section  100   p  performs dead band compensation for the modulated three-phase target application voltages (Vu*″, Vv*″, Vw*″) based on the detected current values (Iu, Iv, Iw), and supplies the thus compensated three-phase target application voltages (Vu*, Vv*, Vw*) to a driving section  101 . 
     With the electric power steering control apparatus as constructed above, owing to the provision of the d-axis target current correction section  100   b , the magnetic field of the motor field magnet is weakened during high-speed steering in which the q-axis current deviation is increased due to voltage saturation so that a decrease in the output torque of the steering assisting motor upon rapid steering can be alleviated, thus making it possible to reduce the increasing steering torque during such rapid steering. 
     Although in the first embodiment, the electric power steering control apparatus has been taken as an example, the present invention may instead be applied to a steer-by-wire steering control apparatus. 
     In the case of such a steer-by-wire steering control apparatus, owing to the provision of the d-axis target current correction section  100   b , the magnetic field of the motor field magnet can be weakened upon high-speed steering in which the q-axis current deviation is increased due to voltage saturation, whereby a decrease in the motor output torque upon rapid steering can be alleviated, thus improving the followability of the actual steering angle with respect to the operator&#39;s induced steering angle during rapid steering. 
     Embodiment 2 
     In a second embodiment of the present invention, the d-axis target current is corrected with a current value which is obtained by referring to a table, which has been prepared in advance using the q-axis current deviation as a parameter, thereby to decrease the magnetic field of the motor field magnet upon high-speed steering in which the q-axis current deviation increases due to voltage saturation, thus alleviating an increase in steering torque during such high-speed steering. 
     FIG. 5 functionally illustrates an example of an electric power steering control apparatus according to the second embodiment of the present invention in which an induction motor is used as a steering assisting motor. In FIG. 5, description will be made with the same or corresponding parts as those in FIG. 2 being identified by the same symbols. 
     In FIG. 5, there is illustrated a functional block diagram of a microcomputer, generally designated at  100 , which performs steering assisting control by executing software incorporated therein. The microcomputer  100  of this second embodiment includes a dq-axis target current calculation section  100   a , a d-axis target current correction section  100   b , a slip angle frequency calculation section  100   l , a power supply angular frequency calculation section  100   m  which calculates a power supply angular frequency from a slip angle frequency ωs and a motor rotational speed ωr of an induction motor  5   a  detected by a speed sensor  104 , an integrator  100   n  which calculates an angle θ from the power supply angular frequency, a uv to dq transformation section  100   d , a current control section  100   e , and a dq to uvw transformation section  100   f.    
     FIG. 6 is a flow chart for explaining the processing performed by the d-axis target current correction section  100   b  in the electric power steering control apparatus according to the second embodiment of the present invention. 
     In step S 1 , it is determined whether the vehicle speed detected by the vehicle speed sensor  6  is not less than a predetermined value, and when the detected vehicle speed is less than the predetermined value, the d-axis target current (Id*) is adopted as a corrected d-axis target current (Id**) in step S 5 , whereas when the detected vehicle speed is not less than the predetermined value, the control process advances to the processing of step S 2 . Then, in step S 2 , it is determined whether the steering torque detected by the torque sensor  3  is not less than a predetermined value, and when the detected steering torque is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 5 , whereas when the detected steering torque is not less than the predetermined value, the control process advances to the processing of step S 3 . 
     In step S 3 , it is determined whether the q-axis current deviation (ΔIq) is not less than a predetermined value, and when the q-axis current deviation (ΔIq) is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 5 , whereas when the q-axis current deviation (ΔIq) is not less than the predetermined value, the control process advances to the processing of step S 6 . In step S 6 , a corrected current value (Ida) is determined by referring to a table, which has been prepared in advance using the q-axis current deviation (ΔIq) as a parameter. In step S 4 , the d-axis target current is corrected by subtracting the corrected current value (Ida) from the d-axis target current (Id*), and the thus corrected d-axis target current (Id**) is supplied to the current control section  100   e.    
     Embodiment 3 
     In a third embodiment of the present invention, the d-axis target current value is corrected in such a manner that it is decreased when an integrated value of the q-axis current deviation is not less than a first predetermined value 1, and increased when the integrated q-axis current deviation is not greater than a second predetermined value 2, whereby the magnetic field of the motor field magnet is weakened upon high-speed steering in which the q-axis current deviation increases due to voltage saturation, thus alleviating an increase in the steering torque during such high-speed steering. 
     FIG. 7 functionally illustrates an example of an electric power steering control apparatus according to the third embodiment of the present invention in which a PM brushless motor is used as a steering assisting motor. In FIG. 7, the same or corresponding parts as those in FIG. 2 are identified by the same symbols while omitting a detailed description thereof. 
     FIG. 8 is a flow chart for explaining the processing performed by a dq-axis target current correction section  100   g.    
     In step S 10 , the q-axis current deviation ΔIq is integrated, and in step S 11 , it is determined whether the integrated value of ΔIq is not less than the first predetermined value 1. When the integrated value of ΔIq is not less than the first predetermined value 1, a d-axis current correction amount Ida is increased in step S 12 , and the correction value is limited to a preset maximum correction value Ida_max in step S 13 . On the other hand, when the integrated value of ΔIq is less than the first predetermined value 1, it is further determined whether the integrated value of ΔIq is not greater than a second predetermined value 2 in step S 14 . When the integrated value of ΔIq is not greater than the second predetermined value 2, the d-axis current correction amount Ida is decreased in step S 15 , and the correction value is limited to a preset minimum correction value Ida_min in step S 16 . Then in step S 17 , a corrected d-axis target current value (Id**) is calculated by subtracting the d-axis current correction value (Ida) from the d-axis target current (Id*). 
     Subsequently, in step S 20 , a maximum d-axis target current (Id_max) is calculated by subtracting the q-axis target current value (Iq*) from a preset maximum current vector value (Ia). In step S 21 , it is determined whether the corrected d-axis target current value (Id**) is greater than the maximum d-axis target current (Id_max). When the corrected d-axis target current value (Id**) is greater than the maximum d-axis target current (Id_max), the maximum d-axis target current (Id_max) is adopted as the corrected d-axis target current value (Id**) in step S 22 . 
     Embodiment 4 
     In a fourth embodiment of the present invention, when the steering torque exceeds a prescribed torque value, the d-axis target current is corrected in such a manner as to decrease the magnetic field of the motor field magnet, whereby the magnetic field is weakened upon high-speed steering in which the steering assisting torque is reduced due to voltage saturation, thus alleviating an increase in the steering torque during such high-speed steering. 
     FIG. 9 functionally illustrates an example of an electric power steering control apparatus according to the fourth embodiment of the present invention in which a PM brushless motor is used as a steering assisting motor. In FIG. 9, the same or corresponding parts as those in FIG. 2 are identified by the same symbols while omitting a detailed description thereof. 
     In FIG. 9, a motor angular velocity calculation section  100   i  determines a motor rotational angular velocity ω through calculations based on the electrical angle θ from the position calculation section  100   c , and supplies it to the d-axis target current correction section  100   b.    
     FIG. 10 is a flow chart for explaining the processing performed by the d-axis target current correction section  100   b  in the electric power steering control apparatus according to the fourth embodiment of the present invention. 
     In step S 30 , it is determined whether the motor angular velocity ω is not less than a predetermined value, and when the motor angular velocity ω is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 36 , whereas when the motor angular velocity ω is not less than the predetermined value, the control process advances to reference torque Ts_ref calculation processing in step  31 . In the reference torque Ts_ref calculation processing in step S 31  (i.e., a reference steering torque generation section), a reference torque Ts_ref is determined by referring to a table, which has been prepared in advance as a function of the vehicle speed detected by the vehicle speed sensor  6  or by the use of the detected vehicle speed as a parameter, alternatively it is determined as a preset constant value instead of referring to such a table. In step S 32 , a comparison is made between the torque sensor signal (Ts) and the reference torque (Ts_ref), and when the torque sensor signal (Ts) is not greater than the reference torque (Ts_ref), the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 36 , whereas when the torque sensor signal (Ts) is greater than the reference torque (Ts_ref), the control process advances to the processing in step S 33 . 
     In step S 33 , a difference ΔTs between the torque sensor signal (Ts) and the reference torque (Ts_ref) is calculated by subtracting the reference torque (Ts_ref) from the torque sensor signal (Ts). Then in the correction current Ida calculation processing in step S 34 , a correction current Ida is calculated by referring to a table, which has been prepared in advance as a function of ΔTs or using ΔTs as a parameter. In step S 35 , a corrected d-axis target current (Id**) is calculated by subtracting the correction current (Ida) from the d-axis target current (Id*), and the thus corrected d-axis target current (Id**) is supplied to the current control section  100   e.    
     Embodiment 5 
     In a fifth embodiment of the present invention, when the motor rotational speed exceeds a predetermined value, the current instruction value for controlling the magnetic field of the motor field magnet is corrected to such a prescribed value as to decrease the magnetic field, whereby the magnetic field is weakened upon high-speed steering, thus alleviating an increase in the steering torque during such high-speed steering. 
     FIG. 11 functionally illustrates an example of an electric power steering control apparatus according to the fifth embodiment of the present invention in which a PM brushless motor is used as a steering assisting motor. In FIG. 11, the same or corresponding parts as those in FIGS. 2 and 9 are identified by the same symbols while omitting a detailed description thereof. 
     FIG. 12 is a flow chart for explaining the processing performed by the d-axis target current correction section  100   b.    
     In step S 40 , it is determined whether the motor angular velocity ω is not less than a predetermined value, and when the motor angular velocity ω is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 43 , whereas when the motor angular velocity ω is not less than the predetermined value, the control process advances to correction current Ida calculation processing in step  41 . In the correction current Ida calculation processing in step S 41 , a correction current Ida is calculated by referring to a table, which has been prepared in advance as a function of the motor angular velocity ω or using the motor angular velocity ω as a parameter. In step S 42 , a corrected d-axis target current (Id**) is calculated by subtracting the correction current (Ida) from the d-axis target current (Id*), and the thus corrected d-axis target current (Id**) is supplied to the current control section  100   e.    
     Embodiment 6 
     In a sixth embodiment of the present invention, when the steering speed exceeds a predetermined value, the current instruction value for controlling the magnetic field of the motor field magnet is corrected to such a prescribed value as to decrease the magnetic field, whereby the magnetic field is weakened upon high-speed steering, thus alleviating an increase in the steering torque during such high-speed steering. 
     FIG. 13 functionally illustrates an example of an electric power steering control apparatus according to the sixth embodiment of the present invention in which a field winding type motor is used as a steering assisting motor. In FIG. 13, the same or corresponding parts as those in FIG. 2 are identified by the same symbols while omitting a detailed description thereof. 
     In FIG. 13, the electric power steering control apparatus of this sixth embodiment includes a steering angle sensor  9 , a steering angular velocity calculation section  100   j  acting as a steering speed detection section for determining a steering angular velocity ω′ based on the output of the steering angle sensor  9 , and a target current calculation section  100   t  for determining a target armature current value (Ia*) and a target field winding current value (If*) to drive a field winding type motor  5   b  based on the torque detection signal of the torque sensor  3 , which detects the steering torque, and the vehicle speed detection signal of the vehicle speed sensor  6 , which detects the vehicle speed. The target current calculation section  100   t  supplies the thus determined target armature current value (Ia*) to the current control section  100   c , and the thus determined target field winding current value (If*) to a target field winding current correction section  100   h  to be described later. 
     FIG. 14 is a flow chart for explaining the processing performed by the target field winding current correction section  100   h.    
     In step S 50 , it is determined whether the steering angular velocity ω′ is not less than a predetermined value, and when the steering angular velocity ω′ is less than the predetermined value, the target field winding current (If*) is adopted as the corrected target field winding current (If**) in step S 53 , whereas when the steering angular velocity ω′ is not less than the predetermined value, the control process advances to correction current Ifa calculation processing in step  51 . In the correction current Ifa calculation processing in step S 51 , a correction current Ifa is calculated by referring to a table, which has been prepared in advance as a function of the steering angular velocity ω′ or using the steering angular velocity ω′ as a parameter. In step S 52 , the corrected target field winding current (If**) is determined by subtracting the correction current (Ifa) from the target field winding current (If*), and the corrected target field winding current (If**) thus determined is supplied to the current control section  100   e.    
     Embodiment 7 
     In a seventh embodiment of the present invention, a d-axis current value (Ida) for setting a working point on a voltage limiting circle is determined through calculations based on a motor rotational speed detection value, a q-axis current instruction value, a stator winding resistance, a stator winding reactance and a motor counter electromotive voltage constant, and when the d-axis current value (Ida) thus calculated is such a current value as to weaken the magnetic field of the motor field magnet more than the d-axis current instruction value (Id*) does, the d-axis current instruction value is corrected in such a manner that the d-axis current value (Ida) calculated above becomes equal to the d-axis current instruction value, thereby weakening the magnetic field upon high-speed steering to alleviate an increase in the steering torque during such high-speed steering. 
     FIG. 15 functionally illustrates an example of an electric power steering control apparatus according to the seventh embodiment of the present invention in which a PM brushless motor is used as a steering assisting motor. In FIG. 15, the same or corresponding parts as those in FIG. 2 are identified by the same symbols while omitting a detailed description thereof. 
     The electric power steering control apparatus according to this seventh embodiment of the present invention substantially includes, in addition to the components of the aforementioned first embodiment, a power supply voltage sensor  10  acting as a power supply voltage detection section, a voltage limitation value generation section  100   k , and a motor angular velocity calculation section  100   i . The voltage limitation value generation section  100   k  generates a voltage limitation value (V_lim) by multiplying the voltage detected by the power supply voltage sensor  10  by a predetermined coefficient, and supplies it to the d-axis target current correction section  100   b.    
     Here, a brief description of the magnetic field weakening control will be made using the following expression (1) and FIG.  16 . 
     A fundamental equation (under-mentioned expression (1)) for the PM brushless motor  5  is well-known as shown below. 
     
       
           V   2 =(φω+ Ri   q   −ωL   d   i   d ) 2 +( Ri   d   +ωL   q   i   q ) 2   (1) 
       
     
     where V represents a terminal voltage supplied to the motor; ω represents the angular velocity of the motor; R represents a stator winding resistance per phase; φ represents an unloaded induced voltage at a unit speed; L d  and L q  represent phase inductances for the d-axis and the q-axis, respectively; Id represents a d-axis current; and Iq represents a q-axis current. 
     FIGS. 16A through 16D are vector diagrams illustrating d-q rotating coordinate axes. 
     As the rotational speed ω of the PM brushless motor  5  increases, the voltage induced therein grows. When the voltage value V, which is a vector sum of the induced voltage ωφ, Ri q  and ωL q i q , reaches the voltage limitation circle as illustrated in FIG. 16A, it becomes impossible for the PM brushless motor  5  to increase its rotational speed to a value equal to or higher than the rotational speed ω which is acquired by the motor  5  when the voltage value V has reached the voltage limitation circle. 
     However, with the electric power steering control apparatus, the rotational speed ω of the PM brushless motor  5  follows the steering speed of the steering wheel, so that the PM brushless motor  5  is forced to rotate at a speed higher than its own rotational speed performance at the time of high-speed steering of the steering wheel. 
     At this time, Ri q  decreases due to an increase in the induced voltage ωφ under the restraint of the voltage value V, as illustrated in FIG.  16 B. As a result, the output torque of the PM brushless motor  5  is decreased, whereby the steering assisting torque is reduced, thus increasing the steering torque. 
     Here, by supplying the d-axis current for weakening the magnetic field of the motor field magnet, there is developed a voltage margin due to Rid and ωL d i d , as illustrated in FIG.  16 C. 
     Consequently, it becomes possible to cause the same amount of q-axis current as in FIG. 16A to flow at the same rotational speed as in FIG. 16B, as illustrated in FIG.  16 D. 
     As described above, a decrease in the output torque of the motor at high-speed rotation thereof can be alleviated by performing magnetic field weakening control, so that an increase in the steering torque during high-speed steering can be reduced. 
     FIG. 17 is a flow chart for explaining the processing performed by the d-axis target current correction section  100   b.    
     In step S 60 , it is determined whether the vehicle speed is not less than a predetermined value, and when the vehicle speed is less than the predetermined value, the d-axis target current (Id*) is adopted as the corrected d-axis target current (Id**) in step S 66 , whereas when the vehicle speed is not less than the predetermined value, the control process advances to Ida calculation processing in step S 61 . In the Ida calculation processing in step S 61 , a d-axis current instruction value (Ida) for setting a working point on the voltage limitation circle is calculated according to a predetermined calculation formula based on the detected motor rotational speed ω, the voltage limitation value (V_lim), the q-axis current instruction value, the stator winding resistance given in advance, the stator reactance given in advance, and the motor counter electromotive voltage constant given in advance. In step S 62 , a maximum d-axis target current (Id_max) is calculated by subtracting a q-axis detection current value (Iq) from a preset maximum current vector value (Ia). 
     In step S 63 , it is determined whether the d-axis current instruction value (Ida) calculated in step S 61  for setting a working point on the voltage limitation circle is greater than the maximum d-axis target current (Id_max). When the d-axis current instruction value (Ida) is greater than the maximum d-axis target current (Id_max), the maximum d-axis target current (Id_max) is adopted as the corrected d-axis target current value (Id**) in step S 64 . On the other hand, when the d-axis current instruction value (Ida) is less than the maximum d-axis target current (Id_max), the d-axis current instruction value (Ida) is adopted as the corrected d-axis target current value (Id**). The corrected d-axis target current (Id**) calculated according to the above-mentioned steps is supplied to the current control section  100   e.    
     Embodiment 8 
     In an eighth embodiment of the present invention, the field winding current instruction value is corrected as a function of an armature current deviation (ΔIa), whereby the magnetic field of the motor field magnet is weakened upon high-speed steering in which the armature current deviation is increased due to voltage saturation, thus alleviating an increase in the steering torque during such high-speed steering. FIG. 18 functionally illustrates an example of an electric power steering control apparatus according to the eighth embodiment of the present invention in which a field winding type motor is used as a steering assisting motor. In FIG. 18, the same or corresponding parts as those in FIGS. 2 and 13 are identified by the same symbols while omitting a detailed description thereof. 
     FIG. 19 is a flow chart for explaining the processing performed by the target field winding current correction section  100   h  in the electric power steering control apparatus according to the eighth embodiment of the present invention. 
     In step S 60 , it is determined whether the vehicle speed detected by the vehicle speed sensor  6  is greater than a predetermined value, and when the vehicle speed is less than the predetermined value, the field current instruction value (If*) is adopted as the corrected field current instruction value (If**) in step S 65 , whereas when the vehicle speed is greater than the predetermined value, the control process advances to the processing in step  61 . Subsequently, in step S 61 , it is determined whether the steering torque detected by the torque sensor  3  is greater than a predetermined value, and when the steering torque is less than the predetermined value, the field current instruction value (If*) is adopted as the corrected field current instruction value (If**) in step S 65 , whereas when the steering torque is greater than the predetermined value, the control process advances to the processing in step S 62 . 
     In step S 62 , it is determined whether the armature current deviation (ΔIa) is greater than a predetermined value, and when the armature current deviation (ΔIa) is less than the predetermined value, the field current instruction value (If*) is adopted as the corrected field current instruction value (If**) in step S 65 , whereas when the armature current deviation (ΔIa) is greater than the predetermined value, the control process advances to the processing in step  63 . In step  63 , a corrected current value (Ida) is determined by referring to a table, which has been prepared in advance using the armature current deviation (ΔIa) as a parameter. Then in step S 64 , the field current instruction value is corrected by subtracting the corrected current value (Ida) from the field current instruction value (If*), and the thus corrected field current instruction value (If**) is supplied to the current control section  100   e.    
     As described in the foregoing, according to the present invention, current is controlled in such a manner that the magnetic field of the motor field magnet is weakened during high-speed steering in which the output torque of the motor decreases. As a result, a decrease in the motor output torque can be alleviated. 
     Moreover, in the case of an electric power steering control apparatus, an increase in the steering torque at the time of high-speed steering can be reduced, as illustrated in FIG. 20 for instance. 
     In addition, in the case of a steer-by-wire steering system, it is possible to improve the followability of an actual steering angle with respect to an operator&#39;s induced steering angle at the time of high-speed steering. 
     While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.