Patent Publication Number: US-2009240389-A1

Title: Electric power steering apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an electric power steering apparatus which generates a steering assisting force in accordance with a steering torque input into a steering system. 
     BACKGROUND ART 
     As an electric power steering apparatus of this type, there is known an electric power steering apparatus having the following structure (see, for example, the patent reference 1). 
     For example, an angular position of an electric motor is detected in a position detect procedure; 
     an abnormality of the position detect procedure is detected by checking whether an amplitude value of a position signal output from the position detect procedure varies according to the rotation angle position of the electric motor to go below a preset value or not in an abnormality detect procedure; 
     after the abnormality of the position detect procedure is detected in the abnormality detect procedure, at the motor rotation angle position where the amplitude value of the position signal becomes equal to or larger than a given level according to the change of the motor rotation angle position, a torque control procedure is executed to output a torque for assisting a steering force; and 
     at the motor rotation angle position where the amplitude value of the position signal becomes equal to or smaller than the given level, a torque zero control procedure is executed to bring the output torque into zero, 
     whereby, even when the assist of the steering force is stopped suddenly in the abnormal state of the position detect procedure to bring the current power steering assisting state into a manual steering state, a steering wheel can be prevented from being returned suddenly due to a reacting force caused by the state of a vehicle. 
     There is also proposed a control apparatus for controlling a brushless motor for a vehicle (see, for example, the patent reference 2). Specifically, in this control apparatus, when a function to detect the position of a rotor becomes abnormal and a brushless motor is in rotation, there is carried out a control operation which switches sequentially directions of currents of electromagnetic coils according to a preset fixed pattern. When the brushless motor is stopped, a drive signal is switched gradually front a low frequency to a high frequency, whereby the brushless motor is started. 
     Further, there is also proposed a vehicle steering apparatus including a PLL circuit having a phase comparator for comparing an output of a reference wave generator for generating a sine wave of which crest value is in accordance with a steering torque, with an output of a difference amplifier for amplifying a difference between the output of the reference wave generator and the current of a brushless motor; and a voltage control oscillator to which an output of the phase comparison output is input and which outputs voltage control to the reference wave generator. Thus, the brushless motor can be driven without using a rotation angle sensor (see, for example, the patent reference 3). 
     On the other hand, as a conventional electric power steering apparatus, there is known an apparatus which, when stopping a motor during a steering assisting operation, causes a shortcircuit between the terminals of the motor for a given time to prevent occurring a so called kickback phenomenon where a return force caused by torsion of a steering system suddenly acts on a steering wheel, thereby relieving the steering load of a driver (see, for example, the patent reference 4). 
     Patent Reference 1: Japanese Patent Examined Publication JP-B-3600805 (Page 1, FIG. 7) 
     Patent Reference 2: Japanese Patent Unexamined Publication JP-A-2005-253226 (Page 1, FIG. 2) 
     Patent Reference 3: Japanese Patent Unexamined Publication JP-A-2003-40119 (Page 1, FIG. 2) 
     Patent Reference 4: Japanese Patent Examined Publication JP-B-3399226 
     DISCLOSURE OF THE INVENTION 
     Problems that the Invention is to Solve 
     However, in the conventional electric power steering apparatus disclosed in the patent reference 1, there is found a problem to be solved. That is, for example, when the vehicle is turning, the steering wheel is steered in a steering further direction, the reacting force of the steering wheel becomes as large as a force incapable of obtaining the assistance of the electric power steering, thereby causing the driver to feel unpleasant. 
     Further, in the conventional brushless motor control apparatus disclosed in the patent reference 2, in such an electric power steering apparatus, there is found a problem to be solved. That is, when the position detect sensor becomes abnormal, if the brushless motor is driven according to a predetermined pattern, there is a possibility that the steering wheel can rotate against an intention of the driver, which makes it impossible to drive the motor according to the intention of the driver. 
     Further, in the conventional steering apparatus disclosed in the patent reference 3, specifically, as a motor used in the electric power steering, there is found a problem to be solved. That is, since it is easily assumed that the steering wheel is rotated when the power supply is off, and also since the position of the steering wheel when the vehicle is stopping cannot be specified, the initial angle of the motor cannot be estimated, which cannot prevent perfectly the driver from feeling strange. Therefore, it can be expected that the conventional steering apparatus disclosed in the patent reference 3 is used as an alternative control apparatus when the detection of the motor rotation angle is abnormal. However, in this case, it is necessary that a PLL circuit or the like is provided in addition to a normal control circuit configuration to form an analog circuit configuration under the precondition that the motor should be driven without using a sensor, or, there is necessary a high-performance arithmetic processing unit which is capable of executing an operation equivalent to the performance of such analog circuit configuration. Thus, there is a problem in which the number of parts is increased and production cost becomes high. 
     Still further, other than the above apparatuses, there are proposed various methods for driving a sensor-less brushless motor using a counter electromotive force proportional to an angular velocity. However, in these methods, it is difficult to control the motor in the vicinity of the speed zero and, to realize such motor control, it is necessary to use a CPU which is capable of a very high speed arithmetic operation. Further, there also exist various conventional examples which require complex and difficult algorithms. Since most of such conventional examples are under the precondition that the brushless motor continues to rotate in a given direction (under the precondition that, to reverse the rotation direction of the brushless motor, there must be used, for example, a method for switching the rotation direction of the brushless motor using a switch or the like), they are not suitable for a master/slave control which is executed in an electric power steering apparatus, or, when they are applied to such control, it is necessary to use a CPU which is capable of a very high speed operation, which results in the increased costs of the apparatus. 
     Therefore, in an electric power steering apparatus which carries out the control operation using an angle detector in its normal state, there is a problem that, when the angle detector becomes abnormal, for example, when there arises a state where the angle information cannot be obtained accurately, the motor cannot be driven in a sensor-less state but a fail-safe must be executed to stop the steering assisting control. 
     Further, in the electric power steering apparatus disclosed in the patent reference 4, when changing a steering assisting state to a manual steering state, only the sudden return of the steering wheel due to a returning force caused by the torsion of the steering system can be restricted; and, therefore, the steering wheel is returned gently to its neutral position. Therefore, there is a problem that it is not possible to prevent perfectly the generation of the above-mentioned kickback phenomenon. 
     The present invention aims at solving the problems to be solved which are found in the above-mentioned various conventional apparatus. Thus, it is an object of the invention to provide an electric power steering apparatus which uses motor relative angle detect means having a simple structure capable of restricting the increased number of parts and the increased cost, thereby being able to prevent a driver from feeling unpleasant. Further, it is a second object of the invention to provide an electric power steering apparatus which, when removing its steering assisting control, can prevent the occurrence of a kick-back phenomenon more properly. 
     Means for Solving the Problems 
     The above-mentioned first object can be attained by the following structures (1)˜(15) of the invention. 
     (1) An electric power steering apparatus, including: 
     an electric motor which applies a steering assisting force to a steering system; 
     steering torque detect means for detecting a steering torque to be transmitted to the steering system; 
     motor control means for calculating a steering assisting command value according to the steering torque detected by the steering torque detect means and for driving and controlling the electric motor according to the calculated steering assisting command value; 
     a motor relative angle information calculating portion which calculates a relative angle information of the electric motor corresponding to a steering amount applied to the steering system by a driver; and 
     motor relative angle detect means including a relative angle information compensating portion for preventing the motor relative angle information calculating portion from being incapable of obtaining the relative angle information to thereby enable to generate the relative angle information all the time, 
     wherein when starting to drive the motor, the motor control means drives and controls the electric motor from an arbitrary actual angle without setting an initial angle based on the relative angle information detected by the motor relative angle detect means. 
     (2) The electric power steering apparatus as set forth in (1), wherein 
     the motor relative angle information compensating portion adds an offset value, which changes a sign in every given cycle when needed, to the relative angle information calculated in the motor relative angle information calculating portion so as to prevent the motor relative angle information calculating portion from being incapable of obtaining the relative angle information. 
     (3) The electric power steering apparatus as set forth in (1), wherein 
     the motor relative angle calculating portion includes relative angle calculation abnormality detect means for detecting a relative angle calculation abnormal state for detecting an abnormality of at least one of the calculated relative angle information and an input value for calculating the calculated relative angle information; and 
     when the relative angle calculation abnormality detect means detects the relative angle calculation abnormal state, the relative angle information is calculated according to other input value where no abnormality is detected. 
     (4) The electric power steering apparatus as set forth in (1), wherein 
     the motor relative angle information calculating portion determines a rotation direction of the electric motor according to the steering torque detected by the steering torque detect means. 
     (5) The electric power steering apparatus as set forth in (1), wherein 
     the motor relative angle information calculating portion includes: 
     correction requiring state detect means for detecting a correction requiring state where a difference between the calculated motor relative angle and an actual angle increases; and 
     relative angle information correcting means for correcting the relative angle information when the correction requiring state detect means detects the correction requiring state. 
     (6) An electric power steering apparatus, including: 
     an electric motor which generates a steering assisting force for a steering system; 
     motor rotation angle detect means for detecting a motor rotation angle of the electric motor; 
     steering torque detect means for detecting a steering torque to be transmitted to the steering system; 
     motor control means for calculating a steering assisting command value according to the steering torque detected by the steering torque detect means and for driving and controlling the electric motor according to the calculated steering assisting command value and the motor rotation angle detected by the motor rotation angle detect means; 
     motor rotation angle abnormality detect means for detecting an abnormality of the motor rotation angle detect means; 
     a motor relative angle information calculating portion for calculating a relative angle information of the electric motor corresponding to the steering amount applied to the steering system by a driver; and 
     motor relative angle detect means including a relative angle information compensating portion for preventing the motor relative angle information calculating portion from being incapable of obtaining the relative angle information to thereby enable to generate the relative angle information all the time, 
     wherein when the motor rotation angle abnormality detect means does not detect the abnormality of the motor rotation angle abnormality detect means, the motor control means selects the motor rotation angle information detected by the motor rotation angle detect means and, when the motor rotation angle abnormality detect means detects the abnormality of the motor rotation angle detect means, the motor control means selects the relative angle information detected by the motor relative angle detect means, so as to drive and control the electric motor according to the selected motor rotation angle information or the relative angle information. 
     (7) The electric power steering apparatus as set forth in (6), wherein 
     when driving and controlling the electric motor according to the relative angle information, the motor control means drives the motor from an arbitrary actual angle without setting an initial angle. 
     (8) The electric power steering apparatus as set forth in (6), wherein 
     the motor relative angle information compensating portion adds an offset value, which changes a sign in every given cycle as needed, to the relative angle information calculated so as to prevent the motor relative angle information calculating portion from being incapable of obtaining the relative angle information. 
     (9) The electric power steering apparatus as set forth in (6), wherein 
     the motor relative angle information compensating portion detects a relative angular velocity and when the detected relative angular velocity comes near to zero, the motor relative angle information compensating portion determines an offset amount and a cycle such that the relative angular velocity positively exceeds an insensitive zone before the motor relative angle calculating portion is capable of obtaining the relative angle information. 
     (10) The electric power steering apparatus as set forth in (6), wherein 
     the motor relative angle information calculating portion includes a relative angle calculation abnormality detect means for detecting a relative angle calculation abnormal state for detecting an abnormality of at least one of the relative angle information calculated and an input value for calculating the relative angle information, and 
     when the relative angle calculation abnormality detect means detects a relative angle calculation abnormal state, the motor relative angle information calculating portion calculates the relative angle information according to other input value where no abnormality is detected. 
     (11) The electric power steering apparatus as set forth in (6), wherein 
     the motor relative angle information calculating portion determines a rotation direction of the electric motor according to the steering torque detected by the steering torque detect means. 
     (12). The electric power steering apparatus as set forth in (6), wherein 
     the motor relative angle information calculating portion includes correction requiring state detect means for detecting a correction requiring state where a difference between the calculated motor relative angle and an actual angle increases, and 
     a relative angle information correcting means for correcting the relative angle information when the correction requiring state detect means detects the correction requiring state. 
     (13) The electric power steering apparatus as set forth in (6), wherein 
     the motor rotation angle detect means outputs a rotation angle detect signal including two systems having sine and cosine wave systems, or including other two or more systems, 
     the motor rotation angle abnormality detect means detects a motor rotation angle abnormality when an amplitude of the sine or cosine wave is out of a given range, 
     the motor relative angle information calculating portion includes:
         correction requiring state detect means for detecting a correction requiring state where a difference between the calculated motor relative angle and an actual angle increases; and   relative angle information correct means for correcting the relative angle information when the correction requiring state detect means detects the correction requiring state,       

     the correction requiring state detect means detects a correction requiring state when the amplitude of the other of normal sine or cosine wave reaches a maximum value a the minimum value, and 
     the relative angle information correct means corrects the relative angle information using the then actual angle when the correction requiring state is detected. 
     (14) The electric power steering apparatus as set forth in (6), wherein 
     the motor rotation angle detect means outputs a rotation angle signal including two systems having sine and cosine wave systems, 
     the motor rotation angle abnormality detect means detects whether a sum of a square value of the sign wave and a square value of the cosine wave is “1” or not, thereby detecting a shortcircuit of these two wave systems, 
     the motor relative angle information calculating portion includes:
         correction requiring state detect means for detecting a correction requiring state where a difference between the calculated motor relative angle and an actual angle increases; and   relative angle information correct means for correcting the relative angle information when the correction requiring state detect means detects the correction requiring state,       

     the correction requiring state detect means detects the correction requiring state when an amplitudes of the short circuited sine or cosine wave reach a minimum value and a maximum value, and 
     the relative angle information correct means corrects the relative angle information by using the then actual angle when the correction requiring state is detected. 
     (15) The electric power steering apparatus as set forth in (6), wherein 
     the motor rotation angle detect means is a pole position sensor for outputting a multi-phase pole position signal, 
     the motor rotation angle abnormality detect means detects the abnormality of one pole position sensor according to the pole position signal output from the pole position sensor, 
     the motor relative angle information calculating portion includes:
         correction requiring state detect means for detecting a correction requiring state where a difference between the calculated motor relative angle and an actual angle increases; and   relative angle information correct means for correcting the relative angle information when the correction requiring state detect means detects the correction requiring state,       

     the correction requiring state detect means detects a correction requiring state when pole position signals are arranged in such a manner that an angle is determined uniquely according to the abnormal state of the pole position sensor in 360 degrees, and 
     the relative angle information correct means corrects the relative angle information using the actual angle of the corresponding pole position signal arrangement when the correction requiring state is detected. 
     Further, the above-mentioned second object can be attained by the following structures (16)˜(24) of the invention. 
     (16) An electric power steering apparatus, including: 
     an electric motor which applying a steering assisting force to a steering system for relieving a steering load of a driver; 
     rotation angle detect means for detecting a motor rotation angle of the electric motor; 
     steering torque detect means for detecting a steering torque; 
     motor control means for driving and controlling the electric motor with reference to the motor rotation angle in order to generate a steering assisting force corresponding to at least the steering torque; and 
     abnormality detect means for detecting an abnormality of the motor rotation angle detected by the rotation angle detect means, 
     wherein the motor control means includes reference angle change means changing a reference angle of the motor rotation angle in such a manner that a rotation state of the electric motor just before occurrence of the abnormality is maintained, when the abnormality detect means detects the abnormality of the motor rotation angle. 
     (17) The electric power steering apparatus as set forth in (16), wherein 
     the reference angle change means sets a motor rotation angle just before the occurrence of the abnormality for the reference angle when the abnormality detect means detects the abnormality of the motor rotation angle. 
     (18) The electric power steering apparatus as set forth in (17), wherein 
     when the current steering torque has a same sign as the reference value and is equal to or smaller than the reference value, the reference angle change means uses a steering torque just before the occurrence of the abnormality for a reference value and maintains the then reference angle. 
     (19) The electric power steering apparatus as set forth in (18), wherein 
     when the current steering torque has the same sign as the reference value and is larger than the reference value, the reference angle change means changes the reference angle in an opposite direction to a steering neutral direction with respect to the then reference angle. 
     (20) The electric power steering apparatus as set forth in (18), wherein 
     when the current steering torque has a different sign from the reference value, the reference angle change means changes the reference angle in the steering neutral direction with respect to the then reference angle. 
     (21) The electric power steering apparatus as set forth in (16), wherein 
     when the abnormality detect means detects the abnormality of the motor rotation angle, the reference angle change means sets a motor rotation angular velocity just before the occurrence of the abnormality for a reference angular velocity, and sets the reference angle according to the reference angular velocity. 
     (22) The electric power steering apparatus as set forth in (21), wherein 
     the reference angle change means includes reference angular velocity reducing means for reducing the reference angular velocity gradually. 
     (23) The electric power steering apparatus as set forth in (16), wherein 
     the motor control means includes gradual change processing means reducing the output of the electric motor gradually when the abnormality detect means detects the abnormality of the motor rotation angle. 
     (24). The electric power steering apparatus as set forth in (23), wherein 
     the gradual change processing means determines a reducing ratio of an output of the electric motor according to the steering torque. 
     EFFECTS OF THE INVENTION 
     According to the invention as set forth in (1)˜(15), the electric motor relative angle information calculating portion calculates the relative angle information of the motor such as the relative angular velocity and relative angle thereof, in accordance with the steering amount of the driver, and the relative angle information compensating portion prevents the relative angle information calculating portion from being incapable of obtaining the relative angle information from the steering amount of the driver to thereby be able to output the relative angle information all the time. 
     Owing to this, even in a state where the initial actual motor angle is not determined, the relative angle of the motor can be detected positively with a simple structure, and the driving and control of the electric motor by the motor control means can be continued to thereby generate a steering assisting force corresponding to the intension of the driver. This can provide the following effects: that is, an increase in the number of parts and an increase in the cost of the system can be prevented and also the steering assisting control can be kept on without making the driver feel strange. 
     According to the invention, when there is provided a correction requiring state where there is a fear that a difference of the motor relative angle from an actual angle continues to increase, the motor relative angle information may be corrected according to the then state, thereby positively preventing the difference of the relative angle information from increasing. 
     Further, according to the invention as set forth in (16)˜(24), when the abnormality of the motor rotation angle detected by the rotation angle detect means is detected, the motor is driven and controlled by changing the reference angle of the motor rotation angle in such a manner that the rotation state of the motor just before occurrence of the abnormality can be maintained. This can provide the effect that the motor is prevented from returning due to the reacting force and thus the occurrence of a kickback phenomenon can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structure view of an embodiment according to the invention. 
         FIG. 2  is a characteristic diagram of a steering torque detect signal which is output from a steering torque sensor. 
         FIG. 3  is a block diagram of a specific structure of a control unit shown in  FIG. 1 . 
         FIG. 4  is a function block diagram of a microcomputer employed in the control unit. 
         FIG. 5  is a block diagram of a specific structure of a current command value calculating portion shown in  FIG. 4 . 
         FIG. 6  is an explanatory view of a steering assisting command value calculating map for showing the relationship between a steering torque used in a steering assistance control processing and a steering assisting command value. 
         FIG. 7  is a typical view used to explain a self-aligning torque. 
         FIG. 8  is a block diagram of a specific structure of an angular velocity·angular acceleration operating portion shown in  FIG. 4 . 
         FIG. 9  is a flow chart of an example of the procedure for a motor rotation angle abnormality detect processing to be carried out by the microcomputer of the control unit. 
         FIG. 10  is an explanatory view of an abnormality check map to be used in the motor rotation angle abnormality processing procedure shown in  FIG. 9 . 
         FIG. 11  is a flow chart of an example of a steering assisting control processing procedure to be carried out by the microcomputer of the control unit. 
         FIG. 12  is a flow chart of an example of the procedure for a relative angle information operating processing to be carried out by the microcomputer of the control unit. 
         FIG. 13  is a characteristic diagram of a difference in the magnetic vector relative angle between the rotor and stator of an electric motor and its relationship to the absolute value of energy generated in the rotor. 
         FIG. 14  is a function block diagram of a second embodiment according to the invention. 
         FIG. 15  is a flow chart of an example of a relative angle operating processing procedure according to a second embodiment of the invention. 
         FIG. 16  is a function block diagram of a motor rotation angle estimating processing. 
         FIG. 17  is a block diagram of an angular velocity·angular acceleration operating portion. 
         FIG. 18  is a flow chart of an example of the procedure for a relative angle detect processing according to a third embodiment of the invention. 
         FIG. 19  is a block diagram of a specific example of a control unit according to another embodiment of the invention. 
         FIG. 20  is a signal waveform diagram of a 3-phase detect signal. 
         FIG. 21  is a signal waveform diagram of an a-phase detect signal when it is fixed at a high level. 
         FIG. 22  is signal waveform diagram of the a-phase 
         FIG. 23  is a flow chart of an example of the procedure for a relative angle correction processing according to another embodiment of the invention. 
         FIG. 24  is a schematic structure view of a vehicle according to an embodiment of the invention. 
         FIG. 25  is a block diagram of an example of a steering assisting control apparatus. 
         FIG. 26  is a block diagram of a specific structure of a control operation unit shown in  FIG. 25 . 
         FIG. 27  is a characteristic diagram of a steering assisting current command value calculating map. 
         FIG. 28  is a flow chart of a control signal output processing to be carried out in a control signal output portion according to a fourth embodiment of the invention. 
         FIG. 29  is a view of a map used to calculate a speed for advancing a control angle. 
         FIG. 30  is a view of a map used to calculate the reducing ratio of a control amount. 
         FIG. 31  is a time chart used to explain the operation of the fourth embodiment of the invention. 
         FIG. 32  is a time chart used to explain the operation of a conventional apparatus. 
         FIG. 33  is a time chart used to explain the operation of a conventional apparatus. 
         FIG. 34  is a time chart used to explain the effect of the fourth embodiment. 
         FIG. 35  is a flow chart of a control signal output processing to be carried out in a control signal output portion according a fifth embodiment of the invention. 
         FIG. 36  is a view of a map used to calculate the reducing ratio of a motor angular velocity. 
         FIG. 37  is a view of a map used to calculate the reducing ratio of a motor angular velocity. 
         FIG. 38  is a time chart used to explain the operation of the fifth embodiment. 
         FIG. 39  is a time chart used to explain the operation of a sixth embodiment according to the invention. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS AND SIGNS 
     
         
           1001 : On-board battery 
           1003 : Control unit 
           1005 : Electric motor 
           1006 : Motor drive circuit 
           1011 : Steering wheel 
           1012 : Steering shaft 
           1013 : Speed reducer 
           1017 : Torque sensor 
           1018 : Resolver 
           1021 : Inverter circuit 
           1022 : FET gate drive circuit 
           1030 : Microcomputer 
           1032 : Motor rotation angle detect circuit 
           1033 : Vehicle speed sensor 
           1042 : Current-command value calculating portion 
           1042 A: Steering assisting torque command value operating portion 
           1042 B: Command value compensating portion 
           1042 C: d-q axis current command value operating portion 
           1044 : Subtracting portion 
           1045 : Current control portion 
           1046 ; Counter electromotive force operating portion 
           1047 : Motor rotation angle operating portion 
           1048 : Angular velocity·angular acceleration operating portion 
           1048   a : Relative angular velocity operating portion 
           1048   b : Sign obtaining portion 
           1049   c : Multiplying portion 
           1048   d : Late limit portion 
           1048   e : Dither processing portion 
           1048   f : Adding portion 
           1048   g : Rotation angle select portion 
           1048   h : Angular velocity operating portion 
           1048   i : Angular velocity select portion 
           1048   j : Angular acceleration operating portion 
           1048   m : Insensitive zone detect portion 
           1048   n : Second rotation angle select portion 
           1048   o : Angular velocity operating portion 
           1048   p ; Second angular velocity select portion 
           1049 : Fail-safe processing portion 
           1070 : Compensating relative angle information operating portion 
           1101   a ˜ 1101   c : Pole position detect sensor 
           2001 : Steering wheel 
           2002 : Steering shaft 
           2003 : Torque sensor 
           2010 : Steering assisting mechanism 
           2011 : Reduction gear 
           2012 : 3-phase brushless motor 
           2013 : Rotor position detect circuit 
           2020 : Steering assisting control unit 
           2021 : Vehicle speed sensor 
           2031 : Steering assisting current command value operating portion 
           2032 : Control signal output portion 
           2034 : d axis command current calculating portion 
           2035 : d-q axis voltage calculating portion 
           2036 : q axis command current calculating portion 
           2037 : 2-phase/3-phase converting portion 
           2042 : PI control portion 
           2043 : PWM control portion 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Now, description will be given below of embodiments according to the invention with reference to the accompanying drawings. 
     Firstly, description will be given of embodiments, that is, first to third embodiments according to the invention in attaining the above-mentioned first object of the invention. 
     First Embodiment 
       FIG. 1  is a structure view of the whole of a first embodiment according to the invention. In  FIG. 1 , reference numeral  1001  designates a normal battery which is mounted on board of a vehicle. A battery voltage Vb, which is output from the battery  1001 , is input through a fuse  1002  into a control unit  1003 . The control unit  1003  includes a motor drive circuit  1006  serving as motor drive means for driving an electric motor  1005  which generates a steering assisting force for a steering system into which the battery voltage Vb to be input through the fuse  1002  is input through a relay  1004  shown in  FIG. 3 . 
     Here, the electric motor  1005  is made of, for example, a brushless motor which is star (Y) connected and is driven by a 3-phase alternating current; and, the electric motor  1005  operates as a steering assisting force generating motor which generates the steering assisting force of an electric power steering apparatus. The electric motor  1005  is connected through a speed reducer  1013  to a steering shaft  1012  to which a steering wheel  1011  is connected. The steering shaft  1012  is connected to a rack and pinion mechanism  1014 . The rack and pinion mechanism  1014  is connected through a connecting mechanism  1015  such as a tie rod to right and left vehicle wheels  1016 . 
     On the steering shaft  1012 , there is mounted a steering torque sensor  1017  which detects a steering torque input to the steering wheel  1011 . On the electric motor  1005 , there is mounted a resolver  1018  for detecting the rotation angle of the motor. A steering torque detect signal detected by the steering torque sensor  1017  and a motor rotation angle detect signal detected by the resolver  1018  are respectively input to the control unit  1003 . 
     Here, the steering torque sensor  1017  is used to detect the steering torque that is applied to the steering wheel  1011  and is transmitted to the steering shaft  1012 . For example, the steering torque sensor  1017  is structured such that it converts the steering torque into the torsion angle displacement of a torsion bar interposed between input and output shafts (neither of which is shown), detects the torsion angle displacement using a magnetic signal and then converts it to an electric signal. 
     The steering torque sensor  1017  is structured in the following manner. That is, as shown in  FIG. 2 , when the steering torque to be input is zero, it outputs a given neutral steering torque detect value T 0 ; for example, when the steering wheel  1011  is turned right from this state, it outputs a value which increases from the neutral steering torque detect value T 0  according to an increase in the steering torque; and, when the steering wheel  1011  is turned left from the state where the steering torque is zero, it outputs a steering torque detect value T which decreases from the neutral steering torque detect value T 0  according to an increase in the steering torque. 
     The motor drive circuit  1006 , as shown in  FIG. 3 , includes an inverter circuit  1021 . The inverter circuit  1021  includes a series circuit in which two field effect transistors Qua and Qub are connected in series to each other, a series circuit which is connected parallel to the above-mentioned series circuit and in which two field effect transistors Qva and Qvb are similarly connected in series to each other, and a series circuit having two field effect transistors Qwa and Qwb. The connecting point of the field effect transistors Qua and Qub of the inverter circuit  1021 , the connecting point of the field effect transistors Qva and Qvb and the connecting point of the field effect transistors Qwa and Qwb are respectively connected to the star-connected excitation coils Lu, Lv and Lw of the electric motor  1005 , and further motor drive currents Imu, Imv and Imw, which are output from the inverter circuit  1021  to the electric motor  1005 , are detected by a motor current detect circuit  1007 . 
     The motor drive circuit  1006  also includes a FET gate drive circuit  1022  which is used to control field effect transistors FET 1 ˜FET 6  respectively included in the inverter circuit  1021 . The FET gate drive circuit  1022  turns on/off the field effect transistors FET 1 ˜FET 6  of the inverter circuit  1021  using PWM (pulse width modulation) signals respectively having duty ratios Du, Dv and Dw which are determined according to current command values Iut, Ivt and Iwt respectively output from a microcomputer  1030  (which will be discussed later), thereby controlling the currents Imu, Imv and Imw which are allowed to flow actually into the electric motor  1005 . Here, depending on the magnitude of the duty ratios Du, Dv and Dw, the FET  1 , FET  3  and FET  5 , which constitutes an upper arm, and the FET 2 , FET 4  and FET  6  constituting a lower arm are respectively PWM driven while having a dead time for avoiding the occurrence of shortcircuit in the arms. 
     The control unit  1003  further includes a microcomputer  1030  used to supply, to the gate drive circuit  1022 , a pulse width modulation signal of which duty ratio allows the electric motor  1005  to generate a steering assisting force. 
     Into this microcomputer  1030 , inputted signals are: 
     phase current detect values Ia˜Ic respectively input from the current detect circuit  1007  detecting the respective phase currents of the electric motor  1005 ; 
     phase terminal voltages Va˜Vc respectively input from a terminal voltage detect circuit  1008  which detects the respective phase terminal voltages of the electric motor  1005 ; 
     motor rotation angle signals sin θ and cos θ, which are respectively supplied from a motor rotation angle detect circuit  1032  receiving the output signal of the resolver  1018  and outputting the motor rotation angle signals through an A/D converter circuit  1031  a steering torque signal detected by the steering torque sensor  1017 ; and 
     a vehicle speed detect value Vs output from a vehicle speed sensor  1033  which detects the speed Vs of the vehicle. 
     Further, into this microcomputer  1030 , as a control power supply, a stabilizing power supply output from a stabilized power supply circuit  1034  which is connected to the fuse  1002  and forms a microcomputer power supply of 5V, for example, is input. 
     Here, the motor rotation angle detect circuit  1032  supplies a carrier signal sin ωt having a given frequency to the resolver  1010  to generate a sine wave signal (sin ωt·sin θ) having a waveform provided by amplitude modulating the carrier signal sin ωt by using a sine wave sin θ and a carrier signal (sin ωt·cos θ) having a wave form provided by amplitude modulating the carrier signal sin ωt by using a cosine wave cos θ, and inputs these sine wave signal (sin ωt·sin θ)—and cosine wave signal (sin ωt·cos θ) into the microcomputer  1030  through A/D converters  1035  and  1036  respectively; and also, it detects, for example, the peak time of a carrier wave sin ωt and inputs a peak detect pulse Pp into the microcomputer  1030 . 
       FIG. 4  is a function block diagram of the structure of the microcomputer  1030 . As shown in  FIG. 4 , the microcomputer  1030  includes: a gradual change control portion  1041 , a current command value calculating portion  1042 , a current output limit portion  1043 , a subtracting portion  1044 , a current control portion  1045 , a counter electromotive force operating portion  1046 , a motor rotation angle operating portion  1047 , and a rotation angular velocity·angular acceleration operating portion  1048 . 
     The gradual change control portion  1041  controls the sudden change of steering torque T input from the steering torque sensor  1017  through a fail-safe signal SF supplied from a fail-safe processing portion  1049  (which will be discussed later) to gradually change the steering torque T. 
     The current command value calculating portion  1042  receives steering torque Ts, the sudden change of which is controlled by the gradual change control portion  1041 , and a vehicle speed Vs detected by the vehicle speed sensor  1033 . The current command value calculating portion  1042  also carries out a vector control operation according to an angular speed ωe and an angular acceleration α respectively input from the angular velocity·angular acceleration detect portion  1048  to thereby calculate 3-phase current command values Ia*˜Ic*. 
     The current output limit portion  1043  limits the current command values Ia*˜Ic* output from the current command value calculating portion  1042  by using a fail-safe signal SF supplied from a fail-safe processing portion  1049  (which will be discussed later). 
     The subtracting portion  1044  carries out a subtraction operation between the current command values Ia*˜Ic* output from the current output limit portion  1043  and phase current detect values Ia˜Ic input therein from the current detect circuit  1007  to thereby calculate their differences ΔIa˜ΔIc. 
     The current control portion  1045  carries out a proportion/integration (PI) control on the differences ΔIa˜ΔIc output from the subtracting portion  1044  to output instruction voltages Va˜Vc to the FET gate drive circuit  1022  of the motor drive circuit  1006 . 
     The counter electromotive force operating portion  1046  receives current detect values Ia˜Ic output from the current detect circuit  1007  and terminal voltages Va˜Vc output from the terminal voltage detect circuit  1008 . The counter electromotive force operating portion  1046  also operates line counter electromotive forces EMFab, EMFbc and EMFca respectively generated between the respective motor coils based on these values and voltages. 
     The motor rotation angle operating portion  1047  operates a motor rotation angle θe expressed as an electric angle based on a sine wave signal (sin ωt·sin θ) and a cosine wave signal (sin ωt·cos θ) respectively input therein from the motor rotation angle detect circuit  1032  and a peak detect pulse Pp. 
     The rotation angular velocity·angular acceleration operating portion  1048  serves as relative angle and actual angle information detect means which calculates an angular velocity and an angular acceleration based on the line counter electromotive forces EMFab, EMFbc and EMFca respectively operated by the counter electromotive force operating portion  1046  and the motor rotation angle θe operated by the motor rotation angle operating portion  1047 . 
     The fail-safe processing portion  1049  serves as motor rotation angle abnormality detect means which receives the steering torque Ts detected by the steering torque sensor  1017 , vehicle speed detect value Vs detected by the vehicle speed sensor  1006  and motor rotation angle θe operated by the motor rotation angle operating portion  1047 . Further, the fail-safe processing portion  1049  also checks abnormalities of the steering torque sensor  1017 , vehicle speed sensor  1033 , resolver  1018 , motor rotation angle detect circuit  1032  and motor rotation angle operating portion  1047  according to the thus received data to thereby carry out a fail-safe processing. 
     Here, the current command value calculating portion  1042 , as shown in  FIG. 5 , includes: a steering assisting torque command value operating portion  1042 A, an command value compensating portion  1042 B and a d-q axis current command value operating portion  1042 C. 
     The steering assisting torque command value operating portion  1042 A calculates a steering assisting current command value I M *with reference to a steering assisting current command value calculating map shown in  FIG. 6  which calculates the steering assisting current command value I M *according to the steering torque Ts input therein from the steering torque sensor  1017  and the vehicle speed detect value Vs. 
     The command value compensating portion  1042 B compensates the steering assisting current command value I M * calculated by the steering assisting torque command value operating portion  1042 A according to an angular velocity ωe and an angular acceleration α (which will be discussed later respectively) input therein from the rotation angular velocity·angular acceleration operating portion  1048 . 
     The d-q axis current command value operating portion  1042 C calculates a d-q axis current command value according to the after-compensation torque command value I*′ compensated by the command value compensating portion  1042 B, and also converts the thus calculated d-q axis current command value to a 3-phase current command value. 
     The steering assisting torque command value operating portion  1042 A calculates the steering assisting current command value I M *expressed as the current command value according to the steering torque Ts and vehicle speed Vs with reference to a steering assisting torque command value calculating map shown in  FIG. 6 . 
     The steering assisting torque command value calculating map, as shown in  FIG. 6 , is composed of a characteristic diagram, in which the steering torque Ts is expressed in a horizontal axis, the steering assisting current command value I M *is expressed in the vertical axis, and parabolic curves are drawn with the vehicle speed Vs as the parameter thereof. Specifically, in the range where the steering torque Ts extends from “0” to a set value Ts 1  existing in the vicinity of “0”, the steering assisting torque command value I M *maintains ‘0’. When the steering torque Ts exceeds the set value Ts 1 , firstly, the steering assisting current command value I M *increases relatively gently with respect to an increase in the steering torque Ts; and, when the steering torque Ts increase further, the steering assisting current command value I M *increases sharply with respect to an increase in the steering torque Ts. That is, the characteristic curve is set such that the inclination thereof decreases as the vehicle speed increases. 
     The command value compensating portion  1042 B includes at least; a convergence compensating portion  1051  for compensating the convergence of a yaw rate according to a motor angular velocity ωe (which will be discussed later) calculated by the rotation angular velocity·angular acceleration operating portion  1048 ; an inertia compensating portion  1052  for compensating the amount of a torque generated due to the inertia of the electric motor  1005  according to the motor angular velocity α calculated by the rotation angular velocity·angular acceleration operating portion  1048  to thereby prevent the sense of inertia or a control responsibility from being degraded; and a SAT estimating feedback portion  1053  for estimating a self-aligning (SAT). 
     Here, the convergence compensating portion  1051  receives the vehicle speed detect value Vs and the motor angular velocity ωe (which will be discussed later) calculated by the rotation angular velocity·angular acceleration operating portion  1048 , and then multiplies the motor angular velocity ωe by a convergence control gain Kv, which is changed according to the vehicle speed Vs, to thereby calculate a convergence compensating value Ic so that the swinging operation of the steering wheel  1001  is braked to improve the convergence of the yawing of the vehicle. 
     Also, the SAT assuming feedback portion  1053  receives the steering torque Ts, the angular velocity ω, the angular acceleration α, and the steering assisting current command value I M *calculated by the steering assisting torque command value operating portion  1042 A, and estimates and operates the self-aligning torque SAT according to the thus received data. The principle of the calculation of the self-aligning torque SAT will be described with reference to  FIG. 7  which shows the state of a torque occurring between the road and the steering wheel. 
     That is, when the vehicle driver steers the steering wheel  1001 , there is generated steering torque T and, according to the steering torque T, the electric motor  1005  generates an assisting torque Tm. As a result of this, vehicle wheels W are turned following the driver&#39;s steering operation and as the reacting force of the turning of the vehicle wheels W, there is generated a self-aligning torque SAT. 
     Also, in this case, due to the inertia J and friction (static friction) Fr of the electric motor  1005 , there is generated a torque that provides resistance against the steering motion of the steering wheel  1001 . When the balance of these forces is taken into consideration, there is obtained the following motion equation (1). 
         J·α+Fr ·sign(ω)+ SAT=Tm+T   (1) 
     Here, the above equation (1) is Laplace transformed with its initial value as zero and, when it is solved with respect to the self-aligning torque SAT, a following equation (2) is obtained. 
         SAT ( s )= Tm ( s )+ T ( s )− J ·α( s )+ Fr ·sign(ω( s )  (2) 
     As can be seen from the above equation (2), when the inertia J and the static friction Fr of the electric motor  1005  are previously obtained, the self-aligning torque SAT can be estimated from the motor angular velocity ω, the rotation angular acceleration α, the assisting torque Tm and the steering torque T. Here, since the assisting torque Tm is proportional to the steering assisting current command value I M *, instead of the assisting torque Tm, there is applied the steering assisting current command value I M *. 
     An inertia compensating value Ii calculated by the inertia compensating portion  1052  and the self-aligning torque SAT calculated by the SAT assuming feedback portion  1053  are added together by an adder  1054 . Thus added output of the adder  1054  and the convergence compensating value Ic calculated by the convergence compensating portion  1051  are added together by an adder  1055  to calculate an instruction compensating value Icom. The instruction compensating value Icom is added to the steering assisting current command value I M *output from the command value calculating portion  1042  by an adder  1056  to calculate an after-compensation torque command value I M *′. This after-compensation torque command value I M *′ is output to the d-q axis current command value operating portion  1042 C. 
     The d-q axis current command value operating portion  1042 C includes: a d axis target current calculating portion  1061 , an induced voltage model calculating portion  1062 , a q axis target current calculating portion  1063 , and a 2-phase/3-phase converting portion  1064 . 
     The d axis target current calculating portion  1061  calculates a d axis target current Id* according to the after-compensation torque command value I M *′ and motor angular velocity ω. 
     The induced voltage model calculating portion  1062  calculates the d axis EMF component ed(θ) and-q axis EMF component eq (θ) of a d-q axis induced voltage model EMF (Electro Magnetic Force) according to the motor rotation angle θ and motor angular velocity ω. 
     The q axis target current calculating portion  1063  calculates a q axis target current Iq* according to the d axis EMF component ed(θ), q axis EMF component eq (θ) which are output from the induced voltage model calculating portion  1062 , the d axis target current Id* and the after-compensation steering assisting torque command value I M *′ which are output from the d axis target current calculating portion  1061  and the motor angular velocity ω. 
     The 2-phase/3-phase converting portion  1064  converts the d axis target current Id* output from the d axis target current calculating portion  1061  and the q axis target current Iq* output from the q axis target current calculating portion  1063  into 3-phase current command values Iu*, Iv* and Iw*. 
     The counter electromotive force operating portion  1046 , firstly, according to the phase terminal voltages Va˜Vc respectively input from the terminal voltage detect circuit  1008 , operates the following (3)˜(5) equations to thereby calculate line voltages Vab, Vbc and Vca. 
         Vab=Va−Vb   (3) 
         Vbc=Vb−Vc   (4) 
         Vca=Vc−Va   (5) 
     Next, according to the thus calculated line voltages Vab, Vbc and Vca as well as the respective phase current detect values Ia˜Ic input from the current detect circuit  1007 , the counter electromotive operating portion  1046  operates the following equations (6)˜(8) to thereby calculate the respective line counter electromotive forces EMFab, EMFbc and EMFca. 
         EMFab=Vab −{( Ra+s·La )· Ia −( Rb+s·Lb )· Ib}   (6) 
         EMFbc=Vbc −{( Rb+s·Lb )· Ib −( Rc+s·Lc )· Ic}   (7) 
         EMFca=Vca −{( Rc+s·Lc )· Ic −( Ra+s·La )· Ia}   (8) 
     Here, Ra, Rb and Rc respectively represents the coil resistances of the motor; La, Lb and Lc represents the inductances of the motor; and, s is a Laplace operator and, here, it represents a differential operator (d/dt). 
     The absolute values of the respective calculated line counter electromotive forces EMFab, EMFbc and EMFca are added together to calculate a counter electromotive force EMF (=|EMFab|+|EMFbc|+|EMFca|). Here, the purpose of the calculation of the counter electromotive force EMF by adding together the absolute values of the respective line counter electromotive forces EMFab, EMFbc and EMFca is to simplify the operation. When enhancing the accuracy of the relative angle operation, to obtain the counter electromotive force EMF, there may be operated a square root of the sum of the squares of the respective line counter electromotive forces EMFab, EMFbc and EMFca, that is, EMF=√(EMFab 2 +EMFbc 2 +EMFca 2 ). Here, the line counter electromotive forces EMFab, EMFbc and EMFca may have such accuracy that can obtain the relative angle of the motor. 
     Also, although, in the above-mentioned equations (6)˜(8), the coil resistances of the motor Ra, Rb and Rc are used as fixed values, since the coil resistances of the motor Ra, Rb and Rc depend on temperature, preferably, the temperature of the motor may be detected to correct the coil resistances of the motor Ra, Rb and Rc. However, even when the coil resistances of the motor Ra, Rb and Rc are used as fixed values and the motor resistance increases or decreases due to varying temperature or the like, there may also be used such fixed values, provided there can be obtained such level of counter electromotive·angle information that is necessary to be able to continue the steering assisting control. 
     Here, it is noted that in order to be able to obtain the angle information, the coil resistances of the motor Ra, Rb and Rc as well as insensitive zone width set values to be set in this case must be set such that they have sufficient margins with respect to temperature variations. 
     Further, the motor rotation angle operating portion  1047  carries out a motor rotating angle calculating processing (not shown) each time when a peak pulse Pp is input therein from the motor angle detect circuit  1032 , thereby calculating sin θ and cos θ; and then, according to the thus calculated sin θ and cos θ, the motor rotation angle operating portion  1047  calculates a motor rotation angle θ  which is an electric-angle. 
     Further, the angular velocity·angular acceleration operating portion  1048 , as shown in  FIG. 8 , includes: a relative angular velocity operating portion  1048   a , a sign obtaining portion  1048   b , a multiplying portion  1048   c , a rate limiting portion  1048   d , a relative angle information offset processing portion  1048   e , an adding portion  1048   f , a rotation angle select portion  1048   g , an angular velocity operating portion  1048   h , an angular velocity select portion  1048   i  and an angular velocity operating portion  1048   j.    
     The relative angular velocity operating portion  1048   a  operates a relative angular velocity ωee according to the counter electromotive force EMF input therein from the counter electromotive force operating portion  1046 . 
     The sign obtaining portion  1048   b  obtains a sign expressing a rotation direction according to the steering torque Ts input therein from the steering torque sensor  1017 . 
     The multiplying portion  1048   c  multiplies together the relative angular velocity ωee operated by the relative angular velocity operating portion  1048   a  and the sign obtained by the sign obtaining portion  1048   b.    
     The rate limiting portion  1048   d  prevents the sudden change of the relative angular velocity ωee output from the multiplying portion  1048   c.    
     The relative angle information offset processing portion  1048   e  serves as a relative angle information compensating portion, which checks whether the relative angular velocity ωee prevented from being sudden change by the rate limiting portion  1048   d  is within an angular velocity area near to zero (that is, in an insensitive zone having a value ±Δω containing ωe=0 and its neighboring values). When ωee&lt;−Δω or ωee&gt;+Δω and also ωee is out of the insensitive zone, the relative angle information offset processing portion  1048   e  outputs the relative angular velocity ωee as it is, and, when ω−Δω≦ωee≦+Δω and ωee is within the insensitive zone, it sets the relative angular velocities ωee in a previously set positive and negative relative angle information offset value ±Δωd alternately at a given interval in order that the relative angular velocity ωee can provide other value than “0”. 
     The adding portion  1048   f  adds the relative angular velocity ωee output from the relative angle information offset processing portion  1048   e  to the previous motor rotation angle θe(n−1) to calculate a relative rotation angle θee. 
     The rotation angle select portion  1048   g  serves as select means for selecting the relative rotation angle θee output from the adding portion  1048   f  and an actual rotation angle θer input therein from the motor rotation angle operating portion  1047  according to a fail-safe signal SF. 
     The angular velocity operating portion  1048   h  differentiates the actual rotation angle θer input therein from the motor rotation angle operating portion  1047  to calculate an actual angular velocity ωer. 
     The angular velocity select portion  1048   i  selects the actual angular velocity ωer input therein from the angular velocity operating portion  1048   h  and the relative angular velocities ωee output from the relative angle information offset processing portion  1048   e  according to a fail-safe signal SF. 
     The angular velocity operating portion  1048   j  differentiates an angular velocity ωe selected by the angular velocity select portion  1048   i  to calculate the angular acceleration α. 
     Here, the counter electromotive force operating portion  1046 , angular velocity operating portion  1048   a , sign obtaining portion  1048   b  and multiplying portion  1049   c  cooperate together to form a relative angle information operating portion. 
     Here, the angular velocity operating portion  1048   a , according to the counter electromotive force EMF input therein from the counter electromotive force operating portion  1046 , operates the following equation (9) to calculate the relative angular velocity ωee. 
       ω ee=EMF/Ke   (9) 
     Here, Ke expresses the counter electromotive force constant [V/rpm] of the motor. 
     In an insensitive zone setting portion (not shown) of the counter electromotive force operating portion  1046 , as the motor coil resistances Ra Rc of the equations (6)˜(8) used to calculate the above-mentioned respective line counter electromotive forces EMFab, EMFbc and EMFca, there are employed resistance model values instead of actual resistance values. Owing to this, there is generated an error in the relative angular velocity ωee, while this error provides an offset error proportional to a motor current. According to this fact, the insensitive zone setting portion sets an insensitive zone which is proportional to a current, thereby removing an estimated error. That is, the reason why the insensitive zone setting portion is able to remove the assumed error is that the relative angular velocity ωee is proportional to a current (a magnitude of a counter electromotive force) and the error is also proportional to a current (a magnitude of a counter electromotive force). Therefore, the set value of the insensitive zone is set for a value which corresponds to a current command value I M *. 
     Further, since the line counter electromotive forces EMFab, EMFbc and EMFca are also influenced by an inductance which is caused to vary due to variations in the motor rotation speed, preferably, the variations in the inductance may be fed back to the counter electromotive force operating portion  1046  according to the rotation speed of the motor to thereby remove the influence of the inductance variations. 
     Still further, the insensitive zone width ±Δω of the relative angle information offset processing portion  1048   e  is a set value which regulates an area where the relative angle is 0 or in the vicinity of 0. Since the motor is driven at a relative angle, when a magnetic field restricting force between the stator and rotor of the motor is large in an area where the motor relative angular velocity is ±Δω, the driver is not able to obtain a next relative angle information (in this case, the counter electromotive force) by steering the steering wheel, that is, there is generated a so called steering (steering wheel) locked phenomenon. 
     Thus, when the relative angular velocity exists in an area that is expressed by ±Δω, the relative angle offset value ±Δω must be set for such value which positively removes an insensitive zone generated due to the restricting force of the magnetic field and the torsion component of the torsion bar before the driver steers the steering wheel and rotates the motor to obtain a proper steering amount. The relative angle offset value ±Δω for removing the insensitive zone is added to the relative angular velocity at a given cycle while reversing the sign. Further, the set value of ±Δω must be within the amount and cycle which does not perform unintended steering assistance of the driver. Here, the angle amount for offsetting the relative angle to the positive and negative directions must be the same in order to prevent the angle from continuing to offset in an unintentional direction. 
     When a fail-safe signal SF input therein from the fail-safe processing portion  1049  is a logical value “0”, the rotation angle select portion  1048   g  selects an actual rotation angle θer input therein from the motor rotation angle operating portion  1047  and, when the fail-safe signal SF is a logical value “1”, it selects a relative rotation angle θee input therein from the adding portion  1048   f.    
     Similarly, when the fail-safe signal SF input therein from the fail-safe processing portion  1049  is a logical value “0”, the angular velocity select portion  1048   i  selects an actual angular velocity ωer input therein from the angular velocity operating portion  1048   h  and, when the fail-safe signal SF is a logical value “l”, it selects a relative angular velocity ωee input therein from the relative angle information offset processing portion  48   e.    
     The fail-safe processing portion  1049  carries out a motor rotation angle abnormality detect processing shown in  FIG. 9 . 
     This motor rotation angle abnormality detect processing is carried out as a timer interrupt processing in every given time (for example, 10 msec.). Specifically, firstly, in Step S 1021 , a sine wave sin θ and a cosine wave cos θ respectively calculated in a motor rotation angle calculating processing (not shown) are read in and next, in Step S 1022 , according to the sine wave sin θ and cosine wave cos θ, it is checked whether the combination of the sine wave sin θ and cosine wave cos θ is normal or abnormal with reference to an abnormality checking map. 
     Here, the abnormality checking map has a structure that, as shown in  FIG. 10 , the sine wave sin θ is expressed in the horizontal axis and the cosine wave cos θ is expressed in the vertical axis respectively, while there are displayed three concentric circles and two quadrangles with the origin G (0, 0) as the center thereof. 
     Firstly, referring to the three concentric circles, there are a circle of (sin θ) 2 +(cos θ) 2 =Pmin on the inner-most side, a circle of (sin θ) 2 +(cos θ) 2 =1 in the center, and a circle of (sin θ) 2 +(cos θ) 2 =Pmax on the outer-most side. A larger quadrangle α is a square one side of which is 2·Pmax, whereas a smaller quadrangle β is a square one side of which is 2 (Pmin/√2). Here, a normal area is expressed by hatched area which is surrounded by the larger quadrangle α and smaller quadrangle β, whereas the remaining area shows an abnormal area. Referring further to the above-mentioned check reference Pmin and Pmax, while the influences of the detect accuracy, the number of the poles of the motor and the like are taken into consideration, Pmax and Pmin can be used to adjust the abnormality detect accuracy. 
     By properly setting Pmax and Pmin, it is possible to detect the failure of a motor during the driving of the motor and the abnormality of the resolver  1018 . A condition of (sin θ) 2 +(cos θ) 2 =1 is a checking reference for normality, and (sin θ) 2 +(cos θ) 2 =Pmin and (sin θ) 2 +(cos θ) 2 =Pmax are used to show the normal range of Pmin&lt;(sin θ) 2 +(cos θ) 2 &lt;Pmax, and thus they are wider than the ordinary checking reference for the normality. 
     Next, when it is found in Step S 1022  that sin θ and cos θ are normal, the processing goes to Step S 1023 , where a fail-safe signal SF of a logical value “0” showing the normality of sin θ and cos θ is output to the angular velocity·angular acceleration operating portion  1048 , thereby ending the timer interrupt processing. When sin θ and cos θ are found abnormal, the processing goes to Step S 1024 , where a fail-safe signal SF of a logical value “1” showing the abnormality of sin θ and cos θ is output to the angular velocity·angular acceleration operating portion  1048 , thereby ending the timer interrupt processing. In this manner, when it is checked whether sin θ and cos θ are normal or abnormal by using the abnormality checking map, it is not necessary to operate (sin θ) 2 +(cos θ) 2  for checking (sin θ) 2 +(cos θ) 2 =1. This can reduce the processing load of the microcomputer  1030  greatly and also can reduce the checking time greatly. 
     The processing shown in  FIG. 9  corresponds to the motor rotation angle abnormality detect means. 
     The microcomputer  1030  carries out a steering assisting control processing shown in  FIG. 11  corresponding to the command value calculating portion  1042  according to the respective input signals. 
     The steering assisting control processing, as shown in  FIG. 11 , firstly, in Step S 1001 , reads the detect values of various sensors such as the steering torque sensor  1017  and vehicle speed sensor  1033  as well as the rotation angle θe, angular velocity ωe and angular acceleration α calculated by the angular velocity·angular acceleration operating portion  1048  and, next, it goes to Step S 1002 , where it calculates the steering assisting torque command value I M *according to the steering torque T with reference to the steering assisting torque command value calculating map shown in  FIG. 6 ; and, after then, the processing goes to Step S 1003 . 
     In Step S 1003 , similarly to the convergence compensating portion  1051 , the motor angular velocity we is multiplied by a compensating coefficient Kv set according to the vehicle speed V to calculate a convergence compensating value Ic and, after then, the processing goes to Step S 1004 . 
     In Step S 1004 , similarly to the inertia compensating portion  1052 , an inertia compensating value Ii is calculated according to the motor angular acceleration α, and next, the processing goes to Step S 1005 , where, similarly to the SAT estimating feedback portion  1053 , the above-mentioned equation (2) is operated according to the motor angular velocity ωe and motor angular acceleration α to calculate self-aligning torque SAT. 
     Next, the processing goes to Step S 1006 , where the convergence compensating value Ic, inertia compensating value Ii and self-aligning torque SAT respectively calculated in Steps S 1003 ˜S 1005  are added to the steering assisting torque command value I M *to calculate the after-compensation steering assisting torque command value I M *′. Then, the processing goes to Step S 1007 , where a d-q axis command value operating processing similar to the d-q axis current command value operating portion  1042 B is executed on the after-compensation steering assisting torque command value I M *′ calculated in Step S 1006  to calculate the d axis target current Id* and the q axis target current Iq*. Next, the processing goes to Step S 1008 , where there is executed a 2-phase/3-phase conversion processing to calculate motor current command values Ia*˜Ic*. 
     Next, the processing goes to Step S 1009 , where the motor currents Ia˜Ic are subtracted from the motor current command values Ia*˜Ic* respectively to calculate current differences ΔIa˜ΔIc. Then, the processing goes to Step S 1010 , where a PI control processing is executed on the current differences ΔIa˜ΔIc to calculate voltage command values Va˜Vc, and, next, the processing goes to Step S 1011 , where, after the thus calculated voltage command values Vu˜Vw are output to the FET gate drive circuit  1022  of the motor drive circuit  1006 , the steering assisting control processing is ended and the processing returns to a given main program. 
     The microcomputer  1030  carries out a relative angular velocity operating processing which is shown in  FIG. 12  and corresponds to the relative angular velocity operating portion  1048   a , sign obtaining portion  1048   b , multiplying portion  1048   c , rate limiting portion  1048   d  and relative angle information offset processing portion  1048   e  of the angular velocity·angular acceleration operating portion  1048 . 
     This relative angular velocity operating processing is executed as a timer interrupt processing in each given time (for example, 1 msec.). Firstly, in Step S 1031 , the counter electromotive force EMF operated by the counter electromotive force operating portion  1046  is read; next, the processing goes to Step S 1032 , where the above-mentioned equation (9) is operated according to the counter electromotive force EMF to calculate the relative angular velocity ωee; and then, the processing goes to Step S 1033 , where the sign of the steering torque Ts is obtained and added to the relative angular velocity ωee and, after then, the processing goes to Step S 1034 . 
     In Step S 1034 , the currently calculated relative angular velocity ωee(n) is subtracted by the previously calculated relative angular velocity ωee(n−1) to calculate a variation amount Δωee. Then, the processing goes to Step S 1035 , where it is checked whether the absolute value of the calculated variation amount Δωee exceeds a variation amount upper limit value Δωes or not. When |Δωee|≦Δωes, it is determined that the variation amount Δωee is small, and the processing goes to Step S 1039 ; and, when |Δωee|&gt;Δωes, it is determined that the variation amount Δωee is too large, and the processing goes to Step S 1036 . 
     In Step S 1036 , it is checked whether the variation amount Δωee is a positive value or a negative value. When Δωee≧0, the processing goes to Step S 1037 , where the variation amount upper limit value Δωes is added to the previously calculated relative angular velocity ωee(n−1) to calculate a current relative angular velocity ωee (n) and then the processing goes to Step S 1039 . When Δωee&lt;0, the processing goes to Step S 1038 , where the variation amount upper limit value Δωs is subtracted from the previously calculated relative angular velocity ωee(n−1) to calculate a current relative angular velocity ωee(n), and then the processing goes to Step S 1039 . 
     In Step S 1039 , it is checked whether the relative angular velocity ωee calculated in Step S 1033  and the current relative angular velocity ωee (n) calculated in Step S 1037  or Step S 1038  are within the insensitive zone to be regulated by ±Δω or not. When the current relative angular velocity ωee (n) is −×Δω≦ωee≦+Δω and is within the insensitive zone, the processing goes to Step S 1040 . 
     In Step S 1040 , a relative angle information offset value Δωd, where an angle variation with respect to the motor relative rotation angle θee is set for a value per adding cycle such as ±2 deg., is set as the current relative angular velocity ωee. After then, the processing goes to Step S 1041 , where “1” is added to the current time coefficient value t to calculate a new time coefficient value t, and then the processing goes to Step S 1042 , where it is checked whether the time coefficient value t exceeds a given value ts (for example, a value corresponding to 20 msec.) or not. When t&gt;ts, the processing goes to Step S 1043 , where the current relative angle information offset value Δωd is multiplied by “−” to reverse the sign and, after then, the processing goes to Step S 1044 . 
     In Step S 1044 , the time coefficient value t is cleared to “0” and then the timer interrupt processing is ended and the processing returns to a given main program. 
     Further, when the check result of the above step S 1039  is ωee&lt;−Δω or ωee&gt;+Δω, it is determined that the relative angular velocity is out of the insensitive zone, and the timer interrupt, as it is, is ended. Further, the processing returns to a given main program. 
     In the processing shown in  FIG. 12 , the processings of Steps S 1031 ˜S 1038  correspond to the motor relative angle information calculating portion, while the processings of Steps S 1039 ˜S 1044  correspond to the relative angle information compensating portion. 
     Next, description will be given below of the operation of the above-mentioned first embodiment. 
     Now, when an ignition switch  37  shown in  FIG. 3  is turned on, power from a battery  1001  is fed into the control unit  1003 , whereby the microcomputer  1030  included in the control unit  1003  starts to execute the motor rotation angle abnormality detect processing shown in  FIG. 9 , steering assisting control processing shown in  FIG. 11 , relative angular velocity calculating processing shown in  FIG. 12 , and the like. 
     In this state, in the steering assisting control processing shown in  FIG. 11  to be executed by the microcomputer  1030 , the steering torque detect value T detected by the steering torque sensor  1017  (Step S 1001 ) is read; a neutral torque T 0  is subtracted from the read steering torque detect value T to calculate a steering torque Ts (Step S 1002 ); next, the vehicle speed detect value Vs is read from the vehicle speed sensor  1033  (Step S 1003 ) and, according to the steering torque Ts and vehicle speed detect value Vs, the steering assisting command value I M *is calculated with reference to the steering assisting command value calculating map shown in  FIG. 6  (Step S 1004 ). 
     At the then time, suppose that the resolver  1018 , motor rotation angle detect circuit  1032  and A/D converters  1035 ,  1036  are normal, when the motor rotation angle abnormality detect processing shown in  FIG. 9  is executed, sin θ and cos θ are read, which are calculated by the motor rotation angle operating portion  1047  shown in  FIG. 4  according to the sine wave signal (sin ωt+sin θ), cosine wave signal (sin θt+cos θ) and peak detect pulse Pp respectively input therein from the motor rotation angle detect circuit  1032  (Step S 1021 ); and, according to the thus read-in sin θ and cos θ, when referring to the abnormality checking map shown in  FIG. 10 , points expressed by such sin θ and cos θ are found present within the normal area shown by hatched area in  FIG. 10  and thus the motor rotation angle is found normal, so that a fail-safe signal SF having a logical value “0” is output to the angular velocity angular acceleration operating portion  1048  (Step S 1023 ). 
     Accordingly, in the angular velocity·angular acceleration operating portion  1048  shown in  FIG. 8 , an actual rotation angle θer calculated by the motor rotation angle operating portion  1047  is selected by the rotation angle select portion  1048   g  and is used as the rotation angle θe; also, an actual angular velocity ωer obtained by the angular velocity operating portion  1048   h  differentiating the actual rotation angle θer is selected by the angular velocity select portion  1048   i  and is used as the angular velocity ωe; further, the angular velocity ωe is differentiated by the angular acceleration operating portion  1048   j  to calculate the angular acceleration α; and, the rotation angle θe, angular velocity ωe and angular acceleration α are respectively output to the current command value calculating portion  1042 . 
     Accordingly, in the steering assisting control processing shown in  FIG. 11  to be executed by the current command value calculating portion  1042 , the processing goes from Step S 1002  to Step S 1003 , where the convergence compensating value Ic is calculated based on the angular velocity ωe. Next, the inertia compensating value I i  for controlling the inertia compensation is calculated according to the angular acceleration α (Step S 1004 ). Further, the self-aligning torque SAT is calculated according to the angular velocity ωe, angular acceleration α, steering torque Ts and steering assisting torque command value I M *(Step S 1005 ). 
     The convergence compensating value Ic, inertia compensating value Ii and self-aligning torque SAT are added to the steering assisting torque command value I M *to calculate the after-compensation steering assisting torque command value I M *′ (Step S 1006 ). According to the thus calculated after-compensation steering assisting torque command value I M *′, rotation angle θe and angular velocity ωe, there is carried out the d-q axis command value operating processing to calculate the target d axis current Id* and target q axis current Iq* (Step S 1007 ). These target d axis current Id* and target q axis current Iq* are 2-phase/3-phase conversion processed to calculate 3-phase motor current command values Ia*, Ib* and Ic* (Step S 1008 ). 
     According to the calculated target-phase motor current command values Ia*, Ib* and Ic* as well as the detected motor phase currents Ia, Ib and Ic, a current feedback processing is carried out to calculate the respective phase voltage command values Va*, Vb* and Vc* of the electric motor  1005  (Step S 1010 ); and, these respective phase voltage command values Va*, Vb* and Vc* are output to the FET gate drive circuit  1022  of the motor drive circuit  1006  (Step S 1011 ). Thus, the FET gate drive circuit  1022  controls and modulates the pulse width of the field effect transistors Qua˜Qwb of the motor drive circuit  1006 , whereby the motor drive circuit  1006  supplies a 3-phase drive current to the electric motor  1005 , thereby allowing the electric motor  1005  to generate a steering assisting force in a direction corresponding to the steering torque applied to the steering wheel  1011 ; and, the thus generated steering assisting force is transmitted through the reduction mechanism  1013  to the output shaft  1012 . 
     At the then time, in a so called rest steering state where the steering wheel  1011  is steered while the vehicle is stopping, since the vehicle speed Vs is zero and the gradient of the characteristic line of the steering assisting command value calculating map shown in  FIG. 6  is large, a large steering assisting command value I M *is calculated with a small steering torque Ts, whereby the electric motor  1005  generates a large steering assisting force to allow the driver to steer the steering wheel  1011  lightly. 
     When the vehicle is started from the stopping mode of the vehicle into a running mode thereof, in a normal steering state for steering the steering wheel  1011  during such running mode of the vehicle, the required steering assisting torque decreases as the vehicle speed increase and thus the steering torque to be transmitted to the steering wheel  1011  also decreases accordingly. Such decreased steering torque is detected by the steering torque sensor  1017  and is input to the microcomputer  1030 . Therefore, the steering assisting command value I M *also decreases and the steering assisting torque to be generated by the electric motor  1005  becomes smaller than the steering assisting torque that is generated in the rest steering state. 
     However, for example, while the vehicle is running, when, in the resolver  1018  and motor rotation angle detect circuit  1032  as well as in the motor rotation angle detect system of the A/D converters  1035 ,  1036 , there occurs an abnormal phenomenon such as breakage, shortcircuit, ground-short or power-short (unintentional shorting to the lower or higher potential point), the sine wave signal (sin ωt+sin θ) and cosine wave signal (sin ω+cos θ) to be input to the microcomputer  1030  from the motor rotation angle detect circuit  1032  become abnormal. The combination of sin θ and cos θ, which are to be calculated in a motor rotation angle calculating processing (not shown) according to the sine wave signal (sin ωt+sin θ) and cosine wave signal (sin ωt+cos θ) as well as the peak detect pulse Pp, becomes abnormal. Thus, when, in the motor abnormality detect processing shown in  FIG. 9 , the abnormality checking map shown in  FIG. 10  is referred to according to sin θ and cos θ, points expressed by sin θ and cos θ are out of the normal area shown by hatched area, so that a fail-safe signal SF of a logical value “1” is immediately output to the angular velocity·angular acceleration operating portion  48 . 
     Accordingly, in the angular velocity·angular acceleration operating portion  1048 , the rotation angle select portion  1048   g  selects the relative rotation angle θee calculated by the adding portion  1048   f , and also the angular velocity select portion  1048   i  selects the relative angular velocity ωee calculated by the relative angle information is offset processing portion  1048   e . At then ti7me, the previous rotation angle θer (n−1) at which the resolver  1018  and the like are normal is supplied to the adding portion  1048   f  as the initial value of the relative rotation angle θee. 
     In this manner, when the rotation angle select portion  1048   g  and angular velocity select portion  1048   i  are switched, in the relative rotation angle calculating processing shown in  FIG. 12  which has been executed before such switching, the rotation angle θe, angular velocity ωe and angular acceleration α are determined by a processing which calculates a relative (rotation) angle according to the counter electromotive force EMF. 
     At the then time, the counter electromotive force operating portion  1046  operates the above-mentioned equations (3)˜(5) to calculate the respective line voltages Vab, Vbc and Vca, next, operates the above equations (6)˜(8) to calculate the respective line counter electromotive forces EMFab, EMFbc and EMFca, and adds them up to calculate the counter electromotive force EMF. 
     In the relative rotation angle calculating processing, the calculated counter electromotive force EMF (Step S 1031 ) is read and, next, the above equation (9) is operated according to the counter electromotive force EMF to calculate the relative angular velocity ωee (Step S 1032 ). 
     Then, the sign of the steering torque Ts detected by the steering torque sensor  1017  is applied to the thus calculated relative angular velocity ωee to thereby calculate the current relative angular velocity ωee (n) having a sign corresponding to the rotation direction of the electric motor  1005 . 
     The calculated current relative angular velocity ωee(n) is subtracted by the previous relative angular velocity ωee(n−1) to calculate a variation amount Δωee (Step S 1034 ). When the absolute value of the variation amount Δωee is equal to or less than the variation amount upper limit value Δωs, the current relative angular velocity ωee (n), as it is, is used as the current value. When the absolute value of the variation amount Δωee is more than the variation amount upper limit value Δωs, it is determined that the variation amount is too large, and thus the variation amount upper limit value Δωs is added to or subtracted from the previous relative angular velocity ωee (n−1) to thereby limit the variation amount of the relative angular velocity ωee. 
     When the calculated relative angular velocity ωee (n) is out of the insensitive zone, the relative angular velocity ωee (n) is determined and is supplied to the adding portion  1048   f  and angular velocity select portion  1048   i.    
     Thus, the adding portion  1048   f  adds the relative angular velocity ωee to the previous rotation angle θer(n−1) to calculate the relative rotation angle θee. 
     The thus calculated relative rotation angle θee is selected by the rotation angle select portion  1048   g  and is output to the current command value calculating portion  1042  as the rotation angle θe. The relative angular velocity ωee (n) is selected by the angular velocity select portion  1048   i  and is output to the current command value calculating portion  1042  as the angular velocity ωe. Further, the angular velocity ωe is differentiated by the angular velocity operating portion  1048   j  to calculate an angular velocity α, and this angular velocity α is also output to the current command value calculating portion  1042 . 
     Accordingly, according to the steering assisting control processing which is shown in  FIG. 11  and is to be executed by the current command value calculating portion  1042 , instead of the actual rotation angle θer which has become abnormal and an actual angular velocity ωer and angular acceleration α which are based on the abnormal actual rotation angle θer, the relative rotation angle θee, relative angular velocity ωee and relative angular acceleration α are applied. A command value compensating processing and a d-q axis command value operating processing, which are respectively based on these pieces of relative angular velocity information, are executed thereby continuing the execution of the steering assisting control processing. 
     In this manner, in a state where the angular velocity·angular acceleration operating portion  1048  is selecting the relative rotation angle θee and relative angular velocity ωee, the relative angular velocity ωee to be calculated by the angular velocity operating portion  1048   a  satisfies −Δω≦ωee≦+Δω and thus goes into the insensitive zone, the processing moves from Step S 1039  shown in  FIG. 12  to Step S 1040  also shown in  FIG. 12 , where a relative angle offset processing is executed on the relative angular velocity ωee. 
     That is, when the relative angular velocity ωee goes into the insensitive zone, in every given time (for example, 20 msec.), there is repeatedly set, as the relative angular velocity ωee, a relative angle information offset value ±Δωd in which an angle variation with respect to the motor relative rotation angle θee is set for ±2 deg. per adding cycle, thereby repeating the operation to set the relative angular velocity ωee for other value than “0”. In this manner, when the relative angular velocity ωee exists in the insensitive zone, as the relative angular velocity ωee, ±Δωd is set, thereby enabling to positively prevent the relative angular velocity ωee from providing “0”. Further, the relative angular velocity ωee set for the relative angle information offset value ±Δωd is added to the previous rotation angle θee (n−1) by the adding portion  48   f , whereby the motor relative rotation angle θee is caused to vary with respect to the previous relative angular velocity θee (n−1) by an angle variation amount of ±2 deg corresponding to the relative angle information offset value ±Δωd of the motor relative rotation angle ωee. 
     That is, in a state where the angle information cannot be used, and also in a state where, for example, the electric motor  1005  generates a high steering assisting force, suppose the relative angular velocity ωee goes into an angular velocity area near to “0”, when the driver steers the steering wheel  1011 , there is a fear that the rotation of the electric motor  1005  is caused to go beyond its limit, resulting in a so called steering (steering wheel) lock. Therefore, in order for the relative angular velocity ωee to exceed the insensitive zone generated before the driver can steer the steering wheel next to obtain a counter electromotive force, the relative angle information offset values ±Δωd, which vary the angle information in the positive and negative direction, are set alternately to thereby be able to positively prevent the relative angular velocity ωee from providing “0”. This makes it possible to continue the steering assisting control processing which is based on the relative angular velocity ωee, while positively preventing the occurrence of the steering lock. 
     Further, when the relative angular velocity ωee is calculated according to the counter electromotive force EMF of the electric motor  1005 , it is difficult to obtain the rotation direction of the electric motor  1005 . For example, although the rotation direction of the electric motor  1005  may be decided according to the state of sight of the phase current of a current and a counter electromotive force respectively to be supplied to the electric motor  1005 , there remains a possibility that a steering assisting force against the intention of the driver can be generated. Therefore, as in the above-mentioned first embodiment, when the motor rotation direction is decided by the direction of the steering torque Ts expressing the direct intention of the driver, it is possible to set a motor rotation direction which corresponds to the intention of the driver. 
     Here, description will be given of the principle that the electric motor  1005  can be driven and rotated according to the relative rotation angle θee. 
     That is,  FIG. 13  shows the relationship of the magnetic vector relative angle difference between the rotor and stator of a brushless motor to the absolute value of energy generated in the rotor. 
     Here, when a fixed current is fed to the stator with the position of the rotor without taking into account of the position of the rotor, as in a “state  1 ” shown in  FIG. 13 , the motor rotates while generating a torque until the magnetic vectors of the rotor and stator coincide with each other. As shown in a “state  2 ”, when the magnetic vectors of the rotor and stator coincide with each other, the rotor is restricted by the magnetic field of the stator and is thereby unable to move (motor torque=0 Nm). That is, there is provided a state where the d axis is perfectly energized. In the “state  2 ”, when the phase current is varied according to the desired rotation direction and rotation speed of the motor, the rotor is rotated while the rotor being restricted by the magnetic field of the stator. The restricting force at the then time is proportional to the phase current. 
     In this manner, according to the first embodiment, when there is generated an abnormality in the rotation angle detect system which detects the rotation angle of the electric motor  1005 , the relative angular velocity ωee is calculated according to the counter electromotive force EMF to calculate the relative rotation angle θee and the relative angular velocity α corresponding to the steering amount of the driver with respect to the steering system, thereby being able to continue the steering assisting control. 
     However, in order to enhance the safety of the steering assisting control further, when there is generated an abnormality in the rotation angle detect system, preferably, by lowering the sensitivity of the steering assisting control, the steering assisting control may be continued. In this case, preferably, the gradual change control portion  1041  and current output limit portion  1043  may be disposed respectively before and behind the current command value calculating portion  1042  in the above-mentioned function block diagram shown in  FIG. 4 . Specifically, when a fail-safe signal SF input from the fail-safe processing portion  1049  has a logical value “0”, the gradual change function of the gradual change control portion  1041  and the limit function of the current output limit portion  1043  may be stopped. Contrary, when the logical value is “1”, the gradual change function and limit function may be fulfilled to limit the current command values Ia*˜Ic*, thereby lowering the steering assisting force that is generated by the electric motor  1005 . 
     Also, in the command value compensating portion  1042 B, too, although the compensating portion is necessary when the rotation angle detect system is normal, one compensating portion which unnecessarily reacts when using the relative rotation angle θee based on the relative angle velocity ωee caused by the counter electromotive force, it is preferable for the one compensating portion to set the output as “0” or be multiplied with a gain less than “1” to provide a value smaller than a value in the normal state, thereby reducing the influence of the compensating portion. On the other hand, one compensating portion which is required to further compensation preferably employs a larger compensation value rather than the normal state. Specifically, the compensation control based on the angle information (the compensation control based on the position of the motor) should be stopped. However, the compensation control, which has the effect to reduce a difference between the relative angle and actual angle, or increases or decreases the gain when compared with the normal state to thereby provide the effect to reduce the difference between the relative angle and actual angle, should be carried out. In other words, the compensation control, which can provide an element to increase the difference between the relative angle and actual angle, should be stopped. 
     Further, in the first embodiment, since the line counter electromotive forces EMFab˜EMFca are calculated according to the line voltages Vab˜Vca and they are added to calculate the motor counter electromotive force EMF, the relative angular velocity ωee can be calculated from the counter electromotive force without depending on the connection of the electric motor  1005  (that is, the Y connection or Δ connection) and, at the same time, the counter electromotive force can be advantageously detected without providing a detect circuit separately. 
     Here, in the first embodiment, description has been given of a case where the line counter electromotive forces EMFab˜EMFca are calculated using the line voltages Vac˜Vca according to the equations (6) (8) and they are added to calculate the motor counter electromotive force EMF. However, the invention is not limited to this embodiment. For example, when a Y connection motor is used as the electric motor  1005 , the mid-point voltage of the electric motor may be detected, the mid-point voltage may be subtracted from the respective motor terminal voltages Va˜Vc to calculate a phase voltage Vih (i=a˜c), the following equation (10) is operated according to the phase voltage Vih to calculate the respective counter electromotive forces ei, and the following equation (11) may be operated according to the calculated counter electromotive forces ea˜ec to thereby calculate the relative angular velocity ωee. In this case, a difference amount between the relative angular velocity ωee and actual angle can be reduced without increasing an operation load and thus, while omitting the rotation angle detect system, the steering assisting control processing can be carried out accurately according to the relative angular velocity ωee. 
         ei=Vih −( Ri+s·Li )· Ii   (10) 
       ω ee= 2×{max(| ea|, |eb|, |ec |)}/Ke  (11) 
     In this case, since the mid-point voltage is ½ of a motor drive circuit application voltage, instead of detecting the mid-point voltage, a ½ value of the motor drive circuit application voltage may be used as a mid-point voltage Vn to thereby calculate phase voltages Vah˜Vch. 
     Further, as shown in the following equation (12), the sum of the respective motor terminal voltages of the electric motor  5  may be found and, while a value obtained by dividing the sum value by the number of the phases of the motor is used as the mid-point voltage Vn, the respective phase voltages may be calculated. 
         Vn =( Va+Vb+Vc+ . . . +Vx )÷number of motor phase  (12) 
     Further, in the first embodiment, description has been given of a case in which the relative angle information offset processing is carried out when the relative angular velocity ωee operated by the relative angular velocity operating portion  1048   a  exists within the insensitive zone. However, the invention is not limited to this embodiment. However, whether the relative angular velocity ωee exists within the insensitive zone or not, the relative angle information offset processing may be always carried out. Also, in this case, when the relative angular velocity ωee is out of the insensitive zone, the relative angle information offset value may be decreased; and, the relative angular velocity ωee exists within the insensitive zone, the relative angle information offset value may be increased. Further, the relative angle information offset value is not limited to the value corresponding to ±2 deg but may be set such that it can exceed the insensitive zone which extends from the time of the speed  0  to the time of the next steering operation. However, when the relative angle information offset value is increased or the adding cycle is increased or both of the two operations are carried out, there is a possibility that the electric motor  1005  can be vibrated. Therefore, when the rotation angle detect system is found abnormal, in order to inform the driver of the abnormal state for recovery thereof, the relative angle information offset value may be increased or the adding cycle may be increased or both of the two operations may be carried out, thereby causing the steering wheel  1011  to vibrate. In this case, the vibration of the steering wheel  1011  may also be increased step by step within the range of the maximum value with the passage of the time from the occurrence of the abnormal state. Further, when the relative angle information offset processing is carried out all the time, the electric motor is also allowed to generate a control sound; and thus, when the relative angle information offset value and the adding cycle are caused to vary to a level where they can be used as an abnormality occurrence notice, they can also be employed as means for notifying the driver of the repair requiring state of the rotation angle detect system. 
     Further, in the first embodiment, description has been given of a case where the relative angle information offset value ±Δωd is set in the relative angular velocity ωee. However, the invention is not limited to this embodiment. However, alternatively, a relative angle information offset value corresponding to the relative angle information offset value ±Δωd may be added to or subtracted from a motor rotation angle θee which is calculated according to the relative angular velocity ωee. 
     Further, in the first embodiment, description has been given of a case where, in a state where the relative angular velocity ωee is in the insensitive zone from the 0 speed area, the relative angular velocity ωee is set in the relative angle information offset value ±Δωd. However, the invention is not limited to this embodiment. Alternatively, for example, as the coil resistances Ra˜Rc to be set by an insensitive zone setting portion (not shown) within the counter electromotive force operating portion  1046 , there may be employed resistance is model values instead of actual resistance values to thereby intentionally reduce the width of an insensitive zone for removing the error of the relative angular velocity ωee; and, information normally to be ignored as angle information may be used intentionally for control. In this case, it is possible to set a value which corresponds to the relative angle information offset value ±Δωd. 
     Still further, in the first embodiment, description has been given of a case where the rotation direction is determined according to the steering torque. However, the invention is not limited to this embodiment. When the reliability of information can be secured, for example, the rotation direction may be preferably determined according to different kinds of information supplied from two or more kinds of information sources, for example, the steering rotation direction information supplied from the steering angle sensor, the state of the phase current and the sight of the counter electromotive force. 
     Still further, in the first embodiment, description has been given of a case where a relative angular velocity ωee is calculated according to the counter electromotive force EMF and this relative angular velocity ωee is added to the previous motor rotation angle θee (n−1) to thereby calculate the motor rotation angle θee. However, since the line counter electromotive forces EMFab, EMFbc and EMFca provide sine waves, when the 0 crossing point of these line counter electromotive forces EMFab, EMFbc and EMFca is detected and, a motor rotation angle θee, which is uniquely determined at the time when the 0 crossing point is detected, is used to correct the motor rotation angle θee in such a manner as shown in the below-mentioned table 1, a more accurate motor rotation angle θee can be calculated. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 0 crossing condition and 
               
               
                   
                 angle (electric angle) 
               
            
           
           
               
               
               
            
               
                 Target 
                 − → + direction 
                 + → − direction 
               
               
                   
               
               
                 0 crossing point of A-B line 
                 180 deg 
                 360 deg (0 deg) 
               
               
                 counter electromotive force EMFab 
               
               
                 0 crossing point of B-C line 
                 120 deg 
                 300 deg 
               
               
                 counter electromotive force EMFbc 
               
               
                 0 crossing point of C-A line 
                  60 deg 
                 240 deg 
               
               
                 counter electromotive force EMFca 
               
               
                   
               
            
           
         
       
     
     Also, in the first embodiment, description has been given of a case where, as the initial value of the relative rotation angle θee, the previous relative rotation angle θer (n−1) where the resolver  1018  and the like were normal is supplied to the adding portion  48   f . However, the invention is not limited to this embodiment. As described above, since the motor can be positively driven according to the relative angle, as the initial value, an arbitrary rotation angle θer can be set. Therefore, until the motor rotation angle is found abnormal, the relative angle may not be calculated; and, when the motor rotation angle is found abnormal or when the sign of the abnormality of the motor rotation angle is obtained, the calculation of the relative angle may be started at the then time and, according to the thus calculated relative angle, the motor may be driven. In this case, the processing load of the operation processing apparatus can be reduced. 
     Further, in the first embodiment, description has been given of a case where the relative speed ωee as the relative angle information is calculated according to the counter electromotive force EMF. However, the invention is not limited to this embodiment. The relative speed ωee may also be calculated according to the angle variation amount of the steering angle that is obtained from the steering angle sensor. Further, in a dual fault state where the counter electromotive force cannot be obtained, the relative angle θee may be calculated directly instead of the steering amount obtained from the steering angle sensor. 
     Second Embodiment 
     Next, description will be given below of a second embodiment according to the invention with reference to  FIGS. 14˜16 . 
     In the second embodiment, there is provided a compensating relative angle information operating portion which, when the relative speed ωee is in a “0” angular velocity area, instead of executing a relative angle information offset processing to compensate the relative angle, calculates a compensating relative angular velocity ωee′ according to the steering torque Ts. 
     That is, in the second embodiment, as shown in  FIG. 14 , there is provided a compensating relative angle information operating portion  1070  which calculates a compensating relative angular velocity ωee′ according to the steering torque Ts detected by the steering torque sensor  1017 . The second embodiment has a similar structure to the previously described first embodiment, except that the compensating relative angular velocity ωee′ operated by the compensating relative angle information operating portion  1070  is supplied to the angular velocity·angular acceleration operating portion  1048 . 
     Here, the compensating relative angle information operating portion  1070  executes a compensating relative angle calculating processing shown in  FIG. 15 . This compensating relative angle calculating processing is executed as a timer interrupt processing in every given time (for example, 1 msec.). Firstly, in Step S 1051 , there is read in the steering torque Ts calculated according to the steering assisting control processing and, after then, the processing goes to Step S 1052 . 
     In this step S 1052 , there is executed an averaging processing which calculates the average value T S     M    of the steering torques Ts corresponding to a given number of previous steering torques including the thus read steering torque Ts and, after then, the processing goes to Step S 1053 . 
     In this step S 1053 , it is checked whether the steering torque average value T S     M    calculated in the above step S 1052  is within a previously set insensitive zone in the steering assisting control, that is, the insensitive zone of the electrically-operated power steering mechanism including a mechanical insensitive zone set due to the reduction gear efficiency, a rack and pinion efficiency and the like of the electric power steering mechanism, or not. When it is found that the average value T S     M    is within the insensitive zone, the processing goes to Step S 1054 , where the steering torque average value T S     M    is changed to “0” and, after then, the processing goes to Step S 1055 . Also, when the average value T S     M    is out of the insensitive zone, the processing as it is goes to Step S 1055 . 
     In Step S 1055 , it is checked whether a variation amount ΔT between the current steering torque average value T S     M    (n), which is composed of the steering torque average value T S     M    calculated in Step S 1052  or the steering torque average value T S     M    changed in Step S 1054 , and the steering torque average value T S     M   (n−1) at the previous sampling exceeds a previously set upper limit value ΔT U  or not. When ΔT&gt;ΔT U , it is determined that the variation amount ΔT is too larger and the processing then goes to Step S 1056 , where a value obtained by adding the upper limit value ΔT U  to the previous steering torque average value T S     M   (n−1) is set as the current steering torque average value T S     M    (n) and, after then, the processing goes to Step S 1057 . When ΔT≦ΔT U , it is determined that the variation amount ΔT is within an allowable range, and the processing as it is goes to Step S 1057 . 
     The processing to be executed in Steps S 1055  and S 1056  are respectively a limit processing for limiting the variation amount ΔT and, in this case, the upper limit value ΔT U  may be a given value or may be an optimum value according to the vehicle speed Vs. 
     In Step S 1057 , there is operated the following equation (13) to calculate a motor relative angle variation Δθ M  and, after then, the processing goes to Step S 1058 . 
       Δθ M   =T   S     M   ( n )· Km/ 2 12   (13) 
     Here, Km expresses a relative angle information gain. 
     In Step S 1058 , the motor relative angle variation Δθ M  calculated in Step S 1057  is added to a motor relative angle θ MP (n−1) calculated in the previous sampling to calculate a current motor relative angle θ MP (n) and, after then, the processing goes to Step S 1059 . 
     In Step S 1059 , the motor relative angle θ MP (n) is converted to, for example, electric angles  0 ˜ 4096  of 12 bits and these electric angles  0 ˜ 4096  are stored into a given storage area of a RAM incorporated in the microcomputer  1030 . After then, the timer interrupt processing is ended. 
     Here, the processing shown in  FIG. 15  and the select portion  1048   n  of the angular velocity·angular acceleration operating portion  1048  correspond to the relative angle information compensating portion. 
       FIG. 16  is a function block diagram of the compensating relative angle calculating processing shown in  FIG. 15 . 
     Here, although the motor relative angle information calculating gain Km may be a constant value, it may also be changed according to the vehicle speed Vs. For this purpose, there may also be provided parameter setting means for varying parameters such as the gain of the motor relative angle information operating gain that can be used to adjust the advancing angle of the motor according to the vehicle speed Vs. 
     It is desirable to change the relative angle information calculating gain Km for adjusting the motor advancing angle according to a limit amount of output of the steering assist force when the output of the steering assist force in the steering assist control process is limited, such as when the electric motor  1005  or control unit  1003  is overheated and the feed amount of power to the electric motor  1005  is reduced for preventing further increase of temperature, or when the vehicle speed sensor  1033  becomes abnormal and a fixed vehicle speed is set to continue the steering assisting control (for example, the gradient in the steering assist command value calculating map shown in  FIG. 5  decreases as the vehicle speed increases and the steering assisting force is limited to a small value in a vehicle speed area where the vehicle speed is slower than the fixed vehicle speed. For this reason, there may also be provided second parameter setting means which is used to vary a parameter capable of adjusting the motor advancing angle such as the gain of the compensating relative angle information operation according to the output limit amount in the output limit state of the steering assisting force. 
     The angular velocity angular acceleration operating portion  1048  is changed as shown in  FIG. 17 . That is, except for following configurations, the configuration of the second embodiment is the same as that of shown in  FIG. 8 . The different configurations are: 
     the relative angle information offset processing portion  1048   e  of the angular velocity·angular acceleration operating portion  1048  is omitted, and instead of this, the relative angular velocity ωee limited by the rate limit portion  1048   d  is supplied directly to the adding portion  1048   f  and is also supplied to the insensitive zone detect portion  1048   m;    
     the output of the adding portion  1048   f  is supplied to one input side of the second rotation angle select portion  1048   n  which can be switched by the detect signal of the insensitive zone detect portion  1048   m;    
     the relative rotation angle θee′ calculated by the compensating relative angle information operating portion  1070  is supplied to the other input side of the second rotation angle select portion  1048   n;    
     the relative rotation angle θee selected by the second rotation angle select portion  1048   n  is supplied to one input side of the rotation angle select portion  1048   g;    
     the angular velocity limited by the rate limit portion  1048   d  is supplied to one input side of the second rotation angle select portion  1048   p ; and 
     the output of an angular velocity operating portion  1048   o  for differentiating the relative rotation angle ωee′ calculated by the compensating relative angle information operating portion  1070  to calculate the compensating angular velocity ωee′ is supplied to the other input side of the second rotation angle select portion  1048   p.    
     Here, the insensitive zone detect portion  1048   m  outputs a detect signal SD of a logical value “0” to the second rotation angle select portion  1048   n  when the relative angular velocity ωee is out of the insensitive zone, and, outputs a detect signal SD of a logical value “1” when the relative angular velocity ωee is in the insensitive zone to the second angular velocity select portion  1048   o.    
     The second rotation angle select portion  1048   n  selects a relative rotation angle θee which is output from the adding portion  1048   f  when the detect signal SD is a logical value of “0”, and, selects the compensating relative rotation angle θee′ calculated by the compensating relative angle information operating portion  1070  when the detect signal SD is a logical value of “1”. 
     The second angular velocity select portion  1048   o  selects the relative angular velocity ωee when the detect signal SD is a logical value of “0”, and, selects the compensating relative angular velocity ωee′ operated by the second angular velocity select portion  1048   o  when the detect signal SD is a logical value of “1”. 
     Next, description will be given below of the operation of the second embodiment. 
     Firstly, in the compensating relative angle information operating portion  1070 , the compensating relative angle calculating processing shown in  FIG. 15  is executed, the steering torque Ts is read every given time according to a timer interrupt processing and, next, an averaging processing is executed on a given number of past steering torques Ts (n)˜Ts (n−31) including a currently read steering torque Ts to calculate a steering torque average value T S     M    (n) (Step S 1052 ). Execution of the averaging processing can positively prevent variations in a numeral LSB, which are caused when a steering torque T output from the steering torque sensor  1017  is converted to a digital signal by the A/D converter  1031 , from being used as a noise component. 
     Further, it is checked whether the calculated steering torque average value T S     M   (n) is within the insensitive zone or not (Step S 1053 ). When it is within the insensitive zone, the calculated steering torque average value T S     M   (n) is set for “0” (Step S 1054 ), thereby positively preventing the electric motor  1005  from being driven and rotated carelessly when the driver does not intend. 
     On the other hand, when the calculated steering torque average value T S     M   (n) is out of the insensitive zone, the processing as it is goes to Step S 1055 . 
     Further, a variation amount ΔT of the calculated steering torque average value T S     M    (n) with respect to the steering torque average value T S     M    (n−1) is calculated in the previous sampling operation, and it is checked whether the calculated variation amount ΔT exceeds a previously set upper limit value ΔT U  or not. When ΔT&gt;ΔT U , it is judged that the variation amount is too large, and a value obtained by adding the upper limit value ΔT U  to the steering torque average value T S     M   (n−1) in the previous sampling operation is set as the current steering torque average value T S     M   (n) (Step S 56 ). When ΔT≦ΔT U , there is used the current steering torque average value T S     M   (n) as it is. Owing to the processing executed in Steps S 1055  and S 1056 , when the variation amount of the steering torque average value T S     M    (n) is large, the variation amount is limited to the upper limit value ΔT U  to thereby enable to limit the sudden change of the compensating relative rotation angle θ MP  when the steering torque T rises suddenly. 
     Further, the set relative rotation angle variation amount Δθ M  is added to the compensating relative rotation angle θ MP  (n−1) in the previous sampling operation to thereby calculate the current compensating relative rotation angle θ MP , the compensating relative rotation angle θ MP  is converted to the electric angles ( 0 ˜ 4096 ) of 12 bits, and these electric angles  0 ˜ 4096  are updated and stored into a given storage area of a RAM incorporated in the microcomputer  1030 . 
     Accordingly, in the angular velocity·angular acceleration operating portion  1048  shown in  FIG. 17 , when the motor rotation angle detect system is normal, similarly to the above described first embodiment, the rotation angle select portion  1048   g  selects an actual rotation angle θer operated by the motor rotation angle operating portion  1047 ; the angular velocity select portion  1048   i  selects an actual angular velocity ωer calculated by the angular velocity operating portion  1048   h , and the angular velocity operating portion  1048   j  differentiates the selected actual angular velocity ωer to calculate an angular acceleration α. The actual rotation angle θer, actual angular velocity ωer and angular acceleration α are supplied to the current command value calculating portion  1042  so as to calculate accurate phase target currents Ia*˜Ic* based on them; calculates differences ΔIa˜ΔIc between the phase target currents Ia*˜Ic* and current detect values Ia˜Ic; a PI control processing is executed on the differences ΔIa˜ΔIc to calculate voltage command values Va*˜Vc*; and, these voltage command values Va*˜Vc* are output to the FET gate drive circuit  1022  of the motor drive circuit  1006 , whereby a 3-phase drive current is supplied to the electric motor  1005  to generate a steering assisting force. 
     Further, when the occurrence of the abnormality of the rotation angle detect system including the resolver  1018  is detected by the fail-safe processing portion  1049 , the fail-safe processing portion  1049  outputs a fail-safe signal SF of a logical value “1” to the rotation angle select portion  1048   g  and angular velocity select portion  1048   i , whereby, similarly to the first embodiment, a relative angular velocity ωee according to the counter electromotive force EMF is selected. 
     Further, when the calculated relative angular velocity ωee is out of the insensitive zone, the insensitive zone detect portion  1048   m  outputs an insensitive zone detect signal SD of a logical value “0” to a second rotation angle select portion  1048   m  and a second angular velocity select portion  1048   p . The second rotation angle select portion  1048   n  selects the relative rotation angle θee calculated by the adding portion  1048   f , and the second angular velocity select portion  1048   p  selects the relative angular velocity ωee output from the rate limit portion  1048   d . According to the relative rotation angle θee, relative angular velocity ωee and relative angular acceleration α respectively calculated according to the counter electromotive force EMF, the current instruction calculating portion  1042  calculates the 3-phase current command values Ia*˜Ic*. The electric motor  1005  is driven and controlled and a steering assisting force is thus generated from the electric motor  1005 , thereby enabling to continue the execution of the steering assisting control processing. 
     In the continuing state of the steering assisting control processing, when the relative angular velocity ωee to be calculated by the relative angular velocity operating portion  1048   a  comes into the insensitive zone. The insensitive zone detect portion  1048   m  outputs a detect signal SD of a logical value “1” to the second rotation angle select portion  1048   m  and second angular velocity select portion  1048   p . Thus, the second rotation angle select portion  1048   m  selects a compensating relative rotation angle θ MP  calculated by the compensating relative angle information operating portion  1070  and stored in the RAM of the microcomputer  1030  and, at the same time, the second angular velocity select portion  1048   p  selects a relative angular velocity ωee′ which is obtained by differentiating a compensating relative rotation angle θee′ (=θ MP ) in the angular velocity operating portion  1048   o.    
     Accordingly, when the relative angular velocity ωee calculated according to the counter electromotive force EMF is in the insensitive zone near to the “0” angular velocity, there are selected the compensating relative rotation angle θ MP  calculated according to the steering torque Ts calculated by the compensating relative angle information operating portion  1070  and the relative angular velocity ωee′; and, a relative angular acceleration α, which is the differentiated value of the relative angular velocity ωee′, is output to the command value calculating portion  1042 , whereby, similarly to the first embodiment, while preventing the occurrence of the steering (steering wheel) lock, the steering assisting control processing can be continued according to the compensating relative rotation angle θ MP , compensating relative angular velocity ωee′ and relative angular acceleration α. 
     Here, in the second embodiment, description has been given of a case where, when the relative angular velocity ωee is in the insensitive zone, the compensating relative angular velocity ωee′ which is calculated according to the steering torque Ts is selected. However, the invention is not limited to this embodiment. That is, there is a possibility that the relative angle θee can vary suddenly when the steering torque Ts is large while the relative angular velocity ωee is in the insensitive zone. Thus, when the steering torque Ts is large while the relative angular velocity ωee is in the insensitive zone, preferably, the compensating relative rotation angle θee′ may be calculated according to the variation amount of the steering torque Ts. 
     Also, in the second embodiment, description has been given of a case in which, when the relative angular velocity ωee calculated according to the counter electromotive is in the insensitive zone, there are applied the compensating relative rotation angle θee′ and compensating relative angular velocity ωee′ which are calculated based on the steering torque T s . However, the invention is not limited to this embodiment. For example, when the relative angular velocity ωee calculated according to the counter electromotive comes into the insensitive zone, whether the steering assisting force generated in the electric motor  1005  is small or not may be determined by checking, for example, whether the steering assisting torque command value I M *calculated in the command value calculating portion  1042  is large or small. When the steering assisting force is small, the steering assisting control processing may be continued according to the relative angular velocity ωee, relative rotation angle θee and relative angular acceleration α respectively calculated according to the counter electromotive force EMF. When the steering assisting force is large, the steering assisting control processing may be continued according to the compensating relative rotation angle θee′, compensating relative angular velocity ωee′ and relative angular acceleration α which are respectively calculated according to the steering torque. 
     Further, in the second embodiment, description has been given of a case in which whether the relative angular velocity ωee is the 0 speed area or not is checked according to whether the relative angular velocity ωee is in the insensitive zone or not. However, the invention is not limited to this embodiment but, for example, even when the angle information ωee obtained according to the counter electromotive force is in inaccurate area (for example, even when the 0 crossing point of the counter electromotive force EMF cannot be determined even if the relative angular velocity ωee comes into the insensitive zone), there may also be selected the compensating relative angular velocity ωee′. 
     Still further, in the second embodiment, description has been given of a case in which, as a correcting value, Δθ M  is set for “0”. However, the invention is not limited to this embodiment but Δθ M  may also be set for a value which can restrict an increase amount of the relative rotation angle θ MP (n) to a very small value. 
     Furthermore, in the second embodiment, description has been given of a case in which the relative rotation angle variation amount Δθ M  is calculated using the steering torque average value T S     M    which is obtained by averaging the steering torques Ts. However, the invention is not limited to this embodiment but the relative rotation angle variation amount Δθ M  may also be calculated while using the steering torques Ts themselves as input values. In short, there can be applied any arbitrary value, provided that it is calculated according to the steering torque Ts. 
     Third Embodiment 
     Next, description will be given below of a third embodiment according to the invention with reference to  FIG. 18 . 
     In the previously described first and second embodiments, since, to calculate the relative angle information of the brushless motor corresponding to the steering amount of the driver, the relative angular velocity constituting the relative angle information is calculated according to the counter electromotive force of the brushless motor, when the counter electromotive force of the brushless motor cannot be detected normally, the relative angle information cannot be obtained; and, therefore, the steering assisting control must be stopped. In view of this, in the third embodiment, there is provided a structure which is able to continue the steering assisting control, even when the counter electromotive force of the brushless motor cannot be detected normally. 
     In the third embodiment, the microcomputer  1030  carries out a relative angle operating processing shown in  FIG. 18 . 
     This relative angle operating processing is executed as a timer interrupt processing in every given time (for example, 10 msec.). Firstly, in Step S 1081 , it is checked whether a motor rotation angle θer to be detected by the resolver  1018  and motor rotation angle operating portion  1047  is normal or not. In this check, a fail-safe signal SF to be output in the above-mentioned motor rotation angle abnormality detect processing shown in  FIG. 9  is read, and the fail-safe signal SF is checked as to whether it is has a logical value “0” or not. 
     When the check result of this step S 1081  shows that the motor rotation angle θer is normal, the processing goes to Step S 1082 , where, using the motor rotation angle θer, the motor angular velocity ωe and angular acceleration α are calculated, after then, the timer interrupt processing is ended, and the processing goes back to a given main program. When the motor rotation angle θer is not normal, the processing goes to Step S 83 . 
     In Step S 1083 , it is checked whether the relative angle information corresponding to the steering amount of the driver can be calculated normally or not. Whether the calculation of the relative angle information is normal or not is determined by checking whether, for example, the motor terminal voltage detected by the motor terminal voltage detect portion  1008  is normal or not. When the calculation of the relative angle information is normal, the processing goes to Step S 1084 , where a relative angle information detect processing similar to the first embodiment is executed, after then, the timer interrupt processing is ended, and the processing goes back to a given main program. When the calculation of the relative angle information is abnormal, the processing goes to Step S 1085 , where a relative angle information detect processing similar to the compensating relative angle information detect processing shown in  FIG. 15  to be executed in the compensating relative angle information operating portion  1070  in the second embodiment is executed, after then, the timer interrupt processing is ended, and the processing goes back to a given main program. 
     According to the third embodiment, when the motor rotation angle detect portion having the resolver  1018  and motor rotation angle operating portion  1047  is normal, the processing goes to Step S 1082 , where there are calculated the motor angular velocity θe and angular acceleration α using the motor rotation angle θer detected in the motor rotation angle detect portion. When the motor rotation angle detect portion is abnormal, it is checked whether the detection of the motor terminal voltage is normal or not. When the detection of the motor terminal voltage is normal, the processing goes to Step S 1084 , where there is executed the relative angle information detect processing according to the first embodiment to thereby calculate the relative angular velocity ωee, relative rotation angle θee and relative angular acceleration α. 
     However, when the detection of the motor terminal voltage is abnormal, since it is impossible to operate accurately the relative angle information according to the first embodiment, the processing goes to Step S 1085 , where, as the relative angle information detect processing, there is executed the compensating angle information operating processing of  FIG. 15  to be executed by the compensating angle information operating portion  1070  according to the second embodiment, thereby calculating the relative rotation angle θ MP  according to the steering torque Ts without using the counter electromotive force EMF; and, using the calculated relative rotation angle θ MP , there are calculated the relative angular velocity ωee and relative angular acceleration α. 
     In this manner, according to the third embodiment, since the calculation of the relative angle information can be carried out in two stages, even when the relative angle information including the relative angular velocity ωee, relative rotation angle θee and relative angular acceleration α cannot be calculated based on the counter electromotive force EMF according to the first embodiment, according to the steering torque Ts, there can be calculated relative angle information which includes the relative rotation angle θee, relative angular velocity ωee and relative angular acceleration α. Owing to this, a range, where the continuation of the steering assisting control in the occurrence of an abnormal state is possible, can be widened, whereby the steering assisting control can be carried out more positively. 
     In the third embodiment, description has been given of a case where, when the detection of the terminal voltage of the motor is normal, there is carried out the relative angle information detect processing similar to the previously described first embodiment. However, the invention is not limited to this embodiment but there may also be carried out the relative angle information detect processing similar to the second embodiment. 
     Also, in the third embodiment, description has been given of a case where, when the relative angle operating processing according to the counter electromotive force cannot be executed, there is executed the relative angle operating processing according to the steering torque. However, the invention is not limited to this embodiment. For example, when the relative angle operating processing according to the counter electromotive force cannot be executed, using a steering angle sensor used in other processing, there may be calculated a relative angle according to the angle variation amount of a steering angle obtained from the steering angle sensor. Also, when the relative angle operating processing according to the steering angle cannot be carried out, there may be carried out the relative angle operating processing according to the steering torque. Further, the combination of these three steering angle operating processing may be decided according to the rate of failures. 
     Also, in the first through third embodiments, description has been given of a case where the abnormality of the motor rotation angle detect system including the resolver  1018  and motor rotation angle detect circuit  1032  is detected according to sin θ and cos θ with reference to the abnormality checking map. However, the invention is not limited to this embodiment. For example, when the sin θ and the cos θ system are short circuited, the shortcircuit may be detected using the relationship that sin 2  θ and cos 2  θ=1. In this case, the amplitudes of sin θ and cos θ vary in a given range while taking the same value; and, when the electric angle is 45°, they are the maximum and, when the electric angle is 225°, they are the minimum. Therefore, while monitoring the peaks of the maximum and minimum, the relative rotation angle θee may be corrected to the electric angles 45° and 225°. 
     Also, in the first through third embodiments, description has been given of a case where sin θ and cos θ are calculated according to the motor rotation angle calculating processing to be executed by the microcomputer  1030 . However, the invention is not limited to this embodiment but sin θ and cos θ may also be calculated within the motor rotation angle detect circuit  1032 . 
     Further, as the motor rotation angle detect means, instead of the resolver  1018 , there may also be employed another structure. For example, as disclosed in Japanese Patent Unexamined Publication JP-A-2004-20548, a permanent magnet constituting an encoder within the bearing of the electric motor  1005  is magnetized by a virtual plane passing through the center of the permanent magnet in such a manner that the south pole and north pole are equally divided in two, two magnetic sensors are disposed at positions which are opposed to the south and north poles of the encoder and are 90° out of phase with respect to them, and there may be applied a rotation state detect device which outputs sin θ and cos θ from these magnetic sensors. In this manner, when sin θ and cos θ are output, for example, as voltages, from the motor rotation angle detect means, the microcomputer  1030  is used to check whether the amplitudes of sin θ and cos θ are in the previously set range or not. When any one of the amplitudes of sin θ and cos θ is out of the previously set range, it may be determined that an abnormality of ground-short or power-short has occurred in the sin θ or cos θ system. In this case, suppose the cos θ system is normal, in the sin θ and cos θ coordinate system, an angle where cos θ provides the maximum value is 0°, an angle where cos θ provides the minimum value is 180°, and an angle where cos θ provides a central value is 90° or 270°. Using this, when cos θ provides the maximum value, the relative rotation angle θee or relative angular velocity ωee may be corrected to “0”. In this case, the maximum value may be detected using a peak detect processing or a peak detect circuit. When a peak value is previously known, the maximum value can be detected by checking whether the maximum value has reached the peak value or not. Further, when the peak value is influenced by temperatures and the like, for example, for the peak values of 0° and 180°, values just before the abnormality occurs may be set for the peak values. 
     In this manner, when any one of sin θ and cos θ is abnormal, the peak value of a normal signal is monitored and an angle where the peak value is provided can be employed as the correction value of the relative rotation angle θee. 
     Also, when, as the rotation position detect means of the motor, as shown in  FIG. 19 , there are applied three pole position sensors  1101   a ,  1101   b  and  1101   c  such as Hall sensors which are used in an ordinary 3-phase brushless motor to detect the pole positions of the a phase, b phase and c phase thereof, since phase detect signals Sa, Sb and Sc output from these pole position sensors  1101   a ,  1101   b  and  1101   c  have a phase difference of 120° as shown in  FIG. 20 , it is possible to detect one pole position sensor  1101   i  (i=a, b, c), which has become abnormal, according to these phase detect signals Sa, Sb and Sc. 
     That is, when an electrically energizing state is expressed by the turn on and off of the respective phase detect signals Sa, Sb and Sc, the electrically energizing states are repeated in such a manner as shown in the lowermost stage in  FIG. 20 , specifically, 1 to 6. 
     In this state, for example, when the a-phase pole position sensor  1101   a  is fixed at a high level, as shown in  FIG. 21 , in the electrically energizing state to be expressed by the on and off, the energizing states “ 4 ”, “ 5 ” and “ 6 ” and the energizing state “ 7 ”, where a new a-phase detect signal Sa, a new b-phase detect signal Sb and a new c-phase detect signal Sc respectively provide a high level, are repeated in a given sequence; and, at the time when the energizing state “ 7 ” appears, an abnormality can be detected. However, in the range of 0°˜180° where the a-phase detect signal Sa originally provides a high level, the repeating pattern is similar to that in a normal state. Here, the energizing state “ 4 ” is a unique energizing state which appears only once in the range of 0°˜360°; and, in this energizing state “ 4 ”, specifically, in the edge portion thereof that provides the energizing state “ 5 ” or “ 6 ”, the angle can be read accurately. Similarly, when the b-phase detect signal Sb and c-phase detect signal Sc are respectively fixed at a high level, there are present energizing states “ 2 ” and “ 1 ” where the angle can be detected uniquely and also, similarly, there are present points where the angle can be recognized accurately. 
     Also, when the a-phase detect signal Sa is fixed at a low level, similarly, as shown in  FIG. 22 , in the range (180°˜360°) where the a-phase detect signal Sa originally provides a low level, the angle can be detected normally; and, the portion of an energizing state “ 3 ” is unique in the range 0°˜360°, and in the edge portions thereof where the energizing state “ 3 ” provide the energizing portions “ 2 ” and “ 1 ”, the angle can be read accurately. Further, when the b-phase detect signal Sb and c-phase detect signal Sc are respectively fixed at a high level, there are present the energizing states “ 5 ” and “ 6 ”, as areas where the angle can be detected uniquely and, similarly, there are present points where the angle can be recognized accurately. 
     As described above, the abnormality of the pole position sensors  1101   a ˜ 1101   c  for detecting the rotation of a body of rotation including the motor pole position can be recognized by detecting the energizing state “ 7 ” or “ 0 ”. Specifically, when the abnormality is detected by the energizing state “ 7 ”, the energizing states “ 1 ”, “ 2 ” and “ 4 ” may be recognized, whereby the angle can be corrected accurately in the switching edge portions of these energizing states. When the abnormality is detected by the energizing state “ 0 ”, the energizing states “ 3 ”, “ 5 ” and “ 6 ” may be recognized, whereby the angle can be corrected accurately in the switching edge portions of these energizing states. 
     Therefore, when there is applied the rotation state detect device for outputting sin θ and cos θ, and when there are used the pole position sensors  1101   a ˜ 1101   c , there are present the points that are capable of accurately recognizing the actual angle, whereby there can be carried out, for example, a relative angle correct processing shown in  FIG. 23 . According to the relative angle correct processing, in Step S 1091 , there is read in a counter electromotive force EMF and then the processing goes to Step S 1092 , where it is checked whether the relative angle is in a correction requiring state or not. In this operation to check whether the correction is necessary or not, when the value of the counter electromotive force EMF or the variation amount ΔEMF thereof is small, it is determined that the correction is not necessary and thus the timer interrupt processing is ended as it is. When the value of the counter electromotive force EMF or the variation amount ΔEMF thereof is large, it is determined that the correction is necessary, and thus the processing goes to Step S 1093 , where it is checked whether the actual angle can be recognized from the detect signals from the rotation state detect device or pole position sensors  1101   a ˜ 1101   c . When the actual angle cannot be recognized, the processing waits until the actual angle can be recognized; and, when the actual angle can be recognized, the processing goes to Step S 1094 , where the actual angle information is set as the relative angle θee and, after then, the timer interrupt processing is ended. In the relative angle correcting processing, the processing to be executed in Step S 1092  corresponds to the correction requiring state detect means, while the processing to be executed in Steps S 1093  and S 1094  correspond to the relative angle information correcting means. 
     Also, in the first through third embodiments, description has been given of a case where the steering assisting control is carried out using the microcomputer  1030  but the invention is not limited to this embodiment, for example, there can also be applied other operation processing devices as well as other hardware which uses an operation circuit, an adding circuit, a comparison circuit and the like. 
     Further, in the first through third embodiments, description has been given of a case where the steering assisting control processing is executed using the microcomputer  1030  and the pulse width control processing is executed using the FET gate drive circuit  1022 . However, the invention is not limited to this embodiment, but both of the steering assisting control processing and pulse width control processing may be executed by the microcomputer  1030  and thus the inverter circuit  21  may be driven and controlled directly by the microcomputer  1030 . 
     Next, description will be given below of embodiments for attaining the second object of the invention, that is, fourth˜sixth embodiments. 
     Fourth Embodiment 
       FIG. 24  is a structure view of the whole of a fourth embodiment of an electric power steering apparatus according to the invention. 
     In  FIG. 24 , reference numeral  2001  designates a steering wheel. A steering force, which is applied to the steering wheel  2001  from the driver, is transmitted to a steering shaft  2002  which includes an input shaft  2002   a  and an output shaft  200   b . One end of the input shaft  2002   a  is connected to the steering wheel  2001 , while the other end thereof is connected through a torque sensor  2003  serving as steering torque detect means to one end of the output shaft  2002   b.    
     A steering force transmitted to the output shaft  2002   b  is then transmitted through a universal joint  2004  to a lower shaft  2005  and is further transmitted through a universal joint  2006  to a pinion shaft  2007 . The steering force transmitted to the pinion shaft  2007  is transmitted through a steering gear  2008  to tie rods  2009  to thereby steer vehicle wheels (not shown). Here, the steering gear  2008  has a rack and pinion type structure including a pinion  2008   a  connected to the pinion shaft  2007  and a rack  2008   b  meshingly engaged with the pinion  2008   a , in which a rotation movement transmitted to the pinion  2008   a  is converted to a linear movement by the rack  2008   b.    
     To the output shaft  2002   b  of the steering shaft  2002 , a steering assisting mechanism  2010  which transmits the steering assisting force to the output shaft  2002   b  is connected. This steering assisting mechanism  2010  includes a reduction gear  2011  connected to the output shaft  2002   b , and a 3-phase brushless motor  2012  serving as an electric motor which is connected to the reduction gear  2011  and generates a steering assisting force for a steering system. 
     The torque sensor  2003  is used to detect the steering torque that is applied to the steering wheel  1  and is transmitted to the input shaft  2002   a . The torque sensor  2003  is structured such that it converts the steering torque to the torsion angle variation of a torsion bar interposed between the input shaft  2002   a  and output shaft  2002   b , and then detects the torsion angle variation using a potentiometer. 
     Also, referring to the structure of the 3-phase brushless motor  2012 , one end of a U-phase coil Lu, one end of a V-phase coil Lv and one end of a W-phase coil Lw are connected to each other to form a star connection; the other ends of the respective coils Lu, Lv and Lw are respectively connected to a steering assisting control unit  2020 ; and, motor drive currents Iu, Iv and Iw are supplied from the steering assisting control unit  2020  to the other ends of the coils Lu, Lv and Lw individually. Further, the 3-phase brushless motor  2012  includes a resolver detecting the rotation position of the rotor and a rotor position detect circuit  2013  serving as rotation angle detect means having an encoder and the like. 
     The rotor position detect circuit  2013  supplies a carrier wave signal sin ωt having a given frequency to the resolver to thereby generate a sine wave signal (sin ωt·sin θ) having a wave form, which is obtained by amplitude modulating the carrier wave signal sin ωt using a sine wave sin θ, and a cosine wave signal (sin ωt·cos θ) having a wave form obtained by amplitude modulating the carrier wave signal sin ωt using a cosine wave cos θ Further, the rotor position detect circuit  2013  A/D converts these sine wave signal (sin ωt·sin θ) and cosine wave signal (sin ωt·cos θ), detects, for example, the positive peak time (peak detect pulse Pp) of the carrier wave sin ωt, executes a motor rotation angle calculating processing each time the peak detect pulse Pp is detected to calculate sin θ and cos θ, and calculates a motor rotation angle (rotor rotation angle) θ according to the calculated sin θ and cos θ. 
     To the steering assisting control unit  2020 , as shown in  FIG. 25   r  input are: a steering torque T detected by the torque sensor  2003  and a vehicle speed detect value Vs detected by the vehicle speed sensor  2021 ; a rotor rotation angle θ detected by the rotor position detect circuit  2013 ; and, motor drive current detect values Iud, Ivd and Iwd respectively output from a motor current detect circuit  2022  which detects the motor drive currents Iu, Iv and Iw supplied to the respective phase coils Lu, Lv and Lw of the 3-phase brushless motor  2012 . 
     The steering assisting control unit  2020  includes: a control operating unit  2023  having, for example, a microcomputer which operates a steering assisting target current value according to the steering torque T, vehicle speed detect value Vs and rotor rotation angle θ to output a motor voltage command values Vu, Vv and Vw; a motor drive circuit  2024  composed of a field effect transistor (FET) for driving the 3-phase brushless motor  2012 ; and, a FET gate drive circuit is  2025  for controlling the gate current of the field effect transistor of the motor drive circuit  2024  according to the phase voltage command values Vu, Vv and Vw output from the control operating unit  2023 . The steering assisting control unit  2020  corresponds to motor control means. 
     The control operating unit  2023 , as shown in  FIG. 26 , includes: a vector control command value calculating circuit  2030  which, after it decides a current command value having vector control components d and q using the excellent characteristic of the vector control, converts the current command value to the respective phase current command values Iu*, Iv* and Iw* corresponding to the respective exciting coils Lu˜Lw, and outputs these phase current command values Iu*, Iv* and Iw*; and, a current control circuit  2040  for carrying out a current feedback processing according to the respective phase current command values Iu*, Iv* and Iw* output from the vector control command value calculating circuit  2030  and the respective motor current detect values Iud, Ivd and Iwd detected by the motor current detect circuit  2022 . 
     The vector control command value calculating circuit  2030 , as shown in  FIG. 26 , includes a steering assisting current command value operating portion  2031 , a control signal output portion  2032 , a d axis command current calculating portion  2034  a d-q axis voltage calculating portion  2035 , a q axis command current calculating portion  2036 , and a 2-phase/3-phase converting portion  2037 . 
     The steering assisting current command value operating portion  2031  receives the steering torque T detected by the torque sensor  2003  and the vehicle speed Vs detected by the vehicle speed sensor  2021  and, calculates a steering assisting current command value I M *according to the thus-input steering torque T and vehicle speed Vs. 
     The control signal output portion  2032  outputs a control angle (an electric angle θe and an electric angular velocity ωe) and a control amount (the current limit value of the steering assisting current command value I M *) according to the rotor rotation angle θ detected by the rotor rotation angle detect circuit  2013 . 
     The d axis command current calculating portion  2034  calculates a d axis command current Id* according to the steering assisting current command value I M *limited by the control amount and the electric angular velocity ωe. 
     The d-q axis voltage calculating portion  2035  calculates a d axis voltage ed(θ) and a q axis voltage eq(θ) according to the electric angle θe. 
     The q axis command current calculating portion  2036  calculates a q axis command current Iq* according to the d axis voltage ed(θ), q axis voltage eq(θ), d axis command current Id* and the steering assisting current command value I M *. 
     The 2-phase/3-phase converting portion  2037  converts the d axis command current Id* output from the d axis command current calculating portion  2034  and the q axis command current Iq* output from the q axis command current calculating portion  2036  to 3-phase current command values Iu*, Iv* and Iw*. 
     The steering assisting current command value operating portion  2031  calculates a steering assisting current command value I M *according to the steering torque T and vehicle speed Vs with reference to a steering assisting current command value calculating map shown in  FIG. 27 . 
     Here, the steering assisting current command value calculating map is composed of a characteristic diagram in which, as shown in  FIG. 27 , the steering torque T is expressed in the horizontal axis, the steering assisting current command value I M *is expressed in the vertical axis, and parabolic curved lines are drawn with the vehicle speed detect value V as the parameter thereof. Further, in this map, when the steering torque T exists in the range of “0” to a set value Ts 1  near to “0”, the steering assisting current command value I M *maintains “0”. When the steering torque T exceeds the set value Ts 1 , initially, the steering assisting current command value I M *increases relatively gradually with respect to an increase in the steering torque T; and, when the steering torque T increases further, the steering assisting current command value I M *increases sharply with respect to this increase. Also, the inclination of the characteristic curve decreases as the vehicle speed increases. 
     Also, in the steering assisting current command value I M *, there is set a current limit value; and, this current limit value is normally set for a normal limit value I MAX0 . Further, the current limit value can be changed by the control signal output portion  2032  and is to be output from the control signal output portion  2032  as a control amount. 
     In the present embodiment, the control amount to be output from the control signal output portion  2032  is set such that, by reducing the control amount, a steering torque to be generated from the motor can be reduced. Here, although there is used the current control value of the motor, it is also possible to use a gain (assisting control gain) which is multiplied by the steering torque. 
     Also, the current control circuit  2040  includes: subtractors  41   u ,  41   v  and  41   w  respectively for subtracting the detect values Iud, Ivd and Iwd of motor phase currents respectively flowing in the respective phase coils Lu, Lv and Lw and detected by the current detect circuit  2022  from the current command values Iu*, Iv* and Iw* supplied from the vector control phase command value calculating portion  2030  to obtain the respective phase current differences ΔIu, ΔIv and ΔIw; a PI control portion  2042  for executing a proportion integration control on the thus found respective phase current differences ΔIu, ΔIv and ΔIw to calculate instruction voltages Vu, Vv and Vw; and, a PWM control portion  2043 , according to the calculated instruction voltages Vu, Vv and Vw, for forming pulse width modulation (PWM) signals PWMua˜PWMwb corresponding to the field effect transistors Qua˜Qwb of the motor drive circuit  2024 . 
     Further, the pulse width modulation signals PWMua˜PWMwb output from the PWM control portion  2043  are then supplied to the FET gate drive circuit  2025 . 
     In this manner, in order to generate a steering assisting force corresponding to the steering torque T and vehicle speed detect value Vs, with reference to the rotor rotation angle θ, there is carried out a steering assisting force control for driving and controlling the motor. The reference angle of the rotor rotation angle θ is an angle which is output from the control signal output portion  2032  as a control angle; and, in the present embodiment, the control signal output portion  2032  executes a control signal output processing (which will be discussed later) and, when the removing condition of the steering assisting force control is not satisfied, the control angle is set for a normal angle and the motor is driven and controlled according to this normal angle. On the other hand, when the removing condition of the steering assisting force control is satisfied, the control angle is changed with respect to the normal angle and, according to the changed control angle, there is carried out an abnormality occurrence time control for driving and controlling the motor. 
       FIG. 28  is a flow chart of the control signal output processing to be executed by the control signal output portion  2032 . This control signal output processing is executed as a timer interrupt processing in every given time. Firstly, in Step S 2001 , the control signal output portion  2032  checks whether the removing condition of the steering assisting force control is satisfied or not. Here, it is checked whether an abnormality occurrence time control flag FL is set for “1” meaning the execution of the abnormality occurrence time control or not. Further, when FL=0 and the removing condition is not satisfied, the processing goes to Step S 2002  and, when FL=1 and the removing condition is satisfied, the processing goes to Step S 2013 . 
     In Step S 2002 , the control signal output portion  2032  carries out an abnormality detect processing for detecting the abnormality of the rotor position detect circuit  2013 . Specifically, the control signal output portion  2032  inputs therein sine wave sin θ and cosine wave cos θ respectively calculated in a motor rotation angle calculating processing (not shown) and checks whether sin θ and cos θ are normal or not. Here, the control signal output portion  2032  operates (sin θ) 2 +(cos θ) 2  and, when (sin θ) 2 +(cos θ) 2 ≠1 it determines that sin θ and cos θ are abnormal; and, with reference to a previously stored abnormality checking map, when the combination of sin θ and cos θ does not exist in a given normal area, it determines that sin θ and cos θ are abnormal. 
     Next, the processing goes to Step S 2003 , where the control signal output portion  2032 , according to the check result of the step S 2002 , checks whether the rotor position detect circuit  2013  is normal or not. Further, when it is found that the rotor position detect circuit  2013  is normal, the processing goes to Step S 2010  (which will be discussed later). 
     In Step S 2004 , the control signal output portion  2032  stores the rotor rotation angle θ detected by the rotor position detect circuit  2013  and a rotor rotation angular velocity θ′ obtained by differentiating the rotor rotation angle θ into a memory. 
     Next, in Step S 2005 , the control signal output portion  2032  carries out the abnormality detect processing on the other parts than the rotor position detect circuit  2013  (such as torque sensor  2003  and vehicle speed sensor  2015 ) and, after then, the processing goes to Step S 2006 . 
     In Step S 2006 , the control signal output portion  2032 , according to the check result of the step S 2005 , checks whether the other parts than the rotor position detect circuit  2013  are also normal or not. Further, when they are found normal, it is determined that the removing condition of the steering assisting force control is not satisfied and, after then, the processing goes to Step S 2007 , where there are set a control angle and a limit amount which are used to carry out a normal steering assisting force control. Specifically, the current rotor rotation angle θ stored into the memory in Step S 2004  is converted to an electric angle Ee, and this electric angle θe is differentiated to calculate an electric angular velocity ωe. Further, they are set as control angles (normal angles). Also, a previously set normal limit value I MAX0  is set as a control amount. 
     Next, the processing goes to Step S 2008 , the control signal output portion  2032  outputs the thus set control angle and control amount and then ends the control signal output processing. 
     On the other hand, in the abovementioned step S 2006 , when the control signal output portion  2032  determines that the other parts than the rotor position detect circuit  2013  are abnormal, the processing goes to Step S 2009 , where there is carried out a steering assisting force control (other abnormality time processing) to be executed when an abnormality occurs in the other parts than the rotor position detect circuit  2013  and, after then, the control signal output processing is ended. 
     In Step S 2010 , the control signal output portion  2032  sets an abnormality occurrence time control flag FL for “1” meaning that a steering assisting force control in the abnormality occurrence time is to be executed and, after then, the processing goes to Step S 2011 . 
     In Step S 2011 , the control signal output portion  2032  converts the rotor rotation angle θ stored in the memory in Step S 2004  to an electric angle θe, differentiates this electric angle θe to calculate an electric angular velocity ωe, and sets them in the initial control angle of the abnormality occurrence control; and, after then, the processing goes to Step S 2012 . 
     In Step S 2012 , the control signal output portion  2032  sets the normal limit value I MAX0  in the initial control amount of the abnormality occurrence time control and, after then, the processing goes to the above-mentioned step S 2008 . 
     Also, in Step  2013 , the control signal output portion  2032  carries out the abnormality detect processing of the torque sensor  2003  and, after then, it goes to Step S 2014 . 
     In Step S 2014 , the control signal output portion  2032 , according to the check result of the above step S 2013 , checks whether the torque sensor  2003  is normal or not and, when the torque sensor  2003  is found normal, it goes to Step S  2015 , where it carries out the steering assisting force control for the torque abnormal time and, after then, it ends the control signal output processing. 
     On the other hand, in the above step S 2014 , when the control signal output portion  2032  determines that the torque sensor  2003  is normal, it goes to Step S 2016 , where it detects the steering torque T. 
     Next, in Step S 2017 , the control signal output portion  2032  executes a control angle update processing according to the steering torque. In the present embodiment, a steering torque at the time when the removing condition of the steering assisting force control is satisfied is used as a reference, and the control angle is to be updated according to a difference between the reference value and a current torque as well as the signs thereof. 
     When the signs of the reference value and current torque are the same and the current torque is larger than the reference value, the control angle is advanced in the opposite direction to the neutral direction of the steering wheel. Also, when the signs of the reference value and current torque are the same and the current torque is smaller than the reference value, the control angle is maintained. 
     Also, when the steering torque has become 0 once or more since the removing condition of the steering assisting force control held, the reference value is changed to “0” and, after then, the control angle is updated according to the reference value “0”. At the then time, when the sign of the steering torque is reversed to the sign of the reference value at the abnormality occurrence time, the control angle is advanced in the neutral direction of the steering wheel. 
     Here, the speed for advancing the control angle is decided as shown in  FIG. 29  according to the difference between the current torque and reference value. That is, the larger the difference between the current torque and reference value is, the higher the speed for advancing the control angle is. Here, in the speed for advancing the control angle, there is set a given limit. 
     Next, in Step S 2018 , the control signal output portion  2032  executes an updating (reducing) processing on the control amount. The reducing ratio of the control amount is decided as shown in  FIG. 30  according to the absolute value of the difference between the current torque and reference value. Here, as the absolute value of the difference between the current torque and reference value increases, the reducing ratio of the control amount decreases, whereby the control time of the abnormality occurrence time control can be extended. 
     By the way, a method for reducing the above reducing ratio can be decided using various functions such as a linear line and a quadratic curve. Also, the reducing ratio of the control amount can also be set constant. 
     When the steering wheel is being steered increasingly such that the steering torque can be larger than the reference value, and also when the steering operation is in process, this control amount reducing processing is not executed but the control amount is to be maintained. 
     Next, in Step S 2019 , the control signal output portion  2032  checks whether the control amount is larger than a given control end check threshold value (for example, 0) or not. Further, when the control amount is larger than “0”, it determines that the abnormality occurrence time control is to be continued, and it moves to the above-mentioned step S 2008 . When the control amount is equal to or less than “0”, it ends the control signal output processing as it is. 
     In  FIG. 28 , the processings of Steps S 2002  and S 2003  correspond to the abnormality detect means, the processings of Steps S 2011  and S 2017  correspond to the reference angle change means, and the processings of Step S 2012  and S 2018  correspond to the gradual change processing means. 
     Next, description will be given below of the operation of the fourth embodiment with reference to a time chart shown in  FIG. 31 . In  FIG. 31 , a reference sign A designates a steering torque and B stands for a control angle. 
     Now, suppose the vehicle is turning along a right curved road while keeping steering, no abnormality is occurring in the torque sensor  2003  and the like, and the removing condition of the steering assisting force control is not satisfied. In this case, the control signal output portion  2032 , in Step S 2003  shown in  FIG. 28 , determines that the rotor position detect circuit  2013  is normal, and it goes to Step S 2004 , where it stores the rotor rotation angle θ and rotor angular velocity θ′ into a memory. Also, since the torque sensor  2003  and vehicle speed sensor  2015  are also normal, the control signal output portion moves from Step S 2006  to Step S 2007 , whereby there is executed a normal steering assisting force control. 
     Therefore, the control operating unit  2023  operates the respective phase current command values Iu*, Iv* and Iw* according to the steering torque T detected by the torque sensor  2003 , the vehicle speed Vs detected by the vehicle speed sensor  2015  and the rotor rotation angle θ detected by the rotor position detect circuit  2013 , and carries out a current feedback processing using the respective phase current command values Iu*, Iv* and Iw* as well as the motor current detect values Iud, Ivd and Iwd detected by the motor current detect circuit  2022  to calculate phase voltage instructions Vu, Vv and Vw. Further, the control operating unit  2023  calculates PWM signals PWMua˜PWMwb according to the thus calculated phase voltage instructions Vu, Vv and Vw and outputs them to the FET gate drive circuit  2025 . 
     The FET gate drive circuit  2025 , according to the PWM signals, controls the gate current of the field effect transistor of the motor drive circuit  2024 . As a result of this, a torque, which is generated by the 3-phase brushless motor  2012 , is converted through the reducing gear  2011  to the rotation torque of the steering shaft  2002 , thereby assisting the steering force of the driver. 
     Suppose, from this state, there occurs an abnormality in the rotor position detect circuit  2013  and the removing condition of the steering assisting control is satisfied. In this case, the control operating unit  2013 , in Step S 2003  shown in  FIG. 28 , determines that an abnormality has occurred in the rotor position detect circuit  2013 , and it goes to Step S 2010 , where it sets the abnormality occurrence time control flag for FL=1. Further, the control operating unit  2023  sets the electric angle θe and electric angular velocity ωe, which are obtained according to the rotor rotation angle θ stored in the memory in the previous sampling processing, for the initial control angles, sets the current limit value I MAX0  for the initial control amount, and then outputs them. Accordingly, there is carried out the steering assisting force control according to the electric angle θe and electric angular velocity ωe which are obtained according to the rotor rotation angle θe just before the occurrence of the abnormality. 
     Therefore, the rotor rotation angle is fixed to the rotor rotation angle just before the occurrence of the abnormality and the rotation state of the rotor is maintained in the rotor rotation state just before the occurrence of the abnormality. 
     After then, suppose the driver steers the steering wheel increasingly in the right direction and, during the period of a time t 1  to a time t 2 , the steering torque becomes equal in sign to a torque at the time t 1 , which provides a reference value, and becomes larger than the reference value. In Step S 2017 , the control signal output portion  2032  advances the control angle at a speed shown in  FIG. 29  according to the difference between the current steering torque and reference value. That is, as shown in  FIG. 31 , during the period of the time t 1  to time t 2 , the control angle increases gradually. Also, since the driver is steering increasingly, in Step S 2018 , the control amount reducing processing is not carried out. Further, according to the thus updated control angle and control amount, there is continued the abnormality occurrence time control assisting control. 
     Also, suppose, during the period of the time t 2  to time t 3 , the steering torque becomes the same in sign as the reference value and becomes equal to or less than the reference value. In Step S 2017 , the control signal output portion  2032  maintains the control angle at the time t 2 . Also, in Step S 2018 , the control signal output portion  2032 , according to the absolute value of the difference between the current steering torque and reference value, reduces the current limit value serving as the control amount at a reducing ratio shown in  FIG. 30 . Further, according to the thus set control angle and control amount, there is continued the control assisting control at the abnormality occurrence time. 
     In this manner, there are repeated the steering angle change processing and control amount reducing processing according to the steering torque and, when the steering torque becomes 0 at a time t 4 , the reference value is changed to “0”. After then, when the sign of the steering torque is reversed and is different from the sign of the reference value at the time t 1 , the control angle is advanced in the neutral direction of the steering wheel at a speed shown in  FIG. 29  according to the difference between the current torque and reference value “0”. 
     Further, when the control amount becomes equal to or less than a control end check threshold value, “No” is given in Step S 2019  and thus the steering assisting force control at the abnormality occurrence time is ended, whereby the power steering is moved to manual steering. 
     Here, as shown in  FIG. 32 , in a state where the driver is steering the steering wheel, when the steering assisting force control is removed at a time t 0  due to the occurrence of an abnormality in the steering assisting mechanism  2010  or the like, unless there is executed such an abnormal occurrence time control as in the present embodiment, as shown in  FIG. 32  ( b ), the steering assisting torque given by the motor disappears suddenly, whereby, due to a return force caused by the elastic deformation of the steering system such as the torsion of the vehicle tire, there is applied to the steering shaft a force to return it to its neutral position, that is, there is generated a so called kickback phenomenon. At the then time, as shown by a solid line in  FIG. 32  ( a ), when a manual input torque by the driver is small, as shown by a solid line in  FIG. 32  ( c ), there is generated the sudden return of the steering wheel. Also, in order to avoid such sudden return of the steering wheel and to secure such a keeping steering state as shown by a broken line in  FIG. 32  ( c ), as shown by a broken line in  FIG. 32  ( a ), there is suddenly necessary a large manual input torque by the driver, which increases the steering load of the driver. 
     In view of this, in order to relieve the steering load of the driver, there is known a technique in which, when the motor is stopped during the steering assisting operation, the terminals of the motor are short circuited across them for a given period of time to thereby restrict the occurrence of the kickback phenomenon. 
     However, in this case, when the steering assisting mechanism  2010  becomes abnormal at the time t 0  in  FIG. 33  and thus the motor must be stopped, since only the sudden return of the steering wheel due to the return force caused by the torsion of the steering system is restricted, unless there is executed a correcting steering by the driver, as shown by a solid line in  FIG. 33  ( c ), the steering wheel is gradually returned to the neutral position. Therefore, in order to avoid such steering wheel return and secure such a keeping steering state as shown by a broken line in  FIG. 33  ( c ), there is inevitably necessary a manual input torque which is applied by the driver. At the then time, as shown in  FIG. 33  ( b ), since the steering assisting torque is gradually reducing, as shown by a broken line in  FIG. 33  ( a ), there is not necessary a sudden manual input torque but there is necessary a large manual input torque similarly to the above-mentioned case shown in  FIG. 32 . 
     On the other hand, according to the present embodiment, when removing a normal steering assisting force control in the occurrence of an abnormality, according to the steering torque, the reference angle of the motor rotation angle is changed in such a manner to maintain the motor rotation angle just before the occurrence of the abnormality, thereby being able to restrict the occurrence of the kickback phenomenon effectively. 
       FIG. 34  is a time chart used to explain the effects to be provided by the present embodiment. In  FIG. 34 , reference sign (a) designates a manual input torque to be given by the driver, (b) a steering assisting torque, (c) a motor control angle, (d) an assisting control amount, and (e) a steering angle, respectively. 
     As shown in  FIG. 34 , when there occurs any abnormality at a time t 11  and thus the removing condition of the normal steering assisting force control is satisfied, there is carried out a steering assisting force control at an abnormality occurrence time until a time t 12 . At the then time, a control angle just before the occurrence of the abnormality is set as an initial control angle and, when the steering torque is the same in sign as the reference value and is smaller than the reference value, the then control angle is held and the rotor rotation angle is fixed to a rotor rotation angle just before the occurrence of the abnormality, thereby being able to provide an operation which can prevent the sudden change of the angle of the steering wheel. 
     Also, when the steering torque is the same in sign as the reference value and is larger than the reference value, the then control angle is advanced in the opposite direction to the neutral direction of the steering, which relieves the steering load that is required as the power steering operation is shifted to the manual steering operation, thereby eliminating the need of a large manual input torque for maintaining such a keeping steering state as shown in  FIGS. 32 and 33 . 
     Further, after the steering torque becomes 0 after the occurrence of the abnormality, the control angle is advanced in the steering neutral direction to stop the electric energization of the motor, which makes it possible to change the power steering operation to the manual steering operation quickly. 
     In this manner, even after the occurrence of the abnormality, the steering assisting torque can be applied according to the steering torque (the manual input torque given by the driver). As a result of this, it is possible to prevent the steering assisting torque in the abnormality occurrence from disappearing suddenly and thus to prevent the sudden return of the steering wheel positively. 
     Therefore, in the fourth embodiment, when the abnormality of the motor rotation angle detected by the rotation angle detect means is detected, the reference angle of the motor rotation angle is changed in such a manner that the rotation state of the electric motor just before the occurrence of the abnormality can be maintained, whereby the return of the motor due to the reacting force can be prevented and thus the occurrence of the kickback phenomenon can also be prevented. Especially, in a vehicle in which a steering torque in the manual steering is large due to the heavy weight thereof, the effect of the present embodiment is large. 
     Also, since the motor rotation angle just before the occurrence of the abnormality is set for the reference angle, there can be provided an operation which fixes the rotor rotation angle to the rotor rotation angle just before the occurrence of the abnormality to thereby be able to prevent the sudden change of the angle of the steering wheel, which makes it possible to properly restrict the occurrence of the kickback phenomenon. 
     Further, since the steering torque just before the occurrence of the abnormality is set for the reference value and also since, when the current steering torque has the same sign as the reference value and is equal to or smaller than the reference value, the then reference angle is held, the motor rotation angle just before the occurrence of the abnormality can be maintained and thus the return of the steering wheel can be restricted properly. 
     Also, since, when the current steering torque has the same sign as the reference value and is larger than the reference value, the reference angle is changed in the opposite direction to the steering neutral direction with respect to the then reference angle, the steering load of the driver while increasing the steering of the steering wheel can be relieved and there can be eliminated the need of a large manual input torque when shifting the power steering operation to the manual steering operation. 
     Further, since, when the current steering torque has a different sign from the reference value, the reference angle is changed in the steering neutral direction with respect to the then reference angle, in the keeping steering state and is in the increasing steering state, the electric energization of the electric motor Can be continued; and, after the manual input torque by the driver disappears, the electric energization of the electric motor can be removed, thereby being able to carry out a proper abnormality occurrence time control. 
     Also, since, when the abnormality of the motor rotation angle is detected by the abnormal detect means, the output of the electric motor is reduced gradually, it is possible to prevent the steering assisting force from reducing down to 0 suddenly. 
     Further, since the reducing ratio of the output of the electric motor is decided according to the steering torque, as the steering torque increases, the reducing ratio is decreased, so that the continuing time of the abnormality occurrence time control can be set long. 
     Here, in the fourth embodiment, description has been given of a case where the desired control angle is set according to the rotor rotation angle just before the occurrence of the abnormality. However, the desired control angle can also be set according to the average value of the rotor rotation angles in a given time before the occurrence of the abnormality. 
     Fifth Embodiment 
     Next, description will be given below of a fifth embodiment according to the invention. 
     According to the fifth embodiment, the control angle is changed in such a manner that the motor rotation angular velocity just before the occurrence of an abnormality can be maintained. 
       FIG. 35  is a flow chart of a control signal output processing procedure to be executed by a control signal output portion  2032  according to the fifth embodiment. 
     Since except for following added steps, the fifth embodiment performs similar processing of the above described  FIG. 28 , the portions thereof for carrying out the same processing are given the same designations, description will be given mainly of the processing that are different from those shown in  FIG. 28 . The added steps are: 
     a step S 2031 , after execution of Step S 2010 , for checking whether a rotor rotation angular velocity θ′ is larger than a given threshold value or not; a step S 2032 , when YES is found in Step S 2031 , for setting an initial control angle according to the rotor rotation angular velocity θ′ just before the occurrence of an abnormality; a step S 2033 , after execution of Step S 2016 , for checking whether the rotor rotation angular velocity θ′ is larger than a given threshold value or not; a step S 2034 , when YES in Step S 2033 , for carrying out a motor angular velocity subtracting processing; and, a step S 2035  for updating the control angle according to the result of the motor angular velocity subtracting processing. 
     In Step S 2031 , the control signal output portion  2032  checks whether the rotor rotation angular velocity θ′ held in the above-mentioned step S 2004  is larger than a given angular velocity threshold value θ′ TH  or not. When θ′≦θ′ TH , the control signal output portion  2032  moves to the above-mentioned step S 2011 ; and, when θ′&gt;θ′ TH , it moves to Step S 2032 . Here, the angular velocity threshold value θ′ TH  is set for a value which can determine that the driver is in a keeping steering state. 
     In Step S 2032 , the control signal output portion  2032  sets the rotor rotation angular velocity θ′ stored in the above-mentioned step S 2004  for the reference angular velocity, sets an electric angle θe and an electric angular acceleration ωe according to the reference angular velocity, and then moves to the above-mentioned step S 2012 . Specifically, the electric angle θe is set such that the rotor rotation angular velocity can coincide with the rotor rotation angular velocity θ′ just before the occurrence of the abnormality stored in the above-mentioned step S 2004 . 
     Also, in Step S 2033 , the control signal output portion  2032  checks whether the rotor rotation angular velocity θ′ (the reference angular velocity) is larger than a given angular velocity threshold value θ′ TH  or not. When θ′≦θ′ TH , the control signal output portion  2032  moves to the above-mentioned step S 2017 ; and, when θ′&gt;θ′ TH , it moves to Step S 2034 . 
     In Step S 2034 , the control signal output portion  2032  carries out a motor angular velocity subtracting processing for reducing the reference angular velocity. In the motor angular velocity subtracting processing, the reducing ratio of the reference angular velocity is calculated with reference to a reducing ratio calculating map shown in  FIG. 36 , and the reference angular velocity is reduced at the calculated reducing ratio. 
     In the reducing ratio calculating map shown in  FIG. 36 , a steering torque is expressed in the horizontal axis, the reducing ratio of the reference angular velocity is expressed in the vertical axis, the direction of the steering torque at the start of the steering assisting force control in the abnormality occurrence time is regarded as positive, and, as the steering torque increases, the reducing ratio decreases. Here, a method for reducing the reducing ratio can be determined using various functions such a linear line and a two-dimension curve. Also, the reducing ratio can also be set constant. 
     Also, in the present embodiment, description has been given of a case where the reducing ratio of the reference angular velocity is calculated with reference to the reducing ratio calculating map shown in  FIG. 36 . However, it is also possible to refer to such a reducing ratio calculating map as shown in  FIG. 37 . In the reducing ratio calculating map shown in  FIG. 37 , the vehicle speed Vs is expressed in the horizontal axis, the reducing ratio of the reference angular velocity is expressed in the vertical axis and, as the vehicle speed Vs increases, the reducing ratio of the reference angular velocity increases. 
     Here, the motor angular acceleration subtracting processing shown in this step S 2034 , when the manual input torque is equal to or larger than a given value, is not carried out but the reference angular velocity is to be held. 
     In Step S 2035 , the control signal output portion  2032  updates the electric angle θe and electric angular velocity ωe according to the reference angular velocity updated in the motor angular velocity subtracting processing in the above-mentioned step S 2034  and, after then, it moves to the above-mentioned step S 2008 . 
     In  FIG. 35 , the processing to be executed in Step S 2034  corresponds to the reference angular velocity reducing means. 
     Next, description will be given below of the operation of the fifth embodiment with reference to a time chart shown in  FIG. 38 . In  FIG. 38 , a reference sign (a) designates a manual input torque to be given by a driver, (b) a steering assisting torque, (c) a motor control angular velocity, (d) an assisting control amount, and (e) a steering angle, respectively. 
     While the driver is steering the steering wheel relatively quickly, when there occurs any abnormality at a time t 21  and the removing condition of a normal steering assisting force control is satisfied, the control signal output portion  2032  moves from Step S 2031  in  FIG. 35  to Step S 2032 , where it sets the rotor rotation angular velocity θ′ just before the occurrence of the abnormality for the reference angular velocity, and sets the electric angle θe and electric angular velocity ωe in such a manner that the reference angular velocity can be maintained. Further, the steering assisting force control at the abnormality occurrence time is started according to these electric angle θe and electric angular velocity ωe. 
     In this manner, since the control angle is set according to the rotor rotation angular velocity just before the occurrence of the abnormality, the occurrence of a sudden motor control angular velocity variation at the abnormality occurrence time can be prevented and also the occurrence of the sudden angular velocity variation of the steering wheel can be prevented. 
     After then, according to the motor angular velocity subtracting processing to be executed in Step S 2034 , the reference angular velocity is reduced at a reducing ratio shown in  FIG. 36 , and the steering assisting force control in the abnormality occurrence time is continued at the electric angle θe and electric angular velocity ωe which are updated according to the gradually reduced reference angular velocity. 
     Further, when the manual input torque becomes equal to or larger than a given value α at a time t 22 , the reference angular velocity is held. Therefore, the motor control angular velocity is held as shown in  FIG. 38  ( c ), and the steering angle increases at a constant speed as shown in  FIG. 38(   e ). 
     Since the steering angle comes near to the angle that is intended by the driver, the driver reduces the manual input torque at a time t 23 , and, when the manual input torque becomes smaller that the given value α at a time t 24 , the reducing control of the reference angular velocity is resumed according to the motor angular velocity subtracting processing of Step S 2034 . 
     After then, when the rotor rotation angle θ′ (reference angular velocity) becomes equal to or smaller than a given angular velocity threshold value θ′ TH , the control signal output portion  2032  determines NO in Step S 2033 , and it carries out the control angle updating processing of Step S 2017  and the control amount reducing processing of Step S 2018 . Therefore, as shown in  FIG. 38  ( d ), the assisting control amount reduces gradually. With the gradual reduction of the assisting control amount, as shown in  FIG. 38  ( b ), the steering assisting torque also reduces gradually and, when the steering assisting torque (control amount) becomes 0 at a time t 25 , the power steering operation is shifted to the manual steering operation completely. 
     In this manner, in the fifth embodiment, since the motor rotation angular velocity just before the occurrence of the abnormality is set for the reference angular velocity and the reference angle of the motor rotation angle is set according to the reference angular velocity, a sudden angular velocity change in the steering wheel when the normal steering assisting force control is removed can be prevented. 
     Also, since the reference angular velocity is reduced gradually, the variation ratio of the steering angle can be decreased gradually, the strange feeling of the driver in the steering operation can be prevented and the running stability of the vehicle can be enhanced. 
     Here, in the fifth embodiment, description has been given of a case where the desired control angle is set according to the rotor rotation angular velocity just before the occurrence of the abnormality. However, the desired control angle can also be set according to the average value of the rotor rotation angular velocities in a given time before the occurrence of the abnormality. 
     Sixth Embodiment 
     Next, description will be given below of a sixth embodiment according to the invention. 
     According to the sixth embodiment, the control angle is fixed in such a manner that a motor rotation angle just before the occurrence of an abnormality can be held for a given period of time. 
     That is, the control signal output portion  2032  according to the sixth embodiment, in the control signal output processing of  FIG. 28  according to the fourth embodiment, deletes the control angle updating processing in Step S 2017  and carries out other processings similarly to those shown in  FIG. 28 . 
     In the present embodiment, as the control amount, instead of the above-mentioned current limit value, there is applied an assisting control gain. 
     As this assisting control gain, there is used, for example, a gain which is multiplied by the proportion integration control in the PI control portion  2040 . Normally, the gain is set for 1 and, in the control amount subtracting processing in Step S 2018 , the gain is controlled to reduce down to a value smaller than 1. Here, the reducing ratio of the assisting control gain, as shown in the above-mentioned  FIG. 30 , can be decided by various functions such as a linear line and a two-dimensional curved line. 
     Next, description will be given below of the operation of the sixth embodiment with reference to a time chart shown in  FIG. 39 . In  FIG. 39 , a reference sign (a) designates an assisting torque, (b) a motor control angle, (c) an assisting control gain, and (d) a steering angle, respectively. 
     When, at a time t 31 , there occurs any abnormality and the removing condition of a normal steering assisting force control is satisfied, the control signal output portion  2032 , in Step S 2011 , sets an electric angle θe and an electric angular velocity ωe according to the motor rotation angle θ just before the occurrence of the abnormality, and starts the steering assisting control in the abnormality occurrence time. Owing to this, the motor rotation angle θ just before the occurrence of the abnormality is held. 
     After then, until the steering assisting control in the abnormality occurrence time is ended, the processing for updating the control angle is not executed. Thus, as shown in  FIG. 39  ( b ), the motor control angle is fixed to a control angle which is set according to the motor rotation angle θ just before the occurrence of the abnormality. 
     On the other hand, from the time t 31  on, the control signal output portion  2032  carries out the control amount reducing processing in Step S 2018  to reduce the control amount at the reducing ratio shown in  FIG. 30 . Accordingly, as shown in  FIG. 33(   c ), the assisting control gain reduces gradually at and after the time t 31  and, with the gradual reduction of the assisting control gain, the steering assisting force also reduces gradually. Owing to this, with the reduction of the assisting control gain, the steering angle, as shown in  FIG. 33(   d ), is gradually returned to the neutral position. 
     At the then time, the reducing ratio of the assisting control gain is set such that, as the absolute value of the difference between the steering torque and reference value (steering torque in the abnormality occurrence time) increases, it decreases, thereby being able to prevent the reduction of the steering assisting force. Therefore, even in a vehicle in which a steering torque is large due to the heady weight thereof or the like, the return of the steering wheel can be prevented effectively. 
     Further, when the control amount (assisting control gain) becomes 0 at a time t 32 , the control signal output portion  2032  ends the steering assisting control in the abnormality occurrence time and then the power steering is switched to the manual steering. 
     In this manner, according to the sixth embodiment, at the time of the occurrence of the abnormality, the reference angle of the motor rotation angle is fixed to the reference angle just before the occurrence of the abnormality and also the output of the electric motor is reduced gradually according to the steering torque, whereby the sudden return of the steering wheel can be prevented using a relatively simple structure. 
     Here, in the fourth to sixth embodiments, description has been given of a case where the control angle in the abnormality occurrence time is set for the initial control angle. However, it can also be set for a previously set arbitrary angle. 
     Also, in the fourth to sixth embodiments, when the assisting torque in the abnormality occurrence time exists near to 0, it can be determined that the influence of the kickback is small and thus it is possible that the abnormality occurrence time control is not executed. 
     Further, in the fourth to sixth embodiments, description has been described of a case where the 3-phase brushless motor is applied as the electric motor. However, there can also be applied a brushless motor system. In this case, the motor rotation angle and motor rotation angular velocity may be calculated from the detect value of the steering sensor, or they may be estimated from the counter electromotive force of the motor. 
     The present application is based on Japanese Patent Application (Patent Application No. 2006-152531) filed on May 31, 2006 and Japanese Patent Application (Patent Application No. 2006-243470) filed on Sep. 7, 2006 and thus the contents thereof are incorporated herein as reference.