Patent Publication Number: US-2003226710-A1

Title: Electric power steering apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an electric power steering apparatus structured such that a steering assist force generated by a motor is applied to a steering system in a motor vehicle or a vehicle, and more particularly to an electric power steering apparatus which improves a steering feeling at a time of stopping a control of a control unit.  
       [0003] 2. Description of the Prior Art  
       [0004] An electric power steering apparatus applying an assisting load to a steering apparatus in a motor vehicle or a vehicle on the basis of a rotational force of a motor is structured such as to apply a driving force of a motor as an assisting load to a steering shaft or a rack shaft by a transmitting mechanism such as a gear, a belt or the like via speed reduction gears. The conventional electric power steering apparatus mentioned above executes a feedback control of a motor electric current in order to accurately generate an assist torque(a steering assist torque). The feedback control is structured such as to adjust a motor applied voltage so that a difference between a command value of an electric current and a detected value of a motor electric current is reduced, and an adjustment of the motor applied voltage is generally performed by adjusting a duty ratio of a PWM(Pulse Width Modulation) control.  
       [0005] In this case, a description will be given of a popular structure of the electric power steering apparatus with reference to FIG. 1.  
       [0006] A shaft  2  of a steering handle  1  is connected to a tie rod  6  of a steered tire wheel via speed reduction gears  3 , universal joints  4   a  and  4   b  and a pinion rack mechanism  5 . A torque sensor(a steering torque detecting means)  10  for detecting a steering torque of the steering handle  1  is provided in the shaft  2 , and a motor  20  assisting the steering force of the steering handle  1  is connected to the shaft  2  via the speed reduction gears  3 . An electric power is supplied to a control unit  30  for controlling the power steering apparatus from a battery  14  via an ignition key  11  and a relay  13 , and the control unit  30  calculates a steering assist command value I of an assist command on the basis of a steering torque T detected by the torque sensor  10  and a vehicle speed V detected by the vehicle speed sensor  12 , and controls the electric current supplied to the motor  20  on the basis of the calculated steering assist command value I.  
       [0007] The control unit  30  is mainly constituted by a CPU. A normal function executed in accordance with a program within the CPU is structured as shown in FIG. 2.  
       [0008] A description will be given of a function and an operation of the control unit  30 . The steering torque T detected by the torque sensor  10  so as to be inputted is phase-compensated by a phase compensator  31  for the purpose of increasing a stability of the steering system, and the phase-compensated steering torque TA is inputted to a steering assist command value calculator  32 . Further, the vehicle speed V detected by the vehicle speed sensor  12  is also inputted to the steering assist command value calculator  32 . The steering assist command value calculator  32  determines the steering assist command value I corresponding to a control target value of an electric current supplied to the motor  20  by referring to a characteristic map(look-up table)  33  on the basis of the inputted steering torque TA and the vehicle speed V.  
       [0009] The steering assist command value I is inputted to a subtractor  30 A, and inputted to a differential compensator  34  of a feed-forward system for increasing a speed of response, a deviation(I−i) calculated by the subtractor  30 A is inputted to a proportion calculator  35 , and a proportional output is inputted to an adder  30 B and input to an integral calculator  36  for improving the characteristic of the feedback system. The outputs of the differential compensator  34  and the integral calculator  36  are added and inputted to the adder  30 B, and a current command value E corresponding to an added result in the adder  30 B is inputted to a motor drive circuit  37  so as to constitute as a motor driving signal. A motor current value “i” of the motor  20  is detected by a motor current detecting circuit  38 , and the detected motor current value “i” is inputted to the subtractor  30 A so as to be fed back.  
       [0010] The electric power steering apparatus mentioned above stops the motor control in the case of turning the ignition key  11  to an OFF-state from an ON-state, the case that a stop signal is generated from a failure diagnosing means(not shown) detecting a failure of the control unit, and the like. When the motor control is stopped in the case of turning off the ignition key  11  as mentioned above, the case that the stop signal is generated from the failure diagnosing means, and the like, the steering torque is rapidly increased and the steering handle  1  becomes rapidly heavy, so that not only a high grade feeling of the steering is deteriorated, but also a steering feeling is significantly deteriorated particularly during the steering.  
       [0011] In conventional, as the electric power steering apparatus solving the problem mentioned above, there are structures shown in Japanese Patent Publication No. 7-94226 B2 and Japanese Patent Publication No. 7-94227 B2.  
       [0012] An electric power steering apparatus described in Japanese Patent Publication No. 7-94227 B2 is provided with an operation stop detecting means for detecting an operation stop command of a motor control signal generating means, and a correcting means for gradually reducing a motor control signal together with a time on the basis of a detecting signal outputted from the operation stop detecting means. In this structure, the correcting means is provided between the motor control signal generating means and the motor driving means.  
       [0013]FIG. 3 shows the structure. This electric power steering apparatus is provided with a steering state detecting means  100  constituted by a steering torque detecting means  101  for detecting the steering torque of the steering system and a steering rotation detecting means  102  for detecting a number of steering rotation, a motor control signal generating means  110  for determining and outputting a motor control signal on the basis of the detecting signal outputted from the steering state detecting means  100 , and a motor driving means  111  for driving the motor on the basis of the motor control signal, and the power steering apparatus is further provided with switches  112  such as a key switch and an external switch, a failure diagnosing means  113  for the apparatus, an operation stop detecting means  114  for detecting an operation stop of the apparatus itself and a correcting means  120  arranged between the motor control signal generating means  110  and the motor driving means  111  for the purpose of gradually reducing the motor control signal together with a time on the basis of detected signals outputted from the operation stop detecting means  114 .  
       [0014] In the structure mentioned above, when the switches  112  for operating and stopping the apparatus itself are in an OFF-state or the stop signal is outputted from the failure diagnosing means  113 , the operation stop detecting means  114  detects that the apparatus is operated or stopped on the basis of the signals. Then, the correcting means  120  reduces the motor control signal together with the time on the basis of the detecting signal outputted from the operation stop detecting means  114  so as to output to the motor driving means  111 , thereby gently stopping the motor.  
       [0015]FIG. 4 shows an example of the operation. At first, a whole of the control system is initialized(Step S 20 ), the failure diagnosing means  113  inputs the steering torque detecting signal from the steering torque detecting means  102 (Step S 21 ), and a failure diagnosis is executed(Step S 22 ). The failure diagnosing means  113  next inputs a current detecting signal from the motor driving means  111 (Step S 23 ) and executes the same failure diagnosis(Step S 24 ), and further the operation stop detecting means  114  inputs a detecting signal from the switches  112 (Step S 25 ). The motor control signal generating means  110  calculates the motor control signal in accordance with the steering torque detecting signal from the steering torque detecting means  101  and  102 (Step S 26 ), and corrects the calculated motor control-signal by the correcting means  120 . Thereafter, the operation stop detecting means  114  judges from the switches  112  whether or not the operation stop state is established(Step S 30 ), and when the operation state is not stopped, the motor driving means outputs the corrected motor control signal so as to drive the motor(Step S 31 ), and is returned to the Step S 21  mentioned above.  
       [0016] On the contrary, at the Step S 30  mentioned above, in the case that it is judged by the operation stop detecting means  114  that the operation stop state is established, the correcting means  120  gradually reduces the motor control signal from the motor control signal generating means  110  in accordance with the time(Step S 32 ), and the failure diagnosing means  113  judges whether or not the motor control signal is “0”(Step S 33 ). If the motor control signal is not “0”, the step goes to the Step S 31 , and if it is “0”, the power steering apparatus(EPS) is stopped and the process is finished(Step S 34 ).  
       [0017] On the contrary, an electric power steering apparatus described in Japanese Patent Publication No. 7-94226 B2 is provided with an initial state detecting means for detecting an operation start state of a motor control signal generating means, and a correcting means for gradually increasing a motor control signal until a determined value together with a time on the basis of a detecting signal outputted from the detecting means. In this structure, the correcting means is provided between the motor control signal generating means and the motor driving means.  
       [0018]FIG. 5 shows the structure. When a switch such as a key switch is turned on, respective detecting signals are outputted from a steering torque detecting means  131  and an initial state detecting means  132 . In a motor control signal generating means  133 , a motor control signal is determined and outputted on the basis of the steering torque signal outputted from the steering torque detecting means  131 . In a correcting means  130 , when a detecting signal showing a start state is inputted from the initial state detecting means  132 , the detecting signal is corrected until the determined value of the motor control signal for the purpose of gradually increasing the motor control signal, and the motor is driven and controlled by a motor driving means  134  on the basis of the corrected motor control signal.  
       [0019] Although a motor control signal monitoring means is not disclosed in Japanese Patent Publication No. 7-94226 B2 and Japanese Patent Publication No. 7-94227 B2, it is necessary to detect an abnormal state of the motor control signal generating means in relation with torque detecting means and the motor driving means. When the motor control signal monitoring means is used with the above described apparatus, in the methods disclosed in the respective publications mentioned above, since the motor control signal is reduced or increased without relation to the motor control signal generating means, there is a problem that it is impossible to function the motor control signal monitoring means during reducing or increasing the motor control signal. That is, when the operation stop state or the initial state is detected, the calculated result of the motor control signal is forcibly changed so as to be reduced or increased together with the time, so that the calculated result of the motor control signal is corrected. Accordingly, a relation between the normal steering detecting signal and the current for actually driving the motor is not established, so that it is unavoidable to cancel the monitoring during the correction.  
       [0020] Further, in the electric power steering apparatus, it is desirable to make the steering force heavier for a comparatively long time so as not to apply a sense of discomfort to a driver at a time of turning off the key switch. On the contrary, since an abnormality exists within a system at a time of detecting the failure, it is dangerous unless the system is quickly stopped, and inversely, if the system is suddenly stopped, the handle is taken and a dangerous state is generated in the vehicle, so that it is desirable to stop the system after making the steering force heavier as fast as possible within a range that the handle is not taken. Accordingly, it is hard to solve both problems by one correcting method.  
       [0021] Still further, in the conventional system in which a weight of the vehicle is small, so much steering assist force is not required and an assist ratio(ratio between the steering force in the case that the electric power steering apparatus functions and the steering force after stopping) is small, it is possible to make the steering force sufficiently heavy so as not to reduce the steering feeling for a comparatively short time. However, in order to prevent the steering feeling in the system in which the weight of the vehicle is heavy and the assist ratio is large from being reduced, the effect of preventing the sense of discomfort can be obtained only by making the steering force heavier for a sufficiently long time, so that there eminently appears the problem mentioned above.  
       SUMMARY OF THE INVENTION  
       [0022] The present invention is made by taking the matters mentioned above into consideration, and an object of the present invention is to provide an electric power steering apparatus which does not stop a function of a motor control signal monitoring means and improves a steering feeling, even in the case of stopping a motor control.  
       [0023] The present invention relates to an electric power steering apparatus provided with a steering state detecting means for detecting a steering state of a steering system, a motor control signal generating means for determining and outputting a motor control signal on the basis of a steering state detecting signal outputted from the steering state detecting means, a motor driving means for driving a motor on the basis of a motor control signal outputted from the motor control signal generating means, and a motor control signal monitoring means for monitoring the motor control signal generating means on the basis of the signals outputted from the steering state detecting means and the motor driving means, and the object mentioned above of the present invention can be achieved by the feature that the electric power steering apparatus is provided with an operation stop detecting means for detecting an operation stop command of the motor control signal generating means, and a steering state signal correcting means disposed between the steering state detecting means and the motor control signal generating means, and the steering state signal correcting means corrects a steering state detecting signal corresponding to an input of the motor control signal generating means in accordance with a time on the basis of the detecting signal outputted from the operation stop detecting means.  
       [0024] Further, the object mentioned above of the present invention can be achieved by the feature that the electric power steering apparatus is provided with a control start detecting means for detecting a control start command of the motor control signal generating means, and a steering state signal correcting means disposed between the steering state detecting means and the motor control signal generating means, and the steering state signal correcting means corrects a steering state detecting signal corresponding to an input of the motor control signal generating means in accordance with a time on the basis of the detecting signal outputted from the control start detecting means.  
       [0025] The present invention relates to an electric power steering apparatus provided with a steering state detecting means for detecting a steering state of a steering system, a motor control signal generating means for determining and outputting a motor control signal on the basis of a steering state detecting signal outputted from the steering state detecting means, and a motor driving means for driving a motor on the basis of a motor control signal outputted from the motor control signal generating means, and the object mentioned above of the present invention can be achieved by the feature that the electric power steering apparatus is provided with an operation stop detecting means for detecting an operation stop command of the motor control signal generating means, a plurality of steering state signal correcting means disposed between the steering state detecting means and the motor control signal generating means, and a correction method selecting means for selecting and utilizing the plurality of steering state correcting means, and the steering state signal correcting means selected by the correction method selecting means corrects a steering state detecting signal corresponding to an input of the motor control signal generating means in accordance with a time on the basis of the detecting signal outputted from the operation stop detecting means.  
       [0026] Further, the object mentioned above of the present invention can be more effectively achieved by the feature that the steering state signal correcting means is selected by the correction method selecting method so as to gradually increase at a time of turning on the ignition key or at a time of returning from the abnormal battery voltage, or the feature that a traveling speed detecting means for inputting a vehicle sped signal is provided in the correction method selecting means and the selection is executed by the correction method selecting means on the basis of the vehicle speed signal, or the feature that the steering state signal correcting means starts the correction on the basis of the signal outputted from the failure diagnosing means before the operation is stopped. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0027] In the accompanying drawings:  
     [0028]FIG. 1 is a schematic view for explaining a general electric power steering apparatus;  
     [0029]FIG. 2 is a block diagram showing an example of a structure of a control unit;  
     [0030]FIG. 3 is a block diagram showing an example of a conventional apparatus;  
     [0031]FIG. 4 is a flow chart showing an example of an operation in accordance with the conventional apparatus;  
     [0032]FIG. 5 is a block diagram showing an example of a conventional apparatus;  
     [0033]FIG. 6 is a block diagram showing a structure of a first embodiment in accordance with the present invention;  
     [0034]FIG. 7 is a flow chart showing an example of an operation in accordance with the present invention;  
     [0035]FIG. 8 is a schematic view showing an embodiment of a correcting means in accordance with the present invention;  
     [0036]FIG. 9 is a schematic view showing another embodiment of a correcting means in accordance with the present invention;  
     [0037]FIG. 10 is a block diagram showing a structure of a second embodiment in accordance with the present invention;  
     [0038]FIG. 11 is a block diagram showing a structure of a third embodiment in accordance with the present invention;  
     [0039]FIG. 12 is a flow chart showing an example of an operation of the third embodiment in accordance with the present invention;  
     [0040]FIG. 13 is a block diagram showing a structure of a forth embodiment in accordance with the present invention;  
     [0041]FIG. 14 is a flow chart showing an example of an operation of the forth embodiment in accordance with the present invention;  
     [0042]FIG. 15 is a view for explaining the operation of the forth embodiment in accordance with the present invention;  
     [0043]FIG. 16 is a block diagram showing a structure of a fifth embodiment in accordance with the present invention;  
     [0044]FIG. 17 is a view showing an example of a characteristic of a correcting means used in the fifth embodiment and a sixth embodiment;  
     [0045]FIG. 18 is a block diagram showing a structure of a sixth embodiment in accordance with the present invention;  
     [0046]FIG. 19 is a block diagram showing a structure of a seventh embodiment in accordance with the present invention; and  
     [0047]FIG. 20 is a block diagram showing a structure of a eighth embodiment in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0048] In accordance with the present invention, the steering state signal correcting means(steering torque input correcting means) is provided between the steering state detecting means (steering torque detecting means) and the motor control signal generating means so as to reduce the steering state detecting signal(steering torque signal) inputted to the motor control signal generating means, thereby correcting the steering state detecting signal corresponding to a motor current command in accordance with a time as a result of calculation executed by the motor control signal generating means while keeping a function of the motor control signal generating means. The steering state detecting signal is corrected after detecting the operation stop state, and the motor control signal is calculated by using the corrected steering state detecting signal.  
     [0049] Further, the electric power steering apparatus is provided with a control start detecting means for detecting a control start command of the motor control signal generating means, thereby increasing the steering state detecting signal in accordance with a time on the basis of the detecting signal.  
     [0050] Still further, in accordance with the present invention, a plurality of steering state signal correcting means(steering torque input correcting means) having different characteristics are provided between the steering state detecting means(steering torque detecting means) and the motor control signal generating means so as to select the steering state signal correcting means(the steering torque input correcting means) by the correction method selecting means and reduce the steering state detecting signal(steering torque signal) inputted to the motor control signal generating means by the selected steering state signal correcting means(the steering torque input correcting means), thereby correcting the steering state detecting signal corresponding to a motor current command in accordance with a time as a result of calculation executed by the motor control signal generating means while keeping a function of the motor control signal generating means. The steering state detecting signal is corrected after detecting the operation stop state, and the motor control signal is calculated by using the corrected steering state detecting signal.  
     [0051] Therefore, in accordance with the present invention, since the monitoring mechanism is assembled in a rear stage of the steering state detecting signal after being corrected, it is possible to monitor during the correction of the steering state detecting signal.  
     [0052] A description will be in detail given below of embodiments in accordance with the present invention with reference to the accompanying drawings.  
     [0053]FIG. 6 is a block diagram showing a first embodiment in accordance with the present invention. The structure is made such that a monitoring function is obtained even when an assist is reduced. A steering torque detecting signal outputted from a steering torque detecting means  41  constituting a steering state detecting means is inputted to a steering torque input correcting means  40  and a failure diagnosing means  42 , and a corrected signal of the steering torque detecting signal which is corrected by the steering torque input correcting means  40  is inputted to a motor control signal generating means  43  and a motor control signal monitoring means  44 . Further, an output of a key switch  45  is inputted to an operation stop detecting means  46 , and a motor driving means  47  drives and controls a motor in accordance with the motor control signal outputted from the motor control signal generating means  43 . The failure diagnosing means  42  monitors whether or not a failure exists in the steering torque detecting means  41 , the motor control signal monitoring means  44  and the motor driving means  47 , and a result of diagnosis in the failure diagnosing means  42  is inputted to the operation stop detecting means  46  and the motor control signal generating means  43 . The operation stop detecting means  46  is structured such as to detect an OFF-operation applied by the key switch  45 , and detects an operation stop command of the motor control signal generating means  43 .  
     [0054] In the structure mentioned above, a description will be given of an operation thereof with reference to a flow chart shown in FIG. 7.  
     [0055] At first, a whole of a control system is initialized(Step S 1 ), and the failure diagnosing means  42  inputs a steering torque detecting signal from a steering torque detecting means  41 (Step S 2 ), and executes a failure diagnosis(Step S 3 ). The failure diagnosing means  42  next inputs a current detecting signal from the motor driving means  47 (Step S 4 ) and executes the same failure diagnosis(Step S 5 ), the motor control signal monitoring means  44  executes a monitoring function(Step S 6 ), and the operation stop detecting means  46  inputs a detecting signal outputted from the key switch  35 (Step S 7 ) and judges whether or not it is an operation stop state(Step S 10 ). If the operation state is not stopped, the motor control signal generating means  43  executes a calculation of the motor control signal in accordance with the steering torque detecting signal outputted from the steering torque detecting means  41  via the steering torque input correcting means  40 (Step S 11 ) and outputs the motor control signal(Step S 12 ), whereby the motor is driven and controlled via the motor driving means  47 . Together with this operation, the step is returned to the Step S 2  mentioned above.  
     [0056] On the contrary, in the Step S 10  mentioned above, in the case that it is judged by the operation stop detecting means  46  that it is the operation stop state, the steering torque detecting signal outputted from the steering torque detecting means  41  is gradually reduced in accordance with a time in the steering torque input correcting means  40 (Step S 13 ), and the motor control signal monitoring means  44  judges whether or not the steering torque correcting signal is “0”(Step S 14 ). If the steering torque correcting signal is not “0”, the step goes to the Step S 11  mentioned above, and if it is “0”, the power steering apparatus(EPS) is stopped and the process is finished(Step S 15 ).  
     [0057]FIG. 8 shows an embodiment of the steering torque input correcting means  40 . This steering torque input correcting means  40  is structured such as to limit the maximum input value with respect to the steering torque input, and is characterized in that it is possible to securely reduce to a target value for a short time. Further, FIG. 9 shows another embodiment. It is possible to limit within a fixed gain with respect to the steering torque input. In this case, it is characterized in that it is possible to smoothly reduce without sense of discomfort.  
     [0058]FIG. 10 shows a second embodiment in accordance with the present invention in correspondence to FIG. 6. In this embodiment, the structure is made such that a monitoring function is provided even when an assist is increased. That is, a control start detecting means  50  for detecting a control start command of the motor control signal generating means  43  is provided, and a detecting signal thereof is inputted to the steering torque input correcting means  40 . Then, the steering torque input correcting means  40  gradually increases the steering torque detecting signal corresponding to the input of the motor control signal generating means  43  in accordance with a time on the basis of the detecting signal outputted from the control start detecting means  50 . The steering torque in the steering torque input correcting means  40  is monitored by the motor control signal monitoring means  44 , and when the steering torque detecting signal becomes a predetermined value, the failure diagnosing means  42  detects the failure.  
     [0059] Further, FIG. 11 shows a third embodiment in accordance with the present invention in correspondence to FIG. 6. In this embodiment, the structure is made such that a correction can be started before stopping the operation. This embodiment is different from the first embodiment mentioned above in view that the result of diagnosis of the failure diagnosing means  42  is inputted to the steering torque input correcting means  40 .  
     [0060] A description will be given of an operation of the third embodiment with reference to a flow chart shown in FIG. 12.  
     [0061] At first, a whole of a control system is initialized(Step S 40 ), and the failure diagnosing means  42  inputs a steering torque detecting signal from a steering torque detecting means  41 (Step S 41 ), and executes a failure diagnosis(Step S 42 ). The failure diagnosing means  42  next inputs a current detecting signal from the motor driving means  47 (Step S 43 ) and executes the same failure diagnosis(Step S 44 ), and the motor control signal monitoring means  44  executes a monitoring function(Step S 45 ). The failure diagnosing means  42  judges whether or not a failure in a torque system is generated(Step S 50 ), and in the case that the failure in the torque system is not generated, the operation stop detecting means  46  inputs a detecting signal outputted from the key switch  45 (Step S 51 ) and judges whether or not it is an operation stop state(Step S 52 ). If the operation state is not stopped, the motor control signal generating means  43  executes a calculation of the motor control signal in accordance with the steering torque detecting signal outputted from the steering torque detecting means  41  via the steering torque input correcting means  40 (Step S 53 ) and outputs the motor control signal(Step S 54 ), whereby the motor is driven and controlled via the motor driving means  47 . Together with this operation, the step is returned to the Step S 41  mentioned above.  
     [0062] On the contrary, in the case that it is judged in the Step S 50  mentioned above that the failure in the torque system is generated and in the case that in the Step S 52  mentioned above, in the case that it is judged by the operation stop detecting means  46  that it is the operation stop state, the steering torque detecting signal outputted from the steering torque detecting means  41  is gradually reduced in accordance with a time in the steering torque input correcting means  40 (Step S 55 ), and the motor control signal monitoring means  44  judges whether or not the steering torque correcting signal is “0”(Step S 56 ). If the steering torque correcting signal is not “0”, the step goes to the Step S 53  mentioned above, and if it is “0”, the power steering apparatus(EPS) is stopped and the process is finished(Step S 57 ).  
     [0063] In each of the embodiments mentioned above, the steering torque detecting means is listed up as the steering state detecting means and the description is given of the correction executed by the steering torque input correcting means corresponding to the correcting means, however, the structure may be made such that the steering state detecting means is constructed by adding a steering rotation detecting means and a steering state signal correcting means is constituted by including the steering torque.  
     [0064] In accordance with the above embodiments according to the present invention, the steering state correcting means is provided between the steering state detecting means and the motor control signal generating means so as to reduce or increase the steering state detecting signal inputted to the motor control signal generating means, thereby reducing or increasing the signal corresponding to the motor current command in accordance with the time as the result of calculation executed by the motor control signal generating means itself while keeping the function of the motor control signal generating means. The motor control signal is calculated by reducing or increasing the steering state detecting signal after detecting the operation stop state or the control start state and using the reduced or increased steering state detecting signal. As mentioned above, since the monitoring mechanism is assembled in the rear stage of the steering state detecting signal after being corrected, it is possible to monitor during the correction.  
     [0065]FIG. 13 is a block diagram showing a first embodiment in accordance with the present invention. A steering torque detecting signal outputted from a steering torque detecting means  60  constituting a steering state detecting means is inputted to two steering torque input correcting means  71  and  72  having different characteristics and a failure diagnosing means  61 , and each of corrected signals of the steering torque detecting signals which are corrected by the steering torque input correcting means  71  and  72  is inputted to a motor control signal generating means  62 . Further, an output of a key switch  63  is inputted to an operation stop detecting means  64 , and a motor driving means  66  drives and controls a motor in accordance with the motor control signal outputted from the motor control signal generating means  62 . Two steering torque input correcting means  71  and  72  are selected by a correction method selecting means  65 , and the correction method selecting means  65  is controlled by the detecting signal of the operation stop detecting means  64 . The failure diagnosing means  61  diagnoses whether or not a failure exists in the steering torque detecting means  60 , the motor control signal generating means  62  and the motor driving means  66 , and a result of diagnosis in the failure diagnosing means  61  is inputted to the operation stop detecting means  64  and the motor control signal generating means  62 . The operation stop detecting means  64  detects an OFF-operation applied by the key switch  63 .  
     [0066] In accordance with the present invention, a plurality of steering correcting means(the steering torque input correcting means  71  and  72 ) are arranged between the steering state detecting means(the steering torque detecting means  60 ) and the motor control signal generating means  62 , and the correction method selecting means  65  for selecting an optimum correcting means for the system on the basis of the detecting signal outputted from the operation stop detecting means  64  is provided, thereby reducing the steering torque detecting signal inputted to the motor control signal generating means  62  in accordance with the state of the system.  
     [0067] As a condition for switching the correcting means  71  and  72 , there are (1) a normal system stopping time due to turning off of the ignition key, (2) a system stopping time corresponding to a fail safe due to a trouble of the steering torque input system, and the like.  
     [0068] Since the case of the item (1) mentioned above is a frequently executed normal sequences it is possible to gently make the steering force heavier mainly in view of applying no sense of discomfort to the driver. At this time, since there is a possibility that the driver executes the steering, it generates a motion different from an intention of the driver if the steering force is simply made heavier in one direction, thereby rather applying the sense of discomfort. Accordingly, it is necessary to make the steering force heavy while reflecting the intention of the driver, so that it is desirable to continue to input the latest torque while making the steering force heavy so as to make the steering force heavy on the basis of the input value. Further, since the case of the item (2) is a sequence which is not generated in a normal case, and the system is already abnormal, it is a main object to safely stop the system, and it is necessary to understand that applying no sense of discomfort to the driver is an additional function, so that it is necessary to make the steering force heavy and finish the system for a time as short as possible within a range capable of allowing the sense of discomfort. Further, since the torque detecting input corresponding to a base for calculating the assist force(the motor control signal) is already the abnormal value and can not be used, it is desirable to execute a process for safely stopping such as setting the input value before the fail is generated to a fixed input, or the like.  
     [0069] Further, in the case of switching the input in addition to the calculation contents of the correction, as the correcting means for switching in the manner mentioned above, it is effective to execute the correction between the steering state detecting means(the steering torque detecting means  60 ) and the motor control signal generating means  42  as shown in FIG. 13. Because, in accordance with the structure of the present invention, elements required for switching and correction are all completed between the steering state detecting means(the steering torque detecting means  60 ) and the motor control signal generating means  62  and the structure after the motor control signal generating means  62  corresponding to a core of the electric power steering apparatus is the same, so that it is possible to put the correction and switching into practice by a simple structure without taking into consideration an influence after the motor control signal generating means  62 .  
     [0070] In the structure shown in FIG. 13, a description will be given of an operation thereof with reference to a flow chart shown in FIG. 14.  
     [0071] At first, a whole of a control system is initialized(Step S 60 ), and the failure diagnosing means  61  inputs a steering torque detecting signal from a steering torque detecting means  60 (Step S 61 ), and executes a failure diagnosis(Step S 62 ). The failure diagnosing means  61  next inputs a current detecting signal from the motor driving means  66 (Step S 63 ) and executes the same failure diagnosis(Step S 64 ), and the operation stop detecting means  64  inputs a detecting signal outputted from the key switch  63 (Step S 65 ) and judges whether or not it is an operation stop state(Step S 70 ). If the operation state is not stopped, the motor control signal generating means  62  executes a calculation of the motor control signal in accordance with the steering torque detecting signal outputted from the steering torque detecting means  60  via the steering torque input correcting means  71  or  72 (Step S 71 ) and outputs the motor control signal(Step S 72 ), whereby the motor is driven and controlled via the motor driving means  66 . Together with this operation, the step is returned to the Step S 61  mentioned above.  
     [0072] On the contrary, in the Step S 70  mentioned above, in the case that it is judged by the operation stop detecting means  64  that it is the operation stop state, an operation stop condition is judged(Step S 73 ). That is, if the ignition key is in an OFF-state, the steering torque input correcting means  71  is selected via the correction method selecting means  65 (Step S 74 ), and if the torque system fail exists, the steering torque input correcting means  72  is selected via the correction method selecting means  65 (Step S 76 ). The steering torque detecting signal outputted from the steering torque detecting means  60  is gradually reduced in accordance with a time in the steering torque input correcting means  71  or  72 , and next it is judged whether or not the steering torque detecting signal is “0”(Step S 75 ). If the steering torque detecting signal is not “0”, the step goes to the Step S 71  mentioned above, and if it is “0”, the power steering apparatus(EPS) is stopped and the process is finished(Step S 77 ).  
     [0073]FIG. 15 shows a relation between the steering torque detecting signal outputted from the steering torque detecting means and a damping property of the steering torque input correcting means  71  and  72 . The steering torque input correcting means  71  is structured such as to damp the maximum input value, and the steering torque input correcting means  72  is structured such as to damp a fixed gain. In the case that the correction method selecting means  65  judges the correction due to the off operation of the ignition key, the normal torque input is used as the input of the motor control signal generating means  62 , the damping at the fixed gain is executed as the correction calculation in the steering torque input correcting means  72 , and a sufficiently long time is given to the correction time. Further, in the case that the correction method selecting means  65  judges the correction due to the failure detection of the torque system, the torque input value before the fail is used as the input of the motor control signal generating means  62 , the damping of the maximum input value is executed as the correction calculation in the steering torque input correcting means  71 , and the correction is finished for a short time.  
     [0074] In this case, it is possible to obtain the same effect by adding a plurality of correcting means and the correction method selecting means to the structure such as the conventional embodiment(FIGS. 3 and 5), however, it is necessary to take into consideration the influence of the motor control signal generating means for the purpose of putting the correction and switching into practice, so that the structure becomes very complex and it is hard to put into practice.  
     [0075]FIG. 16 shows a fifth embodiment in accordance with the present invention in correspondence to FIG. 13. In this embodiment, the structure is made such that three systems of steering torque input correcting means  71  to  73  including the assist increasing time are provided, thereby controlling the correction method selecting means  65  on the basis of the detecting signal of the control start detecting means  80 . That is, the control start detecting means  80  for detecting a control start command of the motor control signal generating means  62  is provided, and a detecting signal thereof is inputted to the correction method selecting means  65 . Then, the steering torque input correcting means  71  to  73  are selected on the basis of the selecting signal outputted from the correction method selecting means  65  so as to gradually increase the steering torque detecting signal corresponding to the input of the motor control signal generating means  62  in accordance with a time. Conditions for switching are an on operation of the ignition key, a fail of the torque system, and an abnormality of a voltage in the battery. The characteristics of the steering torque input correcting means  71  and  73  are structured as shown in FIG. 17, the correcting means  71  executes a correction in accordance with a damping at a fixed gain, the correcting means  72  executes a correction in accordance with a damping at a maximum input value, and the correcting means  73  executes a correction in accordance with a damping at a maximum input value.  
     [0076] Further, FIG. 18 shows a sixth embodiment in accordance with the present invention in correspondence to FIG. 13. In this embodiment, the structure is made such that a traveling speed detecting means  81  is provided as a condition of the correcting method. A vehicle speed signal detected by the traveling speed detecting means  81  is inputted to the correction method selecting means  65 . As an input for switching the correction method, an OFF-operation of the ignition key, a fail(a vehicle speed low) of the torque system and a fail(a vehicle speed high) of the torque system are employed. In this case, the steering torque input correcting means  71  to  73  are structured such as to respectively correct in accordance with the characteristics shown in FIG. 17.  
     [0077] In the structure mentioned above, in the case that the correction method selecting means  65  judges that it is the correction in accordance with the OFF-operation of the ignition key via the operation stop detecting means  64 , the normal torque input is used as the input of the motor control signal generating means  62 , the damping at the fixed gain is executed as the correction calculation in the steering torque input correcting means  71  and a sufficiently long time(some seconds to some tens seconds) is given to the correcting time. Further, in the case that it is judged to be the correction in accordance with the failure detection of the torque system and the vehicle speed is low, the torque input value before the fail is used as the input of the motor control signal generating means  62 , the damping of the maximum input value is executed as the correction calculation in the steering torque input correcting means  73 , and the correction is finished for a comparatively short time(about 10 to 100 milli-seconds). Further, in the case that it is judged to be the correction in accordance with the failure detection of the torque system and the vehicle speed is high, the torque input value before the fail is used as the input of the motor control signal generating means  62 , the damping of the maximum input value is executed as the correction calculation in the steering torque input correcting means  72 , and the correction is finished for a shorter time than the above(about 1 to 10 milli-seconds).  
     [0078] In the sixth embodiment, when the vehicle speed is high, a motion of the vehicle with respect to the handle input is great and there is a low possibility that a large input is given as the handle input, so that the third embodiment is effective in the case that it is intended to early stop the assist. When the correction is executed for a long time, the vehicle moves in an unintended direction. When the vehicle speed is low, the motion of the vehicle with respect to the handle input is small and there is a high possibility that a large input is given as the handle input, so that it is possible to achieve the correcting effect.  
     [0079]FIG. 19 shows a seventh embodiment in accordance with the present invention in correspondence to FIG. 13. In this embodiment, the structure is made such that the correction can be started before the operation is stopped. This embodiment is different from the forth embodiment in view that the result of diagnosis of the failure diagnosing means  61  is inputted to the correction method selecting means  65 .  
     [0080] In this case, as the condition of switching and inputting the correction method, it is possible to consider the OFF-operation of the ignition key, the fail of the torque system, the reduction of the battery voltage and the increase of the battery voltage. The structure of the eighth embodiment in this case is shown in FIG. 20.  
     [0081] In the case that the correction method selecting means  65  judges that it is the correction in accordance with the OFF-operation of the ignition key, the normal torque input is used as the input of the motor control signal generating means  62 , the damping at the fixed gain is executed as the correction calculation and a sufficiently long time is given to the correcting time in the steering torque input correcting means  71 . In the case that it is judged to be the correction in accordance with the failure detection of the torque system, the torque input value before the fail is used as the input of the motor control signal generating means  62 , the damping of the maximum input value is executed as the correction calculation in the steering torque input correcting means  72 , and the correction is finished for a short time. Further, in the case that the battery voltage is reduced, it is detected by the battery voltage detecting means  82 , the selection of the correcting means is executed by the correction method selecting means  65  via the operation stop detecting means  64 , the normal torque input is used as the input of the motor control signal generating means  62 , the damping at the fixed gain is executed as the correction calculation, and the correcting time in the steering torque input correcting means  73  is set to a sufficiently long time. In the case that the battery voltage is increased, in the same manner, it is detected by the battery voltage detecting means  82 , the torque input value before the fail is used as the input of the motor control signal generating means  62 , the damping at the fixed gain is executed as the correction calculation, and the correction in the steering torque input correcting means  72  is finished for a short time.  
     [0082] In accordance with the eighth embodiment, in the case that the battery voltage is reduced, since it is not the normal sequence but corresponds to a comparatively easily generated phenomenon, it is possible to gently make the steering force heavier with mainly paying attention only to the matter giving no sense of discomfort to the driver, and in the case that the battery voltage is increased, there is a risk that the system is damaged due to an abnormal increase which is not generated in the normal using state, however, it is possible to prevent the system breakage mentioned above.  
     [0083] In each of the embodiments mentioned above, the steering torque detecting means is listed up as the steering state detecting means and the description is given of the correction executed by the steering torque input correcting means corresponding to the correcting means, however, the structure may be made such that the steering state detecting means is constructed by adding a steering rotation detecting means and a steering state signal correcting means is constituted by including the steering torque.  
     [0084] In accordance with the forth to eighth embodiments, a plurality of steering state correcting means are provided between the steering state detecting means and the motor control signal generating means so as to reduce or increase the steering state detecting signal inputted to the motor control signal generating means, thereby reducing or increasing the signal corresponding to the motor current command in accordance with the time as the result of calculation executed by the motor control signal generating means itself while keeping the function of the motor control signal generating means. The motor control signal is calculated by reducing or increasing the steering state detecting signal after detecting the operation stop state or the control start state and using the reduced or increased steering state detecting signal. Since the steering state correcting means is selected in an optimum manner in correspondence to the state of the system, it is possible to improve the steering feeling.  
     [0085] Executing the correction on the basis of the motor control signal in accordance with the conventional embodiment corresponds to outputting without respect to the control system, so that the operation during the correction becomes unstable. In particular, in the case that the assist ratio is large(a weight of the vehicle is heavy and a difference between a state with an assist and a state without an assist is large), the effect can be obtained only when the correcting time is set to a long time, so that the control system diverges if the output without respect to the control system is continued during the period, and a danger may be generated because there is a possibility that it is oscillated. In accordance with the present invention, since it is possible to correct while keeping the control block except the torque system by correcting the torque system, it is possible to put a stable correction into practice.  
     [0086] The present invention can be, of course, applied to a column type and a pinion type electric power steering apparatus, and further, can be also applied to a rack assist type electric power steering apparatus.