Abstract:
Control method for an electric power steering apparatus feeds back a motor current, flowing through a motor whose operation is controlled in accordance with a target motor current, so that the driving of the motor is controlled with the fed-back motor current. Peak value of the motor current is detected, and, for feedback control based on the detected peak value, the detected peak value is corrected to decrease. The thus-corrected peak value is fed back to control the motor current to conform to a target motor current value.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to methods for controlling electric power steering apparatus which inpart power of an electric motor to a vehicle steering system to thereby reduce necessary manual steering force.  
         BACKGROUND OF THE INVENTION  
         [0002]    [0002]FIG. 8 hereof schematically illustrates a general setup of a conventional electric power steering apparatus  100  suitable for application to a motor vehicle. The conventional electric power steering apparatus  100  includes a steering wheel  101 , a steering shaft  102  connected integrally to the steering wheel  101 , a manual steering torque generation mechanism  106  provided on the steering shaft  102 , and a rack and pinion mechanism  105  having a pinion  105   a  coupled to the manual steering torque generation mechanism  106  via a connecting shaft  103  having universal joints  103   a  and  103   b . The rack and pinion mechanism  105  includes a rack shaft  107  having a rack tooth  107   a  meshing with the pinion  105 , and the rack shaft  107  can axially reciprocate via the meshing engagement between the rack teeth  107   a  and the pinion  105 . Left and right front wheels  109 , constructed as steerable wheels of the vehicle, are coupled via tie rods  108  to the opposite ends of the rack shaft  107 . Human operator or driver of the vehicle operates the steering wheel  101  to steer the steerable front wheels  109  by way of the manual steering torque generation mechanism  106  and rack shaft  107 .  
           [0003]    To reduce manual steering torque to be generated via the manual steering torque generation mechanism  106 , an electric motor  110  for supplying steering assist torque is provided, for example, coaxially with the rack shaft  107 . Rotational force supplied as the Steering assist torque by the motor  110  is converted into linear force via a ball thread mechanism  111  provided substantially parallel to the rack shaft  107 , which is applied to the rack shaft  107 . Rotor of the steering assisting motor  110  has a helical driving gear  110   a  provided integrally thereon and meshing with a helical gear  111   b  provided integrally on an end of a threaded shaft  111   a  of the ball thread mechanism  111 . The ball thread mechanism  111  has a nut operatively coupled to the rack shaft  107 .  
           [0004]    [0004]FIG. 9 hereof is a block diagram showing a control device employed in the conventional electric power steering apparatus  100 . Within a steering gearbox (not shown in FIG. 8), there is provided a manual steering torque detector section  112  for detecting manual steering torque T acting on the pinion  105   a . The manual steering torque detector section  112  converts the detected manual steering torque T into a manual steering torque detection signal Td and delivers the thus-converted manual steering torque detection signal Td to the control device  114 . Using the manual steering torque detection signal Td as a primary motor-operating signal, the control device  114  operates the steering assisting motor  110  and controls steering assist torque to be produced by the motor  110 . The control device  114  includes a target current setting section  115  and a controller  116 . The target current setting section  115  sets target assisting torque on the basis of the manual steering torque detection signal Td and generates a target motor current signal IT necessary for the motor  110  to produce the target assisting torque.  
           [0005]    [0005]FIG. 10 is a block diagram showing detailed structure of the controller  116  of FIG. 9. As shown, the control section  116  includes an offset calculation section  117 , a motor operation control section  118 , a motor drive section  119 , and a peak current value detection section  120 . The offset calculation section  117  calculates an offset value between the target motor current signal IT output from the target current setting section  115  of FIG. 9 and a peak current signal IM output from the peak current value detection section  120 , and it outputs an offset signal  117   a  indicative of the calculated offset value. The motor operation control section  118  includes an offset current control section  121  and a PWM (Pulse Width Modulated) signal generation section  122 . The offset current control section  121  performs processing, such as proportional, integral and differential (PID) operations, on the offset signal  117   a  supplied from the offset calculation section  117  and thereby generates a driving current signal  121   a  for controlling the motor current to be supplied to the motor  110  in such a manner that the offset signal  117   a  approaches an ideal zero value. The PWM signal generation section  122  generates a PWM signal for PWM-operating the motor  110  on the basis of the driving current signal  121   a  and outputs the thus-generated PWM signal as a drive control signal  122   a .    
           [0006]    Further, in the controller  116 , the motor drive section  119  includes a gate-driving circuit section  123 , and a motor drive circuit section  124  having four powering FETs (Filed Effect Transistors) interconnected via an H-shaped bridge circuit. The gate-driving circuit section  123  selects two of the four FETs on the basis of the drive control signal (PWM signal)  122   a  and in accordance with a current steering direction of the steering wheel  101 , and it drives the gates of the selected two FETs to allow these FETs to perform a switching operation. The peak current value detection section  120  detects a peak value of the motor current (armature current) flowing through the steering assisting motor  110  and outputs a peak current signal IM. In the manner set forth above, the control device  114  PWM-controls the current to be supplied from a battery power supply  126  to the motor  110  and thereby controls the output power (steering assist torque) of the motor  110 , on the basis of the manual steering torque T detected via the manual steering torque detector section  112  of FIG. 9.  
           [0007]    As seen in FIG. 10, the control device  114  achieves enhanced control characteristics of the motor  110  by the controller  116  detecting the peak value of the motor current actually flowing through the motor  110  and performing feedback control of the motor current based on the peak current signal IM. In the aforementioned manner, the manual steering torque T applied by the vehicle driver is detected via the manual steering torque detector section  112  of the manual steering torque generation mechanism  106  shown in FIG. 8, and the control device  114  controls the output power of the motor  110 , on the basis of the detected manual steering torque T, so as to assist the rack shaft  107  in the steering gearbox in moving linearly for desired steerage.  
           [0008]    Control method performed by the conventional control device  114  is explained below. As illustrated in FIG. 10, the steering assisting motor  110  is driven on the basis of feedback control of the motor current performed by the control device  114 . Then, the motor current flowing through the motor  110  when two diagonally-opposed FETS, among the four FETs interconnected by the H-shaped bridge circuit, are in the ON state is detected by the peak current value detection section  120 .  
           [0009]    [0009]FIG. 11 is a block diagram showing the peak current value detection section  120 , motor drive circuit  124 , steering assisting motor  110  and battery  126 . The peak current value detection section  120  generates the peak current signal IM on the basis of a voltage VS(t) across both ends of a shunt resistor  125  connected in series with the motor drive circuit  124 . The peak current value detection section  120  includes a peak holding circuit for holding a peak value VP of the voltage VS(t) input thereto. The peak current signal IM output from the peak current value detection section  120  represents a peak value of the motor current (detected peak current value) ISP. Switch  127  is kept in a closed state during operation of the vehicle, so that voltage of the battery  126  is applied to a capacitor  128 . The capacitor  128  is provided to stabilize the battery voltage to be fed to the motor drive circuit  124 . The peak current value ISP (peak current signal IM) detected by the peak current value detection section  120  is fed back to the offset calculation section  117  for calculation of an offset between the target motor current signal IT and the peak current value ISP, and the driving of the motor  110  is controlled so that the offset is minimized to zero.  
           [0010]    With the electric power steering apparatus where the feedback control is performed using the peak-holding-type peak current value detection section  120 , there would arise the following inconveniences.  
           [0011]    In the peak current value detection section  120  using the peak holding circuit to perform the current detection, the peak value is detected from the motor current varying in response to a duty cycle of the PWM signal. Thus, when the duty cycle of the PWM signal is smaller than 50% so that the motor current takes a small value, a difference between an average value of the actual motor current and the detected peak current value would become considerably great. The following paragraphs set fourth relationship between the duty cycle of the PWM signal and the detected peak current value and inconveniences arising from the relationship.  
           [0012]    [0012]FIGS. 12 and 13 are timing charts each illustrating variations over time of various current values (i.e., value of the motor current, average value of the motor current, shunt current value and detected peak current value) relative to the PWM signal. Specifically, FIG. 12 shows a timing chart when the duty cycle of the PWM signal is greater than 50% and the motor current is relatively great. More specifically, section (a) of FIG. 12 shows a variation over time of the PWM signal  122   a  output from the PWM signal generation section  122 , and section (b) of FIG. 12 shows variations over time of the motor current IMM flowing through the motor  110 , average value Ia of the motor current, shunt current Ish and peak current IM detected by the peak current value detection section  120  (detected peak current value TSP). In this case, a ratio of the peak value of the motor current (i.e., detected peak current value ISP) to the average value Ia of the motor current, namely, ISP/Ia, is very close to a value “1”; that is, the average value Ia and of the motor current and the detected peak current value ISP are close to each other.  
           [0013]    [0013]FIG. 13 shows variations over time of the various current values when the duty cycle of the PWM signal is less than 50% and the motor current is relatively small. Specifically, section (a) of FIG. 13 shows a variation over time of the PM signal  122   a  output from the PWM signal generation section  122 , and section (b) of FIG. 13 shows variations over time of the motor current IMM flowing through the motor  110 , average value Ia of the motor current IMM, shunt current Ish and peak current IM detected by the peak current value detection section  120  (detected peak current value ISP). In this case, the ratio, to the average value Ia of the motor current, of the peak value of the motor current (i.e., detected peak current value XSP), namely, ISP/Ia, is considerably greater than the value “1”; that is, the average value Ia and of the motor current and the detected peak current value ISP are greatly different from to each other. In this case, the detected peak current value ISP to be used for the feedback control, greatly differing from the average value Ia of the motor current, would prevent optimal feedback control of the motor current.  
           [0014]    [0014]FIG. 14 is a graph showing characteristics of the detected peak current value ISP relative to the average value Ia of the motor current, where the horizontal axis represents the average value Ia while the horizontal axis represents the detected peak current value ISP. As shown, when the average value Ia of the motor current is small, the ratio ISP/Ia is greater than the value “1”, presenting a nonlinear characteristic. Therefore, according to the conventional control method performed by the control device  114  based on the feedback control using the peak-holding-type peak current value detection section  120 , there are obtained actual control characteristics as illustratively shown in FIG. 15. In FIG. 15, the horizontal axis represents the target motor current signal IT, while the vertical axis represents the average value Ia of the motor current. From FIG. 15, it is seen that the average motor current value Ia becomes smaller than the value of the target motor current signal IT without coinciding with the latter. Therefore, at the beginning of turning, by the vehicle driver, of the steering wheel, when the target motor signal IT is set at a small value (i.e., a small value range), the average value Ia of the motor current would come short of a predetermined value so that the motor current can not be output as designated by the target motor signal IT, as a result of which a desired steering feel can not be attained.  
           [0015]    Namely, with the conventional control device  114  where the peak motor current, i.e. the detected peak current value ISP, is ted back by the peak current value detection section  120  for the motor current control, there would arise the problem that, when the target motor current signal is set at a small value (i.e., a small value range), the actual steering assisting current (average motor current value) would become smaller than the target steering assisting current (target average motor current value), undesirably providing insufficient torque assist.  
         SUMMARY OF THE INVENTION  
         [0016]    In view of the foregoing prior art problems, it is an object of the present invention to provide a motor operation control method for an electric power steering apparatus which, even when a target motor current signal is set at a small value, can minimize a difference between actual and target steering assisting motor currents and thereby achieve a sufficient steering assist.  
         SUMMARY OF THE INVENTION  
         [0017]    In order to accomplish the above-mentioned object, the present invention provides a control method for an electric power steering apparatus, which comprises the steps of: detecting a peak value of a motor current flowing through a steering assisting motor whose operation is controlled on the basis of a PWM signal generated in accordance with a target motor current signal; correcting the detected peak value; and performing feedback control of the motor current on the basis of the corrected peak value.  
           [0018]    According to the control method of the present invention, the peak value of the motor current (i.e., detected peak current value) is corrected so that the corrected peak current value is used for feedback control of the motor current. Thus, the motor current can be controlled using, as a feedback value, the corrected peak value that is close to an average value of the motor current. Therefore, the steering assisting current of a small value, e.g. the steering assisting current at the beginning of slow turning of the steering wheel during straight travel of the vehicle, can be controlled appropriately in accordance with a desired target motor current, so that an assisted steering feel of a vehicle driver can be enhanced significantly.  
           [0019]    It is preferable that the correcting step correct the detected peak current value to a smaller value. For example, the correction of the detected peak current value may be performed on the basis of a current correction table. More specifically, the current correction table is searched for a current correcting amount corresponding to the detected peak current value, and the detected peak current value is corrected through an arithmetic operation to subtract therefrom the current correcting amount. In the case where the correction of the detected peak current value is performed on the basis of the current correction table, the corrected peak current value can become an optimal value to be used for the feedback control of the motor current, which thereby permits optimal feedback control of the motor current. It is also preferable that the current correction table be prepared on the basis of relationship between various average values of the motor current flowing through the steering assisting motor and various detected peak current values of the motor current, because, in this case, the corrected peak current value can become an optimal value to permit optimal feedback control of the motor current.  
           [0020]    Further, according to a preferred embodiment of the present invention, the correction of the detected peak current value is preferably carried out only when the target motor current signal is set at a small value (i.e., a small value range) such that a ratio of the detected peak current value to the average value of the motor current flowing through the steering assisting motor exceeds a predetermined value. Namely, in this case, the correction of the detected peak current value is performed only when necessary, so that appropriate correction of the detected peak current value and hence optimal feedback control of the motor current is achieved by the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a block diagram showing a general set up of a controller employed in a control device for performing a control method of the present invention;  
         [0023]    [0023]FIG. 2 is a block diagram showing a first specific example of a current detection correcting section shown in FIG. 1;  
         [0024]    [0024]FIG. 3 is a graph explanatory of a table indicating relationship between detected peak current values and current correcting amounts;  
         [0025]    [0025]FIG. 4 is a flow chart of a current correction program that operates the current detection correcting section shown in FIG. 2;  
         [0026]    [0026]FIG. 5 is a graph showing characteristics of a motor current relative to a target motor current when the motor current is controlled in accordance with the control method of the present invention;  
         [0027]    [0027]FIG. 6 is a block diagram showing a second specific example of the current detection correcting section;  
         [0028]    [0028]FIG. 7 is a flow chart of a current correction program that operates the current detection correcting section shown in FIG. 6;  
         [0029]    [0029]FIG. 8 is a schematic view showing a general setup of a conventional electric power steering apparatus;  
         [0030]    [0030]FIG. 9 is a block diagram showing a control device employed in the electric power steering apparatus of FIG. 8;  
         [0031]    [0031]FIG. 10 is a block diagram showing detailed structure of a controller shown in FIG. 9;  
         [0032]    [0032]FIG. 11 is a block diagram showing a peak current detection section, motor drive circuit, steering assisting motor and battery power supply in the controller of FIG. 10;  
         [0033]    [0033]FIG. 12 is a timing chart showing variations over time of a PWM signal and various current values in the conventional apparatus;  
         [0034]    [0034]FIG. 13 is a timing chart showing variations over time of the PWM signal and various current values in the conventional apparatus;  
         [0035]    [0035]FIG. 14 is a graph showing characteristics of a detected peak current value relative to an average value of a motor current; and  
         [0036]    [0036]FIG. 15 is a graph showing characteristics of a motor current relative to a target motor current when the motor current is controlled in accordance with a conventional control method. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    General structure of an electric power steering apparatus to be controlled by a control method of the present invention is basically the same as described earlier in relation to the conventional electric power steering apparatus FIG. 8.  
         [0038]    [0038]FIG. 1 is a block diagram showing a general set up of a controller  1  employed in a control device for performing the control method of the present invention that is intended to control operation of a steering assisting motor of an electric power steering apparatus. Briefly stated, the controller  1  of the electric power steering apparatus shown in FIG. 1 is characterized by including a current detection correcting section  10  in addition to the components of the controller  116  employed in the conventional control device  114  shown in FIGS. 9 and 10.  
         [0039]    The current detection correcting section  10  subtractively corrects a detected peak current value ISP supplied from a peak current value detection section  120 , to thereby output a corrected current value IPM. The corrected current value IPM is delivered to an offset calculation section  117  to he used for feedback control.  
         [0040]    [0040]FIG. 2 is a block diagram showing a first specific example of the current detection correcting section  10 . This current detection correcting section  10  includes a CPU  11 , and a memory  12  where are prestored a current correction table  13  and current correction program  14 .  
         [0041]    The current correction table  13  prestored in the memory  12  is a lookup table prepared on the basis of a graph that represents relationship of various average motor current values Ia to various detected peak current values ISP output from the peak current value detection section  120  (FIG. 1). More specifically, the current correction table  13  represents correspondency between various detected peak current values ISP and current correcting amounts D. For example, the current correction table  13  is prepared by calculating differences of the individual detected peak current values ISP from the corresponding average motor current value Ia as illustrated in the graph of FIG. 14 and then setting the thus-calculated differences as the current correcting amounts D. FIG. 3 is a graph explanatory of the current correction table  13  that defines the current correcting amounts D corresponding to the various detected peak current values ISP.  
         [0042]    The current correction program  14  also prestored in the memory  12  is constructed to calculate a corrected current value IPM from the detected peak current value ISP, as flowcharted in FIG. 4. Once a detected peak current value ISP is input from the peak current value detection section  120  to the current detection correcting section  10  at step ST 10 , the CPU  11  searches the current correction table  13  for a current correcting amount D corresponding to the input detected peak current value ISP at step ST 11 . Then, the CPU  11  proceeds to step ST 12  in order to perform an arithmetic operation, as indicated in Expression (1) below, to determine a corrected current value IPM. 
           IPM=ISP−D   (1) 
         [0043]    Then, the CPU  11  outputs the thus-calculated corrected current value IPM at step ST 13 .  
         [0044]    Next, a description will be given about the control method performed by the control device that includes the controller  1  shown in FIG. 1. The offset calculation section  117  of FIG. 1 calculates an offset value between a target motor current signal IT output from a target current setting section  115  similar to the target current setting section  115  of FIG. 9 and the corrected current value IPM output from the current detection correcting section  10 , and it outputs an offset signal  117   b  indicative of the calculated offset value. Motor operation control section  118 , having received the offset signal  117   b , generates a drive control signal (PWM signal)  122   a  and outputs the drive control signal  122   a  to a motor drive section  119  as in the conventional control device of FIG. 10. Thus, the motor drive section  119  drives a steering assisting motor  110  in the same manner as in the conventional control device of FIG. 10.  
         [0045]    Motor current thus caused to flow through the steering assisting motor  110  is detected by the peak-holding-type peak current value detection section  120  shown in FIG. 1. Peak current value ISP detected by the peak current value detection section  120  is passed to the current detection correcting section  10 . Once the detected peak current value ISP has been received, the current detection correcting section  10  operates in accordance with the current correction program  14 . Namely, the CPU  11  determines and outputs a corrected current value IPM by searching the current correction table  13 , stored in the memory  12 , for a particular current correcting amount D corresponding to the received detected peak current value ISP and then executing the arithmetic operation indicated by Mathematical Expression (1).  
         [0046]    After that, the corrected current value IPM is fed to the offset calculation section  117  that calculates an offset of the corrected current value IPM from the value of the target motor current signal IT. Then, the controller  1  supplies the motor drive section  119  with an ultimate output signal (motor drive signal) such that the offset is minimized, in a similar manner to the conventional controller  116 . The above-described arrangements achieve linear current output characteristics relative to the target motor current.  
         [0047]    Reference numeral L 1  in FIG. 5 represents a curve of control characteristics, i.e. characteristics of the motor current relative to the target motor current, detected when the control method of the present invention is applied. In this figure, the horizontal axis represents the target motor current value, while the vertical axis represents the average motor current value. From the curve L 1 , it can be seen that the inventive control method can achieve linear current output characteristics relative to the target motor current value. Reference numeral L 2  represents control characteristics when the conventional control method is applied, for purposes of comparison.  
         [0048]    Because the control method of the present invention linear current output characteristics relative to the target motor current value as noted above, it is possible to set a desired steering assist at an initial manual steering stage when the vehicle driver has just started turning the steering wheel.  
         [0049]    [0049]FIG. 6 is a block diagram showing a second specific example of the current detection correcting section, which is generally denoted by reference numeral  20 . The second specific example  20  is constructed to subtractively correct the detected peak current only when the target motor current is set at a small value (i.e., a small value range) such that the ratio, to the average value Ia of the current flowing through the motor  110 , of the detected peak current value ISP exceeds a predetermined value, so that the thus-corrected current value ISP is used for the feedback control.  
         [0050]    The current detection correcting section  20  includes a CPU  21 , and a memory  22  where are prestored a current correction table  23 , peak-current-value vs. average-current-value table  24  and current correction program  25 . The current correction table  23  is similar to the current correction table  13  employed in the current detection correcting section  10  of FIG. 2. The peak-current-value vs. average-current-value table  24  is a lookup table storing various average motor current values Ia corresponding to various detected peak current values ISP output from the peak current value detection section  120 .  
         [0051]    The current correction program  25  is constructed to calculate a corrected current value IPM from the detected peak current value ISP, as flowcharted in FIG. 7. Once a detected peak current value ISP is input from the peak current value detection section  120  to the current detection correcting section  20  at step ST 20 , the CPU  21  searches the current correction table  23  for an average motor current value Ia corresponding to the input detected peak current value ISP at step ST 21  and calculates a ratio of the detected peak current value ISP to the average motor current value Ia (SP/Ia) at step S 22 . If the calculated ratio (SP/Ia) is smaller than a predetermined value k, the detected peak current value ISP is output as it is, at step ST 23 . If, on the other hand, the calculated ratio (SP/Ia) is equal to or greater than the predetermined value k, the CPU  21  searches through the current correction table  23  for a current correcting amount D corresponding to the input detected peak current value ISP at step ST 24 . Then, the CPU  21  performs the arithmetic operation as indicated by Mathematical Expression (1) to calculate a corrected current value IPM at step ST  25 , and it outputs the thus-calculated corrected current value IPM at step ST 26 . Here, the predetermined value k is set at a value close to one, e.g. “1.1”, such that the steering feel is not impaired.  
         [0052]    Namely, in the case where the current detection correcting section  20  is employed, the detected peak current value ISP is subtractively corrected only when the target motor current signal is set at a small value (i.e., a small value range) such that the ratio of the detected peak current value ISP to the average motor current value Ia (SP/Ia) exceeds the predetermined value k, and the resultant corrected current value IPM is used for the feedback control. In this case, the correction of the detected peak current value is performed only when necessary, so that appropriate correction of the detected peak current value and hence optimal feedback control of the motor current is achieved.  
         [0053]    Whereas the embodiment has been described as performing subtractive motor current correction using the current correction table storing current correcting amounts each obtained by subtracting an average motor current value from a corresponding detected peak current value, the present invention is not so limited. Namely, where the current correction table is one storing current correcting amounts each obtained by subtracting a detected peak current value from a corresponding average motor current value, additive motor current correction is performed instead of the subtractive motor current correction. Further, where the current correction table is one storing current correcting amounts each obtained by dividing a detected peak current value by a corresponding average motor current value, divisional motor current correction is performed instead of the subtractive or additive motor current correction. Furthermore, where the current correction table is one storing current correcting amounts each obtained by dividing a detected peak current value by dividing an average motor current value by a corresponding detected peak current value, multiplicative motor current correction is performed.  
         [0054]    In summary, the present invention is characterized by correcting the detected peak current value and controlling the motor current using, as the feedback value, the corrected peak value that is close to the average value of the motor current. Thus, the current flowing through the steering assisting motor when the target motor current is set at a small value, i.e. the steering assisting current at the beginning of slow turning of the steering wheel during straight travel of the vehicle, can be controlled appropriately in accordance with a desired target motor current, so that an assisted steering feel can be enhanced significantly. Further, because the present invention allows a motor current, corresponding exactly to the target motor current, to flow through the steering assisting motor, it facilitates setting of an appropriate steering assist.  
         [0055]    The present disclosure relates to the subject matter of Japanese Patent Application No. 2001-339301, filed Nov. 5, 2001, the disclosure of which is expressly incorporated herein by reference in its entirety.