Patent Publication Number: US-6902028-B2

Title: Electric power steering control system

Description:
This is a divisional of application Ser. No. 10/634,955 filed Aug. 6, 2003, which is a Divisional Application of abandoned U.S. application Ser. No. 09/995,675 filed Nov. 29, 2001; the above noted prior applications are all hereby incorporated by reference. 
   This application is based on Application No. 2001-169669, filed in Japan on Jun. 5, 2001, the contents of which are hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electric power steering for reducing a required steering force in a steering system by means of mechanical power of a motor. More particularly, the present invention relates to an electric power steering control system for preventing overheating of the motor and a motor drive circuit. 
   2. Description of the Related Art 
   As an example of an art having an objective of preventing overheating of a motor and a motor drive circuit, there exists an electric power steering device described in Japanese Patent Application Laid-open No. 11-59444. 
   According to the above-mentioned conventional art, a heat sensor is provided on a periphery of the motor or the motor drive circuit portion, a motor current limit value is calculated based on a heating value estimated from a temperature detected by the temperature sensor and a current of the motor, and the motor current is limited by means of this calculated motor current limit value. Accordingly, the overheating of the motor is prevented. 
   According to the above-mentioned conventional art, the heat sensor is provided to a vicinity of a location generating heat, and control for preventing overheating is realized by means of directly detecting an ambient temperature of the location generating heat. However, in an actual system there are cases when it is difficult, for reasons of construction and cost, to place the temperature sensor in the vicinity of the heat-generating part. In such cases, prevention of overheating becomes problematic in the conventional art. Furthermore, in a case when there exists a plurality of heat-generating parts (or parts at which it is desirable to estimate the temperature thereof), a plurality of temperature sensors and I/F circuits need to be installed. Therefore, there was a problem that it was disadvantageous in terms of cost and miniaturization. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the problems described above. Therefore, an object of the present invention is to obtain an electric power steering control system capable of achieving overheating prevention even without a temperature sensor being provided to a vicinity of a heat-generating location. 
   An electric power steering control system according to the present invention comprises a motor for adding a steering assisting force to a steering system; a steering force detection section for detecting steering force in the steering system; a motor current determination section for determining a motor current based on at least the steering force detected by means of the steering force detection section; a temperature detection section for detecting an ambient temperature; a coefficient setting section for setting a coefficient in accordance with a detected temperature obtained by means of the temperature detection section; a motor current determination section for detecting a current being passed to the motor; a maximum current limit value calculation section for calculating a maximum current limit value based on the detected current detected by means of the motor current detection section and the coefficient set by the coefficient setting section; a current limiting section for selecting and outputting as a target current the smaller of either the motor current determined by means of the motor current determination section or the maximum current value calculated by means of the maximum current limit value calculation section; a motor current control section for passing the target current to the motor such that the target current becomes equal to the detected current being detected by the motor current detection section. 
   An electric power steering control system according to the present invention comprises a motor for adding steering assistance force to a steering system; a steering force detection section for detecting steering force of the steering system; a motor current determination section for determining a motor current based on at least the steering force detected by means of the steering force detection section; a temperature detection section for detecting an ambient temperature; a timer for measuring time from when predetermined conditions are established; a control temperature calculation section for calculating a control temperature based on the temperature detected by the temperature detection section and the time measured by the timer; a coefficient setting section for setting a coefficient based on the control temperature calculated by the control temperature calculation section; a motor current detection section for detecting a current being passed to the motor; a maximum current limit value calculation section for calculating a maximum current limit value based on a current detected by the motor current detection section and the coefficient set by the coefficient setting section; a current limiting section for selecting the smaller value between the motor current determined by the motor current determination section and the maximum current limit value calculated by the maximum current limit value calculation section, and outputting this as a target current; and a motor current control section for passing the target current to the motor in such a way that the motor current is equal to the current detected by the motor current detection section. 
   An electric power steering control system according to the present invention comprises a motor for adding steering assistance force to a steering system; a steering force detection section for detecting steering force of the steering system; a motor current determination section for determining a motor current based on at least the steering force detected by means of the steering force detection section; a temperature detection section for detecting an ambient temperature; a timer for measuring time from when predetermined conditions are established; a control temperature calculation section for calculating a control temperature based on the temperature detected by the temperature detection section and the time measured by the timer; a coefficient setting section for setting a coefficient based on the control temperature calculated by the control temperature calculation section and the temperature detected by the temperature detection section; a motor current detection section for detecting a current being passed to the motor; a maximum current limit value calculation section for calculating a maximum current limit value based on a current detected by the motor current detection section and the coefficient set by the coefficient setting section; a current limiting section for selecting the smaller value between the motor current determined by the motor current determination section and the maximum current limit value calculated by the maximum current limit value calculation section, and outputting this as a target current; and a motor current control section for passing the target current to the motor in such a way that the motor current is equal to the current detected by the motor current detection section. 
   Further, in an electric power steering control system according to the present invention, the predetermined condition is that the key switch is on. 
   Further, an electric power steering control system according to the present invention further comprises an engine rotation detection section for detecting the number of engine rotations, wherein the predetermined condition is that the number of engine rotations detected by the engine rotation detection section is greater than a predetermined value. 
   Further, an electric power steering control system according to the present invention further comprises a vehicle speed detection section for detecting a vehicle speed, wherein the predetermined condition is that the vehicle speed detected by the vehicle speed detection section is above a predetermined value. 
   Further, in an electric power steering control system according to the present invention, the predetermined condition is that the steering force detected by the steering force detection section is greater than a predetermined value. 
   Further, in an electric power steering control system according to the present invention, the predetermined condition is that the motor current is greater than a predetermined value. 
   Further, in an electric power steering control system according to the present invention, the coefficient setting unit sets the coefficient in accordance with a detected temperature at the time of activation obtained by means of the temperature detection section. 
   Further, an electric power steering control system according to the present invention further comprises a power supply holding section for holding a power supply until the temperature detected by the temperature detection section drops below a predetermined value after the key switch is turned off. 
   Further, an electric power steering control system according to the present invention further comprises a power supply holding section for holding a power supply until the temperature detected by the temperature detection section drops below a predetermined value after the key switch is turned off, or until a duration of time having elapsed since the key switch was turned off is measured and the elapsed duration of time becomes greater than a predetermined duration of time. 
   Additionally, in electric power steering control system according to the present invention, the control temperature calculation section calculates the control temperature based on a temperature that is the temperature detected by the temperature detection section and corrected by a correction amount set in accordance with characteristics of self-generation of heat, and the duration of time measured by the timer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram depicting a construction of an electric power steering control system according to Embodiment 1 of the present invention; 
       FIG. 2  is a block diagram depicting a construction of the a control apparatus of the electric power steering control system according to Embodiment 1 of the present invention; 
       FIG. 3  is a diagram depicting a control block of the control apparatus of the electric power steering control system according to 1 of the present invention; 
       FIG. 4  is a diagram depicting input and output characteristics of a motor current determination unit of the control apparatus of the electric power steering control system, according to Embodiment 1 of the present invention; 
       FIG. 5  is a diagram depicting a current limiting unit of the control apparatus of the electric power steering control system, according to Embodiment 1 of the present invention; 
       FIG. 6  is a flow chart depicting processing of a calculation of a maximum current limit value for the control apparatus of the electric power steering control system, according to Embodiment 1 of the present invention; 
       FIG. 7  is a diagram depicting a coefficient of a coefficient determining unit of the control apparatus in the electric power steering control system according to Embodiment 1 of the present invention; 
       FIG. 8  is a diagram depicting an example of results of calculations by a maximum current limit value calculation unit of the control apparatus in the electric power steering control system according to Embodiment 1 of the present invention; 
       FIG. 9  is a diagram depicting a control block of a control apparatus in an electric power steering control system according to Embodiment 2 of the present invention; 
       FIG. 10  is a flow chart depicting processing of a timer and a control temperature calculation unit of the control apparatus in the electric power steering control system according to Embodiment 2 of the present invention; 
       FIG. 11  is a timing chart depicting operation of the timer and the control temperature calculation unit of the control apparatus in the electric power steering control system according to Embodiment 2 of the present invention; 
       FIG. 12  is a diagram depicting a block control of an electric power steering control system according to Embodiment 3 of the present invention; 
       FIG. 13  is a flow chart depicting processing of a timer and a control temperature calculation unit of a control apparatus in the electric power steering control system according to Embodiment 3 of the present invention; 
       FIG. 14  is a flow chart depicting processing of a coefficient setting unit of the control apparatus in the electric power steering control system according to Embodiment 3 of the present invention; 
       FIG. 15  is a timing chart depicting operation of the timer, the control temperature calculation unit and the coefficient setting unit of the control apparatus in the electric power steering control system according to Embodiment 3 of the present invention; 
       FIG. 16  is a diagram depicting a coefficient of the coefficient setting unit of the control apparatus in the electric power steering control system according to Embodiment 3 of the present invention; 
       FIG. 17  is a diagram depicting a control block of a control apparatus of an electric power steering control system according to Embodiment 4 of the present invention; 
       FIG. 18  is a flow chart depicting processing of a timer and a control temperature calculation unit of the control apparatus in the electric power steering control system, according to Embodiment 4 of the present invention; 
       FIG. 19  is a timing chart of operations of the timer, the control temperature calculation unit and a coefficient setting unit of the control apparatus in the electric power steering control system, according to Embodiment 4 of the present invention; 
       FIG. 20  is a diagram depicting a control block of a control apparatus of an electric power steering control system, according to Embodiment 5 of the present invention; 
       FIG. 21  is a flow chart depicting processing of a timer and a control temperature calculation unit of the control apparatus in the electric power steering control system, according to Embodiment 5 of the present invention; 
       FIG. 22  is a flow chart depicting processing of the timer and the control temperature calculation unit of the control apparatus in the electric power steering control system, according to Embodiment 5 of the present invention; 
       FIG. 23  is a timing chart depicting operations of the tier, the control temperature calculation unit and the coefficient setting unit of the control apparatus in the electric power steering control system, according to Embodiment 5 of the present invention. 
       FIG. 24  is a diagram depicting a control block of a control apparatus of an electric power steering control system according to Embodiment 6 of the present invention; 
       FIG. 25  is a flow chart depicting processing of a timer and a control temperature calculation unit of the control apparatus in the electric power steering control system, according to Embodiment 6 of the present invention; 
       FIG. 26  is a flow chart depicting processing of the timer and the control temperature calculation unit of the control apparatus in the electric power steering control system, according to Embodiment 6 of the present invention; 
       FIG. 27  is a timing chart depicting operations of the timer, the control temperature calculation unit and the coefficient setting unit of the control apparatus in the electric power steering control system, according to Embodiment 6 of the present invention; 
       FIG. 28  is a flow chart depicting processing of a coefficient setting unit of a control apparatus of an electric power steering control system according to Embodiment 7 of the present invention; 
       FIG. 29  is a diagram depicting a construction of a control apparatus of an electric power steering control system according to Embodiment 8 of the present invention; 
       FIG. 30  is a diagram depicting a construction of a power circuit of the control apparatus in the electric power steering control system according to Embodiment 8 of the present invention; 
       FIG. 31  is a flow chart depicting processing of a micon in the control apparatus in the electric power steering control system according to Embodiment 8 of the present invention; 
       FIG. 32  is a flow chart depicting processing of a micon in a control apparatus in an electric power steering control system according to Embodiment 9 of the present invention; 
       FIG. 33  is a flow chart depicting processing of a timer and a control temperature calculation unit of a control apparatus in an electric power steering control system according to Embodiment 10 of the present invention; and 
       FIG. 34  is a diagram depicting characteristics of a correction amount of the control apparatus in the electric power steering control system according to 10 of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiment 1 
   Explanation will now be made of an electric power steering control system according to Embodiment 1 of the present invention, making reference to the drawings.  FIG. 1  is a diagram depicting a construction of the electric power steering control system according to Embodiment 1 of the present invention. Note that the same reference numerals in each of the drawings indicate the same or equivalent parts. 
   In  FIG. 1 ,  1  is a handle;  2  is a torque sensor (i.e., steering force detection section) for detecting steering force of a steering system;  3  is a vehicle speed sensor (i.e., vehicle speed detection section) for detecting a vehicle speed;  4  is an engine rotation signal [i.e., engine rotation sensor (i.e., engine rotation detection section)] for obtaining the number of engine rotations;  5  is a key switch;  6  is a motor for adding a steering assisting force to the steering system;  7  is a control apparatus which is installed in a deep area of a panel in front of a driver seat and a passenger seat of the car and which controls the motor  6  based on information from the torque sensor  2 , the vehicle speed sensor  3 , the engine rotation signal  4 , and the key switch  5 ;  8  is a deceleration apparatus for transmitting output of the motor  6  to the steering system and the like;  9  is a rack and pinion mechanism for converting a rotational force into a horizontal force;  10  is a tie rod for transmitting the horizontal force to a steering wheel to be described hereinafter. 
     FIG. 2  is a diagram depicting a construction of a control apparatus of the electric power steering control system according to Embodiment 1 of the present invention; and  11  is the tie rod. 
   In  FIG. 2 , the torque sensors  2  through the control device  7  are the same as those of FIG.  1 . Reference numeral  12  is a battery for providing electric power to the control device  7 . Further,  71  is an I/F circuit for inputting a signal from the torque sensor  2 ;  72  is an I/F circuit for inputting a signal from the vehicle speed sensor  3 ;  73  is an I/F circuit for inputting the engine rotation signal  4 ;  74  is an I/F circuit for inputting a signal from the key switch  5 ;  75  is the temperature sensor (the temperature detection section) for detecting the temperature;  76  is an I/F circuit for inputting a signal from the temperature sensor  75 ;  77  is a motor drive circuit for driving the motor  6 ;  78  is a current detection circuit for detecting the current being passed to the motor  6 ; and  79  is a micon for performing control of the electrical power steering. 
   Next, explanation will be made of an operation of the electric power steering control system according to Embodiment 1 of the present invention, making reference to the drawings. 
     FIG. 3  is a diagram depicting a control block of processing carried out by the micon inside the control apparatus of the electric power steering control system according to Embodiment 1. 
   Explanation will now be made of the processing carried out by the micon  79 . In  FIG. 3 , reference numeral  101  is a vehicle speed signal detected by the vehicle speed sensor  3 ;  102  is a torque signal detected by the torque sensor  2 ; and  103  is a temperature signal detected by the temperature sensor  75 . 
   Further, in  FIG. 3 ,  104  is a motor current determination unit (i.e., the motor current determination section) for determining, based on a vehicle speed signal  101  (VSP) and a torque signal  102  (TRQ), the motor current for assisting the steering force;  105  is a current limiting unit (i.e., the current limiting section) for applying a limit being the maximum current limit value (Limit), explained below, to the motor current (Iml) determined by the motor current determination unit  104 ;  106  is a motor current control unit (i.e., the motor current control section) for passing the target current (Imt) indicated by the current limiting unit  105  to the motor  6  in a controlled fashion such that the target current (Imt) is equivalent to the detected current (Imd) which is detected by an motor current detection unit explained below; and  107  is the motor current detection unit (i.e., the motor current detection section) for detecting the motor current and corresponds to the current detection circuit  78  of FIG.  2 . 
   Additionally, in  FIG. 3 ,  108  is a coefficient setting unit (i.e., the coefficient setting section) for setting a coefficient for calculation of the maximum current limit value described below in accordance with the temperature signal  103  (i.e., Temp); and  109  is a maximum current limit value calculation unit (i.e., the maximum current limit value calculation section) for calculating the maximum current limit value (i.e., Limit) based on the detected current (i.e., Imd) detected by means of the motor current detection unit  107  and the coefficient which is determined by means of the coefficient setting unit  108  (i.e., the maximum current limit value calculation section). 
     FIG. 4  is a diagram depicting input output characteristics of the motor current determination unit. 
   Here explanation will be made of the motor current determination unit  104 . The motor current determination unit  104  has the input and output characteristics shown in FIG.  4  and determines the motor current (Iml) in accordance with the torque (TRQ) and the vehicle speed (VSP). By having the characteristics shown in  FIG. 4  a result is produced such that at a time of steering to the right the motor current is passed to the right direction, so less steering force is required. Further, at a time of steering to the left, on the other hand, the motor current is passed to the left direction, so less steering force is required. Additionally, altering the motor current in accordance with the vehicle speed (VSP) produces a result that the steering assisting force appropriate for each vehicle speed (ex, low vehicle speed through high vehicle speed) is generated. 
     FIG. 5  is a diagram depicting a construction of the current limiting unit. 
   Next, explanation will be made of the current limiting unit  105 . The current limiting unit  105  has a construction as shown in  FIG. 5 , and selects and outputs as a target current (i.e., Imt) the smaller of either the motor current (i.e., Iml) determined by means of the motor current determination unit  104  or the maximum current limit value (i.e., Limit) calculated by means of the maximum current limit value calculation unit  109 . 
     FIG. 6  is a flow chart depicting processing of the maximum current limit value calculation unit  109 . 
   Next, explanation will be made of the maximum current limit value calculation unit  109 . According to  FIG. 6 , at first when the control device  7  is activated at step  121 , an initial value is set for the maximum current limit value (i.e., Limit). Next, at step  122  an the current increase rate (kim) is calculated; at step  123  the current increase rate (kim) is added to the maximum current limit value (i.e., Limit); and steps  122  to  123  are repeated thereafter. 
     FIG. 7  is a diagram depicting characteristics (i.e., coefficients) of the current increase rate of the maximum current limit value calculation unit. 
   At step  122  the characteristics of the current increase rate (kim) are as in  FIG. 7  by the detected current of the motor (i.e., Imd). Further, the characteristic (i.e., coefficient) of the current increase rate (kim) is constructed such that when the temperature is low the coefficient is as indicated by (1) (n.b. the encircled numbers in the diagrams are represented in parenthesis in the specification for reasons of convenience), and as the temperature rises the coefficient changes to (2) and (3). It is the coefficient setting unit  108  which makes the coefficient change in accordance with the temperature. 
     FIG. 8  is a diagram depicting a result (i.e., an attenuation characteristic) of the calculation performed by the maximum current limit value calculation unit. Note that this  FIG. 8  depicts an example in which the coefficient does not change. When the detected temperature changes, smooth curved lines as shown in  FIG. 8  are not produced. 
   According to the construction described above, the efficient setting unit  108  sets the coefficient based on a temperature Temp detected by means of the temperature sensor  75 , and the maximum current limit value calculation unit  109  calculates the maximum current limit value Limit based on the coefficient set by the coefficient setting unit  108 . When the handle is turned and held as it is in that position the maximum current limit value (i.e., Limit) attenuates as shown in FIG.  8  and limits the motor current. Accordingly, it becomes possible to prevent overheating of the motor  6  and the control device  7 . 
   Further, when the coefficient is switched as shown in  FIG. 7  ( 1 )-( 3 ) by means of the coefficient setting unit  108 , the attenuation characteristics of the maximum current limit value also change as shown in  FIG. 8  ( 1 )-( 3 ) such that the motor current is less at a time of high temperature ( 3 ) than at a time of low temperature ( 1 ). Accordingly, overheating prevention being adapted to the temperature becomes possible. 
   Embodiment 2 
   Explanation will now be made of an electric power steering control system according to Embodiment 2 of the present invention, making reference to the drawings. 
   In this Embodiment 2, the control block in  FIG. 3  of the above-described Embodiment 1 is modified as shown in FIG.  9 . 
   In  FIG. 9 , the same reference numerals perform the same operations as those of  FIG. 3 , so explanation thereof is omitted, and explanation will be made of other parts. 
   In  FIG. 9 ,  110  is a key switch (signal);  111  is timer for measuring a duration of time from when the key switch  110  is turned on; and  112  is a control temperature calculation unit (i.e., a control temperature calculation section) for calculating the control temperature (i.e., Temp) in accordance with the temperature signal  103  and the timer  111 . 
     FIG. 10  is a flow chart depicting an operation of the control temperature calculation unit. 
   Explanation will now be made of operations of the temperature signal  103 , the key switch  110 , the timer  111 , and the control temperature calculation unit  112 , making use of FIG.  10 . 
   In  FIG. 10 , first, at step  130  a temperature correction value TempC and the timer are initialized to zero ( 0 ). 
   Next, at step  131  the signal  103  from the temperature sensor  75  is assigned to TempM. 
   Next, at step  132  the status of the key switch  110  is determined, and if the key switch is off then step  133  is carried out. At step  133  the timer  111  for measuring the on time of the key switch is cleared to zero and the procedure advances to step  138 . 
   In the case when the key switch  110  is on, at step  132  the procedure splits off to YES, and at step  134  the timer  111  is incremented. Next, at step  135  the value indicated by the timer  111  is compared against a predetermined value Time 1 , and in the case when the value of the timer  111  is smaller the procedure splits off to No and advances to step  138 . 
   In the case when the timer  111  is greater than the predetermined value Time1, the procedure splits off to YES at step  135  and at step  136  the timer  111  is cleared to zero. 
   Then, at step  137  the predetermined value T 1  is added to the temperature correction value TempC and the procedure advances to step  138 . 
   At step  138 , the temperature correction value TempC is subtracted from the TempM which has saved the signal  103  of the temperature sensor  75 , and the result is assigned to the control temperature Temp. 
   After that, at step  139  the control temperature Temp is compared against the predetermined value T 2 , and in the case when the control temperature Temp is equal to or greater than the predetermined value T 2  the procedure splits off to NO and returns to step  131 . On the other hand, in the case when the control temperature Temp is less than the predetermined value T 2  at step  139 , the procedure splits off to YES and the predetermined value T 2  is then assigned to the Temp at step  140 . 
   According to the processing as depicted in  FIG. 10 , the temperature correction value TempC is zero immediately after the activating of the control device  7 ; therefore, at step  138  the value TempM detected by means of the temperature sensor  75  is assigned to the control temperature Temp. After that, when the key switch  110  is turned on the temperature correction value TempC increases by increments of the predetermined value T 1  for each time a predetermined duration of time Time 1  elapses. At step  138  the control temperature Temp drops as this temperature correction value TempC increases. Note that at steps  139  and  140  the control temperature Temp is clipped so that it drops only as far as the predetermined value T 2 . This situation is depicted by the timing chart of FIG.  11 . 
   Other parts of  FIG. 9  operate similarly to those of Embodiment 1 described above. Therefore, the coefficient setting unit  108  sets the coefficient based on the control temperature Temp calculated by means of the control temperature calculation unit  112 , and the maximum current limit value calculation unit  109  calculates the maximum current limit value Limit based on the coefficient set by the coefficient setting unit  108 . 
   In the case of Embodiment 2 of the present invention, the operations as described above produce effects as follows. In a case when the temperature signal  103  detected by means of the temperature sensor  75  indicates a high temperature immediately after starting, the control temperature Temp gradually drops and the maximum current limit is relaxed in accordance with the elapsing of time from when the key switch was turned on. That is, even when a temperature inside the passenger compartment of the vehicle (i.e., an ambient temperature) rises, when the driver boards the vehicle the control anticipates that the driver will normally operate the air conditioning or open a window to lower the temperature inside the passenger compartment. 
   In a case when it is necessary due to constructional or other such considerations to attach the temperature sensor at a position which is apart from the place where the temperature must be measured, or also in a case when a heat-generating body or an entity which is relatively hot exists in the vicinity of the temperature sensor and it becomes difficult to detect the ambient heat of the place where the temperature must be measured, a construction such as that of Embodiment 2 enables the ambient temperature of the object portion to be predicted and the maximum current limit value to be calculated in accordance therewith. 
   Embodiment 3 
   Explanation will now be made of the electric power steering control system according to Embodiment 3 of the present invention, with reference to the drawings. 
   In this Embodiment 3, the control block in  FIG. 3  of the above-described Embodiment 1 is modified as shown in FIG.  12 . 
   In  FIG. 12 , the same reference numerals perform the same operations as those of  FIG. 3 , so explanation thereof is omitted, and explanation will be made of other parts. 
   In  FIG. 12 ,  150  is the number of engine rotations obtained from the engine rotation signal  4  (i.e., the engine rotation detection section);  151  is a timer for measuring a duration of time from when the number of engine rotations  150  reaches a predetermined value or greater;  152  is a control temperature calculation unit for calculating the control temperature Temp in accordance with the temperature signal  103  and the timer  151 ;  153  is a coefficient setting unit for setting a coefficient of the maximum current limit value calculation unit  109  based on the control temperature Temp and the temperature signal  103 . 
   Next, explanation will be made of operations of the temperature signal  103 , the engine rotation signal  150 , the timer  151  and the control temperature calculation unit  152 , making use of FIG.  13 . 
   In  FIG. 13 , step  132  in  FIG. 10  of Embodiment 2 has been changed to step  161 . That is, the only change is that the on/off status of the key switch in  FIG. 10  has been changed to the number of engine rotations being greater than/less than the predetermined value NE1. 
   Next, explanation will be made of the coefficient setting unit  153 . The coefficient setting unit  153  has a flag for operating in accordance with to the temperature signal  103  and operates as shown in FIG.  14 . 
   First, at step  171  a flag F_mode is cleared to 0. Next, the detected temperature is checked, and in the case when the detected temperature is greater than a predetermined value T 3  the procedure splits off to YES at step  172  and at step  173  the flag F_mode is set to 1. When the temperature drops and is below the predetermined value T 3  the procedure splits off to NO at step  172 . 
   Next, at step  174  the detected temperature and the predetermined value T 4  are compared, and if the detected temperature is greater than the predetermined value T 4  then the procedure splits off to NO and returns to step  172 . In the case when the detected temperature is less than the predetermined value T 4  the procedure splits off to YES at step  174 , and at step  175  the flag F_mode is cleared to 0, and thereafter, steps  172  to  175  are repeated. 
   At this point, when the predetermined values T 3  and T 4  are set such that T 3 &gt;T 4 , when the temperature rises and exceeds T 3  the flag F_mode is set to 1, and once it is so set, the flag F_mode is maintained at 1 until the temperature becomes less than the predetermined value T 4 , and when the temperature becomes less than the predetermined value T 4  the flag F_mode is cleared to 0. 
   The operations of  FIGS. 13 and 14  are depicted as a timing chart as shown in FIG.  15 . 
   Further, in a case when the flag F_mode is 0, the coefficient setting unit  153  sets the coefficients (1)-(3) in  FIG. 16  in accordance with the control temperature Temp. When the control temperature is high the coefficient (3) is selected, and as the control temperature drops the coefficient is switched from (3) to (2) to (1). However, in the case when the flag F_mode is 1, the coefficient setting unit  153  is configured to select coefficient (4) regardless of the control temperature Temp. This produces a result of the following operations. 
   First, when the control device  7  activates the flag F_mode is cleared to 0 and the temperature detected by means of the temperature sensor  75  is set to as the control temperature Temp. The flag F_mode is 0, therefore, the coefficient setting unit  153  selects a coefficient from (1)-(3) in  FIG. 16  in accordance with the control temperature Temp. Then, the maximum current limit value calculation unit  119  calculates the maximum current limit value Limit based on the coefficient set by the coefficient setting unit  153  and the motor current Imd, thus limiting the motor current by means of the current limiting unit  105 . 
   After that the engine starts, and when the number of engine rotations  150  becomes greater than the predetermined value NE 1  the timer  151  begins incrementing, and the control temperature Temp decreases by increments of the predetermined value T 1  each time the predetermined time duration Time  1  elapses until the control temperature Temp drops to the predetermined value T 2 . 
   As this takes place, the coefficient selected by the coefficient setting unit  153  changes, and thus the maximum current limit is relaxed. 
   When use continues in this state and the value detected by the temperature sensor  75  rises due to some cause and exceeds the predetermined value T 3 , the flag F_mode is set to 1. When the flag F_mode becomes 1 the coefficient setting unit  153  selects the coefficient (4) in FIG.  16 . This coefficient (4) is the coefficient which most quickly limits the motor current, so the rising of the temperature may be suppressed. Further, when the temperature drops and the temperature detected by the temperature sensor  75  becomes less than the predetermined value T 4 , the flag F_mode is cleared to 0 and the maximum current limit value calculation which is suitable for the original control temperature Temp is reset again. 
   Operations in accordance with Embodiment 3 are as described above; therefore, in this embodiment it is forecast that the ambient temperature (i.e., the temperature inside the passenger compartment) will drop due to operation of the air conditioner or such when the driver boards the vehicle and starts the engine, and the maximum current calculation is performed in accordance with that effect. Accordingly, unnecessary limitation of the current is not performed, so the overall control is pleasant in feeling. Additionally, in a case when the temperature rises and the temperature detected by target current temperature sensor  75  rises in contrast to the forecast that the temperature would drop, it is possible to force a switch of the control coefficient and urge the system to perform the calculation of the maximum current limit value so as to immediately suppress the rising of the temperature. 
   Embodiment 4 
   Explanation will now be made of an electric power steering control system according to Embodiment 4 of the present invention, making reference to the drawings. 
     FIG. 17  is a control block diagram of Embodiment 4, in which the engine rotation signal  150  of the control block of Embodiment 3 shown in  FIG. 12  is changed to a vehicle speed  101 . 
   With respect to the controls, only one a part thereof has been changed. That is,  FIG. 18  is a flow chart of this Embodiment 4 in which the step  161  of Embodiment 3 shown in  FIG. 13  has been changed as indicated at step  191 . 
   Therefore, as for the operations thereof, in Embodiment 3 the timer begins operating from the time of the starting of the engine; however, in Embodiment 4 the timer only begins operating from the time when the vehicle speed  101  becomes greater than a predetermined value of SP1. Therefore, Embodiment 4 operates as shown in the timing chart of FIG.  19 . 
   Operation in accordance with Embodiment 4 are as described above, so when a driver boards and begins to run the vehicle, a drop in the ambient temperature is predicted and a calculation of the maximum current limit is performed in accordance with this result. Accordingly, unnecessary limitation of the current is not performed, so the overall control is pleasant in feeling. Additionally, in a case when the temperature rises in contrast to the prediction of a drop in temperature such that the temperature detected by the temperature sensor  75  rises, it is possible to force a switch of the control coefficient, and cause the calculation of a maximum current limit value so as to immediately suppress the rise in the temperature. 
   Embodiment 5 
   Explanation will now be made of an electric power steering control system according to Embodiment 5 of the present invention, making reference to the drawings. 
     FIG. 20  is a control block diagram of Embodiment 5, in which the engine rotation signal  150  of the control block of Embodiment 3 shown in  FIG. 12  is changed to a torque  102 . 
   Next, explanation will be made of the controls making reference to FIG.  21  and FIG.  22 . At step  201  in  FIG. 21 , the control temperature calculation unit  152  clears the flag F_TRQ to zero. Next, at step  202  an absolute value of the torque signal  102  and a predetermined value TRQ 1  are compared against each other, and in the case when the absolute value of the torque is less than the predetermined value TRQ 1  the procedure splits off to NO and returns to step  202 . In the case when the absolute value of the torque is greater than the predetermined value TRQ 1  the procedure splits off to YES, and at step  203  the flag F_TRQ is set to 1 and the procedure returns to step  202 . 
   Next, explanation will be made of the flow chart of FIG.  22 . In  FIG. 22 , step  161  of the flow chart of  FIG. 13  of Embodiment 3 has been changed to step  204 . 
   Performing of control as depicted in FIG.  21  and  FIG. 22  enables the following operation. Immediately after the control device  7  is activated the flag F_TRQ is in the cleared state of zero, so at step  204  the procedure splits off to NO and the timer  151  is turned to the cleared state of zero. After that the driver steers the handle  1 , and when the absolute value of the torque becomes greater than the predetermined value TRQ 1  the flag T_TRQ is set to 1 at step  203 . 
   Once the flag F_TRQ is set to 1, the flag F_TRQ remains set at 1 thereafter even if the absolute value of the torque drops below the predetermined value of TRQ1. When the flag F_TRQ is set to 1 the procedure splits off to YES at step  204 , and the timer  151  performs its incremental operation. This enables operations as depicted in the timing chart of FIG.  23 . 
   Operations according to Embodiment 5 are as described above; therefore, when the driver boards the vehicle, once he or she steers the handle a drop in the ambient temperature is predicted, and the calculation of the maximum current limit is performed in accordance with this result. Accordingly, unnecessary limitation of the current is not performed, so the control is pleasant in feeling. Additionally, in a case when the temperature rises in contrast to the prediction of a drop in temperature such that the temperature detected by the temperature sensor  75  rises, it is possible to force a switch of the control coefficient, and cause the calculation of a maximum current limit value so as to immediately suppress the rise in the temperature. 
   Embodiment 6 
   Explanation will now be made of an electric power steering control system according to Embodiment 6 of the present invention, making reference to the drawings. 
     FIG. 24  is a control block diagram of Embodiment 6, in which the engine rotation signal  150  of the control block of Embodiment 3 shown in  FIG. 12  is changed to a motor current Imd. 
   Next, explanation will be made of operations making reference to FIG.  25  and FIG.  26 . At step  211  in  FIG. 25 , the control temperature calculation unit  152  clears the flag F_Im to zero. Next, at step  212  the motor current Imd obtained through the timer  151  and a predetermined value Imd 1  are compared against each other, and in the case when motor current Imd is less than the predetermined value Imd 1  the procedure splits off to NO and returns to step  212 . In the case when the motor current Imd is greater than the predetermined value Imd 1  the procedure splits off to YES, and at step  213  the flag F_Im is set to 1 and the procedure returns to step  212 . 
   Next, explanation will be made of the flow chart of FIG.  26 . In  FIG. 26 , step  161  of the flow chart of  FIG. 13  of Embodiment 3 has been changed to step  214 . 
   Performing of control as depicted in FIG.  25  and  FIG. 26  enables the following operation. Immediately after the control device  7  is activated the flag F_Im is in the cleared state of zero, so at step  214  the procedure splits off to NO and the timer  151  is turned to the cleared state of zero. After that the driver steers the handle  1 , and when the motor current becomes greater than the predetermined value Imd 1  the flag F_Im is set to 1 at step  213 . Once the flag F_Im is set to 1, the flag F_Im remains set at 1 thereafter even if the motor current drops below the predetermined value Imd 1 . 
   When the flag F_Im is set to 1 the procedure splits off to YES at step  214 , and the timer performs its incremental operation. Accordingly, this enables operations as depicted in the timing chart of FIG.  27 . 
   Operations according to Embodiment 6 are as described above; therefore, when the driver boards the vehicle, once he or she steers the handle and the current is passed to the motor  6 , a drop in the ambient temperature is predicted, and the calculation of the maximum current limit is performed in accordance with this result. Accordingly, unnecessary limitation of the current is not performed, so the control is pleasant in feeling. Additionally, in a case when the temperature rises in contrast to the prediction of a drop in temperature such that the temperature detected by the temperature sensor  75  rises, it is possible to force a switch of the control coefficient, and cause the calculation of a maximum current limit value so as to immediately suppress the rise in the temperature. Further, in Embodiment 6 the detected current of Imd is used for the motor current; however, an equivalent effect may be obtained by using the target current Iml or Imt shown in  FIG. 3  as well. 
   Embodiment 7 
   Explanation will now be made of an electric power steering control system according to Embodiment 7 of the present invention, making reference to the drawings. 
   The control block diagram of this Embodiment 7 is the same as the diagram used in connection with the above-mentioned Embodiment 1. 
   In operation, as well, Embodiment 7 differs from Embodiment 1 only with respect to temperature detection  103 . 
   Explanation will be made of the temperature detection, making reference to FIG.  28 . In  FIG. 28 , when the control device  7  is activated the coefficient setting unit  108  first saves as the Temp the temperature detected at step  221 . After that, this detected temperature Temp which has been saved is then held. This manner of construction enables the coefficient setting unit  108  to set the coefficient based on the temperature at the time of the activation of the control device  7 . 
   A construction such as the one described above enables the following effects. The control device  7  has each of the circuits shown in  FIG. 2  other than the motor drive circuit  77  built therein. Due to this, when an electrical power source is turned on for the control device  7  the temperature of the control device  7  rises even if the motor current is not flown (hereinafter, this is referred to as self-generation of heat). When the temperature sensor  75  is installed to the inside portion of the control device  7  the temperature detected by the temperature sensor  75  rises due to the self-generation of heat, and it becomes impossible to accurately detect the ambient temperature. 
   According to Embodiment 7 the coefficient is set using the temperature immediately after activation. Immediately after activation there is almost no self-generation of heat; therefore, the detected temperature is the same as the ambient temperature. Therefore, it becomes possible to set the coefficient in accordance with the ambient temperature. 
   According to Embodiment 7 the temperature that was measured once at the time of activation is held; however, it is also possible to hold a value of an average temperature during a fixed period of time after activation when the influence of the self-generation of heat is small, and the coefficient for the calculation of the maximum current limit value may be set according to this held temperature. 
   Embodiment 8 
   Explanation will be now be made of an electric power steering control system according to Embodiment 8 of the present invention, making reference to the drawings. 
     FIG. 29  is a diagram depicting a construction of a control apparatus according to Embodiment 8 of the present invention. In this diagram the same reference numerals as those in  FIG. 2  refer to parts which are the same as in Embodiment 1; therefore, explanation will be made of parts other than these. 
   In  FIG. 29  reference numeral  80  is a power circuit (a part of a power supply holding unit). 
     FIG. 30  is a diagram depicting an interior construction of the current according to Embodiment 8. 
   In  FIG. 30 , Q 1 , Q 2  and Q 3  are transistors, and  81  is a 5-volt power circuit for generating a steady 5 volts of voltage from a battery voltage VB (12 volts) obtained through the transistor Q 3 . 
   A construction such as that of  FIG. 30  enables a key switch  5  to turn on, and when the transistor Q 1  is turned on or the transistor Q 2  is turned on by means of a signal (VCONT) outputted from the micon  79 , the transistor Q 3  is turned on and the battery voltage VB is supplied to the 5-volt power circuit  81 . Accordingly, it becomes possible for the 5-volt power circuit  81  to supply 5 volts of electrical voltage Vcc to the micon  79 . 
   Next, explanation will be made of controls of the power circuit  80 , making reference to the flow chart of FIG.  31 . In this figure, at step  231  the power supply holding unit (not shown; i.e., the part of the power supply holding section) inside the micon  79  performs an on/off determination of the key switch  5  based on information outputted from the key switch I/F circuit in FIG.  29 . If the key switch  5  is on, then the procedure splits off to YES at step  231 . After splitting off to YES at step  231 , the VCONT signal being outputted from the micon  79  is set to high at step  234  and the transistor Q 2  is turned on. After that the procedure returns to step  231 . 
   On the other hand, when the key switch  5  is off at step  231  the procedure splits off to NO. Next, at step  232  the value detected by the temperature sensor  75  and a predetermined value T 5  are compared against each other, and in the case when the detected temperature is greater than the predetermined value T 5  the procedure splits off to YES and the transistor Q 2  is turned on at step  234 . In the case when the detected temperature is less than the predetermined value T 5  the procedure splits off to NO at step  232  and the transistor Q 2  is turned off at step  233 . After that, the procedure returns to step  231  and the same processing is repeated. 
   According to the construction described above, first, when the key switch  5  is turned on the transistor Q 1  is turned on, and due to this the transistor Q 3  is turned on; therefore, the battery voltage VB is supplied to the 5-volt power circuit  81 , and when the 5-volt electrical power source Vcc is supplied to the micon  79  the micon  79  is activated. When the micon  79  is activated the processing depicted in  FIG. 31  is carried out. At this point, the key switch  5  is in the on state, so according to the processing in  FIG. 31  the transistor Q 2  is turned on. Next, when the key switch  5  is turned off the transistor Q 1  is turned off; however, the micon  79  has already turned the transistor Q 2  on, so the transistor Q 3  is in the on state. Since the transistor Q 3  is in the on state the power supply is provided to the micon  79  and it is possible for the micon  79  to continue operating. 
   In other words, after the key switch  5  is turned of f, the power supply holding section, which is constructed of the power circuit  80  and the power supply holding unit inside the micon  79 , holds the electrical source Vcc (5 volts) for the micon  79  until the temperature detected by the temperature sensor  75  drops below the predetermined value T 5 . 
   Although detailed depiction thereof is not made, it is normal to stop the motor drive after the key switch is turned off, and therefore, the temperature of the control device  7  drops. When the temperature of the control device  7  drops the temperature detected by the temperature sensor  75  also drops accordingly. When this temperature drops below the predetermined value T 5  the procedure splits off to NO at step  232  of  FIG. 31 , and at step  233 , Q 2  is turned off. The key switch  5  is already turned off and the transistor Q 1  is already in the off state; therefore, when the transistor Q 2  turns off this causes the transistor Q 3  to turn off. The power supply to the micon  79  is stopped and the control device  7  stops completely. 
   When Embodiment 8 is joined together with Embodiment 3, for example, the following effects are obtained. According to Embodiment 3, when the engine starts it is considered that the driver has boarded the vehicle, and the process is performed for gradually lowering the control temperature Temp. Accordingly, even in the case when the temperature detected by the temperature sensor  75  is high the control temperature Temp is dropping. Therefore, the limit set by the maximum current limit value calculation is relaxed, and good electrical power steering may be realized. However, when the key switch  5  is turned off while the temperature detected by the temperature sensor  75  is still high and the electrical source of the control device  7  is cut off and then the system is immediately reactivated, the micon  79  performs processing once again from the beginning Accordingly, control is started once again beginning with the high temperature detected by the temperature sensor  75 , which is not desirable. 
   However, when joined together with Embodiment 8, the control device  7  continues performing control when the temperature detected by the temperature sensor  75  is still high after the key switch is turned off. Even if the key switch is turned off and then immediately turned on, the control device  7  does not perform processing again from the beginning. Therefore, the maximum current limit value calculation is not performed using a high temperature. Additionally, after the key switch is turned off, if the temperature detected by the temperature sensor  75  drops and goes below the predetermined value, T 5  then the electrical source of the control device  7  is cut off. When it is reactivated the next time the processing is performed again from the beginning; however, at this point the temperature of the temperature sensor  75  has dropped, so a current limiting value calculation is not performed using a high temperature. In this way in Embodiment 8 it is possible to avoid performing a calculation of the motor current limit value at a high temperature, so good electrical power steering may be realized. 
   Embodiment 9 
   Explanation will now be made of an electric power steering control system according to Embodiment 9 of the present invention, making reference to the drawings. 
   In Embodiment 9 the flow chart in  FIG. 31  pertaining to Embodiment 8 is changed to FIG.  32 . 
   Explanation will now be made of FIG.  32 . At step  241  the electrical source holding unit inside the micon  79  determines whether the key switch  5  is ON or OFF based key switch information inputted from the key switch I/F circuit  74  in  FIG. 29 , and in the case when the key switch is on the procedure splits off to NO. Next, at step  242  the key off timer is cleared to zero and at step  243  the VCONT signal (high) is outputted so as to turn on the transistor Q 2 . Then the procedure returns to step  241 . 
   On the other hand, in the case when the key switch  5  is turned off, the procedure splits off to YES at step  241  and at step  244  the key off timer performs increments. Next, at step  245  the key off timer is checked, and when the key off timer is below a fixed time Time 2  the procedure splits off to NO. At step  246  the temperature detected by the temperature sensor  75  and the predetermined value T 5  are compared against each other, and when the detected temperature is greater than the predetermined value T 5  the procedure splits off to NO and advances to step  243 . 
   When the key off timer is greater than the predetermined value Time  2  at step  245  or when the temperature is less than the predetermined value T 5  at step  246  the procedure advances to step  247 , and the VCONT signal (low) is outputted so as to turn the transistor Q 2  off. After that the process returns to step  241 . 
   According to the process described above, when the temperature is greater than the predetermined value T 5  after the key switch is turned off the transistor Q 2  is turned on and the power supply is provided to the micon  79 , and accordingly, it becomes possible to continue control. Further, when the temperature drops and goes below the predetermined value T 5  the transistor Q 2  is turned off, the provision of the power supply to the micon  79  is stopped and the control device  7  may be completely stopped; therefore, it is possible to obtain an effect equivalent to that of Embodiment 8. 
   Additionally, according to Embodiment 9, when a fixed duration of time elapses after the key off timer is used to turn the key switch off, according to step  247  the transistor Q 2  is turned off and enables the control device  7  to be turned off. Because of this process, when the temperature sensor  75  fails and a state continues in which the detected temperature value exceeds the predetermined value T 5  it is still possible to cut off the electrical source and stop the control device  7  when the predetermined duration of time elapses. Therefore, even if the temperature sensor  75  fails it is possible to prevent the battery from going dead. 
   Embodiment 10 
   Explanation will now be made of an electric power steering control system according to Embodiment 10 of the present invention, making reference to the drawings. 
   In Embodiment 10 the flow chart of  FIG. 13  for Embodiment 3 is altered as depicted in FIG.  33 . The reference numerals in  FIG. 33  which are the same as those in  FIG. 13  indicate processes which are the same as Embodiment 3. Note that Embodiment 10 may be applied not only to Embodiment 3, but also to Embodiments 1 and 2. 
   Explanation will be made of the points in  FIG. 33  which are different from FIG.  13 . First, after the control device  7  is activated, at step  130  the control temperature calculation unit  152  performs initialization of a corrected temperature value TempC and of the timer, and then at step  251  clears an activation timer to zero. Next, at step  252  a value equal to the temperature detected by the temperature sensor  75  less a correction amount COR is substituted for TempM. Next, at step  253  the above-mentioned activation timer is incremented. Thereafter, the processing from step  161  to step  140  is the same as in Embodiment 3 described above, and after that, the process returns to step  252  and repeats the same processing. 
   Next, explanation will be made of the correction amount COR of step  252 . This correction amount COR is a value which changes with time, as shown in  FIG. 34 , and this time is the time measured by the above-mentioned activation timer. 
   Typically, when the control device  7  is activated the temperature rises with time due to the energy consumed by the circuit inside the control device  7 , even if the motor current is not transmitted. Due to this self-generation of heat, there develops a discrepancy between the temperature at the portion which needs to be measured and the detected temperature; therefore, calculation of the appropriate maximum current limit value becomes difficult. However, according to Embodiment 10, by setting the characteristics of the correction amount COR in  FIG. 34  in accordance with the characteristics of the self-generation of heat it becomes possible to make a calculation of an appropriate maximum current limit value.