Abstract:
An electrically operated power steering apparatus for an automotive vehicle having a steering wheel to be operated by the vehicle operator includes a motor and a power supply restricting device. The motor applies a drive force thereof to a torque transmitting system so as to assist a steering torque applied to the steering wheel by the vehicle operator. The power supply restricting device utilizes a temperature of a heated portion of the electrically operated power steering apparatus which emits heat as a result of a supply of an electric power to the motor, at a reference point of time, as an initial temperature of the heated portion, utilizes a plurality of electric power-related values each of which is related to at least one of a current and a voltage value of the motor, as a plurality of physical quantities related to temperature increases of the heated portion each of which is an increase of the temperature of the heated portion from that at the reference point of time, and restricts the electric power supply to the motor such that an actual value of the temperature of the heated portion does not exceed a predetermined upper limit thereof.

Description:
This application is based on Japanese Patent Application No. 11-156675 filed Jun. 3, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrically operated power steering apparatus in which a drive force of a motor assists a required value of steering torque applied to a steering wheel operated by an operator of an automotive vehicle. 
     2. Discussion of the Related Art 
     An electrically operated power steering apparatus for an automotive vehicle is generally constructed to include (a) a torque transmitting system transmitting steering torque which is applied to a steering wheel by a vehicle operator, to a steerable wheel of the vehicle laying on a road surface, so as to assist the steering torque, (b) a motor applying a drive force thereof to the torque transmitting system, and (c) a controlling device controlling an electric power supply to the motor. 
     Japanese Patent Publication No. 10-100913 discloses an example of a conventional type of the electrically operated power steering apparatus identified above. In this example, the temperature of a winding of a motor is estimated on the basis of a voltage and a current value of the motor, without the provision of a temperature sensor detecting a temperature of the motor. The estimated temperature results in preventing overheat of the motor. 
     However, in general, what can be accurately estimated in relation to the temperature of a winding of a motor by the use of a voltage and a current value of the motor is an increase of temperature of the motor winding at each one of a plurality of discrete points of time after a reference point of time. The increase is calculated from the temperature of the heated portion obtained at the reference point of time. The increase is a relative value, not an absolute value of temperature of the motor winding. In addition, the example explained above is not designed to detect or estimate an absolute value of temperature of the motor winding at the reference point of time. Consequently, this example fails to obtain the temperature of the motor with an adequately high precision. 
     In the case where a temperature sensor is arranged sufficiently near a winding of a motor, the temperature of the motor can be sequentially detected at a high precision. However, in this case, since such a temperature senor is generally expensive, there arises a problem that the substantial increase in the cost of an electrically operated power steering apparatus is unavoidable. 
     In addition, a problem of generation of heat as a result of a supply of an electric power to a motor arises about the motor in an electrically operated power steering apparatus, but the same problem can also arise about other electrical parts of the apparatus. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an electrically operated power steering apparatus in which the temperature of a heated portion of the apparatus which emits heat as a result of a supply of electric power to a motor can be more precisely obtained in lower cost. 
     The object may be achieved according to any one of the following modes of this invention. Each of these modes of the invention is numbered like the appended claims, and depends from the other mode or modes, where appropriate. This type of explanation about the present invention is for better understanding of some ones of a plurality of technical features and a plurality of combinations thereof disclosed in this specification, and does not mean that the plurality of technical feature and the plurality of combinations in this specification are interpreted to encompass only the following modes of this invention: 
     (1) An electrically operated power steering apparatus for an automotive vehicle having a steering wheel to be operated by an operator of the vehicle and a steerable wheel thereof laying on a road surface, comprising: 
     a torque transmitting system transmitting a steering torque which is applied to the steering wheel by the operator, to the steerable wheels; 
     a motor applying a drive force thereof to the torque transmitting system so as to assist the steering torque; 
     a controlling device controlling an electric power supply to the motor, thereby permitting reduction in a required value of the steering torque with the assist of the drive force of the motor; and 
     a power supply restricting device utilizing a temperature of a heated portion of the electrically operated power steering apparatus which emits heat as a result of the electric power supply to the motor, at a reference point of time, as an initial temperature of the heated portion, utilizing a plurality of electric power-related values each of which is related to at least one of a current and a voltage value of the motor, as a plurality of physical quantities related to temperature increases of the heated portion each of which is an increase of the temperature of the heated portion from that at the reference point of time, and restricting the electric power supply to the motor such that an actual value of the temperature of the heated portion does not exceed a predetermined upper limit thereof. 
     In general, the temperature of the heated portion, if not changed or gently changed, forms a constant relationship thereof with an ambient temperature of the heated portion. In addition, the ambient temperature of the heated portion can be detected without using a temperature sensor exclusively used for detection of the temperature of the heated portion. Accordingly, the temperature of the heated portion, if not changed or gently changed, can be easily detected or estimated. 
     The use of at least one of a current and a voltage value of the motor permits estimation of an increase of the temperature of the heated portion from that at a reference point of time. In addition, control of the motor is usually effected with feedback of at least one of a current and a voltage value of the motor. Therefore, a sensor detecting at least one of a current and a voltage value of the motor is usually provided with the electrically operated power steering apparatus. Consequently, in many cases, at least one of a current and a voltage value of the motor can be easily detected. 
     There exists a technology of using the temperature of the heated portion detected or estimated in the manner mentioned above, as an initial temperature of the heated portion. There also exists a technology of using at least one of a current and a voltage value of the motor as a physical quantity related to an increase of the temperature of the heated portion from that at a time when the initial temperature of the heated portion has been obtained. These two technologies can cooperate with each other to estimate the temperature of the heated portion at each one of a plurality of discrete points of time after the initial temperature of the heated portion has been obtained. 
     Based on the above findings, the apparatus according to this mode ( 1 ) utilizes the temperature of the heated portion at a reference point of time as an initial temperature of the heated portion, and utilizes at least one of a current and a voltage value of the motor as a physical quantity related to an increase of the temperature of the heated portion from that at the reference point of time. Further, the apparatus according to this mode ( 1 ) restricts an electric power supply to the motor so as to prevent an actual value of the temperature of the heated portion from exceeding a predetermined upper limit of the temperature of the heated portion. 
     Consequently, in the apparatus according to this mode ( 1 ), it is not indispensable to employ a temperature sensor which exclusively detects the temperature of the heated portion with a high precision and which is expensive. Therefore, a substantial increase in the cost of the apparatus resulting from the addition of a function of obtaining the temperature of the heated portion to an electrically operated power steering apparatus can be easily avoided. 
     Further, in the apparatus according to this mode ( 1 ), a determination as to whether the restriction on the electric power supply to the motor is necessary or not is performed using both of an initial value and a subsequent increase of the temperature of the heated portion from the initial value, both of which are reflected by the actual condition of the heated portion. As a result, the presence of an unnecessary restriction on the electric power supply at lower temperature of the heated portion can be easily avoided, and the absence of a necessary restriction on the electric power supply at higher temperature of the heated portion can also be easily avoided. 
     The apparatus according to this mode ( 1 ) may be adapted to include a temperature sensor capable of precisely detecting the temperature of the heated portion as long as the temperature is substantially in a stable condition thereof. The apparatus according to this mode ( 1 ) may be also adapted to include a temperature sensor capable of precisely detecting the temperature of the heated portion not only in a stable condition thereof but also in a transitional condition thereof. 
     In the apparatus according to this mode ( 1 ), the heated portion may be defined as the motor, a switching element connected to the motor and a power supply to the motor, at least one of a plurality of media for transferring current from the power supply to the motor, including such as a wire, a connector, etc., for example. The heated portion also may be defined as at least one of the plurality of media which is especially required to be prevented from being overheated. 
     In the apparatus according to this mode ( 1 ), the torque transmitting system is generally constructed to include (a) a steering shaft rotatable with the steering wheel, (b) an axially movable steering rod permitting the orientation of the steerable wheel to change, and (c) a coupling device operatively coupling the steering shaft and steering rod such that a rotary motion of the steering shaft is converted into a linear motion of the steering rod. In this arrangement, the motor is engaged to at least one of the steering shaft, steering rod and coupling device so as to apply to the at least one of these three elements a drive force of the motor for assisting the steering torque of the steering wheel applied by the vehicle operator. 
     In the apparatus according to this mode ( 1 ), the restriction on the electric power supply may be effected by reducing an actual and absolute value of current (i.e., electric current) of the motor to a certain value. The certain value is smaller than a nominal value of current of the motor available when the restriction on the electric power supply is unnecessary, but is not equal to zero. The restriction on the electric power supply also may be effected by reducing the actual and absolute value to zero. 
     (2) The apparatus according to the above mode ( 1 ), wherein the power supply restricting device comprises: 
     a temperature estimating means for repeating obtaining one of the plurality of electric power-related values after the reference point of time, for obtaining a sum of the plurality of electric power-related values which have been already obtained, each time a new one of the plurality of electric power-related values has been obtained, the obtained sum being defined as an integrated value of the already obtained plurality of electric power-related values, for estimating the temperature increase of the heated portion on the basis of the obtained integrated value, and for estimating the temperature of the heated portion at each one of a plurality of discrete points of time after the reference point of time, on the basis of the initial temperature and the estimated temperature increase of the heated portion; and 
     a power supply restricting means for restricting the electric power supply to the motor such that the actual value of the temperature of the heated portion does not exceed the predetermined upper limit, on the basis of the estimated temperature of the heated portion. 
     In the apparatus according to this mode ( 2 ), an increase of the temperature of the heated portion is estimated on the basis of an integrated value of a plurality of electric power-related values. As a result, the temperature increase of the heated portion is estimated by the adequate consideration of a time-dependent change in the electric power-related value. Therefore, in the apparatus according to this mode ( 2 ), the precision in estimation of the temperature increase of the heated portion is improved, resulting in another improvement in estimation of the temperature of the heating portion. 
     (3) The apparatus according to the above mode ( 2 ), wherein the power supply restricting means comprises a restricting amount determining means for, when the estimated temperature of the heated portion has reached a reference temperature formulated to be lower than the predetermined upper limit, restricting the electric power supply to the motor, and for repeating determining a restricting amount by which the electric power supply to the motor is to be restricted, on the basis of the estimated temperature of the heated portion at a corresponding one of a plurality of discrete points of time. 
     In the apparatus according to this mode ( 3 ), a restricting amount by which the electric power supply is to be restricted is repeatedly determined after the commencement of restriction on the electric power supply to the motor, on the basis of the temperature of the heated portion estimated at a corresponding one of a plurality of discrete points of time. Therefore, it can surely be avoided that an actual temperature of the heated portion exceeds the predetermined upper limit thereof. 
     (4) The apparatus according to the above mode ( 1 ), wherein the power supply restricting device comprises: 
     an allowable supply time period determining means for utilizing an initiation point of time of a holding operation of the steering wheel during which the vehicle operator is holding the steering wheel substantially at one steering position thereof which is other than a neutral position thereof, and for determining a time period which is estimated to pass from the initiation point of time of the holding operation until the temperature of the heated portion has reached the reference temperature, on the basis of the initial temperature of the heated portion, a reference temperature of the heated portion at which the restriction on the electric power supply to the motor is to be initiated, and the electric power-related value obtained at the initiation point of time of the holding operation, the determined time period being defined as an allowable supply time period for the electric power supply to the motor; and 
     a supply restricting means for starting restricting the electric power supply to the motor when the determined allowable time period has passed. 
     In a holding operation during which the vehicle operator holds the steering wheel at one steering angle thereof, a change in the magnitude of the electric power supply to the motor, i.e., the electric power-related value of the motor is not as large as in a steering operation during which the vehicle operator operates the steering wheel so as to increase a steering angle thereof. Accordingly, if the magnitude of the electric power-related value at an initiation of the holding operation can be recognized, an increase of the temperature of the heated portion at each one of a plurality of discrete points of time after the initiation of the holding operation can be represented as a function of time, during the holding operation. 
     Based on this finding, in the apparatus according to this mode ( 4 ), an allowable power supply time period for the motor is determined as a time period which is estimated to pass from an initiation of a holding operation until the temperature of the heated portion has reached a reference temperature, on the basis of the initial temperature of the heated portion, and the electric power-related value obtained at the initiation of the holding operation. Further, when the determined allowable time period has passed, the restriction on the electric power supply to the motor is initiated. 
     Therefore, the apparatus according to this mode ( 4 ) can easily prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit without an indispensable performance of integration of the plurality of electric power-related values. 
     In the apparatus according to this mode ( 4 ), the term “a holding operation” may be defined as an operation during which the rate of change in a steering angle of the steering wheel or an amount of change in the steering angel per a certain time period is not larger than a reference value. The term “a holding operation” also may be defined as an operation during which the rate of change in the electric power-related value or an amount of change in the electric power-related value is not larger than a reference value. 
     (5) The apparatus according to the above mode ( 4 ), further comprising a second allowable time period determining means for, at a change point of time when a time-dependent change of the electric power-related value occurs, whose amount is not less than a predetermined reference value thereof, during the holding operation, estimating the temperature increase which is an increase of the temperature of the heated portion from that at the initiation point of time of the holding operation, on the basis of an integrated value of the plurality of electric power-related values obtained during a period from the initiation point of time of the holding operation to the change point of time, and for estimating a time period which is expected to pass from the change point of time until the temperature of the heated portion has reached the reference temperature, on the basis of a sum of the estimated temperature increase and the initial temperature of the heated portion, and the electric power-related value obtained at the change point of time, the estimated time period being defined as a second allowable supply time period for the electric power supply to the motor. 
     There exists a fact that the electric power-related value, i.e., a physical quantity related to the temperature of the heated portion can vary even during a holding operation of the steering wheel by the vehicle operator. There also exists a fact that the temperature of the heated portion when a time-dependent change in the electric power-related value can be estimated on the basis of an integrated value of a plurality of electric-power related values which have been obtained since the initiation of the holding operation, and the initial temperature of the heated portion. In light of these facts, the apparatus according to this mode ( 5 ) determines a time period which is estimated to pass from the occurrence of the time-dependent change until the temperature of the heated portion has reached a reference temperature as a second allowable supply time period for the motor. 
     Accordingly, the apparatus according to this mode ( 5 ) can prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit thereof due to a time-dependent change in the electric-power-related value during a holding operation of the steering wheel by the vehicle operator. 
     (6) The apparatus according to the above mode ( 4 ) or ( 5 ), wherein the allowable time period determining means comprises: 
     a first means for determining an allowable increase of the temperature of the heated portion on the basis of a difference between the reference temperature and the initial temperature of the heated portion; and 
     a second means for determining the allowable supply time period corresponding to both the electric power-related value obtained at the initiation point of time of the holding operation and the determined allowable increase of the heated portion, according to a predetermined relationship among the electric power-related value obtained at the initiation point of time of the holding operation, the allowable increase, and the allowable supply time period. 
     (7) The apparatus according to the above mode ( 6 ), wherein the second means comprises a means for determining the allowable supply time period such that the allowable supply time period decreases as the allowable increase decreases, and decreases as the electric power-related value at the initiation point of time of the holding operation increases. 
     (8) The apparatus according to any one of the above modes ( 1 )-( 7 ), wherein the power supply restricting device comprises an initial temperature determining means for determining an ambient temperature of the heated portion at the reference point of time, as the initial temperature of the heated portion. 
     (9) The apparatus according to any one of the above modes ( 1 )-( 8 ), further comprising a torque detecting device detecting the steering torque, the torque detecting device including a temperature sensor detecting a temperature of the torque detecting device, the power supply restricting device comprising an initial temperature obtaining means for obtaining the initial temperature of the heated portion on the basis of the temperature detected by the temperature sensor. 
     In the apparatus according to this mode ( 9 ), the same temperature sensor performs both detection of the temperature of the heated portion and acquisition of an initial temperature of the heated portion. Accordingly, the apparatus according to this mode ( 9 ) can eliminate the total number of temperature sensors installed in an automotive vehicle, compared with the case where detection of the temperature of the heated portion and acquisition of an initial temperature of the heated portion are separately performed by respective temperature sensors. As a result, the apparatus according to this mode ( 9 ) can reduce increase in the cost of the apparatus due to the addition of a function for obtaining the temperature of the heated portion. 
     (10) The apparatus according to the above mode ( 9 ), wherein the temperature sensor is located near the heated portion in the electrically operated power steering apparatus. 
     (11) The apparatus according to the above mode ( 9 ) of ( 10 ), wherein the temperature sensor detects the temperature of the torque detecting device as a temperature to be changed according to a substantially constant correlation thereof with an ambient temperature of the heated portion. 
     (12) The apparatus according to any one of the above modes ( 9 )-( 11 ), wherein the controlling device comprises a means for controlling the electric power supply to the motor on the basis of the steering torque detected by the torque detecting device. 
     (13) The apparatus according to the above mode ( 1 ), wherein the power supply restricting device comprises: 
     a first allowable supply time period determining means for using an initiation point of time of one continuous steering operation of the steering wheel by the vehicle operator, and for determining a time period which is expected to pass from the initiation point of one continuous steering operation until the temperature of the heated portion has reached a reference temperature at which the restriction on the electric power supply to the motor is to be initiated, on the basis of the initial temperature of the heated portion, a reference temperature formulated to be lower than the predetermined upper limit, and the electric power-related value obtained at the initiation point of time of one continuous steering operation, the determined time period being defined as a first allowable supply time period for the electric power supply to the motor; and 
     a second allowable supply time period determining means for, at each one of a plurality of discrete points of time after the initiation point of time of one continuous steering operation, estimating the temperature increase which is an increase of the temperature of the heated portion from that at the initiation point of time of one continuous steering operation, on the basis of an integrated value of at least one of the plurality of electrical power-related values which has been obtained since the initiation point of time of one continuous steering operation, and for determining a time period which is expected to pass from each one of the plurality of discrete points of time until the temperature of the heated portion has reached the reference temperature, on the basis of a sum of the estimated temperature increase and the initial temperature of the heated portion, and the electric power-related value obtained at a corresponding one of the plurality of discrete points of time, the determined time period being defined as a second allowable supply time period for the electric power supply to the motor; and 
     a supply restricting means for starting restricting the power supply to the motor when the first or second allowable supply time period determined by the first or second allowable supply time period determining means has passed. 
     The apparatus according to this mode ( 13 ) can prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit, according to the principle corresponding to a principle which is employed in the apparatus according to the above mode ( 4 ). 
     The apparatus according to this mode ( 13 ) may be used, irrespective of whether one continuous steering operation set forth in this mode ( 13 ) is defined as the holding operation set forth in the above mode ( 4 ). 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
     FIG. 1 is a cross sectional front view illustrating a mechanical arrangement of an electrically operated power steering apparatus constructed according to a first embodiment of this invention; 
     FIG. 2 is a cross sectional front view exclusively illustrating in enlargement a gear box  60  of FIG. 1; 
     FIG. 3 is an electric circuit diagram illustrating a torque detecting device  80  equipped with the electrically operated power steering apparatus of FIG. 1; 
     FIG. 4 is a block diagram illustrating a software arrangement of the electrically operated power steering apparatus of FIG. 1; 
     FIG. 5 is a flow chart illustrating a motor temperature estimation routine executed by a computer  100  of FIG. 4; 
     FIG. 6 is a graph representing a relationship between a coil temperature θ C  and an initial temperature θ M0  of the motor utilized in the motor temperature estimation routine of FIG. 5; 
     FIG. 7 is a flow chart illustrating a reference motor-current-value determination routine executed by the computer  100  of FIG. 4; 
     FIG. 8 is a flow chart illustrating a desired motor-current-value determination routine executed by the computer  100  of FIG. 4; 
     FIG. 9 is a flow chart illustrating a motor drive routine executed by the computer  100  of FIG. 4; 
     FIG. 10 is a graph for explaining changes with time τ in a motor-current-value I, a coil temperature θ C  and a motor temperature θ M  in the electrically operated power steering apparatus of FIG. 1; 
     FIG. 11 is a flow chart representing a desired motor-current-value determination routine executed by a computer in an electrically operated power steering apparatus constructed according to a second embodiment of the present invention; 
     FIG. 12 is a graph illustrating a relationship among an allowable temperature increase Δθ, an actual motor-current-value I act  at the initiation of a holding operation of a steering wheel by a vehicle operator, and an allowable supply time period T 0  utilized in the desired motor-current-value determination routine of FIG. 11; 
     FIG. 13 is a graph for explaining changes with time τ in a motor-current-value I by execution of the desired motor-current-value determination routine of FIG. 11, and 
     FIG. 14 is a flow chart representing a desired motor-current-value determination routine executed by a computer in an electrically operated power steering apparatus constructed according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings, like numerals are used to indicate like elements throughout. 
     Referring first to FIG. 1, there will be described a mechanical arrangement of a first embodiment of the present invention in the form of an electrically operated power steering apparatus (hereinafter referred to simply as “a steering apparatus”) for an automotive vehicle. The steering apparatus includes a steering shaft (not shown) rotatable with a steering wheel  10 . The steering shaft is fixed at one of ends thereof which is remote from the steering wheel  10 , to one end of a torsion bar  20 . 
     The steering apparatus further includes a pair of tie rods  22 ,  22  pivotably connected to a pair of knuckle arms (not shown), respectively. The pair of knuckles are attached to a pair of steerable wheels  24 ,  24  (e.g., a front set of steerable wheels) of the vehicle, respectively. The pair of tie rods  22 ,  22  are connected to each other through a steering rod  26  extending in a lateral direction of the vehicle such that the pair of tie rods  22 ,  22  are bendable and rotatable relative to the steering rod  26 . 
     The steering apparatus further includes a hollow main housing  30  fixedly mounted at a body of the vehicle. The main housing  30  is passed through by the steering rod  26  with a radial clearance therebetween so that the steering rod  26  is axially movable relative to the main housing  30 . 
     The main housing  30  further accommodates a motor  40  and a motion converting mechanism in the form of a ball screw  42 . 
     The motor  40  is constructed to include a stator  44  fixed to the main housing  30  and a cylindrical rotor  46  with the stator  44  and rotor  46  being rotatably fitted to each other. The rotor  46  is supported on the main housing  30  via a plurality of bearings  48  such that the rotor  46  is rotatable but is not axially movable relative to the main housing  30 . A motor coil  50  is wound around the stator  44 , and a cylindrical magnet  52  is fixed to the rotor  46  at its outer circumference. The rotor  46  is rotated as a result of interaction of an electromagnetic force of the motor coil  50  and a magnetic force of the magnet  52 . 
     The ball screw  42  identified above is in the form of a combination of a nut  54  and a shaft  56  wherein the nut  54  and shaft  56  are rotatably fitted to each other via a plurality of balls. The nut  54  is coaxially fixed to the rotor  46 , and the shaft  56  is integrally formed at the steering rod  26  described above. In the ball screw  42 , a rotary motion of the nut  54  is converted into an axial motion of the shaft  56 . 
     The steering apparatus further includes a gear box  60 . As shown in the enlarged view of FIG. 2, the gear box  60  is equipped with (a) a gear box housing  62  fixedly mounted within the vehicle body, and (b) a pinion shaft  66  rotatably supported on the gear box housing  62  through a bearing  64 . A pinion  68  is coaxially and integrally formed at the pinion shaft  66 . The steering rod  26  forms a plane portion on its outer circumferential surface  26 . The plane portion extends in parallel with the steering rod  26 , forming a rack  70 . The rack  70  meshes with the pinion  68  explained above, whereby the rack  70  is axially moved due to a rotation of the pinion  68 . That is, they cooperate with each other to constitute a so-called rack and pinion mechanism. Consequently, the steering rod  26  is axially moved due to a rotary motion of the pinion  68  and a rotary motion of the motor  40 . The pinion  68  is fixed to a remaining end of the torsion bar  20  described above so as to permit the pinion  68  to rotate with the steering wheel  10  mentioned above. 
     When the vehicle operator applies a steering torque to the steering wheel  10 , the torsion bar  20  is then twisted accordingly. In addition, there is established a constant relationship between the magnitude of the steering torque and an angle of twist of the torsion bar  20 . Consequently, the magnitude of the steering torque can be detected from the angle of twist of the torsion bar  20 . In the present embodiment, for obtaining the angle of twist of the torsion bar  26 , the torsion bar  26  extends through a rotatable member in the form of a sleeve  74 . One of opposite ends of the sleeve  74  is fixed to one of opposite ends of the torsion bar  20  which is remote from the other end at which the torsion bar  20  is connected to the pinion shaft  66 . The other end of the sleeve  74  is rotatably fitted with the pinion shaft  66 . The sleeve  74  is rotatably supported at the gear box housing  82  described above via a bearing  76 . 
     The gear box  60  is equipped with a torque detecting device  80  detecting the steering torque applied to the steering wheel  10  using the sleeve  74  described above. The torque detecting device  80  accommodates a first member  82  and a second member  84 . The first member  82  is coaxially fixed to the sleeve  74  on its outer side so that the first member  82  rotates with the sleeve  74 . On the other hand, the second member  84  is fixed to the pinion shaft  66  at a position at which the second member  84  is coaxially opposing to the pinion shaft  66  and which is close to the pinion shaft  66 . The first member  82  has a plurality of teeth (not shown) forming one circular line on a first member end face which is one of opposite end faces of the first member  82  opposing to the second member  84 . Similarly, the second member  84  has a plurality of teeth (not shown) forming one circular line on a second member end face which is one of opposite end faces of the second member  84  opposing to the first member  82 . Consequently, depending upon a change in a relative angular position between the first and second member end face, an area (hereinafter referred to as “an overlapping area”) of a portion where a tip of each tooth on the first member end face and a tip of each tooth on the second member end face overlap each other is changed. 
     The torque detecting device  80  explained above is farther equipped with a ring-shaped coil  90  for detecting a torque, coaxially with the first and second member  82 ,  84 . The coil  90  is fixed to the gear box housing  62  described above on the outside of the first and second member  82 ,  84  at a position close to the first and second member end face mentioned above, via a small clearance. The coil  90  is surrounded by a member  91  for facilitating a magnetic path to be formed therein on the outside of the coil  90 . When a magnetic flux is generated within the coil  90 , the magnetic flux passes through the first and second member  82 ,  84  together, with a permeance of the magnetic flux being changed depending upon the overlapping area described above. This means that an inductance of the coil  90  is changed relying on the overlapping area. Eventually, the inductance of the coil  90  is changed depending upon the magnitude of the steering torque of the steering wheel  10 . 
     The torque detecting device  80  further includes a torque detecting circuit  92  shown in FIG.  3 . As shown in FIG. 2, the torque detecting device  80  is mounted at the main housing  62  described above. As shown in FIG. 3, the torque detecting circuit  92  is equipped with a resistor  93  connected to the coil  90  explained above in series therewith, and an oscillation circuit  94  connected to the resistor  93 . The oscillation circuit  94  outputs a predetermined pulse signal to the coil  90  through the resistor  93 . The torque detecting circuit  92  further includes a torque signal generating circuit  95 . The torque signal generating circuit  95  receives a signal output from the coil  90  in response to the pulse signal from the oscillation circuit  94 , and then generates a torque signal representing the inductance of the coil  94 , that is, the steering torque of the steering wheel  10 . Additionally, the torque signal generating circuit  95  outputs the generated torque signal to a motor controller  96  (See FIG.  4 ). 
     The torque detecting device  92  further includes a temperature detecting circuit  98  connected to the coil  90  explained above. The temperature detecting circuit  98  detects the temperature of the coil  90  on the basis of a resistance thereof, and outputs a coil temperature signal representing the detected temperature of the coil  90 . The motor controller  96  calculates a provisional value of the steering torque on the basis of the received torque signal, and then corrects the previously calculated provisional value of the steering torque into a final value which does not depend on the temperature of the coil  90 , on the basis of the received coil temperature signal. 
     It is added that, although temperature compensation is thus performed in order that a change in the finally detected steering torque due to change in the temperature of the coil  90  to be compensated, using a computer in a software manner in the present embodiment, the temperature compensation may be performed using an electrical circuit in a hardware manner by accepting an arrangement where an additional coil of the same type with the coil  90  is disposed close to the coil  90  as a coil for detecting the temperature of the coil  90 , and a component of the torque signal output from the coil  90  depending upon the temperature thereof is cancelled by a coil temperature signal output from the additional coil. 
     A software arrangement of the present steering apparatus is illustrated in FIG.  4 . The steering apparatus includes the motor controller  96  mentioned above. The motor controller  96  is principally constructed by a computer incorporating a central processing unit (CPU), a read-only memory (ROM), and a random-access memory (RAM). To the input side of the motor controller  96 , there are connected the torque detecting device  80  described above, and a motor-current-value sensor  110 . The motor-current-value sensor  110  detects an actual current value flowing through the motor coil  50 . To the output side of the motor controller  96 , there is connected the motor coil  50  of the motor  40  as explained above. 
     The ROM explained above has stored various control programs. These programs include (a) a motor-temperature estimation routine executed to estimate the temperature (hereinafter referred to simply as “motor temperature ”) of the motor coil  50 , (b) a reference motor-current value determination routine executed to determine a reference motor-current-value as being equal to a desired motor-current-value used in the case where restriction on an electric power supply to the motor  40  is unnecessary, (c) a desired motor-current-value determination routine executed to the desired motor-current-value so as to selectively restrict the electric power supply to the motor  40 , and (d) a motor drive routine executed to supply a drive signal to the motor  40  for driving the motor  40 . These routines will be described below in this descriptive sequence. 
     The motor-temperature estimation routine is illustrated in the flow chart of FIG.  5 . 
     Described conceptually first, the present routine is formulated by especially taking account of a fact that there exists a constant correlation between the coil temperature θ C  detected by the temperature detecting circuit  98 , and an initial motor-temperature θ M0  which is the temperature of the motor  40  at initiation of a driving operation of the motor  40  during one continuous steering operation of the steering wheel  10  by the vehicle operator. The present routine is executed to estimate the initial motor-temperature θ M0  from the coil temperature θ C . That is, in the present embodiment, the initiation of one continuous drive operation of the motor  40  corresponds to “a reference point of time.” The present routine is further executed to sequentially detect an actual motor-current-value I act  using the motor-current-value sensor  110  after the initiation of drive operation of the motor  40 . The execution is to calculate an integrated value ε of a plurality of actual motor-current-values I act , and to estimate a current motor-temperature θ M  on the basis of the calculated integrated value ε. The estimation is performed using a constant correlation between the integrated value ε and the motor-temperature θ M . 
     Described in detail, this routine is cyclically executed by the computer  100 . Each cycle of execution of this routine begins in step S 1  to read the actual motor-current-value I act  from the motor-current-value sensor  110 . This routine proceeds to step S 2  where a determination is made as to whether the actual motor-current-value I act  is substantially equal to zero. That is, this step is implemented to determine whether it is at initiation of a drive operation of the motor  40 . If the actual motor-current-value I act  is substantially equal to zero, the determination is affirmative (YES), and then one cycle of execution of this routine is terminated. 
     To the contrary, unless the actual motor-current-value I act  is substantially equal to zero, the determination is negative (NO), and then this routine proceeds to S 3  where the coil temperature θ C  is read out from the temperature detecting circuit  98 . In step S 4 , the initial motor-temperature θ M0  is then estimated on the basis of the previously read coil temperature θ C . In the present embodiment, the ROM has stored a relationship as shown in the graph of FIG. 6 between the coil temperature θ C  and the initial motor-temperature θ M0 , in the form of a table, a map, an expression, etc. According to the stored relationship, the initial motor-temperature θ M0  is estimated from the coil temperature θ C . 
     In the step S 5  of FIG. 5, the actual motor-current-value I act  is then read out from the motor-current-value sensor  110 . In step S 6 , a present value of the integrated value ε is updated by adding an absolute value of the actual motor-current-value I act  to the present value of the integrated value ε. It is noted that the integrated value ε is designed to be initialized as zero when an electric power is first applied to the computer  100 . 
     In step S 7 , estimation is preformed as to a motor-temperature increase Δθ which is an increase of the motor-temperature θ M  from that at the initiation of the drive operation of the motor  40 , on the basis of the present value of the integrated value ε. In the present embodiment, the ROM has stored a relationship between the integrated value ε and the motor-temperature increase Δθ, in the form of a table, a map, an expression, etc. According to the stored relationship, the motor-temperature increase Δθ is estimated from the integrated value ε. 
     In step S 8 , a present value of the motor-temperature θ M  is then estimated by adding the estimated motor-temperature increase Δθ to the estimated initial motor-temperature θ M0 . The estimated motor-temperature θ M  is stored in the RAM described above. 
     In step S 9 , the actual motor-current-values I act  is then read out from the motor-current-value sensor  110 . In step S 10 , a determination is then made as to whether a condition that the actual motor-current-values I act  is substantially equal to zero has been consecutively repeated a predetermined number of times. This step is implemented to determine whether the drive operation of the motor  40  has been terminated or not. If the actual motor-current-values I act  is not substantially equal to zero, the determination is negative, and then this routine proceeds back to step S 5 . If the condition above-mentioned has been consecutively repeated the predetermined number of times, as a result of repeated execution of a loop including steps S 5 -S 10 , the determination in step S 10  is affirmative, and then this routine proceeds to step S 11  where the present value of the integrated value ε is initialized as zero for subsequent execution of this routine. Then, one cycle of execution of this routine is terminated. 
     The reference motor-current-value determination routine is represented in the flow chart of FIG.  7 . This routine is cyclically executed by the computer  100 , like the motor-temperature estimation routine as previously described. In each cycle of execution of this routine, step S 21  is initially implemented to read the steering torque t from the torque detecting device  80 . This routine then proceeds to step S 22  where the reference motor-current-value I REF  is determined on the basis of the previously read steering torque t. The reference motor-current-value I REF  is a current value which is allowed to be supplied to the motor coil  50  when the motor-temperature θ M  has not exceeded a predetermined upper limit temperature θ LIMIT . In the present embodiment, the ROM has stored a relationship between the steering torque t and the reference motor-current-value I REF , in the form of a table, a map, an expression, etc. According to the stored relationship, the reference motor-current-value I REF  is determined from the steering torque t. The determined reference motor-current-value I REF  is stored in the RAM described above. Then, one cycle of execution of this routine is terminated. 
     The desired motor-current-value determination routine mentioned above is illustrated in the flow chart of FIG.  8 . This routine is cyclically executed by the computer  100 , like the other routines as already described. In each cycle of execution of this routine, step S 41  is initially implemented to read a present value of the estimated motor-temperature θ M  from the RAM. This routine then proceeds to step S 42  where a determination is made as to whether the estimated motor-temperature θ M  is not lower than a predetermined reference temperature θ REF  lower than the upper limit temperature θ LIMIT . If the estimated motor-temperature θ M  is lower than the predetermined reference temperature θ REF , the determination is negative, and then this routine proceeds to step S 43 . In this step, a present value of the reference motor-current-value I REF  is read out from the RAM, and then the reference motor-current-value I REF  as such is utilized as a present value of the desired motor-current-value I*. The desired motor-current-value I* is stored in the RAM. Then, one cycle of execution of this routine is terminated. 
     To the contrary, if the estimated motor-temperature θ M  is not lower than the predetermined reference temperature θ REF , the determination in step S 42  is affirmative, and then this routine proceeds to step S 44  where the restriction on the electric power supply to the motor  40  is effected. Described in more detail, a present value of the reference motor-current-value I REF  is read out from the RAM, the reference motor-current-value I REF  is then multiplied by a predetermined correction factor k larger than “0” and smaller than “1.” The predetermined correction factor k is determined such that it decreases as a difference between the estimated motor-temperature θ M  at that time and the upper limit temperature θ LIMIT  decreases, whereby the motor-temperature θ M  is prevented from exceeding the upper limit temperature θ LIMIT  after initiation of the restriction on the electric power supply to the motor  40 . A product of the reference motor-current-value I REF  and the correction factor k is stored as a present value of the desired motor-current-value I* in the RAM. Then, one cycle of execution of this routine is terminated. 
     The motor drive routine is illustrated in the flow chart of FIG.  9 . This routine is cyclically executed by the computer  100 , like the other routines as already described. In each cycle of execution of this routine, step S 61  is initially implemented to read a present value of the desired motor-current-value I* from the RAM. This routine then proceeds to step S 62  to read the actual motor-current-value I act  from the motor-current-value sensor  110 . In the step S 63 , a motor drive signal suitable to be supplied to the motor coil  50  for substantial coincidence of the actual motor-current-value I act  with the desire motor-current-value I* is determined by feedback of the actual motor-current-value I act . In step S 64 , the determined motor drive signal is then supplied to the motor coil  50  thereby driving the motor  40 . Then, one cycle of execution of this routine is terminated. 
     Changes with time τ in the reference motor-current-value I REF , the desire motor-current-value I*, the coil-temperature θ C  (not by its actual value but by its detected value), and the motor-temperature θ M  are illustrated in the graph of FIG.  10 . Referring to FIG. 10, when the motor-temperature θ M  is raised to the reference motor-current-value I REF , the restriction on the electric power supply to the motor coil  50  is initiated, whereby the desire motor-current-value I* is reduced below the reference motor-current-value I REF . Consequently, an increasing gradient of the motor-temperature θ M  becomes more gentle, and as a result, the motor-temperature θ M  is prevented from exceeding the upper limit temperature θ LIMIT . 
     It will be understood from the foregoing description of the present embodiment that the motor coil  50  constitutes an example of a “heated portion” of the steering apparatus, a portion of the motor controller  96  assigned to execute the reference motor-current-value determination routine of FIG. 7, to implement step S 43  of the desired motor-current-value determination routine of FIG. 8, and to execute the motor drive routine of FIG. 9 cooperates with the torque detecting device  80  and the motor-current-value sensor  110  to constitute an example of a “controlling device” of the steering apparatus, and a portion of the motor controller  96  assigned to execute the motor-temperature estimation routine of FIG.  5  and steps S 41 , S 42 , and S 44  of the desired motor-current-value determination routine of FIG. 8 constitutes an example of a “power supply restricting device” of the steering apparatus. Moreover, a portion of the motor controller  96  assigned to execute the motor-temperature estimation routine of FIG. 5 constitutes an example of a “temperature estimating means” of the steering apparatus, and a portion of the motor controller  96  assigned to implement steps S 42  and S 44  of FIG. 8 constitutes an example of a “power supply restricting means” of the steering apparatus. Furthermore, the coil  90  for detecting the steering torque and the temperature detecting circuit  98  cooperate with each other to constitute an example of a “temperature sensor” of the steering apparatus, and a portion of the motor controller  96  assigned to implement steps S 3  and S 4  of FIG. 5 constitutes an example of an “initial temperature determining means” of the steering apparatus. 
     There will next be described an electrically operated power steering apparatus for an automotive vehicle, constructed according to a second embodiment of this invention. However, since the second embodiment is similar to the first embodiment in many elements except only ones associated with a motor-temperature estimation routine and a desired motor-current-value determination routine, only these different elements will be described in detail, while those similar elements will be identified by the same reference signs as used in relation to the first embodiment, for omission of detailed and redundant description on those similar elements in description of the second embodiment. 
     In the present embodiment, an initiation of one continuous holding operation during which the steering torque is rarely changed, which operation is a part of one continuous steering operation, is defined as a “reference point of time” of the steering apparatus. There exists a fact that it is reasonably possible to assume that an actual motor-current-value I act  is placed in a stable condition thereof at an initiation of the holding operation. There also exists a fact that it is reasonably possible to estimate that a plurality of actual motor-current-values I act  obtained from an initiation to a termination of one continuous holding operation are substantially equal to the actual motor-current-value I act  obtained at the initiation of the same holding operation. In light of these two facts, the steering apparatus according to the present embodiment determines an allowable supply time period as a time period which is estimated to pass while the motor-temperature θ M  is raised from the initial motor-temperature θ M0  to the reference temperature θ REF , on the basis of the actual motor-current-value I act  obtained at the initiation of the holding operation. Further, after the determined allowable supply time period T 0  has passed, the steering apparatus according to the present embodiment is adapted to start restricting the electric power supply to the motor coil  50 , thereby preventing the motor-temperature θ M  from exceeding the upper limit temperature θ LIMIT . 
     Thus, in the present embodiment, in addition to a assumption that the motor-temperature θ M  is a parameter defined as a function of time τ, the determined allowable supply time period T 0  is employed in place of the reference temperature θ REF , and as a result, a motor-temperature estimation routine is not utilized like in the first embodiment. 
     A desired motor-current-value determination routine used in the present embodiment is illustrated in the flow chart of FIG.  11 . This routine is cyclically executed by the computer  100 , like the other routines as already described. In each cycle of execution of this routine, step S 101  is initially implemented to read a currently detected value I act(n)  of the actual motor-current-value I act  from the motor-current-value sensor  110 . This routine then proceeds to step S 102  to subtract a previously detected value I act(n−1)  of the actual motor-current-value I act  from the currently detected value I act(n)  which has been previously read, thereby calculating an amount ΔI of change in the actual motor-current-value I act . In step S 103 , a determination is then made as to whether a first condition which is met when the currently detected value I act(n)  is not substantially zero and a second condition which is met when an absolute value of the calculated amount ΔI of change is substantially equal to zero are met at the same time. If these two conditions are not met concurrently, the determination is negative, and then this routine proceeds back to step S 101 . Afterwards, if these two conditions are met concurrently after repeated execution of a loop including steps S 101 -S 103 , the determination in step S 103  is affirmative, and then this routine proceeds to step S 104 . 
     In step S 104 , the coil temperature θ C  is read from the temperature detecting circuit  98 . This routine then proceeds to step S 105  where the initial motor-temperature θ M0  is estimated on the basis of the previously read coil temperature θ C  in the same manner with the first embodiment. Then, in step S 106 , an allowable temperature-increase Δθ defined as an allowable increase of the motor-temperature θ M  from the initial motor-temperature θ M0 , by subtracting the estimated initial motor-temperature θ M0  from the reference temperature θ REF . 
     In step S 107 , the allowable supply time period T 0  is then determined. In the present embodiment, the ROM has stored a relationship between the allowable supply time period T 0 , the allowable temperature-increase Δθ, and the actual motor-temperature θ M  at the initiation of one continuous holding operation by the vehicle operator, in the form of a table, a map, an expression, etc. According to the stored relationship, the allowable supply time period T 0  is determined from the determined allowable temperature-increase Δθ and the actual motor-temperature θ M  at the initiation of the holding operation. In the present embodiment, as shown in the graph of FIG. 12, the relationship is formulated such that the allowable supply time period T 0  is reduced as the allowable temperature-increase Δθ is raised, and is reduced as the actual motor-temperature θ M  at the initiation of the holding operation is raised. 
     In step S 108 , a passed time T period to be calculated from the initiation of the holding operation is initialized to be zeroed. This routine then proceeds to step S 109  wherein a determination is made as to whether a present value of the passed time T is not shorter than the determined allowable supply time period T 0 . If the present value of the passed time T is shorter than the determined allowable supply time period T 0 , the determination is negative, and then, in step S 110 , the present value of the passed time T is updated by adding a predetermined cycle time period of this routine to the present value of the passed time T. Afterwards, in step S 111 , a present value of the reference motor-current-value I REF  is read from the RAM, and then the reference motor-current-value I REF  itself is used as a desired motor-current-value I*. The desired motor-current-value I* is stored in the RAM. This routine then returns to step S 109 . 
     If the present value of the passed time T becomes not shorter than the allowable supply time period T 0  during repeated execution of a loop including steps S 109 -S 111 , the determination in step S 109  is affirmative, and then, in step S 112 , the restriction on the electric power to the motor  40  is effected. More specifically, a present value of the reference motor-current-value I REF  is read out from the RAM, the reference motor-current-value I REF  is multiplied by a predetermined correction factor k (here a fixed constant value) larger than “0” and smaller than “1.” The result of multiplication is used as a new value of the desired motor-current-value I*. The desired motor-current-value I* is stored in the RAM. 
     In step S 113 , the actual motor-current-values I act  is then read out from the motor-current-value sensor  110 , and a determination is then made as to whether a condition that the actual motor-current-values I act  is substantially equal to zero has been consecutively repeated a predetermined number of times. This step is implemented to determine whether the holding operation of the steering wheel  10  by the vehicle operator has been terminated or not. If the condition above-mentioned has not yet been consecutively repeated the predetermined number of times, the determination is negative, and then this routine proceeds back to step S 112 . To the contrary, if the condition above-identified has been consecutively repeated the predetermined number of times, the determination is affirmative, and then one cycle of execution of this routine is terminated. 
     Changes with time τ in a relationship between the reference motor-current-value I REF  and the desired motor-current-value I* is illustrated in the graph of FIG.  13 . Upon initiation of a holding operation of the steering wheel  10 , the allowable supply time period T 0  is determined on the basis of the actual motor-current-values I act  and the allowable temperature-increase Δθ. If the allowable supply time period T 0  has passed since the initiation of the holding operation, the desired motor-current-value I* is reduced below the reference motor-current-value I REF . The reduction means to restrict the electric power supply to the motor  40 , thereby preventing the motor-temperature θ from exceeding the upper limit temperature θ LIMIT . 
     It will be understood from the foregoing description of the present embodiment that the motor coil  50  constitutes an example of a “heated portion” of the steering apparatus, a portion of the motor controller  96  assigned to execute the reference motor-current-value determination routine of FIG. 7, to implement step S 111  of the desired motor-current-value determination routine of FIG. 11, and to execute the motor drive routine of FIG. 9 cooperates with the torque detecting device  80  and the motor-current-value sensor  110  to constitute an example of a “controlling device” of the steering apparatus, and a portion of the motor controller  96  assigned to implement steps S 101 -S 110 , S 112  and S 113  of the desired motor-current-value determination routine of FIG. 11 constitutes an example of a “power supply restricting device” of the steering apparatus. Moreover, a portion of the motor controller  96  assigned to implement steps S 101 -S 107  of FIG. 11 constitutes an example of an “allowable supply time period determining means” of the steering apparatus, and a portion of the motor controller  96  assigned to implement steps S 108 -S 110 , S 112  and S 113  of FIG. 11 constitutes an example of a “power supply restricting means” of the steering apparatus. Furthermore, the coil  90  for detecting the steering torque and the temperature detecting circuit  98  cooperate with each other to constitute an example of a “temperature sensor” of the steering apparatus, and a portion of the motor controller  96  assigned to implement steps S 3  and S 4  of FIG. 5 constitutes an example of an “initial temperature determining means” of the steering apparatus. 
     There will next be described an electrically operated power steering apparatus for an automotive vehicle, constructed according to a third embodiment of this invention. However, since the third embodiment is similar to the second embodiment in many elements except ones associated with a desired motor-current-value determination routine, only this routine will be described in detail, while those similar elements will be identified by the same reference signs as used in the second embodiment, for omission of detailed and redundant description on those similar elements in description of the third embodiment. 
     In the second embodiment, the allowable supply time period T 0  is determined only once at initiation of a continuous holding operation of the steering wheel  10  during the continuous holding operation. In the present embodiment, additionally, at a point of time when an amount of time-dependent change in the actual motor-current-values I act  becomes not less than a predetermined reference value during the continuous holding operation, an actual increase Δθ act  of the actual motor-temperature θ from that at the initiation of the continuous holding operation, on the basis of an integrated value of a plurality of actual motor-current-values I act  obtained from the initiation of the continuous holding operation to the occurrence of the excessive amount of time-dependent change identified above. Furthermore, in the present embodiment, a second allowable supply time period T 0  is determined as a time period which is expected to pass since the occurrence of the excessive amount of time-dependent change until the actual motor-temperature θ M  has reached the reference temperature θ REF , on the basis of a sum of the estimated increase Δθ and the initial motor-temperature θ M0 , and the actual motor-current-values I act  at the initiation of the continuous holding operation. 
     A desired motor-current-value determination routine used in the present embodiment is illustrated in the flow chart of FIG.  14 . This routine is cyclically executed by the computer  100 . Each cycle of execution of this routine is initiated with step S 201  in which a determination is made as to whether a continuous holding operation of the steering wheel  10  has been initiated, in the same manner as steps S 101 -S 103  in the second embodiment. If a continuous holding operation has been initiated, the determination is affirmative, and then the computer  100  implements steps S 202 -S 205  in the same manner as steps S 101 -S 107  in the second embodiment. 
     In step S 206 , the passed time T as already described in relation to the second embodiment is reset to zero similarly with step S 108  in the second embodiment, and then this routine proceeds to step S 207  where a determination is made as to whether a present value of the passed time T is not shorter than the allowable supply time period T 0  previously determined in step S 205 . If the present value of the passed time T is shorter than the determined allowable supply time period T 0 , the determination is negative, and then this routine proceeds to step S 208  where an amount ΔI of change of a currently detected value I act(n)  from a previously detected value I act(n−1)  of the actual motor-current-value I act  is not less than a reference value A. If the amount ΔI of change is less than the reference value A, the determination is negative, and then the computer  100  implements steps S 209  and S 210  in the same manner as steps S 110  and S 112  in the second embodiment. This routine then proceeds back to step S 207 . 
     After repeated implementation of steps S 207 -S 210 , if the determination in step S 208  is affirmative, and then this routine proceeds to step S 211 . In this step, an integrated value of a plurality of actual motor-current-values I act  obtained from a time when the determination in step S 210  has become affirmative (i.e., at the initiation of continuous holding operation) to a time when the determination in step S 208  has become affirmative (i.e., at the occurrence of the excessive amount ΔI of change) is calculated. Further, in this step, the actual increase Δθ act  of the actual motor-temperature θ at the occurrence of the excessive amount ΔI of change from that at the initiation of the continuous holding operation is calculated on the basis of the calculated integrated value. The estimation is performed in the same manner as in step S 203 . 
     In step S 212 , the actual motor-temperature θ M  is then estimated as a sum of the initial motor-temperature θ M0  and the estimated actual increase Δθ act , and an allowable increase Δθ is determined by subtracting the estimated actual motor-temperature Δ M  from the reference temperature Δ REF . This routine then proceeds back to step S 205  wherein an new allowable supply time period T 0  is determined on the basis of the determined allowable increase Δθ and a present value of the actual motor-temperature θ M  (i.e., a sum of the estimated increase Δθ and the initial motor-temperature θ M0 ) and according to a predetermined relationship between the allowable increase Δθ and the actual motor-temperature θ M , represented by a graph similar to the graph of FIG.  12 . This routine then proceeds to step S 206  wherein the passed time T is reset to zero. Afterwards, the computer  100  implements steps including step S 207  and the following ones, in the same manner as in the foregoing explanation. 
     Then, if a present value of the passed time T has become not less than the present value of the allowable supply time period T 0 , the determination in step S 207  is affirmative, and then the computer  100  implements steps S 213  and S 214  in the same manner as in steps S 112  and S 113 . Then, one cycle of execution of this routine is terminated. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.