Abstract:
A motor vehicle steering angle detector for detecting a steering angle of a steering mechanism of a motor vehicle. The detector includes: a steering angle detecting section which receives an initial value applied thereto when an ignition key of the motor vehicle is turned on, and detects a relative steering angle indicative of a direction and degree of a steering operation; a steering angle midpoint computing section which repeatedly performs a computation during traveling of the motor vehicle to determine a steering angle midpoint value; an absolute steering angle calculating section which calculates an absolute steering angle on the basis of the steering angle midpoint value and the relative steering angle; and a rewritable nonvolatile memory for storing therein a latest steering angle midpoint value. Before the steering angle midpoint is newly determined immediately after the turn-on of the ignition key, the steering angle midpoint value stored in the nonvolatile memory is set as a tentative steering angle midpoint value for the calculation of the absolute steering angle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a motor vehicle steering angle detector for detecting the steering angle of a steering mechanism of a motor vehicle. The invention further relates to a power steering system employing such a steering angle detector, more particularly, to a power steering system which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.  
           [0003]    2. Description of Related Art  
           [0004]    Power steering systems are conventionally utilized which assist the operation of a steering wheel by supplying a working oil from an oil pump into a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.  
           [0005]    When the steering is not turned, no steering assist force is required. Therefore, a so-called “stop-and-go control” is performed, wherein the electric motor is deactuated when the steering assumes a straight travel steering state virtually at a steering angle midpoint and, in response to detection of a steering angle change greater than a predetermined level, the electric motor is actuated.  
           [0006]    Detection of the steering angle midpoint is based, for example, on an output of a steering angle sensor provided in association of the steering mechanism. The steering angle sensor is adapted to detect a steering angle change with respect to an initial steering angle, i.e., a relative steering angle. The initial steering angle is a steering angle observed when an ignition key switch is turned on. Accordingly, the initial steering angle does not always coincide with the steering angle midpoint, but often corresponds to a steering angle observed when the steering wheel is turned. Therefore, steering angle data for the steering angle midpoint is determined, for example, by sequentially sampling steering angle data for determination of frequency of occurrence of steering angle data, and employing the most frequent steering angle data as the steering angle midpoint data.  
           [0007]    The sampling of the steering angle data (relative steering angle data) for the determination of the steering angle midpoint is carried out, for example, on condition that a steering angular speed is virtually zero or the steering wheel is held still (with no steering operation) for at least a predetermined period.  
           [0008]    However, the detection of the steering angle midpoint requires a relatively long period of time after the turn-on of the ignition key switch. During this period, a control operation for deactuation of the electric motor cannot be performed, making the energy saving of the power steering system difficult.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a motor vehicle steering angle detector which is capable of detecting a steering angle immediately after an ignition key is turned on, thereby contributing, for example, to improvement of energy saving of a power steering system.  
           [0010]    It is another object of the present invention to provide a power steering system which is capable of starting a control operation for bringing an electric motor into an energy saving mode at a relatively early time point after an ignition key is turned on for improvement of energy saving thereof.  
           [0011]    In accordance with the present invention, there is provided a steering angle detector for a motor vehicle, which comprises: a steering angle detecting section which receives an initial value applied thereto when an ignition key of the motor vehicle is turned on, and detects a relative steering angle indicative of a direction and degree of a steering operation; a steering angle midpoint computing section which repeatedly performs a computation during traveling of the motor vehicle to determine a steering angle midpoint value which corresponds to an output value of the steering angle detecting section indicative of a steering angle midpoint; an absolute steering angle calculating section which calculates an absolute steering angle on the basis of the steering angle midpoint value determined by the steering angle midpoint computing section and the relative steering angle detected by the steering angle detecting section; a rewritable nonvolatile memory for storing therein a latest steering angle midpoint value determined by the steering angle midpoint computing section; and a section for setting the steering angle midpoint value stored in the nonvolatile memory as a tentative steering angle midpoint value for the calculation of the absolute steering angle before the steering angle midpoint is newly determined by the steering angle midpoint computing section immediately after the turn-on of the ignition key. The detector preferably further comprises a validity judging section for judging validity of the tentative steering angle midpoint value.  
           [0012]    According to the present invention, the steering angle midpoint value determined by the steering angle midpoint computing section is stored in the rewritable nonvolatile memory, and retained therein even after the ignition key is turned off. Since the steering angle midpoint value determined in the previous traveling of the motor vehicle is retained in the nonvolatile memory, the absolute steering angle calculating section performs the absolute steering angle calculation process by employing the steering angle midpoint value as the tentative steering angle midpoint value immediately after the ignition key is next turned on.  
           [0013]    When a steering operation is performed during an off-period of the ignition key, however, a relationship between an actual steering angle of a steering mechanism and the steering angle midpoint value stored in the nonvolatile memory is changed from that observed immediately before the turn-off of the ignition key. This means that the tentative steering angle midpoint value read out of the nonvolatile memory is not always valid. Therefore, it is preferred to provide the validity judging section for judging the validity of the tentative steering angle midpoint value.  
           [0014]    The steering angle midpoint computing section determines the steering angle midpoint value by sampling outputs of the steering angle detecting section each indicative of a relative steering angle, counting the outputs to determine the frequency of occurrence of outputs for each output value for preparation of a histogram, and defining the most frequent output value as the steering angle midpoint value. The judgment of the validity of the tentative steering angle midpoint value is more easily achieved than the determination of the steering angle midpoint value. Therefore, the process to be performed on condition that a valid steering angle midpoint value is determined or on condition that a correct absolute steering angle is determined can be started at a relatively early time point after the turn-on of the ignition key of the motor vehicle.  
           [0015]    The validity judging section may be adapted to judge that the tentative steering angle midpoint value is valid if the motor vehicle travels a distance not smaller than a first predetermined distance with the absolute steering angle being kept within a predetermined steering angle range including the tentative steering angle midpoint value. In this case, the validity judging section maybe adapted to judge that the tentative steering angle midpoint value is invalid unless the tentative steering angle midpoint value is judged to be valid before a traveling distance of the motor vehicle after the turn-on of the ignition key reaches a second predetermined distance greater than the first predetermined distance.  
           [0016]    With this arrangement, the tentative steering angle midpoint value is judged to be valid if the motor vehicle travels a distance not smaller than the first predetermined distance with the absolute steering angle being kept within the predetermined steering angle range including the tentative steering angle midpoint value. That is, if the relationship between the tentative steering angle midpoint value and the actual steering angle of the steering mechanism is not significantly changed from that observed immediately before the previous turn-off of the ignition key, the absolute steering angle calculated by the absolute steering angle calculating section on the basis of the tentative steering angle midpoint value is kept within the predetermined steering angle range including the tentative steering angle midpoint value when the motor vehicle travels straight. If the steering operation is performed during the ignition key off-period, the relationship between the tentative steering angle midpoint value and the actual steering angle of the steering mechanism is significantly changed. Accordingly, the absolute steering angle calculated by the absolute steering angle calculating section is not kept within the predetermined steering angle range including the tentative steering angle midpoint value even if the motor vehicle travels straight. Therefore, the tentative steering angle midpoint value is judged to be invalid unless the tentative steering angle midpoint value is judged to be valid before the traveling distance of the motor vehicle after the turn-on of the ignition key reaches the relatively great second predetermined distance.  
           [0017]    Further, if the steering angle midpoint value is newly determined by the steering angle midpoint computing section after the turn-on of the ignition key, the steering angle midpoint value thus determined is preferably employed instead of the tentative steering angle midpoint value.  
           [0018]    With this arrangement, upon the determination of the steering angle midpoint value by the steering angle midpoint computing section, the steering angle midpoint value thus determined is employed instead of the tentative steering angle midpoint value, and stored in the nonvolatile memory. This thereafter makes it possible to correctly determine the absolute steering angle.  
           [0019]    The detector preferably further comprises a rewritable nonvolatile relative-steering-angle memory for storing therein a relative steering angle value detected by the steering angle detecting section immediately before the turn-off of the ignition key, and an initial relative steering angle setting section for setting the relative steering angle value stored in the rewritable nonvolatile relative-steering-angle memory as an initial output value of the steering angle detecting section after the turn-on of the ignition key.  
           [0020]    With this arrangement, when the ignition key is turned on, the relative steering angle value detected at the previous turn-off of the ignition key is set as the initial output value of the steering angle detecting section. Therefore, a relationship between the output value of the steering angle detecting section and the tentative steering angle midpoint value observed immediately before the turn-off of the ignition key can be maintained. Thus, the absolute steering angle can correctly be calculated immediately after the turn-on of the ignition key, as long as the tentative steering angle midpoint is valid.  
           [0021]    The storage of the relative steering angle in the rewritable nonvolatile relative-steering-angle memory immediately before the turn-off of the ignition key is achieved, for example, by storing an output of the steering angle detecting section in the rewritable nonvolatile relative-steering-angle memory when the speed of the motor vehicle detected by a vehicle speed sensor is zero.  
           [0022]    The rewritable nonvolatile relative-steering-angle memory and the nonvolatile steering-angle-midpoint memory may be provided in different storage areas in the same storage device.  
           [0023]    In accordance with the present invention, there is further provided an electric power steering system for generating a steering assist force to be applied to a steering mechanism by utilizing a hydraulic pressure generated by a pump driven by an electric motor, the electric power steering system comprising: a motor vehicle steering angle detector having the aforesaid features; and an energy saving mode transition controlling sections for bringing the electric motor into an energy saving mode essentially on condition that an absolute steering angle detected by the motor vehicle steering angle detector is kept within a predetermined steering angle range including a steering angle midpoint value for a predetermined time period.  
           [0024]    According to the present invention, the essential condition for bringing the electric motor into the energy saving mode is that the steering angle of the steering mechanism is kept within the predetermined steering angle range including the steering angle midpoint value for the predetermined time period. When the steering angle midpoint value is not sure immediately after the turn-on of the ignition key, therefore, the control operation for bringing the electric motor into the energy saving mode can speedily be started by employing a tentative steering angle midpoint value. This improves the energy saving of the power steering system.  
           [0025]    The term “energy saving mode” herein means a mode in which the electric motor is operated with a lower power consumption, for example, a mode in which the electric motor is deactuated or a mode in which the electric motor is driven at an idling rotation speed which is lower than a minimum rotation speed (stand-by rotation speed) thereof employed for a steering assist operation.  
           [0026]    The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a conceptual diagram illustrating the basic construction of a power steering system according to one embodiment of the present invention;  
         [0028]    [0028]FIG. 2 is a block diagram illustrating a functional construction for determination of an absolute steering angle by an electronic control unit;  
         [0029]    [0029]FIG. 3 is a flow chart for explaining a process for determination of a steering angle midpoint;  
         [0030]    [0030]FIG. 4 is a flow chart for explaining a process for judgment of validity of a tentative steering angle midpoint value; and  
         [0031]    [0031]FIG. 5 is a flow chart for explaining a motor deactuation control process. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    [0032]FIG. 1 is a conceptual diagram illustrating the basic construction of a power steering system according to one embodiment of the present invention. The power steering system is provided in association with a steering mechanism  1  of a motor vehicle for applying a steering assist force to the steering mechanism  1 .  
         [0033]    The steering mechanism  1  includes a steering wheel  2  to be operated by a driver, a steering shaft  3  coupled to the steering wheel  2 , a pinion gear  4  provided at a distal end of the steering shaft  3 , and a rack shaft  5  having a rack gear  5   a  meshed with the pinion gear  4  and extending transversely of the motor vehicle. Tie rods  6  are connected to opposite ends of the rack shaft  5 , and further connected to knuckle arms  7  which respectively support left and right front wheels FL and FR as steerable vehicle wheels. The knuckle arms  7  are respectively provided rotatably about king pins  8 .  
         [0034]    With this arrangement, when the steering wheel  2  is operated to rotate the steering shaft  3 , the rotational motion is converted into a linear motion transverse to the motor vehicle by the pinion gear  4  and the rack shaft  5 . The linear motion is converted into rotational motions of the knuckle arms  7  about the kingpins  8 , thereby turning the left and right front wheels FL, FR.  
         [0035]    A torsion bar  9  which is adapted to be twisted in accordance with the direction and magnitude of a steering torque applied to the steering wheel  2  and a hydraulic pressure control valve  23  which is adapted to change its valve aperture in accordance with the direction and magnitude of the torsion of the torsion bar  9  are incorporated in the steering shaft  3 . The hydraulic pressure control valve  23  is connected to a power cylinder  20  for applying a steering assist force to the steering mechanism  1 . The power cylinder  20  includes a piston  21  provided integrally with the rack shaft  5 , and a pair of cylinder chambers  20   a  and  20   b  split by the piston  21 . The cylinder chambers  20   a  and  20   b  are connected to the hydraulic pressure control valve  23  via oil supply/return lines  22   a  and  22   b , respectively.  
         [0036]    The hydraulic pressure control valve  23  is disposed in an oil circulation line  24  which extends through a reservoir tank  25  and an oil pump  26 . The oil pump  26  is driven by an electric motor  27 , so that a working oil contained in the reservoir tank  25  is pumped up and supplied to the hydraulic pressure control valve  23 . An excess of the working oil is returned to the reservoir tank  25  from the hydraulic pressure control valve  23  via the oil circulation line  24 .  
         [0037]    When a torsion is exerted on the torsion bar  9  in one direction, the hydraulic pressure control valve  23  supplies the working oil to one of the cylinder chambers  20   a ,  20   b  of the power cylinder  20  via one of the oil supply/return lines  22   a ,  22   b . When a torsion is exerted on the torsion bar  9  in the other direction, the hydraulic pressure control valve supplies the working oil to the other of the cylinder chambers  20   a ,  20   b  via the other of the oil supply/return lines  22   a ,  22   b . When virtually no torsion is exerted on the torsion bar  9 , the hydraulic pressure control valve  23  is in a so-called equilibrium state, so that the working oil is not supplied to the power cylinder  20  but circulated in the oil circulation line  24 .  
         [0038]    When the working oil is supplied to either one of the cylinder chambers of the power cylinder  20 , the piston  21  moves transversely of the motor vehicle. Thus, a steering assist force acts on the rack shaft  5 .  
         [0039]    An exemplary construction of the hydraulic pressure control valve  23  is disclosed in detail, for example, in Japanese Unexamined Patent Publication No. 59-118577 (1984).  
         [0040]    The electric motor  27  comprises, for example, a DC motor, and is controlled by an electronic control unit  30  via a driver circuit  28 . The driver circuit  28  comprises, for example, a power transistor bridge circuit, and is adapted to supply electric power to the electric motor  27  from an on-board battery  40  as a power source in accordance with a control signal applied from the electric control unit  30 .  
         [0041]    The electronic control unit  30  includes a microprocessor supplied with electric power from the on-board battery  40  for operation. The microprocessor includes a CPU  31 , a RAM  32  which provides a work area for the CPU  31 , a ROM  33  storing therein control data and operation programs for the CPU  31 , an EEPROM (electrically erasable/programmable ROM)  35  for storing therein a steering angle midpoint value and steering angle data (which will be described later), and buses  34  interconnecting the CPU  31 , the RAM  32 , the ROM  33  and the EEPROM  35 .  
         [0042]    Steering angle data outputted from a steering angle sensor  11  is applied to the electronic control unit  30 . The steering angle sensor  11  is provided in association with the steering wheel  2 , and outputs a signal indicative of the direction and degree of the rotation of the steering wheel  2 . In the electronic control unit  30 , an initial steering angle data value is determined when an ignition key switch is actuated for startup of an engine, and steering angle data (relative steering angle data) corresponding to a relative steering angle with respect to the initial value and having a sign corresponding to the direction of a steering operation is generated on the basis of the detection signal of the steering angle sensor  11 . The CPU  31  performs a steering angle midpoint determination process (which will be described later) to determine a steering angle midpoint which corresponds to a steering angle observed when the motor vehicle is in a straight traveling state, on the basis of the steering angle data. Further, the CPU  31  determines an absolute steering angle which corresponds to the direction of the vehicle wheels FR, FL, on the basis of the steering angle midpoint and the steering angle data outputted from the steering angle sensor  11 .  
         [0043]    The steering angle midpoint determined by the CPU  31  is stored in the EEPROM  35 . The determination of the steering angle midpoint is repeatedly performed during an on-period of the ignition key switch and, thereupon, the steering angle midpoint stored in the EEPROM  35  is updated.  
         [0044]    The steering angle data indicative of the relative steering angle is stored in the EEPROM  35  in a control cycle (or in a predetermined control cycle) on condition that a vehicle speed Vf detected by a vehicle speed sensor  13  (which will be described later) is not higher than a predetermined level.  
         [0045]    Since the information stored in the EEPROM  35  is retained even after the turn-off of the ignition key switch, the steering angle midpoint and the steering angle data can be read out of the EEPROM  35  so as to be used for the calculation of the absolute steering angle immediately after the ignition key switch is next turned on. In this case, the steering angle data read out of the EEPROM  35  is employed as the initial steering angle data value.  
         [0046]    Electric current data outputted from an electric current detection circuit  12  for detecting an electric current flowing through the electric motor  27  is also applied to the electric control unit  30 . The electric current data has a value directly proportional to an electric current consumed by the electric motor  27  (motor current).  
         [0047]    Vehicle speed data outputted from the vehicle speed sensor  13  is also applied to the electronic control unit  30 . The vehicle speed sensor  13  may be adapted to directly detect the speed Vf of the motor vehicle or, alternatively, adapted to calculate the speed Vf of the motor vehicle on the basis of output pulses of vehicle wheel speed sensors provided in association with the vehicle wheels.  
         [0048]    The electronic control unit  30  controls the electric motor  27  for driving thereof on the basis of the steering angle data, the electric current data and the vehicle speed data.  
         [0049]    [0049]FIG. 2 is a block diagram illustrating a functional construction for the determination of the absolute steering angle by the electronic control unit  30 . When the steering wheel  2  is operated, the steering angle sensor  11  outputs a signal indicative of the direction and degree of the steering operation. The signal from the steering angle sensor  11  is converted into relative steering angle data in a digital form by a relative steering angle data generating section  42 .  
         [0050]    The relative steering angle data generating section  42  has, for example, a function as an up-down counter. When the steering angle sensor  11  detects a right turn operation of the steering wheel  2 , the relative steering angle data generating section  42  increments a count value in accordance with the degree of the right turn operation and, when the steering angle sensor  11  detects a left turn operation of the steering wheel  2 , decrements the count value in accordance with the degree of the left turn operation. The relative steering angle data generated by the relative steering angle data generating section  42  is applied to a steering angle midpoint computing section  43  and to an absolute steering angle computing section  44 .  
         [0051]    When the vehicle speed Vf becomes zero, the electronic control unit  30  stores the relative steering angle data generated by the relative steering angle data generating section  42  in the EEPROM  35  via a line  42   a . Immediately after the ignition key switch  41  is turned on, the electronic control unit  30  applies the relative steering angle data stored in the EEPROM  35  (immediately before the previous turn-off of the ignition key switch) as an initial relative steering angle data value to the relative steering angle data generating section  42  via a line  42   b.    
         [0052]    The steering angle midpoint computing section  43  determines the frequency of occurrence of relative steering angle data for each data value for preparation of a histogram, and sets the most frequent relative steering angle value as a steering angle midpoint value. The steering angle midpoint value thus set is stored in the EEPROM  35 . As long as the ignition key switch  41  is actuated, the steering angle midpoint computing section  43  repeatedly perform the computation for the determination of the steering angle midpoint value. Upon determination of a new steering angle midpoint value, the steering angle midpoint value stored in the EEPROM  35  is updated with the new steering angle midpoint value.  
         [0053]    The absolute steering angle computing section  44  calculates the absolute steering angle of the steering wheel  2  on the basis of the steering angle midpoint value stored in the EEPROM  35  and the relative steering angle data applied from the relative steering angle data generating section  42 . That is, the absolute steering angle is determined, for example, by subtracting the steering angle midpoint value from the relative steering angle data. The absolute steering angle thus determined is applied, for example, to a motor deactuation controlling section  45  for deactuation control of the electric motor  27 .  
         [0054]    Immediately after the turn-on of the ignition key switch  41 , the steering angle midpoint computing section  43  cannot determine the steering angle midpoint value because the relative steering angle data has not been sampled in an amount sufficient for the determination of the steering angle midpoint. In this embodiment, the absolute steering angle computing section  44  calculates the absolute steering angle by employing the steering angle midpoint value stored in the EEPROM  35  as a tentative steering angle midpoint value immediately after the turn-on of the ignition key switch  41 .  
         [0055]    However, there is a possibility that the steering wheel  2  is operated during the off-period of the ignition key switch  41 . Accordingly, it is impossible to ensure that a relationship between the tentative steering angle midpoint value and an actual steering angle of the steering mechanism  1  is kept unchanged during a period from the previous turn-off of the ignition key switch  41  to the next turn-on of the ignition key switch  41 . Therefore, the validity of the tentative steering angle midpoint value is judged by a validity judging section  46 .  
         [0056]    Thevalidityjudgingsection 46 judges the validity of the tentative steering angle midpoint value (which is equivalent to the validity of the absolute steering angle calculated on the basis of the tentative steering angle midpoint value), and information indicative of the judgment result is applied to the motor deactuation controlling section  45 . When the steering angle midpoint is determined by the steering angle midpoint computing section  43  after the turn-on of the ignition key switch  41  and applied to the validity judging section  46  via a line  43   a , information indicating that the steering angle midpoint value is valid (or the absolute steering angle outputted from the absolute steering angle computing section  44  is valid) is transferred from the validity judging section  46  to the motor deactuation controlling section  45 .  
         [0057]    [0057]FIG. 3 is a flow chart for explaining the steering angle midpoint determination process to be performed by the steering angle midpoint computing section. The steering angle midpoint determination process is performed on condition that the motor vehicle is currently in a traveling state. The detection of the traveling state of the motor vehicle is based on a condition such that the speed Vf of the motor vehicle is not lower than a predetermined level (e.g., 5 km/h).  
         [0058]    For the determination of the steering angle midpoint, the electronic control unit  30  detects the steering wheel being held still on the basis of a condition such that a steering angular speed determined by differentiating the relative steering angle data with time is lower than a predetermined level (e.g., 8 (degrees/second)). Then, the electronic control unit  30  constantly checks if the steering wheel is held still for a predetermined period (e.g., 1 second) (Step S 1 ).  
         [0059]    If the steering wheel is held still for the predetermined period or longer (YES in Step S 1 ), the electronic control unit  30  samples the relative steering angle data outputted from the steering angle sensor  11  for the determination of the steering angle midpoint (Step S 2 )  
         [0060]    On the basis of the sampling result, the electronic control unit  30  counts the steering angle data outputs to determine the frequency of occurrence of outputs for each steering angle data value for preparation of a histogram. Relative steering angle data having the highest frequency of occurrence in the histogram, i.e., the most frequent relative steering angle data, is set as steering angle data for the steering angle midpoint (Step S 3 ). For higher accuracy of the determination of the steering angle midpoint, however, Step S 3  is preferably performed only after a predetermined number of steering angle data outputs (e.g., three outputs) are sampled.  
         [0061]    [0061]FIG. 4 is a flow chart for explaining the process to be performed by the validity judging section  46  for the judgment of the validity of the tentative steering angle midpoint value. When the ignition key switch  41  is turned on, the validity judging section  46  performs an integration process on outputs of the vehicle speed sensor  13  to start measuring a traveling distance Di of the motor vehicle (Step S 11 ). While continuously performing the traveling distance measuring operation, the validity judging section  46  judges if the absolute steering angle outputted from the absolute steering angle computing section  44  has a value within a predetermined steering angle range (e.g., an angle range about twice the minimum resolution of the steering angle sensor  11 ) around the steering angle midpoint (Step S 12 ). In this case, the steering angle midpoint value determined immediately before the previous turn-off of the ignition key switch  41  is stored in the EEPROM  35 , and the absolute steering angle is determined by employing this steering angle midpoint value as the tentative steering angle midpoint value. The predetermined steering angle range around the steering angle midpoint means a predetermined angle range with the tentative steering angle midpoint value centered therein.  
         [0062]    If the absolute steering angle is judged to be within the predetermined steering angle range around the midpoint (YES in Step S 12 ), the validity judging section  46  starts measuring a validity judgment distance De (Step S 13 ). If the validity judgment distance De reaches, for example, 100 m (first predetermined distance) with the absolute steering angle being kept within the predetermined steering angle range around the steering angle midpoint (YES in Step S 14 ), the tentative steering angle midpoint value is judged to be valid (Step S 15 ). If it is judged in Step S 14  that the validity judgment distance De does not reach 100 m, the process goes to Step S 17 .  
         [0063]    If the absolute steering angle is out of the predetermined steering angle range around the steering angle midpoint (NO in Step S 12 ), the validity judgment distance De is reset to zero (Step S 16 ). Then, it is judged if the traveling distance Di continuously measured from the turn-on of the ignition key switch  41  reaches, for example, 1000 m (second predetermined distance) (Step S 17 ). A process sequence from Step S 12  is repeatedly performed until the traveling distance Di reaches 1000 m and, if the traveling distance Di reaches 1000 m (YES in Step S 17 ), the tentative steering angle midpoint value is judged to be invalid (Step S 18 ).  
         [0064]    [0064]FIG. 5 is a flow chart for explaining the motor deactuation control process to be performed by the motor deactuation controlling section  45 . The motor deactuation controlling section  45  judges if the electric motor  27  is to be deactuated, on the basis of the absolute steering angle applied from the absolute steering angle computing section  44 . At this time, the motor deactuation controlling section  45  obtains the information indicative of the validity of the steering angle midpoint (including the tentative steering angle midpoint value) from the validity judging section  46  and, only when the steering angle midpoint is valid, performs the deactuation control of the electric motor  27 .  
         [0065]    The motor deactuation controlling section  45  first judges if the steering angle midpoint (tentative steering angle midpoint value) is valid (Step S 21 ). If the steering angle midpoint is valid, the motor deactuation controlling section  45  judges if the steering angular speed determined by differentiating the absolute steering angle is zero or close to zero (Step S 22 ). If the steering angular speed is not zero nor close to zero, a process sequence from Step S 21  is repeatedly performed.  
         [0066]    If the steering angular speed is zero or close to zero, the motor current detected by the electric current detection circuit  12  is referred to. If the motor current exceeds a predetermined deactuation judgment current level (NO in Step S 23 ), the condition for the motor deactuation is not satisfied, so that the process returns to Step S 21 .  
         [0067]    If the condition that the motor current is not higher than the deactuation judgment current level is satisfied (YES in Step S 23 ), it is judged if the absolute steering angle has an absolute value within a deactuation range which is a minute steering angle range with the steering angle midpoint centered therein (Step S 24 ). If the absolute steering angle is out of the deactuation range (NO in Step S 24 ), a process sequence from Step S 21  is repeatedly performed. If the absolute steering angle is within the deactuation range (YES in Step S 24 ), power supply to the electric motor  27  is stopped (Step S 25 ). Thereafter, a motor actuation control process is performed to actuate the electric motor  27  again.  
         [0068]    Where the steering wheel  2  is kept at a very small steering angle, for example, when the motor vehicle travels along a gentle curve, conditions in Steps S 22 , S 23  and S 24  in FIG. 5 may all be satisfied. In this case, the steering assist force is abruptly reduced if the electric motor  27  is deactuated. This abruptly increases the heaviness of the steering operation performed by the driver, deteriorating the steering feeling. One exemplary approach to this problem is to reduce the rotation speed of the electric motor  27  continuously or stepwise for the deactuation of the electric motor  27  rather than suddenly deactuate the electric motor  27  in the motor deactuation process (step S 25 ).  
         [0069]    In accordance with this embodiment, as described above, the steering angle midpoint value stored in the EEPROM  35  is tentatively employed for the calculation of the absolute steering angle immediately after the ignition key switch  41  is turned on for startup of the engine. Then, the validity of the tentative steering angle midpoint value is judged through the simple process (Steps S 11  to S 18  in FIG. 4) and, if the tentative steering angle midpoint value is judged to be valid, the motor deactuation control process is started.  
         [0070]    Thus, the motor deactuation control can be started even before the steering angle midpoint is determined by the steering angle midpoint computing section  43 . Therefore, the motor deactuation control is started at a relatively early time point after the turn-on of the ignition key switch  41  for improvement of the energy saving of the power steering system.  
         [0071]    While one embodiment of the present invention has thus been described, the invention may be embodied in any other ways. In the embodiment described above, the electric motor  27  is deactuated if the steering assist is not necessary but, instead, the electric motor  27  may be controlled so that the rotation speed thereof is reduced to an idling rotation speed which is lower than a minimum rotation speed (stand-by rotation speed) required for the steering assist (see Step S 25  in FIG. 5). Thus, the rotation speed of the electric motor  27  can speedily be increased when the steering assist is required for improvement of the responsiveness of the steering assist.  
         [0072]    While the present invention has been described in detail by way of the embodiment thereof, it should be understood that the foregoing disclosure is merely illustrative of the technical principles of the present invention but not limitative of the same. The spirit and scope of the present invention are to be limited only by the appended claims.  
         [0073]    This application corresponds to Japanese Patent Application No. 2000-359717 filed to the Japanese Patent Office on Nov. 27, 2000, the disclosure thereof being incorporated herein by reference.