Abstract:
An electric parking brake system has an electric motor, an output shaft, a frictional member, a drive circuit and a controller. The output shaft is reciprocated by the electric motor. The output shaft presses the frictional member against a rotor of a vehicle such that the frictional member applies brake to a wheel of the vehicle with a predetermined baking force. The drive circuit supplies a voltage to the electric motor to drive the electric motor. The controller controls the drive circuit. For a predetermined period that is required for the frictional member to generate the predetermined braking force, the controller causes the drive circuit to supply a predetermined constant voltage to the electric motor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an electric parking brake system for vehicles, and a method for controlling the electric parking brake system.  
           [0002]    In recent years, electric parking brake systems have been introduced. Such a brake system is operated by an actuator having an electric motor as a power source. An electric parking brake system converts rotational torque generated by an electric motor into linear torque of an output shaft with a reduction gear, thereby pressing a brake pads of a disk brake, which are connected to the output shaft, against a disc rotor, or pressing shoes of a drum brake against a drum. In this manner, the electric parking brake system generates braking force.  
           [0003]    Japanese Laid-Open Patent Publication No. 2001-39279 discloses a method for controlling braking force. In this method, a voltage applied to or a current supplied to electric motors are finely controlled for changing the rotational torque generated by the electric motors. The braking force is controlled, accordingly.  
           [0004]    However, in the method disclosed in Japanese Laid-Open Patent Publication No. 2001-39279, the rotational torque of the electric motors and the braking force of the parking brake system need to be directly detected. Alternatively, the braking force needs to be estimated based on the current supplied to the electric motors. This complicates the system and increases the costs. Also, when there are abrupt changes of the load on the electric motors, inertial force is generated based on the rotation of the motors. This causes the generated braking force to be unstable.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, it is an objective of the present invention to provide a simply constructed electric parking brake system that stably generates braking force, and to a method for controlling the electric parking brake system.  
           [0006]    To achieve the above object, the present invention provides an electric parking brake system for a vehicle. The electric parking brake system has an actuator, a frictional member, a drive circuit and a controller. The actuator includes an electric motor and an output shaft. The output shaft is reciprocated by the electric motor. The frictional member is capable of approaching and separating from a rotor that integrally rotates with a wheel of the vehicle. The output shaft presses the frictional member against the rotor such that the frictional member applies brake to the wheel with a predetermined baking force. The drive circuit supplies a voltage to the electric motor to drive the electric motor. The controller controls the drive circuit. For a predetermined period that is required for the frictional member to generate the predetermined braking force, the controller causes the drive circuit to supply a predetermined constant voltage to the electric motor.  
           [0007]    The present invention also provides a method for controlling an electric parking brake system for a vehicle. The electric parking brake system uses an actuator having an electric motor to press a frictional member against a rotor that rotates integrally with a wheel of the vehicle, thereby applying brake to the wheel with a predetermined braking force, the method includes controlling a drive circuit to supply a constant voltage to the electric motor for a predetermined period that is required for the frictional member to generate the predetermined braking force.  
           [0008]    The present invention also provides another electric parking brake system for a vehicle. The electric parking brake system has an actuator, a frictional member, a drive circuit and a controller. The actuator includes an electric motor and an output shaft. The output shaft is reciprocated by the electric motor. The frictional member is capable of approaching and separating from a rotor that integrally rotates with a wheel of the vehicle. The output shaft presses the frictional member against the rotor such that the frictional member applies brake to the wheel with a predetermined baking force. The drive circuit supplies a voltage to the electric motor to drive the electric motor. The controller controls the drive circuit. The controller determines a voltage to be supplied to the electric motor based on a state of the vehicle. The controller controls the drive circuit to supply the determined voltage to the electric motor for a predetermined period.  
           [0009]    The present invention also provides another method for controlling an electric parking brake system for a vehicle. The electric parking brake system uses an actuator having an electric motor to press a frictional member against a rotor that rotates integrally with a wheel of the vehicle, thereby applying brake to the wheel with a predetermined braking force. The method includes determining a voltage to be supplied to the electric motor based on a state of the vehicle; and controlling the drive circuit to supply the determined voltage to the electric motor for a predetermined period.  
           [0010]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0012]    [0012]FIG. 1 is a diagrammatic view illustrating an electric parking brake system according to a first embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a diagrammatic view illustrating a brake and a driving portion of the electric parking brake system shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is a graph showing the relationship between a power source voltage and a duty ratio of PWM;  
         [0015]    [0015]FIG. 4 is a graph showing the relationship of a voltage supply period To the power source voltage and to a traveled distance of an output shaft;  
         [0016]    [0016]FIG. 5 is a diagram showing the structure of the memory in the ECU;  
         [0017]    [0017]FIG. 6 is a diagrammatic view illustrating an electric parking brake system according to a second embodiment of the present invention;  
         [0018]    [0018]FIG. 7 is a diagrammatic view illustrating a brake and a driving portion of the electric parking brake system shown in FIG. 6;  
         [0019]    [0019]FIG. 8 is a graph showing the relationship of the voltage supply period To the power source voltage and to the traveled distance of the output shaft according to the second embodiment;  
         [0020]    [0020]FIG. 9 is a diagram showing the structure of the memory in the ECU according to the second embodiment; and  
         [0021]    [0021]FIG. 10 is a graph showing the relationship between the motor temperature and a voltage adjustment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    A first embodiment of the present invention will now be described with reference to FIGS.  1  to  5 .  
         [0023]    [0023]FIG. 1 is a diagrammatic view illustrating a vehicle  2  having an electric parking brake system  1 . The electric parking brake system  1  is a floating caliper type disc brake system. The electric parking brake system  1  includes two braking portions  11 , two actuators, a drive circuit  13  for supplying electricity to the actuators  12 , and an electronic control unit (ECU)  14  connected to the drive circuit  13 .  
         [0024]    The braking portions  11  are each provided at one of the rear wheels  15 . Rotors, which are discs  17  in this embodiment, are fixed to a rear axle  16 . Each disc  17  corresponds to one of the braking portions  11 . Each braking portion  11  is connected to one of the actuators  12 , and stops rotation of or locks the corresponding disc  17  with force generated by the corresponding actuator  12 .  
         [0025]    As shown in FIG. 2, each braking portion  11  includes a brake caliper  23 , an outer brake pad  24 , an inner brake pad  25 , and a piston  26 . The brake pads  24 ,  25  function as frictional members.  
         [0026]    The brake caliper  23  is supported by a bracket (not shown) that rotatably supports the rear axle  16 , such that the brake caliper  23  is movable in a predetermine range in the axial direction of the rear axle  16 . The brake pads  24 ,  25  are arranged in the brake caliper  23 , and face the sides (an outer side and an inner side) of the disc  17  fixed to the axle  16 , respectively. Specifically, the outer brake pad  24  is located toward the outer side of the brake caliper  23 , and the inner brake pad  25  is located toward the inner side of the brake caliper  23 . The inner brake pad  25  is movable in a direction perpendicular to the longitudinal direction of the disc  17 .  
         [0027]    The piston  26  is located toward the inner side of the brake caliper  23  relative to the inner brake pad  25 . The piston  26  reciprocates to cause the inner brake pad  25  to contact and separate from the disc  17 . When the inner brake pad  25  is pressed against the disc  17 , the reaction force moves the brake caliper  23  toward the inner side (rightward as viewed in FIG. 2). In accordance with the movement of the brake caliper  23 , the outer brake pad  24  is pressed against the disc  17 .  
         [0028]    The actuator  12  includes an electric motor  27  and an output shaft  28 . The actuator  12  is activated when electricity is supplied to the electric motor  27  from the drive circuit  13 . The actuator  12  converts forward and reverse rotations of the electric motor  27  into reciprocation of the output shaft  28  with a motion converter. In this embodiment, the output shaft  28  is directly coupled to the piston  26 . When the electric motor  27  of the actuator  12  rotates and the output shaft  28  reciprocates, the piston  26  is driven by the actuator  12 . In accordance with the reciprocation of the piston  26 , the brake pads  24 ,  25  contact and separate from the disc  17 .  
         [0029]    A distance sensor  29  is located in the vicinity of the output shaft  28 . The distance sensor  29  detects a traveled distance of the output shaft  28  during operation of the actuator  12 , and sends a detection signal to the ECU  14 .  
         [0030]    Referring to FIG. 1, the drive circuit  13  receives commands from the ECU  14  and transforms a supply voltage V of an on-vehicle electric power source  31  to a predetermined voltage V0. The drive circuit  13  supplies the predetermined voltage V0 to the electric motors  27  of the actuators  12 . The voltage V is transformed to the predetermined voltage V0 through the PWM control. The ECU  14  monitors the power supply voltage V of the electric power source  31 . When the power supply voltage V exceeds the predetermined voltage V0, the ECU  14  commands the drive circuit  13  to lower the duty ratio.  
         [0031]    When the power supply voltage V is less than the predetermined voltage V0, the ECU  14  commands the rive circuit  13  to increase the duty ratio to 100% (see FIG. 3). At this time, the ECU  14  turns a warning lamp  32  (see FIG. 1) to warn occupants of the vehicle  2  that the electric power source  31  is about to be exhausted. The warning lamp  32  is located in a passenger compartment (not shown) and functions as warning means.  
         [0032]    The ECU  14  includes storing means, which is a memory  33 . In addition to the predetermined voltage V0, the memory  33  stores data necessary for controlling the drive circuit  13 . An inclination sensor  35  is connected to the ECU  14 . The inclination sensor  35  detects the gradient of the road surface on which the vehicle  2  is located, that is, the inclination angle θx of the vehicle  2 , and sends a detection signal to the ECU  14 .  
         [0033]    The operation of the electric parking brake system  1  will now be described.  
         [0034]    Referring to FIG. 4, a supply period T during which the predetermined voltage V0 is supplied to the actuator  12  is varied for generating a braking force sufficient for preventing the vehicle  2  from moving.  
         [0035]    Specifically, when the vehicle  2  needs to be prevented from moving, the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motor  27  of each actuator  12  during a predetermined period tx, which is sufficient for stopping the vehicle  2 . The actuator  12  converts rotation of the electric motor  27  into linear motion of the output shaft  28  during the predetermined period tx, so that the piston  26  coupled to the output shaft  28  is moved. In accordance with the motion of the piston  26 , the corresponding brake pads  24 ,  25  are moved toward and pressed against the disc  17 . Accordingly, the vehicle  2  is prevented from moving.  
         [0036]    During a period until the brake pads  24 ,  25  contacts the disc  17 , that is, a period Tm from when the ECU  14  outputs a braking command to when the brake starts being applied (idle running period), the electric motor  27  receives a relatively low load. In this state, the voltage supplied to the electric motor  27  is constant. Therefore, drive torque generated by rotation of the electric motor  27  is substantially entirely used for moving the output shaft  28 , or for moving the brake pads  24 ,  25 . The traveled distance X of the output shaft  28  is proportional to the idle running period Tm. For given distances between each disc  17  and the corresponding brake pads  24 ,  25 , the idle running period Tm is constant.  
         [0037]    After the brake pads  24 ,  25  contact the disc  17 , the drive torque of the electric motor  27  is substantially entirely converted into a force for pressing the brake pads  24 ,  25  against the disc  17 . That is, the drive torque is converted into braking torque. The braking force generated by the electric parking brake system  1  is increased in proportion to the duration of a pressing period Tt. The braking force generated by the electric parking brake system  1  is changed in accordance with the predetermined period tx (the sum of the idle running period Tm and the pressing period Tt).  
         [0038]    As shown in FIG. 5, in addition to the predetermined voltage V0, the memory  33  of the ECU  14  stores a control table  37 . The control table  37  defines the predetermined period tx for generating a sufficient braking force for preventing the vehicle  2  from moving. The predetermined period tx includes a plurality of data periods (t1, t2). The inclination angle Ox includes a plurality of inclination angle data (θ1, θ2). Each data period corresponds to one of the inclination angle data.  
         [0039]    Based on the detection signal related to the inclination angle θx of the vehicle  2  sent from the inclination sensor  35 , the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motors  27  during the predetermined period tx.  
         [0040]    For example, if an inclination data θ1 is sent from the inclination sensor  35 , the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motor  27  during a first data period t1. If an inclination data θ2 is sent from the inclination sensor  35 , the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motors  27  during a second data period t2. The data periods T0 are stored in the control table  37  and each correspond to one of the inclination angles θx. The data periods T0 are obtained through experiments in advance.  
         [0041]    When the power supply voltage V is less than the predetermined voltage V0, the ECU  14  sets the duty ratio to 100% as described above. Then, the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motors  27  for a predetermined period Tx.  
         [0042]    An operation for releasing the parking brake will now be described. The ECU  14  commands the drive circuit  13  to supply a voltage that is opposite the voltage for applying brake to the electric motors  27 . Each electric motor  27  rotates in a reverse direction in relation to rotation for applying brake. Accordingly, the output shaft  28  is moved with the piston  26  in a direction for separating the brake pads  24 ,  25  from the discs  17 . Accordingly, the parking brake is released.  
         [0043]    When the parking brake is released, the ECU  14  monitors the traveled distance X of the output shafts  28 , which is sent from the distance sensors  29 . When the traveled distance X reaches a predetermined distance X0 (see FIG. 4), the ECU  14  commands the drive circuit  13  to stop supplying voltage to the electric motor  27 . That is, the control for releasing the parking brake is executed by moving the output shafts  28  by the predetermined distance X0 (see FIG. 5), which is previously stored in the memory  33 , in a direction for moving the brake pads  24 ,  25  away from the discs  17 .  
         [0044]    This embodiment provides the following advantages.  
         [0045]    In response to commands from the ECU  14 , the drive circuit  13  supplies the predetermined voltage V0, which is generated by transforming the power supply voltage V of the electric power source  31 , to the electric motors  27  of the actuators  12 . Accordingly, the electric motors  27  receive a constant voltage. This stabilizes the drive torque of the electric motors  27 . As a result, a stable braking force is generated. Since the predetermined voltage V0 is supplied to the electric motors  27 , motion converters of the actuators  12  do not receive excessive load. This lowers the level of required strength of the motion converters.  
         [0046]    The braking force is controlled by changing the supply period T, during which the predetermined voltage V0 is supplied to the actuators  12 . In this embodiment, no torque sensors are needed for controlling the braking force. This simplifies the configuration.  
         [0047]    Each distance sensor  29  detects the amount of movement (traveled distance) of the corresponding output shaft  28  during operation of the actuator  12 , and sends a detection signal to the ECU  14 . While releasing the parking brake, the ECU  14  monitors the traveled distance X of each output shaft  28 . When the traveled distance X reaches the predetermined distance X0, the ECU  14  commands the drive circuit  13  to stop supplying voltage to the electric motors  27 . Accordingly, when the parking brake is released, the distance between each of the brake pads  24 ,  25  and the corresponding disc  17  is always the same. Therefore, the idle running period Tm for the subsequent application of the parking brake will be the same as that of the current braking. In other words, a stable braking force is always generated. Further, the power that the electric motor  27  is required to generate is reduced.  
         [0048]    When receiving an inclination data θx, which indicates that the road surface on which the vehicle  2  stays is inclined, from the inclination sensor  35 , the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the electric motors  27  during the predetermined time tx that corresponds to the inclination angle θx. As a result, a braking force required for preventing the vehicle  2  from moving is generated in accordance with the condition of the road surface by a simple configuration.  
         [0049]    When the power supply voltage V is less than the predetermined voltage V0, the ECU  14  sets the duty ratio to 100%. After the value of the voltage reaches the predetermined voltage V0, the ECU  14  commands the drive circuit  13  to supply the predetermined voltage V0 to the motors  27  for the predetermined period tx. As a result, even if the voltage of the electric power source  31  is low, a stable braking force is generated.  
         [0050]    When the power supply voltage V is less than the predetermined voltage V0, the warning lamp  32  in the passenger compartment (not shown) is lit. Accordingly, occupants of the vehicle  2  is warned of the low voltage of the electric power source  31 .  
         [0051]    A second embodiment of the present invention will now be described with reference to FIGS. 6 and 10. The differences from the embodiment of FIGS.  1  to  5  will mainly be discussed. Like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of FIGS.  1  to  5  and detailed explanations are omitted.  
         [0052]    As shown in FIG. 6, a drive circuit  13  of an electric parking brake system  40  includes an electric current sensor  41 . Instead of the distance sensor  29 , each actuator  12  has a pulse generator  42 . The current sensor  41  detects a current I supplied to the electric motors  27 , and sends the detected value to the ECU  14 . The pulse generator  42  of each actuator  12  is provided at a rotation shaft (not shown) of the corresponding electric motor  27 , and generates pulses according to rotational state of the electric motor  27  (see FIG. 7). The pulse generators  42  are connected to the ECU  14 , which functions as controlling means. The ECU  14  monitors pulses generated by the pulse generators  42 . Each pulse generator  42  includes a ring magnet and a Hall IC.  
         [0053]    The operation of the electric parking brake system  40  will now be described.  
         [0054]    A control of braking force generated by the electric parking brake system  40  of this embodiment includes two stages, or initial braking and re-pressing. After the initial braking, the re-pressing is performed for compensating for influences of the temperature of the electric motors  27 . Specifically, due to a temperature raise, the torque of the electric motors  27  is lowered, which reduces the braking force. When applying the parking brake, the ECU  14  first supplies a first voltage V1 to the electric motors  27  for a first period T1. Then, to compensate for the reduced braking force, the ECU  14  supplies a second voltage V2 to the motors  27  for a second period T2.  
         [0055]    For example, even if the voltage supplied to the electric motors  27  is constant, when the temperature of the motor  27  is high, the resistance of the coils in the motors  27  is increased, and the magnetization of the magnets in the motors  27  is lowered. Accordingly, the torque generated by each electric motor  27  is deceased compared to that in a reference temperature, which is an ordinary temperature. For example, if the temperature coefficient of resistance α is 0.4%, and the temperature coefficient β of the magnets is −0.2%, the torque generated by the electric motor  27  is calculated to be lowered to 71% of the ordinary temperature when the temperature of the motor  27  increased from 20° C. (ordinary temperature) to 80° C. As the torque generated by the electric motors  27  is reduced, the braking force generated by the electric parking brake system  40  is reduced. Therefore, after the initial braking in which the first voltage V1 is supplied to the electric motors  27  for the first period T1, the re-pressing is performed by supplying the second voltage V2 to the electric motors  27  for the second period T2. Accordingly, the decrease in the braking force due to the temperature raise of the electric motors  27  is compensated for.  
         [0056]    As shown in FIG. 9, a memory  45  of the ECU  14  stores a control table  47 . In addition to the first voltage V1, the control table  47  stores the initial braking period T1. The ECU  14  supplies current to the electric motors  27  based on the control table  47 .  
         [0057]    Specifically, when applying the parking brake, the ECU  14  commands the drive circuit  13  to supply the first voltage V1 to the electric motor  27  of the actuator  12  during the initial braking period T1. The actuator  12  converts rotation of the electric motor  27  into linear motion of the output shaft  28  during the initial braking period T1, so that the piston  26  coupled to the output shaft  28  is moved. Subsequently, the brake pads  24 , 25  are moved toward, and pressed against the disc  17 .  
         [0058]    The ECU  14  detects pulses generated by the pulse generators  42 . When pulse changes disappear, the ECU  14  determines that the motors  27  are in lockup state, and sets the current I from the current sensor  41  at the time as a lockup current It. The ECU  14  estimates the temperature of the electric motor  27  based on the lockup current It and determines the second voltage V2, which is supplied in the re-pressing.  
         [0059]    Specifically, during the initial braking, the current I supplied to the electric motors  27  is lowered as the load on the motors  27  decreases when the motors  27  are started. Thereafter, the current I is substantially constant until the brake pads  24 ,  25  contacts the disc  17 , or during the idle running period. As the load is increased by pressing the brake pads  24 ,  25  against the disc  17 , the current I starts increasing. When the brake pads  24 ,  25  cannot be moved further, the current I converges to a certain value. At this time, the electric motor  27  does not rotate, and the changes of the pulses generated by the pulse generator  42  is therefore zero. In this state, the ECU  14  determines that the motor  27  is locked up, and sets the current I of this state as the lockup current It.  
         [0060]    Thereafter, the ECU  14  computes the temperature of the electric motors  27  based on the lockup current It. Specifically, the ECU  14  computes an operational resistance R2 of the electric motor  27  based on the lockup current It and the first voltage V1. Further, the ECU  14  computes the rate of increase of the resistance by comparing the operational resistance R2 with an ordinary temperature resistance R1 (see FIG. 9). The ordinary temperature resistance R1 is previously stored in the memory  45  and used as a reference resistance. Based on the rate of increase of the resistance and the temperature coefficient of resistance α, the ECU  14  computes the temperature of the electric motors  27 .  
         [0061]    For example, suppose that the ordinary temperature resistance is 1Ω, the first voltage V1 is 8V, and the lockup current It is 6.45 A. In this case, since the operational resistance R2 is 1.24Ω, the rate of increase of the resistance of the electric motor  27  ((R2−R1)/R1) is 0.24. A value (60) obtained by dividing 0.24 by the temperature coefficient of resistance a (0.4) is added to the ordinary temperature (20). Accordingly, the temperature of the electric motor  27  during operation is computed as 80° C.  
         [0062]    Then, the ECU  14  determines the value of the second voltage V2 supplied in a re-pressing for compensating for influences of the temperature. During the second period T2, which is the re-pressing period, the ECU  14  commands the drive circuit  13  to supply the second voltage V2 to the electric motors  27  of the actuators  12 . The influence of the temperature of the electric motors  27  is estimated based on the temperature coefficient of resistance α and the temperature coefficient β of the magnets, which are stored in the memory  45 . The influence of the temperature of the electric motors  27  is represented by the ratio of torque generated by the electric motors  27  in an ordinary temperature to the torque generated by the electric motors  27  during operation. The second voltage V2 is computed by multiplying the reciprocal of the estimated influence of the temperature of the electric motors  27  by the first voltage V1 (see FIG. 10). For example, when the first voltage V1 is 8V during the initial braking, and the temperature of the electric motors  27  during operation is 80° C., the torque generated by each electric motor  27  is 71% of that in an ordinary temperature. That is, the influence of the temperature of the electric motors  27  is 0.71/1. In this case, the second voltage V2 is computed by multiplying the first voltage V1 (8V) of the initial braking by the reciprocal of the influence of the temperature (1/0.71). The second voltage V2 is thus 11.2 V.  
         [0063]    The voltage supplied to the electric motors  27  is transformed through the PWM control. The drive circuit  13  receives the PWM duty ratio from the ECU  14 , and transforms the power supply voltage V to the first and second voltages V1, V2 and supplies the voltages V1, V2 to the electric motors  27  of the actuators  12 . For example, if the first voltage V1 of the initial braking is 8V, and the power supply voltage V is 12V, the ECU  14  commands the drive circuit  13  to set the PWM duty ratio to 66.7% during the initial braking, and commands the drive circuit  13  to set the PWM duty ratio to 93.3% during the re-pressing. Based on commands from the ECU  14 , the drive circuits  13  supply the second voltage V2 to the electric motors  27  during the second period T2, thereby performing the re-pressing. Accordingly, the braking is completed. In this embodiment, the second period is determined by multiplying the first period T1 by a predetermined coefficient. The second period T2 is proportionate to the first period T1 and corresponds to the temperature of the electric motor  27 .  
         [0064]    An operation for releasing the parking brake will now be described. The ECU  14  commands the drive circuit  13  to supply a voltage that is opposite the voltage for applying the parking brake, or the first voltage V1 in this embodiment (see FIG. 8), to the electric motor  27 . Accordingly, the parking brake is released.  
         [0065]    Specifically, when releasing the parking brake, the ECU  14  monitors pulses generated by the pulse generators  42  provided at the rotation shafts (not shown) of the electric motors  27 . At this time, since the load on the motors  27  is low, the torque generated by rotation of each motor  27  is substantially entirely used for moving the corresponding output shaft  28 , that is, for moving the brake pads  24 ,  25 . The traveled distance X of the output shafts  28  is proportionate to the number of rotation of the electric motors  27 . Therefore, by monitoring the pulses generated by the pulse generators  42 , the ECU  14  obtains the traveled distance X of the output shafts  28 . When the count value of the pulses reaches a predetermined count value A previously stored in the memory  45 , the ECU  14  determines that the traveled distance X of the output shafts  28  is a predetermined distance X0, and commands the drive circuit  13  to stop supplying current to the electric motors  27 . As a result, the release of the parking brake is completed.  
         [0066]    This embodiment provides the following advantages.  
         [0067]    The braking force generated by the electric parking brake system  40  is controlled in the following manner. First, the first voltage V1 is supplied to the electric motors  27  for the first period T1. Then, to compensate for a decrease of the braking force due to a decrease of the torque of the motors  27  caused by temperature changes, the second voltage V2 is supplied to the electric motors  27  for the second period T2.  
         [0068]    The voltage supplied to the electric motors  27  is adjusted based on the temperature of the electric motors  27 . Therefore, even if the temperature of the electric motors  27  changes, a stable motor torque and a stable braking torque are generated.  
         [0069]    The ECU  14  monitors pulses generated by the pulse generators  42 . When there is no changes in pulses, the ECU  14  determines that the electric motors  27  are in the lockup state and estimates the temperature of the electric motors  27  based on the lockup current It, thereby determining the predetermined voltage V2 to be supplied during re-pressing.  
         [0070]    Accordingly, the temperature of the electric motors  27  is estimated without providing temperature sensors. In other words, the temperature of the electric motors  27  is detected with a simple configuration.  
         [0071]    The second voltage V2 is determined by multiplying the reciprocal of the ratio of the generated torque of the electric motors  27  in an ordinary temperature to the generated torque of the electric motors  27  during operation by the first voltage V1. Therefore, during re-pressing, the second voltage V2 corresponding to the temperature of the electric motors  27  is supplied to the electric motors  27 . As a result, stable braking force is generated.  
         [0072]    The ECU  14  releases the parking brake in the following manner. That is, when the count value of the pulses generated by the pulse generators  42  reaches a predetermined count value previously stored in the memory  45 , the ECU  14  determines that the traveled distance X of the output shafts  28  is a predetermined distance X0, and commands the drive circuit  13  to stop supplying current.  
         [0073]    Accordingly, the traveled distance of the output shafts  28  is computed without additionally providing traveled distance sensors. Thus, the traveled distance of the output shafts  28  is detected with a simple configuration. Also, the positions of the brake pads  24 ,  25  are stabilized when the parking brake is released. This minimizes the idle running distance in the subsequent application of the parking brake. As a result, time required for applying the parking brake is shortened and stabilized. This minimizes the required output of the electric motors  27 .  
         [0074]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0075]    In the embodiments of FIGS.  1  to  10 , the electric parking brake systems  1 ,  40  may be of a fixed caliper type. Also, drum brakes may be used instead of the disk brakes.  
         [0076]    In the embodiments of FIGS.  1  to  10 , the braking portions  11  may be provided for the front wheels of the vehicle  2 .  
         [0077]    In the embodiments of FIGS.  1  to  10 , the actuators  12  and the braking portions  11  may be located elsewhere, and the output shaft  28  of each actuator  12  may be coupled to the piston  26  of the corresponding braking portion  11  with a wire or a hydraulic pipe.  
         [0078]    In the embodiment of FIGS.  1  to  5 , the distance sensors  29  may detect the moved amount of the pistons  26  or of the brake pads  24 ,  25 .  
         [0079]    In the embodiment of FIGS.  1  to  5 , the ECU  14  may have a sensor for monitoring the amount of depression of the foot brake, and the supply period T may be determined based on a control table corresponding to the depression amount of the foot brake.  
         [0080]    In the embodiments of FIGS.  1  to  10 , the braking portions  11  may include the foot brake and the electric parking brake  1 . Alternatively, the foot brake and the electric parking brake  1  may be independent from each other.  
         [0081]    In the embodiments of FIGS.  1  to  10 , when the power supply voltage V is less than the predetermined voltage V0, a voice guidance or a warning sound may be produced with a speaker.  
         [0082]    In the embodiments of FIGS.  1  to  10 , the drive circuit  13  may be replaced by other types of voltage control means for motor.  
         [0083]    In the embodiments of FIGS.  1  to  10 , the memory  33  (the memory  45 ) may store the predetermined period tx (the first period T1), and the control table  37  (the control table  47 ) may store the predetermined voltage V0 (the first voltage V1). That is, the braking force generated by the electric parking brake system  1  (the electric parking brake system  40 ) may be controlled by setting the voltage supplied to the electric motors  27  to the predetermined voltage V0 (the first voltage V1), and supplying the voltage for the predetermined period tx (the first period T1), as circumstances demand.  
         [0084]    In the embodiment of FIGS.  6  to  10 , a temperature sensor may be provided in each electric motor  27 , and the voltage supplied to the electric motors  27  may be adjusted based on the detection result of the temperature sensors.  
         [0085]    In the embodiment of FIGS.  6  to  10 , the control table  47  stores the first period T1, which is the initial braking period corresponding to the state of the vehicle  2 . However, the control table  47  may store the second period T2. The control of the initial braking may be performed by supplying the first voltage V1 to the electric motors  27  for the first period T1 regardless of the state of the vehicle  2 . The second voltage V2 may be supplied to the electric motors  27  for the second period T2 after there are no changes in the pulses generated by the pulse generators  42 , that is, after the electric motors  27  are locked up.  
         [0086]    In the embodiments of FIGS.  1  to  10 , the predetermined voltage V0 (the first voltage V1) and the predetermined period tx (the first period T1) may be stored in the control table  37  (the control table  47 ), and the ECU  14  may determine the predetermined voltage V0 (the first voltage V1) and the predetermined period tx (the first period T1) based on the state of the vehicle.  
         [0087]    In the embodiments of FIGS.  6  to  10 , the temperature of the electric motors  27  may be detected prior to applying the parking brake, and the voltage to be supplied may be adjusted in advance for compensating for the influence of the temperature.  
         [0088]    In the embodiment of FIGS.  6  to  10 , the predetermined voltage may also be controlled not only when the temperature of the electric motors  27  is higher than the ordinary temperature, but also when the temperature of the electric motors  27  is less than the ordinary temperature.  
         [0089]    In the embodiment of FIGS.  6  to  10 , the current sensor  41  in the drive circuit  13  may be replaced by a shunt resistor for detecting the current I supplied to the electric motors  27 .  
         [0090]    In the embodiment of FIGS.  6  to  10 , a table storing the second voltage V2 for re-pressing that has been obtained through experiments in advance may be used.  
         [0091]    The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.