Patent Publication Number: US-2017349153-A1

Title: Motor drive control unit

Description:
TECHNICAL FIELD 
     The present invention relates to a motor drive control unit applied to driving of a motor for driving a pump which is provided to a vehicle braking device. 
     BACKGROUND ART 
     Vehicle braking devices based on conventional art include a pump and a motor for driving the pump. The pump charges a brake fluid from a master cylinder (hereinafter referred to as M/C) side and discharges the brake fluid to a wheel cylinder (hereinafter referred to as W/C) side. Such a conventional vehicle braking device is configured to automatically generate a braking force. Specifically, such a conventional vehicle braking device generates a W/C pressure by driving the pump with use of the motor and automatically generates a braking force. Therefore, it has been desired to improve responsiveness so that a braking force is secured in a shorter period as urgency is higher when automatic braking for automatically generating a braking force is applied. 
     For example, in the conventional vehicle braking device, automatic braking is applied if it is determined that a front obstacle is present, based on identification information which is supplied from a sensor or the like used for identifying the front obstacle. In this case, primary braking in which a brake force is increased with a comparatively slight increase gradient is applied at an initial stage of the automatic braking, and then secondary braking in which a braking force is increased with a comparatively steep increase gradient is applied. If an obstacle suddenly appears in front of and relatively near the vehicle from a direction lateral to the traveling direction of the vehicle, rather than approaching from a distance, a braking force needs to be increased with a steeper increase gradient than the increase gradient of the conventional secondary braking. However, the conventional vehicle braking device cannot generate a braking force in a shorter period and thus cannot provide high responsiveness. 
     To address such a disadvantage, Patent Literature 1 proposes a motor control apparatus enabling a vehicle braking device to achieve higher responsiveness. According to this motor control apparatus, while supply of the driving current (motor current) to the motor is suppressed by performing high frequency control such as PWM control, the duty ratio is set higher at the initial stage of starting automatic braking control than in the subsequent steady state that is after generation of a predetermined braking force. This makes it possible to increase a braking force with a steep increase gradient at the initial stage in which automatic braking control is started with high duty ratio, thereby enabling generation of a braking force in a shorter period. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 2009-131128 A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     According to the technique of causing duty ratio to be high only at the initial stage of starting automatic braking control while suppressing duty ratio in a steady state as in the invention disclosed in Patent Literature 1, it is true that responsiveness at the initial stage can be obtained, but a high braking force with high responsiveness cannot be obtained. That is, if high responsiveness is merely required to be obtained, duty ratio has to be high only at the initial stage of starting automatic braking control as with the invention of Patent Literature 1. However, since this is based on the premise of suppressing duty ratio in a steady state, a high braking force cannot be obtained. 
     To obtain a higher braking force while securing high responsiveness, it is necessary to control motor current so as to achieve a fully energized state in which the motor is driven by being continuously supplied with a motor current, instead of controlling motor current by performing only high frequency control. However, in the case where the motor is driven in a fully energized state, a startup current (motor current at startup) increases at the initial stage of starting automatic braking control. This causes a decrease in voltage of a battery which supplies current to the motor. This may consequently cause malfunction in the control systems of various electrical components used in the vehicle. The decrease in voltage of the battery caused by the increase in startup current depends on the state or temperature of the battery. It is therefore difficult to accurately estimate a decrease in voltage of the battery without taking account of these factors. 
     In view of the foregoing points, an object of the present invention is to provide a motor drive control unit capable of suppressing, at an initial stage of starting automatic braking control, an increase in startup current supplied to a motor, while supplying current to the motor for full energization after the automatic braking control is started. 
     Solution to Problem 
     To attain the object, a motor drive control unit recited in claim  1  includes a switching element ( 62 ,  63 ), and control means ( 70 ). The switching element is provided in a supply path through which a motor current is passed from a power source ( 61 ) to the motor ( 60 ) to control on/off switching of the supply path. The control means performs motor control in which current supply to the motor is controlled by controlling on/off switching of the switching element, and includes startup control means ( 320 ) and normal control means ( 370 ). The startup control means performs high frequency control with respect to the switching element at startup when current supply to the motor is started to apply automatic braking. The normal control means continuously turns on the switching element after the high frequency control is performed at the startup, and achieves a fully energized state in which current supply to the motor is continuously performed. 
     Advantageous Effects of the Invention 
     As described above, while output of the motor is increased by controlling the motor in the fully energized state so that a high braking force with high responsiveness is obtained, a startup current is prevented from becoming excessive by performing the high frequency control only at startup. Thus, decrease in battery voltage is minimized and thus the occurrence of malfunction is minimized in the control systems of various electrical components used in the vehicle. 
     Note that the reference sign in parentheses for the each means indicates an example of correspondence with the specific means in an embodiment described later. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a hydraulic circuit configuration of a vehicle braking device including a motor drive control unit according to a first embodiment of the present invention. 
         FIG. 2  is a view illustrating a circuit configuration of a drive circuit of a motor  60 . 
         FIG. 3  is a view illustrating a state where an obstacle  82  is present in an identification range  81  in front of a vehicle  80 . 
         FIG. 4  is a flow chart showing automatic braking control performed in the case where the obstacle  82  is present relatively far from the vehicle  80 . 
         FIG. 5  is a view showing a relationship between a relative distance from the vehicle  80  to the obstacle  82  and a period until the vehicle  80  reaches the obstacle  82 . 
         FIG. 6  is a time chart showing a change in braking force when the automatic braking control shown in  FIG. 4  is performed. 
         FIG. 7  is a flow chart showing automatic braking control performed when the obstacle  82  has suddenly appeared near the vehicle  80 . 
         FIG. 8  is a time chart showing a change in braking force when the automatic braking control shown in  FIG. 7  is performed. 
         FIG. 9  is a flow chart showing a startup program for emergency braking. 
         FIG. 10  is a view showing a relationship between battery voltage and motor current. 
         FIG. 11  is a time chart showing the case where motor control described in the first embodiment is performed. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description will discuss embodiments of the present invention with reference to the drawings. In the embodiments described below, the same or equivalent parts are given identical reference signs. 
     First Embodiment 
     The following description will discuss an embodiment of the present invention shown in the drawings. The following discusses a vehicle braking device including a motor drive control unit according to an embodiment of the present invention.  FIG. 1  is a hydraulic circuit diagram illustrating a basic configuration of a vehicle braking device  1  according to the present embodiment. The first embodiment describes, as an example, a vehicle equipped with a hydraulic circuit for front and rear piping. However, the vehicle can be equipped with, for example, X-type piping. 
     As illustrated in  FIG. 1 , a brake pedal  11  which is a brake operation member to be operated by a driver is connected via a brake booster  12  to an M/C  13  which is a source of brake fluid pressure. When the brake pedal  11  is depressed, a depressing force is boosted by the brake booster  12  and then master pistons  13   a  and  13   b  provided in the M/C  13  are depressed by the depressing force thus boosted. This causes the same M/C pressure to be generated in each of a primary chamber  13   c  and a secondary chamber  13   d  which are defined by the master pistons  13   a  and  13   b . The M/C pressure is then transmitted to each of W/Cs  14 ,  15 ,  34 , and  35  through a brake fluid pressure control actuator  50 . The M/C  13  includes a master reservoir  13   e  having paths for communication with the primary chamber  13   c  and the secondary chamber  13   d.    
     The brake fluid pressure control actuator  50  is configured by a first piping system  50   a  and a second piping system  50   b  and includes a block, not illustrated, which is made of aluminum or the like and in which brake piping is formed, with various components assembled for integration. The first piping system  50   a  is a rear system which controls a brake fluid pressure applied to a left rear wheel RL and a right rear wheel RR. The second piping system  50   b  is a front system which controls a brake fluid pressure applied to a left front wheel FL and a right front wheel FR. 
     The first and second piping systems  50   a  and  50   b  have similar basic structures. The following description will therefore deal with the first piping system  50   a , and description of the second piping system  50   b  will be omitted. 
     The first piping system  50   a  includes a pipeline A which serves as a main pipeline and through which the M/C pressure described above is transmitted to the W/C  14  provided in the left rear wheel RL and the W/C  15  provided in the right rear wheel RR so that a W/C pressure is generated. 
     The pipeline A is provided with a first differential pressure control valve  16 . The pipeline A is controlled to be in a communicating state or a differential pressure state by the first differential pressure control valve  16  which controls a differential pressure between a first pipeline on the M/C  13  side which is an upstream side and a second pipeline on the W/Cs  14  and  15  side which is a downstream side. In the first differential pressure control valve  16 , the valve position is adjusted such that the first differential pressure control valve  16  is brought into the communicating state during normal braking in which the driver operates the brake pedal  11  (i.e., when neither automatic braking control, such as collision avoidance, nor vehicle motion control, such as skid prevention control, is performed). When current is caused to flow through a solenoid coil provided in the first differential pressure control valve  16 , the valve position of the first differential pressure control valve  16  is adjusted such that the first differential pressure control valve  16  is brought into the differential pressure state in which the differential pressure becomes larger as larger current is caused to flow. 
     In the case where the first differential pressure control valve  16  is in the differential pressure state, the brake fluid can flow from the W/Cs  14  and  15  side to the M/C  13  side only when the brake fluid pressure on the W/Cs  14  and  15  side is higher than the M/C pressure by not less than a predetermined value. Accordingly, the brake fluid pressure on the W/Cs  14  and  15  side is always maintained so as not to be higher than the M/C pressure on the M/C  13  side by more than the predetermined value. Furthermore, a check valve  16   a  is provided in parallel to the first differential pressure control valve  16 . 
     The pipeline A branches into two pipelines A 1  and A 2  on the W/Cs  14  and  15  side which is a downstream side of the first differential pressure control valve  16 . The pipeline A 1  is provided with a first pressure increase control valve  17  which controls an increase in brake fluid pressure applied to the W/C  14 . The pipeline A 2  includes a second pressure increase control valve  18  which controls an increase in brake fluid pressure applied to the W/C  15 . 
     The first and second pressure increase control valves  17  and  18  are each constituted by a normally open two-position double solenoid valve capable of controlling a communicating state and a non-communicating state. Specifically, in a state where control current applied to solenoid coils of the respective first and second pressure increase control valves  17  and  18  is zero (i.e., in a state where the solenoid coils are not energized), the first and second pressure increase control valves  17  and  18  are controlled to be in the communicating state. In a state where control current is applied to the solenoid coils of the respective first and second pressure increase control valves  17  and  18  (i.e., in a state where the solenoid coils are energized), the first and second pressure increase control valves  17  and  18  are controlled to be in the non-communicating state. 
     The rear system also includes a pipeline B serving as a decompression pipeline to connect a pressure regulator reservoir  20  to a point between the first pressure increase control valve  17  and the W/C  14 , and to a point between the second pressure increase control valve  18  and the W/C  15  in the pipeline A. The pipeline B includes a first pressure reduction control valve  21  and a second pressure reduction control valve  22 . The first and second pressure reduction control valves  21  and  22  are each constituted by a normally closed two-position double solenoid valve capable of controlling a communicating state and a non-communicating state. Specifically, in a state where control current applied to solenoid coils of the respective first and second pressure reduction control valves  21  and  22  is zero (i.e., in a state where the solenoid coils are not energized), the first and second pressure reduction control valves  21  and  22  are controlled to be in the non-communicating state. In a state where control current is applied to the solenoid coils of the respective first and second pressure reduction control valves  21  and  22  (i.e., in a state where the solenoid coils are energized), the first and second pressure reduction control valves  21  and  22  are controlled to be in the communicating state. 
     A pipeline C serving as a reflux pipeline is provided between the pressure regulator reservoir  20  and the pipeline A serving as the main pipeline. The pipeline C is provided with a self-sucking pump  19  that is driven by a motor  60  to charge the brake fluid from the pressure regulator reservoir  20  and discharge the brake fluid towards the M/C  13  or the W/Cs  14  and  15 . The motor  60  is driven with the energization controlled by a drive circuit illustrated in  FIG. 2 . A configuration of the drive circuit of the motor  60  will be described later. 
     A pipeline D serving as an auxiliary pipeline is provided between the pressure regulator reservoir  20  and the M/C  13 . Through the pipeline D, the pump  19  charges the brake fluid from the M/C  13  and discharges the brake fluid to the pipeline A. In this manner, when vehicle motion control is performed, the brake fluid is supplied to the W/Cs  14  and  15  side so that the W/C pressure of a target wheel is increased. 
     The foregoing description has dealt with the first piping system  50   a . The second piping system  50   b  is similar in configuration to the first piping system  50   a  and includes components similar to those of the first piping system  50   a . Specifically, the second piping system  50   b  includes a second differential pressure control valve  36  and a check valve  36   a  corresponding to the first differential pressure control valve  16  and the check valve  16   a , respectively. The second piping system  50   b  includes a third pressure increase control valve  37  and a fourth pressure increase control valve  38  corresponding to the first and second pressure increase control valves  17  and  18 , respectively. The second piping system  50   b  includes a third pressure reduction control valve  41  and a fourth pressure reduction control valve  42  corresponding to the first and second pressure reduction control valves  21  and  22 , respectively. The second piping system  50   b  includes a pump  39  corresponding to the pump  19  and a pressure regulator reservoir  40  corresponding to the pressure regulator reservoir  20 . The second piping system  50   b  further includes pipelines E to H corresponding to the pipelines A to D, respectively. Note, however, that the first piping system  50   a  and the second piping system  50   b  may differ from each other such that the W/Cs  34  and  35  of the second piping system  50   b  that serves as the front system and supplies the brake fluid to the W/Cs  34  and  35  are larger in capacity than the W/Cs  14  and  15  of the first piping system  50   a  that serves as the rear system and supplies the brake fluid to the W/Cs  14  and  15 . In the case where the first and second piping systems  50   a  and  50   b  are thus configured, a larger braking force can be generated on the front side. 
     The vehicle braking device  1  further includes an electronic control unit for braking control (hereinafter referred to as brake ECU)  70  corresponding to control means. The brake ECU  70  is constituted by a microcomputer including CPU, ROM, RAM, I/O, and the like. The brake ECU  70  performs a process, including various calculations, according to a program stored in the ROM or the like, and performs braking control, including automatic braking control and various types of vehicle motion control. For example, an obstacle detection device, or, for example, the brake ECU  70  uses as a basis detection signals supplied from other sensors, such as an obstacle sensor, to calculate various physical values occurring in the vehicle, a deceleration necessary for avoiding collision with an obstacle, and the like. Various components of the brake fluid pressure control actuator  50  are controlled based on the results of the calculations, so that the automatic braking control is performed to generate a braking force equivalent to a desired deceleration for a controlled wheel, or that the vehicle motion control is performed. 
     Referring to  FIG. 2 , the following description will discuss a configuration of a drive control apparatus of the motor  60 .  FIG. 2  illustrates a circuit configuration of the drive control apparatus of the motor  60 . The drive control apparatus is constituted by a part of the brake ECU  70  which part controls the circuit configuration illustrated in  FIG. 2  and by the circuit illustrated in  FIG. 2 . 
     As illustrated in  FIG. 2 , in a supply path of a motor current supplied from a battery  61  which is a power source, switching elements  62  and  63  which control on/off switching of the supply path are connected in parallel to a diode  62 a on an upstream side of the motor  60  and a diode  63   a  on a downstream side of the motor  60 , respectively. In this manner, the basic circuit of the drive circuit of the motor  60  is configured. The switching elements  62  and  63  are driven based on a control signal supplied from the brake ECU  70 . When both of the switching elements  62  and  63  are turned on, current is supplied from the battery  61  to the motor  60  so that the motor  60  is driven. 
     The brake ECU  70  carries out an operation for performing various types of brake fluid pressure control, including automatic braking control and various types of vehicle motion control. Accordingly, the brake ECU  70  recognizes the fact that one of the various types of brake fluid pressure control has been started and also recognizes various control variables of the brake fluid pressure control. The brake ECU  70  therefore drives various control valves or drives the motor  60  on the basis of a request from the brake fluid pressure control thus performed to operate the pumps  19  and  39  so as to charge and discharge the brake fluid. 
     In this case, the motor current can be controlled by performing high frequency switching with respect to one of the switching elements  62  and  63  by performing high frequency control, such as PWM control, while the other one of the switching elements  62  and  63  is turned on. Thus, motor control can be performed in both of a control mode in which the motor current is supplied for achieving a fully energized state by continuously turning on both of the switching elements  62  and  63  and a control mode in which the high frequency control is performed with respect to the motor current by performing high-speed switching. According to these control modes of motor control, the amount of a brake fluid to be discharged by the pumps  19  and  39  can be adjusted according to a control request value, thereby achieving desired brake fluid pressure control. 
     In the case of performing high frequency switching with respect to the switching elements  62  and  63 , another element, such as a reflux diode for absorbing switching surges, can be connected in parallel to the motor  60 . Since such an element is common, it is omitted from  FIG. 2 . 
     As illustrated in  FIG. 2 , a terminal voltage, i.e., a battery voltage, on the upstream side of the motor  60  in the motor drive circuit is inputted to the brake ECU  70 . This allows the brake ECU  70  to monitor the battery voltage as one of vehicle conditions. As illustrated in  FIG. 2 , the motor  60  includes a temperature sensor  71  as rotation speed correction means, so that a motor temperature can be detected through the temperature sensor  71 . 
     The vehicle braking device  1  including the motor drive control unit according to the present embodiment is configured as described above. The following description will discuss an example of an operation conducted by the vehicle braking device  1  having such a configuration. The present embodiment is characterized by automatic braking control for avoiding collision with an obstacle when the obstacle is detected on the basis of a detection signal supplied from an obstacle sensor, not illustrated. In the present embodiment, the normal braking operation of the driver depressing a brake pedal and various types of vehicle motion control, such as skid prevention control, for stabilizing the vehicle are similar to those of a conventional technique. Therefore, only the automatic braking control is described herein. 
     As illustrated in  FIG. 3 , when an obstacle  82  is detected ahead of a vehicle  80  during traveling, based on a detection signal supplied from an obstacle sensor, automatic braking control is performed according to the distance to the obstacle  82 . For example, in the case where the distance from the vehicle  80  to the obstacle  82  is long and urgency is not high, such as the case where, as illustrated in  FIG. 3 , the obstacle  82  has stopped at a position A in an identification range  81  of the obstacle sensor, automatic braking control is performed in a control mode similar to a conventional control mode. 
     Specifically, as shown in  FIG. 4 , until the obstacle  82  is detected, the vehicle  80  is in a normal state shown in step  100 . That is, until then, the vehicle  80  is in a state where braking control can be performed in response to depression of the brake pedal by the driver or braking control can be performed in response to a request from vehicle motion control, such as skid prevention control. If the obstacle  82  is detected in this state, a process of starting a collision alarm is performed in step  110  and the brake ECU  70  instructs an alarm section, not illustrated, to issue a collision alarm. The alarm section can be a display or an alarm lamp which visually notifies the driver of a collision risk, or can be a voice guidance device which audibly notifies the driver of a collision risk. With use of the alarm section, the driver is notified that the obstacle  82  is present in front of the vehicle and there is a collision risk. 
     The process then proceeds to step  120  and the ECU  70  applies primary braking at an initial stage of automatic braking to increase the braking force with a comparatively slight increase gradient. The process then proceeds to step  130  and the ECU  70  applies secondary braking to increase the braking force with a comparatively steep increase gradient. That is, as illustrated in  FIG. 5 , in the case where no automatic braking control is performed, a relative distance from the own vehicle to the obstacle  82  is long and thus it takes a relatively long time until the own vehicle collides with the obstacle  82 . As illustrated in  FIG. 6 , therefore, collision with the obstacle  82  can be avoided by increasing the braking force with a comparatively slight increase gradient as in the primary braking and then generating a desired braking force with a comparatively steep increase gradient as in the secondary braking. 
     Subsequently, when the vehicle stops, with the secondary braking being applied until then, the automatic braking control is terminated, as shown in step  140 . 
     In the case where the obstacle  82  is present at a position B illustrated in  FIG. 3 , that is, for example, in the case where the obstacle  82  enters the identification range  81  of the obstacle sensor from a direction lateral to the traveling direction of the vehicle, the distance from the own vehicle to the obstacle  82  is short and urgency is high. In such a case, the ECU  70  performs automatic braking control with high responsiveness so that a desired braking force is generated in a shorter period. 
     Specifically, as shown in  FIG. 7 , in the case where an obstacle is detected while the vehicle  80  is in a normal state as in step  200  and the distance to the obstacle is short and thus high responsiveness is required, the following process is performed. First, in step  210 , a process of starting a collision alarm is performed as in step  110  shown in  FIG. 4  to notify the driver that the obstacle is present in front of the vehicle and there is a collision risk. 
     The process then proceeds to step  220  and an instruction for starting emergency braking is issued. This causes, as shown in  FIG. 8 , a braking force with a steep increase gradient to be generated from the initial stage of starting automatic braking. The increase gradient in this case is set to be steeper than the increase gradient at the time of applying the secondary braking described above. That is, in the case where the obstacle  82  is present at the position B and no automatic braking control is performed, as illustrated in  FIG. 5 , the relative distance from the own vehicle to the obstacle is short and thus it takes a shorter period until the own vehicle collides with the obstacle. Therefore, unless a braking force is generated with a steep increase gradient as shown in  FIG. 8 , collision with the obstacle cannot be avoided. Thus, a braking force is ensured to be generated with a steep increase gradient so that a collision with the obstacle can be avoided even if the distance from the vehicle to the obstacle is short. 
     Subsequently, when the vehicle stops, with the secondary braking being applied until then, the automatic braking control is terminated as shown in step  230 . 
     However, to generate a braking force with a steep increase gradient as described above, the motor  60  needs to be started with high responsiveness. If the motor current is increased with a steep increase gradient for full energization of the motor to achieve high responsiveness, the startup current unavoidably increases and this leads to a decrease in battery voltage. This may cause malfunction in the control systems of various electrical components used in the vehicle. 
     According to the present embodiment, therefore, after the instruction for starting the emergency braking is issued as shown in step  220  and until the vehicle stops in step  230 , various processes of a startup program for emergency braking shown in  FIG. 9  are performed. 
     Specifically, in step  300 , the vehicle conditions are measured, and then in step  310 , high frequency control for startup is started according to the vehicle conditions thus measured. 
     The “vehicle conditions” refers to the battery voltage or the temperature of the motor. A decrease in battery voltage to not more than a predetermined voltage may cause malfunction in the control systems of the various electrical components used in the vehicle. The resistance of a motor coil depends on the temperature of the motor, specifically, the temperature of the motor coil, leading to a change in the rotation speed of the motor when the motor current is constant. For these reasons, the battery voltage and the temperature of the motor are measured as the vehicle conditions. 
     In step  320 , the high frequency control of repeatedly turning on/off switching the motor by PWM control is performed as startup motor control, and a duty ratio (ratio of on time in a predetermined period) in the PWM control is determined. For example, as shown in  FIG. 10 , duty ratio is changed according to the battery voltage so as to be decreased as the battery voltage decreases. In this manner, the battery voltage is prevented from being abnormally decreased by the increase in the startup current. 
     The process then proceeds to step  330  and the PWM control is started as the startup motor control determined in step  320 . Since the high frequency control based on the PWM control is performed at the startup in this manner, while the motor current is increased with a steep increase gradient at the start of increasing the motor current, the increase gradient can be suppressed to prevent an abnormal decrease of the battery voltage by the excessive startup current. Since the duty ratio in the PWM control is set according to the battery voltage, the motor current is increased with a predetermined increase gradient according to the change in the battery voltage. 
     Subsequently, the process proceeds to step  340  where it is determined whether the rotation speed of the motor  60  has reached a threshold. The threshold is set to a rotation speed which does not exceed an allowable upper limit of the motor current. With the threshold being set to a value less than an allowable upper limit, the battery voltage is prevented from decreasing to a level so low as to cause malfunction in the control systems of other various electrical components used in the vehicle, even if the startup current increases when control of the motor  60  is switched from the high frequency control to fully-energized-state control. That is, if the rotation speed of the motor  60  is insufficient and the control of the motor  60  is switched from the high frequency control to the fully-energized-state control, the startup current starts increasing and unavoidably reaches the allowable upper limit. Therefore, by determining, in step  340 , whether the rotation speed of the motor  60  has reached the threshold, the battery voltage is prevented from being decreased by the switching of the control of the motor  60  to the fully-energized-state control. 
     The rotation speed of the motor  60  can be estimated by performing an operation based on the driving voltage (battery voltage in the present embodiment) applied to the motor  60  and the time elapsed from the start of current supply. Since the resistance of the motor coil depends on the temperature of the motor coil, the rotation speed of the motor  60  can be more precisely calculated by calculating the resistance of the motor coil on the basis of the detection result of the temperature sensor  71  described above, and then correcting the rotation speed of the motor  60  according to the change in resistance of the motor coil. 
     As a matter of course, the motor  60  may include a rotation speed sensor and the rotation speed of the motor  60  may be directly measured based on the detection result of the rotation speed sensor. When the motor  60  is provided with a brush, torque ripple is caused in the motor current with the rotation of the motor  60 . Accordingly, the rotation speed of the motor  60  can be calculated based on the torque ripple. 
     If a negative determination is made in step  340 , the rotation speed of the motor  60  is still insufficient. In such a case, the process proceeds to step  350  and the vehicle conditions are measured again. Subsequently, the process proceeds to step  360  where the duty ratio is changed as necessary by resetting the duty ratio in the PWM control performed at the startup, based on the result of the measurement in step  350 . For example, as shown in  FIG. 10 , if the battery voltage decreases, the duty ratio is changed, for example decreased, to prevent abnormal decrease of the battery voltage. 
     After the duty ratio in the PWM control is reset as described above, step  340  is repeated. If an affirmative determination is made in step  340 , the process proceeds to step  370 . In this case, since the rotation speed of the motor  60  is sufficient, control of supplying a motor current for achieving a fully energized state is started as normal control. 
       FIG. 11  is a time chart showing the motor control described above. In addition to the motor current (indicated by the solid line) flowing when the motor control is performed,  FIG. 11  shows, for reference, motor current (indicated by the dashed line) flowing when fully-energized-state control is performed from the startup of the motor  60 .  FIG. 11  shows, as an example, the case where the motor  60  includes a brush. Accordingly, in this example, a torque ripple is caused in the motor current that is a change other than the change due to the high frequency control. The torque ripple, therefore, is not due to the high frequency control. The battery voltage in an actual vehicle presumably has a wave form different than shown in  FIG. 11  due to other factors, such as electrical charging caused by the start of an alternator. However, such other factors are disregarded in  FIG. 11 . 
     If the motor  60  is controlled in the fully energized state from the startup, the motor current can be increased with a steep increase gradient as indicated by the dashed line in  FIG. 11 , but the startup current also increases and reaches an allowable upper limit, leading to a decrease in battery voltage. 
     According to the present embodiment, the high frequency control based on the PWM control is performed at the startup of the motor  60 . Therefore, as indicated by the solid line in  FIG. 11 , increase in startup current can be suppressed, while the motor current is increased with a steep increase gradient. Thus, lowering of the battery voltage is minimized. 
     If the rotation speed of the motor  60  exceeds the threshold, control of the motor  60  is switched from the high frequency control to the fully-energized-state control. Although this may increase the motor current to some extent, even in such a case, the increase in motor current neither causes the motor current to reach the allowable upper limit nor causes the battery voltage to decrease. 
     As has been described, according to the present embodiment, output of the motor  60  is increased by controlling the motor  60  in the fully energized state, so that a high braking force with high responsiveness is obtained. Furthermore, the startup current is prevented from becoming excessive by performing the high frequency control only at the startup. Accordingly, decrease in battery voltage is prevented and thus malfunction is prevented from occurring in the control systems of the various electrical components used in the vehicle. 
     Other Embodiments 
     The present invention is not limited to the embodiment described above, but can be altered as appropriate within the scope of the claims. 
     For example, according to the above embodiment, by comparing the rotation speed of the motor  60  with the threshold in step  340 , it is determined whether an increase in startup current can be prevented. Alternatively, it can be configured such that an increase in the amount of a startup current is calculated on the basis of the rotation speed of the motor  60 , and then the control of the motor  60  is switched from the high frequency control to the fully-energized-state control if the startup current thus calculated is not more than the allowable upper limit. That is, a counter-electromotive force of the motor  60  can be calculated from the rotation speed of the motor  60 , and based on the counter-electromotive force, an increase in the amount of the motor current can be calculated for the case where the control of the motor  60  is switched to the fully-energized-state control. Then, the startup current after the switching can be calculated from the increase in the amount of the motor current. It may therefore be only necessary to determine whether the startup current is not more than the allowable upper limit. 
     The above embodiment deals with an example in which the switching elements  62  and  63  are provided upstream and downstream of the motor  60 , respectively. However, switching element does not need to be provided both upstream and downstream of the motor  60  but may be provided either upstream or downstream, and control of the switching element may be switched from the high frequency control to the fully-energized-state control. Furthermore, in the case of providing the switching elements  62  and  63  upstream and downstream of the motor  60 , respectively, the high frequency control can be performed for either one of the switching elements  62  and  63 . 
     According to the above embodiment, the duty ratio is set according to the battery voltage, but may instead be set according to the motor temperature, or according to both the motor temperature and the battery voltage. When the motor temperature is low, the resistance of the motor coil becomes low, and this tends to increase an inrush current. In the state where the motor temperature is low, the battery temperature is also expected to be low and thus charging/discharging capability of the battery is expected to be lowered as well. Therefore, by creating a setting where lower temperature causes lower duty ratio, the battery voltage can be prevented from being decreased by the increase in inrush current at the startup. 
     The steps shown in the drawings correspond to respective means for performing various processes. That is, the parts performing the processes in step  320 ,  330 , and  370  correspond to the startup control means, the determination means, and the normal control means, respectively. 
     REFERENCE SIGNS LIST 
       1 : Vehicle braking device;  19 ,  39 : Pump;  60 : Motor;  61 : Battery;  62 ,  63 : Switching element;  70 : Brake ECU;  80 : Vehicle;  81 : Identification range;  82 : Obstacle