Patent Publication Number: US-2021188254-A1

Title: Electric vehicle and control method for electric vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-230083 filed on Dec. 20, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to automatic parking control for an electric vehicle. 
     2. Description of Related Art 
     An electric vehicle including a parking assist device that assists a user&#39;s parking when parking the vehicle at a parking location is well-known. The parking assist device may adjust, for example, vehicle speed during executing automatic parking control by controlling driving force and braking force of the vehicle. 
     For such a parking assist device, International Publication No. 2018/230175 discloses a technology in which the vehicle speed is increased during parking according to behavior from previous parking (for example, elapsed time or travel distance), thereby reducing the discomfort felt by a driver familiar with using the automatic parking control. 
     SUMMARY 
     When the electric vehicle is stopped, driving torque may be limited in order to prevent a drive motor from being overheated due to a large driving torque being generated in the drive motor, for example, in a state where the rotation of the wheels is limited by the braking device. However, if the driving torque is limited during executing of the automatic parking control, for example, the vehicle may fall backward on the slope due to insufficient driving torque on a slope, or parking may take a longer time. 
     The present disclosure is intended to address the shortcomings described above. An objective of the present disclosure is to provide an electric vehicle and a control method for an electric vehicle, which are respectively capable of promptly completing parking while preventing the vehicle from falling backward when executing automatic parking control. 
     An electric vehicle according to one aspect of the present disclosure includes a power storage device, a drive electric motor configured to apply driving torque to the electric vehicle using electric power of the power storage device, a braking device configured to operate by receiving hydraulic pressure, and a control device configured to limit the driving torque such that the driving torque does not exceed an upper limit value which is set such that the drive electric motor is not overheated, when the electric vehicle is stopped while the hydraulic pressure is supplied to the braking device. The control device is configured to, while executing automatic parking control for moving the electric vehicle toward a target location without an operation of a user, cancel limitation of the driving torque in a case where the driving torque is applied to the electric vehicle that has stopped. 
     Consequently, it is possible to prevent the vehicle from falling backward due to insufficient driving torque during the automatic parking control on the slope. Therefore, the parking can be promptly completed. 
     In the aspect, the control device may cancel, while executing the automatic parking control, the limitation of the driving torque until a predetermined period elapses in a case where the driving torque is applied to the electric vehicle that has stopped. 
     With this configuration, while automatic parking control is executed, the limitation of the driving torque is canceled until the predetermined period elapses in a case where the driving torque is applied to the electric vehicle that has stopped. Thus, it is possible to prevent the vehicle from falling backward due to insufficient driving torque during the automatic parking control, for example, on the slope. Therefore, the parking can be promptly completed. 
     Further, in the aspect, the control device may gradually change, while executing the automatic parking control, the driving torque such that the driving torque is equal to or less than the upper limit value in a case where the electric vehicle does not move until the predetermined period elapses. 
     Consequently, it is possible to prevent the electric vehicle from falling backward by gradually changing the driving torque in a case where the vehicle does not move while the automatic parking control is executed. Further, it is possible to prevent the drive electric motor from being overheated by reducing the driving torque such that the driving torque is equal to or less than the upper limit value. 
     Further, in the aspect, the control device may increase, while executing the automatic parking control, the driving torque and reduce the hydraulic pressure supplied to the braking device in a case where the driving torque is applied to the electric vehicle that has stopped. 
     Consequently, it is possible to promptly complete the parking while preventing the vehicle from falling backward, for example, on the slope. 
     A control method for an electric vehicle according to another aspect of the present disclosure is a control method for an electric vehicle. The electric vehicle includes a power storage device, a drive electric motor configured to apply driving torque to the electric vehicle using electric power of the power storage device, and a braking device configured to operate by receiving hydraulic pressure. The control method includes a step of limiting the driving torque such that the driving torque does not exceed an upper limit value which is set such that the drive electric motor is not overheated, when the electric vehicle is stopped while the hydraulic pressure is supplied to the braking device, and a step of canceling, while executing automatic parking control for moving the electric vehicle toward a target location without an operation of a use, limitation of the driving torque in a case where the driving torque is applied to the electric vehicle that has stopped. 
     With the present disclosure, it is possible to provide an electric vehicle and a control method for an electric vehicle, which are respectively capable of promptly completing parking while preventing the vehicle from falling backward when executing automatic parking control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  is a diagram schematically showing a configuration of an electric vehicle; 
         FIG. 2  is a diagram showing a part of functional blocks set in an ECU; 
         FIG. 3  is a flowchart showing one example of processing executed by an automatic parking control unit; 
         FIG. 4  is a flowchart showing one example of processing executed by an upper limit value setting unit; 
         FIG. 5  is a time chart showing one example of an operation of the ECU; and 
         FIG. 6  is a time chart showing another example of the operation of the ECU. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to drawings. In the drawings, the same or equivalent components will have the same reference signs assigned, and descriptions thereof will be omitted. 
     Hereinafter, a case where an electric vehicle according to an embodiment of the present disclosure is a hybrid vehicle will be described as one example.  FIG. 1  is a diagram schematically illustrating a configuration of an electric vehicle  1  (hereinafter simply referred to as a “vehicle  1 ”). As shown in  FIG. 1 , the vehicle  1  includes a first motor generator (hereinafter referred to as a “first MG”)  10 , a second motor generator (hereinafter referred to as a “second MG”)  12 , an engine  14 , a power split device  16 , a drive wheel  28 , a brake actuator  29 , a braking device  31 , a power control unit (PCU)  40 , a system main relay (SMR)  50 , a power storage device  100 , a monitoring unit  200 , an electronic control unit (ECU)  300 , and an electric power steering (EPS)  360 . 
     Each of the first MG  10  and the second MG  12  is a three-phase alternating current rotating electric motor, e.g., a permanent magnet synchronous motor including a rotor in which permanent magnets are embedded. Each of the first MG  10  and the second MG  12  functions as both an electric motor and a power generator. The first MG  10  and the second MG  12  are connected to the power storage device  100  via the PCU  40 . 
     The first MG  10  may, for example, be driven by an inverter included in the PCU  40  when the engine  14  is started, and rotates an output shaft of the engine  14 . Further, the first MG  10  receives the power of the engine  14  and generates power during power generation. The electric power generated by the first MG  10  is stored in the power storage device  100  via the PCU  40 . 
     The second MG  12  may, for example, be driven by an inverter included in the PCU  40  when the vehicle  1  is traveling. The power of the second MG  12  is transmitted to the drive wheel  28  via a power transmission gear (not shown), such as a differential gear or a reduction gear. Further, the second MG  12  may, for example, be driven by the drive wheel  28  during braking, and the second MG  12  operates as the generator to perform regenerative braking. The electric power generated by the second MG  12  is stored in the power storage device  100  via the PCU  40 . In the present embodiment, the second MG  12  corresponds to a “drive electric motor”. Even though only one drive wheel  28  is shown in  FIG. 1 , at least two drive wheels  28  are actually provided in the vehicle  1 . 
     The engine  14  is a well-known internal combustion engine that burns fuel (gasoline or light oil), such as a gasoline engine or a diesel engine, so as to output power, and is configured such that operating states (such as a throttle opening degree (an intake amount), a fuel supply amount, and ignition timing) are electrically controlled by an ECU  300 . The ECU  300  may control, for example, a fuel injection amount, ignition timing and an intake air amount of the engine  14  such that the engine  14  operates at a target rotation speed and a target torque set based on a state of the vehicle  1 . 
     The power split device  16  splits the power of the engine  14  into a path transmitted to the drive wheel  28  and a path transmitted to the first MG  10 . The power split device  16  may be configured by, for example, a planetary gear mechanism. 
     The braking device  31  is provided for each wheel (including the drive wheel  28 ), and is configured to generate a friction braking force on the wheel using a hydraulic pressure supplied from the brake actuator  29 . The braking device  31  includes a disc rotor  31   a  and a brake caliper  31   b . The disc rotor  31   a  is fixed to a wheel and is configured to be integrally rotatable with the wheel. The brake caliper  31   b  includes a wheel cylinder and a brake pad (neither shown). The wheel cylinder is operated by the hydraulic pressure supplied from the brake actuator  29 . The brake pad is pressed against the disc rotor  31   a  to limit the rotation of the disc rotor  31   a  during operating the wheel cylinder. The higher the hydraulic pressure applied to the wheel cylinder is, the higher a pressing force of the brake pad against the disc rotor  31   a  is. 
     The brake actuator  29  is configured to supply the hydraulic pressure to each wheel cylinder of each wheel according to a control signal from the ECU  300 . The brake actuator  29 , for example, supplies the hydraulic pressure to the braking device  31  of each wheel regardless of an operation of a brake pedal, or supplies the hydraulic pressure, to the braking device  31  of each wheel, corresponding to a depression amount of the brake pedal. 
     The PCU  40  is a power conversion device that performs power conversion between the power storage device  100  and the first MG  10 , or performs power conversion between the power storage device  100  and the second MG  12 , according to the control signal from the ECU  300 . The PCU  40  may include, for example, the inverter that converts direct current power from the power storage device  100  into alternating current power so as to drive the first MG  10  or the second MG  12 , and a converter that adjusts a voltage level of the direct current power supplied from the power storage device  100  to the inverter (neither shown). 
     The SMR  50  is electrically connected between the power storage device  100  and the PCU  40 . Closing/opening of the SMR  50  is controlled according to the control signal from the ECU  300 . 
     The power storage device  100  is a rechargeable direct current power supply, and may be, for example, a secondary battery, such as a nickel-metal hydride battery or a lithium-ion battery containing a solid or liquid electrolyte. As the power storage device  100 , a capacitor, such as an electric double layer capacitor, can also be employed. The power storage device  100  supplies the electric power for generating a travel driving force of the vehicle  1  to the PCU  40 . In addition, the power storage device  100  is charged with the electric power generated by using the first MG  10  and the engine  14 , charged with the electric power generated by the regenerative braking of the second MG  12 , or discharged by a driving operation of the first MG  10  or the second MG  12 . 
     The monitoring unit  200  includes a voltage detection unit  210 , a current detection unit  220 , and a temperature detection unit  230 . The voltage detection unit  210  detects the voltage VB between terminals of the power storage device  100 . The current detection unit  220  detects the current IB input to and output from the power storage device  100 . The temperature detection unit  230  detects the temperature TB of the power storage device  100 . Each detection unit outputs the detection result to the ECU  300 . 
     The EPS  360  may include, for example, an electric actuator that applies a steering force to a steering wheel. The EPS  360  uses the electric actuator to assist the steering force generated by a user&#39;s steering operation, or applies the steering force to the steering wheel using the electric actuator regardless of the user&#39;s steering operation, according to the control signal from the ECU  300 . The steering wheel may be the drive wheel  28 , or may be another driven wheel provided in the vehicle  1 . 
     The ECU  300  is an electronic control unit having a central processing unit (CPU)  301 , and a memory (including, for example, a read-only memory (ROM) or a random access memory (RAM))  302 . The ECU  300  controls each device (the engine  14 , the brake actuator  29 , the PCU  40 , the SMR  50  and the like) in the vehicle  1  such that the vehicle  1  is in a desired state based on a signal received from the monitoring unit  200 , an automatic parking execution switch  350 , a vehicle speed sensor  352 , a shift position sensor  354  or a hydraulic brake pressure sensor  356 , or information, such as maps or programs stored in the memory  302 . Various controls executed by the ECU  300  are not limited to processing executed by software, and may be performed by dedicated hardware (an electronic circuit). 
     The ECU  300  may calculate, for example, a state-of-charge (SOC) indicating remaining capacity of the power storage device  100 , while the vehicle  1  is operated, using the detection result of the monitoring unit  200 . As a method for calculating the SOC, various well-known algorithms, such as an algorithm using current value integration (Coulomb count) or an algorithm using estimation of open circuit voltage (OCV), can be employed. 
     The automatic parking execution switch  350 , the vehicle speed sensor  352 , the shift position sensor  354 , the hydraulic brake pressure sensor  356  and a camera  358  are connected to the ECU  300 . 
     The automatic parking execution switch  350  may be, for example, a button or a lever. In a case where the automatic parking execution switch  350  receives an ON operation (for example, an operation of pressing the button or an operation of moving the lever to a predetermined position) performed by the user, the automatic parking execution switch  350  is configured to transmit, to the ECU  300 , a signal indicating that the ON operation is received. 
     The vehicle speed sensor  352  detects the speed of the vehicle  1  (hereinafter referred to as “vehicle speed”). The vehicle speed sensor  352  transmits a signal indicating the detected vehicle speed to the ECU  300 . 
     The shift position sensor  354  detects a gear shift position selected by the user from a plurality of gear shift positions. The plurality of gear shift positions may include, for example, a parking position, a reverse position (hereinafter referred to as a “R position”), a neutral position, and a drive position (hereinafter referred to as a “D position”). The shift position sensor  354  transmits a signal indicating the detected gear shift position to the ECU  300 . 
     For example, in a case where the D position is set as the gear shift position, the ECU  300  controls each device (for example, the PCU  40  and the engine  14 ) in the vehicle  1  such that the vehicle  1  can move forward. 
     Similarly, for example, in a case where the R position is set as the gear shift position, the ECU  300  controls each device (for example, the PCU  40  and the engine  14 ) in the vehicle  1  such that the vehicle  1  can move backward. 
     Further, the ECU  300  controls the PCU  40  so as to generate the driving torque equivalent to creep torque in the second MG  12  in a case where a traveling position, such as the D position or the R position, is selected and the vehicle speed is equal to or less than a threshold, even in a state where an accelerator pedal is not depressed. 
     The hydraulic brake pressure sensor  356  detects the hydraulic pressure supplied to the braking device  31  (hereinafter referred to as a “hydraulic brake pressure”). The hydraulic brake pressure sensor  356  transmits a signal indicating the detected hydraulic brake pressure to the ECU  300 . 
     The cameras  358  are provided, for example, on a front side and a rear side of the vehicle  1 , and are configured to be able to capture image of the front and the rear of the vehicle  1 . The camera  358  transmits a signal indicating a captured image to the ECU  300 . 
     In the vehicle  1  having such a configuration, in a case where the accelerator pedal and the brake pedal are depressed in parallel while the vehicle  1  is stopped, the electric power is supplied to the second MG while the rotation of the drive wheel  28  is limited. Therefore, the second MG  12  may become overheated. Thus the ECU  300  executes torque limit control for setting an upper limit value of the driving torque generated in the second MG  12  in a case where a predetermined execution condition is satisfied. 
     Examples of the predetermined execution condition include, for example, a condition in which the vehicle  1  is stopped, a condition in which the vehicle  1  is in a brake-on state where the hydraulic brake pressure is greater than a threshold, and a condition in which the gear shift position is a traveling position (D position or R position). 
     The upper limit value of the driving torque of the second MG  12  is set, for example, such that the motor is not overheated even if a predetermined time elapses in a case where current flows through the second MG  12  in a state where the rotation of the drive wheel  28  is limited. 
     It is possible to prevent the second MG  12  from being overheated in a case where, for example, the accelerator pedal and the brake pedal are depressed in parallel by the user while the vehicle  1  is stopped, by executing the torque limit control in a case where the predetermined execution condition is satisfied. 
     Further, in a case where the ON operation is performed on the automatic parking execution switch  350  while the vehicle  1  is stopped, the automatic parking control is executed to move the vehicle  1  toward the target location without the operation of the user. The operation including at least one of a driving operation, a braking operation, a steering operation, and a shifting operation, which is required until the vehicle  1  is parked in a parking space, is automatically performed by executing the automatic parking control. 
     For example, when the user turns on the automatic parking execution switch  350  in a state where the vehicle is stopped next to an entrance of the parking space surrounded by a boundary line, a predetermined parking operation is performed such that the vehicle  1  is parked in the parking space. 
     The predetermined parking operation may include, for example, a first operation and a second operation. The first operation includes the steering operation in which the vehicle is steered in a first direction away from the parking space when the vehicle is moving forward, the driving operation in which the vehicle  1  moved forward by a predetermined distance in a state where the D position is selected, and the braking operation in which the vehicle  1  is stopped. The second operation, after the first operation is completed, includes the steering operation in which the vehicle is steered in a second direction opposite to the first direction, the operation in which the gear shift position is shifted from the D position to the R position, the driving operation in which the vehicle  1  is moved backward so as to enter the parking space in a state where the R position is selected, and the braking operation in which the vehicle  1  is stopped. 
     The boundary line set as the parking space may be recognized by, for example, image processing executed on the image captured by the camera  358 , and various operations (the driving operation, the braking operation, or the steering operation) are performed such that the vehicle  1  enters the parking space based on the recognition result. 
     It is possible to move the vehicle  1  to the parking space without the operation of the user by executing the automatic parking control as described above. 
     When the vehicle  1  is stopped while the automatic parking control is executed, the vehicle  1  is in the brake-on state in which the hydraulic brake pressure is higher than the threshold such that the vehicle  1  does not move due to the driving torque which is equivalent to the creep torque. In a case where the vehicle  1  is started while the automatic parking control is executed, the brake actuator  29  is required to be controlled such that the hydraulic brake pressure gradually decreases as the driving torque increases in order to prevent the vehicle  1  from falling backward in a parking lot including a slope. 
     However, when the vehicle  1  is in the brake-on state in which the hydraulic brake pressure is greater than the threshold in a case where the driving torque of the second MG  12  is applied to the vehicle  1  that has stopped, the upper limit value is set for the driving torque of the second MG  12  by the torque limit control described above. Therefore, since the driving torque for starting the vehicle  1  in the parking lot including a slope is insufficient, the vehicle  1  may fall backward or it may take a longer time to complete the predetermined parking operation due to a decreased moving speed. 
     In the present embodiment, the ECU  300  cancels, while executing the automatic parking control, the limitation of the driving torque in a case where the driving torque is applied to the vehicle  1  that has stopped. 
     Consequently, it is possible to prevent the vehicle from falling backward due to insufficient driving torque during the automatic parking control on the slope. Therefore, the parking can be promptly completed by the predetermined parking operation. 
     A part of a configuration of functional blocks set in the ECU  300  as software or hardware and the operation thereof will be described hereinbelow with reference to  FIG. 2 .  FIG. 2  is a diagram showing a part of the functional blocks set in an ECU  300 . 
     The ECU  300  includes an automatic parking control unit  400 , a torque adjustment unit  402 , a torque limiting unit  404 , a torque command unit  406 , a hydraulic pressure setting unit  408 , a hydraulic pressure command unit  410 , and an upper limit value setting unit  412 . 
     The automatic parking control unit  400  may execute, for example, the automatic parking control that performs the predetermined parking operation when the ON operation of the automatic parking execution switch  350  is received. The automatic parking control unit  400  sets, while executing the automatic parking control, various required amounts for performing various operations (the driving operation, the braking operation, the steering operation, and the shifting operation) that constitute the predetermined parking operation. The various required amounts may include, for example, a required driving torque and a required hydraulic brake pressure. The automatic parking control unit  400  may set, for example, the required driving torque such that the driving torque of the second MG  12  gradually increases until the vehicle speed reaches a target vehicle speed when the vehicle  1  is started. Furthermore, the automatic parking control unit  400  may set, for example, the required hydraulic brake pressure such that the hydraulic brake pressure gradually decreases when the vehicle  1  is started. 
     The various required amounts may include, for example, a required steering force. The automatic parking control unit  400  sets the required steering force such that the steering wheel is steered in a steering direction based on the predetermined parking operation (steering operation). A steering control unit (not shown in  FIG. 2 ) controls the EPS  360  such that the set required steering force is generated. 
     Furthermore, the automatic parking control unit  400  may set a forward drive request or a backward drive request based on the predetermined parking operation. The gear shift control unit (not shown in  FIG. 2 ) selects the D position as the required gear shift position when the forward drive request is set, and selects the R position as the required gear shift position when the backward drive request is set. Switching to the required gear shift position may be automatically performed using an actuator or the like, or may be performed by prompting the switching by offering a display guidance or a voice guidance to the driver. 
     Further, the automatic parking control unit  400  turns on a cancellation request flag for canceling the limitation of the driving torque in a case where a request to start the vehicle  1  is issued for performing the driving operation. The automatic parking control unit  400  turns off the cancellation request flag in a case where the predetermined period has elapsed from the time when the cancellation request flag was turned on. The automatic parking control unit  400  determines that there is a request to start the vehicle  1  in a case where, for example, the vehicle speed is zero and the required driving torque is greater than a threshold. 
     The torque adjustment unit  402  adjusts a plurality of pieces of required driving torque set in a plurality of functional blocks including the automatic parking control unit  400  to set a single piece of required driving torque. The torque adjustment unit  402  sets, for example, the greatest required driving torque from among various pieces of required driving torque as the required driving torque after adjustment. Further, the adjustment is not limited to the method described above, and the torque adjustment unit  402  may set, for example, a required driving torque set in the functional block having a high priority as the required driving torque after adjustment. 
     The torque limiting unit  404  compares the required driving torque after adjustment with the upper limit value of the driving torque calculated by the upper limit value setting unit  412 , to be described below, so as to set the final value of required driving torque. The torque limiting unit  404  sets the upper limit value as the final value of required driving torque in a case where, for example, the required driving torque after adjustment exceeds the upper limit value. The torque limiting unit  404  sets the required driving torque after adjustment as the final value of required driving torque in a case where the required drive torque after adjustment is equal to or less than the upper limit value. 
     The torque command unit  406  generates a control command for generating the final value of required driving torque set in the torque limiting unit  404 , and transmits the generated control command to the PCU  40 . 
     The hydraulic pressure setting unit  408  acquires a current hydraulic brake pressure from the hydraulic brake pressure sensor  356 . The hydraulic pressure setting unit  408  sets the final value of required hydraulic brake pressure using the required hydraulic brake pressure set by the automatic parking control unit  400 , and the acquired current hydraulic brake pressure. The hydraulic pressure setting unit  408  may set the final value of required hydraulic brake pressure, for example, such that the current hydraulic brake pressure gradually approaches the required hydraulic brake pressure. 
     The hydraulic pressure command unit  410  generates a control command for generating the required hydraulic brake pressure set in the hydraulic pressure setting unit  408 , and transmits the generated control command to the brake actuator  29 . 
     The upper limit value setting unit  412  sets the upper limit value for preventing the second MG  12  from being overheated in a case where, for example, a predetermined condition is satisfied. Examples of the predetermined condition include a condition in which the cancellation request flag is in an OFF state, in addition to the predetermined execution condition of the torque limit control described above. The upper limit value may be a predetermined value, or may be set based on, for example, a temperature or a load history of the second MG  12 . The upper limit value setting unit  412  cancels setting of the upper limit value in a case where, for example, the predetermined condition is not satisfied. In this case, the upper limit value setting unit  412  sets, for example, an upper limit value that is greater than the upper limit value set as the predetermined condition is established (for example, greater than the required driving torque that can be set in the automatic parking control unit  400 ). 
     One example of processing executed by the automatic parking control unit  400  will be described hereinbelow with reference to  FIG. 3 .  FIG. 3  is a flowchart showing one example of the processing executed by the automatic parking control unit  400 . 
     In step (hereinafter step is simply referred to as “S”)  100 , the automatic parking control unit  400  determines whether the automatic parking control is currently being executed. 
     The automatic parking control unit  400  sets an automatic parking control execution flag to the ON state by, for example, performing the ON operation of the automatic parking execution switch  350 . Therefore, the automatic parking control unit  400  determines that the automatic parking control is currently being executed in a case where the automatic parking control execution flag is in the ON state. Further, the automatic parking control unit  400  sets the automatic parking control execution flag to the OFF state in a case where the automatic parking control is completed or interrupted. In a case where it is determined that the automatic parking control is currently being executed (YES in S 100 ), the process proceeds to S 102 . 
     In S 102 , the automatic parking control unit  400  sets the various required amounts. Since the various required amounts are as described above, the detailed descriptions thereof will be omitted. 
     In S 104 , the automatic parking control unit  400  determines whether a start request is issued for the vehicle  1 . Since the method for determining whether the start request is issued is as described above, the detailed descriptions thereof will be omitted. In a case where it is determined that the start request is issued (YES in S 104 ), the process proceeds to S 106 . 
     In S 106 , the automatic parking control unit  400  sets the cancellation request flag to the ON state. At this time, the automatic parking control unit  400  measures, for example, the elapsed time from the time when the cancellation request flag is set to the ON state using a timer (not shown) or the like. 
     In S 108 , the automatic parking control unit  400  determines whether a predetermined time has elapsed from the time when the cancellation request flag was set to the ON state. In a case where it is determined that the predetermined time has elapsed (YES in S 108 ), the process proceeds to S 110 . 
     In S 110 , the automatic parking control unit  400  sets the cancellation request flag to the OFF state. In S 112 , the automatic parking control unit  400  determines whether the vehicle  1  is unable to be started. The automatic parking control unit  400  determines that the vehicle  1  is unable to be started in a case where, for example, the vehicle speed is less than or equal to the threshold. In a case where it is determined that the vehicle  1  is unable to be started (YES in S 112 ), the process proceeds to S 114 . 
     In S 114 , the automatic parking control unit  400  executes a cancellation process of canceling the starting of the vehicle  1 . In particular, the automatic parking control unit  400  sets, while executing the cancellation process, the required driving torque such that the required driving torque gradually decreases until the required driving torque is equal to or less than a first upper limit value. The automatic parking control unit  400  may set the required driving torque such that, for example, the required driving torque gradually decreases to zero. Further, the automatic parking control unit  400  sets the required driving torque such that, for example, the driving torque linearly decreases (by a predetermined amount). 
     Further, the automatic parking control unit  400  sets, while executing the cancellation process, the required hydraulic brake pressure such that the required hydraulic brake pressure gradually increases until the required hydraulic brake pressure becomes a target hydraulic brake pressure. The target hydraulic brake pressure may be, for example, the hydraulic brake pressure at the time when the hydraulic brake pressure starts to be reduced in order to start the vehicle  1 . The automatic parking control unit  400  sets the required hydraulic brake pressure such that, for example, the hydraulic brake pressure linearly increases (by a predetermined amount). The automatic parking control unit  400  ends the cancellation process in a case where the required driving torque has a value equivalent to the creep torque, and the required hydraulic brake pressure reaches the target hydraulic brake pressure. 
     In addition, in a case where it is determined that the automatic parking control is not currently being executed (NO in S 100 ), where it is determined that no start request is issued (NO in S 104 ), or where it is determined that the vehicle has started (NO in S 112 ), the process ends. In a case where it is determined that the predetermined time has not elapsed (NO in S 108 ), the process returns to S 108 . 
     One example of processing executed by the upper limit value setting unit  412  will be described hereinbelow with reference to  FIG. 4 .  FIG. 4  is a flowchart showing one example of the processing executed by the upper limit value setting unit  412 . 
     In S 200 , the upper limit value setting unit  412  determines whether the gear shift position is a traveling position. The upper limit value setting unit  412  determines that the gear shift position is the traveling position in a case where, for example, the gear shift position is the D position or the R position. In a case where it is determined that the gear shift position is the traveling position (YES in S 200 ), the process proceeds to S 202 . 
     In S 202 , the upper limit value setting unit  412  determines whether the vehicle  1  is stopped. The upper limit value setting unit  412  determines that the vehicle  1  is stopped in a case where the vehicle speed is equal to or less than the threshold. In a case where it is determined that the vehicle  1  is stopped (YES in S 202 ), the process proceeds to S 204 . 
     In S 204 , the upper limit value setting unit  412  determines whether the vehicle is in the brake-on state. The upper limit value setting unit  412  determines that the vehicle is the brake-on state in a case where the current hydraulic brake pressure is higher than a threshold. In a case where it is determined that the vehicle is in the brake-on state (YES in S 204 ), the process proceeds to S 206 . 
     In S 206 , the upper limit value setting unit  412  determines whether the cancellation request flag is in the OFF state. In a case where it is determined that the cancellation request flag is in the OFF state (YES in S 206 ), the process proceeds to S 208 . 
     In S 208 , the upper limit value setting unit  412  sets the upper limit value of the driving torque of the second MG  12 . Further, when the gear shift position is not the traveling position (NO in S 200 ), when the vehicle is not stopped (NO in S 202 ), when the vehicle is not in the brake-on state (NO in S 204 ), or when the cancellation request flag is in the ON state (NO in S 206 ), the process proceeds to S 210 . 
     In S 210 , the upper limit value setting unit  412  cancels setting of the upper limit. The upper limit value setting unit  412  may set, as a new upper limit value, a value greater than the required driving torque that can be set in the automatic parking control unit  400 . 
     One example of the operation of the ECU  300  mounted on the vehicle  1 , which is the electric vehicle according to the present embodiment, based on the structures and flowcharts described above, will be described hereinbelow.  FIG. 5  is a timing chart showing one example of the operation of the ECU  300 . A horizontal axis in  FIG. 5  indicates time. A vertical axis in  FIG. 5  indicates the automatic parking control execution flag, the cancellation request flag, the vehicle speed, the driving torque, the hydraulic brake pressure, and the gear shift position. 
     LN 1  in  FIG. 5  indicates a change in the automatic parking control execution flag. LN 2  in  FIG. 5  indicates a change in the cancellation request flag. LN 3  in  FIG. 5  indicates a change in the vehicle speed. LN 4  in  FIG. 5  indicates a change in the driving torque. LN 5  in  FIG. 5  indicates a change in the hydraulic brake pressure. LN 6  in  FIG. 5  indicates a change in the gear shift position. 
     For example, it is assumed that the automatic parking execution switch  350  is turned on and the automatic parking control is currently being executed. In this case, the automatic parking control execution flag is maintained as being in the ON state as indicated by LN 1  in  FIG. 5 . Further, it is assumed that the vehicle speed is zero (the vehicle is stopped) as shown by LN 3  in  FIG. 5 , the driving torque is Tq( 0 ) equivalent to the creep torque as shown by LN 4  in  FIG. 5 , and the hydraulic brake pressure is Pb( 0 ) (in a constant state) as shown by LN 5  in  FIG. 5 . Further, as shown by LN 6  in  FIG. 5 , the gear shift position is assumed to be the D position. The drive operation shown in  FIG. 5  is assumed to be performed as, for example, the driving operation included in the first operation of the predetermined parking operation. 
     At this time, the gear shift position is the D position which is the traveling position as shown by LN 6  in  FIG. 5  (YES in S 200 ), the vehicle is stopped as shown by LN 3  in  FIG. 5  (YES in S 202 ), the vehicle is in the brake-on state as shown by LN 5  in  FIG. 5  (YES in S 204 ), and the cancellation request flag is in the OFF state as shown by LN 2  in  FIG. 5  (YES in S 206 ), thus an upper limit value Tq( 1 ) of the driving torque of the second MG  12  is set (S 208 ). 
     At time t( 0 ), while the automatic parking control is executed (YES in S 100 ), if various required amounts are set to perform the driving operation (S 102 ), the start request is issued (YES in S 104 ), thus the cancellation request flag is set to the ON state (S 106 ). Since the cancellation request flag is in the ON state (NO in S 206 ), setting of the upper limit value Tq( 1 ) of the driving torque of the second MG  12  is canceled (S 210 ). 
     Since the required hydraulic brake pressure is set to gradually decrease while setting the various required amounts, the hydraulic brake pressure decreases by a predetermined amount over time as shown by LN 5  in  FIG. 5 , from the hydraulic brake pressure Pb( 0 ) to zero at time t( 5 ). 
     Further, while setting the various required amounts, the required driving torque is set to gradually increase until the vehicle speed reaches the target vehicle speed. Therefore, the driving torque increases by a predetermined amount over time when the driving torque starts to increase at time t( 1 ) after time t( 0 ). Further, a timing at which the driving torque starts to increase may be the same as a timing at which the hydraulic brake pressure starts to decrease, or may be earlier than the timing at which the hydraulic brake pressure starts to decrease, and the timing can be appropriately set. 
     The setting of the upper limit value of the driving torque of the second MG  12  is canceled, thus the driving torque continuously increases even after the driving torque reaches the upper limit value Tq( 1 ) at time t( 2 ), as shown by LN 4  in  FIG. 5 . 
     When the driving force acting on the vehicle  1  exceeds the force that limits the movement of the vehicle  1  due to the increased driving torque of the second MG  12  at time t( 3 ), the vehicle  1  starts to move. Therefore the vehicle speed increases as shown by LN 3  in  FIG. 5 . 
     The vehicle speed becomes constant at time t( 4 ) as shown by LN 3  in  FIG. 5 . When it reaches time t( 5 ) as the predetermined time has elapsed from time t( 0 ) (YES in S 108 ), the cancellation request flag is in the OFF state as shown by LN 2  in  FIG. 5  (S 110 ). If the vehicle  1  has started to move, it is determined that the vehicle can be started (NO in S 112 ), and therefore the cancellation process is not executed. 
     Further, at time t( 5 ) when the cancellation request flag is in the OFF state, when the driving torque of the second MG  12  reaches Tq( 2 ) as shown by LN 4  in  FIG. 5 , in a case where the vehicle speed reaches the target vehicle speed, the driving torque is maintained so as to be constant thereafter. Further, as shown by LN 5  in  FIG. 5 , when the hydraulic brake pressure reaches zero, the hydraulic brake pressure is continuously maintained so as to be constant thereafter. The second operation is performed after the other operations included in the first operation are performed as well as the driving operation. 
     When the second operation is performed and the vehicle  1  is moved backward by switching from the D position to the R position, the same operation as the driving operation described above is performed. That is, setting of the upper limit value of the driving torque of the second MG  12  is canceled as the cancellation request flag is in the ON state. 
     Another example of the operation of the ECU  300  mounted on the vehicle  1 , which is the electric vehicle according to the present embodiment, will be described hereinbelow.  FIG. 6  is a timing chart showing another example of the operation of the ECU  300 . A horizontal axis in  FIG. 6  indicates time. A vertical axis in  FIG. 6  is the same as the vertical axis in  FIG. 5 . Therefore, the detailed descriptions thereof will be omitted. 
     LN 7  in  FIG. 6  indicates a change in the automatic parking control execution flag. LN 8  in  FIG. 6  indicates a change in the cancellation request flag. LN 9  in  FIG. 6  indicates a change in the vehicle speed. LN 10  in  FIG. 6  indicates a change in the driving torque. LN 11  in  FIG. 6  indicates a change in the hydraulic brake pressure. LN 12  in  FIG. 6  indicates a change in the gear shift position. 
     The changes shown by LN 7 , LN 8 , LN 12  in  FIG. 6  are the same as the changes shown by LN 1 , LN 2 , LN 6  in  FIG. 5 , respectively. The changes shown by LN 9  to LN 11  in  FIG. 6  up to time t( 3 ) are the same as the changes shown by LN 3  to LN 5  in  FIG. 5  up to time t( 3 ), respectively. Therefore, the detailed descriptions thereof will be omitted. 
     In a case where, for example, the slope is steep at time t( 3 ), the driving torque of the second MG  12  does not exceed the force that limits the movement of the vehicle  1 , and therefore the vehicle  1  does not start to move. Thus, as shown by LN 9  in  FIG. 6 , the vehicle  1  continues to stop after time t( 3 ). 
     When it reaches time t( 5 ) as the predetermined time has elapsed from time t( 0 ) (YES in S 108 ), the cancellation request flag is in the OFF state as shown by LN 8  in  FIG. 6  (S 110 ). Since the vehicle  1  does not start to move, it is determined that the vehicle is unable to be started (YES in S 112 ), and therefore the cancellation process is executed (S 114 ). 
     Therefore, as shown by LN 10  in  FIG. 6 , the driving torque of the second MG  12  gradually decreases after time t( 5 ) so as to be equal to or less than the upper limit value Tq( 1 ). Further, as shown by LN 11  in  FIG. 6 , the hydraulic brake pressure gradually increases after time t( 5 ) until the hydraulic brake pressure reaches Pb( 0 ). 
     According to the electric vehicle of the present embodiment, while the automatic parking control is executed, the limitation of the driving torque is canceled in a case where the driving torque is applied to the vehicle  1  that has stopped. Thus, it is possible to prevent the vehicle  1  from falling backward due to insufficient driving torque during the automatic parking control on the slope or the like. Therefore, the parking can be promptly completed. Therefore, it is possible to provide an electric vehicle and a control method for an electric vehicle, which are respectively capable of promptly completing parking while preventing the vehicle from falling backward when executing automatic parking control. 
     Furthermore, while the automatic parking control is executed, the limitation of the driving torque is canceled until the predetermined period elapses in a case where the driving torque is applied to the vehicle  1  that has stopped. Thus, it is possible to prevent the vehicle  1  from falling backward due to insufficient driving torque during the automatic parking control on the slope or the like. 
     Moreover, it is possible to prevent the vehicle  1  from falling backward by gradually changing the driving torque in a case where the vehicle  1  does not move while the automatic parking control is executed. Further, it is possible to prevent the second MG  12  from being overheated by reducing the driving torque such that the drive torque is equal to or less than the upper limit value. 
     Further, while the automatic parking control is executed, the driving torque increases and the hydraulic pressure supplied to the braking device  31  is reduced in a case where the driving torque is applied to the vehicle  1  that has stopped. Therefore, it is possible to prevent the vehicle  1  from falling backward on the slope while promptly completing the parking. 
     Modified examples will be described hereinbelow. In the embodiment described above, the configuration of a hybrid vehicle has been described as the example of the vehicle  1 . However, the vehicle  1  is not limited to a hybrid vehicle as long as it is an electric vehicle. The vehicle  1  may be, for example, an electric vehicle equipped with one or more motor generators as a driving source. 
     Furthermore, in the embodiment described above, the automatic parking control is executed by turning on the automatic parking execution switch. However, instead of turning on the automatic parking execution switch, the automatic parking control may be performed by touch on the automatic parking execution switch displayed on a touchscreen display. 
     Further, in the embodiment described above, the predetermined parking operation is exemplified in that the vehicle  1  is moved forward while steering in a direction in which the vehicle enters the parking space in a state in which the vehicle is stopped in parallel with the entrance of a parking space that is surrounded by the boundary line, and then the vehicle  1  moves backward with the steering direction reversed, thereby parking in the parking space. However, the parking operation is not particularly limited thereto. For example, the predetermined parking operation may include an operation in which the vehicle is parked in the parking space in a state where the vehicle is parked adjacent to a parking space in which parallel parking is possible, or may include an operation in which the vehicle  1  is moved to the outside of the parking space in a state where the vehicle  1  is stopped in the parking space. 
     Further, in the embodiment described above, it is exemplified that the driving torque is linearly changed, however, the driving torque may be gradually changed so as to gradually increase or decrease. For example, the driving torque may be changed non-linearly. 
     Further, in the embodiment described above, it is exemplified that the hydraulic brake pressure is linearly changed, however, the hydraulic brake pressure may be gradually changed so as to gradually increase or decrease. For example, the hydraulic brake pressure may be changed non-linearly. 
     The modified examples may be implemented by combining all or some of these examples as appropriate. The embodiments disclosed are to be considered as illustrative and not restrictive. The scope of the present disclosure is defined by the terms of the claims, not the description described above, and includes any modifications within the scope and meanings equivalent to the terms of the claims.