Patent Publication Number: US-2022212546-A1

Title: Vehicle control apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2021-000383 filed on Jan. 5, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The technology relates to a vehicle control apparatus that controls a traveling motor serving as a power source of a drive system. 
     Vehicles such as electric vehicles and hybrid vehicles include traveling motors serving as power sources of drive systems. The traveling motor receives electric power at a stator coil from a lithium-ion battery via an inverter. Reference is made to Japanese Unexamined Patent Application Publication (JP-A) Nos. H9-70195, 2020-5373, 2007-325417, and 2017-220971. 
     SUMMARY 
     An aspect of the technology provides a vehicle control apparatus to be applied to a vehicle. The vehicle control apparatus is configured to perform control of a traveling motor of the vehicle that serves as a power source of a drive system of the vehicle. The vehicle control apparatus includes an inverter, a torque setting unit, a torque correction unit, an inverter control unit, and a motor lock determination unit. The inverter includes a plurality of switching elements. The inverter is configured to supply electric power to the traveling motor through the switching elements. The torque setting unit is configured to set a first torque command value of the traveling motor on the basis of an operation amount of an accelerator of the vehicle. The torque correction unit is configured to correct the first torque command value to a second torque command value to be used to suppress vibration of the drive system by performing feedback of the result of the control of the traveling motor to the first torque command value. The inverter control unit is configured to generate a drive signal for the switching elements on the basis of the second torque command value and a carrier signal. The motor lock determination unit is configured to determine whether the traveling motor is in a motor lock state in which the traveling motor operates within a lock range. In a case where the traveling motor is determined to be in the motor lock state, the torque correction unit is configured to set a feedback gain to be smaller than a threshold gain at the time of correcting the first torque command value to the second torque command value. In the case where the traveling motor is determined to be in the motor lock state, the inverter control unit is configured to set a frequency of the carrier signal to be lower than a threshold frequency at the time of generating the drive signal. 
     An aspect of the technology provides a vehicle control apparatus to be applied to a vehicle. The vehicle control apparatus is configured to perform control of a traveling motor of the vehicle that serves as a power source of a drive system of the vehicle. The vehicle control apparatus includes an inverter and circuitry. The inverter includes a plurality of switching elements. The inverter is configured to supply electric power to the traveling motor through the switching elements. The circuitry is configured to set a first torque command value of the traveling motor on the basis of an operation amount of an accelerator of the vehicle. The circuitry is configured to correct the first torque command value to a second torque command value to be used to suppress vibration of the drive system by performing feedback of the result of the control of the traveling motor to the first torque command value. The circuitry is generate a drive signal for the switching elements on the basis of the second torque command value and a carrier signal. The circuitry is determine whether the traveling motor is in a motor lock state in which the traveling motor operates within a lock range. Upon determining that the traveling motor is in the motor lock state, the circuitry is configured to set a feedback gain to be smaller than a threshold gain at the time of correcting the first torque command value to the second torque command value. Upon determining that the traveling motor is in the motor lock state, the circuitry is configured to set a frequency of the carrier signal to be lower than a threshold frequency at the time of generating the drive signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the technology and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments and, together with the specification, serve to explain the principles of the technology. 
         FIG. 1  is a block diagram illustrating an exemplary configuration of a vehicle including a vehicle control apparatus according to one example embodiment of the technology. 
         FIG. 2  is a diagram illustrating an exemplary torque map used to set a target torque command value. 
         FIG. 3  is a block diagram illustrating an exemplary inverter and an exemplary motor controller according to one example embodiment of the technology. 
         FIG. 4  is a diagram illustrating an exemplary configuration of a command value correction unit that executes damping control. 
         FIG. 5  is a diagram illustrating an exemplary lock range. 
         FIG. 6  is a flowchart of an exemplary procedure of lock cancelling control. 
         FIG. 7  is a flowchart of the exemplary procedure of the lock cancelling control. 
         FIG. 8  is a diagram illustrating an exemplary transition of the operational state of a traveling motor. 
         FIG. 9A  is a diagram illustrating a change in electric current flow at a carrier frequency. 
         FIG. 9B  is a diagram illustrating a change in electric current flow at a carrier frequency. 
         FIG. 10  is a diagram illustrating another example of the lock range. 
     
    
    
     DETAILED DESCRIPTION 
     While the traveling motor is in a motor lock state in which the traveling motor operates at a high-torque and in a low-rotation lock range, electric current flowing to a stator coil can be locally increased. As the temperature of the traveling motor locally increases in the motor lock state, it is necessary to urge the driver to perform an accelerator operation or another driving operation for cancelling the motor lock state. 
     It is desirable to provide a vehicle control apparatus that urges the driver to perform an accelerator operation or another driving operation while the traveling motor is in the motor lock sate. 
     In the following, some example embodiments of the technology are described in detail with reference to the accompanying drawings. Note that the following description is directed to illustrative examples of the disclosure and not to be construed as limiting to the technology. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the technology. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. Throughout the present specification and the drawings, elements having substantially the same function and configuration are denoted with the same reference numerals to avoid any redundant description. In addition, elements that are not directly related to any embodiment of the technology are unillustrated in the drawings. 
     Exemplary Vehicle Configuration 
       FIG. 1  illustrates an exemplary configuration of a vehicle  11  including a vehicle control apparatus  10  according to an example embodiment of the technology. As illustrated in  FIG. 1 , the vehicle  11  may include a drive system  12 . The drive system  12  may include a traveling motor  13  serving as a power source. The traveling motor  13  illustrated in  FIG. 1  may be a three-phase AC motor, such as a synchronous motor or an induction motor. The traveling motor  13  may include a rotor  14  to which wheels  18  are coupled via a motor output shaft  15 , a differential mechanism  16 , and a wheel drive shaft  17 . The traveling motor  13  may further include a stator  19  to which an inverter  20  is coupled. To the inverter  20 , a battery  21  may be coupled. The battery  21  may be a lithium-ion battery, for example. The traveling motor  13  may further include a rotation sensor  22 . For example, the rotation sensor  22  may be a resolver that detects the rotation speed on the basis of the rotation angle of the rotor  14 . 
     The vehicle  11  may further include an electric hydraulic brake system  30  that applies the brakes to the wheels  18 . To the electric hydraulic brake system  30 , a brake pedal  31  may be coupled. The electric hydraulic brake system  30  may further include a master cylinder  33  provided with an electric motor  32 . The electric hydraulic brake system  30  may further include a caliper  35  and a hydraulic pressure circuit  36 . The caliper  35  may apply the brake to a disc rotor  34  of each wheel  18 . The hydraulic pressure circuit  36  may control the hydraulic brake pressure to be supplied to each caliper  35 . When the master cylinder  33  is thrusted by depressing on the brake pedal  31  or by the electric motor  32 , the hydraulic brake pressure may be transferred from the master cylinder  33  to the caliper  35  via the hydraulic pressure circuit  36 , so that the caliper  35  may apply the brake to the disc rotor  34  of each wheel  18 . 
     Main Controller 
     As illustrated in  FIG. 1 , the vehicle control apparatus  10  may include a main controller  40 . The main controller  40  may be a microcomputer, for example. The main controller  40  may include a target torque setting unit  41 , a brake control unit  42 , and a meter control unit  43 . The target torque setting unit  41  sets a target torque command value Tm 1  of the traveling motor  13  on the basis of an accelerator operation performed by the driver, for example. In one embodiment, the target torque setting unit  41  may serve as a “torque setting unit”. In one embodiment, the target torque command value Tm 1  may serve as a “first torque command value”. The brake control unit  42  may control the electric hydraulic brake system  30  depending on traveling conditions, for example. The meter control unit  43  may control information displayed on a meter panel  44  depending on traveling conditions, for example. To the main controller  40 , an accelerator sensor  46 , a brake sensor  47 , a vehicle speed sensor  48 , and a gradient sensor  49  may be coupled. The accelerator sensor  46  may detect the amount of depression of the accelerator pedal  45  (hereinafter referred to as an accelerator operation amount Acc). The brake sensor  47  may detect the amount of depression of the brake pedal  31 . The vehicle speed sensor  48  may detect the traveling speed of the vehicle  11 . The gradient sensor  49  may detect the gradient of a road surface. The gradient sensor  49  may be an acceleration sensor, for example. 
       FIG. 2  illustrates an exemplary torque map used to set the target torque command value Tm 1 . As illustrated in  FIG. 2 , the torque map may have characteristic lines L 1  to L 4  indicating the target torque command values Tm 1  for the respective accelerator operation amounts Acc. That is, in a case where the accelerator operation amount Acc is 0%, the target torque command value Tm 1  may be set on the basis of the characteristic line L 1 , and in a case where the accelerator operation amount Acc is 25%, the target torque command value Tm 1  may be set on the basis of the characteristic line L 2 . Likewise, in a case where the accelerator operation amount Acc is 50%, the target torque command value Tm 1  may be set on the basis of the characteristic line L 3 , and in a case where the accelerator operation amount Acc is 100%, the target torque command value Tm 1  may be set on the basis of the characteristic line L 4 . For example, in a case where the accelerator operation amount Acc is 50% and where the rotation speed ωm of the rotor  14  is “Sa”, the target torque command value Tm 1  may be set to “Ta”. Although the torque map illustrated in  FIG. 2  has the four characteristic lines L 1  to L 4 , this is a non-limiting example. Alternatively, the torque map may have five or more characteristic lines. 
     Motor Controller 
     As illustrated in  FIG. 1 , the vehicle control apparatus  10  may include a motor controller  50 . The motor controller  50  may be a microcomputer, for example. The motor controller  50  may control the traveling motor  13  via the inverter  20 . The motor controller  50  and the main controller  40  may be connected to each other via an in-vehicle network  51 , such as a controller area network (CAN), in a mutually communicable manner.  FIG. 3  illustrates examples of the inverter  20  and the motor controller  50 . 
     As illustrated in  FIG. 3 , the inverter  20  may be a three-phase bridge circuit that includes a plurality of switching elements SW 1  to SW 6 . The inverter  20  supplies electric power to the traveling motor  13 . These switching elements SW 1  to SW 6  may be driven by pulse width modulation (PWM) control. DC power outputted from the battery  21  may be converted into AC power while passing through the switching elements SW 1  to SW 6 . The AC power outputted from the inverter  20  may be supplied to a U-phase field coil Cu, a V-phase field coil Cv, and a W-phase field coil Cw of a stator coil  52  of the traveling motor  13 . Supplying the AC power to the U-phase field coil Cu, the V-phase field coil Cv, and the W-phase field coil Cw causes the stator  19  to generate a rotating magnetic field, which rotates the rotor  14 . 
     As illustrated in  FIG. 3 , the motor controller  50  may include a command value correction unit  53  and an inverter control unit  54 . The command value correction unit  53  may correct the target torque command value Tm 1  to a damping torque command value Tm 2  to perform damping control (to be described later) for suppressing vibration of the drive system  12 . In one embodiment, the command value correction unit  53  may serve as a “torque correction unit”. In one embodiment, the damping torque command value Tm 2  may serve as a “second torque command value”. The inverter control unit  54  may include a carrier output unit  55 , a duty ratio output unit  56 , a pulse setting unit  57 , and a gate drive unit  58 . The carrier output unit  55  may output a carrier signal Sc having a predetermined frequency. The duty ratio output unit  56  may output command duty ratios Du, Dv, and Dw that are set on the basis of the damping torque command value Tm 2 . The pulse setting unit  57  may compare the carrier signal Sc outputted from the carrier output unit  55  with the command duty ratios Du, Dv, and Dw outputted from the duty ratio output unit  56  to set pulse signals Pu, Pv, and Pw that are driving signals of the switching elements SW 1  to SW 6 . On the basis of the pulse signals Pu, Pv, and Pw, the gate drive unit  58  may drive the switching elements SW 1  to SW 6 . To set the command duty ratios Du, Dv, and Dw, the duty ratio output unit  56  may receive electric currents from the U-phase field coil Cu, the V-phase field coil Cv, and the W-phase field coil Cw in the current sensor  59 , and the rotation angle of the rotor  14  from the rotation sensor  22 . 
     Damping Control 
     Damping control of the traveling motor  13  performed by the motor controller  50  will now be described. The damping control of the traveling motor  13  may be torque control of the traveling motor  13  for suppressing torsional vibration of the wheel drive shaft  17  of the drive system  12 .  FIG. 4  illustrates an exemplary configuration of the command value correction unit  53  that executes the damping control. A motor system  60  illustrated in  FIG. 4  may be a control system including the inverter control unit  54 , the inverter  20 , and the traveling motor  13 . 
     As illustrated in  FIG. 4 , the command value correction unit  53  of the motor controller  50  may include a feedforward processing unit  61  and a feedback processing unit  62  in order to execute the damping control of the traveling motor  13 . The feedforward processing unit  61  may execute a feedforward process on the target torque command value Tm 1  received from the main controller  40  to attenuate a resonant component of the wheel drive shaft  17 . The feedback processing unit  62  may perform feedback of the rotation speed ωm of the rotor  14  acquired as the result of the control of the traveling motor  13  to the target torque command value Tm 1  having been subjected to the feedforward process. The feedback processing unit  62  may thereby correct the target torque command value Tm 1  to the damping torque command value Tm 2  to cancel the vibration of the wheel drive shaft  17 . 
     The feedback processing unit  62  may include a filter processing section  63 , a torque converting section  64 , and a gain processing section  65 . The filter processing section  63  may perform a predetermined filtering process on the rotation speed ωm to extract a resonant component ω 1  of the wheel drive shaft  17  from the rotation speed ωm. Thereafter, the torque converting section  64  may perform a predetermined conversion process on the resonant component ω 1  of the rotation speed ωm to convert the resonant component ω 1  of the rotation speed ωm into a resonant component t 1  of a motor torque. Thereafter, the gain processing section  65  may multiply the resonant component t 1  of the motor torque by a predetermined feedback gain K to calculate a torque correction value t 2 . The torque correction value t 2  may be used to cancel vibration of the wheel drive shaft  17 . Thereafter, a correction processing unit  66  in the command value correction unit  53  may correct the target torque command value Tm 1  having been subjected to the feedforward process to the damping torque command value Tm 2  using the torque correction value t 2 . 
     As described above, the traveling motor  13  may be controlled on the basis of the pulse signals Pu, Pv, and Pw generated on the basis of the damping torque command value Tm 2  to cancel the vibration of the wheel drive shaft  17 . This suppresses vibration of the drive system  12  generated while the vehicle  11  is traveling. As the feedback gain K takes a larger value, the function of the feedback process of attenuating the resonant component of the wheel drive shaft  17  of the drive system  12  to cancel the vibration of the wheel drive shaft  17  is more strengthened. This effectively suppresses the vibration of the drive system  12  but lowers the responsivity of the motor torque. As the feedback gain K takes a smaller value, the function of the feedback process is more weakened. This makes the drive system  12  more prone to vibration but enhances the responsivity of the motor torque while. 
     Motor Lock State 
     Described next is a motor lock state of the traveling motor  13 .  FIG. 5  illustrates an exemplary lock range. As illustrated in  FIG. 5 , the lock range may be a high-torque and low-rotation range in which the rotation speed ωm of the traveling motor  13  is less than a threshold rotation speed Sb and the motor torque of the traveling motor  13  is greater than a threshold torque Tb. While the traveling motor  13  is in the motor lock state and operating in the lock range, the electric current flowing into the stator coil  52  can be locally increased. For example, the electric current flowing into the U-phase field coil Cu, the V-phase field coil Cv, and the W-phase field coil Cw can be locally increased while the traveling motor  13  is in the motor lock range. This can locally increase the temperature of the stator coil  52 . 
     To address the concern, the motor controller  50  includes a motor lock determination unit  67 , as illustrated in  FIG. 3 . The motor lock determination unit  67  determines whether the traveling motor  13  is operating within the lock range or not, that is, whether the traveling motor  13  is in the motor lock state or not. The motor lock determination unit  67  may determine whether the traveling motor  13  is operating within the lock range on the basis of the motor torque and the rotation speed of the traveling motor  13 . If it is determined that the traveling motor  13  has been in the motor lock state for a predetermined period of time, the command value correction unit  53  in the motor controller  50  actively reduces the damping torque command value Tm 2  to protect the traveling motor  13  from being heated by an excessive electric current. This reduces the motor torque of the traveling motor  13 . As a result, the traveling motor  13  is brought out of the lock range, as indicated by an arrow α 1 , to protect the traveling motor  13 . 
     Meanwhile, the traveling motor  13  may be supposed to operate within the lock range when the driver adjusts the accelerator operation amount to stop the vehicle  11  on a climbing road surface. That is, in the lock range, a motor torque force Fm that moves the vehicle  11  forward and a gravitational force Fg that moves the vehicle  11  backward are balanced to stop the vehicle  11 . In such a situation where the traveling motor  13  is determined to be in the motor lock state and where the motor torque is lowered while the vehicle  11  is stopped by the accelerator operation, the vehicle  11  can be moved backward against the intention of the driver. To address such a concern, if the traveling motor  13  is determined to be in the motor lock state, the vehicle control apparatus  10  according to the example embodiment actively vibrates the vehicle  11  by executing lock cancelling control to be described later, rather than immediately lowering the motor torque. That is, the vehicle control apparatus  10  generates vibration of the vehicle  11  to notify the driver of the vehicle  11  that the vehicle  11  is in the motor lock state, and urges the driver to perform an accelerator operation or a brake operation. The vehicle control apparatus  10  thereby cancels the motor lock state of the traveling motor  13 . 
     Lock Cancelling Control 
     The lock cancelling control performed by the vehicle control apparatus  10  will now be described.  FIGS. 6 and 7  are flowcharts illustrating an exemplary procedure of the lock cancelling control. The flowcharts illustrated in  FIGS. 6 and 7  are connected with each other at portions A and B.  FIG. 8  illustrates an exemplary transition of the operational state of the traveling motor  13 . In the flowing description, the frequency of the carrier signal Sc may be referred to as a carrier frequency. 
     As illustrated in  FIG. 6 , the feedback gain may be set to a predetermined feedback gain K 1 , and the carrier frequency may be set to a predetermined carrier frequency FH in Step S 10 . Thereafter, in Step S 11 , it may be determined whether the traveling motor  13  is in the motor lock state. If it is determined in Step S 11  that the traveling motor  13  is in the motor lock state (Step S 11 : YES), the procedure may proceed to Step S 12 . In Step S 12 , it may be determined whether the road gradient is an upward gradient greater than a predetermined gradient SL 1 . If it is determined in Step S 12  that the gradient of the road surface is the upward gradient greater than the predetermined gradient SL 1  (Step S 12 : YES), the procedure may proceed to Step S 13 . In Step S 13 , it may be determined whether the traveling motor  13  has been in the motor lock state for longer than a predetermined period of time Ti 1 . If it is determined in Step S 13  that the traveling motor  13  has been in the motor lock state for longer than the predetermined period of time Ti 1  (Step S 13 : YES), the procedure may proceed to Step S 14 . In Step S 14 , it may be determined whether the amount of accelerator operation performed by the driver is less than a predetermined amount Ac 1 . If it is determined in Step S 14  that the amount of accelerator operation is less than the predetermined amount Ac 1  (Step S 14 : YES), that is, if the accelerator pedal  45  has a stamping margin, the procedure may proceed to Step S 15 . In Step S 15 , an instruction may be displayed on the meter panel  44  to urge the driver to perform an accelerator operation or a brake operation for cancelling the motor lock state. 
     That is, in a case where the traveling motor  13  has been in the motor lock state for longer than the predetermined period of time, where the road surface on which the vehicle  11  is traveling has the upward gradient, and where the accelerator pedal  45  has the stamping margin, it may be supposed that the vehicle  11  is stopped on a climbing road surface by the accelerator operation. In this case, the temperature of the stator coil  52  can excessively increase. To address such a concern, in Step S 15 , an instruction to further depress the accelerator pedal  45  or depress the brake pedal  31  may be displayed on the meter panel  44  to urge the driver to perform the accelerator operation or the brake operation for cancelling the motor lock state. When the driver further depresses the accelerator pedal  45  after recognizing the instruction displayed on the meter panel  44 , the motor torque of the traveling motor  13  may be increased, as indicated by an arrow β 1  in  FIG. 8 , to cancel the motor lock state of the traveling motor  13 . When the driver depresses the brake pedal  31  after recognizing the instruction displayed on the meter panel  44 , the motor torque of the traveling motor  13  may be lowered, as indicated by an arrow β 2  in  FIG. 8 , to cancel the motor lock state of the traveling motor  13 . 
     After the instruction to further depress the accelerator pedal  45  or to perform another driving operation is displayed in order to urge the driver in Step S 15 , the procedure may proceed to Step S 16  as illustrated in  FIG. 6 . In Step S 16 , it may be determined whether the traveling motor  13  is in the motor lock state. If it is determined in Step S 16  that the traveling motor  13  is in the motor lock state (Step S 16 : YES), the procedure may proceed to Step S 17 . In Step S 17 , it may be determined whether the motor lock state has been maintained for longer than a predetermined period of time Ti 2 . If it is determined in Step S 17  that the motor lock state has been maintained for longer than the predetermined period of time Ti 2  (Step S 17 : YES), the procedure may proceed to Step S 18 , as illustrated in  FIG. 7 . In Step S 18 , the feedback gain may be decreased to a predetermined feedback gain K 2 , and the carrier frequency may be lowered to a predetermined carrier frequency FL. In contrast, if it is not determined that the traveling motor  13  is in the motor lock state in Step S 11  (Step S 11 : NO) or Step S 16  (Step S 16 : NO), the procedure may exit from the routine without changing the feedback gain and the carrier frequency. 
     In Step S 18 , the feedback gain may be decreased to the predetermined feedback gain K 2  so that the drive system  12  is actively vibrated by the traveling motor  13  and the torque responsivity of the traveling motor  13  is enhanced. Note that the predetermined feedback gain K 2  may be smaller than the default feedback gain K 1  and smaller than a predetermined threshold gain to vibrate the drive system  12 . As described above, as the feedback gain takes a smaller value, the function of the feedback process for suppressing vibration of the drive system  12  is more weakened. Thus, the damping torque command value Tm 2  of the traveling motor  13  may be set to such a value that vibrates the drive system  12 . In other words, the drive system  12  may be actively vibrated by setting a small feedback gain. Setting a small feedback gain also enhances the responsivity of the motor torque in preparation for the next accelerator operation. 
     Additionally, the carrier frequency may be lowered to the predetermined carrier frequency FL in Step S 18  so that the drive system  12  is actively vibrated by the traveling motor  13 . Note that the predetermined carrier frequency FL used in Step S 18  may be lower than the default carrier frequency FH and lower than a predetermined threshold frequency to vibrate the drive system  12 .  FIGS. 9A and 9B  illustrate changes in electric current flows at respective carrier frequencies.  FIG. 9A  illustrates the transition of an electric current flow when a carrier signal Sc 1  having the carrier frequency FH is used.  FIG. 9B  illustrates the transition of an electric current flow when a carrier signal Sc 2  having the carrier frequency FL is used. In  FIGS. 9A and 9B , the electric currents flowing in the U-phase field coil Cu of the traveling motor  13  are exemplified. However, the same electric current may flow in the other field coils such as the V-phase field coil Cv and the W-phase field coil Cw. 
     As illustrated in  FIGS. 9A and 9B , a pulse signal Pu may be set to a high level when the carrier signal Sc 1  or Sc 2  is lower than a command duty ratio Du, while the pulse signal Pu may be set to a low level when the carrier signal Sc 1  or Sc 2  is higher than the command duty ratio Du. Accordingly, the number of switching of the inverter  20  becomes smaller when the carrier signal Sc 2  having a low frequency is used than when the carrier signal Sc 1  having a high frequency is used. This causes the electric current flowing in the U-phase field coil Cu to change coarsely. In other words, by lowering the carrier frequency to the predetermined carrier frequency FL, it is possible to set such a pulse signal Pu that increases the torque pulsation or the torque ripple of the traveling motor  13 , and to actively vibrate the drive system  12  using the increased torque pulsation. Further, by lowering the carrier frequency to the predetermined carrier frequency FL, it is possible to approximate a switching control sound of the inverter  20  closely to the human audible range. 
     As described above, the feedback gain may be decreased to the predetermined feedback gain K 2 , and the carrier frequency may be lowered to the predetermined carrier frequency FL in Step S 18  of  FIG. 7 . This allows the traveling motor  13  to actively vibrate the drive system  12 . For example, in a case where the driver performs no accelerator operation for cancelling the motor lock state despite of the fact that the instruction to urge the driver to perform the accelerator operation or another driving operation is displayed on the meter panel  44 , the vehicle  11  is actively vibrated by the traveling motor  13 . This makes the driver recognize the instruction displayed on the meter panel  44 . Additionally, the switching control sound of the inverter  20  is approximated closely to the human audible range by lowering the carrier frequency to the predetermined carrier frequency FL. This makes the driver recognize the instruction displayed on the meter panel  44  with the sound as well as the vibration. The driver is thereby urged to perform the accelerator operation or the brake operation for cancelling the motor lock state, as illustrated by the arrows β 1  and β 2  in  FIG. 8 . 
     When the driver further depresses the accelerator pedal  45  after recognizing the instruction displayed on the meter panel  44 , the feedback gain may be increased to the predetermined feedback gain K 1 , and the carrier frequency may be maintained at the predetermined carrier frequency FL until the motor lock state is cancelled. For example, as described above with reference to  FIG. 7 , in a case where the feedback gain is decreased to the predetermined feedback gain K 2 , and where the carrier frequency is lowered to the predetermined carrier frequency FL in Step S 18 , the procedure may then proceed to Step S 19 . In Step S 19 , it may be determined whether the accelerator pedal  45  is further depressed or not, i.e., whether the accelerator operation amount is greater than a predetermined threshold operation amount. In a case where it is determined in Step S 19  that the accelerator pedal  45  is further depressed (Step S 19 : YES) and where it is determined in Step S 20  that the accelerator pedal  45  has been depressed for longer than the predetermined period of time Ti 2  (Step S 20 : YES), the procedure may proceed to Step S 21 . In Step S 21 , the feedback gain may be increased to the predetermined feedback gain K 1 , and the carrier frequency may be maintained at the predetermined carrier frequency FL. 
     In other words, in Step S 21 , it may be supposed that the driver is depressing the accelerator pedal  45  after recognizing the instruction displayed on the meter panel  44  due to the vibration of the vehicle  11 . That is, the purpose of the vibration of the vehicle  11  may be accomplished in Step S 21 . Thus, the feedback gain may be increased to the predetermined feedback gain K 1  to suppress the vibration of the vehicle  11 . Additionally, in Step S 21 , the traveling motor  13  may be maintained in the motor lock state. Thus, the carrier frequency may be maintained at the predetermined low carrier frequency FL to reduce the number of switching and suppress heating of the inverter  20 . Note that the predetermined threshold operation amount, which is compared with the accelerator operation amount in Step S 19  in order to determine whether the accelerator pedal  45  is further depressed, may be set larger than the accelerator operation amount required to cancel the motor lock state of the traveling motor  13 . 
     Thereafter, in Step S 22 , it may be determined whether the motor lock state has been cancelled. If it is determined in Step S 22  that the motor lock state has been cancelled (Step S 22 : YES), the procedure may proceed to Step S 23 . In Step S 23 , the carrier frequency may be returned to the predetermined carrier frequency FH, and the procedure may exit from the routine. In contrast, if it is not determined in Step S 19  that the accelerator pedal  45  is further depressed (Step S 19 : NO), the procedure may proceed to Step S 24 . In Step S 24 , it may be determined whether the brake pedal  31  is depressed or not. In a case where it is determined in Step S 24  that the brake pedal  31  is depressed (Step S 24 : YES) and where it is determined in Step S 25  that the motor lock state has been cancelled (Step S 25 : YES), the procedure may proceed to Step S 23 . In Step S 23 , the feedback gain may be returned to the predetermined feedback gain K 1 , and the carrier frequency may be returned to the predetermined carrier frequency FH. The procedure may then exit from the routine. Note that the predetermined feedback gain K 1  set in Step S 23  may be larger than a predetermined threshold gain to suppress the vibration of the drive system  12 . Additionally, the carrier frequency FH set in Step S 23  may be higher than a predetermined threshold frequency to suppress the vibration of the drive system  12  and heating of the inverter  20 . 
     Conclusion 
     As described above, when it is determined that the traveling motor  13  is in the motor lock state, the feedback gain is set smaller than the predetermined threshold gain, and the carrier frequency is set lower than the predetermined threshold frequency. This causes the traveling motor  13  to actively vibrate the drive system  12 , making the driver feel strange and urging the driver to perform the driving operation for cancelling the motor lock state. Further, by setting the carrier frequency lower than the predetermined threshold frequency, it is possible to approximate the switching control sound of the inverter  20  closely to the human audible range. This makes the driver feel strange and urges the driver to perform the driving operation for cancelling the motor lock state. Accordingly, it is possible to make the driver perform the accelerator operation or the brake operation for cancelling the motor lock state of the traveling motor  13 . 
     In the above description, the driver may perform the accelerator operation or the brake operation after recognizing the generation of the motor lock state due to the vibration of the drive system  12 ; however, there may be some situations where the driver does not perform the accelerator operation nor another driving operation. In such situations, the damping torque command value Tm 2  may be actively decreased by the command value correction unit  53  of the motor controller  50  to protect the traveling motor  13  from being excessively heated. Such a decrease in the damping torque command value Tm 2  can cause the vehicle  11  to move backward against the intention of the driver. To address such a concern, the electric hydraulic brake system  30  may be controlled by the brake control unit  42  of the main controller  40 , and the electric hydraulic brake system  30  may apply the brakes to the wheels  18 . As described above, the motor controller  50  has a protection function that protects the traveling motor  13  in the motor lock state from being heated. 
     Further, as described above, the motor controller  50  includes the command value correction unit  53  and the inverter control unit  54 . The command value correction unit  53  and the inverter control unit  54  set the feedback gain to be smaller than the predetermined threshold gain and the carrier frequency to be lower than the predetermined threshold frequency when the traveling motor  13  is in the motor lock state. In other words, the motor controller  50  has a notification function to notify the driver of the motor lock state by vibrating the drive system  12 . By providing the protection function and the notification function to the single motor controller  50  as described above, the execution timing of the protection function is delayed until just before the traveling motor  13  is protected from being excessively heated. This reduces the time in which the motor torque is limited and thus secures driving performance. In contrast, if the protection function and the notification function to be executed prior to the protection function are installed in separate controllers, it is necessary to set the execution timing of the protection function with a time margin because it is necessary to consider the communication delay between controllers. On the other hand, according to the present embodiment in which the protection function and the notification function are installed in a single controller, the execution timing of the protection function is delayed until just before the traveling motor  13  is protected from being excessively heated. This reduces the time in which the motor torque is limited and thus secures driving performance. 
     The example embodiments described above may be non-limiting examples and may be modified in various ways without departing from the gist of the technology. In the example embodiments described above, the vehicle  11  to which the vehicle control apparatus  10  is applied may be an electric vehicle including the traveling motor  13  as a sole power source. However, this is a non-limiting example. Alternatively, the vehicle  11  may be a hybrid vehicle including both a traveling motor and an engine as power sources. Further, in the example embodiments described above, the main controller  40  includes the target torque setting unit  41 , and the motor controller  50  includes the command value correction unit  53 , the inverter control unit  54 , and the motor lock determination unit  67 . However, this is a non-limiting example. For example, the target torque setting unit  41 , the command value correction unit  53 , the inverter control unit  54 , and the motor lock determination unit  67  may be included in a single controller or a plurality of controllers. 
     In the example illustrated in  FIG. 5 , the lock range may be a range in which the traveling motor  13  has a rotation speed lower than the predetermined threshold rotation speed Sb and a motor torque greater than the predetermined threshold torque Tb. However, this is a non-limiting example. Alternatively, the threshold rotation speed Sb compared with the rotation speed of the traveling motor  13  may be changed depending on the motor torque. Still alternatively, the threshold torque Tb compared with the motor torque of the traveling motor  13  may be changed depending on the rotation speed.  FIG. 10  illustrates another example of the lock range. As illustrated in  FIG. 10 , a characteristic line (threshold) Lx may be set on the basis of the motor torque and the rotation speed, and the range in which the motor torque is higher than the characteristic line Lx and the rotation speed is lower than the characteristic line Lx may be set as the lock range. Note that the characteristic line Lx is not necessarily a straight line, and may be alternatively a curved line. 
     In the above description, the feedback gain K 1  or K 2  and the carrier frequency FH or FL may be used. However, this is a non-limiting example. Alternatively, the feedback gain or the carrier frequency may be changed depending on the rotation speed or the motor torque of the traveling motor  13 . Further, in the flowchart described above, the feedback gain is set to be smaller and the carrier frequency is set to be lower in a case where the traveling motor  13  is in the motor lock state and where the road surface has an upward gradient. However, this is a non-limiting example. Alternatively, the feedback gain may be set to be smaller and the carrier frequency may be set to be lower in a case where the vehicle  11  is determined to be traveling on a flat road and where the traveling motor  13  is determined to be in the motor lock state. 
     According to the example embodiment of the technology, when the traveling motor is determined to be in the motor lock state, the torque correction unit sets the feedback gain, which is used to correct the first torque command value to the second torque command value, to be smaller than the threshold gain, and the inverter control unit sets the frequency of the carrier signal to be lower than the threshold frequency. Accordingly, it is possible to urge the driver to perform the accelerator operation or another driving operation by vibrating the drive system when the traveling motor is in the motor lock state. 
     At least one of the target torque setting unit  41 , the command value correction unit  53 , the inverter control unit  54 , or the motor lock determination unit  67  illustrated in  FIGS. 1 and 3  is implementable by circuitry including at least one semiconductor integrated circuit such as at least one processor (e.g., a central processing unit (CPU)), at least one application specific integrated circuit (ASIC), and/or at least one field programmable gate array (FPGA). At least one processor is configurable, by reading instructions from at least one machine readable non-transitory tangible medium, to perform all or a part of functions of the target torque setting unit  41 , the command value correction unit  53 , the inverter control unit  54 , and the motor lock determination unit  67 . Such a medium may take many forms, including, but not limited to, any type of magnetic medium such as a hard disk, any type of optical medium such as a CD and a DVD, any type of semiconductor memory (i.e., semiconductor circuit) such as a volatile memory and a non-volatile memory. The volatile memory may include a DRAM and a SRAM, and the nonvolatile memory may include a ROM and a NVRAM. The ASIC is an integrated circuit (IC) customized to perform, and the FPGA is an integrated circuit designed to be configured after manufacturing in order to perform, all or a part of the functions of the target torque setting unit  41 , the command value correction unit  53 , the inverter control unit  54 , and the motor lock determination unit  67  illustrated in  FIGS. 1 and 3 .