JP6684438B2 - Electric power steering device and power supply state switching method - Google Patents

Description

  The present invention relates to an electric power steering device and a power supply state switching method for generating a steering assist force (assist force) by an electric motor.

  In Patent Document 1 below, a main power supply and an auxiliary power supply are provided, and when the absolute value of the steering speed is less than or equal to a predetermined value, electric power is supplied to the motor drive circuit from only the main power supply, and the absolute value of the steering speed is at a predetermined value. There is disclosed an electric power steering device (EPS) that supplies electric power from a main power supply and an auxiliary power supply to a motor drive circuit when the electric power exceeds.

JP, 2009-78737, A

  In the invention described in Patent Document 1, the main power supply and the power supply are not only used when entering a garage or when parking, but also when a course is suddenly changed to avoid an obstacle or when turning a relatively steep curve in a mountain road. It becomes possible to supply power to the motor drive circuit from the auxiliary power supply. However, when traveling on a road surface having a small coefficient of friction such as a snow road, the steering speed is likely to be high, so even when a large steering assist force is not required, electric power is supplied from the main power supply and the auxiliary power supply to the motor drive circuit. It becomes easy to be supplied. Therefore, the invention described in Patent Document 1 has a problem that wasteful power is easily consumed.

  An object of the present invention is to provide an electric power steering device and a power supply state switching method capable of suppressing wasteful power consumption.

The invention according to claim 1 is a drive circuit (52) for supplying electric power to an electric motor (18), and a main power supply (31) and an auxiliary power supply (44) capable of supplying electric power to the drive circuit. And a first power supply state in which power is supplied to the drive circuit only by the main power supply, and a second power supply state in which power is supplied to the drive circuit using both the main power supply and the auxiliary power supply. A switching circuit (43) for switching and a control means (33) for controlling the switching circuit, wherein the control means includes an input voltage of the drive circuit, a rotation speed of the electric motor, and a motor flowing in the electric motor. And a motor torque generated by the electric motor is calculated as an actual motor torque (Tm) on the basis of an acquisition unit that acquires a current and a motor current acquired by the acquisition unit. And Tatoruku calculating means (33), based on the input voltage and motor rotational speed obtained by the obtaining unit, the maximum motor torque calculation means for calculating a maximum motor torque _(Tm-Lim) which can be generated by the electric motor ( 33) and when the power supply state to the drive circuit is the first power supply state, a difference (ΔTm) between the two obtained by subtracting the actual motor torque from the maximum motor torque is a first predetermined value. When the value is less than α (α> 0), the switching circuit is controlled so that the power supply state to the drive circuit is switched to the second power supply state, and the power supply state to the drive circuit is changed to the second power supply state . When the difference (ΔTm) is the second predetermined value β (β> 0) or more in the power supply state, the power supply state to the drive circuit is the first power supply state. An electric power steering system including means (33) for controlling the switching circuit so as to switch to a power supply state . In addition, although the alphanumeric characters in the parentheses represent corresponding constituent elements and the like in the embodiments described later, the scope of the present invention is not limited to the embodiments. The same applies in this section below.

A state in which the difference ( _ΔTm = Tm− _lim −Tm) between the maximum motor torque (Tm− _lim ) and the actual motor torque (Tm) is equal to or larger than the first predetermined value α when the switching circuit is in the first power supply state. Is a state in which the actual motor torque (Tm) is smaller than the maximum motor torque (Tm- _lim ) that can be generated from the electric motor, so it is considered that there is a margin in the steering assist force that can be generated. To be Therefore, in such a case, the switching circuit cannot switch to the second power supply state.

  On the other hand, when the switching circuit is in the first power supply state, the state in which the difference (ΔTm) is less than the first predetermined value α means that the actual motor torque is different from the maximum motor torque that can be generated from the electric motor. Are equal to or close to each other, it is considered that there is no margin in the steering assist force that can be generated. Therefore, in such a case, the switching circuit is switched to the second power supply state.

With this configuration, when the switching circuit is in the first power supply state, the switching circuit cannot switch to the second power supply state when the steering assist force that can be generated has a margin. Can be suppressed.
The invention according to claim 2 is the electric power steering apparatus according to claim 1, wherein the second predetermined value β is larger than the first predetermined value α.

  The invention according to claim 3 further includes storage means for storing, for each input voltage of the drive circuit, a rotation speed / torque characteristic of the electric motor according to the input voltage, and the maximum motor torque calculation means. Among the rotation speed / torque characteristics for each input voltage stored in the storage means, the motor rotation speed / torque characteristic corresponding to the input voltage acquired by the acquisition means is the motor rotation speed acquired by the acquisition means. The electric power steering system according to claim 1, wherein a torque corresponding to a number is calculated as the maximum motor torque.

The invention according to claim 4 is the electric power steering apparatus according to any one of claims 1, 2 and 3, wherein the auxiliary power source is a capacitor.
According to a fifth aspect of the present invention, a drive circuit for supplying electric power to the electric motor, a main power supply and an auxiliary power supply capable of supplying electric power to the drive circuit, and the drive circuit only by the main power supply Electric power steering including a first power supply state for supplying power and a switching circuit for switching between a second power supply state for supplying power to the drive circuit using both the main power supply and the auxiliary power supply A power supply state switching method in an apparatus, comprising: a first step of acquiring an input voltage of the drive circuit, a rotation speed of the electric motor, and a motor current flowing in the electric motor; and a motor acquired by the first step. based on the current, a second step of calculating a motor torque generated by the electric motor as an actual motor torque, the first step Based on the input voltage and motor rotational speed obtained Te, a third step of calculating the maximum motor torque can be generated by the electric motor, the power supply state to the drive circuit is in the first power supply state In this case, if the difference between the two obtained by subtracting the actual motor torque from the maximum motor torque is less than a first predetermined value α (α> 0) , the power supply state to the drive circuit is the second When the switching circuit is controlled so as to switch to the power supply state, and the power supply state to the drive circuit is the second power supply state, the difference cannot exceed the second predetermined value β (β> 0). For example, a fourth step of controlling the switching circuit so that the power supply state to the drive circuit is switched to the first power supply state , the power supply state switching method.

According to this method, as in the first aspect of the invention, when the switching circuit is in the first power supply state, the switching circuit is in the second power supply state when the steering assist force that can be generated has a margin. Since switching cannot be performed, it is possible to suppress wasteful power consumption.
The invention according to claim 6 is the power supply state switching method according to claim 5, wherein the second predetermined value β is larger than the first predetermined value α.

  According to a seventh aspect of the present invention, the electric power steering apparatus further includes a storage unit that stores, for each input voltage of the drive circuit, a rotation speed / torque characteristic of the electric motor according to the input voltage. In the third step, among the rotation speed / torque characteristics for each input voltage stored in the storage means, in the rotation speed / torque characteristic according to the input voltage acquired in the first step, the first step is performed. 7. The power supply state switching method according to claim 5, wherein a torque corresponding to the motor rotation speed acquired in step is calculated as the maximum motor torque.

  The invention according to claim 8 is the power supply state switching method according to any one of claims 5, 6 and 7, wherein the auxiliary power supply is composed of a capacitor.

FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing an electrical configuration of the electric power steering device of FIG. FIG. 3 is a flowchart showing a procedure of a power supply state switching process executed by the power supply control ECU 33. FIG. 4 is a graph showing motor rotation speed / torque characteristics corresponding to the input voltage VD acquired in step S1 of FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus according to an embodiment of the present invention.
An electric power steering device (EPS) 1 includes a steering wheel 2 as a steering member for steering a vehicle, and a steering mechanism that steers the steered wheels 3 in association with the rotation of the steering wheel 2. 4 and a steering assist mechanism 5 for assisting the driver in steering. The steering wheel 2 and the steering mechanism 4 are mechanically connected via a steering shaft 6 and an intermediate shaft 7.

The steering shaft 6 includes an input shaft 8 connected to the steering wheel 2 and an output shaft 9 connected to the intermediate shaft 7. The input shaft 8 and the output shaft 9 are rotatably connected to each other via a torsion bar 10.
A torque sensor 11 is arranged near the torsion bar 10. The torque sensor 11 detects the steering torque T applied to the steering wheel 2 based on the relative rotational displacement amount of the input shaft 8 and the output shaft 9. In this embodiment, as the steering torque T detected by the torque sensor 11, for example, the torque for steering to the right is detected as a positive value and the torque for steering to the left is determined as a negative value. It is assumed that the detected steering torque becomes larger as the absolute value thereof becomes larger.

  The steered mechanism 4 is a rack and pinion mechanism including a pinion shaft 13 and a rack shaft 14 as a steered shaft. The steered wheels 3 are connected to the respective ends of the rack shaft 14 via tie rods 15 and knuckle arms (not shown). The pinion shaft 13 is connected to the intermediate shaft 7. The pinion shaft 13 is adapted to rotate in association with steering of the steering wheel 2. A pinion 16 is connected to the tip (lower end in FIG. 1) of the pinion shaft 13.

  The rack shaft 14 extends linearly in the left-right direction of the automobile. A rack 17 that meshes with the pinion 16 is formed at an intermediate portion in the axial direction of the rack shaft 14. The pinion 16 and the rack 17 convert the rotation of the pinion shaft 13 into the axial movement of the rack shaft 14. The steered wheels 3 can be steered by moving the rack shaft 14 in the axial direction.

When the steering wheel 2 is steered (rotated), this rotation is transmitted to the pinion shaft 13 via the steering shaft 6 and the intermediate shaft 7. Then, the rotation of the pinion shaft 13 is converted into an axial movement of the rack shaft 14 by the pinion 16 and the rack 17. As a result, the steered wheels 3 are steered.
The steering assist mechanism 5 includes an electric motor 18 for steering assist, and a reduction mechanism 19 for transmitting the output torque of the electric motor 18 to the steering mechanism 4. The electric motor 18 is, for example, a three-phase brushless motor. The electric motor 18 is provided with a rotation speed sensor 23 for detecting the rotation speed N of the electric motor 18.

  The reduction mechanism 19 is composed of a worm gear mechanism including a worm shaft 20 and a worm wheel 21 that meshes with the worm shaft 20. The worm shaft 20 is rotationally driven by the electric motor 18. The worm wheel 21 is rotatably connected to the steering shaft 6 integrally. The worm wheel 21 is rotationally driven by the worm shaft 20.

  When the worm shaft 20 is rotationally driven by the electric motor 18, the worm wheel 21 is rotationally driven, and the steering shaft 6 rotates. The rotation of the steering shaft 6 is transmitted to the pinion shaft 13 via the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. As a result, the steered wheels 3 are steered. That is, the steered wheels 3 are steered by rotating the worm shaft 20 by the electric motor 18.

  The vehicle is provided with a vehicle speed sensor 24 for detecting the vehicle speed V. The steering torque T detected by the torque sensor 11, the rotation speed N of the electric motor 18 detected by the rotation speed sensor 23, the vehicle speed V detected by the vehicle speed sensor 24, and the like are determined by the EPS ECU (ECU: Electronic Control Unit) 12. Entered in. The EPS ECU 12 controls the electric motor 18 based on these inputs or the like to perform so-called assist control.

A case where electric power is supplied to the motor drive circuit 52 (see FIG. 2) in the EPS ECU 12 only by the main power supply 31 and a case where electric power is supplied by using both the main power supply 31 and the auxiliary power supply device 32. There is. The auxiliary power supply device 32 is controlled by the power supply control ECU 33. The EPS ECU 12 and the power supply control ECU 33 are connected via a communication line.
FIG. 2 is a circuit diagram showing an electrical configuration of the electric power steering device 1.

  The EPS ECU 12 includes a motor control circuit 51 including a microcomputer and a motor drive circuit (inverter circuit) 52 that is controlled by the motor control circuit 51 and supplies electric power to the electric motor 18. The output signal of the current sensor 53 for detecting the motor current Im flowing through the electric motor 18 is input to the EPS ECU 12. The motor current Im detected by the current sensor 53 is given to the power supply control ECU 33 via the EPS ECU 12.

  The motor control circuit 51 includes a steering torque T detected by the torque sensor 11, a vehicle speed V detected by the vehicle speed sensor 24, a rotation speed N detected by the rotation speed sensor 23, and a motor detected by the current sensor 53. The motor drive circuit 52 is drive-controlled based on the current Im. Specifically, the motor control circuit 51 sets a target current value based on the steering torque T and the vehicle speed V, so that the motor current Im flowing through the electric motor 18 becomes equal to the target current value. To drive. The rotation speed N detected by the rotation speed sensor 23 is given to the power supply control ECU 33 via the EPS ECU 12.

The auxiliary power supply device 32 is connected to the main power supply 31 in series. The auxiliary power supply device 32 includes a relay 41, a charging circuit 42, a discharging circuit (switching circuit) 43, and a capacitor 44 as an auxiliary power supply.
The relay 41 is arranged between the positive electrode side terminal of the main power supply 31 and the charging circuit 42. A connection point between the relay 41 and the charging circuit 42 is indicated by P1. The charging circuit 42 is a circuit for charging the capacitor 44. The charging circuit 42 includes a pair of switching elements 42A and 42B connected in series, and a boosting coil 42C connected between a connection point P3 and a connection point P1 of these switching elements 42A and 42B. The switching elements 42A and 42B are composed of n-channel MOSFETs.

  The source of the upper switching element 42A is connected to the drain of the lower switching element 42B. The drain of the upper switching element 42A is connected to the output terminal (connection point P2) of the capacitor 44. The source of the lower switching element 42B is grounded. The connection point P1 is connected to the input terminal of the capacitor 44 (connection point P5). By alternately turning on the pair of switching elements 42A and 42B, the output voltage (battery voltage VB) at the connection point P1 can be boosted and applied to the output terminal of the capacitor 44, so that the capacitor 44 can be charged.

  The discharging circuit 43 is connected to the charging circuit 42 in series. The discharge circuit 43 includes a pair of switching elements 43A and 43B connected in series. The switching elements 43A and 43B are n-channel MOSFETs. The source of the upper switching element 43A is connected to the drain of the lower switching element 43B. The drain of the upper switching element 43A is connected to the output terminal (connection point P2) of the capacitor 44. The source of the lower switching element 43B is connected to the input terminal (connection point P5) of the capacitor 44. The connection point P4 of the pair of switching elements 43A and 43B is connected to the motor drive circuit 52 in the EPS ECU 12.

When the lower switching element 43B is on and the upper switching element 43A is off, electric power is supplied to the motor drive circuit 52 only from the main power supply 31. The state in which electric power is supplied to the motor drive circuit 52 only by the main power supply 31 in this way may be referred to as the “first electric power supply state”.
On the other hand, when the lower switching element 43B is off and the upper switching element 43A is on, discharging from the capacitor 44 to the motor drive circuit 52 is possible. As a result, electric power is supplied to the motor drive circuit 52 from both the main power supply 31 and the capacitor 44. Further, when the switching element 43B on the lower stage side and the switching element 43A on the upper stage side are alternately turned on, the voltage VD applied to the motor drive circuit 52 can be changed depending on the ratio of the on times of both. In this way, the lower switching element 43B is turned off and the upper switching element 43A is turned on, or the lower switching element 43B and the upper switching element 43A are alternately turned on to turn on the main power supply. The state in which electric power is supplied to the motor drive circuit 52 by using both 31 and the capacitor 44 may be referred to as a "second electric power supply state".

The input voltage VD of the motor drive circuit 52 is detected by the voltage sensor 48. The detection value of the voltage sensor 48 is input to the power supply control ECU 33. An ignition state detection signal (not shown) indicating the state of the ignition key is input to the power supply control ECU 33.
The power supply control ECU 33 is composed of a microcomputer. The microcomputer includes a CPU and a memory (a memory such as a ROM, a RAM, or a non-volatile memory) that stores the program thereof. A nonvolatile memory (not shown) stores, for each input voltage VD of the motor drive circuit 52, a rotation speed / torque characteristic (NT characteristic) of the electric motor 18 according to the input voltage VD. For example, in the nonvolatile memory (not shown), the input voltage VD of the motor drive circuit 52 is used as the applied voltage to the electric motor 18, and the applied voltage VD to the electric motor 18 is changed when the applied voltage is changed. The rotation speed / torque characteristic (NT characteristic) of the motor 18 is stored in association with the applied voltage VD.

  The power supply control ECU 33 controls ON / OFF of the relay 41 based on the ignition state detection signal. The power supply control ECU 33 receives the input voltage VD of the motor drive circuit 52 detected by the voltage sensor 48, the rotation speed N of the electric motor 18 detected by the rotation speed sensor 23, and the motor current Im detected by the current sensor 53. Based on, the power supply state is switched between the first power supply state and the second power supply state.

  When the power supply state is set to the first power supply state, the power supply control ECU 33 sets, for example, the lower switching element 43B in the discharge circuit (switching circuit) 43 to ON and the upper switching element. Set 43A off. Further, the power supply control ECU 33 alternately turns on the pair of switching elements 42A and 42B in the charging circuit 42 as needed. As a result, electric power is supplied to the motor drive circuit 52 only by the main power supply 31, and the capacitor 44 is charged as needed.

  When the power supply state is set to the second power supply state, the power supply control ECU 33 turns off the pair of switching elements 42A and 42B in the charging circuit 42. In addition, the power supply control ECU 33 sets, for example, the upper switching element 43A in the discharge circuit (switching circuit) 43 to ON and the lower switching element 43B to OFF. As a result, electric power is supplied to the motor drive circuit 52 by both the main power supply 31 and the capacitor 44. In this case, the power supply control ECU 33 may alternately turn on the pair of switching elements 43A and 43B in the discharge circuit 43.

FIG. 3 is a flowchart showing a procedure of a power supply state switching process executed by the power supply control ECU 33. The process of FIG. 3 is repeatedly executed every predetermined calculation cycle.
The power supply control ECU 33 acquires the motor current Im detected by the current sensor 53, the input voltage VD of the motor drive circuit 52 detected by the voltage sensor 48, and the motor rotation speed N detected by the rotation speed sensor 23 (step S1). The power supply control ECU 33 calculates the actual motor torque Tm (= Kt · Im) by multiplying the motor current Im acquired in step S1 by the motor torque constant Kt of the electric motor 18 (step S2).

Next, when the input voltage VD and the motor rotation speed N are the input voltage VD and the motor rotation speed N acquired in step S1, respectively, the power supply control ECU 33 can generate the maximum motor torque that can be generated from the electric motor 18. Tm- _lim is _calculated (step S3). Specifically, as shown in FIG. 4, the power supply control ECU 33 controls the motor corresponding to the input voltage VD acquired in step S1 of the motor speed / torque characteristics for each input voltage VD in the nonvolatile memory. In the rotation speed / torque characteristic (curve S), the torque corresponding to the motor rotation speed N acquired in step S1 is obtained as the maximum motor torque Tm- _lim .

  Next, the power supply control ECU 33 determines whether or not the state determination flag F is set (F = 1) (step S4). The state determination flag F is reset (F = 0) when the power supply state is set to the first power supply state, and set (F = 1) when the power supply state is set to the second power supply state. ) Flag. The initial value of the state determination flag F is 0.

When it is determined that the state determination flag F is reset (F = 0) (step S4: NO), that is, when it is determined that the power supply state is set to the first power supply state The power supply control ECU 33 moves to step S5. In step S5, the power supply control ECU 33 subtracts the actual motor torque Tm calculated in step S2 from the maximum motor torque Tm- _lim obtained in step S3, and the difference _ΔTm (= Tm- _lim -Tm) _between them. ) (See FIG. 4) is less than a first predetermined value α (α> 0).

When the difference ΔTm is equal to or larger than the first predetermined value α (step S5: NO), the power supply control ECU 33 ends the current process without switching the power supply state. Therefore, in this case, the first power supply state is maintained.
When it is determined in step S5 that the difference ΔTm_ is less than the first predetermined value α (step S5: YES), the power supply control ECU 33 sets the state determination flag F (F = 1). After that (step S6), the power supply state is switched to the second power supply state (step S7). As a result, electric power is supplied to the motor drive circuit 52 by both the main power supply 31 and the capacitor 44. Then, the power supply control ECU 33 ends the processing of this time.

When it is determined in step S4 that the state determination flag F is set (F = 1) (step S4: YES), that is, the power supply state is set to the second power supply state. If it is determined that, the power supply control ECU 33 proceeds to step S8. In step S8, the power supply control ECU 33 subtracts the actual motor torque Tm calculated in step S2 from the maximum motor torque Tm- _lim obtained in step S3, and a difference _ΔTm (= Tm- _lim -Tm) _between them. ) Is greater than or equal to the second predetermined value β (β> 0). In this embodiment, the second predetermined value β is set to a value (β> α) larger than the first predetermined value α. However, the second predetermined value β may be set to the same value as the first predetermined value α.

When the difference ΔTm is less than the second predetermined value β (step S8: NO), the power supply control ECU 33 ends the current process without switching the power supply state. Therefore, in this case, the second power supply state is maintained.
When it is determined in step S8 that the difference ΔTm_ is greater than or equal to the second predetermined value β (step S8: YES), the power supply control ECU 33 resets the state determination flag F (F = 0). After that (step S9), the power supply state is switched to the first power supply state (step S10). As a result, electric power is supplied to the motor drive circuit 52 only by the main power supply 31. Then, the power supply control ECU 33 ends the processing of this time.

In the above embodiment, when the power supply state is in the first power supply state, the difference between them obtained by subtracting the actual motor torque Tm from the maximum motor torque _Tm-Lim DerutaTm_ first predetermined value or more α , The power supply state cannot be switched. When the difference ΔTm_ is less than the first predetermined value α when the power supply state is the first power supply state, the power supply state is switched to the second power supply state.

  When the power supply state is the first power supply state, the state in which the difference ΔTm_ is equal to or larger than the predetermined value α means that the actual motor torque is smaller than the maximum motor torque that can be generated from the electric motor 18. Therefore, it is considered that the steering assist force that can be generated has a margin. Therefore, in such a case, the power supply state cannot be switched to the first power supply state. As described above, when the discharge circuit 43 is in the first power supply state, the discharge circuit 43 cannot be switched to the second power supply state when the steering assist force that can be generated has a margin. Consumption can be suppressed.

  On the other hand, when the power supply state is the first power supply state, the difference ΔTm_ being less than the predetermined value α means that the actual motor torque is the maximum motor torque that can be generated from the electric motor 18. Since they are equal to or close to each other, it is considered that there is no margin in the steering assist force that can be generated. Therefore, in such a case, the power supply state is switched to the second power supply state. That is, the discharge circuit (switching circuit) 43 is switched to the second power supply state. Accordingly, when the power supply state is the first power supply state, the power supply state can be switched to the second power supply state when the steering assist force that can be generated has no margin.

When the power supply state is the second power supply state and the difference ΔTm_ between the two obtained by subtracting the actual motor torque Tm from the maximum motor torque Tm- _lim is less than the second predetermined value β, the power is The supply status cannot be switched. When the difference ΔTm_ is equal to or larger than the second predetermined value β when the power supply state is the second power supply state, the power supply state is switched to the first power supply state. That is, when the power supply state is the second power supply state and the steering assist force that can be generated has a margin, the power supply state is switched to the first power supply state. In this embodiment, the second predetermined value β is set to a value larger than the first predetermined value α, so that when the power supply state is the second power supply state, it is sufficient for the steering assist force that can be generated. When there is a margin, it becomes possible to switch the power supply state to the first power supply state.

In the above-described embodiment, the auxiliary power supply is composed of the capacitor, but the auxiliary power supply may be an auxiliary power supply other than the capacitor.
In addition, various design changes can be made within the scope of the matters described in the claims.

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 12 ... EPS ECU, 18 ... Electric motor, 23 ... Rotation speed sensor, 31 ... Main power supply, 32 ... Auxiliary power supply device, 33 ... Power supply control ECU, 44 ... Capacitor (auxiliary power supply), 48 ... Voltage sensor, 51 ... Motor control circuit, 52 ... Motor drive circuit, 53 ... Current sensor

Claims (8)

A drive circuit for supplying electric power to the electric motor,
A main power supply and an auxiliary power supply capable of supplying power to the drive circuit;
To switch between a first power supply state in which power is supplied to the drive circuit only by the main power supply and a second power supply state in which power is supplied to the drive circuit using both the main power supply and the auxiliary power supply. Switching circuit of
And a control means for controlling the switching circuit,
The control means is
An acquisition unit that acquires the input voltage of the drive circuit, the rotation speed of the electric motor, and the motor current flowing in the electric motor;
An actual motor torque calculation means for calculating a motor torque generated by the electric motor as an actual motor torque based on the motor current acquired by the acquisition means;
A maximum motor torque calculation means for calculating a maximum motor torque that can be generated by the electric motor based on the input voltage and the motor rotation speed acquired by the acquisition means;
When the power supply state to the drive circuit is the first power supply state, the difference between the two obtained by subtracting the actual motor torque from the maximum motor torque is a first predetermined value α (α> 0). When it is less than, the switching circuit is controlled so that the power supply state to the drive circuit is switched to the second power supply state, and the power supply state to the drive circuit is the second power supply state. , A means for controlling the switching circuit so that the power supply state to the drive circuit is switched to the first power supply state when the difference becomes a second predetermined value β (β> 0) or more. Power steering device.   The electric power steering device according to claim 1, wherein the second predetermined value β is larger than the first predetermined value α. A storage unit that stores, for each input voltage of the drive circuit, a rotation speed / torque characteristic of the electric motor according to the input voltage;
The maximum motor torque calculation means is the acquisition means in the rotation speed / torque characteristics corresponding to the input voltage acquired by the acquisition means among the rotation speed / torque characteristics for each input voltage stored in the storage means. The electric power steering device according to claim 1 or 2, which is configured to calculate a torque corresponding to the motor rotation speed obtained by the above as the maximum motor torque.   The electric power steering apparatus according to claim 1, wherein the auxiliary power source is a capacitor. A drive circuit for supplying power to the electric motor, a main power supply and an auxiliary power supply capable of supplying power to the drive circuit, and a first power supply state in which power is supplied to the drive circuit only by the main power supply. And a switching circuit for switching between a second power supply state in which power is supplied to the drive circuit by using both the main power source and the auxiliary power source, and a power supply state switching method in an electric power steering apparatus. hand,
A first step of acquiring an input voltage of the drive circuit, a rotation speed of the electric motor, and a motor current flowing through the electric motor;
A second step of calculating a motor torque generated by the electric motor as an actual motor torque based on the motor current acquired in the first step;
A third step of calculating a maximum motor torque that can be generated by the electric motor based on the input voltage and the motor rotation speed acquired in the first step ;
When the power supply state to the drive circuit is the first power supply state, the difference between the two obtained by subtracting the actual motor torque from the maximum motor torque is a first predetermined value α (α> 0). When it is less than, the switching circuit is controlled so that the power supply state to the drive circuit is switched to the second power supply state, and the power supply state to the drive circuit is the second power supply state. And a fourth step of controlling the switching circuit to switch the power supply state to the drive circuit to the first power supply state when the difference becomes equal to or larger than a second predetermined value β (β> 0). , Power supply state switching method.   The power supply state switching method according to claim 5, wherein the second predetermined value β is larger than the first predetermined value α. The electric power steering apparatus further includes a storage unit that stores, for each input voltage of the drive circuit, a rotation speed / torque characteristic of the electric motor according to the input voltage.
In the third step, among the rotation speed / torque characteristics for each input voltage stored in the storage means, in the rotation speed / torque characteristics corresponding to the input voltage acquired in the first step, the first step The power supply state switching method according to claim 5, wherein a torque corresponding to the motor rotation speed acquired by is calculated as the maximum motor torque.   8. The power supply state switching method according to claim 5, wherein the auxiliary power source is a capacitor.

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