Steering control device

1. A steering control device for controlling current supply to a motor to generate an assist torque in correspondence to a steering torque, the steering control device comprising:

a target steering torque calculation unit configured to calculate a target steering torque which is a target value of the steering torque;

a servo controller configured to calculate a base assist torque so that the steering torque follows the target steering torque;

a servo limit value calculation unit configured to calculate a servo limit value which limits an output of the servo controller, a polarity of which is defined to be positive or negative in accordance with a direction of steering;

a correction control calculation unit configured to calculate a correction control value in accordance with a steering state value which is a physical value reflecting a steering state;

a current command value calculation unit configured to calculate a current command value to be supplied to the motor based on an assist torque command value determined by adding the correction control value to the base assist torque;

a current limit value calculation unit configured to calculate a current limit value with respect to the current command value; and

a torque converter configured to output a torque limit value determined by converting the current limit value into a torque to the servo limit value calculation unit,

wherein the servo limit value calculation unit is configured to calculate the servo limit value having an absolute value larger than an absolute value of the torque limit value in at least a part of a region of the steering state value or the correction control value.

2. The steering control device according to claim 1 , wherein:

a difference between the absolute value of the servo limit value and the absolute value of the torque limit value in the at least a part of the region is a value which compensates a decrease in the absolute value of the base assist torque caused by addition of the correction control value.

3. The steering control device according to claim 1 , wherein:

the servo limit value calculation unit is configured to set, as the absolute value of the servo limit value over an entire region of the steering state value or the correction control value, a value calculated by adding a positive enlargement value to the absolute value of the torque limit value.

4. The steering control device according to claim 1 , wherein:

the servo limit value calculation unit is configured to calculate the servo limit value based on the torque limit value and the correction control value.

5. The steering control device according to claim 4 , wherein:

the servo limit value calculation unit is configured to set the absolute value of the servo limit value to be equal to or larger than the absolute value of the torque limit value and equal to or smaller than a value calculated by adding a positive enlargement value to the absolute value of the torque limit value.

6. The steering control device according to claim 5 , wherein:

the servo limit value calculation unit is configured to set the servo limit value by subtracting the correction control value from the torque limit value with respect to an upper limit value and a lower limit value of the servo limit value when the correction control value is a negative value and a positive value, respectively.

7. The steering control device according to claim 1 , wherein:

the servo limit value calculation unit is configured to calculate the servo limit value based on the torque limit value and a steering speed or a steering angle, which is the steering state value.

8. The steering control device according to claim 7 , wherein:

the servo limit value calculation unit is configured to set the absolute value of the servo limit value to be equal to or larger than the torque limit value and equal to or smaller than a value calculated by adding a positive enlargement value to the absolute value of the torque limit value; and

the servo limit value calculation unit is configured to set, as the absolute value of the servo limit value, a value calculated by adding a value, which is a product of the absolute value of the steering speed or the steering angle and a positive coefficient, to the absolute value of the torque limit value.

9. The steering control device according to claim 1 , wherein:

the correction control calculation unit is configured to calculate the correction control value by convergence control, which attenuates the assist torque in accordance with the steering speed, which is the steering state value, and a vehicle speed.

10. The steering control device according to claim 1 , wherein:

the correction control calculation unit is configured to calculate the correction control value by active return control, which moves a steering wheel toward a neutral position in accordance with the steering angle, which is the steering state value, and a vehicle speed.

11. The steering control device according to claim 1 , wherein:

the servo controller is configured to limit, by the servo limit value, a previous value of an output of an integrator which integrates a difference between the steering torque and the target steering torque, or an integrated value of the previous value of the output of the servo controller.

12. A steering control device for controlling current supply to a motor to generate an assist torque in correspondence to a steering torque, the steering control device comprising:

a processing unit with a memory storing control processing, the processing unit being configured by execution of the control processing to:

calculate a target steering torque which is a target value of the steering torque;

calculate a base assist torque so that the steering torque follows the target steering torque;

calculate a servo limit value which limits an output of a servo controller, a polarity of which is defined to be positive or negative in accordance with a direction of steering;

calculate a correction control value in accordance with a steering state value which is a physical value reflecting a steering state;

calculate a current command value to be supplied to the motor based on an assist torque command value determined by adding the correction control value to the base assist torque;

calculate a current limit value with respect to the current command value; and

output a torque limit value determined by converting the current limit value into a torque,

wherein the processing unit is configured to calculate the servo limit value having an absolute value to be larger than an absolute value of the torque limit value in at least a part of a region of the steering state value or the correction control value.

13. The steering control device according to claim 12 , wherein:

a difference between the absolute value of the servo limit value and the absolute value of the torque limit value in the at least a part of the region is a value which compensates a decrease in the absolute value of the base assist torque caused by addition of the correction control value.

14. The steering control device according to claim 12 , wherein the processing unit is configured to:

set, as the absolute value of the servo limit value over an entire region of the steering state value or the correction control value, a value calculated by adding a positive enlargement value to the absolute value of the torque limit value.

15. The steering control device according to claim 12 , wherein the processing unit is configured to:

calculate the servo limit value based on the torque limit value and the correction control value;

set the absolute value of the servo limit value to be equal to or larger than the absolute value of the torque limit value and equal to or smaller than a value calculated by adding a positive enlargement value to the absolute value of the torque limit value; and

set the servo limit value by subtracting the correction control value from the torque limit value with respect to an upper limit value and a lower limit value of the servo limit value when the correction control value is a negative value and a positive value, respectively.

16. The steering control device according to claim 12 , wherein the processing unit is configured to:

calculate the servo limit value based on the torque limit value and a steering speed or a steering angle, which is the steering state value;

set the absolute value of the servo limit value to be equal to or larger than the torque limit value and equal to or smaller than a value calculated by adding a positive enlargement value to the absolute value of the torque limit value; and

set, as the absolute value of the servo limit value, a value calculated by adding a value, which is a product of the absolute value of the steering speed or the steering angle and a positive coefficient, to the absolute value of the torque limit value.

17. The steering control device according to claim 12 , wherein the processing unit is configured to:

calculate the correction control value by convergence control, which attenuates the assist torque in accordance with the steering speed, which is the steering state value, and a vehicle speed.

18. The steering control device according to claim 12 , wherein the processing unit is configured to:

calculate the correction control value by active return control, which moves a steering wheel toward a neutral position in accordance with the steering angle, which is the steering state value, and a vehicle speed.

19. The steering control device according to claim 12 , wherein the processing unit is configured to:

limit, by the servo limit value, a previous value of an output of an integrator which integrates a difference between the steering torque and the target steering torque, or an integrated value of the previous value of the output of the servo controller.