CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on, and claims priority from, Japanese Patent Application Serial Number 2019-057056, filed on Mar. 25, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety.

BACKGROUND

Field

The present disclosure relates to a steering control system, and more particularly, to a steering control system for a steer-by-wire vehicle.

Background Art

Japanese Patent Application Laid-Open No. 2018-047784 discloses a technique relating to a steering control apparatus for performing processing for avoiding that a steering operated so that a steering angle exceeds an upper limit value. In this technique, when the magnitude of the maximum value of the turning angle of the wheel or the steering angle of the steering becomes equal to or larger than the limit start threshold value, the reaction force actuator is operated to rapidly increase the limiting reaction force for limiting further increase in the magnitude of the steering angle.

Further, Japanese Patent Application Publication No. 2015-123864 discloses a technique relating to a steering control apparatus capable of adjustment a steering angle ratio characteristic representing a relationship between a steering angle of a steering wheel and a turning angle of turning wheels. In this technique, at least two characteristics are set as the steering angle ratio characteristic, i.e., the steering angle ratio characteristic at the time of driving and the steering angle characteristic at the time of stopping.

SUMMARY

In a configuration in which the steering angle ratio characteristic is adjusted in accordance with a change in the state of the vehicle, as in the technique of Japanese Patent Laid-Open No. 2015-123864, it is possible to optimize the steering angle ratio characteristic in accordance with the state of the vehicle. However, when the steering angle ratio characteristic changes, a steering limit of the steering corresponding to the turning limit at which the turning angle becomes the maximum changes. Therefore, as in the art of Japanese Patent Application Laid-Open No. 2018-047784, when applying the technology of JP-2015-123864 in the configuration of rapidly increasing the anti-torque at the steering limit, the opposite torque suddenly changes when the state of the vehicle being steered at the steering limit changes, the driver may perceive an unexpected steering feeling. As described above, in steering control of a steer-by-wire vehicle in which the steering angle ratio characteristic can be adjusted according to the state of the vehicle, there is a problem in controlling the reaction torque when the state of the vehicle changes at the steering limit.

The present disclosure has been made in view of the above-mentioned problems, and an object thereof is to provide a steering control system capable of suppressing a sudden change in reaction torque when a steering angle of a steer-by-wire vehicle is steered to a steering limit.

In order to solve the above problems, the first disclosure is applied to a steering control system for a steer-by-wire vehicle. The steering control system includes a steering motor for turning wheels of the vehicle, a reaction force motor for applying a reaction torque to a steering of the vehicle, and a controller for calculating a turning angle of the wheels for controlling the steering motor and a reaction torque for controlling the reaction force motor based on a state of the vehicle. The controller is configured to calculate the turning angle according to the steering angle based on a first characteristic representing a relationship of a magnitude of the turning angle to the steering angle of the steering, and calculate the reaction torque according to the steering angle based on a second characteristic representing a relationship of a magnitude of the reaction torque to the steering angle. The first characteristic and the second characteristic are configured to be adjustable according to the state of the vehicle. The controller is configured to change the first characteristic from a characteristic corresponding to a first state to a characteristic corresponding to a second state in response to a change in the state of the vehicle from the first state to the second state. The controller is configured to maintain the second characteristic at the characteristic corresponding to the first state when the state of the vehicle changes from the first state to the second state in the case where the steering angle is steered to a steering limit corresponding to an upper limit of the turning angle.

The second disclosure has the following further features in the first disclosure.

The controller is configured to, when the steering angle is returned to a predetermined steering angle in a third state after the state of the vehicle is changed from the first state to the second state at the steering limit, change the second characteristic from the characteristic corresponding to the first state to the characteristic corresponding to the third state.

In order to solve the above-mentioned problems, the third disclosure is applied to a steering control system for a steer-by-wire vehicle. The steering control system includes a steering motor for turning wheels of the vehicle, a reaction force motor for applying a reaction torque to a steering of the vehicle, and a controller for calculating a turning angle of the wheels for controlling the steering motor and a reaction torque for controlling the reaction force motor based on a state of the vehicle. The controller is configured to calculate the turning angle according to the steering angle based on a first characteristic representing a relationship of a magnitude of the turning angle to the steering angle of the steering, and calculate the reaction torque according to the steering angle based on a second characteristic representing a relationship of a magnitude of the reaction torque to the steering angle. The first characteristic and the second characteristic are configured to be adjustable according to the state of the vehicle. The controller is configured to change the first characteristic from a characteristic corresponding to a first state to a characteristic corresponding to a second state in response to a change in the state of the vehicle from the first state to the second state. The controller is configured to, when the steering angle is steered to a steering limit corresponding to an upper limit of the turning angle and the state of the vehicle changes from the first state to the second state, change the second characteristic from the characteristic corresponding to the first state to the characteristic corresponding to the second state at a change speed slower than a change speed of the state of the vehicle.

The forth disclosure has the following further features in any one of the first to third disclosures.

The state of the vehicle includes a vehicle speed. The second state has a higher vehicle speed than the first state. When the state of the vehicle changes from the first state to the second state at the steering limit, the controller is configured to change the first characteristic so that the magnitude of the turning angle with respect to the steering angle decreases.

The fifth disclosure has the following further features in any one of the first to third disclosures.

The state of the vehicle includes a vehicle speed. The second state has a lower vehicle speed than the first state. When the state of the vehicle changes from the first state to the second state at the steering limit, the controller is configured to change the first characteristic so that the magnitude of the turning angle with respect to the steering angle increases.

According to the steering control system of the first disclosure, even when the state of the vehicle changes from the first state to the second state and the first characteristic changes at the steering limit, the second characteristic is maintained as the characteristic corresponding to the first state. As a result, since the reaction torque can be maintained even when the steering limit changes, it is possible to suppress a sudden change in the steering angle of the steering and to solve the driver's discomfort.

According to the steering control system of the second disclosure, when the steering angle is returned to the prescribed steering angle in the third state after the state of the vehicle is changed from the first state to the second state in the steering limit, the second characteristic is changed from the characteristic corresponding to the first state to the characteristic corresponding to the third state. The second characteristic is that the smaller the steering angle, the smaller the difference in the reaction torque corresponding to the state of the vehicle. Therefore, according to the present disclosure, it is possible to make the second characteristic follow the characteristic corresponding to the state of the vehicle while suppressing the sudden change of the reaction torque.

According to the steering control system of the third disclosure, even when the state of the vehicle changes from the first state to the second state and the first characteristic changes at the steering limit, the second characteristic changes from the characteristic corresponding to the first state to the characteristic corresponding to the second state at the change speed slower than the change speed from the first state to the second state. As a result, even when the state of the vehicle changes and the steering limit changes, the variation of the reaction torque accompanying the change becomes slow, so that the driver may turn or turn back of the steering without any sense of discomfort.

According to the fourth disclosure, it is possible to suppress further turning of the steering angle of the steering when the vehicle accelerates at the steering limit.

According to the fifth disclosure, when the vehicle decelerates at the steering limit, it is possible to suppress the steering angle of the steering from being suddenly turned back.

