Vehicle steering system

A vehicle steering system comprising:

• an electric motor to provide a steering force to a steering operation mechanism of a vehicle;
• a setter configured to set an angle control target torque to cause an angle difference between a target steering angle and an actual steering angle to approach zero;
• an estimator configured to estimate a road load torque received from a road by an object to be driven by the electric motor;
• an automatic steering controller configured to set a target automatic steering torque in accordance with the angle control target torque set by the setter and the road load torque estimated by the estimator, and configured to control the electric motor in accordance with the target automatic steering torque so as to exercise automatic steering control;
• a manual steering controller configured to control the electric motor in accordance with a target assist torque responsive to a steering torque so as to exercise manual steering control;
• and
• an automatic steering deactivator configured to make switching from the automatic steering control to the manual steering control in accordance with a steering operation performed by a driver during the automatic steering control exercised by the automatic steering controller, wherein
• the automatic steering deactivator includes a transition controller configured to exercise transition control when a transition control start requirement is satisfied, the transition control start requirement including at least a requirement that an absolute value of the steering torque be equal to or greater than a first predetermined value, and
• the transition controller includes
• a first controller configured to, at a time when the transition control start requirement is satisfied, save the angle control target torque set by the setter, the angle control target torque being saved in a form of a transition control start angle control target torque,
• a second controller configured to calculate a transition control target automatic steering torque in accordance with the transition control start angle control target torque and the road load torque estimated by the estimator, configured to assign weights to the transition control target automatic steering torque and the target assist torque using an absolute value of a value responsive to the angle difference so as to calculate a target motor torque, and configured to control the electric motor in accordance with the target motor torque, and
• a third controller configured to, when the absolute value of the value responsive to the angle difference is equal to or greater than a second predetermined value, terminate the transition control so as to make switching from the automatic steering control to the manual steering control.