Power steering system

1. A power steering system that allows a motor to boost steering torque, and transmits the steering torque to a wheel via a steering transmission mechanism, comprising:

an impact detection device that detects an impact of an object on a rack housing that houses a rack bar constituting the steering transmission mechanism,

wherein the impact detection device includes

an angle detection device that detects a first rotational angle of a rotary member constituting the steering transmission mechanism, and

an angular speed calculating device that calculates an angular speed of the rotary member based on the detected first rotational angle, and

wherein the impact detection device detects the impact of a peripheral component on the steering transmission mechanism based on the calculated angular speed.

2. The power steering system according to claim 1 , wherein

the angle detection device includes a rotational angle sensor that detects a second rotational angle of the motor that assists a rack bar constituting the steering transmission mechanism in moving in an axial direction.

3. The power steering system according to claim 1 , wherein

the angle detection device includes a torque sensor that detects a steering torque based on a difference between rotational angles of a steering input shaft and an output shaft, which are connected to each other via a torsion bar constituting the steering transmission mechanism.

4. The power steering system according to claim 1 , wherein

the impact detection device detects the impact by discriminating a vibration due to the impact of the peripheral component on the steering transmission mechanism from a vibration that occurs in the steering transmission mechanism due to a backward input from the wheel.

5. The power steering system according to claim 4 , wherein

the vibration of the steering transmission mechanism due to the impact of the peripheral component is discriminated from the vibration of the steering transmission mechanism due to the backward input from the wheel, based on a changing pattern and/or frequency characteristics of an angular speed signal output from the angular speed calculating device.

6. The power steering system according to claim 4 , wherein

the vibration of the steering transmission mechanism due to the impact of the peripheral component is discriminated from the vibration of the steering transmission mechanism due to the backward input from the wheel by using an output signal from the torque sensor for detecting the steering torque.

7. The power steering system according to claim 4 , wherein

the vibration of the steering transmission mechanism due to the impact of the peripheral component is discriminated from the vibration of the steering transmission mechanism due to the backward input from the wheel by using a difference between the output signals each of which is sent from one of a pair of the rotational angle sensors one of which is provided for the steering input shaft, and the other of which is provided for the output shaft, the steering input shaft and the output shaft being connected to each other via the torsion bar.

8. The power steering system according to claim 1 , wherein

the impact detection device detects a displacement of the rack housing, relative to a vehicle body.

9. The power steering system according to claim 1 , wherein,

when the impact detection device detects the impact of the object on the rack housing, a mode of controlling a steering assist torque to be applied to the rack bar is changed.

10. The power steering system according to claim 9 , wherein

the change of the control mode includes a change of the operational state of an electric motor between a stopped state and a driven state.

11. The power steering system according to claim 1 , wherein the motor to boost steering torque is an electric motor.