The present invention relates to a power steering system for generating a steering assist force by means of a hydraulic power cylinder that is operated in accordance with a steering torque of a steering wheel or the like.
This kind of power steering system is disclosed in Unexamined Japanese Patent Publication No. 2002-145087 which is assigned to the same assignee of this application.
The power steering system includes a steering shaft attached to a steering wheel, an output shaft connected to a lower end portion of the steering shaft, a rack and pinion mechanism provided to a lower end portion of the output shaft, a hydraulic power cylinder connected to the rack, a reversible pump for selectively supplying working oil to one of axially opposite first and second oil pressure chambers of the hydraulic power cylinder by way of corresponding one of first and second communication passages, and an electromagnetic valve for selectively opening and closing a bypass passage connecting between the first and second communication passages.
When the steering wheel is steered to the left or right during running of the vehicle, a detecting mechanism detects a steering torque for such a steering operation and supplies a signal by way of a control circuit to the electromagnetic valve to close the same while causing the reversible pump to rotate in the forward direction or in the reverse direction thereby supplying working oil in one of the oil pressure chambers and one of the communication passages to the other of the oil pressure chambers and the other of the communication passages.
Further, the power steering system is provided with an abnormal condition monitoring circuit for monitoring an abnormal condition of an electric motor for driving the reversible pump. The abnormal condition monitoring circuit is configured to determine that the electric motor is abnormal when the rotation speed of the electric motor is zero and causes the electromagnetic valve to open thereby performing a failsafe function. The abnormal condition monitoring circuit can also detect that the reversible pump is abnormal, because the reversible pump is connected to the electric motor.