This invention relates to a rear wheel steering device for a vehicle.
Conventionally, a four-wheel steering device which steers rear wheels, as well as front wheels, has been known in the art, as disclosed by the Japanese Patent Application Laying Open Gazette No. 59-26365, for example.
Such a device as mentioned above is provided with a rear wheel steering shaft which steers rear wheels by its displacement, neutrally holding means which constantly biases the rear wheel steering shaft to the neutral position, a motor which is linked with the rear wheel steering shaft and changes a steering angle ratio (ratio of the rear wheel steering angle to the front wheel steering angle) by displacing the rear wheel steering shaft, and oil pressure assist means which assists displacement of the rear wheel steering shaft by the motor by oil pressure force.
In the above device, in the case of four-wheel steering device of vehicle speed sensing, it is so controlled that the rear wheels are steered in the reverse phase (direction) at a low speed and in the same phase (direction) at a high speed in relation to the front wheels and at the vehicle speed=0, the steering angle ratio shows the maximum value in reverse phase (direction).
However, when a vehicle stops, due to delay in working of the motor which changes a steering angle ratio it sometimes occurs that the motor stops before a standard position where the steering angle ratio of the maximum value in reverse phase (direction) is attained and in the case where the engine stops in such fashion, oil pressure of the oil pressure means is relieved.
In the above case, when control is resumed by engine driving it is necessary in the first place to work the motor to attain the standard position even at the initial check which precedes the start of the rear wheel steering control.
At the above stage, however, oil pressure is not yet supplied to the oil pressure assist means which assists driving of the motor. Moreover, since a buffer mechanism comprising a spring which absorbs and buffers steering force to be transmitted to the rear wheel steering shaft at fail-safe, etc. is provided at the midway part of the system by which steering force is transmitted to the rear wheel steering shaft so that at the time of usual fail-safe two-wheel steering (only front wheels are steered) is made possible without transmitting handle steering force to the rear wheels, the above-mentioned standard position must be attained only by the force of the motor against the spring force, etc. of the buffer mechanism, without getting assist force by oil pressure. In order to enable us to carry out such operation of the motor accurately at any place including a cold district (-40.degree. C., for example), it is required to make the capacity of a motor extremely large. This raises the problem of requirement of large size motors. On the other hand, if it is so designed that the rear wheel steering control can be resumed immediately without working the motor to attain the standard position, there is a danger that a big disagreement is caused between the controlled steering angle ratio by the control system and the actual steering angle ratio.