1. Field of the Invention
The present invention relates to a height control device for vehicle to be used for maintaining always constant a distance between a wheel-side member and a frame-side member of a vehicle.
2. Description of the Prior Art
Conventionally in a vehicle, for example, a load carrying vehicle such as a truck, a height control device for vehicle is used to maintain always constant a vehicle height under a load condition and an unload condition. There are various types in such a height control device for vehicle. For example, Japanese Utility Model Laid-open Publication No. 60-72712 discloses a height control device for vehicle including a vehicle height control valve mounted to a vehicle body side, a link mechanism for connecting the vehicle height control valve to an axle case, an expansion mechanism mounted to the link mechanism, and driving means for operating the expansion mechanism. In this device, a vehicle height is detected by the link mechanism, and is adjusted by the vehicle height control valve to be mechanically operated by the link mechanism.
In another height control device for vehicle excluding such an expansion mechanism and driving means as mentioned above, a vehicle height control valve is mounted to a sprung member of a vehicle, e.g., a vehicle frame, and a link mechanism is provided to connect the vehicle height control valve to an unsprung member of the vehicle, e.g., an axle case. In this height control device for vehicle, the link mechanism is operated by relative displacement between the vehicle frame and the axle case under the load condition and the unload condition, and the vehicle height control valve is operated by the movement of the link mechanism to adjust the vehicle height. That is, when the vehicle is loaded, the link mechanism adjusts the vehicle height control valve to an air induction side, thereby including the air into an air spring connected to a load carrying platform of the vehicle to lift the platform. Reversely, when the vehicle is unloaded, the link mechanism adjusts the vehicle height control valve to an air discharge side, thereby discharging the air from the air spring to lower the platform of the vehicle.
However, in both the height control devices for vehicle as mentioned above, since the link mechanism is directly connected to the vehicle height control valve mounted to the sprung member of the vehicle and to the unsprung member of the vehicle, a high sensibility is undesirably provided to the vehicle height control valve. Accordingly, the vehicle height control valve is sensitively operated to open and close in response to vertical motion of tires to induce and discharge the air with respect to the air spring even during normal running of the vehicle. That is, the vehicle height is changed with high-frequency vibration during running of the vehicle. Therefore, particularly in the load carrying vehicle, vertical vibration of the platform of the vehicle is amplified by the above operation during running of the vehicle.
Further, U.S. Pat. No. 4,105,216 has proposed that momentary height change from a target height level is ignored not to conduct height control by means of electrical signal processing techniques. However, this proposal is directed to an electronic height control device, and it has not solved the problem in the mechanical height control device for vehicle.