The suspension settings are controlled by hydraulic valves operated by solenoids. Each solenoid is driven in a proportional control by a selected current level. A dithering signal is superimposed on the solenoid current. This causes the valve element to oscillate slightly and thus free it from any sticking characteristics. The valve is easily adjusted in either direction and is thus free from hysteresis effects. The dithering signal is derived by varying the mark-space ratio of a pulse width modulated signal.
