mean value and has a high oscillation frequency to each target current i obj determined according to the target position the period t dith of the dithering square wave is too small to disturb the hydraulic circuit 21 driven by the solenoid valves 30 but allows to inhibit the occurrence of static friction phenomena within the solenoid valves 30 in other words each solenoid valve 30 is kept u201c in fibrillation u201d with minor fast and small scale oscillations around the target position for preventing the moving parts of the solenoid valve 30 from u201c sticking u201d by increasing the breakout static friction as shown in fig4 the main microcontroller 35 determines the target current i obj it determines the dithering square wave i dith which normally has a zero mean value and adds the dithering square wave i dith to the target current i obj for each solenoid valve 30 the main microcontroller 35 communicates the addition of the target current i obj and the dithering wave square i dith to the digital input 39 of the corresponding supporting microcontroller 37 such that the supporting microcontroller 37 drives the coil 34 of the solenoid valve 30 to track such addition as apparent from fig5 the supporting microcontroller 37 drives the coil 34 of the solenoid valve 30 by means of the known control technique called u201c chopper u201d which provides the application to the terminals of the coil 34 of the solenoid valve 30 of a positive voltage which determines an increase of the current i which crosses the coil 34 and alternatively of a zero or negative voltage which determines.