Differential area electrohydraulic doser actuator

An electrohydraulic doser actuator comprising a housing having a bore therewithin;

• piston means contained by said bore and axially movable therein, said piston means having at least one hydraulic fluid pressure-responsive surface area, said surface area defining a variable volume hydraulic fluid control chamber in a portion of said bore;
• means for exerting a constant force upon said piston means in a direction which causes said piston means to axially move to reduce the volume of said control chamber;
• valve means operatively connected to said control chamber for selectively venting a quantity (dose) of pressurized hydraulic fluid either to or from said control chamber thereby axially moving said piston means in opposite directions within said bore to either increase or reduce respectively said volume of said control chamber;
• control means for controlling said valve means to vary said quantity (dose) of said hydraulic fluid vented to or from said control chamber thereby effecting axial movement of said piston means to desired axial positions;
• said valve means including a first valve to vent said fluid quantity (dose) to said control chamber and a second valve to vent said fluid quantity (dose) from said control chamber, said valves having only on-off operational states, being either fully open or fully closed, respectively, whereby said quantity (dose) of hydraulic fluid depends, in part, on the amount of time said orifices are in said on state, each of said first and second valves having a normally closed position thereby establishing in said position a hydraulic lock on said piston means thereby maintaining said last desired axial position.