Velocity control of unbalanced hydraulic actuator subjected to over-center load conditions

The electro-hydraulic actuation system comprising:

• an unbalanced hydraulic actuator capable of motion in retraction and extension directions during movement of a load;
• a pump for providing a flow of fluid to the actuator, a fluid volume output of the pump controlling a velocity of the actuator during motion in the retraction and extension directions;
• an electric motor for driving the pump, speed and direction of the electric motor affecting the fluid volume output of the pump;
• a controller for controlling the speed and direction of the electric motor;
• and
• a feedback device operable for sensing a system condition and for providing a feedback signal indicative of the sensed system condition to the controller, the controller being responsive to the feedback signal for determining an occurrence of an over-center load condition and for modifying the speed of the electric motor in response to the occurrence in an attempt to maintain the velocity of the actuator;
• wherein the actuator includes a piston/rod assembly that divides the actuator into first and second chambers and moves relative to a housing of the actuator during motion in the retraction and extension directions, one of the first and second chambers being a high pressure chamber during movement of the piston/rod assembly relative to the housing, upon the occurrence of an over-center load condition the high pressure chamber switching to the other of the first and second chambers, the feedback device being responsive to the switching of the high pressure chamber for providing the feedback signal to the controller;
• wherein the system further includes a charge pump system, a shuttle valve that is responsive to a pressure differential between the first and second conduits for connecting the charge pump system in fluid communication with one of the first and second chambers, upon the occurrence of an over-center load condition the shuttle valve switching positions to connect the charge pump system in fluid communication with the other of the first and second chambers, the feedback device being adapted to sense a position of the shuttle valve;
• and
• wherein the controller determines the occurrence of an over-center load condition when a direction of movement of the piston/rod assembly relative to the housing remains unchanged when the valve shifts positions.