Patent ID: 7739941

Claim:
A hydraulic drive system comprising components that cooperate with one another to deliver reciprocating motion and to provide piston strokes of consistent length in each actuation stroke, said system comprising: (a) an actuator comprising a piston disposed within a cylinder and reciprocable between two cylinder heads in each actuation stroke, whereby said piston divides said cylinder into respective first and second hydraulic fluid chambers and a piston stroke is defined by said piston traveling from a first predetermined position near one of said cylinder heads to a second predetermined position near the other one of said cylinder heads; (b) at least one piston rod comprising a first end connected to said piston and a second end extending through one of said two cylinder heads and out of said cylinder; (c) a flow switching device comprising a flow switching member that is actuatable between at least two positions by an actuator that is activatable by an electronic signal to reverse the direction of hydraulic fluid flow to or from said first and second hydraulic fluid chambers so that hydraulic fluid flows into one of said first or second hydraulic fluid chambers when hydraulic fluid is flowing out of the other one of said first or second hydraulic fluid chambers; (d) a hydraulic pump comprising a discharge outlet and a suction inlet; (e) high pressure conduits for respective fluid connections between each one of said first and second hydraulic fluid chambers and respective fluid couplings of said flow switching device, and between an inlet of said flow switching device and said discharge outlet; (f) low pressure conduits for connecting an outlet of said flow switching device to a hydraulic fluid reservoir and said hydraulic fluid reservoir to said suction inlet, or for connecting said outlet of said flow switching device directly to said suction inlet; (g) a shuttle valve and a fluid passage through said piston wherein said shuttle valve is operable to close said fluid passage when said piston is moving during one of said piston strokes, and to open said fluid passage when said piston is at the end of one of said piston strokes; and (h) a controller that is programmed to: determine when said piston has reached the end of each piston stroke based upon at least one of hydraulic pump speed, hydraulic fluid pressure, or elapsed time, all measured during each piston stroke; and send an electronic signal to said flow switching device to command said flow switching member to be actuated from one position to another position to reverse the hydraulic fluid flow when said controller determines that said piston has reached the end of each piston stroke.