US Patent 7051526 Closed-system electrohydraulic actuator

An electrohydraulic actuator (10) includes a fluid-powered actuator (11) having a piston (15) in a cylinder (16). The piston divides the cylinder into two opposed chambers (18, 19). The actuator includes a fixed-displacement reversible motor-driven pump (13) having a first port communicating with one of the chambers and having a second port communicating with the other of the chambers in a closed hydraulic system. A pre-charged accumulator (14) communicates with the pump first port. One of the actuator chambers is maintained at a fixed pre-charged pressure of the accumulator, and the operation of the pump controls the pressure in, and volume of, the other chamber.