The design and operation of a hydraulic actuator that can track precise trajectories under large accelerations and under heavy load conditions is presented. The actuator is based on a master and slave-piston arrangement and compensates for varying temperature and supply pressures without significant deterioration. A slave piston, capable of exercising a large force, tracks faithfully the motion of a needle or master spool. The actuator has been used to provide individual control over the intake and exhaust engine valves in a medium duty engine, replacing the mechanical camshaft. Simulation and test data shows the actuator to have exceptional tracking characteristics for viscosities between 0.02 and 1.5 Ns/m2, and pressures above 5MPa during warm operation. A linear motor is used to prescribe the needle motion. The low inertia of the needle and its electronic interface allows for a highly flexible electronically controlled valve system. The power consumption from the hydraulic and electrical systems is shown to be smaller than its mechanical counterpart.

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