Stepper motor-operated or less frequently DC motor-operated hydraulic cylinder with integrated control unit (following control valve) and mechanical feedback via spindle or toothed rack that can be integrated or fitted externally. High response proportional valve (a four edge valve) and spool form in this manner a highly accurate following system that can be operated digitally (Figure E 21).
Any change in velocity or direction of the drive motor will result in a corresponding change in the spool speed (control of rapid traverse and operating speed), while the number of steps define the position. With such electro-hydraulic linear amplifiers position accuracies between 0.002 and 0.01 mm can be achieved.
Figure E 21: Block diagram (a) and principle (b) of the linear amplifier