Abstract: (a) Hydraulic servo motor comprising a jack (1, 14) intended to be connected by its rod (3.sub.1, 3.sub.2, 15.sub.1) to a moving member to be displaced and accurately positioned, said jack being completely filled with liquid;(b) Servo motor characterized in that the displacement of the piston (2) is implemented by direct liquid transfer from one chamber to the other chamber of the jack by at least one electric vibrator pump (6) supplied by an alternating current power source and in that a so-called electro valve (7), normally open when not in operation, is connected in order to make the two jack chambers communicate;(c) The invention relates to hydraulic servo motors.