Abstract: A reversible mechanical unit of the "slave" type comprising a motor element and position detector is controlled by an assembly which delivers command signals relating to the positions of the motor element. Means are provided for delivering an error signal .epsilon. which is proportional to the difference between the signals delivered respectively by the detector and the control assembly. A signal delivered by a generator and having an amplitude which is proportional to the signal .epsilon. produces direct action on the torque of the motor element; the error signal and the torque are thus in one-to-one correspondence, the error signal being fed back into the control assembly.