1 - An electromechanical actuator comprising: - a magnetic circuit (1, 3) having a fixed armature capable of conducting a main magnetic flux between a first end and a second end, and having a mobile armature engaged between the first end and the second end of the fixed armature, the mobile armature being capable of being displaced in a movement relative to the fixed armature between a working state and a resting state toward which it is restored by restoring means, the magnetic circuit (1, 3) being capable of conducting the majority of said main magnetic flux, a leakage magnetic flux passing through the space surrounding the magnetic circuit (1, 3), - a control coil arranged so as, when it is supplied from a DC electrical energy source, to generate a magnetic flux comprising said main magnetic flux conducted by the magnetic circuit (1, 3) and comprising said leakage magnetic flux passing through the space surrounding the magnetic circuit (1, 3), - a magnetic field sensor placed so as to be subjected to said magnetic flux generated by the control coil, - the magnetic field sensor being structured so as to produce an output signal having a first signal value and a second signal value, the output signal switching over to its first signal value if the detected magnetic field becomes greater than a first magnetic field threshold, the output signal switching over to its second signal value if the detected magnetic field becomes less than a second magnetic field threshold, the first magnetic field threshold being greater than the second magnetic field threshold, - a control switch connected in series between said DC electrical energy source and said control coil and driven by the signal coming from the magnetic field sensor, so as to be in an off state when said detected magnetic field is greater than the first magnetic field threshold and so as to be in an on state when said detected magnetic field is less than the second magnetic field threshold, - a freewheel diode connected in parallel on the control coil, - so that the magnetic field sensor and the control switch supply the control coil with a chopped and modulated voltage when the assembly is connected to said DC electrical energy source, wherein: - the magnetic field sensor is placed in the space surrounding the magnetic circuit (1, 3) so that the leakage magnetic flux passes through it, - the magnetic field sensor is placed, in relation to the magnetic circuit (1, 3), in a position in which the leakage magnetic field is highly variable when the mobile armature is displaced in the vicinity of its working state. - a magnetic circuit (1, 3) having a fixed armature capable of conducting a main magnetic flux between a first end and a second end, and having a mobile armature engaged between the first end and the second end of the fixed armature, the mobile armature being capable of being displaced in a movement relative to the fixed armature between a working state and a resting state toward which it is restored by restoring means, the magnetic circuit (1, 3) being capable of conducting the majority of said main magnetic flux, a leakage magnetic flux passing through the space surrounding the magnetic circuit (1, 3), - a control coil arranged so as, when it is supplied from a DC electrical energy source, to generate a magnetic flux comprising said main magnetic flux conducted by the magnetic circuit (1, 3) and comprising said leakage magnetic flux passing through the space surrounding the magnetic circuit (1, 3), - a magnetic field sensor placed so as to be subjected to said magnetic flux generated by the control coil, - the magnetic field sensor being structured so as to produce an output signal having a first signal value and a second signal value, the output signal switching over to its first signal value if the detected magnetic field becomes greater than a first magnetic field threshold, the output signal switching over to its second signal value if the detected magnetic field becomes less than a second magnetic field threshold, the first magnetic field threshold being greater than the second magnetic field threshold, - a control switch connected in series between said DC electrical energy source and said control coil and driven by the signal coming from the magnetic field sensor, so as to be in an off state when said detected magnetic field is greater than the first magnetic field threshold and so as to be in an on state when said detected magnetic field is less than the second magnetic field threshold, - a freewheel diode connected in parallel on the control coil, - so that the magnetic field sensor and the control switch supply the control coil with a chopped and modulated voltage when the assembly is connected to said DC electrical energy source, - the magnetic field sensor is placed in the space surrounding the magnetic circuit (1, 3) so that the leakage magnetic flux passes through it, - the magnetic field sensor is placed, in relation to the magnetic circuit (1, 3), in a position in which the leakage magnetic field is highly variable when the mobile armature is displaced in the vicinity of its working state.