Control circuit of a stepper motor capable of detecting an external magnetic field

An electronic control circuit (20) of a stepper-type motor (2), the motor comprising a rotor (3) provided with a permanent magnet (6), which is bipolar and has a magnetisation axis perpendicular to the axis of rotation (7) of this rotor, and a stator (4) which defines a magnetic circuit, an opening (8) forming a housing for the permanent magnet, two isthmi (12a & 12b) at the periphery of the opening and diametrically opposed in a first direction (14) perpendicular to the axis of rotation, and two rest positions for the rotor in which the permanent magnet is oriented in a second direction (16), angularly offset from the first direction, respectively in both senses of direction;

• the permanent magnet generating, in the two rest positions, a first magnetic flux (FA) passing through the two isthmi respectively in both senses of direction, a first rest position corresponding to a positive sense of direction and the second rest position corresponding to a negative sense of direction of the first magnetic flux;
• the motor further comprising a coil (18) mounted on the magnetic circuit so as to be able to generate, when supplied with a positive electrical pulse (+IB (t)) or a negative electrical pulse (−IB (t)), a second magnetic flux passing through the two isthmi respectively in said positive sense of direction or in said negative sense of direction;
• the electronic control circuit determines the position of the rotor at rest, from among said first and second rest positions, and includes an electrical pulse generator (22) that is arranged to selectively supply the coil with positive and negative electrical pulses;
• wherein the electronic control circuit (20) further comprises a circuit (24) for detecting an external magnetic field formed by a circuit (26) for measuring said electrical current, a circuit (28) for comparing the measured electrical current with a reference current (IRef), a time measurement circuit (32) arranged to measure a rise time (TM) between triggering of one of said electrical pulses and the next moment in time when the electrical current flowing in the coil reaches the reference current, and a circuit (36) for processing the rise time, this circuit being arranged to determine whether the measured rise time indicates the presence of an external magnetic field (HExt) passing through the two isthmi.