Image Number 5 for United States Patent #4734631.
A step motor control system which can control a step motor to rotate at a variable high speed like a brushless motor. A position signal indicating a position of a rotor of a step motor relative to a specific one of field windings is detected as a voltage signal which is successively compared with a reference voltage in order to detect a specific angular position of the rotor at which selective energization and deenergization of the field windings are started for a next stepping operation of the motor.The position signal may be extracted as a voltage signal from an electric current flowing through a last energized field winding by a resistor connected thereto. From the voltage signal, a back EMF induced in the field winding due to its self inductance is subtracted to detect a voltage position signal or EMF induced in the field winding by a permanent magnet of the rotor. The back EMF is produced by an electronic simulation means including an integrating circuit having a same time constant with the field winding.