Patent ID: 7667424

Claim:
A servo device comprising, a M-phase brushless motor driven by a M-phase drive signal; a brushless motor drive signal generator for creating the M-phase drive signal to drive said M-phase brushless motor; a position detector for detecting the position of an output shaft transmitting a drive output of said M-phase brushless motor via a reduction mechanism; a rotational speed detector for detecting the rotational speed of said M-phase brushless motor; a PWM control signal generator for producing a PWM control signal to control said M-phase drive signal created by said brushless motor drive signal generator, based on input control information, positional information detected by said position detector, and rotational speed information detected by said rotational speed detector; and a signal switching circuit for reversing the polarity of the output of the rotational speed detector according to the rotational direction of the brushless motor; wherein said rotational speed information selects a stator coil terminal of said brushless motor by the selection switching circuit, which is controlled based on a M-phase drive signal output from said brushless motor drive signal generator, and is created by a terminal voltage of said stator coil.