Patent ID: 8587296

Claim:
A rotation angle detecting device that includes a detection rotor that rotates in accordance with rotation of a rotor and that has a plurality of magnetic poles, and a sensor that outputs three sinusoidal signals which have a phase difference of a predetermined degree in accordance with rotation of the detection rotor, the rotation angle detecting device detecting a rotation angle of the rotor based on the sinusoidal signals, comprising: a signal reading unit that reads the sinusoidal signals in each predetermined computation cycle; and a detecting unit that detects a magnetic pole transition in at least one of the sinusoidal signals on the basis of the sinusoidal signals read by the signal reading unit, wherein when the sinusoidal signal in which a magnetic pole transition is detected is a first sinusoidal signal, one of the other two sinusoidal signals is a second sinusoidal signal and the other one of the other two sinusoidal signals is a third sinusoidal signal, the detecting unit detects a magnetic pole transition in the first sinusoidal signal based on a current value of one of the second sinusoidal signal and the third sinusoidal signal, an absolute value of a difference between an immediately preceding value and the current value of the one of the second and third sinusoidal signals being larger than an absolute value of a difference between an immediately preceding value and the current value of the other of the second and third sinusoidal signals, an immediately preceding value of the first sinusoidal signal and a current value of the first sinusoidal signal.