Brushless motor speed detection device

摘要

A delay signal rising after the passage of a predetermined time from the rising or falling of a position signal is formed by means of a delay circuit. While an offset circuit is used for sequentially monitoring the size relation between an induced electromotive voltage and a coil neutral point voltage, an offset is applied to the induced electromotive voltage when a regular speed detection signal is output from a comparator at the time the induced electromotive voltage and the coil neutral point voltage cross each other so as to raise the induced electromotive voltage up to the vicinity of a supply voltage Vcc or otherwise lower the induced electromotive voltage up to the vicinity of the ground potential. The inversion of the compared input voltage subjected to the waveform disturbance produced in the induced electromotive voltage due to the switching of energizing driving coils, speed error correction and the like is thus prevented and the offset signal is canceled after waveform disturbances are overlooked according to the delay signal and prior to the detection of the next speed detection signal.