Rotational pulse generating circuit for motors

Abstract

A motor rotational pulse generating circuit for a motor is provided which generates a correct pulse signal even at an initial turning-on stage of the motor, by adjusting a filter cutoff frequency in response to motor rotational condition. The motor rotational pulse generating circuit includes a rotational pulse generation circuit which generates ripple pulses based on a signal being inputted from the DC motor , in which a ripple is superposed whose frequency is in proportion to a rotation number of the DC motor . A filter makes a cutoff frequency variable on the basis of a clock signal issued from a PLL circuit . An oscillation frequency at an oscillator VCO is determined by the ripple pulses and a motor rotation condition signal inputted by way of circuits to inclusive. A microcomputer operates, when the motor is turned on, to cause the oscillator VCO to issue a preliminary clock signal. The microcomputer measures the oscillation frequency on the basis of the resultant preliminary clock signal to judge whether or not an initial cutoff frequency of the filter is correct. If the result is negative, a switching circuit is controlled in switching mode to adjust an attenuance of an attenuator , thereby correcting the initial cutoff frequency.