Control system with improved robustness to disturbances

Claim 1

A control system for controlling a brushless motor comprising: position sensing means for producing multiphase position sensor signals indicative of a rotational position of the brushless motor; sensing means for producing a sensed signal having a frequency indicative of a rotational speed of the brushless motor, wherein the frequency of said sensed signal is substantially higher than a frequency of the position sensor signals; speed detecting means for producing a detected signal of a digital number representative of the rotational speed of the brushless motor at intervals of a detecting cycle period proportional to a period of the sensed signal; error signal producing means for producing an error signal when the detected signal indicates that the rotational speed of the brushless motor deviates from a target rotational speed by more than a specified amount; a memory having stored therein a plurality of memorized values at a plurality of sequential storing positions corresponding to a plurality of sequential rotational positions of the brushless motor; cyclic up-down counter means for counting pulses of at least one of the multiphase position sensor signals according to a rotating direction of the brushless motor, wherein a count value is obtained corresponding to a rotational position of the brushless motor; initial position detection means for detecting a timing at which the count value of said counter means changes after an absolute value of the error signal becomes less than a predetermined value, and for detecting from the count value at the thus detected timing a storing position in said memory corresponding to a rotational position of the brushless motor; storing means for renewing, sequentially and periodically at intervals of a renewing cycle period proportional to the detecting cycle period of the speed detecting means, each of the plurality of memorized values in said memory by a first mixed value which is obtained by a means for mixing the error signal with at least one of the memorized values which are at least an interval of L renewing cycle periods older, where L is an integer at least equal to 2 and the interval of L renewing cycle periods is equal to the rotational period of the brushless motor, wherein said storing means determines a storing position from which the renewing is to be started from the storing position detected by said initial position detecting means; control signal producing means for producing a control signal from a second mixed value which is obtained by a means for mixing the error signal with at least one of the memorized values; and driving means responsive to the control signal for providing to the brushless motor multiphase currents proportional to the control signal.