Fine phase frequency multipiler for a brushless motor and corresponding control method

Abstract

A circuit to establish an accurate instantaneous position of a DC motor rotor includes an input terminal to receive a rotor position signal. A first counter circuit counts to a value between two successive rotor position signals at a slow clock rate, and stores this count value in a register. Then a second counter begins counting from zero at a system clock rate, which is faster than the slow clock rate. The first counter circuit and the second counter circuit are evaluated by a comparator, and when the counters equal one another, the instantaneous position signal is generated. Alternately, the second counter can be a down-counter that is initially loaded with the count value, and the instantaneous position signal is generated when the second counter reaches zero. In both versions, another sub-circuit can be added which, for a first cycle after the rotor position signal is received, causes the instantaneous position signal to be generated at a time calculated to equal the lag time from the actual position of the motor rotor and when the rotor position signal is received. In the first version, the comparator outputs the signal when the second counter reaches the offset count value, instead of when the second counter reaches the count value generated by the first counter. In the second version, the offset value is loaded into the count-down timer the first cycle after the rotor position signal is received.