High frequency pulse width modulation

Claim 1

A high frequency pulse width modulation circuit for controlling a door drive motor atop an elevator car in response to a low frequency microprocessor, comprising: a controller (16) for providing a state signal indicative of "door open" or "door close"; ROM means (18) for storing a velocity signal indicative of the desired velocity profile of the door drive motor; a microprocessor (10) having a slow duty cycle, responsive to the state signal for providing the velocity signal from the ROM to a first line (20); high frequency oscillator means (26) for providing a high frequency signal; counter means (24) for providing an incrementing count signal on a second line (28) several times during the duty cycle of the microprocessor in response to the high frequency signal; comparator means (30), connected at its inputs to the first line (20) and to the second line (28), for providing an output signal on a third line (32) which is High when the velocity signal is greater than the incrementing count signal and which is Low when the incrementing count signal is greater than or equal to the velocity signal, so that the High:Low ratio of the output signal is proportional to the velocity signal from the microprocessor; amplification means (34), connected at its input to the third line (32) for providing an output at power levels for operating the door drive motor (12) in response to the comparator means output, wherein the door drive motor speed is proportional to the High:Low ratio of the output signal.