Motor rotational speed control system and method having duty cycle variable modulation mechanism

A motor rotational speed control method having a duty cycle variable modulation mechanism, comprising processes of:

• (a) using a motor driver to drive a motor according to an on-time signal;
• (b) receiving a target duty cycle command by the motor driver;
• (c) setting, by the motor driver, a duty cycle of a latest one of a plurality of waveforms of the on-time signal to be equal to a target duty cycle instructed by the target duty cycle command;
• (d) detecting a rotational speed of the motor by a rotational speed detector;
• (e) determining, by the motor driver, whether or not the rotational speed of the motor is larger than or equal to a rotational speed limit value, in response to determining that the rotational speed of the motor is not larger than or equal to the rotational speed limit value, sequentially performing the processes (f) to (g) and then performing the process (l), and in response to determining that the rotational speed of the motor is larger than and not equal to the rotational speed limit value, sequentially performing the processes (h) to (j);
• (f) modulating, by the motor driver, the duty cycle difference between the duty cycle of each of the plurality of waveforms that are generated in the on-time signal within each of a plurality of time intervals and the duty cycle of each of the plurality of waveforms that are generated in the on-time signal within a next one of the plurality of time intervals to be equal to an initial duty cycle difference;
• (g) driving the motor according to the on-time signal having the duty cycle difference being equal to the initial duty cycle difference, by the motor driver;
• (h) setting, by the motor driver, the duty cycle difference between the duty cycles of each subsequent two of the plurality of waveforms of the on-time signal to be equal to a duty cycle modulation difference, wherein the duty cycle modulation difference is smaller than the initial duty cycle difference;
• (i) driving the motor according to the on-time signal having the duty cycle difference being equal to the duty cycle modulation difference, by the motor driver;
• (j) determining, by the motor driver, whether or not the rotational speed of the motor is larger than or equal to the rotational speed limit value, in response to determining that the rotational speed of the motor is larger than or equal to the rotational speed limit value, performing the process (k), and in response to determining that the rotational speed of the motor is not larger than and not equal to the rotational speed limit value, performing the process (l);
• (k) reducing the duty cycles of subsequent ones of the plurality of waveforms of the on-time signal for driving the motor, by the motor driver;
• (l) determining, by the motor driver, whether or not the duty cycle of one of the plurality of waveforms of the on-time signal that is currently used for driving the motor is equal to a target duty cycle, in response to determining that the duty cycle of the one of the plurality of waveforms of the on-time signal is equal to the target duty cycle, not performing the process (m) in which the on-time signal is further modulated, and in response to determining that the duty cycle of the one of the plurality of waveforms of the on-time signal is not equal to the target duty cycle, performing the process (m);
• and
• (m) determining, by the motor driver, whether or not the duty cycle of the one of the plurality of waveforms of the on-time signal is larger than the target duty cycle, in response to determining that the duty cycle of the one of the plurality of waveforms of the on-time signal is larger than the target duty cycle, reducing the duty cycles of the subsequent ones of the plurality of waveforms of the on-time signal for driving the motor, and in response to determining that the duty cycle of the one of the plurality of waveforms of the on-time signal is not larger than the target duty cycle, increasing the duty cycles of the subsequent ones of the plurality of waveforms of the on-time signal for driving the motor.