PWM概念及设计参考(英文).doc

PWM概念及设计参考（英文）
在一般的机器人套件中，动力通常使用直流马达，而控制直流马达最常用的就是PWM（脉宽调制）方式，这里给出了PWM的基本概念以及设计中需要注意的事项。
PWM or Pulse Width Modulation refers to the concept of rapidly pulsing the digital signal of a wire to simulate a varying voltage on the wire. This method is commonly used for driving motors, heaters, or lights in varying intensities or speeds.
A few terms are associated with PWM:
· Period--how long each complete pulse cycle takes
· Frequency--how often the pulses are generated. This value is typically specified in Hz (cycles per second).
· Duty Cycle--refers to the amount of time in the period that the pulse is active or high. Duty Cycle is typically specified as a percentage of the full period.

In the above diagram, the duty cycle is shown at 50%. The pink line shows the average output and you can see that at 50% duty cycle, the output averages is roughly 6V or 50% of full power. Below is a diagram of what a 25% duty cycle PWM signal looks like:

Note
The human ear can hear frequencies up to roughly 20kHz. When using PWM at frequencies below this, the device being driven can often be heard to buzz. Higher frequencies avoid this.

Note
When controlling motors, much greater PWM efficiency is achieved at frequencies above 20-30 kHz. This is because the current (induction) in the windings of the motors doesn't get a chance to collapse and leave the motor when the OFF-period is short. The collaps of this induction field takes some time; driving the motors at high PWM frequencies keeps this induction current in the motors at all times, resulting in much higher efficiencies.