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PWM Control

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inktomi

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Hi, I have implemented a PID controller on a microcontroller. The PID controller controls the duty cycle of a PWM signal as follows.

duty_cycle = duty_cycle + ( Kp * e[n] + Kd * {e[n] - e[n-1]} )

where, duty_cycle is a register that controls the duty cycle,
Kp and Kd are the proportional and derivative gains respectively,
and e[n] is the current error and e[n-1] is the previous error.

Since the PID output does not produce the output directly, but rather specifies the change, from what I gather it's called the 'velocity-form' PID.

Notice that I didn't use an integral term, cause I'm thinking I don't need it since duty_cycle is acting like an integrator. However, I'm doing some research and I found statement saying that you cannot use it in velocity-form without an integral term.

Am I doing something wrong? Intuitively, I think that if I add an integral term I will always get overshoot.

Please correct me if I'm wrong.
Thank you
 
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