Hi,
For simplicity, let's concentrate on the proportional gain alone (no Ki or Kd) to start with, OK?

Let's then assume the following:
* You have a proportional gain of 1, ie pid_Kp=$0100
* You're setpoint value is 480
* You have 2.1A flowing in your circuit resulting in an ADC value of 480

Now, when you calculate the error, pid_Error = SetPoint - ADC the error is 0 and therefor pid_Out will be 0. Since you're feeding pid_Out straight to the PWM module the dutycycle will be set to 0.

Now, because the dutycycle is 0 the current drops, rapidly. By the time you sample the current again it has dropped to 0 or close to it (I'm guessing a bit), so the ADC value is 0 and pid_Error will be 480. Because you have a proportional gain of 1 pid_Out would be 480 but it gets clamped to 240. That's where your bouncing output comes from.


If your 'idle' point requires a dutycycle of 240 then you need to set that up first. THEN you sample the current and run the error thru the PID-loop and ADD the pid_Out to the 'idle' dutycyle. For example, 240 is nominal dutycycle value, when the ADC returns 475 it means the current is on the low side, the error is 5 and pid_Out becomes 5 which you add to your 'idle' resulting in a dutycycle value of 245. If the ADC value is 490 (too much current) the error will be -10, the the pid_Out will be -10, which you add to your 'idle value' resulting in a dutycycle of 230, lowering the current.

Again, the above ilustrates what happens with a proportional gain of 1 only, when you add in the integral and derivative terms it'll act a little different.

There is no built in way to have it clamp in a soft way as you describe, you have to do that "manually" if you need.

/Henrik.