PID-filter routine (2nd try).

[HenrikOlsson]

Hi Mike,
There should be no need to edit the include file for this, I don't think.

All you need to do is calculate the error and send it to the PID-filter, it determines if the error is positive or negative, calculates the "drive" and returns it in the variable PID_Out. You never need to pass any "direction-bit" TO the PID-routine.

Now, if you're driving something like a heater where you can only deliver power in "one direction" (you can just add more or less heat, you can't remove it) then you simply look at the sign-bit of the PID_Out variable (PID_Out.15) and if it's set (meaning negative "drive" becuse the temperature is too high) you simply set the output to 0.

OR (and this is probably a better aproach)

Let's say you know that under normal conditions you need a duty cycle of ~30% to keep the temperature fairly constant. Now you look at the sign of the value in the PID_Out variable and depending on its state you either add or subtract the ABS value of PID_Out from whatever value corresponds to 30% duty cycle. Here you need to make sure you clamp the end value between 0 and 1023 before moving it to the duty cycle register.

Does this make sense?

/Henrik.

[mlmccauley]

Thanks for the reply.

Yes, makes sense to me. The process does accept a variable input, not on-off. It's a DC drive that I'm generating a reference voltage for using hardware PWM on a 18F2320 @ 40MHz. Sounds like this should work out fine.

Once again, thanks!

Mike

[HenrikOlsson]

Yes, what I meant with set the output to 0 was that if the ouput of the PID filter goes negative you "clamp" the dutycycle at 0 since you can't effectively have a negative drive with that system.

In any case the second aproach should work better. Just a note though, in your first message you wrote:

Likewise, I'll need to take the output of the PID include that gets passed back to the main process, and add/subtract the new output with the last output based on the direction bit, currently B.15.

You don't add/subtract the PID filter output from the last output, you add/subtract it from the initial "steady state value". I guess you had that figured out but just in case.

Let me know how it goes or if there's anything I can do.

/Henrik.

[mlmccauley]

The routine works like a champ!

Did the first trial run-up today. No problem at all with mono-polar operation. A few tweaks on the constants and it's working great. Need to make a small hardware change in external circuitry, and it'll be wound up.

Again, thanks! A very handy tool to hang onto.