Tuning PDF loop (pseudo derivative feedback)

and down, measure the peaks and distance between the peaks, and calculate K p, Ki, and Kd.

intrigued by it. There isn't a lot of information out there about it, but nearly everyone that's used it has good things to say.

it slightly simpler. An auto-tune algo like the one I just mentioned would be great, or at the very least, some general guide for manual tuning. A m ethod that doesn't mention poles, zeros, or transforms would be nice becaus e even though I have an EE degree, my strong point isn't controls.

n't know which.

Excellent! (I'm very interested also and will try this in the next control task.) I think the PDF term is a bit of a confusing as a name. What he r eally does is just use negative feedback around the 'plant'. So (though I haven't done it and don't know for sure.) You are still going to have to ga in up the error signal, then integrate it with some time constant, and appl y negative feedback as the damping term. I'd start with near the same gain and integral time as in the 'normal' PI controller. and then add the nega tive feedback.

So 'normal' PI looks like

error->gain--+--P(gain=1)----+->to plant--- +--intergral----+ (where the P and I terms are summed.)

and the PDF will look like,

error->gain--+--intergral---+->to plant---+-- +-

ol task.) I think the PDF term is a bit of a confusing as a name. What he really does is just use negative feedback around the 'plant'. So (though I haven't done it and don't know for sure.) You are still going to have to gain up the error signal, then integrate it with some time constant, and ap ply negative feedback as the damping term. I'd start with near the same ga in and integral time as in the 'normal' PI controller. and then add the ne gative feedback.

I have it implemented, I just want to know how to tune it. See my original post :-)

I found pseudocode that I converted to C and it works well, but tuning, yea h that's the problem.

eprints.soton.ac.uk/66198/1/mbennett_thesis.pdf

Well how do you tune a PI loop? (I get close and then do a step response changing the reference by a small amount and looking at the output.)

George H.

It's easy enough to add real derivative control to Phelan's method. I've read some of his original articles (or the one?). On the one hand, the hype to value ratio is a bit high for my taste. On the other hand, the method itself is a useful one, it's just the text surrounding it that gets tedious.

Phelan's layout does make implementing integrator anti-windup convenient.

Look around and see if you can find a Z-N method for just Ki and Kp. Better yet, try Astrom-Hagglund. Ziegler-Nichols is basically a highly refined seat of the pants method, and tends to cough up underdamped systems. Astrom-Hagglund is basically Ziegler-Nichols but with different constants that result in more conservatively tuned systems.

Astrom and Hagglund wrote a book just on PID control -- I bet they have every flavor of Ziegler-Nichols-like tuning method in there, including ones that exclude a derivative in the controller.

I also mention that in the original post. I don't give specifics but this is a great article on it:

However PID is not PDF so I can't use the same coefficients.

Hmmm, I already have that implemented for PI control. But P is not exactly the same as the D term in PDF, although close. I'm terrible at the math s urrounding this. I was thinking that at minimum, there was some tuning met hodology for PDF, but it's apparently not common enough.

se changing the reference by a small amount and looking at the output.)

s is a great article on it:

Nice thanks, This looks useful

Hmm well I havne't tried, but I don't expect the coefficients to be that mu ch different.. your system has some gain it needs and some time constant, t hat's pretty much fixed. I really need to build a small (fast) thermal test bed to try it out. I tr ied making an LT spice file to play with... but I never felt comfortable wi th implementing the thermal time of the system. I had to make at least 3 R C's in series to get the thing to oscillate (which totally makes sense) but I model the system with a single RC... so how do I relate those things.. ( sorry talking out loud.)

George H.

ly the same as the D term in PDF, although close. I'm terrible at the math surrounding this. I was thinking that at minimum, there was some tuning m ethodology for PDF, but it's apparently not common enough.

I've got a copy of Phelans book.. but it's at home... And I never understoo d his notation (in depth) There is no D (derivative term) in PDF. Just an integral and damping. Hey you might want to read Bob Pease's "What's all this PID stuff anyway" So what's a bit weird is that if you do a PI loop t he I term becomes the 'gain' and the P term is the damping.. Then for PDF h e keeps the integral (gain) but uses negative feed back for the damping. A nd (for the way I think about it) in PI the P term is one. And for PDF the negative feed back term is -1 (I think.)

George H.

If this page:

is correct, then the action of the loop differs in its response to input, not its response to a disturbance, nor in its stability properties.

So the numbers you get should be as applicable for Phelan's loop as for a "traditional" PID.

Somehow I remember Phelan doing something different -- didn't he have a PD controller whose output was integrated? At any rate, if you get your scaling right then in the linear case it all works the same.

From what I can tell, he maintains the integral term just as it is in PID. Then he subtracts away the (input * Kd). Not the error, but the raw input. Seems to work ok but again, that's with guesses for coefficients.

I don't know if anyone has seen this, but this is how I first learned of it.

I do not know anything about PID or control "systems". But i have a fancy MicroOmega PID controller as a part of a toaster oven temperature controlled system. PID systems are billed as being the best thing since Langendorf sliced bread). Well, then why the heck is it that they all seem to take forever to stabilize, ponderously ring after the "step" turn-on, and take five times forever to stabilize at a temperature DIFFERENT than the set-point?

Seems to me, if proper parameters were used in a proper control system (most likely _NOT_ PID), the controlled environment would see the turn-on step with only a slight overshoot, NO ringing, followed with an almost immediate settling TO the set-point. Am guessing a damping factor of 0.95; there is a paper somewhere that gets into that and calculates the ideal under-damped value.

Exactly. PID isn't a very good control algo, but everyone knows of it so they keep using it.

I beg to differ.

Ripping some PID algorithm off of a web page and implementing it without knowing what you're doing isn't going to bring you joy. Most control loops use some form of PI or PID control loop, with various decorations.

Choosing the right decorations to your PID loop, and tuning it correctly, make a huge difference.

"I don't know anything about making XXX's work, and my XXX works like crap" is more a statement about one's level of knowledge about the thing in question than about the thing itself. Following that with "so therefore all XXX's are crap" says a lot more about one's own ability to think things through than it does about whatever it is that you're slamming.

I have it too. The main nugget I took away was that you win by wrapping a P loop around the plant to make it fast and simple, and then putting an integrating loop around the whole works. The trick works great, but his "pseudo-derivative" nomenclature is claptrap.

Cheers

Phil Hobbs

Using local loops has the advantage of improving the bandwidth (and phase) of the individual "chunks". ...Jim Thompson

Which is too bad, If he'd just called it negative feedback it might have had more impact.

As Tim said PI vs PDF is basically the same with respect to 'outside' changes. But in response to setpoint changes the PDF is a bit better. and then Phelan makes some semi-outrageous claims about how much better PDF is where he compares the exponential 'power' response of the PI to a more delayed PDF, with the cavet that they both stay in the linear region. Anyhow I'll be doing one soon, I guess I'll tune it by looking at the step response. (to answer the OP's question)

George H.

How come everyone hates Phelan so much? Adding proportional feedback to a s umming node after the integrator is equivalent to differentiating and summi ng before the integrator, without actually building a differentiator, hence the pseudo. Supposedly PDF is more immune to nose disturbances in the feed back sensors. Wasn't Phelan a mechanical engineer, so he's going to be doin g things that seem odd to an EE.

I don't hate Phelan at all, but reading his stuff you do get the impression that his head was too big to fit through the door. He was always spouting off about how everybody else's approaches were obsolete and so on, and publishing "universal" controller schematics that look like some plumber's nightmare. Apparently he pissed off everybody in the field back in the day, so they ignored him more or less completely.

Which is too bad, because some of his stuff was great.

Cheers

Phil Hobbs

Perfect response, thanks Phil. I don't hate Phelan at all. I'm eager to try it. He does come across as a bit of a blow hard.

It does give one pause that negative feedback (PDF, whatever you call it) seems to be ignored in the control community.

George H.