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Tuning PDF loop (pseudo derivative feedback)

Started by Unknown January 15, 2014
On Wednesday, January 15, 2014 8:12:47 PM UTC-5, Phil Hobbs wrote:
> On 1/15/2014 8:06 PM, bloggs.fredbloggs.fred@gmail.com wrote: > > > On Wednesday, January 15, 2014 5:40:29 PM UTC-5, Phil Hobbs wrote: > > > > > >> 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 > > > > >> http://electrooptical.net > > > > > > How come everyone hates Phelan so much? Adding proportional feedback > > > to a summing node after the integrator is equivalent to > > > differentiating and summing before the integrator, without actually > > > building a differentiator, hence the pseudo. Supposedly PDF is more > > > immune to nose disturbances in the feedback sensors. Wasn't Phelan a > > > mechanical engineer, so he's going to be doing 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 > > > > > > -- > > Dr Philip C D Hobbs > > Principal Consultant > > ElectroOptical Innovations LLC > > Optics, Electro-optics, Photonics, Analog Electronics > > > > 160 North State Road #203 > > Briarcliff Manor NY 10510 > > > > hobbs at electrooptical dot net > > http://electrooptical.net
Well, he was a professor at Cornell. He died in 2010: http://theuniversityfaculty.cornell.edu/memorials/phelan.pdf
On Wed, 15 Jan 2014 15:35:45 -0800, George Herold wrote:

> On Wednesday, January 15, 2014 5:40:29 PM UTC-5, Phil Hobbs wrote: >> On 1/15/2014 11:25 AM, George Herold wrote: >> >> > On Wednesday, January 15, 2014 10:59:34 AM UTC-5, hon...@yahoo.com >> > wrote: >> >> On Wednesday, January 15, 2014 9:46:23 AM UTC-6, Tim Wescott >> >> > I've got a copy of Phelans book.. but it's at home... And I never >> >> > understood 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 the I term becomes the 'gain' and the P >> > term is the damping.. Then for PDF he keeps the integral (gain) but >> > uses negative feed back for the damping. And (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. >> > >> > >> 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. > 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)
If you really want to make the loop work well, and if servo response (as opposed to just disturbance rejection) is what you're after, then you want to tune the loop for both. The 'traditional' PID loop basically has the feedback and feedforward terms equal: anything that shows up on the command shows up in the error, which goes through the one-and-only loop filter, and thence to the plant. Phelan's "huge" breakthrough was to set the proportional feedforward to zero. What you want to do is to build a loop that lets you adjust the feedforward independently of the feedback. Build a Phelan-esque loop, but inject some of the command into the summing junction just before the plant. Then tune the loop for good stability and disturbance rejection. Then DON'T TOUCH THAT. Then play with the feedforward gain until you have the step/ramp/whatever response that you like. There's fancier things you can do, particularly if you know what you're servoing to ahead of time, but you can go a long way just by tuning the feedforward. Note that this works with derivative control, too: you can put both the proportional and derivative just in the feedback, or you can make them equal in feedback and feedforward, or you can use feedforward that's somewhere in between full and none. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Wed, 15 Jan 2014 17:06:08 -0800, bloggs.fredbloggs.fred wrote:

> On Wednesday, January 15, 2014 5:40:29 PM UTC-5, Phil Hobbs wrote: > >> 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 >> >> >> >> >> >> -- >> >> Dr Philip C D Hobbs >> >> Principal Consultant >> >> ElectroOptical Innovations LLC >> >> Optics, Electro-optics, Photonics, Analog Electronics >> >> >> >> 160 North State Road #203 >> >> Briarcliff Manor NY 10510 >> >> >> >> hobbs at electrooptical dot net >> >> http://electrooptical.net > > How come everyone hates Phelan so much? Adding proportional feedback to > a summing node after the integrator is equivalent to differentiating and > summing before the integrator, without actually building a > differentiator, hence the pseudo. Supposedly PDF is more immune to nose > disturbances in the feedback sensors. Wasn't Phelan a mechanical > engineer, so he's going to be doing things that seem odd to an EE.
If you go and reinvent sliced bread, then go around insisting to everyone that you have an invention that's the best thing since sliced bread, we'll get tired of you, too. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Wed, 15 Jan 2014 17:38:12 -0800, George Herold wrote:

> On Wednesday, January 15, 2014 8:12:47 PM UTC-5, Phil Hobbs wrote: >> On 1/15/2014 8:06 PM, bloggs.fredbloggs.fred@gmail.com wrote: >> >> > On Wednesday, January 15, 2014 5:40:29 PM UTC-5, Phil Hobbs wrote: >> > How come everyone hates Phelan so much? Adding proportional feedback >> > to a summing node after the integrator is equivalent to >> > differentiating and summing before the integrator, without actually >>> building a differentiator, hence the pseudo. Supposedly PDF is more >> > immune to nose disturbances in the feedback sensors. Wasn't Phelan a >> > mechanical engineer, so he's going to be doing 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 obsolet >> 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. >> > 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.
It's not. It's just that it's not known as "Phelen's Super New Cool Method", or PDF. It's just another well-known (and known before Phelan) way of arranging a loop. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
In article <f742c473-e1db-4ba7-b0ac-dabd07038839@googlegroups.com>, 
hondgm@yahoo.com says...
> > On Wednesday, January 15, 2014 11:55:51 AM UTC-6, Robert Baer wrote: > > > > 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.
How's that? PID has every step needed to control the loop, more so than what you were referring to earlier.. You can set up a PID to make it work any way you wish.. however, there are differences in how they are implemented depending on the type of output the device has.. A good PID device has analog output and thus the derivative can behave as analog instead of the crappy digital time method used. Most likely what you are referring to is a basic feed back that involves gain error and integration only, no D equations. Those can be found in the basic closed loop systems like drives for the main speed loop. PID's are an added function for input control between a process variable and control variable from the outside world.. Jamie
On Wednesday, January 15, 2014 9:10:49 PM UTC-5, Tim Wescott wrote:
> On Wed, 15 Jan 2014 17:06:08 -0800, bloggs.fredbloggs.fred wrote: >=20 >=20 >=20 > > On Wednesday, January 15, 2014 5:40:29 PM UTC-5, Phil Hobbs wrote: >=20 > >=20 >=20 > >> I have it too. The main nugget I took away was that you win by >=20 > >> wrapping >=20 > >>=20 >=20 > >> a P loop around the plant to make it fast and simple, and then putting >=20 > >>=20 >=20 > >> an integrating loop around the whole works. The trick works great, bu=
t
>=20 > >>=20 >=20 > >> his "pseudo-derivative" nomenclature is claptrap. >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> Cheers >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> Phil Hobbs >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> -- >=20 > >>=20 >=20 > >> Dr Philip C D Hobbs >=20 > >>=20 >=20 > >> Principal Consultant >=20 > >>=20 >=20 > >> ElectroOptical Innovations LLC >=20 > >>=20 >=20 > >> Optics, Electro-optics, Photonics, Analog Electronics >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> 160 North State Road #203 >=20 > >>=20 >=20 > >> Briarcliff Manor NY 10510 >=20 > >>=20 >=20 > >>=20 >=20 > >>=20 >=20 > >> hobbs at electrooptical dot net >=20 > >>=20 >=20 > >> http://electrooptical.net >=20 > >=20 >=20 > > How come everyone hates Phelan so much? Adding proportional feedback to >=20 > > a summing node after the integrator is equivalent to differentiating an=
d
>=20 > > summing before the integrator, without actually building a >=20 > > differentiator, hence the pseudo. Supposedly PDF is more immune to nose >=20 > > disturbances in the feedback sensors. Wasn't Phelan a mechanical >=20 > > engineer, so he's going to be doing things that seem odd to an EE. >=20 >=20 >=20 > If you go and reinvent sliced bread, then go around insisting to everyone=
=20
>=20 > that you have an invention that's the best thing since sliced bread,=20 >=20 > we'll get tired of you, too. >=20 >=20 >=20 > --=20 >=20 >=20 >=20 > Tim Wescott >=20 > Wescott Design Services >=20 > http://www.wescottdesign.com
He seems to have independently discovered the economy of using phasor algeb= ra instead formulating systems of differential equations, computing the LaP= lace transform and solving, for the case of sinusoidal steady state computa= tions. I'm pretty sure Steinmetz predates him by about 50 years on that one= .
Someone posted  this link earlier in the thread
http://www.controlguru.com/pages/table.html

It is very good

Read the section about the two ways to create the derivative term, either from the error or from the PV directly.

Mark
hondgm@yahoo.com wrote:

> I know about the auto-tuning algorithm for PID: cycle the temperature up > and down, measure the peaks and distance between the peaks, and calculate > Kp, Ki, and Kd. > > But recently I've been experimenting with what Phelan called PDF, and am > 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. > > The problem is, how is it tuned? It only has a Ki and Kd, so that makes > 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 method that doesn't mention poles, zeros, or transforms would > be nice because even though I have an EE degree, my strong point isn't > controls. > > Either I forgot everything in my controls class or never learned it, I > don't know which.
There is a information about the algorithm here:- <http://stablesimulations.com/technotes/pdf.html> <http://www.google.com/url?q=http%3A%2F%2Fstablesimulations.com%2Ftechnotes%2Fpdf.html&sa=D&sntz=1&usg=AFQjCNG0NQj7yIorDiTW_idpePOQEP01Rg> You might find some clues in those. -- ******************************************************************** Paul E. Bennett IEng MIET.....<email://Paul_E.Bennett@topmail.co.uk> Forth based HIDECS Consultancy.............<http://www.hidecs.co.uk> Mob: +44 (0)7811-639972 Tel: +44 (0)1235-510979 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
On Wednesday, January 15, 2014 9:12:35 PM UTC-5, Tim Wescott wrote:
> On Wed, 15 Jan 2014 17:38:12 -0800, George Herold wrote: > > > > On Wednesday, January 15, 2014 8:12:47 PM UTC-5, Phil Hobbs wrote:
<snip>
> > 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. > > It's not. It's just that it's not known as "Phelen's Super New Cool > Method", or PDF. It's just another well-known (and known before Phelan) > way of arranging a loop. >
Hi Tim, Hey thanks for your other response. So what do people in cotrol theory call it? I found nothing similar in the control guru webpage linked to above. (well I only did a quick browse, looking at signal flow diagrams.) this, http://www.controlguru.com/pages/table.html George H.
> -- > > > > Tim Wescott > > Wescott Design Services > > http://www.wescottdesign.com
On Thu, 16 Jan 2014 06:23:29 -0800, George Herold wrote:

> On Wednesday, January 15, 2014 9:12:35 PM UTC-5, Tim Wescott wrote: >> On Wed, 15 Jan 2014 17:38:12 -0800, George Herold wrote: >> >> >> > On Wednesday, January 15, 2014 8:12:47 PM UTC-5, Phil Hobbs wrote: > <snip> >> > 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. >> >> It's not. It's just that it's not known as "Phelen's Super New Cool >> Method", or PDF. It's just another well-known (and known before >> Phelan) >> way of arranging a loop. >> > Hi Tim, Hey thanks for your other response. > So what do people in cotrol theory call it? > I found nothing similar in the control guru webpage linked to above. > (well I only did a quick browse, looking at signal flow diagrams.) this, > http://www.controlguru.com/pages/table.html
I don't think it has a name. At least in aerospace and motion control circles, showing it to someone elicits a very blase "oh yea, I do that sometimes, too." -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com