Greetings to all,
in an earlier post (#23) I mentioned that:
the length of time (the period) between successive oscillations is a VERY important characteristic to consider when preparing to tune a control loop
then in post #27, Thomas Sullens asked: “why?”
the simple answer would be:
“read this and you’ll see” ... but that would be way too easy ...
I don’t have time to go into a LOT of detail ... but maybe this will help ... disclaimer: the following is a general beginner-level discussion ... there are exceptions to all of these “rules” ...
basically each “closed-loop” control system has certain unique characteristics ... let’s use my six individual Hotrod stations as examples ... specifically, Hotrod #1 has slightly different operating characteristics from Hotrod #6 ... even though I built all six of the Hotrods at the same time and to exactly the same specifications ... now if all of the Hotrods COULD have been made to be functionally IDENTICAL, then one universal set of tuning parameters (our Proportional, Integral, and Derivative settings - PID) would suffice to properly tune ALL of the Hotrods ... (and each student could simply copy the tuning values from his neighbor) ... but such is not the case ... simple variables inherent in the manufacture of each individual device give that device its own unique characteristics ...
some important characteristics of a heating device such as the Hotrod include:
1. how fast can its temperature start responding after the controller changes the drive to the heater?
2. and once the temperature has started to respond, how fast can the temperature actually rise?
3. how big a change in temperature will I get for a 1% change in drive to the heater?
4. how will a heavy load on the system affect the operation when compared to a light load?
these are the same types of questions that you would ask yourself whenever you’re deciding which new car to buy ... you crank up the car and press on the gas pedal ... does the engine rev right up – or does it hesitate just a little? ... once the engine starts to respond, does it quickly rev up to match the gas pedal position – or does it give a “mushy” response that takes quite awhile to come up to speed? ... does a little bit of gas pedal movement result in a big change in engine speed – or do you really have to stomp on it to make the engine spin? ... and no matter WHAT the engine sounds like in the dealer’s parking lot, you’re NOT going to buy anything until you’ve checked its response out on the freeway ...
so the basic idea is that you can learn a lot about a car’s engine just by checking its response to the gas pedal ... a “VVVRRROOOMMMM-VVVRRROOOMMMM” response is obviously a lot different from a “putt-putt-putt” response ...
so now back to tuning the Hotrod ... the same types of questions that I listed above (and others that we don’t have time to discuss here) can be answered by running what is commonly called an “open loop” test and graphing the Hotrod’s response ...
the graph in
this post should give you some ideas about how such a test might be run and how the results might be graphically analyzed ... by knowing the length of the chart’s X-axis and Y-axis and then setting up some simple ratios of “how-many-millimeters-for-this-compared-to-how-many-millimeters-for-that” then we can come up with some fairly precise numbers for our analysis ...
the important thing to remember is that the purpose of this particular test is simply to gain some insight into the operating characteristics of the system ... in other words, “how will this thing react when I try to control it automatically?” ... you really need to have some type of “feel” for the system’s characteristics before you can successfully control it ...
now the “open loop” test is very popular and it’s basically easy to set one up ... but it does have a drawback ... the “open loop” test is (by definition) always done in the manual mode ... specifically, the PID controller is NOT being tested at the same time we’re testing the process itself ...
so now we’ll take up another commonly performed procedure: the “closed loop” test and try to answer Thomas Sullens’ specific question ...
once our Hotrod system has been actually connected to the PID controller, we might do the “closed loop” test ... what we’re going to do is force the system to oscillate ... we’ll graph those oscillations and then analyze the data ... hopefully this will give us even more insight into how the system will respond when we eventually try to control it ... one big advantage that the “closed loop” test has over the “open loop” test that we previously discussed, is that now the PID controller will be actively participating in the test ... so this is a lot more like the freeway “road test” for your new car than the simple “gun the gas” test performed in the dealer’s parking lot ...
but one disadvantage of the “closed loop” test is that not all industrial systems should be tested like this ... some machinery will actually tear itself right off the wall if forced into oscillations ...
anyway ... the picture in
this post should give you a good idea of what a “closed loop” test might look like ... once again ... by knowing the length of the chart’s X-axis and Y-axis and then setting up some simple ratios of “how-many-millimeters-for-this-compared-to-how-many-millimeters-for-that” then we can come up with some fairly precise numbers for our analysis ...
now once someone has looked at enough of these “open loop” and “closed loop” tests ... and they’ve gotten a good bit of experience under their belt ... then (I’m told) that they develop a sort of “seat-of-the-pants feel” for how to go about tuning a control loop ... at least that’s what I’ve been told ... so far I personally haven’t reached that stage yet ... and this late in life, I doubt that I ever will ...
so now the question is: once we’ve performed our “open loop” and/or our “closed loop” test and we've measured and analyzed the graphs, what do we do next? ...
here I’d suggest that you read the classic “Ziegler-Nichols” paper which was written way back in 1942 ... far before PLCs and computers had come along ... you can find a copy here at
Mr. Driedger’s excellent website ...
in all honesty I should warn you that many (most?) people nowadays (including my distinguished colleague Mr. Peter Nachtwey) don’t put too much faith in this old “dark ages” stuff ... but personally I find it fascinating and very helpful ... in my humble opinion, the main benefit of studying the Ziegler-Nichols method is that it literally FORCES my students to view the graphs from the “open loop” and the “closed loop” tests as something much more than just “squiggly lines” on a paper ... from my own personal experience, once I started seriously studying the various details on these graphs, I started gaining a LOT more insight into what each one of the individual features actually MEANS about the system to be tuned ... in short, I was learning “what-to-look-for” on the trends ...
once you’ve worked your way through the old Ziegler-Nichols paper, then I’d suggest that you take a look at this
previous example showing how I applied that old “dark ages” approach to an actual graph to come up with some “ball park” tuning values ...
and actually you’ll find that there are TWO different approaches given by Ziegler and Nichols in that old paper ... the first is based on taking a “closed loop” test ... they generally referred to this as their “ultimate sensitivity” method ... the second is based on taking an “open loop” test ... they generally referred to this as their “reaction curve” method ...
there’s lots of good stuff here folks ... in my opinion, if these old techniques were able to help people tune loops back in the old days before computers and PLCs, why would they suddenly become totally useless today? ... the elemental laws of physics didn’t change just because we started using computers and PLCs for our controllers ... the old-timers had to rely on springs, and levers, and pressures, and all sorts of gadgets and what-nots for their controllers ... and these old voodoo tricks helped them get by ... in my humble opinion, this old stuff is still well worth studying ... I know that it’s helped me ...
in summing up I’d like to discuss Thomas Sullens’ comment:
And I hope you can get Peter straighten out????
I’m quite sure that Thomas intended for this to be in a humorous vein ... but I’d like everyone to know that I consider Peter to be WAY ahead of me in this process control subject ... but ... he and I DO have very different approaches to the same subject ... his approach seems to be much more “mathematically” oriented ... mine is more “visually” oriented ... to be honest, when Peter talks about “pole placement” and the “s domain” frankly I don’t understand a word he’s saying ... I just don’t have (nor do I desire to have) the “elegant math” background that Peter enjoys ... personally, I think that my “kung-fu” is much better than his ... but I’m also sure that Peter is firmly convinced that his approach is better than mine ... so be it ...
the major point is that even though Peter and I might seem to “disagree” from time to time, I’m extremely glad that he and I (and many others) can get together here on Phil’s excellent forum and discuss our different viewpoints ...