PID Without a PhD, Finally

Thanks - passed on to folk who might benefit.

One thing - in the text for Figure 5, you say "magnet is attached to the stage, which moves with an acceleration proportional to the coil current."

I've done exactly this with a large voice coil (4" diameter, 4" throw) with a constant *voltage*, and you get a *speed* proportional to the voltage, although the current quickly settles to a constant (-ish). I'm sure I don't need to explain why - but it might be worth mentioning back EMF at some point. It was a surprise to me, initially.

Clifford Heath.

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Clifford Heath
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Embedded Systems Design (or whatever they call themselves) kept moving this around -- so I've revamped it, updated it, and posted it on the web.

Take a gander. Please comment on anything you like/don't like. I'm not sure if the way that I'm setting off the math is a Really Good Idea or a Really Bad Idea -- I'm trying to make it easy for the math-averse to skip over it, without breaking up the flow too much for folks who can read math without breaking stride.

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Tim Wescott 
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Tim Wescott

"In addition, the electrical requirements to build a good current-output amplifier and non-contact transducer interface can be chal-lenging. You can expect that if you are doing a project like this you are a member of a fairly talented team (Note 4)."

"Note 4: Or that you're heading toward a very educational disaster."

Hahahahaha :)

It's great! So much better than the Process Control text our professor wrote and used in our class.

Michael

Reply to
mrdarrett

Page 20, Figure 18 has an... interesting... X-axis.

Michael

Reply to
mrdarrett

That's as it came out of Scilab. Argh -- Scilab can be coerced to make more human-readable output, but it's a pain.

Thanks, though.

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Tim Wescott 
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Tim Wescott

Glad you like it. I feel that embedding little tid-bits like that in a document helps to keep people on their toes. Without, I hope, detracting too much from the content.

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Tim Wescott 
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Tim Wescott

I could have gotten the required PhD in the meantime...

Haha no just kidding that would be lame-o. Looks great! :)

Reply to
bitrex

You leave out math but include C code, I think that's defeating the purpose. If you can't explain it in non-technical words or a simple picture... well, try harder :-)

You spend only one page on tuning, and give no examples of "If you see this, try this" which is much more useful to the practical user.

You don't mention feed-forward terms at all.

In short, it looks much like all the other PID papers out there, and wouldn't have helped me "get" PID back when I was trying to figure it out.

The section on I - the last paragraph should go first, since the point of the I term is to correct for long-term errors, and you don't even mention it in the first paragraph.

The second paragraph in D should be first, and is probably the most useful paragraph in the paper, if you don't already understand PID.

The other thing I always hated about PID papers is that they never gave solid examples of the difference between controlling speed and controlling position, or the more complicated case of torque/speed/position nested PID loops for CNC control.

I'm reminded of a YouTube video about how NOT to weld - the guy would intentionally do it wrong various ways so you could compare his results with your results, so you could figure out what you were doing wrong and how to fix it. Something like that for PID would be more useful than showing an ideal situation.

Reply to
DJ Delorie

I don't know if color is something that will be preserved, but that bright green for links actually hurts for me to look at. Hard to miss though.

The PDF page numbering is off 1 from the text page numbers. I'm pretty sure there is a way to deal with that.

Page 2 (text page number) paragraph 2, "Some command is given to a controller, and the determines a drive signal to be applied to the plant." Is "the" the word you want?

Page 4, equation 1, you might explain the basis of this equation. I assume the voltage actually controls the torque. Friction force is a result of velocity and the acceleration is from the excess force until the motor speeds up. Contrasted to the equation for the frictionless platform.

Page 6, paragraph 1, figure 5, you write "The magnet is attached to the stage", but the diagram calls it a "platform". That is not hugely distracting, but if I know nothing about optical systems (which I don't) I might be thinking the "stage" is something different and start looking for what it means (I did).

Just for completeness, you might include the "position transducer" in your diagram. I assume it measures the position of the platform rather than the magnet.

Page 6, paragraph 2, showing my ignorance I don't understand, "With this arrangement the force on the magnet is independent of the stage motion." Doesn't a magnet moving in the coil create a current/voltage that interacts with the applied current/voltage? Are we assuming the "good current-output amplifier" deals with this?

Page 6, equation 2, I don't see where Vp is defined. It does not seem to be used anywhere else in the paper.

Page 8, equation 3, you define Th twice but don't say what units. I assume it needs to be absolute temperature, Kelvin? Two time constants are given, but no explanation for why two or what is different about them. I don't know about others, but I have a hard time considering an equation I don't understand. It keeps me from getting an understanding of how the controller would work.

I need to go now. Please take this for what it is worth, free advice.

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Rick
Reply to
rickman

I don't have a problem with that. In fact I often make graphs in LTSpice that way to make the data easily visible rather than obscured by the axis.

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Rick
Reply to
rickman

Good points.

The intended audience is writers of software for embedded processors, so the 'C' code may be easier to understand than English for some readers. This was implied when it was in its original context -- I think I'll at least put in an expanded forward.

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Tim Wescott 
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Tim Wescott

No - I think I wanted "this".

Hmm. I'm trying to keep the math stuff short -- people write entire white papers on the behavior of motors alone. Maybe put all the math into a several-page appendix?

To unpack a bit, the difference between the applied voltage and the motor's back-emf, divided by the armature resistance, determine the motor's armature current. Torque is armature current times the motor's torque constant, and acceleration is torque divided by the motor's moment of inertia. All of that is rolled into the kv and the time constant, in this case.

You're right, the nomenclature should be consistent. I don't know about other people, but this is incredibly difficult for me. It's a subset of the "don't toss jargon around" rule. (Sometimes, the multiple-language problem is a consequence of incomplete solutions to removing jargon, at least for me).

Good point.

The good current-output amplifier, and the frictionless support of the platform, deals with this.

Vp comes out of the position transducer that I left off of the drawing. Yup, need to change something!

Well, I pulled the time constants out of my ear. Or my donkey (I _do not_ get that cliche :P ). Or something.

Te temperature of the load (Th) is determined by both the driving input (Vd) and the ambient temperature (Ta). Again, I need to think about making that more clear without making the math-averse reader run away screaming.

I appreciate your time and trouble you've taken to read the thing and write your comments.

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Tim Wescott 
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Tim Wescott

...

Yes, putting all the math in appendices would be great, I agree!

Michael

Reply to
mrdarrett

I think maybe I should write a white paper on how motors work. The principle of voice-coil motor behavior is pretty much the same as motors that turn in circles.

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Tim Wescott 
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Tim Wescott

The boxed equations don't have any equals signs. I imagine that's stuff that's understood, but maybe not for everyone.

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John
Reply to
quiasmox

I don't think tucking away the math into an appendix is the answer. Maybe derivations or something not essential to understanding the problem. All I would need is an understanding of the meaning of the terms of the equation. Typically each term of the equation comes from some specific "thing" in the problem. I don't get what those "things" are in these equations.

I'm not sure I understand this. I'd need to write out the equations. The back-emf is proportional to what, the rate of change of the current? Or the speed of the motor? It's been a long time and I forget. I think saying it "is rolled into the kv and the time constant" is a bit simplified. Again, I'd like to know the meaning of each term in the equation. d theta/dt is the speed of the motor, but why is that in this spot of the equation? Does this represent the friction? Then I would get that the friction force balances with the motor torque and the acceleration would become zero.

Then where did this equation come from? I find it very hard to follow the rest of the approach if I can't understand the equation. I guess I'm not the target audience.

Seems to me the equation would be

dTh/dt = ka * (Ta - Th) + kh * Vd

kh * Vd is the heat applied by the heater with kh the constant for the heater watts vs the thermal mass of the container. ka * (Ta -Th) is the heat entering (or leaving) the container with ka accounting for the thermal mass and the amount of surface area, etc. The heat moving into/out of the container is proportional to the temperature rate of change, no?

How does this get to be a second order differential equation?

I appreciate you writing this. I'm sure I will learn a lot from reading it.

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Rick
Reply to
rickman

I just spotted an error. If the heater is controlled by volts, it would have to be voltage squared to get power. Unless the volts is out of the ADC and the heater is controlled by a voltage to watts control, lol.

dTh/dt = ka * (Ta - Th) + kh * Vd^2

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Rick
Reply to
rickman

Need to give this up tonight... make that DAC, not ADC.

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Rick
Reply to
rickman

Do too.

I've gotten this report before, and I think it's something about the way that LaTeX renders pdf files, and then the way that Adobe renders those pdf files on some screens. I would really appreciate it if you could email me a screen shot so that I can file a bug report with the LaTeX folks, or ask for a work-around.

tim AT wescottdesign DOT com.

Thanks in advance.

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Reply to
Tim Wescott

{message trimmed}

Ok, I downloaded the file instead of looking at it in the browser, and the equals signs are fine. I've sent you an email with the screenshots from within the browser.

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John
Reply to
quiasmox

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