PID tuning. Lag = ~ integrating time

You drive the heater and look at the thermistor temperature. I'm a huge fan of step responses for this job because the high frequency info is all there in the first ~ 3x the rise time. (For a 1-pole response the rise time is 2.2 time constants.) In your situation you'd want to do it at a few different drive levels, because as Tim says, the gain and phase will be different for the two heat sources.

Putting one thermistor by the transistor and another by the resistor will also allow you to null out the effect of the resulting temperature gradient at the position of your sample by changing the relative gain applied to the two. The simplest way is to wire them in series, but then the nulling trick only works at one temperature. (I talk about this in my thermal control chapter someplace.) You can make the null position more stable with respect to the set point by linearizing the thermistors independently and then forming the overall error signal using a linear combination.

Cheers

Phil Hobbs

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(Wrong way up.)

1/(tau_i/tau_leak + j omega f tau_i).

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Dr Philip C D Hobbs 
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Phil Hobbs

Huh, so sine waves (of power) (with some DC offset) into my heater and just measure the temperature as a function of time?

I always listen to Phil.

Grin well we teach what we know.... There's much more for the kids to do so this is just a small part. It doesn't have to be a "great tune".

George H.

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George Herold

Right, I guess I'm more conservative now too. I had some TEC's in a loop that got too "ringy" in production. (I never figured out if it was the TEC's, how hard they were clamped, or something else... doesn't matter now.)

George H.

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George Herold

Right I did the step response like that... here's the link again.

formatting link

I've got 'scope shots for longer times, but it looks like a single RC. (I run out of 'scope time base for weak links.)

Grin.. no offense Phil, but I often feel like you're three steps ahead of me. you're running around and I'm still trying to figure out how to stand up and not fall over. I want the "Junior" Phil Hobbs book. (The Junior comes about 'cause back in grad school, I was studying Thermo on my own. I had Reif's classic on Thermo, but I really liked the thermo book that he did for the Berkeley Series. We called it Reif Jr. just my speed.)

George H.

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George Herold

As long as the temperature isn't getting out of hand, yes.

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Tim Wescott

OK that's easy. I want to add an input (back panel) that will let people look at the step response. Since that will also allow them to do a sine wave response also, I'm going to mark that down as, "giving them the tools to characterize the system." :^)

George H.

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George Herold

If you just want to characterize the plant, then you drive the heater and measure the temperature.

The classic way to equip a circuit for a control systems analyzer is to scatter a few summing junctions around (like in the article). Put the output over the return, and you have the loop gain. Put the return over the analyzer drive, and you have the loop response. Put the output over the analyzer drive, and you have the loop sensitivity. Or drive where convenient, and look at the actual command to the driver (heater in your case) and the return from the measured process variable (temperature in your case), and you have the plant transfer function.

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Tim Wescott

(The Junior comes

ermo

?

Kudos to him for making the effort to do that.

I just signed the contract for BEOS's third edition. I'm planning to shop t he new book around once I make some more progress on it, due to, um, differ ences of opinion on the reasonableness of a few of the terms.

One of the reasons I have to write concisely is that it covers a fairly bro ad range of topics, and the proverbial "a mile wide and an inch deep" appro ach is pretty well useless.

The closest thing I know of is Mark Johnson's "Photodetection and Measureme nt". It's a great introductory book that covers maybe 20% of the topics.

Cheers

Phil Hobbs

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Phil Hobbs

thermo

the new book around once I make some more progress on it, due to, um, diff erences of opinion on the reasonableness of a few of the terms.

road range of topics, and the proverbial "a mile wide and an inch deep" app roach is pretty well useless.

ment". It's a great introductory book that covers maybe 20% of the topics. Yeah that's a nice book. I wasn't really serious about Hobbs Jr. You do tend to drill down deep into a subject. (which is a good thing.) But if I'm not at your starting depth, then I've gotta back up and dig out your references on the subject... etc...

I never bought the 2nd ed. of BEOS, (maybe the second hand price will come down when you release the 3rd. :^) How much do you expect to be newer, updated stuff in the 3rd? (Maybe I can spring for it.)

Hey you also mumbled about a "twenty projects" book. Is that still in the works?

George H.

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George Herold

The draft third edition is revised throughout, and has probably 50-75 pages of new stuff. It'll be out in a year or so, God willing. The second edition had about 100 pages more than the first, but Wiley shrunk the font and the margins, so it doesn't look like as much as it is.

That's the new one that I couldn't get acceptable contract terms on from W. (They insist on being indemnified, and it's not your father's indemnity clause, either.)

To send it to another publisher, I need to have more to show than I do at the moment.

Cheers

Phil Hobbs

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Phil Hobbs

Imdemnity, meaning if someone blasts their eye out with a laser 'because' of your book they want you to be on the hook?

George H.

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George Herold

That's your common or garden indemnity. This one is even uglier. Their lawyer argued with me for half an hour on the phone before finally admitting she didn't have the authority to change it, so I walked.

Cheers

Phil Hobbs

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Phil Hobbs

That's nuts.

Quote from _my_ lawyer "Indemnity means insurance. You're not an insurance company, Tim". Nuff said.

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Tim Wescott

Yup. I don't grant indemnities, but 've insisted on receiving them sometimes, when I'm building something that's dangerous in unskilled hands. Once was when I was building a proto that weighed about 40 pounds and emitted a 1-W YAG laser beam (1.064 um in the near IR), focused about 2 feet outside the box. I could just picture somebody damaging an eye or dropping it on his foot, filing a worker's comp claim, and then their insurance company coming after me.

The client was amenable when I explained that I wasn't looking for a get-out-of-jail-free card, just the same level of liability protection that their regular employees had. That project went dormant for a year or so but may be coming back to life.

Cheers

Phil Hobbs

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Phil Hobbs

I'm not sure how it could be uglier, but maybe you can't talk about it.

Way back when someone came to us with a kinda nice diode pumped Q-switched laser.. (lased green). He had a few experiments to hang on the end, but mostly it would be about how the laser worked. But the power was just to scary for us to put into a student lab. (I can't remember the numbers.)

George H.

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George Herold

.

Another nice fishhook was that they could handle the case any way they chos e, including unlimited authority to settle using my money, without my havin g any input.

Cheers

Phil Hobbs

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Phil Hobbs

ed.

ose, including unlimited authority to settle using my money, without my hav ing any input.

Oh nice, and I assume that isn't just money they owe you for royalties, but your house and car too. (Or your business if you have a LLC)

Do people sue publishers regularly? To see it from their side, I'm guessing they don't make a lot of money from one of your books. (Doesn't sell like AoE3.)

George h.

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George Herold

Well, there's libel and plagiarism, plus things like publishing somebody's IP when you're under NDA.

There's never been a claim lodged against BEOS in the 17 years it's been out, which is why I'm not too worried about that one, but the new book will have a lot of stuff from consulting gigs. Of course I'm going to run each section past the relevant customer before publishing it. My primary threat model is Customer B reading Customer A's section and thinking that I'm blabbing B's IP, but it could be just somebody with a lot of money who doesn't like me. (Fortunately I know of none such, but one never can tell.)

Cheers

Phil Hobbs

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