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Closing Control Loops

--snip--

If you were to write an article about this (beyond "think hard about your problem and deal with sensor failures") I'd be delighted to read it.

I haven't really addressed this kind of thing in detail, because I've mostly worked with loops that lack redundant sensors and which were inherently mechanically safe. I _do_ have an abhorrence of controllers with modes, because the mode change always seems to be awkward at best. I suspect that a modeless controller is going to be inherently better positioned to deal with sensors coming and going (not perfectly, just better, often).

I don't think it's just that people don't understand the limits of their models. I think that people are so used to academic problems that they forget that their models might possibly not be sufficiently accurate. There's at least a few places in the book where the phrases "if your model is accurate enough" or "if your linear model applies" appear. I also made a point in the chapter on dealing with nonlinearities of showing what happens when you blithely use a linear model to a design a controller for a nonlinear system (a big scary/embarrassing oscillation, in the example).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott
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In this case if there was a Roman camp it was _really_ obscure: it's Worcester Massachusetts, no doubt named after the 'real' Worcester. Any 'original' name would have been from the local Native American tribes.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

Who were slaughtered in their sleep by the English settlers...

Reply to
Fred Bloggs

The English stole it from the Nipmuck Indians with some funny money and cloth. The Indians had no concept of exclusive ownership, they thought the English were buying the right to share, and killed more than a few of them when they found what they were about.

Reply to
Fred Bloggs

[snip]

Indeed! Though I don't use the material on any regular basis (*), I had four semesters on non-linear control theory in grad school. Great fun, the real world!

(*) Though I recently was involved in a laser beam "wobulator"... a mirror driven by a power amplifier... great fun tuning it up for stability ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.
Reply to
Jim Thompson

You should apply for this one next go-round. There might be big bucks in it for you. The deal is to stabilize the internal mirror pointing system against acoustic wave interference induced into the turret by platform motion through the air.

formatting link

Reply to
Fred Bloggs

When I went to Woopie Tech, it was "guys only". The nearest "gulls" were at some sort of prep school just down the street that ran through the center of campus.

And now I have to add my favorite rant: I hope you discussed at some length the fact that every control loop needs an auditor, a second, independent measurement device that is not involved in closing the loop. The reason for this is that once you have closed the loop, you cannot use that measurement to know anything about what's really going on in the process over the long term. If the controller is functioning properly, the measurement (at the controller) will be forced to track the setpoint no matter what is actually happening in the real world. Providing such auditors used to be standard practice in the process control industry until about 15 years ago when engineers lost control of their plants and bean counters started "cost-reducing" everything. In fact, we used to have RTDs designed with that in mind: one RTD to control and one to audit in the same probe.

Also, of course, we had the clever E&I technicians. Operators would complain that the controller measurements weren't the same as the auditor measurements, so some dork with a 12-inch screwdriver would "re-calibrate" the RTD-to-current-loop converters to make the auditors "read right."

Reply to
John E. Hadstate

If sensors are in the same place, they ought to read the same value. Audit probes did me the most good when they were located somewhat apart from the control probe.

Jerry

--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

No!! Between the sensors and the computer is usually a device ("converter" or "amplifier") to produce a 4-20 mA (or

10-50 mA) signal from whatever the sensor produces (resistance, millivolts, etc.) Back in the computer room, there is often another converter to change the 4-20 mA signal to something like 0-10 Volts or 1-5 Volts. These devices almost always have "Span" and "Zero" adjustments accessible to "the dork with the 12-inch screwdriver". Consequently, without a serious threat to "break fingers", these converters are often used to make the computer say whatever the operators want it to say.
Reply to
John E. Hadstate

BIG SNIP

Oh yes. 30 years ago, when I was in Control Engineering at a large, nearly dead company that makes photographic film, we had a constant battle wuth the Instrument Techs, who on average knew absolutely ZERO.

Standard procedure was to slip the pens on the shaft until the pen and pointer agreed. What they agreed ON was irrelevant. Calibrations were all "end-to end;" there was no telling anything from the 4-20 mA loop signal. 50 percent of PV would indicate something near 50 on the instrument, but the loop might be at 16mA!

We had an area full of high-end environmental chambers, which were capable of great precision when properly set up. USUALLY, however, you would look in the chamber and find a $1.99 desktop thermometer/hygrometer that the operator put in as a check on the $10,000 instruments whish were ALWAYS completely fouled up. Head of the temperature calibration lab was a kindergarten teacher with no technical education or credentials, who hired technicians in her image.

Strange thing, that company still can't understand why their costs are so high. Lost big money for 5 quarters in a row now.....

Reply to
BFoelsch

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

and Warrik (RI) ...

Reply to
Fred Marshall

I used to take the New Haven... RR from Boston to DC, then switch to the C&O on into Huntington. Trying to figure out those conductor calls was quite a challenge.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
     It\'s what you learn, after you know it all, that counts.
Reply to
Jim Thompson

The train wicket guy in the UK who sold me the ticket to 'Lie-stir' looked at me funny, but sold the ticket to Leicester nonetheless.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I disn't say they /would/ read the same, but that thay /ought/ to. When two thermometers in the same stirred pot of soup indicate different temperatures, at least one of them is wrong. Loop or no loop.

Jerry

--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

Montreal streetcars (remember them) used to roll along St. Catherine until they arrived at "GeeGuy". Sometimes "Laroogeeguy". Those are hard 'g's.

--
"If you want to post a followup via groups.google.com, don\'t use
 the broken "Reply" link at the bottom of the article.  Click on 
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 "Reply" at the bottom of the article headers." - Keith Thompson
More details at: 
Also see
Reply to
CBFalconer

... snip ...

Or you have an unusually busy population of Maxwells demons.

--
"If you want to post a followup via groups.google.com, don\'t use
 the broken "Reply" link at the bottom of the article.  Click on 
 "show options" at the top of the article, then click on the 
 "Reply" at the bottom of the article headers." - Keith Thompson
More details at: 
Also see
Reply to
CBFalconer

I don't know if I could really class my thoughts and experiences as an 'article' (so many different meanings to article as in length and depth of content). I could probably summarise a few of the major ones in a 'War stories' page sometime.

.....

Common 'solutions' I hear too often from people (either customers or those 'advising' them) are

"just put a big computer in there that will solve the problem"

or "there are GigaBytes of bandwidth available in the RF spectrum"

or "This will be solved by the promised new version of product X"

If you want I could outline describe in email the worst closed loop complete system with lots of open loop technology in the way that people were asking to change things to make it better. I would asked to think about what could be done with modifications to a company's products to give an answer to the problem, they did not like my response, and got all three of the above responses.

--
Paul Carpenter          | paul@pcserviceselectronics.co.uk
    PC Services
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Reply to
Paul Carpenter

I cot confused in Bawston when the MTA dispatcher told me to take the Pack kah (rhymes with kaka). It took a while to sort that out.

Jerry

--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

I must say that I've had this experience with folks who should have known better. When asked: "How are you going to do that?" they would say: "well, there will be a computer inside". They had NO idea what they were going to do or how they would do it. If the SNR was infinitely low, "well, there will be a computer inside".

Fred

Reply to
Fred Marshall

Sort of the general case of the "put a PID controller in there", which somehow answers all the questions about sensor and actuator response etc. ;-)

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

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