Tim Wescott wrote:> Fred Bloggs wrote: >... snip ...>> >> TW must be talking about WPI http://www.wpi.edu/ , pronounced >> Wuss-duh in those parts. >> > If it was me it was supposed to be off the record. > > My Thesis advisor once told me that the first time he visited WPI > he spent about an hour on the interstate -- he was looking for > Wu'sta. He passed by Wor-ches-ter several times before it sunk in...What happened when he needed fish from Gloss-tuh? -- "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: <http://cfaj.freeshell.org/google/> Also see <http://www.safalra.com/special/googlegroupsreply/>
Closing Control Loops
Started by ●February 24, 2006
Reply by ●February 26, 20062006-02-26
Reply by ●February 26, 20062006-02-26
On Sat, 25 Feb 2006 20:34:04 -0800, Tim Wescott <tim@seemywebsite.com> wrote: [snip]> >My Thesis advisor once told me that the first time he visited WPI he >spent about an hour on the interstate -- he was looking for Wu'sta. He >passed by Wor-ches-ter several times before it sunk in...Pronounced just like the sauce ;-) And sort of like Gloucester. Then there's Wooster, OH ;-) ...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 by ●February 26, 20062006-02-26
CBFalconer wrote:> Tim Wescott wrote: > >>Fred Bloggs wrote: >> > > ... snip ... > >>>TW must be talking about WPI http://www.wpi.edu/ , pronounced >>>Wuss-duh in those parts. >>> >> >>If it was me it was supposed to be off the record. >> >>My Thesis advisor once told me that the first time he visited WPI >>he spent about an hour on the interstate -- he was looking for >>Wu'sta. He passed by Wor-ches-ter several times before it sunk in... > > > What happened when he needed fish from Gloss-tuh?Does he slaver it with war-sester-shire saucr? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 26, 20062006-02-26
Paul Carpenter wrote:> On Saturday, in article > <lZ6dnS0pVYzZA53ZnZ2dnUVZ_v-dnZ2d@web-ster.com> > tim@seemywebsite.com "Tim Wescott" wrote: >--snip-->>>Hopefully your book will not be like that. >>> >> >>I tried to keep it grounded in reality, to live up to the "Applied" in >>the title. Most of the examples are simplified, so that I can talk to >>the point that I'm trying to make, but I try to keep the other issues >>visible. > > > I realise that there has to be simplification, but a lot of theory does > not USUALLY cover the applied like what to do when an external sensor > or feedback in the loop says there is a fault and how to make sure the > loop (and the system) return to the correct state (whatever that may be). > > An example would be printing shop guillotines that usually require two > spaced out buttons to be pressed at the same time for the whole of the cut > process. On the basis that if both hands are on the buttons then the > operator's hands are not in the machinery! Now dpending on how the system > operates there are basically three safety stop states - stop, move back > a little or return to start position. > > I have seen some control systems really screwed by an 'abort' function > that need complete restarts or worse after that, basically because the > control loop hardware/software could not effectively reset correctly. >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 think there's a place for simplistic models -- I've done motion >>controllers where the emphasis was on getting a mechanism from point A >>to point B without toasting any electronics or breaking/jamming the >>mechanism, and accuracy was taken care of by other parts of the system. >> In those cases a very simple model that ignored such complications >>like backlash and friction worked very well -- as long as I remembered >>what the limitations of the model were. I tried to keep up a continuing >>theme that you need to remember what assumptions you made about your >>model, and what those assumptions mean. > > > Quite often I have seen customers who do not understand the limitations > and assumptions in their systems let alone any model they think they have > created. > > Some of the major control loop problems I have seen is applying open loop > blocks to an overall system that is closed loop. Mainly because they > did not understand the limits of the blocks they were using, most > notably delays, determinicity and sampling restraints. >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 Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●February 26, 20062006-02-26
On Sun, 26 Feb 2006 09:12:37 -0800, Tim Wescott <tim@seemywebsite.com> wrote:>Paul Carpenter wrote:[snip]>> >> Some of the major control loop problems I have seen is applying open loop >> blocks to an overall system that is closed loop. Mainly because they >> did not understand the limits of the blocks they were using, most >> notably delays, determinicity and sampling restraints. >> >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).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 by ●February 26, 20062006-02-26
Tim Wescott wrote:> 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). >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. http://www.dodsbir.net/sitis/archives_display_topic.asp?Bookmark=19430
Reply by ●February 26, 20062006-02-26
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:waCdnbrQguUnspzZRVn-rw@web-ster.com...> Fred Bloggs wrote: > > -snip- > >>> I love >>> "Readership: PRIMARY MARKET: ... >>> SECONDARY MARKET: Engineering students at the *more >>> practical* engineering and technical schools; ..." >>> emphasis added ;/ >>> >>> Lets out my Alma Mater. >> >> >> TW must be talking about WPI http://www.wpi.edu/ , >> pronounced Wuss-duh in those parts. >> > If it was me it was supposed to be off the record. > > My Thesis advisor once told me that the first time he > visited WPI he spent about an hour on the interstate -- he > was looking for Wu'sta. He passed by Wor-ches-ter several > times before it sunk in... >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 by ●February 26, 20062006-02-26
John E. Hadstate wrote:> 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."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 by ●February 26, 20062006-02-26
"Jerry Avins" <jya@ieee.org> wrote in message news:fvudnYRrKteSY5zZnZ2dnUVZ_tydnZ2d@rcn.net...> John E. Hadstate wrote: > >> 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." > > 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. >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 by ●February 26, 20062006-02-26
"John E. Hadstate" <jh113355@hotmail.com> wrote in message news:zjnMf.31285$Ly6.7326@bignews5.bellsouth.net...> > "Jerry Avins" <jya@ieee.org> wrote in message > news:fvudnYRrKteSY5zZnZ2dnUVZ_tydnZ2d@rcn.net... >> John E. Hadstate wrote: >> >>> 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." >> >> 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. >> > > 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. > > >Yes, I am use LabView so I am have no problims. Evin cumputer get burn up coz ov over monkey mistak. :) :).... Also hav 'honest' backup what no-wun am figur owt. Bonus, am also iso9005.02.03 Nise Coffeeee plus Doner Kebab...... Easy. DNA
