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Shameless Plug

Started by Tim Wescott November 29, 2004
Tim Wescott wrote:

>I fear that my mind was poisoned long ago by a German instructor who >pointed out that modern linguistic theory doesn't much recognize a >"right way" and a "wrong way" -- it just records prevalent usage, and >tries to keep out of the way of the steamroller.
I tend to agree, but the "wrong way" hinders communication if it is *too* different from the "right way." If one decides to use a few non-standard fleemishes and the reader can still gloork the meaning from the context, but there ix a limit; If too many ot the vleeps are changed, it becomes harder and qixer to fllf what the wethcz is blorping, and evenually izs is bkb longer possible to ghilred frok at wifx. Dnighth? Ngfipht yk ur! Uvq the hhvd or hnnngh. Blorgk? Blorgk! Blorgkity-blorgk!!!!
Tim Wescott wrote:

>Guy Macon wrote: > >> "Why do some controller boards have an option to reverse the >> phase of the D? What is that good for?" > >OK, I'll bite -- what _is_ it good for? I've never done closed-loop >control with prepackaged controllers and I've never seen that done >elsewhere. I can certainly see reversing the phase of the whole thing, >or reversing the phase of the D term if it's coming from some other >feedback source (which would imply a second input) but I _can't_ see the >point in intentionally establishing an unstable zero in your control system.
My experience is more hands-on than theory, but here is the answer I gave my classes: In N years of setting up servos, I have never once found a use for it, nor have I found any literature that explains when it might be of some use. I think that somewhere back in the early days someone was told to put in a jumper that reverses the phase of the entire servo (quite handy when someone miswired a section that is really hard to get to), got it wrong, and some other manufacturers have been copying the "feature" ever since. If one of the theory boys has a better answer, I am all ears. -- Guy Macon <http://www.guymacon.com>
Guy Macon wrote:
> Tim Wescott wrote: > > >>I fear that my mind was poisoned long ago by a German instructor who >>pointed out that modern linguistic theory doesn't much recognize a >>"right way" and a "wrong way" -- it just records prevalent usage, and >>tries to keep out of the way of the steamroller. > > > I tend to agree, but the "wrong way" hinders communication if it is > *too* different from the "right way." If one decides to use a few > non-standard fleemishes and the reader can still gloork the meaning > from the context, but there ix a limit; If too many ot the vleeps > are changed, it becomes harder and qixer to fllf what the wethcz > is blorping, and evenually izs is bkb longer possible to ghilred > frok at wifx. Dnighth? Ngfipht yk ur! Uvq the hhvd or hnnngh. > Blorgk? Blorgk! Blorgkity-blorgk!!!! > >
Well, that's why if I know there's two markedly different ways if saying the same thing I'll footnote it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Guy Macon wrote:

> Tim Wescott wrote: > > >>Guy Macon wrote: >> >> >>> "Why do some controller boards have an option to reverse the >>> phase of the D? What is that good for?" >> >>OK, I'll bite -- what _is_ it good for? I've never done closed-loop >>control with prepackaged controllers and I've never seen that done >>elsewhere. I can certainly see reversing the phase of the whole thing, >>or reversing the phase of the D term if it's coming from some other >>feedback source (which would imply a second input) but I _can't_ see the >>point in intentionally establishing an unstable zero in your control system. > > > My experience is more hands-on than theory, but here is the answer > I gave my classes: > > In N years of setting up servos, I have never once found a use for > it, nor have I found any literature that explains when it might be > of some use. I think that somewhere back in the early days someone > was told to put in a jumper that reverses the phase of the entire > servo (quite handy when someone miswired a section that is really > hard to get to), got it wrong, and some other manufacturers have > been copying the "feature" ever since. > > If one of the theory boys has a better answer, I am all ears. >
Being able to reverse the sign of the whole thing is good -- one of the old curmudgeonly engineers from whom I learned practical control liked to say that when designing one of these things you should count up all the sign changes in the loop -- then throw in one extra for the one you missed. His circuits always had at least one spot where you could rearrange the inputs to an op amp and reverse the sense of a signal. I follow that now: there's always at least one place in my software where one can insert a '-' and change the sign of the whole thing. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Wed, 01 Dec 2004 07:14:01 +1300, the renowned Bruce Durdle
<bmdurdle@clear.net.nz> wrote:

>Yestedy i culdnt spel "Enginie - Now i are wun.
I like the French word for engineer better: "ing&#2013265929;nieur" Sounds like it has more to do with ingenuity than pistons and connecting rods. Of course, the Latin root "ingenium" (ability?) is the same for both words. 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
Spehro Pefhany wrote:

> On Tue, 30 Nov 2004 18:49:28 +0000, the renowned Guy Macon > <http://www.guymacon.com> wrote: > > >>Tim Wescott wrote: >> >> >>>Guy Macon wrote: >>> >>> >>>> "Why do some controller boards have an option to reverse the >>>> phase of the D? What is that good for?" >>> >>>OK, I'll bite -- what _is_ it good for? I've never done closed-loop >>>control with prepackaged controllers and I've never seen that done >>>elsewhere. I can certainly see reversing the phase of the whole thing, >>>or reversing the phase of the D term if it's coming from some other >>>feedback source (which would imply a second input) but I _can't_ see the >>>point in intentionally establishing an unstable zero in your control system. >> >>My experience is more hands-on than theory, but here is the answer >>I gave my classes: >> >>In N years of setting up servos, I have never once found a use for >>it, nor have I found any literature that explains when it might be >>of some use. I think that somewhere back in the early days someone >>was told to put in a jumper that reverses the phase of the entire >>servo (quite handy when someone miswired a section that is really >>hard to get to), got it wrong, and some other manufacturers have >>been copying the "feature" ever since. >> >>If one of the theory boys has a better answer, I am all ears. > > > According to Lipt&#2013265921;k, it's primarily used to introduce a lag into the > control loop when the process is very fast or noisy. > > > Best regards, > Spehro Pefhany
I'll search for that. Have you ever used it? It seems like introducing a low-pass would be a much better way to kill noise and add lag at the same time. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Tue, 30 Nov 2004 18:49:28 +0000, the renowned Guy Macon
<http://www.guymacon.com> wrote:

> >Tim Wescott wrote: > >>Guy Macon wrote: >> >>> "Why do some controller boards have an option to reverse the >>> phase of the D? What is that good for?" >> >>OK, I'll bite -- what _is_ it good for? I've never done closed-loop >>control with prepackaged controllers and I've never seen that done >>elsewhere. I can certainly see reversing the phase of the whole thing, >>or reversing the phase of the D term if it's coming from some other >>feedback source (which would imply a second input) but I _can't_ see the >>point in intentionally establishing an unstable zero in your control system. > >My experience is more hands-on than theory, but here is the answer >I gave my classes: > >In N years of setting up servos, I have never once found a use for >it, nor have I found any literature that explains when it might be >of some use. I think that somewhere back in the early days someone >was told to put in a jumper that reverses the phase of the entire >servo (quite handy when someone miswired a section that is really >hard to get to), got it wrong, and some other manufacturers have >been copying the "feature" ever since. > >If one of the theory boys has a better answer, I am all ears.
According to Lipt&#2013265921;k, it's primarily used to introduce a lag into the control loop when the process is very fast or noisy. 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
On Tue, 30 Nov 2004 11:08:27 -0800, the renowned Tim Wescott
<tim@wescottnospamdesign.com> wrote:

> >I'll search for that. Have you ever used it? It seems like introducing >a low-pass would be a much better way to kill noise and add lag at the >same time.
I've not used it. I may have run into a case, once, where it would have come in handy. The controller really had to be detuned to make it stable. 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
"Tim Wescott" <tim@wescottnospamdesign.com> wrote
> Guy Macon wrote: > > "Why do some controller boards have an option to reverse the > > phase of the D? What is that good for?" > OK, I'll bite -- what _is_ it good for?
True negative derivative is called (negative) velocity feedback and is used to decrease response time at the expense of overshoot. The frequency response of the system goes up. The system becomes stiff but 'nervous'. Can be used to stabilize systems as the same stiffness can be had with a lower proportional gain. This is just what one would expect as plain-ole' derivative feedback is used to lessen overshoot at the expense of slower response time. Think of D as oil: it can be used to slow something down by being thick and greasy or it can speed something up by lubricating it. Speculation: if the controller is configurable to use derivative on error _or_ derivative on process then the sign of the derivative at the summing junction must change, most modern controllers do this automatically in software but I suppose there are those that don't. The same is true if feed-forward on SP is available.
> but I _can't_ see the point in intentionally establishing an > unstable zero in your control system.
Not so simple ... it doesn't necessarily make the system unstable: think Nyquist diagram.
> > Another gotcha that sometimes trips up software engineers: > > non-monotonic ADCs causing a "bad spot" that has positive > > feedback.
I'm not sure what this has to do with software, except to know to shout 'Hardware Error!' and go for a cup of coffee. FWIW: dual slope and V/F are preferred conversion techniques for process control as there are no 'bad spots'. Also work a charm if synched to the line frequency.
> Yes, this could be _very_ counter-intuitive to my target audience.
Er, maybe a new audience is needed? The only requirements for control are monotonicity and repeatability, everything else is icing on the cake (though some sort of linearity is _really_ nice to have). -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/
On Tue, 30 Nov 2004 10:56:08 -0800, Tim Wescott wrote:
> Guy Macon wrote: >> Tim Wescott wrote: >> >>>I fear that my mind was poisoned long ago by a German instructor who >>>pointed out that modern linguistic theory doesn't much recognize a >>>"right way" and a "wrong way" -- it just records prevalent usage, and >>>tries to keep out of the way of the steamroller. >> >> I tend to agree, but the "wrong way" hinders communication if it is >> *too* different from the "right way." If one decides to use a few >> non-standard fleemishes and the reader can still gloork the meaning from >> the context, but there ix a limit; If too many ot the vleeps are >> changed, it becomes harder and qixer to fllf what the wethcz is >> blorping, and evenually izs is bkb longer possible to ghilred frok at >> wifx. Dnighth? Ngfipht yk ur! Uvq the hhvd or hnnngh. Blorgk? >> Blorgk! Blorgkity-blorgk!!!! >> > Well, that's why if I know there's two markedly different ways if saying > the same thing I'll footnote it.
I'd think that at least in the case of PID, the "right" way would be the one that most accurately reflects what the thing is doing. Just from a high-school algebra edjamacaited bench tech, it sounds like "derivative" is more the effect you're going after - a "differential" is something else entirely - it's not even on the same axis, as it were. But anyway, now that I know which word "should" be used, do I have to go to school for four years to grasp the transfer function, or is it basically a critically-damped filter with a cutoff freq of .01 Hz? And, of course, you've got the asymmetric hysterisis and all that stuff to contend with - basically, the control circuit has to predict the future. ;-) Thanks! Rich