On Saturday, May 21, 2016 at 10:04:20 AM UTC+12, Tim Wescott wrote:
> On Fri, 20 May 2016 16:29:39 -0500, Tim Wescott wrote:
>
> > On Fri, 20 May 2016 10:56:33 -0700, gyansorova wrote:
> >
> >> On Friday, May 20, 2016 at 5:57:43 PM UTC+12, Tim Wescott wrote:
> >>> On Thu, 19 May 2016 15:40:51 -0700, gyansorova wrote:
> >>>
> >>> > On Friday, May 20, 2016 at 7:49:55 AM UTC+12, Tim Wescott wrote:
> >>> >> On Thu, 19 May 2016 10:54:03 -0700, gyansorova wrote:
> >>> >>
> >>> >> > On Thursday, May 19, 2016 at 4:58:38 PM UTC+12, Tim Wescott
> >>> >> > wrote:
> >>> >> >> On Wed, 18 May 2016 14:44:23 -0700, gyansorova wrote:
> >>> >> >>
> >>> >> >> > On Thursday, May 19, 2016 at 9:35:10 AM UTC+12, Phil Hobbs
> >>> >> >> > wrote:
> >>> >> >> >> On 05/18/2016 05:12 PM, Tim Wescott wrote:
> >>> >> >> >> > On Wed, 18 May 2016 10:05:16 +1000, Clifford Heath wrote:
> >>> >> >> >> >
> >>> >> >> >> >> On 18/05/16 01:54, Tim Wescott wrote:
> >>> >> >> >> >>> On Tue, 17 May 2016 08:15:42 -0500, John S wrote:
> >>> >> >> >> >>>> For your next demo, use an electromagnet to lift a metal
> >>> >> >> >> >>>> ball and hold it suspended. Sense the height with a
> >>> >> >> >> >>>> light sensor. Use PID to achieve stability.
> >>> >> >> >> >>> I've done that. You need a honkin' big electromagnet to
> >>> >> >> >> >>> make it work with a plain steel load.
> >>> >> >> >> >>
> >>> >> >> >> >> Could you use a smaller electromagnet below, partly
> >>> >> >> >> >> canceling the pull of a rare-earth magnet from above?
> >>> >> >> >> >
> >>> >> >> >> > Yes. IMHO, a thingie that's hanging suspended from a point
> >>> >> >> >> > looks more impressive than a thingie that's hanging
> >>> >> >> >> > suspended between two points.
> >>> >> >> >> >
> >>> >> >> >> > A thingie that's floating _above_ a point is more
> >>> >> >> >> > impressive yet, but if you do that then all of a sudden you
> >>> >> >> >> > have to control it in three dimensions instead of one (or
> >>> >> >> >> > perhaps six if you can't make it inherently stable in
> >>> >> >> >> > rotation). You can float an aluminum pan above an
> >>> >> >> >> > electromagnet and have it be stable, but you can't do the
> >>> >> >> >> > same thing with plain old magnets.
> >>> >> >> >> >
> >>> >> >> >> >
> >>> >> >> >> You can, actually, if the object is sufficiently diamagnetic,
> >>> >> >> >> such as pyrolytic graphite. I have a little demo on the
> >>> >> >> >> shelf over my lab bench that levitates a small sheet of
> >>> >> >> >> graphite over four NdFeB magnets arranged in a quadrupole. I
> >>> >> >> >> posted a video a few years back.
> >>> >> >> >>
> >>> >> >> >> When physicists visit, I give them a spiel about room
> >>> >> >> >> temperature superconductors and the Meissner effect...the
> >>> >> >> >> size of the double-take goes linearly with how much physics
> >>> >> >> >> they know. ;)
> >>> >> >> >>
> >>> >> >> >> Cheers
> >>> >> >> >>
> >>> >> >> >> Phil Hobbs
> >>> >> >> >>
> >>> >> >> >> --
> >>> >> >> >> Dr Philip C D Hobbs Principal Consultant ElectroOptical
> >>> >> >> >> Innovations LLC Optics, Electro-optics, Photonics, Analog
> >>> >> >> >> Electronics
> >>> >> >> >>
> >>> >> >> >> 160 North State Road #203 Briarcliff Manor NY 10510
> >>> >> >> >>
> >>> >> >> >> hobbs at electrooptical dot net http://electrooptical.net
> >>> >> >> >
> >>> >> >> > PID is quick and nasty but good to get something working.
> >>> >> >> > Proper classical way is by lag-lead controllers and
> >>> >> >> > integrators all cascaded and by using a Bode plot, phase
> >>> >> >> > margin etc.
> >>> >> >>
> >>> >> >> Terminology, terminology.
> >>> >> >>
> >>> >> >> Give me a transfer function of your "lag-lead controllers and
> >>> >> >> integrators all cascaded".
> >>> >> >>
> >>> >> >> It's of the form
> >>> >> >>
> >>> >> >> a2 * s^2 + a1 * s + a0
> >>> >> >> H(s) = ----------------------
> >>> >> >> s ( s + b )
> >>> >> >>
> >>> >> >> right?
> >>> >> >>
> >>> >> >> So -- that's the transfer function for a PID, with a
> >>> >> >> band-limited derivative term. Plain ol' PID. Nuthin' special
> >>> >> >> about it except for the fancified language you want to use to
> >>> >> >> describe it.
> >>> >> >>
> >>> >> >> Same thing. Different words. Words are nothing. Reality is
> >>> >> >> everything. PID = "lag-lead controllers and integrators all
> >>> >> >> cascaded",
> >>> >> >> only "PID" is shorter.
> >>> >> >>
> >>> >> >> Now, a PID that's _tuned_ using the seat-of-the-pants method --
> >>> >> >> that can be improved on with the classical Bode plot method
> >>> >> >> using gain & phase margins. But it's still a PID -- or a
> >>> >> >> lead-lag controller with an integrator, all cascaded, if that's
> >>> >> >> the only way you can retain your sanity. But no matter what
> >>> >> >> words you use, the resistors and caps do not change: the
> >>> >> >> description does not change the circuit,
> >>> >> >> or the code, or the pneumatic cylinders, or whatever you use to
> >>> >> >> implement the controller.
> >>> >> >>
> >>> >> >> --
> >>> >> >>
> >>> >> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com
> >>> >> >>
> >>> >> >> I'm looking for work -- see my website!
> >>> >> >
> >>> >> > Wrong! It is more than ordinary PID. Of course you can emulate a
> >>> >> > PID that way as you point out, but that is just the starting
> >>> >> > point. Much os this stuff is not generally not understood by the
> >>> >> > great unwashed and does not appear in the text books. Here goes.
> >>> >> >
> >>> >> > What is an ideal Bode plot? That is the first question to ask
> >>> >> >
> >>> >> > Second question is what limits the bandwidth of say an
> >>> >> > electro-mechanical system?
> >>> >> >
> >>> >> > An idea Bode plot would be a vertical straight line 9except the
> >>> >> > close look system woudl be unstable)
> >>> >> >
> >>> >> > Limiting factor on bandwidth is structural resonance(s).
> >>> >> >
> >>> >> >
> >>> >> > Hence you need multiple integrators at low frequency to make your
> >>> >> > Bode plot as steep as possible. Likewise you can use multiple
> >>> >> > phase-advances too but with caution since you disrupt the
> >>> >> > structural resonance. (phase advance has high frequency gain of
> >>> >> > course).
> >>> >> >
> >>> >> > So a PID is rather limited. The order of a controller could be
> >>> >> > much higher depending on just how many integrators you can
> >>> >> > squeeze in at low frequency and how many phase advances. If you
> >>> >> > can't fit an integrator in then you need a phase-lag which has
> >>> >> > less phase problems.
> >>> >> >
> >>> >> > Anyway, this is the art of the control engineer which has been
> >>> >> > around since the 70s or so but is kind of lost with todays PID
> >>> >> > thinking. Nothing wrong with PID of course if you are happy with
> >>> >> > the result and like second best, but you can do so much better.
> >>> >> >
> >>> >> > There is not enough time or space to explain all of it. I only
> >>> >> > know it myself because an old hard disk designer explained it to
> >>> >> > me thirty years ago. I and you know the theory already, it's just
> >>> >> > the art of it all that is pretty smart.
> >>> >> > It's like lego, putting in extra terms here and there and
> >>> >> > watching your phase budget - push up bandwidth like a violin
> >>> >> > string. None of it is in textbooks because such people don't
> >>> >> > write text books, they like keeping it trade secret!
> >>> >>
> >>> >> Semantics again. With the exception of a double integrator (which
> >>> >> isn't always a good idea) you're describing my "PID with
> >>> >> decoration"
> >>> >> and saying "that's not PID".
> >>> >>
> >>> >> And just as a note: multiple lead-lags only work to the extent that
> >>> >> the high-frequency behavior of the plant is consistent and
> >>> >> well-mannered. You can easily design monstrously frustrating lab
> >>> >> queen if you don't pay attention to this. Systems that only work
> >>> >> at room temperature, or when they're freshly maintained, or when
> >>> >> the operator doesn't bang on them too hard are not systems that
> >>> >> make the people who sign checks happy.
> >>> >>
> >>> >> --
> >>> >>
> >>> >> Tim Wescott Wescott Design Services http://www.wescottdesign.com
> >>> >>
> >>> >> I'm looking for work -- see my website!
> >>> >
> >>> > Double integrators are always a good idea. The have an enormous gain
> >>> > at low frequencies and hence reject disturbances.
> >>>
> >>> Whilst, and at the same time, causing enhanced disturbance response
> >>> somewhere else. See Bode's Sensitivity Integral for details.
> >>>
> >>> > What you are talking about is process control of relatively slow
> >>> > processes - which is fair enough.
> >>>
> >>> I'm so happy for you that you're psychic. But maybe you need to brush
> >>> up on the mind-reading. Read on...
> >>>
> >>> > I am talking high speed elec mech drives like a hard disk head.
> >>> > Speed is of the essence and you must maximize the bandwidth.
> >>>
> >>> I'm talking about high precision electromechanical drives that
> >>> actually have to work over the full military temperature range, in
> >>> high vibration environments, while mounted onto a variety of
> >>> platforms.
> >>>
> >>> --
> >>> Tim Wescott Control systems, embedded software and circuit design I'm
> >>> looking for work! See my website if you're interested
> >>> http://www.wescottdesign.com
> >>
> >> Well you are not doing your best with a PID, that is for sure. I assume
> >> speed is not an issue in terms of transient response and it's a don't
> >> care much about disturbance rejection. PID is ok for those that know
> >> little and want a fast and simple solution - I haven't a problem about
> >> that. You don't want to be using it for fast electro-mech drives
> >> though.
> >> Sure it will work, not the best solution by a long way though. Then
> >> when you get to system with more than one input and output then you
> >> have to throw away the PID and the lag-lead methods too.
> >
> > You have no clue what you are talking about. Try actually reading what
> > I've been saying.
>
> Let me rephrase that. You are obviously a good controls engineer, but
> you have trouble reading what other people write. Perhaps you've never
> worked outside of the industry you're in, and it has caused ossification
> of some of your brain cells. Try actually reading what I've been saying.
>
> Note the part that hasn't been changed. Reading comprehension often
> improves if you actually read what you're "reading".
>
> --
>
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com
>
> I'm looking for work -- see my website!
I don't like arguing with other engineers because they are usually as smart if not smarter than yourself! I have a great respect for your work, let's just leave it at that.