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
Stability and Insanity
Started by ●May 17, 2016
Reply by ●May 18, 20162016-05-18
Reply by ●May 18, 20162016-05-18
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.netPID 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.
Reply by ●May 18, 20162016-05-18
On Tue, 17 May 2016 10:54:48 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Tue, 17 May 2016 08:15:42 -0500, John S wrote: > >> On 5/17/2016 1:41 AM, Tim Wescott wrote: >>> I've just spent over two weeks getting ready to do my next video. It >>> was a combination of one of those vast underestimations one >>> occasionally makes, combined with falling into a bit of an obsession. >>> >>> I am, at this point, not only wondering if it was worth it, but >>> questioning my sanity in carrying on even when the going went beyond >>> tough to just plain crazy. >>> >>> At any rate, a good video needs a visual aid, and I decided that my >>> video needed to demonstrate stability with a pendulum. Moreover, it >>> needed a pendulum that could be worked electronically. So, I've >>> >>> * Disassembled a hard drive for it's head positioner. This took a day >>> or two. >>> >>> * Decided that wasn't good enough and wound my own custom coil (220 >>> feet of #40 wire, woo hoo!). This took a false start (18 feet of #34 >>> wire) and several days. >>> >>> * Mounted the coil into a custom pendulum, running on Real Ball >>> Bearings. Several more days, and if you touch it wrong the Q goes down >>> from about 80 to about 10, then you have to fiddle with it for several >>> minutes so the moving parts don't rub. >>> >>> * Built an oscillator that uses the pendulum as its resonator (this is >>> where stability comes in -- is an oscillator stable? How is it stable? >>> What if it's showing chaotic behavior?). This was astonishingly >>> frustrating, and didn't finally work until I carefully modeled the >>> pendulum as a resonator AND took the coil inductance into account in >>> the circuit. This part too about a week. >>> >>> And for all that, I now have the time base for an exceptionally >>> inaccurate electro-mechanical clock! Check out the picture. That's >>> one cycle of the pendulum, running off of a "tick-toc" circuit that (A) >>> minimizes the load on the pendulum (to give a high loaded Q, essential >>> for wringing as much accuracy as possible out of a pendulum, never mind >>> that it's made of wood, masking tape, and car parts that I picked up >>> off the floor), and (B) has to be started by hand (I wanted to >>> demonstrate a hard limit cycle). >>> >>> http://wescottdesign.com/movies/stability_teaser.gif >>> >>> More on all of this when I post the video. >>> >>> >> 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 saw an article that did this 30 or so years ago. They used a hollow >> steel ball with a map of the earth painted on. Can't remember the >> diameter of the ball, but maybe 1". > >I've done that. You need a honkin' big electromagnet to make it work >with a plain steel load. > >The executive desk-toys with the floating globes use big (30mm dia x >10mm) rare-earth magnets, and float the ball a little bit below the >neutral point. I believe that they use hall effect sensors to detect the >magnet proximity.I'm coming in a little late, but last week I was at ISEF (a very large international high-school level science fair), and one of the kids was levitating a small (about 1/2" dia) steel sphere. He had a hall effect sensor underneath and an electromagnet above. He ran into trouble with random spin in the ball that would make it difficult to control in one dimension. The spin was presumably due to eddy currents in the ball since it was solid. He solved that by 3D-printing a cage that would stabilize the ball after he glued two toothpicks to it. That stopped the spin and the levitation worked pretty well after that. His innovation was really that he used a self-developed conrol technique after he couldn't get PID to work. A couple other judges who were PhD candidates in control at ASU pointed out that his new technique was also PID, but it was pretty cool that he had derived he whole thing himself and got it into a form that worked. Anyway, I'm guessing that's why hollow spheres are usually used.
Reply by ●May 18, 20162016-05-18
Eric Jacobsen wrote:> > I'm coming in a little late, but last week I was at ISEF (a very large > international high-school level science fair), and one of the kids was > levitating a small (about 1/2" dia) steel sphere. He had a hall > effect sensor underneath and an electromagnet above. He ran into > trouble with random spin in the ball that would make it difficult to > control in one dimension. The spin was presumably due to eddy > currents in the ball since it was solid. He solved that by > 3D-printing a cage that would stabilize the ball after he glued two > toothpicks to it. That stopped the spin and the levitation worked > pretty well after that. > > His innovation was really that he used a self-developed conrol > technique after he couldn't get PID to work. A couple other judges > who were PhD candidates in control at ASU pointed out that his new > technique was also PID, but it was pretty cool that he had derived he > whole thing himself and got it into a form that worked. > > Anyway, I'm guessing that's why hollow spheres are usually used. >I'd've guessed it was just because it takes less power than levitating a heavier solid one. "Look, it levitates a sphere AND keeps my hands warm!" -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix.
Reply by ●May 19, 20162016-05-19
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!
Reply by ●May 19, 20162016-05-19
On 18.05.2016 6:28, herrmannsfeldt@gmail.com wrote:> On Tuesday, May 17, 2016 at 5:05:57 AM UTC-7, Evgeny Filatov wrote: > > (snip) > >> "In Drosophila and many other animals, including humans, the heart of >> the circadian clock is a delayed negative feedback loop based on >> transcription regulators: accumulation of certain gene products switches >> off the transcription of their own genes, but with a delay, so that the >> cell oscillates between a state in which the products are present and >> transcription is switched off, and one in which the products are absent >> and transcription is switched on." > > I have heard about genes with really long introns, which slows down transcription, > and the suggestion that the reason is timing, but no details on what is being timed. > That might be the one, though. > > -- glen >Frankly, I don't know biology. Am only exposed to it because of my wife's occupation. ;) Gene
Reply by ●May 19, 20162016-05-19
On 05/17/2016 05:26 PM, Tim Wescott wrote:> On Tue, 17 May 2016 13:49:43 -0400, Phil Hobbs wrote: > >> On 05/17/2016 01:30 PM, Tim Wescott wrote: >>> On Tue, 17 May 2016 12:02:12 -0500, John S wrote: >>> >>>> On 5/17/2016 11:57 AM, Tim Wescott wrote: >>>>> On Tue, 17 May 2016 11:52:12 -0500, John S wrote: >>>>> >>>>>> On 5/17/2016 10:54 AM, Tim Wescott wrote: >>>>>>> On Tue, 17 May 2016 08:15:42 -0500, John S wrote: >>>>>>> >>>>>>>> On 5/17/2016 1:41 AM, Tim Wescott wrote: >>>>>>>>> I've just spent over two weeks getting ready to do my next video. >>>>>>>>> It was a combination of one of those vast underestimations one >>>>>>>>> occasionally makes, combined with falling into a bit of an >>>>>>>>> obsession. >>>>>>>>> >>>>>>>>> I am, at this point, not only wondering if it was worth it, but >>>>>>>>> questioning my sanity in carrying on even when the going went >>>>>>>>> beyond tough to just plain crazy. >>>>>>>>> >>>>>>>>> At any rate, a good video needs a visual aid, and I decided that >>>>>>>>> my video needed to demonstrate stability with a pendulum. >>>>>>>>> Moreover, it needed a pendulum that could be worked >>>>>>>>> electronically. So, I've >>>>>>>>> >>>>>>>>> * Disassembled a hard drive for it's head positioner. This took >>>>>>>>> a day or two. >>>>>>>>> >>>>>>>>> * Decided that wasn't good enough and wound my own custom coil >>>>>>>>> (220 feet of #40 wire, woo hoo!). This took a false start (18 >>>>>>>>> feet of #34 wire) and several days. >>>>>>>>> >>>>>>>>> * Mounted the coil into a custom pendulum, running on Real Ball >>>>>>>>> Bearings. Several more days, and if you touch it wrong the Q >>>>>>>>> goes down from about 80 to about 10, then you have to fiddle with >>>>>>>>> it for several minutes so the moving parts don't rub. >>>>>>>>> >>>>>>>>> * Built an oscillator that uses the pendulum as its resonator >>>>>>>>> (this is where stability comes in -- is an oscillator stable? >>>>>>>>> How is it stable? >>>>>>>>> What if it's showing chaotic behavior?). This was astonishingly >>>>>>>>> frustrating, and didn't finally work until I carefully modeled >>>>>>>>> the pendulum as a resonator AND took the coil inductance into >>>>>>>>> account in the circuit. This part too about a week. >>>>>>>>> >>>>>>>>> And for all that, I now have the time base for an exceptionally >>>>>>>>> inaccurate electro-mechanical clock! Check out the picture. >>>>>>>>> That's one cycle of the pendulum, running off of a "tick-toc" >>>>>>>>> circuit that (A) >>>>>>>>> minimizes the load on the pendulum (to give a high loaded Q, >>>>>>>>> essential for wringing as much accuracy as possible out of a >>>>>>>>> pendulum, never mind that it's made of wood, masking tape, and >>>>>>>>> car parts that I picked up off the floor), and (B) has to be >>>>>>>>> started by hand (I wanted to demonstrate a hard limit cycle). >>>>>>>>> >>>>>>>>> http://wescottdesign.com/movies/stability_teaser.gif >>>>>>>>> >>>>>>>>> More on all of this when I post the video. >>>>>>>>> >>>>>>>>> >>>>>>>> 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 saw an article that did this 30 or so years ago. They used a >>>>>>>> hollow steel ball with a map of the earth painted on. Can't >>>>>>>> remember the diameter of the ball, but maybe 1". >>>>>>> >>>>>>> I've done that. You need a honkin' big electromagnet to make it >>>>>>> work with a plain steel load. >>>>>>> >>>>>>> The executive desk-toys with the floating globes use big (30mm dia >>>>>>> x 10mm) rare-earth magnets, and float the ball a little bit below >>>>>>> the neutral point. I believe that they use hall effect sensors to >>>>>>> detect the magnet proximity. >>>>>>> >>>>>>> >>>>>> Oops! Sorry! I didn't know you were already doing it. >>>>> >>>>> Well, the project is on long-term hold, but I did get as far as >>>>> getting one of those desk toys and starting to take it apart. >>>>> >>>>> >>>> Would it be inappropriate to demonstrate PID? Just curious. >>> >>> I suspect that it would be a good way to demonstrate a number of >>> principles in control systems, PID controllers included. >>> >>> A lot of people seem to separate "PID" from other controllers -- >>> somewhere in this thread someone made a comment about "oh, that >>> wouldn't work with a PID controller". Yet most advanced controllers >>> really boil down to a PID controller with: >>> >>> * A different way of arriving at the gains; >>> * various linear and nonlinear decorations; >>> * and a fancy name. >>> >>> But -- that's a rant for another day. >>> >>> >> Well, there are various lead-lag type tricks that require basically >> lowpass filtering the D term. PLLs and diffusion-dominated temperature >> controls don't like terms with huge noise gain. >> >> When I was a post-doc, in about 1988, I did a motion controller for a >> piezo bimorph used in a scanning force microscope. It had a notch >> filter for the lowest resonance, two integrators (with lead-lag to make >> the loop stable) and two more poles to get rid of wideband crap that >> would excite the higher resonances. Worked great--when tweaked for best >> settling time, the loop BW was about 30% of the resonant frequency, vs. >> 3% for the previous version. (The resonance had a Q of about 30, so >> with just one integrator and no notch, the BW had to be backed off a >> really long way to prevent oscillation.) >> >> Would you include that in "decorated PID"? > > By the time you've got that much stuff stuck on the loop then I may just > insist that there's a PID controller (or at least PI) at the core. > > You're making me realize that the line is blurry -- but I've certainly > made loops involving a band-limited derivative (which is just another way > of making a lead-lag filter) and a notch, and still considered it a "PID". ><snip> In Floyd Gardner's PLL book ("Phaselock Techniques") he takes a somewhat similar view, namely that it's only what happens inside the loop bandwidth that qualifies the loop as first, second, or higher order. So in his scheme my motion controller would be a second order loop (two integrators and a plant without much inertia) plus various out-of-band decorations (the notch and the two extra poles). So if you leave out the case of multiple integrators, and confine the discussion to stuff at frequencies near the unity gain cross or lower, I can see your point. Multiple integrators are also useful in temperature controllers--you put the first integrator inside the controlled volume, to eliminate temperature drift, and use the second one to force the first one's bias point to be constant. (The first integrator A1 will force its input to stay still, but you need the second one (A2) to force A1's _output_ to stay still as well. That guarantees that A1's dissipation is constant, so it doesn't perturb the system. Of course getting rid of windup takes more thought in a system like that. It wasn't a huge issue in the piezo controller. Designing stuff sure is fun. Right now I'm mostly doing optical antenna and tunnel junction simulations, which is also fun but gets old as a steady diet. Somebody else is doing the processing on this one--processing, characterization and taking SEM pictures does a lot to relieve the monotony. (That's one reason I'm doing some proprietary products in between.) 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
Reply by ●May 19, 20162016-05-19
On Wed, 18 May 2016 21:46:07 +0000, Eric Jacobsen wrote:> On Tue, 17 May 2016 10:54:48 -0500, Tim Wescott <tim@seemywebsite.com> > wrote: > >>On Tue, 17 May 2016 08:15:42 -0500, John S wrote: >> >>> On 5/17/2016 1:41 AM, Tim Wescott wrote: >>>> I've just spent over two weeks getting ready to do my next video. It >>>> was a combination of one of those vast underestimations one >>>> occasionally makes, combined with falling into a bit of an obsession. >>>> >>>> I am, at this point, not only wondering if it was worth it, but >>>> questioning my sanity in carrying on even when the going went beyond >>>> tough to just plain crazy. >>>> >>>> At any rate, a good video needs a visual aid, and I decided that my >>>> video needed to demonstrate stability with a pendulum. Moreover, it >>>> needed a pendulum that could be worked electronically. So, I've >>>> >>>> * Disassembled a hard drive for it's head positioner. This took a >>>> day or two. >>>> >>>> * Decided that wasn't good enough and wound my own custom coil (220 >>>> feet of #40 wire, woo hoo!). This took a false start (18 feet of #34 >>>> wire) and several days. >>>> >>>> * Mounted the coil into a custom pendulum, running on Real Ball >>>> Bearings. Several more days, and if you touch it wrong the Q goes >>>> down from about 80 to about 10, then you have to fiddle with it for >>>> several minutes so the moving parts don't rub. >>>> >>>> * Built an oscillator that uses the pendulum as its resonator (this >>>> is where stability comes in -- is an oscillator stable? How is it >>>> stable? >>>> What if it's showing chaotic behavior?). This was astonishingly >>>> frustrating, and didn't finally work until I carefully modeled the >>>> pendulum as a resonator AND took the coil inductance into account in >>>> the circuit. This part too about a week. >>>> >>>> And for all that, I now have the time base for an exceptionally >>>> inaccurate electro-mechanical clock! Check out the picture. That's >>>> one cycle of the pendulum, running off of a "tick-toc" circuit that >>>> (A) >>>> minimizes the load on the pendulum (to give a high loaded Q, >>>> essential for wringing as much accuracy as possible out of a >>>> pendulum, never mind that it's made of wood, masking tape, and car >>>> parts that I picked up off the floor), and (B) has to be started by >>>> hand (I wanted to demonstrate a hard limit cycle). >>>> >>>> http://wescottdesign.com/movies/stability_teaser.gif >>>> >>>> More on all of this when I post the video. >>>> >>>> >>> 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 saw an article that did this 30 or so years ago. They used a hollow >>> steel ball with a map of the earth painted on. Can't remember the >>> diameter of the ball, but maybe 1". >> >>I've done that. You need a honkin' big electromagnet to make it work >>with a plain steel load. >> >>The executive desk-toys with the floating globes use big (30mm dia x >>10mm) rare-earth magnets, and float the ball a little bit below the >>neutral point. I believe that they use hall effect sensors to detect >>the magnet proximity. > > I'm coming in a little late, but last week I was at ISEF (a very large > international high-school level science fair), and one of the kids was > levitating a small (about 1/2" dia) steel sphere. He had a hall effect > sensor underneath and an electromagnet above. He ran into trouble with > random spin in the ball that would make it difficult to control in one > dimension. The spin was presumably due to eddy currents in the ball > since it was solid. He solved that by 3D-printing a cage that would > stabilize the ball after he glued two toothpicks to it. That stopped > the spin and the levitation worked pretty well after that. > > His innovation was really that he used a self-developed conrol technique > after he couldn't get PID to work. A couple other judges who were PhD > candidates in control at ASU pointed out that his new technique was also > PID, but it was pretty cool that he had derived he whole thing himself > and got it into a form that worked. > > Anyway, I'm guessing that's why hollow spheres are usually used.Get that kid into a graduate program! -- 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
Reply by ●May 19, 20162016-05-19
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!
Reply by ●May 19, 20162016-05-19
On Thu, 19 May 2016 10:37:38 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Wed, 18 May 2016 21:46:07 +0000, Eric Jacobsen wrote: > >> On Tue, 17 May 2016 10:54:48 -0500, Tim Wescott <tim@seemywebsite.com> >> wrote: >> >>>On Tue, 17 May 2016 08:15:42 -0500, John S wrote: >>> >>>> On 5/17/2016 1:41 AM, Tim Wescott wrote: >>>>> I've just spent over two weeks getting ready to do my next video. It >>>>> was a combination of one of those vast underestimations one >>>>> occasionally makes, combined with falling into a bit of an obsession. >>>>> >>>>> I am, at this point, not only wondering if it was worth it, but >>>>> questioning my sanity in carrying on even when the going went beyond >>>>> tough to just plain crazy. >>>>> >>>>> At any rate, a good video needs a visual aid, and I decided that my >>>>> video needed to demonstrate stability with a pendulum. Moreover, it >>>>> needed a pendulum that could be worked electronically. So, I've >>>>> >>>>> * Disassembled a hard drive for it's head positioner. This took a >>>>> day or two. >>>>> >>>>> * Decided that wasn't good enough and wound my own custom coil (220 >>>>> feet of #40 wire, woo hoo!). This took a false start (18 feet of #34 >>>>> wire) and several days. >>>>> >>>>> * Mounted the coil into a custom pendulum, running on Real Ball >>>>> Bearings. Several more days, and if you touch it wrong the Q goes >>>>> down from about 80 to about 10, then you have to fiddle with it for >>>>> several minutes so the moving parts don't rub. >>>>> >>>>> * Built an oscillator that uses the pendulum as its resonator (this >>>>> is where stability comes in -- is an oscillator stable? How is it >>>>> stable? >>>>> What if it's showing chaotic behavior?). This was astonishingly >>>>> frustrating, and didn't finally work until I carefully modeled the >>>>> pendulum as a resonator AND took the coil inductance into account in >>>>> the circuit. This part too about a week. >>>>> >>>>> And for all that, I now have the time base for an exceptionally >>>>> inaccurate electro-mechanical clock! Check out the picture. That's >>>>> one cycle of the pendulum, running off of a "tick-toc" circuit that >>>>> (A) >>>>> minimizes the load on the pendulum (to give a high loaded Q, >>>>> essential for wringing as much accuracy as possible out of a >>>>> pendulum, never mind that it's made of wood, masking tape, and car >>>>> parts that I picked up off the floor), and (B) has to be started by >>>>> hand (I wanted to demonstrate a hard limit cycle). >>>>> >>>>> http://wescottdesign.com/movies/stability_teaser.gif >>>>> >>>>> More on all of this when I post the video. >>>>> >>>>> >>>> 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 saw an article that did this 30 or so years ago. They used a hollow >>>> steel ball with a map of the earth painted on. Can't remember the >>>> diameter of the ball, but maybe 1". >>> >>>I've done that. You need a honkin' big electromagnet to make it work >>>with a plain steel load. >>> >>>The executive desk-toys with the floating globes use big (30mm dia x >>>10mm) rare-earth magnets, and float the ball a little bit below the >>>neutral point. I believe that they use hall effect sensors to detect >>>the magnet proximity. >> >> I'm coming in a little late, but last week I was at ISEF (a very large >> international high-school level science fair), and one of the kids was >> levitating a small (about 1/2" dia) steel sphere. He had a hall effect >> sensor underneath and an electromagnet above. He ran into trouble with >> random spin in the ball that would make it difficult to control in one >> dimension. The spin was presumably due to eddy currents in the ball >> since it was solid. He solved that by 3D-printing a cage that would >> stabilize the ball after he glued two toothpicks to it. That stopped >> the spin and the levitation worked pretty well after that. >> >> His innovation was really that he used a self-developed conrol technique >> after he couldn't get PID to work. A couple other judges who were PhD >> candidates in control at ASU pointed out that his new technique was also >> PID, but it was pretty cool that he had derived he whole thing himself >> and got it into a form that worked. >> >> Anyway, I'm guessing that's why hollow spheres are usually used. > >Get that kid into a graduate program!I talked to him for about 10-15 minutes. Brilliant kid, very articulate, definitely going places. The best part is, there were a number of kids in the same category that had better projects. I'm always inspired by this stuff. There are some genuinely brilliant kids out there. Cheers, Eric
