On Wednesday, July 15, 2015 at 3:03:21 PM UTC-4, John Larkin wrote:> > Will that simultaneously simulate my electronics? Something has to do > both.Yes, they're called electro-thermal simulation. Mentor has one, there are others, not a whole lot that I can see.>> > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
I don't understand thermostats
Started by ●July 14, 2015
Reply by ●July 15, 20152015-07-15
Reply by ●July 15, 20152015-07-15
On Wed, 15 Jul 2015 11:53:54 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Wednesday, July 15, 2015 at 2:31:31 PM UTC-4, John Larkin wrote: >> On Wed, 15 Jul 2015 19:10:32 +0200, Habib Bouaziz-Viallet >> <habib@nowhere.com> wrote: >> >> >On 15/07/2015 18:50, John Larkin wrote: >> >> On Wed, 15 Jul 2015 18:25:22 +0200, Habib Bouaziz-Viallet >> >> <habib@nowhere.com> wrote: >> >> >> >>> John, >> >>> >> >>> where is the setpoint on your looped system ? a PID corrector should >> >>> theoretically help to reach the plant output to the assigned setpoint >> >>> with an optimum time (without ringing behavior ...). >> >>> >> >>> Habib. >> >>> >> >> >> >> The Spice model defaults to the Schmitt threshold, 0.5 volts. >> >> >> >> >> >I had suspected one of the two thresholds acts as the system setpoint. >> >> An LT Spice default Schmitt has trip points + and - Vh from 0.5 volts. >> >> >If your system is fully characterized as a first order model and if you >> >did not need some optimum response then the system should work fine ... >> >As i understand your needs you have no need to implement a PID (or a PD) >> >corrector ; or i missed something (always my English understanding ... :( ) >> >> Your English is fine, about as good as mine. A PID controller takes >> more parts and has power dissipation in the amp and the square-law >> problem, or needs PWM drive. A PID might need big capacitors in the >> integral and would need tuning, too. >> >> >> > >> >BTW : What is R3/C3 ? did not understand the reference of this RC model >> > >> >Habib. >> >> I was just playing with a system that has multiple thermal masses and >> thermal conductivities. The conductivities are actually diffusive, so >> an RC is only an approximation. Well, the masses are diffusive, too. >> C3 could be the heater mass and the big one, C2, the thing to be >> heated. I used lots of RC delays to see what the dumb thermostat loop >> would do. >> >> I can probably get by with three thermal masses (three caps) >> corresponding to heater, gadget, and sensor, connected by thermal >> conductivities/resistors. The heater mass might exceed the gadget >> mass. What's interesting is that the bang-bang loop works pretty well, >> even with zero hysteresis. Saves parts. >> >> I'd just never thought about thermostats much before. Now I need to >> try to characterize the physics of the actual setup, including losses >> to ambient everywhere. I need numbers, of course. >> >> The thing below might be better. It has a current/heat source dumping >> into the heater mass, no R3 that you asked about. >> >> Incidentally, this is pretty close for simulation: >> >> 1 farad == 1 gram aluminum >> >> 1 amp == 1 watt of heat >> >> 1 volt == 1 deg C >> >> 1 ohm == 1 degC/watt >> >> 1 second == 1 second >> >> >> >> Version 4 >> SHEET 1 880 680 >> WIRE -320 -80 -368 -80 >> WIRE -272 -80 -320 -80 >> WIRE -272 -48 -272 -80 >> WIRE -368 -16 -368 -80 >> WIRE -320 0 -320 -80 >> WIRE 496 48 -320 48 >> WIRE -368 160 -368 64 >> WIRE -256 160 -368 160 >> WIRE -96 160 -176 160 >> WIRE 64 160 -96 160 >> WIRE 144 160 64 160 >> WIRE 320 160 224 160 >> WIRE 384 160 320 160 >> WIRE 496 160 496 48 >> WIRE 496 160 448 160 >> WIRE -96 208 -96 160 >> WIRE -368 224 -368 160 >> WIRE 64 224 64 160 >> WIRE 320 224 320 160 >> WIRE -368 336 -368 288 >> WIRE -96 336 -96 288 >> WIRE 64 336 64 288 >> WIRE 320 336 320 288 >> FLAG -368 336 0 >> FLAG 64 336 0 >> FLAG 320 336 0 >> FLAG -272 -48 0 >> FLAG -96 336 0 >> SYMBOL Digital\\schmtinv 384 96 R0 >> WINDOW 0 -6 -11 Left 2 >> WINDOW 3 -26 23 Left 2 >> SYMATTR InstName A1 >> SYMATTR Value Vh=5m >> SYMBOL cap 304 224 R0 >> WINDOW 0 67 13 Left 2 >> WINDOW 3 61 47 Left 2 >> SYMATTR InstName C1 >> SYMATTR Value 100� >> SYMBOL cap 48 224 R0 >> WINDOW 0 66 13 Left 2 >> WINDOW 3 66 45 Left 2 >> SYMATTR InstName C2 >> SYMATTR Value 5m >> SYMBOL cap -384 224 R0 >> WINDOW 0 69 15 Left 2 >> WINDOW 3 68 46 Left 2 >> SYMATTR InstName C3 >> SYMATTR Value 5m >> SYMBOL res -160 144 R90 >> WINDOW 0 -40 52 VBottom 2 >> WINDOW 3 -36 53 VTop 2 >> SYMATTR InstName R1 >> SYMATTR Value 2K >> SYMBOL res 240 144 R90 >> WINDOW 0 -45 55 VBottom 2 >> WINDOW 3 -39 53 VTop 2 >> SYMATTR InstName R2 >> SYMATTR Value 10K >> SYMBOL g -368 80 R180 >> WINDOW 0 -53 4 Left 2 >> WINDOW 3 -68 -24 Left 2 >> SYMATTR InstName G1 >> SYMATTR Value 100� >> SYMBOL res -112 192 R0 >> WINDOW 0 54 44 Left 2 >> WINDOW 3 48 74 Left 2 >> SYMATTR InstName R3 >> SYMATTR Value 10K >> TEXT -24 0 Left 2 !.tran 200 uic >> TEXT -48 -80 Left 2 ;THERMOSTAT >> TEXT -56 -40 Left 2 ;JL July 15, 2015 >> TEXT -344 312 Left 2 ;heater mass >> TEXT 96 312 Left 2 ;gadget mass >> TEXT 336 312 Left 2 ;sensor mass >> TEXT -72 312 Left 2 ;heat loss >> >> >> >> -- >> >> John Larkin Highland Technology, Inc >> picosecond timing precision measurement >> >> jlarkin att highlandtechnology dott com >> http://www.highlandtechnology.com > >Do you have any idea of how much heat you will need? >1 watt, 10, 100?Not yet. A few watts likely.> >I did a P-only control loop, where things were fairly tightly coupled. >(heater, thing, sensor) and that worked fine.I can't afford to let the loop go linear here, or I'll toast the amp. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 15, 20152015-07-15
On Wednesday, July 15, 2015 at 3:15:21 PM UTC-4, John Larkin wrote:> On Wed, 15 Jul 2015 11:30:15 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Wednesday, July 15, 2015 at 11:56:59 AM UTC-4, John Larkin wrote: > >> On Wed, 15 Jul 2015 08:33:49 -0700 (PDT), George Herold > >> <gherold@teachspin.com> wrote: > >> > >> >On Tuesday, July 14, 2015 at 4:29:26 PM UTC-4, John Larkin wrote: > >> >> Imagine a mass that we want to heat with some closed-loop controller. > >> >> It's C2 below. Voltage represents temperature. Thermal systems are > >> >> diffusive, which we represent as a bunch of RC lags. Assume the > >> >> voltage at C4 is the temperature sensor. > >> >> > >> >> If I were to design a PID controller, I'd have to really think about > >> >> it, or fiddle some, to keep it stable. But if I do a dumb on/off > >> >> thermostat, it seems to always work. I can tweak the hysteresis and > >> >> vary the p-p temperature excursions and the switching frequency, but > >> >> it's always stable. Or maybe it's always unstable. But it works. > >> >> > >> >> As Vh gets smaller, the oscillation frequency converges to some > >> >> limiting value, which is I guess the ultimate performance of a > >> >> thermostat for this physics. To get any less temperature excursion, I > >> >> guess I'd have to do a real PID loop. Curious. > >> >> > >> >> This may have something to do with the fact that there is no thermal > >> >> equivalent to an inductor. > >> >> > >> >> Version 4 > >> >> SHEET 1 880 680 > >> >> WIRE 0 0 -288 0 > >> >> WIRE 528 0 80 0 > >> >> WIRE -288 160 -288 0 > >> >> WIRE -224 160 -288 160 > >> >> WIRE -96 160 -144 160 > >> >> WIRE -16 160 -96 160 > >> >> WIRE 96 160 64 160 > >> >> WIRE 192 160 96 160 > >> >> WIRE 304 160 272 160 > >> >> WIRE 416 160 304 160 > >> >> WIRE 528 160 528 0 > >> >> WIRE 528 160 480 160 > >> >> WIRE -288 224 -288 160 > >> >> WIRE -96 224 -96 160 > >> >> WIRE 96 224 96 160 > >> >> WIRE 304 224 304 160 > >> >> WIRE -288 336 -288 288 > >> >> WIRE -96 336 -96 288 > >> >> WIRE 96 336 96 288 > >> >> WIRE 304 336 304 288 > >> >> FLAG -288 336 0 > >> >> FLAG -96 336 0 > >> >> FLAG 96 336 0 > >> >> FLAG 304 336 0 > >> >> SYMBOL Digital\\schmtinv 416 96 R0 > >> >> WINDOW 0 2 -10 Left 2 > >> >> WINDOW 3 -30 23 Left 2 > >> >> SYMATTR InstName A1 > >> >> SYMATTR Value Vh=0.002 > >> >> SYMBOL cap 80 224 R0 > >> >> WINDOW 0 67 13 Left 2 > >> >> WINDOW 3 64 44 Left 2 > >> >> SYMATTR InstName C1 > >> >> SYMATTR Value 1m > >> >> SYMBOL cap -112 224 R0 > >> >> WINDOW 0 63 18 Left 2 > >> >> WINDOW 3 64 53 Left 2 > >> >> SYMATTR InstName C2 > >> >> SYMATTR Value 5m > >> >> SYMBOL cap -304 224 R0 > >> >> WINDOW 0 60 22 Left 2 > >> >> WINDOW 3 66 53 Left 2 > >> >> SYMATTR InstName C3 > >> >> SYMATTR Value 1m > >> >> SYMBOL res -128 144 R90 > >> >> WINDOW 0 0 56 VBottom 2 > >> >> WINDOW 3 32 56 VTop 2 > >> >> SYMATTR InstName R1 > >> >> SYMATTR Value 1K > >> >> SYMBOL res 80 144 R90 > >> >> WINDOW 0 0 56 VBottom 2 > >> >> WINDOW 3 32 56 VTop 2 > >> >> SYMATTR InstName R2 > >> >> SYMATTR Value 1K > >> >> SYMBOL res 96 -16 R90 > >> >> WINDOW 0 0 56 VBottom 2 > >> >> WINDOW 3 32 56 VTop 2 > >> >> SYMATTR InstName R3 > >> >> SYMATTR Value 1K > >> >> SYMBOL cap 288 224 R0 > >> >> WINDOW 0 67 13 Left 2 > >> >> WINDOW 3 64 44 Left 2 > >> >> SYMATTR InstName C4 > >> >> SYMATTR Value 1m > >> >> SYMBOL res 288 144 R90 > >> >> WINDOW 0 0 56 VBottom 2 > >> >> WINDOW 3 32 56 VTop 2 > >> >> SYMATTR InstName R4 > >> >> SYMATTR Value 1K > >> >> TEXT -208 -56 Left 2 !.tran 50 uic > >> >> TEXT 192 -88 Left 2 ;THERMOSTAT > >> >> TEXT 192 -48 Left 2 ;JL July 14, 2015 > >> >> TEXT -72 80 Left 2 ;===== thermal lags ===== > >> >> > >> >> > >> >> > >> >> > >> >> -- > >> >> > >> >> John Larkin Highland Technology, Inc > >> >> picosecond timing precision measurement > >> >> > >> >> jlarkin att highlandtechnology dott com > >> >> http://www.highlandtechnology.com > >> > > >> >Hmm it seems to me you have a gain knob in there too. Whatever the power is > >> >coming out when the thermostat is on. (1V in this case... I don't know which spice line changes the amplitude.) > >> > >> It's all normalized to 1 volt, which I could arbitrarily call 100 > >> degrees C or something. The gain is 1/Vh (well, 0.5/Vh the way LT > >> Spice defines hysteresis) so if I set Vh =0 the schmitt becomes an > >> infinite gain, which actually doesn't change things much. > > > >Calling the hysteresis the gain is a bit funny, though I understand what you mean. > >In theory smaller hysteresis leads to a smaller excursion of the temperature > >from the set point. At some point I don't think smaller hysteresis will do anything > >you'll be stuck with what ever time delay is in the loop. > >(I set you R3 to 1 and C3 to 1p, and lower hysteresis does nothing.) > > Yup, low hysteresis corresponds to high gain, ultimately a pure > comparator with no hysteresis. Thst seems to work, with the p-p temp > excursions depending on the thermal geometry.Before I read the other posts, I want to say I was thinking about this on the ride home. The bang-bang control, comes at the oscillation of the P-only control.. but from the other side. (The other side of stability space...) Which is cool. As long as you don't care about the time constant, and some oscillation, then you've got a useful system... I still say you've got a knob that is the max power of your bang-bang. (and too much power will give oscillations.) I've been dreaming about building my own thermal test bed. I'll now have to include bang-bang. George H.> > > > > > >> > >> > >> > > >> >With 3 RC's and enough gain you should be able to make it oscillate. > >> > >> It seems to always oscillate! That's the point of a thermostat. But > >> the oscillation seems benign. > > > >Sure.. It's a bang-bang oscillator vs. one with too much gain and phase shift. > >I guess I was thinking about the oscillations you get > >with a P-only controller when you crank the gain up too high. > > The bang-bang always oscillates, so you don't worry about it! But > unlike a PID, the temperature excursion magnitude is limited, whereas > a PID might bang the process rail-to-rail if it's tuned wrong. > > > > >> > >> >(I always had to have at least three RC's to simulate thermal loops > >> >w/ spice...otherwise no oscillations.) > >> > >> With hysteresis set nonzero, a single RC oscillates. That's the > >> classic triangle wave generator. > >> > >> I need to build two temperature controllers, on opposite sides of a > >> round PC board about 1" in diameter, around the electro-optical > >> gadgets. The thermostat approach is appealing... very simple and the > >> amplifier won't fry like a linear controller would. Similar to your > >> recent situation, where PWM would be nice but has side effects, the > >> side effect in my case being parts count and loop stability. A > >> bang-bang loop, maybe with zero hysteresis, would sure be easy, but > >> the temperature excursions would be dominated by the thermal > >> properties (masses, conductivities) of the "process", which is hard to > >> model. The controllers on either side of the board will interact too, > >> more fun. > > > >I'm sorry to say that besides the thermostat in my house, I don't know much > >about them for thermal control. If your set point was higher, so that the > >heater had to be on for a longer fraction of the time it seems like there > >might be larger temperature excursions. So picking the power level so > >that it's on about 1/2 the time might be optimal. > >(But again I'm totally guessing) > > > >> > >> My customer insists on two controllers, but maybe I can talk him into > >> one, with lots of thermal vias side to side. > > > >Why two? (at only 1" apart?) Are they worried about thermal gradients? > >It's usually worth while to spend some time thinking about how the heat > >will flow and putting the heaters and sensors in the "right" places. > > No rational reason. I plan to talk him out of it. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●July 15, 20152015-07-15
On Wednesday, July 15, 2015 at 3:40:55 PM UTC-4, Phil Hobbs wrote:> On 07/15/2015 03:10 PM, John Larkin wrote: > > On 15 Jul 2015 18:27:31 GMT, Glen Walpert <nospam@null.void> wrote: > > > >> On Wed, 15 Jul 2015 09:20:47 -0700, John Larkin wrote: > >> > >> <clip> > >>> It could be that my heater (actually a bunch of surfmount > >>> resistors surrounding the optical widgets) will have more > >>> thermal mass than the gadgets being heated. I'm trying to get a > >>> mockup to test. Even when I have one, figuring out the equivalent > >>> circuit will be a chore. Electronics is so easy to drive and > >>> probe; mechanical and thermal systems aren't. Imagine designing a > >>> racing car engine, and wondering what the temperatures and flows > >>> are like inside; we sure have it easy. > >>> > >>> It is tempting to control the temperature of the heater, and not > >>> the object to be heated. Much better dynamics, almost first > >>> order. > >> > >> Or you could control both; an inner loop controlling heater > >> temperature and an outer loop controlling the heated object > >> temperature (the output of which is the setpoint for the inner > >> heater temperature controller). This should give you the better > >> dynamics without any loss of accuracy. > >> > >> Not really pertinent to your control system, actually responding to > >> another comment about optimal control with PID, but I do not think > >> PID can ever achieve optimal transient response in a temperature > >> controller (or pretty much any other controller, possibly excepting > >> some which need to be too fast for digital control). PID can be > >> tuned for the best response possible from a system with a PID > >> controller; fastest response with specified (possibly zero) > >> overshoot. But some sort of model reference control system can > >> always do better, at the cost of additional complexity. > > > > PID isn't optimum, but "optimum" is hard to quantify anyhow. In my > > current situation, I don't care much about overshoot or transient > > response. > > In a system with a significant thermal diffusion contribution, PID will > spoil your whole day. In diffusion, the phase shift continues to grow > without bound as the signal rolls off, so dialling up the D term will > make a nice oscillator. It can help some in cases where the thermal > mass approximation works accurately. > > >> For a temperature controller you would create a math model of the > >> thermal system which will accurately model the temperature at the > >> sensor as a function of heat load or ambient temperature, heater > >> power input, possibly other variables influencing the system, and > >> time. If the system has significant nonlinearities they should be > >> included in the model. > > > > Mechanical/thermal systems are essentially linear. A resistive heater > > is a square-law device, like the problem George posted about. PWM or > > bang-bang eliminate that nonlinearity. > > Most of my thermal control loops are designed using a plant model > consisting of an integrator and a time delay in cascade. You trigger a > scope when the heater turns on, and you can read both the delay and the > slope right off the trace. Generally in small TEC-based loops, it won't > even need tweaking IME.One thing (I think) I've found out about TEC's is that how tightly they are clamped in place, can change the gain. (little pieces of crud between the surfaces can also ruin a day.) George H.> > > >> The model parameters can be determined from measured response to a > >> step or other stimulus. Then when you have a transient to respond > >> to the controller calculates how many watt-seconds of energy are > >> needed to reach the set point and turns the heater on or off fully > >> for the calculated time, then backs off to the calculated steady > >> state power. > > > > Yeah, the nuisance is to quantify the physics, ideally at the design > > level. A thermal breadboard might be pragmatic; I could probably do > > that in a fraction of the time I could learn and run thermal > > modeling software. > > > > The error > >> signal is not the difference between set point and measured > >> temperature, it is the difference between measured temperature and > >> model predicted measured temperature, and this error signal can be > >> used to adjust heater on/off time or power level and also to adjust > >> the model for more accurate response on the next transient, which I > >> have seen referred to as adaptive model reference control. > >> > >> While not too many heaters need this level of optimization, there > >> are situations where the improvement is worth the effort, and it > >> can be a lot more fun than boring old PID <yawn> or bang-bang > >> <double-yawn>. > > > > A little boredom is fine if I can get it done quick. It's pretty much > > a charity job. I prefer systems that respond in picoseconds to those > > that settle in minutes. > > Ditto, except femtoseconds. ;) > > 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 ●July 15, 20152015-07-15
On Wednesday, July 15, 2015 at 4:32:26 PM UTC-4, Habib Bouaziz-Viallet wrote:> On 15/07/2015 20:31, John Larkin wrote: > > The conductivities are actually diffusive, so > > an RC is only an approximation. > > > > Incidentally, this is pretty close for simulation: > > > > 1 farad == 1 gram aluminum > > > > 1 amp == 1 watt of heat > > > > 1 volt == 1 deg C > > > > 1 ohm == 1 degC/watt > > > > 1 second == 1 second > > John, > > You mention in your very first post that C2 is the mass to heat with > R2(�C/W) the thermal resistance i.e. some Y Watt dissipated with respect > to X �C temp rise. quite homogeneous in math point of view. > > Nevertheless in your global model, the variable "watt of heat" should be > *only* unidirectional, is it so ? I think not ... > > Modeling the heat transfer phenomenon by equivalent electrical > components is quite difficult AFAIK. > > Habib. > > PS : this is a very smart idea, however.I use the electrical model for thermal stuff all the time. It works just fine (to the ~20% level). I didn't find the modelling very hard at all. I wanted to define a new unit, the thermal ohm... Tohm (degree K [voltage]/Watt[current]) I'm not sure if it should have a "th" sound or just "tee". George H.
Reply by ●July 16, 20152015-07-16
On Wed, 15 Jul 2015 19:14:27 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Wednesday, July 15, 2015 at 4:32:26 PM UTC-4, Habib Bouaziz-Viallet wrote: >> On 15/07/2015 20:31, John Larkin wrote: >> > The conductivities are actually diffusive, so >> > an RC is only an approximation. >> > >> > Incidentally, this is pretty close for simulation: >> > >> > 1 farad == 1 gram aluminum >> > >> > 1 amp == 1 watt of heat >> > >> > 1 volt == 1 deg C >> > >> > 1 ohm == 1 degC/watt >> > >> > 1 second == 1 second >> >> John, >> >> You mention in your very first post that C2 is the mass to heat with >> R2(�C/W) the thermal resistance i.e. some Y Watt dissipated with respect >> to X �C temp rise. quite homogeneous in math point of view. >> >> Nevertheless in your global model, the variable "watt of heat" should be >> *only* unidirectional, is it so ? I think not ... >> >> Modeling the heat transfer phenomenon by equivalent electrical >> components is quite difficult AFAIK. >> >> Habib. >> >> PS : this is a very smart idea, however. > >I use the electrical model for thermal stuff >all the time. >It works just fine (to the ~20% level). > >I didn't find the modelling very hard at all. >I wanted to define a new unit, >the thermal ohm... Tohm (degree K [voltage]/Watt[current]) >I'm not sure if it should have a "th" sound or just "tee". >We tend to call that "theta." -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 16, 20152015-07-16
John Larkin <jlarkin@highlandtechnology.com> wrote:> On Wed, 15 Jul 2015 07:25:16 +0100, piglet <erichpwagner@hotmail.com> > wrote: > >> On 15/07/2015 02:10, John Larkin wrote: >>> the differential equation are there. An RLC will ring, ditto. Thermal >>> systems can't ring, because the inductor equivalent doesn't exist. >>> It's like making a circuit out of just resistors and capacitors. >>> >> >> But even circuits without L can ring if there is enough accumulated R-C >> phase shift? >> > > Not unless you build a feedback loop with gain, which you can't do > with purely thermal stuff. >Would a heat pump count as a thermal gain element?
Reply by ●July 16, 20152015-07-16
On Thursday, July 16, 2015 at 4:01:34 AM UTC+2, George Herold wrote:> On Wednesday, July 15, 2015 at 3:40:55 PM UTC-4, Phil Hobbs wrote: > > On 07/15/2015 03:10 PM, John Larkin wrote: > > > On 15 Jul 2015 18:27:31 GMT, Glen Walpert <nospam@null.void> wrote: > > > > > >> On Wed, 15 Jul 2015 09:20:47 -0700, John Larkin wrote: > > >> > > >> <clip> > > >>> It could be that my heater (actually a bunch of surfmount > > >>> resistors surrounding the optical widgets) will have more > > >>> thermal mass than the gadgets being heated. I'm trying to get a > > >>> mockup to test. Even when I have one, figuring out the equivalent > > >>> circuit will be a chore. Electronics is so easy to drive and > > >>> probe; mechanical and thermal systems aren't. Imagine designing a > > >>> racing car engine, and wondering what the temperatures and flows > > >>> are like inside; we sure have it easy. > > >>> > > >>> It is tempting to control the temperature of the heater, and not > > >>> the object to be heated. Much better dynamics, almost first > > >>> order. > > >> > > >> Or you could control both; an inner loop controlling heater > > >> temperature and an outer loop controlling the heated object > > >> temperature (the output of which is the setpoint for the inner > > >> heater temperature controller). This should give you the better > > >> dynamics without any loss of accuracy. > > >> > > >> Not really pertinent to your control system, actually responding to > > >> another comment about optimal control with PID, but I do not think > > >> PID can ever achieve optimal transient response in a temperature > > >> controller (or pretty much any other controller, possibly excepting > > >> some which need to be too fast for digital control). PID can be > > >> tuned for the best response possible from a system with a PID > > >> controller; fastest response with specified (possibly zero) > > >> overshoot. But some sort of model reference control system can > > >> always do better, at the cost of additional complexity. > > > > > > PID isn't optimum, but "optimum" is hard to quantify anyhow. In my > > > current situation, I don't care much about overshoot or transient > > > response. > > > > In a system with a significant thermal diffusion contribution, PID will > > spoil your whole day. In diffusion, the phase shift continues to grow > > without bound as the signal rolls off, so dialling up the D term will > > make a nice oscillator. It can help some in cases where the thermal > > mass approximation works accurately. > > > > >> For a temperature controller you would create a math model of the > > >> thermal system which will accurately model the temperature at the > > >> sensor as a function of heat load or ambient temperature, heater > > >> power input, possibly other variables influencing the system, and > > >> time. If the system has significant nonlinearities they should be > > >> included in the model. > > > > > > Mechanical/thermal systems are essentially linear. A resistive heater > > > is a square-law device, like the problem George posted about. PWM or > > > bang-bang eliminate that nonlinearity. > > > > Most of my thermal control loops are designed using a plant model > > consisting of an integrator and a time delay in cascade. You trigger a > > scope when the heater turns on, and you can read both the delay and the > > slope right off the trace. Generally in small TEC-based loops, it won't > > even need tweaking IME. > > One thing (I think) I've found out about TEC's is that how > tightly they are clamped in place, can change the gain. > (little pieces of crud between the surfaces can also > ruin a day.)My 1996 micro-degree paper (of which you've got a copy) has got an equation for TEC performance, where the summed thermal resistances between the TEC and the object being controlled as well as between the TEC and the output heatsink influence performance - if either is significant they can really lower performance. We used graphite cloth as a tidier and better-performing) substitute for zinc-oxide loaded silicone grease. -- Bill Sloman, Sydney
Reply by ●July 16, 20152015-07-16
On 2015-07-15, tabbypurr@gmail.com <tabbypurr@gmail.com> wrote:> On Wednesday, 15 July 2015 01:55:01 UTC+1, Dave Platt wrote: >> In article <gcabqatiaij1slj29lu0h1bk8ck5gkfcog@4ax.com>, >> John Larkin <jlarkin@highlandtechnology.com> wrote: >> >> >But systems composed of heat sources and masses and thermal conductors >> >don't ring or overshoot, whereas mechanical systems do ring and >> >overshoot and oscillate. >> >> Seems to me that by limiting the system to "heat sources and masses >> and thermal conductors", you have eliminated any way for the >> temperature of the "downstream" parts of the system to affect the heat >> source. Hence, no feedback... and hence the classic conditions for >> oscillation cannot be met. >> >> > You have to add some non-thermal element, >> >like electronics or some mechanical gidget, or gain somehow, to make a >> >thermal system ring. >> >> Or like a thermostat? >> >> Put a "bang-bang" thermostat into the system to control the heat >> source, and you've introduced feedback... and at this point, the >> system sure-and-for-gosh is oscillating! It cycles above and below >> the setpoint temperature, at a rate set by the thermal mass of the >> system, the rate of loss-of-heat, and the rate-of-added-heat when the >> heater is on. Plot the temperature vs. time with the right axis >> scales, and you'll see something akin to a distorted sine or triangle >> or sawtooth wave. >> >> That's really not very different from the oscillations you'd get in a >> mechanical system... like a ping-pong ball in a column, with a >> fixed-force air jet which is turned on every time the ball falls below >> a specific height. >> >> >It does look like, for a nontrivial system, the p-p temperature >> >excursion and frequency will be limited by the process, all the way >> >down to zero hysteresis, which is equivalent to an ideal comparator. >> >So it's kinda hard to get wrong. >> >> FSVO "right" and "wrong". >> >> If the thermometer has too much thermal mass of its own, or isn't >> tightly coupled to the air in the room, it'll be slow in sensing the >> increase (or decrease) in temperature in the process area, and you'll >> end up with loads of overshoot. >> >> Household thermostats often have an "anticipator" built in, to limit >> this effect. It's a small heater, located near the sensing element, >> which goes on at the same time as the main house heater. This helps >> the thermostat "anticipate" the amount by which the main heater is >> probably warming the room air, and reduces the delay in shutting off >> when the right temperature is reached. > > The heater inside bimetal stats is for compensation of mechanical stat hysteresis, not anticipation of heating system overshoot.I've got a Honeywell bimetal thermostat in my junkbox with a 1K hysterisis I use it as a room themometer. -- umop apisdn
Reply by ●July 16, 20152015-07-16
On 2015-07-15, John Larkin <jlarkin@highlandtechnology.com> wrote:> have one, figuring out the equivalent circuit will be a chore. > Electronics is so easy to drive and probe; mechanical and thermal > systems aren't. Imagine designing a racing car engine, and wondering > what the temperatures and flows are like inside; we sure have it easy.Imagine trying to build an engine with 5% tolerance parts. -- umop apisdn
