The anticipator in a mechanical thermostat actually acts as an inductor. Actually almost like a Schmitt trigger.
I don't understand thermostats
Started by ●July 14, 2015
Reply by ●July 15, 20152015-07-15
Reply by ●July 15, 20152015-07-15
"John Larkin" wrote in message news:n6cbqah2q1fl9mba34ed30dnroq8mb9tge@4ax.com...>A simple mechanical system, like a rod sticking out of the ground, or >a solid sphere, can ring if you whack it. The three basic elements of >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.All systems are nonlinear. Those rigidly physicsy ones sometimes hold out the longest, though. E&M fields give way to photon doubling (and other multiplications) when a reaction mass is present (e.g., KDP crystal), or pair production at high enough frequencies (>511keV) or field intensities (emission from quantum foam, virtual particles made real). I don't know exactly what intensity this occurs at, but I think we're getting close, with the peak power, EUV+ range, and sharpening and focusing techniques we have these days. Supposedly, thermal conductivity becomes interesting at relativistic energies. The rate of heat diffusion goes way up, and more heat is transferred at the head of the wave: a literal thermal shock wave. As far as I know, we have no way of generating such an effect. It seems likely, on an unobservably small scale, with something like the LHC or Tevatron. No, not even fusion nukes or any proposed fusion reactors are hot enough to achieve this (~1-100MeV/particle is relativistic electron territory, but nuclei (> 1GeV) are just getting warmed up by then). Perhaps such an experiment could be realized by accelerating a macroscopic lump of matter (a few grams?) to a sizable fraction of the speed of light (so that its energy/particle is in the GeV... and its total energy somewhere in the "stop the orbit of the Moon in its tracks" range, I think?), and observing the process of emission as it tunnels into some hapless astronomical object riddled with sensors. So to be strictly true, there are inductors in thermal systems. You just have to warp the fabric of space-time itself to find them. Very small value inductances, indeed. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●July 15, 20152015-07-15
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. NT
Reply by ●July 15, 20152015-07-15
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. -- 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 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.comHmm 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.) With 3 RC's and enough gain you should be able to make it oscillate. (I always had to have at least three RC's to simulate thermal loops w/ spice...otherwise no oscillations.) George H.
Reply by ●July 15, 20152015-07-15
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.> >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.>(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. My customer insists on two controllers, but maybe I can talk him into one, with lots of thermal vias side to side. Electronic design seems to be 50% thermal design and 50% packaging, with about zero time spent scribbling schematics. -- 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 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. > > > > > >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. > > >(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. > > My customer insists on two controllers, but maybe I can talk him into > one, with lots of thermal vias side to side. > > Electronic design seems to be 50% thermal design and 50% packaging, > with about zero time spent scribbling schematics. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing laser drivers and controllers > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.comSeems most of the time a hysteretic temperature controller regulates the temperature of a mass coupled to the thermodynamics of the configuration but with much less heat capacity than the mass to be temperature regulated. The heat capacity is used as the low pass filtering of the temperature excursions of the critical element of the system.
Reply by ●July 15, 20152015-07-15
On Wed, 15 Jul 2015 09:04:45 -0700 (PDT), bloggs.fredbloggs.fred@gmail.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. >> >> >> > >> >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. >> >> >(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. >> >> My customer insists on two controllers, but maybe I can talk him into >> one, with lots of thermal vias side to side. >> >> Electronic design seems to be 50% thermal design and 50% packaging, >> with about zero time spent scribbling schematics. >> >> >> -- >> >> John Larkin Highland Technology, Inc >> picosecond timing laser drivers and controllers >> >> jlarkin att highlandtechnology dott com >> http://www.highlandtechnology.com > >Seems most of the time a hysteretic temperature controller regulates the temperature of a mass coupled to the thermodynamics of the configuration but with much less heat capacity than the mass to be temperature regulated. The heat capacity is used as the low pass filtering of the temperature excursions of the critical element of the system.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. -- 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 14/07/2015 22:29, John Larkin wrote:> 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, 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.
Reply by ●July 15, 20152015-07-15
On Wed, 15 Jul 2015 18:25:22 +0200, Habib Bouaziz-Viallet <habib@nowhere.com> wrote:>On 14/07/2015 22:29, John Larkin wrote: >> 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, > >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. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
