I had suspected one of the two thresholds acts as the system setpoint. 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 ... :( )
BTW : What is R3/C3 ? did not understand the reference of this RC model
Habib.
r.
t t I lower is
ow which spice line changes the amplitude.)
temperature of a mass coupled to the thermodynamics of the configuration b ut 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 excu rsions of the critical element of the system.
One thing for certain is that no one uses SPICE for this. Maybe you can get a free eval copy of something like this:
They /should/ enable a time varying control input, if not at least a steady state, but like I've said in the past, if engineers have any say in the ar chitecture, it will be lacking.
It's always a 3D simulation. One big problem is the requirement for input d ata you may not have, that's why your best hope is to get a system specific to electronics.
Google "thermal modeling for electronics cad' for others.
Go back to selling women's shoes.
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.
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. 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. 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 or bang-bang .
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.)
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.
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)
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.
George H.
An LT Spice default Schmitt has trip points + and - Vh from 0.5 volts.
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.
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 aluminum1 amp == 1 watt of heat
1 volt == 1 deg C1 ohm == 1 degC/watt
1 second == 1 secondVersion 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
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
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?I did a P-only control loop, where things were fairly tightly coupled. (heater, thing, sensor) and that worked fine. (Fet as heater, constant voltage, control current.) There's a little gain dependent offset, but not too bad, and I cared more about stability than any exact temperature.
George H.
I do. It works fine, once I get the numbers, the thermal masses and losses and conductivities.
Maybe you can get a free eval copy of something like this:
Will that simultaneously simulate my electronics? Something has to do both.
Try living in a world without engineers.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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.
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.
The
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
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.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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.
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.
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
.
Actually, if your system contains a convecting element, you can get interes ting behaviour. Convection doesn't happen if the Rayleigh number is less th an 500, and if it's less 100,000 the convection will be laminar. Convection involves mass transfer, so it has a well-defined time lag, that decreases as the temperature gradient rises.
You may need to add a little imagination - or at least physical insight - t o your armoury to appreciate thi.
From your rather restricted perspective that may well be true, at least unt il you start demonstrating something to as customer, which is when the inte resting effects traditionally chose to manifest themselves.
-- Bill Sloman, Sydney
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.
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.
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
Inductance and capacitance work by storing energy in an electric field or a magnetic field with contrasting mathematics. Heat can not be converted to other forms of energy efficiently. Your equations only represent one form of energy storage, heat. Use an engine to efficiently convert the heat to some other form of energy storage and you can use more complex math which may allow for oscillations.
-- Rick
I'd not be so certain of that. A biochemist once remarked to me that he was using SPICE to model enzymes and glandular secretions. It's like a big spreadsheet, really: there's LOTS of uses.
But way, way easier to debug than a big spreadsheet.
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
John,
You mention in your very first post that C2 is the mass to heat with
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.
That's very cool. When I get a mockup to test, I'll see how well that model fits the measurements.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Den tirsdag den 14. juli 2015 kl. 22.29.26 UTC+2 skrev John Larkin:
snip
here with what is rougly a 1st order deltasigma
Version 4 SHEET 1 2332 932 WIRE 0 0 -288 0 WIRE 352 0 80 0 WIRE 944 0 352 0 WIRE 352 32 352 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 352 160 352 112 WIRE 352 160 304 160 WIRE 656 160 352 160 WIRE 656 208 592 208 WIRE 944 208 944 0 WIRE 944 208 832 208 WIRE -288 224 -288 160 WIRE -96 224 -96 160 WIRE 96 224 96 160 WIRE 304 224 304 160 WIRE 592 288 592 208 WIRE -288 336 -288 288 WIRE -96 336 -96 288 WIRE 96 336 96 288 WIRE 304 336 304 288 WIRE 592 432 592 368 WIRE 0 512 -288 512 WIRE 544 512 80 512 WIRE -288 672 -288 512 WIRE -224 672 -288 672 WIRE -96 672 -144 672 WIRE -16 672 -96 672 WIRE 96 672 64 672 WIRE 192 672 96 672 WIRE 304 672 272 672 WIRE 400 672 304 672 WIRE 480 672 400 672 WIRE 544 672 544 512 WIRE -288 736 -288 672 WIRE -96 736 -96 672 WIRE 96 736 96 672 WIRE 304 736 304 672 WIRE -288 848 -288 800 WIRE -96 848 -96 800 WIRE 96 848 96 800 WIRE 304 848 304 800 FLAG -288 336 0 FLAG -96 336 0 FLAG 96 336 0 FLAG 304 336 0 FLAG -288 848 0 FLAG -96 848 0 FLAG 96 848 0 FLAG 304 848 0 FLAG 400 672 t FLAG 656 256 0 FLAG 592 432 0 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 SYMBOL cap 80 736 R0 WINDOW 0 67 13 Left 2 WINDOW 3 64 44 Left 2 SYMATTR InstName C5 SYMATTR Value 1m SYMBOL cap -112 736 R0 WINDOW 0 63 18 Left 2 WINDOW 3 64 53 Left 2 SYMATTR InstName C6 SYMATTR Value 5m SYMBOL cap -304 736 R0 WINDOW 0 60 22 Left 2 WINDOW 3 66 53 Left 2 SYMATTR InstName C7 SYMATTR Value 1m SYMBOL res -128 656 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1K SYMBOL res 80 656 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 1K SYMBOL res 96 496 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 1K SYMBOL cap 288 736 R0 WINDOW 0 67 13 Left 2 WINDOW 3 64 44 Left 2 SYMATTR InstName C8 SYMATTR Value 1m SYMBOL res 288 656 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 1K SYMBOL res 368 128 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R9 SYMATTR Value 1K SYMBOL Digital\\dflop 736 112 R0 SYMATTR InstName A2 SYMBOL voltage 592 272 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 1 0 10u 10u 1m 2m) SYMBOL Digital\\inv 480 608 R0 WINDOW 3 -103 8 Left 2 SYMATTR InstName A1 SYMATTR Value vh=0.002 TEXT -208 -56 Left 2 !.tran 100 uic TEXT -280 448 Left 2 ;THERMOSTAT TEXT -280 480 Left 2 ;JL July 14, 2015 TEXT -72 80 Left 2 ;===== thermal lags ===== TEXT -72 592 Left 2 ;===== thermal lags ====
If you had a Peltier or some other heater/cooler, it would be bidirectional. You could add diodes to the model if you want to force heating only.
Not so bad, if you can measure the process and get the numbers.
You could even use the lossy transmission line model to approximate diffusive conduction.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The thermal analysis work for mainframes was done on electronics simulators for decades.
She does.
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