On 07/15/2015 10:01 PM, 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 <firstname.lastname@example.org> 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.) >Yup. For one-offs I sometimes lap the TEC and heat sink together with fine valve grinding compound. Thins down the bond line pretty well. If you get the hard-solder ones (Ferrotec/Tellurex/Marlow), you can soft-solder them to the load and heat sink, at least for sizes less than 15 mm or thereabouts. That really helps. With larger TECs, or arrays of them, the shear from CTE mismatch will crack the bismuth-telluride pillars. (The alumina is in compression, so it doesn't usually crack.) 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
I don't understand thermostats
Started by ●July 14, 2015
Reply by ●July 16, 20152015-07-16
Reply by ●July 17, 20152015-07-17
On 16/07/2015 22:10, Jasen Betts wrote:> On 2015-07-15, John Larkin <email@example.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. >Chip design isn't so bad. The whole engine might be too large or too small by up to 30% but at least the parts usually match each other within 0.1% or so.