); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Hi All, I am not trying to start an argument, but I would like to point out that PID controllers are only good at controlling a certain class of system. For a system that has a coal truck that must dump its coal down a hole, the system has mass, velocity and distance, all the qualities to get a perfectly damped system with PID control. For thermal control, the function is more likely to be a Bessel Function, and a Z transform filter is more likely to find a match. In any case, PID controllers are often to be found in totally unsuitable situations giving worse control than even a bang-bang controller. The thermal block controllers work well because of the dominant integrating effect of the block, the time delay for a heat front to propagate through the block is the only concern for instability. When instability is a problem I relocate the thermistor closer to the heater, giving a marginal degree of under- control. Because the block is well insulated it soon becomes very close to isothermal. cheers, Neville Michie
On 11/10/2007, at 3:08 PM, Bruce Griffiths wrote: > Neville Michie wrote: >> I have made several ovens for oscillators over the years. >> The recipe is: >> get a piece of aluminium big enough to contain the oscillator, >> voltage regulator and first stage amplifier. >> With a mill remove the shapes of each component. >> Bolt a large power transistor, large power fets are best, to the >> outside of the block as a heater, and it is run >> off the unregulated input power. Judicious selection of a component >> decides the start-up current. >> Make a plate to cover the excavation for the components, and bolt it >> down. >> The circuit can be made with discrete transistors in the most >> unstable looking amplifier ever seen, >> alternating NPN and PNP transistors, connected directly to each other >> with load resistors. >> The main temperature sensor is a resistor bridge with a high value >> glass encapsulated thermistor. >> These are available a several trade houses. The amplifier is also >> temperature sensitive, but is within the thermal loop. >> The thermistor bridge gives a very large signal ~ 50mV per degree. >> Gain may have to be backed off if thermal >> oscillations do not die down, but the metal block acts as an >> integrator and the circuits are very easy to get >> high gain and sensitivity. >> The whole block is packed in two inches of foam insulation, my 1MHz >> oscillator only draws about 80 mA at 12 volts. >> The temperature is set to 40 C. >> The stability of the oscillator is very good, but as I have not yet >> got a disciplined oscillator going I dont know which is drifting, >> the HP 10811 in my frequency counter or the 1MHz oscillator. After a >> year, the difference is currently 0.3 ppm. >> cheers Neville Michie >> >> > Neville > > A correctly tuned PID control loop should allow even tighter > temperature > control. > A "boostrapped" oven like that used by Wenzel should be even better. > > (http://www.wenzel.com/documents/Sub-pico%20Multiplier.pdf). > > > Despite Wenzel's claims this type of oven isn't new it was used for > portable standard cell enclosures decades ago. > > Bruce > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
