That is not always as easy as it sounds. The thermal equivalent of a "rigid body" does not exist. If you apply heat to a block of metal at one end, it takes a while for it to propagate to the other end. In fact, a long thin rod looks a lot more like a transmission line than an isothermal block.
This matters because if you try and increase the loop gain, the wrap-up of the phase shift soon reaches 180 degrees, and the thing becomes unstable as negative FB at low frequencies becomes positive FB at higher frequencies. -John =============== > Hi > > Actually, overshoot is pretty easy to eliminate on a conventional OCXO by > picking a good location for the thermistor. The heater will always run > "hot", but the rest of the stuff does not have to. > > Bob > > > On Sep 21, 2011, at 6:37 PM, Magnus Danielson wrote: > >> On 09/22/2011 12:30 AM, Chuck Harris wrote: >>> Perry Sandeen wrote: >>>> GM List, >>>> ..............................................................................The >>>> >>>> ovens are proportionally controlled. On start-up all ovens, >>>> proportional or not, >>>> will have over-shoot. Some more, some less. An inescapable fact of >>>> life. >>> >>> Imagine that the set point is variable, and can be set below the >>> desired >>> temperature. Then imagine that the set point can approach the desired >>> temperature more closely as it gets closer to the desired temperature. >>> >>> ... And you will have discovered (100 years late) the PID controller. >>> >>> PID controllers do not have to overshoot the desired temperature. It is >>> not an inescapable fact of life. 30 years ago I was designing PID >>> controllers, >>> with a little microprocessor magic, that could quickly arrive at the >>> set >>> point >>> temperature and never, I repeat, never, exceed that temperature. >>> Someone's >>> internal organs would have become toast if it did. >> >> Overshot is fairly easy to avoid for a well controlled PID loop simply >> by setting the damping factor properly. >> >> PIDs is nice in that you can control loop bandwidth and damping factor >> fairly well. Overshot properties vs. damping factor is a well researched >> field and already tabulated before I was born. >> >> Cheers, >> Magnus _______________________________________________ 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.
