-----Original Message----- From: Michael Rigby-Jones To: 'Lawrence Lile' Date: Tuesday, May 18, 1999 9:48 AM Subject: RE: PID algorithms in PIC C >OTOH, the oven can't be any more on than on, so how does I help when the >oven is too cold? > s'easy - when the oven is on, it is on with a duty cycle between 0% and 100% which achieves proportional control. Howver, unlike a motor that can turn both ways, a heating element can heat but it cannot cool. So when we want the oven cooler we just have to wait, whereas if we want it hotter we can duty cycle a measured, precise amount of heat. The I-term can help achieve a setpoint with minimum error but only in one direction. In the other PID application floating around the List today, using the peltier junction, which can both heat and cool, you'd probably only toss the I-term, if at all, when P=0, like Tjaart suggested. Maybe never. > > >> In my oven, when it is too hot I turn the output to the heaters off, and >> also set the I term to zero. The controller can't GO any lower than off, >> so >> I term doesn't accomplish anything, and I don't want positive values of I >> stacking up and canceling the negative values of I which will be seen >> after >> the oven cools below the setpoint. I think you need to dump the I term >> every once in a while. When P=0 would work great if you have proportional >> control of your output. >> >> >> >> -----Original Message----- >> From: Tjaart van der Walt >> To: PICLIST@MITVMA.MIT.EDU >> Date: Monday, May 17, 1999 11:54 PM >> Subject: Re: PID algorithms in PIC C >> >> Good advice. I have noticed another thing that can make your >> system much more immune to the low-frequency I-oscillations : >> Reset the I-value to zero whenever P=0. >> >> -- >> Friendly Regards /"\ >> \ / >> Tjaart van der Walt X ASCII RIBBON CAMPAIGN >> mailto:tjaart@wasp.co.za / \ AGAINST HTML MAIL >> |--------------------------------------------------| >> | Cellpoint Systems SA | >> | http://www.cellpt.com | >> |--------------------------------------------------| >> | http://www.wasp.co.za/~tjaart/index.html | >> |Voice: +27-(0)11-622-8686 Fax: +27-(0)11-622-8973| >> | WGS-84 : 26010.52'S 28006.19'E | >> |--------------------------------------------------|