> The derivative term is used to speed up the response for an extremely slow > loop ie. temperature. If derivative were used on a fast changing process > variable the output would be out of control. This is because derivative > acts on the "rate of change of the input", the higher the rate of change, > the more the output is boosted. When the input stops changing, the output > returns to its PI value and derivative is disabled. When the temp is > rising the > d term will boost the output up to provide an inrush of cool air until the > temp > settles then the output will drop its "boost" and continue controlling > with the > PI terms. > > -- > JUSTIN GRIMM > Eclipse Energy Systems > http://www.geocities.com/SiliconValley/Ridge/1839/ > I'll admit, this makes sense. However, the system I was using had a thermal time constant of maybe 4 or 5 seconds, and we needed accuracy over and above response time. Adding a derivavtive term did not in any way improve the response time of the system, and in fact slightly worsened the stability. This seemed to be because the proportional term had enough gain to drive the output to maximum for relatively modest errors. The sytem being controlled was a tunable laser, heated or cooled by a thermo electric cooler and it had to keep the laser within 0.1 C of it's setpoint over the range 0 to 80 C which it did achieve. Apart from switch on, there should be no sudden large errors , just a small long term drift which can be compensated by the P and I terms. Another reason we chose not to use Derivative is that the TEC can take a lot of current (up to 1.5 amps) and the addition of a D term would mean small errors could cause large current spikes, something we wished to avoid. What I am trying to say is that Derivative *may* not be needed, depending on the application. If you could accept the response may be slowed, the reduction in complexity of the code and setting up may be worth it. Regards Mike Rigby-Jones