PID control in Lookout - Gas mixing with 4 flow controllers and 4 PID's

[vtiu]

We are using Lookout Ver. 4.1 and Fieldpoint modules. We want to use 4 PID controllers to control 4 gas flow controllers and build a gas mixing station. The problem is, the gas composition is analysed by a series of detectors and there is a time delay of about 45 secs before a  change in the composition is registered. Therefore the controller is too late in closing the appropriate valve. The PID overshoots and takes time to come back. We can live with small fluctuations around the setpoints of say ±1% but not more and overshooting is definately a problem. We realise that the PID's must work simutaneously and that also causes some problem. We would apprciate any help. Thanks

[Jochen]

A deadtime (time delay) is the worst thing for a control system to handle. A standard PID controller is probably not the best approach to deal with this type of problem and there are several methods (e. g. model based control - Smith predictor) to deal with this problem.

In Lookout your options are a bit limited to implement anything else but PID. In Lookout you could try to de-tune the controller to make it very slow so it doesn't get time to overshoot. Additionally you shoudl be very careful with KI. Maybe you will get the best results using a PD controller.
The best approach however would be to use an intelligent network module like the FP-2000 to run the control algorithm onboard. An FP-2000 can be programmed with LabVIEW Real-Time which allows you to implement your own control algorithms including model based approaches.

I hope that helps,

Jochen Klier
National Instruments Germany