hi all,

i don't have a clear idea about it, one of the options.

this is about putting filter in the control system. the idea is to put first order lag. i am not sure currently if the dcs supports it (i think in all likelihood, it should).

fundamentally, how does a first order lag differ from putting damping in the instrument itself?

as was discussed in an earlier thread (the demerits of damping), would then a filter be a good option?

 

> fundamentally, how does a first order lag differ from putting damping in the instrument itself?

Most instruments implement their filter as an exponential filter which gives a response suitable for use with PID tuning algorithms. I believe that the damping and filter you talk about are the same but their usefulness is different.

If you put the filter in the instrument you have no easy way of seeing what the real instrument measurement is (perhaps you have a high frequency oscillation rather than pure noise) so if you need to do further troubleshooting the instrument filter will be removing fidelity from the system. If you filter in the PLC/DCS then you are able to monitor the raw signal data but only take control actions based on the filtered data.

The other problem with having filters in the instrument is that if the electronics get changed there is a chance the filter will be removed. If you do it in code, then the filter stays there until someone changes the code.

Chris Jennings

 

Didn't we have this conversation before? "Filtering" and "Damping" are the same thing.

If you want to get all technical about it you can go look up second order filters, butterworth filters etc. But basically the previous conversation still stands - Don't filter your measurement until you understand why its noisy in the first place.

Rob