Signal Filters in DCS 3

[controlnovice]

So, the filtering of signals should be in the DCS. That is what I was leaning toward.

Should different process signals have different filters?

Should Pressure have a second order filter? Temperature not have a filter? Flow have a first order filter? level?

Or to make things more simple in the DCS, just make all analog input have the same filter?

 

[CARF]

Or to make things more simple in the DCS, just make all analog input have the same filter?

Of course not, the main idea of filtering is to get rid of unwanted noise without changing the underlying signal containing information about flow, pressure, level, pH.
Over-filtering (too long filter times) will choke your signal to death (lose information). Especially signals feeding PID-controllers are important. The control performance will go down if you filter the input signal too much. Most of the time I kick out ALL filters, unless I have a very good reason to apply (noisy signal).

Just some rule of thumb:

- pressure, flow: 1st order 5 - 30 seconds
- level: 1st order around 30 seconds (filter the waves out, not the real level
- pH, temperature: no filter (very slow signals, no noise)

2nd order? Don't do this, keep it simple!

Hope this helps,
CARF

 

[ilan7]

Filtering the signals is due to:
1. Measuring parameter.
2. Nature of the Control loop the signal is the input for.

Measuring: When measuring Temperature, PH, the changes expected to be very slow. In this case, we use relativly long time filter - 10 to 20 sec.

Other inputs - depend on the signal and the control loop. If the control loop needs to control accurate level or pressure, filtering must be limit to few tenthes of a second or not at all.
Same with signals involved with shut downs like over pressure or overspeed. Filtering is important to get read of fault alarm, but need to be short to catch a true failure in time.

 

[CARF]

Nice link:

Choosing the Best Control Loop Filter

In general, the best type of filter is a second order Butterworth, but a simple first order filter can do much to reduce controller output jitter and save the life of your valve or variable frequency drive. Since most systems have first order filters built in, it is usually a good choice.

http://www.automationtechies.com/sitepages/pid1479.php

Does any of you have more good links or books on "Signal Filtering in Industrial Process Control systems"?

Thanks for sharing knowledge,
CARF

 

[buzzp]

I don't think it is a good idea to apply filters generically. Every application/installation will be different. You need to know what your trying to filter, the bandwidth of the important info, the repsonse time, etc to properly apply a filter without creating more expense/work.

 

[nbucska]

Try to design the system for low noise, use filter only
to reduce unavoidable noise.

use e.g. proper grounding, local preamplifier, twisted
or coax. wire, digita (not analog) transmission,low-noise sensor, software filtering with proper algorithm,
eliminating local noise, reduce pick-up with shielding,etc.


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032