damping

E

Thread Starter

engineer

hi,

i am using a transmitter for level that has option for damping setting. the process is very noisy. should i use dampnig? how much time (sec)should be fine as the damping factor?

well, whatever i do, i will check it's effect on measurement and control on DCS, but when is damping used in general?

fortune- chaos is increasing.
 
No - Don't put damping in the transmitter, it's a really bad idea. Filtering the input signal at the transmitter simply hides problems in your process.

Why is your measurement noisy? Is the process really varying that much and why? Is the transmitter installation bad? A level transmitter based on weight measurement can have noise if the vessel is vibrating. Is this the case and why?

If you've checked all the above (and more) and the problem doesn't go away, then you will probably need need to use some filtering in your PID controller. You display and trend the unfiltered measurement but you control on the filtered measurement. Keep this filter time as short as possible. It is a common mistake to put too much filtering on a PID loop. You only want to remove the high frequency noise that makes your P action or D action changes too aggressive. Use a Derivative Filter if your loop has one and you are using D.

A better technique to filter measurements for control is to oversample the MV and filter that. Eg, if your PID loop runs at 1 second - sample the MV at 200mSec and apply a 200mSec filter to it. That way you can filter out noise without significantly affecting the time constant of the control loop.

Rob
www[.]lymac.co.nz
 
C

Chris Jennings

I tend not to apply filtering or damping at the instrument because then you can't see what is actually happening. If you apply the filtering in the DCS/PLC then you can still trend the unfiltered signal but the control can be performed on the filtered signal. I like to use an exponential filter so that it is a first order response, using something like a average block doesn't work as well due to the difficulty in matching it to normal PID tuning values. Most instruments provide an exponential filter as their standard filtering option. You need to understand the response time of the loop and how it has been tuned to make sure that your filter setting won't slow the loop response down.

You should look at what is causing the noise, is it process or interference?
 
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