Low-pass filter

[Flex727]

Has anyone ever tried to implement a rudimentary low-pass filter in ladder logic? Is it even practical?

[Ausman]

More info please, Flex! 

cheers, Aus

[Flex727]

I think it's a futile request. Sometimes vibration sensors can be affected by noise, or higher frequency vibration that isn't useful to measure. For instance, with a large pump or motor, you're probably looking at seeing a problem with a rotational bearing, or something similar, which will result in a low frequency vibration. Any high frequency vibration might mask the signal you're looking for or cause false positives. I know it can be a common practice to place an electrical low-pass filter between the sensor and the ultimate measuring device, but I was thinking maybe there was a software way to accomplish this. I suspect there is really very little (or no) frequency information available to the PLC on the 4-20mA analog input, but thought I would check it out anyway.

[hotwires]

They have the 50 or 60 Hz noise rejection settings on Unistream 8 ch AI, I suspect it is a software based notch filter.  A ladder based filter while not impossible would be a bit over my head. Good notion, Nobody likes false alarms or measurements! 

[AlexUT]

Hi,

Low-pass filter in Ladder is more like to DSP (Digital Signal Processing), which require special hardware module, or lot of resources.

Some PLCs have a Filter option in Hardware Configuration, and other PLCs can use Filter option from FB's Menu.

Please look an next link from Control Engineering:

Ladder logic 402: Data manipulation and filtering. Frank Lamb.

http://www.controleng.com/single-article/ladder-logic-402-data-manipulation-and-filtering/e589114a10276058909584cea125cbcf.html

Please remember, that conversion time for analog inputs will be number of system scans.

B.R.

 

[Flex727]

Great source. Thank you, AlexUT!