RS5000, input filtering

Can someone tell me the correct way to filter an analog input to get stable readings on my HMI? We recently configured an input for motor amps and we want to filter it so that it is not jumping all over on the screen.
Thanks

The first place to start, is on the module property pages, the "Channel Configuration". Standard Logix 5000 analog input modules have both notch filtering, and digital low pass filtering on a per channel basis.

Second, you can use the old standby in logic:
Filt = (Unfilt - Filt)/Constant + Filt
where Constant can run from 5 to 100 or so, trigger that every 100ms or so.

Third, you can probably filter the value on the drive output.

But, do you really want it filtered? Is the signal actually that variant? I generally like to datalog and trend unfiltered, but display a filtered value for the operators.

Sorry, I hit my mouse before I completed my thought.

However, your last suggestion is what we are attempting to do, capture the raw input, meassage it somehow and send a filtered value to the HMI screen,

Dan

You could try using the moving average function in function block, or load up a FIFO on timed intervals in ladder and do the math to average how ever many samples you want.

dangrandon said:

Sorry, I hit my mouse before I completed my thought.

However, your last suggestion is what we are attempting to do, capture the raw input, meassage it somehow and send a filtered value to the HMI screen,

Dan

Click to expand...

Uhhh, and? I listed three ways. The first and the third act on the actual raw I/O tag. The second requires that you read a new tag on the HMI, the 'Smoothed' value.

You want logic for method 2?

BST XIO MyTmr.DN TON MyTmr 100 0 NXB 

XIC MyTmr.DN SUB RawValue SmoothedValue TempReal DIV TempReal 20.0 TempReal 

ADD TempReal SmoothedValue SmoothedValue BND

I don't know what more to say.

rdrast said:

Filt = (Unfilt - Filt)/Constant + Filt
where Constant can run from 5 to 100 or so, trigger that every 100ms or so.

Click to expand...

rdrast, what would you call this?

It's one of the many digital implementations of a first order infinite input response low pass filter.

Keith

kamenges said:

It's one of the many digital implementations of a first order infinite input response low pass filter.

Keith

Click to expand...

Aye, and has been used for many years by lots of people, primarily since its implementation is pretty simple in just about any PLC.

It has a secondary advantage of being easy to understand (functionally) with basic math skills.

There are many other, more developed and specific purpose filter algorithms out there, the Logix processors have 5 separate filter blocks, but honestly, I've never had to use them.