Analog Input on HMI screen not Stable

Hi All
We have a pressure transmitter (a very cheap one which can not be calberated), whose input is taken from the channel input and send into a SCP instruction for scaling before being displyed on PV1000 screen as a pressure reading for the process. The problem is when the process is running, this value on the PV screen is jumping up and down all the time, and it is not stable. Is there a way if i can make this value stable (get rid of the jumping so fast) by taking the average value or something before display it on the screen? All suggestion will be appreciated. Thanks in advance

you might want to check this current thread.


Brian.

Which PLC are you using?

You should be able to do a running average, the longer the average time, the more stable the reading, but the longer the average time the more response lag.
You will have to balance the two.

See this thread for a discussion on the topic of filtering values for more stable display:

http://www.plctalk.net/qanda/showthread.php?t=20246

hi Guys thanks for your input. I am using SLC 5/04 processor with PV1000 HMI screen.

I am not the best at this but I have a few thoughts. How is it scaled? Are you directly taking the analog and converting it to a pressure reading? What value are you using for the Input?

Example: Lets say you are using 0-4095 for the input and reading 0-100 psi then you will have 40.96 units input per unit output BUT if your resolution is 14 bit then it would be 16384/100 = 16.384...which means that for every 16.384 units of change the display will change, in the above case it was 40.96 units of change.

This is just to show that you can minipulate the data primarily for display purposes but this process will affect the resolution that may be required for the process...IT depends, you can separate them but then the display may not accurately represent the process...again IT depends.

Another option, for the display part, may be to use a timer and just take the value every second or so. May look at using one of the ticks available with the clock status bits.

All of the above assume there is no problem with the process as programmed and you just want to stabilize the display aspect. I did not mention it but if you are not using a floating point destination then consider it, depending on how you define the resolution you may be able to move the fluctuation to the decimal points. IF you are displaying 0-100 then modify the program and display for 100.0.

If you would like to average the incoming numbers this may help:

Alaric said:

See this thread for a discussion on the topic of filtering values for more stable display:

http://www.plctalk.net/qanda/showthread.php?t=20246

Click to expand...

What does this have to do with keeping the display stable? If you mount the display in concrete is should be stable then. Maybe it needs a gyroscope. Who makes the most stable HMIs?

Its gone now, not to return for another year or so.

I screwed up

Example: Lets say you are using 0-4095 for the input and reading 0-100 psi then you will have 40.96 units input per unit output BUT if your resolution is 14 bit then it would be 16384/100 = 16.384...which means that for every 16.384 units of change the display will change, in the above case it was 40.96 units of change.

Click to expand...

The units should have been 40.96 and 163.84...ie the higher the resolution the value will have to change more before a change occurs.

How long has the system been running? Has it ever been stable?

The AB cards have a filter built in to the cards which can be set up in the card parameters. Changing this value has been enough to stabilize a lot of noisy AI points for me. I have had a few cases where the input changed drastically - pressure in a vent stack from the wind blowing hard across the top of it, and weigh belt inputs where slugs of product go across the weigh point instead of a steady flow of material. In those cases I used a filtering algorithm in the ladder logic.