Unstable Analogue Value

Hi, I wonder if anyone can help with an issue that I have with an AB CompactLogix system. The PLC has an analogue I/O module 1769-IF4XOF2, into which is fed a 4-20mA signal from a 0-25bar pressure sensor. This value is used in the program and is also routed via one of the output channels to a PID controller which operates a control valve. The problem is that the value is very unstable - it constantly jumps up and down by about 0.5bar. We've tried bypassing the PLC by connecting the sensor direct to the PID controller and this then gives a steady reading, so the PLC is obviously introducing the variation.

My program uses a MOV instruction to read the input value (Local3:ICh1Data) and transfer the value into the output (Local3:O.Ch0Data). I'm not very familiar with AB PLC's as I use Siemens mostly, so I'm wondering if perhaps transferring the signal from an input directly to an output in one instruction is where the problem lies? Could I have missed a vital configuration setting?

Try trending these two channels and see if they both are unstable. That might reval something.
I personally would bring the input channel into an tag, and then move the tag to the output just to buffer the data.
Is it possible you have a grounding problem between these three connections?

I usually filter the pressure values. That and only use PI control.

Good Luck,

Yosi

what type of power suppky is powering the sensor?
there are two types of power supplies, switchers and linear.

if you are using a switching power supply and it has a bad ripple noise, that may be part of your problem.

ground the shield of the sensor at only one location. i try to use the plc chassis.


hope this helps,
james

Even if you fix the noise problem......

If the PID controller was one of our controllers I would insist the that feed back come directly to our controller to avoid phase lag and sample jitter. The PLC could read the information from our controller over Ethernet.

Thanks for all your replies. I'm coming round to the opinion that it may be better to split the signal from the sensor direct to both the PLC input and to the PID controller, and do away with the need for the output. I will look into the possibility of grounding problems.

Looking through the manual for the analogue I/O module I found the following cryptic note under the heading of Input Data File:

"Input words 6 and 7 contain the Output Data Echo/Loopback
information for output channels 0 and 1 respectively. Bits 0
through 6 and Bit 15 of words 6 and 7 should always be set to
zero in your control program. If they are not set to 0, the invalid
data flag (Ex) will be set for that channel by the module.
However the channel will continue to operate with the
previously converted value."

Could this be relevant to my problem? - I haven't done anything like this in my program.

Aw you describe it, it sounds more like a missing common ground problem.Is the DC supply common to both sensor and PLC or do you have differnt supplies for the two?

The sensor and PLC input are fed from the same DC PSU, it's a 2 wire loop powered type. I'm told that the supply is earthed, although I've not yet been on site to see the problem for myself.

Noise on analog signal

http://automation-control-blog.blogspot.ca/2014/08/the-secret-of-getting-rid-of-noise-on.html

The above link will show you how to insert a capacitor to filter the noise on your analog signal. It has helped me.

Thanks for the link. Unfortunately the server here blocks all blogs - however helpful they be to be to me in doing my work...

I'll look at tonight when I get home.