Analog Input Raw Value Fluctuation in S7200

[nil9178]

Hi,
The following problem have been noticed in recent projects which we have executed using S7200 PLC.
We have been using 226 CPU along with EM235(4AI/1AO) and EM231(4AI) for quite a number of projects. What we have noticed during is that, say we have given constant 16mA to one of the Analog Input channel of EM235. In this case the Raw Count should be constant at 25600. But very frequently this number changes by + or - 1500 to 2000 counts.
We also changed the 4 to 20mA souce & this is not working even the source is set to some constant value.
The same is noticed even with EM231 Analog Input Cards. We tried changing the Analog Filter settings. But this is not working.
Need some help.............

Regards
Nil

 

[BryanG]

I would be looking at Power Supply stability then checking the grounding of the screened cable that you should have used for the inputs and finally I would ground the negative of the analogue inputs. I have used a lot of these analogue units without problem so I don't think it is a design issue.

Bryan

 

[nil9178]

Hey Bryan,
Whay ever u said has been taken care. No problem with ground nor with earthing. We have faced this kind of problem even at sites.
At times when client insists on AB or Fanuc PLC, we found that there is no fluctuation at all.
I am facing this problem only with 200 PLC. Infact we have worked with S7300,400 & PCS7, and never we had any kind of problem.
Cheers
Nil

 

[BryanG]

Can you post drawings of how you are connecting to the Analogue modules.

Bryan

 

[Gil47]

I always use a separate power supply for analog signal, and digital signals, as relays switching can and do introduce a spikes on the DC power supply, when switching.

If the wiring to any analog has spare cores, these should be tied to earth at one end also.

If relays are on the same DC power supply, snubbing diodes fitted to all relay coils can help in lessening the effects of spikes.

 

[HJTRBO]

I have not much experience with analogue I/O but is there any filtering options or sampling rate adjustments available?
If everything you say is good, and of course you have a crow on the incoming signal haven't you? Lol. And everything is good with AB products and the Siemens card is not defective then maybe its just a sensitivety issue.

 

[falgunvasava]

Just try joining M and ground,it works, the value sticks.
i don't know if it is right method or not but it works for sure.
i have been successful in a recent project.