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Analog (EM231) problems

Created by: KJLRWW at: 5/6/2009 2:45 PM (10 Replies)

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5/6/2009 2:45 PM	Rate (0)
KJLRWW Posts: 5 Rating: (0)	Hi All I currentely have a CPU226 with a EM231 (6ES72310HC220XA0). We are using channel A and B for a 4-20mA Var transducer input. We have a HMI connected to the CPU and this is displaying the scaled VAR value. We are having an issue when the 4-20mA signal is sent to the card, we are having wild fluctuations on the reading. It reads the correct value for a few seconds and then drops to bottom of scal for a few milli seconds and then goes to top of scale for a few milli seconds. This is playing havoc with our control system. Has anyone encountered this problem on S7-200 and how can we fix. Thanks in advance.
KJLRWW Posts: 5 Rating: (0)
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5/6/2009 5:14 PM	Rate (0)
Ayman Elotaify Platinum Expert Joined: 5/28/2008 Last visit: 11/6/2024 Posts: 4516 Rating: (839)	Hello, Please check that cables of each transducer is shielded and connected to the ground. Regards
	Ayman Elotaify
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5/6/2009 5:17 PM	Rate (0)
safety4ever Experienced Member Joined: 12/5/2006 Last visit: 2/6/2024 Posts: 73 Rating: (4)	Hi KJLRWW, a very common issue with Analog variation is the proper connection of the M terminal orgrounding interferences. I found this usefull link related to your problem it might help you fix your issue. http://support.automation.siemens.com/WW/view/en/2415890 Regards

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5/7/2009 7:15 AM	Rate (0)
Arunakat1 Posts: 217 Rating: (52)	Hello, A little bit more,you can adjust the filtering time of the analog input. If the correct value stays more time (and there is some amount of gap between noise and actual value) then this will be effective but it may slow down the response of AI. For that go to system block then to input filters and select analog inputs or click the link given by safty4ever. Before this please check the grounding and cabling as said by Ayman Elotaiy earlier. Best regards.
Arunakat1 Posts: 217 Rating: (52)
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5/7/2009 7:26 AM	Rate (0)
KJLRWW Posts: 5 Rating: (0)	Hi Ayman - Both 4-20mA signals are in shielded cables and one side of the cable is grounded. HI Safety4ever - I did see that comment regarding the common mode voltages. We have a solid earth in the panel and the analog card and CPU are earthed to a common earth. It has been suggested by some PLC people that I have talked to that we might need to earth the return on the 4-20mA signal to the same earth as the analog card. Our normal operation is to leave the 4-20mA signal floating as it is isolated on transducer side. Can anyone comment as to whether the grounding of the return should be problamatic? We have installed about 5 systems with the same configuration and no problems and all of a sudden 2 new instalations are having these problems. Has anything changed with the card?
KJLRWW Posts: 5 Rating: (0)
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5/7/2009 7:29 AM	Rate (0)
KJLRWW Posts: 5 Rating: (0)	Hi Arunakat1 Thanks for the response. I am going back to site tomorrow so will try the filtering option. The problem is that it doesnt look like noise as the signal is droping to bottom of range and then going to top of range and then settling on correct value for a second. It isnt jumping around an set value that would be seen with noise. Thanks
KJLRWW Posts: 5 Rating: (0)
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5/7/2009 4:17 PM	Rate (1)
safety4ever Experienced Member Joined: 12/5/2006 Last visit: 2/6/2024 Posts: 73 Rating: (4)	To confirm the answer to your Question KJLRWW whether you should be grounding the return of your sensor or not. If you look at the S7 200 manual http://support.automation.siemens.com/WW/view/en/1109582 If you go in the appendix Page 428/554 the drawing for the 0CH22 shows that the M is grounded, now why is it not the case with your other systems now this requires a bit more analysis on site and studying the wiring conditions of each system individually also the components installed and their revisions. Regards

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5/11/2009 7:34 AM	Rate (0)
KJLRWW Posts: 5 Rating: (0)	Thanks All. I was on site yesterday and it turns out the PLC was powered by a temp UPS supply. We connected the standard VT supply and the problem was sorted...
KJLRWW Posts: 5 Rating: (0)
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5/12/2009 6:11 AM	Rate (1)
Arunakat1 Posts: 217 Rating: (52)	Hello, Nice to hear every thing is OK. And many thanks for your feedback, which is very valuable to us. Best regards.
Arunakat1 Posts: 217 Rating: (52)
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5/12/2009 7:26 AM	Rate (0)
Arunakat1 Posts: 217 Rating: (52)	Hello, I didn’t mean (feedback) rating my answers. I mean that the feedback of you (How you correct your problem) is very valuable for us when we face same kind of problem. Best regards
Arunakat1 Posts: 217 Rating: (52)
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