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problem of noise with a 1212C

Created by: Jean V. at: 5/30/2013 9:18 AM (9 Replies)

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5/30/2013 9:18 AM	Rate (0)
Jean V. Posts: 20 Rating: (0)	Goodmorning, I have also a problem of noise with a 1212C. The analog input varies much more than the current measured with a multimeter. For example, if the current measured is 4.9mA, the value displayed (after conversion, of course) in the "force table" shows sometimes 4.8mA, 4.9mA, 5mA, or higher even 5.3mA, and this happens very fast, however the current displayed on the multimeter does not vary. My questions are: Is this kind of variations normal? It seems to me that they are too big. Is there a solution for the analog inputs not to vary so much? Like a capacity, as explained by Huggy.
Jean V. Posts: 20 Rating: (0)	Last edited by: O_Moderator at: 5/31/2013 10:01 AM new subject after splitting from /tf/WW/en/Posts/87771#top
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5/30/2013 9:50 AM	Rate (0)
BryanG2001 Posts: 113 Rating: (22)	Try searching here for 'S7-1200 analog noise', you will find plenty of information.
BryanG2001 Posts: 113 Rating: (22)
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6/1/2013 8:30 PM	Rate (1)
Tomy Master Gold Member Joined: 5/4/2009 Last visit: 11/23/2024 Posts: 914 Rating: (40)	Dear friend, I have faced a similar problem before, the solution of your problem is could be by adhering some wiring rules like for example signal cables must be shielded and the shield must be connected to ground before it reaches signal module, also twisted pairs for high speed digital signals, try to isolate your controller from high EMI sources and try to isolate signal cables from power cables. For more information you should refer to manual of S7-1200 controller "Installation Chapter" and your wiring must comply with the manual. Have Luck Tomy.
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6/1/2013 8:49 PM	Rate (0)
Tomy Master Gold Member Joined: 5/4/2009 Last visit: 11/23/2024 Posts: 914 Rating: (40)	Ohh one another thing, If you do all the wiring (including good grounding) by the book and the problem stays, I am afraid that you should use signal isolator that will do the buffering job (including signal filtering) for you. Have luck Tomy
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6/2/2013 11:28 AM	Rate (0)
IBN-Service Posts: 8946 Rating: (999)	Jean V. ... Is this kind of variations normal? It seems to me that they are too big. ... NO, that is not normal. Is your program working correct? Also, please have a look at tomy's advices.
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6/2/2013 6:33 PM	Rate (1)
BryanG2001 Posts: 113 Rating: (22)	'The value displayed after conversion' varies a lot, how much does the raw analog input vary? If the raw analog value is stable then you have a problem in your programming. If the raw analog value is unstable then you probably have a problem in the wiring or a faulty analog unit. A multimeter reading is fairly heavily damped so you may not see noise, also the multimeter will be isolated and so won't see problems due to inconsistent commoning of the negative.
BryanG2001 Posts: 113 Rating: (22)
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6/7/2013 6:48 PM	Rate (0)
Jean V. Posts: 20 Rating: (0)	Tomy Master try to isolate your controller from high EMI sources and try to isolate signal cables from power cables. Tomy. The problem is that I can't isolate the meter (PT100) from the power cables because this meter needs to be powered by the same cables as the analog output of the PT100. Concretely, I must supply the PT100 with 24V and the PT100 gives me the current in the same circuit. So, the power supply is in serie (is it correct in Engish ?) with the analog output. I send in attachment the manual with the wiring (page 38).Sorry, this is in French. Maybe this kind of wiring doesn't fit with the Siemens system... Faithfully yours, Attachment SONDE PT100 (TMR31).pdf (177 Downloads)
Jean V. Posts: 20 Rating: (0)
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6/7/2013 7:00 PM	Rate (0)
Jean V. Posts: 20 Rating: (0)	BryanG2001 how much does the raw analog input vary? If the raw analog value is stable then you have a problem in your programming. If the raw analog value is unstable then you probably have a problem in the wiring or a faulty analog unit. Thank you for your advice on the multimeter, I didn't know. The raw analog value vary proportianally with the converted value (if I remember well). I have presently no access with the CPU (I work on another project), but I want to know why it happened so. Moreover, I tried to visualize the values with an empty program, and the result was the same. Consequently, I think this would rather be a problem of wiring. Faithfully yours,
Jean V. Posts: 20 Rating: (0)
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6/7/2013 7:06 PM	Rate (0)
Jean V. Posts: 20 Rating: (0)	Thank you for the connection of the shield to the ground. I shall try, the next time.
Jean V. Posts: 20 Rating: (0)
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6/7/2013 8:53 PM	Rate (0)
BryanG2001 Posts: 113 Rating: (22)	OK, so this is a PT100 teperature sensor with a built in 4-20mA transmitter. Because of the construction of PT100 sensors they normally have electrical isolation from their surroundings, but can be a bit slow to respond. The bit that concerns me here is the description: 'The Easytemp® TMR31, TMR35 has a new kind of thin-layer sensor element that is soldered directly into the sensor tip.' That makes me wonder if the device is electrically isolated, if not then that could be your problem, any voltage difference between the location of the sensor and the negative at the PLC will cause the 4-20mA signal to vary. This is normally solved by adding a 4-20mA loop isolator. Something like this: http://www.status.co.uk/product/21_provides-isolated-power-for-a-transmitter-derived-from-a-powered-loop.html The other possible thing is that the variations you are seeing are quite normal. The sensor is a fast acting type so maybe the temperature at the sensor tip is varying rapidly. If that is the case then you can programme a simple averaging function to smooth the reading.
BryanG2001 Posts: 113 Rating: (22)
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