New and strange issues with analog signals discovered these past two days
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The last edit to this post was made by Pulsing melody on 2009-11-29 at 22:38. Our extruder’s PLC has two analog signals that come from transmitters located on the MCC cabinets; these transmitters generate 4–20mA signals which are sent to the PLC’s AI card (Siemens SM331-7KF02-0AB0). When the connection at MCC was disconnected, a voltage of 14V was measured. It has been found that if both signals are connected to the PLC terminals, no data is displayed on the Japanese control panel, with the values reaching their maximum levels; however, if only one of the two signals is connected, data can be displayed on the Japanese control panel. What’s even more strange is that these two signals interfere with each other; when I connect one of the signals using two wires, and either disconnect one wire from the other signal or disconnect both wires, the value of the signal that is connected properly changes. Later, I asked the construction team to disconnect the positive terminal of one of the signal circuits and connect an ammeter in series with the negative terminal; as a result, a current of over 90 milliamps was detected. At this point, only the negative terminal of that signal is connected. There should be no current; this issue exists in the other signal as well, though not to such an extent. Let’s see where the problem lies The Japanese say that the wiring inside their cabinets is fine; it is tested before leaving the factory.
The electromagnetic interference in the MCC area is very strong; it may be caused by electromagnetic interference. Shielded cables should be used, and proper shielding grounding must be implemented on the control room side.
I agree with what was said above: ensure proper grounding and shielding of the signal wires to minimize interference as much as possible. Other on-site display instruments (that receive 4~20MA) can also be tried to see if they can display the data properly, before making a further judgment.
I agree with this view; it’s possible that the grounding in the control room isn’t properly connected. It is recommended to check this carefully
Thank you all. I’ve identified the problem: both signals originate from the same module in the MCC, and that module can only output one analog signal at a time. However, the design calls for two outputs. As a result, when both signals are sent to the PLC, problems arise due to interference between them. Now one signal path has been removed, and only the other signal path is used.
Thank you to the original poster for bringing things to a conclusion and letting us know the real reason behind it. I also encountered a situation on site where the output current of an electromagnetic flowmeter was as high as over 90 mA, but I suspect that my multimeter was faulty… because I couldn’t measure it using another multimeter
It is likely that the interference on the wires coming from the MCC cabinet is too high; shielded cables should be used, with proper grounding of those cables. In the case of high-frequency interference, dual-end grounding should be employed, while single-end grounding is sufficient in other cases.