Unstable PLC current input
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For Siemens PLC analog inputs, the field device provides a 4-20mA output from a mass flow meter; when the flow rate is 0, the PLC shows fluctuations between 0 and 4 mA. A 4–20mA signal sent from a signal generator on-site shows normal operation. Once connected to the instrument, the display becomes abnormal, with unstable fluctuations.
Modify the PLC configuration to remove the signal below 4mA. Check whether the supply voltage for the mass flow rate is normal? Is it suspected that the PLC card does not have sufficient load-bearing capacity? Or is the impedance of the mass flow meter too high? For current-source type signals, the lower the instrument impedance, the better.
A 4–20mA signal was sent to the field using a signal generator, and the display showed normal operation; therefore, there is an issue with the output of the mass flow meter. The output circuit of the flow meter was checked, and if it could not be repaired, a new one was installed.
It seems there’s still a problem with the mass flow meter; let’s try another one
What does this have to do with PLCs? The signal isn’t provided by the PLC