Hi Y'all, thanks in advance for any advice.

I've got 23 IFM PN2670 pressure sensors going back to three Siemens S7-300 analog input(331-7kf02-0ab0) card in 2 wire configuration. The sensors and 3 analog cards are all powered by the same 24v 7.5A power supply. I have a 25 conductor cable(shielded on the outside but not in between the 25 wires) routing power to the sensors and routing their signals back. The signal cables all go to the negative terminals of each cards channels. The 0V of each sensor is tied. All 23 sensors are totally wired up, but only 4 are connected to pressure.

I can have up to four sensors powered and running signal back to my PLC with pressure readings within 10 PSI of what the sensors read locally, but once I add a fifth sensor, the signals all jump up a few psi, and it gets increasingly worse the more sensors I add. Once I have all 23 sensors powered on and sending signal to the PLC, each sensor remotely reads about 3000 PSI despite only four being connected to pressure. The readings get increasingly worse with any combination of my sensors.

I tried running a new, 8 conductor, shielded, twisted pair, larger gauge cable and used it to add the fifth and sixth sensor signals, but still ran into the same issue of the pressure readings getting worse at the addition of the fifth sensor, and then even worse at the addition of the sixth. I then used this new cable to route power to the sensors and had the original cable just route the first 6 signals back to the plc, and still got the same issue.

I don't think I have a ground loop because the 0V of the analog cards and sensors are all tied together, I don't think I have an external noise interference issue because the first four sensors read fine on their own, I don't think it's an internal noise issue because I got the same issue when running additional signals through a separate, better shielded cable, and I don't think it's an issue with noise interference from the 24V power because I got the same issue when I ran power in a separate cable from my signals. I also don't think it's an issue with wire insulation because I get the same signal issue when running signal in a separate cable.

I have similar pressure transducers of the same brand in other applications wired identically to the PLC and reading accurately, and when I do just one sensor at a time in my current application it reads fine on the PLC end, so I doubt my issue is with my wiring set up at my analog card.

I'm running out of ideas as to what my issue could be and would really appreciate any input/advice. Thanks!

 

Also, I have confirmed the signal output mode of the sensors is configured to 4-20 mA

 

The "Getting Started with 4-20mA" manual for the 6ES7331-7KF02-0AB0 module is here:

https://cache.industry.siemens.com/...1_ai_8x12_bit_getting_started_en-US_en-US.pdf

Note that there's a 'measuring range D' for 2-wire current signals, one for each pair of input channels (page 16)

Page 22 states that "2-wire" is for a transducer supplied with power from the analog input module.

You are using an stand-alone DC power supply for loop power.

The symptoms of the problem you describe is a classic common mode, ground loop problem, which manifests itself as an offset on the signal(s), because the ground loop current adds or subtracts from the regulated 4-20mA current, depending on polarity.

It is very common for one or two signals to work, but the common mode is exceeded by adding an additional signal, all of them show an offset.

The magnitude of the offset relates to magnitude of ground loop.

Why do you not use the available DC power from the AI module for loop power for the transmitters?

 

The S7-300 analog input card should be setup as 4 wire configuration, for 3 or 4 wire devices. Wire OUT2 of IFM PN2670 to the positive terminals of each cards channels. Wire 0V to the negative terminals of each cards channels.