Where industrial type "conductive sensor"??

[keithkyll]

What if I ask whether any industrial sensor that measure OHM and send signal to PLC? (Please don't tell me an ohmmeter or a multimeter)

A meter that does a 4-wire Kelvin resistance measurement.
Read this.
See this one from Duncan. You need model 5002 with lower current. Sold as Cropico in the UK.
My first choice would be Keithley, but expensive.

Low cost, but no PLC interface is this one from Extech. About $500.00. Similar from China on eBay.

 

[daba]

RTD = Resistance Temperature Detector

 

[Paul Eagen]

You could use a sourcing plc card. sourcing card is looking for 0v. No voltage means no arcing and safe.

 

[Lancie1]

What I want is to detect whether a metal strip (5" long) sits on both point A and point B. I need a little voltage on point A. If the metal strip sits on both point A and B, then this system becomes a close loop.

Your answers are ranging over a wide area. That shows that your question is too broad with not enough details. If you are withholding information due to patent concerns, then you have to live with the poor results here.

Technically, to detect a closed contact between A and B with a metal strip and having voltage on A, all you need to do is connect the (-) side of your voltage source to the Common of a PLC digital input module, and B to a digital Input terminal on the same module, then have the PLC program look to see if the input goes ON. That is all.