I need to measure the resistance of a wire. i am using siemens S7-300 PLC with Profibus Distributed I/O. The Resistance is between 2-3Ohm. needed accuracy of 0.5%
thank you.
Why not pass a known, stable current through the wire, and measure the voltage drop? You will have to do a little experimenting to get exactly what you desire, but this should work quite well.
Is this 2-3 Ohm (0.5% accuracy needed) a sensor you want to monitor? Or, is this the resistance for a connecting wire that you want to compensate? A little more details about your sensor / process / application might bring an easier solution. If you want, you can contact me at [email protected]
If you want to stay with your solution I would recommend an analog to digital converter. 0.5% accuracy means 1 part in 200. Technically an 8 bit AD converter (1/256) should do the work. A typical 12 bit AD is more than enough.
One solution could be an AI card 6ES7 331-7NF10-0AB0 and 1.6 A current source (with very good stability). Put the card into 1-5 V mode then 3.2V - 2ohm and 4.8V - 3ohm. You should achieve 0.3% accuracy from the card plus error introduced by the current source. Other solution is to use a digital multimeter with RS232 interface + the CP340 card for S7 + some programming of comms interface in Step 7.
