I have a refrigerated facility that is currently controlled by a Teletrol Integrator 386. The system is becoming increasingly unstable and is need of replacement. I was hoping to use all of the same sensors and replace the controller and IO with Allen-Bradley components. All temperature sensors are thermistors and Allen-Bradley does not have a solution similar to the RTD cards that they offer. Does anybody know if there is a solution that integrates easily with Allen-Bradley equipment (eg. Devicenet, Ethernet/IP)? Any help would be greatly appreciated.

Thanks,
Lee Hyatt

 

Lee,

If the 1756 IR6I card is not suited to your application, I have used in the past a PTC to analog module available from Phoenix or similiar. Scaling can generally be performed on the module but it may suit your purpose to scale via the PLC.

Cheers

 

I would simply use a current source and a "normal" voltage input analog card. An alternative is a bridge circuit, again with a voltage input card. Solve for thermistor resistance (equations in any electronics text or Ohm's law for the current source) then do a lookup table or the equations provided by the thermistor manufacturers. (Hint: Omega publishes a fairly good one). Since you are dealing with a limited range of temperatures, a bridge can be set up for very high resolution and a lookup table is attractive. These are both instrumentation quality methods and the cost is far lower than a specialized signal conditioner that does the same thing internally. If you provide a nominal resistance at your target temperature, range needed and perhaps vendor I could draw up a suitable bridge circuit for a garden variety AB analog card. For
control without indication, you may not even need to solve for temperature as long as you know the voltages for your setpoints. In short, this is not a problem.

Regards

cww

 

Try the 1746-NR4 card for the SLC 500 platform. It handles RTD and resistance. Also check out the UNIVERSIAL ANALOG card 1746sc-NI8U.

 

I think you might well be able to use the RTD module. It puts out a constant current source and measures the voltage drop across the RTD. You would need to scale the result in your PLC but that should not be especially difficult given the fine math functions of AB PLCs.