How to Build a Current Loop System with CompactRIO? Hardware: Compact FieldPoint>>Analog I/O Modules
Problem: I want to build a 4 - 20 mA current loop system with CompactRIO (cRIO). Which module should I use and configurations are possible? Solution: When picking your cRIO module for a current loop application, there are a few aspects that are important to note since not just any basic Analog Input module will suffice. Isolation - Without properly isolating the DAQ device, a ground loop can form. Ground loops may cause offset errors in your measurements. Also, large common mode voltages caused by ground potential differences can dammage any DAQ Device. Shunt Resistor - Most all DAQ devices measure voltage. A highly effective and accurate method of measuring current is to place a precision shunt resistor between the leads of the DAQ amplifier to convert the current signal into a voltage measurement. The CompactRIO modules that contain these two aspects and are suitable for current loop systems are: NI 9203
NI 9219 Two configurations for cRIO are both without an external power supply. Meaning, the current loop system needs to have its own power supply as in Figure 1. For the NI 9203, there is Channel to Earth isolation. This allows for all the COMs to be tied together as in Figure 1.  Figure 1 The NI 9219 has Channel-to-Channel isolation. Each LO will need to be tied back to the power source as in Figure 2 unlike the NI 9203 where all the commons are tied together.  Figure 2 Related Links: Developer Zone Tutorial: 4 – 20mA Current Loop Tutorial Attachments:
Report Date: 02/21/2008 Last Updated: 04/01/2008 Document ID: 4IKHF3SG |
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