I have a ControlLogix 5000 series processor running ControlNet. I want to measure the current useage of fuel valves on an industrial turbine engine. I have installed several "shunts" inline of know resistances (0.1 ohms) and would like to measure voltage drop across this resistor to calculate amperage use on various parts of the fuel system while in use. A reading of 10mV equates to 1A. This is a 24vDC fuel valve system. I have spare analog input modules (1794IE8). This module has a common 24vDC backplane. I want to measure a value of less than 1 volt drop but the shunt voltage is 24vDC. How can I do this? Is there another module (ControlNet) that I could use to do this? Thanks for any feedback.

 

I would use a converter to take the mV signal to something useful like 0-10 or 4-20mA. The Flex analog input card you have will accommodate either. Take a look at Action Instruments or Moore Industries.

 

Why not put each differential through a signal conditioner and get out an isolated 0-5/10VDC or 4-10ma signal. I guarantee there is an analog module for the CLX that can take one of those formats. Also, check your scaling if you choose to feed into the modules directly. 1A through 0.1 ohm will develop 100mv, not 10mv.

 

There are DC-DC isolators that will convert between voltage ranges. Phoenix and various other similar companies make these. I am not familiar with the analogue input module you are using, but this sort of signal conditioner should be able to convert your milli-volt signal to 0-10V or 4-20ma, or whatever else your module is using.

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Michael Griffin
London, Ont. Canada
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Brad---

This is not a good application for the IE8 module, but since it's there and has spare channels, put an instrumentation amplifier between the shunts and the IE8. The manufacturer of the shunts probably has a product to sell or recommend.

Hope this helps!

Larry Lawver
Rexel / Central Florida

 

To measure the voltage across the shunts with Flex i/o you would need to use a 1794-IF4I to take advantage of the isolation between channels.