4-20mA signal ELVIS

[RyanG]

Hello, Everybody...

    (First of all, I'd like to say that I did a search for "4-20mA AND ELVIS" and got 0 results before posting)

    I've got a question (addressed below in bold) about using the NI ELVIS board with Lab*View.    I have an assignment as an independent study for one of my professors at Penn State University using Lab*View & the NI ELVIS prototype board.  My assignment is this:

    I am to create two lab assignments using a pre-built "thermal simulator" (more info on that later); one assignment that focuses on data acquisition and graphing temperature vs time, and the other assignment building on top of the first and using PID-type controls to throttle the temperature given a specific set point.

    More about the pre-built thermal simulator:   This device consists of a soldering iron with the tip removed, which is connnected to a PID controller that has the ability to change the duty cycle (on-time) of the solderin iron.  There are two thermocouples (type J or K, there is an inconsistancy in the documentation for the unit).  The thermocouples are each connected to 4-20mA analog output generators, w/ 4mA = 0*F and 20mA = 1000*F.  So, at room temperature = 73*F, the output would read 7.3 %.

    Now for my question:   I'm having trouble using the NI ELVIS unit together with Lab*View to pull in the 4-20 mA signal coming from the thermal simulator.  I just cannot get coherent data.  Can anyone write a tutorial about using the ELVIS unit with analog current outputs (preferably 4-20mA)?  There is little documentation available at school, and unfortunately it all covers the installation of the labview software.   Somebody please message me on this.  I don't know if it's foolish, but I'm going to put my email address up here.  DrPhantum@comcast.net      If it's unadvisable to post my email, I'm sure one of the mods will edit it for me.

Thanks everybody!  Wish me luck!

~Ryan, PA

[Wayne.C]

Ryan,

You want to measure current with an analog input that measures voltage.  That means you need to pass the unknown current through a known resistance and measure the voltage drop across the resistor with your voltage input.  ( OHMS law strikes again. )  You can then calculate the current in your program.  Size the resistor such that the max voltage drop will not exceed the max input voltage of the AI.  250 ohms is a commonly used value since it produces 5V @ 20mA. 

ELVIS is a package that includes a M series DAQ card.  Suggest a search on DAQ and 4-20mA.

[TheWoost]

Hi,

Centerbolt, the NI ELVIS has circuitry onboard that allows for direct measurement of current (up to 250mA) without the need for external resistors to provide current to voltage conversion.

Ryan, I would recommend referring to the following KnowledgeBase article that gives information about current measurement with the NI ELVIS:
How Can I Improve the NI ELVIS Current Measurement Accuracy?

All manuals and help files for the NI ELVIS can be found here:
NI ELVIS Manuals

I don't know if you've already found it, but there is a shipping example in LabVIEW that would be a good place to start in taking current measurements from the device.  The name of the example is "NI ELVIS Basic DMM.vi" and it can be found in the NI Example Finder (in LabVIEW, go to Help>>Find Examples) under Hardware Input and Output>>DAQmx>>NI ELVIS.

Regards,
Andrew W
National Instruments

[altenbach]

I would suggest to take this over to the academic board:

Academic Products (LabVIEW SE, NI-ELVIS)