Speed of AC RMS Measurement with Lab View controlling NI USB-6008 DAQ

[VA Cyclist]

Dear Board,

I am attempting to control a motorized Variac's output voltage using a couple of relays for up and down control, actuated by an Agilent 34970A. I sample the variac voltage through a transformer, and use a lab view program to measure AC RMS volts and compare the measurement to an input command to regulate the output by actuating the relays. I have a 3 volt window programmed, with overshoot and undershoot compensation. The program works, but is not nearly fast enough, as the output swings some 50V or so, around the command voltage. I was thinking that the rate of AC measurement is a key. I also have not tried it in .exe format. Would that make it faster, or is it a function of the speed of RMS measurement? Would another DAQ, like the NI USB-6008 help? Thank you for any insights.

Sincerely,

Mike

[Dan_K]

Hello Mike,

Thank you for your interest in National Instruments Data Acquisition products. For more information on performing basic RMS calculations in LabVIEW check out the Developer Zone Example: Basic DC-RMS Measurements.  In terms of choosing a data acquisition device the main thing that you will need to be careful of is the voltage specifications of the device you are considering.  For your application the USB-9229 would work very well, as it can handle ±60 V DC signals and has an acquisition rate of 50 kS/s per channel.  This should give you the voltage swing you need along with a more than sufficient sampling rate.