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

[johnsold]

Mike,

What part of your system creates the excessive delays?  The relays?  The Agilent 34970A?  The  data acquisition? The analysis and control software?  The OS?

The built application (.exe) should run at the same speed as the VI running in the development environment.  There are exception, but they are unlikely to be relevant in your situation.

How fast does the motor turn the Variac?

Are you using the 34970 for both voltage measurements and relay control?  How long does it take to make a measurement and how long does it take to turn on a relay?  Which measurement modes are you using?  From the data sheet it appears that you can get readings a slow as one every 7 seconds or as fast as 100 per second.  Which module are you using to control the relays?

The USB-6008 would give much less accuracy and probably would not be any faster than the fastest modes of the 34970A.

Posting your program along with answers to the questions above will help.

Lynn 

[Cburg]

Hi Lynn,

It looks like it was a matter of using the measure function vs the configure, then read. I also set the AC measurement speed to MAX. It runs at 200ms per sample now, and is controlling the variac within 2 volts!

Thanks for the help!

Best regards,

Mike

[johnsold]

Mike,

Glad you figured it out.

Instruments like that often have multiple modes and corresponding speeds, but it is sometimes quite difficult to determine which combination will work well for a particular application.

Lynn 

[wb2nvy]

I'm trying to do something very similar, and I've created the attached vi, but it's very slow. I want to make multiple voltage measurements as quickly as possible. Apparently I can configure 1 time then do multiple quick "measures" in a loop? Is there a "measure.vi" available for doing that?

[johnsold]

I do not have the HP subVIs, but with a name like ...EZ Voltage.vi, I suspect that this is not the most efficient way to read voltages.  It is probably a quick and dirty way to grab one reading to check out the setup.  Open it up and see it if is not doing some configuration stuff every time it is called.

Lynn 

[wb2nvy]

It looks like the original poster, Mike, accomplished exactly what I'm trying to do. Is there any way I can contact Mike & get his inputs?

Keith

[GenU1Chevy]

Hello,

I'm having the same issue.  I need my HP34970A to scan faster through the AC measurements but I can't figure out how to fix it.  I've tired the configure voltage icon but it seems to mess things up or I'm not doing it right.  Can anyone help me or show me an example that wroks for measuring AC with this system?

Thanks,

Jeff K.