How can I set up a VI to run an experiment at a given Hz?

[Blackavar]

I want to take actuate an air flow valve at a given rate per second... Is there any way to do that with decent accuracy through LabVIEW software?

[Dennis_Knutson]

This isn't so much a LabVIEW question as an OS question. If you're using strictly software timimg with Windows, Mac, Linux, etc., you don't have a deterministic OS and software timimg can be wildly variable. You can use LabVIEW RT or hardware timing.

[Valen Li]

Hello Dennis,
I have a couple more questions about your comment. When you say "The software timing can be wildly varible for a OS that is not deterministic." What parameters will affect the timing? And at what scales(ns, ms, or sec)? Where can I find more technical informations on this topic? Thank you very much.

Valen

[Blackavar]

I would also like to know how accurate and rapid hardware timing is and how I would access said timing through LabVIEW, if possible, and if not, then outside of it would also be useful to know. Thank you!

[Dennis_Knutson]

Greg McKaskle addressed some of your questions about hardware timimg in your other post here. If you have something like a DAQ board, you use the board's timing circuitry to generate a very precise signal. You program it and just let it run. An exteranl instrument like a function generator can do the same sort of thing.

The software timing issue arises because an OS like Windows, is usually trying to do a lot of different things in the background. A program like Microsoft FindFast will start and slow another application down. A user trying to start some application or even just
clicking with the mouse will briefly interupt all other programs. LabVIEW RT works great because it runs on dedicated hardware and there is no interaction with the OS. Some people of removed components from the OS to minimize the operations or used dual processor systems. It really all depends on your exact timing requirements. If you want to run at 400 Hz +/- 50%, then a software approach might work. If you really need 400 Hz +/- .01%, then look at providing the timing with hardware.

[Blackavar]

That's just the problem, though. I was first introduced to LabVIEW and this hardware about a month ago. I feel like I'm making very good progress in understanding and utilizing the program and the hardware, but I've never attempted to set up a hardware clock, or seen how. Is it a built in function of the 6020E DAQ-Pad or the DIO-24? I'm also using an SCB-68 and a CB-50LP... What would work best, and how would I configure it? Sorry to trouble you with something that's probably fairly trivial..

[Dennis_Knutson]

I don't think the DIO-24 would help you much but the 6020 has 2 analog outputs you could try. There's an example VI called Function Generator.vi. Try setting it up for a square wave at whatever amplitude and frequency you need. If you have access to an external frequency counter you can check the accuracy of the board's hardware clock by measuring the output and see if it's satisfactory.

[Blackavar]

I don't know if we have one, but we do have several waveform generators. What's VCG? It has a VCG in port on the front, and I'm not sure what that is.. Could that be useable?

[Dennis_Knutson]

The waveform generator could be used as the source of your air flow valve actuator. It may or may not have a more accurate clock than your daqpad. Your instrument manual should tell you what VCG stands for and what the timimg accuracy of the instrument is.