Valve Control

[Ian_D]

Currently, I have an M-8259 DAQ board with LabVIEW 8.2. I am PWMing 7 different valves at different duty cycles, frequencies and phases. I do this by generating 7 different square waves in LabVIEW and then feeding that into one "update digital channel" VI and then looping that several times.  When I look at the signal on an oscilloscope the frequency and duty cycle are off from the values I inputted into the square wave.  The frequency is slower and the duty cycle is slightly off.  I was wondering if the DAQ board is fast enough to handle all these digital outputs or is it the computer? I am using a dell with 2 gigs of ram. Also, if it is the board are there other boards out there that could control these digital outputs better?

Thanks for your help. 

[Paul_C.]

Hi Ian_D,

I couldn't find the 8259 as a National Instruments product. Did you mean the 6259?  Also, from your post, it sounds like your using a software timed pulse width modulation (PWM) signal.  What are the different duty cycles, frequenies, and phases that you're using?  Is the "update digital channel" a custom vi or found in an example? 

In regards to your questions about PWM, I wanted to mention to you that we have a very good article about doing PWM with DAQmx here.

There are also a few examples under digital generation (Help » Find Examples » Hardware Input and Output » DAQmx) about continuously writing to a digital port using an external clock.  The clock will clock out one bit of your digital wave form every clock cycle.  This clock can be a custom signal you create with a counter output or one of the internal timebases.

I'd also like to link you to another forum post that I found that discusses similair questions about PWM.  It can be found here.

I hope this helps,
Paul C.