vi timing or NI 9481 delay fault

[tone5]

Hello

I have written a vi to control a test rig which will have to run fatigue tests which last about a month. Basically the vi monitors the signal from a loadcell and switches a hydraulic valve when the load target is reached. Switching the valve causes the load to reverse direction.

I'm using is a NI 9237 (bridge analogue input module) for monitoring the load signal and a NI 9481 (electromechanical relay module) for switching the valve.

My vi seems to work fine and controls the rig ok for a while. However sometimes there is a delay in the signal sent to the valve and consequently the load exceeds the allowable limit and the rig shuts down (it is fitted with overload protection). It can take anything up to 5 hours for this to occur.

I have also noticed that if I lock or unlock (by pressing ctrl alt delete) the pc the vi seems to pause breifly, which alls the load to increase too much and again the rig shuts down. The vi runs fine while the pc is locked.

I have attached a seperate datalogger to the output terminals of the NI 9481 and this confirms that the shutdowns occurs due to a delay in the opening/closing of the terminals on the NI 9481. Of course the problem may be within the vi and thus causing the delayed response from the NI 9481.

The test rig can tolerate a delay of about 200ms before it shuts down due to overlaod. The main control loop in my vi runs every 2ms (using the wait vi).

Can anyone suggest how I can solve this problem? Has anyone experienced inconsistent responses from a NI 9481?

Thanks

[RavensFan]

I'd suggest moving your control scheme off of PC-based control under Windows and go to something with more real-time reliability such as a real-time OS, a compact Fieldpoint, or a compact RIO.

If a 200 msec hiccup causes system problems, I wouldn't trust a Windows-based PC.  Same thing if you need a 2 millisecond control loop.  Too many things can happen in Windows to cause it to go off and do something else for a few seconds (virus scanning, handling the OS lockdown screen) thus leaving your time critical application starved for attention.