Acquiring velocity using optical (infrared) screens

[Powderman]

Thanks for the replies guys.  I have a VI written that is basically catching the bullet flying over the first chronograph screen.  What I would like to do now is copy it and run the copy simultaneously and then run a counter/edge timer VI to get the time between the 2 screens.

Can someone tell me in simplest terms how to make a copy of my VI and run both at the same time?

Thanks,

Powderman

[Frisbee]

Making your bullet-catching VI re-entrant and calling it twice from another VI will allow two unique instances of your bullet catching program to run at the same time.

VIs can be made re-entrant in the "Execution" category of "VI Properties", which can be found in the "File" menu.

The following link outlines the concept of reentracy:
http://zone.ni.com/reference/en-XX/help/371361J-01/lvconcepts/reentrancy/

[Powderman]

Ok...we've gotten to the point where we are getting the voltage signal from both screens(start and stop).

I'm to the point now where I need to compare the times and compute a velocity...for what its worth, the screens are 10 feet apart.

I was going to try and use the measure 2 edge seperation VI but I believe that it only uses 1 digital input while I have 2 analog inputs.  Does anyone have another example VI I could look at or another idea?

I've attached a screenshot of the VI we are running.

Thanks,

Powderman

[John_P1]

Converting the signals to TTL and using the counter to measure the difference between the two edges is still by far the best way to make the measurement (10 ns resolution).

If the projectile is travelling really slowly and the signal generated stays high for long enough I *guess* you could try to measure this with two analog inputs, but the 500 kHz per channel only gives you a 2 us sample resolution.  You can't configure two different analog triggers on a single 6351.  I would strongly advise to do the measurement the way I first described.

Best Regards,

[Powderman]

I've tried searching but haven't been able to find much on converting Non-TTL signals to TTL signals using X-series devices...is this possible with the PCIe-6351?

Thanks for the help.

Powderman

[John_P1]

No, you need to convert the signal before wiring it into the X Series*.  I'd start by looking for an open-collector comparator that you can simply pull up to 5V (keep in mind that the X Series uses >20kOhm pull-down resistors on the digital inputs).

*You may convert a single analog input signal (up to ±10V) to TTL logic levels using the analog trigger circuitry of the X Series.  However, you can only use this method for a single signal at a time, so for your use-case it probably is best to just use external circuitry.  

Best Regards,