How to configure the Peak Detection VI to calculate velocities?

[jagrs26]

In a Ballistics Range in a laboratory somewhere in Ohio...

I have a rapid fire gun that shoots 3 rounds at chosen delay intervals of 1ms to 15sec.  The bullets passes through 2 sets of speed traps that are configured with chronographs.  I have 2 speed traps for redundancy and verification so each bullet will have 2 velocities.  The chronographs display one velocity at a time and when delay intervals are less than 2 seconds it is impossible to get the velocities of all three bullets.  With BNC splitters I ran cables from the chronographs to a anolog board on my PC and have voltages of 2ms long pulses from each screen.  The screens are set up start screen 1 and start screen2 right next to eachother, then 5ft seperation, then stop screen 1 and stop screen 2 right next to eachother.  I've configured the screens to send voltages to the PC in the following manner, Start screen 1 (3Volts), Start screen 2 (4.6Volts), Stop screen 1 (5.8Volts), and Stop screen 2 (8Volts).

I'm having trouble configuring a program that will collect the voltages using a peak detection VI and calculating the velocities.  I would like to collect it in to an array or matrix and clear it when I commence my next round of firing.  I would also like to see all 12 peaks on a graph at the same time, a display of all 6 velocities, their time location and their amplitude.

Please help me cause I've been at this for a while now and keep getting errors.

Thank you.

[tbd]

Hi jagrs26,

      If I understand your application, then when a bullet is fired, you want to see four edges/peaks that will occur relatively close-together.  In order to get good precision for measured-time between edges, you'll probably want to acquire as rapidly as your DAQ hardware will allow.  BUT there are big time-intervals between bullets - maybe 15 sec, and that will create much unused data (to display) if three bullets are always measured as a single data acquisition.

      If you only had one trap (two screens separated by 5ft), I'd recommend NOT using analog DAQ, but, instead, 'acquire' counter-values on Analog triggers - when bullets pass a screen.  The delta between counter-values => time.  However, with two traps, I guess you could get two simultaneous triggers for either start or stop.

      How about triggering three times - once for each bullet - when inter-bullet gaps are greater that 1sec?  If your hardware supports analog-triggering, you could trigger on either first screen (start) keeping some pre-trigger scans and simply acquire for a millisecond or so, then wait for the next trigger.  However,

Before getting too deep into a specific solution - what hardware are you using? 

Cheers!

[jagrs26]

TBD,

Thank you for your reply.

The Speed traps are Oehler model 37 and the chronographs are Oehler 35P.  The DAQ is a USB 6210.  The USB 6210 can only take +/-10 volts and the Oehler sends out 12 volts so I used resistors to break the voltages from each speed trap down to 3, 4.6, 5.8 and 8 volts respectively.  I decided to use different voltages because when the delay time between each bullet fired is microseconds apart, to simulate rapid fire, the second or third bullet may cross the first screen before the first bullet crosses the last screen and this will make it difficult to differentiate between the peaks and determine which peaks belongs to which screen.

I found some examples in the help section that was able to capture the peaks and display them.  I also found some examples demonstrating the use of peak detector but none of the examples of the peak detector is configured to help me.

Thank you.

[tbd]

Hi Jagrs,

      If I understand your description (perhaps a jpeg or some data could be helpful) what's really needed is a "step detector" capable of identifying steps on the leading and/or trailing edge of a waveform.  At 250KHz (for analog inputs), events (pulses) that happen within about 4us of each other may appear to happen at exactly same time.  Even if events happen further apart than 4us, I'm guessing capacitance in the trigger-line will tend to smooth-out steps, making "short" steps difficult to identify - especially if there's any noise present.

Sorry if I've got this wrong - maybe this is a straight-forward signal-analysis problem.  Again, actual data might help.

One thing I'm having trouble with is: Is the Oehler outputting a 2ms(millisecond) pulse with each bullet, and bullets may be micro-seconds(uS) apart(?)  What does the Oehler do if detects a bullet,  starts a 2ms pulse - and another bulltet passes-by?

Cheers.