Using NI CompactDAQ to Crash Test Safety Helmets - NI case study

[Helen19411]

Hi all, 

I have a question regarding the counter function in LabVIEW. The example below shows a case study for crash test sefaty helmets using CompactDAQ. 

http://sine.ni.com/cs/app/doc/p/id/cs-785

They say:

As the helmet is falling, a sensor passes through a velocity gate, and we use the counter/timer function to measure the helmet velocity just prior to impact. This triggers the data acquisition system, which measures the acceleration that the head feels during impact.

My question is: If they used a photoeletric sensor, what is the purpose of using a counter? Why don't just simulate the start trigger from DAQmx trigger.vi?

[Blokk · Accepted solution]

I guess the counter/timer function is to measure the elapsed time between two positions: there should be two sensor gates? To calculate the velocity, you can divide the distance between the two gates with the elapsed time.

[NIRebecca · Accepted solution]

Hi Helen,

The counter/timer is a basic unit of hardware functionality seen on most measurement devices and they are often used for time critical functions like the measurement of velocity, as hardware clocks can run much faster and more accurately than software loops. See: http://www.ni.com/pdf/products/us/4daqsc386-393_386-387.pdf

From having a very careful look through the video, I think the set up is NI9481 NI9401, NI9233. So I would imagine the NI9481 initalises the sequence, the NI 9401 is used for implementing custom, high-speed counter/timers, which in turn allows the NI 9233 to take high-accuracy audio frequency measurements from IEPE sensors. The following documents using the NI9401 for hardware-timed digital input/output tasks: http://zone.ni.com/reference/en-XX/help/370466V-01/mxdevconsid/digiocseries/

It may be useful to have a read through the discussion at the following link which talks about using an onboard counters to generate a sample clock for your task: http://digital.ni.com/public.nsf/allkb/51754212AD10BDCE862573BD007BFDD2?OpenDocument

I hope this is of some help in understanding why you might choose counter/timer methods. 

Rebecca