Measurement of impact deflection?

[Grunchy]

Can anybody help, I need to measure the maximum deflection of a large flat panel under an impact. There's nothing really extreme about the situation, it's a mass of 9 lbs impacting at about 50 fps (34 mph or so) and total deflection is on the order of about 3".

Assuming the deceleration force is constant, that's about 150 g's deceleration (about 1,300 lbs) and time duration of 10 ms. I'm afraid that if I used a regular dial gauge, it would gain too much momentum during the impact and would register overshoot, furthermore it would probably wreck the mechanism. Perhaps it would work if I set the tip 2.5" from the panel, so it experiences only the last 1/6 of the impact event when the speed is much lower.

Any ideas/suggestions, anyone?
Thanks in advance.

 

[IRstuff]

This is cheesy, but should be cheap. Affix a pencil or pen to the end of the plate and arrange the structure next to a white wall and run your experiment.

Other more scientific approach, strain gauge affixed to plate where it bends the most, run to data acquisition board, collect data during impact. Then, statically deflect plate to get same strain on gauge and measure amount of deflection.

Depending on how much deflection there is, you might be able to use a LVDT device to measure the deflection directly.

A solid state accelerometer mounted on the end of the plate? I think Analog Devices and other suppliers have accelerometers that will easily cover your range.

TTFN

 

[Grunchy]

OK.. actually those are all equally scientific suggestions

The pen/pencil idea is good, but I need resolution to accuracy of 0.010" (I didn't say that before). I'm also worried about the pen bouncing and leaving the paper surface. Awesome solution though!

The strain gauge method also won't be accurate enough because the plate becomes plastically deformed during the impact.

The accelerometer is probably fine, but I want to minimize cost & don't already have such equipment. Furthermore I need to measure multiple data points because the impact can occur anywhere within a 5" diameter circle. What a dilemma!

In the end, I decided to use 3/16" drill rod suspended thru pieces of 1/4" mechanical tubing which are firmly supported by a solidly mounted 2x4 board. When the drill rod is impacted it will be driven thru a big block of foam for friction damping. So, all I need is a before-and-after measurement of how close the drill rod is to the surface of the 2x4, that'll give me deflection.

I figure I'll use lots of foam and just do my best to minimize overshoot.

Thanks for your help!