Shock Testing, Round #1

One of the required ratings for our rugged VIUD is shock or acceleration and so we will need to build a test rig for this purpose. There are basically two ways of measuring the acceleration an object undergoes:

1. Install accelerometers on the object and measure the acceleration directly.
2. Take high-speed video of the object and indirectly measure the acceleration.

Our VIUD is far too small to install accelerometers on it which leaves us with the only other choice. 

High-Speed Video Camera

Using high-speed video necessitates buying a capable video camera but after much searching it seems that all of the dedicated high-speed cameras are far out of my price range of a few $100. Most vendors never list prices on their websites and instead say "Contact Us for Quote" which is just code for "really expensive", at least many $1000s if not $10,000s. 

Fortunately, after some further research I found out that Casio has a line of consumer cameras, the High Speed Exilim models, that support high speed video capture. Some models, including the ZR200, support frame rates up to 1000 fps at a resolution of 224x64 pixels: small in size but hopefully fast enough for our purposes. The price of around $220 is also nice although I hit a small issue when trying to purchase the camera: Casio doesn't sell their cameras at all in Canada. Fortunately I was able to eventually find a vendor in Japan that would sell me one (a lot of American vendors in general don't ship outside the USA).

An example video from one of the first few shock tests is below (prototype #3 at an impact velocity of 130 km/hr) :

The low resolution is immediately apparent but it is still usable for our purposes. Not surprisingly it takes pretty brightly lite environment to get a good video image at 1000 fps. The black & white pattern in the background is a scale (5 cm per large band) to give us the real world measurements within the video. More shock testing videos can be seen in our VIUD YouTube channel.

Shock Test Rig

The idea for the shock test rig is to make a custom air gun that fires the VIUD at a desired rate of speed at some target. The basic design of an air gun is pretty simple:

• Barrel for firing the projectile
• Valve to fire the gun
• Source of air under pressure

Unfortunately, my first naive attempt failed wonderfully: I simply hooked up an air compressor to a 1/2" ball valve attached to a 4 ft length of 1" pipe. Technically you could say it works if you considering "working" as barely being able to push the VIUD out of the barrel. The air compressor and its 1/4" hose simply can't supply the air fast enough into the barrel to give the projectile enough velocity.

The air gun design needs to be changed to hold a volume of compressed air close to the barrel with the largest possible pipe diameter connecting them (ideally the same size as the barrel if not larger). I first considered using a length of 2" or larger pipe but could not easily source the material locally. The next option considered was a small portable air tank. Initially this seemed like it would not work as all air tanks I looked at used a small 1/4" hose but on closer inspection the actual connection into the tank was a larger 1/2" NPT which is hopefully large enough to give our air gun some punch. I was also lucky enough to find a 5 gallon air tank on sale at my favourite store Princess Auto for only $25. I must confess to purchasing much more than just the air tank during my visit there (a very dangerous place to visit).

With a better design the assembly is relatively simple and uses all off-the-shelf parts available at any large hardware store. Total cost of everything to make the air gun was under $150 not counting the air compressor which I already had.

The Finished Air Gun for Shock Testing

The basic parts and features as shown in the above image are as follows:

• Barrel - 4 ft, 1" pipe with Copper pipe insert
• Firing Device - 1" Ball Valve 
• Air Chamber - 5 gallon tank with a 1/2" NPT connection
• Air Source - Electric air compressor 
• Control - Pressure gauge, regulator valve and shut off valve

I added a Copper pipe into the original barrel mainly to provide a smoother surface: the inside of the steel pipe was very rough and was scratching up the VIUD pretty badly during the first few runs. It also served to narrow the barrel's ID a little bit resulting in around 30% higher velocities at a given air pressure.

After a few trial runs it was obvious that the shock testing rig design works very well. Projectile velocity even at the lowest pressure setting of 15 psi is around 110 km/hr (60 m/hr) which is fast enough to be a little scary. This should be more than enough to reach the VIUD's terminal velocity of 55 m/s (200 km/hr, 120 m/hr) and beyond should we need it.

Measuring Acceleration

With our shock testing rig and high-speed video camera we're all set with the exception of having to figure out exactly how to measure our VIUD's acceleration from the video. My first attempt was to simply save individual frames from the movie, paste them into Inkscape (a 2D drawing editor), measure the movement and somehow convert that into an acceleration. This was very tedious and didn't work well at all so it wasn't long before I was searching the Internet for an alternative.

It was a quick search as the first Google result led me to an article about measuring the acceleration of a jumping Arboreal Lizard and that quickly led me to Tracker, a free application used for measuring velocities and accelerations from videos. The interface is a little wonky (some things just stop working randomly) and the documentation is mostly the "figure it out yourself" type but it does exactly what I need and was pretty easy to pick up its basic usage.

Tracker: A Free Video Analysis and Modelling Tool for Physics Education

Shock Testing

The procedure for shock testing is pretty straight forward:

1. Choose an air pressure (higher pressure for higher exit speeds)
2. Choose a target type (wood, stone, etc...)
3. Record the impact using the high-speed camera
4. Analyse the video using Tracker to determine the speed and acceleration
5. Test the USB to ensure it still works (basic error checking and bit change tests)

Using one of the third prototypes for testing we have the following results so far:

Pressure	Target	Speed	Max Acceleration	Tests Passed	Notes
15 psi	Wood	80 km/hr	900-1000 g	Passed	Minor scratches from inside of barrel
15 psi	Wood	80 km/hr	1100 g	Passed
15 psi	Wood	80 km/hr	1200 g	Passed
20 psi	Wood	110 km/hr	1500 g	Passed
25 psi	Wood	130 km/hr	1800 g	Passed
15 psi	Wood	100 km/hr	1300-1500 g	Passed	Added internal Copper sleeve to barrel
15 psi	Stone	100 km/hr	1500 g	Passed
20 psi	Wood	125 km/hr	1900 g	Passed
25 psi	Wood	150 km/hr	2100 g	Passed
25 psi	Wood	145 km/hr	2000 g	Passed
30 psi	Wood	160 km/hr	2100 g	Passed
35 psi	Wood	180 km/hr	2500 g	Passed
40 psi	Wood	190 km/hr	1900 g	Passed	Broke the wood target!

So far the VIUD passes all shock tests with flying colours which is good news indeed. One surprising thing is the large accelerations involved. Even with video at 1000 fps the impact occurs in less than one frame which means that even these large accelerations may well be underestimates. This also makes it difficult or impossible to distinguish the difference between target materials. In theory a stone target should result in considerable more deceleration than a wood target but we would probably need a 10,000 fps or higher video camera to discern the difference.

Amazingly the last test broke all three layers of the 2" thick wood target. The first 1/2" MDF layer broke completely through likely due to the combined effect of all previous tests. The next two 3/4" Pine layers broke into multiple pieces along their grain. Looks like I'll need to make a sturdier target, probably out of multiple layers of MDF, hardwood and plywood.

Broken 2" Thick Wood Target After a Dozen Shock Tests (Side View on the Right)

An interesting note is that the few rugged USB competitors that do explicit shock ratings are only rated at 40-50 g which means our shock ratings exceed theirs by an amazing factor of 50!