13 June 2012

Balloon-borne camera, part 2 - Building a stable camera platform

As I mentioned in my last post, the major problem we were having was how to stabilize the camera so you could get a decent video of it. A little searching online led me to kite aerial photography enthusiasts, who prefer the picavet as a way of suspending a camera from the kite line so that the camera naturally levels itself. I decided to construct one and see if it gave us any better results.

Two websites were particularly helpful to me and deserve special mention - Charles Benton's Kite Aerial Photography page and the KAPER Picavet page. This last one in particular had useful images and diagrams.

Briefly, a picavet is basically a cross suspended from the kite line (not from the kite itself) by a rope strung through the four corners of the cross. Each corner has an eye through which the rope runs, along which the picavet can slide. The camera is attached to the picavet from below. This rig uses its own weight to slide down to the bottom of the rope, where it hangs perpendicular to the ground. It continuously readjusts itself as the kite line moves. The principle on which it works deserves its own post, so I'll leave it at that for now.

This diagram from the KAPER site shows the major parts:

Picavet diagram. Points A and B attach to the kite line. The camera hangs underneath the cross.

We wanted to take a very low-budget approach using materials found around the museum. This picture shows some of what I collected:

String, washer, balsa wood, and glue gun
Cup hooks

On the top starting from the left, you see a spool of reasonably strong string (I can snap it by pulling, but I have to pull really hard), a tiny washer, two rectangular sticks of balsa wood (30 cm x 2 cm), and a hot glue gun. Other materials included some E-Z hang cup hooks for hanging things from walls (bottom), and (not shown) pliers and two cheapo carabiners like the ones you get in the checkout aisle at a convenience store.You'll see the pliers and carabiners in later pictures.

Note: If you plan on lifting a fancy camera that will be expensive to replace, I really suggest you use stronger materials, like aluminum for the cross, bolts to connect the cross pieces, sturdy kite line for the string, and real climbing carabiners. My approach works well for our 30 g picavet and 30 g camera, but doesn't really scale.

Alright, now we're ready to put it together. The first step is to make the cross out of balsa wood. I used a ruler to mark the center of each piece, and did my best to align them perpendicularly. I drew lots of guide lines on the wood to help me. Carefully holding the pieces in place, squirt glue from the glue gun down the edges where the pieces meet (all four edges - flip the cross over to get the last two). There are probably more secure methods of joining the cross-pieces, but this one works well enough for our light load and is quick and simple to do. Feel free to substitute your own glue, but I would be very hesitant to use arts and crafts glue. Stick to stuff meant for hardware, like cyanoacrylate-based adhesives.

The last step is to screw the cup hooks into each end of the cross. Before you do that, you need to close the hooks so that the end makes an eye (obviously, an alternative is to simply use eye bolts instead of hooks, but I used what I was able to most easily find). Just crush the hooks with pliers, like in the following two pictures. Crush them and crush them well, so the string won't slip out later when your camera is 50m in the air (you can also avoid this by using thicker string or rope).

Not crushed.

Screw one into each end of the picavet cross. When you're done, turn them all to face the same direction. It should look something like this:
Picavet cross, with eyes oriented left-to-right. You can also see where I glues the cross pieces together. Ignore the hole in the middle - that was a failed experiment! I promise to explain it later.
On a final note, feel free to replace these materials with anything else you feel is better or easier for you to obtain. Eye bolts, for example, would have been a good replacement for the cup hooks, but I found the cup hooks first so that's what I went with.

Congrats, the base of the picavet is finished! Next time: stringing it and hanging it from the kite line.

Disclaimer: Although I am a volunteer at the Canada Science and Technology Museum, the content on this blog in no way reflects an official communication of the museum. 

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