Monday, January 14, 2008

The poor man's space-age materials

Although I do not need such strong composites as I was previously using due to the downscale of the puppet I still want to go through the discovery process that I found very valuable as a DIY "instructable" as they are now called.

I must mention that I still plan to use the material I prepared, except now I will utilize it to carve and build the entire body of the robotic-puppet instead of just the limbs.

Original requirements.

Regarding the construction of the limbs, a promising construction technique came from an unexpected source. A Near Space article 1 on boom construction for near-space aircraft and ballonSat 2 frames and rigs described a simple, inexpensive and very strong method for a lightweight composite boom, which I adopted after some preliminary tests.

The following pictures show some of the materials that I am currently using to construct the limbs of the Creator. After some comparison tests with other methods and materials, I have found this to be the optimal structural material that will comply with the many requisites of the performing object.

Limb Test Construction Process

Limbs are currently made of a composite material consisting of 3/4” extruded polystyrene foam sandwiched between either 1/34” triply birch plywood, and/or 1/4” balsa wood. Styrofoam, as is commonly known, has a high compressive strength foam very resistant to water penetration which is a consideration in a high humidity environment such as Florida where we currently live and work (the swamp!)3.

Weight-strength-cost are important considerations. Given the size of the puppet (approximately 5.5 meters) and the fact that it will be controlled via servo motors, it is imperative that the structure be as light as possible and at the same time be able to withstand the forces applied to the different joints.
The puppet must be easily assembled and disassembled for transportation and performance and be able to survive the abuse associated with these events.

The foam and plywood are glued with 5 min epoxy and held together between 2x4’s to distribute the pressure evenly.

I use Jorgenson’s wooden pressure clamps and can’t say enough about them. These ingenious clamps can vary the angle of pressure precisely. Using almost any other type of clamp will result in the sandwiched elements sliding out of alignment due to the viscosity of the glue while drying.

I found that both the birch plywood and the balsa wood, when epoxied to the styrofoam have an enormous resistance to fracture, you cannot break them with your hands alone. The difference is in terms of surface, since the plywood is smooth and hard and the balsa is very soft and easily damaged.
In terms of weight they end up the same due to the different thickness-weight ratio. At every joint an additional element like a plastic or metal tube or bushing that will support the axis, bearing and constrain the joint must be securely attached.

So far I am satisfied with the light weight and rigidity of this construction technique.

1- Nuts and Volts, September 2006, p.78

2- BalloonSAT Basics
- Sounding balloons 1200-1500 grams filled with helium
- Rise to altitudes over 100,000 feet
- Parachutes attached to payloads
- Modules include:
- Communications - flight and ground, radio, TNC and GPS
- Science payloads – weather data, Geiger counter, particulate sampling, ozonesondes,
- Imaging payloads – digital or analog cameras
- Flight Computer – data transmission or storage

3- Talk about humidity. Since we moved from the Apple to the swamp we have had to deal with 80-90% humidity on any given day, excluding storm or hurricane season where nothing applies! Since my "dirty workshop" (where wood cutting, sanding, spraying etc. happens) is located on a shed in the backyard, even though I insulated the whole structure, humidity seeps in at an incredible rate. I bought a pump-dehumidifier and the thing extracts around 5 gallons of water every 24 hours! If it not were for that essential piece of equipment you can imagine the state of materials and tools. In fact, what prompted me to buy such a thing was the sorry sight of my precious tools rusting and everything else deteriorating no matter how much you clean and oil etc.
This is the "clean corner" of the "dirty-shop"

The most absurd thing is that nobody sells dehumidifiers in Florida! on the contrary, they sell humidifiers!!??? what is up with these gators! Can't wait to go to higher ground :-)

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