The UCSD SPOT ball
The SPOT ball is a gimbal system designed to ensure that the SPOT is facing skyward. This will allow the SPOT to send and receive satellite signals, and improve the chances of finding a balloon which has drifted outside of cell phone or HAM radio coverage. The idea is simple: two gimbals, one inside the other, with perpendicular axes of rotation. These gimbals are mounted inside a hamster ball in order to prevent objects from jamming them. The hamster ball is not meant to carry significant load, merely to protect the gimbals from foreign objects. The hamster ball lid allows convenient access to the SPOT, such that it can be easily turned on prior to launch.
lid allows access to SPOT buttons.
This SPOT ball was constructed from materials readily available at a hardware store. There are many ways to construct this, but this guide covers how the above SPOT ball was made.
SPOT ball construction diagram
A small piece of 5 in outer diameter PVC tubing (wall thickness is .25 in). You only need a piece half an inch long, but you will probably have to buy more than this (it depends on the store). Cut a half inch section for the outer gimbal ring, and save the rest if you intend to build future SPOT balls.
An inner ring of about 4 inches outer diameter. This ring's outer diameter must be less than the outer gimbal's inner diameter, and it helps if the ring is slightly smaller than the SPOT. This will be the inner gimbal ring. A yellow lid was used for the SPOT ball shown above, with the inner portion cut away to allow access to the SPOT. The SPOT will work through some material, but reducing the amount of material between the antenna and the sky will help ensure it works.
A threaded rod about 5.5 inches long. 1/4 inch diameter will suffice.
Washers to fit on the threaded rod (used for spacing purposes)
Two lock nuts that fit the threaded rod.
Two 1/4 in diameter machine screws, 1.5 inches long each.
Four lock nuts to fit the machine screws.
Zip ties (metal toothed is recommended.)
Tools you will need:
A saw (if you must trim your own section of PVC).
Sandpaper or a power sander
Drill with drill bits matching the diameter of your threaded rod and machine screws.
1.) Cut a section of 5 inch diameter PVC so that it is 1/2 inches long. Debur and sand the edges smooth. This is your outer ring.
2.) Drill two holes through the outer ring wall: the axes of these holes should be coincident, and pass through the center of the ring. These holes should be large enough to allow the threaded rod to pass through, with a snug fit.
3.) Drill two holes through the inner ring wall. These axes of these holes should be coincident, and through the center of the ring. This will be the axis of rotation of the inner ring. These holes should be big enough that the threaded rod can pass through, but the inner ring should be able to rotate freely around the rod.
4.) Place the inner ring inside the outer ring, and line up the holes. Put the threaded rod through all four holes. Make sure the inner ring can rotate freely on the threaded rod. Place some washers on the rod between the inner ring and outer ring until the inner ring cannot slide on the. The goal is to make sure the inner ring stays in the center of the outer ring, while still allowing the inner ring to rotate freely on the threaded rod.
5.) Once the washers are in place, put lock nuts on both ends of the threaded rod outside the outer ring.
6.) Drill two more holes in the outer ring: their axes should be coincident, and should pass through the center of the ring. They should be perpendicular to the other holes in the outer ring. The 1.5 inch long machine screws should fit through these holes, and the outer gimbal ring should be able to rotate easily on the screws.
7.) Drill two holes in the hamster ball, making sure their axes are coincident and pass through the center of the ball. The machine screws should fit snugly. Find a place on the ball which does not have slots in it for this: near the seam used to join the two hemispheres works well. These will be used to support the entire gimbal system.
8.) You will need to cut a portion of the hamster ball in order to install the gimbal system. Remove the ball lid, and note how the ball has slots in it (intended to allow the hamster fresh air. Some hamster balls may be constructed differently). Make a cut extending one of these slots all the way to the lid hole. Do the same with the slot adjacent to it. You should now be able to bend back part of the plastic enough to allow your gimbal rings to slide into the ball.
9.) Before installing your gimbal rings into the ball, secure your SPOT to the inner ring. Use the zip ties to fasten the SPOT underneath the inner ring, such that it is centered on the ring. The SPOT must be under the inner ring's axis of rotation in order for the system to work. Make sure you do not block the SPOT buttons. Zip tie placement will vary, depending on your SPOT model and the exact dimensions of your gimbal system.
10.) Place the machine screws through the holes in the hamster ball. Thread two lock nuts onto each machine screw inside the ball, but do not tighten them down yet. The screws should still be able to slide around.
11.) Now you are ready to install the gimbals in the ball. Slide the gimbals into the ball, and line up the unoccupied holes of the outer ring to the screws hamster ball. The screws should be loose enough to allow this. Now tighten one lock nut on each screw to hold the screw to the hamster ball. (This may be awkward, requiring one hand to hold pliers inside the ball while the other uses a screwdriver.)
12.) Adjust the other lock nut on each screw such that the outer gimbal ring is in the correct position: it should be centered in the hamster ball, and free to rotate on the machine screws.
13.) Test the SPOT ball by rolling it around: the SPOT should always end up pointing skyward. Adjust as necessary, and remember to test your SPOT ball prior to launch to make sure your SPOT is working, and replace the SPOT batteries before launch.
Remember that the hamster ball should not be used to carry significant load: a string tied through the slots on the ball should hold the ball, but the plastic ball could fracture under load.