GPS Tracker Gimbal for High Altitude Balloon Payload

Earlier this year, we had a project to capture near-space footage for our client to use in a promotional video.

There is an established community for high-altitude ballooning, or near-space ballooning. Typically, a high-altitude balloon launch is where a payload, usually a camera or instrumentation, is sent up in an unpiloted fashion using a weather balloon. After reaching a certain altitude, the balloon bursts, and the payload falls unguided back to Earth, slowed only by a parachute. Round trip can take 4-8 hours.

In earlier times, you’d rig a camera and timing mechanism, write your name and address on your payload and hope someone would (hopefully) send it back to you. These days, we have GoPros and GPS trackers.

The GPS tracker we chose was the inexpensive, compact, satellite-connected Spot Trace, which is popular in the high-altitude ballooning community. Due to limitations in the design of the Spot Trace, a specific side always needs to face the sky. When the payload lands, there is no guarantee that it will land upright as it can easily be knocked over. Therefore, we needed to design a gimbal mechanism to keep the appropriate side of the GPS tracker facing the sky.

There are three concentric circular frames which can rotate about two axes. The hexagonal recesses are for M3 nuts. The GPS mount is screwed to the center frame. The GPS is placed in the mount with logo side in. The GPS is kept in the proper orientation by gravity. There is a clamping plate to help you attach to your payload. There is an optional battery frame allowing for tight integration of the battery and gimbal. This allowed for better battery and gimbal positioning which led to better balance.

Clamping plates are 40mmX50mmX6mm. The clamping plate holes are a loose 3.75mm diameter 15mm x 30mm apart with a 90 degree countersink. The mounting faces (clamps removed) are 156mm apart. The diameter of the outer ring is 152mm. The battery frame adds 29mm to the height with a 40mmx114mm footprint. Hole spacing is 98mm x 15mm. It can straddle a 71mm x 25mm battery. It adds about 60 grams of mass.

The weight with GPS and screws installed is about 280 grams without the battery frame.

Additional documentation, including CAD files and images, can be found here. Assemble and use at your own risk. For any launch, look up and follow your local regulations.

BOM: (Prices may have changed and are approximate)

Off-the-Shelf Components

To Print:

(We used a Form 2 SLA printer with Grey V4 resin. Note that support material could double the needed volume of material)

Tools:

• Diagonal Cutter for Hard Wire (Knipex 70 01 160) ~$20

• 1/16in Drill Bit (McMaster P/N: 29115A711) ~$2.73

• Pin Vice (McMaster P/N: 8455A12) ~$15

• Push-On Tool (McMaster has one for $58) (For control, I created an insert for a set of Cleco clamp pliers ~$2+$7) (if I were going to make a lot of these, I could imagine making a jig for a press or pliers wrench to precisely locate the nut)

• File/Sandpaper

• 2mm Allen Key (for flat head screws) (McMaster P/N: 7813A43) ~$12

• 2.5mm Allen Key (for socket cap screws) (McMaster P/N: 7813A44) ~$1

Assembly Instructions:

1. Inspect and clean up the 3D printed parts. If you printed the spacers, make sure the top and bottom are smooth, flat and parallel.

2. Using the 1/16” drill bit in the pin vice, clean up 1/16” holes by running the 1/16” drill bit through them a few times.

3. Super glue M3 nuts into hexagonal holes being careful not to glue yourself or the threads. Wearing gloves, I like to use a long screw with a nut on the end, add the glue and then pull the nut into the hole so it seats properly. (I like embedding nuts in 3D printed nuts in the plastic due to strength, but if I were designing this again, I’d likely move to brass inserts: trading holding strength for ease of assembly)

4. Cut 6 in of rod into four - 1.5in sections using diagonal cutters. Roll your sections across a flat surface to inspect for straightness.

5. Push push-on nut just onto the end of each rod using a push-on nut tool (or an appropriate size hole [3.3mm or ⅛” ] in piece of metal) and trim excess (It is a bit tricky. You may want to use a stop so you don’t push it on too far)

6. Slide the rod with push-on nut through the inner ring, spacer, middle ring.

7. Push on another push-on nut being careful not to squeeze the parts (causing friction). (Maybe use something you can pull out as a spacer)

8. Repeat on the opposite side. Repeat for the outer and middle ring.

9. Screw GPS mount to center ring with M3-8mm socket cap screws.

10. Charge GPS and turn it on.

11. Snap in the GPS on mount, logo facing in.

12. Mount completed gimbal to battery base (optional).

13. Mount the assembly onto the payload with flat head M3 screws (sized appropriately for your wall thickness).

After running with the version described above, I continued to iterate and designed a lighter-weight version in case we needed to launch again. In the future, an FEA model could be used to optimize for weight and durability.

Lighter-Weight Replacement Parts:

​This version is a little smaller and mounted from only one side. Without the battery frame, approximately 130mm height clearance is needed. In the other frames 125mm (off center) - 135 (centered). The weight of the lighter assembly with the GPS installed and no Battery Frame nor screws is about 180 grams.

NK Labs, LLC is a product development engineering firm in Cambridge, MA. We have broad experience in the design of consumer electronics, robotics, and other innovative products. Please consider us for your next engineering design project!

#GPS #gimbal #highaltitude #balloon #3Dprint #SLA