Batcopter, mission successful

These Brazilian Free-tailed bats (also called Tadarida) come together in the millions in caves all over Texas, leaving every night in swarms so big they can be detected by doppler radar. Somehow, they manage to fly through this dense self-clutter without colliding– our goal is to better understand this behavior. For the AIRFOILS project, the IML team created the previously mentioned Batcopter. The goal was to fly a UAV along the dense clutter and record the bats’ trajectories, using three ground-based high-speed FLIR cameras and an airborne 3D HD GoPro camera. The hope is to extract fundamental control laws of flying behavior in order to achieve better autonomous UAV flight.



The original quadcopter frame was built out of aluminum, with the frame arms sourced from Home Depot chromed aluminum towel racks. The idea was to be able to locally source major parts in case of breakage. The netting was held in place through a box made of fiberglass kite rods, held to the quadcopter frame by bamboo rods. (Bamboo is amazingly strong, light, and can be found at any local garden store.). The overall weight of the frame was some ~600 grams (heavy, but that’s what happens when an airframe is built strictly out of parts available at the local hardware store) and the netting frame was ~400 grams. Add in the ~300g for the 3D GoPro camera (not the best HD camera, but what other 3D camera could have survived the abuse?), and there’s a lot of weight for just four motors.

While this design performed acceptably in Boston, the difference in temperature and altitude drove the AXI motors over their limits. Even with larger 12×4.5 props, the Batcopter v1.0 could not maintain orientation in the strong winds. This was likely due to the fact that the CopterControl is not performing model-specific control. There was very little room for error, and the lack of coupling between motor and controller made it impossible to react sufficiently quickly. This is definitely something that the will be significantly improved by more advanced ESCs.

After several days of trials and errors, it was finally determined that the aluminum design was at its limits. A new approach was decided upon, one that would use the 4 carbon fiber rods that had been packed along with the other spare parts. The decision was made to build a V-frame that could eventually be upgraded to a hexacopter, in case additional motors to complement the quadcopter’s 4 could be found.

In addition to the weight savings brought about by the shift away from aluminum, it became clear that another 400g could be shaved off by radically changing the way the netting was held to the frame. Along with the other supplies brought to Texas were two large pieces of blue packing foam. These pieces could serve to hold the netting far away from the props, completely removing the need for the fiberglass structure.

The next morning, the bat team drove 50km into the Uvalde, TX, the closest town, in order to find additional supplies. At the local hardware store, a very helpful and knowledgeable salesman assisted in the brainstorming for ways to attach the carbon fiber rods together. It was decided that the best strategy was to use twine impregnated with white glue; the twine would be strong and the white glue could be dissolved in case the approach was not working. It was light, strong, somewhat rigid, and would build up the joint so a heavy-duty zip-tie could be added for additional clamping power.

After a very successful, if long, visit to the hardware store, a second stop was made at a local hunting store, where several old carbon-fiber arrow shafts were donated to the project. These shafts would later serve an integral part of the structure.

Once back at the cabin, work was started in earnest to build the Batcopter v2.0. While working in the 35C heat is never easy, progress came quickly. Using clamps, a jig was fashioned that would hold the rods exactly 40cm apart. In addition, after a few abortive tries a good winding strategy was found. A couple hours later, the makings of a frame were clear. It basically looked like a tic-tac-toe grid made out of carbon fiber.

The motors were attached to some FRP that had been brought down for just this sort of tinkering, and then the FRP was attached to the booms via zip-ties, with some sticky double-sided tape added to keep them from walking down the beam. It was not a design to be repeated, given the choice, but one must work with the tools one has on hand.

After having built the square frame, it was clear that the joints and small carbon fiber rod were nowhere near rigid enough. They were strengthened with two arrow shafts cutting diagonally across the center, which nicely provided a crossing place where the heart of the Batcopter, the OpenPilot CopterControl, could be mounted. However, note that one arrow shaft was attached to the top of the 10mm rod, and the other to the bottom. Thus, there was a 10mm gap between them at the center. This would later prove a stumbling block.

The motors mounted and the frame completed, it was now necessary to mount the circuitry. This was not obvious, as as mentioned above, the center mounting area was encumbered by the fact that the two arrow shafts where 1cm apart. Fortunately, one of the lab assistants had had the foresight to send down some plates for attaching zip ties to flat surfaces. Two of them together spanned the gap, with just enough space missing for to slide in the fiberglass board where the CopterControl was originally mounted on the Batcopter v1.0.

The battery was mounted on a bamboo boom behind the electronics. The fact that it off-balanced the QC didn’t make a big difference, except that the rear motors had to work harder than the front ones.

Finally, the netting needed to be added. This was done through the creative use of a hacksaw on the blue foam. That stuff is tough, but light. The cut foam blocks were impaled onto the ends of the frame, and then kept from sliding around with some gorilla tape. Steel wire was passed from the front to the back of each major rail, and in the middle supported by some bamboo. The approach worked perfectly, both protecting the QC from shock and providing a large structure for the netting to rest on, so it would not get pushed into the props by the bats.

Finally, Batcopter v2.0 was ready to fly. A rough estimate was that at least 500g was shaved off the final flying weight.

Come 3PM, the hottest part of the day, bat team set out to the caves. The initial plan was to fly without the GoPro and GPS, in order to thoroughly test the new Batcopter before risking equipment, but results were so encouraging that the decision was made to start making flights with GPS and GoPro cameras. After a few tentative motions, it was clear that the Batcopter v2.0 was a revolutionary leap over v1.0. In other words, it flew like a dream. The batcopter lept into the air and started flying along the dense clouds of bats.

During the flights, three major things happened:

1) A rotor got caught in the netting and the quadcopter crashed inverted into the hard-scrable Texas wilderness, buckling one of the smaller CF rods. Due to the robust nature of the electronics and controls programming, a broken frame did not slow down the tests.

2) The motors and ESCs eventually overheated in the Texas sun, at which point the experiment was brought to a successul end.

3) Somewhere, this magnificent infrared photo was taken. Science meets science.

The next day, it was settled that the quadcopter would be flown in its hobbled state, in order to have a record of just how good the CopterControl was. The video is here. The only reason it crashed this last time was because the rotor caught on one of the delaminated carbon fiber strands. However, this time, the crash permanently disabled the Batcopter v2.0, breaking its spine where the two arrowshafts met. Oh, well, it’s another trophy for the “Version Wall” at the BU UAV lab.

