Wednesday, May 8, 2013

You can't always get the the drone you want, but if you try a laser you'll get what you need

 The "perfect" small unmanned aircraft, commonly called a drone, might still be several generations away. But like Moore's law, those generational cycles are getting shorter and shorter.

Chris Anderson of 3DRobotics suspects we're closing in on the drone equivalent of the Mac: a relatively affordable, accessible, and most importantly, practical piece of technology that can be deployed every day.

Tremendous headway has been made with multirotor technology (the heicopters, quadrotors, hexcopters, octocopters, and what have you). The market is quickly becoming flush with a variety of these aircraft, to the point where several options are available for each price bracket.

There's everything from $300 hobbyist rigs from big-name RC and electronics manufacturers, to $1,000 semipro setups from DJI and 3DRobotics, to $10,000 rigs that can loft, pan and tilt a DSLR or DV camera. The differences between each step may be as simple as stronger frames, larger motors and higher-capacity batteries.

A drone of your very own, from novice to pro. Sometimes no assembly required.

For the time being, however, it's still useful to have the technical know-how to put one together. It's even more useful to know how to fabricate a drone, or fabricate parts to suit your specific application.

The HeliPal Storm Drone is one of the many quadcopters stationed in the middle of the pack. It's got enough power to fly a GoPro or iPhone. In fact, it was designed specifically with those video devices in mind.

It comes stock with a control board with a six-axis gyro. It doesn't, however, have GPS functionality that would allow it to hold a position. It also doesn't have semi-autonomy, which would allow it to navigate waypoints or follow a target.

With the help of the Champaign-Urbana Community Fab Lab, I was able to design and fabricate a new deck for the quadrotor, sized for the 3DRobotics ArduPilot Mega 2.5 autopilot.

Doing this required open-source graphic design software (Inkscape), a 50 watt Epilog laser engraver, and a sheet of 1/4th inch-thick acrylic.

The process involved first making measurements of the existing mounting plate, drawing the plate in Inkscape, then exporting the drawing as a PDF with 0.01" lines. The Epilog driver makes the laser engraver show up much like an ordinary printer on the PC, albeit with options such as speed, power, and efficiency.

Standard 3M screws were used to fasten the circuit boards to the drone. But in an effort to minimize vibration to the autopilot, rubber o-rings were used as a cushion between the APM, mounting screws, and the mounting plate.

It remains to be seen whether the acrylic will provide enough structural integrity to withstand vigorous flight, though that will be tested as soon as a new motor is sourced. One motor on this aircraft was damaged in a previous flight, which needs to be replaced.

There also is a number of small, fixed-wing aircraft on the market, but there's not nearly as much variety. This might be due to the certain limitations of fixed wing aircraft (more open space for landing, takeoff, and flying), though fixed-wing aircraft remain the best option for mapping and scanning large areas.

Comprehensive drone mapping systems remain fairly expensive. A Gatewing or Sensefly will set you back $20,000 to $30,000.

More options are becoming available, but these tend to be niche products that are produced in small batches, and therefore can't take advantage of economies of scale. The best bang for your buck is to modify an existing fixed-wing aircraft system.

Enter the custom APM 2.5, GPS, and battery module. It's also fabricated from a laser cutter, but this time the material is 1/8th inch-thick birch.

A press-fit box generating program made the initial template, but that template was exported to Inkscape to add the appropriate mounting holes and other features.

It is designed to secure lithium polymer batteries of up to six cells (25.2v), along with the 3DRobotics APM 2.5, and the uBlox LEA-6 GPS receiver.

Like the quadrotor autopilot mount, this solution uses 3mm rubber o-rings to reduce vibration transfer. But it also uses several birch o-rings, which are cut from the same slab of wood.

Here's a close-up of the engraving detail, with the logos from my grant, the university, and the federal agency responsible for funding the grant. The module is designed to be strapped to the interior of the fuselage with velcro straps for ease of servicing.

I'll upload a time-lapse video showing the design and fabrication process at a later date. Most of the birch's surface area was actually used for one of my sensor journalism projects, which calls for another post.