Student drone launches from portable base

Article By : Martin Rowe

Seniors at WPI show off their engineering projects to the public. The Oddisy Drone Dispatch System lets a drone stay outside, even in wet weather.

Each year on the third week of April, seniors at Worcester Polytechnic Institute (WPI) present their Major Qualifying Projects (MQPs) to the WPI community and to the public. No classes take place this day, seniors dress in business clothing, and each team gives a 15-minute presentation. Some 20 teams from the WPI Robotics Department participated, showing their robots that fly, collect fish, sail, crawl, sort objects, and roll.

One such project, the Oddisy Drone Dispatch System, has commercial potential in that the drone comes in a weather-protective box that opens and closes to allow takeoffs and landings while keeping the drone safe and dry. Designed and built by Noah Hillman, Nick Sorensen, Marek Travnikar, and Steven Viola, the Oddisy can fly over and inspect farms, solar panels, roofs, and even help first responders assess dangerous situations without having to be brought indoors after every flight. Travnikar spoke with EDN about how it works.

WPI Robotics Oddisy posterFigure 1. Undergraduate students at WPI designed and built the Oddisy Drone Dispatch System as their Major Qualifying Project, a requirement for graduation.

While the ideas for some MQPs come from WPI faculty, the idea for Oddisy came from the students. They recognized that while drones can fly over and inspect many things, you can’t just leave them in the field, even if the drone were to make the same flight every day. Transport can be awkward depending on the drone’s size and the size of, say, a vehicle. With that in mind, the students designed a system where you can leave the drone between flights and recharge its batteries. The video below shows the case opening to expose the drone.

Students purchased the drone’s components—airframe, motors, propellers, motor controllers, etc.—for about $300. There was no need to manufacture these parts. They designed the base station, control boards, power-distribution board, and wrote the software. While most of the mechanical design went into the base, students had to figure out how to mount six PCBs on the drone (Figure 2).

WPI Oddisy project electronicsFigure 2. The drone’s six PCBs provide sensing, communications, power, and control

The flight safety handler handles the precise control needed to land the drone in the base station. It takes data from the IMU, GPS, and the motor controller and distributes data to the Raspberry Pi. Furthermore, the flight safety handler provides some redundancy in case of a failure—it’s capable of taking over control of the drone should the Raspberry Pi hang.

In addition to delivering power to the motors, the power-distribution board supplies power to the drone’s systems while maintaining safety. That is, it can deliver power to the other boards but not to the motors during development and initial testing.

Why two IMUs? It’s to provide flight data more precisely than is possible with a single IMU. Think of it as providing rough and then more precise data. For example, the GPS provides data at 4 Hz while the second IMU updates at a rate of 100 Hz. The higher data rate was needed to handle the drone positioning during landings.

Precise landing
Because the drone and its housing operate autonomously, the base enclosure must open and close for launch and landing. While launching isn’t all that difficult, landing requires precise X, Y, and Θ positioning. Students selected a GPS for the drone that has a resolution of about 1.5 cm. “It’s differential GPS, with real-time kinematics (RTK),” said Travnikar. “Instead of relying on timing of the GPS signal to get its location, RTK performs wavelength counting and phase measurement of the signal frequency coming from the satellite to get a precise location. A second GPS in the base station makes atmospheric distortion measurements and communicates that data to the drone to use to compensate for atmospheric conditions. That’s how we can get 1.5-cm resolution.”

Having a magnetometer provides angle information relative to the ground. That’s needed not only for flight but for landing because only one of six possible rotational positions lets the drone line up with a charging connector (Figure 3).

WPI Oddisy base stationFigure 3. While at rest in the base station, the drone’s batteries can recharge.

On liftoff, the drone’s electronics record the location of the base so it knows where to return. That’s essential because the base could be set up in a different location for each deployment. “We had the option of using a camera to snap a picture of the base station’s location and use image software to locate the base on return, but we didn’t need it. The drone can find the base station using the other sensors,” said Travnikar. “The drone can use any camera as needed for vision, infrared heat detection, and so on.”

For takeoffs and landings, the drone must communicate with the base station for the doors to open and the latches to release. That takes place over a 433 MHz link. Messages tell the base to open and close the doors and latches. “The base is a slave to the drone,” said Travnikar. “It has its own Raspberry Pi board for controlling the doors, latches, lights, and the onboard lithium battery charger that charges the drone.” In its current design, the base station needs AC mains power, both for itself and to charge the drone’s battery. “It could be retrofitted for battery operation,” said Travnikar. “All it needs is 28 VDC.” Nick Sorensen, who designed the power-distribution board for the drone, also designed the lithium-battery charger that provides constant current for quick charging and constant voltage for trickle charging.

Base station design
Travnikar’s role in the project focused mostly on the base station mechanical design. A hexagon-shaped “ring” consisting of curved guides align the drone to its rest position on landing (see video below). To provide a clear line-of-sight from the base to the drone, the doors had to swing open more than enough for the drone to enter and exit. The doors open to a “flat” position to eliminate any chance of them colliding with the sides of the base station. When closed, the doors cover the drone and a slightly peaked shape let’s water flow off. Figure 4 shows the base station with one open door.

WPI oddisy base stationFigure 4. The Oddisy base station shown with one open door. Servo motors open and close the doors.

“Inclined roofs on hexagons provide some really wild angles,” noted Travnikar. “We had to manufacture components with angles such as 95.67 degrees. I had to design tools specifically to bend the sheet metal to those angles.”

The students went through several potential designs for moving the doors before settling on one. They originally thought of using motors that open and close automotive windows, adding limit switches. Unfortunately, limit switches provide no feedback as to the speed of the doors. Instead, the team chose off-the shelf servo motors, which provide position and speed information and even let one of the doors open and close at a different rate than the other, depending if the doors are opening or closing. “We also get force-feedback data. If something impedes the doors opening or closing, the system will know and stop the door motion.”

The video below, taken by the students, shows the Oddisy flying over the WPI football field.


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