sUAS Design for Sensory and Swarm Missions

This project outlines my efforts to design a workhorse UAV for Northeastern UAV (NUAV). Historically, our student group has used a heavy-lift octocopter to perform autonomous missions. However, the Tarot 1000 UAV was bulky and required a large operational team. It was especially frustrating when payloads and tests did not require such a powerful UAV. Moreover, the club was in dire need of a smaller, fully autonomous vehicle for testing stereo cameras and swarm missions. Out of inconvenience and the need for a robust airframe, the initiative to design a multipurpose developer drone was born! 

Version 1 UAV

Development began in late December of 2019 with the intention of testing power systems to accommodate between 8-10″ tri-blade propellers. The commercial electronics chosen were: a Pixhawk Mini for flight control, NVIDIA Jetson Nano for onboard computation, and an Intel Realsense D435 depth camera. The design was pushed to be as flat as possible to allow for space-efficient methods of payload storage aboard a larger UAV in the future. This would be useful for the aerial deployment that was the primary goal for these new airframes. Another driving factor of the design was the overall cost; the new airframe needed to be cost-effective but versatile. As a result, carbon fiber and 3D printed parts were the primary materials used in the initial design.

The first version of this swarm drone was experimental and needed to permit multiple motor and propeller configurations. As a result, the base plate had to include mounting points for all of the current and future electronics, motor arms, as well as connection points for the batteries. Additional mounting points were included in the base plate to allow a variety of payloads including both the Zed 2 and Intel D435i stereo cameras. To complement the base plates, I designed a custom PCB.

V1 Frame Assembly with PCB Model

V1 PCB

The top plate of the drone was similarly designed with the intention of being water jet cut out of carbon fiber sheets. Aluminum standoffs were used to separate the plates to make room for a rear-mounted Jetson Nano flight computer and forward depth camera(s). Up to this point, the designs had been similar to commercially produced UAVs. The 3D printed arms were used in order to allow quick production of different lengths to accommodate the different test propeller sizes. This ended up being the main flaw of the V1 design, but it will be mentioned again later on.

V1 Arms mounted in the Frame Assembly

V1 Arm Internal Side View: Wire Channel and ESC Placement

The V1 PCB also had some flaws. Although the design was essentially just connectors, many unnecessary items were forced into it. The primary function of the PDB was to distribute power from the batteries to all of the other electronics components. Two parallel batteries were connected to PDB using XT60 connectors. The ESC connections were designed to be removable, but they also used XT60 connectors which in hindsight were over-specked since it needed to be designed for a wide range of voltage and current configurations (different motor, propeller, and battery configurations).

The figures below show the first version prototype. Assembly was similar to many of the other Pixhawk builds that I had done prior. Arducopter firmware was loaded onto the 3DR Pixhawk mini and an FPV camera was included in case manual flight was needed.

Pixhawk and Motor Wiring

Assembled V1 Ready for First Flight

The first hover test showed bad oscillations in the pitch and roll axis. The stock Arducopter PID values were tuned to reduce the oscillations and improve yaw stability using the Zeigler-Nicholas method. Once the vehicle had been tuned, endurance testing to compare motor/propeller configurations and Realsense data collection tests ensued.

V1 Swarm Drone Fitted with a RealSense Camera

Onboard Fractal Aruco Marker Pose Estimation

Four tests were performed on 8″, 9″, 10″ and 13″ propellers. For each test, a single 6s 2200 mAh LiPo battery was used to perform a basic 4-waypoint autonomous mission. The vehicle was set to fly the circuit continuously until the battery was discharged below nominal voltage (22.2 V). Voltage degradation results were plotted with respect to time as shown:

From the tests, we concluded that the 13″ propeller configuration using T-Motor 4012-480 Kv brushless motors provided the longest flight time. It should be noted that this was by no means the perfect evaluation of the motors and propellers. Only a single run of each configuration was recorded and each run was performed using a different battery. If this was design for a commercial product, then more extensive testing would have been performed. That being said, this test did provide a district answer to the question: what motor and propeller combination should we use for our UAV? In addition to the quantitative results, the 13″ configuration proved to have the best qualitative results. The longer arms and slower propeller rotation speed noticeably induced fewer oscillations as well as increased stability and responsiveness in manual flight. Although, the flight performance at a whole was sub par from other platforms of the same size that I had flown.

At this point, the project had spanned from December 2019 – March of 2020 . Though successful in flight, tests showed that the V1 had number of issues that would make its mass production impractical. The first of these was its poor flight performance. As mentioned, the ‘squashed X’ motor configuration gave the drone poor flight characteristics and made it very difficult to manually control due to a lack of pitch authority. Second, the design of the parametric, 3D printed arms became unnecessarily complex and as inferred previously, did not perform well in crashes even when made out of PC. Thus the arms took long to print and were not strong enough. Resulting from these factors, I began the design of the V2 swarm drone.

Failure of 3D Printed Arms

Version 2 UAV

The main targets for the second design of the UAV were to improve overall reliability and cut back on some of the unnecessary features of the first version. From this, the frame was designed as a ‘true X’ where all of the motors were in a symmetric square shape rather than the previous rectangular. This meant longer arms, thus sacrificing slightly in terms of form factor in order to maintain the same FOV for the front-mounted Realsense camera. The redesigned base plate would be less spacious. The first version base plate had room for future internal electronics, but that created many empty voids. The new version would only have mounting for the electronics that we knew that we needed. Fewer standoffs were used to make removing the top plate easier for when it would undoubtedly need to be serviced. Many improvements were also made to the stereo camera mount including a downward cutout to allow a single camera to look ahead and downwards as well as frame-mounted vibration grommets to decrease the height of the damped camera mount. Cutouts were also made in the top plate to allow a bottom-mounted GPS to protrude past its surface. This eliminated the need to attach components to the top plate that had fragile wires which were often damaged during maintenance of the V1 design. Lastly, the height of the frame was reduced by a further 5 mm in pursuit of a more deployable platform for use on swarm missions. Finally, the arms would be constructed out of square carbon tube to make them lightweight but durable.

Main Sketch Top View

V2 Frame Top

Frame with Carbon Tubes

The next step was the PCB. The V1 PCB had many unused signal and power traces so simplification was needed. The PCB would mainly be for power distribution to the motors and key 5 V components while only routing crucial signal wires such as those for the ESCs. The following simple PCB was designed with Altium:

V2 PCB

Full V2 Electronics Assembly

V2 Full CAD Assembly

With the completion of the frame and its components, the manufacturing of the first prototype began with waterjet cutting the frame plates and FDM printing parts.

Laser Cut Wood Frame (Pre-assembly)

Motor Mount (Before Nylon Models)

Electronics, PCB, and Arm Clamp Assembly

Early Assembly with Landing Gear

Swarm Drone V2: Final Assembly Render

I first laser-cut wood to test the plate dimensions and allow the hardware team to make a few minor revisions before waterjet cutting out of carbon. Tolerances were also tested and iterated with the motor and arm clamps. I had to experiment with materials for the motor and arm clamps. The final materials used were Nylon and Alloy 910 respectively. The wiring was relatively clean given the placement of regulators and use of the carbon tube arms to internalize motor and ESC wiring. The overall weight of the airframe was ~2.3 kg (including battery, propellers, and the Intel Realsense). This was approximately the same weight as the V1 but with a much more accessible design. Following a successful assembly, I conducted flight tests using a similar waypoint mission to the V1 efficiency tests to evaluate the performance of the new design.

With a stock ArduCopter tune, the vehicle was completely stable! Flight tests showed consistent tracking of waypoint missions with no oscillation and clean turning. The V2 design had successfully eliminated the poor handling characteristics of the V1 design while retaining all functionality. The endurance tests showed a flight time of 11-12 minutes which was up to a 3 minute improvement on the first design (within the same weight). The improved flight time was most likely also due to the improved stability as the PID loop was using less power to keep flight stable.

With the functionality of the initial prototype verified, the beginning of the fall 2020 semester marked a manufacturing spree to produce another 5 vehicles. Over the course of 1 month, myself and the other co-founder of NUAV fixed design bugs, integrated electronics, and ultimately completed 6 vehicles in total to support autonomous tests.

RealSense Depth Camera and Jetson Nano Payloads Wired

Assembled Drone On Test Day

Swarm Drone Fleet Undergoing Checks

Bodies Assembled After Machning

First Prototype Test Flight

Motor/ESC/Arm Subassemblies

Downward Facing Camera (Articulating Mount)

Team Picture on Test Day!

Moving forward, this airframe will be an essential tool for NUAV members to learn software and hardware development skills in pursuit of our Swarm Carrier Project: to deploy swarms of UAVs from aerial platforms for rapid surveying missions. You can read more about this initiative in my post on  Drone Deployment from an Aerial Platform

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