Above and Beyond

The vehicle’s full name is Powered-flying Ultra-underactuated LiDAR (light detection and ranging) Sensing Aerial Robot (PULSAR), and the development team describe it as an agile and self-rotating UAV whose three-dimensional position is fully controlled by actuating only one motor to obtain the required thrust and moments.

Mr Nan Chen, PhD candidate of Mechanical Engineering and lead author of the paper which has been published in Science Robotics said: “The greatest advantage of UAVs lies in their ability to break free from the constraints of terrain, swiftly reaching places that are hard for humans to access, and providing efficient real-time environmental observation. UAVs are used in various applications, such as aerial photography, express transportation, search and rescue, and building mapping. There is a lot of potential in the industry for multiple further applications.”

The team, from the Mechatronics and Robotic Systems (MaRS) Laboratory led by Dr Fu Zhang, have been working on UAVs since 2018, aiming to achieve a simple and reliable structure design and fully autonomous navigation. Autonomous UAVs typically have visual sensors to perceive obstacles and explore environments, but their perception capability is limited by a small sensor FoV. The MaRS team solved this problem by leveraging self-rotation to extend the sensor FoV without consuming extra power.

“Before PULSAR, we already had the Gemini II, a compact and efficient dual-rotor UAV with a servoless design,” said Mr Chen. “While, theoretically only one actuator is enough to realise a powered flight, in fact, when using one motor, the counter-torque of the motor cannot be fully counteracted and must result in an uncontrollable self-rotation of the UAV body.”

Self-rotation motion

The team first asked themselves how a self-rotation motion could be utilised. The question they wanted to answer was: since the FoV of LiDAR often tends to limit the efficiency of 3D reconstruction of environments, wouldn’t combining self-rotation motion with LiDAR enable scanning for a larger FoV?

“Therefore, we designed PULSAR whose revolution derives from the facts that fully autonomous flight is achieved with a minimal number of actuators, plus the self-rotation motion very inherently leads to a larger FoV of LiDAR,” said Mr Chen.

“While the self-rotation motion can extend the LiDAR FoV, the firm support of an efficient and robust algorithm of LiDAR localisation is indispensable, because the high-speed rotation movement is a substantial challenge for such algorithms,” he added.

“The LiDAR localisation algorithm employed by PULSAR is FAST-LIO2 – which is an efficient LiDAR-inertia odometry also developed by our MaRS laboratory. Notably, FAST-LIO2 demonstrates remarkable robustness to the aggressive motion. In the future, we plan to use UAVs like PULSAR as a platform to explore more advanced and more robust LiDAR localisation algorithms.”

In addition to the extended FoV, the use of a single actuator has other advantages, as it reduces the energy conversion loss of the propulsion system during flights, meaning that PULSAR consumes 26.7 per cent less power than widely used quadrotor UAVs with a similar propeller area, while retaining a good level of agility.

The onboard LiDAR sensor enables the PULSAR’s ability to perform autonomous navigation in unknown environments, and to detect obstacles – whether static or dynamic – in panoramic views without any external instruments.

The team have been experimenting with the PULSARs in environment exploration and multidirectional dynamic obstacle avoidance with the extended FoV via self-rotation, which could lead to increased perception capability, task efficiency, and flight safety.

Intricate systems

Asked about his own particular fascination for UAVs, Mr Chen said: “The applications for UAVs are undoubtedly multiple and exciting, and at the same time they are also intricate systems that demand a compact structure to reduce overall weight, an efficient propulsion system to extend flight duration, stable and reliable control, mapping, and planning methods to ensure flight safety, and efficient algorithms to accommodate limited onboard computing resources. In these aspects, UAVs still have many issues that are worth exploring and solving.

“Our next step will be to combine the characteristics of both self- rotation and single-actuation with the advantages of fixed-wing UAVs, for supporting long-distance environmental observation tasks,” he added, “because while PULSAR has higher efficiency than most quadrotor UAVs, it is still hard to compare with the fixed-wing UAVs in the aspects of flight efficiency and flight speed.”

Watch PULSAR in action