The Story Behind Our Perching Rotorcraft: A Collaborative Journey

An undergraduate senior project soon took flight, propelling the research forward. Over several years and through multiple PhD students' efforts, the perching rotorcraft project overcame COVID challenges to culminate in a passive mechanism enabling a fourfold increase in mission time.
The Story Behind Our Perching Rotorcraft:  A Collaborative Journey
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How One Student's Senior Project Took Flight

What started as an undergraduate senior project for Yi-Hsuan (Nemo) Hsiao in 2019 soon took on a life of its own, propelling the research forward in unexpected ways. Nemo's initial work with advisors Pakpong Chirarattananon and Zuankai Wang on developing a perching rotorcraft showed promise, but needed further testing to reach publication status.

Nemo stayed on as a research assistant to continue working on the project. During his PhD interview with Professor Kevin Yufeng Chen (MIT), Nemo's early progress captivated Kevin's interest. Kevin joined the collaborative effort as well as admitted Nemo to MIT's PhD program in Fall 2021.

But just as the work was gaining altitude, a global pandemic threatened to crash-land the research. When COVID hit, Nemo could not complete the manuscript from overseas. So two PhD students, Songnan Bai and Huaiyuan Jia, took the lead, pushing ahead to finish what Nemo had started. The research also benefitted from biomimetic adhesive pads provided by Professor Zuankai Wang and Yongsen Zhou.

Through perseverance, teamwork and crossing disciplines, this project went from a humble senior design to an international research effort. And the journey itself highlights what it truly takes to advance scientific research - a willingness to take risks, ride the ups and downs, and turn roadblocks into launch pads.


A Passive Lightweight Mechanism for Perching

The perching rotorcraft strategy combines wet biomimetic adhesive pads, propeller thrust assistance, and the "ceiling effect" proximity phenomenon [1]. This allowed the 32-gram rotorcraft to repeatedly perch on walls and ceilings with minimal added mass (only 1 gram heavier).

The developed prototype incorporates two pairs of wet adhesive pads: ceiling pads and wall pads, as well as three rigid poles to prevent propeller collisions. Each pair of bioinspired adhesive pads, decorated on PDMS bases, are separately engaged for ceiling or wall perching.

To remain stationary while perching, the required collective thrust is radically reduced from the nominal flight condition thanks to the presence of the adhesion force. Meanwhile, the proximity to surfaces substantially decreases aerodynamic power and power consumption.

Figure 1: Overview of the developed prototype and the perching strategy. The scale bar is 2 cm.

For ceiling perching, the ceiling adhesive pads lower the required collective thrust, contributing to power conservation. Static equilibrium necessitates a moment arm dictated by the adhesion force and mechanical advantage.

For wall perching, the weight is balanced by shear adhesion produced by the wall adhesive pads. Static equilibrium is achieved when the moment arm of the collective thrust exceeds that of the robot's weight.

Characterizing the wet adhesive and analyzing stabilization conditions enabled autonomous perching maneuvers. Compared to flying, the rotorcraft conserved up to 50% power when perching on ceilings and up to 85% power when perching on walls.

The mussel-inspired wet adhesive displays advantages over dry adhesives and microspines. Unlike dry adhesives that lose stickiness on damp surfaces, the wet adhesive retains effectiveness on damp substrates and sticks firmly to non-smooth materials.

During ceiling perching, a four-stage framework enables the robot to reversibly perch and conserve energy over a wide range of substrates by evaluating the maximum adhesion pressure on-the-fly. Position feedback is substituted by onboard feedback.

Figure 2: Ceiling and wall perching. (a) The four-stage ceiling perching strategy involving the manipulation of four rotor thrusts. Takeoff is accom- plished through peeling, requiring no additional mechanism or actuators. (b) The four-stage wall perching maneuver. 

The devised strategy was implemented for the robot to demonstrate surface locomotion on four materials in dry and wet conditions. In the dry acrylic case, the robot briefly generated preload during Stage II. The collective thrust was gradually decreased until peel off was detected.

For challenging surfaces with lower adhesion and creep, the adhesive pads became unable to support the weight after a few seconds. This was addressed by reverting the center of thrust to reduce the load on the adhesive.

During wall perching, the robot momentarily applied substantial compressive preload to the adhesive, taking into account actuation limits. Once perched, minimal thrust markedly lowered power consumption.

The adhesive pads remained under slight compression during wall perching, making creep resistance less relevant. As a result, the robot was able to lower power expenditure by 20-30% when perching on walls.

Prolonged endurance tests revealed that ceiling and wall perchings conserve approximately 40% and 80% of power, respectively. The mission times of the robot increased by up to fourfold thanks to combined adhesive forces and proximity effects.


Figure 3: Power consumption and operational endurance. (a) Average input power of the robots (i) from hovering flights, (ii) during ceiling perching, (iii) during wall perching, and (iv) during the ceiling and wall perchings in endurance flight tests. (b) Total operational times of the robots in extended hovering and perching flights (endurance test). 

This passive lightweight mechanism increased the mission time up to 4-fold. While the wet adhesive offers advantages over dry adhesives and microspines, future improvements could enhance reusability and lifetime.

The findings demonstrate the potential benefits of perching rotorcraft for extending operation times of small drones. The synergistic combination of aerodynamic forces, thrust assistance and wet adhesion in a lightweight mechanism could inspire future designs.

Learn more about our research on bio-inspired robots and micro aerial vehicles:
ris.bme.cityu.edu.hk

Reference

[1] Y. H. Hsiao and P. Chirarattananon, “Ceiling effects for hybrid aerial-surface locomotion of small rotorcraft,” IEEE/ASME Transactions on Mechatronics, vol. 24, iss. 5, 2019.

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