Crash-perching on vertical poles with a hugging-wing robot

Abstract

Perching with winged Unmanned Aerial Vehicles has often been solved by means of complex control or intricate appendages. Here, we present a simple yet novel method that relies on passive wing morphing for crash-landing on trees and other types of vertical poles. Inspired by the adaptability of animals' and bats' limbs in gripping and holding onto trees, we design dual-purpose wings that enable both aerial gliding and perching on poles. With an upturned nose design, the robot can passively reorient from horizontal flight to vertical upon a head-on crash with a pole, followed by hugging with its wings to perch. We characterize the performance of reorientation and perching in terms of impact speed and angle, pole material, and size. The robot robustly reorients at impact angles above 15° and speeds of 3 m/s to 9 m/s, and can hold onto various pole types larger than 28% of its wingspan in diameter. We demonstrate crash-perching on tree trunks with an overall success rate of 71%. The method opens up new possibilities for the use of aerial robots in applications such as inspection, maintenance, and biodiversity conservation.

Figures (6)

• Figure 1: A big brown bat (eptesicus fuscus) holding onto a tree trunk using its wings and claws (left), a great grey owl (strix nebulosa) fledging on its first day out of the nest wrapping its wings around a tree trunk to rest during climbing (center), and the PercHug robot perching vertically on a tree by hugging (right). Photo credits: bat_photo ? Citation 'bat_photo' on page undefined, owl_photo ? Citation 'owl_photo' on page undefined.
• Figure 2: a Operating principle of PercHug depicting the key steps of the perching maneuver: (1) gliding, (2) primary impact, (3) reorientation and wing release, (4) secondary impact, and (5) wing-wrapping. The red arrows represent the expected magnitudes of the impact forces, proportionally drawn. b Isometric view of PercHug showing different elements of the robotic platform. c Side view and physical properties of the robot. d Pre-loaded segmented wing interface in an open configuration. e Side view of the outermost wing segment highlighting the hooks. f Latching wing release mechanism (blue and red). g Backup bistable trigger (green).
• Figure 3: a Illustration of the concepts of unsuccessful reorientation, where the UAV bounces off the wall after impact, and a successful one in which it reaches a vertical orientation while making secondary contact with the wall. b Time evolution of the UAV's translational velocity $u$, pitch angle $\theta$, and pitch rate $\dot{\theta}$ for a sample trial (see \ref{['fig:kinematics']} and \ref{['sec:flight kinematics']} for the definitions of the state variables). The red region shows the duration of the primary impact, and the blue line corresponds to the time of maximum pitch for a successful reorientation. c Characterization results of the UAV reorienting from horizontal to vertical configuration. The plots show variations in success rate and mean primary impact force with impact angle and speed for four different types of noses.
• Figure 4: a Free-body diagram used for static modeling of the robot perched on a pole, shown in isometric and top views (refer to \ref{['sec:model']} for further details). b The theoretical minimum and maximum pole diameters the robot can perch on. c Simulation results of the static model showing variations in net squeezing force by the wings, maximum static payload capacity, and friction split with pole size and material. The static friction coefficients correspond to the poles used for the actual static experiments, while the diameter range is defined by the minimum and maximum values.
• Figure 5: a Close-up pictures of the surfaces of the poles used in the static perching experiments. b List of poles and their specifications in order of increasing friction coefficient. The "*" symbol denotes poles with a diameter smaller than the model's predicted minimum value of [detect-all]265. These cases were analyzed since the model was found to be valid even outside the previously mentioned diameter range, provided that the considered diameter is close to the limit. c Model prediction and results of the real-world static experiments. The measured values start at the weight of the prototype alone ([detect-all]325) and increase in increments of [detect-all]100 (see \ref{['sec:static perching experiments']} for more details). The insignificant discrepancies between experiment and simulation results in cases II, XII, and XIV can be attributed to minor errors due to the non-uniformity of tree barks (XII and XIV) and possible discrepancy of the stiffness coefficient of the torsion springs for small poles (case II) that exceed the manufacturer values for linear operation.