Aerial insects demonstrate fast and precise heading control when they perform body saccades and rapid escape maneuvers. While insect-scale micro-aerial-vehicles (IMAVs) have demonstrated early results on heading control, their flight endurance and heading angle tracking accuracy remain far inferior to that of natural fliers. In this work, we present a long endurance sub-gram aerial robot that can demonstrate effective heading control during hovering flight. Through using a tilted wing stroke-plane design, our robot demonstrates a 10-second flight where it tracks a desired yaw trajectory with maximum and root-mean-square (RMS) error of$\boldsymbol{14.2^{\circ}}$and$\boldsymbol{5.8}^{\mathrm{o}}$. The new robot design requires 7% higher lift forces for enabling heading angle control, which creates higher stress on wing hinges and adversely influences robot endurance. To address this challenge, we developed novel 3-layered wing hinges that exhibit 1.82 times improvement of lifetime. With the new wing hinges, our robot demonstrates a 40-second hovering flight - the longest among existing sub-gram IMAVs. These results represent substantial improvement of flight capabilities in soft-actuated IMAVs, showing the potential of operating these insect-like fliers in cluttered natural environments.

Flight is an energetically expensive task. While aerial insects can effortlessly fly through natural environments, achieving power autonomous flights in insect-scale robots remains a major challenge. In prior works, we developed soft-actuated insect-scale aerial robots that demonstrated unique capabilities such as in-flight collision recovery and somersaults. However, the soft dielectric elastomer actuators (DEAs) have low efficiency (600 V). These properties represent formidable obstacles for soft aerial robots to achieve power autonomous flights. In this work, we developed a 127 mg boost circuit that can convert a 7.7 V DC input into a 600 V and 400 Hz output for driving a 120 mg DEA. It has an equivalent capacitance and resistance of 20 nF and 5$\mathbf{k}\Omega$, respectively. The DEA is assembled into a 158 mg aerial robot, which can demonstrate liftoff while carrying the boost circuit as a payload. Although the robot remains tethered to an off-board power supply, this result represents a first step towards achieving power autonomy in soft aerial robots.

Accurate and agile trajectory tracking in sub-gram Micro Aerial Vehicles (MAVs) is challenging, as the small scale of the robot induces large model uncertainties, demanding robust feedback controllers, while the fast dynamics and computational constraints prevent the deployment of computationally expensive strategies. In this work, we present an approach for agile and computationally efficient trajectory tracking on the MIT SoftFly [1], a sub-gram MAV (0.7 grams). Our strategy employs a cascaded control scheme, where an adaptive attitude controller is combined with a neural network (NN) policy trained to imitate a trajectory tracking robust tube model predictive controller (RTMPC). The NN policy is obtained using our recent work [2], which enables the policy to preserve the robustness of RTMPC, but at a fraction of its computational cost. We experimentally evaluate our approach, achieving position Root Mean Square Errors (RMSEs) lower than 1.8 cm even in the more challenging maneuvers, obtaining a 60% reduction in maximum position error compared to [3], and demonstrating robustness to large external disturbances.

Flying insects are remarkably agile and robust. As they fly through cluttered natural environments, they can demonstrate aggressive acrobatic maneuvers such as backflip, rapid escape, and in-flight collision recovery. Current state-of-the-art subgram microaerial-vehicles (MAVs) are predominately powered by rigid actuators such as piezoelectric ceramics, but they have low fracture strength (120 MPa) and failure strain (0.3%). Although these existing systems can achieve a high lift-to-weight ratio, they have not demonstrated insect-like maneuvers such as somersault or rapid collision recovery. In this article, we present a 665 mg aerial robot that is powered by novel dielectric elastomer actuators (DEA). The new DEA achieves high power density (1.2 kW/kg) and relatively high transduction efficiency (37%). We further incorporate this soft actuator into an aerial robot to demonstrate novel flight capabilities. This insect-scale aerial robot has a large lift-to-weight ratio (>2.2:1) and it achieves an ascending speed of 70 cm/s. In addition to demonstrating controlled hovering flight, it can recover from an in-flight collision and perform a somersault within 0.16 s. This work demonstrates that soft aerial robots can achieve insect-like flight capabilities absent in rigid-powered MAVs, thus showing the potential of a new class of hybrid soft-rigid robots.

Remoras employ their adhesive discs to rapidly attach to and detach from a wide range of marine surfaces. By analyzing high-speed images of remoras' (Echeneis naucrates) hitchhiking behavior, we describe the fish's detachment mechanism as a lip curling up to break the seal between the disc and substrate. By mimicking the kinematic and morphological properties of the biological disc, we fabricated a multi-material biomimetic disc (whose stiffness spans four orders of magnitude) that is capable of both attachment and detachment. Detachment is realized by a flexible cable-driven mechanism that curls the anterior region of the silicone soft lip, allows leakage under the disc, and equalizes the internal pressure to the external pressure. The disc lamellae with attached carbon fiber spinules can be rotated by hydraulic soft actuators whose internal pressure is precisely tuned to the ambient underwater pressure. During attachment, increasing the rotational angle of the lamellae and the preload of the disc significantly enhanced the adhesive forces. We found that curling up the soft lip and folding down the lamellae rapidly reduced the pulling force of the disc by a factor of 254 compared to that under the attached state, which lead to detachment. Based on these mechanisms, underwater maneuvers involving repeated attachment and detachment were demonstrated with an integrated ROV unit that had a self-contained actuation and control system for the disc. This study lays a foundation for the development of fully untethered robotic systems for underwater hitchhiking in real-world marine environments.

Here we experimentally quantify the effects of wing morphological and inertial parameters on flapping flight performance. Through running at-scale, passive pitching experiments with different wing designs, we compare the relative importance of wing inertia, wing shape, and wing-actuation pairing. We find wing inertia strongly influences the coupling between stroke and pitch dynamics, which directly impacts lift production and efficiency. Flapping resonance frequency is reduced as wing aspect ratio or area moment increases. Further, wing leading edge design strongly influences chordwise center of pressure, which further impacts pitching dynamics. Based on our experimental results we propose a new wing design and measure 37% increase in mean lift relative to a previous work.

Here we present a suite of theoretical, computational, and experimental studies culminating in the first aerial and aquatic capable insect-scale robot. We develop a computational fluid dynamics (CFD) simulation to model fluid-wing interaction in air and water. From CFD and a system dynamics analysis we predict that a multi-modal flapping strategy will enable locomotion in both air and water for a single device. We validate the CFD predictions by running at-scale, robotic wing-flapping experiments. Finally, we demonstrate for the first time a flying and swimming capable flapping-wing insect-like robot.

We implement a 2D computational model to investigate the unsteady aerodynamic effects not captured by classical quasi-steady models. We compare numerical simulation results, experimental measurements and quasi-steady predictions to demonstrate the strength of the numerical tool in identifying unsteady fluid mechanisms and improving propulsive efficiency of flapping wing robots. In particular, this study quantifies the effect of the relative phase between wing degrees of freedom δ on lift and drag production. The computational model also identifies unsteady effects such as wake capture and downwash that are not accounted for in classical quasi-steady models. To examine the accuracy of our computational model, we fabricate millimeter-scale wings through the SCM fabrication processes and measure flapping kinematics and dynamics. The experiments show 2D computational model is 44% more accurate than the quasi-steady model and can be further used to improve wing morphology for better aerodynamic performance.

The design of flapping wing robots and the study of flapping wing flyers requires a detailed knowledge of how wings interact with the surrounding fluid. However, the unsteady nature of fluid-structure interactions during flapping wing flight render analytical design of wing shapes and motion kinematics difficult. We propose that flapping wing micro aerial vehicle (MAV) design will benefit from a complimentary, datadriven approach in which wing shape, material properties, and stroke-kinematics may be varied rapidly. Here, we present a high-throughput experimental apparatus for fabrication and optimization of MAV wings for flapping flight. This apparatus incorporates the collection and analysis of multiple sensor modalities including force, electrical power, resultant fluid flow, and wing kinematics into the experiment control loop. This “analysis-in-the-loop” methodology enables multivariate optimization routines for flapping flight of unmanned aerial vehicles. We demonstrate the validity of this approach through optimization experiments on wing kinematics, fluid flow, lift and power consumption.

Articulation based on flexure hinges is increasingly popular in microrobotics because of the absence of Coulomb friction, ease of manufacturability, fluid motion, durability, and large angular ranges. However, the inherent flexibility of these hinges makes modeling very complex and specific to the particular engineering applications for which they were developed. In this paper we describe the development and testing of a simplified, versatile method for modeling the stress on a flexure hinge under multi-axis loads in order to maximize hinge lifespan. We also discuss other stress concentration reducing features and design rules that can be applied to more general flexure hinge designs to further extend hinge lifespan.

This paper presents a wing characterization method for insect-scale flapping-wing robots. A quasi-steady model is developed to predict passive wing pitching at mid-stroke. Millimeter scale wings and passive hinges are manufactured using the SCM fabrication processes. Flapping experiments at various frequencies and driving voltages are performed to extract kinematics for comparison with the quasi-steady predictions. These experiments examine the validity of the quasi-steady model and demonstrate the robustness of the wing characterization method. In addition, because time-averaged lift and drag are strongly correlated with flapping kinematics, quasi-steady prediction of wing kinematics directly leads to predictions of lift and drag generation. Given a flapping frequency and a driving voltage, the model computes the hinge stiffness that leads to optimal flapping kinematics. This reduces the number of flapping experiments required for wing characterization by a factor of four.