Powerful, soft combustion actuators for insect-scale robots.

Insects perform feats of strength and endurance that belie their small stature. Insect-scale robots-although subject to the same scaling laws-demonstrate reduced performance because existing microactuator technologies are driven by low-energy density power sources and produce small forces and/or displacements. The use of high-energy density chemical fuels to power small, soft actuators represents a possible solution. We demonstrate a 325-milligram soft combustion microactuator that can achieve displacements of 140%, operate at frequencies >100 hertz, and generate forces >9.5 newtons. With these actuators, we powered an insect-scale quadrupedal robot, which demonstrated a variety of gait patterns, directional control, and a payload capacity 22 times its body weight. These features enabled locomotion through uneven terrain and over obstacles.

Controlled flight of a microrobot powered by soft artificial muscles.

Flying insects capable of navigating in highly cluttered natural environments can withstand in-flight collisions because of the combination of their low inertia1 and the resilience of their wings2, exoskeletons1 and muscles. Current insect-scale (less than ten centimetres long and weighing less than five grams) aerial robots3-6 use rigid microscale actuators, which are typically fragile under external impact. Biomimetic artificial muscles7-10 that are capable of large deformation offer a promising alternative for actuation because they can endure the stresses caused by such impacts. However, existing soft actuators11-13 have not yet demonstrated sufficient power density to achieve lift-off, and their actuation nonlinearity and limited bandwidth create further challenges for achieving closed-loop (driven by an input control signal that is adjusted based on sensory feedback) flight control. Here we develop heavier-than-air aerial robots powered by soft artificial muscles that demonstrate open-loop (driven by a predetermined signal without feedback), passively stable (upright during flight) ascending flight as well as closed-loop, hovering flight. The robots are driven by multi-layered dielectric elastomer actuators that weigh 100 milligrams each and have a resonance frequency of 500 hertz and power density of 600 watts per kilogram. To increase the mechanical power output of the actuator and to demonstrate flight control, we present ways to overcome challenges unique to soft actuators, such as nonlinear transduction and dynamic buckling. These robots can sense and withstand collisions with surrounding obstacles and can recover from in-flight collisions by exploiting material robustness and vehicle passive stability. We also fly two micro-aerial vehicles simultaneously in a cluttered environment. They collide with the wall and each other without suffering damage. These robots rely on offboard amplifiers and an external motion-capture system to provide power to the dielectric elastomer actuators and to control their flight. Our work demonstrates how soft actuators can achieve sufficient power density and bandwidth to enable controlled flight, illustrating the potential of developing next-generation agile soft robots.

Untethered flight of an insect-sized flapping-wing microscale aerial vehicle.

Heavier-than-air flight at any scale is energetically expensive. This is greatly exacerbated at small scales and has so far presented an insurmountable obstacle for untethered flight in insect-sized (mass less than 500 milligrams and wingspan less than 5 centimetres) robots. These vehicles1-4 thus need to fly tethered to an offboard power supply and signal generator owing to the challenges associated with integrating onboard electronics within a limited payload capacity. Here we address these challenges to demonstrate sustained untethered flight of an insect-sized flapping-wing microscale aerial vehicle. The 90-milligram vehicle uses four wings driven by two alumina-reinforced piezoelectric actuators to increase aerodynamic efficiency (by up to 29 per cent relative to similar two-wing vehicles5) and achieve a peak lift-to-weight ratio of 4.1 to 1, demonstrating greater thrust per muscle mass than typical biological counterparts6. The integrated system of the vehicle together with the electronics required for untethered flight (a photovoltaic array and a signal generator) weighs 259 milligrams, with an additional payload capacity allowing for additional onboard devices. Consuming only 110-120 milliwatts of power, the system matches the thrust efficiency of similarly sized insects such as bees7. This insect-scale aerial vehicle is the lightest thus far to achieve sustained untethered flight (as opposed to impulsive jumping8 or liftoff9).

An insect-inspired collapsible wing hinge dampens collision-induced body rotation rates in a microrobot.

Some flying insects frequently collide their wingtips with obstacles, and the next generation of insect-inspired micro air vehicles will inevitably face similar wing collision risks when they are deployed in real-world environments. Wasp wings feature a flexible resilin joint called a 'costal break' that allows the wingtip to reversibly collapse upon collision, helping to mitigate wing damage over repeated collisions. However, the costal break may provide additional benefits beyond reducing wing wear. We tested the hypothesis that a collapsible wing tip can also dampen sudden and unpredictable body rotations caused by collisions. We designed a wing buckle hinge for an insect-scale microrobot, inspired by the costal break in wasp wings, and performed wing collision tests in a yaw-based magnetic tether system. We found that a collapsible wing tip reduced collision-induced airframe yaw rates by approximately 40% compared to a stiff wing, and that the effect was most pronounced for collisions that occurred early in the wing stroke. Our results suggest that a collapsible wingtip may simplify flight control requirements in both insects and insect-scale microrobots. We also introduce a scalable hinge design for engineering applications that recreates the nonlinear strain-weakening behaviour of a costal break.

Dynamics and flight control of a flapping-wing robotic insect in the presence of wind gusts.

With the goal of operating a biologically inspired robot autonomously outside of laboratory conditions, in this paper, we simulated wind disturbances in a laboratory setting and investigated the effects of gusts on the flight dynamics of a millimetre-scale flapping-wing robot. Simplified models describing the disturbance effects on the robot's dynamics are proposed, together with two disturbance rejection schemes capable of estimating and compensating for the disturbances. The proposed methods are experimentally verified. The results show that these strategies reduced the root-mean-square position errors by more than 50% when the robot was subject to 80 cm s-1 horizontal wind. The analysis of flight data suggests that modulation of wing kinematics to stabilize the flight in the presence of wind gusts may indirectly contribute an additional stabilizing effect, reducing the time-averaged aerodynamic drag experienced by the robot. A benchtop experiment was performed to provide further support for this observed phenomenon.

A biologically inspired, flapping-wing, hybrid aerial-aquatic microrobot.

From millimeter-scale insects to meter-scale vertebrates, several animal species exhibit multimodal locomotive capabilities in aerial and aquatic environments. To develop robots capable of hybrid aerial and aquatic locomotion, we require versatile propulsive strategies that reconcile the different physical constraints of airborne and aquatic environments. Furthermore, transitioning between aerial and aquatic environments poses substantial challenges at the scale of microrobots, where interfacial surface tension can be substantial relative to the weight and forces produced by the animal/robot. We report the design and operation of an insect-scale robot capable of flying, swimming, and transitioning between air and water. This 175-milligram robot uses a multimodal flapping strategy to efficiently locomote in both fluids. Once the robot swims to the water surface, lightweight electrolytic plates produce oxyhydrogen from the surrounding water that is collected by a buoyancy chamber. Increased buoyancy force from this electrochemical reaction gradually pushes the wings out of the water while the robot maintains upright stability by exploiting surface tension. A sparker ignites the oxyhydrogen, and the robot impulsively takes off from the water surface. This work analyzes the dynamics of flapping locomotion in an aquatic environment, identifies the challenges and benefits of surface tension effects on microrobots, and further develops a suite of new mesoscale devices that culminate in a hybrid, aerial-aquatic microrobot.