Aimy Wissa

Abstract Origami-enabled robots are scalable, compliant, and easy-to-fabricate soft robots. In this study, we investigate the locomotion of an origami-enabled multisegment robot. Such robots offer energetic advantages and enable complex missions by combining several simple robotic segments. This article evaluates the effect of actuation frequency and substrate friction on the straight-line locomotion of the single- and multisegment robots, and their ability to turn. Moreover, we report our evaluation of the multisegment robot’s robustness to failure when one or more segments are disabled. During straight-line locomotion, the one-segment system can crawl with minimal penalties on low-friction substrates. In contrast, the multisegment system, when the distribution of mass is even, is insensitive to both substrate friction and frequency. The turning locomotion results reveal trade-offs between efficiency and maneuverability. Systems with high efficiency during straight-line locomotion have reduced displacement and a high cost of transport (COT), a measure of the energy required for locomotion while turning. When a segment fails during straight-line motion, the robot can continue operating with up to two segments disabled. During turning locomotion, the robot can operate only with one segment disabled, with the penalties to COT and displacement depending on the distribution of functioning segments. Thus, with the ability to crawl effectively on a range of substrates and at varying frequencies, both as a single- and multisegment system, this robot is a viable option for applications where redundancy and modularity are much needed.

Grasshoppers seamlessly alternate between flapping and gliding, adapting to changing conditions and conserving energy. This study examines the hindwings of the Schistocerca americana grasshoppers and determines the key elements of the wing features that can enable insect-scale flyers to use gliding as a mode of flight. Wing-specific elements include planform shape, camber profile and corrugation patterns. The study begins with a morphological study of S. americana hindwings and characterizes their aerodynamics through water channel experiments of grasshopper-inspired wing models. We then design, fabricate and evaluate, through flight testing, a grasshopper-inspired glider. Results reveal that while a corrugated wing model has the highest aerodynamic efficiency at low angles of attack, its aerodynamic efficiency decreases at higher angles of attack. In contrast, the glider with the wing model that captures the wing planform shape and has a simplified camber profile exhibits consistent aerodynamic efficiency across a wide range of angles of attack and repeatable flight performance. Therefore, we have identified that the wing planform and a simplified camber profile are key parameters when designing insect-scale gliding robots. The results lay the groundwork for advancing insect-scale robots that exploit gliding flight, offering new opportunities for untethered locomotion with low energy expenditure.

Abstract Click beetles (Coleoptera: Elateridae) have evolved a unique clicking mechanism that enables them to rapidly convert stored elastic potential energy into kinetic energy. However, the biological reason for why this mechanism evolved remains unclear. Several hypotheses have been proposed, such as self-righting, jumping away from threats, or deterring predators. We focused on studying the functionality of clicking as a means of escaping constraints. To investigate how the clicking mechanism influences interactions with the surrounding environment, we created a 3-dimensional dynamic simulation model. This model simplifies the beetle’s structure into several robotic components, including links, revolute joints, and torsional springs. We observed its response over time under various initial conditions and parameters, considering both prescribed and free joint movement. The simulation model showed that when fully constrained, forward and backward movement - and eventual escape - typically required multiple clicks. In contrast, in scenarios involving partial constraint, a single click frequently resulted in successful disengagement. This suggests that clicking plays a more important role in escaping partial constraints than in overcoming full confinement. The number of clicks required for escape was influenced by parameters such as surface stiffness and clamping force. We designed and fabricated a test rig that enabled precise constraint of live beetles in controlled orientations to replicate ecologically relevant postures encountered in natural environments. This setup allowed us to study the clicking behavior when constrained, through direct observation and kinematic analysis. We found that when fully constrained, click beetles cannot use the clicking mechanism to create enough lateral movement to escape the 4-sided constraint. However, when only partially constrained, the mechanism helps the click beetle break free. The combination of simulation and live beetle experiments enabled a more detailed investigation into the forces exerted during the click beetle’s clicking motion and its impact on surrounding materials. This research provides further insights into the design and construction of legless robotic mechanisms capable of locomotion across diverse terrain (bio-inspired design) while also helping to answer evolutionary questions in biology (engineering-informed biology).

Abstract The Toyota Mothership project explores a cutting-edge high-altitude aerial platform inspired by a large-scale, advanced kite. This innovative concept aims to utilize exceptional endurance and station-keeping abilities for diverse missions, including communication relay, atmospheric data collection, and airborne wind energy generation. The Mothership kite is a multifunctional, flexible, and inflatable kite with an unconventional platform shape and cross-section profile. This study aims to use wind tunnel experiments to characterize the aerodynamics of the kite and investigate the efficacy of covert feather-inspired flight control effectors to ensure that the kite maintains stability for an extended flight duration and can perform the maneuver required for energy harvesting. By investigating the aerodynamics of the kite cross section as well as the effects of covert-inspired flaps on the upper and lower sides of the wing section, we: (1) characterize the aerodynamic forces and moments on the baseline configuration of an unconventional airfoil, (2) quantify the aerodynamic effects of covert-inspired flaps on an unconventional airfoil, and (3) determine the efficacy of covert-inspired flaps in enabling stability augmentation and the required energy harvesting maneuver. Results show that covert-inspired flaps can effectively modulate lift and drag, producing rolling and yawing moments for in-flight control, validating their use as viable flight control effectors and supporting their implementation on airborne wind energy systems.

Aquatic ecosystems vital to biodiversity and climate change-such as coral reefs, kelp forests, and mangrove forests-are often cluttered with natural obstacles. To navigate these complex habitats, fish have evolved relatively small body sizes and outstanding maneuverability. In contrast, most unmanned underwater vehicles currently deployed for ocean monitoring are bulky and slow, limiting their ability to access these environments. Developing small and agile underwater robots that mimic native fish species provides a unique opportunity for automated sampling of dynamic aquatic ecosystems. In this paper, we present BlueGuppy, a miniature, low-cost, and untethered fish-like robot (9.5×2.4×3.0cm, 33.1 g) capable of maneuvering with a single actuator. It achieves swimming speeds of up to 2.8 body lengths per second and can execute tight turns with small circles 1.4 body lengths in radius. BlueGuppy can generate a net thrust even in the presence of an incoming flow, but the flow field around BlueGuppy only mirrors that of biological organisms when it is free-swimming, underscoring the importance of untethered robots for biomimetic research. We explored the maneuverability of BlueGuppy by tuning its kinematics. By varying its flapping frequencies and temporal bias, BlueGuppy can access a wide range of speeds and turning curvatures. The combination of speed, maneuverability, and simplicity establishes BlueGuppy as a unique platform in the literature with tremendous potential for both uncovering the biomechanics of schooling fish and advancing the state-of-the-art in autonomous ocean sampling.

System identification (SysID) techniques were applied to nonlinear simulation data for Toyota's Airborne Wind Energy Harvesting (AWEH) kite with bioinspired control effectors to develop accurate aerodynamic models. Orthogonal phase-optimized multisine inputs were designed to simultaneously excite all four bio-inspired control surfaces. Aerodynamic forces and moments were reconstructed using a modified flight dynamics model, accounting for bridle tension effects inherent to the tethered kite configuration. Stepwise regression and the equation-error approach were used to determine the aerodynamic model structure and estimate nondimensional stability and control derivatives. The results highlight the successful analysis of this type of airborne energy harvesting system, laying a foundation for flight testing and further refinement of aerodynamic and control models for AWEH systems. Additionally, the paper explores interesting aspects of identifying the aerodynamics of cable-tethered kites.

Bird wings are equipped with multiple flow control devices, one of which is the covert feathers. The covert feathers act as high-lift aeroelastic flow control devices for stall mitigation. This study investigates the performance characteristics of covert-inspired passive torsionally hinged flaps mounted on the suction side of a finite rectangular wing with AR= 4.67 at Re = 200, 000. Wind tunnel experiments including force and flow field measurements are used to analyze the effect of flap location, hinge stiffness, and flap inertia on the wing aerodynamic forces (i.e., lift and drag) and the flow physics of the wing-flap system. Two post-stall α ranges are defined in the study: the partial stall regime (19◦ ≤ α ≤ 26◦) and the global stall regime (28◦ ≤ α ≤ 36◦). Results show that the flaps can reduce drag by up to 15% and improve lift by up to 5% in the partial stall regime. The leading-edge flaps are effective in reducing drag in this regime since they can lessen the effect of induced drag at the wing tip, where the downwash effect is prominent, and decrease pressure drag at the wing areas with separated flow by bringing the shear layer closer to the wing surface, leading to a substantial overall drag reduction. In the global stall regime, the flaps reduce drag by up to 5% and improve lift by up to 20%. The leading- edge flap mitigates flow separation in this regime by interacting with the shear layer. However, the lift increase for the flap cases leads to an induced drag penalty, counteracting the reduction in pressure drag, resulting in a minimal overall drag reduction. Furthermore, we show that the flap inertia and hinge stiffness control the flap deflection angle which in turn controls to the change in the lift and drag forces relative to the baseline case.

Passive suction-side flaps, inspired by a bird's covert feathers, have been shown to increase the critical angle of attack for subscale aircraft in flight experiments. In this paper, a data-driven aerodynamic model for the coefficient of lift was derived to explore the effects of such bio-inspired flow control devices at high angles of attack. This model was derived from flight test data during a power-on stall maneuver that achieves high angles of attack. The model structure was derived iteratively using stepwise regression and the Bayesian Information Criterion to balance goodness of fit and statistical significance. Once identified, the vehicle's lift coefficient derivatives were estimated from the flight data. By comparing the estimated aerodynamic derivatives of an aircraft with the flow control devices to that of a baseline vehicle without them, it is evident that the suction side covert flaps increase lift at a given angle of attack. Additionally, it is observed that the covert flaps enhance pitch damping, which contributes to a stabilizing effect that mitigates the frequency and severity of stall. This initial study identifies a data-driven aerodynamic model for maneuvering in the stall regime, estimates parameters for that model, and compares derived models to provide evidence that bio-inspired covert flaps are an effective flow control device to improve aircraft performance near stall conditions.

Multiple rows of feathers, known as the covert feathers, contour the upper and lower surfaces of bird wings. These feathers have been observed to deploy passively during high angle of attack maneuvers and are suggested to play an aerodynamic role. However, there have been limited attempts to capture their underlying flow physics or assess the function of multiple covert rows. Here, we first identify two flow control mechanisms associated with a single covert-inspired flap and their location sensitivity: a pressure dam mechanism and a previously unidentified shear layer interaction mechanism. We then investigate the additivity of these mechanisms by deploying multiple rows of flaps. We find that aerodynamic benefits conferred by the shear layer interaction are additive, whereas benefits conferred by the pressure dam effect are not. Nevertheless, both mechanisms can be exploited simultaneously to maximize aerodynamic benefits and mitigate stall. In addition to wind tunnel experiments, we implement multiple rows of covert-inspired flaps on a bird-scale remote-controlled aircraft. Flight tests reveal passive deployment trends similar to those observed in bird flight and comparable aerodynamic benefits to wind tunnel experiments. These results indicate that we can enhance aircraft controllability using covert-inspired flaps and form insights into the aerodynamic role of covert feathers in avian flight.

Abstract Covert-inspired flaps are novel feather-inspired aerodynamic control surfaces that enable stability augmentation and maneuvering in small-scale Uncrewed Aerial Vehicles (sUAVs), especially for tailless configurations. For the first time, this paper uses time-averaged particle image velocity (PIV) to reveal the effect of static covert-inspired flaps on the flow field when simultaneously deflected on a wing’s upper (suction) and lower (pressure) surfaces. Compared to a flap deployed on a single side, the simultaneously deflected flaps enhance the modulation range of the aerodynamic response (i.e., lift, drag, and pitching moment), making them more effective as control surfaces. Results reveal two categories explaining why the modulation range of the simultaneous deflection response is larger than each side deflection alone. Limit-bounded cases, where the response is within the bounds of the single-sided experiments, and limit-expanding cases, where the interaction between the suction and pressure sides flaps is crucial. Limit-expanding cases are associated with flow fields with increased wake size or flow features that cannot be reconstructed from the suction-only or pressure-only flow fields. Using the velocity fields, we also show that superposition better predicts post-stall responses than pre-stall responses because the flow features present in the wake of the post-stall flow are similar between suction-only, pressure-only, and simultaneous deflection experiments. Finally, we show that the post-stall flow is more sensitive to pressure side flaps than suction side flaps due to significant flow changes occurring at the pressure side when the pressure side flap is deflected, which can increase the size of the wake and significantly alter the response. The results from the flow fields support the data-driven aerodynamic models that express the lift, drag, and pitching moment as a function of the flow and flap parameters.

Our ability to measure and image biology at small scales has been transformative for developing a new generation of insect-scale robots. Because of their presence in almost all environments known to humans, insects have inspired many small-scale flying, swimming, crawling, and jumping robots. This inspiration has affected all aspects of the robots’ design, ranging from gait specification, materials properties, and mechanism design to sensing, actuation, control, and collective behavior schemes. This article highlights how insects have inspired a new class of small and ultrafast robots and mechanisms. These new robots can circumvent motors’ force-velocity tradeoffs and achieve high-acceleration jumping, launching, and striking through latch-mediated spring-actuated (LaMSA) movement strategies. In the article, we apply a solution-driven bioinspired design framework to highlight the process for developing LaMSA-inspired robots and systems, starting with understanding the key biological themes, abstracting them to solution-neutral principles, and implementing such principles into engineered systems. Throughout the article, we emphasize the roles of modeling, fabrication, materials, and integration in developing bioinspired LaMSA systems and identify critical future enablers such as integrative design approaches. Graphical abstract

This study presents a novel spatially disrupted flow control system inspired by the covert feathers on bird wings. The system is a passive flow control system consisting of multiple feather-inspired flaps that dynamically interact with the surrounding flow to mitigate stall. Incorporating covert-inspired flaps on the suction side of the airfoil resulted in a substantial increase in lift (up to 50%) and a substantial reduction in drag (up to 30%) in post-stall conditions. Using wind tunnel experiments and time-resolved particle image velocimetry, the physical mechanisms responsible for post-stall lift improvements and drag reduction are identified as (1) shear layer interaction and (2) pressure dam effect. In the first mechanism, flap deployment reduces the geometric adverse pressure gradient that the flow encounters, reducing the degree of flow separation. In the second mechanism, the deployed flap acts as a barrier, preventing the relatively high pressure downstream from propagating upstream of the airfoil. The flow control mechanism employed was a function of the location of the flap. Flaps near the leading edge interacted mainly with the shear layer, while flaps near the trailing edge induced a pressure dam effect. Increasing the number of flaps along the chord increased the gain in lift and the reduction in drag. However, additive performance enhancements were sensitive to spatial distribution and flow control mechanisms. The shear layer interaction mechanism is found to be additive; that is, the deployment of additional flaps increases the lift gain, whereas the pressure dam effect is not.

Bio-inspired flow control strategies can provide a new paradigm of efficiency and adaptability to overcome the operational limitations of traditional flow control. This is particularly useful to small-scale uncrewed aerial vehicles since their mission requirements are rapidly expanding, but they are still limited in terms of agility and adaptability when compared to their biological counterparts, birds. One of the flow control strategies that birds implement is the deployment of covert feathers. In this study, we investigate the performance characteristics and flow physics of torsionally hinged covert-inspired flaps mounted on the suction side of a NACA2414 airfoil across different Reynolds numbers, specifically 200,000 and 1,000. These two Reynolds numbers are representative of different avian flight regimes where covert feathers have been observed to deploy during flight, namely cruising and landing/perching. We performed experiments and simulations where we varied the flap location, the hinge stiffness, and the moment of inertia of the flap to investigate the aerodynamic performance and describe the effects of the structural parameters of the flap on the aerodynamic lift improvements. Results of the study show up to 12% lift improvement post-stall for the flapped cases when compared to the flap-less baseline. The post-stall lift improvement is sensitive to the flap's structural properties and location. For instance, the hinge stiffness controls the mean deflection angle of the flap, which governs the resulting time-averaged lift improvements. The flap moment of inertia, on the other hand, controls the flap dynamics, which in turn controls the flap's lift-enhancing mechanism and how the flap affects the instantaneous lift. By examining the time-averaged and instantaneous lift measurement, we uncover the mechanisms by which the covert-inspired flap improves lift and highlights similarities and differences across Reynolds numbers. This article highlights the feasibility of using covert-inspired flaps as flow control across different flight missions and speeds.

Flying fishes are renowned for their ability to glide hundreds of meters through the air, after a taxi and takeoff behavior that is possible because of their asymmetric tail shape. To taxi, the ventral lobe of the tail, which is almost double the length of the dorsal lobe, is the only part of the fish that is in the water. The hydrodynamic effects of this asymmetric, or heterocercal tail, have never been studied. Herein, we developed a bioinspired robotic model organism based on the functional morphology of flying fishes, to experimentally control for the effect of the shape of the tail on thrust production. Using both force transducers and particle image velocimetry, we found that there was no significant difference in thrust generation between the heterocercal flying fish tails and the homocercal tails, where both tail fin lobes are equal in length, as is typical of other fishes. However, the current study employed rigid fin designs, and we anticipate that the addition of flexible fins that are more similar to the material properties of fish fins may have different results.

In nature, click-beetles use a unique hinge structure between their prothorax and mesothorax that acts as a latch-mediated spring actuation system to produce a high acceleration that can result in a jump. This mechanism enables them to jump a height of several times their body length without using their legs when the beetle is unconstrained. To study the beetle jump trajectory, we designed simplified beetle-inspired prototypes and a launching platform. The simplified prototypes are fundamentally two masses connected by a spring. The masses simulate the portion of a click beetle’s body located anteriorly (M1) and posteriorly (M2) to the clicking mechanism, and the spring simulates the elastic energy storage element. The launcher uses a quick-reaction release mechanism and magnetic actuator to simulate the unlatching process. In trajectory analysis, the parameters that are most important are initial velocity at take-off and the take-off angle since both the click beetles and the prototypes are governed by ballistic motion. We determined that morphological features such as elytra (body) curvature and the ratio of the two body masses affect these two dynamic parameters. Our findings provide further insight into the design and fabrication of legless jumping robotic mechanisms and apply engineering models and experimental tools to answer key biological questions.

Recent advances in imaging, manufacturing and microelectronics have made insect-scale robotics a reality. Many flying insect-scale robots rely on continuous wing flapping for locomotion. However, the energetic cost of flapping is prohibitive and usually results in most of these robots being tethered to off-board power and electronics. In this article, we present an insect-scale glider. The glider is inspired by grasshoppers, an order of insects that have been observed to glide intermittently between bursts of flapping flight, presumably to reduce their energy consumption. We characterized the hindwing morphology of collected grasshoppers. We also present preliminary wind tunnel experiments of hindwing models based on the morphology characterization, and free-flight testing results of the grasshopper-inspired glider. Results show that the glider has a lift-to-drag ratio of 2.9, producing a glide ratio better than projectile motion. However, this lift-to-drag ratio is less than those measured from actual grasshoppers during gliding. Future studies will focus on including more grasshopper wing design features associated with aerodynamic improvements, such as wing corrugations.

In the original version of this review, the citation information in the legend to Figure 1 and in Table 1 was incorrect. In addition the title for Table 1 had a typographical error. The revised versions are below and the items have now been corrected in the PDF and HTML versions of the review.

Birds are agile flyers that can maintain flight at high angles of attack (AoA). Such maneuverability is partially enabled by the articulation of wing feathers. Coverts are one of the feather systems that has been observed to deploy simultaneously on both the upper and lower wing sides during flight. This study uses a feather-inspired flap system to investigate the effect of upper and lower side coverts on the aerodynamic forces and moments, as well as examine the interactions between both types of flaps. Results from wind tunnel experiments show that the covert-inspired flaps can modulate lift, drag, and pitching moment. Moreover, simultaneously deflecting covert-inspired flaps on the upper and lower sides of the airfoil exhibit larger force and moment modulation ranges compared to a single-sided flap alone. Data-driven models indicate significant interactions between the upper and lower side flaps, especially during the pre-stall regime for the lift and drag response. The findings from this study are also biologically relevant to the observations of covert feathers deployment during bird flight. Thus, the methods and results summarized here can be used to formulate new hypotheses about the coverts role in bird flight and develop a framework to design covert-inspired flow and flight control devices for engineered vehicles.

Abstract Flow control is the attempt to favorably modify a flow field’s characteristics compared to how the flow would have developed naturally along the surface. Natural flyers and swimmers exploit flow control to maintain maneuverability and efficiency under different flight and environmental conditions. Here, we review flow control strategies in birds, insects, and aquatic animals, as well as the engineered systems inspired by them. We focus mainly on passive and local flow control devices which have utility for application in small uncrewed aerial and aquatic vehicles (sUAVs) with benefits such as simplicity and reduced power consumption. We also identify research gaps related to the physics of the biological flow control and opportunities for device development and implementation on engineered vehicles.

Millions of years of evolution have allowed animals to develop unusual locomotion capabilities. A striking example is the legless-jumping of click beetles and trap-jaw ants, which jump more than 10 times their body length. Their delicate musculoskeletal system amplifies their muscles' power. It is challenging to engineer insect-scale jumpers that use onboard actuators for both elastic energy storage and power amplification. Typical jumpers require a combination of at least two actuator mechanisms for elastic energy storage and jump triggering, leading to complex designs having many parts. Here, we report the new concept of dynamic buckling cascading, in which a single unidirectional actuation stroke drives an elastic beam through a sequence of energy-storing buckling modes automatically followed by spontaneous impulsive snapping at a critical triggering threshold. Integrating this cascade in a robot enables jumping with unidirectional muscles and power amplification (JUMPA). These JUMPA systems use a single lightweight mechanism for energy storage and release with a mass of 1.6 g and 2 cm length and jump up to 0.9 m, 40 times their body length. They jump repeatedly by reengaging the latch and using coiled artificial muscles to restore elastic energy. The robots reach their performance limits guided by theoretical analysis of snap-through and momentum exchange during ground collision. These jumpers reach the energy densities typical of the best macroscale jumping robots, while also matching the rapid escape times of jumping insects, thus demonstrating the path toward future applications including proximity sensing, inspection, and search and rescue.