Design and Simulation of a Bio-Inspired Deployable Mechanism Achieved by Mimicking the Folding Pattern of Beetles' Hind Wings

In this paper, a beetle with excellent flight ability and a large folding ratio of its hind wings is selected as the biomimetic design. We mimicked the geometric patterns formed during the folding process of the hind wings to construct a deployable mechanism while calculating the sector angles and dihedral angles of the origami mechanism. In the expandable structure of thick plates, hinge-like steps are added on the thick plate to effectively avoid interference motion caused by the folding of the thick plate. The kinematic characteristics of two deployable mechanisms were characterized by ADAMS 2018 software to verify the feasibility of the mechanism design. The finite element method is used to analyze the structural performance of the deployable mechanism, and its modal response is analyzed in both unfolded and folded configurations. The aerodynamic generation of a spatially deployable wing is characterized by computational fluid dynamics (CFD) to study the vortex characteristics at different frame rates. Based on the aerodynamic parameters obtained from CFD simulation, a wavelet neural network is introduced to learn and train the aerodynamic parameters.

Empowering artificial muscles with intelligence: recent advancements in materials, designs, and manufacturing

Drawing on foundational knowledge of the structure and function of biological muscles, artificial muscles have made remarkable strides over the past decade, achieving performance levels comparable to those of their natural counterparts. However, they still fall short in their lack of inherent intelligence to autonomously adapt to complex and dynamic environments. Consequently, the next frontier for artificial muscles lies in endowing them with advanced intelligence. Herein, recent works aimed at augmenting intelligence in artificial muscles are summarized, focusing on advancements in functional materials, structural designs, and manufacturing techniques. This review emphasizes memory-based intelligence, enabling artificial muscles to execute a range of pre-programmed movements and refresh stored actuation states in response to changing conditions, as well as sensory-based intelligence, which allows them to perceive and respond to environmental changes through sensory feedback. Furthermore, recent applications benefiting from intelligent artificial muscles, including adaptable robotics, biomedical devices, and wearables, are discussed. Finally, we address the remaining challenges in scalability, dynamic reprogramming, and the integration of multi-functional capabilities and discuss future perspectives of augmented intelligent artificial muscles to support further advancements in the field.

Wing extension-flexion coupled aeroelastic effects improve avian gliding performance

During flight, birds instigate remarkably large changes in wing shape, commonly termed 'wing morphing'. These changes in shape, particularly extension-flexion, have been well documented to influence the production of aerodynamic forces. However, it is unknown how wing stiffness changes as a result of the structural rearrangements needed for morphing. We address this gap in knowledge through mechanical testing of in situ flight feathers in anaesthetized pigeons and found that while the most distal portion of the feathered wing remained unaffected, proximal areas saw an increase in out-of-plane stiffness due to wing folding. Following this, we used computational fluid-structure interaction simulations to evaluate how this morphing-coupled change in stiffness might modulate local flow patterns to affect aerodynamic performance. We found that flexible wings perform better than entirely rigid wings as an increase in near-wall vorticity delayed flow separation. Furthermore, an increase in stiffness in a folded wing during high-speed flight prevented the reduction in lift seen in more flexible cases caused by aeroelastic flutter modes destructively interfering with shed leading-edge vortices. Collectively, these results reveal that mechanical changes coupled with wing morphing can provide a speed-dependent mechanism to enhance flight performance.

Seamless Integration of Adaptive Sensing Layers in Ornithopter Structures for Enhanced Motion Monitoring

Integrating perception function in structural components of smart robots for motion monitoring is highly needed but still challenging due to the deformation limitations of rigid sensing materials and the viscoelastic hysteresis of flexible sensing materials. The seamless integration of a lightweight and adaptive sensing composite layer on the structure support is reported at the leading edge of a bird-like ornithopter for motion monitoring. This perception-integrated structural component is designed by firmly wrapping a carbon fiber reinforced plastics (CFRP) rod core in sequence with a polyacrylonitrile (PAN) nanofiber network, an MXene/carbon nanotubes (CNT) thin conductive layer, and a thermoplastic polyurethane nanofiber protection sheath. The piezoresistive MXene/CNT sensing layer effectively adapts to trace structural changes of the CFRP rod with the help of the PAN nanofiber network, achieving the bend deformation perception of the structural component. More importantly, the perception-integrated structural component enabled flight attitude reproduction and structural damage warning in an actual flight of the ornithopter. This work provides promising solutions for advancing the development of future smart ornithopters, paving the way for more intelligent, compact, and lightweight designs.

The feather's multi-functional structure across nano to macro scales inspires hierarchical design

Bird feathers are finely tuned structures with key features at every length scale, from nanometre to metre, furnishing a unique multi-functional hierarchical design that can inspire material scientists, biologists and designers alike. Feathers are not only a crucial component in equipping birds with flight, but are also responsible for thermoregulation, coloration and crypsis, water repellency, silencing and sound production, sensing, directional fastening and even self-healing. Despite this broad multifunctionality, all feathers are formed from the same basic template using a universal building block: the feather keratin protein. Consequently, feather diversity across approximately 10 000 bird species arises from subtle differences in architecture rather than variations in chemical composition. To understand these underlying hierarchical mechanisms, we systematically review feather properties across all length scales, connecting development and morphogenesis to biomechanics and integrated structure-property-function relationships. This systematic distillation of the feather's complex design into comprehensive principles will enkindle new biohybrid, biomimetic and bioinspired material solutions.

Particle-armored liquid robots

It is challenging to emulate biological forms and functions with artificial machines: Fluidity and adaptability seen in cellular organisms, characterized by their ability to deform, split, merge, and engulf, are hard to recapitulate with traditional rigid robotic structures. A promising avenue to tackle this problem is harnessing the supreme deformability of liquids while providing stable yet flexible shells around them. Here, we report a highly robust liquid-particle composite, named a Particle-armored liquid roBot (PB), featuring a liquid blob coated with unusually abundant superhydrophobic particles. The enhanced deformability and structural stability of our millimetric PBs enable a range of versatile robotic functions, such as navigating through complex environments, engulfing and transporting cargoes, merging, and adapting to various environments. We use both theoretical analysis and experimental approaches to develop a framework for predicting the shape evolution, dynamics, and robotic functions of PBs. The forms and functions of our liquid robots mark an essential hallmark toward miniature biomachines that perform like cells.

Aerodynamic analysis of complex flapping motions based on free-flight biological data

The wings of birds contain complex morphing mechanisms that enable them to perform remarkable aerial maneuvers. Wing morphing is often described using five wingbeat motion parameters: flapping, bending, folding, sweeping, and twisting. However, the specific impact of these motions on the aerodynamic performance of wings throughout the wingbeat cycle, and their potential to inform engineering applications, remains insufficiently explored. To bridge this gap and better incorporate the properties of coupled motions into the design of biomimetic aircraft, we present a numerical investigation of four flapping-based coupled motions during different flight phases (i.e. take-off, level flight, and landing) using a pigeon-like airfoil model. The wingbeat motion data for these four coupled motions were based on real flying pigeons and divided into: flap-bending, flap-folding, flap-sweeping, and flap-twisting. We used computational fluid dynamic simulations to study the effects of these coupled motions on the flow field, generation of transient aerodynamic forces, and work done by different motions on flapping. It was found that, first, the flap-bending motion causes unstable changes in the effective angle of attack (AoA), which affects the attachment of the leading-edge vortex (LEV), thereby producing more lift at smaller bending angles. Next, the flap-folding motion causes the LEV to attach to the wing earlier and regulates the detachment of vortices. Significant changes in the folding angle are used to influence lift generation and the flap-sweeping motion has minimal effect on the flow field structure across the three flight phases. Finally, flap-twisting motion leads to notable changes in the effective AoA, allowing for dynamic adjustments to control aerodynamics at different stroke stages, resulting in less drag during take-off and more drag during landing. This study enhances the understanding of the aerodynamic performance of bird with coupled motions in different flight phases and provides theoretical guidance for the design of bionic flapping-wing aircraft with multi-degree-of-freedom wings.

Acrobatics at the insect scale: A durable, precise, and agile micro-aerial robot

Aerial insects are exceptionally agile and precise owing to their small size and fast neuromotor control. They perform impressive acrobatic maneuvers when evading predators, recovering from wind gust, or landing on moving objects. Flapping-wing propulsion is advantageous for flight agility because it can generate large changes in instantaneous forces and torques. During flapping-wing flight, wings, hinges, and tendons of pterygote insects endure large deformation and high stress hundreds of times each second, highlighting the outstanding flexibility and fatigue resistance of biological structures and materials. In comparison, engineered materials and microscale structures in subgram micro-aerial vehicles (MAVs) exhibit substantially shorter lifespans. Consequently, most subgram MAVs are limited to hovering for less than 10 seconds or following simple trajectories at slow speeds. Here, we developed a 750-milligram flapping-wing MAV that demonstrated substantially improved lifespan, speed, accuracy, and agility. With transmission and hinge designs that reduced off-axis torsional stress and deformation, the robot achieved a 1000-second hovering flight, two orders of magnitude longer than existing subgram MAVs. This robot also performed complex flight trajectories with under 1-centimeter root mean square error and more than 30 centimeters per second average speed. With a lift-to-weight ratio of 2.2 and a maximum ascending speed of 100 centimeters per second, this robot demonstrated double body flips at a rotational rate exceeding that of the fastest aerial insects and larger MAVs. These results highlight insect-like flight endurance, precision, and agility in an at-scale MAV, opening opportunities for future research on sensing and power autonomy.

Adaptive morphing of wing and tail for stable, resilient, and energy-efficient flight of avian-inspired drones

Avian-inspired drones feature morphing wing and tail surfaces, enhancing agility and adaptability in flight. Despite their large potential, realising their full capabilities remains challenging due to the lack of generalized control strategies accommodating their large degrees of freedom and cross-coupling effects between their control surfaces. Here we propose a new body-rate controller for avian-inspired drones that uses all available actuators to control the motion of the drone. The method exhibits robustness against physical perturbations, turbulent airflow, and even loss of certain actuators mid-flight. Furthermore, wing and tail morphing is leveraged to enhance energy efficiency at 8 m/s, 10 m/s, and 12 m/s using in-flight Bayesian optimization. The resulting morphing configurations yield significant gains across all three speeds of up to 11.5% compared to non-morphing configurations and display a strong resemblance to avian flight at different speeds. This research lays the groundwork for the development of autonomous avian-inspired drones that operate under diverse wind conditions, emphasizing the role of morphing in improving energy efficiency.

Bird-inspired reflexive morphing enables rudderless flight

Gliding birds lack a vertical tail, yet they fly stably rudderless in turbulence without needing discrete flaps to steer. In contrast, nearly all airplanes need vertical tails to damp Dutch roll oscillations and to control yaw. The few exceptions that lack a vertical tail either leverage differential drag-based yaw actuators or their fixed planforms are carefully tuned for passively stable Dutch roll and proverse yaw. Biologists hypothesize that birds stabilize and control gliding flight without rudders by using their wing and tail reflexes, but no rudderless airplane has a morphing wing or tail that can change shape like a bird. Our rudderless biohybrid robot, PigeonBot II, can damp its Dutch roll instability (caused by lacking a vertical tail) and control flight by morphing its biomimetic wing and tail reflexively like a bird. The bird-inspired adaptive reflexive controller was tuned in a wind tunnel to mitigate turbulent perturbations, which enabled PigeonBot II to fly autonomously in the atmosphere with pigeon-like poses. This work is a mechanistic confirmation of how birds accomplish rudderless flight via reflex functions, and it can inspire rudderless aircraft with reduced radar signature and increased efficacy.

A twist of the tail in turning maneuvers of bird-inspired drones

A banked turn is a common flight maneuver observed in birds and aircraft. To initiate the turn, whereas traditional aircraft rely on the wing ailerons, most birds use a variety of asymmetric wing-morphing control techniques to roll their bodies and thus redirect the lift vector to the direction of the turn. Nevertheless, when searching for prey, soaring raptors execute steady banked turns without exhibiting observable wing movements apart from the tail twisting around the body axis. Although tail twisting can compensate for adverse yaw, functioning similarly to the vertical tail in aircraft, how raptors use only tail twisting to perform banked turns is still not well understood. Here, we developed and used a raptor-inspired feathered drone to find that the proximity of the tail to the wings causes asymmetric wing-induced flows over the twisted tail and thus lift asymmetry, resulting in both roll and yaw moments sufficient to coordinate banked turns. Moreover, twisting the tail induces a nose-up pitch moment that increases the angle of attack of the wings, thereby generating more lift to compensate for losses caused by the banking motion. Flight experiments confirm the effectiveness of tail twist to control not only low-speed steady banked turns but also high-speed sharp turns by means of coordinated tail twist and pitch with asymmetric wing shape morphing. These findings contribute to the understanding of avian flight behaviors that are difficult to study in controlled laboratory settings and provide effective control strategies for agile drones with morphing aerial surfaces.

3D printed feathers with embedded aerodynamic sensing

Bird flight is often characterized by outstanding aerodynamic efficiency, agility and adaptivity in dynamic conditions. Feathers play an integral role in facilitating these aspects of performance, and the benefits feathers provide largely derive from their intricate and hierarchical structures. Although research has been attempted on developing membrane-type artificial feathers for bio-inspired aircraft and micro air vehicles (MAVs), fabricating anatomically accurate artificial feathers to fully exploit the advantages of feathers has not been achieved. Here, we present our 3D printed artificial feathers consisting of hierarchical vane structures with feature dimensions spanning from 10-2to 102mm, which have remarkable structural, mechanical and aerodynamic resemblance to natural feathers. The multi-step, multi-scale 3D printing process used in this work can provide scalability for the fabrication of artificial feathers tailored to the specific size requirements of aircraft wings. Moreover, we provide the printed feathers with embedded aerodynamic sensing ability through the integration of customized piezoresistive and piezoelectric transducers for strain and vibration measurements, respectively. Hence, the 3D printed feather transducers combine the aerodynamic advantages from the hierarchical feather structure design with additional aerodynamic sensing capabilities, which can be utilized in future biomechanical studies on birds and can contribute to advancements in high-performance adaptive MAVs.

Avian-inspired embodied perception in biohybrid flapping-wing robotics

Avian feather intricate adaptable architecture to wing deformations has catalyzed interest in feathered flapping-wing aircraft with high maneuverability, agility, and stealth. Yet, to mimic avian integrated somatic sensation within stringent weight constraints, remains challenging. Here, we propose an avian-inspired embodied perception approach for biohybrid flapping-wing robots. Our feather-piezoelectric mechanoreceptor leverages feather-based vibration structures and flexible piezoelectric materials to refine and augment mechanoreception via coupled oscillator interactions and robust microstructure adhesion. Utilizing convolutional neural networks with the grey wolf optimizer, we develop tactile perception of airflow velocity and wing flapping frequency proprioception. This method also senses pitch angle via airflow direction and detects wing morphology through feather collisions. Our low-weight, accurate perception of flapping-wing robot flight states is validated by motion capture. This investigation constructs a biomechanically integrated embodied perception system in flapping-wing robots, which holds significant promise in reflex-based control of complex flight maneuvers and natural bird flight surveillance.

Gust Response and Alleviation of Avian-Inspired In-Plane Folding Wings

The in-plane folding wing is one of the important research directions in the field of morphing or bionic aircraft, showing the unique application value of enhancing aircraft maneuverability and gust resistance. This article provides a structural realization of an in-plane folding wing and an aeroelasticity modeling method for the folding process of the wing. By approximating the change in structural properties in each time step, a method for calculating the structural transient response expressed in recursive form is obtained. On this basis, an aeroelasticity model of the wing is developed by coupling with the aerodynamic model using the unsteady panel/viscous vortex particle hybrid method. A wind-tunnel test is implemented to demonstrate the controllable morphing capability of the wing under aerodynamic loads and to validate the reliability of the wing loads predicted by the method in this paper. The results of the gust simulation show that the gust scale has a significant effect on the response of both the open- and closed-loop systems. When the gust alleviation controller is enabled, the peak bending moment at the wing root can be reduced by 5.5%∼47.3% according to different gust scales.

Agile perching maneuvers in birds and morphing-wing drones

Avian perching maneuvers are one of the most frequent and agile flight scenarios, where highly optimized flight trajectories, produced by rapid wing and tail morphing that generate high angular rates and accelerations, reduce kinetic energy at impact. While the behavioral, anatomical, and aerodynamic factors involved in these maneuvers are well described, the underlying control strategies are poorly understood. Here, we use optimal control methods on an avian-inspired drone with morphing wing and tail to test a recent hypothesis derived from perching maneuver experiments of Harris' hawks that birds minimize the distance flown at high angles of attack to dissipate kinetic energy before impact. The resulting drone flight trajectories, morphing sequence, and kinetic energy distribution resemble those measured in birds. Furthermore, experimental manipulation of the wings that would be difficult or unethical with animals reveals the morphing factors that are critical for optimal perching maneuver performance of birds and morphing-wing drones.

Characterization of Wing Kinematics by Decoupling Joint Movement in the Pigeon

Birds have remarkable flight capabilities due to their adaptive wing morphology. However, studying live birds is time-consuming and laborious, and obtaining information about the complete wingbeat cycle is difficult. To address this issue and provide a complete dataset, we recorded comprehensive motion capture wing trajectory data from five free-flying pigeons (Columba livia). Five key motion parameters are used to quantitatively characterize wing kinematics: flapping, sweeping, twisting, folding and bending. In addition, the forelimb skeleton is mapped using an open-chain three-bar mechanism model. By systematically evaluating the relationship of joint degrees of freedom (DOFs), we configured the model as a 3-DOF shoulder, 1-DOF elbow and 2-DOF wrist. Based on the correlation analysis between wingbeat kinematics and joint movement, we found that the strongly correlated shoulder and wrist roll within the stroke plane cause wing flap and bending. There is also a strong correlation between shoulder, elbow and wrist yaw out of the stroke plane, which causes wing sweep and fold. By simplifying the wing morphing, we developed three flapping wing robots, each with different DOFs inside and outside the stroke plane. This study provides insight into the design of flapping wing robots capable of mimicking the 3D wing motion of pigeons.

CGull: A Non-Flapping Bioinspired Composite Morphing Drone

Despite the tremendous advances in aircraft design that led to successful powered flights of aircraft as heavy as the Antonov An-225 Mriya, which weighs 640 tons, or as fast as the NASA-X-43A, which reached a record of Mach 9.6, many characteristics of bird flight have yet to be utilized in aircraft designs. These characteristics enable various species of birds to fly efficiently in gusty environments and rapidly change their momentum in flight without having modern thrust vector control (TVC) systems. Vultures and seagulls, as examples of expert gliding birds, can fly for hours, covering more than 100 miles, without a single flap of their wings. Inspired by the Great Black-Backed Gull (GBBG), this paper presents "CGull", a non-flapping unmanned aerial vehicle (UAV) with wing and tail morphing capabilities. A coupled two degree-of-freedom (DOF) morphing mechanism is used in CGull's wings to sweep the middle wing forward and the outer feathered wing backward, replicating the GBBG's wing deformation. A modular two DOF mechanism enables CGull to pitch and tilt its tail. A computational model was first developed in MachUpX to study the effects of wing and tail morphing on the generated forces and moments. Following the biological construction of birds' feathers and bones, CGull's structure is mainly constructed from carbon-fiber composite shells. The successful flight test of the proof-of-concept physical model proved the effectiveness of the proposed morphing mechanisms in controlling the UAV's path.

Steady as they hover: kinematics of kestrel wing and tail morphing during hovering flights

Wind-hovering birds exhibit remarkable steadiness in flight, achieved through the morphing of their wings and tail. We analysed the kinematics of two nankeen kestrels (Falco cenchroides) engaged in steady wind-hovering flights in a smooth flow wind tunnel. Motion-tracking cameras were used to capture the movements of the birds as they maintained their position. The motion of the birds' head and body, and the morphing motions of their wings and tail were tracked and analysed using correlation methods. The results revealed that wing sweep, representing the flexion/extension movement of the wing, played a significant role in wing motion. Additionally, correlations between different independent degrees of freedom (DoF), including wing and tail coupling, were observed. These kinematic couplings indicate balancing of forces and moments necessary for steady wind hovering. Variation in flight behaviour between the two birds highlighted the redundancy of DoF and the versatility of wing morphing in achieving control. This study provides insights into fixed-wing craft flight control from the avian world and may inspire novel flight control strategies for future fixed-wing aircraft.

Integrated Actuation and Sensing: Toward Intelligent Soft Robots

Soft robotics has received substantial attention due to its remarkable deformability, making it well-suited for a wide range of applications in complex environments, such as medicine, rescue operations, and exploration. Within this domain, the interaction of actuation and sensing is of utmost importance for controlling the movements and functions of soft robots. Nonetheless, current research predominantly focuses on isolated actuation and sensing capabilities, often neglecting the critical integration of these 2 domains to achieve intelligent functionality. In this review, we present a comprehensive survey of fundamental actuation strategies and multimodal actuation while also delving into advancements in proprioceptive and haptic sensing and their fusion. We emphasize the importance of integrating actuation and sensing in soft robotics, presenting 3 integration methodologies, namely, sensor surface integration, sensor internal integration, and closed-loop system integration based on sensor feedback. Furthermore, we highlight the challenges in the field and suggest compelling directions for future research. Through this comprehensive synthesis, we aim to stimulate further curiosity among researchers and contribute to the development of genuinely intelligent soft robots.

Quantitative analysis of the morphing wing mechanism of raptors: Bionic design of Falco Peregrinus wing skeleton

The wing is one of the most important parts of a bird's locomotor system and is the inspiration origination for bionic wing design. During wing motions, the wing shape is closely related to the rotation angles of wing bones. Therefore, the research on the law of bone movement in the process of wing movement can be good guidance for the design of the bionic morphing wing. In this paper, the skeletal posture of the peregrine falcon wing during the extension/flexion is studied to obtain critical data on skeletal posture. Since an elbow joint and a wrist joint rotate correlatively to drive a wing to flex/extend, the wing skeleton is simplified as a four-bar mechanism in this paper. The degree of reproduction of wing skeleton postures was quantitatively analyzed using the four-bar mechanism model, and the bionic wing skeleton was designed. It is found that the wing motions have been reproduced with high precision.

Quantitative analysis of the morphing wing mechanism of raptors: Analysis methods, folding motions, and bionic design of Falco Peregrinus

Raptors can change the shape and area of their wings to an exceptional degree in a fast and efficient manner, surpassing other birds, insects, or bats. Some researchers have focused on the functional properties of muscle skeletons, mechanics, and flapping robot design. However, the wing motion of the birds of prey has not been measured quantitatively, and synthetic bionic wings with morphing abilities similar to raptors are far from reality. Therefore, in the current study, a 3D suspension system for holding bird carcasses was designed and fabricated to fasten the wings of Falco Peregrinus with a series of morphing postures. Subsequently, the wing skeleton of the falcon was scanned during extending motions using the computed tomography (CT) approach to obtain three consecutive poses. Subsequently, the skeleton was reconstructed to identify the contribution of the forelimb bones to the extending/folding motions. Inspired by these findings, we propose a simple mechanical model with four bones to form a wing-morphing mechanism using the proposed pose optimisation method. Finally, a bionic wing mechanism was implemented to imitate the motion of the falcon wing-divided into inner and outer wings with folding and twisting motions. The results show that the proposed four-bar mechanism can track bone motion paths with high fidelity.

The Aerodynamic Effect of Biomimetic Pigeon Feathered Wing on a 1-DoF Flapping Mechanism

This study focused on designing a single-degree-of-freedom (1-DoF) mechanism emulating the wings of rock pigeons. Three wing models were created: one with REAL feathers from a pigeon, and the other two models with 3D-printed artificial remiges made using different strengths of material, PLA and PETG. Aerodynamic performance was assessed in a wind tunnel under both stationary (0 m/s) and cruising speed (16 m/s) with flapping frequencies from 3.0 to 6.0 Hz. The stiffness of remiges was examined through three-point bending tests. The artificial feathers made of PLA have greater rigidity than REAL feathers, while PETG, on the other hand, exhibits the weakest strength. At cruising speed, although the artificial feathers exhibit more noticeable feather splitting and more pronounced fluctuations in lift during the flapping process compared to REAL feathers due to the differences in weight and stiffness distribution, the PETG feathered wing showed the highest lift enhancement (28% of pigeon body weight), while the PLA feathered wing had high thrust but doubled drag, making them inefficient in cruising. The PETG feathered wing provided better propulsion efficiency than the REAL feathered wing. Despite their weight, artificial feathered wings outperformed REAL feathers in 1-DoF flapping motion. This study shows the potential for artificial feathers in improving the flight performance of Flapping Wing Micro Air Vehicles (FWMAVs).

Bio-inspired flapping wing robots with foldable or deformable wings: a review

Traditional flapping-wing robots (FWRs) obtain lift and thrust by relying on the passive deformation of their wings which cannot actively fold or deform. In contrast, flying creatures such as birds, bats, and insects can maneuver agilely through active folding or deforming their wings. Researchers have developed many bio-inspired foldable or deformable wings (FDWs) imitating the wings of flying creatures. The foldable wings refer to the wings like the creatures' wings that can fold in an orderly manner close to their bodies. Such wings have scattered feathers or distinct creases that can be stacked and folded to reduce the body envelope, which in nature is beneficial for these animals to prevent wing damage and ensure agility in crossing bushes. The deformable wings refer to the active deformation of the wings using active driving mechanisms and the passive deformation under the aerodynamic force, which functionally imitates the excellent hydrodynamic performance of the deformable body and wings of the creatures. However, the shape and external profile changes of deformable wings tend to be much smaller than that of folding wings. FDWs enable the FWRs to improve flight degree of flexibility, maneuverability, and efficiency and reduce flight energy consumption. However, FDWs still need to be studied, and a comprehensive review of the state-of-the-art progress of FDWs in FWR design is lacking. This paper analyzes the wing folding and deformation mechanisms of the creatures and reviews the latest progress of FWRs with FDWs. Furthermore, we summarize the current limitations and propose future directions in FDW design, which could help researchers to develop better FWRs for safe maneuvering in obstacle-dense environments.

Development and evaluation of haltere-mimicking gyroscope for three-axis angular velocity sensing using a haltere-mimicking structure pair

This paper presents a three-axis biomimetic gyroscope, mimicking the haltere of Diptera. Diptera use a club-shaped mechanosensory organ called the haltere to get the three-axis angular velocity information, namely roll, pitch and yaw axes, for flight control. One pair of halteres is physically connected to the wings of Diptera that vibrate in antiphase to the flapping wings in ambient air. They sense the Coriolis force and relay angular velocity information to the Diptera. As an alternative to the conventional micro-electro-mechanical system gyroscopes which are widely used in robotics, many research groups have attempted to mimic the haltere. However, no previous study succeeded in measuring all three-axis components of angular velocity, due to various shortcomings. In this paper, we developed the first three-axis haltere-mimicking gyroscope. Two perpendicularly positioned haltere-mimicking structures that can vibrate at a 180° amplitude were mechanically integrated into a robot actuator. Two accelerometers, placed at the tip of each structure, were employed to measure the Coriolis force. The performance of the novel biomimetic gyroscope was measured in all rotational directions, using a motion capture system as the ground truth. One-axis input experiments were performed 240 times at different input magnitudes and directions, and the measured orientation error was less than ±2.0% in all experiments. In 80 three-axis input experiments, the orientation error was less than ±3.5%.

Asymmetric Airfoil Morphing via Deep Reinforcement Learning

Morphing aircraft are capable of modifying their geometry configurations according to different flight conditions to improve their performance, such as by increasing the lift-to-drag ratio or reducing their fuel consumption. In this article, we focus on the airfoil morphing of wings and propose a novel morphing control method for an asymmetric deformable airfoil based on deep reinforcement learning approaches. Firstly, we develop an asymmetric airfoil shaped using piece-wise Bézier curves and modeled by shape memory alloys. Resistive heating is adopted to actuate the shape memory alloys and realize the airfoil morphing. With regard to the hysteresis characteristics exhibited in the phase transformation of shape memory alloys, we construct a second-order Markov decision process for the morphing procedure to formulate a reinforcement learning environment with hysteresis properties explicitly considered. Subsequently, we learn the morphing policy based on deep reinforcement learning techniques where the accurate information of the system model is unavailable. Lastly, we conduct simulations to demonstrate the benefits brought by our learning implementations and validate the morphing performance of the proposed method. The simulation results show that the proposed method provides an average 29.8% performance improvement over traditional methods.

Wirelessly Powered 3D Printed Hierarchical Biohybrid Robots with Multiscale Mechanical Properties

The integration of flexible and stretchable electronics into biohybrid soft robotics can spur the development of new approaches to fabricate biohybrid soft machines, thus enabling a wide variety of innovative applications. Inspired by flexible and stretchable wireless-based bioelectronic devices, we have developed untethered biohybrid soft robots that can execute swimming motions, which are remotely controllable by the wireless transmission of electrical power into a cell simulator. To this end, wirelessly-powered, stretchable, and lightweight cell stimulators were designed to be integrated into muscle bodies without impeding the robots' underwater swimming abilities. The cell stimulators function by generating controlled monophasic pulses of up to ∼9 V in biological environments. By differentiating induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) directly on the cell stimulators using an accordion-inspired, three-dimensional (3D) printing construct, we have replicated the native myofiber architecture with comparable robustness and enhanced contractibility. Wirelessly modulated electrical frequencies enabled us to control the speed and direction of the biohybrid soft robots. A maximum locomotion speed of ∼580 μm/s was achieved in robots possessing a large body size by adjusting the pacing frequency. This innovative approach will provide a platform for building untethered and biohybrid systems for various biomedical applications.

A Manta Ray Robot with Soft Material Based Flapping Wing

Recent research on robotic fish mainly focused on the bionic structure design and realizing the movement with smart materials. Although many robotic fish have been proposed, most of these works were oriented toward shallow water environments and are mostly built with purely rigid structures, limiting the mobility and practical usability of robotic fish. Inspired by the stability of the real manta ray, a manta ray robot design is proposed with soft material made flapping wing based on an open-source ROV (Remotely Operated Vehicle). The flapping wing structure with three different materials mimics the wide pectoral fins of real manta rays, which have bones, muscles, and skin. Furthermore, its modular design makes it easy to install and disassemble. The kinematic and hydrodynamic analysis of the manta ray robot are simulated in this paper. The actual manta ray robot is fabricated and several sets of test are performed in the pool. The robot can swim forward continually and stably with a simple rolling and pitching pattern.

Bioinspired morphing wings: mechanical design and wind tunnel experiments

Bioinspired morphing wings are part of a novel research direction offering greatly increased adaptability for use in unmanned aerial vehicles. Recent models published in the literature often rely on simplifications of the bird wing apparatus and fail to preserve many of the macroscopic morphological features. Therefore, a more holistic design approach could uncover further benefits of truly bioinspired bird wing models. With this issue in mind, a prototype inspired by crow wings (Corvusgenus) is developed, which is capable of planform wing morphing. The prototype imitates the feather structure of real birds and replicates the folding motion with a carbon fiber reinforced polymer skeleton with one controllable degree of freedom. The mechanism supplies a smooth airfoil lifting surface through a continuous morphing motion between a fully extended and a folded state. When extended, it has an elliptic planform and emarginated slots between primary remiges. In the folded state, the wingspan is reduced by 50% with a 40% reduction in surface area and the aspect ratio decreases from 2.9 to 1.2. Experimental data from a subsonic wind tunnel investigation is presented for flow velocities ranging from 5 to 20 m s^-1, corresponding to Reynolds numbers between 0.7 × 10⁵-2.8 × 10⁵. The wing is analyzed in the three static states (folded, intermediate, and extended) through aerodynamic coefficients and flow visualizations along the surface. The bioinspired design enables the wing to capture several phenomena found on real bird wings. Through its morphing capabilities and intrinsic softness, the wing can sustain large angles of attack with greatly delayed stall and maintain optimal performance at different velocities.

Agilicious: Open-source and open-hardware agile quadrotor for vision-based flight

Autonomous, agile quadrotor flight raises fundamental challenges for robotics research in terms of perception, planning, learning, and control. A versatile and standardized platform is needed to accelerate research and let practitioners focus on the core problems. To this end, we present Agilicious, a codesigned hardware and software framework tailored to autonomous, agile quadrotor flight. It is completely open source and open hardware and supports both model-based and neural network-based controllers. Also, it provides high thrust-to-weight and torque-to-inertia ratios for agility, onboard vision sensors, graphics processing unit (GPU)-accelerated compute hardware for real-time perception and neural network inference, a real-time flight controller, and a versatile software stack. In contrast to existing frameworks, Agilicious offers a unique combination of flexible software stack and high-performance hardware. We compare Agilicious with prior works and demonstrate it on different agile tasks, using both model-based and neural network-based controllers. Our demonstrators include trajectory tracking at up to 5g and 70 kilometers per hour in a motion capture system, and vision-based acrobatic flight and obstacle avoidance in both structured and unstructured environments using solely onboard perception. Last, we demonstrate its use for hardware-in-the-loop simulation in virtual reality environments. Because of its versatility, we believe that Agilicious supports the next generation of scientific and industrial quadrotor research.

Whirl Tower Demonstration of an SMA Blade Twist System

This paper focuses on the development and demonstration of a novel blade morphing system within a whirl tower facility. The scope is to investigate the behavior of the proposed architecture under representative loads, demonstrating its capability to alter the blade original shape in operation under centrifugal, aerodynamic, and internal forces. The morphing concept was developed inside the European project “Shape Adaptive Blades for Rotorcraft Efficiency”, SABRE, and consists of a shape memory alloy system able to change the original twist law and, in this way, enhance rotor performance at certain specific regimes, such as hover and vertical flight. These phases, indeed, are generally penalized with respect to other more extended flight regimes (cruise). The work starts with an overview of the research in the field of morphing, with specific reference to the researches envisaging rotary wing demonstrations. Then, an overview of the morphing twist concept is provided, with particular attention paid to those features particularly suited for the whirl tower representative test environment. The laboratory characterization and commissioning operations are illustrated. Then, the task of the installation of the prototype on the whirl tower facility is described together with the testing modality adopted. Finally, the results of the test campaign are illustrated and critically discussed, providing the reader with insights and possible future steps to be taken in further research. The impact on the morphing capability of the following different parameters was investigated: the number of the prototype segments switched on, the speed and thus the centrifugal actions, and the angles of attack. The stiffening effect due to centrifugal actions was quantified through the measurement of the actual twist and the internal deformation. The link between speed, angle of attack at root, and twist and flap angles was also tracked, building a database useful for the comprehension of the phenomenon, and for the assessment of numerical predictive models. The achieved results highlighted the capability of the system to produce a twist angle matching the target of 8° per blade radius; this figure is related to a potential power saving of 10% in hover and vertical flight and an improvement of about 1% on the over-all efficiency of the rotorcraft.

Submillimeter-scale multimaterial terrestrial robots

Robots with submillimeter dimensions are of interest for applications that range from tools for minimally invasive surgical procedures in clinical medicine to vehicles for manipulating cells/tissues in biology research. The limited classes of structures and materials that can be used in such robots, however, create challenges in achieving desired performance parameters and modes of operation. Here, we introduce approaches in manufacturing and actuation that address these constraints to enable untethered, terrestrial robots with complex, three-dimensional (3D) geometries and heterogeneous material construction. The manufacturing procedure exploits controlled mechanical buckling to create 3D multimaterial structures in layouts that range from arrays of filaments and origami constructs to biomimetic configurations and others. A balance of forces associated with a one-way shape memory alloy and the elastic resilience of an encapsulating shell provides the basis for reversible deformations of these structures. Modes of locomotion and manipulation span from bending, twisting, and expansion upon global heating to linear/curvilinear crawling, walking, turning, and jumping upon laser-induced local thermal actuation. Photonic structures such as retroreflectors and colorimetric sensing materials support simple forms of wireless monitoring and localization. These collective advances in materials, manufacturing, actuation, and sensing add to a growing body of capabilities in this emerging field of technology.

A bioinspired revolving-wing drone with passive attitude stability and efficient hovering flight

Among small rotorcraft, the use of multiple compact rotors in a mechanically simple design leads to impressive agility and maneuverability but inevitably results in high energetic demand and acutely restricted endurance. Small spinning propellers used in these vehicles contrast with large lifting surfaces of winged seeds, which spontaneously gyrate into stable autorotation upon falling. The pronounced aerodynamic surfaces and delayed stalls are believed key to efficient unpowered flight. Here, the bioinspired principles are adopted to notably reduce the power consumption of small aerial vehicles by means of a samara-inspired robot. We report a dual-wing 35.1-gram aircraft capable of hovering flight via powered gyration. Equipped with two rotors, the underactuated robot with oversized revolving wings, designed to leverage unsteady aerodynamics, was optimized for boosted flight efficiency. Through the analysis of flight dynamics and stability, the vehicle was designed for passive attitude stability, eliminating the need for fast feedback to stay upright. To this end, the drone demonstrates flight with a twofold decrease in power consumption when compared with benchmark multirotor robots. Exhibiting the power loading of 8.0 grams per watt, the vehicle recorded a flight time of 14.9 minutes and up to 24.5 minutes when equipped with a larger battery. Taking advantage of the fast revolving motion to overcome the severe underactuation, we also realized position-controlled flight and illustrated examples of mapping and surveillance applications with a 21.5-gram payload.

Robotics as a Comparative Method in Ecology and Evolutionary Biology

Comparative biologists have typically used one or more of the following methods to assist in evaluating the proposed functional and performance significance of individual traits: comparative phylogenetic analysis, direct interspecific comparison among species, genetic modification, experimental alteration of morphology (for example by surgically modifying traits), and ecological manipulation where individual organisms are transplanted to a different environment. But comparing organisms as the endpoints of an evolutionary process involves the ceteris paribus assumption: that all traits other than the one(s) of interest are held constant. In a properly controlled experimental study, only the variable of interest changes among the groups being compared. The theme of this paper is that the use of robotic or mechanical models offers an additional tool in comparative biology that helps to minimize the effect of uncontrolled variables by allowing direct manipulation of the trait of interest against a constant background. The structure and movement pattern of mechanical devices can be altered in ways not possible in studies of living animals, facilitating testing hypotheses of the functional and performance significant of individual traits. Robotic models of organismal design are particularly useful in three arenas: (1) controlling variation to allow modification only of the trait of interest, (2) the direct measurement of energetic costs of individual traits, and (3) quantification of the performance landscape. Obtaining data in these three areas is extremely difficult through the study of living organisms alone, and the use of robotic models can reveal unexpected effects. Controlling for all variables except for the length of a swimming flexible object reveals substantial non-linear effects that vary with stiffness. Quantification of the swimming performance surface reveals that there are two peaks with comparable efficiency, greatly complicating the inference of performance from morphology alone. Organisms and their ecological interactions are complex, and dissecting this complexity to understand the effects of individual traits is a grand challenge in ecology and evolutionary biology. Robotics has great promise as a "comparative method," allowing better-controlled comparative studies to analyze the many interacting elements that make up complex behaviors, ecological interactions, and evolutionary histories.

Control of leading-edge separation on bioinspired airfoil with fluttering coverts

In this work, the aerodynamic role of the artificial covert feathers (i.e., coverts) on an airfoil is experimentally studied in a wind tunnel to investigate the flow control effect on the leading-edge separation. We apply flexible featherlike devices on a high-angle-of-attack airfoil. We use a hot-wire anemometer to measure the velocity profiles and turbulent fluctuations in the downstream wake flow. As a baseline of flow separation, a two-dimensional NACA 0018 airfoil model is set at the angle of attack of 15 ° at the chord-based Reynolds number of 1.0×10^{5}, causing strong leading-edge and trailing-edge shear layers and a low-speed wake flow area in between as large as 0.35 chord length. When deployed on the upper wing surface, the flexible coverts adaptively flutter under the influence of the local unsteady airflow. Hot-wire measurement results show that the leading-edge coverts effectively suppress the flow separation and reduce the size of the wake flow area. The change of power spectral density shows that the predominant peaks as the fundamental and harmonic frequencies are both attenuated due to the suppression of unsteady motions of the shear layers. On the other hand, the fluttering coverts at the trailing edge modify the trailing-edge shear layer by redistributing the turbulent kinetic energy to the high-frequency components. By simultaneous double-point measurement, we find that the leading-edge and trailing-edge shear layers are drawn closer to each other, and the two shear layers show an increased peak in the coherence spectrum. Further multiscale wavelet analysis shows that the perturbations at the 60% chord length increase the large-scale amplitude modulation of small-scale turbulence and therefore they stabilize the leading-edge and trailing-edge shear layers. Meanwhile, the flow intermittency outside of the wake flow area is attenuated as well. The effective flow control effects in the present work are in good agreement with the previous direct observations of bird flight in literature that the coverts on the upper wing surface play an important role in flow separation control during high-angle-of-attack flight. These findings advance the understanding of aerodynamic contribution of the covers on bird wings and reveal the engineering potential of bioinspired coverts for flow separation control of aircrafts and unmanned air vehicles.

Design and Implementation of Morphed Multi-Rotor Vehicles with Real-Time Obstacle Detection and Sensing System

Multirotor unmanned aerial vehicles (MUAVs) are becoming more prominent for diverse real-world applications due to their inherent hovering ability, swift manoeuvring and vertical take-off landing capabilities. Nonetheless, to be entirely applicable for various obstacle prone environments, the conventional MUAVs may not be able to change their configuration depending on the available space and perform designated missions. It necessitates the morphing phenomenon of MUAVS, wherein it can alter their geometric structure autonomously. This article presents the development of a morphed MUAV based on a simple rotary actuation mechanism capable of driving each arm’s smoothly and satisfying the necessary reduction in workspace volume to navigate in the obstacle prone regions. The mathematical modelling for the folding mechanism was formulated, and corresponding kinematic analysis was performed to understand the synchronous motion characteristics of the arms during the folding of arms. Experiments were conducted by precisely actuating the servo motors based on the proximity ultrasonic sensor data to avoid the obstacle for achieving effective morphing of MUAV. The flight tests were conducted to estimate the endurance and attain a change in morphology of MUAV from “X-Configuration” to “H-Configuration” with the four arms actuated synchronously without time delay.

Materials with Electroprogrammable Stiffness

Stiffness is a mechanical property of vital importance to any material system and is typically considered a static quantity. Recent work, however, has shown that novel materials with programmable stiffness can enhance the performance and simplify the design of engineered systems, such as morphing wings, robotic grippers, and wearable exoskeletons. For many of these applications, the ability to program stiffness with electrical activation is advantageous because of the natural compatibility with electrical sensing, control, and power networks ubiquitous in autonomous machines and robots. The numerous applications for materials with electrically driven stiffness modulation has driven a rapid increase in the number of publications in this field. Here, a comprehensive review of the available materials that realize electroprogrammable stiffness is provided, showing that all current approaches can be categorized as using electrostatics or electrically activated phase changes, and summarizing the advantages, limitations, and applications of these materials. Finally, a perspective identifies state-of-the-art trends and an outlook of future opportunities for the development and use of materials with electroprogrammable stiffness.

Bioinspired, Shape-Morphing Scale Battery for Untethered Soft Robots

Geometrically multifunctional structures inspired by nature can address the challenges in the development of soft robotics. A bioinspired structure based on origami and kirigami can significantly enhance the stretchability and reliability of soft robots. This study proposes a novel structure with individual, overlapping units, similar to snake scales that can be used to construct shape-morphing batteries for untethered soft robots. The structure is created by folding well-defined, two-dimensional patterns with cutouts. The folding lines mimic the hinge structure of snakeskin, enabling stable deformations without mechanical damage to rigid cells. The structure realizes multi-axial deformability and a zero Poisson's ratio without off-axis distortion to the loading axis. Moreover, to maximize areal density, the optimal cell shape is designed as a hexagon. The structure is applied to a stretchable Li-ion battery, constructed to form an arrangement of electrically interconnected, hexagonal pouch cells. In situ electrochemical characterization and numerical simulation confirm that the shape-morphing scale battery maintains its performance under dynamic deformation with a 90% stretching ratio and 10-mm-radius bending curve, guaranteeing a long-lasting charging/discharging cycle life during cyclic bending and stretching (exceeding 36,000 cycles). Finally, the shape-morphing energy storage device is applied to movable robots, mimicking crawling and slithering, to demonstrate excellent conformability and deformability.

Gull-inspired joint-driven wing morphing allows adaptive longitudinal flight control

Birds dynamically adapt to disparate flight behaviours and unpredictable environments by actively manipulating their skeletal joints to change their wing shape. This in-flight adaptability has inspired many unmanned aerial vehicle (UAV) wings, which predominately morph within a single geometric plane. By contrast, avian joint-driven wing morphing produces a diverse set of non-planar wing shapes. Here, we investigated if joint-driven wing morphing is desirable for UAVs by quantifying the longitudinal aerodynamic characteristics of gull-inspired wing-body configurations. We used a numerical lifting-line algorithm (MachUpX) to determine the aerodynamic loads across the range of motion of the elbow and wrist, which was validated with wind tunnel tests using three-dimensional printed wing-body models. We found that joint-driven wing morphing effectively controls lift, pitching moment and static margin, but other mechanisms are required to trim. Within the range of wing extension capability, specific paths of joint motion (trajectories) permit distinct longitudinal flight control strategies. We identified two unique trajectories that decoupled stability from lift and pitching moment generation. Further, extension along the trajectory inherent to the musculoskeletal linkage system produced the largest changes to the investigated aerodynamic properties. Collectively, our results show that gull-inspired joint-driven wing morphing allows adaptive longitudinal flight control and could promote multifunctional UAV designs.

Recent advances in evolutionary and bio-inspired adaptive robotics: Exploiting embodied dynamics

This paper explores current developments in evolutionary and bio-inspired approaches to autonomous robotics, concentrating on research from our group at the University of Sussex. These developments are discussed in the context of advances in the wider fields of adaptive and evolutionary approaches to AI and robotics, focusing on the exploitation of embodied dynamics to create behaviour. Four case studies highlight various aspects of such exploitation. The first exploits the dynamical properties of a physical electronic substrate, demonstrating for the first time how component-level analog electronic circuits can be evolved directly in hardware to act as robot controllers. The second develops novel, effective and highly parsimonious navigation methods inspired by the way insects exploit the embodied dynamics of innate behaviours. Combining biological experiments with robotic modeling, it is shown how rapid route learning can be achieved with the aid of navigation-specific visual information that is provided and exploited by the innate behaviours. The third study focuses on the exploitation of neuromechanical chaos in the generation of robust motor behaviours. It is demonstrated how chaotic dynamics can be exploited to power a goal-driven search for desired motor behaviours in embodied systems using a particular control architecture based around neural oscillators. The dynamics are shown to be chaotic at all levels in the system, from the neural to the embodied mechanical. The final study explores the exploitation of the dynamics of brain-body-environment interactions for efficient, agile flapping winged flight. It is shown how a multi-objective evolutionary algorithm can be used to evolved dynamical neural controllers for a simulated flapping wing robot with feathered wings. Results demonstrate robust, stable, agile flight is achieved in the face of random wind gusts by exploiting complex asymmetric dynamics partly enabled by continually changing wing and tail morphologies.

Check-Valve Design in Enhancing Aerodynamic Performance of Flapping Wings

A flapping wing micro air vehicle (FWMAV) demands high lift and thrust generation for a desired payload. In view of this, the present work focuses on a novel way of enhancing the lift characteristics through integrating check-valves in the flapping wing membrane. Modal analysis and static analysis are performed to determine the natural frequency and deformation of the check-valve. Based on the inference, the check-valve opens and closes during the upstroke flapping and downstroke flapping, respectively. Wind tunnel experiments were conducted by considering the two cases of wing design, i.e., with and without a check-valve for various driving voltages, wind speeds and different inclined angles. A 20 cm-wingspan polyethylene terephthalate (PET) membrane wing with two check-valves, composed of central disc-cap with radius of 7.43 mm, supported by three S-beams, actuated by Evans mechanism to have 90° stroke angle, is considered for the 10 gf (gram force) FWMAV study. The aerodynamic performances, such as lift and net thrust for these two cases, are evaluated. The experimental result demonstrates that an average lift of 17 gf is generated for the case where check-valves are attached on the wing membrane to operate at 3.7 V input voltage, 30° inclined angle and 1.5 m/s wind speed. It is inferred that sufficient aerodynamic benefit with 68% of higher lift is attained for the wing membrane incorporated with check-valve.

Bioinspired wing and tail morphing extends drone flight capabilities

The aerodynamic designs of winged drones are optimized for specific flight regimes. Large lifting surfaces provide maneuverability and agility but result in larger power consumption, and thus lower range, when flying fast compared with small lifting surfaces. Birds like the northern goshawk meet these opposing aerodynamic requirements of aggressive flight in dense forests and fast cruising in the open terrain by adapting wing and tail areas. Here, we show that this morphing strategy and the synergy of the two morphing surfaces can notably improve the agility, maneuverability, stability, flight speed range, and required power of a drone in different flight regimes by means of an avian-inspired drone. We characterize the drone's flight capabilities for different morphing configurations in wind tunnel tests, optimization studies, and outdoor flight tests. These results shed light on the avian use of wings and tails and offer an alternative design principle for drones with adaptive flight capabilities.

Unmanned Aerial Vehicle Pitch Control Using Deep Reinforcement Learning with Discrete Actions in Wind Tunnel Test

Deep reinforcement learning is a promising method for training a nonlinear attitude controller for fixed-wing unmanned aerial vehicles. Until now, proof-of-concept studies have demonstrated successful attitude control in simulation. However, detailed experimental investigations have not yet been conducted. This study applied deep reinforcement learning for one-degree-of-freedom pitch control in wind tunnel tests with the aim of gaining practical understandings of attitude control application. Three controllers with different discrete action choices, that is, elevator angles, were designed. The controllers with larger action rates exhibited better performance in terms of following angle-of-attack commands. The root mean square errors for tracking angle-of-attack commands decreased from 3.42° to 1.99° as the maximum action rate increased from 10°/s to 50°/s. The comparison between experimental and simulation results showed that the controller with a smaller action rate experienced the friction effect, and the controllers with larger action rates experienced fluctuating behaviors in elevator maneuvers owing to delay. The investigation of the effect of friction and delay on pitch control highlighted the importance of conducting experiments to understand actual control performances, specifically when the controllers were trained with a low-fidelity model.

A stunt flying hawk-inspired drone

A winged drone demonstrates aggressive and agile flight by morphing its wings and tail.

Individual variation and the biomechanics of maneuvering flight in hummingbirds

An animal's maneuverability will determine the outcome of many of its most important interactions. A common approach to studying maneuverability is to force the animal to perform a specific maneuver or to try to elicit maximal performance. Recently, the availability of wider-field tracking technology has allowed for high-throughput measurements of voluntary behavior, an approach that produces large volumes of data. Here, we show how these data allow for measures of inter-individual variation that are necessary to evaluate how performance depends on other traits, both within and among species. We use simulated data to illustrate best practices when sampling a large number of voluntary maneuvers. Our results show how the sample average can be the best measure of inter-individual variation, whereas the sample maximum is neither repeatable nor a useful metric of the true variation among individuals. Our studies with flying hummingbirds reveal that their maneuvers fall into three major categories: simple translations, simple rotations and complex turns. Simple maneuvers are largely governed by distinct morphological and/or physiological traits. Complex turns involve both translations and rotations, and are more subject to inter-individual differences that are not explained by morphology. This three-part framework suggests that different wingbeat kinematics can be used to maximize specific aspects of maneuverability. Thus, a broad explanatory framework has emerged for interpreting hummingbird maneuverability. This framework is general enough to be applied to other types of locomotion, and informative enough to explain mechanisms of maneuverability that could be applied to both animals and bio-inspired robots.

Rapid two-anchor crawling from a milliscale prismatic-push-pull (3P) robot

Many crawling organisms such as caterpillars and worms use a method of movement in which two or more anchor points alternately push and pull the body forward at a constant frequency. In this paper we present a milliscale push-pull robot which is capable of operating across a wide range of actuation frequencies thus enabling us to expand our understanding of two-anchor locomotion beyond the low-speed regime. We designed and fabricated a milliscale robot which uses anisotropic friction at two oscillating contact points to propel itself forward in a push-pull fashion. In experiments we varied the oscillation frequency,f, over a wide range (10-250 Hz) and observe a non-linear relationship between robot speed over this full frequency range. At low frequency (f< 100 Hz) forward speed increased linearly with frequency. However, at an intermediate push-pull frequency (f> 100 Hz) speed was relatively constant with increasing frequency. Lastly, at higher frequency (f> 170 Hz) the linear speed-frequency relationship returned. The speed-frequency relationship at low actuation frequencies is consistent with previously described two-anchor models and experiments in biology and robotics, however the higher frequency behavior is inconsistent with two-anchor frictional behavior. To understand the locomotion behavior of our system we first develop a deterministic two-anchor model in which contact forces are determined exactly from static or dynamic friction. Our experiments deviate from the model predictions, and through 3D kinematics measurements we confirm that ground contact is intermittent in robot locomotion at higher frequencies. By including probabilistic foot slipping behavior in the two-anchor friction model we are able to describe the three-regimes of robot locomotion.