Existing robots designed for locomotion in granular media typically excel at a single purpose—either surface travel or subsurface digging—while lacking the ability to perform both within the same platform. In contrast, nature offers various examples of burrowing organisms that exhibit multi-functional digging behaviors by separating their body into two essential parts: a digger for substrate intrusion and rest of the body as anchor for stabilization and controlling digger orientation. Inspired by these biological strategies, we present an extension to an existing Screw Propelled Vehicle (SPV) that incorporates an adjustable body anchor to reduce drag and enable orientation control. This integration allows the robot to transition between horizontal crawling and vertical digging. We also investigate the effect of local fluidization (LF), a bio-inspired technique that temporarily reduces the resistive forces in granular media. Experimental results show that integrating LF improves surface propulsion performance in terms of speed and depth with increment of over 5x compared to the baseline configuration. These findings support the hypothesis that bio-inspired design principles—specifically body–anchor separation and local fluidization—significantly enhance both the functionality and efficiency of granular locomotion robots, providing a pathway toward more versatile, autonomous, and high-performance subsurface exploration.

In most instances, flapping wing robots have emulated the “synchronous” actuation of insects in which the wingbeat timing is generated from a time-dependent, rhythmic signal. The internal dynamics of asynchronous insect flight muscle enable high-frequency, adaptive wingbeats with minimal direct neural control. In this paper, we investigate how the delayed stretch-activation (dSA) response of asynchronous insect flight muscle can be transformed into a feedback control law for flapping wing robots that results in stable limit cycle wingbeats. We first demonstrate - in theory and simulation - the mechanism by which asynchronous wingbeats self-excite. Then, we implement the feedback law on a dynamically-scaled robophysical model as well as on an insect-scale robotic flapping wing. Experiments on large- and small-scale robots demonstrate good agreement with the theory results and highlight how dSA parameters govern wingbeat amplitude and frequency. Lastly, we demonstrate that asynchronous actuation has several advantages over synchronous actuation schemes, including the ability to rapidly adapt or halt wingbeats in response to external loads or collisions through low-level feedback control.

Flapping wing insects benefit from a compliant thorax that provides elastic energy exchange and resiliency to wing collisions. In this paper, we present a flapping wing robot that uses an underactuated compliant transmission inspired by the insect thorax. We developed a novel fabrication method that combines carbon fiber (CF) laminate and soft robotics fabrication techniques for transmission construction. The transmission design is optimized to achieve desired wingstroke requirements and to allow for independent motion of each wing. We validate these design choices in bench-top tests measuring transmission compliance and kinematics. We integrate the transmission with laminate wings and two types of actuation, demonstrating elastic energy exchange and limited lift-off capabilities Lastly, we tested collision mitigation through flapping wing experiments that obstructed the motion of a wing. These experiments demonstrate that an underactuated compliant, transmission can provide resilience and robustness to flapping wing robots.

In this work we present the design of a swimming robot that is inspired by the body shape modulation of small microorganisms. Amoebas are small single celled organisms that locomote through deformation and shape change of their body. To achieve similar shape modulation for swimming propulsion in a robot we developed a novel flexible appendage using tape springs. A tape spring is an elongated strip of metal with a curved cross-section that can act as a stiff structure when loaded against the curvature, while it can easily buckle when loaded with the curvature. We develop a tape spring appendage that is capable of freely deforming its perimeter through two actuation inputs. In the first portion of this paper we develop the kinematics of the appendage mechanisms and compare with experiment. Next we present the design of a surface locomoting robot that uses two appendages for propulsion. From the appendage kinematics we derive the local connection vector field for locomotion kinematics and study the optimal gait for forward swimming. Lastly, we demonstrate robot swimming performance in open water conditions. The novel appendage design in this robot is advantageous because it enables omnidirectional movement, the appendages will not tangle in debris, and they are robust to collisions and contact with structures.

Underactuated designs of robot limbs can enable these systems to passively adapt their joint configuration in response to external forces. Passive adaptation and reconfiguration can be extremely beneficial in situations where manipulation or locomotion with complex substrates is required. A common design for underactuated systems often involves a single tendon that actuates multiple rotational joints, each with a torsional elastic spring resisting bending. However, a challenge of using those joints for legged locomotion is that limbs typically need to follow a cyclical trajectory so that feet can alternately be engaged in stance and swing phases. Such trajectories present challenges for linearly elastic underactuated limbs. In this paper, we present a new method of underactuated limb design which incorporates hysteretic joints that change their torque response during loading and unloading. A double-jointed underactuated limb with both linear and hysteretic joints can thus be tuned to create a variety of looped trajectories. We fabricate these joints inside a flexible legged robot using a modified laminate based 3D printing method, and the result shows that with passive compliance and a mechanically determined joint sequence, a 2-legged minimalist robot can successfully walk through a confined channel over uneven substrates.

In this paper we present the design, fabrication, testing, and control of a 0.4 g milliscale robot employing a soft polymer flexure transmission for rapid ground movement. The robot was constructed through a combination of two methods: smart-composite-manufacturing (SCM) process to fabricate the actuators and robot chassis, and silicone elastomer molding and casting to fabricate a soft flexure transmission. We actuate the flexure transmission using two customized piezoelectric (PZT) actuators that attach to the transmission inputs. Through high-frequency oscillations, the actuators are capable of exciting vibrational resonance modes of the transmission which result in motion amplification on the transmission output. Directional spines on the transmission output generate traction force with the ground and drive the robot forward. By varying the excitation frequency of the soft transmission we can control locomotion speed, and when the transmission is oscillated at its resonance frequency we achieve high speeds with a peak speed of 439 mm/s (22 body lengths/s). By exciting traveling waves through the soft transmission, we were able to control the steering direction. Overall this paper demonstrates the feasibility of generating resonance behavior in millimeter scale soft robotic structures to achieve high-speed controllable locomotion.

Continuum robots that move through undulatory actuation must be composed of body materials that can enable flexible movement yet also provide resistive forces to the surrounding fluid, granular, or solid environments. This need for “f1exible-yet-stiff” materials is notably important in robot designs that use passive propulsive elements such as tails and wings. Here we explore a laminate design paradigm for “f1exible-yet-stiff” robotic materials through sliding layer laminates (SLLs). We present design principles motivated by theory and experiment and illustrate a taxonomy of SLL enabled morphable materials capable of up to 7 fold change in stiffness. Lastly, we demonstrate the applicability of SLLs to undulatory continuum robots: a swimming robot with a passive tail. We target two desired robot locomotor behaviors: fast open water swimming, and steady swimming through narrow channels emulating underwater caverns and pipes. We demonstrate how tuning the stiffness of the robot tail maximizes thrust generation in these two locomotion modes. Soft tails are optimal in confined swimming because they generate short amplitude high wavenumber oscillations, while stiff tails in confined environments either collide with the walls or do not generate sufficient thrust. However, stiff tails are far better in unconfined environments which enable large stroke amplitudes requiring high stiffness. Through this demonstration we show that stiff or soft tail designs alone are incapable of effective locomotion in complex underwater environments challenge.

Onboard vision sensing is a current challenge in micro-scale robotics. Small flapping-wing robots such as the RoboBee present significant constraints on power, weight, and image quality for an onboard vision sensor. Here we report the integration of a 1 × 1 × 1.7 mm camera capable of video capture in flight. Inspired by gaze stabilization in insects, we designed and fabricated a one degree of freedom mechanism attached to the top of the RoboBee that achieves output angles of -41° to +60°. We perform open-loop roll maneuvers and demonstrate initial control of the gaze angle during flight. This represents the first use of a camera in free flight at this scale.

Small, lightweight micro-aerial vehicles (MAVs) must rely on a limited number of actuators for flight stability and control. A method for six-degree of freedom control in a dual-actuator MAV has been previously proposed which employs stroke amplitude, bias, and split-cycle timing modulation. This control scheme is the basis of actuation for stable, controlled flapping wing flight of the Harvard Robobee. The role of passive wing pitching dynamics are currently unexplored in their effects on yaw-dynamics during free flight. Here we demonstrate in simulation and experiment the critical role wing pitching dynamics play in yaw control of a dual-actuated MAV using the split-cycle control scheme. We find that yaw-authority sensitively depends on the functional form of the wing hinge joint and that pitching dynamics of wing hinges with linear stiffness may compromise yaw control. To solve this we present a design method for laminate based non-linear hinges and demonstrate that non-linear hinge stiffness improves yaw torque generation during split-cycle actuation.

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

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