Table of contents

The active transport of fluids by pumps plays an essential role in engineering and biology. Due to increasing energy costs and environmental issues, topics like noise reduction, increase of efficiency and enhanced robustness are of high importance in the development of pumps in engineering. The study compares pumps in biology and engineering and assesses biomimetic potentials for improving man-made pumping systems. To this aim, examples of common challenges, applications and current biomimetic research for state-of-the art pumps are presented. The biomimetic research is helped by the similar configuration of many positive displacement pumping systems in biology and engineering. In contrast, the configuration and underlying pumping principles for fluid dynamic pumps (FDPs) differ to a greater extent in biology and engineering. However, progress has been made for positive displacement as well as for FDPs by developing biomimetic devices with artificial muscles and cilia that improve energetic efficiency and fail-safe operation or reduce noise. The circulatory system of vertebrates holds a high biomimetic potential for the damping of pressure pulsations, a common challenge in engineering. Damping of blood pressure pulsation results from a nonlinear viscoelastic behavior of the artery walls which represent a complex composite material. The transfer of the underlying functional principle could lead to an improvement of existing technical solutions and be used to develop novel biomimetic damping solutions. To enhance efficiency or thrust of man-made fluid transportation systems, research on jet propulsion in biology has shown that a pulsed jet can be tuned to either maximize thrust or efficiency. The underlying principle has already been transferred into biomimetic applications in open channel water systems. Overall there is a high potential to learn from nature in order to improve pumping systems for challenges like the reduction of pressure pulsations, increase of jet propulsion efficiency or the reduction of wear.

Soft robotic snakes promise significant advantages in achieving traveling curvature waves with a reduced number of active segments as well as allowing for safe and adaptive interaction with the environment and human users. However, current soft robot platforms suffer from a lack of accurate theoretical dynamic models and proprioceptive measurements, which impede advancements toward full autonomy. To address this gap, this paper details our recent results on the design, fabrication, and experimental evaluation of a new-generation pressure-operated soft robotic snake platform we call the WPI SRS, which employs custom magnetic sensors embedded in a flexible backbone to continuously monitor the curvature of each of its four bidirectional bending segments. In addition, we present a complete and accurate dynamic undulatory locomotion model that accounts for the propagation of frictional moments to describe linear and rotational motions of the SRS. Experimental studies indicate that on-board sensory measurements provide accurate real-time curvature feedback, and numerical simulations offer a level of abstraction for lateral undulation under ideal conditions.

We present a soft robotic skin that can recognize and localize texture using a distributed set of sensors and computational elements that are inspired by the Pacinian corpuscle, the fast adapting, uniformly spaced mechanoreceptor with a wide receptive field, which is responsive to vibrations due to rubbing or slip on the skin. Tactile sensing and texture recognition is important for controlled manipulation of objects and navigating in one's environment. Yet, providing robotic systems or prosthetic devices with such capability at high density and bandwidth remains challenging. Each sensor node in the presented skin is created by collocating computational elements with individual microphones. These nodes are networked in a lattice and embedded in EcoFlex rubber, forming an amorphous medium. Unlike existing skins consisting of passive sensor arrays that feed into a central computer, our approach allows for detecting, conditioning and processing of tactile signals in-skin, facilitating the use of high-bandwidth signals, such as vibration, and allowing nodes to respond only to signals of interest. Communication between nodes allows the skin to localize the source of a stimulus, such as rubbing or slip, in a decentralized manner. Signal processing by individual nodes allows the skin to estimate the material touched. Combining these two capabilities, the skin is able to convert high-bandwidth, spatiotemporal information into low-bandwidth, event-driven information. Unlike taxel-based sensing arrays, this amorphous approach greatly reduces the computational load on the central robotic system. We describe the design, analysis, construction, instrumentation and programming of the robotic skin. We demonstrate that a 2.8 square meter skin with 10 sensing nodes is capable of localizing stimulus to within 2 centimeters, and that an individual sensing node can identify 15 textures with an accuracy of 71%. Finally, we discuss how such a skin could be used for full-body sensing in existing robots, augment existing sensing modalities, and how this material may be useful in robotic grasping tasks.

The detailed mechanical design of a digital mask projection stereolithgraphy system is described for the 3D printing of soft actuators. A commercially available, photopolymerizable elastomeric material is identified and characterized in its liquid and solid form using rheological and tensile testing. Its capabilities for use in directly printing high degree of freedom (DOF), soft actuators is assessed. An outcome is the ∼40% strain to failure of the printed elastomer structures. Using the resulting material properties, numerical simulations of pleated actuator architectures are analyzed to reduce stress concentration and increase actuation amplitudes. Antagonistic pairs of pleated actuators are then fabricated and tested for four-DOF, tentacle-like motion. These antagonistic pairs are shown to sweep through their full range of motion (∼180°) with a period of less than 70 ms.

A biologically inspired neural control system has been developed that coordinates a tetrapod trotting gait in the sagittal plane. The developed neuromechanical system is used to explore properties of connections in inter-leg and intra-leg coordination. The neural controller is built with biologically based neurons and synapses, and connections are based on data from literature where available. It is applied to a planar biomechanical model of a rat with 14 joints, each actuated by a pair of antagonistic Hill muscle models. The controller generates tension in the muscles through activation of simulated motoneurons. The hind leg and inter-leg control networks are based on pathways discovered in cat research tuned to the kinematic motions of a rat. The foreleg network was developed by extrapolating analogous pathways from the hind legs. The formulated intra-leg and inter-leg networks properly coordinate the joints and produce motions similar to those of a walking rat. Changing the strength of a single inter-leg connection is sufficient to account for differences in phase timing in different trotting rats.

Cephalopods (i.e., octopuses and squids) are being looked upon as a source of inspiration for the development of unmanned underwater vehicles. One kind of cephalopod-inspired soft-bodied vehicle developed by the authors entails a hollow, elastic shell capable of performing a routine of recursive ingestion and expulsion of discrete slugs of fluids which enable the vehicle to propel itself in water. The vehicle performances were found to depend largely on the elastic response of the shell to the actuation cycle, thus motivating the development of a coupled propulsion-elastodynamics model of such vehicles. The model is developed and validated against a set of experimental results performed with the existing cephalopod-inspired prototypes. A metric of the efficiency of the propulsion routine which accounts for the elastic energy contribution during the ingestion/expulsion phases of the actuation is formulated. Demonstration on the use of this model to estimate the efficiency of the propulsion routine for various pulsation frequencies and for different morphologies of the vehicles are provided. This metric of efficiency, employed in association with the present elastodynamics model, provides a useful tool for performing a priori energetic analysis which encompass both the design specifications and the actuation pattern of this new kind of underwater vehicle.

Pneumatic artificial muscles (PAMs) are actuators known for their high power to weight ratio, natural compliance and light weight. Due to these advantages, PAMs have been used for orthotic devices and robotic limbs. Small scale PAMs have the same advantages, as well as requiring greatly reduced volumes with potential application to prostheses and small scale robotics. The bladder of a PAM affects common actuator performance metrics, specifically: blocked force, free contraction, hysteresis, and dead-band pressure. This paper investigates the effect that bladder thickness has on static actuation performance of small scale PAMs. Miniature PAMs were fabricated with a range of bladder thicknesses to quantify the change in common actuator performance metrics specifically: blocked force, free contraction, and dead-band pressure. These PAMs were then experimentally characterized in quasi-static conditions, where results showed that increasing bladder wall thickness decreases blocked force and free contraction, while dead-band pressure increases. A nonlinear model was then applied to determine the structure of the stress–strain relationship that enables accurate modeling and the minimum number of terms. Two nonlinear models are compared and the identified parameters are analyzed to study the effect of the bladder thickness on the model.

The ionic polymer–metal composite (IPMC) is a soft material based actuator and sensor and has a promising potential in underwater application. This paper describes a hybrid biomimetic underwater vehicle that uses IPMCs as sensors. Propelled by the energy of waves, this underwater vehicle does not need an additional energy source. A physical model based on the hydrodynamics of the vehicle was developed, and simulations were conducted. Using the Poisson–Nernst–Planck system of equations, a physics model for the IPMC sensor was proposed. For this study, experimental apparatus was developed to conduct hydrodynamic experiments for both the underwater vehicle and the IPMC sensors. By comparing the experimental and theoretical results, the speed of the underwater vehicle and the output of the IPMC sensors were well predicted by the theoretical models. A maximum speed of 1.08 × 10⁻¹ m s⁻¹ was recorded experimentally at a wave frequency of 1.6 Hz. The peak output voltage of the IPMC sensor was 2.27 × 10⁻⁴ V, recorded at 0.8 Hz. It was found that the speed of the underwater vehicle increased as the wave frequency increased and the IPMC output decreased as the wave frequency increased. Further, the energy harvesting capabilities of the underwater vehicle hosting the IPMCs were tested. A maximum power of 9.50 × 10⁻¹⁰ W was recorded at 1.6 Hz.

Digital implementations of control laws typically involve discretization with respect to both time and space, and a control law that can achieve a task at coarser levels of discretization can be said to require less control attention, and also reduced implementation costs. One means of quantitatively capturing the attention of a control law is to measure the rate of change of the control with respect to changes in state and time. In this paper we present an attention-minimizing control law for ball catching and other target tracking tasks based on Brockett's attention criterion. We first highlight the connections between this attention criterion and some well-known principles from human motor control. Under the assumption that the optimal control law is the sum of a linear time-varying feedback term and a time-varying feedforward term, we derive an LQR-based minimum attention tracking control law that is stable, and obtained efficiently via a finite-dimensional optimization over the symmetric positive-definite matrices. Taking ball catching as our primary task, we perform numerical experiments comparing the performance of the various control strategies examined in the paper. Consistent with prevailing theories about human ball catching, our results exhibit several familiar features, e.g., the transition from open-loop to closed-loop control during the catching movement, and improved robustness to spatiotemporal discretization. The presented control laws are applicable to more general tracking problems that are subject to limited communication resources.

This work aims to demonstrate the feasibility of a novel approach for the development of 3D self-assembled polydimethylsiloxane structures, to be used as engineered flexible matrices for bio-hybrid actuation. We described the fabrication of engineered bilayers, organized in a 3D architecture by means of a stress-induced rolling membrane technique. Such structures were provided with ad hoc surface topographies, for both cell alignment and cell survival after membrane rolling. We reported the results of advanced finite element model simulations, predicting the system behavior in terms of overall contraction, induced by the contractile activity of muscle cells seeded on the membrane. Then, we tested in vitro the structure with primary cardiomyocytes to evaluate the real bio-actuator contraction, thus validating the simulation results. At a later stage, we provided the samples with a stable fibronectin coating, by covalently binding the protein on the polymer surface, thus enabling long-term cultures with C2C12 skeletal muscle cells, a more controllable cell type. These tests revealed cell viability and alignment on the rolled structures, but also the ability of cells to differentiate and to form multinucleated and oriented myotubes on the polymer surface, also supported by a fibroblast feeder layer. Our results highlighted the possibility of developing 3D rolled PDMS structures, characterized by different mechanical properties, as novel bio-hybrid actuators.

Most robotic applications that contain shape memory alloy (SMA) actuators use the SMA in a linear or spring shape. In contrast, a novel robotic finger was designed in this paper using SMA plates that were thermomechanically trained to take the shape of a flexed human finger when Joule heated. This flexor actuator was placed in parallel with an extensor actuator that was designed to straighten when Joule heated. Thus, alternately heating and cooling the flexor and extensor actuators caused the finger to flex and extend. Three different NiTi based SMA plates were evaluated for their ability to apply forces to a rigid and compliant object. The best of these three SMAs was able to apply a maximum fingertip force of 9.01N on average. A 3D CAD model of a human finger was used to create a solid model for the mold of the finger covering skin. Using a 3D printer, inner and outer molds were fabricated to house the actuators and a position sensor, which were assembled using a multi-stage casting process. Next, a nonlinear antagonistic controller was developed using an outer position control loop with two inner MOSFET current control loops. Sine and square wave tracking experiments demonstrated minimal errors within the operational bounds of the finger. The ability of the finger to recover from unexpected disturbances was also shown along with the frequency response up to 7 rad s⁻¹. The closed loop bandwidth of the system was 6.4 rad s⁻¹ when operated intermittently and 1.8 rad s⁻¹ when operated continuously.

The flight performance of insects is strongly affected by the deformation of the wing during a stroke cycle. Many insects therefore use both active and passive mechanisms to control the deformation of their wings in flight. Several studies have focused on the wing kinematics, and plenty is known about the mechanism of their passive deformability. However, given the small size of the vein-joints, accurate direct mechanical experiments are almost impossible to perform. We therefore developed numerical models to perform a comparative and comprehensive investigation of the mechanical behaviour of the vein-joints under external loading conditions. The results illustrate the effect of the geometry and the presence of the rubberlike protein resilin on the flexibility of the joints. Our simulations further show the contribution of the spikes to the anisotropic flexural stiffness in the dorsal and ventral directions. In addition, our results show that the cross veins, only in one joint type, help to transfer the stress to the thicker longitudinal veins. The deformation pattern and the stress distribution in each vein-joint are discussed in detail. This study provides a strong background for further realistic modelling of the dragonfly wing deformation.

An accurate knowledge of the unsteady aerodynamic forces acting on a bio-inspired, flapping-wing micro air vehicle (FWMAV) is crucial in the design development and optimization cycle. Two different types of experimental approaches are often used: determination of forces from position data obtained from external optical tracking during free flight, or direct measurements of forces by attaching the FWMAV to a force transducer in a wind-tunnel. This study compares the quality of the forces obtained from both methods as applied to a 17.4 gram FWMAV capable of controlled flight. A comprehensive analysis of various error sources is performed. The effects of different factors, e.g., measurement errors, error propagation, numerical differentiation, filtering frequency selection, and structural eigenmode interference, are assessed. For the forces obtained from free flight experiments it is shown that a data acquisition frequency below 200 Hz and an accuracy in the position measurements lower than ± 0.2 mm may considerably hinder determination of the unsteady forces. In general, the force component parallel to the fuselage determined by the two methods compares well for identical flight conditions; however, a significant difference was observed for the forces along the stroke plane of the wings. This was found to originate from the restrictions applied by the clamp to the dynamic oscillations observed in free flight and from the structural resonance of the clamped FWMAV structure, which generates loads that cannot be distinguished from the external forces. Furthermore, the clamping position was found to have a pronounced influence on the eigenmodes of the structure, and this effect should be taken into account for accurate force measurements.

In recent years there has been a growing interest in the field of dynamic walking and bio-inspired robots. However, while walking and running on a flat surface have been studied extensively, walking dynamically over terrains with varying slope remains a challenge. Previously we developed an open loop controller based on a central pattern generator (CPG). The controller applied predefined torque patterns to a compass-gait biped, and achieved stable gaits over a limited range of slopes. In this work, this range is greatly extended by applying a once per cycle feedback to the CPG controller. The terrain's slope is measured and used to modify both the CPG frequency and the torque amplitude once per step. A multi-objective optimization algorithm was used to tune the controller parameters for a simulated CB model. The resulting controller successfully traverses terrains with slopes ranging from +7° to −8°, comparable to most slopes found in human constructed environments. Gait stability was verified by computing the linearized Poincaré Map both numerically and analytically.

The babassu nut is the fruit of the babassu palm Orbignya speciosa. The combination of hardness and impact strength is difficult to acquire for artificial materials, making the babassu nut a promising source for biomimetic inspiration. Unnotched Charpy impact tests, Shore D hardness tests and scanning electron microscopy were used for mechanical and microscopical analysis of the pericarp. Four major principles were found for a biomimetic approach: a hard core ((1); endocarp) is embedded in a soft outer layer of high impact strength ((2); epicarp) and is reinforced with fibres of variable fineness (3), some of which are oriented radial to the core (4). Biomimetic fibre-reinforced composites were produced using abstracted mechanisms of the babassu nut based on regenerated cellulose fibres (lyocell, L) with two different fineness values as reinforcement embedded in a polylactide (PLA) core matrix and polypropylene (PP) based outer layers. The biomimetic fibre composite reaches a significantly higher impact strength that is 1.6 times higher than the reference sample produced from a PLA/PP/L-blend. At the same time the hardness is slightly increased compared to PP/L.

The directional properties of bat ears as receivers is a current area of interest in ultrasound research. This paper presents a new approach to analyse the relationship between morphological features and acoustical properties of the external ear of bat species. The beam pattern of Rousettus leschenaultii's right ear is measured and compared to that of receiver structures whose design is inspired by the bat ear itself and made of appropriate geometric shapes. The regular shape of these receivers makes it possible to control the key reception parameters and thus to understand the effect on the associated beam pattern of the parameters themselves. Measurements show one receiver structure has a beam pattern very similar to that of R. leschenaultii's ear, thus explaining the function of individual parts constituting its ear. As it is applicable to all bat species, this approach can provide a useful tool to investigate acoustics in bats, and possibly other mammals.

In this paper, the effect of torso flexibility on the energetics of quadrupedal bounding is examined in a template setting. Two reductive sagittal-plane models, one with a rigid, non-deformable torso and one with a flexible, unactuated torso are proposed. Both models feature non-trivial leg mass and inertia to capture the energy associated with repositioning the legs after liftoff as well as the energy lost due to impacts. Bounding motions that minimize the cost of transport are generated for both models via a simple controller that coordinates leg recirculation. Comparisons reveal that torso compliance promotes locomotion efficiency by facilitating leg recirculation in anticipation of touchdown at speeds that are sufficiently high. Furthermore, by considering non-ideal torque generating and compliant elements with biologically reasonable efficiency values, it is shown that the flexible-torso model can predict the metabolic cost of transport for different animals, estimated using measurements of oxygen consumption. This way, the proposed model offers a means for approximating the energetic cost of transport of running quadrupeds in a simple and direct fashion.

The increasing interest in the development of small flying air vehicles has given rise to a strong need to thoroughly understand low-speed aerodynamics. The barn owl is a well-known example of a biological system that possesses a high level of adaptation to its habitat and as such can inspire future small-scale air vehicle design. The combination of the owl-specific wing geometry and plumage adaptations with the flexibility of the wing structure yields a highly complex flow field, still enabling the owl to perform stable and at the same time silent low-speed gliding flight. To investigate the effects leading to such a characteristic flight, time-resolved stereoscopic particle-image velocimetry (TR-SPIV) measurements are performed on a prepared natural owl wing in a range of angles of attack ${0}^{\circ }\leqslant \alpha \;\leqslant$${6}^{\circ }$ and Reynolds numbers 40 000 $\leqslant \;{{Re}}_{c}\leqslant 120\; 000$ based on the chord length at a position located at 30% of the halfspan from the owl's body. The flow field does not show any flow separation on the suction side, whereas flow separation is found on the pressure side for all investigated cases. The flow field on the pressure side is characterized by large-scale vortices which interact with the flexible wing structure. The good agreement of the shedding frequency of the pressure side vortices with the frequency of the trailing-edge deflection indicates that the structural deformation is induced by the flow field on the pressure side. Additionally, the reduction of the time-averaged mean wing curvature at high Reynolds numbers indicates a passive lift-control mechanism that provides constant lift in the entire flight envelope of the owl.

Polystyrene (PS) microspheres are synthesized by dispersion polymerization, and a close-packed two-dimensional (2D) array of the PS microspheres is formed by the self-assembly method through dip drawing under magnetic stirring. This array is then used to fabricate a 2D polydimethylsiloxane concave mold by soft lithography. The mold is employed to produce convex polymethylmethacrylate-based compound eye-replicating films of different hemispherical heights by thermopressing. The optical properties of the ommatidia on these biomimetic compound eye-replicating films are investigated, and the films are used with a charge-coupled device camera to construct a biomimetic visual system. The visual distance and field of view of this system are measured. The film with the greatest hemispherical height results in the biomimetic visual system with the highest visual distance and the widest field of view. In addition, it is found that the quality of the optical images is not dependent on the hemispherical height of the biomimetic films. The ability of the biomimetic visual system to detect moving object in real time is also studied.

In this paper, we study the behavioral response of rats to a robotic rat during multi-rat interaction. Experiments are conducted in an open-field where a robotic rat called WR-5 is put together with three laboratory rats. WR-5 is following one rat (target), while avoiding the other two rats (outside observers) during interaction. The behavioral characteristics of each target rat is evaluated by scoring its locomotor activity and frequencies of performing rearing, body grooming and mounting actions. Additionally, the frequency of being mounted by other rats is also measured. Experimental results show that the target becomes more active after interaction. The rat species, with more active behavioral characteristics, is more susceptible to being adjusted by the robot. The increased time spent by the outside observers in the vicinity of the robot indicates that a biomimetic robot has the promise for modulating rat behavior even without direct interaction. Thus, this study provide a novel approach to shaping the sociality of animals living in groups.

We show that the forward flight speed affects the stability characteristics of the longitudinal and lateral dynamics of a flying hawkmoth; dynamic modal structures of both the planes of motion are altered due to variations in the stability derivatives. The forward flight speed u_e is changed from 0.00 to 1.00 m s⁻¹ with an increment of 0.25 m s⁻¹. (The equivalent advance ratio is 0.00 to 0.38; the advance ratio is the ratio of the forward flight speed to the average wing tip speed.) As the flight speed increases, for the longitudinal dynamics, an unstable oscillatory mode becomes more unstable. Also, we show that the up/down (w_b) dynamics become more significant at a faster flight speed due to the prominent increase in the stability derivative Z_u (up/down force due to the forward/backward velocity). For the lateral dynamics, the decrease in the stability derivative L_v (roll moment due to side slip velocity) at a faster flight speed affects a slightly damped stable oscillatory mode, causing it to become more stable; however, the t_half (the time taken to reach half the amplitude) of this slightly damped stable oscillatory mode remains relatively long (∼12T at u_e = 1 m s⁻¹; T is wingbeat period) compared to the other modes of motion, meaning that this mode represents the most vulnerable dynamics among the lateral dynamics at all flight speeds. To obtain the stability derivatives, trim conditions for linearization are numerically searched to find the exact trim trajectory and wing kinematics using an algorithm that uses the gradient information of a control effectiveness matrix and fully coupled six-degrees of freedom nonlinear multibody equations of motion. With this algorithm, trim conditions that consider the coupling between the dynamics and aerodynamics can be obtained. The body and wing morphology, and the wing kinematics used in this study are based on actual measurement data from the relevant literature. The aerodynamic model of the flapping wings of a hawkmoth is based on the blade element theory, and the necessary aerodynamic coefficients, including the lift, drag and wing pitching moment, are experimentally obtained from the results of previous work by the authors.

The wings of the hawkmoth Manduca sexta are lined with mechanoreceptors called campaniform sensilla that encode wing deformations. During flight, the wings deform in response to a variety of stimuli, including inertial-elastic loads due to the wing flapping motion, aerodynamic loads, and exogenous inertial loads transmitted by disturbances. Because the wings are actuated, flexible structures, the strain-sensitive campaniform sensilla are capable of detecting inertial rotations and accelerations, allowing the wings to serve not only as a primary actuator, but also as a gyroscopic sensor for flight control. We study the gyroscopic sensing of the hawkmoth wings from a control theoretic perspective. Through the development of a low-order model of flexible wing flapping dynamics, and the use of nonlinear observability analysis, we show that the rotational acceleration inherent in wing flapping enables the wings to serve as gyroscopic sensors. We compute a measure of sensor fitness as a function of sensor location and directional sensitivity by using the simulation-based empirical observability Gramian. Our results indicate that gyroscopic information is encoded primarily through shear strain due to wing twisting, where inertial rotations cause detectable changes in pronation and supination timing and magnitude. We solve an observability-based optimal sensor placement problem to find the optimal configuration of strain sensor locations and directional sensitivities for detecting inertial rotations. The optimal sensor configuration shows parallels to the campaniform sensilla found on hawkmoth wings, with clusters of sensors near the wing root and wing tip. The optimal spatial distribution of strain directional sensitivity provides a hypothesis for how heterogeneity of campaniform sensilla may be distributed.

Natural substrates like sand, soil, leaf litter and snow vary widely in penetration resistance. To search for principles of appendage design in robots and animals that permit high performance on such flowable ground, we developed a ground control technique by which the penetration resistance of a dry granular substrate could be widely and rapidly varied. The approach was embodied in a device consisting of an air fluidized bed trackway in which a gentle upward flow of air through the granular material resulted in a decreased penetration resistance. As the volumetric air flow, $Q$, increased to the fluidization transition, the penetration resistance decreased to zero. Using a bio-inspired hexapedal robot as a physical model, we systematically studied how locomotor performance (average forward speed, ${v}_{x}$) varied with ground penetration resistance and robot leg frequency. Average robot speed decreased with increasing $Q$, and decreased more rapidly for increasing leg frequency, $\omega$. A universal scaling model revealed that the leg penetration ratio (foot pressure relative to penetration force per unit area per depth and leg length) determined ${v}_{x}$ for all ground penetration resistances and robot leg frequencies. To extend our result to include continuous variation of locomotor foot pressure, we used a resistive force theory based terradynamic approach to perform numerical simulations. The terradynamic model successfully predicted locomotor performance for low resistance granular states. Despite variation in morphology and gait, the performance of running lizards, geckos and crabs on flowable ground was also influenced by the leg penetration ratio. In summary, appendage designs which reduce foot pressure can passively maintain minimal leg penetration ratio as the ground weakens, and consequently permits maintenance of effective locomotion over a range of terradynamically challenging surfaces.

Inspired by the limb structures of animals/insects in motion vibration control, a bio-inspired limb-like structure (LLS) is systematically studied for understanding and exploring its advantageous nonlinear function in passive vibration isolation. The bio-inspired system consists of asymmetric articulations (of different rod lengths) with inside vertical and horizontal springs (as animal muscle) of different linear stiffness. Mathematical modeling and analysis of the proposed LLS reveal that, (a) the system has very beneficial nonlinear stiffness which can provide flexible quasi-zero, zero and/or negative stiffness, and these nonlinear stiffness properties are adjustable or designable with structure parameters; (b) the asymmetric rod-length ratio and spring-stiffness ratio present very beneficial factors for tuning system equivalent stiffness; (c) the system loading capacity is also adjustable with the structure parameters which presents another flexible benefit in application. Experiments and comparisons with existing quasi-zero-stiffness isolators validate the advantageous features above, and some discussions are also given about how to select structural parameters for practical applications. The results would provide an innovative bio-inspired solution to passive vibration control in various engineering practice.

In the field of human motor control, the motor synergy hypothesis explains how humans simplify body control dimensionality by coordinating groups of muscles, called motor synergies, instead of controlling muscles independently. In most applications of motor synergies to low-dimensional control in robotics, motor synergies are extracted from given optimal control signals. In this paper, we address the problems of how to extract motor synergies without optimal data given, and how to apply motor synergies to achieve low-dimensional task-space tracking control of a human-like robotic arm actuated by redundant muscles, without prior knowledge of the robot. We propose to extract motor synergies from a subset of randomly generated reaching-like movement data. The essence is to first approximate the corresponding optimal control signals, using estimations of the robot's forward dynamics, and to extract the motor synergies subsequently. In order to avoid modeling difficulties, a learning-based control approach is adopted such that control is accomplished via estimations of the robot's inverse dynamics. We present a kernel-based regression formulation to estimate the forward and the inverse dynamics, and a sliding controller in order to cope with estimation error. Numerical evaluations show that the proposed method enables extraction of motor synergies for low-dimensional task-space control.

Providing unpredictable trajectories for patrol robots is essential when coping with adversaries. In order to solve this problem we developed an effective approach based on the known protean behavior of individual prey animals—random zig-zag movement. The proposed bio-inspired method modifies the normal robot's path by incorporating sudden and irregular direction changes without jeopardizing the robot's mission. Such a tactic is aimed to confuse the enemy (e.g. a sniper), offering less time to acquire and retain sight alignment and sight picture. This idea is implemented by simulating a series of fictive-temporary obstacles that will randomly appear in the robot's field of view, deceiving the obstacle avoiding mechanism to react. The new general methodology is particularized by using the Arnold's cat map to obtain the timely random appearance and disappearance of the fictive obstacles. The viability of the proposed method is confirmed through an extensive simulation case study.

Fish schooling is a remarkable biological behavior that is thought to provide hydrodynamic advantages. Theoretical models have predicted significant reduction in swimming cost due to two physical mechanisms: vortex hypothesis, which reduces the relative velocity between fish and the flow through the induced velocity of the organized vortex structure of the incoming wake; and the channeling effect, which reduces the relative velocity by enhancing the flow between the swimmers in the direction of swimming. Although experimental observations confirm hydrodynamic advantages, there is still debate regarding the two mechanisms. We provide, to our knowledge, the first three-dimensional simulations at realistic Reynolds numbers to investigate these physical mechanisms. Using large-eddy simulations of self-propelled synchronized swimmers in various rectangular patterns, we find evidence in support of the channeling effect, which enhances the flow velocity between swimmers in the direction of swimming as the lateral distance between swimmers decreases. Our simulations show that the coherent structures, in contrast to the wake of a single swimmer, break down into small, disorganized vortical structures, which have a low chance for constructive vortex interaction. Therefore, the vortex hypothesis, which is relevant for diamond patterns, was not found for rectangular patterns, but needs to be further studied for diamond patterns in the future. Exploiting the channeling mechanism, a fish in a rectangular school swims faster as the lateral distance decreases, while consuming similar amounts of energy. The fish in the rectangular school with the smallest lateral distance (0.3 fish lengths) swims 20% faster than a solitary swimmer while consuming similar amount of energy.

Simple mechanical models emulating fish have been used recently to enable targeted study of individual factors contributing to swimming locomotion without the confounding complexity of the whole fish body. Yet, unlike these uniform models, the fish body is notable for its non-uniform material properties. In particular, flexural stiffness decreases along the fish's anterior-posterior axis. To identify the role of non-uniform bending stiffness during fish-like propulsion, we studied four foil model configurations made by adhering layers of plastic sheets to produce discrete regions of high (5.5 × 10⁻⁵ Nm²) and low (1.9 × 10⁻⁵ Nm²) flexural stiffness of biologically-relevant magnitudes. This resulted in two uniform control foils and two foils with anterior regions of high stiffness and posterior regions of low stiffness. With a mechanical flapping foil controller, we measured forces and torques in three directions and quantified swimming performance under both heaving (no pitch) and constant 0° angle of attack programs. Foils self-propelled at Reynolds number 21 000–115 000 and Strouhal number ∼0.20–0.25, values characteristic of fish locomotion. Although previous models have emphasized uniform distributions and heaving motions, the combination of non-uniform stiffness distributions and 0° angle of attack pitching program was better able to reproduce the kinematics of freely-swimming fish. This combination was likewise crucial in maximizing swimming performance and resulted in high self-propelled speeds at low costs of transport and large thrust coefficients at relatively high efficiency. Because these metrics were not all maximized together, selection of the 'best' stiffness distribution will depend on actuation constraints and performance goals. These improved models enable more detailed, accurate analyses of fish-like swimming.

Insect wing shapes are diverse and a renowned source of inspiration for the new generation of autonomous flapping vehicles, yet the aerodynamic consequences of varying geometry is not well understood. One of the most defining and aerodynamically significant measures of wing shape is the aspect ratio, defined as the ratio of wing length (R) to mean wing chord ($\bar{c}$). We investigated the impact of aspect ratio, AR, on the induced flow field around a flapping wing using a robotic device. Rigid rectangular wings ranging from AR = 1.5 to 7.5 were flapped with insect-like kinematics in air with a constant Reynolds number (Re) of 1400, and a dimensionless stroke amplitude of $6.5\bar{c}$ (number of chords traversed by the wingtip). Pseudo-volumetric, ensemble-averaged, flow fields around the wings were captured using particle image velocimetry at 11 instances throughout simulated downstrokes. Results confirmed the presence of a high-lift, separated flow field with a leading-edge vortex (LEV), and revealed that the conical, primary LEV grows in size and strength with increasing AR. In each case, the LEV had an arch-shaped axis with its outboard end originating from a focus-sink singularity on the wing surface near the tip. LEV detachment was observed for $\mathrm{AR}\gt 1.5$ around mid-stroke at $\sim 70\%$ span, and initiated sooner over higher aspect ratio wings. At $\mathrm{AR}\gt 3$ the larger, stronger vortex persisted under the wing surface well into the next half-stroke leading to a reduction in lift. Circulatory lift attributable to the LEV increased with AR up to AR = 6. Higher aspect ratios generated proportionally less lift distally because of LEV breakdown, and also less lift closer to the wing root due to the previous LEV's continuing presence under the wing. In nature, insect wings go no higher than $\mathrm{AR}\sim 5,$ likely in part due to architectural and physiological constraints but also because of the reducing aerodynamic benefits of high AR wings.