Human-human learning is usually conducted utilizing multiple modalities (verbal, visual, kinesthetic etc), which is proven to significantly improve the quality of communication between the teacher and the learner. Towards extending this idea to human-robot learning, we propose here a multi-modal scheme for robot Learning by Demonstration (LbD), that combines visual observation of the human hand from a depthless RGB camera, with enhanced direct kinesthetic teaching of the robot. The proposed method, exploits the visually perceived action of the human hand, to provide haptic guidance during kinesthetic teaching, facilitating corrections along the axes with the maximum uncertainty. The method is experimentally evaluated using an RGB camera and a UR10e robot, and compared against single-modal approaches, i.e. visually trained behavior and trained through kinesthetic teaching. Our method is shown to outperform the latter, in terms of both the quality of skill transfer, and the cognitive/physical load required for the training.

Bio-inspired undulatory fin propulsion holds considerable potential for endowing robotic underwater vehicles with low-speed manoeuvrability and stable station-keeping. Robotic fins typically comprise a number of serially arranged and individually actuated “fin rays”, interconnected by a membrane-like flexible surface. Propulsive forces are generated by the propagation of a traveling wave along the mechanism, via appropriately timed ray oscillations. The present paper describes a dynamic model for an elementary two-ray fin system, analyzed as a standard robot mechanism with additional contributions arising from the elastic deformation of the flexible membrane and from the hydrodynamic forces. The model's main aspects, particularly with regard to the hydrodynamic effects, are explored via simulation studies, as well as via experiments with a robotic prototype. The developed model can serve a number of purposes towards optimizing the mechanical design, the control strategies, and the propulsive efficacy of robotic undulatory fins.

The present paper considers the modeling, control, development and experimental validation of bio-inspired multiarm underwater robotic swimmers actuated by compliant actuating elements, in the context of the soft robotics paradigm. Each one of the swimmer's compliant arms is actuated at its base by a pair of antagonistic compliant shape memory alloy (SMA) springs. The base joint of each such arm displays hysteretic behavior and asymmetries, which are compensated via the modified Prandtl-Ishlinskii (MPI) model of the joint's response, in conjunction with angular position feedback from a potentiometer. This closed-loop control scheme achieves fast and efficient tracking of a sculling joint motion profile. Experimental results based on a pair of such submerged arms, integrated in a catamaran hull, indicate the feasibility of this actuation and control scheme, providing propulsive speeds up to approximately 0.5 arm lengths per second (~50 mm/sec) and propulsive forces up to 30 mN. The experimental studies presented, regarding the effect of the arm kinematic parameters on propulsive speed and force, are in qualitative agreement with previous results of our group for rigid-actuator multi-arm underwater swimmers.

This paper presents an 8-arm compliant robot, able to propel itself underwater by movements of its arms, either alone or interconnected via a passively-compliant web. The robot is inspired by the morphology and outstanding locomotor capabilities of the octopus, and is fabricated primarily from compliant materials. This robotic swimmer is first investigated computationally via dynamical models capturing the arm and web compliance, and indicating the effect of various kinematic parameters of the system on its motion. The performance of the robotic prototype is, then, tested experimentally, to demonstrate this novel mode of underwater propulsion by combining various patterns of sculling movements of the arms and web. Speeds of 0.5 body lengths per second and propulsive forces of up to 10.5 N were achieved, with a cost of transport as low as 0.62.

The locomotion of miniature medical robots in the interior of the human body, possibly in contact with the tissue of organs and vessels, is a challenging problem in robotics, which, if properly addressed, might favorably impact several significant applications. This paper investigates the potential of employing relatively low-frequency vibrations, generated by an on-board eccentric motor, to reduce friction for such robots, moving by some other primary mechanism over and in contact with liver tissue. This study focuses on the computational modeling and the experimental assessment of the interaction between vibratory-actuated miniature robots and compliant tissue. The computational model developed to analyze the effect of such vibrations, for a robot moving over a substrate with normal-wise viscoelasticity, is employing a modified Dahl model with viscous damping for the tangential friction. Experiments with a vibratory-actuated platform pulled over ex-vivo bovine liver tissue indicate that the friction reduces as the frequency of vibrations increases, in a manner consistent with the model's predictions, reaching a reduction of up to 40%.

Inspired by the octopus arm morphology and exploiting recordings of swimming octopus, we investigate the propulsive capabilities of an 8-arm robotic system under various swimming gaits, including arm sculling and arm undulations, for the generation of forward propulsion. A dynamical model of the robotic system, that considers fluid drag contributions accurately evaluated by CFD methods, was used to study the effects of various kinematic parameters on propulsion. Experiments inside a water tank with an 8-arm robotic prototype successfully demonstrated the sculling-only gaits, attaining a maximum speed of approximately 0.2 body lengths per second. Similar trends were observed, as in the simulation studies, with respect to the effect of the kinematic parameters on propulsion.

The multi-arm morphology of octopus-inspired robotic systems may allow their aquatic propulsion, in addition to providing manipulation functionalities, and enable the development of flexible robotic tools for underwater applications. In the present paper, we consider the multi-arm swimming behavior of the octopus, which is different than their, more usual, jetting behavior, and is often used to achieve higher propulsive speeds, e.g., for chasing prey. A dynamic model of a robot with a pair of articulated arms is employed to study the generation of this mode of propulsion. The model includes fluid drag contributions, which we support by detailed Computational Fluid Dynamic analysis. To capture the basic characteristics of octopus multi-arm swimming a sculling mode is proposed, involving arm oscillations with an asymmetric speed profile. Parametric simulations were used to identify the arm oscillation characteristics that optimize propulsion for sculling, as well as for undulatory arm motions. Tests with a robotic prototype in a water tank provide preliminary validation of our analysis.

This paper addresses the problem of vision-based closed-loop control for undulatory robots. We present an image-based visual servoing scheme, which drives the robot to a desired location specified by a target image, without explicitly estimating its pose. Instead, the control relies on the computation of the epipolar geometry between the current and target images. We analyze controllability and stability of the proposed control scheme, which is validated by simulation studies using the SIMUUN computational tools. Preliminary experiments, involving the Nereisbot undulatory robotic prototype, are also presented.

The present paper explores the effect of the mechanism-substrate frictional interface on the locomotion characteristics of robotic mechanisms employing traveling waves for propulsion. For these investigations, an extended class of undulatory robotic locomotors is considered, termed pedundulatory, which augment lateral body undulations by coordinated dorso-ventral oscillations of multiple pairs of lateral paddle-shaped appendages (parapodia). We examine how, the same robotic prototype, allows the implementation of four distinct bio-inspired undulatory and pedundulatory modes of locomotion, by modifying the motion control strategy depending on the mechanism-substrate frictional interface. These modes employ retrograde or direct body waves, either standalone (giving rise to eel-like and ochromonas-like undulatory locomotion modes, respectively), or combined with appropriately coordinated substrate contact by the parapodial appendages (giving rise to centipede-like and polychaete-like pedundulatory modes, respectively). These four modes are investigated and comparatively assessed, both in simulation and via extensive experiments on granular substrates with the Nereisbot prototype. Our results validate the identified locomotion principles and also highlight the enhanced performance and gait repertoire of pedundulatory systems, compared to purely undulatory ones.

The polychaete annelid marine worms propel themselves in a variety of challenging locomotion environments by a unique form of tail-to-head body undulations, combined with the synchronized action of numerous parapodial lateral appendages. This combined parapodial and undulatory mode of locomotion is termed pedundulatory in the present work. Robotic analogues of this type of locomotion are being studied, both in simulation, and via experiments with biomimetic robotic prototypes, which combine undulatory movements of their multi-link body with appropriately coordinated parapodial link oscillations. Extensive experimental studies of locomotion on sand demonstrate the potential of the pedundulatory robotic prototypes, especially their rich gait repertoire and their enhanced performance compared to robotic prototypes relying only on body undulations

A biological paradigm of versatile locomotion and effective motion control is provided by the polychaete annelid worms, whose motion adapts to a large variety of unstructured environmental conditions (sand, mud, sediment, water, etc.), and could thus be of interest to replicate by robotic analogs. Their locomotion is characterized by the combination of a unique form of tail-to-head body undulations (opposite to snakes and eels), with the rowing-like action of numerous lateral appendages distributed along their long segmented body. Focusing on the former aspect of polychaete locomotion, computational models of crawling and swimming by such tail-to-head body undulations have been developed in this paper. These are based on the Lagrangian dynamics of the system and on resistive models of its interaction with the environment, and are used for simulation studies demonstrating the generation of undulatory gaits. Several biomimetic robotic prototypes have been developed, whose undulatory actuation achieves propulsion on sand and other granular unstructured environments. Extensive experimental studies demonstrate the feasibility of robot propulsion by tail-to-head body undulations in such environments, as well as the agreement of its qualitative and quantitative characteristics to the predictions of the corresponding computational models.

Polychaete annelid worms provide a biological paradigm of versatile locomotion and effective motion control, adaptable to a large variety of unstructured environmental conditions (water, sand, mud, sediment, etc.). The undulatory locomotion of their segmented body is characterized by the combination of a unique form of tail-to-head body undulations, with the rowing-like action of numerous lateral appendages distributed along their body. Computational models of polychaete-like crawling and swimming have been developed, based on the Lagrangian dynamics of the system and on resistive models of its interaction with the environment, and used for simulation studies demonstrating the generation of undulatory gaits. Several lightweight robotic prototypes have been developed, whose undulatory actuation achieves propulsion on sand. Extensive experiments demonstrate that the propulsion of these robots is characterized by essential features of polychaete locomotion, in agreement with the corresponding simulations.

To investigate the undulating median fin propulsion and its potential for implementation in man-made underwater vehicles, a "fin actuator" has been developed. The device consists of eight parallel bellows actuators or PBAs (fin rays) arranged in a series and interconnected via a flexible material (fin membrane). The PBAs are pneumatically driven and allow for bending movements in any orientation plane. Position data are provided by flexible electrogoniometers. The forces generated from the fin actuator undulations are measured by a pair of tri-axial force sensors. Initial testing has concentrated on 2D fin ray motions, whereby each PBA moves laterally, with the deflection angle performing a sinusoidal variation. Preliminary measurements, for a range of the propulsive waveform parameters, demonstrate reversible thrust generation and good agreement with the theoretical predictions of a momentum-based model.