Inspired by the octopus and other animals living in water, soft robots should naturally lend themselves to underwater operations, as supported by encouraging validations in deep water scenarios. This work deals with equipping soft arms with the intelligence necessary to move precisely in wave-dominated environments, such as shallow waters where marine renewable devices are located. This scenario is sub-stantially more challenging than calm deep water since, at low operational depths, hydrodynamic wave disturbances can represent a significant impediment. We propose a control strategy based on Nonlinear Model Predictive Control that can account for wave disturbances explicitly, optimising control actions by considering an estimate of oncoming hydrodynamic loads. The proposed strategy is validated through a set of tasks covering set-point regulation, trajectory tracking and mechanical failure compensation, all under a broad range of varying significant wave heights and peak spectral periods. The proposed control methodology displays positional error reductions as large as 84% with respect to a baseline controller, proving the effectiveness of the method. These initial findings present a first step in the development and deployment of soft manipulators for performing tasks in hazardous water environments.

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Elasticity has been linked to the remarkable propulsive efficiency of pulse-jet animals such as the squid and jellyfish, but reports that quantify the underlying dynamics or demonstrate its application in robotic systems are rare. This work identifies the pulse-jet propulsion mode used by these animals as a coupled mass-spring-mass oscillator, enabling the design of a flexible self-propelled robot. We use this system to experimentally demonstrate that resonance greatly benefits pulse-jet swimming speed and efficiency, and the robot's optimal cost of transport is found to match that of the most efficient biological swimmers in nature, such as the jellyfish Aurelia aurita. The robot also exhibits a preferred Strouhal number for efficient swimming, thereby bridging the gap between pulse-jet propulsion and established findings in efficient fish swimming. Extensions of the current robotic framework to larger amplitude oscillations could combine resonance effects with optimal vortex formation to further increase propulsive performance and potentially outperform biological swimmers altogether.

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Capability of a pulsed-jetting, aquatic soft robot to perform turning manoeuvres by means of a steerable nozzle is investigated experimentally for the first time. Actuation of this robot is based on the periodic conversion of slowly-charged elastic potential energy into fluid kinetic energy, giving rise to a cyclic pulsed-jet resembling the one observed in cephalopods. A steerable nozzle enables the fluid jet to be deflected away from the vehicle axis, thus providing the robot with the unique ability to manoeuvre using thrust-vectoring. This actuation scheme is shown to offer a high degree of control authority when starting from rest, yielding turning radii of the order of half of the body length of the vehicle. The most significant factor affecting efficiency of the turn has been identified to be the fluid momentum losses in the deflected nozzle. This leads, given the current nozzle design, to a distinct optimum nozzle angle of 35°.

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A soft hydraulic actuator is presented which uses elastic energy storage for the purpose of pulsed-jet propulsion of soft unmanned underwater vehicles. The actuator consists of a flexible membrane that can be inflated using a micropump and whose elastic potential energy may be released on demand using a controllable valve, in a manner inspired by the swimming of squids and octopuses. It is shown that for equivalent initial elastic energy, the drop in peak thrust is linearly proportional to the decrease in nozzle cross section. Peak hydraulic power amplification of the soft actuator of approximately 75% is achieved with respect to that of the driving pump, confirming that passive elasticity can be exploited in aquatic propulsion to replicate the explosive motion skills of agile sea-dwelling creatures.

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Cephalopods use large-scale structural deformation to propel themselves underwater, changing their internal volume by 20–50%. In this work, the hydroelastic response of a swimmer comprised of a fluid-filled elastic-membrane is studied via an analytic formulation of two coupled non-linear dynamic equations. This model of the self-propelled soft-body dynamics incorporates the interplay of the external and internal added-mass variations. We compare the model against recent experiments for a body which abruptly reduces its cross-section to eject a single jet of fluid mass. Using the model we study the impact of size-change excitation on sustained swimming speeds and efficiency.

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The design, calibration and testing of an experimental rig for measuring 2-DOFs unsteady loads over aquatic robots is discussed. The presented apparatus is specifically devised for thrust characterization of a squid-inspired soft unmanned underwater vehicle, but its modular design lends itself to more general bioinspired propulsion systems and the inclusion of additional degrees of freedom. A purposely designed protocol is introduced for combining calibration and error compensation upon which force and moment measurements can be performed with a mean error of 0.8% in steady linear loading and 1.7% in unsteady linear loading, and mean errors of 10.2% and 9.4% respectively for the case of steady and dynamic moments at a sampling rate of the order of 10 Hz. The ease of operation, the very limited cost of manufacturing and the degree of accuracy make this an invaluable tool for fast prototyping and low-budget projects broadly applicable in the soft robotics community.

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Previous studies have demonstrated that added-mass variation can play a significant role on thrust generation. In this respect, a simplified mechanical system such as an aquatic shape-changing linear oscillator lends itself to this study because it allows to segregate the contribution of added-mass variation from other terms. We present the design of an experimental apparatus which highlights the capability of a deformable oscillator to drive sustained resonance by exploiting the thrust produced by shape-change alone. These results will have significant implications in aquatic propulsion and in the design of deformable, self-propelled underwater vehicles.

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Aquatic organisms capable of undergoing extensive volume variation of their body during locomotion can benefit from increased thrust production. This is enabled by making use of not only the expulsion of mass from their body, as documented extensively in the study of pulsed-jet propulsion, but also from the recovery of kinetic energy via the variation of added mass. We use a simplified mechanical system, i.e. a shape-changing linear oscillator, to investigate the phenomenon of added-mass recovery. Our study proves that a deformable oscillator can be set in sustained resonance by exploiting the contribution from shape variation alone which, if appropriately modulated, can annihilate viscous drag. By confirming that a body immersed in a dense fluid which undergoes an abrupt change of its shape experiences a positive feedback on thrust, we prove that soft-bodied vehicles can be designed and actuated in such a way as to exploit their own body deformation to benefit of augmented propulsive forces.

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A submerged body subject to a sudden shape change experiences large forces due to the variation of added-mass energy. While this phenomenon has been studied for single actuation events, application to sustained propulsion requires the study of periodic shape change. We do so in this work by investigating a spring-mass oscillator submerged in quiescent fluid subject to periodic changes in its volume. We develop an analytical model to investigate the relationship between added-mass variation and viscous damping, and demonstrate its range of application with fully coupled fluid-solid Navier-Stokes simulations at large Stokes number. Our results demonstrate that the recovery of added-mass kinetic energy can be used to completely cancel the viscous damping of the fluid, driving the onset of sustained oscillations with amplitudes as large as four times the average body radius r0. A quasi-linear relationship is found to link the terminal amplitude of the oscillations X to the extent of size change a, with X/a peaking at values from 4 to 4.75 depending on the details of the shape-change kinematics. In addition, it is found that pumping in the frequency range of 1-a/2r0 < ω2/ω2n < 1 + a/2r0, with ω/ωn being the ratio between frequency of actuation and natural frequency, is required for sustained oscillations. The results of this analysis shed light on the role of added-mass recovery in the context of shape-changing bodies and biologically inspired underwater vehicles.@en