A new kind of underwater vehicle is developed by taking inspiration from cephalopods. Its actuation routine is scrutinized via a suitable model. Similar to octopuses and squids, these vehicles consist of an elastic, hollow shell capable of undergoing sequential stages of ingestion and ejection of ambient fluid, which is driven by the recursive inflation and deflation of the shell. The shell actively collapses, and in this way it expels water through a funnel; then it passively returns to the inflated shape, drawing ambient fluid into the cavity. By doing so, a pulsed-jet propulsion routine is performed that enables the vehicle to propel itself in water. Due to their soft nature, the actuation of these vehicles is largely dependent on the subtle management of the elastic response of the shell throughout the propulsion routine. A kinematic model of the actuation mechanism, thoroughly corroborated by experimental validation, is devised which elucidates the relationship between the active (collapse) and passive (refill) stages of the actuation. Upon association with the dynamics of the vehicle, this model permits the derivation of the generic performance profiles of this new kind of vehicle. It is acknowledged that, for given design specifications, an optimal swimming speed exists in coincidence with the coordinated operation between the crank mechanism driving the shell contraction and the onset of elastic energy, which determines the speed of inflation of the shell. These results are invaluable in the definition of rigorous design criteria and derivation of ad-hoc control laws for a new breed of optimized soft-bodied, pulsed-jet, unmanned underwater vehicles.

We introduce an octopus-inspired, underwater, soft-bodied robot capable of performing waterborne pulsed-jet propulsion and benthic legged-locomotion. This vehicle consists for as much as 80% of its volume of rubber-like materials so that structural flexibility is exploited as a key element during both modes of locomotion. The high bodily softness, the unconventional morphology and the non-stationary nature of its propulsion mechanisms require dynamic characterization of this robot to be dealt with by ad hoc techniques. We perform parameter identification by resorting to a hybrid optimization approach where the characterization of the dual ambulatory strategies of the robot is performed in a segregated fashion. A least squares-based method coupled with a genetic algorithm-based method is employed for the swimming and the crawling phases, respectively. The outcomes bring evidence that compartmentalized parameter identification represents a viable protocol for multi-modal vehicles characterization. However, the use of static thrust recordings as the input signal in the dynamic determination of shape-changing self-propelled vehicles is responsible for the critical underestimation of the quadratic drag coefficient.

Predictive control methods can substantially improve the performance of Unmanned Underwater Vehicles (UUVs), particularly in shallow water environments or near the free surface where wave induced disturbance are of magnitude comparable to the vehicle characteristic inertia. To facilitate the adoption of these methods, a fast estimation of the time evolution of hydrodynamic forces acting on a vehicle is required. To this end, we perform experiments in a wave tank with an ROV to validate the use of Linear Wave Theory (LWT) to capture the time history of surge, heave and pitch wave induced forces and moments. Validation is performed for various sea states, reconstructed with a mean correlation of 0.9138 in comparison to experimental measurements, displaying a maximum normalised mean error deviation between simulation and experimental data of 0.16 and 0.27 respectively for surge and heave forces, and 0.34 for pitch moment. The effectiveness of employing real-time wave disturbance forecasting for the purpose of anticipatory control is then assessed by incorporating the predicted loads within a Model Predictive Controller. Results display a mean RMS positional error reduction of 47.32% in comparison to a standard PD controller. This presents evidence that accurate, near real-time predictions of the wave-generated forces and moments on an ROV can be produced, laying the foundation for developing model-based predictive control strategies that better suit operation in harsh environments.

Prokaryotic flagellum is considered as the only known example of a biological “wheel,” a system capable of converting the action of rotatory actuator into a continuous propulsive force. For this reason, flagella are an interesting case study in soft robotics and they represent an appealing source of inspiration for the design of underwater robots. A great number of flagellum-inspired devices exists, but these are all characterized by a size ranging in the micrometer scale and mostly realized with rigid materials. Here, we present the design and development of a novel generation of macroscale underwater propellers that draw their inspiration from flagellated organisms. Through a simple rotatory actuation and exploiting the capability of the soft material to store energy when interacting with the surrounding fluid, the propellers attain different helical shapes that generate a propulsive thrust. A theoretical model is presented, accurately describing and predicting the kinematic and the propulsive capabilities of the proposed solution. Different experimental trials are presented to validate the accuracy of the model and to investigate the performance of the proposed design. Finally, an underwater robot prototype propelled by four flagellar modules is presented.

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.