CFD-OctoProp - Computational Fluid Dynamics Aided Design of the Propulsion and Locomotion Systems of a Bioinspired Robot Octopus
The overarching aim of the CFD-OctoProp project was to develop an innovative kind of underwater robot inspired by cephalopods (i.e. squids and octopi) in order to exploit the advantages provided by the locomotion strategy these aquatic animals rely on. Cephalopods were taken as the source of inspiration for designing novel underwater vehicles because they display attributes which can provide a critical asset in submersibles design with respect to the state of the art technology. Cephalopods are peculiar in that they thrust themselves in water by means of pulsed-jetting. This mode of generating thrust, which has been demonstrated to be extremely effective and efficient even when compared to standard propellers or fish-like caudal flapping, occurs by sequences of abrupt inflation and collapse of the animals “mantle” (i.e. the head). This routine is enabled by the fact that, thanks to the lack of skeletal structures, cephalopods can undergo large deformations of their body. The outstanding swimming performances of cephalopods are associated either with the nature of the jet expelled and with fluid-dynamics phenomena arising from the variation of shape which the body undergoes while swimming. This suggests that cephalopods could indeed be regarded as the perfect paradigm for the ideal underwater robot. First, because they are almost entirely devoid of rigid structures, which makes them a great source of inspiration for developing soft-bodied robots, i.e. robots composed for the most part of materials such as elastomers and distinguished for their high degree of resilience and compliance. Secondly, because they sport outstanding swimming performances both in terms of speed and manoeuvring skills.
Finally, cephalopods such as the octopi are also renowned for their dexterous manipulation capabilities and strength regardless of their lack of a skeletal supportive structure. Being capable of designing a fully deformable soft-vehicle which performs a swimming routine analogous to that of cephalopods not only provides an advantage in terms of locomotion performances, but it also guarantees resilience to impacts and unprecedented flexibility. These features altogether determine a set of unique assets which no other underwater vehicle possesses nowadays. The reduced risk of damage, the capability to adjust through cramped spaces and narrow apertures along with the enhanced manoeuvring skills and manipulation capabilities are a few examples where cephalopod-like, soft-bodied robots could represent a disruptive solution in marine applications. ding swimming performances of cephalopods are therefore associated either with the nature of the jet expelled or with the phenomena dependent on the variation of shape which the body undergoes while swimming.
The CFD-OctoProp project succeeded in realizing the first soft-bodied unmanned underwater vehicle ever built propelled by pulsed-jetting. The robot is made for as much as 90% of its volume of rubber-like materials and propels itself in water by contracting an elastic shell and expelling finite slugs of fluid across a nozzle, in a very resembling way to that of cephalopods. This allows the robot to benefit of all the assets provided by cephalopod-inspired propulsion, that is the combination of the jet expelled and the phenomena dependent on shape variation. Differently from a standard propeller, in which thrust is generated in a continuous fashion and is therefore impractical for operation at low speed, the kind of propulsion strategy employed by the soft robots benefits from the discontinuous nature of the pulsations. This endows the vehicle with the capability to abruptly accelerate or decelerate, thus providing enhanced manoeuvrability for precise control and operations in cramped environments.
Because of the highly unconventional actuation mechanism and design of the vehicle, significant effort had to be put in the development of suitable mathematical models which could explore and predict the many relevant parameters and phenomena entailed with its complex dynamics. Three models with different scopes were coded; these entailed: a model for predicting the performance of the robot in terms of the mechanical parameters, a model for estimating hydrodynamic coefficients in order to develop the control of the vehicle and a model for addressing the flow parameters and structural characteristic under strain of the elastomeric material during the vehicle displacement in water. During the CFD-OctoProp project, these models were progressively refined and extensively exploited in the process of optimization of the mechanical design of the pulsed-jet thruster by investigating the non-linear effects arising from the actuation mechanism and the elastic nature of the vehicle. This eventually culminated in the definition of a revised design of the original prototype which benefited from augmented thrust production capability and a superior degree of structural flexibility. This innovative soft-bodied unmanned underwater vehicle, patented and presented at several international conferences in robotics and ocean engineering, is capable of propelling itself in water via impulsive bursts of acceleration and, being almost entirely made of elastic materials, it is unaffected by impacts. Because these features naturally lend themselves to overcoming challenging tasks which require navigation in confined spaces such as offshore infrastructure inspection and archaeological exploration, this new vehicle is a possible candidate for employment in forbidding or sensitive offshore environments for which traditional robotics or human divers are unsuitable .
This newly-patented underwater vehicle lends itself to be exploited both as a stand-alone robot with agile manoeuvring skills as well as a thrusting unit inside a larger robotic platform. The chance to design a whole new underwater vehicle which incorporated the soft-bodied pulsed-jet thruster, a crawling unit and soft manipulators in a single, self-contained, soft robotic platform was eventually addressed. By capitalizing on the results from the prototypes and models of the CFD-OctoProp project and those from the OCTOPUS Integrating project (http://www.octopusproject.eu/), a completely innovative soft-bodied underwater vehicle with swimming, crawling and manipulation capabilities was designed, manufactured and successfully tested both in the laboratory and in the open sea. This is the first soft-bodied underwater robot ever built which exploits unconventional mode of waterborne propulsion as well as innovative legged-locomotion skills and infinite degrees of freedom continuum manipulators. This new prototype will benefit from unprecedented skills thanks to which it will perform agile navigation in submerged environments forbidden to standard underwater robots; it will be capable of crawling over the irregular sea bottom and stick on to those submerged structures upon which inspection and maintenance has to be carried out by exploiting the compliance of its soft manipulators. This mode of operation according to which the working procedure will take place by direct contact with the submerged structures can potentially provide solutions to some of the most challenging problems encountered, for instance, during construction or inspection operations of marine renewable energy plants (e.g. tidal turbines, wave-energy converters, floating and fixed offshore wind turbines) where even infrastructure maintenance operations must contend with strong ocean currents and high wave energy.