GiorgioSerchiEtAl2012_TransMech

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IEEE/ASME TRANSACTIONS ON MECHATRONICS 1

Biomimetic Vortex Propulsion: Toward the NewParadigm of Soft Unmanned Underwater Vehicles

Francesco Giorgio Serchi, Andrea Arienti, and Cecilia Laschi, Senior Member, IEEE

Abstract—A soft robot is presented which replicates the abilityof cephalopods to travel in the aquatic environment by means ofpulsed jet propulsion. In this mode of propulsion, a discontinuousstream of fluid is ejected through a nozzle and rolls into a vortexring. The occurrence of the vortex ring at the nozzle-exit plane hasbeen proven to provide an additional thrust to the one generatedby a continuous jet. A number of authors have experimented withvortex thrusting devices in the form of piston-cylinder chambersand oscillating diaphragms. Here, the focus is placed on designinga faithful biomimesis of the structural and functional character-istics of the Octopus vulgaris. To do so, the overall shape of thisswimming robot is achieved by moulding a silicone cast of an ac-tual octopus, hence offering a credible replica of both the exteriorand interior of an octopus mantle chamber. The activation cycle re-lies on the cable-driven contraction/release of the elastic chamber,which drives the fluid through a siphon-like nozzle and eventuallyprovides the suitable thrust for propelling the robot. The proto-type presented herein demonstrates the fitness of vortex enhancedpropulsion in designing soft unmanned underwater vehicles.

Index Terms—Autonomous underwater vehicles (AUVs), bioin-spiration, biorobotics, propulsion, soft robots, thruster, unmannedunderwater vehicles (UUVs).

I. INTRODUCTION

W ITH the ever growing importance of offshore technolo-gies and maritime transport, autonomous underwater

vehicles (AUVs) and remotely operated vehicles (ROVs) havebeen progressively employed throughout a wider range of ap-plications. Unmanned underwater vehicles (UUVs) have earneda very positive reputation for their suitability at dealing withcomplex tasks in poorly accessible, often unsafe environments.Among the great variety of such vehicles, two major classes ofUUVs are distinguished, which Krieg and Mohseni [1] identifyas the “torpedo-like” and the “box” type. To the first class, long-range, high-speed travelers pertains, while the second familybroadly comprehends efficient low-speed maneuvering vehicles.

In addition to standard UUVs, a growing fascination for bioin-spired technologies has lead, in recent times, to the development

Manuscript received November 15, 2011; revised April 30, 2012 and July 27,2012; accepted September 11, 2012. Recommended by Guest Editor S. Kim.This work was supported by the European Commission in the frame of the ICT-FET OCTOPUS Integrating Project and the CFD-OctoProp project EuropeanReintegration Grant. F. G. Serchi and A. Arienti contributed equally to thiswork.

The authors are with the Research Centre on Sea Technologies and MarineRobotics, Scuola Superiore Sant’Anna, Pisa 57126, Italy (e-mail: [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMECH.2012.2220978

of a number of aquatic robots which employ alternative strate-gies for the locomotion (e.g., [2] and [3]). Among these, a broadrange of variants are found in terms of the swimming patternadopted, as thoroughly accounted for in [4]. These new fam-ily of bioinspired robots make use of discrete links of rigid orsemirigid materials joint together in such a way to exploit theflexibility of the overall structure. In this respect, the design ofthese robots relies on standard mechanical principles assembledin a somewhat uncommon fashion with the purpose of replicat-ing the functionality of the biological source of inspiration. Atotally new and alternative approach to robot design has latelyappeared in the form of soft robotics. Despite a rigorous andformal definition of soft robot partially remains a topic of opendebate, the basic elements, which the concept of soft roboticsstems from, are the intrinsic compliance and hyper-redundancy.Soft robots are manufactured with flexible materials which ex-ert minimal resistance to compressive and shearing forces. Thisproperty of the constitutive materials allows soft robots to un-dergo extensive strains, thus making them less prone to damageand to causing harm, as well as making them suitable for anentire new spectrum of applications. An exhaustive treatmenton this topic is out of the scope of this paper and the reader isreferred to the comprehensive review of [5]. Soft robots haveso far acquired only limited credibility, this being due, on onehand, to the complexity of designing soft robots which performat least as effectively as their hard counterparts and, on the otherhand, in not having fully recognized how the benefit providedby a soft structure can be exploited to its whole extent. Thisstands true for the terrestrial as well as for the aquatic envi-ronment. However, cephalopods bring evidence that, in water,organisms lacking rigid structures do not appear to suffer anymajor limitation when compared to organisms with an endoor exoskeleton. Quite the contrary, it occurs that cephalopodsmanifest outstanding performances in terms of both manipu-lation and locomotion. The case of cephalopods strongly sug-gests they might be a fruitful source of inspiration for designingaquatic soft robots. Indeed, active research is currently beingdedicated to the development of soft manipulators which re-semble the octopus arm [6]–[8]; however, only a limited efforthas been put so far in producing cephalopod-inspired propulsiondevices [1], [9]–[13]. This paper focuses on the development ofa completely new type of UUV which faithfully draws inspi-ration from the swimming technique adopted by cephalopods.The robot presented herein is unique in that it tightly matchesthe shape of a real octopus vulgaris, is entirely made of flexiblematerial (with the exception of one gearmotor), and employs anefficient propulsion mechanism which is very closely reminis-cent of that one of cephalopods.

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2 IEEE/ASME TRANSACTIONS ON MECHATRONICS

II. BIOINSPIRED PROPULSION

Commercially available UUVs of the “box” type commonlyrely on several thrusters in order to provide localized acceler-ation for enhanced control. The majority of thrusters found onUUVs are standard propellers. Azimuthal, podded, and ductedpropellers are also fairly common in this field of application.All these, however, suffer from low effectiveness when it comesto operating at few or single rotations of the propeller as it oc-curs when impulsive, but brief accelerations are required [1].This represents a significant drawback which can dramaticallylimit the controllability of the UUVs in unpredictable, wave orcurrent dominated environments. The latest trend in underwa-ter propulsion for addressing the issue of maneuverability liesin the inspiration from aquatic animals and the strategies theyhave adopted throughout evolution. Early research has devotedgreat interest in swimming fish. Lately, a growing attention hasbeen addressed toward the propulsion via the employment offins (e.g., [14]). Finally, cephalopod-like pulsed jet propulsionhas very recently been suggested as an appealing alternative tothe other ways of locomotion in water. Pulsed jet propulsionhas been broadly investigated either from a biological point ofview and from a mostly fluid mechanical perspective startingfrom the early work on cephalopod locomotion by the authorsof [15] and [16] and that on laminar vortex rings by the au-thors of [17]–[19]. The research in this field progressed in afairly discontinuous fashion until a renewed interest arose fromthe work of the authors in [20]–[22] who provided evidence ofthe existence of a universal scaling factor for vortex ring for-mation. The acknowledgment that optimal formation time andenhanced thrust of the vortex ring coincided [23] has promoteda renovated appeal for the potential exploitation of pulsed jettechnology for underwater propulsion. In recent times, the inter-est in pulsed jet propulsion has fostered a wealth of new researchin the fluid mechanics of vortex rings [24]–[27] and cephalopodlocomotion [28]–[31]. Most importantly, a number of underwa-ter vehicles propelled by various kinds of vortex thrusters havebeen developed and tested [1], [9], [10], [12], [13], [32].

A. Pulsed Jet Propulsion

The pulsation cycle in cephalopods is driven by the interplayof the mantle muscle groups. The octopus has longitudinal,radial, and circular muscles, the concurrent contraction of whichpermits the pressurization and stiffening of the mantle chamber.Differential activation of one group over the others gives rise tochanges in the shape of the mantle [33].

Ideally, a pulsation cycle is initiated by the refilling of themantle cavity. This is driven by the expansion of the mantlechamber due to the antagonistic contraction of radial muscles,on one hand, and circular and longitudinal muscles, on the other,along with the incompressibility of the muscular tissue. Thediameter of the chamber increases and water is selectively in-gested via a pair of valves located underneath the funnel [seeFig. 1(a)]. Inflow across the siphon is impaired by the collapseof the siphon itself due to pressure drop inside the nozzle.

The ejection of the fluid stored inside the mantle cavity oc-curs at this stage. When the activation of the circular muscles

Fig. 1. Schematic depiction of the pulsation sequence as seen from an exter-nal side view of the octopus mantle. (a) Mantle expands and water is ingestedthrough the ingestion valves and, to a minor extent, through the siphon. (b) Man-tle contracts and water is expelled through the siphon, generating a structuredvortex ring.

is prominent over the others, the diameter of the chamber de-creases [33], and a fluid slug is expelled through the funnel [seeFig. 1(b)]. During this process, the ingestion valves are sealed,thus forcing the fluid to accelerate across the nozzle and, at thesame time, avoiding any significant pressure loss.

As the slug of fluid reaches the edge of the nozzle, the bound-ary layer generated inside the wall of the funnel rolls up andgives rise to a vortex ring [20]. Krueger and Gharib [26] bringevidence that pulsed jet mode provides a significant benefit interms of thrust in comparison to the case of a continuous jetmode. The reason for the enhanced thrust lies in the occurrenceof the vortex ring which is responsible for what Krueger [34]refers to as the nozzle-exit overpressure. This additional termappears to participate to the overall thrust with a very remark-able contribution which, for the case of optimal formation time,can be as large as 20% [23], [26]. The existing work on vortexenhanced propulsion suggests that a strong potential lies behindthis yet unexplored research field. Nevertheless, only a limitedeffort has been made so far in order to thoroughly assess theactual employability of this mean of propulsion. The most re-markable advances in pulsed jet propulsion are presented in thefollowing.

B. Existing Vortex Thrusters

In recent times, pulsed jet vortex thrusters have been de-signed and tested on actual prototypes [10], [12], [13], [32],[35]. Mohseni [32] and Krieg and Mohseni [10] developeda diaphragm-driven zero net-mass flux actuator. The expan-sion and contraction of the diaphragm drives the ingestion andexpulsion of fluid which produces the vortex ring. Krieg andMohseni [10] bring evidence that in vortex-driven propulsion,the time required for thrust to reach its steady-state value is sig-nificantly shorter than it is for standard propellers. This strongly


GIORGIO SERCHI et al.: BIOMIMETIC VORTEX PROPULSION: TOWARD THE NEW PARADIGM OF SOFT UNMANNED UNDERWATER VEHICLES 3

supports the idea that pulsed jet propulsion could be an ex-tremely valid alternative to propellers for low-speed maneuver-ing and station-keeping tasks.

Moslemi and Krueger [11], [12] developed the self-propelledpiston-driven UUV Robosquid. In this case, a piston/cylinderapparatus is implemented along with a set of check valves whichguarantee the unidirectionality of the fluid through the nozzle ina similar fashion to what is observed in real squids.

Finally, Ruiz et al. [13] developed an underwater vehicledriven by a propeller. The device presented by Ruiz et al. [13]can either generate a continuous or a discontinuous jet by thecontrolled occlusion of the propeller slipstream. A series of testswere performed with the aim of experimentally comparing thevehicle efficiency in the case of steady-jet and pulsed-jet modesof propulsion. Results from the experiment of Ruiz et al. showa 40% increase in Froude efficiency.

The outcome of the experimental research on vortex thrusterpropelled vehicles demonstrates that pulsed jet propulsion canindeed represent a feasible alternative to standard propeller.In what follows, the recent advancement in vortex enhancedpropulsion is applied in the context of soft robotics with thescope of developing a new type of soft unmanned underwatervehicle (SUUV) for which cephalopods represent the perfectsource of inspiration.

III. SOFT VORTEX THRUSTERS: CONCEPT AND DESIGN

The aim of this study is to introduce a new kind of UUV. Thenovelty of the prototype presented herein lies in its soft nature,hence the definition of soft unmanned underwater vehicle, orSUUV, along with a bioinspired propulsion mechanism.

Soft robots promise and have already proved to be suitedfor providing advantages in a number of ways over standardrigid robots [5], among which the reduced risk of damage andcapability to adjust through narrow apertures are only two ex-amples. These advantages could indeed be exploited throughouta wide variety of marine applications. The authors believe thatSUUVs could benefit from a tremendous asset in comparisonto standard UUVs thanks to their reduced weight, their prone-ness to comply with complex surroundings, and their intrinsicsafety when interacting with a diver. These features will pro-vide a remarkable benefit in several applications of offshoreengineering such as pipeline and ship hull inspection, wreckexploration, fisheries and aquaculture survey, and underwaterarcheology. In addition, recent developments in the design ofsoft robotic arms (i.e., [6]–[8], [36]) have anticipated that softrobots will soon benefit from manipulation capabilities compa-rable to those of rigid robots. This suggests that, once equippedwith the appropriate manipulators, an SUUV could accomplishdifficult endeavors including marine growth removal, damagedpropeller cropping, and even underwater welding, which nowa-days mainly remain a prerogative of expert divers.

In developing this first prototype, inspiration was drawnfrom the aquatic animal which better suits the design scope,namely the octopus. The choice of the octopus rather than othercephalopods is motivated by the lack of collagen layers in itsmantle thickness [33]. This on one hand makes the octopus a less

impressive sea dweller, as opposed to collagen-endowed squidsand cuttlefish [37], but, on the other hand, it makes the octo-pus the perfect paradigm of softness. As far as the propulsionmechanism is concerned, the authors have managed to achievea functional synthesis of the swimming routine of cephalopods,hence benefiting from the advantage provided by the vortex-enhanced pulsed jet propulsion of these animals. This is doneby manufacturing a replica of a real mantle of a cephalopod,more precisely an octopus vulgaris, and endow this with an ap-paratus which imitates the motor functions of these swimmingorganisms. In addition, since the robot is fashioned accordingto accurate biomimicry, it could be employed as a test bed forthe study of the biological locomotion in octopuses.

The design concept presented here represents a first tentativeapproach of incorporating the principles of soft robotics intothe development of an SUUV and, in this respect, this studyprovides a new step into what can be referred to as marine softrobotics.

A. Mantle Chamber

In an effort to conceive a functional replica of a real oc-topus, the first step involves the design of a robot which isstructurally resemblant to its biological counterpart. This offersthe advantage of ensuring that the robot will benefit from anefficient hydrodynamic shape suited for performing the com-pression/expansion unsteady mode of swimming. The similar-ity, however, should not only account for the exterior of theoctopus, but for the inside, as well. Observation of an octopusdemonstrates that the interior of the mantle chamber is shaped insuch a way to guarantee that the fluid is smoothly pushed towardthe funnel during compression of the chamber, as schematicallyportrayed in Fig. 1. This said, the most effective way of produc-ing a faithful replica of the mantle of an octopus is achieved bymanufacturing a cast of an actual specimen.

A 1.2 kg (wet body weight) octopus vulgaris was acquiredfrom the local market. Because the focus of this study is themantle alone, the arms were cut off. Liquid polyurethane waspoured inside the mantle cavity, holding the mantle with thefunnel facing upward. The polyurethane was allowed to creepthrough the cavities of the mantle and was left to condensate for40 min. The body of rigid polyurethane within the mantle cham-ber will be referred to as “PU-1” [see Fig. 2(b)]. The ensemblecomposed of PU-1 and the octopus mantle was then immersedinto a container capable of comfortably lodging the whole setwith the funnel region still facing upward. An additional amountof liquid polyurethane was poured in the container around thepolyurethane-filled octopus mantle. This second stage of castingis performed in two parts. A first cast is poured up to the level oflargest cross section of the mantle. The octopus mantle and PU-1are initially held in place by means of external hangers and laterby the quickly increasing viscosity of the external polyurethane.Once this first cast has solidified, a film of silicone is drippedover it. Then, a second cast of polyurethane is poured onto thesilicone film up to the top of the mantle. In this manner, theexternal body of polyurethane can conveniently be opened tofacilitate the extraction of the inner elements. The second cast



Fig. 2. Elements composing the polyurethane mould. (a) Block is assem-bled and filled with silicone. (b) Various pieces obtained throughout the pro-cess of casting the original octopus are presented. Element 1 corresponds toPU-1, which is the first mould of the inside of the octopus, while elements 2–4correspond to PU-2, namely the outside of the octopus. The gap in betweenPU-2 and PU-1 constitutes the actual octopus mantle.

of polyurethane is allowed to solidify. At this stage, the externalcast, referred to as “PU-2,” contains the octopus mantle [seeFig. 2(b)]. This, in turn, encloses the PU-1. The remnants ofthe octopus, comprising the mantle tissues and internal organs,are removed by opening PU-2. At this stage, PU-1 and PU-2compose the inside and the outside of the original animal, whilethe gap in between them represents the actual octopus mantle[see Fig. 2(a)].

A detailed image of the octopus mantle was achieved by per-forming an MRI of the polyurethane block filled with jelly andCAD processing the MRI data (see Fig. 3). Finally, a total of280 g of EcoFlex00-30 silicone is poured in the gap betweenPU-1 and PU-2 and is allowed to polymerize at ambient tem-perature for 12 h. By extracting the silicone cast from thepolyurethane mould, an exact replica of the original octopusmantle is obtained (Fig. 4).

The final silicone cast (see Fig. 4) manifests a number ofinteresting features. A first qualitative analysis of the thicknessof the mantle wall shows a smooth but appreciable variationacross the body [see Fig. 3(b)]. The wall thickness varies from athin sheet in the ventral region to a more thick-walled layer in thedorsal area. A similar pattern is found along the axial direction,as portrayed in Fig. 3(a). Qualitative tests in fresh water seemto suggest that such a pattern of the wall depth supports thecontractile function of the mantle. Indeed, according to thisconfiguration, the silicone mock-up demonstrates a distinctiveaptitude for elastic response to inward radial strain. As a result,

Fig. 3. Mantle chamber as achieved from a CAD processed Magnetic Reso-nance Imaging of a jelly mould: (a) view from above across the section EF ,(b) lateral view across the median section AB , (c) frontal view across the crosssection CD.

Fig. 4. Ecoflex00-30 silicone cast of an octopus mantle: (a) frontal view,(b) side view, (c) ventral view, and (d) dorsal view.

these mantle walls are extremely efficient at recalling ambientfluid through the apertures and passively sustaining the refillphase.

B. Actuation

In order to replicate the biological propulsion of cephalopods,it is necessary to mimic the mechanism by which the slug of fluidis accelerated through the funnel and eventually gives rise to thevortex ring. In real cephalopods, the driving pressure is gener-ated by the radial compression of the mantle chamber whichprimarily occurs thanks to the shortening of the circumferen-tial muscles located within the mantle thickness [33]. This isthe very essence of the muscular hydrostat, according to whicha radial expansion of few millimeters translates into a tenfoldcompression in volume. A number of viable options have beentaken into account for the design of a suitable actuator includingelectroactive polymers and shape memory alloy.



Fig. 5. Actuator components and activation sequence. (a) Side view of the sil-icone mantle and the actuator. The numbers, respectively, refer to 1 the batteries,2 the PCL structure which joins the batteries to the gearmotor, 3 the gearmotor,4 the PCL fairlead connected to the main structure, 5 the cables, and 6 the rodadapted onto the shaft of the gearmotor. (b) Cross-sectional view of the mantleat the initial stage of the actuation sequence: the external silicone walls areundeformed and the rod is angled −90◦. (c) Same cross section as in (a) duringmaximum tension on the cables: the rod is angled 90◦ and the external siliconewalls are folded inward.

However, it is the authors understanding that the existingresearch on artificial muscles does not yet offer reliable solutionsfor replicating the functionality of the mantle, as discussed inSection II-A. On the other hand, none of the existing vortexthrusters (see Section II-B) are regarded as suitable candidatesfor implementation into a soft bodied, light-weight robot. Allthis poses significant limitation both to the adoption of standardtechnology and to the design of a suitable actuator for the scopeof this study.

Given the unsuitability of the available technology to repli-cate the exact mechanism of isotropic mantle compression, afunctional surrogate for the muscular hydrostat is developed. Inthe current prototype, the compression of the mantle chamberis executed by means of a series of cables fitted, at one end, tothe exterior of the mantle walls and, at the other end, to a rodlocated at the center of the mantle cavity. The rod is adapted tothe shaft of a gearmotor, the rotation of which cyclically pullsand releases the cables [see Fig. 5(b) and (c)]. Two gearmotors,the Solarbotics GM11A and the GM12A, have been employedfor this purpose. Reported motor specifications are provided in

TABLE ISPECIFICATIONS FOR THE 56.8:1 GM11A GEARMOTOR

TABLE IISPECIFICATIONS FOR THE 100:1 GM12A GEARMOTOR

Fig. 6. Details from the CAD of the actuator components: (a) exploded and(b) assembled view. The numbers refer to the various components as illustratedin the caption of Fig. 5.

Tables I and II. The power source for the gearmotor is sup-plied by common lithium-ion batteries. These are immersed inthe silicone in the same place where the organs are found inthe real octopus. The batteries and the gearmotor are the onlyrigid components of the whole structure. The gearmotor is heldapproximately at the center of the mantle cavity, kept in placeby a polycaprolactone (PCL) mould, see element 2 in Fig. 6(a)and (b). The ribbon-shaped PCL cast wraps around the gear-motor [element 3 in Fig. 6(a)] and reaches out to the batteries,as portrayed in Fig. 5(a). In addition, in order for the cables tobe equally pulled regardless of their position, a kind of fixedpulley is employed through which the cables are gathered [ele-ment 4 in Fig. 6(a)]. The fixed pulley is made of PCL and actsas those devices which in the nautical industry are referred toas fairleads. This is placed underneath the rod and it is held inplace by an extrusion of the PCL ribbon which joins the batteryto the gearmotor [see Fig. 6(a) and (b) for clarification]. Thefixed pulley allows all the cables to be pulled from the samespot, for the same amount of time, and to the same extent. Theactuator drives the contraction, while the resilience of the sili-cone provides the force which pulls back the cables as they getreleased by the gearmotor. In this respect, the robot differs fromthe biological counterpart in the way the mantle walls undergodeformation. In the animals, the deformation of the mantle cav-ity occurs uniformly over the cross section of the mantle, while



in the robot, the contraction is performed in a discrete mannerat distinct spots over the mantle surface, as dictated by the ar-rangements of the cables. This implies that the collapse of themantle chamber in the robot takes place through the bucklingof the silicone structure in the proximity of the cable fittings, asdepicted in Fig. 5(c).

As opposed to the current design, a continuous approachcould entail circumferentially placing one single cable at a crosssection of the mantle within the silicone thickness and coilingthe cable in a way which, ideally, would better approximatethe shortening of the circumferential muscle. However, such aconfiguration does not guarantee that buckle or wrinkling ofthe silicone mantle would not occur, given the unpredictablereactions due to the friction between the cable and the silicone.Moreover, the tension on the cable during the coiling required forachieving a certain degree of cross-sectional reduction would besignificantly higher than in the case of the radially pulled cables,because the silicone walls would have to be compressed in thedirection tangential to the cable. Finally, the capability of the sili-cone walls to return to the unstrained state would be significantlyhindered by the friction between the cable and the silicone.

The choice for a discrete, radially oriented cable arrangementis thus justified by the low torque required for the crank mech-anism to perform the collapse of the structure, as well as bythe effectiveness of the mantle to elastically recover its inflatedstate. The buckling associated with the traction of the cablesdoes not visibly disrupts the slenderness of the body nor does itprevent the overall mechanism from offering a consistent rep-resentation of the pulsation cycle of a swimming octopus. Thecomponent of the robot which practically deals with the inflowand outflow of fluid is addressed in the following section.

C. Ingestion Valves

Octopuses ingest ambient water via two valves located under-neath the funnel. These valves are sealed during compressionof the mantle chamber so that the fluid is forced through thesiphon. During expansion of the mantle chamber, the siphoncollapses and the ingestion valves are flung open, thus permit-ting the inflow and refill of the mantle (see Fig. 1). A functionalreplica of this entire process in a robot does either require to pre-cisely control the opening/closing sequence of the valves andthe nozzle or, alternatively, to exploit the passive behavior of thematerial which these components are made of.

In order to address this issue, a funnel-shaped cast of PCLwas manufactured with two openings (see Fig. 7). This com-ponent of the robot will be referred to as the mantle–siphonconjunction and serves the purpose of directing the flow fromthe mantle toward the nozzle, as well as providing the lodgingfor the nozzle and the ingestion valve. The lower aperture iscrescent shaped and acts as ingestion valve; the upper openingis designed to accommodate an elastic silicone siphon or a rigidplexiglass siphon of circular cross section (see Fig. 7). A thinfilm of silicone is glued over the crescent-shaped valve on theinside of the mantle–siphon conjunction. This silicone film getslifted during the phase of cable release, when the mantle cav-ity undergoes a pressure drop. When the cables are pulled and

Fig. 7. Prototype in its final shape comprising of the mantle with the actuatorand the mantle–siphon conjunction with the outflow orifice and the crescent-shaped ingestion valve: (a) side view, (b) frontal view, and (c) ventral view.

pressurization occurs in the mantle chamber, the silicone filmdrops down, sealing the crescent-shaped aperture.

A qualitative analysis of the working process of the valve andnozzle reveals that the effectiveness of the valve at regulatingthe ingestion of ambient fluid is partially hindered by the em-ployment of a rigid siphon. This is because, in the case of arigid nozzle, the inflow through the nozzle rather than throughthe valve is facilitated. In contrast, the combination of siliconenozzle and valve appears as a very effective solution. In thiscase, the pressure drop in the mantle cavity causes the elasticnozzle to collapse. The extent to which the collapse of the noz-zle occurs is dependent, to a large extent, on the thickness of thesilicone cylindrical wall of the nozzle. Indeed, even a minimalstrain of the nozzle-exit cross section is sufficient for the valve tobecome the preferential path of the inflow. These observationssuggest that, provided a sufficient pressure drop occurs in themantle cavity, an efficient approximation of the inflow/outflowmechanism adopted by cephalopods can be achieved by adjust-ing the valve surface area, the valve orientation, and the nozzlethickness.

IV. PROTOTYPE TESTING

In its final shape, the robot is composed of the elastic mantle,the mantle–funnel conjunction, and the internal actuator (seeFig. 7). Two ion-lithium batteries are immersed in the thickerlayer of silicone stored in the upper portion of the mantle. Apotentiometer and a switch are also included in order to com-fortably adjust the voltage across the gearmotor. By regulatingthe torque and angular velocity of the gearmotor, the frequencyof mantle compression and the force by which the cables arepulled can be varied. The robot is 160 mm long from the fore-most to the rearmost point, 95 mm wide, and 80 mm tall atthe largest cross section. The whole prototype, including elec-tronic and mechanical components, weights 333.5 g, 212.0 g of



TABLE IIISUMMARY OF THE TESTS PERFORMED

which are silicone. The rigid components thus represent onlythe 36.5%, in mass, of the whole robot.

In order to evaluate the swimming performances as acombination between the pulsation frequency and funnelcharacteristics, the velocity of the prototype is tested during itsdisplacement along a straight track in water (see Table III). Inthis set of laboratory tests, three different nozzles are employed:namely two rigid plexiglass funnels and one soft siliconefunnel. The pair of nozzles of the same material, respectively,have a diameter of 30.0 and 14.5 mm. The silicone nozzle hasa 30.0-mm diameter. Each nozzle is mounted at the orificeof the siphon–mantle conjunction and a number of runs areperformed at different pulsation frequencies. Two differentgearmotors have been employed in order to perform the testsover different regimes of pulsation frequency. The GM12A (seeTable II) was used for pulsation regimes in the range from 1 to2.5 pps, while the GM11A (see Table I), was used in the rangefrom 3 to 6.5 pps. Estimate of the expelled volume is achievedby measuring the volume of water stored within the mantlechamber when the cables are pulled (i.e., the mantle walls arecontracted), which provides a value of 59 mL. This correspondsto 17% of the whole volume contained inside the mantle cavityunder unstrained condition. In terms of mass percentage, theprototype is capable of ideally expelling as much as 18% of itsentire mass. This value is fairly close to the 10% estimated byTrueman and Packard for octopuses with their arms still attachedto the mantle [15]. From the estimate of the expelled volume, atheoretical vortex formation time is estimated to be equal to 2.8and 25, respectively, for the 30.0-mm and 14.5-mm siphons.

It is worth mentioning that, because the SUUV is not neutrallybuoyant in water, an ad-hoc float is attached to the dorsal partof the robot. The float is shaped in such a way to minimize drag

Fig. 8. Velocity profiles for (a) the 30.0-mm plexiglass and (b) silicone siphon.Values of pulsation frequency are annotated directly on the profiles. In (a), tests1–8 of Table III are presented, while in (b), tests 9–15 of Table III are presented.

at the free surface and allow the robot to travel below the freesurface. In addition, the float is allowed to slide along a wire heldat the distal sides of the tank. The wire helps the robot maintaina straight line during testing without exerting any appreciablefriction. The major drawback of this setup lies in the oscillationof the float as a result of the discontinuous accelerations of therobot. This in turn gives rise to small waves at the free surfacewhich generate a persistent noise in the recordings and, moreimportantly, slow down the robot as it gets closer to the end-wall of the tank. The test rig entails a 118-cm-long, 50-cm-deep,and 50-cm-wide tank filled with tap water. The experiments arerecorded with a 25 frames/s digital camera and then processedwith an image tracking software from which displacement andvelocity temporal profiles are extracted. These are presented inFigs. 8–10.

The complex dynamics involved in the activation of the softrobot is apparent in the set of results shown. It occurs that higherfrequencies of pulsation coincide with lower effectiveness ofthe thrusting mechanism (see Figs. 8 and 9). In this respect,the major limiting factor is represented by the mechanism ofpassive inflation of the mantle chamber. The stresses within theelastic wall of the mantle cause the expansion of the chamberwhich, in turn, drives the ingestion of ambient water. Being thismechanism purely passive, the recharge time is fixed and solelydetermined by the geometry and material characteristics of thechamber. Hence, in order for the robot to perform a functionalcycle of pulsation, the speed of revolution of the rod fitted to theshaft has to be slow enough for the mantle walls to inflate and,



Fig. 9. Velocity profiles for the 14.5-mm plexiglass siphon. Values of pulsationfrequency are annotated directly on the profiles. Here, tests 16–22 of Table IIIare presented.

Fig. 10. Average velocity as a function of pulsation frequency for all the testsof Table III.

in this way, refill the chamber. By increasing the speed of themotor, the chamber is given less time to reinflate, therefore lesswater will be available for expulsion at the following pulsation,thus depleting the thrust. In the case of the plexiglass siphon withlarger diameter, at a frequency of 6.5 pps, hardly any refillingof the mantle occurs and the robot starts to travel backward[see Fig. 8(a)], solely under the effect of the jerking of thesilicone walls. The higher average velocities obtained with the14.5-mm diameter siphon are associated with the water issuingfaster from the nozzle, because at such high values of vortexformation time (see Table III), no additional thrust is supportedby the nozzle-exit overpressure, as stated in [23] and [34]. Theinverse correlation between frequency and speed of the robotis found across the whole set of experiments and is clearlyhighlighted in Fig. 10, where the average velocity of each run isplotted against the respective frequency of pulsation.

These results also underline the importance of the interplaybetween the ingestion valve and the siphon. It is observedthat when the silicone siphon is employed, the robot travelsfaster than when the analogous plexiglass siphon is mounted[see Fig. 8(a) and (b)]. In this case, the difference in per-formances is likely attributed to a closer approximation withthe inflow/outflow mechanism of the living cephalopods. Whenthe rigid siphon is employed, the refill phase entails an inflowthrough the ingestion valve, as well as through the siphon. This,

however, is not a desirable condition since the backward flowthrough the nozzle not only destroys the boundary layer alongthe cylindrical wall of the funnel, but it also convects vorticousfluid remnant of the previous pulsation back into the funnel.This, according to Krieg and Mohseni [10], can represent a sig-nificant limitation to the onset of nozzle-exit overpressure andto the generation of additional thrust hence provided. On theother hand, the silicone siphon is always subject to some degreeof strain during mantle inflation, which partially impedes thebackflow through the nozzle. By doing so, the siphon fostersthe inflow through the ingestion valve and supports the optimalconditions for producing the additional thrust provided by theonset of overpressure at the nozzle exit.

This analysis is further supported by the comparison betweentest13 and 14 (see Table III). These two tests were both per-formed at 3.4 pps, for being the transition point between highand low frequencies. The difference between these tests lies inthe way the nozzle was fit to the mantle–siphon conjunction. Inthe first case, the nozzle was simply stretched onto the originalorifice of the mantle–siphon conjunction. In the second case, ashort plexiglass extension of the orifice of the mantle–siphonconjunction was added and the nozzle was fit onto this. Themajor difference between these two arrangements lies in theadditional flexibility of the silicone nozzle in the second case.In the first case, the nozzle is partially stretched; therefore, onlya short portion of the nozzle is allowed to collapse due to pres-sure drop in the mantle cavity. In the second case, the wholenozzle is allowed to collapse, hence better responding to pres-sure variation in the mantle cavity. The fact that, in test14, ahigher speed of the robot is recorded reinforces the argumentthat unidirectionality of the flow enhances the thrust.

V. CONCLUSION

A novel prototype of soft robot has been presented whichexploits the propulsive strategy of cephalopods to swim in theaquatic environment. In this respect, the robot is the first one ofits kind in that it combines the concepts of soft robotics with theprinciples of vortex-enhanced pulsed jet propulsion.

A significant effort has been put in designing a robot whichcould provide a first insight of the yet unexplored field of un-derwater soft robotics, as well as being used for the study ofthe propulsion of cephalopods. In order to comply with thesestringent biomimetic requirements, an unconventional manufac-turing process has been employed where a polyurethane mouldof an actual octopus vulgaris has been used for casting an iden-tical silicone replica of the original specimen.

The robot was meant to propel itself in water by contractingthe mantle chamber and expelling fluid in a discontinuous fash-ion across a nozzle, in analogy with what cephalopods do. Thecurrent prototype relies on a cable driven actuation, by meansof which several spots over the surface of the silicone mantleare pulled inward by a gearmotor located at the center of themantle cavity. The process which accounts for the refill of themantle cavity is also addressed here by designing an ingestionvalve which operates similarly to those found in cephalopods.The cable-driven actuation, along with the inertia of the silicone



mantle, and the interplay between the ingestion valve and acollapsible nozzle give rise to a swimming routine which isextremely resemblant of the biological one.

A number of tests have been performed in order to assess theactual swimming performances of this first prototype. The ma-jor outcome of these experiments is the acknowledgment of theinverse correlation between frequency of pulsation and velocityof the robot. It occurs that a fast sequence of pulsation causesthe inflation of the mantle to become less effective at refillingthe mantle cavity, which in turn is reflected in an impoverishedcapability of the mechanism to generate thrust. In addition, thedata demonstrate the importance of the interplay between thesiphon and the ingestion valve. The analysis of the velocityprofiles recorded suggests that the employment of a soft, col-lapsible nozzle may offer a significant benefit over a rigid one.This is justified by the role of the soft siphon in regulating theinflow through the valve and the outflow through the nozzle,hence promoting the onset of nozzle-exit overpressure. Indeed,these findings are in evident agreement with what is observedin real cephalopods. As far as performances are concerned, theexperiments demonstrate that a marked discrepancy occurs be-tween the robot and the actual octopuses. Early work from [15]suggests that octopuses jetting at a frequency of 1.67 pps withan expelled volume of 53 mL at 1.26 m/s can travel as fast as18 cm/s, while current experiments show that, when jetting at1.5 pps, the soft robot expels water at a maximum jet velocityof 53 cm/s and travels at a maximum speed of 4 cm/s.

The experiments performed herein are insightful of what de-sign criteria require rigorous treatment in future development.On one hand, the effectiveness of the production of thrust liesin the capability of the crank mechanism to deliver fast jets withsignificant volume of water as close as possible to the optimalvortex formation time. On the other hand, the major limitingfactor is represented by the passive inflation of the chamberduring recharge phase. The drawbacks represented by the re-fill phase being passive can be attenuated by targeted designspecifications. Because the bending moment of an elastic plateis directly proportional to the bending rigidity which, in turn,varies as the product of Young’s modulus and the thickness ofthe plate to the third power, small variations in the geometryof the mantle and its rigidity correspond to ample variations inthe refill time. The key factor in the design process hence liesin the accurate dimensioning of the composition and geometryof the mantle (i.e., recharge time) against the volume of fluidwhich can be expelled and the torque delivered by the crankmechanism (i.e., thrust generated). The authors hope with thisstudy to have posed the foundation for the development of anew exciting branch of research on what could be referred to asSUUVs.

ACKNOWLEDGMENT

The authors would like to thank Dr. M. Calisti for his valuablehelp in the motion tracking of the soft robot.

REFERENCES

[1] M. Krieg and K. Mohseni, “Dynamic modeling and control of biologicallyinspired vortex ring thrusters for underwater robot locomotion,” IEEETrans. Robot., vol. 26, no. 3, pp. 542–554, Jun. 2010.

[2] C. Zheng, S. Shatara, and T. Xiaobo, “Modeling of biomimetic robotic fishpropelled by an ionic polymer-metal composite caudal fin,” IEEE/ASMETrans. Mechatronics, vol. 15, no. 3, pp. 448–459, Jun. 2010.

[3] J. Yu, R. Ding, Q. Yang, M. Tan, W. Wang, and J. Zhang, “On a bio-inspired amphibious robot capable of multimodal motion,” IEEE/ASMETrans. Mechatronics, vol. 17, no. 5, pp. 1–10, Oct. 2011.

[4] J. E. Colgate and K. M. Lynch, “Mechanics and control of swimming: Areview,” IEEE J. Ocean. Eng., vol. 29, no. 3, pp. 660–673, Jul. 2004.

[5] D. Trivedi, C. D. Rahn, W. M. Kier, and I. D. Walker, “Soft robotics:Biological inspiration, state of the art, and future research,” Appl. Bion.Biomech., vol. 5, pp. 99–117, 2008.

[6] M. Cianchetti, V. Mattoli, B. Mazzolai, C. Laschi, and P. Dario, “A newdesign methodology of electrostrictive actuators for bio-inspired robotics,”Sens. Actuat. B, Chem., vol. 142, pp. 288–297, 2009.

[7] M. Cianchetti, A. Arienti, M. Follador, B. Mazzolai, P. Dario, andC. Laschi, “Design concept and validation of a robotic arm inspired bythe octopus,” Mater. Sci. Eng. C, vol. 31, pp. 1230–1239, 2011.

[8] M. Calisti, M. Giorelli, G. Levy, B. Mazzolai, B. Hochner, C. Laschi, andP. Dario, “An octopus-bioinspired solution to movement and manipulationforsoft robots,” Bioinspirat. Biomimet., vol. 6, pp. 1–10, 2011.

[9] M. Krieg, A. Pitty, M. Salehi, and K. Mohseni, “Optimal thrust character-istics of a synthetic jet actuator for application in low speed maneuveringof underwater vehicles,” in Proc. MTS/IEEE Oceans Conf., Washington,DC, 2005, pp. 2342–2347.

[10] M. Krieg and K. Mohseni, “Thrust characterization of a bio-inspired vortexring generator for locomotion of underwater robots,” IEEE J. Ocean Eng.,vol. 33, no. 2, pp. 123–132, Apr. 2008.

[11] A. A. Moslemi and P. S. Krueger, “Effect of duty cycle and stroke ratioon propulsive efficiency of a pulsed jet underwater vehicle,” presented atthe 39th AIAA Fluid Dyn. Conf., San Antonio, TX, 2009.

[12] A. A. Moslemi and P. S. Krueger, “Propulsive efficiency of a biomorphicpulsed-jet underwater vehicle,” Bioinspirat. Biomimet., vol. 5, pp. 1–14,2010.

[13] L. A. Ruiz, R. W. Whittlesey, and J. O. Dabiri, “Vortex-enhanced propul-sion,” J. Fluid Mech., vol. 668, pp. 5–32, 2011.

[14] F. Liu, K.-M. Lee, and C.-J. Yang, “Hydrodynamics of an undulating finfor a wave-like locomotion system design,” IEEE/ASME Trans. Mecha-tronics, vol. 17, no. 3, pp. 554–562, Jun. 2012.

[15] E. R. Trueman and A. Packard, “Motor performances of somecephalopods,” J. Exp. Biol., vol. 49, pp. 495–507, 1968.

[16] W. Johnson, P. D. Soden, and E. R. Trueman, “A study in met propulsion:An analysis of the motion of the squid, Loligo Vulgaris,” J. Exp. Biol.,vol. 56, pp. 155–165, 1972.

[17] D. I. Pullin, “Vortex ring formation at tube and orifice openings,” Phys.Fluid, vol. 22, pp. 401–403, 1978.

[18] D. Auerback, “Experiments on the trajectory and circulation of the startingvortex,” J. Fluid Mech., vol. 183, pp. 185–198, 1987.

[19] A. Glezier, “The formation of vortex rings,” Phys. Fluid, vol. 31, pp. 3532–3542, 1988.

[20] M. Gharib, E. Rambod, and K. Shariff, “A universal time scale for vortexring formation,” J. Fluid Mech., vol. 360, pp. 121–140, 1998.

[21] M. Rosenfeld, E. Rambod, and M. Gharib, “Circulation and forma-tion of laminar vortex rings,” J. Fluid Mech., vol. 376, pp. 297–318,1998.

[22] K. Mohseni and M. Gharib, “A model for universal time scale of vortexring formation,” Phys. Fluid, vol. 10, pp. 2436–2438, 1998.

[23] P. S. Krueger and M. Gharib, “The significance of vortex ring formation tothe impulse and thrust of starting jet,” Phys. Fluid, vol. 15, pp. 1271–1281,2003.

[24] J. O. Dabiri and M. Gharib, “A revised slug model boundary layer cor-rection for starting vorticity flux,” Theor. Comput. Fluid Dyn., vol. 17,pp. 293–295, 2004.

[25] J. O. Dabiri and M. Gharib, “Fluid entrainment by isolated vortex rings,”J. Fluid Mech., vol. 511, pp. 311–331, 2004.

[26] P. S. Krueger and M. Gharib, “Thrust augmentation and vortex ring evo-lution in a fully pulsed jet,” AIAA J., vol. 43, pp. 792–801, 2005.

[27] M. Shusser, M. Rosenfeld, J. O. Dabiri, and M. Gharib, “Effect of time-dependent piston velocity program on vortex ring formation in a pis-ton/cylinder arrangement,” Phys. Fluids, vol. 18, pp. 033601-1–033601-6,2006.



[28] J. T. Thompson and W. M. Kier, “Ontogenetic changes in mantle kinemat-ics during escape-jet locomotion in the oval squid, Sepioteuthis lessonianalesson, 1830,” Biol. Bull., vol. 201, pp. 154–166, 2001.

[29] J. T. Thompson and W. M. Kier, “Ontogeny of squid mantle function:Changes in the mechanics of escape-jet locomotion in the oval squid,Sepioteuthis lessoniana lesson, 1830,” Biol. Bull., vol. 203, pp. 14–26,2002.

[30] E. J. Anderson and M. A. Grosenbaugh, “Jet flow in steadily swimmingadult squid,” J. Exp. Biol., vol. 208, pp. 1125–1146, 2005.

[31] I. K. Bartol, P. S. Krueger, W. J. Stewart, and J. T. Thompson, “Hydro-dynamics of pulsed jetting in juvenile and adult brief squid Lolligunculabrevis: Evidence of multiple jet ‘modes’ and their implications for propul-sive efficiency,” J. Exp. Biol., vol. 212, pp. 1889–1903, 2009.

[32] K. Mohseni, “Zero-mass pulsatile jets for unmanned underwater vehiclemaneuvering,” presented at the 3rd AIAA Unmanned Unlimited Tech.Conf., Workshop Exhib., Chicago, IL, 2004.

[33] J. M. Gosline and M. E. DeMont, “Jet-propelled swimming squids,” Sci.Amer., vol. 252, pp. 96–103, 1985.

[34] P. S. Krueger, “An over-pressure correction to the slug model for vortexring circulation,” J. Fluid Mech., vol. 545, pp. 427–443, 2005.

[35] M. Krieg, P. Klein, R. Hodgkinson, and K. Mohseni, “A hybrid class un-derwater vehicle: Bioinspired propulsion, embedded system, and acousticcommunication and localization system,” Marine Technol. Soc. J., vol. 45,pp. 153–164, 2011.

[36] C. Laschi, B. Mazzolai, V. Mattoli, M. Cianchetti, and P. Dario, “Design ofa biomimetic robotic octopus arm,” Bioinspirat. Biomimet., vol. 4, pp. 1–8,2009.

[37] M. Krieg and K. Mohseni, “New perspectives on collagen fibers in thesquid mantle,” J. Morphol., vol. 273, pp. 586–595, 2012.

Francesco Giorgio Serchi received the M.Sc. degreein marine sciences from the University of Pisa, Pisa,Italy, in 2006, and the Ph.D. degree in computationalfluid dynamics from the Centre for ComputationalFluid Dynamics (CFD), University of Leeds, Leeds,U.K., in 2011.

He was a Marie Curie Early Stage Training Fel-low with the University of Leeds. He is currently withthe Research Centre on Sea Technologies and MarineRobotics, Scuola Superiore SantAnna, Pisa, where heis involved in the CFD-OctoProp project on under-

water bioinspired locomotion.

Andrea Arienti is currently working toward the Un-dergraduate degree in mechanical engineering at theUniversity of Pisa, Pisa, Italy.

He is working on his thesis at The BioRoboticsInstitute, Scuola Superiore SantAnna, Pisa, as partof the OCTOPUS Integrating Project (FP7, ICT2007.8.5 Embodied Intelligence). His main researchinterests include the fields of bioinspired and softrobotics, underactuated structures and design of smartactuation systems, underwater robotics, and bioin-spired propulsion systems.

Cecilia Laschi (M’00–SM’12) received the M.Sc.degree in computer science from the University ofPisa, Pisa, Italy, in 1993, and the Ph.D. degree inrobotics from the University of Genoa, Genoa, Italy,in 1998.

She is currently an Associate Professor ofbiorobotics at the Biorobotics Institute, Scuola Supe-riore Sant’Anna, Pisa, Italy. From 2001 to 2002, shewas a Japan Society for the Promotion of Science Vis-iting Researcher at Waseda University, Tokyo, Japan.She has been and currently is involved in many Na-

tional and European Union-funded projects. She has authored or coauthoredmore than 40 papers in ISI journals. She has been a Guest Co-Editor of Spe-cial Issues on Autonomous Robots of the IEEE TRANSACTIONS ON ROBOTICS,Applied Bionics and Biomechanics, and Advanced Robotics. Her research in-terests include the field of biorobotics and is currently involved in research onhumanoid and biomimetic robotics.

Dr. Laschi is a member of the Editorial Board of Bioinspiration & Biomimet-ics, Applied Bionics and Biomechanics, and Advanced Robotics. She is a mem-ber of the IEEE Engineering in Medicine and Biology Society, and of the IEEERobotics and Automation Society, where she serves as an AdCom member.

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