A Soft Modular End-Effectorfor Underwater Manipulation.

Domenico Mura1, Manuel Barbarossa1, Giacomo Dinuzzi1,Giorgio Grioli1, Andrea Caiti2,3, Manuel G. Catalano1

Abstract—Current underwater end-effector technology haslimits in terms of finesse and versatility. Because of this, theexecution of several underwater operations - as archaeologicalrecovery and biological sampling - often still requires directintervention of human operators, exposing them to the risksof working in a difficult environment. This paper proposes thedesign and implementation of an under-actuated and compliantunderwater end-effector, which embodies both grasp capabilitiescomparable to those of a scuba’s real hand and the large graspingenvelope of grippers. The proposed end-effector, whose design isbased on the the Pisa/IIT SoftHand, is composed of a modularwatertight chamber with pressure compensation - hosting theelectronics and motor - and of a set of two soft terminal devices -one with an anthropomorphic hand form and one with a gripper-like form, respectively for medium/small and large object manip-ulation - which implement an adaptive grasping function. Thesedevices have been tested in laboratory to withstand a pressure of50 bar without damage nor degradation in performance and arereadily interchangeable through a custom fast toolchange system.The two parts are connected via a magnetic drive coupling totransfer actuator torque to the wet fingers. Field results, obtainedwith the end-effector controlled underwater by a human operator(10 meters depth), show good grasping performance in bothdexterity and force tasks. Moreover, preliminary lab testing showsthe possibility of implementing basic - yet meaningful - intrinsicforce sensing for the reconstruction of basic grasp interactions.

I. INTRODUCTION

Performing underwater fine operations with current roboticmanipulation technology is still a very challenging task. Incontrast to human divers, ADSs (Atmospheric Diving Suits),ROVs (Remotely Operated Vehicles) and AUVs (AutonomousUnderwater Vehicles) can readily and safely access the depths,yet they demonstrate limited manipulation abilities. Com-mercially available underwater systems are designed to per-form simple and heavy mechanical work (i.e. construction,or pipeline maintenance). Generally, they use claw-like end-effectors, capable of opening/closing and wrist-rotation move-ments only. Such technological limit keeps any deep water1

fine manipulation task still a goal on the horizon. This isparticularly true for underwater archaeological recovery andbiological sampling. However, these fields presents imperativetasks, some of which are described below.

1Dept. of Soft Robotics for Human Cooperation and Rehabilitation, IstitutoItaliano di Tecnologia, Genoa, Italy

2Centro di Ricerca “E. Piaggio”, Univ. di Pisa, Pisa, Italy.3Dept. of Information Engineering, Univ. di Pisa, Pisa, Italy.Correspond to: manuel.catalano at iit.it1With the term “deep water” we address here any sea region which lies

beneath diver depth, i.e. 60 m ca.

Figure 1. The developed system grasping a coin underwater.

Shipwrecks below diver depth presents an unusual, moreprimitive view on ancient cultures. Historical data indicatethat the seafloor far offshore contains 20%-23% of all wrecks[1], preserved for possibly thousands of years like in a timecapsule. Deep near-shore waters, in particular, hold a highnumber of shipwrecks containing well-preserved artifacts [2].Biological sampling operations find a typical application incoral reef studies. Reefs occurring at depths greater than 30 m,which are protected from both human and natural disturbances[3], remain dramatically under-studied when compared toother highly diverse habitats. Another example consist of theunexploited biomasses identified in the mesopelagic zone (i.e.the water column between 200 and 1000 m.) [4]. This largelyunknown zone is known to play a significant role in the globalcarbon cycle and, if exploited at sustainable levels withoutimpacting upon biodiversity, this biomass could be used toproduce more high quality ingredients for human food chain.Biologists work to modify research design, collection methodsand tools to best suit their needs, as delicate collection,manipulation, or measurement of deep sea organisms. Thecapability of performing these tasks in situ would open upa vast potential to moving forward the understanding of seaecology - and of its modifications due to climate change.

Soft robotic hands and grippers appears to be ideally suitedfor this kind of operations: they grasp and manipulate delicate,complex objects by conforming to the object shape and

(a) (b) (c) (d) (e)

(f) (g) (h) (i) (j)

Figure 2. State of the art in underwater manipulation. (a-f) Complex postures of the hand performed by scuba divers in archaeological recovery (a2 - b3),biological sampling (c4 - d5) and offshore IRM (e6 - f7). (g8) A modern ADS pincer-like end-effector. (h9 - i10) Typical deep water ADS operation. (j11)Extra-vehicular activity (EVA) conducted underwater with pressurized gloves.

distributing grasping forces. Soft systems also offer improvedsafety in interacting with humans and animals, because of theirnatural compliance and backdrivability [5]. As a matter offact, among the few underwater robots currently capable ofarchaeological recovery (e.g. the OceanOne by Khatib et al.[6], the TRIDENT I-AUV [7], [8] and the MARIS ProjectAUV [9]) or biological sampling (e.g. [10]) soft grippers forspecimen manipulation are employed. However, the richnessand complexity of the sensory and motor functions of thehuman hand still remain unmatched by current underwater softgripper prototypes.

This paper is organized as follows. After addressing theproblem in Sec. II, Sec. III presents the proposed solution,describing the manipulation system mechanical design inSubsec. III-A and the fast toolchange system design in Subsec.III-B. Sec. IV presents a simplified system model for controlpurposes, while Subsec. IV-A focuses on the theory usedto enable a simple fault detection procedure. Experimentalvalidations are presented and the obtained results discussedin Sec. V. Finally conclusions are drawn in Sec. VI.

II. PROBLEM DEFINITION

Underwater archaeology consists mostly in locating, mon-itoring and preserving archaeological sites. Yet, sometimes,cultural heritage artifact rescue from such sites becomesmandatory, either for preservation or to prevent dispossession.

2www.megalehellas.net 7www.antwerpunderwatersolutions.com3www.nottingham.ac.uk/archaeology/underwater4www.livingoceansfoundation.org 10adas.org.au5www.calacademy.org 6www.airliquide.com 8www.nutyco.com9www.oceanworks.com 11www.nasa.gov

Another typical task is preventive conservation, that aim toprevent or reduce potential damage to artifacts. As withland archaeology, recovery and conservation techniques areessentially manual, relying on simple equipment and operatordexterity to handle man-made objects with different shapes,such as jewelry, dishes, weapons, and tools. In underwaterbiological sampling, most specimens consist in soft-bodiedand fragile organisms, like anemones, sponges, and corals. So,scientists need to approach them with the greatest possiblecare. Regarding e.g. coral sampling, the most sustainablemethod is to manually collect loose fragments already brokenoff the parent colony, or to gently snip a small branchoff the parent colony. More complex underwater biologicaloperations consist in tissue biopsy techniques, which usesyringes, fluorescence measurements with lamps and lasers,and RNA stabilizer application. Some of these tasks arerepresented in Fig. 2 (a-d). In general, many other underwateroperations require a gentle grasp or working with tools madefor the human hand, as in subsea IRM (Inspection, Repair andMaintenance) of offshore structure components, e.g. pipelinesand umbilicals (see Fig. 2 (e-f)).

The industrial nature of existing underwater robotic manip-ulation technology does not allow to perform such fine activi-ties: attempt at object grasping often results in damage throughunwitting application of excessive contact forces, which intro-duces expensive delays or complete failure in task execution.Human skills are still needed, and mostly provided by scubadivers in shallow water, or by ADS operators, equipped witheither unpractical pressurized gloves or primitive lobster-likeclaws, as presented in Fig. 2 (g-j). These approaches presentrisks for human life and are highly inefficient.

(a) (b)

Figure 3. a) The developed system scheme. Interchangeability of the terminal devices, allowed by the magnetic coupling drive and the toolchange system, isexploited. Tool components are highlighted in blue and control components in red. b) Core system and terminal devices sections. Components are numeratedas follows: 1a) SoftHand terminal device, 1b) Soft Gripper terminal device, 2a-2b) inner and outer magnetic coupling disks, 3) tool coupling device, 4) ferriticstainless steel ring, 5) tool pulley, 6) superior plate, 7) cast acrylic tube, 8) magnetic encoder group, 9) PCB, 10) central support, 11) motor, 12) inferiorflange, 13) inferior end cap, 14) cable penetrator hole, 15) motor coupling device, 16) motor coupling device springs.

III. PROPOSED SOLUTION

The Pisa/IIT SoftHand [11] is an under-actuated, adaptivesoft robotic hand, which is a flexible-joint robot [14], i.e.its compliance is concentrated in the joints. It is designedwith the purpose of being robust and easy to control as anindustrial gripper, while exhibiting high grasping versatilityand a form factor similar to that of the human hand. It has19 joints, but needs only one actuator to activate its graspingcapabilities. Such simplification is enabled through the theoryof adaptive synergies, resulting in a series of considerableadvantages both for control and design simplification. ThePisa/IIT SoftHand implements one adaptive synergy, actuatedwith a transmission system that uses one tendon, pulleys, anda single gear motor. The SoftHand demonstrates excellentgrasping skills in many different situations: in [11] a totalof 107 objects of different shape was successfully grasped bythe hand controlled by a human operator. Examples includea bottle, a pen, a cup, a hammer, a book, coins, etc. Thisgrasping capability and the soft robotic mechanical design ofthe SoftHand appear to well suit fine underwater operations,considering in particular that i) the tendon-driven design itselfhas already been proven reliable in underwater use (as in[6], inspired by [12]), ii) the SoftHand joints are able towithstand the even severe disarticulations hydrostatic pressurecould provide without losing their adaptivity (see e.g. Fig. 15in [11]), and iii) no closed spaces are present in the graspingmechanism, so no pressure differential exists which can causedeformations and ruptures.

Thus we propose a solution based on the SoftHand tech-nology, i.e. a set of two soft terminal devices, to be usedin a waterproof end-effector. Such soft devices present i)the original SoftHand anthropomorphic hand form and ii) a

gripper-like form with middle phalanges longer than the hand.They are intended for medium/small and large object graspingrespectively. The soft gripper terminal device is designed hav-ing in mind an industrial diving scenario. In this environment itcould be used to stabilize a ROV/AUV by firmly catching holdto a structure in a docking procedure. Then a second robot armcould employ the SoftHand to perform more fine maintenanceoperations. Finally, more complex underwater manipulationtasks could require an operator/robot to use both the developedtools (e.g. if a robot employed in an archaeological operationhas to remove large debris in order to access the smallerartifacts to be retrieved with finesse). So we designed a customtoolchange system in order to enable fast and simple switchingbetween different terminal devices.

A. Design and Implementation

The system design concept, shown in Fig. 3a, is a modularsystem composed of a waterproof servo-actuator core unit, aset of soft terminal devices readily interchangeable through atoolchange system, and an optional waterproof handle devicefor use by a human operator (e.g. as an ADS end-effector).

One of the main design issues in submarine operationsis electronic waterproofing. Two common techniques are:1) building a water and pressure tight enclosure, to placeelectronics in it; 2) encasing of all the electronics in someresin, a methodology known as potting. The first approach waschosen for the core module, as it allows easier modificationand intervention on the system. Thus we designed a watertightenclosure that constitutes the central body of the underwatermanipulation system. A section of the complete assemblyis presented in Fig. 3b: its maximum diameter is 95 mm,its max length is 170 mm and its mass about 2 kg. Theenclosure components consist of a cast acrylic tube (7), an o-

(a) Tool A (SoftHand) (b) Dis-engagement (c) Switch (d) Engagement (e) Tool B (gripper)

(f) (g) (h) (i) (j)

(k) (l) (m) (n) (o)

Figure 4. The toolchange procedure phases (a-e) and the respective toolchange mechanism configuration in the longitudinal (f-g) and frontal (k-o) plane.Note the mechanism spring (in yellow) position during the procedure. Springs, tool coupling device and motor coupling device are depicted with the samecolors of Fig. 3b. The toolchange system flanges are depicted with different colors (light blu and green) to remark their different position.

ring sealed flange (13), and openings for cable penetrators (14)to allow communication/electrical power supply from the outerenvironment. To further protect the enclosure from hydrostaticpressure the hull was filled with mineral oil, thus reducing thepressure excursion between inner and outer environments.

A second design choice is related to the necessity oftransmitting the motor torque from inside the watertight en-closure to the outer underwater surrounding where the wetmanipulation system interacts with the objects to be grasped.A waterproof shaft sealing between those two environmentsis not trivial to obtain: the rotating motion of the shaft cancause momentary pathways for fluids to leak through and, withenough pressure/depth, water will eventually penetrate pastthe seal and into the motor housing. A more reliable solutionto this issue is to make the dynamic coupling between theactuated rotor (motor) and the operative rotor immaterial. Thisis obtained by using a magnetically coupled drive consisting oftwo different sets of magnets, one on the motor side and one onthe tool side. Magnetic couplings (MCs) are currently prizedfor their effectiveness when submerged in water for two mainadvantages [13], [14]: i) the technology is intrinsically a torquelimiter that can help saving the system electromechanics. Thecoupling will slip in the case of a severe rupture of the loadtorque. Moreover, this failure mode can be easily detectedthrough voltage and current monitoring of the motor (see

also Subsec. IV-A); ii) they can be manufactured as dust-proof, fluid-proof and rust-proof, engineered to handle extremeoperating conditions. Even if such coupling renders the devicemore heavy, this is less relevant in an underwater environment.Considering thus the trade-off between the coupling/deviceweight and its functionality, the design choice opted for a disc-type MC12 (2a and 2b) capable of transmitting a torque of 1Nm with a gap between its two plates of about 6 mm.

The actuator (11) is a 12 V DC gear motor Maxon DCX22 with 83:1 gear ratio. For motor position control we usetwo magnetic encoders (8). The encoder magnets are placedon two gears, with a different number of teeth, connectedto a third gear mounted on the motor shaft. The combinedreadings of the two encoders enable the reconstruction of theabsolute position of the motor shaft over a range of severalturns thanks to the different gear ratio between the motor andthe two sensors. Preliminary experiments proved that bothi) being flooded in mineral oil and ii) proximity with thecoupling magnetic field do not interfere with the sensors.Encoders and actuator are connected to the SoftHand PCB,which communicates with the outer environment by meansof the cables passing though the penetrator. The employedbus carries power/ground and two lines which implements an

12By MagneticTechnologies, http://www.magnetictech.com.

(a)

0 2 4 6 8 10 12

Motor Angle q (rad)

400

500

600

700

800

Re

s.

Fre

q.

(ra

d/s

)

R(q)

max R

= 737 rad/s

min R

= 656 rad/s

(b) (c)

Figure 5. a) Two-inertia representation of the system and analyzed oscillator subsystem. JM and JL are respectively the combined motor/first coupling halfand second coupling half/load inertias; τM , τC and τL are the motor, coupling and load torques; q and qL are the motor and load angles; KC and KL are thecoupling and load torsional spring stiffness coefficients. b) Resonance frequency ωR(q) (in red) of the oscillator of Subfigure 5a with respect to q. c) Depictionof the SoftHand terminal device exerting a lifting force equal to F (lift force value definition).

RS485 communication (see [15] for details).Thanks to the manipulation system modular design and to

the immaterial coupling provided by the MC, various terminaldevices (robotic hands, grippers and/or tools) can be readilyconnected to the outer motor shaft by using a custom-madetoolchange system (described in the next subsection). Thedeveloped core system, along with the two terminal devicescurrently designed, are shown in Fig. 3. The maximum lift,pinch grasp and power grasp forces exerted by the SoftHandare respectively 400 N, 20 N and 76 N. The lift force valuerefers to the force that the robotic hand/gripper exerts in orderto hang a corresponding weight, as in Fig. 5c. The grippermaximum lift force results in about 150 N.

B. Toolchange System

The toolchange system consists, essentially, in a snap mech-anism between the motor group and each tool group. A coupleof springs (16) is fastened to the motor group coupling device(15). Such springs realize the snap mechanism, depicted inFig. 4; placing them on the motor group make a unique motorcoupling mechanism suitable for a set of tools, each equippedwith its MC half. The coupling of different tools is renderedfast and simple thanks to both the MC magnetic attractiveforce and the spring load, while the uncoupling is possible bymeans of a custom made tool housing. Once the motor groupis positioned and ensured inside the housing, the spring work(i.e. the snap mechanism between tool and motor group) isunactivated. In such configuration the tool group (3) resultsfastened to the tool housing, both longitudinally and axially.In particular the longitudinal fastening is accomplished bya set of magnets that connects to a ferritic stainless steelring (4) fastened to the MC half residing in the tool group(2a); the same magnets block the tool MC half, allowingto maintain the tool angle of a previously uncoupled toolwhen recoupling with it. The axial fastening makes use ofspecial guides obtained in the tool housing which act as aprismatic joint for the tool group. So the motor group canbe uncoupled by the tool group - that remains secured tothe tool housing - and retrieved by the operator/robot simplyby applying an axial force on it. The coupling/uncoupling

procedure is schematically depicted in Fig. 4, along with thetoolchange mechanism configuration in each phase.

IV. MAGNETIC COUPLING CONTROL

MCs can be modeled by a two-bodies system (see e.g. [16])where a connection, due to a magnetic spring element, exists.Such an elastic element may cause oscillations during positioncontrol that could, in turn, degrade control performance. Thishigh-compliance characteristic is known to represent a possibledrawback when applied to robot motion control, and can limitthe use of magnetic coupling to systems with a relatively low-bandwidth dynamic transients. Here, we verify that the latteris our case, i.e. oscillations provided by the MC are negligible.

Consider the representation of our device shown in Fig. 5a,where the recalled two-bodies model has been adopted for theMC. It is to point out that this is a roughly simplified model: nodamping nor feedback control, which are obviously presentsin the real system, are taken into account. In this manner, weare considering the worst operative conditions. The couplingbehaves as a nonlinear soft torsional spring that, if linearizedabout the origin as in [16], has a stiffness coefficient KC,lin =pTmax, where p= 10 is the number of coupling magnetic polesand Tmax is its maximum torque. The soft device equivalentstiffness coefficient is non-linear and depends on the actualmotor angle q. Its linearized expression gives KL,lin(q) = K1+K2q, where K1 and K2 are numerical parameters, previouslyidentified (see eq. (2) in Subsec. V-A). Such a varying stiffnesscoefficient is taken into account in the simplified subsystemalso shown in Fig. 5a, which is an harmonic oscillator withimpulse response/transfer function:

H(s) =Θ(s)F(s)

=1JL

1

s2 +KC,lin+KL,lin(q)

JL

; (1)

Refer to Table I for the numerical values of the quantitiesdefined here. Eq. (1) is evaluated for each q0 ≤ q ≤ q?

where q0 = 0 and q? are the motor angles that correspondsrespectively to an open hand and to the hand max closure.

The resonance frequency ωR(q) =√

KC,lin+KL,lin(q)JL

of the eq.(1) function is computed for increasing values of q and its

Figure 6. The terminal device deformations against sea stones and a table.

trend is shown in Fig. 5b. It can be seen, from such a figure,that even in the worst conditions where the damping of the realsystem is neglected i) the lower ωR(q) results in ' 650 rad/s,so employing the motor under this frequency will provide nosensible oscillations in position control, and ii) that ωR(q)itself is almost constant, so it can be filtered easily.

A. Fault Detection

As recalled in Subsec. III-A, one of the advantages of drivetrains incorporating magnetic gears/couplings is its torquelimiting nature: an excessive torque results in a pole-slippingcondition, as opposed to the mechanical damage that is char-acteristic of normal gearbox systems. However, from a servocontrol perspective, the effect of a magnetic gear pole-slippingis a loss of control of the load/tool. Consequently, it is ofinterest to have a pole-slip detection method.

This can be done by analyzing the absorbed motor currentIm. The algorithm used to identify the pole slipping is based onthe grasp force reconstruction algorithm presented in Subsec.V-A. This detection could trigger abort/survival modes, or asoftware procedure to reset automatically the motor angularposition at rest q0, so to recover the tool functionality.

Pole-slipping can be induced even from the tool side ofthe magnetic coupling, e.g. if a traumatic event occurs at theterminal device fingers. However the finger robust, compliantnature helps preventing such event. The robust design of bothterminal devices is shown in Fig. 6, were various kinds offinger deformations are reported.

V. EXPERIMENTAL RESULTS

The developed end-effector was validated carrying out bothlaboratory and field testings, which are presented in thefollowing subsections.

A. Grasp Force Estimation

To aid the operator and/or the automated control systemof a ROV/AUV a disturbance observer [17] can be used forgrasp force estimation. Consider the following motor dynamicequation:

Jnq = KtnIre f − τd = KtnIre f − τmodel− τint =

= KtnIre f − τmodel−J(q)T wext ;(2)

where q, Ktn, Ire f and Jn denote the motor angular acceleration,torque constant, current and inertia, respectively, J(q) is themanipulator Jacobian that depends on motor position q andwext represents the wrench exerted by the end-effector onthe environment (see e.g. [18]). The disturbance torque τdcombines all the internal and external disturbance torques andis assumed to be formed by two main components: the torqueneeded to close the hand τmodel when there is no interactionwith the environment (i.e. without grasping any object) and theinteraction torque τint . So the identification procedure aims atobtaining an estimate of τint , relating it to the current absorbedby the motor. To do so, a calibration procedure is performedto identify the term τmodel , which can be decomposed intothree components: the elastic torque generated by the handtendons during closure τte, the gravitational effect τg, and thefrictional torque τ f , which is mostly due to friction in theactuator gearbox. Considering that τg can be mostly neglected,especially for underwater operations, modeling τ f with aHayward-Dahl friction model [19], τmodel is defined as:

τmodel = Dq+K1(q−q0)+K2(q−q0)‖q−q0‖++K f (q−q f );

(3)

where qo is the motor angular position at rest (hand open), qis the motor velocity, D is the viscous damping, K1 and K2 arecoefficients of a simplified elastic model (τte = Ke(q− q0) 'K1(q−q0)+K2(q−q0)‖q−q0‖), K f is related to the frictioncoefficient and q f is the adhesion point of the Hayward model.

Supposing that the hand does not come in contact withthe object to be grasped (i.e., τint = 0 in eq. (2)) during thecalibration procedure, the hand model torque can be computedfrom the motor current and its motion response:

τmodel = KtnIre f − Jnq; (4)

Such a calculation requires motor current and accelerationsensing. The latter would be sensitive to noise if computedfrom feedback position differentiation, so the equivalent con-tribution determined by the PID that controls the actuator iscomputed in feedforward instead. To identify the parametersof eq. (3), the hand was driven with several velocity referencetrajectories from the fully open to fully closed position andreverse. Next, the resulting current, position, and velocityprofiles are used to estimate the parameters (their numericalvalues are reported in Table I) by means of conventional leastsquares identification algorithm. This leads us to the estimateddisturbance torque τmodel and the following equality holds:

τmodel ' KmodelIcal ; (5)

where the calibration current Ical , which is the current themotor needs to close the empty terminal device, can nowbe determined. Taking Ical as the current offset, experimentswhere the hand grasps various objects were carried over. Nowwe can establish the following relation:

τint ' Kint Ires ' J(q)T wext ; (6)

(a) (b)

(c) (d)

(e)

(f)

Figure 7. Current residuals obtained during underwater grasping tasks (a-d) with the shown soft object (e) and stiff object (f). Object dimensions are 60x60x120mm. Subfigures a) and c) refers to the SoftHand terminal device, subfigures b) and d) refers to the soft gripper. Both measured (in blue) and saturated (foranalysis simplification, in red) current residual are reported. The soft gripper adsorbs more current than the SoftHand, by virtue of the former longer phalanges.

0 5 10 15 20 25 30

time [sec]

-500

0

500

1000

1500

2000

cu

rre

nt

[mA

]

Pole Slipping Detection

misured current res.

saturated current res.Pole Slipping Event

Figure 8. Fault detection experiment with the soft gripper. The excessivemotor adsorbed current Im that provokes the pole-slipping condition at t =14 s results of about 1500 mA. Both measured (in blue) and saturated (foranalysis simplification, in red) motor current are reported. Saturation thresholdof ± 200 mA is depicted as a black dashed line. Compare this figure with Fig.7, where the same experiment is performed without excessive torque demand.

where Ires = Im− Ical is the current residual measured by thecalibrated motor and is recognized as the current that the handneeds to close and grasp an object (recall that Im is the totaladsorbed motor current); so it is zero if the hand closes emptyand is proportional to the resistance it encounters grasping anobject, e.g. to the grasped object stiffness. This behavior isshown in Fig. 7: for the same closure reference and objectdimension the current residual is larger when the graspedobject is stiffer, denoting a larger interaction force, and revertsto its empty closed hand value (' 0) if the object is pulled out.

In order to exploit the effects of pole-slipping, experimentssimilar to those presented in Subsec. V-B were conducted,when applying a load torque demand that is beyond thecapability of the magnetic coupling (which could happen ifan over-aggressive control action is applied to the motor).Fig. 8 shows the resulting Im with a dead zone threshold. Theexcessive load torque occurs when a Im ' 1500 mA is appliedat t ' 14 s: as expected a discontinuity on Im is observed.

Then the motor stays still until t ' 20 s, when the hand returnsopen. However now Im results quite different compared to itsopen hand original value (which is set to be ' 0 mA after acalibration procedure), i.e. before the pole-slipping occurring.If we compare Fig. 8 with Fig. 7, where Im is shown in caseof successful grasps, it is clear that an uncoupling can bediagnosed through this current monitoring. In this manner asimple fault detection algorithm without any sensor on the loadside can be implemented.

B. Laboratory Experiments

To get an estimate of the end-effector grasping force,laboratory experiments with the estimation algorithm of Sec.V-A have been carried: after calibration, different (stiff orsoft) objects are grasped and released by the system insidea water tank. The current residual resulting from these tasksis recorded and reported. An example of the grasp force exper-iment process and its resulting current residual is presented inFig. 7, where the expected current variations between objectsof different stiffness (also shown) can be seen. The experimentis performed for both terminal devices. A rubber foam block(Young modulus ' 0.015 GPa) is used as soft specimen, whilea small aluminum bar (Young modulus ' 69 GPa) representsthe stiff specimen. The two object shape and dimensions arevery similar. During the grasp phase the SoftHand yields amean current residual of ' 250 mA for the soft object and of' 320 mA for the stiff one. Regarding the soft gripper, thesame quantities results respectively in ' 400 mA and ' 470mA. As expected, the current residual is proportional to i) thegrasped object stiffness, i.e. to the total grasp force exerted bythe end-effector, and to ii) the grasp Jacobian of the contact,which is larger for the soft gripper, by virtue of its longermiddle phalanges.

Moreover, laboratory experiments of the developed fast

toolchange system have been carried out: a completetoolchange procedure is shown in Fig. 4.

Manipulation task of big objects, difficult to grasp with theSoftHand, have been simulated employing the soft grippercontrolled by a human operator. Such tasks are shown in thefirst two rows of subfigures of Fig. 10.

Finally we performed a pressure test to better assess thepressure-tolerant behavior of our mechanical structure. Thetwo terminal devices underwent a static depth test in a pressurechamber (provided to us by a company specialized in underwa-ter robotic R&D, the Graal Tech Srl in Genoa). We inspectedthem after the test and they showed no sign of degradation ordamage after being subjected to a pressure of 50 bar (' 500m depth). Their performance too appeared to be unaffected bythe pressure test, as we were able, rightly after removing thedevices from the chamber, to grasp even complex objects withthe SoftHand one (thanks also to the fast toolchange system).Video footage of the test is shown in the attached multimediamaterial.

C. Field Experiments

Underwater experimental validation of the system has beencarried on at various depths with the two terminal devicesdepicted in Fig. 3. The manipulation system was controlledby a human operator, similarly as in [11]. In Fig. 9 and 10field experiments at a depth of ' 1 m are presented, employingrespectively the SoftHand and soft gripper device.

In such experiments, considering the SoftHand terminaldevice, archaeological recovery (Fig. 9 (a-j)) and biologicalsampling (k-t) operations were simulated, along with a forceoperation (Fig. 9 (u-y)). The end-effector successfully graspedcomplex objects like stones, vase shards, coins, reproducedcoral shards and reproduced aquatic plant stems, with theoperator reporting a positive feedback about the ease of use. Itcan be seen, from Fig. 9, that natural, intuitive and human-likegrasping gestures were achieved for every object. In the samefield experiments several grasping tasks were simulated withthe soft gripper (similarly to the SoftHand). Results are shownin the last panel row of Fig. 10.

Another field experiment, taking place in the sea nearPisa (Cecina location) at a depth of ' 10 m, is presentedin the attached multimedia material. There, two professionalscuba operators employ the soft hand terminal device via themechanical handle. They perform several manipulation tasks,similar to those of the 1 m depth experiment. The goal of suchan experiment is to point out that the manipulation tasks areaccomplished with ease by the scubas, although it is the firsttime they use the device.

VI. CONCLUSIONS

In this paper a new under-actuated and compliant robotic un-derwater end-effector, based on the technology of the Pisa/IITSoftHand, has been designed and developed. The manipulationsystem has a modular design that consists of: i) a watertightchamber with pressure compensation, hosting the electronicsand motor, ii) a set of two soft yet robust terminal devices with

Parameter Value Parameter Value

D 0.7719 mNms Jn 2.44 Nms2

K1 0.2619 mNm KC,lin 14 Nm

K2 0.0002 mNm JL 3.25×10−4 Nms2

K f 1614 mNm Ktn 9.18×10−3 Nm/A

Table INUMERICAL VALUES OF SECS. IV AND V-A PARAMETERS.

the form factor of a SoftHand and of a gripper implementingan adaptive grasping function, iii) a custom fast toolchangesystem, and iv) a mechanical handle for human control ofthe end-effector. Moreover, the system terminal devices arereadily interchangeable through a magnetic drive coupling thatconnects i) and ii), making the dynamic coupling betweenthe actuated-operative rotors immaterial. This modular natureand the custom toolchange system opens the possibility ofa simple use by an ADS operator or by a ROV/AUV roboticarm, and will allow, in the future, to extend the set of tools thatcan be mounted by including also power tools and the like.A grasping force estimation algorithm has been tested withvarious objects through preliminary lab experiments, alongwith a fault detection procedure. Laboratory testing assessedthe pressure-tolerant nature of the two terminal devices, whichwere able to withstand a pressure of 50 bar without visibledamage nor degradation in performance. Field results, ob-tained underwater with a human operator up to a depth of' 10 m, demonstrate that this new robotic end-effector couldbe well suited for those operations requiring a fine, adaptablegrasp (e.g. archaeological recovery and biological sampling)that are currently challenging to be performed underwater. Inthe authors’ view this could aid to remove human operatorsfrom those underwater tasks that still require their directintervention, i.e. from the risks of working in such difficultenvironments.

VII. ACKNOWLEDGMENT

We thank Andrea Caffaz and Enrico Fontanesi at Graal TechSrl13 for their help in the pressure chamber tests, and for theirgeneral comments that greatly improved the manuscript.

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(a) (b) (c) (d) (e)

(f) (g) (h) (i) (j)

(k) (l) (m) (n) (o)

(p) (q) (r) (s) (t)

(u) (v) (w) (x) (y)

Figure 9. Experimental validation of the end-effector with the SoftHand terminal device at a depth of ' 1 m. First four rows of subfigures show dexterousoperation and the last row shows a force operation. In particular: (a-e) archaeological recovery, grasping a coin; (f-j) archaeological recovery, grasping vaseshards; (k-o) biological sampling, grasping a reproduced coral; (p-t) biological sampling, grasping a reproduced aquatic plant stem; (u-y) force operation,turning a valve. First column subfigures show grasped object dimensions and for every experiment.

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