Design of a biomimetic robotic octopus arm

DESIGN OF A BIOMIMETIC ROBOTIC OCTOPUS ARM Cecilia Laschi*, Barbara Mazzolai*, Matteo Cianchetti*, Virgilio Mattoli, Lorenzo Bassi-Luciani, Paolo Dario* ARTS (Advanced Robotics Technology and Systems) and CRIM (Centre of Research In Microengineering) Laboratories Scuola Superiore Sant'Anna, Pisa, Italy Pisa, 56127 Italy *also with the Italian Institute of Technology (IIT) Abstract This paper illustrates the rationale and the design of a robotic arm inspired by the octopus arm. The octopus arm (tentacle) presents peculiar features, like the capability of bending in all directions, of producing fast elongations, and of varying the stiffness. Such features are very attractive from a robotics viewpoint and pose demanding requirements on robot kinematics and control, and especially on the materials and the actuators. In the octopus, these unique motor capabilities are obtained thanks to the hydrostatic characteristics of the muscles and to their arrangement in the tentacle. By taking inspiration from them, we designed a robot arm completely soft and compliant, composed of muscles that can contract and that are arranged according to the geometry of the octopus tentacle, so as to reproduce the tentacle motor performance. In this paper we present the design criteria of the robotic tentacle and the fabrication of a generic tentacle muscle, based on a silicone material and on an EAP (Electro-Active Polymer). We designed the EAP with a particular geometry that increases the contraction range and force, and we show how this design, based on a special geometry for the arrangement of the muscles, allows to build the robotic tentacle. 1. Introduction The octopus is a paradigmatic example for bioinspired robotics (Beer et al., 1997; Vincent, 2005; Brooks, 1991). The octopus is an invertebrate sea animal with amazing motor capabilities and intelligent behavior (Hochner et al., 2003; Hochner et al., 2006), which appear due to the peculiar morphology of the octopus body, and especially of its limbs (tentacles). The motor capabilities of the tentacles are far beyond any existing robots, for their dexterity and for the variability of their stiffness. Based on this consideration, the authors started an investigation on the octopus tentacle (Mazzolai et al., 2007), aimed at: 1) a deeper understanding of the biomechanics, kinematics, dynamics, control, and behaviour of the octopus tentacle, and 2) new design principles for actuation, sensing, and manipulation control, for robots with increased performance, in terms of dexterity, control, flexibility, and applicability. 2. Anatomy of the Octopus Tentacle The octopus has eight tentacles, composed of muscles plus soft tissue and it lacks any skeletal structure. The octopus tentacle provides an extreme case of an arm with virtually unlimited degrees of freedom that can elongate, shorten, bend, or twist at any point (Rokni and Hochner, 2002). Each tentacle is composed of special muscles named hydrostats, whose main characteristic is that their volume is constant during contractions (Kier and Smith, 1985; Kier, 1988). The result is that if the diameter of a hydrostat increases, its length decreases, and vice versa. Presented at the Biological Approaches for Engineering Conference, held 17-19 March 2008 at University of Southampton, Southampton, UK

The octopus arms are almost entirely constructed of densely packed muscle fibers along their transverse, longitudinal, and oblique axes (Sumbre et al., 2001; Smith and Kier, 1989). Fig. 1 (a) shows a simplified scheme of the octopus tentacle muscular structure. From biological literature (Kier, 1988) it is known that torsion (twisting around the long axis) of the arm results from contraction of the external and medial oblique muscles. Elongation of the arm can be obtained by contraction of the transverse muscles, as their orientation decreases the cross-sectional area. Shortening of the arm results from contraction of the longitudinal muscles: the cross-sectional area increases, re-elongating the transverse muscles. Thus, the transverse and longitudinal muscles have a reciprocal antagonistic action. Bending of the arm can be obtained by contraction of longitudinal muscles on one side of the arm and, simultaneously, by contraction of the transverse muscle in order to resist the longitudinal compression forces caused by contraction of the longitudinal muscles (Walker et al., 2005). 3. Design of a Robotic Tentacle In the design of the robotic tentacle, we consider specifically the longitudinal muscles and the transverse muscles, and their reciprocal actions. We design the longitudinal muscles as cylindrical muscles running all along the tentacle length (Fig. 1 (b)) and we design the transverse muscles as 4 quarters of arcs, arranged on a plane, perpendicular to the longitudinal muscles (Fig. 1 (c)). The robotic tentacle is then obtained by 4 longitudinal muscles and a number of transverse muscles in parallel, whose number depends on the total tentacle length (Fig. 1 (d)). We know from biology that the tentacle has hydrostatic muscles, which do not change their volume during the contraction. The relation between the diameter of a cylinder and its length, with constant volume, is shown in Fig. 2: when L is greater than Φ, then small reductions of Φ correspond to large increases of L. In the geometry of the octopus tentacle, usually L/Φ > 1 and typically L/Φ > 10. This is very important, because small contractions of the transverse muscles can produce great elongations, passively. This affects the requirements for the muscles, which need to be highly compliant, passive elongation, and do not need to perform large contractions. 4. Design and development of an EAP actuator for the tentacle muscles We analysed many different technologies for developing an actuator to embed in our tentacle muscles and we chose an electro-active polymer (EAP) (Bar-Cohen, 2004), based on dielectric elastomers (Pelrine et al., 2002) because it allows to achieve theoretically the performances required, in terms of force and power density (even if lower than other not polymeric actuators).

(a)

(b)

(c)

(d)

Fig. 1. (a) Arrangement of the hydrostatic muscles in the octopus tentacle: the longitudinal muscles (L) extend along the whole tentacle length; the transversal muscles (T) connect the external tissues and, when contracted, make the tentacle diameter reduce and the length increase. The external oblique muscles (O) around the whole tentacle allow torsion. The central axis of the arm is occupied by the axial nerve cord (N), which includes both nerve cell bodies and axons, and it is wrapped by medial oblique muscles. (b), (c), (d) Design of the muscular elements of the robotic tentacle: longitudinal muscles (b), transverse muscles (c), and integration in an arm (d).

Presented at the Biological Approaches for Engineering Conference, held 17-19 March 2008 at University of Southampton, Southampton, UK

L/Φ = 10

V L= πΦ

L/Φ = 1

( 2)

2

(a)

(b)

Fig. 2. (a) Mathematical relation between the diameter Φ and the length L of a cylinder of constant volume. (b) Graphical representation of the relation between Φ and L for a cylinder with a volume of 3 94,200 mm . The straight lines indicate the constant ratios between the length and the diameter of 1 and 10. (c) Relative increase of length as a function of the relative reduction of the diameter. We can see that a 5% contraction of the transverse muscles gives an elongation of 10% of the longitudinal muscles, while a 10% contraction of the transverse muscles gives an elongation of 23.5%. Due to this, the speed of elongation is also higher than the speed of contraction of the transverse muscles. (c)

The mechanical driving force of dielectric EAPs derives from Coulomb charge attraction. When a capacitor is charged by application of an electric field, the electrodes tend to move closer squeezing the elastomeric dielectric material that is between them. The energy density of the electric field is:

1 u = ε rε 0 E 2 2

, where εr and ε0 are the dielectric constants and E the electric field.

dU ε r ε 0V 2 = A dz z2 where dU is first derivative of the energy of the dielectric field. This latter expression shows that better The force between the electrodes is: F = −

performance can be obtained with thin layers (low z) and large areas (high A). Since the contraction direction is perpendicular to the area of the electrodes, in order to exploit this principle, for our aims, it is necessary to stack the elementary units in order to obtain extended device able to contract along the longer axis. A new actuator structure has been thus developed; the structure is built by folding many times a thin and soft polymeric substrate (the dielectric material) covered on the 2 sides by thin metal films that work as electrodes (see Fig. 4 (a)). The polymeric material used is a particular type of silicone; it was chosen for its electrical and mechanical characteristics, by using the equation of force and after a series of characterization tests. The Fabrication of the EAP actuator for the tentacle muscles is illustrated in Fig. 3 Pictures of the working principle and of the prototype are given in Fig. 4 (b) and (c). Manufacturing processes have to be optimized, but already allow to obtain functional devices with the right design. Fig. 4 (d) shows a preliminary functional test. Data from contraction tests follow well enough a theoretical model based on the equations above, while further studies on materials can improve the actuator performance, in terms of contraction capability.

Fig. 3. Fabrication steps: the first step of the fabrication of the EAP is the molding of a film of silicone with a thickness of 300 µm. The silicone was the “ECOFLEX™ 00-30 Platinum Cure Silicone Rubber”, produced by SMOOTH-ON inc. The electrodes were built directly by sputtering a thin gold film (90 nm) onto the silicone film (300 µm). The silicone film was kept stretched during the sputtering in order to guarantee the integrity of the electrodes during the folding and in the working condition (the percentage of pre-stretching required was previously calculated). After the electrodes deposition, the film was relaxed obtaining a good gold adhesion to the substrate, as verified by scotch tape test. The structure obtained was cut, folded and embedded in a silicon matrix to obtain the final shape of a stack, able to self-contract along its long axis.

Presented at the Biological Approaches for Engineering Conference, held 17-19 March 2008 at University of Southampton, Southampton, UK

5 mm

(a) (b) (c) (d) Fig. 4. (a) The geometry of the proposed EAP. (b) Picture of a detail of thin gold film (90 nm) onto the silicone film (300 µm). (c) view of the top of the EAP prototype, showing the folding. (d) preliminary contraction tests.

5. Conclusions and future work The work presented here is part of the ongoing study, design and development of a novel robotic octopus tentacle. The prototype of one tentacle muscle, presented here, is based on a silicone material and on a new EAP actuator, characterized by a particular folded geometry aimed at increasing the contraction range and force. The EAP actuator is based on a thin silicone film sputtered with gold, which works as an electrode. The robotic tentacle has been designed considering specifically the longitudinal muscles and the transverse muscles of the octopus, and their reciprocal actions. The ongoing work is devoted to fabricate and to control the tentacle muscles with the EAP technology proposed, and to arrange them according to the biological tentacle muscular geometry. This allows to obtain a completely compliant robot body, with no rigid structures, able of fast elongation, multidirectional bending, and varying stiffness. Acknowledgments This work has been partly supported by the Italian Institute of Technology Network. References Bar-Cohen Y., Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential, and Challenges, SPIE Press, Bellingham, WA, 2004. Beer R. D., Quinn R. D., Chiel H. J., and Ritzmann R. E., “Biologically inspired approaches to robotics”, Communications of the ACM, 40(3):, pp. 30-38, 1997. Brooks, R.A., New Approaches to Robotics. Science, 253:1227-1232, 1991. Hochner B., Brown E. R., Langella M., Shomrat T., Fiorito G. “A Learning and Memory Area in the Octopus Brain Manifests a Vertebrate-Like Long-Term Potentiation”, J Neurophysiol 90: 3547-3554, 2003. Hochner B., Shomrat T., Fiorito G., “The Octopus: A Model for a Comparative Analysis of the Evolution of Learning and Memory Mechanisms”, Biol. Bull. 210: 308-317, 2006. Kier W.M., “The musculature of squid arms and tentacles: ultrastructural evidence for functional differences”. J Morphol 185: 223–239, 1985. Kier W. M, Smith K. K. “Tongues, tentacles and trunks: the biomechanics of movement in muscular-hydrostats”. Zool J Linn Soc, 83: 307-324, 1985. Kier W. “The arrangement and function of Molluscan muscle”, The Mollusca, Vol.II, 1988. Mazzolai B., C. Laschi, M. Cianchetti, F. Patanè, L. Bassi-Luciani, I. Izzo, P. Dario, “Biorobotic Investigation on the Muscle Structure of an Octopus Tentacle”, accepted for: International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC07), Lyon, France, August 23-26, 2007,. Pelrine R., R. Kornbluh, Q. Pei, S. Stanford, S. Oh, and J. Eckerle, “Dielectric elastomer artificial muscle actuators: Toward biomimetic motion,” SPIE Smart Structures and Materials, Electroactive Polymer Actuators and Devices, vol.4695, pp. 126–137, 2002. Rokni D. and Hochner B., “Ionic currents underlying fast action potentials in the obliquely striated muscle cells of the octopus arm”. J Neurophysiol 88: 3386–3397, 2002. Smith K. K., Kier W. M., “Trunks, tongues, and tentacles: moving with skeletons of muscle”, American Scientist, Vol. 77, 1989. Vincent J. F. V. Stealing ideas from nature. Deployable Structures. Edited by S. Pellegrino. Vienna: Springer, pp. 51-58, 2005.

Presented at the Biological Approaches for Engineering Conference, held 17-19 March 2008 at University of Southampton, Southampton, UK