IOP PUBLISHING BIOINSPIRATION & BIOMIMETICS

Bioinsp. Biomim. 2 (2007) S42–S49 doi:10.1088/1748-3182/2/2/S05

Artificial annelid robot driven by softactuatorsKwangmok Jung1, Ja Choon Koo1, Jae-do Nam2, Young Kwan Lee2

and Hyouk Ryeol Choi1

1 School of Mechanical Engineering, Sungkyunkwan University, Chunchun-dong 300, Jangan-gu,Suwon, Korea2 School of Applied Chemistry, Sungkyunkwan University, Chunchun-dong 300, Jangan-gu, Suwon,Korea

E-mail: hrchoi@me.skku.ac.kr

Received 6 November 2006Accepted for publication 14 February 2007Published 5 June 2007Online at stacks.iop.org/BB/2/S42

AbstractThe annelid provides a biological solution of effective locomotion adaptable to a large varietyof unstructured environmental conditions. The undulated locomotion of the segmented bodyin the annelid is characterized by the combination of individual motion of the musclesdistributed along the body, which has been of keen interest in biomimetic investigation. In thispaper, we present an annelid-like robot driven by soft actuators based on dielectric elastomer.To mimic the unique motion of the annelid, a novel actuation method employing dielectricelastomer is developed. By using the actuator, a three-degree-of-freedom actuator module ispresented, which can provide up–down translational motion, and two rotationaldegree-of-freedom motion. The proposed actuation method provides advantageous features ofreduction in size, fast response and ruggedness in operation. By serially connecting theactuator modules, a micro-robot mimicking the motion of the annelid is developed and itseffectiveness is experimentally demonstrated.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Locomotion, which has been one of the most significantissues in robotics, becomes even important in advanced roboticapplications, e.g. locomotion in unstructured environments. Indealing with the aforementioned problems, robotic researchersmainly concentrate on mimicking natures such as humans oranimals recently. Typically, there are many reports on annelid-like robots, such as the inchworm or earthworm [1–5]. Theannelid is one of the most popular mechanisms in robotic fieldsand it is employed in various areas such as in-pipe inspectionrobots, wall climbing robots, etc, because the locomotion ofthe annelid is the most simple and effective to move in arbitraryenvironments among the lower animals [6].

However, it is almost impossible to realize a robot capableof mimicking the biological locomotion of the annelid withexisting technologies. For instance, traditional actuators suchas electromagnetic motors, pneumatic actuators etc, do notmeet the requirements as biomimetic actuators because their

intrinsic properties are truly different from those of biologicalmuscles. Shape memory alloy (SMA), often regarded as oneof the candidates for artificial muscle, has been employedfor a small sized annelid-like robot [1, 3, 4]. SMA may beadequate for the micro-robot because it has a simple actuationprinciple and structure. However, it should be noted thatSMA actuators have low bandwidth and efficiency because oftheir working mechanism, i.e. heating and cooling. Amongpotential candidates, electroactive polymers (EAP) seem tohave great potential to be a new means of actuation [7]. In spiteof the technical difficulties their application areas are rapidlyexpanding especially in robotic fields since the actuationmechanism of the polymers is similar to the human muscle.Among the various kinds of EAPs, dielectric elastomers canbe considered to be prospective because they are very softand their deformation is much greater than that of any otherexisting one. The deformation of dielectric elastomers canbe used in various ways to produce actuation [8–13]. Thestretched film-type, the rolled and the bow tie actuators are

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Artificial annelid robot driven by soft actuators

Figure 1. Earthworm [14].

Figure 2. Sectional view of the earthworm [14].

the typical configurations of the actuator employing dielectricelastomer [9]. Basically, it has the advantages of low price,light weight, ease of fabrication and adaptability to a varietyof actuator configurations which can provide the competitiveedge over the other alternatives. In this paper, an annelid-like robot, which is constructed by artificial muscle based on adielectric elastomer actuator (abbreviated as DEA afterwards),is developed. To mimic annelid motions, the actuator itselfshould have characteristics similar to the actuating scheme ofthe annelid animals. Thus, a new design of DEA actuator isproposed. It has advantageous features of reduction in size,fast response, ease and ruggedness of operation. It is noted thatthey are soft with muscle-like features and structural simplicityof the actuator capable of generating multiple degree-of-freedom motions. The proposed annelid robot is the resultof systematic researches considering these features.

The present paper is organized as follows. In section 2,biological inspirations from the annelid are addressed. Insections 3, 4, 5 and 6, the fundamentals of DEA, design ofDEA actuator, fabrication and experimental evaluations aregiven. The annelid robot is described in detail in section 7with demonstrations, and conclusions are given finally.

2. Biological inspirations from an annelid

As illustrated in figures 1 and 2, the earthworm, a typicalannelid, has two distinctive muscular systems for locomotion,both of which run the whole length of the body. Thesetwo muscles are circular muscles and longitudinal muscles,respectively. The circular muscles surround each segment and

Figure 3. The vermiculation of an earthworm [15].

longitudinal muscles run from segment to segment for theentire length of the earthworm. When the circular musclesare contracted, the diameter of the body is reduced, makingthe earthworm thin. When the longitudinal muscles arecontracted, the length of the body is reduced, making theearthworm short. The contractions of an earthworm’s musclesresemble a wave, contracting and relaxing a few segmentsat a time. Figure 3 illustrates the vermiculation of theearthworm. Traction is achieved through bristle hairs calledsetae that are distributed about the earthworm’s segments. Thesetae are imbedded in the earthworm’s longitudinal muscles;therefore, when the longitudinal muscle contracts and relaxes,the distance between the setae of different segments fluctuates.To move, an earthworm elongates its body and anchors itsanterior with its setae and pulls the rest of its body forward.The locomotion of the earthworm is very simple and allowsit to move effectively in its surroundings. Additionally, itcan move on rough terrains and creep in thin tubes or acrossobstacles because it is very flexible.

In order to reproduce these muscle combinations, a newactuation method is needed since the conventional actuationmethod cannot meet the requirements. To keep morphologicalsimilarity such as metameric structures composed of numerousring-like segments, distributed actuation is necessary with lightoverheads. In addition, kinematically it should provide thelongitudinal motion as well as rotation, which is representedas three degrees of freedom in segment space. Moreover thereshould be possibilities of manufacturing a robot with a totallynew fabrication method that enables mass production of arobot through innovative processes such as injection moldingor stamping etc. Overall, a new domain of actuation needs tobe investigated from the more fundamental and built up into adevice.

3. Fundamental principles of actuation

The basic operational mechanism of DEA has been introducedin various publications despite some limitations of theformulation [8, 9, 11, 12]. The electromechanical transductionof a pair of parallel plates is the main principle of operationof the actuator. When an electric potential is applied acrossthe elastomer film, the elastomer film is compressed along thethickness direction while expanded along the lateral direction

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K Jung et al

Figure 4. Simulated strain curve along thickness direction.

Table 1. Material properties of silicone (KE441 by Shinetsu).

Items Properties

Elastic modulus (MPa) 2Breakdown voltage (kV mm−1) 20Relative permittivity 2.8

so that a mechanical actuation force is produced. The effectivemechanical pressure in the thickness direction is derived as

σz = −ε0εrE2 = −ε0εr

(V

t

)2

(1)

where E is an applied electric field, t is thickness of elastomer,and ε0 and εr are the electric permittivity of free space andrelative permittivity of the polymer, respectively.

In order to avoid the time-dependent behavior of thedielectric elastomer actuator, pre-tension should be removedand only a pure compressive force induced by the Maxwellstress should be used for actuation. For the first step of thenon-prestrained actuator design, the deformation of dielectricelastomer caused by the Maxwell stress is calculated. Thegoverning equation should be modified for the vertical strainδz according to the compression stress σz:

t = t0(1 + δz) (2)

σz

Y= δz = − 1

Yε0εr

[V

(1 + δz)t0

]2

= − 1

Yε0εr

(V

t0

)2 (1

1 + δz

)2

(3)

δ3z + 2δ2

z + δz = − 1

Yε0εr

(V

t0

)2

(4)

where Y denotes the elastic modulus and t0 is the initialthickness.

Figure 4 shows the vertical strain δz curve according tovoltage increase about the silicone KE441 (ShinEtsu), thematerial properties of which are shown in table 1. As shownin figure 4, the estimated compressive strain amount is about

(a) (b)

(c)

Figure 5. Conceptual schematic view of the proposed actuator.

1–3.5%, although that is dependent on the material propertiesand the applied input voltage. Since most dielectric elastomersare incompressible, that is if the actuator is assumed to be athin circular disc, the radial strain δr is derived as

(1 + δr)2 (1 + δz) = 1 (5)

δr = 1√1 + δz

− 1. (6)

Approximation of equation (6) yields

δr ≈ − 12δz. (7)

Equation (7) states that the usable radial strain is only halfthe vertical strain. For this reason either material with a highdielectric constant or very high input voltage can be used toyield high actuator performance. However, neither seems tobe very practical since the polymeric materials commerciallyavailable have limited dielectric characteristics and the electriccircuit devices handling such high voltages are also limited.Therefore, a new actuating method has to be developed for thenon-prestrained actuator.

4. Proposed design of biomimetic actuator

The operational concept of the non-prestrained dielectricactuator is illustrated in figure 5. As shown in figure 5(a), athin dielectric elastomer sheet is confined by rigid boundaries.Once a compressive force is applied to the sheet it expands suchthat it induces a buckling situation on the sheet and causes itto become either convex or concave. This would create anefficient actuation without prestrain. The relation between theradius of curvature r, the angle θ and the transverse strain δa

can be derived as follows:

b = a(1 + δa) = rθ (8)

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Artificial annelid robot driven by soft actuators

Figure 6. Construction of the proposed actuator.

(a) (b)

Figure 7. Prototype of the actuator.

Figure 8. Experimental set-up.

r sin

(θ

2

)= a

2(9)

θ

sin(θ/2)= 2(1 + δa). (10)

From the Taylor series expansion, sin(θ/2) can be [θ(24−θ2)/

48], approximately. The angle θ can be derived as follows:

θ =√

24δa/(1 + δa). (11)

Table 2. Specifications of the prototype actuator for experiments.

Items Specifications

Total diameter d(mm) 5.8Effective actuation diameter dr(mm) 5.1Total thickness t (mm) 0.75Average thickness of each layer (µm) 50Stacked layers 12Total weight of actuation unit (g) 0.02

And the displacement h is to be

h = r

[1 − cos

(θ

2

)](12)

where

r =√

(1 + δa)3a2

24δa

. (13)

5. Fabrications

Figure 6 provides a schematic illustration of the non-prestrainactuator construction; its actual dimension is listed in table 2.The construction uses KE441 (ShinEtsu) silicone, which has

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K Jung et al

(a) (b)

Figure 9. Experimental results of displacement and force.

(a)

(b)

Figure 10. Frequency response of the actuator proposed.

lower viscosity than VHB4905. The spin coated elastomerfilm was coated with carbon electrodes and stacked to makemultiple layers. Each layer of dielectric elastomer has anapproximate thickness of 50 µm and the total thickness of theactuator (t) is 0.75 mm. To make an insulating area between

electrodes, both sides of the dielectric elastomer have a non-electrode area. The diameter of the membrane (d) is slightlylarger than the diameter of the fixed frame (df ) and mightcreate either a concave or convex circular membrane that couldprovide more stable control over the deformation in the desired

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Artificial annelid robot driven by soft actuators

(a) (b)

Figure 11. Response characteristic of the actuator.

Figure 12. Actuator module.

direction during actuation. Only expansion occurs in the areawith electrodes dr ; thus the actual strain δa should be calculatedfrom δr . That can be derived as

δa = δi + δr

dr

d(14)

where δa denotes a converted strain for total diameter andδi is the virtual initial strain given by the initial conditionδi = d/df − 1. δr is given by equation (6) and thevertical displacement is derived by equation (12). Figure 7shows the actual fabricated prototype of a dielectric elastomeractuator.

6. Preliminary experiments

To validate the mathematical modeling and evaluate theperformance of the actuator proposed, several experimentswere conducted. As shown in figure 8, the displacements andforces of the actuator are measured with a laser displacementsensor (Keyence) and a strain gauge sensor. A test and ananalysis have been compared in figure 9. The simulationand the experiments have shown good agreement. There isa small error between the calculated result and experimentthat might be caused by the disparity and difference inthickness of each layer, externally coated shield layer andthe fabrication process. For complete measurements of theactuator performance, the frequency response of the actuator

Figure 13. Earthworm robot.

was also tested for both displacement and force. As shown infigure 10, the soft-actuator generates fairly large displacementand force. The weight of the actuator is only 0.02 g andthe diameter is 6.5 mm (including frame) with a thicknessof 0.75 mm. Furthermore, the actuator shows fast responsetime for square wave form inputs as shown in figure 11. Thiswould indicate that the developed actuator can work for use inpractical applications.

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K Jung et al

(a) (b)

(c) (d)

Figure 14. Movie frame for locomotion : robot speed—about 1 mm s−1 at 5 Hz.

7. Artificial annelid robot

To build up an artificial creature from biological inspiration,modification and simplification are necessary to optimize thekinematic structure and the actuation system. The ultimategoal would be to develop a robotic mover with simpleand minimal components that fulfils the requirements oflocomotion. According to preliminary observation of thelocomotion of the annelid, it is noted that the longitudinalmuscle primarily contributes to the locomotion, but thatof the circular muscle is just supplementary. In addition,the undulatory robotic locomotion requires three-degree-of-freedom modules for realizing the biomimetic movement ofthe annelid at least. To mimic the longitudinal muscle,the proposed actuator is designed as a module shown infigure 12. Actuators are embedded in a PCB substratewith a pattern of electrodes for supplying the power to theactuators. An artificial annelid is constructed by connectingthe modules serially. The actuator module consists of 12actuators on both sides of a printed circuit board, whichis a patterned electric wire to supply the electric energy.This actuator module works as both a power plant for themovement and a body skeleton of the robot. In other words,the robot can be built by simple stacking of the actuatormodules without any additional mechanical structure. Theactuator module shown in figure 12 has 20 mm diameter,3 mm thickness and 0.4 g weight. In figure 13 a fully assembledinchworm robot is shown. This robot has eight actuatormodules (96 actuators). Four wires in total are used to supplyelectric power and are connected to each module. In order toconnect each module, small silicone cylinders, which have a1 mm diameter and a height between 0.2 and 0.4 mm, are used

Figure 15. The earthworm robot covered with artificial skin.

to make point–point connections between each of the modules.They are bonded by silicone adhesives. The earthworm robothas front and rear sectors, each sector having four actuatormodules. Each of the sectors is operated sequentially to createthe motion of the earthworm. Specifications of the earthwormrobot are shown in table 3. The speed of the earthwormrobot depends on the operating frequency of each actuator.Figure 14 shows four movie frames during a 4.5 s locomotion.The robot shows a speed of about 1 mm s−1 with 5 Hzactuation.

The earthworm robot is covered with artificial skin toprotect the inner organs as shown in figure 15. The skin isfabricated by a 3D molding method with a silicone CF19-2186(Nusil). The thickness of skin is only 100 µm. Additionally, ithas wrinkles at the position of each septa to help the contractionand expansion of the robot.

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Artificial annelid robot driven by soft actuators

Table 3. Specifications of the earthworm robot developed.

Items Specifications

Size (D × L mm) 20 × 45Speed (mm s−1) at 10 Hz 2.5Weight (g) 4.7Load (g) >10

8. Conclusions

This paper presents an earthworm robot that uses a novelactuator. The robot simply consists of polymer and plasticmaterials except for the electric wire, and showed thepossibility of constructing cheap robots. To construct theearthworm robot, a novel soft actuator based on a dielectricelastomer has been proposed. The soft actuator has been builtto avoid the time-dependent behavior of previous approaches.Also, it is not necessary to construct strong frames to supporta high pre-tension and the additional elastic body to keep thebalance of restoration force. Therefore, we can construct anactuator with small size, light weight and simple structure.

It not only enables efficient actuation but also makesit possible to manufacture a robot through a totally newfabrication method that enables mass production of a robotthrough processes such as injection molding or stamping. Inaddition, there are possibilities of easily mimicking the naturaland delicate motions such as animal skin motions, wrinklingand eyebrow movement without using many actuators. Thedesign of the segment addressed in this paper illustrates arealization of embedding actuators in the robot without usingcomplicated mechanisms or their substitutes.

Acknowledgments

This research was performed for the Intelligent RoboticsDevelopment Program, one of the 21st Century Frontier R&DPrograms funded by the Ministry of Science and Technologyof Korea.

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