IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 10, NO. 1, FEBRUARY 2005 77
Design and Fabrication of a LocomotiveMechanism for Capsule-Type Endoscopes Using
Shape Memory Alloys (SMAs)Byungkyu Kim, Sunghak Lee, Jong Heong Park, Member, IEEE, and Jong-Oh Park
Abstract—Endoscopes are medical devices to diagnose var-ious kinds of diseases throughout the whole gastrointestinaltracks. Generally, they are divided into conventional push-typeendoscopes and more recently developed wireless capsule-typeendoscopes. The conventional endoscopes cannot reach the smallintestines and generate pain and discomfort to patients due to thestiffness of their body. Such disadvantages do not exist in wirelesscapsule-type endoscopes. However, commercialized capsule-typeendoscopes move passively by peristaltic waves (and the gravity),which makes it impossible for doctors to diagnose the areas ofhis or her interest more thoroughly and actively. To address thisproblem of passivity, a locomotive mechanism is proposed forwireless capsule-type endoscopes. Prototypes with micro brushlessdc motors, ionic polymer metal composite actuator, and shapememory alloy (SMA) wires are designed and fabricated for pre-liminary tests. Based on the tests, spring-type SMA actuatorsare selected to be microactuators for capsule endoscopes. Thus,two-way linear actuators using a pair of SMA springs are de-veloped based on a static analysis on them. Moreover, a simpleand effective clamping device is developed based on biomimeticapproach. A prototype endoscope with four pairs of SMA springsand four clampers was developed. It has 13 mm in diameter and33 mm in total length, with a hollow space of 7.6 mm in diameterto house other parts that are needed for endoscopy such as acamera, an RF module, sensors, e.g., for endoscopic ultrasound,and a battery. A sequential control of the four actuators improvesthe efficiency of locomotion up to four times. To validate theperformance of the proposed locomotive mechanism, a series ofexperiments were carried out including in-vitro tests. The resultsof the experiments indicate that the proposed locomotion mecha-nism is effective to be used for micro capsule-type endoscopes.
Index Terms—Capsule-type endoscopes, clampers, sequentialcontrol, shape memory alloy (SMA), two-way SMA springs.
I. INTRODUCTION
MORE and more people suffer from gastrointestinaldiseases due to changes in their diets, environmental
pollution, and mental stress. Gastrointestinal troubles include
Manuscript received April 3, 2003; revised December 5, 2003. This work wassupported by the Intelligent Microsystem Center (which carries out one of theR&D Projects sponsored by the Korea Ministry of Science and Technology)under Contract Project Code MS-02-142-01.
B. Kim is with the Microsystem Research Center, Korea Institute of Scienceand Technology, Seoul 130-650, Korea (e-mail: [email protected]).
S. Lee is with the Research Center, Hyundai Motor Co., Kyungki-Do 445-706, Korea (e-mail: [email protected]).
J. H. Park is with the School of Mechanical Engineering, Hanyang University,Seoul, 133-791, Korea (e-mail: [email protected]).
J.-O. Park is with the Intelligent Microsystem Center, Korea Institute of Sci-ence and Technology, Seoul 130-650, Korea (e-mail: [email protected]).
Digital Object Identifier 10.1109/TMECH.2004.842222
intestinal cancers, intestinal tumor, peptic disease, and inflam-matory bowel. These diseases cannot be diagnosed reliablyby indirect methods such as X-Rays or computed tomographyscanners. On the other hand, direct methods such as endoscopesare much more reliable than and thus preferred to the indirectones.
Gastrointestinal endoscopes for diagnosis can be classifiedinto two types: push type and wireless capsule type. Theconventional push-type endoscope is most commonly used inmost hospitals and operated by the hands of skilled individualoperator. Since its tube needs some structural strength tobe pushed, it has somewhat high stiffness, causing pain anddiscomfort to the patient. Moreover, it cannot reach the smallintestine for diagnosis [1].
These problems prompted the development of robotic endo-scopes including wireless-capsule type endoscopes. As far asnoncapsule-type endoscopes are concerned, inchworm types asin [2], [3] are promising. Capsule-type endoscopes have advan-tages over noncapsule-type ones in that their small size intro-duces far less pain to their patients. The very first capsule-typeendoscope called the M2A was developed and commercializedin 2001 by Given Imaging Inc., Israel [4]. It is 10 mm in di-ameter and 27-mm long with a charge coupled devices (CCD)camera, an RF module, illuminating LEDs, and a battery inte-grated. It can be swallowed and can transmit wireless still andmoving images from the gastrointestinal track. Due to the devel-opment of the wireless-capsule endoscopes, it is now possibleto diagnose small intestines, which can not be achieved by con-ventional endoscopes, and to reduce pain and discomfort of thepatient.
Another wireless type endoscope called Norika V3 is beingdeveloped by RF System Co., Japan [5]. It does not have anybattery module in its body, and thus requires a smaller volumethan the M2A . It rather receives the energy externally throughtransmitted microwaves. Besides being able to transmit images,it can change the viewpoint of the camera with a rotating mech-anism based on rotor coils, which is not possible for the M2A .
The newly developed capsule-type endoscopes have an ad-vantage in reducing pain and discomfort of the patient due to itswireless nature. However, they must move passively from themouth to anus by the peristaltic waves. Thus, no active diag-nosis is possible due to the lack of a locomotive mechanism.
In this paper, we propose a locomotive mechanism for cap-sule-type endoscopes based on shape memory alloy (SMA) ac-tuators. Once a capsule-type endoscope is swallowed, it passesthrough a gastrointestinal track which has soft and locally de-formable tissues, slippery surfaces, and secretion of mucus, that
78 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 10, NO. 1, FEBRUARY 2005
TABLE ICHARACTERISTICS OF MICROACTUATORS
is a hostile environment to any locomotive mechanism. To beable to move around in such an environment and to house allthe necessary parts inside a small capsule, a successful endo-scope should have a kind of clamping and releasing mecha-nism for locomotion, and small and efficient actuators to allowa hollow space inside the endoscope, and a locomotion mecha-nism that is also simple and easy for assembly and debugging.This paper addresses these design requirements with the use ofSMA-spring actuators and biomimetic clamping devices.
In Section II, candidates for small actuators are consideredincluding the SMAs. The design and analysis for the SMAactuators are explained in Section III. Fabrication-related is-sues and experiments are covered in Section IV, followed byconclusions in Section V.
II. SHAPE MEMORY ALLOYS (SMAs)
A. Microactuators for Capsule Endoscopes
Microactuators are most critical components in developing asuccessful locomotive mechanism for capsule-type endoscopesbecause they have to meet tough design specifications in termsof size, power, force, motion range, and safety. The characteris-tics of typical microactuators are summarized in Table I.
Preliminary tests were done using a micro brushless dc(BLDC) motor and ionic polymer metal composite (IPMC) [6].Fig. 1(a) shows a prototype endoscope using a micro BLDCmotor of 3 mm in diameter. It converts rotational motion ofthe motor into linear motion by linkages. However, the torqueis limited when the motor size is small enough to fit into acapsule. Moreover, the inside of the capsule becomes quitecomplicated due to many linkages for clamping.
Fig. 1(b) shows a prototype endoscope using IPMC actuators.The mechanism for clamping is quite simple. However, IPMCactuators can produce only 0.1–4.0 g of force [6] depending onthe thickness of the actuator, which makes it infeasible to usethe IPMC for endoscopes, at least now.
The third actuator we tried was SMA. It has the so-calledshape memory effect. They possess the ability to undergo shapechange at a low temperature and to retain this deformation untilthey are heated, at which point they return to their original shape.Shape memory effect occurs as the result of a change in theatomic crystal structure of the alloys by a temperature change:austenite phase at a high temperature and the martensite phasein a low temperature [7]. Even though a temperature hysteresisexists due to atomic-scale processes, it is not important in thecurrent work because the control of the SMA doesn’t have tobe so precise to be used for a locomotive mechanism of endo-scopes. SMA-wire actuators are one of the typical types of SMAactuators used. In this work, spring-type SMA actuators, called
Fig. 1. Mechanisms for capsule-type endoscopes using (a) BLDC motor,(b) IPMC, and (c) SMA wires.
“SMA springs,” are used instead since they can deform rathersignificantly.
B. SMA Actuators
An actuator with a bare SMA wire shows different character-istics depending on its diameter. SMA wires of various diameterranging from 25 to 375 m were studied. Their exerted forcesvaried from 7 to 2000 g depending on the diameter. For the capa-bility of bi-linear motion, a “pulling force” should be externallyapplied at the wire. Thus, the difference between the SMA re-covery force and the external pulling force is the maximum pro-ducible force by the SMA wire. Typical values of the maximumlinear strain for the most SMAs are about 8%. To keep the re-liable performance of SMA actuators reliable, a lower strain oftypically 3%–5% is mostly used. SMA wires with the diameterof 100 and 150 m perform best with respect to power consump-tion. A prototype SMA-wire actuator combined with a clampingand a locomotive mechanism is shown in Fig. 1(c).
Based on our findings that SMA-wire actuators can produceonly a small displacement and that it is difficult to provide an ap-propriate pulling force, a different type of SMA actuator, SMAsprings, are fabricated as an alternative actuator. They have thecapability to make a quite large strain [8] and to make it easy
KIM et al.: DESIGN AND FABRICATION OF LOCOMOTIVE MECHANISM FOR CAPSULE-TYPE ENDOSCOPES 79
Fig. 2. Fabrication of an SMA spring. (a) Constrained SMA wire on a mandrel.(b) SMA spring after “shape setting” heat treatment.
to provide an appropriate pulling force when they are used inpairs.
Even though the recovery force generated by an SMA springis smaller than that of an SMA wire, it maximum allowablestrain increases 80 folds compared with an SMA wires. SMAsprings are fabricated through a process called “shape settingheat treatment” at 400–500 C for about 30 min. SMA springsshown in Fig. 2 were made using a standard M1 bolt as a man-drel of 1 mm in diameter and 0.25 mm in pitch.
III. DESIGN OF LOCOMOTIVE MECHANISM
A. Basic Concept of Locomotion
The locomotive mechanism for capsule endoscopes is de-signed with a simple mechanism. The actuation part is arrangedaround the surface of the capsule to make the capsule bodyhollow for system integration. It consists of two-way linear ac-tuators, clampers and a capsule body. Fig. 3 shows the locomo-tive principle of the capsule. To drive a mechanism, the clamperslides forward when a front linear actuator is contracted. Afterthe clamper finished sliding forward, the clamper clamps thecontact surface and the body moves forward during the contrac-tion of the rear linear actuator. Finally, the clamper releases thecontact surface and slides forward as the front linear actuator iscontracting. As this procedure is performed repeatedly, the bodycan move forward. This concept, that imitates the locomotion ofthe insects, enables the capsule to have the simple mechanismand to minimize the volume of locomotion devices for the cap-sule endoscopes [9].
B. Design for Locomotive Mechanism
The locomotive mechanism for the capsule endoscope is de-signed as shown in Fig. 4. Four locomotive modules installedon the surface of the capsule help increase the efficiency of lo-comotion and enable the capsule body to move under any en-
vironment. The micro hook of the capsule was protruded fromthe body by 400–600 m. So it minimizes the damages to theorgans and the friction in a sliding direction. The body is hollowfor the further system integration such as an RF module, a bat-tery, sensors, for example for ultrasound [10], and a camera. Inaddition, for the maximization of the stroke of two-way SMAsprings, they are designed to span the full length of the capsulebody.
C. Biomimetic Clamping Device
The clamper plays an important role in performing the loco-motion besides the actuator. It is an effective device to overcomea rough in-body environment with clamping and releasing func-tions. We designed and fabricated the micro-sized and simple-structured biomimetic clamping device imitating from insectsor plants in nature [9]. Fig. 5(a) shows the clamping devicesof insects. The devices help insects stick to or move easily inslippery surfaces by means of increasing frictions or contactingforce. These devices can be very useful in flexible and slipperyin-body environments. Fig. 5(b) shows the biomimetic clampingdevice using micro hook whose diameter is 180 m. It can per-form passive clamping and releasing functions and enables thecapsule move in a single direction. In addition, it is hardly dan-gerous or painful to in-body organs [11].
D. Two-Way SMA Springs
1) Two-Way Linear Actuator: To achieve the mechanismproposed in this paper, we used two-way SMA springs asbi-directional linear actuators. Since a SMA spring deformsitself up to its ’memorized’ length at high temperature, it needsa deform force to return to the previous length and thus tomake bi-directional motions possible. Generally, the methodsto apply the deform force can be divided into the bias SMAsprings, bias steel springs and bias gravity. The bias gravitymethod is efficient and ideal because it provides a minimumconstant deform force and is constructed in a simple structure.However, it is difficult to design and fabricate a mechanismwith a bias gravity due to the limitation of the space. Bothbias steel and SMA springs are installed in the same method,but the latter can produce about twice the stroke of the first.Therefore, we chose the bias SMA spring method in which atwo-way linear actuator is embodied with two spring type SMAactuators [12]. With an increase in the number of the actuators,the bias SMA spring method has advantages in producing thelarge displacement and in the availability of active control.
2) Mathematical Modeling and Analysis: Before the designof the locomotive mechanism, a mathematical modeling andanalysis of the two-way SMA springs are performed. Fig. 6is the force versus displacement characteristics of the two-waySMA spring unit [8]. The stiffness of each spring changes de-pending on its temperature. When it is cool, or at room tem-perature, its linear stiffness is . When it gets hot by ohmicheat generated by the current flowing though the spring wire,the stiffness becomes . When there is no force applied at thesprings, the length of each is assumed to be . It is assumedthat the right and left springs are identical.
In Fig. 6, point represents an equilibrium point when thetwo springs remain cool and thus the pulling forces exerted
80 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 10, NO. 1, FEBRUARY 2005
Fig. 3. Principle of locomotion.
Fig. 4. Locomotive mechanism for capsule-type endoscopes.
Fig. 5. Clamping devices. (a) Clamping device of an insect. (b) Biomimeticclamping device.
by the left and right springs are of the same magnitude butin the opposite directions. At point , the connector is at an
Fig. 6. Static analysis for a two-way SMA spring set.
equilibrium state with the left spring being hot and the rightspring being cooled. Similarly, point represents an equilib-rium point when the left spring is cooled and the right spring isheated. By repeated cycles of cooling and heating of the springs,the connector of the springs can move to the left or to the right,generating bi-directional or two-way motions.
For the further analysis of the spring unit, let us assume thateach of the springs is elongated by at equilibrium point .Then, the total length of the two-way SMA spring assembly, ,becomes
(1)
The left and right spring characteristics with heating andcooling can be represented by
(2)
(3)
(4)
(5)
where the subscripts indicate whether corresponding spring isheated ( ) or cooled ( ) and the superscripts denote the location
KIM et al.: DESIGN AND FABRICATION OF LOCOMOTIVE MECHANISM FOR CAPSULE-TYPE ENDOSCOPES 81
of the corresponding spring, either left ( ) or right ( ). And,’s and ’s denote force and spring constant, respectively.Using the set of linear equations of (2)–(5) and the equilib-
rium constraints of
it is possible to find the displacements at equilibrium points,and :
(6)
(7)
It is important that the range of the spring operations shouldbe always within the limit of the strength of the SMA wires. Forthat, let us define the maximum deflection, of each SMAspring, using its maximum allowable strain, , as
(8)
Note that points and respectively represent the state ofthe minimum and the maximum deformation for the spring onthe left-hand side. Thus, the maximum deformation can be ex-pressed by
(9)
The force exerted by a coil spring, , when it is deformed by, is represented by
(10)
where is the number of turns of the wire, is the shear mod-ulus, is the diameter of the SMA wire, and is the diameterof the spring [13]. Equation (10) indicates that spring stiffness
is proportional to the shear modulus resulting in
(11)
By the way, the length of the spring under no deformation canbe expressed as
(12)
where is the pitch of the SMA spring when no force is ex-erted.
From (8), (9), and (12),
from
(13)From (6), (7), and (11), theoretical stroke of the two-way
SMA springs can be obtained as
(14)
From (1), (13), and (14), stroke rate defined as the ratio ofthe stroke to the length of the actuator, can be represented as
(15)
Fig. 7. Measured characteristics of an SMA spring. (a) Static characteristics.(b) Dynamic characteristics.
Suppose the left and right springs are in equilibrium at point. Simultaneous heating of the right spring and cooling of the
left would increase the pulling force of the spring and forcesthe spring connector move to the right and eventually to point
. The difference in the pulling forces by the springs can berepresented by
(16)
The maximum of the force difference occurs when andthus
(17)
Now, all the parameters that affect the performance of thetwo-way SMA spring unit are identified: , , , , , and
. Shear modulus is a material property which can be deter-mined experimentally, and parameters , and are the de-sign parameters that are to be determined under the constraintsof the size and power of the actuator. Parameters and arealso design parameters. As in (1) and (13), we can choose therange of and for the desired length of the two-way SMAsprings. However, the stroke rate depends on , but not on
82 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 10, NO. 1, FEBRUARY 2005
Fig. 8. Fabrication of an actuator based on a two-way SMA spring set.
. To achieve the efficient stroke, it is important to maximize. Equation (15) indicates that stroke rate monotonically in-
creases as increases.
E. Static and Dynamic Experiments
Shear modulus and maximum strain of the SMAsprings can vary depending on the way of heat treatment. Thus,it is necessary to evaluate them experimentally in order to gettheir precise numerical values. In addition, it is necessary todetermine the proper time spans to heat the SMA spring andto let it be cooled as control parameters of locomotion. Theseparameters were measured by a series of dynamic experiments,where a linear stage, load cells and a data acquisition boardwere used.
Fig. 7(a) indicates the result of the static experiment whichshows the relationship between the deflection of an SMA springand the exerted force, at high or low temperature. It shows thatspring becomes soft as the deflection becomes large. At hightemperature, it shows a linear characteristic up to a point whereits deformation is 6 mm. At the low temperature, more or lessgradual softening of the spring can be observed.
Since the larger force is generated when the wire is hot ratherthan it being cooled, and the spring force of the hot wire be-comes significantly saturated when the deformation is largerthan 6 mm, the maximum allowable deformation of the springis set to be 6 mm. This means that the maximum strain of theSMA springs, i.e., , is set to be 2.0 because the length ofthe SMA spring without any deformation is 3.0 mm.
For the sake of simplicity, a linear characteristic of the wire isassumed and the linear stiffness is estimated with the curve-fit-ting method. The estimated stiffness ( ) was 15.56 g per mil-limeter and 2.78 g per mm, respectively, at the high and lowtemperature. The measured shear modulus ( ) was 43 376 and7,464 MPa, respectively, at the high and low temperature.
Fig. 7(b) shows the results of the dynamic experiment. It de-picts how the exerted force changes as a function of time whenthe flowing current is switched off. More than 10 s is needed inorder for the exerted force to be settled down. Based on theseresults, a compromise was made between the actuation speedand the maximum (minimum) exerted force, in that the on-timeand off-time of the actuator are selected to be more than 2 s and5 s, respectively.
Fig. 9. Prototype with a locomotive mechanism.
IV. EXPERIMENTS
A. Fabrication of Prototype
An actuator with a pair of SMA wires that is capable ofbi-linear motions for endoscopes is assembled (see Fig. 8).Both ends of each of the two SMA springs are attached to anelectrode. One of the electrodes is shared by the two springs.The SMA wire used is 150 m in diameter. Their highesttemperature becomes approximately 70 C . To operate thetwo-way SMA springs, current in the range of 250–400 mA isrequired, which in turn requires the applied voltage to be lessthan 2 V due to very low resistance in the wires.
Through the analysis and experiment of design parameters,the SMA spring of 1 mm in diameter, 18 turns and 4.5 mmin original length is fabricated. As maximum strain ( ) is setto 2, the computed initial deflection ( ) is 5.25 mm. There-fore, the total length becomes 19.5 mm excluding the length ofthe common electrode. By calculating, the maximum stroke is7.5 mm and the stroke rate is 38%. It shows that the shape-set-ting heat treatment is effective in improving the strain, con-sidering the maximum strain of SMA wire is 5%. The max-imum force which can be produced by two-way SMA springsis 135.9 g of force. That is 40% of 330 grams of force that theSMA wire can produce. However, considering that the weightof the commercially used capsule endoscope is 10 g, it is not agreat loss.
A prototype endoscope is assembled with a moving mod-ules and a pair of SMA springs, which is shown in Fig. 9. Theprototype is a capsule type made of the acetylene material toreduce the damage to digestive organs and friction. The outer
KIM et al.: DESIGN AND FABRICATION OF LOCOMOTIVE MECHANISM FOR CAPSULE-TYPE ENDOSCOPES 83
Fig. 10. Sequential control of actuators.
diameter is set to 13 mm. The total length of the capsule be-comes 33 mm and the interior space for system integration is7.6 mm in diameter. The weight of the prototype is minimizedup to 3.07 grams of force due to the weight of other compo-nents to be added as parts of a system. A total of nine pieces ofthe electric wires including the common ground for supplyingthe current are gathered inside of the capsule. The enamelledelectric wires of 50- m diameter are used to reduce the stiff-ness of the wires. These electric wires can supply eight piecesof the SMA springs with currents independently. The theoret-ical moving speed, which is based on the stroke and the cycletime of a moving module, is 110 mm/min when controlling thefour pieces of the moving modules sequentially.
B. Control Systems
In order to control the length of an SMA precisely, a closedloop control with sensors is needed. Many types of controllershave been developed for that purpose [14]. However, in theview of the environment of digestive organs and the mechanismof movement, the precise-controlling of the each SMA has fewadvantages. The installment of the controller circuit, whichconsists of a large size of sensors and complicated structure,is unnecessary considering the size of the capsule endoscope.Therefore, the open-loop control was used.
The driver circuit supplies power to contract and relax theSMA and prevents the overheating of the SMA . This driver cir-cuit is actuated and regulated by the control circuit. Generallyit is divided into the active current regulator for switching theproper current on and off and the pulsewidth modulation (PWM)circuits [15]. The active current circuit has the advantage thatit has a simple structure and capable of preventing from over-heating. However, it has the disadvantage that it has low powerefficiency. To the contrary, although PWM has a complicatedstructure, it is capable of getting effective power operation andis suitable for the battery and the solar powered system. In thispaper, the active current regulator is used for the driver circuitbecause the prototype is not yet wireless capsule by the battery.
Labview is used to control the eight pieces of SMA springssequentially. A data acquisition board, NI-6713 of NationalInstruments Co. transmits signals to eight channels to controllocomotion devices inter-dependently. Each signal is then
Fig. 11. Setup for experiments.
amplified by an OPA547 current amplifier of Burr-Brown Co.which flows electric current to one of the SMA springs used inthe endoscope. Since the SMA wires can not be soldered due tothe fact that they are made of the alloy of Nickel-Titanium, theyare attached mechanically by pressing the copper tube againstthem and electrically by bonding with conductive epoxy glue.
It takes more than 2 s to activate an SMA spring unit andtakes more than 5 s to cool it off. The cooling time duringwhich the spring is cooled is rather longer than the activationtime for heating. To make a single cycle of the two-way SMAsprings, which is a set of a forward and backward move, it takestwo subcycles of activation and cooling. Since it takes a largeamount of time to activate and cool of the actuator, the effi-ciency of locomotion is low. To address this problem, sequen-tial control of multiple actuators was used. As can be seen inFig. 10, one module of the two-way SMA spring is activated(with its clamp open) while the other is cooled off (with itsclamp closed). Sequentially operating four different modules ofthe two-way SMA springs improves the locomotion efficiencyup to four folds, compared with operations of a single module.
In Fig. 10, a backward move of a clamper with its clampclosed during its “backward moving” stage would force the cap-sule body move forward. During this, the clamp works as a foothold. On the other hand, a forward move of a clamper with itsclamp open in its “forward moving” stage would not move thecapsule body because the clamper simply slides over the wall on
84 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 10, NO. 1, FEBRUARY 2005
Fig. 12. Experiment on silicon rubber pads with (a) single-surface, (b) two-surface, and (c) three-surface contact.
which the endoscope moves. An important principle in control-ling the clampers is that a forward thrust of the endoscope bodyshould be made only by single clamper module at any given mo-ment. It is allowed however that more than one clamper simulta-neously move forward while a single clamper moves backward.
C. Experimental Setup
To validate the performance of the proposed locomotivemechanism, a series of experiments were done in environmentssimilar to intestinal organs of human: locomotion test onflexible and slippery silicon rubber pads and in-vitro tests. Toconfirm the improvements in locomotive efficiency when morethan a single clamper is sequentially controlled, we performedexperiments with a single-surface contact, two-surface contact,and three-surface contact, changing the number of contactsurfaces and the number of active clampers. Fig. 11 shows theexperiment setup. To measure the displacement of the endo-scope, A L60/01 laser displacement sensor of Keyence Co. andPCI-MIO-16E-4 data acquisition board of National InstrumentCo. were used.
D. Locomotion on Silicone Rubber Pads
Motion characteristics and the improvements in the locomo-tion efficiency with the sequential control are verified in a seriesof experiments on flexible and slippery silicon rubber pads.Fig. 12(a) shows how the endoscope moves with a single-sur-face contact. The permitted current to the SMA spring is
between 300 and 400 mA . The actuation and the rest timeare 2 and 6 s, respectively. Thus, make a complete cycle takesapproximately 16 s. The reason why the rest time is set to be6 s not 5 s is that more time is needed to cool off the SMAsprings heated with current of 400 mA . Movement becomesfaster as the permitted current gets higher. Experimentally,the speed of the prototype is 9.19 mm/min at 300 mA, and13.43 mm/min at 400 mA. However, when they operate at400 mA, the SMA springs become quite hot and their lifeexpectancy become shorter, which made us to operate theSMA springs at 300 mA. Sometimes, it is observed that thebody actually moves backward slightly during the rest timebecause the friction between the clamper and the pads forcesthe clamper drag the endoscope. Fig. 12(b) shows how theendoscope moves in sequential control of two pieces of movingmodules. The actuation and the rest time are set to be 2 and5 s, respectively. The total time to complete a single cycle isapproximately 14 ss. The speed becomes almost doubled at17.49 mm/min. The body moves continuously forward as thetwo moving modules operate sequentially, with a 180 offset.
Fig. 12(c) shows the locomotion with a three-surface con-tact in which the endoscope makes contacts at its bottom andboth sides with the rubber pads. Three pieces of moving mod-ules are sequentially operated and the moving speed is improvedto be 23.0 mm/min. The body has three chances to move for-ward in a single cycle due to the sequential operations of themoving modules, each with a phase shift of 120 . Due to fail-ures in clamping, the moving stroke sometimes becomes only
KIM et al.: DESIGN AND FABRICATION OF LOCOMOTIVE MECHANISM FOR CAPSULE-TYPE ENDOSCOPES 85
Fig. 13. In-vitro test under (a) a single-surface contact and (b) three-surfacecontact.
twice rather than three times that of a single moving module.Clamping failures and increased friction due to the increase inthe contact surface prevent a linear increase in the speed withrespect to the number of moving modules.
E. In-Vitro Test
The performance of the movement was verified in a seriesof in-vitro tests, where pig’s colons were used, which has verysimilar characteristics to the human’s. The experiments wereconducted under a single-surface contact and a three-surfacecontact. The average speed with a single-surface contact and athree-surface contact is approximately 8.5 and 14.7 mm/min,respectively. Compared to the speed on the silicon rubber paddescribed in the previous subsection, the efficiency of locomo-tion under a single-surface contact is slightly decreased. The lo-comotion speed is significantly decreased with a three-surfacecontact. It is believed that the increase in clamping failures asso-ciated with easily deformable colon’s shape was the main factorin the decrease in the speed. Fig. 13(a) shows the experimentwith a single-surface contact when the moving module locatedat the bottom operates. Fig. 13(b) shows the experiment resultswith a three-surface contact when three pieces of the movingmodules sequentially operate at the bottom and the two sides ofthe endoscope. Based on the in-vitro tests, the possibility to usea locomotive mechanism to move through a slippery, flexibleand deformable gastrointestinal track is shown. In near future,in-vivo tests with live pigs will be performed with an aim toapply this endoscope to the human beings.
F. Issues on Power and Heat
The proposed microrobot consumes significant power so thatit is difficult to use a commercial battery as its power source.However, it is expected that fuel cells to be developed soon willdeliver enough power for the microrobot. As an alternative wayto deal with the power-related issue, a different type of SMAthat requires smaller power is currently under test with a com-mercially available alkaline manganese button cell battery. Thiswill be addressed in the coming research publications.
A rise in the temperature at the clampers is monitored duringthe in-vitro tests using a temperature sensor (model: TEMP-0001) by Xenon Inc. that uses infrared light. In approximately3 min after starting the experiment, the temperature is increasedby 12 C and the rate of its increase becomes flattened at zero.From this observation, it is concluded that the rise in the tem-perature at the local intestine wall around the contact point witha clamper of the endoscope is limited such that no significantharm to the intestine is made. Using an SMA actuator with asmaller diameter that requires lower current and thus generatesless heat would greatly reduce any concern regarding the heat.
V. CONCLUSION
A locomotive mechanism to be used for capsule endoscopeswas designed and fabricated based on SMA springs and microhooks. For a selection of proper actuators, prototype endoscopesusing a few kinds of microactuators including micro BLDC mo-tors, IPMC, and the SMAs-based were designed and fabricated,and tested to measure their feasibility. It turns out that the pro-posed actuator based on a pair of SMA springs is a better choicethan any others among the microactuators we selected . Experi-ments showed the proposed biomimetic clampers using microhooks are proper for capsule endoscopes because of its sim-plicity and effectiveness in clamping/releasing functions. It waspossible to increase the speed and the efficiency of the locomo-tion by means of sequential control with two or more clampingmodules.
The capsule body of the prototype was designed and fabri-cated with a hollow space for further integration with other com-ponents such as a camera, a battery and an RF module. For theevaluation of the locomotion device, a series of experiments onthe locomotion over silicon rubber pads and in-vitro tests weredone. In these experiments, it turned out that the locomotivemechanism is effective not only quantitatively but also qualita-tively in the gastrointestinal environment. In future works, in-tegrated micro capsules that consists a micro camera, an RFsystem, batteries and a locomotive mechanism will be devel-oped and tested for in-vivo test.
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Byungkyu Kim received the Ph.D. degree inmechanical engineering from the University ofWisconsin, Madison, in 1997.
From 1990 to 1993, he was a Research Scientistin the Robotics Center, Korea Institute of Scienceand Technology (KIST), Seoul, Korea, where heworked on robot application for automobiles au-tomation. From 1997 to 2000, he was a TechnicalStaff Member at the Center for X-ray Lithography(CXrL), University of Wisconsin, where he devel-oped a computer code for thermal modeling of a
mask membrane and wafer during beam exposure. Since 2000, he has been withMicrosystem Center of KIST as a Senior Research Scientist. He was in chargeof the developing the microrobot for colonoscopy. Currently, he is working forsystem integration of the microcapsule-type endoscope. His research interestincludes microelectromechanical systems actuator, simulation, and medicalapplications of microrobots.
Sunghak Lee received the B.A. and M.S. degreesin mechanical engineering from Hanyang University,Seoul, Korea, in 1997 and 2003, respectively.
During his study for the M.S. degree, he workedat the Korea Institute of Science and Technology(KIST), Seoul, Korea, under the Joint CooperationProgram between Hanyang University and KIST.He is currently with Hyundai Motor Company,Kyungki-do, Korea. His research interests includedynamics and control of mobile systems.
Jong Hyeon Park (M’96) received the B.A. degreein mechanical engineering from Seoul NationalUniversity, Seoul, Korea, in 1981, and the M.S. andPh.D. degrees from the Massachusetts Institute ofTechnology (MIT), Cambridge, in 1983 and 1991,respectively.
Since 1992, he has been with the School of Me-chanical Engineering, Hanyang University, Seoul,Korea, where he is currently a Professor. He wasa KOSEF-JSPS Visiting Researcher with WasedaUniversity, Tokyo, Japan, in 1999 and 2003, a
KOSEF-CNR Visiting Researcher with Scuola Superiore Sant’Anna, Pisa,Italy, in 2000, and a Visiting Researcher with MIT, Cambridge, in 2002–2003.From 1991 to 1992, and from 2001 to 2003, he was with Brooks Automation,Chelmsford, MA, working on developing semiconductor equipments. Hisresearch interests include biped robots, robot dynamics and control, bio-robots,and haptics.
Dr. Park is a member of the IEEE Robotics and Automation Society and apermanent member of KSME, ICASE, KPE, and KSAE.
Jong-Oh Park received the Dr.-Ing. degree inmechanical engineering from University of Stuttgart,Stuttgart, Germany, in 1987.
From 1982 to 1987, he was a Visiting ResearchScientist at the Institut Produktionstechnki Au-tomatisierung. From 1987 to present, he has beena Principal Research Scientist of MicrosystemResearch Center, Korea Institute of Science andTechnology (KIST), Seoul, Korea, where he is theDirector of the Intelligent Microsystem Center. Hisresearch interest includes intelligent microsystem,
human-friendly teleoperation, and virtual reality.
