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Funicular Flexible Crawler for Colonoscopy*
Jun-ya Nagase, Member, IEEE, Fumika Fukunaga, Keiji Ogawa, and Norihiko Saga, Member, IEEE

Abstract— The number of people with cancer of the large
intestine has been increasing steadily. According to The Global
Cancer Observatory, its prevalence is second among all cancer
types in world statistics over the past five years. Early detection
and treatment of diseases by colonoscopy can enable complete
recovery. Nevertheless, conventional colonoscopes require
extremely high skill of the doctors who use them. Moreover, theirinvasiveness in the colon is high. Therefore, many robots tofacilitate colonoscope-related operations have been developed
recently. Unfortunately, no practical system exists which can truly
overcome the proven effectiveness and lower costs presented byconventional colonoscopes. We therefore propose a novel
autonomous propulsion mechanism for colonoscopy named the“Funicular Flexible Crawler.” It is suitable for propulsion throughnarrow spaces because it is driven by multiple small diameter
crawler units connected to a flexible shaft. Experimentation witha large intestine phantom confirmed that the flexible crawler can
run from the rectum to the cecum. As described herein, this
propulsion mechanism is applicable for self-traveling colonoscopy.
Index Terms— Colonoscopy, crawler belt, flexible shaft, large
intestine

I. INTRODUCTIONIN Japan, colorectal cancer is the third most common cause ofcancer death in men and the first most common cause in
women[1]. Early detection of such diseases supports complete
recovery. The five-year survival rate is 95% if cancer can befound while it remains in the large intestine. To conduct
examinations for colon cancer detection, a doctor inserts acolonoscope through the anus. The large intestine is non-fixed,with high flexibility and complicated shape. Therefore, doctors
must have extremely developed skills to insert a conventional
colonoscope, which consists of a distal bending tip, acolonoscope body, tendon knobs, connector, biopsy forceps,
and high-resolution mini-camera at the end of the distal bending
tip. A doctor must use both hands to insert such colonoscopes
into the large intestine: the strong hand advances the endoscope
while the supporting hand bends the tip to direct it. Typically,
patients feel pain and discomfort during procedures.
Conventional colonoscopy has mainly two colonoscope
insertion methods as depicted in Fig. 1: a loop method and anon-loop method [2], [3]. These methods are commonly used at*Research supported by Grant-in-Aid for Scientific Research (C)(No.
17K01429).
*Jun-ya Nagase and Keiji Ogawa are with Mechanical and Systems
Engineering, Science and Technology, Ryukoku University, Shiga, Japan.
Fumika Fukunaga was with Mechanical and Systems Engineering, Scienceand Technology, Ryukoku University, Shiga, Japan. She is now with Johnson
and Johnson, Tokyo, Japan. Norihiko Saga is with Human System
Interaction, Kwansei Gakuin University, Hyogo, Japan. (correspondence email: nagase@rins.ryukoku.ac.jp).

clinical sites. The loop method is likely to be painful because of
hyperextension of the intestines. The non-loop method
suppresses hyperextension and causes less pain, but it is very
difficult to master. Therefore, doctors must have extremely highskills for colonoscope insertion. The insertion time and load on
the large intestine strongly depend on their skill.
Given such circumstances, capsule endoscopes have beendeveloped as painless inspection devices [4]–[7]. Capsule-type
endoscopes can be swallowed, after which they move from themouth to the anus by peristaltic waves. Consequently, diagnosisof small intestine disorders is now possible. Examinations arepainless and safe, but no active diagnosis or MIS is possible
because a locomotive mechanism is lacking [8]. For that reason,
their application is limited.
Several developed inchworm-like endoscopic robots are
driven by shape memory alloy (SMA) actuator, DC motor, or apneumatic actuator [9]~[11]. Theoretically, through rhythmic
control of their actuators, these robots can move forward andbackward similarly to an inchworm. Nevertheless, such robots
move very slowly through the intestinal tract because the colonis slippery and loosely constrained [9], [10]. Endotics systemshowed superiority in pain reduction versus conventional
colonoscopy, but a relatively low cecal intubation rate need to
be overcome [11].
Conventional wired endoscopes are still necessary for
medical procedures for precise diagnosis, polyp removal, andso on. They are more reliable and are usually used to diagnose
the colon. Therefore, several active wired endoscopes havebeen studied. One report by Alcaide et al. [12] describes
locomotion of their robot, which mimics an earthworm’s
peristaltic motion and which uses SMA actuators. Nakamura etal. [13] also developed a peristaltic crawling robot using fiberreinforced artificial muscles for colonoscopy. However, they
have difficulty moving quickly in phantoms of large intestines.
Wakimoto and Suzumori [14] developed a propulsion device
using travelling waves driven by a pneumatic actuator andattempted to apply it to an endoscope. However, it cannot moveindependently.
For this study, we aimed at implementation of a semiautonomous colonoscopic device with high running
performance and safety in a large intestine. We developed acylindrical crawler robot [15],[16] as presented in Fig. 2. The
tracked crawler mechanism propels the robot by driving
multiple crawler belts in axial symmetry to a cylindrical framewith a single actuator via a single worm. Consequently, it has amuch simpler structure than that of a conventional tracked
crawler mechanism with complicated structures and many
actuators [17]–[19]. Our earlier device [20] can propel itself innarrow pipes of 50 mm inner diameter. Herein, we propose a2funicular flexible crawler mechanism with 16.4 mm outer
diameter based on our earlier crawler used for colonoscopy.
Section II describes the structure and driving mechanism of
the funicular flexible crawler. Section III explains the basic
characteristics of the crawler unit for propulsion. Section IV
describes the steering characteristics of crawler unit for turning.
Section V explains the running characteristics of the funicular
flexible crawler.
II. FUNICULAR FLEXIBLE CRAWLER STRUCTURE AND
DRIVING MECHANISM
Fig. 3 presents the structure of the funicular flexible crawler:
The flexible crawler. The flexible crawler includes an actuation
unit, crawler units for propulsion, a crawler unit for turning, aflexible inner shaft, and a flexible outer shaft. The actuation unit
has a geared motor. The geared motor output shaft is connected
to a flexible inner shaft through a coupling. Multiple crawler
units used for propulsion are arranged in the flexible inner shaft.
Loop Method

hyper
extension

Colonoscope

Fig. 4 portrays a section of the crawler unit used for
propulsion. The unit comprises a worm, crawler belts, and aframe. Bearings at both ends support the worm, arranged inside
the frame. Both worm ends are connected to a flexible innershaft. Two crawler belts are spaced asymmetrically around thelongitudinal axis of the cylindrical frame. Each crawler belt iswound around the frame: both ends are connected, producing aloop. Additionally, teeth having a rake angle corresponding to
the lead angle of the worm tooth are formed on the crawler belt
outer surface. A crawler unit for turning is connected to the
distal end of the flexible inner shaft. The crawler has a single
crawler belt. The frame installed in the crawler unit supports
one end of the worm. Other structural features of the crawler
unit for turning are the same as those of the crawler unit for
propulsion. A hollow flexible outer shaft is arranged between
each crawler unit and between the motor frame and the crawler
unit. It is positioned around the flexible inner shaft.
Next, we describe the flexible crawler driving mechanism.
First, by driving the gearmotor installed in the actuator unit,
torque is generated on the flexible inner shaft. Then the worms
connected to the flexible inner shaft rotate so that the crawler
belt engaged with the worm rotates longitudinally. Finally,
propulsive force is generated in the flexible crawler byfeel pain

Motor

Worm

Non Loop Method
Colonoscope

Crawler belt
collapse colonic folds

Linearization(suppress hyperextension)

Fig. 1. Conventional colonoscopy(a) Prototype(b) Section
Fig. 2. Amoeba-inspired cylindrical crawler mechanism locomotion
proposed earlier.
Frame

Geared motor

Crawler belt

Flexible outer shaft

Crawler unit for
propulsion (CrP)
Flexible inner shaft
Fig. 3. Funicular flexible crawler proposed in this study.

Crawler unit for
turning (CrT)3frictional force acting on the ground contact surface from thecrawler belts.
Multiple crawlers can be arranged on the shaft. Therefore, a
propulsive force can be generated throughout the colonoscope.
The crawler belts are soft rubber belts that are curved to theoutside of the frame. Therefore, the crawler can move easily
along the irregular intestinal inside wall. Furthermore, a single
gear motor is located at one end of the flexible shaft on theoutside of the crawler. Therefore, during colonoscopy, one canarrange the gear motor outside the body so that the crawler unit
can be greatly miniaturized. For the flexible crawler prototype
described in Section V, 16.4 mm outer diameter was achieved:
it is the narrowest crawler structure ever reported.
For conventional wired colonoscopes, pushing force of thehand must be transmitted directly to the end of the scope during
insertion. Therefore, the colonoscope has high bending rigidity,
putting a large burden on the large intestine. By contrast, thisflexible crawler can run along the contours of the large intestine
while generating a propulsive force on the endoscope by acrawler unit arranged on several flexible shafts. It can therefore
alleviate the load on the large intestine.

Flexible outer shaft

Crawler belt

Flexible inner shaft

Frame
Worm

Attachment
Bearing

Fig. 4. Section of a crawler unit.

Motor

Pipe(ø28)
Crawler unit
Attachment

III. CRAWLER UNIT FOR PROPULSION

Force gage

A. Relation between crawler belt length and traction force
The traction force generated in each crawler unit section
propels the flexible crawler. The crawler traction force isbelieved to be attributable to the crawler belt properties. For thisreason, this study must investigate the relation between the
crawler belt properties and the traction force. However, thelarge intestine structure is extremely complex. It has many folds
in its inner wall. Moreover, the inner diameter and the shapediffer depending on the location in the apparatus. Therefore,
quantitative evaluation of the traction force characteristics inthe large intestine is difficult. First, we examined the relation
between belt length and traction force using an acrylic pipe toassess the fundamental characteristics of the traction force.
A schematic diagram of the experimental device for traction
force measurement and the crawler unit used in the experimentis portrayed in Fig. 5. For this experiment, the traction force ofthe crawler unit driving in an acrylic pipe was measured using
a force gauge (FGP-5; Nidec Corp.) fixed to the ground, as
portrayed in Fig. 5. The 28 mm inner diameter acrylic pipe isconnected vertically to the force gauge tip. A lubricant was usedin the pipe to simulate a wet condition. The gear motor is fixed
to the wall at the top of the pipe. The crawler unit is connectedto the gear motor through a flexible shaft. The crawler unit’s
total length is 44 mm. The outermost diameter of the crawler is
16.4 mm. The 8-mm-wide silicone rubber crawler belt is in an
epoxy resin frame.
The respective rubber hardnesses of durometer type A were50, 60, 70, and 80. Generally speaking, Young’s modulus
increases when durometer type A increases. The respective
numbers of belt teeth were 5, 6, 7, 8, 9, 10, 11, and 12. The pitch
lengths of all belts were 7.3 mm. The belt length is proportional
to the number of belt teeth because the crawler belt length is thepitch length multiplied by the number of belt teeth.

Vise

Belt tooth

Pitch length
Fig. 5. System for measuring crawler traction force.
Rubber hardness(dutometer A)

Fig. 6. Belt teeth number and traction force on each rubber
hardness (Type A durometer).

The maximum traction force was measured five times foreach belt. Then the average of five measurements was taken as
the traction force measurement result. Fig. 6 presents therelation between the number of the belt teeth and traction force
for respective rubber hardnesses (Type A durometer): The
black, blue, gray and red lines respectively represent rubber
hardnesses of 80, 70, 60, and 50. Slippage between the belt andthe pipe occurred in this experiment. The traction force, which
increases with belt hardness, has a local maximum value withrespect to the belt length. The belt length exhibiting thestrongest traction force was independent of the belt hardness.
B. Elastic force of the crawler belt
The frictional force between the elastically deformed crawler

4

Force gage

Plate
Crawler belt
Fig. 7. System for measuring the crawler unit elastic force.
6

1.4

Elastic force
1.2

Traction force

1.0

4

0.8

3
0.6

2

0.4

1

Traction force [N]5

Elastic Force [N]belt and the ground contact surface generates the flexible
crawler traction force. Therefore, the traction force of thecrawler unit probably depends on the crawler belt elastic force.
The elastic force characteristics of the crawler belt were
measured to investigate the mechanism by which the belt elastic
force influences the crawler unit traction force.
Fig. 7 presents the experimental system used to measure thecrawler unit elastic force. In the experiment, with the crawler
unit fixed at both ends with a vise, the radial elastic force wasmeasured when the plate was pressed against the crawler belt.
The pushing amount of the crawler belt was set as equal to theradial deformation amount of the belt when the crawler unit was
inside a ø28 mm pipe. The respective numbers of the belt teeth
were 5, 6, 7, 8, 9, 10, 11, and 12. The rubber hardness ofdurometer type A was 80. The elastic force was measured fivetimes for each belt. Then the average of five measurements was
taken as the elastic force measurement result.
Fig. 8 presents the measurement results. As with the traction
characteristics, the elastic force exhibited a characteristic of
having a local maximum value with respect to the belt length.
Furthermore, as with the traction characteristics, the belt withseven teeth showed the strongest elastic force. Apparently, thetraction force characteristics of the crawler unit are dominatedby elastic force characteristics.
Next, we present some theoretical discussion of the elastic
force characteristics. The crawler belt is made of rubber.
Therefore, its material nonlinearity is strong. The crawler belt
deforms considerably. Therefore, its geometrical nonlinearity isalso strong. The crawler belt characteristics considering such
nonlinearity cannot be analyzed theoretically. In this study,
using a simplified linear model of the crawler belt, as presented
in Fig. 9, we discuss the crawler belt elastic force having a local
maximum value with respect to the belt length.
The relation between belt length l and radial direction elastic
force W can be expressed as the following equations referredfrom the literature [21].

0.2

0

0.0

5

6

7

8

9

10

11

12

Number of belt teeth
Fig. 8. Elastic and traction forces measured for different numbersof belt teeth.
tNon-loaded
crawler Belt

Dy

Pipe
Deformed
Crawler Beltrprw

Worm Shaft

𝐷𝑦
𝑊 = 𝛼𝐸 3
𝑙

(1)

8𝜋 3
𝐼
𝜋 2
( − )
4 𝜋

(2)

𝛼=

(3)

From Eqs. (1)–(3), the relation between the belt length l and theelastic force W for pipe radius rp can be derived as follows.

𝑊=

𝛼𝐸 1 1{ + (𝑡 + 𝑟𝑤 − 𝑟𝑝 )}𝑙2 𝜋 𝑙is satisfied.
𝑙=where E denotes Young’s modulus, Dy represents belt
deformation in the radial direction, and I signifies the momentof inertia of area. Belt deformation Dy in the radial direction is
written geometrically as the following equation.
𝑙
𝐷𝑦 = + 𝑡 + 𝑟𝑤 − 𝑟𝑝𝜋Fig. 9. Schematic diagram of the crawler belt and worm shaft fortheoretical analysis.

(4)Regarding Eq. (4), when the belt length l satisfies Eq. (5), Eq. (6)3𝜋(𝑟 − 𝑡 − 𝑟𝑤 )2 𝑝

𝜕𝑊
= 0 and
𝜕𝑙

𝜕2𝑊
<0
𝜕𝑙 2

(5)
(6)

Therefore, the elastic force has a local maximum value withrespect to the belt length. In addition, the belt length that
produces the local maximum value is not related to physicalproperties such a Young’s modulus. These qualitative
characteristics are consistent with the experiment results.
Solely from these results and theoretical discussion, theoptimal crawler belt length cannot be ascertained for thefunicular flexible crawler that runs in the large intestine.
However, these experiment results suggest the importance ofascertaining the adequate crawler belt length to obtain high5elastic force and thereby realize a funicular flexible crawler thathas high propulsion force.

θr

y
x

Colonic fold
Head crawler(a) One belt(b) Two belts

Fig. 10. Head crawler in a bending area of the large intestine.

0s

0s

10 s

15 s

15 s
(a) One belt

30 s

(7)

where fl represents the magnitude of the frictional force vector
acting on the right side of the crawler belt from the colon. Also,θl denotes the angle between the x axis and the direction of thefrictional force vector acting on the right side of the crawler belt
from the colon. In this case, if θl is greater than 0 deg and lessthan 90 deg, then the thrust force is obtainable in the right
direction through frictional force fl. Therefore, it can passthrough the bent area curved to the right.
For two belts arranged on the left side and the right side, aspresented in Fig. 10(b), the head crawler thrust force ftwo_x in thex axis direction can be expressed as the following equation.
𝑓𝑡𝑤𝑜_𝑥 = 𝑓𝑙 cos 𝜃𝑙 + 𝑓𝑟 cos 𝜃𝑟θlx

Next, the crawler unit for turning will be described. The
crawler unit for turning is arranged at the flexible crawler head:it steers the crawler, enabling it to enter deeply into bending
areas of the large intestine. This study evaluated the bending
path performance of head crawlers with two belts and one belt.
Results show how crawlers can pass through a bent portion of
the rectum of the large intestine model (bending to the right).
First, we discuss the bending path performance of headcrawlers theoretically. Fig. 10 portrays the schematic diagramthat the crawler runs at the bending area of colon in the cases of
one belt and two belts. In this situation, we assume that only the
crawler belt contacts with the large intestine to simplify the
discussion.
In the case of a single belt arranged only on the left side of
the frame, as presented in Fig. 10(a), the head crawler thrustforce fone_x in the x axis direction can be expressed as
𝑓𝑜𝑛𝑒_𝑥 = 𝑓𝑙 cos 𝜃𝑙θlyIV. CRAWLER UNIT FOR TURNING

Fr

Fl

Fl

(8)

Therein, fr represents the magnitude of the frictional force
vector acting on the right side of the crawler belt from the colon.
Also, θr stands for the angle between the x axis and the directionof the frictional force vector acting on the right side of thecrawler belt from the colon. When the left side belt and the rightside belt are placed symmetrically with respect to the y axis,
fl=fr and θl = -θr. Then ftwo_x becomes 0, rendering passage
through the bent portion difficult.
Fig. 11 portrays the state of the respective paths in the casesof one belt and two belts. Experiment results show that the headcrawler with two belts was unable to pass through the bent area.
That with one belt passed through the bent section in about 15s.
Based on the results presented above, only one belt of the top
crawler was used. For bending to the left, if the shaft is rotated
180 deg by the operator's hand, then it can pass through the bent
area on the same principle.
V. FUNICULAR FLEXIBLE CRAWLER
A. Prototype
Fig. 12 exhibits the flexible crawler prototype with a crawler(b) Two belts
Fig. 11. Turn at the curved colon: (a) one belt and (b) two belts.

unit for propulsion and for turning. Table I presents its
specifications. The prototype crawler has 1400 mm total length.
A crawler unit for propulsion and a crawler unit for turning
have respective outer diameters of ø16.4 mm and ø12.7 mm
when the crawler belt is deformed maximally. The flexible shaft
outer diameter is ø8 mm. This prototype consists mainly of
three crawler units for propulsion, a crawler unit for turning,
and an actuation unit. Because more than four crawler units
for propulsion cause twist deformation of the flexible shaft
because of the motor torque, three crawler units wereselected for propulsion.
The crawler unit frame was molded of epoxy resin using
stereolithography (Form2; Formlabs Inc.). In this crawler, theslip between the belt and the pipe/phantom was observed
under all belt teeth conditions. The slip increases theinsertion time and load on the colon. Therefore, the seven
teeth belt that provides maximum friction was selected toreduce the slippage. The worm was an aluminum rod (A2017)
produced using an NC milling machine. The flexible inner shaft(LB-2.1-2.5-2.9-2-0.2; Hagitec Inc.) and flexible outer shaft(LB-7.0-7.6-8.0- 2.0-0.2; Hagitec Inc.) comprise a sleeve and astainless steel thin sheet coil. The sleeve, formed by weaving
narrow stainless steel wires, was arranged on the outer thinsheet coil. The actuation unit included a DC geared motor

6(GP13ASL; Maxon Motor AG). The interval between the
crawler unit for propulsion and the crawler unit for turning, andthe interval between crawler units for propulsion were,respectively, 150 mm and 100 mm.

Crawler unit for
propulsion (CrP)

B. Traction force
The crawler unit moves forward by the frictional force
between the crawler belts and its ground contact surface.
Therefore, the maximum traction force f of a crawler unit canbe expressed as
𝑓 = 𝜇𝑁(9)where μ denotes the friction coefficient between the crawler belt
and its ground contact surface. N denotes normal force acting
on the ground surface from the crawler belt.
The flexible crawler generates greater traction force byconnection of multiple crawler units to the flexible shaft as thetotal frictional force between the crawler belt and its groundcontact surface increases. Traction force Ft of the flexible
crawler with multiple crawler units is expressed as Eq. (10),𝑛𝐹𝑡 = ∑ 𝑓𝑖

Geared motor

(10)

𝑖=1

where n represents the number of the crawler units connected
to a flexible shaft.
We verified whether the traction force of the flexible crawler
actually increases, or not, by increasing the number of crawler
units. Fig. 13 presents an experimental system used to measure
the flexible crawler traction force. The experiment, conductedwith a ø28 mm pipe fixed in the ground, measured the flexible
crawler traction force using a force gage when driving in thepipe. The crawler belt of the crawler unit for turning wasremoved to measure the traction force of the crawler unit for
propulsion. In addition, A lubricant was used in the pipe tosimulate a wet condition.
Regarding measurements, the maximum traction force wasmeasured five times for each condition. Then the average offive measurements was taken as the traction force measurement
result. Fig. 14 presents the measurement result. The respective
traction forces of one unit, two units, and three units were 0.67
N, 1.36 N, and 2.06 N. Results verified that the traction force ofthe flexible crawler increased almost linearly with increasing
numbers of crawler units.

Crawler unit for
turning (CrT)

Flexible shaft
Fig. 12. Prototype of a funicular flexible crawler.
TABLE I. FUNICULAR FLEXIBLE CRAWLER PROTOTYPE SPECIFICATIONS
Total length

1400 mm

Weight

187 g

M otor

DCX14L 10.5W 103:1, M axon M otor AG

Interval between crawler unit for propulsion
and crawler unit for turning
Interval between crawler units for
propulsion
Outer diameter of crawler unit for
propulsion

150 mm
100 mmø16.4 mm

Outer diameter of crawler unit for turningø12.7 mm

Flexible shaft
Outer diameter of outer flexible shaft

ø8 mm

Outer diameter of inner flexible shaft
Crawler belt

ø2 mm

M aterialsilicone rubber

Width
Height of belt part9.0 mm
1.5 mmHeight of tooth part1.0 mmGeared motor
Crawler unit for propulsion (CrP)
Pipe(ø28)

Crawler unit for turning (CrT)パ

StringForce gage

ひ フォ

Fig. 13. System for measuring traction force of the funicular
flexible crawler.
2.5

Traction force [N]

2.5

C. Moving velocity
This flexible crawler was designed with multiple crawler units
arranged on the flexible shaft to suppress the moving velocity
loss caused by friction between the flexible shaft and its contact
surface. Therefore, we verified whether the moving velocity
loss is actually suppressed by increasing the number of crawler
units, or not.
Fig. 15 presents an experimental system used to measure theflexible crawler speed. Three flexible crawlers were used in thisexperiment: one with a crawler unit for turning and a crawler
unit for propulsion, one with a crawler unit for turning and two
crawler units for propulsion, and one with a crawler unit for
turning and three crawler units for propulsion. The experiment

2.06

2
2.0

1.5

1.36

1.5

1
1.0

0.67
0.5

0.5

0

0

1
2
3
1ユニット
2ユニット
3ユニット
Number of crawler units for propulsion
Fig. 14. Traction force of the funicular flexible crawler vs. crawler
units used for propulsion.

7

D. Phantom Insertion Experiment
Examining the entire large intestine requires insertion of thecolonoscope tip up to the cecum. In addition, the actual large
intestine is extremely soft and movable, with a complicatedshape. For that reason, medical students and young doctors
often use the silicone rubber large intestine phantom for
insertion training. Its shape and compliance resemble those of
actual large intestines. For this study, we conducted insertion
experiments using a large intestine phantom (M40 11361-000,
Kyoto Kagaku Inc.) to verify the usefulness of this flexible
crawler. Results of this experiment verified the relation between
the crawler unit number and the insertable distance.
The silicone rubber large intestine phantom consists of an anus
part, rectum part, the sigmoid colon part, the descending colonpart, the transverse colon part, the ascending colon part, and thececum part. Regarding the phantom specification used for thisstudy, the phantom total length from the anus to the cecum isabout 1000 mm. The phantom inner diameter between therectum and the cecum is 28–55 mm. The anus part innerdiameter is 20 mm. The phantom thickness is about 1.4 mm. A
lubricant was used in the pipe to simulate a wet condition.
The flexible crawler was inserted from the anus to the cecum.
In the phantom insertion experiment, the operator knows which
direction to rotate the device by the LED light installed in the
crawler head. Four flexible crawlers were used with the moving
velocity experiment described above. The experiment was
conducted three times for each condition. The flexible crawler
runs inside the phantom using the traction force generated by
the crawler unit for propulsion. At the bent area, it steers using
the steering mechanism by the crawler unit for turning as
described in section IV. The gear motor rotation rate was set as2000 r/min for all flexible crawlers.
Fig. 17 and Table II present experiment results obtained forphantom insertion. Results suggest that increasing the numberof crawler units is effective for improving the propulsion
performance inside the large intestine phantom. In addition,

500 mmTarget position

Start position

Fig. 15 Running the funicular flexible crawler in an L -shaped pipe.
200

Time to reach the target [s]was performed with the same prototype, but without engaging
all crawlers. In the experiment, conducted with a T-shaped pipeof ø28 mm fixed to the ground, time to reach the target of eachflexible crawler was measured as it moved through the pipe.
Measurement was started when the head crawler unit reached
the end of a straight pipe positioned in front of a tee. It was
terminated when the head crawler unit reached 500 mm fromthe tee. A lubricant was used in the pipe to simulate a wet
condition. The rotating speed of the gear motor was set as 2000
r/min for all flexible crawlers. Time to reach the target wasmeasured five times for each condition. Then the average offive measurements was taken as the time to reach the target
measurement result.
Fig. 16 presents the measurement results. Results verify thatthe time to reach the target decreased concomitantly with anincreasing number of crawler units used for propulsion.
Multiple crawler units reduced the slip between the crawler
belts and the pipe. The results presented above demonstrate that
increasing the number of crawler units is effective for
improving propulsion performance.

180

160
140

120
100

80
60

40
20

0

1

2

3

Number of crawler units for propulsion
Fig. 16. Time to reach the target vs. the number of crawler units.
Transverse
colon

Ascendingcolon

Descending
colon

Cecum
Tip of the flexible
crawler (6min 12s
from anus)

RectumSigmoid
colon
Anus
Insertion

Fig. 17. Phantom insertion experiment of a prototyped funicular
flexible crawler.
TABLE II
EXPERIMENT RESULT OF PHANTOM INSERTION: CRT AND CRP DEPICTCRAWLER UNIT FOR TURNING AND CRAWLER UNIT FOR PROPULSION.
Number of units
CrT
CrP
1

0

1

1

1

2

1

3

RectumSigmoidcolon

Descending Transverse
colon
colon

Ascending
colon

Cecum

8
results demonstrated reported that the flexible crawler headreached the cecum when three crawler units were used for
propulsion. The flexible crawler head reached the cecum inabout 6 min. That performance seems to be sufficient because
conventional colonoscopy lasts approximately 45 min for an
entire procedure [22]. Future works will conduct an ex vivo
insertion test to demonstrate the effectiveness of the flexible
crawler for colonoscopy.
VI. CONCLUSIONAs described herein, we proposed the funicular flexible
crawler mechanism and described its expected applicability tocolonoscopy as a first step for this study. Our conclusions andfuture works can be summarized as follows:(1) The proposed crawler comprises a flexible shaft attached toa multiple crawler unit and a geared motor. Miniaturizationof the outer diameter (ø16.4 mm) of the device wasachieved.
(2) Experiment and theoretical analysis confirmed a qualitative
relation between the crawler belt length and the elastic force.
Based on this result, it is expected that a model withconsideration of the deformability of colon tissues and therelative movement of the lumen is constructed for
optimization of the belt.
(3) In-pipe running experiments show that the number ofcrawler units increases the traction force and the moving
velocity. The relation between the belt slip velocity andthe traction force, or between the belt slip velocity andthe friction force of the scope in the pipe, must be
examined more deeply in future studies.
(4)

Phantom insertion experiments demonstrated that
increasing the number of crawler units is effective for
improving propulsion performance. The flexible crawler
with a crawler unit for turning and three crawler units for
propulsion was able to run from the rectum to the cecum.

In the future, practical design for diagnosis, and clinicalevaluation are also expected. Regarding practical design, theflexible crawler has no device for diagnosis or for MIS yet.
Nevertheless, because the flexible crawler is a wired scope andnot a capsule, adding such a device would presumably be easierto accomplish than if one were to use a capsule. It is importantto realize that the single bend on the tip can manage left andright angles after applying roll of the entire scope. That canrealize colon inspection and can reduce pain for the patient andenable diagnosis behind the bends. Such a steering system
must be constructed for future study. Regarding clinicalevaluation, the crawler belt edge might damage colon tissue by
the scope or belt rotation. It is important to demonstrate that theflexible crawler does not damage actual colon tissue.

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