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Abstract—This paper describes a tube actuator aimed for a robot to inspect in the narrow and curved path. The tube is made of silicone, and its surroundings are restrained. This actuator has mainly two functions: enabling advancing of the robot, and enabling it to curve into the desired direction. The advancing motion is promoted by pressuring, which draws out the folded tube, and the curving motion is achieved by changing the restraining condition of surroundings of the tube. Preliminary experiments showed that the tube can curve 90 degrees under the pressure of 0.2MPa. Furthermore, to curve the tube into desired direction, the restraining material can be made of water-soluble film or thermoplastic film, which changes its mechanical properties under certain conditions. Using this actuator, the developed robot was able to advance through narrow path and curve through the path in directions of three dimensions.

I. INTRODUCTION At a disaster site, rescuing rapidly the victims underneath a

debris environment is required. As methods for the inspection, the use of a pole equipped with a camera on its head, or a fiber scope are applied. However, a pole equipped with a camera has a problem that its straight shape makes the inspection in curved path impossible. On the other hand, the flexibility of a fiber scope enables it to be inserted in curved path, but it is difficult to inspect in narrow path since the more deeply it is inserted, the more sliding friction between its surface and the debris increases and prevents it from advancing. Therefore, a rescue robot that can inspect though narrow and curved path is required.

Up to now, a robot that is able to advance without increasing the sliding friction has been proposed by Tadokoro et al. [1] It has thin hair on its surface, and the driving force is generated at any part of its surface by vibrating the hair. Therefore, the relationship between the driving force and the sliding friction stays constant, even if the robot proceeds into narrow path more deeply. However, it is difficult to steer the robot into the desired direction. Even if its head is turned in the desired direction, the advancing direction of the robot not necessarily corresponds with the desired direction, since its

Akihisa Mikawa is with Department of Mechanical and Control

Engineering, Tokyo Institute of Technology, 2-12-1-S5-19 Ookayama Meguro-ku, Tokyo, Japan (e-mail: a-mikawa@cm.ctrl.titech.ac.jp).

Hideyuki Tsukagoshi is with Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1-S5-19 Ookayama Meguro-ku, Tokyo, Japan (e-mail: htsuka@cm.ctrl.titech.ac.jp).

Ato Kitagawa is with Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1-S5-16 Ookayama Meguro-ku, Tokyo, Japan (e-mail: kitagawa@cm.ctrl.titech.ac.jp).

driving force is generated at all the body except its head. On the other hand, the robot which generates the driving force with its head has been proposed by the Authors [2]. This enables to steer the robot into the desired direction by using the tension of wire. However, it is difficult to control the robot in the complicated path since there is a limit within the range which the tension is transmitted.

I.D.Walker et al. [3] has proposed a similar method to the above without using the tension of wire to steer the robot which has several degrees of freedom, and its shape is similar to the arm of an octopus. However, the degree of freedom of the pneumatically driven robot generally depends on the number of the valves able to be used. Therefore, a lot of valves are needed in order to realize a complicated movement. As result, it is concerned that the size of the robot becomes large and that it is difficult to advance in narrow path. In addition, in the method to squeeze the robot in the path, it is difficult to restrain the generation of sliding friction between its surface and the debris.

In this paper, a robot that has main two functions is proposed: enabling steering into the desired direction by controlling a few of valves, and enabling advancing without increasing the sliding friction. Due to these two functions, a smaller robot can be built and it is possible to advance into the curved and narrow path deeply. In this research, the objective is focused on the robot using pipe lines (e.g. sewage line) as routes to approach the disaster site, and searching for victims (Fig. 1).

II. METHOD OF ADVANCING As a possible solution for advancing without increasing the

sliding friction, the method of drawing out a folded tube is proposed. In this method, it is possible to decrease the sliding friction to the half compared with when simply inserting a tube into the debris, since the tube that have already been drawn out sits almost still. Moreover, drawing out two or more tubes arranged face to face as shown in Fig. 2 makes the restraint of the generation of the sliding friction possible.

Tube Actuator with Drawing out Drive Aimed for the Inspection in the Narrow and Curved Path

Akihisa Mikawa, Hideyuki Tsukagoshi, Ato Kitagawa

Fig. 1. Inspecting in the pipe line

2010 IEEE/ASME International Conference onAdvanced Intelligent MechatronicsMontréal, Canada, July 6-9, 2010

978-1-4244-8030-2/10/$26.00 ©2010 IEEE 1368

One possible method to generate the force to draw out the

folded tube is to pressurize it and to use the fluid energy inside it. If the route for the fluid is cut off, the fluid energy is transformed into the driving force at the cut off point. In order to cut off the route, two methods can be thought. One is to bend a tube flattened by heat-treatment as in Fig. 3a which has been developed by the Authors [4], and another is to pinch a tube with two rollers, named “pinch roller” as in Fig. 3b proposed by the Authors [5]. In this case, the robot consists of a head unit equipped with a small camera and a microphone, and of a tube to generate the driving force. The tube is bent inside the head unit, and it is fixed in such a way to not separate from the head unit. Thus, the driving force generated at the bending point of the tube is transmitted to the head unit, and the advancing motion of the robot is achieved. On the other hand, the retreating motion is achieved by supplying pressure into the opposite side of the tube.

III. METHOD OF STEERING

A. Steering by the difference of the pressure As a possible method to steer the robot which advances by

drawing out the two tubes, generation of a difference in the pressure supplied into them is proposed. The tube supplied with higher pressure generates more driving force. Therefore, it is drawn out more than the tube supplied with low pressure, and both tubes curve in such a way that the tube with high pressurization becomes the outer side of the curve (Fig. 4). Thus, steering the robot into the desired direction is achieved by controlling the pressure supplied into the two tubes.

However, in this case, the shape of the curving tubes changes according to the variation of the pressure supplied into them. Therefore, a mechanism to keep their shape is required in order to steer the robot two or more times. Then, as a possible material to keep its shape, the thermoplastic which state changes from solid to gel by heating is proposed. In this research, the thermoplastic which presents elasticity under thestate of solid, and the property to change the state under the low temperature is required. Therefore, the thermoplastic that

includes polyester, which turns into gel under the temperature of 60 degrees Celsius is adopted. Moreover, as the case to encapsulate the thermoplastic material, a tube named “zigzag tube”, with its folding lines arranged vertically against the flat face is adopted (Fig. 6).

There are two reasons why the zigzag tube is adopted. One is to enable to curve into directions of three dimensions, and the other is to enable to change the state of the thermoplastic material faster compared with the tube which its shape is similar to a pipe, and it is able to curve into directions of three dimensions as well as the zigzag tube, since it is possible to make the area in contact to the heating source larger. If the heater is installed inside the head unit, changing the rigidity of the tube before and after drawing out is possible since the thermoplastic material inside the zigzag tube solidifies by the temperature of the surroundings after drawing out. Therefore, it is possible to keep the curving shape by making the rigidity of the zigzag tube high after drawing out it, and as result, steering the robot two or more times is achieved. Thus, the method using zigzag tube seems to be optimal solution. However, the zigzag tube can not generate the driving force by pressuring since the thermoplastic is encapsulated in it. Then, drawing out it coupled with the flat tube by pressurizing the flat tube can be adopted, and the preliminary experiment based on drawing out a set of the zigzag tube and the flat tube was performed. As result, it is confirmed that the flat tube could not generate enough force to advance the head unit, since loss of energy occurs at each bent point of the zigzag tube. In addition, it is expected that the robot becomes large with a complicated structure (Fig. 7), since at least six tubes and three valves are required in order to steer the robot three-dimensionally. Hence, a robot with a more simple structure is required.

Fig. 2. Drawing out two or more tubes

(a) (b)

Fig. 3. The two methods to cut off the route for the fluid: a)bend a flat tube; b)pinch a tube with two rollers

Heating

Fig. 5. Change of the state of the thermoplastic material

High pressure

Low pressure

Fig. 4. Steering by the difference of the pressure supplied into two tubes

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B. Steering by curving one tube actively As a robot with the most simplified structure, a robot with

several degrees of freedom, which consists of one tube and one valve is proposed. In order to curve one tube actively, changing the length of its both sides is required. Then, a tube actuator that has the structure with three layers is proposed. The first layer, i.e. the layer that is nearest to the center of the tube actuator consists of a tube that has the property to expand axially and radially by pressurizing. The second layer consists of a film to restrain the inner tube from expanding into all directions, and the third layer, i.e. the surface of the tube actuator consists of a cloth that has the property to expand only axially. If the restraint of the second layer is released, the first layer expands into all directions at the released point, and its expansion is transformed to the axial stretch by the third layer. As result, the tube actuator curves actively with the outer side of the curve being the expanded point (Fig. 8).

Using this method, a preliminary experiment has been conducted, which consists of using not a film but a cloth to restrain the expansion to all directions, with one of its parts replaced with a cloth that has the property to expand axially (Fig. 9), and a tube covered with it was pressurized (Fig. 10). The curve angle depends on not only the pressure level but also the length of a cloth with the property to stretch axially. According to Fig. 11, the tube curved by 90 degrees under the pressure of 0.2 MPa-gauge (gauge pressure hereafter) in case that the length of the cloth is 50mm. In this experiment, the tube is made of silicone, and its inner diameter is 8mm and the outer diameter is 10mm. Here, the cloth to restrain the expansion is made of nylon, and the cloth which expands axially is made of polyurethane and its width is 25mm.

In order to use this tube actuator as the method of steering, it must have the property to curve actively at any point. In other

words, the restrain of the second layer must be released at the desired point. Therefore, the second layer needs to have enough force to restrain the expansion of the first layer, and to have the property that its strength dilutes by a signal sent from the controller. Then, as possible materials of the second layer, the attention is focused on two materials: water-soluble film and thermoplastic.

The water-soluble film has the property to dissolve in water (Fig. 12). Then, the water-soluble film was adopted as the second layer, and the experiment to curve a tube at the desired point was conducted by applying water to it partially with the tube pressurized. In this experiment, as the first layer, silicone tube which inner diameter is 8mm and outer diameter is 10mm is adopted. Fig. 13 shows the result of this experiment.

This experiment confirmed that the tube curved at the point where water was applied. However, a long period of time was taken to curve it. This is because it is necessary a triple layer of water-soluble film in order to strengthen the force to restrain the expansion of silicone tube. As result, it is thought that tearing the film becomes difficult. In addition, it is impossible to revert the process after dissolving this film in water. Hence, the robot would only advance once, making it impossible to be used in several inspections.

Tear film

Pressurize

Tube that expands in all directions

Film that restrains expansion of all directions

Cloth that expands axially

Cloth that restrains expansion of all directions

Cloth that expands axially

Rucking

Flat tube

Zigzag tube

Fig. 6. Image of zigzag tube

Fig. 7. Image of a robot steered into directions of two dimensions by the difference of the pressure supplied into two tubes

Zigzag tube Tube that generates the driving force

Fig. 9. Image of the tube covered with the cloth whose one part has the property to stretch axially

Fig. 8. Structure of the tube actuator and the principle to curve

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On the other hand, the film made of the thermoplastic material has three advantages: enabling generating enough force to restrain the tube expansion even though it is thin, enabling changing its strength rapidly by heating, and enabling reverting the process, i.e. solidification with decrease of the applied heat. Then, the experiment to curve the tube, which is covered with it, at the desired point was conducted in the way to heat it partially by using a small heater. Fig. 14 shows the result of this experiment. In this experiment, though its thickness was 0.1mm or less, it was confirmed to generate enough force to restrain the expansion of the first layer that consists of silicone tube which inner diameter is 8mm and outer diameter is 10mm. Therefore, at most 5 seconds ware taken to release the restraint by changing the state of the thermoplastic material. Moreover, enough curve angle was confirmed by supplying with the pressure of 0.3 MPa after heating the range which length is 50mm and width is 25mm. Compared with the result of the preliminary experiment which consists of the tube that has two layers curved by 90 degrees under the pressure of 0.2 MPa, it is thought that the tube that has three layers generates losses in transforming the expansion of the tube into the axial stretch. This is because there is the space between the second layer and the third layer. Therefore, the expansion of the tube does not transmit to the third layer efficiently. However, it is thought that the pressure level of 0.3

MPa is within the realistic range. Therefore, the film made of the thermoplastic is adopted as the second layer. In this case, the outer diameter of this tube actuator with three layers is 12mm.

IV. PROTOTYPE OF ROBOT Fig. 15 shows the overall view of the prototype of the robot,

and Table I and Fig. 16 show its specification and its structure. In this robot, the pinch rollers are installed in the head unit since cutting off the route for the fluid by them is better in the case of the tube which its shape is similar to a pipe. In addition, the guide box is installed in order to lead the tube into the pinch roller, and a small heater is installed in its internal perimeters. In order to shorten the time that is needed to steer the robot in the desired direction, a small cartridge heater is adopted. With this, it is possible to conform the advancing direction to the desired direction in about 10 seconds by turning on the heater and releasing the restraint before drawing out the tube. Furthermore the camera with microphone is installed in the head unit in order to inspect inside the path. In the case to steer the robot, the operator chooses the advancing direction based on the image sent from the camera. Moreover, it is covered with passive wheels in order to reduce its sliding friction with the surroundings by transforming this friction to the rolling friction.

Heating

Fig. 14. Motion of a tube covered with the film made of thermoplastic

Apply water

Fig. 12. Tearing water-soluble film by applying water

Fig. 13. Motion of a tube covered with water-soluble film

Fig. 10. Fundamental motion of curving tube

Fig. 11. Angle of curving tube

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Camera with microphone

TABLE I

SPECIFICATION OF THE PROTOTYPE OF THE ROBOT

Head unit size

Length : 92.5mm Width : 47mm Height : 45mm Mass : 148.6g

Tube actuator size Inner diameter : 8mm Outer diameter : 12mm

The advancing experiment was conducted by pressurizing

the silicone tube covered with nylon-braid (Fig. 17). This experiment confirmed that the robot advanced in a straight pipe line which its length is 700mm in about 2 seconds under the pressure of 0.25 MPa. In addition, the digital force gauge was fixed on the head unit, and the force which was generated by pressurizing the tube and transmitted to the head unit was measured. Fig. 18 shows the relationship between the pressure and the driving force generated by the tube. In this graph, the value under the ideal condition is the simple multiplication of the internal pressure of the tube and its cross-sectional area, and the loss based on the resistance caused by drawing out the tube, air leakage at the cut off point, and the friction between the head unit and the inside of the pipe line is not considered. According to this graph, the driving force decreases to the half of the ideal value because of the resistance generated when drawing out the tube. In addition, it is confirmed that there is a threshold value when the pinch rollers start to rotate. It is thought that the resistance to draw out the tube is generated by pinching it tightly. Thus, the driving force is nearly zero in the area where the pressure is low. On the other hand, in the area where the pressure is high, the increasing rate of the driving

force declines. This is because the higher the pressure level supplied into the tube is, the more force to expand the interval of the two rollers is generated. Therefore, it causes air leakage and prevents the driving force from increasing. However, judging from the result that the robot moved 700mm in about 2 seconds, it is thought that the tube generates enough force to advance the head unit.

In the case of a curving pipe line, which has a single route, it was confirmed that the robot advanced inside it while curving passively following its curving shape (Fig. 19). In other words, advancing into the curving path which its route was only one was achieved by drawing out the tube simply without releasing the restraint generated by the second layer.

On the other hand, in the case that there are several routes in which the robot is able to advance, it is necessary to choose the desired route actively. Then, the experiment to curve into the desired direction was conducted by drawing out the tube covered with the film made of the thermoplastic material without the nylon-braid. In this experiment, just before the robot reached the curving point, the tube was heated by the heater installed in the guide box in order to steer the robot by curving the tube actively. However, the stable driving force was not generated since cutting off the route for the fluid by pinching the tube had was difficult because of the uneven thickness of the film. Then, as the second layer, instead of the film, the nylon-braid which had a hole in order to release the restraint at the decided point was adopted. Due to this second layer, enough driving force to advance the head unit was generated, and steering the robot into the desired direction was confirmed (Fig. 20).

Fig. 18. Driving force of pinch roller

Fig. 17. Advancing experiment in the straight pipe line

Fig. 16. Structure of the prototype of the robot

Fig. 15. Overall view of the prototype of the robot

Passive wheel

Pinch roller

Heater

Guide box

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V. CONCLUSION In order to develop a tube actuator that aims at inspecting in

narrow and curved path, drawing out the folded tube is proposed as a method to reduce the sliding friction between its surface and the debris. Moreover, steering by curving the tube is proposed.

As a possible solution to curve one tube actively, the method to change the restraining condition around the tube that has three layers is proposed, and it was experimentally confirmed that it is effective to adopt water-soluble film and thermoplastic film as the second layer.

In the final experiment, the possibility that a rescuer is able to choose the advancing direction based on the image sent from the camera installed in the head unit is shown. Therefore, it is thought that this rescue robot is effective as a method of inspection at the disaster site.

However, at the present stage, the first layer (silicone tube) expands when pressurizing again after using this actuator once, since thermoplastic which is heated in order to release the restraint becomes solidified conforming to the shape of the first layer which expands. Therefore, as a future work, a mechanism to enable reversibility of the shape will be proposed.

REFERENCES [1] K. Isaki, A. Niitsuma, M. Konyo, F. Takemura, and S. Tadokoro,

Development of an Active Flexible Cable by Ciliary Vibration Drive for Scope Camera, Proceedings of the 2006 IEEE/RS J International Conference on Intelligent Robots and System, pp.3946-3951, Beijing, 2006

[2] I. Kiryu, H. Tsukagoshi, and A. Kitagawa, Grow-Hose-I: a Hose Type Rescue Robot Passing Smooth though Narrow Rubble Spaces, Proceedings of the 7th JFPS International Symposium on Fluid Power, Vol.3, pp.821-824, Japan, 2008

[3] W. McMahan, V. Chitrakran, M. Csencsits, D. Dawson, I. D. Walker, B. A. Jones, M. Pritts, D. Dienno, M. Grissom, and C. D. Rahn, Field Trials and Testing of the OctArm Continuum Manipulator, Proceeding on the 2006 IEEE International Conference on Robotics and Automation, pp.2336-2341, Orland, Florida, 2006

[4] Y. Mori, H. Tukagoshi, A. Kitagawa, and M. Hirai, A Fluid Power Actuator that Drives along the Flat Tube and its Application for Rescue Operation, Proceedings on Spring Conference of Japan Fluid Power System Society 2009, pp.164-166, Japan, 2009 (in Japanese)

[5] H. Tsukagoshi, S. Nozaki, K. Nishizawa, and A. Kitagawa, Development and Motion Analysis of the Inscribed Pinch-roller Type Fluid Power Motor Driven by Tap Water Pressure, Journal of JFPS, Vol.35, No.6, pp.109-116, 2004 (in Japanese)

Fig. 20. Active curving experiment

Fig. 19. Passive curving experiment

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