7TH INT SYMP ON FLUID CONTROL, MEASUREMENT AND VISUALIZATION

DEVELOPMENT OF A FLEXIBLE PNEUMATIC CYLINDER WITH A FLEXIBLE TUBE Tetsuya AKAGI*, Shujiro DOHTA** and Katsuro OKABE** *Tsuyama National College of Technology, 624-1, Numa, Tsuyama, 708-8509, Japan **Okayama University of Science, 1-1, Ridai-cho, Okayama, 700-0005, Japan (E-mail : [email protected]) Keywords: Flexible Pneumatic Cylinder, Pneumatics, Power Assist, Soft Actuator ABSTRACT In this paper, we proposed and tested two types of flexible pneumatic cylinder that consists of a flexible tube as a cylinder or rod. We investigated the characteristics of the actuator. We have confirmed that the tested cylinders are useful to apply as an actuator that can be attached to the human body with the advantages of being flexible and lightweight. 1 INTRODUCTION Virtual reality technology has been flourishing as an interactive information technology. Recently, virtual reality systems that feed back to human senses have only been realized as visual and acoustic feedback. However, they are not enough to realize a force feedback system for hands, arms, legs and so on. Furthermore, due to the aging society and the decreasing birthrate in Japan, an increasing problem of providing nursing care for the elderly has occurred. As a result, it is necessary to develop systems to aid in nursing care. The actuators necessary for such a feedback system, as for power assisted nursing care of the elderly[1][2], need to be flexible not to injure the human body. The purpose of our study is to develop a flexible and lightweight actuator which can be safely used when attached to the human body. We propose new types of flexible pneumatic actuators that can be used even if the actuator is deformed by external force. We tested several types of flexible pneumatic cylinder. One of the proposed flexible pneumatic cylinders consists of a flexible tube as a cylinder, a nylon string as a rod and an elastic cylinder head. We call it a “Rod type flexible pneumatic cylinder”. The other is a “Rodless type flexible pneumatic cylinder” which consists of steel balls as a cylinder head. In this study, in order to confirm the availability of the flexible cylinder proposed in this study, we investigated the relation between generated force and supply pressure for several radius of curvature of the cylinder tube. 2 ROD TYPE FLEXIBLE PNEUMATIC CYLINDER 2.1 Construction and Operating Principle Figure 1 shows the construction of an rod type flexible pneumatic cylinder. The tested actuator is a kind of elastic pneumatic cylinder which has flexibility. In order to realize flexibility, the actuator consists of a flexible tube as a cylinder, a nylon string as a rod, one-touch joint type tube connector and an elastic cylinder head (piston). Using the one-touch joint type tube connector, it is very easy to connect the tube and adjust the stroke of the actuator. By using the flexible tube as a cylinder, the cylinder expands toward the radial direction of the tube according to the input pneumatic pressure. Therefore, we need a device to keep the sealing between the cylinder head and tube inner wall. In our previous study[3], we tested several types of flexible cylinder. 1

Tetsuya AKAGI, Shujiro DOHTA and Katsuro OKABE

Tube connector

Nylon string

Flexible tube Inlet

Flexible piston

Fig. 1 – Construction of rod type flexible pneumatic cylinder

Pressurized

Expansion

Compression

φ1.3

φ12 φ8

NBR rubber Nylon string piston Flexible tube

Stopper Y-ring type gasket (a) – Construction of cylinder head (b) – Operating principle for sealing Fig. 2 – Construction and operating principle of rod type flexible pneumatic cylinder head

Figure 2(a) shows the tested cylinder head which is one type of cylinder head that we developed[3]. We call it a “Unite type cylinder head”. The cylinder head consists of a NBR rubber cast piston and two Y-ring type gaskets which have outer diameters of 8.0 mm and inner diameters of 5.0 mm. The Y-ring type gaskets are set on the circumferential faces of the rubber piston. On both sides of the piston, there are two stoppers connected with a nylon string which has an outer diameter of 1.3 mm. The piston can slide along the nylon string between the two stoppers. Between the inner wall of the cylinder and cylinder head, silicone grease is applied to reduce friction. The actuator is very lightweight compared with any conventional pneumatic cylinder. The mass of the actuator with stroke of 1 m is about 0.1 kg. Figure 2(b) shows the operating principle for sealing between the cylinder head and the tube. The operating principle of the flexible cylinder is as follows: When a supply pressure is applied to one side of the cylinder, the elastic cylinder tube expands and the inner diameter of the tube becomes larger. At that time, the rubber cast piston slides and is pushed toward the opposite side of the tube. The rubber piston and Y-ring type gaskets expand to the radial direction and this keeps the sealing between the cylinder head and tube wall. Then the actuator generates force according to the supply pressure. 2.2 Experimental Results We investigated the relation between supplied pressure and generated force of the tested flexible pneumatic cylinder. Figure 3 shows relations between input pressure and generated force of the tested cylinder. In this figure, symbols ◆, ●, ▲, ■ and ● show the measuring data in the cases where the cylinder tube is kept straight and the cylinder head is located on the point of 45, 90, 135 and 180 degrees from the straight position as shown in Fig.3. Broken lines show the theoretical value assuming that there is no friction between the cylinder head and tube. From Fig.4, we can find that relations between input pressure and generated force of the cylinder are almost linear even if the cylinder tube is bent. In the results using a straight cylinder tube, the generated force becomes maximum. It can be found that the generated force decreases as the measuring angle θ in the bent cylinder tube becomes larger. We thought that it was caused by the nylon string (cylinder rod) contacting with the inner wall of the cylinder tube; increasing contact area according to angleθ, friction becomes larger. It can be found that the output force generated by using the cylinder is about 20 N in spite of the whole mass of less than 0.1 kg with the stroke of 1 m. We can recognize that a ratio of generated force to weight of the tested cylinder is larger than conventional pneumatic cylinders in spite of a long stroke of more than 1 2

DEVELOPMENT OF A FLEXIBLE PNEUMATIC CYLINDER WITH A FLEXIBLE TUBE

Generated force [N]

m. However, the tested cylinder has two demerits in which it can not generate pushing force and the generated force decreases according to the bending of the cylinder. 25

rB=100mm

20

:Straight : 45deg. : 90deg. :135deg. :180deg.

15 10 5 0

0

100 200 300 400 Input pressure [kPa]

Fig. 3 - Relation between input pressure and generated force of rod type flexible pneumatic cylinder

3 RODLESS TYPE FLEXIBLE PNEUMATIC CYLINDER 3.1 Construction and Operating Principle of Ball Type Figure 4 shows the construction of a rodless type flexible pneumatic cylinder. We call it a “Ball type”. The cylinder consists of a flexible tube which has an inner diameter of 8.0 mm and outer diameter of 12.0 mm as a cylinder and gasket, two stainless steel balls which have diameters of 9.0 mm as a cylinder head and a duralumin slide stage that can slide along the outside of the flexible tube. The slide stage has fifteen small stainless steel balls (which have diameters of 2.0 mm) set on the inner bore of the stage in order to press and deform the tube toward the center of the tube. In addition, between the cylinder tube and cylinder head, there is silicon grease to reduce the friction. The operating principle of the actuator is as follows. When a supply pressure is applied to one side of the cylinder, the elastic cylinder tube expands and the inner diameter of the tube becomes larger, then outer diameter of the balls are larger than the maximum inner diameter of the tube, the two inner steel balls of the tube are pushed. At the same time, they push small balls of the slide stage and move the stage while they make the tube deformed. The two balls are not connected to each other, that is, each ball can move independently. Therefore, the balls can move along the curved tube in the same manner of a straight tube, and it is easier for us to decompose or compose the elements of the cylinder compared with the case of a rod type cylinder. In addition, because no rubber parts exist in the cylinder, we expect that the rodless type cylinder is more durable than the rod type. Steel ball(2mm)

Flexible tube B

A

φ8

13.2

A'

ΦD

B-B'

Duralumin plate

B'

A-A'

Steel ball(9mm)

Fig. 4 – Construction of rodless type flexible pneumatic cylinder (ball type)

3.2 Estimation of Holding and Actuation of Cylinder In such a kind of actuator, the design parameters such as the distance φD between the center of the small balls that press the tube from outside to the center and a diameter of the inner steel ball 3

Tetsuya AKAGI, Shujiro DOHTA and Katsuro OKABE

Table 1 – Estimation for holding and actuation 　 　　Holding 　 　Actuation φD[mm] φ8.5 φ9.0 φ9.5 φ8.5 φ9.0 φ9.5 12.4 ◎ ◎ ◎ △ × × 12.6 ◎ ◎ ◎ △ × × 12.8 ◎ ◎ ◎ ○ × × 13 ○ ◎ ◎ ○ × × 13.2 ○ ◎ ◎ ◎ ◎ ◎ 13.4 × ◎ ◎ ◎ ◎ ○ 13.6 × × ◎ ◎ ◎ ○ 13.8 × × ◎ － ◎ ○

Min. pressure [kPa]

affect the condition of the friction and holding. Table 1 shows the estimation for holding the inner balls and slide stage and actuation when the pressure is applied to the cylinder. In the experiment of estimation, we used three kinds of steel ball which have diameter of 8.5, 9.0 and 9.5mm. DistanceφD is changed every 0.2 mm from 12.4 to 13.8 mm. As a method of estimation of holding, we judge the possibility of holding them when the pressure of 500 kPa is applied to the cylinder. And we also judge the possibility of holding when we give external force to the slide stage under the condition when supplied pressure is 500 kPa. As the results of holding in Table 1, each symbol ◎, ○ and × shows the case that satisfies both conditions, the case that satisfies one condition without the external force and the case of failure, respectively. As the estimation of the actuation, we observed that the slide stage can move or not when the pressure of 500 kPa is applied to the cylinder. As the results of actuation in Table 1, each symbol ◎, ○, △ and × shows the case when the stage can move smoothly, the case when it can move with the condition that the inner ball is caught in the hole of the stage temporarily (we call this condition “locked” for short), the case when it can move while appearing “locked” frequently and the case of failure, respectively. From Table 1, it can be seen that the combinations covered with bold frames are satisfied with both conditions of holding and actuation. Figure 5 shows the relation between the distance φ D and the minimum pressure for actuation of the tested cylinder. In Fig.5, each symbol ●, ▲ and ■ shows experimental results using inner steel balls with a diameter of 8.5 mm, 9.0 mm and 9.5 mm, respectively. Vertical lines show the scatter in measured values. From Fig.5 and Table 1, we can see that the cylinder by using distance φD of 13.4 mm and inner steel ball with a diameter of 9.0 mm required lowest driving pressure of 180 kPa. 500

φ8.5mm φ9.0mm φ9.5mm

400 300 200 100 0 12.4

12.8 13.2 13.6 Distance φD[mm]

14

Fig. 5 – Minimum pressure for actuation (ball type)

Photo.1 shows transient views of the slide stage when a stepwise pressure of 500 kPa is applied to the cylinder. It can be seen that the slide stage can move smoothly on the curved cylinder tube which has a stroke of 250 mm without deformation of the cylinder stroke. We can find that the tested cylinder can realize high moving speed such as more than 1 m/s. It is the same as that of a typical commercial pneumatic cylinder.

0ms

191ms

319ms

445ms

Photo. 1 – Movement of rodless type flexible pneumatic cylinder (ball type)

4

DEVELOPMENT OF A FLEXIBLE PNEUMATIC CYLINDER WITH A FLEXIBLE TUBE

4 IMPROVED RODLESS TYPE FLEXIBLE PNEUMATIC CYLINDER 4.1 Construction and Operating Principle of Roller Type A ball type flexible pneumatic rodless cylinder has the advantages of flexibility, light weight and high moving speed. However, the minimum pressure for actuation of the cylinder is larger than that of a conventional pneumatic cylinder. In order to reduce the minimum driving pressure depended on the friction between cylinder tube and slide stage or cylinder head, we propose an improved rodless type flexible pneumatic cylinder. Figure 6 shows the construction of the improved cylinder. We call it a “Roller type”. Differing from the ball type, the improved type has two rollers whose diameter is 4.0 mm instead of fifteen steel balls in the slide stage. The operating principle of the roller type is the same as the ball type. Steel balls (9mm) ΦD

Brass roller (Φ4mm)

Steel balls (3mm) Duralumin plate(3mm) Flexible tube Fig. 6 – Construction of rodless type flexible pneumatic cylinder (roller type)

4.2 Estimation of Holding and Actuation of Cylinder In order to estimate the holding and actuation of the improved cylinder, we investigated them using the parameters such as the center distance φD between two rollers and the diameter of inner steel ball. The methods of estimation for holding and actuation are the same as in the case of the ball type mentioned in section 3.2. Table 2 shows the results of the estimation. In the table, the symbols have the same meaning as in Table 1. It is remarkable to see that all tested cylinders can satisfy the condition for actuation without being “locked”. Compared with the ball type, the roller type slide stage gives the deformation in a vertical direction, that is, the inner ball is not easily caught in the space between the two rollers. Figure 7 shows the relation between distance φD and minimum pressure for actuation of the roller type. The figure shows the influence of the diameter of the inner steel ball. We can find that the minimum pressure of the roller type cylinder is130 kPa , we can reduce 32 % of friction compared with the ball type.

Table 2 – Estimation for holding and actuation Holding Actuation φD[mm] φ9.0 φ9.5 φ9.0 φ9.5 14.2 ◎ ◎ ◎ ◎ 14.4 ◎ ◎ ◎ ◎ 14.6 ◎ ◎ ◎ ◎ 14.8 × ○ ◎ ◎

Min. pressure[ｋPa]

250 200 150 100 φ9mm φ9.5mm

50 0 14

14.2 14.4 14.6 14.8 Distance φD [mm]

15

Fig. 7 – Minimum pressure for actuation (roller type)

5

Tetsuya AKAGI, Shujiro DOHTA and Katsuro OKABE

4.3 Experimental Results Photo.2 shows the experimental equipment for measuring the generated force of the tested cylinder. Both ends of the cylinder are connected with digital force gauges, the slide stage which has a gap with respect to the axis of force gauges is fixed on the table. Figure 8 shows the relation between input pressure and generated force that is the sum of force generated on both ends of the cylinder. In Fig.8, each line shows experimental results using the ball type and roller type, respectively. Each symbol shows the difference of the gap as shown in Photo.2. We can see that the generated forces are almost the same even if the cylinder strokes are given deformation. It can be said that the roller type is superior to the ball type from the point of view of less friction.

Force gauge (Pull) Inlet

Regulator Fixed stage Pressure sensor Photo. 2 - Experimental equipment

Generated force[N]

Flexible cylinder (Stroke:300mm) Force gauge (Push)

Gap 0～30mm

16

: Ball type : Roller type

12

Gap : 0mm :15mm :30mm

8 4 0 0

100 200 300 400 500 Input pressure [kPa] Fig. 8 - Relation between input pressure and generated force

5 CONCLUSIONS This study aiming at the development of a flexible and lightweight pneumatic actuator which can be safely attached to the human body can be summarize as follows: 1) We proposed and tested two types of flexible pneumatic cylinder. One is a “Rod type flexible pneumatic cylinder” which consists of a rubber piston and nylon rod. The other is a “Rodless type flexible pneumatic cylinder” that the cylinder tube is useful to use as a gasket and a cylinder rod. 2) We investigated the generated force of the rod type flexible cylinder. As a result, we can find that generated force is about 20 N in spite of the whole mass of less than 0.1 kg with the stroke of 1 m. And, we can confirm that the tested cylinder can work even when the cylinder tube is bent. 3) As for the rodless type flexible cylinder, we proposed and tested two types of the cylinder; those being ball and roller types. As the results of investigation for actuation, we can find suitable values of design parameters of the cylinder. We can confirm that the tested cylinder can realize higher moving speed of about 1 m/s which is the same as a typical commercial pneumatic cylinder. 4) By comparing the performance of ball and roller type rodless cylinders, we can find that the roller type can move more smoothly and reduce more friction between the cylinder tube and slide table or cylinder head than the ball type We can expect that the flexible cylinder is useful to apply in various fields such as assisting in nursing care tasks, medical equipment, human-friendly robots and so on. REFERENCES 1. 2. 3.

K. Yamamoto et al, Powered Suit for Assisting Nurse Labor Employing Muscle Sensor and Sliding Rotary Actuator, Proc Symp. FLUCOME’97, Japan, Vol. 1, pp 497-501, 1997. K. Yamamoto et al, Power Assisting Suit Utilizing Muscle Hardness Sensor Using Load Cell and Rotary Actuator Using Pressure Cuff, Proc. Symp. FLUCOME2000, FL_106, Canada, pp 1-6. T. Akagi, S.Dohta et al ,Development of a Flexible Pneumatic Actuator with a Flexible Tube, Proc INTERMAC2001 Joint Tech. Conf, Japan, F-1093, pp 1-10, 2001. 6