IROS2019国际学术会议论文集2538

Abstract Soft contraction actuator has always been an important research field in soft robotics. This paper presents a novel soft contraction actuator which can realize high-ratio contraction when pressurized by pneumatic power. This actuator can be easily made by soft materials including rubber tube, one-way extensible cloth, inextensible cloth and wire. 2 types of structures are presented based on different working circumstances. When pressurized, the actuator can curve and then coil to a helical shape due to the different extensibilities of its opposite sides, a high contraction length and large contraction force can be achieved synchronously. Experiments are conducted to show the relationship between contraction force and contraction ratio of this actuator. The high contraction ratio of this actuator is revealed by comparing with McKibben actuator under the same condition, the maximal contraction ratio reaches 78% and the actuation stress is 8.36kPa. I. INTRODUCTION Conventionally, soft robots refer to a kind of robotic devices made of soft and extensible materials, with inherent or structural compliance 1. Compared with their rigid-structured counterparts, most of the soft actuators are driven by pneumatic or hydraulic power, usually designed to realize complex deformation including stretching, contraction, and bending. With inherent compliance, soft actuators present high adaptability to the changing environments and can ensure safe and compliant interaction with different objects 2. Because of their unique functionality and performance, different kinds of soft actuators have been designed and fabricated to support humans lives, a soft exosuit was developed for walking assistance 3, a soft robotic glove was used for hand rehabilitation 4, a soft robotic splint was developed for wrist rehabilitation 5. Some soft contraction actuators are also known as pneumatic artificial muscle (PAM) for having the same working principle with human muscle. Driven by pneumatic power, they can realize contraction movement by controlling the air pressure inside their actuator chambers 6, thus have been supposed to be widely used in bionics research, medical rehabilitation and other areas where soft interaction and contraction movement are both needed. The Mckibben pneumatic actuator was invented in 1950s and has been widely used since then, though it can generate a large contraction force along its tubular body, its contraction ratio is Peizheng Yuan is with Tokyo Institute of Technology, Tokyo, 152-8550 Japan. (phone: 080-8729-8204; fax: 03-5734-3085; e-mail: yuan.p.aam.titech.ac.jp). Ginjiro Kawano is with Tokyo Institute of Technology, Tokyo, 152-8550 Japan. Hideyuki Tsukagoshi is with Tokyo Institute of Technology, Tokyo, 152-8550 Japan. low, conventionally smaller than 40% 7. Some contraction actuators use soft bellows and can only realize high contraction ratio by applying negative pressure to their internal chambers, like the APAM (Antagonistic Pneumatic Artificial Muscle) 8 and vacuum-bellow actuator 9, But using negative pressure causes a small controllable pressure range, limited to 101 kPa. Though some helical actuators have been developed yet, they have different flaws. For example, some of them are not designed for contraction, but for expanding 10, or just for shape-changing 11. And for those helical actuators that have contraction abilities, because of only using inextensible materials 12, or letting the actuator working initially in a helical shape 13, their contraction ratio remains low. So how to design a soft actuator that can realize high contraction ratio (larger than 50%) under positive air pressure remains a problem in soft robotics. In this paper, we present a novel soft pneumatic actuator, which can realize a high-ratio contraction and a considerable contraction force at the same time when pressurized by pneumatic power through a helical deformation. It can be simply fabricated by assembling some simple and cheap soft materials, including rubber tube, one-way extensible cloth, inextensible cloth, and inextensible wires (Fig. 1(a). A typical example of the working performance of the helical actuator is shown in Fig. 1(b), where a helical actuator with 11.6mm outer diameter tied with 200 g load coils under 0.3 MPa air pressure. The shrinkage length is 60% of the initial length of the actuator, meaning 60% contraction ratio is realized. Figure 1. (a) Helical actuator can be fabricated by simple and cheap materials. (b) Helical actuator can realize 60% contraction ratio with 200 g load under 0.3 MPa. Soft Pneumatic Helical Actuator with High Contraction Ratio Peizheng Yuan, Ginjiro Kawano, Hideyuki Tsukagoshi 2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) Macau, China, November 4-8, 2019 978-1-7281-4003-2/19/$31.00 2019 IEEE8294 II. ACTUATOR DESIGN AND DRIVING PRINCIPLE A. Two Structures of the Helical Actuator Based on different application circumstances, we propose 2 kinds of structures and fabrication methods for the helical actuator. The first kind of structure of the helical actuator is shown in Fig. 2. The one-way extensible cloth refers to the cloth made by braiding elastic string and non-stretchable fiber perpendicularly so that one direction is extensible while the vertical direction is not. The fiber angle means the acute angle between fibers weaving direction (inextensible direction) and the axial direction of the tube, this angle influences the shape of the helical actuator after pressurized. As it is shown in Fig. 2, the fabrication process which requires rubber tube, one-way extensible cloth, and inextensible wire can be conducted as follows: 1. Place the inextensible wire inside the tube on one side of the inner surface and place the tube on a piece of one-way extensible cloth. 2. Sew the cloth together into a sleeve along the tube. 3. Fix inlet and block respectively at 2 ends of the tube and fix the wire, tube, cloth and inlet (block) together, ensure the wire is attached on the same side of the inner surface of the tube. Figure 2. The first structure of the helical actuator. In the first kind of structure, only one kind of cloth is used. The tube covered with cloth curves when its inside is pressurized, because one side of the tube is stretchable, while the other side attached with inextensible wire is non-stretchable. But in our experiment, we found that the wire inside doesnt have enough toughness to endure the tension inside it. When the pressure increases, the possibility of the wire being pulled apart becomes larger. So we think the first kind of structure may not have enough durability in extreme situation like industrial application. To improve the durability of the helical actuator, we propose the second kind of structure and fabrication method of the helical actuator. In this structure, the wire inside the tube is replaced by a piece of inextensible cloth that has the same size as the extensible clothes. The structure is shown in Fig. 3, and the fabrication process can be also conducted within the following 3 steps: 1. Place the one-way extensible cloth and the inextensible cloth on the opposite sides of the tube. 2. Sew 2 kinds of cloth together into a sleeve along the tube. 3. Fix inlet and block respectively at 2 ends of the tube and fix the tube, cloth and inlet (block) together. Figure 3. The second structure of the helical actuator. In this case, the inextensible cloth will play the same role with the inextensible wire in the first structure, the durability of the actuator is increased, but this fabrication method requires more materials and one more sewing operation. B. Driving Principle We first explain the driving principle based on the structure of the first type of helical actuator with 90 fiber angle. As introduced in the last section, the rubber tube is covered with a piece of one-way extensible clothe. When pressurized with air, as the air pressure inside the rubber tube goes high, the tube will tend to expand its volume both in axial and circumferential directions. But the expansion in circumferential direction will be blocked because the one-way extensible cloth is non-stretchable in circumferential direction. On the other hand, in the axial direction, because only half side is fixed with inextensible wire, only half side can expand, the other side cant. The two sides of the actuator now have different lengths, so it will curve. When the air pressure increases, the tube tends to expand its volume bigger, thus the length becomes longer on the extensible side, the actuator will coil (Fig. 4(a). As for the second structure of the actuator, in circumferential direction the expansion is also blocked. In the axial direction, the inextensible wire attached inside the tube will play the same role as the inextensible cloth, so it will perform the same reaction as the firs type actuator does when pressurized. In Fig. 4(b) and Fig. 4(c), both types of actuators contraction movements are shown. Figure 4. (a) Driving principle of the helical actuator. (b) Shinkage of the first type of helical actuator. (c) Shinkage of the second type of helical actuator. 8295 When a traction force is applied as the end of the helical actuator, first the helix will lengthen in the axial direction, then the stress generated in the extensible direction will produce an axial force component, thus the actuator can generate a contraction force, just as Fig. 1(b) shows. III. CHARACTERISTICS In this section, we analyze the relationship between the fiber angle of the one-way extensible cloth with the helical shape of the actuator. Because 2 kinds of structures of the helical actuator have the same working principle, we conduct our analysis based on the second structure of the actuator. When fabricating the actuator, we found that the fiber angle of the extensible cloth can influence the shape of the helix after the actuator is pressurized. In general, because of the use of soft weaving materials, the fiber angle will always influence the deformation of the soft actuator, finite element simulations are often used to analyze this kind of influence 14. But in this research, we carry out experiments to reveal the relationship. In the extensible cloth, the inextensible fiber is weaved vertically to the extensible direction, and the fiber angle means the angle between the fiber direction and the axial direction of the sleeve. In general condition, the fiber angle is 90, meaning that the extensible direction of the cloth is along with the axial direction of the sleeve. But when we change the fiber angle less than 90 to make a sleeve, the actuator deforms to a new spiral under the same pressure. Because the expansion direction of the rubber tube is not consistent with the extensible direction of the cloth, a pinch angle will appear, the axial length will increase, and the helical radius will also change. In order to analyze how the fiber angle can influence the helical shape, it is necessary to understand how the shape of a three-dimensional space curve is influenced by curvature and torsion. Conventionally, a helical curve in 3-dimensional space can be described using the following equation: cos ( )sin at r tat bt = , (1) where a is the spiral radius and b is the increasing speed of the curve so it is related to the pinch angle of the spiral. In this case, the curvature and torsion can be calculated using the following equation: 2222 ,ab = + . (2) And on the country, we can also get the following equation showing that both a and b are determined by and , suggesting that the shape of the spiral can be determined by the curvature and torsion . 2222 , ab abab = + (3) When the input air pressure is given and kept constant, curvature and torsion will be determined by the fiber angle of the extensible cloth, though this functional relationship can be quantitatively derived, we dont discuss it here. We know that a and b are influenced by fiber angle, so the helical radius and axial length of the actuator will be also influenced by the fiber angle , which is shown in Fig. 5. Figure 5. Fiber angle can influence the helical radius and the axial length of the actuator. In order to show how the fiber angle influence the helical shape, we conduct a group of experiments: Several actuators of the second kind of structure are made by the same rubber tube (outer diameter =11.6 mm, length=250 mm) covered by extensible cloth, only fiber angles are respectively different. We record the axial length and the helical radius of all actuators after they are pressurized by the same air pressure (0.3 MPa). The results of this experiment are shown in Fig. 6. From the results, we can see that as the fiber angle increases from 0 to 90, the axial length of the actuator will increase then decrease, the helical radius will decrease then increase. In some special situations, when =0, the actuator can get the biggest radius and shortest axial length; when =45, the actuator can get the longest length and smallest radius and when =90, the actuator can form a spiral with short axial length and also small radius, meaning a high contraction ratio and small radial size, which is exactly what we need. Figure 6. Experiment results of the relationship between helical length, radius and fiber angle of the helical actuator. 8296 Based on this consideration, in general condition, the actuator is made by extensible cloth with fiber angle equaling 90. In the following contents, we will use this kind of actuator as default. After we decide the best fiber angle for the helical actuator, we test the contraction ability of the helical actuator by adding different loads to the actuator after pressurized and record the corresponding contraction ratios, the results are shown in Fig. 7. Figure 7. (a) Different contraction ratios of the fist type of helical actuator with different loads and 0.3 MPa pressure. (b) Contraction ratio of the second type of helical actuator with 200g load and 0.3MPa pressure. We can see that with 200 g load and 0.3 MPa, both types of the helical actuator can realize 60% contraction ratio. If no load is applied, a maximal 78% contraction ratio can be realized on the first type helical actuator. At the same time, we can see that when the load changes, the contraction ratio also changes. The gravity of the load is equal to the contraction force of the helical actuator by force balance, thus there is a correlation between the contraction ratio and the contraction force of the helical actuator, which is to be discussed in the next section. IV. EXPERIMENTS In this section, experiments are conducted to show the relationship between the actuators contraction force and the contraction ratio. Then the results are compared with the McKibben actuator to reveal the high contraction ability of the helical actuator. The typical experimental environment is shown in Fig. 8. The actuator is first pressurized to deform into a helical shape, then it is fixed at one end. On the other end, a force gauge is connected to the actuator. A ruler is placed along the moving direction. When traction the force gauge to a certain position, the actuator will lengthen and the contraction force will be equal to the reading number of the force gauge. At the same time, we record the reading number of the ruler to get the current length of the actuator, so we can get the contraction force correspondent to different actuator lengths. We define pulling the force gauge to the left as upstream, the opposite direction as downstream, to get precise data, every traction experiment is conducted twice including upstream traction and downstream traction. Figure 8. Main test device and experiment method. In our experiment, we found that hysteresis phenomena exist when we move the force along opposite directions, an example is shown in Fig. 9, where a helical actuator made of 3mm tube is pressurized under 0.3MPa air pressure. We found that in the same position, the force data in upstream is always bigger than the force data in downstream. To minimize this error, we use the average value of upstream and downstream data as the true contraction force at each length. Figure 9. Hysteresis exists in the experiments, thus average force is used as the true value in data analysis. We tested 3 helical actuators and compared their contraction abilities, 2 helical actuators using the first kind of structure with different fiber angle, and 1 helical actuator using the second type of structure. The contraction ratio is used to eliminate the influence caused by different length of these actuators, which can be calculated using following equation: 100% original lengthcurrent length contractionratio original length = . (4) Because we firstly pressurize the actuator to make them compress, then apply a traction force, the compression ratio 8297 will decrease from the maximal value as the traction force increases. All the experiments are conducted under 0.3 MPa air pressure, and all the actuators have 11.6 mm outer diameter and 8 mm inner diameter. The comparison result is shown in Fig. 10. Figure 10. Experiments are conducted using 3 different helical actuator to reveal the relationship between the contraction force and the contraction ratio. From Fig. 10, we can see that when the pinch angle reaches 45, the helical actuator will possess the biggest axial length, so its maximal contraction ratio is the lowest, but we can see the gradient of its plot is the sharpest, and the maximal contraction force is the biggest, about 60 N force can be generated when contraction ratio reaches 0. On the other hand, when the pinch angle is 90, both types of helical actuators can possess a balance between contraction force and contrac