IROS2019国际学术会议论文集2520

Soft Polymer-Electrolyte-Fuel-Cell Tube Realizing Air-Hose-Free Thin McKibben Muscles Hiroyuki Nabae1, Akio Kodaira1, Tetsuya Horiuchi2, Kinji Asaka2, Gen Endo1, and Koichi Suzumori1 AbstractAmong the various pneumatic actuators, thin McKibben muscles are particularly attractive owing to their high performance in terms of high contraction ratio, high fl exibility, and productivity. One of the challenges for thin McKibben muscles and other pneumatic actuators is the fact that they generally need air hoses and air sources like compressors. This necessarily leads to bulky systems. To solve this problem, we propose and study a new approach to driving pneumatic actuators that exploits electrolysis/synthesis of water by a polymer electrolyte fuel cell (PEFC). This method could in-principle have high applicability to thin McKibben muscles. However, one challenge must still be addressed for the realization of an electrically driven thin McKibben muscle: the development of a tube-shaped soft PEFC. This paper proposes an electrically driven thin McKibben muscle with a tube-shaped soft PEFC realized through a fl owing non-electrolytic plating method. First, the proposed thin McKibben muscle is briefl y described. Then, a novel method for plating a tube-shaped soft PEFC using refl ux is introduced, and its fundamental operation is tested. After the evaluation of the PEFC, we report a prototype realization of the proposed thin McKibben muscle. The developed prototype has a length of over 170 mm, a diameter of 4 mm, and high fl exibility for bending. Finally, we elaborate on a driving experiment through which fundamental characteristics were experimentally evaluated. The prototype thin McKibben muscle succeeded in electrically controlling contraction and expansion motion with a fl exible and bendable structure. I. INTRODUCTION Robotics and mechatronics technologies are experiencing a turning point in various fi elds thanks to soft actuators. In particular, pneumatic actuators are enabling practical usages of soft actuators owing to their simplicity and long historical background. Among the various pneumatic actuators, thin McKibben muscles exhibit high performance in terms of high contraction ratio (over 20% in general), high fl exibility, and productivity. A thin McKibben muscle is one of the McKibben-type artifi cial muscles1 that has a very thin diameter and is advantageousin terms of bending softness2, 3. Additionally, it presents a high potential for practical applications, e.g., supporting orthosis4, as a rehabilitation device5, as an active cloth6, for robots7, 8, 9, etc. In spite of this high potential for practical applications, there This work was supported by KAKENHI Grant-in-Aid for Scientifi c Research on Innovative Areas ”Science of Soft Robot” project funded by JSPS under Grant Number 18H05470. 1H. Nabae, A. Kodaira, G. Endo, and K. Suzumori are with School of Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan, 2T. Horiuchi and K. Asaka are with the National Institute of Advanced Industrial Science and Technology, Osaka 563-8577 Electrical wires PEFC tube McKibben muscle (a)(b) Fig.1.ElectricallydrivenMcKibbenmuscles:(a)previously developed24; and (b) prototype proposed in this paper. Note that (a) has a short and hard PEFC mainly owing to rigid metal electrodes and a body with a large diameter and short length. By contrast,(b) features a very thin and long body with high fl exibility. exist challenges for accelerating their activities. They are ac- tually general problems for pneumatic actuators. Specifi cally, one of the challenges that pneumatic actuators have to face is that they generally need air hoses and air sources like compressors. This necessarily leads to bulky systems. Some previous research works have struggled with this problem, proposing and developing small and portable pneumatic sources10 including phase transitions of carbon dioxide on a triple point11, gasabsorption/releasing of hydrogen storing alloys 12, and irreversible chemical reactions 13, 14, 15, 16, 17, mechanical pumps18, 19, 20, etc. Most of these proposals suffer from diffi cult electrical con- trol. Taking into account this scenario, we propose and study a new approach to driving pneumatic actuators that exploits a reversible gas/liquid chemical reaction: electrolysis/synthesis of water by a Polymer Electrolyte Fuel Cell (PEFC)21. The proposed method succeeds not only in achieving an electrically controlled pressure but also in providing an energy regenerative cycle of control22, which is important for practical usages but diffi cult to accomplish for other approaches. Note that the pneumatic drive by PEFC has demonstrated control from 0 to 0.5 MPaG21 and operation of a pneumatic actuator at practical speed22. This principle was also applied to a hose-free pneumatic robot. In this case, the prototype can be driven without air-hose for actuation and with wireless control from an outer side23. Integration with a pneumatic actuator is an open topic for this technology.The proposed method is based on a simple structure composed of a proton-exchange membrane with electrodes on both 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 IEEE8281 Thin McKibben Muscle Soft PEFC Tube Electrical wire Fig. 2. Schematic fi gure of the proposed electrically driven thin pneumatic McKibben muscle with soft PEFC tube. The proposed actuator has a dual cylindrical structure and is mainly composed of a thin McKibben muscle, a tube-shaped soft PEFC, and electrical wires for applying a voltage to the electrodes. sides. This method has the potential to be integrated with pneumatic actuators and really implements an electrically driven pneumatic actuator. We have already tried to combine the proposed gas generator/absorber with the McKibben- type artifi cial muscle in Fig. 1(a) 24, which is one of the representative pneumatic actuators. The prototype succeeded concerning driving, but the PEFC has a hard body owing to rigid metal electrodes, and it is short compared to the large diameter of the actuator. This means losing the main advantage of pneumatic soft actuators. Therefore, a soft PEFC that can be inserted into thin McKibben muscles are defi nitely desired. Specifi cally, a tube shape featuring an isotropic mechanical bending property is required for easy production. In this paper, we propose a novel pneumatic thin McK- ibben muscle that can be electrically driven and fl exibly bent with a soft PEFC, as shown in Fig. 1(b). First, the proposed thin McKibben muscle is briefl y described and technical contributions are briefl y summarized. Then, a novel method using refl ux for plating a tube-shaped soft PEFC is introduced, and its fundamental operation is tested. After the evaluation of the PEFC, we report a prototype realization of the proposed thin McKibben muscle. Finally, fundamental characteristics are experimentally evaluated by the developed prototype. II. DESIGN OVERVIEW OF PROPOSED THIN MCKIBBEN MUSCLE In this section, the proposed thin McKibben muscle is briefl y described, and technical contributions other than a soft PEFC tube are explained. The schematic fi gure of the proposed actuator is depicted in Fig 2. The proposed actuator has a dual cylindrical structure and is mainly composed of a thin McKibben muscle, a tube-shaped soft PEFC, and electrical wires for applying a voltage to the electrodes. The tube-shaped soft PEFC is inserted into the thin McKibben muscle, and its wall separates the inner space of the thin McKibben muscle into two rooms not to mix generated oxygen and hydrogen. Wires with different length (a) Parallel wire routing (inside of a tube) (b) Helical wire routing (outside of a tube) Electrical wirePt electrode Proton exchange tube Fig. 3. Schematic fi gure of helical and parallel wire routing. Helical routing for outside and parallel routing for the inside of the PEFC tube to apply a uniformed distribution of voltage. One wire is used for helical routing, and a total of six wires are used for parallel wire routing: three pairs of wires with different lengths, respectively. A. HELICAL AND PARALLEL WIRE ROUTING Because of its high aspect ratio, it is diffi cult to apply a certain voltage to the whole body of the soft PEFC tube through normal methods via wire connections. Thus, we design a wire routing method named helical and parallel wire routing, illustrated in Fig. 3. A bare electrical wire just helically contacts the outside electrode to uniformly apply a voltage to the whole electrode without causing any effect of surface resistance. For the inside electrode, six bare electrical wires are inserted in parallel into the tube. These wires contact the inside electrode taking advantage of their restoring force. The optimal number of internal wires depends on parameters of the actuator like wires stiffness, diameter, tube size, etc. On the one hand, a greater number of internal wires works better because it reduces the electrical impedance. On the other hand, too many wires cause an increase of stiffness of the artifi cial muscle. Taking these factors into account, we set the number of internal wires to six for the prototype in this paper. The six wires consist of three pairs of different length, thereby achieving a uniform voltage distribution. B. PARAMETER DESIGN BASED ON AMOUNT OF SUB- STANCE In the gas generation process, water is electrolyzed into hydrogen and oxygen by an applied voltage as follows: 4H+ 4e 2H2(1) 2H2O O2+ 4e+ 4H+(2) Protons move to the negative electrode via the proton- exchange-membrane and hydrogen is generated on the negative electrode while the oxygen is generated on the positive electrode. On the contrary, the absorption process 8282 Thin McKibben Muscle Soft PEFC Tube H!O H! O! 2H!O 2H! + O! A A! (Volume ratio: A : A! = 1 : 2) (Electrolysis) (Synthesis) Fig. 4.Cross sectional view of the proposed thin McKibben muscle. The left side is an initial state whereas the right side is a driven state. The area ratio A1 and A2 is designed to be two according to the gas generation ratio of oxygen and hydrogen. is operated by a shortened circuit according to the following equations: 2H2 4H+ 4e(3) O2+ 4e+ 4H+ 2H2O(4) The volumes of generated /absorbed gases are different depending on the electrodes. To avoid overstress on the PEFC due to the difference of pressures between the two rooms, the volume ratio of the outer side to the inner side should be the same as the gas generation ratio of hydrogen gas and oxygen gas. We decided that the outside generates oxygen whereas the inside generates hydrogen. The ratio of cross-sectional area between outside and inside should be 1:2 because a larger area of electrodes is expected to produce a faster chemical response. III. TUBE-SHAPED SOFT PEFC A tube-shaped PEFC is essential for the proposed concept of an electrically driven thin McKibben muscle having high softness (because of easy production, isotropic mechanical property, etc.). However, it is diffi cult to form plating on a tubed shaped ion-exchange membrane through a method designed for plate-shaped PEFC25, 26, and plating is necessary for soft electrodes. In this section, a fabrication method of the tube-shaped soft PEFC is described, and fundamental evaluation is experimentally conducted. A. Flowing non-electrolytic plating This subsection explains a proposed plating method for soft PEFC tubes named fl owing non-electrolytic plating. In plating, key aspects to take into account are the small inner diameter of the tube and the deformation in bending motions. Another point is to combine the fl owing process inside the tube with alcohol-assisted non- electrolytic plating. On the surface of the tube, we plated Pt by alcohol- assisted non-electrolytic plating. This procedure is suggested by HS Wang et al27. Note that Pt is a hard material, and after plating on a soft material, it is easy to lose electrical conductivity precisely by deformation of base soft material. To solve this problem, alcohol-assisted non-electrolytic plat- ing is suggested. This tube is made from Nafi on, which is Pump Tube Solution of 25% ethanol in water Reduction liquid Pump A) Pump B) Pump C) Put additional reduction liquid Fig. 5. Fabrication process of plating with internal and external refl ux method. expanded by water. Its expanding ratio is about 20% by water, and about 50% by a solution of 25% ethanol in water. After non-electrolytic plating in the solution of 25% ethanol in water and subsequent removal of ethanol, this Pt plating is hard to break by deformation because it is plated in the largely expanded state of the tube. Our plating procedure is the same as that of HS Wang et al.s study. However, HS Wang et al. just plated the surface of Nafi on fi lm. In our case, we need to plate not only the outer surface but also the inner surface of the tube. To plate inside, we suggest the fl owing non-electrolytic plating method shown in Fig. 5. The setup for the proposed plating method was mainly composed of a tube pump (SJ-1211II-L, ATTO CORPORATION, fl ow rate is 90ml/hour), water container (50 50 300 mm), tubes, and two beakers. In one side of the beaker, a reduction liquid composed of 90 ml pure water, 1 ml 30 wt% ammonia liquid and 4 ml 1.5 wt% Pt(NH3)4Cl2 water solution was added. The tank with the tube was fi lled with the solution of 25% ethanol in water. Every time before fl owing reduction liquid, we added one type of liquid in the reduction process, and two types of liquids in the growth process. In the reduction process, 0.6 ml of 5 wt% NaBH4 liquid was added. In the growth process, we added two types of liquids, one is 1 ml of 5 wt% NH2OHHCl liquid, and the other is 0.5 ml of 20 wt% NH2NH2H2O liquid. It takes one hour to fl ow two types of liquid for the one-way process. After one hour, the process is repeated; however, this time the liquid is fl owed in the opposite direction. After two iterations of this two-stage process, the entire process is complete. 8283 - - - - - - - - - Electronic scale Soft PEFC Tube X stage with a micrometer Rod Fig. 6. Measurement setup for bending modulus. The setup mainly consists of an electronic scale, an X stage with a micrometer, and a rod. The reaction force when bending the prototype was measured by the electronic scale with a rod on. The prototype is set to the X stage with a micrometer and pushed down to the rod by the micrometer. 00.511.52 Displacement mm 0 0.05 0.1 0.15 0.2 Force N Fig. 7.Relationship between the reaction force with bending defor- mation and the displacement of the tip in the pushing direction of the developed soft PEFC tube. B. FUNDAMENTAL EVALUATION This subsection describes a measurement of bending modulus as well as an electrolysis experiment as a basic evaluation of the soft PEFC tube. 1) Measurement of bending modulus: Youngs modulus is measured in this subsection to confi rm that the developed PEFC tube is soft enough for the practical realization of the proposed concept. The measurement setup is depicted in Fig. 6. The setup mainly consists of an electronic scale (AJ-12K, SHINKO DENSHI CO.,LTD.), an X stage with a micrometer, and a rod. The reaction force when bending the prototype was measured by the electronic scale with a rod on. Note that the length of the test piece is 14.3 mm.The prototype is set to the X stage with a micrometer and pushed down to the rod by the micrometer. The results of the measurement are shown in Fig. 7, which plots the relationship between displacement and bending force. From the results in Fig. 7, the bending modulus and Youngs modulus are calculated as 9.6 105Nm2and 0.36 GPa, respectively. The obtained bending modulus is small enough for free bending in use with thin McKibben muscles. 050100150 0 0.3 0.6 0.9 1.2 Current A 050100150 Time s 0 5 10 Gas volume mL Fig. 8.Current and estimated gas volume under 1 atm in the electrolysis experiment. The gas volume is estimated by the measured current value. 2) Electrolysis experiment: For an electrolysis experi- ment, a constant voltage of 3 V is applied to the prototype, which is put into a cylindrical container fi lled with water. Current fl owing through is measured by a current probe. Given that two molecules and one oxygen molecule are generated for four electrons, the total gas volume (under 1 atm) V is estimated from the current value I according to the following equation: . V = 3Vm 4Fd Z Idt(5) where Vmand Fddenote the molar volume and the Faraday constant, respectively. Fig. 8 shows the current value and es- timated gas volume in the electrolysis experiment. Although the current profi le has a peak at fi rst, the value keeps almost constant for 0.3 A and the fl ow rate under 1 atm is approx. 0.058 mL/s. IV. APPLYNG TO THIN MCKIBBEN MUSCLE This section describes the application of the tube-shaped soft PEFC to a thin McKibben muscle. The driving perfor- mance of the prototype is evaluated by a driving experiment. A. ASSEMBLY PROCESS Fig. 9 explains an assembly process of the proposed actuator. The details are described below. i) Helical and parallel wire routings are set to the PEFC tube and an end of the tube is sealed. ii) The PEFC tube is put into a water-fi lled container and fi lled with water by vacuuming. iii) After sealing the other end of the PEFC tube, the tube is inserted into a thin McKibben muscle in which one of the ends is sealed previously. iv) The unit is fi lled with water by the same vacuuming process as in the second step. The other end of the unit is fi nally sealed to avoid gas and water leakage. The thin McKibben muscle consists of a silicone tube whose outer diameter is 4 mm. The close-up pictures of braiding fi bers and an end cap (wired side) are shown in