Study Concerning the Energy-to-Mass Ratio in Pneumatic Muscles

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1 Study Concerning Energy-to-Mass Ratio in Pneumatic s Tudor Deaconescu, Andrea Deaconescu Abstract The utilization of s in actuation of industrial systems is still in its early stages, hence studies on constructive solutions which include an assessment of ir functional performance with a focus on one of most important characteristics-energy efficiency are required. A quality indicator that adequately reflects energy efficiency of an actuator is energyto-mass ratio. This ratio is computed in paper for various types and sizes of s manufactured by Festo, and is subsequently compared to similar ratios determined for two categories of pneumatic cylinders. Keywords Pneumatic cylinders, s, energy-tomass ratio, muscle stroke. I. INTRODUCTION FTER electric current compressed air is second most A important energy generator in industrial actuations. Applications of compressed air go back to 19 th century, when first pneumatic jack hammers were built to be used in mining. The increasingly large scale of compressed air industrial applications is due to its specific benefits, including [1]: compressed air can be generated in any place and quantity; high energy density, reduced weight and easy transportation; pneumatic energy is easily stored; compressed air is non-flammable and carries no risk of explosion; pneumatic systems maintenance requires minimum effort, etc. The disadvantages of compressed air utilisation concern: achieving high actuation forces requires large size working equipment; water contained by compressed air corrodes components of installations; shocks occur at end of piston stroke causing destruction of pneumatic motor; high energy consumption. The most frequently deployed effector elements of pneumatic systems are linear or rotation motors with a piston, a membrane or blades. It is role of se motors to transform pneumatic energy supplied by compressed Tudor Deaconescu (Prof. Dr. Eng.) andandrea Deaconescu (Prof. Dr. Eng.) are with Transilvania University of Brasov, Faculty of Technological Engineering and Industrial Management, Department of Industrial Engineering and Management, Bd. Eroilor 29, Romania, RO (phone: ; fax: ; tdeacon@ unitbv.ro, deacon@ unitbv.ro). air into a linear displacement or a rotation. The construction of linear pneumatic motors with membranes typically includes two casings clamping an oilresistant rubber membrane. Compared to piston motors, membrane ones have a number of significant benefits: lighter; no high manufacturing accuracies required; easy sealing, by very membrane; high sensitivity; longer service life. Despite all se advantages, a large-scale utilization of membrane pneumatic motors is limited by fact that actuation force is not constant during length of stroke. The utilization of membranes in construction of pneumatic actuation elements has known a continued development, particularly in relation to industrial robots. Results worth to mentioning were obtained by researchers from Orthopedic Centre of Heidelberg, Germany in 1948, pneumatic arm developed by American J.L. McKibben, stepping robot WAP 1 built by Waseda University Tokyo, Japan in 1969 [2]. The series of se achievements has been recently expanded by pneumatic muscle, which comes in form of a tube that contracts when fed pressure. The history of this type of starts in 1872, when professor Franz Reuleaux described first flexible pneumatic actuator [2]. It was A. H. Morin who in 1953 patented a first variant of actuator in that a lattice of textile fibers is built into a cylindrical rubber tube [3]. Since n numerous constructive solutions of artificial pneumatic muscles were developed, like ones put forward by Yarlott and Mass (1972), Takagi and Sakaguchi (1986), Kukolj (1988), Paynter (1988) or Daerden (1999) [4]-[8]. Anor constructive variant of a is one developed by Festo of Germany [9]. This paper undertakes a study of Festo s, with a focus on a characteristic known as energy-to-mass ratio. Conducting such a study is necessary because in literature on s high value of this quantity is presented as an advantage, without however offering concrete data or a comparison with values of this ratio in case of or types of pneumatic actuators [10]-[12]. Following introduction, Section II of paper presents concisely some of characteristics of s, followed, in Section III by a description of methodology used to determine values of energy-to-mass ratios for three types of Festo s. Section IV includes a comparison of values of ratios computed previously with those of cylinders of same dimensions. 1737

2 Conclusions are presented in Section V. II. CHARACTERISTICS OF THE PNEUMATIC MUSCLESS Artificial muscles broadly mimic functioning of human muscle fiber (Fig. 1) ), and benefit from a number of characteristics like shock absorbing capacity and shock resistance, low weight, reduced overall size and reducedd mass per power unity, elasticity (spring-like behavior) due on one hand to air compressibility and to variation of force with displacement, on or, easy connectivity and safe operation. Fig. 1 Pneumatic artificial muscles mimic a biological muscle Compared to classical piston pneumatic motors, pneumatic muscles have certain advantages like: at an identical diameter, considerably higher force; superior media resistance; for many applications, reduced compressed air consumption; any actuator length easy to produce; no stick-slip characteristics; silent positioning etc. The main European manufacturer of s is Festo AG & Co of Germany. The main constructive element of artificial muscles is a chloroprene flexible tube covered by a sealed envelope made from inelastic aramid fibers displayed in diamond patterns, thus forming a 3D-lattice. (Fig. 2) [ 9]. The interior diameters of chloroprenee tubes are of 10, 20 or 40 mm, resulting in three muscle types, tube wall thickness varying between one and two millimeters. In small size muscles, number of fibers is 60 displayed in two layers, while larger muscles have up to 240 fibers. The diameter of each fiber is from 0.1 to 0.3 mm [13], [14]. The precise position of each fiber is remarkablee in construction of se muscles, so that despite ir large number, no fiber touches anor. Fig. 2 Construction and structure of a Festo The mass of artificial muscles is directly dependent on ir length, but, due to materials used for ir construction this mass is significantly smaller than that of a pneumatic cylinder of similar size. Fig. 3 presents dependencies of masses of three types of muscles manufactured by Festo on ir lengths. The graph was plotted based on information extracted from SIM application, provided by manufacturer. Fig. 3 Dependency of Festo mass on muscle length The linear functions that best describe dependency of a mass on its length are: The indices attached to masses denote interior diameter of chloroprene tube of each type of muscle. The mass of s is a particularly important characteristic where computing energy-to-mass ratio is concerned. Section III of paper presentss methodology for computing values of se ratios for three types of Festo muscles and shows variation of this characteristic (1) (2) (3) 1738

3 versus actuator length. III. THE ENERGY-TO-MASS RATIO The energy-to-mass ratio is a quality indicator used for comparing various types of actuators from viewpoint of ir energy efficiency. In 2005 D. Plettenburg provided an expression for this indicator [15]: where F(s) is force developed by depending on its axial contraction s, and m is its mass. This paper puts forward a relationship for computing energy-to-mass ratio that considers specific axial contraction = s/l of, as well as its length l. When inflated by compressed air pneumatic muscle shortenss its length; this shortening is stroke of, denoted by s. With se notations, equation becomes: Fig. 4 Nomogramm F = f() corresponding to a 20 mm diameter For each type, dimension of min and max denote specific minimumm and maximum admissible axial contractions of se actuators, respectively. Similar results to those provided by SIM application are obtained also by analyzing nomograms corresponding to each type of. Fig. 4 exemplifies such a nomogram for a 20 mm diameter [16]. Starting from information in Table I, curves presented in Fig. 5 were plotted, showing dependency of forces developed by three types of s versus ir specific axial contractions, dependencies described by (6)-(8). The polynomial functions above have determination coefficients equal to 1 (R 2 = 1), what confirms thatt se relationships describe studied phenomenon with maximum accuracy. (4) Prerequisite to computing energy-to-mast describes variation of ratio is determining equation that force developed by a in dependence on its specific axial contraction. In this regard, based on information provided by SIM application it could be noted that for any given pressure (e.g. 0.6 MPa), force developed by, regardless of its length, varies within same limits. Table I features se limits for three types of muscles. TABLE I VARIATION LIMITS OF THE FORCES DEVELOPED BY A PNEUMATIC MUSCLE diameter [mm] min Maximumm force F max [N] max Minimumm force F min [N] (7) (8) Knowing relationships for forces furr on expressions for energy developed by pneumatic muscles are determined. Figs. 6-8 show working range for each type and dimension of underlying determined mamatical relationships for energies. (5) (6) 1739

4 ,, ,... (12) (13) (14) Fig. 5 Dependency of force developed by a on its specific axial contractionn The equations of energy developedd by pneumatic muscles are: (9) (10) (11) Upon introducing (1)-(3) and (9)-(11) into (5) energy- can be computed. to-mass ratios for three dimensions of s Fig. 9 features curves that describe dependency of energy-to-mass ratio on length of pneumatic muscle for each muscle diameter. Equations (12) (14) as welll as curves of Fig. 9 reveal thatt energy-to-mass ratios of pneumatic muscles increase with muscle length, i.e. with magnitude of capable stroke. The above results gain relevance when compared to or pneumatic actuators. Such comparison is conducted in Section IV of paper, selected actuators being two single-acting cylinders. IV. THE ENERGY-TO-MASS RATIO IN SINGLE-ACTINGG CYLINDERS The two pneumatic cylinders selected for determining energy-to-mass ratio belong to ESNU and to compact range ISO (model AEN), respectively, are single- a acting, with a selected piston diameter of 20 mm, and maximum stroke of 25 mm. Fig. 6 Working range of Festo MAS-10-N Fluid 1740

5 For pneumatic cylinders, Table II features characteristics of interest for computation of energy-to- mass ratio. TABLE II CHARACTERISTICS OF THE ESNU AND AEN PNEUMATIC CYLINDERS Cylinder type ESNU AEN Developed force at 0.6 MPa [N] Mass corresponding to a 0 mmm stroke [kg] Additional mass for every 10 mm stroke [kg] Fig. 7 Working range of Festo MAS-20-N Fluid Fig. 8 Working range of Festo MAS-40-N Fluid (16) The computational relationships of energy-to-mass ratios of se cylinders are:,..,.. (17) (18) Starting from se data, linear functions that describe dependency of mass of such a pneumatic cylinder on conducted stroke (s) can be determined (15) Starting from (17) and (18), Fig. 11 showss dependencies of energy-to-mass ratios on strokes of two pneumatic cylinders. In order to observe differences between energy efficiencies of a and a pneumatic cylinder, 1741

6 respectively, Table III features a comparison of computed values of energy-to-mass ratios of a 20 mm diameter and of previously analyzed cylinders. Fig. 9 Dependency of energy-to-mass ratios on lengths of s ESNU AEN Fig. 10 ESNU and AEN (Festo) type pneumatic cylinders Fig. 11 Dependency of energy-to-mass ratios on stroke of pneumatic cylinders TABLE III COMPARISON BETWEEN THE VALUES OF THE ENERGY-TO-MASS RATIOS Actuator type Stroke [mm] Pneumatic muscle 20 ESNU AEN R E/m The data in Table III allow plotting of variation diagrams of energy-to-mass ratio versuss stroke of three actuators. Fig. 12 presents se dependencies. The analysis, Fig. 12, confirms assertions found in literature, in studies on s, namely that this type of pneumatic actuator ensures a higher energy-to- dimensions. Consequently, from viewpoint of energy efficiency utilization of pneumatic muscless is mass ration than or types of pneumatic motors of similar recommended in applications requiring generation of a certain force while keeping weight of assembly at a minimum. V. CONCLUSION The study presented and discussed in paper, concerning devising of a methodology for computing energy-to- that, mass ratio for different pneumatic actuators demonstrates although yet insufficiently known and deployed, pneumatic muscles offer numerous benefits. In-depth knowledge of ir performance will enable replacement of single-acting cylinders by s in an increasing number of applications. 1742

7 Fig. 12 Comparison between evolutions of energy-to-mass ratios versus stroke of three types of actuators REFERENCES [1] E. Pashkov, Y. Osinskiy, A. Chetviorkin, Electropneumatics in Manufacturing Processes. Isdatelstvo SevNTU Sevastopol, [2] S. Hesse, The Fluidic in Application. Blue Digest on Automation, Esslingen, 2003 [3] A.H. Morin, Elastic Diaphragm. U.S. Patent No , [4] J.M. Yarlott, H. Mass, Fluid Actuator. U.S. Patent No , [5] T. Takagi, Y. Sakaguchi, Pneumatic Actuator for Manipulator. U.S. Patent No , [6] M. Kukolj, Axially Contractible Actuator. U. S. Patent No , [7] H.M. Paynter, Hyperboloid of Revolution Fluid-Driven Tension Actuators and Method of Making. U.S. Patent No , [8] Daerden, F., Conception and Realization of Pleated Pneumatic Artificial s and Their Use as Compliant Actuation Elements. Ph.D. Thesis, Vrije Universiteit Brussels, Brussels, Belgium, [9] Fluidic DMSP/MAS, Festo Product Flyer, 2015, n.pdf [10] N. Tsagarakis, D.G. Caldwell D.G, Improved modelling and assessment of actuators, in Proc. of IEEE International Conference on Robotics & Automation, April 2000, San Francisco, USA, pp [11] N. Nakamura, M. Sekiguchi, K. Kawashima, T. Fujita, T. Kagawa, Developing a robot arm using pneumatic artificial rubber muscles, in Bath Workshop on Power Transmission & Motion Control, [12] G.A. Medrano-Cerda, C.J. Bowler, D.G. Caldwell, Adaptive position control of antagonistic actuators, in Proc. of IEEE/RSJ International Conference on Intelligent Robots and Systems, Pittsburgh, USA, 1995, pp [13] Festo: Precisely to point: membrane technology, (2014) corp/14047.htm [14] M. Santora, Membrane technology flexes its muscle precisely, Pneumatic Tips. A Fluid Power World Resource, [15] D.H. Plettenburg, Pneumatic Actuators: a Comparison of Energy-to- on Rehabilitation Robotics, Chicago, IL, USA, 2005, pp [16] Festo Fluidic DMSP/MAS, corp/assets/pdf/info_501_en.pdf Mass Ratio s, in Proc. of 2005 IEEE 9th International Conference 1743