Keywords: Mars rover, Flange, Deformation analysis, Stress distribution

Transcription

1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 574, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland The Static Analysis for the Flange of a Prototype Mars Rover Based on a FEA Software Yang CAO 1,3,a, Hong-bing TAN 2,b, Masakatsu G. FUJIE 1,c and Qi-xin CAO 2,d 1 Graduate School of Advanced Science and Engineering, Waseda University, Tokyo, Japan 2 School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China 3 College of Engineering Science and Technology, Shanghai Ocean University, Shanghai, China a a15caoyang@fuji.waseda.jp, b hongbing.tan@gmail.com, c mgfujie@waseda.jp, d qxcao@sjtu.edu.cn Keywords: Mars rover, Flange, Deformation analysis, Stress distribution Abstract. A reliable flanges for steering racks of a Mars rover prototype needs the structural analysis of detail design. This paper presents the static analysis result from a FEA (finite element analysis) software. We set the preloading force of screwed connection and the load of vehicle on the flange. The result of analysis shows the extent of deformation is within the acceptable range and the permanent deformation and fracture does not happen. Introduction The importance of the exploration of Mars is now well recognized. Increasing countries have tried to explore Mars by several methods since the first mission of Soviet Union [1]. One of the methods is Mars landing which is through releasing the rover on the surface of Mars. Since the first landing rover in 1997, JPL has successfully landed four rovers on the surface of Mars, including the Sojourner rover [2] in 1997 and the Spirit [3] and the Opportunity [4] in Then the newest successful landing mission is the Curiosity in 2012 [5]. According to many cases [6, 7], the classic design of Mars rover is based on the Spirit and Sojourner, which adopt the structure of six-wheeled rocker called as "Rocker-bogie" Suspension System [8]. This system consists of the left suspension, the right suspension, main body and wheel system. And the suspension system is how the wheels are connected to and interact with the rover body [9].Having summarized the basic features of Mars rover vehicle structural system, combined with the status quo of China's Mars rover landing on Mars for the first time mission requirements, we proposed a six-wheel rocker - rover steering rack structure (Fig.1). Mars rover has three basic movement patterns (Fig.2): (1) Go forward and back. (2) Situ rotation (zero turning radius of rotation). (3) Large radius rotation (rotation around a specified point). To satisfy (2) and (3) patterns, the mobile system of the Mars rover we have developed is consist of 6 independent drive wheels, including the front and rear four independent steering racks (Fig. 3 a).for the manipulation of these steering racks, we put a servo motor inside every flange of rack (Fig. 3 b). Mechanically, the flange is the most important part of the steering rack, because it needs to be tough enough to be under vehicle load and avoids excessive deformation which may be harmful to the motor inside. The purpose of this paper is to present a stress analysis in order to assure the flange that it can satisfy sufficient load capacity and protect the motor inside simultaneously. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/04/16,06:13:10)

2 254 Recent Research on Mechanical Engineering, Mechatronics and Automation Fig.1 The Mars rover with six-wheel rocker - rover steering rack structure a. Go forward and back b. Situ rotation c. Large radius rotation Fig. 2. Three movement patterns Rear steering racks Front steering racks a. The front and rear b. Four independent steering racks c. A servo motor for steering Fig. 3. The steering racks

3 Applied Mechanics and Materials Vol Materials and Methods The study was conducted using a 3D finite element analysis method by Solidworks 2012 structural analysis program (Solidworks Corporation, Concord, MA, USA).As Fig.4 is shown, the load from vehicle happens in the two screwed holes of the flange which are to fix the flange and main part of the Mars rover together. So, the load is transmitted through the two screws and then presses on the wall of screwed holes. On the other hand, we connected the flange with steering rack by 6 M4 screwed holes. We designed the stress analysis according to following characteristics. Weight (The main part of Mars rover) : 40 [kg] Materials of flange : 2024Aluminium alloy Screw type : M4 Materials: alloy steel There were three steps included in stress analysis of this study. The first step was to calculate the screw preloading torque T, and the screw preloading calculation formula is shown below: T K F d (1) F 0.5 s As (2) The Kin Eq.1 is bolt pre-tightening force coefficient and its value is The din Eq.1 is nominal diameter of M4 screw. The Fin Eq.1 represents the value of bolt pre-tightening force that it is calculated from Eq.2. The s in Eq.2 represents yield limit that its value is [MPa], which can be obtained according to mechanical property of M4 screw used in this study. The A sin Eq.2 represents nominal that its value is 8.78, which can be also obtained according to mechanical property of M4 screw used in this study. Finally, value of the screw preloading torque T could be calculated as [Nm]. Solidworks structural analysis program software has a specialized function that is to set screw connection element (Fig.5). The torque T obtained from above calculation was used as a parameter of this screw connection function. The second step of stress analysis was to fix the bottom surface of the flange and added screw connection function with T to 6 M4 screwed holes which were arranged in an annulus (Fig. 6). The last step was to set a 150 [N]static axial load, which was a conservative estimated force applied to two screwed holes. Thus, we can calculate the stress distributions (Fig. 7). Screwed holes Screwed holes a. Before transparency b. After transparency Fig. 4. Two screwed holes for connection between flange and vehicle body

4 256 Recent Research on Mechanical Engineering, Mechatronics and Automation 6 M4 screwed holes Fig. 5. The effect of screw connection function Fig. 6. Green arrows assumed as fixed direction 150[N] a. Pink arrow represents the load application b. Stress distributions Fig. 7. Static axial load Fig. 8. The analysis results of deformation

5 Applied Mechanics and Materials Vol Result Fig. 9. Distribution of von Mises stresses (MPa) at the flange Results were shown by considering URES displacement [10] and von Mises criteria [11].Calculated numerical data were transformed into color graphics to better visualize mechanical phenomena in the models [12]. The analysis of the URES displacement values reveal that maximum deformation concentrations is located at the top part of flange, which is toward to the main body of vehicle (Fig.8). The maximum URES displacement value shown in Fig.8 is 2.371e -3 [mm]. The analysis of von Mises stress values reveal that maximum stress concentrations is located at three screwed holes which is further than the other three according to the distance from the main body of vehicle(fig.9). It can be seen that, the stress values were concentrated upon small piece of the area inside the screwed holes (Fig.9). The maximum von Mises stress value shown in Fig.9 is [N/m 2 ] or [MPa]. Discussion The use of Solidworks structural analysis program software was in order to ensure the quality and performance of our design before we commit to production. Actually, there are many exited finite element analysis software developed by different companies. Therefore, we believed the result would be different by calculation of other software. The reason why we used the Solidworks software to analysis deformation and stress concentrations was same company s production as CAD program we used to design Mars rover. So, we could analyze the original CAD file directly and avoid file format conversion, which might cause loss of features. Furthermore, this structural analysis program can add screw features without drawing up thread in the CAD stage. Judging the analysis result shown in Fig.8, I can conclude that the maximum deformation value 2.371e -3 [mm] is under safety range, because the nearest distance between servo motor and flange in wall is 4[mm] (Fig.3 b). The materiel used in flange was 2024 Aluminium alloy, and its yield limit value is 96 [MPa] [13]. Obviously, the design of flange won t cause permanent deformation and fracture theoretically though observation of Fig.9. In addition, we added the screwed property to the holes of flange, so that is why the stress values are concentrated upon small piece of the area inside the screwed holes (Fig.9), because it is screw thread.

6 258 Recent Research on Mechanical Engineering, Mechatronics and Automation As a result, in statics opinion, this flange structure was accepted to the whole design of Mars rover. However, Mars rover is a dynamic device, therefore the load upon on flange will be dynamically changing. In future step, we should exert dynamic force on the flange in order to test its reliability under dynamic situation. Conclusion Within the static structural analysis of this study, the following conclusions were drawn: 1. The deformation caused by load from main part of rover does not affect or collide the inside motor. 2. The flange is tough enough to be under vehicle load without permanent deformation and fracture Acknowledgment This research is part of the project supported by the National Natural Science Foundation ( ). It has also been partially supported by Shanghai Education Commission (12ZZ014), Doctoral Fund of Ministry of Education of China ( ) and. Great thanks are addressed to them by the research team. References [1] R.Stuart (2008). "Journey Through the Galaxy" Mars Program: Mars ~ SJR Design. Retrieved [2] R.Team, Characterization of the Martian surface deposits by the mars pathfinder rover, sojourner. Science 5, 278(5344): [3] W.Squyres,R.E. Arvidson, J.F. Bell III, J. Brückner, N.A. Cabrol, et al., 2004a. The spirit rover's athena science investigation at gusev crater, mars. Science 6, 305(5685): [4] W.Squyres, R.E. Arvidson, J.F. Bell III, J. Brückner, N.A. Cabrol, et al., 2004b. The opportunity rover's athena science investigation at meridiani planum, mars. Science 3, 306(5702): [5] A.K.Richard, Hang On! Curiosity is plunging onto mars. Science 22, 336(6088): [6] K.Iizuka, Y. Sato, Y. Kuroda and T. Kubota, Experimental study of wheeled forms for lunar rover on slope terrain. 9th IEEE International Workshop on Advanced Motion Control, pp: [7] R.A. Lindemann, C.J. Voorhees, Mars exploration rover mobility assembly design, test and performance. IEEE International Conference on Systems, Man and Cybernetics, 1: [8] R. Volpe, J. Balaram, T. Ohm, The Rocky Mars Rover Prototype, IEEE International Conference on Robotics and Automation, April , Minneapolis MN [9] Surface Operations: Rover, [10] URES displacement: [11] M. Sevimay, A. Usumez, G. Eskitascioglu (2005), The influence of various occlusal materials on stresses transferred to implant supported prostheses and supporting bone: a three-dimensional finite-element study. J Biomed Mater Res B ApplBiomater73: [12] O. Eraslan, O. Inan, The effect of thread design on stress distribution in a solid screw implant: a 3D finite element analysis, Journal: Clinical Oral Investigation, Volume 14, Issue 4, pp [13] ALCOA MILL PRODUCTS, INC., ALLOY 2024 SHEET AND PLATE.

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