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Development of An Omniwheel-based Holonomic Robot
Platform for Rough Terrain
Christopher McMurrough∗
Heracleia Human Cente...
Figure 1: Layout of wheel module with key mea-
surements shown in inches
inexpensive, readily available components that is...
tem for each drive module. Each motor and wheel pair is
allowed to slide upward along a pair of vertical rails against a
s...
Figure 4: Assembled mechanical platform
Figure 5: ROS software control architecture
IIS 1238660, and IIS 1329119. Any opin...
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Development of An Omniwheel-based Holonomoic Robot Platform for Rough Terrain

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In this paper, an ongoing effort to develop a robust omnidirectional robotic platform for outdoor operation on non-smooth surfaces is presented. The design of an off-road, low-cost omniwheel is presented along with a suspension system that will allow the platform to traverse rough terrain. A control architecture based on the open-source Robotic Operating System (ROS) is also provided.

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Development of An Omniwheel-based Holonomoic Robot Platform for Rough Terrain

  1. 1. Development of An Omniwheel-based Holonomic Robot Platform for Rough Terrain Christopher McMurrough∗ Heracleia Human Centered Computing Laboratory The University of Texas at Arlington Harris Enotiades† Heracleia Human Centered Computing Laboratory The University of Texas at Arlington Scott Phan‡ Heracleia Human Centered Computing Laboratory The University of Texas at Arlington Stephen Savoie§ Assistive Robotics Laboratory The University of Texas at Arlington Research Institute (UTARI) Fillia Makedon¶ Heracleia Human Centered Computing Laboratory The University of Texas at Arlington ABSTRACT In this paper, we present an ongoing effort to develop a ro- bust omnidirectional robotic platform for outdoor operation on non-smooth surfaces. The design of an off-road, low- cost omniwheel is presented along with a suspension system that will allow the platform to traverse rough terrain. We also provide a control architecture based on the open-source Robotic Operating System (ROS). Categories and Subject Descriptors I.2.9 [Artificial Intelligence]: Robotics—Autonomous ve- hicles, Propelling mechanisms General Terms Design, Experimentation, Performance, Reliability Keywords Omniwheel, holonomic platforms, omnidirectional platforms, assistive robotics 1. INTRODUCTION Traditional vehicle platforms, such as car-like Ackermann steering or tank-like skid-steering, are used in the vast ma- jority of mobile ground robots. These mechanically simple ∗ email: mcmurrough@uta.edu † email: harris.enotiades@mavs.uta.edu ‡ email: scott.phan@mavs.uta.edu § email: ssavoie@uta.edu ¶ email: makedon@uta.edu Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from Permissions@acm.org. PETRA’13, May 29 - 31, 2013, Rhodes Island, Greece. Copyright c 2013 ACM 978-1-4503-1973-7/13/05... $15.00 http://dx.doi.org/10.1145/2504335.2504398 systems provide a relatively efficient means of forward lo- comotion, but have limited degrees of motion freedom due to their non-holonomic configuration. Because of this, tradi- tional platforms are typically constrained by a tight coupling between angular and linear motion. Holonomic platforms, on the other hand, are not limited by these constraints and are able to move in any direction inde- pendent of their orientation. These highly mobile platforms, while more mechanically complex, can perform some tasks that are not suitable for non-holonomic platforms. Holo- nomic platforms have been used commercially in factory and warehouse settings for material handling [10]. In [4], an omnidirectional wheelchair platform is presented which allows disabled workers to move boxes with a compact fork- lift mechanism. Other holonomic wheelchair platforms are discussed in [11] and [5]. Wheel-based platforms generally achieve holonomic motion by using specially designed omnidirectional or mecanum wheels [2] that permit sliding in certain directions, or by using stan- dard wheels with complicated steering mechanisms. Omni- wheel approaches feature complicated wheel assemblies, but require less actuators and far simpler mechanical linkages than their counterparts. Some hybrid approaches have been presented, such as [1]. Modeling and control of wheel-based platforms has been explored in previous work, and is gen- erally well understood. Kinematic modeling for feedback control of such platforms is detailed in [6] and [3], and a dynamic model that accounts for wheel slip is presented in [12]. Considerations for energy efficient driving and control during motor failure is discussed in [8]. Omnidirectional and mecanum wheel based platforms, while useful on smooth surfaces, tend to be limited in outdoor op- eration. Outdoor solutions have been presented using com- plex steering mechanisms [9], but approaches using omnidi- rectional platforms are sparse. Given the trade-offs between approaches, an off-road omniwheel platform could be useful for high mobility wheelchairs, agricultural robots, material handling, etc. We present an omniwheel design that uses
  2. 2. Figure 1: Layout of wheel module with key mea- surements shown in inches inexpensive, readily available components that is relatively simple to fabricate and assemble. A holonomic platform de- sign using 4 omniwheel assemblies, each with independent suspension for off-road use, is also presented. Finally, an ar- chitecture for the control system currently being developed is discussed. 2. OMNIWHEEL DESIGN The omniwheel utilized by our platform is designed to pro- vide traction on rough surfaces when moving in the direc- tion of wheel rotation, while providing near frictionless cast- ing in directions along the axis of the drive shaft (i.e., the wheel should freely slide sideways). To facilitate this, we de- signed an assembly that utilizes commercially available 100 millimeter polyurethane wheels that are commonly found on recreational scooters. These wheels were chosen due to their relatively low-cost, integrated bearings for smooth ro- tations, and durability in outdoor environments. Each of the 16 wheels are mounted on either side of a piece of aluminum extrusion using a single carriage bolt. The resulting 8 wheel pairs are bolted to a central aluminum disc along with a wheel hub. The wheel hub can be changed to accommodate a wide range of motors and drive shafts. The wheel design, shown in Figure 1, has an overall diame- ter of 15.0 inches. This diameter supports the arrangement of 16 wheels such that the gaps between wheels are mini- mized in order to provide the smoothest operation possible. Other omniwheel designs often employ barrel shaped orbital wheels in multiple layers to eliminate this gap, though this approach does not work well on rough surfaces and is gen- erally difficult to fabricate. Our design can be fabricated with simple machine shop equipment, such as a hand-held hacksaw and power drill. The fully assembled wheel mod- ule is shown in Figure 2. The wheel assembly is driven by brushed DC motors which can be salvaged from discarded electric wheelchairs. (a) Front view of wheel assembly (b) Side view of wheel assembly mounted on motor Figure 2: Completed wheel module after fabrication 3. PLATFORM SUSPENSION AND LAYOUT In order for omniwheel platforms to move successfully, each drive wheel must maintain contact with the ground when ac- tuated. This design consideration is commonly overlooked in most approaches, since it is generally assumed that the sur- face of the operating environment is both flat and smooth. Our approach uses an independent vertical suspension sys-
  3. 3. tem for each drive module. Each motor and wheel pair is allowed to slide upward along a pair of vertical rails against a spring. The spring applies a constant force in the downward direction, such that the drive module is normally pressed against the vehicle chassis when the wheel is on flat ground. When the platform is on rough terrain, the wheel modules will move upward toward the springs, which absorbs the shock and causes the wheel to maintain contact with the ground. The rail and spring drive suspension assembly is shown in Figure 3(a). Our platform is designed to utilize 4 drive modules mounted in a square configuration. Each drive module is positioned on one side of a chassis base with a 90 degree offset from the adjacent motors. When mounted on a 24.0 inch square chassis base, the resulting platform is 37.0 inches in length and width. The arrangement of the drive assemblies and suspension rails on the chassis base is shown in Figure 3(b). The fully assembled mechanical platform is shown in Figure 4. 4. CONTROL SOFTWARE Our system utilizes the open-source Robot Operating System [7] software to facilitate code modularity and reduce devel- opment time. Each core software process is implemented as a ROS node, which compiles and executes independently and publishes or subscribes to ROS topics. The resulting architecture, shown in Figure 5, shows the minimum set of software processed needed to control the platform using the methods discussed in related work. New nodes can be added to accommodate additional hardware without affect- ing existing nodes, which will be necessary as the platform is adapted to various applications. Our architecture uses standard ROS topics and messages, which are shown in Fig- ure 5. The control software is currently under development, and will be uploaded to the public ROS repositories once complete. 5. RESULTS Preliminary testing of the platform was performed on both a smooth indoor surface and on mixed outdoor terrain. The outdoor terrain consisted of grass, tightly packed dirt, and rough patches resulting from light soil erosion. The platform was controlled manually using a USB joystick attached to a netbook running ROS. In all cases, the vehicle maintained full control authority over all degrees of motion freedom (2D position and heading). While the omniwheels produce some vibration, the vertical suspension system absorbs the major- ity of the mechanical disturbances before they are transmit- ted to the payload area containing the electronics. Figure 6 shows the platform during testing in an area consisting of mixed grass and dirt. 6. CONCLUSION & FUTURE WORK In this paper, we presented an ongoing effort to develop a holonomic omniwheel based platform capable of outdoor, off-road operation. A novel wheel omniwheel design, assem- bled from inexpensive and readily available components, is shown along with details of the vehicle suspension system. A platform control software architecture intended for imple- mentation on the ROS platform was discussed. (a) Motor module spring rail sus- pension (b) Drive assembly arrangement Figure 3: Platform suspension and motor arrange- ment In future work, we plan on performing a formal analysis of the vehicle performance on several types of terrain in order to investigate its potential for various applications, such as agricultural robotics and mobile manipulation. We also plan on sensorizing the platform such that advanced navigation, obstacle avoidance, and autonomous behaviors can be imple- mented. We believe that omnidirectional platforms can be useful for many outdoor applications, and hope to demon- strate the utility of such approaches with our system. 7. ACKNOWLEDGMENTS This work is supported in part by the National Science Foun- dation under award numbers CNS 0923494, CNS 1035913,
  4. 4. Figure 4: Assembled mechanical platform Figure 5: ROS software control architecture IIS 1238660, and IIS 1329119. Any opinions, findings, and conclusions or recommendations expressed in this publica- tion are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Additional support has been provided by The University of Texas at Arlington Research Institute (UTARI). 8. REFERENCES [1] K.-s. Byun, S.-j. Kim, and J.-b. Song. Design of a four-wheeled omnidirectional mobile robot with variable wheel arrangement mechanism. In Proceedings 2002 IEEE International Conference on Robotics and Automation (Cat. No.02CH37292), volume 1, pages 720–725. IEEE, 2002. [2] O. Diegel, A. Badve, G. Bright, J. Potgieter, and S. Tlale. Improved Mecanum Wheel Design for Omni-directional Robots. In 2002 Australasian Conference on Robotics and Automation, number November, pages 27–29, 2002. [3] L. Huang, Y. S. Lim, D. Li, and C. E. L. Teoh. Design and Analysis of a Four-Wheel Omnidirectional Mobile Figure 6: Platform during off road testing Robot. In 2nd International Conference on Autonomous Robots and Agents, Palmerston North, New Zealand, 2004. [4] J. W. Kang, B. S. Kim, and M. J. Chung. Development of omni-directional mobile robots with mecanum wheels assisting the disabled in a factory environment. In 2008 International Conference on Control, Automation and Systems, pages 2070–2075. IEEE, Oct. 2008. [5] L. Kitagawa, T. Kobayashi, T. Beppu, and K. Terashima. Semi-autonomous obstacle avoidance of omnidirectional wheelchair by joystick impedance control. Proceedings 2001 IEEE/RSJ International Conference on Intelligent Robots and Systems. Expanding the Societal Role of Robotics in the the Next Millennium (Cat. No.01CH37180), 4:2148–2153, 2001. [6] P. Muir and C. Neuman. Kinematic modeling for feedback control of an omnidirectional wheeled mobile robot. In Proceedings. 1987 IEEE International Conference on Robotics and Automation, volume 4, pages 1772–1778. Institute of Electrical and Electronics Engineers, 1987. [7] M. Quigley, B. Gerkey, K. Conley, J. Faust, T. Foote, J. Leibs, E. Berger, R. Wheeler, and A. Y. Ng. ROS: an open-source Robot Operating System. Proc. Open-Source Software workshop of the International Conference on Robotics and Automation (ICRA), 2009. [8] R. Rojas. Holonomic Control of a Robot with an Omni-directional Drive, 2006. [9] M. Udengaard and K. Iagnemma. Design of an omnidirectional mobile robot for rough terrain. In 2008 IEEE International Conference on Robotics and Automation, pages 1666–1671. IEEE, May 2008. [10] Vetex Inc. SIDEWINDER Lift Truck, 2011. [11] M. Wada. Development of a 4WD omnidirectional wheelchair. 2008 SICE Annual Conference, pages 1767–1771, Aug. 2008. [12] R. Williams, B. Carter, P. Gallina, and G. Rosati. Dynamic model with slip for wheeled omnidirectional robots. IEEE Transactions on Robotics and Automation, 18(3):285–293, June 2002.

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