Development of An Omniwheel-based Holonomic RobotPlatform for Rough Terrain

Christopher McMurrough∗Heracleia Human Centered

Computing LaboratoryThe University of Texas at

Arlington

Harris Enotiades†Heracleia Human Centered

Computing LaboratoryThe University of Texas at

Arlington

Scott Phan‡Heracleia Human Centered

Computing LaboratoryThe University of Texas at

Arlington

Stephen Savoie§Assistive Robotics Laboratory

The University of Texas atArlington Research Institute

(UTARI)

Fillia Makedon¶Heracleia Human Centered

Computing LaboratoryThe University of Texas at

Arlington

ABSTRACTIn this paper, we present an ongoing effort to develop a ro-bust omnidirectional robotic platform for outdoor operationon non-smooth surfaces. The design of an off-road, low-cost omniwheel is presented along with a suspension systemthat will allow the platform to traverse rough terrain. Wealso provide a control architecture based on the open-sourceRobotic Operating System (ROS).

Categories and Subject DescriptorsI.2.9 [Artificial Intelligence]: Robotics—Autonomous ve-hicles, Propelling mechanisms

General TermsDesign, Experimentation, Performance, Reliability

KeywordsOmniwheel, holonomic platforms, omnidirectional platforms,assistive robotics

1. INTRODUCTIONTraditional vehicle platforms, such as car-like Ackermannsteering 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

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systems provide a relatively efficient means of forward lo-comotion, but have limited degrees of motion freedom dueto their non-holonomic configuration. Because of this, tradi-tional platforms are typically constrained by a tight couplingbetween angular and linear motion.

Holonomic platforms, on the other hand, are not limited bythese 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 tasksthat are not suitable for non-holonomic platforms. Holo-nomic platforms have been used commercially in factoryand warehouse settings for material handling [10]. In [4],an omnidirectional wheelchair platform is presented whichallows disabled workers to move boxes with a compact fork-lift mechanism. Other holonomic wheelchair platforms arediscussed in [11] and [5].

Wheel-based platforms generally achieve holonomic motionby 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, butrequire less actuators and far simpler mechanical linkagesthan their counterparts. Some hybrid approaches have beenpresented, such as [1]. Modeling and control of wheel-basedplatforms has been explored in previous work, and is gen-erally well understood. Kinematic modeling for feedbackcontrol of such platforms is detailed in [6] and [3], and adynamic model that accounts for wheel slip is presented in[12]. Considerations for energy efficient driving and controlduring motor failure is discussed in [8].

Omnidirectional and mecanum wheel based platforms, whileuseful 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 betweenapproaches, an off-road omniwheel platform could be usefulfor high mobility wheelchairs, agricultural robots, materialhandling, etc. We present an omniwheel design that uses

Figure 1: Layout of wheel module with key mea-surements shown in inches

inexpensive, readily available components that is relativelysimple to fabricate and assemble. A holonomic platform de-sign using 4 omniwheel assemblies, each with independentsuspension for off-road use, is also presented. Finally, an ar-chitecture for the control system currently being developedis discussed.

2. OMNIWHEEL DESIGNThe 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., thewheel should freely slide sideways). To facilitate this, we de-signed an assembly that utilizes commercially available 100millimeter polyurethane wheels that are commonly foundon recreational scooters. These wheels were chosen due totheir relatively low-cost, integrated bearings for smooth ro-tations, and durability in outdoor environments. Each of the16 wheels are mounted on either side of a piece of aluminumextrusion using a single carriage bolt. The resulting 8 wheelpairs are bolted to a central aluminum disc along with awheel hub. The wheel hub can be changed to accommodatea 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 arrangementof 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 orbitalwheels in multiple layers to eliminate this gap, though thisapproach does not work well on rough surfaces and is gen-erally difficult to fabricate. Our design can be fabricatedwith simple machine shop equipment, such as a hand-heldhacksaw and power drill. The fully assembled wheel mod-ule is shown in Figure 2. The wheel assembly is driven bybrushed 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 LAYOUTIn order for omniwheel platforms to move successfully, eachdrive wheel must maintain contact with the ground when ac-tuated. This design consideration is commonly overlooked inmost 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-

tem for each drive module. Each motor and wheel pair isallowed to slide upward along a pair of vertical rails against aspring. The spring applies a constant force in the downwarddirection, such that the drive module is normally pressedagainst the vehicle chassis when the wheel is on flat ground.When the platform is on rough terrain, the wheel moduleswill move upward toward the springs, which absorbs theshock and causes the wheel to maintain contact with theground. The rail and spring drive suspension assembly isshown in Figure 3(a).

Our platform is designed to utilize 4 drive modules mountedin a square configuration. Each drive module is positionedon one side of a chassis base with a 90 degree offset fromthe adjacent motors. When mounted on a 24.0 inch squarechassis base, the resulting platform is 37.0 inches in lengthand width. The arrangement of the drive assemblies andsuspension rails on the chassis base is shown in Figure 3(b).The fully assembled mechanical platform is shown in Figure4.

4. CONTROL SOFTWAREOur system utilizes the open-source Robot Operating System[7] software to facilitate code modularity and reduce devel-opment time. Each core software process is implementedas a ROS node, which compiles and executes independentlyand publishes or subscribes to ROS topics. The resultingarchitecture, shown in Figure 5, shows the minimum setof software processed needed to control the platform usingthe methods discussed in related work. New nodes can beadded to accommodate additional hardware without affect-ing existing nodes, which will be necessary as the platformis adapted to various applications. Our architecture usesstandard 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 oncecomplete.

5. RESULTSPreliminary testing of the platform was performed on botha smooth indoor surface and on mixed outdoor terrain. Theoutdoor terrain consisted of grass, tightly packed dirt, andrough patches resulting from light soil erosion. The platformwas controlled manually using a USB joystick attached to anetbook running ROS. In all cases, the vehicle maintainedfull control authority over all degrees of motion freedom (2Dposition and heading). While the omniwheels produce somevibration, the vertical suspension system absorbs the major-ity of the mechanical disturbances before they are transmit-ted to the payload area containing the electronics. Figure6 shows the platform during testing in an area consisting ofmixed grass and dirt.

6. CONCLUSION & FUTURE WORKIn this paper, we presented an ongoing effort to develop aholonomic omniwheel based platform capable of outdoor,off-road operation. A novel wheel omniwheel design, assem-bled from inexpensive and readily available components, isshown 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 ofthe vehicle performance on several types of terrain in orderto investigate its potential for various applications, such asagricultural robotics and mobile manipulation. We also planon sensorizing the platform such that advanced navigation,obstacle avoidance, and autonomous behaviors can be imple-mented. We believe that omnidirectional platforms can beuseful for many outdoor applications, and hope to demon-strate the utility of such approaches with our system.

7. ACKNOWLEDGMENTSThis work is supported in part by the National Science Foun-dation under award numbers CNS 0923494, CNS 1035913,

Figure 4: Assembled mechanical platform

Figure 5: ROS software control architecture

IIS 1238660, and IIS 1329119. Any opinions, findings, andconclusions or recommendations expressed in this publica-tion are those of the author(s) and do not necessarily reflectthe views of the National Science Foundation. Additionalsupport has been provided by The University of Texas atArlington Research Institute (UTARI).

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Figure 6: Platform during off road testing

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