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.

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
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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. 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. 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. 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).
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