Journal of Robotics, Networking and Artificial Life
Vol. 6(3); December (2019), pp. 167–170
DOI: https://doi.org/10.2991/jrnal.k.191202.003; ISSN 2405-9021; eISSN 2352-6386
https://www.atlantis-press.com/journals/jrnal

Research Article

Spherical Mobile Robot Driven by Single Omni Wheel
Liu Wei*, Wang Runjiao, Duan Lunqin, Wang Yanyan, Wang Jiwu
Mechanical Engineering Department, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, P. R. China

ARTICLE INFOABSTRACTArticle History

This paper presents a new spherical mobile robot driven by single omni wheel. The physical model is that an arc track is built
inside the sphere shell and can rotate around a spherical diameter freely. It supports one driving omni wheel and keeps thedriving omni wheel touching the inside of the sphere shell. The touching friction force can drive the sphere rolling forward orbackward. Steering drive can move the omni wheel running along the arc track, that leads the spherical mobile robot turning
left or right. The omni wheel can run sphere shell in straight way and turn its direction freely at same time. The mechanismof the spherical mobile robot is described in detail, and its motion and dynamics coordinate systems are built to analysis. The
preliminary analysis shows that the robot has a variety of good motion capabilities, such as obstacle crossing, jumping, small
radius steering etc.

Received 27 October 2018
Accepted 31 December 2018
Keywords
Spherical mobile robot
omni wheel
arc track
eccentric gravitational moment© 2019 The Authors. Published by Atlantis Press SARL.
This is an open access article distributed under the CC BY-NC 4.0 license (http://creativecommons.org/licenses/by-nc/4.0/).

1. INTRODUCTION
The sphere is concise shape with the smallest surface area, the largest
volume, and rolling in all direction. It is the most excellent shapeexisting in the universe, and the best rolling body that every animal
likes to play with. The typical characteristics of the sphere are: (1)when no friction and air resistances, an ideal sphere with uniform
density will be kept moving forward straightly and smoothly by
the inertia force and moment of inertia; (2) when the mass centroid close to the sphere center, the position attitude is in stable,
and when the mass centroid away from the center, the sphere has
a strong tendency to self-stabilization, etc. The characteristics ofspheroid inspire the enthusiasm of scholars to invent many kinds ofspherical robots. Since the spherical robot concept first appeared,
it was considered as a new technology to lead the robot industrial
revolution such as the implementation in planet-moon exploration, military usage, and inevitably arising the demand of the toy
entertainment industry [1,2].
Through reviewing literature and patents, there are several typesof representative spherical robots, and according to their physical driving principles, the current spherical robots with omnidirectional motion performance could be divided into two typicalcategories: (1) Inertia moment driving type [2], the principle is torely on the rotating inertia moment to drive the omnidirectionalrolling of a spherical robot, but the inertia moment needs the rapidrotation like spinning gyroscope to acquire enough driving force.
(2) Eccentric torque drive type, its principle is to rely on the motionof eccentric mass centroid producing the eccentric torque on thefulcrum, and the eccentric torque drives the spherical robot rolling.
The invention of drive mechanism with eccentric torque driving
Corresponding author. Email: liuw@bjtu.edu.cn*

the spherical robot has been very rich [3–9]. One of the earliest
and concise spherical robots is invented and researched by Halmeet al. [3]. Its driving mechanism is made up of a driving wheel, asteering shaft and a balance wheel. One of recent and most concise
spherical robots is reported by Wei et al. [9]. It has two independent driving omni wheels located at each end of the inside vertical
diameter of the sphere shell, and the two wheels’ axes are in orthogonal position. Both omni wheels can roll on the inner surface ofthe spherical shell by their friction forces. The lower omni wheel isresponsible for walking drive, and the upper one for steering drive.
Although there are many mechanisms to realize the eccentric
torque drive principle, their physical model can be describedbriefly as shown in Figure 1. Supposing the only resistance is therolling resistance torque Tf , the torque T caused by mass centroid isconsidered to drive or steer the robot rolling. The distance r whichfrom the sphere center to the mass centroid m directly determines
the quantity of the driving torque T, that is, T = m⋅g⋅r⋅sin(a). When
T > Tf , the spherical robot will start to move and increase speed.
When T = Tf , the spherical robot will move uniformly. When T < Tf ,the spherical robot will reduce speed until stop.
This paper proposes a novel and concise spherical robot mechanism with only one omni wheel to drive the eccentric torque overcome the rolling resistance torque. The spherical robot can realize
the omnidirectional motion in any direction. The omni wheel isarranged by a series of small rollers around the circumference
which are perpendicular to the driving direction. When omni
wheel drives, its rollers can slide laterally on the wheel periphery.
The lateral rolling of small rollers does not interfere its circumferential driving function. The mechanism is shown in Figure 2. The
spherical robot driven by one omni wheel will be analyzed as following in several aspects as mechanism composition, motion anddynamics analysis etc.

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Figure 1 | Principle of eccentric torque drive.

Figure 2 | Omni wheel.

Figure 3 | Drive mechanism of single omni wheel.

Figure 4 | Driving pose and steering pose.

2. MECHANISM
The spherical mobile robot contains spherical shell, arc trackand drive assembly which include chassis, support wheels, driving motor, steering motor, side wheels, springs etc. Its interior
structures are shown in Figure 3. The structure shows the masscentroid is in the lowest position now. An omni wheel is locatedunder the arc track and touches the inner surface of the spherical shell. The support wheels, driving motor, steering motor,side wheels, springs and omni wheel are fixed in the chassis. The
support wheels roll along the arc track by the track force which
are acquired by the friction from the springs pressure, or by theengaged gear track and wheel. The side wheels keep the omni
wheel stable in the arc track plane and let the drive assembly
moving along the arc track freely.

3. FUNCTION ANALYSIS
The robot has two moving functions, rolling function and turning
function. The rolling function is shown in Figure 4. When driving
motor starts up, the spring force and the gravity of drive assembly
under arc track provide friction to let the moving parts climbing
the inner spherical shell, and then the eccentric mass provides theeccentric torque to roll the spherical shell forward or backward.
The side wheels can keep the drive assembly in stable and reduce
the friction to arc track when the drive assembly moving along thearc track, that is the omni wheel can always touch the inner surfaceof the spherical shell. When steering motor starts up, the support
wheels run along the arc track to drive the chassis and its drive
assembly left or right, and at the same time the omni wheel willslide laterally, so the eccentric mass provides the eccentric torqueto turn the spherical shell turning left or right. Driving motor andsteering motor can operate simultaneously, they don’t interfere withFigure 5 | The relationship of the coordinate systems.

each other. The turning function is shown in Figure 4. The driving
motor turns the mass center a angle and produces the eccentric
torque to roll the spherical robot forward. The steering motor turns
the mass center b angle and produces the eccentric torque to turnthe spherical robot left.

4. COORDINATE SYSTEMS
Built the coordinate systems is the first step to do the analysis of
the kinematics and dynamics. The coordinate system W-XYZ is
attached on the ground, and the coordinate system S-XYZ is established on the center of spherical robot. The A-XYZ is the arc track
coordinate system located at the sphere center. The O-XYZ is theomniwheel coordinate system. The O-XYZ slides or rotates along
the A-XYZ. The A-XYZ rotates around the S-XYZ. The S-XYZ rollson the W-XYZ. The relationship built between the coordinate systems is shown in Figure 5. That is the fundamental to simplify theanalysis of the kinematics and dynamics.

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5. CONCLUSIONThe mechanism of the spherical robot takes advantage of the characteristics of the omni wheel, and it combines the driving and steering
together in the simplest way. The spherical robot lowers the center ofdrive assembly gravity which can generate the maximized eccentric
driving torque. The simulation with the mechanical dynamic analysis software ADAMS shows that the mechanism can facilitate themotion toward any direction successfully, and it has a variety of goodmotion capabilities, such as obstacle crossing, jumping, small radius
steering etc. As the ends of arc track limit the lateral sliding of theomni wheel, its steering function is a little bit weak and more behaviors of the spherical mobile robot will be explored further.

CONFLICTS OF INTEREST
The authors declare they have no conflicts of interest.

[2]
[3]

[4]
[5]
[6]
[7]

ACKNOWLEDGMENTS
This research was funded by China National High Technology
Research and Development Plan (863 Plan) 2013AA041302.

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Authors Introduction
Dr. Liu Wei

Mr. Lunqin Duan

He received his PhD from Beijing Jiaotong
University, P. R. China in 2005. He is an associate professor in the Mechanical Engineering
Department, School of Mechanical, Electronicand Control Engineering, Beijing Jaotong
University. His main research interests aremechanical design, optimization applications
and computer graphics. In recent years, heinterests in the novel spherical robot.

Mr. Runjiao Wang

Lunqin Duan graduated from Beijing
Jiaotong University with a master’s degree
in mechanical design and theory. His
research interests include mechanicaltransmission and structure design, especially the research on the transmission performance of herringbone gears.

Mr. Yanyan Wang
Runjiao Wang is currently studying
toward the master’s degree in Engineering
at School of Mechanical, Electronic andControl Engineering, Beijing Jiaotong
University, P. R. China. His research interests include mechanical design and theory,
with a particular focus on simulation-based
mechanical optimization design.

Yanyan Wang has graduated with themaster’s degree in School of Mechanical,
Electronic and Control Engineering,Beijing Jiaotong University, P. R. China.
His research interests include mechanicaland electrical equipment system design,with a particular focus on study on root
fracture of wind turbine blade.

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Prof. Jiwu Wang
Wang Jiwu is now working in Beijing
Jiaotong University, P. R. China. His main
research is in the field of Mobile robot,
Intelligent control, Image processing etc.