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Abstract
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An omnidirectional mobile robot equipped with four
independent omnidirectional wheels can perform 3 DOF
motion and has one redundant DOE This redundancy can
be used to drive the mechanism enabling the wheel
arrangement to vary, which can function as a continuously
-variable transmission (CVT). In this research, a special
mechanism called a variable wheel arrangement
mechanism is proposed to increase the range of velocity
ratio for the CVT from the wheel velocities to robot
velocity, which may improve performance of the mobile
robot. The mobile robot with this mechanism was
constructed and various tests have been conducted to
demonstrate the validity and feasibility of the proposed
mechanism.4981
1. Introduction
Omnidirectional mobile robots are capable of moving
in an arbitrary direction without changing the direction of
wheels, because they can achieve 3 DOF motion on a
two-dimensional plane. Various types of omnidirectional
mobile robots have been proposed so far; universal
wheels [l, 21, ball wheels [3], off-centered wheels [4] are
popular among them.
The omnidirectional mobile robots using
omnidirectional wheels composed of passive rollers or
balls usually have 3 or 4 wheels. The three-wheeled
omnidirectional mobile robots are capable of achieving 3
DOF motions by driving 3 independent actuators [5, 61,
but they may have stability problem due to the triangular
contact area with the ground, especially when traveling on
a ramp with the high center of gravity owing to the
payload they carry. It is desirable, therefore, that four-
wheeled vehicles be used when stability is of great
concern [7]. However, independent drive of four wheels
creates one extra DOF. To cope with such a redundancy
problem, the mechanism capable of driving four
omnidirectional wheels using three actuators was
suggested [8].
Another approach to a redundant DOF is to devise
some mechanism which uses this redundancy to change
wheel arrangements [9, 101. Since the relationship
between the robot velocity and wheel velocities depends
on wheel arrangement, varying wheel arrangement can
function as a transmission. Furthermore, it can be
considered as a continuously-variable transmission (CVT), because the robot velocity can change continuously by
adjustment of wheel arrangements without employing a
gear train. The CVT can provide more efficient motor
driving capability as its range of velocity ratio get wider.
The mobile robot proposed by Wada [9], however, has a
limited range to ensure stability of the vehicle.
In this paper, a new mechanism so-called variable
wheel arrangement mechanism (VWAM) is proposed in
which robot stability is guaranteed regardless of wheel
arrangement and thus the range of velocity ratio is greatly
extended. The four-wheeled omnidirectional mobile robot
involving this mechanism combined with the continuous
altemate wheels developed in our laboratory [ 111 has been