Abstract
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 四轮全方位移动机器人英文文献和翻译:http://www.751com.cn/fanyi/lunwen_1818.html