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developed. In the remaining sections, the kinematic and
dynamic analysis of the proposed mobile robot is
performed and various test results are presented
demonstrate the validity and feasibility of the proposed
mechanism.
2. Variable Wheel Arrangement Mechanism
In this section, the structure and operational principle
of the proposed variable wheel arrangement mechanism
(VWAM) will be presented. In addition, comparison of
this mechanism with the variable footprint mechanism
(VFM) proposed by Wada is discussed below. Fig. 1 Variable footprint mechanism for CVT [9].
Figure 1 shows the variable footprint mechanism in
which two beams can rotate at a pivot joint P in the
middle [9]. Note that the two beams are constrained to
rotate in a symmetric fashion with a single DOF by means
of differential gears at the pivot. The ball wheels and
motors are mounted at each end of the beams. Meanwhile,the variable wheel arrangement mechanism (VWAM)
developed in this research is illustrated in Fig. 2. The
wheel module consists of the omnidirectional wheel
called a continuous altemate wheel developed in our
laboratory [ll] (see Fig. 6), an inpidual motor and
steering link. Notice that the four wheel modules can
rotate about each pivot point CI, .., C, located at the
comers of the robot body, but they are constrained to have
a synchronized steering motion of 1 DOF by the VWAM
comprising the connecting links and linear guide. In Figures 1 and 2, the steering angle 4 is defined as the
angle from the zero position in which the beams (Fig. 1)
or the lines (i.e., C,C3 or C2C4) connecting the centers of
diagonally opposed wheels (Fig. 2) coincide with the
diagonal lines of the robot body. The wheelbases, the
distances between the centers of two adjacent wheels on
the x- and y-axis, at the configuration 4 = 0 are denoted as
dxo and dyo. If the robot body is square, then dxo = dyo.
In Fig. 1 the rotation center of the wheel module is
located at the intersection P of the two beams. As the
steering angle 4 becomes large, therefore, one side of the
rectangle whose vertices are wheel-ground contact points
may get excessively smaller than the other side, thus
leading to increased instability. Hence the steering angle
was limited to the range between -17.5' and +17.5' ,
which causes the range of velocity ratio (defined in
Section 4) to be limited. On the contrary, since the wheel
modules in Fig. 2 rotate about each pivot joint C,, .., C4
placed at the comers of a robot platform, the robot is
structurally stable even for a large steering angle. As a
result of this feature, the steering angle can be
substantially large, and thus the range of velocity ratio
increases accordingly. Figure 3 shows various wheel .
arrangements Using the variable wheel arrangement