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points between the robot and the ground, and it
contributes to the mobility on the slope or soft terrain.
The other merit is adaptability for steps. Front passive
wheel is relatively small, and its adaptability is limited.
By using leg function, the robot can move over higher
steps.
Here, we consider a gait for front legs. First we
assume the robot moves straight, and we consider the leg
motion in the horizontal plane. The leg moving
trajectory is seen as a dotted line in Fig.6. We define
forward end point of the trajectory as 0, and the backward
end as 1. The robot is also assumed to maintain static
stability during locomotion. Therefore, the robot must
always have more than three contact points with the
ground. This means that duty factor p (fraction of a
locomotion cycle that each leg spends in contact with the
ground) must be between 0.5 and 1.0. Considering the
symmetry of walking, phase difference between front two
legs q3 is set to 0.5. In order to continue steady walking,
it is assumed that each leg repeats forward and backward
motion. Now we set a reference position of leg motion.
For the left leg, we select forward end point of the
trajectory as a reference position. In the case of the
right leg, reference position is automatically calculated
from duty factor p and phase difference q3, and its value is
1/(2p). After front two legs are positioned at their reference positions, each leg is drived along the trajectory.
Of course, leg motion must be synchronized to the
driving of the rear wheels.
In order to
simplify the problem, we assume that turning center is
constant during a leg motion cycle that starts from the
reference position. By this assumption, a transfer from
straight movement to turning, or from turning to turning
with different curvature is always done at the reference
position. Now we define s as robot moving distance in a
leg motion cycle. This means that robot body turns
against the turning center by s/r in the coordinate system
fixed to the ground. On the other hand, a leg turns
against the turning center by pdr in reverse direction in
the body coordinate system. Turning radiuses of front
two legs, r, and r2, are calculated by following equations.
Next, turning gait is considered. where, f is half of y-coordinate difference of two reference
positions, and d and e are x-coordinates of the left
reference position and the right reference position,
respectively. Wheel driving distances during the leg
motion cycle, w3 and w4 are expressed as follows. w
leg, and rear is wheel locomotion. Although a relatively
large wheel is used, principally a wheel cannot climb a
step whose height is larger than half of the wheel’s
diameter. When the robot encounters a large step, a
step mode is utilized. Locomotion algorithm in the step
mode is shown in Fig.7. After the robot stand in front
of a step (a), front wheel is moved on the step by leg