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a step, the robot is equipped with an ultrasonic range
sensor. The sensor head is attached at the tip of an arm
that rotates around the vertical axis on the body. The
altitude of the terrain around the robot is measured by the
sensor. Control is performed by a personal computer.
The whole control system is shown in Fig.3. In order to
proyide the command to the robot, the control system is
equipped with a command controller. Operator can give
four analog and six digital signals to the computer as the
control commands.
4. Locomotion algorithm
Walk’n Roll has several locomotion modes by
selecting wheeled function or legged function.
4.1 Wheel mode
In this mode, four wheels are used for locomotion,
and fast and efficient locomotion is realized on relatively
flat terrain. Rear wheels are driven, and front wheels
are free and perform steering action. When front wheel
is positioned just bellow the first joint, steering action is
done by controlling the first joint only. The other two
joints and rear legs can be used as active suspension. Next, we consider relation between steering angle
and wheel driving. Now we consider a situation as
shown in Fig.4. We set a coordinate system fixed to the
robot body, and its x-axis is moving direction and the y-
axis is a line connecting two rear wheels. A wheeled
vehicle whose front wheels perform steering action has a
turning center on the line connecting rear wheels. In
this figure, 81 and 82 denote the steering angle. When
the coordinate of the turning center is r, 81 and 82 are
calculated as following equations.
e, = tan-'(a/(r-b>)
e, = tan-'{a/(r+b))
Here, a is wheel base, and b is half of front wheel tread.
A ratio of travelling distances of two rear wheels m/w4 is
given by next equation.
w,/w, =(r-c)/(r+c) (3)
Here, c is half of rear wheel tread. Above analysis is
done when the turning center is outside of the robot.
However, It is possible for the robot to turn with the
turning center inside of two rear wheels as shown in
Fig.5. Obtained results are the same as equations (1)-
(3). It should be noticed that steering and wheel driving
directions are different in both sides. When absolute
value of r is smaller than c, wheel driving directions are
different each other. In the case of the steering, the
directions are different when the absolute value of r is
smaller than b. 4.2 Hybrid mode
In this mode, front passive wheels are locked, and
front two legs are used to walk. This mode is
cooperative locomotion of front legs and rear driving
wheels. Two merits appear by using front legs to walk.
One is traction. When the robot moves by the wheel
mode, traction is generated only rear wheels. In the
hybrid mode, a front leg drags the robot body during its
support phase. Traction is generated at all the contact