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3. Proposed locomotion mechanism
We set basic requirements for our experimental
mobile robot as follows.
1) On flat terrain, fast and efficient locomotion is realized with simple control.
The robot can move in the discontinuous contact
locomotion environment.
For, the movement on steep slope, traction is
generated at the all contact points between the robot
and terrain.
In order to increase the reliability and decrease the
cost, the robot has less number of actuators than
normal legged robot.
satisfy above requirements, we propose a new
configuration as shown in Fig.1. Many mobile robots
developed earlier have an active wheel at the tip of their
leg. Though such a mechanism shows high adaptability
to the environment, it requires large number of actuators.
In our proposed scheme, different kinds of mechanism are
employed at the front end and rear end in order to
decrease the number of actuators. The robot has two
front legs and two rear legs. Front leg has a serial link
structure, and has three rotational joints. A relatively
small lockable passive wheel is attached at the tip of each
front leg. The first joint, that links the bcdy and the
thigh, has vertical axis and the second and third joints
have horizontal axis. End of the front leg (passive
wheel) can be positioned arbitrarily in its work space.
Rear leg has a rotational joint, and also has a relatively
large active wheel at the tip of the leg. The joint axis is
horizontal, and its movable range is over one
round. Accordingly, the active wheel moves on a
circular trajectory in the lateral plane. Totally,
this configuration has ten degrees of freedom, and
it is smaller number than that of a normal
quadruped robot, twelve. On a flat terrain, four
wheels are used for locomotion. Wheels contribute fast and efficient locomotion. When front passive wheel is
positioned at just bellow the first joint axis, steering
action is performed by controlling the first joint only. In
addition, legs can function as active suspensions. In the
case of rough terrain, the front passive wheels are locked,
and front two legs are used to walk. Rear active wheels
are still used for locomotion. The robot can generate
traction at the all the contact points between the robot and
the ground. When the robot encounters a large step where rear wheel can not climb, the rear leg is moved
from the rear side to the front side through the air by leg
joint motion.
Fig.2 shows a photograph of constructed mobile robot,
and it is called ‘Walk’n Roll”. Basic specifications are
also shown in Table 1. The robot is separated from the
power source, motor amplifiers, and a control computer,
and they are connected by the cables. DC servo motors
are used to drive the joints and rear wheels. Solenoids
are used to lock the front wheels. Rotary encoder is
attached at each joint and active wheel. In order to detect