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Fig. 2. Leg-wheel type robot ASTERISK H.

Table 1. Specifications of ASTERISK H.

Dimensions 815 706 85 mm

Dimensions620

537

334 mm(Home Position)

Leg Length 330 mm

Motor Dynamixel DX-117

Signal CommunicationRS485 :

Max Baud Rate 1Mbps

Weight 3.4 kg

OS NetBSD

Running Speed 30 cm/s

its ankles, indicating that traveling by wheels and gait by

legs are completely separated in the concept.

Adachi et al. developed Walkn Roll, having both ac-

tive and passive wheels [7] and 2 passive-wheel front legswith 3 DOF and 2 active-wheel back legs with 1 DOF. The

passive wheels incorporate a lock and the robot moves

in 3 modes based on the environment wheel, hybrid,

and step. Yoneda et al. proposed semierect movement for

traversing rough terrain by a leg-wheel hybrid quadruped

robot [8] that used instructions from an operator for mov-

ing the center of gravity (COG) when stepping over ob-

jects; it was thus unable to achieve fully autonomous

movement using sensor information.

The complexity of leg-wheel robot mechanics has

caused research to focus on mechanisms and movement

rather than on hybrid leg-wheel locomotion using sensor

information. We propose hybrid rough-terrain locomotionalternating legs and wheels based on sensor information,

and demonstrate that movement suitable in given environ-

ments can be generated automatically.

3. ASTERISK H

The integrated limb mechanism robot ASTERISK,

having both arm and leg functions [9], moves efficiently

by selecting functions based on the environment and en-

ables components to be downsized through function inte-

gration. Integration involves isotropic leg design for legarrangement of the robot. The developed robot, which

Joint1

Joint2(-95 : +95)

(-100 : +100)

(-75 : +75)

(-120 : +120)

[deg]

Joint4

Joint3

Side ViewLink1 Link2

Joint2 Joint3 Joint4Joint1

Wheel

BodyLink3 Link4

Fig. 3. Configuration of joints of ASTERISK H.

has legs in 6 directions disposed radially from the center,

provides working space and traveling capability in all di-

rections. The vertical symmetry of ASTERISK H (Fig. 2,

Table 1) provides equal workability for upper and lower

limbs and its predecessors integrated limbs and isotropicdesign.

Providing each leg with 4 joints (Fig. 3) enables the

robot to select leg positioning and steering angle for om-

nidirectional movement. Wheels on legs are active and

motor-driven, with encoders on axes providing rotational

information. Servomotors provide torque information.

Combining encoder and torque information enables the

robot to detect the surface on which it moves. Servomo-

tors and sensors are wired and powered by a daisy chain

via RS485 and controlled by a built-in CPU card.

The leg-wheel hybrid locomotion we propose does not

rely on friction, which may vary, but uses sensors to de-

tect load variations in supporting legs encountering stepsand generates gait movement to traverse obstacles. Its tri-

pod gait is typical for hexapod robots, enabling fast, stable

traversal of objects.

4. Leg-Wheel Object Traversal

4.1. Leg Action

Leg robots are statically stable when a vertical line

from the COG goes through the polygon formed by sup-

porting legs. The tripod gait is static, but we evaluated

object traversal using a normalized energy stability mar-gin [10], which is a robot walking stability index focusing

on mechanical energy and consider movement stability on

rough terrain including steps.

The traversal performance we require is such that it

travels over steps that wheels cannot manage, i.e., steps

higher than the robots wheel radius. We confirmed that

ASTERISK H traversed steps over twice the height of the

wheel radius.

4.2. Wheel Action

Under conditions required to traverse a step using awheel (Fig. 4) let us use a model in which angle formed

404 Journal of Robotics and Mechatronics Vol.20 No.3, 2008