IEEE Robotics & Automation Magazine - December 2017 - 41

Rear
Support Wheel

Vertically
Projected Body

Vw 3

LL
Z

VPw 4
Y

Support
Polygon

Stability
Margin

Front

VPw 3

COG of the
P-WA + Driver

X
Rear

Leg-Motion
Wheel

Support
Wheel

Front

Vw 4

Support
Wheel

(a)

(b)

Figure 5. The (a) leg motion and (b) stability margin.

V w4 ( t ) =

2 A r (t )
V (t ) f,
L L (t ) Pw4

(5)

where VP is the velocity of the center of the wheel-arm, P,
and this value is derived from the angular velocity of the
front steering; VPw3 is the velocity of P if only the rear-left
wheel advances by pivoting at the rear-right wheel; and VPw4
is the velocity of P if only the rear-right wheel advances by
pivoting at the rear-left wheel. VP is composed of VPw3
and VPw4 .
In addition, L L /L R is the distance between the center of
the front-wheel arm, P, and the point of the rear-left/rearright wheels on the vertically projected body. This is the leg
motion of the P-WA. The steering and rolling mechanisms are
installed in both front and rear bodies, because every wheel is
needed to perform a leg motion. Figure 3(c) shows the pitching mechanism of the seat. A footrest is installed in the
front part of the seat to avoid a conflict with the front
wheels, front-wheel arm, and parts of the body when
doing a leg motion or moving on rough terrain.
Figure 3(d) shows an auxiliary wheel mechanism, which is
for a pivoting turn of the P-WA. When the auxiliary wheel
leaves from the bottom of the body and touches the ground,
the front wheels are lifted from the ground, and, by rotating
the rear wheels in opposite directions, the P-WA pivots.
The System
The electrical and computer system of the P-WA is shown in
Figure 6. The main central processing unit (CPU) is
SH-7785 (600 MHz) by RENESAS, Japan, and the joystick
controller for a driver is connected via Ethernet. In addition,
a posture sensor is installed under the seat, and three photoelectronic sensors are deployed with different angles at each
wheel (Figure 6). Commands for actuators are sent to motor
driver boards from the CPU through interface boards. To

hold the shape of the P-WA without power, the pitching,
rolling, and footrest actuators have a holding brake.
The power source comes
from a lithium polymer
battery and is adjusted to
A footrest is installed in
several voltages such as 5,
12, 24, and 48 V. The conthe front part of the seat
trol power is insulated
against the drive power,
to avoid a conflict with the
and some emergenc y
switches are installed in
front wheels, front-wheel
the power generator circuit. When an emergency
arm, and parts of the body
switch is pushed down,
the power controller bowhen doing a leg motion or
ard immediately shuts
down the main relay for
moving on rough terrain.
safety. To participate in
the Cybathlon, our device
underwent an electromagnetic compatibility test at an
industrial test field.
The Software
The software of the P-WA consists of three modes (Figure 7).
The light-emitting diode (LED)-tape indicator on its body
[e.g., blue-lighted LED tape in Figure 2(b)] tells a driver the
status of the P-WA. The three software modes are as follows:
● Waiting mode:
● After booting the system, this mode runs.
● All actuators are tested for correct operation only once.
● In this mode, the device does not move by a joystick.
● Drive mode:
● A driver operates the P-WA with a joystick and commands from the driver are the speed and direction of
the P-WA.
DECEMBER 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2017