IEEE Robotics & Automation Magazine - December 2017 - 34

Center of Mass
Rear Wheel
Front Wheel

Vertical Position (m)

1.5

Tracks-Rear Axis
Tracks-Front Axis
Support Polygon

35°
(b)

0.5

29°
-1.5

-1

-0.5
0
Horizontal Position (m)
(a)

0.5

1.5
(c)

Figure 10. (a) The typical courses of the wheelchair and the user center of mass, front/rear wheel axes, and front/rear track axes during
stair climbing. The middle position of the wheelchair presents the case with minimum stability margin (approximately 7 cm), where the
blue cross denotes the projection of the center of mass on the support polygon. The maximum tested inclinations for (b) backward (35° of
inclination) and (c) forward (29° of inclination) driving are also shown.

mechanism enables the platform to automatically adjust to
the inclination so that the wheels are in constant contact with
the stairs to provide additional safety. In this mode, the user
can climb any number of stairs [see Figure 9(e) and (f)]. At
the last step, the back of
the wheelchair is lowered
to ensure a smooth transiThe wheelchair
tion from inclined to level
driving, while additional
development was
rotation transfers the
tracks parallel to the
organized as a student
ground [see Figure 9(c)
and (d)]. Once over the
project, during which
stairs, the user lowers the
seat to the initial position
students could gain
and can use the rotate
mode to reorient the
experience under the
wheelchair and continue
his or her driving [see
supervision of senior
Figure 9(a) and (b)].

mentors (professors and

Rough Terrain Mode
While driving on a rough or demanding terrain, the tracks are
lowered to ground level to provide additional support and
traction. The velocity of the wheels is synchronized with the
tracks' velocity. The combination of wheels and tracks provides maximum friction and power to move the wheelchair
over an obstacle.

teaching assistants).

Competition

Experimental Results
Figure 10 shows the trajectories of the wheelchair's center of mass,
wheels, and tracks during stair climbing. A set of stairs with
the same parameters as in the Cybathlon competition was
built in the laboratory for testing and training. Data were

measured using an Optotrak optical tracking system. Optical
markers were attached to the tracks and the wheelchair chassis. Markers were also attached to the stairs to calculate the
exact relative position between the wheelchair and the stairs.
The trajectories shown in Figure 10(a) were then calculated
from the marker trajectories. In addition to the stairs, a ramp
with an adjustable inclination was built to test the chair's
stability and traction on an incline. Experiments on the ramp
have shown that the maximum inclination is 35° for backward driving and 29° for forward driving. In both cases, the
stability margin is approximately 25 cm, indicating high stability of the wheelchair. Higher inclinations were not tested
due to insufficient traction on steeper inclinations.

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

Team Organization/Structure
The wheelchair development was organized as a student project, during which students could gain experience under the supervision of senior mentors (professors and teaching assistants).

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