IEEE Robotics & Automation Magazine - December 2017 - 28

crowded streets. Steering has to allow a small turning circle
and utilize a simple mechanical design that does not require a
significant amount of space. For the wheelchair to slalom
between poles placed 140 cm apart at the competition, the
maximum turning circle must be 260 cm (taking into
account that the poles have a 20-cm round base). The
maneuverability has to be sufficient to drive through a
120-cm wide corridor. As steering with only one pair of
wheels does not meet
the aforementioned criteria, four-wheel steerThe prototype was
ing is required. The team
selected independent
designed using a compact
steering for each wheel
via an electric motor besystem that allows the
cause, unlike skid steering and Ac k e r m a n n
seat to recline, moving
steering mechanisms,
independent steering is
the center of gravity
precise and requires only
a small implementabackward while
tion space.
During initial testing,
maneuvering obstacles.
the wheels proved inadequate for a safe and comfortable climb over the stairs, so the decision was made to add
tracks. The track system must provide safe and comfortable
climbing over any number of stairs, not just the three stairs
used on the Cybathlon track. In addition, it needs to be compact enough to fit under the chassis between the wheels and

Systems

g
h

j
i

Figure 2. The power, control, and main drive system: (a) the
power management box with a battery pack; (b) the electronic
fuses; (c) the main controller box with the electrical connections;
(d) the in-wheel motor with Hall sensors; (e) the controller for
the in-wheel motor; (f) the steering motor with an incremental
encoder; (g) the steering transmission; (h) the steering axis with
an absolute encoder; (i) the wheel velocity sensor (magnetic
encoder); and (j) the wheel suspension mechanism.

IEEE ROBOTICS & AUTOMATION MAGAZINE

retractable to allow driving over 15-cm-high obstacles. To
comply with the Cybathlon rules, the wheelchair must climb
three 17-cm-high stairs with a pitch of 31°. At the same time,
the system must retract enough to allow the user to drive up a
ramp with a 20° inclination. To meet these requirements, the
team designed retractable rubber tracks with a dedicated Chebyshev mechanism.
In general, when driving over stairs or slopes, the wheelchair tilts more than 30°, which impacts the comfort of the
driver and decreases the stability of the wheelchair. To solve
this problem, the prototype was designed using a compact system that allows the seat to recline, moving the center of gravity
backward while maneuvering obstacles. A limitation of the
adopted approach is that it requires the wheelchair to face
backward while climbing stairs. Based on this limitation, the
wheelchair had to be turned 180° at the top of the stairs, which
was possible due to the independent four-wheel steering.
The wheelchair consists of 15 degrees of freedom (DoF)
actuated by 14 actuators: 4 active DoF with in-wheel motors,
4 active DoF with steering motors, 2 active and 1 passive DoF
of the Chebyshev linkage, and 2 active DoF for the left and
right tracks. The seat system uses two actuators connected in
parallel that actuate 2 mechanically coupled DoF (the seat's
inclination and translation).
The wheelchair is controlled by an off-the-shelf industrial
controller: a joystick is used to govern the wheelchair's velocity and direction, and a touchscreen enables switching
between the different driving and control modes. A LiFePo4
48-V battery pack powers the motors and electronics, while
an auxiliary power unit supplies all critical electronics when
the main battery is disconnected. The wheelchair is equipped
with safety switches for powering off the system and a master
switch for powering off the battery.

DECEMBER 2017

Drive System
The main drive system is depicted in Figure 2. Four active
wheels with a 0.2-m radius, each with 2 DoF, one for traction (actuated by an in-wheel motor) and one for steering,
provide the main driving capability. The interaxial distances
are 0.58 m in the front/rear directions and 0.68 m in left/
right directions. Each wheel is attached to the chassis via a
passive spring-based suspension. Each in-wheel motor
(Taizhou Quanshun Motor, China, Qs 10x3.0 inch 500 W
205 28 H; single-shaft hub motor, size U270 × 190 mm,
weight approximately 13 kg) guarantees 60 N∙m of peak
traction torque at 48 V. The in-wheel motors are controlled through off-the-shelf motor controllers (RoboteQ
MBL1660, United States) that receive commands from the
main control unit via a CANopen bus and provide corresponding power to the in-wheel motors. The motor controller is equipped with several safety features, such as stall
detection, speed limit, torque limit, and auxiliary power
input. Safety limits are implemented on maximum velocity
(2.5 m/s) and acceleration/deceleration (2.5 m/s2). In case of

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