IEEE Robotics & Automation Magazine - December 2017 - 29

a main power failure, electromagnetic braking is activated.
Each traction motor is equipped with two position sensors:
Hall sensors are embedded in the in-wheel motor, while
magnetic encoders (LM13, RLS, Slovenia) are mounted
externally. The two sensors enable smooth operation with
sinusoidal control of the in-wheel motor and provide redundancy in case of sensor malfunction.
The prototype has individually steered wheels. Steering is
actuated with a dc motor [Maxon Motor Switzerland, RE 50,
Ø50 mm, graphite brushes, 200 W; planetary gearhead GP
62 A, Q62 mm, 8-50 N∙m, transmission ratio of 236:1;
encoder HEDL 5540, 500 cycles per turn (CPT), three channels, with line driver RS 422] connected via a timing belt
(transmission ratio of 32:22) to the wheel steering axis. The
steering torque is limited to 100 N∙m, the maximum steering
angle is limited to ±45°, and the steering angular velocity is
limited to 45°/s. Steering actuators equipped with a dc motor
control module are directly controlled from the main control
unit. Each steering actuator is equipped with two angle-measurement sensors. An incremental optical encoder attached to
the dc motor is used to control the steering angle. The additional absolute magnetic encoder (RMB20V, RLS, Slovenia) is
fixed to the wheel steering axis for system initialization and as
a redundant safety measure.
The main drive system provides 240 N∙m of total torque.
With an assumed mass of 250 kg for the wheelchair and the
user, the wheels provide more than enough torque for climbing ramps of over 20° inclination, and, with a good grip, the
wheelchair can overcome steeper obstacles.
Track System
Two parallel rubber tracks (Thistle Special Belting, United
Kingdom) are attached to an adjustable Chebyshev linkage
mounted in the middle of the wheelchair (Figure 3). During wheel-based driving, the tracks are retracted and lifted
from the ground. They can be lowered when driving over
steep obstacles and rough terrain to provide additional
traction. In this mode, the tracks are leveled with the
wheels. This configuration enables the synchronous actuation of the wheels and rubber tracks, guarantees safety, and
prevents the wheelchair from rolling over even on steep
and uneven terrain.
Each rubber track is actuated independently (Beckhoff
servomotor AM8131-0F21-0000, gearbox with a transmission ratio of 40:1). Therefore, the wheelchair can also be
steered while driving on tracks. The motor torque is transmitted to the rubber track via a chain (transmission ratio of
28:16). The total rated traction force on each rubber track is
1.4 kilonewtons (kN) (peak force of approximately 3 kN).
With the combined tracks' traction force, the wheelchair can
climb obstacles of up to 35° of inclination (the safety limit
related to system stability). Motors are equipped with velocity sensors and brakes that are released automatically when
power is disconnected. Track actuators are controlled and
powered by the main control unit via a servomotor controller module.

The adjustable Chebyshev linkage is actuated with two linear actuators (carts) running on the same rail. This enables
the tracks' position adjustment in 2 DoF: the distance from
the ground and the forward-backward position (Figure 4).
The tracks' inclination relative to the wheelchair chassis is
passively adjustable, allowing the tracks to follow the configuration of the terrain below. The tracks' basic position (when
the tracks are parallel to the chassis) is defined by cart positions p 1 and p 2 (see Figure 4):
p = p2 - p1
r 2 - 1 ^ p + qh2 + h 1 + h 2,
4

where h is the distance from the chassis to the tracks. A
threaded spindle (SKF SD 14X4R, 4-mm pitch) actuated
with the same dc motor (Maxon motor RE 40 Q40 mm,
graphite brushes, 200 W; planetary gearhead GP 42 A
Q42 mm, transmission ratio 26:1; encoder HEDL 5540, 500
CPT, three channels, with line driver RS 422, Maxon Motor,

c
a

Figure 3. The track system: (a) the tracks' guiding mechanism;
(b) the tracks' actuator with an absolute encoder; (c) the
rubber tracks; (d) the Chebyshev-like tracks' height adjustment
mechanism; (e) the rail and cart mechanism for the tracks' height
and position adjustment; (f) the tracks' position adjustment
actuator with an incremental encoder; and (g) the cart wire
potentiometer.

Front

Rear

r
q

Figure 4. The adjustable Chebyshev linkage kinematics.

DECEMBER 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

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