IEEE Robotics & Automation Magazine - December 2017 - 30

where j is the seat inclination and d is the change of seat
position. The seat is actuated with two parallel nonbackdrivable linear actuators (Timotion series TA2, Taiwan). Each
motor generates 750 N of thrust force at the maximum speed
of 11 mm/s. The motors are equipped with Hall-based position sensors having a resolution of 0.1 mm/count. The parallel configuration provides 1,500 N of force and mechanical as
well as sensory redundancy. Both actuators are directly controlled from the main control unit equipped with a dc motor
control module.

l3
ϑ l2

ϕ
α β

l4
d0 + d

l1
s

s0 + s

c
d
d

e
e
f

Figure 5. The seat system: (a) the seat, (b) arm rest, (c) footrest,
(d) seat position linear guide, (e) seat inclination mechanism
and coupling with position adjustment, and (f) seat position and
inclination actuator.

Switzerland) moves each linear actuator. The actuator generates 5 kN of peak force with a maximum movement speed of
approximately 20 mm/s. The actuator is equipped with two
position sensors. An optical encoder is mounted directly
onto the dc motor, and a wire potentiometer measures the
displacement of the cart for initialization and redundancy.
All active DoF of the tracks' mechanism are directly
controlled from the main controller.
Seat System
The pilot's seat, with integrated user interface in the armrest
for control of the wheelchair, provides comfort and safety for
the user (Figure 5). The right-side armrest is equipped with a
joystick and a touchscreen, while an emergency button is
inserted into the left-side armrest.
The seat can be actively moved via two mechanically coupled DoF that enable simultaneous backward translation and
reclining of the seat relative to the chassis. The backward
translation shifts the overall center of gravity to the rear of the
wheelchair when ascending and descending steep obstacles
(the user faces in a downward direction while traversing
obstacles with inclinations steeper than approximately 20°). In
most circumstances, the reclining seat keeps the user leveled.
Seat kinematics is defined as follows (see Figure 5):
{

= a - b - arccos

= arcsin

l 21 + l 52 - ^s 0 + sh2
2l 1 l 5

l 2 sin { - l 4
l3

d = l 3 cos j - l 2 cos { - d 0,
30

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

Power Management
All wheelchair systems are powered from a power pack,
which is capable of outputting several different voltage
levels: 48 V (maximum current 120 A), 24 V, 5 V, and
12 V for auxiliary power source. The power pack is contained in an aluminum housing mounted beneath the
chair system and includes the main power battery cells,
the auxiliary battery cells for the low-power uninterruptible power supply (UPS), the battery management
systems, the dc-dc converters, the power relays, the
fuses, a liquid crystal display status indicator, the thermometer, and the current sensors. The main power
source is a battery pack, which contains 14 LiFePo4 cells
(AMP20M1HD-A, A123 Systems) with 3.3-V nominal
voltage per cell, resulting in a battery nominal voltage of
46.2 V. The cell capacity is 20 Ah at nominal voltage.
The battery of the main power supply is charged with an
external charger through a battery management system
module programmed to limit the maximum battery
voltage to 49.7 V. All wheelchair motors are powered
with the main power supply. The main controller requires a 24-V power supply source, and most sensors
operate at 5 V; hence, the battery voltage is converted to
lower voltage outputs by two dc-dc converters. The battery system also includes power relays for switching off
the battery pack subsystems. The dead man's switch and
emergency buttons are connected to the main power
relay and switch the power on/off for all motors. A lowpower UPS powers the wheelchair electronics when the
main power supply is disconnected. The UPS specifically provides power to the in-wheel motor controllers. In
case of a main battery failure, the UPS enables magnetic breaking by short-circuiting all three phases in the
motor controllers.
Control System
The wheelchair control is implemented on a standard industrial controller (Beckhoff CX5130-0120) with expansion
modules for analog/digital inputs/outputs and motor control. The control system block diagram is shown in Figure 6.
The blue blocks in Figure 6 represent the controller modules, the gray blocks represent various motors, the light red
blocks represent the power system, the dark red blocks
represent the safety system, and the orange blocks represent
user interfaces.

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