IEEE Robotics & Automation Magazine - December 2018 - 36

System Description
Mechanical Design
The prototype presented in this article has six rotors arranged
evenly in a circle (Figure 1) in a traditional helicopter configuration. The choice of six rotors represents a compromise
between simplicity and
the ability to generate sufThe ability of the system to ficient thrust to compensate for gravity in any
configuration on SE(3).
independently orient the
Clearly, the hexacopter
thrust generated from each configuration has a larger
wrench envelope compared to a quadcopter.
rotor reduces the internal
The rotor arms are constructed of carbon-fiber
forces and, therefore,
tubes, and each rotor can
rotate around the arm's
increases the efficiency.
axis. Only the motor and
its housing rotate. This
rotation is achieved using
brushless dc motors inserted at the end of the arms, as can
be seen in Figure 2. The arms themselves are rigidly
attached to the platform's main frame, which gives the system increased structural rigidity compared to designs that
rotate the entire arm.
The tilting motors are 12 mm in diameter and equipped
with three Hall sensors and a high-ratio gearbox for precise
position estimation and control. The tilting motors have a
maximum rotation speed of 7.85 rad/s and a maximum
torque of 0.5 N∙m. This allows for fast control of the tilting
angles. The decision to use this motor setup over radio

control (RC) servo motors was made because such servos are
typically limited to 180c of rotation, which would limit the
platform's flight envelope. A further advantage of the brushless motors is their narrow profile, which allows them to be
hidden inside the aircraft's tubular arms. With this setup, the
rotors are capable of rotating 720c around their axes. The limiting factor in this design is the winding of the thrust motor
cables around the arms.
Hardware
The main electronic and mechanical components of the
system are as follows. For thrust generation, 9-in DJI propellers and KDE2315XF-885 motors were used. This propulsion system is capable of providing a maximum of
13.7 N of thrust per motor. The system requires a high
thrust-to-weight ratio so that it can hover at large inclination angles, in cases where most rotors cannot orient
their force vector to directly oppose gravity. Faulhaber
1226S012B1 brushless dc motors were used in the rotortilting system. Each of these tilting motors requires a
motion controller that measures the current position of the
motor shaft and applies the required voltage to control its
position. To power the motors and the electronic components at different voltages, a six-cell lithium-polymer battery and accompanying power distribution boards were
used. Table 1 shows the full list of the electronic components incorporated in this platform.
To house these electronics, a special core was designed
using customized parts. The core consists of two 1.5-mmthick carbon-fiber plates, assembled using aluminum screws.
Aluminum spacers keep the plates a fixed distance apart to
provide room for the electronics. They also act to clamp the
carbon tubes on which the rotor units are mounted. Inside the

Table 1. The Voliro's components and their
specific names and models.

Figure 1. The final version of the Voliro platform.
Motor and Propeller
Sleeve

Carbon Tube

Connection
Shaft/Motor
Mounting
dc Motor for Tilting

Motor Mounting

Figure 2. A section view of a rotor unit.

IEEE ROBOTICS & AUTOMATION MAGAZINE

december 2018

Component

Name

Thrust motors

KDE2315XF-885

ESC

KDEXF-UAS35

Tilt motor

1226S012B1

Motion control board

MCBL 3002

Battery

Swaytronic LiPo 6S

PDB

Piko PDB

BEC 12 V

Micro BEC

BEC 5 V

Reely UBEC

Onboard computer

UP board

Flight controller

Pixhawk

RC receiver

Futaba R3008SB

Wi-Fi antenna

Dlink DWA-1725 GHz

Note: ESC: electronics speed controller; PDB: power distribution board;
BEC: battery eliminator circuit.

IEEE Robotics & Automation Magazine - December 2018

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2018