IEEE Robotics & Automation Magazine - December 2018 - 59

As a consequence of coupling the aerial-locomotion system
to the ground-locomotion system, we must ensure that displacements of the ground-locomotion system do not affect the
stability of the aerial-locomotion system. Due to the presence
of constraints, movements of the ground-locomotion system
affect the dynamic behavior of the aerial system. This introduces disturbances in the form of forces applied to the tip of
the manipulator, which results in a rotation and displacement
of the aerial system. To deal with this undesired effect, we used
a modified controller based on our previous work [2]. This
contact controller stabilizes the relative orientation of the multirotor to the end effector, thus offsetting the effects of disturbances caused by the coupling with the end effector.
This stabilization re sults in automatic tracking of the
ground locomotion.
In practice, this implies that any time the motion of the end
effector disturbs the multirotor, the multirotor will respond by
stabilizing itself in a new equilibrium position. This self-stabilization allows us to control the position of the multirotor
implicitly by controlling the position of the ground-locomotion system. An elastic element provides the rotational decoupling necessary to facilitate this effect. The rotational
compliance introduces a spherical constraint that allows relative displacements between the multirotor and the end effector. This is needed to allow the contact controller to react to
the displacement within its bandwidth. Besides stabilizing the
system, the contact controller serves the task of providing the
normal forces required for successful ground locomotion and
tool operation.
End-Effector Design
The end effector, which is detailed in
Figure 3, was designed to move along
the surface and perform surface operations (in this case brushing). Both
these functions require a normal force
to be applied to the end effector.
Therefore, in crafting the end effector,
designers place special emphasis on
making it robust yet lightweight.
The body of the end effector was
formed by the base and top platforms,
which were rigidly connected by three
metal spacers. To enable the end
effector to move along the surface,
three actuated omnidirectional wheels
were attached to the base platform at
120° angles. Each of these wheels was
actuated in one direction and contained freely rotating barrels that
allowed movement in the other direction. This combination of three independently driven wheels resulted in
full controllability of the end effector's
pose on the surface, assuming sufficient friction.

The brushing system, as detailed in the bottom part of
Figure 3, represented the surface operation functionality. It is
possible to use different types and sizes of tools, which may
require different surface pressure, depending on the operation. To control this surface pressure, the tool was mounted
on a parallel structure suspended by three compression
springs. The parallel structure comprised three hinge beams
connecting the motor mount to linear slider bearings, which
slid over the linear guides. These linear guides are, in fact, the
spacers between the top and bottom platforms. The beams'
material stiffness allowed minor rotational misalignment of
the tool. In the uncompressed state, the springs press the
brushing system against the base platform so that the brush
sticks out. When compressed, with all wheels in contact with
the surface, the springs apply a constant force on the brushing system that is independent of the drone's contact force.
The prototype of the end effector (Figure 4) weighed 0.15 kg
and carried a flat, soft brush having a diameter of 3 cm. The
suspension was designed to apply a force of 6 N in the compressed state, which translates to an applied pressure of 8,500
Pa. Four high-power 300:1 Pololu Micro Metal gearmotors
were used to actuate the wheels and the brush, both controlled
by an Adafruit Feather board. Open-loop control was applied
for the ground locomotion, with Cartesian body-velocity commands ranging from [- 1, 1] as input. The flexible joint connecting the end effector and the manipulator was implemented
as a male-to-male M5 rubber shock mount with a compression
load of 200 N, shown in the top part of Figure 4.

Top Platform

Electric Gear Motor
Base Platform
Brushing System
Omnidirectional Wheel
Electric Gear Motor

Hinge Beam
Compression
Spring
Slider Bearing
Linear Guide
Motor Mount
Brush

Figure 3. An illustration of the end-effector design.

December 2018

*

IEEE ROBOTICS & AUTOMATION MAGAZINE

*

59
IEEE Robotics & Automation Magazine - December 2018

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