Post on 13-May-2023
Abstract: In this paper, a new tethered aerial robot for
maneuvering around the buildings for an urban search and
rescue (USAR) mission is introduced. The robot can enter
from a hatch in order to search the indoor area of a collapsed
house or maneuver around a building for search and rescue.
This new mechanism is composed of an airship as a flying and
hovering platform, and a Small Aerial Vehicle (SAV) as a high
maneuverable robot. A light weight tether, which is strong
enough, is used to connect the airship and the SAV. The airship
transports the tethered SAV (T-SAV) to the desired location,
lowers it and then supports it to gather information, using his
high maneuverability. In contrast to other proposed platforms,
this robot operates with very low noise, making it suitable for
search and rescue missions to avoid drown out victims’ low
voice. Moreover, the robot’s actuator’s vibrations are weak
enough to avoid the occurrence of another collapse. This paper
explains the design, modeling and controlling of the T-SAV.
Finally, the first prototype of the T-SAV is explained.
I. INTRODUCTION
There are a many efforts to develop the aerial robots for
Search and Rescue or Security and Rescue operations [1],
[2] based on their advantages over the ground robots [3].
The aerial urban search and rescue has several difficult
problems such as maneuvering between buildings, ascending
and approaching a building, or monitoring inside [1]. Such a
system should have sufficient stability, good station keeping
capability and high maneuverability in tight environments
[1]. Furthermore, such a platform needs to have low altitude
flying capability for close monitoring and inspection.
Consequently, it is important that the flying vehicle has low
noise to avoid suppressing victims’ weak voice [4].
Furthermore, the vehicle should not generate too much
vibration to avoid the occurrence of another collapse [3].
There are research studies about aerial robots for indoor
search and rescue missions [5], [6]. In these studies, it is
assumed that the robot easily moves in a limited-size
environment and it can be operated remotely. But when a
large scale disaster hits an area, such as the 2011 Japan
tsunami, reaching the destroyed buildings is hard or
impossible. Consequently, it is important to have an aerial
robot that can fly to the disaster area and shift to the
hovering mode at a safe altitude for surveillance and
monitoring.
To overcome the difficulty of maneuvering among
buildings and entering tight environments, the tethered aerial
vehicles are proposed. In such a tethered architecture, a
smaller robot is connected to a bigger one to provide it with
power, control, and computation power.
One group of the well known tethered robots is the
unmanned underwater robots referred to as remotely
operated vehicles (ROVs). In ROVs, tethers were used to
remotely tele-operate it, to provide the power and
communication, and to avoid losing the robots [7].Tethers
also have been used on walking robots to enable them to
walk on vertical or high slope surfaces without falling [8]. In
Stratosail™ project [9], [10] a tethered mechanism is used
for trajectory control.
McKerrow et. al proposed a search and rescue aerial robot
based on the tethered architecture [11], [12]. The host
platform is a robotic aerial vehicle that can fly to the area
and shift to hovering at a safe altitude above the targeted
object. The smaller tethered aerial vehicle is designed to
lower from the platform via a long cable to survey the target
on the ground. This tethered aerial vehicle is a long flying
pendulum that uses gravity to achieve basic vertical stability
mode by locating the center of gravity (COG) at the lower
end of the agent’s body. Two ducted fans are placed toward
the top of the robot near the tether point [11]. The system
acts like a double pendulum with two modes of natural
movement. Due to the second mode of motion it may
oscillate around its center of gravity while it swings or flies
in a circle [11]. This tethered SAV has low maneuverability
and undesirable natural mode in his motion. Its main design
goal is to have the orientation close to the vertical in
collapsed structures, but entering the desired structures is
THE DESIGN, MODELING AND CONTROL OF A TETHERED
AERIAL ROBOT FOR SERACH AND RESCUE MISSIONS
S.M.M. Dehghan, M. Zarezadeh, N. Farhadian, H. Moradi
Fig. 1. The sketch of the proposed robot approaching a building
after an accident at high elevation.
H. Moradi, S.M.M. Mehdi Dehghan, M. Zarezadeh, and N.
Farhadian are with Control and Intelligent Processing Center, at the
school of Electrical and Computer Engineering, University of Tehran,
Iran. This work has been done in the Advanced Robotics and Intelligent
Systems laboratory. Corresponding author: smm.dehghan@ut.ac.ir.
very hard and there are a lot of problem to do this, such as
finding and locating any vertical opening or hatch, adjusting
the location of the hovering agent and maneuvering to enter
the structure.
In this study, we propose a new tethered aerial vehicle
constructed of an airship as the flying and hovering platform
[13], [14] and a small plane, which we call it Tethered Small
Aerial Vehicle1 or T-SAV. As mentioned earlier, an airship
provides long flying time, good hovering and station keeping
behavior with almost zero noise and turbulence. Finally, its
failure has low damage because of airship’s low weight and
slumping improbability. Using a plane as the SAV, in
comparison to the SAVs used in [11], [12], has the
advantages of high maneuverability among obstacles and in
tight environments. Furthermore, it does not suffer from the
complicated dynamics and the oscillation behavior around
its COG [11], [12].
This tethered aerial system can fly longer distance to
reach the desired point, can increase the flight duration
because of using the airship power resources in comparison
to helicopters [15] or similar systems [16] in hover-and-stare
application.
II. MOTION MODES OF TETHERED AERIAL ROBOT
At the beginning of a flight, the SAV is attached to the
airship and the system is in cruise mode. In this situation, the
robot dynamics is similar to the systems which try to use the
aerostatics forces accompany with aerodynamics forces [17].
When the airship reaches to the desired point, switches its
cruise mode to the station keeping or hovering mode.
In the station keeping mode, the SAV is lowered and
begins its mission. In the first version of our SAV, two fans
are placed on the wings of a small airplane in equal distance
1 This new abbreviation is used to distinguish this fixed wing aerial
vehicle from a Micro Aerial Vehicle (MAV) that is usually smaller and also
an UAV which is typically bigger
from the coincident center of gravity (COG) and center of
rotation (COR). In other word, the midpoint of the fans,
COG and COR are coincident. These fans produce the trust
for linear motion and the torque for rotation of the SAV.
Based on this architecture, the T-SAV acts as a spherical
(two DOF) pendulum (Fig. 2). The length of the tether can
also be controlled to increase or decrease the altitude of the
SAV.
When the SAV goes to monitoring mode, it would fly
between buildings, gets close to windows and hovers near
them, moves vertically or horizontally adjacent to walls,
drops a sensor or a robot such as scout [18] into indoor
spaces, and circles around buildings.
In this study, we assume that the tether would not get in
contact with any object such as a building. We understand
that this is a restricting assumption, that limits the
application of the proposed platform, and we will address it
in our future work.
III. T-SAV DYNAMICS
The dynamics of attached airship and SAV is modeled in
[17]. With good station keeping behavior [13], [14], we can
assume the Tethered Point (i.e. TP in Fig. 3) is fixed to
model the dynamics of the T-SAV (this assumption is valid
and has not any effect on the generality of the model). Also
the tether is assumed to be straight and without weight.
With these assumptions, the T-SAV is modeled as a
spherical (two DOF) pendulum with its weight centered at
TP. Consequently, the SAV can move on a section of
surface of a sphere. The position of the SAV is determined
by θ and φ angles in the spherical coordinates with θ as the
tether angle with vertical axis (z) and φ is rotation of the
tether around z (Fig. 3). Angle is used to describe SAV
relative rotation with respect to the tether.
The x axis of the local frame is in the same direction with
the SAV’s nose, y axis is along the right wing and z axis is
Fig. 2. T-SAV Schematics: It can maneuver on a section of a sphere
centered at the attachment point to the airship with radius equal to the
tether’s length.
Fig. 3. The coordinate frames and the forces applied on the T-SAV.
defined in the right coordinates. If the COG, COR and tether
point assumed to be coincident, then the SAV’s surface is
perpendicular to the tether in all situation.
The SAV’s thrust, T, is the average of fans thrusts. Torque
which changes is proportion to the difference of fans
thrusts.
If the dynamics of T-SAV is described in two decoupled
motions, motion of spherical pendulum with concentrated
mass at TP and SAV rotating around its contact point to the
tether, the Lagrangian of system is written for θ and φ
coordinates, those affected from transformed component of
T along the tether and its perpendicular direction. The
Lagrangian is defined as the difference between potential
and kinetic energy. The kinetic energy of this pendulum is
summation of two term, kinetic energy of receding from
vertical axis and kinetic energy of rotating around vertical
axis. The only potential energy is gravity that is impacted by
changing the altitude of SAV.
(1)
Since two generalized coordinates, θ and φ, are used,
therefore two equations are generated according to
Lagrangian formulation:
(2)
(3)
Substituting (1) into (2) and (3), the equations of motion
of the T-SAV are obtained regardless of SAV rotation
around tether point:
(4)
(5)
As the equation (5) shows, when θ is zero, there is a
division by zero in calculation. This is the result of this
fact that when θ is zero, φ’s variation is not affecting the
pendulum’s state. But in the proposed robot, there is a SAV
at TP so when φ is changed; SAV's orientation and the thrust
vector in airship frame are altered. Therefore we assume is
zero in this situation to eliminate the division by zero and the
previous changes the SAV’s angle. In standard pendulum
modeling, is assumed to be zero.
IV. T-SAV CONTROL
The controller design for an airship [19]-[21], the path
following as needed in cruise mode [22] and also the
hovering and station keeping [13], [14] are studied in many
researches. Consequently, in this study we focus on the
control and maneuver of T-SAV.
The system is described in (4) and (5) cannot be linearized
around the equilibrium point, i.e. . Thus the controller design with linear state feedback is
not possible. Consequently, feedback linearization method, a
nonlinear control approach, is used in which and
inputs are (6) and (7).
(6)
(7)
The resulting linearized system is represented in (8).
(8)
in which the state variables are , input
variables are and output variables are θ and φ.
This system has normal form and relative degrees {2, 2}
with no internal dynamics. Furthermore, the stability of the
linearized system by feedback linearization is equivalent to
stability of the original nonlinear system. Linear state
feedback methods or robust control methods can be used to
design controller for this new system. This controller can
asymptotically stabilize the system at any point. By
calculating the controller inputs and , T and can be
calculated to control this nonlinear system.
One of the primary tasks of T-SAV is to move from any
point in its workspace to another point. To follow a given
trajectory from a starting pose to a goal pose, the integral
method is used to change the designed regulator method to a
tracker in order to track the desired trajectory based on the
reference signals θ and φ. The designed controller system
can eliminate the fixed disturbances in regulator and tracker
systems.
Since there are two fans, it is obvious that two internal
controller should be used to track the desired values of T and
.
V. T-SAV SIMULATION AND PROTOTYPE DEVELOPMENT
In this section, the results of model simulation and its
control are presented first. In the second subsection, the
results of prototype development are discussed.
A. T- SAV and controller simulation
1) Stabilization of T-SAV after release
After releasing T-SAV from the airship, it may have
undesirable initial state, non-zero value of φ and θ and non-
zero values of their angular velocities, while T-SAV should
be stabilized around zero (equilibrium) state. As Fig. 4
shows, the robot starts at a non-equilibrium state
and the controller controls
to the desired equilibrium point, i.e. .
2) Ascending from equilibrium to a desired point and
start pose keeping
In many situations, T-SAV should ascend/descend to a
desired point, e.g. close to a window to look through it, from
the current point and hover there to gather information.
Consequently, a simulation was designed to show that the
robot can be controlled to move from a given point to a
desired point and keep its pose at the desired point. As
shown in Fig. 5, the robot starts from equilibrium to
θ
point and keeps its pose.
3) Maneuvering around a building
For maneuvering around a building with a fixed distance
from its walls, T-SAV should increase φ angle gradually
while θ is fixed. In the maneuver shown in Fig. 6, T- SAV
begins from equilibrium point, moves to a point with θ
and simultaneously increases φ. After getting to θ
,
point, the robot starts the pose keeping task.
4) Altitude increasing/decreasing with fixed distance to
wall
T-SAV should have the ability of controlling the tether
length to increase/decrease the SAV’s altitude when the
distance to desired wall is fixed. In this maneuver, the
tether’s length is decreased and simultaneously θ is
increased (Fig. 7). This maneuver is simulated in Fig. 8.
The robot begins to decrease the tether length from time 27
until time 47 at a constant speed.
B. Development of the T-SAV prototype
A small prototype of the proposed T-SAV has been built
with two small electrical fans on its 13cm wings (Fig 9). The
T-SAV is hanging from a fixed point using a 50cm tether.
The fans are controlled through wireless communication
using a computer.
Three experiments were conducted to test the system. In
the first experiment, the fans were activated to move the T-
SAV away from the vertical axis, i.e. increasing θ, without
changing φ. Although, due to the typical differences in fans
and prototyping inaccuracies in the construction, the T-SAV
started rotating around the vertical axis, i.e. Z in Fig. 3,
however it was possible to keep the pose fixed by changing
the speed of the fans.
In the second experiment, the effects of moving the
airship while the T-SAV is tracking a given task are studied.
To do so, the fixed hanging point was moved at low speed.
As a result, the T-SAV did not become unstable or behaved
differently.
The last experiment was conducted to determine the
Fig. 4. T-SAV had undesirable initial state, non-zero value of φ and θ
and non-zero values of their angular velocities (
)
after releasing from the airship but it can be stabilized around zero
(equilibrium) state after a short time.
Fig. 5. Ascending from equilibrium to
point and
pose keeping.
0 10 20 30 40 50 60 700
5
10
15
20
25
30
35
40
45
50
Time
Teth
a(D
egre
e)
Actual
Desired
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
80
90
100
Time
Phi(
Deg
ree)
Actual
Desired
effects of changing the length of the tether on the behavior
of the robot. Thus the length of the tether was manually
changed in order to observe the behavior of the robot. As a
result, only the distance to the vertical axis changed and no
undesirable dynamics appeared. This test also showed that
the length of the tether can be used to increase the
maneuverability of the T-SAV.
The 2nd prototype of the TSAV has been built (Fig. 10)
using an off-the-shelf airplane. The actual airplane design
will allow to take advantage of the lift created by the wings.
Fig. 11 shows a sequence of positions of the TSAV when
it is in circular motion. The centrifugal force created by this
motion provides bigger workspace suitable for monitoring
larger area.
VI. CONCLUSION AND FUTURE WORKS
In this paper, a new mechanism was introduced to
increase the capabilities of airships by incorporating Small
Aerial Vehicles (SAV) attached to them using a tether. Such
a tethered SAV (T-SAV) would peer through windows and
transmits video and other information to a remote operator.
The proposed system satisfies the majority performance
characteristics for USAR robots [1] such as Human-System
Interaction, logistics, operating environment, and safety.
The system was modeled and a controller developed to
control the robot for a given desired maneuver. The
Fig.7. Schematics of altitude increasing with fixed distance to vertical
axis maneuver. This is a useful maneuver in many search and rescue
mission such as defining which floor catch a disaster or localizing
victim's mobile signals.
Fig. 6. Maneuvering around a building until reaching to
,
point. T- SAV begins from equilibrium point, moves to a point
with θ
and simultaneously increases φ. After getting to θ
,
point, the robot starts pose keeping task.
0 10 20 30 40 50 600
5
10
15
20
25
30
35
40
45
50
Time
Teth
a(D
egre
e)
Desired
Actual
0 10 20 30 40 50 600
20
40
60
80
100
120
140
160
180
200
Time
Phi(
Deg
ree)
Desired
Actual
Fig.8. Vertical motion: T-SAV tried to increase its altitude while its
distance to the vertical axis is fixed. In this maneuver, the tether’s
length is decreased and simultaneously θ is increased. The robot
began to decrease the tether length from time 27 until time 47 at
constant speed.
0 10 20 30 40 50 60 70 800
5
10
15
20
25
30
35
40
45
50
Time
mete
r
Distance to vertical axes
Tether lenght
Fig.9. The first prototype of the proposed T-SAV. The two fans are
mounted on the wing and the control board is placed on the fuselage.
The T-SAV is made of hard foam to reduce the weight of the SAV.
The battery required to operate the T-SAV, was used in two different
configurations: a) on the T-SAV to make itself inclusive, 2) off board
connected through the tether to reduce the weight and increase the
maneuverability of the robot. In other words, the battery can be placed
on the airship and the power goes down to the SAV through the
tether.
simulation results and the initial prototype show the
effectiveness of this system.
The future work is toward generalizing the model by
eliminating simplifying assumption such as decoupled
motion of the SAV and 2DOF pendulum. Furthermore, a
more robust control system is needed to use feedback
linearization and incorporate the length of the tether. Finally,
the interaction of the tether with obstacles and its impact on
the behavior of the SAV and its control should be studied.
ACKNOWLEDGEMENT
The authors would like to thank Mayhar Karami for his
cooperation in prototype building. They also like to thanks
ms. Lrila Kashani for her comments on the writing.
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Fig.10. The second prototype of the proposed T-SAV. In this
prototype, the battery and control board are used off board and
connected through the tether. Also, the configuration of the T-SAV
becomes more similar to the real airplane to check airfoil effect on the
maneuverability.
Fig.11. The sequence of figures shows another maneuver that is
appeared due to the centrifugal force when the T-SAV is tangent to the
rotation circle. This maneuver did not consider in the simulation
before, however, it can be used to cover wider area for monitoring, and
search tasks.