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.