Situational awareness based flight control of a three-rotor mini-aircraft

Abstract

This paper focuses on a critical component of the situational awareness (SA), the control of autonomous vertical flight for an unmanned aerial vehicle (UAV). Autonomous vertical flight is a challenging but important task for tactical UAVs to achieve high level of autonomy under adverse conditions. With the SA strategy, we proposed a two stage flight control procedure using two autonomous control subsystems to address the dynamics variation and performance requirement difference in initial and final stages of flight trajectory for a nontrivial nonlinear three-rotor mini-aircraft model. This control strategy for chosen mini-aircraft model has been verified by simulation of hovering maneuvers using software package Simulink and demonstrated good performance for fast SA in real-time search-and-rescue operations.

Full text

Situational Awareness Based Flight Control of a
Three-Rotor Mini-Aircraft
IGOR ASTROV, ANDRUS PEDAI
Department of Computer Control
Tallinn University of Technology
Ehitajate tee 5, Tallinn 19086
ESTONIA
igor.astrov@dcc.ttu.ee, andrus.pedai@dcc.ttu.ee
Abstract: - This paper focuses on a critical component of the situational awareness (SA), the control of autonomous
vertical flight for an unmanned aerial vehicle (UAV). Autonomous vertical flight is a challenging but important task for
tactical UAVs to achieve high level of autonomy under adverse conditions. With the SA strategy, we proposed a two
stage flight control procedure using two autonomous control subsystems to address the dynamics variation and
performance requirement difference in initial and final stages of flight trajectory for a nontrivial nonlinear three-rotor
mini-aircraft model. This control strategy for chosen mini-aircraft model has been verified by simulation of hovering
maneuvers using software package Simulink and demonstrated good performance for fast SA in real-time search-and-
rescue operations.
Key-Words: - Flight control, three-rotor aircraft, simulation, situational awareness, unmanned aerial vehicle.
1 Introduction
Situation awareness has been formally defined as “the
perception of elements in the environment within a
volume of time and space, the comprehension of their
meaning, and the projection of their status in the near
future” 1 . As the term implies, situation awareness
refers to awareness of the situation. Grammatically,
situational awareness (SA) refers to awareness that only
happens sometimes in certain situations.
SA has been recognized as a critical, yet often
elusive, foundation for successful decision-making
across a broad range of complex and dynamic systems,
including emergency response and military command
and control operations 2 .
The term SA have become commonplace for the
doctrine and tactics, and techniques in the U.S. Army
3]. SA is defined as “the ability to maintain a constant,
clear mental picture of relevant information and the
tactical situation including friendly and threat situations
as well as terrain”. SA allows leaders to avoid surprise,
make rapid decisions, and choose when and where to
conduct engagements, and achieve decisive outcomes.
The tactical unmanned aerial vehicle (TUAV) is one
of the key tools to gather the information to build SA for
all leaders. The TUAV is the ground maneuver
commander's primary day and night system. The TUAV
provides the commander with a number of capabilities
including:
 Enhanced SA.
 Target acquisition.
 Battle damage assessment.
 Enhanced battle management capabilities (friendly
situation and battlefield visualization).
The combination of these benefits contributes to the
commander's dominant SA allowing him to shape the
battlefield to ensure mission success and to maneuver to
points of positional advantage with speed and precision
to conduct decisive operations. Some conditions for
conducting aerial reconnaissance with TUAVs are as
follows.
 Time is limited or information is required quickly.
 Detailed reconnaissance is not required.
 Extended duration surveillance is not required.
 Target is at extended range.
 Threat conditions are known; also the risk to ground
assets is high.
 Verification of a target is needed.
 Terrain restricts approach by ground units.
A mini-TUAV offers many advantages, including
low cost, the ability to fly within a narrow space and the
unique hovering and vertical take-off and landing
(VTOL) flying characteristics.
The current state of TUAVs throughout the world is
outlined 4 . A novel design of a multiple rotary wing
platform which provide for greater SA in the urban
terrain is then presented.
Autonomous vertical flight is a challenging but
important task for TUAVs to achieve high level of
autonomy under adverse conditions. The fundamental
requirement for vertical flight is the knowledge of the
height above the ground, and a properly designed
Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
71
ISBN: 978-954-92600-1-4

controller to govern the process.
In 5 , a three stage flight control procedure using
three autonomous control subsystems for a nontrivial
nonlinear helicopter model on the basis of equations of
vertical motion for the center of mass of helicopter was
proposed. The proposed control strategy has been
verified by simulation of hovering maneuvers using
software package Simulink and demonstrated good
performance for fast SA.
This paper concentrates on issues related to the area
of 5 , but demonstrates another field for application of
these ideas, i.e., research technique using control system
modeling and simulation on the basis of equations of
motion for the center of mass of small three-rotor TUAV
for fast SA.
In this paper our research results in the study of
vertical flight (take-off and hovering cases) control of
small three-rotor TUAV which make such SA task
scenario as "go-search-find-return" possible are
presented.
The contribution of the paper is twofold: to develop
new schemes appropriate for SA enhancement using
TUAVs by hybrid control of vertical flight of small
three-rotor TUAVs in real-time search-and-rescue
operations, and to present the results of hovering
maneuvers for chosen model of a three-rotor TUAV for
fast SA in simulation form using the
MATLAB/Simulink environment.
2 Three-Rotor TUAV Model
The three-rotor TUAV is composed of three rotors. It is
clear that one of the advantages of trirotors with respect
to quadrotors is that they require one motor less which
can lead to a reduction in weight, volume and energy
consumption. The two main rotors in the forward part of
the trirotor rotate in opposite directions and are fixed to
the aircraft frame. The tail rotor can be tilted using a
servomechanism.
The dynamics of the three-rotor TUAV for the case of
low speeds of motion can be represented by the
following equations 6 - 7
m
xsin
4

(1)
m
ysincos
4

(2)
g
m
zcoscos
4

(3)


 JWWWJJW
(4)
where
,
,
sin01
cossincos0
coscossin0
W
,
cos00
coscossinsinsin0
sincossincoscos0
W

(5)
,
3
2
1
),(2121 ffl
,cos)( 332112 flffl
,sin
333 fl
,cos
3214 fff
zyx ,,
are coordinates of center of mass in the earth-
frame;
,,
are yaw, pitch and roll angles;
is the tilting angle of third rotor;
)3,2,1(ifi
is the thrust generated by the i-th rotor;
1
l
is the distance from the centre of mass to the centre of
line between the first and second rotors;
2
2l
is the distance between the first and second rotors;
3
l
is the distance from the centre of mass to the third
rotor;
J
is the inertia matrix;
g
is the gravity constant;
m
is the mass of the TUAV.
Note that exists an orientation of body frame in which
the inertia matrix in (4) simplifies to:
),,( zyx IIIdiagJ
(6)
For simplicity we consider the matrix
J
in (6) as unit
matrix, i.e.
),1,1,1(diagJ
(7)
where
.1 2
kgmIII zyx
Substituting (7) into (4), we obtain


 WWWW
(8)
If we apply the properties of vector product to (8), we
obtain
Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
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ISBN: 978-954-92600-1-4



 WW
(9)
From (9), we have


 WWW 11
(10)
where
.
0sincos
0coscoscossin
cossinsinsincos
cos
1
1
W
(11)
We can regroup the three dynamics in (10) as:
321 sincossec tgtgtg 

(12)
cossincos 21

(13)
secsinseccossec 21
tg

(14)
From (1)-(3), (12)-(14) we can see that the attitude
vector
T
zyx ),,(
for given model of TUAV can be
computed.
The numerical values for three-rotor TUAV’s
constant parameters of (1)-(5) for a case of small
elevation above sea level are given by 6 :
,5.0 kgm
,33.0,24.0,07.0 321 mlmlml
2
/81.9 smg
.
3 Control System
It is possible to consider the thrusts
1
f
and
2
f
in (5) as
constant functions of time with one value and tilting
angle as a constant angle. Hence, we have
,0)()( 21 tftf
(15)
0)(
1t
. (16)
With selection of (15)-(16), control problem is now
turned into a hybrid constrained control problem with
using only one thrust
3
f
as control input for controlling
the coordinate
z
of altitude with respect to reference
input
0
z
.
The control system configuration to regulate the input
variable
3
f
is thus designed, to have the next structure
(see Fig. 1)
))(( 2
0
13 zztzztKf 

(17)
where
21,tt
are constants to be determined.
It is possible to consider the variable
3
f
as a “fast”
function of time. Hence, assuming that
0
3
f

, from (17),
we find
0
112 ztztztz 
(18)
The following coefficients of (18) are obtained from
8 , for overshooting with value of
%5
z
d
dt
t
t
t
z
23
,
92
2
1
(19)
where
z
d
t
is desired transition time of coordinate
z
.
For a hovering flight, angles of roll, pitch, and yaw
must be zeros. Therefore, the equation (3) becomes
,)()( 4gtctz

(20)
where
m
c1
. (21)
Differentiating both sides of (20) with respect to time,
we obtain
)()( 3tfbtz 

(22)
where
m
bcos
. (23)
Combining (17) and (22), we have
)),()(()( 3tztibKtz 
(24)
where
).())(()( 2
0
13 tzttzztti 
(25)
Defining
)()( tatz

in (24), we obtain
)()()( 3tbKatbKita

(26)
The variable
)(ta
in (26) can be described in a
common way through next expression as indicated in 9
,))(()( )(
03
)(
0tA
tAedbKieata
(27)
where
tbKdtA
0
)(
. (28)
Let us consider the behavior of the considered control
system (see Fig. 1) for the time
t
of time interval
z
d
tt
during the hovering.
Hence, assuming that
,0
0
a
,)( 00 ztzz
,05.0
,0)(tz

,)( 0
13 constztti
from (27)-(28), we find
)1()( 3bKt
eita
(29)
Assume now that for the desired transition time for
control of acceleration
)(ta
lies in the zone of
Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
73
ISBN: 978-954-92600-1-4

overshooting with value of
%5
, then, from (29), it
follows that
bK
tz
d)ln(

(30)
Therefore, using (23) and (30), and the ratio of
coordinate-to-acceleration transition times
zd
z
d
t
t
N

and
that
3)ln(
, we obtain
cos
3
z
d
tNm
K
(31)
4 Simulation Results
Consider the control of three-rotor TUAV model (1)-(3),
(12)-(14) for the case of take-off and hovering
maneuvers by hybrid constrained system of two control
subsystems.
The goal of the following simulations is twofold.
First, we verify that these control subsystems are able to
control the take-off and hovering trajectories. Second,
we observed the effect of enhancing SA because the
variety of such trajectory parameters as desired transition
times, ratios of coordinate-to-acceleration transition
times and heights of hovering easily can be changed the
possible take-off and hovering trajectories of three-rotor
TUAV.
Constant thrust forces of the first and second rotors,
constant tilting angle of the third rotor, initial conditions,
desired height positions, ratios of coordinate-to-
acceleration transition times and desired transition times
for control subsystems are chosen to be:
,1.2
21 Nff
deg,78
,0)0()0()0( mzyx
,10,5.2 0
2
0
1mzmz
,5,20 21 NN
.20,6 21 stst dd
Simulation results of the offered block scheme with
two control subsystems (see Fig. 1) are shown in Fig. 3.
Fig. 2 shows the height trajectory of flight control.
We simulated the block diagrams of subsystems as
parts of hybrid control system and take into account that
the full take-off and hovering trajectories were separated
into initial and final phases with boundary point in the
first lag position.
Some advantages of this example are as follows.
 Possibility to consider a terrain restriction in the
places of hovering.
 Smooth trajectory of flight and possibility of lag in
two different selected height positions.
 Using of two control subsystems to control the take-
off and hovering trajectories of flight.
Fig. 1. Block diagram of hybrid control system.
Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
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ISBN: 978-954-92600-1-4

010 20 30 40 50 60
-2
0
2
4
6
8
10
12
Time (s)
Height (m)
Fig. 2. Height trajectory of flight control.
-10
0
10
20
30
-20
0
20
40
60
80
100
120
140
160
180
-2
0
2
4
6
8
10
12
Y (m)
X: 21.94
Y: 135.4
Z: 10.01
X (m)
Z (m)
Fig. 3. 3-D motion of the 3-rotor TUAV.
These results support the theoretical predictions well
and demonstrate that this research technique would work
in real-time flight conditions.
5 Conclusions
A new research technique is presented in this paper for
enhanced SA in possible TUAV’s missions.
Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
75
ISBN: 978-954-92600-1-4

The need for highly reliable and stable hovering for
VTOL class TUAVs has increased morbidly for critical
situations in real-time search-and-rescue operations for
fast SA.
For fast, stable and smooth hovering maneuvers, we
proposed a two stage flight strategy, which separates the
flight process into initial and final phases. Two control
schemes are designed for this flight strategy. The
effectiveness of the proposed two stage flight strategy
has been verified in field of flight simulation tests for
chosen model of the three-rotor TUAV using software
package Simulink.
From the simulation studies of flight tests, the
following can be observed:
 The block diagram of flight control is very useful for
graphic representation of the flight trajectory.
 The received control subsystems are autonomous and
completely shared in time.
 The trajectory tracking display forms give a
researcher an immediate view of a three-rotor TUAV
motion with a range of such trajectory parameters as
transition times, ratios of coordinate-to-acceleration
transition times and heights of hovering for the
different phases of flight. This allows us to investigate
the sensitivity of the hybrid control system, providing
a medium for such development and evaluation and
enhancing the researcher’s understanding of hovering
maneuvers.
Although many of the details inevitably relate with
this particular system, there is sufficient generality for
this research technique to be applied to others models of
TUAVs during hovering maneuvers.
From the applications viewpoint, we believe that this
two stage flight strategy using flexible and effective
hybrid control furnish a powerful approach for
enhancing SA in applications to VTOL class TUAVs.
Future work will involve further validation of the
performance of the proposed research technique and
exploring other relevant and interesting TUAV’s
missions.
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Proceedings of the 12th WSEAS International Conference on AUTOMATIC CONTROL, MODELLING & SIMULATION
ISSN: 1790-5117
76
ISBN: 978-954-92600-1-4