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A Hybrid Controller for Inflight Stability and Maneuverability of an Unmanned Aerial Vehicle in Indoor Terrains

Authors:
A Hybrid Controller for Inflight Stability and Maneuverability of an Unmanned
Aerial Vehicle in Indoor Terrains
C. Todd1, H. Koujan2, S. Fasciani3
1Faculty of Computer Science, Hawaii Pacific University, 1164 Bishop Street, Honolulu, Hawaii, 96813, USA
2, 3Dept. Information Science and Engineering, University of Wollongong in Dubai, Knowledge Village, Dubai, UAE
1cghourani@hpu.edu, 2hk963@uowmail.edu.au, 3stefanofasciani@uowdubai.ac.ae
1http://www.hpu.edu, 2, 3 http://www.uowdubai.ac.ae
Abstract
A hybrid controller is designed and tested in simulation-
based experiments, overcoming several research
challenges associated with stability and control of UAV
flight in a GPS-denied environment, achieving faster more
accurate autonomous indoor flight. There exists a need for
design and modeling of a robust, innovative flight
controller that offer more stable, autonomous flight in
challenging indoor environments. Drone drifts caused by
air turbulence pose as a significant challenge for
autonomous aerial systems that are GPS-denied. A hybrid
flight controller, that utilizes PID, PIDD2 and FLC
techniques in addition to KF, EKF and Complimentary
Error Minimization algorithms, has been developed in a
simulation software and validated. Major contributions of
the work include: multiple switchable flight modes at the
Position and Tracking Controller (PTC) level, and
multiple quadcopter flying configuration at the Motor
Mixer level. A novel drift correction mechanism utilizes
the drone states and an EKF estimator generates a
compensation signal with integration of the PTC. FLC
adds an extra layer of control by determining the drone’s
flight mode and flying configuration for the optimal flight
output and stability performance. Contributions of this
work may further offer increase in accuracy for connected
processes enabling UAV flight, including SLAM, path
planning, collision detection and avoidance and,
subsequently, overall flight performance. 1
Keywords: Autonomous UAV, Flight Controller, Drift
Correction, Hybrid Controller, Fuzzy Logic and PID
Controller.
1. Introduction
Unmanned Aerial Vehicles (UAVs) are those capable of
flight controlled by an onboard computer and/or remotely
from the ground. The UAV can be designed to support
numerous tasks such as surveillance, scientific research,
1 This study has been implemented on a Simulink
platform in Matlab.
construction and military operations [1]. UAVs can be
classified based on their lift mechanisms such as fixed
wings, rotorcrafts or Vertical Take Off and Landing
(VTOL) [2]. A quadcopter is a multirotor aircraft built
using four thrust sources, generally fixed angle propellers
driven by DC motors [2]. Compared to other types of
aerial vehicles, quadcopters offer several advantages: the
single rigid body with four fixed angle propellers provides
a simple mechanical structure, while other aircrafts such
as helicopters use one rotor and require complex
mechanical components to change the rotor orientation
while providing thrust to maneuver [3]. A quadcopter can
execute movements with higher accuracy than helicopters,
essential when operating in indoor environments [3].
Quadcopters can fly at low speed, whereas fixed-wing
aircrafts do not present such flexibility in aerodynamics
and require high air pressure for flight capability that
occurs only at a high speed [3]. However, quadcopters
present major drawbacks in terms of power efficiency, due
to continuous, rapid acceleration of the four motors and
flight stability. Effective control strategies may be
introduced for stability improvements [4].
In this work, a novel approach to controller design is
proposed toward enhanced UAV (Quadcopter) flight
stability and maneuverability, overcoming several
research challenges within this field. A Quadcopter
possesses structural characteristics that change with its
dynamic model and each model poses unique design
challenges. Appropriate modeling of aerodynamics and
dexterous control are the main concerns in this field. To
establish autonomous flight accuracy of pose estimation,
obstacle location and classification are key issues [5][6].
Indoor environments raise special concerns in terms of
nature of failures and safety issues. Current UAV flight
controller techniques can handle sparse and large indoor
areas, but embedded and/or moving objects of various
type and shape still place significant restrictions on indoor
drone flight capability. This context requires sophisticated
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
27
and advanced control algorithms to circumnavigate
obstacles, for path planning and localization.
Sophisticated control algorithms may be designed to
enable a UAV to circumnavigate indoor embedded
obstacles, providing dexterity in control strategy. Indoor
spaces deny the UAV’s accessibility to signals provided
by the Global Positioning System (GPS), and as such,
alternative solutions, for drone localization and/or as input
for control, are needed. A control system that
communicates with path planning and collision avoidance
is required to output correct flight path decisions for the
drone’s controller and also to facilitate UAV inflight
stability, which is subject several disturbances in indoor
contexts [1], [4], [6], [7], [8]. Air turbulence reflected from
objects close to the drone, including when taking off or
landing, causes lateral drifts that affect the accuracy of the
autonomous functions [1], [7]. Light-weight UAVs
introduce additional challenges in dynamic modeling
because the assumption made for larger drones, such as
structural rigidity and symmetry, are no longer valid and
non-linear parameters in the mathematical model of the
Quadcopter frame should be introduced [6]. Simultaneous
changes in the drone’s motors speeds add another level of
complexity that may determine in instability [8].
For UAV autonomous control, research challenges are of
a technical nature and are application-specific, such as
relating to drift, instability and inaccuracies due to
inappropriate fine-tuning of the flight controller. This
work overcomes associated research challenges by
designing a hybrid flight controller system offering
stability and high accuracy in indoor autonomous flight. A
control algorithm that achieves better flight stability for
indoor autonomous quadrotors as compared to existing
solutions [9], [10], [11], [12] is designed, implemented and
validated using a hybrid model of Proportional, Integral,
Derivative (PID) and Fuzzy Logic Control (FLC), in
Matlab Simulink. UAV design exhibits improved
performance offering faster response time and accuracy of
flight control, in comparison to existing techniques [1], [2].
This system overcomes disturbances of drift in which
better results of mapping, localization, collision detection
and avoidance may be obtained, within GPS-denied
environments.
Section (2) focuses on the hybrid controller design and
implementation in Simulink, in Matlab. Section (3)
presents the results of the study. Section (4) provides a
discussion of the research findings and Section (5)
emphasizes major conclusions and recommendations of
the work.
2. Controller Design and Implementation
In the applied method, it is assumed that the drone has full
operability of all propellers and that the battery provides
supply power for the duration of the flight. Environmental
issues such as lateral drifts during drone-obstacle air
turbulence interference are considered for controller
design. A standard quadcopter is considered, as in [1], [7],
[13], with rigid body construction and four thrust sources
(motors) placed on each wing, equi-spaced from the frame
center, as in Figure 1. Propeller rotation direction plays an
important role in achieving quadrotor basic stability [7];
each two opposing fans, separated by 180
o
, should rotate
in the same direction, with direction opposite to that of the
other fan pair. For example, if propellers 1 and 3 are
rotating clockwise, propellers 2 and 4 should rotate anti-
clockwise [13]. Quadrotor motion is described with its tilt
on three axes of rotation w
x
,w
y
and w
z
[13]. The UAV
rotation around the w
z
(Yaw) axis results in turning to the
left or right, and it can be achieved by reducing the speed
of one of the counter rotating propeller pairs [1]. Rotation
of the quadrotor about the w
y
(Pitch) axis is performed by
rising the angular speed of motor 4 and decreasing the
speed of motor 2 [1]. Rotation about the w
x
(Roll) axis is
controlled by the same operation as for Pitch but with fans
1 and 3 [1]. The UAV's take-off and landing mechanism
simultaneously increases or decreases the speed of all
propellers [2]. Basic maneuvers of a quadrotor assist in
understanding stability requirements and flight control of
a UAV of this type. The controller design proposed
controls six degrees of freedom including yaw, pitch, roll,
x-transition, y-transition and attitude of the four motors
positioned on the quadrotor. This is achieved through
three layers of control.
Figure 1. Quadcopter: six Degree of Freedom, rigid frame with four
equi-spaced thrust sources.
PID control is combined with FLC to provide a hybrid
solution for greater quadrotor control and stability during
indoor autonomous flight. Several PID controllers are
embedded enabling multiple inputs for more complex
functions demanded by the drone [9]; a technique that
offers a higher control accuracy of each degree of freedom
of the quadcopter [8], [10]. The FLC requires that control
functions are written using linguistics for all possible
control scenarios, forming the control data sets [5].
Compared to other methods, FLC do not require an
accurate input signal: smaller errors and overshoots of
system output, and faster time to reach steady state
produce better results in terms of final stability [2], [14].
The Backstepping control technique is not considered in
this design due to the high computational complexity,
which is not suitable for UAVs controlled by onboard low
power computational platforms. Backstepping is a
recursive complementary tool that has been used with
other control techniques to achieve better stability than
singular control techniques [5], [10], [15]. A major
challenge in designing and implementing the
Backstepping controller is the greater complexity and
accuracy in terms of the mathematical and dynamic
models required to fully integrate them in a quadrotor. The
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
28
Backstepping controller is substituted by the Kalman
Filter (KF) and the Extended Kalman Filter (EKF) that
enable estimating the result of a process with a high degree
of confidence. The EKF algorithm is designed to estimate
the state of a non-linear systems affected by unpredictable
noises. EKF uses less computational resources and it
requires less information about the estimated system than
a Backstepper controller [16]. The aforementioned
techniques have been implemented to minimize the
measurement errors and to estimate the drift of the
quadcopter as part of the drift correction mechanism.
A hybrid controller that exploits the advantages of all the
three controllers: PID, FLC and EKF has been designed,
developed and tested in this work. PID controllers are
designed to control independently pitch, roll, yaw and
altitude; one controller is dedicated to each degree of
freedom. The FLC is designed to then achieve further
flight stability; it continuously adjusts the flight mode and
the motor control mixer configuration. Further, the FLC
operates as the main controller that determines the
movement output of the UAV based on two parameters;
the input signal and the estimated disturbance existing in
the drone’s environment. An overview of the proposed
system is illustrated in Figure 2.
Figure 2. Configuration of the FLC, PID and EKF controllers within the
hybrid controller design.
The drift correction algorithm (Figure 3) designed for this
flight controller, utilizes an EKF to estimate the real pitch
and roll of the drone away from the control processes.
Then, the estimated angles are fed back to the flight
controller as an error signal after subtracting them from the
input command signals. This error, the difference between
the two signals, represents the drift of the quadcopter that
will be corrected automatically by the flight controller as
it computes the other needed corrections.
Figure 3. Flowchart of the algorithm for Drift Correction.
Modeling of Drone Control: The controller utilizes
multiple techniques in control and filtering including PID,
Proportional, Integral, Derivative (Second Order) PIDD
2
,
FLC, KF, complementary filtering and Extended Kalman
Filter (EKF). The flight controller is divided into five
components: Position and Tracking Controllers, a Motor
Mixer, a Drift Correction mechanism and the FLC. Novel
techniques are included in each component of the UAV
flight control and stabilization: the Position Controller
compensates for drift correction by application of the EKF
in addition to applying existing techniques including basic
manipulation of the desired x-, y- and z- coordinate, which
represent the desired rotational rate as output. The
Tracking Controller receives the output from the Position
Controller and together with the proposed control
strategies, determines the correction signal required and
adjusts the quadcopter's attitude. Existing techniques
implement only a single control methodology for the
Tracking Controller for all flight conditions. In this work
the Tracking Controller has two flight modes with
dynamic switching capability for mode selection, to
overcome induced stability and fast-response issues. The
motor control mixer then maps the angle correction
signals determined by the Tracking Controller into the
required throttle for each of the four motors on the UAV
frame. The quadcopter can fly in two different kinematic
configurations: X configuration, in which the drone’s X-
Y axes of movements are 45 degrees apart between each
of the four motors and the Plus (+) configuration, in which
the drone’s body frame is the same as its inertial one. In
this work, there is an option to switch between
configurations based on the desired output which is a
novel approach.
For autonomous drone flight, drifts are key concerns. To
address these, the EKF has been designed and
implemented to estimate environmental drifts. This drift
correction mechanism estimates pitch and roll angles in
isolation from control process noise and interference;
these estimates are then compared to the planned angles
for drift calculation. The compensation commands are
then generated by the Position Controller. PID and
associated derivatives are implemented in the Flight
Controller for individual motor control. To improve
system stability, the FLC is implemented to determine the
optimal setting required to drive the most stable output:
given the multiple operating flight modes and
configurations of the Flight Controller, the FLC operates
to drive mode and configuration selection for greatest
stability.
Mechanical Modeling: Mathematical modeling of
quadcopter aerodynamics enables calculation of the
required practical parameters for controller
implementation. Two conventional coordinate systems
are for the quadcopters are the Plus (+) configuration, with
the x- and y- axes as the arms supporting the two
consecutive motors (90 arm separation). According to the
Plus convention the first motor must rotate anti-clockwise
and the second motor clockwise. In the Ex (X)
configuration, the x-axis is aligned by +45 from the arm
of the first motor towards the second; that is, the Plus
configuration rotated by +45Changes in configuration
will cause differences in modelling and alignment of
motors during this process. In this work, the both
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
29
configurations are enabled with switching between each
mode.
Assuming rigidity of the quadcopter's structure and
propellers, that the UAV structure is symmetrical, and the
center of gravity and fixed body frame coincide, the
moment of inertia of the quadcopter mass may be defined
by the following matrix (Equation 1):
 00
0
 0
00

Equation 1
By applying approximations [11], [12], the momentum of
the quadcopter's structure, actuator mass and arm mass are
determined by Equations 2, 3 and 4.
,,,
Equation 2
21
41
3
21
41
3

1
41
12
21
221
41
3

,,,
Equation 3
21
41
3
21
41
3

1
41
12
21
221
41
3
 ,
,,
Equation 4
4

4

where mx is the mass of the sub-notation x which could be
referring to the motor (M), drone’s arm (A) or drone’s hub
body (H), dx is the radius from what is referred to in the
sub-notation x to the Centre of Mass, hx is the height of
sub-notation x from its base, Lx is the total length from the
base to sub-notation x, and rx is the radius of the structure
referred to in the sub-notation x.
The main forces and moments that affect the drone are
from gravity or due to the thrust and torque initiated by
the four rotors [3]. The thrust generated by the propellers
are conventionally forces that are perpendicular to the x-y
plane of the body frame in the positive direction of the z
axes [12]. Drone maneuverability is constrained by motor
thrust, which is the key driving force for its movements
[11]. The generated thrust, T, by an actuator on the
quadcopter is described by Equation 5.

Equation 5
where is the thrust coefficient provided by the
manufacturer of the rotor or measured manually, is the
air density, is the propeller's rotation cross-sectional
area, is the radius of the rotor and is its angular
velocity. Simplifying Equation 5, Equation 6 represents
the lumped thrust coefficient, .
 Equation 6
The total thrust provided by all four propellers is
represented by Equation 7. Varying the thrusts of motors
2 and 4 results in rotation of the drone about the x-axis.
This rotation is coined the rolling torque, L, as represented
by Equation 8.

Equation 7
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
30




Equation 8
where
is the arm length of the drone. The pitch torque,
M, is the drone’s rotation around y-axis and results due to
a change in the thrust of motors 1 and 3. Pitch torque is
calculated by application of Equation 9.




Equation 9
The yawing torque, N, is the drone’s rotation around the z-
axis. It is the resultant of the difference in the developed
torques between all four motor rotations (counter-
clockwise and clockwise) and is expressed as Equation 10.






Equation 10
The torque generated by each motor, Q, can be expressed
using the lumped torque coefficient, and is represented by
Equation 11.
_

_
Equation 11
Practical tests enable determination of the parameters in
Equations 1 to 11. All inertial forces and torques of the
quadcopter may then be grouped into one, inertial, matrix
(Equation 12).
Σ

0
0

0
0


Equation 12
where d is just another convention notation for the
aforementioned notation l.
For modeling of the mechanical system, all parameters and
forces that affect the aerodynamics of the UAV may be
captured in one matrix. The resulting matrix,
,
,
(Equation 13) accounts for all of the aerodynamic,
gyroscopic, and thrust moments created by the rotors’
systems on the quadcopter for the Plus configuration. The
lift force is represented by Equation 14. Additional effects
such as blade flapping and frame aerodynamic drag not
included in the model.
,





















Equation 13
PID Controller Model: The quadcopter PID flight
controller consists of two sub-systems: the first
determines the desired quadcopter rotational rate driving
position control, and the second measures the actual
quadcopter rotational speed for calculation of the required
motor output to achieve the desired rotational movement,
as shown in Figure 4. The Inertial Measurement Unit
(IMU) attains actual rotational position information
pertaining to the rotational speed of the drone in addition
to the angle of deviation from any of the drone axes. This
actual angle measurement is compared with the desired
angle of the drone, which represent the input of the first
PID controller. The difference of desired rotational rate to
meet the requirements is sent to the second PID controller
and compared with the actual rotational rate to provide
motors with a throttle control signal (voltage).
Figure 4. Diagram of the two sub-systems for PID control of motor
angular rotation.
Dynamic alternation is achieved in-flight between
different stability and structural modes. The Tracking
Controller processes the quadcopter commanded desired
position in two stages: first it finds the feed forward error
of the quadcopter's position to determine the desired
rotational velocities, and then it finds the required
correction to be performed by the drone for its rotational
states and altitude. The Motor Mixer Controller translates
the required corrections into throttle commands that are
distributed to each of the four motors and are dependent
on the quadcopter configuration. Both Plus and Ex
configurations integrated in the system, requiring a
switching strategy to provide maximum drone
maneuverability. The State Finder manipulates all
parameters, state equations and forces acting on the drone
for current use by the system.
The Tracking Controller determines the correction needed
for pitch, roll and yaw of the drone, in addition to the
altitude, for velocity control; by comparing actual and
desired position and attitude readings throughout several
layers of PID control. An improved controller is achieved
over existing solutions [9], [10], [11], [12] with novelty in
utilization of two modes (configurations) in combination
with FLC for tuning and a PIDD
2
controller with a KF to
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
31
filter out noise. This approach further provides a more
accurate measurement of the angle of the quadcopter.
Variation of the Tracking Controller from existing designs
lie in the dual mode switching capabilities dependent on
the flight environment, in addition to the KF
implementation to calculate more exactly the error signal
of the feed-forward PIDD
2
. The KF effectively eliminates
noise due to measurement and control to increase the
accuracy of the correction signal made by the controller.
After receiving the desired x and y coordinates, altitude
and yaw, the Tracking Controller calculates the desired
pitch and roll angles from these inputs, applying Euler’s
Equations to compute the transformation of the drone’s
position and velocities between its inertial- and body-
frames. First the error is determined for the x and y
coordinates of the body-frame. This error is then mapped
to a desired velocity in the same frame, which is
transferred to the desired rotational rate. For calculation of
error and its mapping to desired rotational rates, the yaw
(Psi angle, ) is needed since a quadcopter can maneuver
towards a point with different Yaw angles. The x and y
components of the yaw are scaled by the difference
between the current coordinates and intended ones to find
the desired velocity in the x and y axes of the body frame,
as in Equation 15 and 16 [3]. Figure 5 shows the Simulink
design of the Tracking Controller.
,


∗cos

sin




∗cos


∗sin
Equation 15
U
,


∗cos

sin




∗cos


∗sin
Equation 16
Figure 5. Simulink design for modeling of the mathematical
manipulation needed to determine the desired velocity correction in x
and y axes of the body-frame using the signals coming from the
commands and quadcopter state manipulator.
Actual pitch and roll rates are controlled by PID
controllers that use the desired velocities and actual
velocities, U and V, measured by the IMU. The feed-
forward PID technique is applied since it offers faster
response than a feedback PID system for control of electric
motors [15], [17]. The feed-forward subsystem calculates
the required adjustment needed by each rotational drone
axis; this is sent as a command signal to the next
subsystem to direct drone movement by the quantified
angle about the respective axis. For calculation of the
angle of required rotation Theta, the difference between
the pitch rate and the linear velocity is calculated along the
x-axis of the inertial-frame. This provides the velocity
error, which is sent to a feed-forward PD controller to
compute the required Theta, limited within the physical
characteristics of the drone and input to the next section
of the controller. For calculation of the angle of rotation
about the y-axis, the roll rate, the desired roll rate
comprises the positive input while the negative input is the
linear velocity in the y-axis of the inertial-frame. A PD
controller enables the required compensation in terms of
roll to be found. The sign inversion corrects the calculated
error due to mathematical operations.
The pitch , roll , yaw and altitude correction signals
are computed in the following subsystem of the controller
using feed-forward PID controllers. This process can be
done in one of two modes: the Acrobatic mode for fast,
sharp maneuvers or the Extra Stable mode for additional
stability. In Acrobatic mode, first the error is determined
and is then fed into the proportional and integral
components of the controller (Figure 6). This error is the
difference between desired and actual the drone’s
rotational angle measurements. The derivative component
comprises a scaling factor since rotational rates of the
drone may be determined through analysis of the IMU
readings and application of Euler’s Equations; substituted
as the derivative of the corresponding angle. All four
required correction signals are generated via identical
controllers with the exception of altitude because this has
a gravity offset input to achieve hovering at a specific
height. Further, the altitude controller finds the derivative
of the linear velocity on the z-axis to calculate the vertical
acceleration of the drone which assists in its stabilization
at the defined gravity offset.
Figure 6. Simulink implementation of the Aerobatic flight controller
mode.
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
32
The Extra Stable mode for flight control detailed in Figure
7 enables higher flight stability with a tradeoff in slower
response time due to the KF applied to the input signal for
noise reduction, providing accurate estimates of pitch ()
and roll () angles. The derivative component in the PID
controller projects the future error of the plant signal and
also enables the controlled system to stabilize about a
targeted point. This type of controller is a PIDD
2
due to
the second derivative component and prove useful in flight
systems such as this in achieving with greater accuracy
and stability, the desired attitude or position. Angle
correction calculations in this mode are performed using a
similar PIDD
2
structure, with altitude correction requiring
an additional component for consideration of initial
conditions, such as if the drone had a specified starting
altitude (gravity offset).
Figure 7. Simulink implementation of the Extra Stable flight controller
mode.
The KF is implemented as a Matlab function box, which
is a customized Simulink block. The state model of this
KF, A, includes the angle and it first two derivatives, and
is initialized to zero; A = [1 dt -dt; 0 1 0; 0 0 1]. This
assumes the filter has a coordinated start with the flight
controller when the drone is at rest and assumes a balanced
start position. The prediction of the error covariance
matrix, P, is initialized as: P = [1000 0 0; 0 1000 0; 0 0
1000]. The KF recursively updates the values of P to
within reasonable limits from the initially high values. The
scaling matrix, B, of t he co ntr ol no ise i s as sume d to b e zer o
for the purposes of simulation. P is then updated with
consideration of values of A, according to the following
Matlab script: P = A*P*A.’ Filter observations, z, are then
equated to the block input signal, u, which is the angle
measured by the drone. The matrix, C, that maps the
current drone state to the measurements read by the IMU,
which in this case is set to: C = [1; 0; 0], since the filter
estimates only the angle and other derivatives are not
needed. Calculation of the prior state estimate is then
performed using Equation 16, with x initialized to 0; x =
[0; 0; 0]. The filter gain is implemented in a two-step
process, by applying Equation 17 to determine S, the
denominator of the gain equation, and then Equation 18 to
calculate gain K. R is a matrix that contains the sensitivity
values of the sensors used to implement the KF, however
for simulation R is initialized to zero since the simulated
IMU signal has no sensitivity; R = [0 0; 0 0]. Finally, the
filter finds the posterior state estimates (x) and updates the
error covariance matrix (P) using Equations 19 and 20
respectively. The output of this block is the estimated
angle by the KF based on the inputted measured angle
from the flight controller.
y = z – C x x
Equation 16
S = C x P x C' + R
Equation 17
K = P x C' / S
Equation 18
x = x + (K x y)
Equation 19
P = (I – (K x C)) x P
Equation 20
Figure 8. Simulink model of the control mixing signal processing for
Quadcopter Plus Configuration.
Figure 9. Simulink model of the control mixing signal processing in Ex
Configuration.
A control signals mixer has been implemented to provide
functions that scale and map the input signal to one that is
a meaningful command to the actuator, such as correction
signals are translated into throttle commands per motor to
drive the quadcopter to the required position. To provide
the mixing equations for each of the two configurations
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
33
Plus and Ex: changes in speed and required rotational
direction of the four propellers map to changes in elevation,
pitch roll and yaw. This control mixer varies from existing
solutions due to the ability to utilize both Plus and Ex
configurations. Further, this model enables dynamic in-
flight configuration toggling which widens design
capabilities for drone path planning systems and provides
a greater range of possible maneuvers, particularly suitable
for indoor environments. Motor control signal mixers use
the angles correction signals, which have been calculated
by the tracking controller as shown in Figures 8 and 9.
Quantizer blocks are used to quantize the mixer control
signals since real-time processing is a discrete process.
Drift Correction: Lateral drifts have adverse effects on
quadcopter indoor SLAM, path planning and control
techniques due to drift calculation complexity. Drifts
experienced indoors are primarily due to reflected air
turbulences from obstructions surrounding the drone such
as furniture. EKF algorithms provide estimation of the
state of non-linear systems suffering from highly
unpredictable noises [16]. In this system, EKF is applied
for the drift correction algorithm, illustrated in Figure 10,
to estimate the actual pitch and roll of the drone as abstract
from the control process. This is also used as feedback
signal for the flight controller angle estimator. These are
subtracted from the input command signals and their
difference estimates the error representing quadcopter
drift, which is corrected automatically by the flight
controller. The gyroscope and accelerometer have been
simulated for calculation of drone pitch and roll angle
without interference of noises from the control processes.
Figure 10. Drift Correction implementation on Simulink: pitch and roll
measurement readings of the drone are input to the EKF filter which
then estimates the actual angles. The estimated angles are fed back to
the flight controller where they are subtracted from the desired angle to
give a new angle correction signal that compensates for the non-linear
drifts.
Fuzzy Logic Controller: The flight controller has a
configuration and several modes that provide improved
stability performance over existing systems [2], [14],
based on user preferences, yet as flight conditions change
and command inputs vary, controller stability
performance can degrade. To compensate for these
stability variances experienced during changes in flight
environment and issued commands, a FLC has been
designed and implemented to provide overall improved
controller stability performance. During high disturbance
greater demands on drift correction are required and the
flight controller achieves better flight stability using the
Extra Stable mode. When requiring fast and sharp
movements the Aerobatic mode outperforms its
counterpart. Due to such demands, commanded pitch and
roll angles provide function in determining the flight
tracking controller mode selected. Further, Ex
configuration is able to sustain greater forces due to
disturbance than the Plus configuration and as such the
former configuration is used to attain greater flight
stability in high disturbance conditions. A Mamdani
model for the FLC is implemented in Simulink (Figure
11), with commanded pitch and roll angles as input to
determine the flight tracking controller mode. In addition,
the disturbance is input in the form of values within the
covariance matrices of the drift correction EKF to
determine the required quadcopter configuration. The four
membership functions of pitch and roll inputs are the
same; this function is shown graphically (Figure 12 and
Figure 13). Membership is assigned based on the current
pitch or roll angle that varies from -90 to 90 degrees; if the
angle is between -90 and -45 or 45 and 90 it is
characterized as high, otherwise it is awarded low. There
are two membership functions for disturbance, as shown
graphically (Figure 14); if the disturbance is less than 20%
of the maximum EKF noise covariance values it is
labelled low, while if the disturbance is greater than this
value (20%) it is categorized as high.
Figure 11. Overall design of the FLC of the flight controller.
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
34
Figure 12. Membership Functions of the Roll input.
Figure 13. Membership Functions of Pitch input.
Figure 14. Membership Functions of the Disturbance input.
3. Results
System design and implementation for quadcopter control
has been implemented and tested in Simulink and Matlab.
System position control is achieved using the Tracking
Controller and Mixing Controller. Noises attributed to
measurement and control processes are minimized by the
KF and Complementary Filter. Flight drifts are calculated
by the EKF which are then compensated by the Tracking
Controller. The FLC establishes the correct flight mode
and drone configuration for optimal stability and dexterity
performance amid varying environmental conditions
(disturbances, input commands). Results were collected
and analyzed from the Position Controller, Tracking
Controller with KF, Complementary Filter of the IMU
Output, Drift Correction using EKF and FLC Controllers.
Performance is evaluated in terms of accuracy, flight
stability and error minimization.
Position and Tracking Controller: Controllers were
subject to testing during drone flight path change to
determine the behavior of the first part of the position
controller, in which the desired position is mapped into
drone’s body frame velocity to calculate the error in the
body frame. Figure 15 shows the commanded x-coordinate
(in blue) and current x-coordinate measurement (in red),
in addition to the output of the position controller (in
yellow) for velocity of the body-frame along the x-axis.
Testing carried out on the second stage of the controller
where error in the body-frame velocities is translated into
a desired angle. Figure 16 shows the commanded Theta
angle represented (in red) and measured Theta angle (in
blue) captured from the test. Test results for the tracking
controller in Aerobatic Mode are shown in Figure 17 with
the commanded Theta (pitch) angle (in blue), the
correction signal (in yellow), and the measured pitch (in
red). Figure 18 shows test results for the tracking
controller in Extra Stable Mode with the commanded
Theta angle (in blue), measured Theta angle (in red) and
correction signal (in yellow).
Figure 15. Commanded signal (blue), current measurement (red) and
output of the Position Controller for body-frame velocity (yellow);
Time versus Position (Radians).
Figure 16. Output of the second stage of the position
controller: commanded (red) and measured (blue) signal;
Time versus Position (Radians).
Figure 17. The Theta correction signal of the Aerobatic mode (yellow),
the commanded Theta signal (blue) and measured Theta (red); Time
versus Position (Radians).
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
35
Results captured in Figures 15, 16, 17 and 18 show stable
position and tracking controller performance. Drone
angles followed the commanded values driven by the
correction signals. These were successfully mapped into
the proper form of input at the different levels of the
controller. Results in Figure 17 reveal that the Aerobatic
mode of the tracking controller has a stable bounded
output and the drone is following the fast changing theta
commands, however, it has a small time lag that could be
reduced with further tuning of the PID parameters. Results
in Figure 18 related to the Extra Stable mode, show that
the controller successfully rejects the control noises
present in the time range [2; 4.5]. It is possible to observe
that in the same time range the Theta angle is not affected
by noise. The noise rejection is a result of using the KF
inside this mode. Although the controller caused the drone
to exceed the command values when the time is equal to
0.8, it manages to correct this minimizing the difference
with the commanded signal. This issue is a result of the
KF error covariance matrix which provides less accurate
state estimates at the first iterations of the KF. Analysis of
the controllers reveals stable and accurate performance,
meeting design objectives.
Figure 18. Output showing behavior of the Extra Stable mode of the
tracking controller; Time versus Position (Radians); Time versus
Position (Radians).
Drift Correction: Sample test results of the EKF filter for
drift correction are shown in Figure 19. The graph is a
result of simulating two consecutive harsh maneuvers on
the Pitch rotational axes, with results of measured pitch
angle, estimated pitch angle and their difference. This test
is intended to validate EKF design and for the completion
of the drift correction process by feeding back the
calculated drift to the following stage of position
controller. Figure 20 shows the simulation results for
drone trajectory as a function of the entire flight controller:
in the presence, and absence, of the drift correction
mechanism. Figure 20a (left-most graph) represents the
actual drone path (blue circles) versus the commanded
path (red line) with drift correction active, while Figure
20b reveals drone path and commanded path for drift
correction set to inactive.
Figure 19. The behavior of the EKF for drift correction: these results
show estimated pitch (in blue), measured pitch angle (in red) and the
difference between these (in yellow).
Figure 20. Simulation results of the output of the flight controller
trajectory (planned path in red, actual path in blue) in the x-y plane
with the presence of simulated wind disturbance, with (a) and without
(b) drift correction.
FLC and System Results: Results of the FLC are
generated in a simulation-based environment, utilizing
functions within the Fuzzy Logic Toolbox embedded in
MATLAB. Sample results representing the output model
for variation in roll and pitch angle are shown in Figure
21. Sample results representing output configuration for
variation in roll and pitch angle are visualized in Figure
22, and with variation in percentage of disturbance in
Figure 23.
Figure 21. Results showing output “mode” as a function
of variations in roll and pitch angles.
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
36
Figure 22. Results of output “configuration” as a function of variations
in roll and pitch angles.
Figure 23. Results of output configuration based on the percentage of
the disturbance.
4. Discussion
Position and Tracking Controller: The function of the
Position Controller and Tracking Controller is to calculate
and generate the correction control signals for the UAV.
The Tracking Controller is designed to operate with two
modes, wherein each mode should produce a correction
signal in terms of the rotational angles, pitch, roll and yaw,
as well as the altitude of the drone. As is evident in the
results presented, this functionality is achieved. Both
controllers exhibit fast, accurate and stable output for
drone flight control correction, with the second mode
exhibiting greater stability due to incorporation of a KF
that increases its reliability in the presence of noise;
appropriately minimizing the impact of this noise on flight
control.
It is noted that Position Controller output is input to the
tracking controller. Both controllers utilize the PID control
technique and its customization. The controllers prove
effective in determining the desired rotational rates when
the input data are x, y, and y coordinate points. The output
of the first stage is the desired correction in velocity of the
body frame. The correction signal is adequately applied to
the entire flight controller and enables successful control
in the x, y, and z state coordinates of the drone. In the
second stage of the controller, desired rotational rates are
effectively input, along with the correction values of the
first stage, in addition to measured angles of the drone.
This resulted in appropriate, dynamic changing of the
angle of the drone in accordance with the desired rates of
change. Both stages of the Position Controller, in addition
to the Tracking Controller, exhibited correct, responsive
behavior for UAV flight control correction. The
implementation of multiple flying modes of a robust
controller widens the capabilities of a UAV by enabling it
to fly and maneuver in challenging environments, as
opposed to mono-mode flight controllers which require
manual tuning for significant changes in flight capability
and control technique.
Drift Correction: Drifts, caused by disturbances due to
obstructions, are one of the major challenges faced by
autonomous quadcopters during indoor flight. Non-linear
lateral drifts are approximated in this work using a
prediction model; EKF is applied in this capacity to
provide an optimal solution for state estimation of this
non-linear system of drifts. EKF is implemented for this
stage of the flight controller for drift estimation and in
response, to correct the UAV behavior after encountering
such drifts. In this application, the EKF proved effective
in estimating the pitch and roll angles of the UAV as
isolated from the interference caused by the other
controller in order to avoid the effect of additive noise.
The state estimations of the EKF differed from the filtered
measured angles and the difference between the two
values was considered to be the estimated drift. As evident
from the results, the implementation of the EKF provides
a close estimate for the pitch and roll angles to those of the
measured. This scenario was conducted in a simulated
environment that did not introduce extreme surrounding
environmental noise and as such did not cause significant
drifts.
The expected behavior of the EKF is validated with the
results obtained, as shown in Figures 19 and 20.
Measured angles by the drone still contain process and
control noise and as such have a larger magnitude than the
values estimated by the EKF. However, this difference is
sufficiently small. Overall, the drift correction mechanism
reveals its capability of generating effective compensation
for the lateral drifts: with an actual drone path suitably
following the commanded (planned) trajectory (Figure
19b).
FLC: The flight controller has two flying modes and two
kinematic configurations. The performance of each option
of selected mode and configuration varies based on the
input data and the flight environment. Due to this
variability there is a need for another layer of control with
the function to determine the flight mode and
configuration of the UAV. The FLC was developed to
provide this function. The FLC has three input signals of
pitch, roll and disturbance, and two output values: mode
and configuration. The FLC utilizes a set of 23 rule to
determine the outputs based on the given inputs. Results
show that by introducing the FLC greater flight stability is
achieved by enabling selection of the most appropriate
mode and configuration. The addition of the extra layer of
control provides further self-governing capabilities, in
which the FLC decides the suitable flight mode based on
drone sensor readings and measured noise signals from
the other controllers. The FLC controller successfully
integrates the efforts of the other developed controllers,
over existing methods [2], [14], [16].
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
37
5. Conclusions and Recommendations
A hybrid flight controller has been designed and tested in
a simulation environment that achieves a high degree of
stability and dexterity in maneuverability within an indoor
location that is subject to non-linear drifts. For the Position
Controller, a KF is applied within the mode that offers
high in-flight stability, effectively rejecting induced noise
and subsequently reducing the errors associated with
Controller processing. Flight Control modes implemented
achieve sound results in terms of stability within extreme
disturbance conditions, in addition to sharp, fast, and
accurate maneuvers under normal conditions. The Motor
Control Mixer effectively maps the angle correction
signals into a required throttle for each of the four motors,
enabling flight in two separate configurations, dependent
on desired throttle output. Non-linear drift estimation is
effectively implemented using the EKF, in which
compensation commands by the Position Controller are
executed based on estimated drift calculation for pitch and
roll angles. In addition to the PID and associated
derivatives for motor control, the FLC proves useful in
offering a further layer of flight control stability.
The proposed system can be further improved: by adding
further Flight Controller modes for selection to enable
greater specificity in flight behavior for different flight
conditions. An automatic PID and PIDD2 fine tuning
mechanism may help to achieve even greater results in
terms of system stability. Sensor fusion techniques,
Bayesian Network and Central Limit Theorem, may be
utilized in the IMU to achieve higher accuracy in angle
measurements, which may lead to better state estimates of
rotational angles and rates. KF and EKF design may be
extended to include yaw and altitude UAV states,
requiring addition of magnetometer and barometer sensors
to attain these measurements. The FLC may be extended
to process greater numbers of input, output and rule sets to
provide more heightened control over the Flight Controller,
particularly if the addition of Flight Controller modes is
realized.
6. References
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Kaufmann. Autonomous Flight using a Smartphone
as On-Board Processing Unit in GPS-Denied
Environments, Proceedings of International
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Multimedia, pages 341-350, 2013.
[2] A. Sharma, A. Barve. Controlling of Quad-Rotor
UAV using PID Controller and Fuzzy Logic
Controller, International Journal of Electrical,
Electronics and Computer Engineering, vol. 1, issue
2, pages 38-41.
[3] P.E. Pounds, D.R. Bersak, A.M. Dollar, Stability of
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Added Payload Mass under PID Control,
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[4] P. Salaskar, S. Paranjape, J. Reddy, A. Shah,
Quadcopter–Obstacle Detection and Collision
Avoidance, International Journal of Engineering
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[5] S. Bouabdallah, Siegwart R, Full Control of a
Quadrotor, IEEE/RSJ International Conference on
Intelligent Robots and Systems, pages 153-158, 2007.
[6] A.L. Salih, M. Moghavvemi, H.A. Mohamed, K.S.
Gaeid, Flight PID Controller Design for a UAV
Quadrotor, Scientific Research and Essays, vol. 5,
issue 23, pages 3660-3667, 2012.
[7] L. Heng, L. Meier, P. Tanskanen, F. Fraundorfer, M.
Pollefeys, Autonomous Obstacle Avoidance and
Maneuvering on a Vision-Guided MAV using On-
Board Processing, Robotics and Automation (ICRA),
2011 IEEE international conference on, pages 2472-
2477, 2011.
[8] J.C.V. Junior, J.C. De Paula, G.V. Leandro, M.C.
Bonfim, Stability Control of a Quad-Rotor using a
PID Controller, Brazilian Journal of Instrumentation
and Control, vol. 1, issue 1, pages 15-20, 2013.
[9] B. Kada, Y. Ghazzawi, Robust PID Controller Design
for an UAV Flight Control System, Proceedings of
the World Congress on Engineering and Computer
Science, vol. 2, issue 1-6, 2011.
[10] L.C. Chien, Fuzzy Logic in Control Systems: Fuzzy
Logic Controller- Part I, IEEE Transactions on
Systems, Man, and Cybernetics, vol. 20, issue 2,
pages 404-418, 2011.
[11] I. Sa, P. Corke, Estimation and Control for an Open-
Source Quadcopter, Proceedings of the Australasian
Conference on Robotics and Automation, pages 7-9,
2011.
[12] M. Achtelik, T. Zhang, K. Kühnlenz, M. Buss,
Visual Tracking and Control of a Quadcopter using a
Stereo Camera System and Inertial Sensors,
Mechatronics and Automation, IEEE, ICMA
International Conference, pages 2863-2869, 2009.
[13] M. Hehn, R. D’Andrea, Real-Time Trajectory
Generation for Quadcopters, Robotics, IEEE
Transactions on, vol. 31, issue 4, pages 877-892,
2011.
[14] A.K. Gaurav, A. Kaur, Comparison Between
Conventional PID and Fuzzy Logic Controller for
Liquid Flow Control: Performance Evaluation of
Fuzzy Logic and PID Controller by using
MATLAB/Simulink, International Journal of
Innovative Technology and Exploring Engineering
(IJITEE), vol. 1, issue 1, pages 84-88, 2012.
[15] Y. Al-Younes, M.A. Jarrah, Attitude Stabilization of
Quadrotor UAV using Backstepping Fuzzy Logic
and Backstepping Least-Mean-Square Controllers,
Mechatronics and Its Applications, ISMA 5th
International Symposium, pages 1–11, 2008.
[16] C.N. Hamdani, E.A.K. Rusdhianto, D.E. Iskandar.
Perancangan Autonomous Landing Pada Quadcopter
Menggunakan Behavior-Based Intelligent Fuzzy
Control, Journal Teknik ITS, vol. 2, issue 2, pages
E63-E68, 2013.
[17] L. Meier, P. Tanskanen, F. Fraundorfer, M. Pollefeys,
Pixhawk: a System for Autonomous Flight using
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(ICRA), 2011 IEEE international conference on,
pages 2992-2997, 2011.
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
38
Biographies
Dr. Catherine Todd received
her B.Eng. (Electrical) and
PhD. (Electrical) from the
University of Wollongong,
Australia. Currently she is
working as an Assistant
Professor of Engineering at
Hawaii Pacific University.
Her research interests include
medical image processing, control systems and real-
time simulation.
Dr. Stefano Fasciani is
currently an Assistant
Professor of Engineering at
University of Wollongong in
Dubai. He is doing research
work in sound engineering.
His main research interests
are in voice processing,
noise cancellation, HCI, acoustic and electronic
engineering.
Mr. Hussien Koujan is
working in Industry as an
Electrical Engineer at
Altorath International
Engineering Consultants,
having graduated from the
University of Wollongong
in Dubai with a Bachelor
of Engineering
(Electrical).
Automatic Control and System Engineering Journal, ISSN 1687-4811, Volume 17, Issue 1, ICGST, Delaware, USA, June 2017
39
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