The helicopter is one of the most complex flying machines due to its versatility and
maneuverability to perform many types of tasks. Classical helicopters are usually
equipped with a main rotor and a tail rotor. However, other types exist which use a twin
rotor. Our specific project is concerned with the design and control of a miniature
rotorcraft, known as a quad-rotor helicopter [1]
. Quad rotors are symmetrical vehicles with four equally sized rotors at the end of four
equal length rods. Unlike their counter parts, quad rotors make use of multiple rotors
allowing for a greater amount of thrust and consequently a greater amount of
maneuverability.
Fixed-wing vehicles have long-range since they are energy efficient, but they lack the
maneuverability required for many UAV tasks. For example, Blimps are easy to control
when there are fewer disturbances like wind, and lift comes from natural buoyancy, but
their maneuverability is limited. The helicopters have advantages over conventional
fixed-wing aircraft and blimps on surveillance and inspection tasks, since they can take-off and land in limited space and can easily hover above targets. Moreover, helicopters
have the advantage of maneuverability. Unfortunately, this advantage makes helicopters
very hard to control, requiring sophisticated sensors and fast on-board computation [2]
.
Unmanned aerial vehicles are aircrafts capable of flight without an on-board operator.
Such vehicles can be controlled remotely by an operator on the ground, or autonomously
via a pre-programmed flight path. Today, unmanned aerial vehicles (UAVs) are an
important part of scientific study both in military and space studies. As a substitute for
human piloted vehicles they are advantageous to protect human life in multiple dangerous
environments. Their reliabilities in tough circumstances are much higher than their
counter parts.
UAVs can be classified into two major groups: heavier-than-air and lighter-than-air.
These two groups self divide in many other that classify aircrafts according to
motorization, type of liftoff and many other parameters. Vertical Take-Off and Landing
(VTOL) UAVs like quadrotors have several advantages over fixed-wing airplanes. They
can move in any direction and are capable of hovering and fly at low speeds. In addition,
the VTOL capability allows deployment in almost any terrain while fixed-wing aircraft
require a prepared airstrip for takeoff and landing. Given these characteristics, quadrotors
can be used in search and rescue missions, meteorology, penetration of hazardous
environments (e.g. exploration of other planets) and other applications suited for such an
aircraft. Also, they are playing an important role in research areas like control
engineering, where they serve as prototypes for real life applications.
The design of unmanned aerial vehicles involves the integration of various steps such as
design, selection of sensors and developing controllers. These steps can not be treated
separately. For example, one can not design a vehicle without considering the sensory
input or the controllers that will be implemented, as these steps are closely related to each
other [2]
. Stabilizing and guidance of these hovering platforms are common and basic
tasks that have to be accomplished before assigning a mission to the vehicle.
Because of the ambitious nature of project goals, development of the UAV can be easily
divided into five major stages:
• Vehicle conceptual design
• Analysis and component-level design and selection
• Fabrication, assembly, hardware testing, and re-design
• Simulation development and verification
• Control and estimation development for future implementation
Though these five stages occasionally overlapped and sometimes interfered with one
another, they can be discussed independently.
Among the specific significant engineering challenges that researchers are focusing on
for the development of successful UAV are: it should be ultra-compact, lightweight,
high-power and high-energy-density propulsion and power sources; untraditional
concepts for lift generation; flight stabilization and control for aerodynamic
environments with very low Reynolds numbers; secure, low-power onboard
electronic processing and communications with sufficient bandwidth for real-time
imaging; micro-gyroscopes and inertial measurement units (IMU) and very small
onboard guidance, navigation, and geo-location systems. To be really useful, a UAV
needs to carry a short-range day/night area imaging system with a sufficient resolution.
The system must feature an accurate geo-location capability. A sufficient vehicle range
and real-time communications are also desired.
With the advent of new technologies ranging from global positioning systems to faster,
smaller, and lighter computer processors, there has been a surge in development of
unmanned vehicles. Unmanned and autonomous vehicles are currently in development
for use in air, over land, and in the water by both private and government agencies.
Definition and Basic Concepts OF QUADCOPTER
A quadrotor, or quadrotor helicopter, is an aircraft that becomes airborne due to the lift
force provided by four rotors usually mounted in cross configuration, hence its name. It is
an entirely different vehicle when compared with a helicopter, mainly due to the way
both are controlled. Helicopters are able to change the angle of attack of its blades,
quadrotors cannot.
The first question one is asked about the quadrotor is how it stands out from the
traditional one. Hence a short introduction about the quadrotor construction and steering
principle is necessary. The quadrotor is a mechatronic system with four rotors that
provide the lift and control. With respect to hover, the main difference is best explained
by considering how the helicopters compensate from gyroscopic torques. Traditional
helicopters basically compensate from the torque generated by the main rotor through the
tail rotor. However the tail rotor compensation conducts a sideways displacement of the
helicopter, thus counter steering by tilting the main rotor blades is necessary. In this way
hover is an ongoing and complex process. The quadrotor has four propellers driven by
four motors in a cross configuration which are fixed to a certain spin axis. The spinning
directions of the rotors are set in pairs to balance the torques, therefore eliminating the
need for a tail rotor. While the front and the rear motor rotate counter-clockwise, the left
and the right motor rotate clockwise, as long as the rotors rotate at the same speed the
gyroscopic effects are nearly eliminated and the quad rotor essentially hovers, this
proving to be a less complex maneuver to retain. With regard to application and
functionality the quad rotor helicopter has the same immediate advantages as the
traditional helicopter. One additional advantage of the quad rotor compared to a
traditional helicopter is the simplified rotor mechanics. By varying the speed of the
single motors, the lift force can be changed and vertical and/or lateral motion can be
created. However a number of issues especially regarding the mechanical construction
prove to be interesting from a control perspective point of view. The first and foremost
issue concerns the modeling of the quad rotor as this proves to be a different and more
feasible task.
Quad Rotor Operation
Quadrotor is an under-actuated, dynamic vehicle with four input forces and six degrees of
freedom. Unlike regular helicopters that have variable pitch angle rotors, a quadrotor
helicopter has four fixed-pitch angle rotors. The quadrotor is very well modeled with a
four rotors in a cross configuration. This cross structure is quite thin and light, however it
shows robustness by linking mechanically the motors (which are heavier than the
structure). Each propeller is connected to the motor through the electronic speed
controller. All the propellers axes of rotation are fixed and parallel. Furthermore, their air
flow points downwards (to get an upward lift). These considerations point out that the
structure is quite rigid and the only things that can vary are the propeller speeds.
Each rotor in a quad rotor is responsible for a certain amount of thrust and torque about
its center of rotation, as well as for a drag force opposite to the rotorcraft’s direction of
flight. These props would provide the thrust necessary to counter gravity while also
providing sufficient residual thrust for control of roll and pitch (and subsequently forward
and lateral velocity), yaw, and vertical velocity. The basic motions of a Quad rotor are
generated by varying the rotor speeds of all four rotors, thereby changing the lift forces.
The helicopter tilts towards the direction of the slow spinning rotor, which enables
acceleration along that direction. Therefore, control of the tilt angles and the motion of
the helicopter are closely related and estimation of orientation (roll and pitch) is critical.
As spinning directions of the rotors are set to balance the moments. This principle is used
to produce the desired yaw motions. In order to define an aircraft’s orientation (or
attitude) around its center of mass, aerospace engineers usually define three dynamic
parameters, the angles of yaw, pitch and roll. This is very useful because the forces used
to control the aircraft act around its center of mass, causing it to pitch, roll or yaw. The
generalized coordinates for a rotorcraft are:
q=(x, y, z, θ, φ, ψ) (1.1)
Where (x, y, z) denote the position of the center of mass of the rotorcraft relative to the
frame, and (θ, φ, ψ) are the three Euler angles which represent the orientation of the craft
[3]
. Figure shows the yaw, pitch and roll rotations of a quadrotor.
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