Bangura, Moses
Description
Quadrotors are aerial vehicles with a four motor-rotor assembly
for generating lift and controllability. Their light weight, ease
of design and simple dynamics have increased their use in aerial
robotics research. There are many quadrotors that are
commercially available or under development. Commercial
off-the-shelf quadrotors usually lack the ability to be
reprogrammed and are unsuitable for use as research platforms.
The open-source code developed in this...[Show more] thesis differs from other
open-source systems by focusing on the key performance road
blocks in implementing high performance experimental quadrotor
platforms for research: motor-rotor control for thrust
regulation, velocity and attitude estimation, and control for
position regulation and trajectory tracking. In all three of
these fundamental subsystems, code sub modules for implementation
on commonly available hardware are provided. In addition, the
thesis provides guidance on scoping and commissioning open-source
hardware components to build a custom quadrotor. A key
contribution of the thesis is then a design methodology for the
development of experimental quadrotor platforms from open-source
or commercial off-the-shelf software and hardware components that
have active community support. Quadrotors built following the
methodology allows the user access to the operation of the
subsystems and, in particular, the user can tune the gains of the
observers and controllers in order to push the overall system to
its performance limits. This enables the quadrotor framework to
be used for a variety of applications such as heavy lifting and
high performance aggressive manoeuvres by both the hobby and
academic communities. To address the question of thrust control,
momentum and blade element theories are used to develop
aerodynamic models for rotor blades specific to quadrotors. With
the aerodynamic models, a novel thrust estimation and control
scheme that improves on existing RPM (revolutions per minute)
control of rotors is proposed. The approach taken uses the
measured electrical power into the rotors compensating for
electrical loses, to estimate changing aerodynamic conditions
around a rotor as well as the aerodynamic thrust force. The
resulting control algorithms are implemented in real-time on the
embedded electronic speed controller (ESC) hardware. Using the
estimates of the aerodynamic conditions around the rotor at this
level improves the dynamic response to gust as the low-level
thrust control is the fastest dynamic level on the vehicle. The
aerodynamic estimation scheme enables the vehicle to react almost
instantaneously to aerodynamic changes in the environment without
affecting the overall dynamic performance of the vehicle.
To quantify the resulting improvements in maintaining a desired
thrust setpoint using the proposed thrust modelling and control
scheme over current state-of-the-art rotor speed control, static
and dynamic flight tests are carried out in downdrafts and
updrafts of varying strengths. In the static tests, the new
scheme is able to determine the changes in axial gust thereby
changing the speed of the rotor to maintain the desired thrust
setpoint. The dynamic flight test is demonstrated by a path
tracking experiment where a quadrotor is flown through an
artificial wind gust and the trajectory tracking error measured.
The proposed approach for thrust control demonstrably reduced
tracking errors compared to the classical RPM rotor control.
Non-linear dynamic models for the drag forces on individual
rotors of a quadrotor are examined and a combined or lumped drag
force model is derived. Combining this drag force model with
measurements from the strapdown inertial measurement unit (IMU)
and a complementary filter that uses barometer height estimates,
the full body-fixed frame velocity measurements are obtained.
Adding measurements from an inertial navigation system and a
magnetometer, a coupled non-linear complementary filter in both
the body-fixed and inertial frames is proposed. The resulting
observer is a velocity aided attitude observer that provides
estimates of both linear velocities in inertial and body-fixed
frames and attitude of the vehicle. The observer is robust to GPS
dropouts and can be used in indoor environments without an indoor
navigation system such as Vicon motion capture system.
A hierarchical control structure for quadrotors is proposed with
high-level position/trajectory tracking control and velocity
control, mid-level attitude control and low-level motor thrust
control based on the time scale separation of each dynamic
sublevel. The attitude error dynamics under the proposed attitude
control law are shown to be non-autonomous. Using passivity based
Lyapunov analysis and showing that the linear dynamics are
uniformly completely observable (UCO), the attitude dynamics are
shown to be locally exponentially stable. Under the proposed
position/trajectory control law, the error dynamics form a block
diagonal matrix with eigenvalues in the left half plane. Hence,
the proposed position/trajectory tracking controller is locally
exponentially stable. The proposed velocity controller which uses
the estimated velocity is also shown to be asymptotically stable
and its linear dynamics are uniformly completely observable.
Hence by the notion of input-to-state stability, the quadrotor is
exponentially stable under the proposed control laws. The dynamic
properties (acceleration, jerk and snap) of the vehicle are used
to algebraically determine feedforward terms (angular velocity
and acceleration) which are used in the mid-level attitude
controller to enable precise trajectory tracking.
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