Real-time model predictive control for quadrotors

This paper presents a solution to on-board trajectory tracking control of quadrotors. The proposed approach combines the standard hierarchical control paradigm that separates the control into low-level motor control, mid-level attitude dynamics control, and a high-level trajectory tracking with a model predictive control strategy. We use dynamic reduction of the attitude dynamics and dynamic extension of the thrust control along with feedback linearisation to obtain a linear system of McMillan degree three that models force controlled position and trajectory tracking for the quadrotor. Model predictive control is then used on the feedback equivalent system and its control outputs are transformed back into the inputs for the original system. The proposed structure leads to a low complexity model predictive control algorithm that is implemented in real-time on an embedded hardware. Experimental results on different position and trajectory tracking control are presented to illustrate the application of the derived linear system and controllers

Aerodynamic Power Control for Multirotor Aerial Vehicles

In this paper, a new motor control input and controller for small-scale electrically powered multirotor aerial vehicles is proposed. The proposed scheme is based on controlling aerodynamic power as opposed to the rotor speed of each motor-rotor system. Electrical properties of the brushless direct current motor are used to both estimate and control the mechanical power of the motor system which is coupled with aerodynamic power using momentum theory analysis. In comparison to current state-of-the-art motor control for multirotor aerial vehicles, the proposed approach is robust to unmodelled aerodynamic effects such as wind disturbances and ground effects. Theory and experimental results are presented to illustrate the performance of the proposed motor control.This work was supported by Australian Research Council through Discovery Grant DP120100316 and National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (No. 2009-0083495, 2013-013911)

Correction: Lassa hemorrhagic fever in a late term pregnancy from northern Sierra Leone with a positive maternal outcome: case report

Lassa fever (LF) is a devastating viral disease prevalent in West Africa. Efforts to take on this public health crisis have been hindered by lack of infrastructure and rapid field deployable diagnosis in areas where the disease is prevalent. Recent capacity building at the Kenema Government Hospital Lassa Fever Ward (KGH LFW) in Sierra Leone has lead to a major turning point in the diagnosis, treatment and study of LF. Herein we present the first comprehensive rapid diagnosis and real time characterization of an acute hemorrhagic LF case at KGH LFW. This case report focuses on a third trimester pregnant Sierra Leonean woman from the historically non-endemic Northern district of Tonkolili who survived the illness despite fetal demise. Employed in this study were newly developed recombinant LASV Antigen Rapid Test cassettes and dipstick lateral flow immunoassays (LFI) that enabled the diagnosis of LF within twenty minutes of sample collection. Deregulation of overall homeostasis, significant hepatic and renal system involvement, and immunity profiles were extensively characterized during the course of hospitalization. Rapid diagnosis, prompt treatment with a full course of intravenous (IV) ribavirin, IV fluids management, and real time monitoring of clinical parameters resulted in a positive maternal outcome despite admission to the LFW seven days post onset of symptoms, fetal demise, and a natural still birth delivery. These studies solidify the growing rapid diagnostic, treatment, and surveillance capabilities at the KGH LF Laboratory, and the potential to significantly improve the current high mortality rate caused by LF. As a result of the growing capacity, we were also able to isolate Lassa virus (LASV) RNA from the patient and perform Sanger sequencing where we found significant genetic divergence from commonly circulating Sierra Leonean strains, showing potential for the discovery of a newly emerged LASV strain with expanded geographic distribution. Furthermore, recent emergence of LF cases in Northern Sierra Leone highlights the need for superior diagnostics to aid in the monitoring of LASV strain divergence with potentially increased geographic expansion.Organismic and Evolutionary BiologyOther Research Uni

Aerodynamics and Control of Quadrotors

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 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

Thrust Control for Multirotor Aerial Vehicles

This paper presents a novel control algorithm to regulate the aerodynamic thrust produced by fixed-pitch rotors commonly used on small-scale electrically powered multirotor aerial vehicles. The proposed controller significantly improves on the disturbance rejection and gust tolerance of rotor thrust control compared to state-of-the-art RPM (revolutions per minute) rotor control schemes. The thrust modelling approach taken is based on a model of aerodynamic power generated by a fixed- pitch rotor and computed in real-time on the embedded electronic speed controllers using measurements of electrical power and rotor angular velocity. Static and dynamic flight tests were carried out in downdrafts and updrafts of varying strengths to quantify the resulting improvement in maintaining a desired thrust setpoint. The performance of the proposed approach in flight conditions is demonstrated by a path tracking experiment where a quadrotor was flown through an artificial wind gust and the trajectory tracking error was measured. The proposed approach for thrust control demonstrably reduced tracking error compared to classical RPM rotor control.This research was supported by the Australian Research Council through Discovery Grant DP120100316 “Integrated High-Performance Control of Aerial Robots in Dynamic Environments

Nonlinear Dynamic Modeling for High Performance Control of a Quadrotor

In this paper, we present a detailed dynamic and aerodynamic model of a quadrotor that can be used for path planning and control design of high performance, complex and aggressive manoeuvres without the need for iterative learning techniques. The accepte

Velocity Aided Attitude Estimation for Aerial Robotic Vehicles Using Latent Rotation Scaling

International audienceFlight performance of aerial robotic vehicles is critically dependent on the quality of the state estimates provided by onboard sensor systems. The attitude estimation problem has been extensively studied over the last ten years and the development of low complexity, high performance, robust non-linear observers for attitude has been one of the enabling technologies fueling the growth of small scale aerial robotic systems. The velocity aided attitude estimation problem, that is simultaneous estimation of attitude and linear velocity of an aerial platform, has only been tackled using the non-linear observer approach in the last few years. Prior contributions have lead to non-linear observers for which either there is no stability analysis or for which the analysis is extremely complex. In this paper, we propose a simple relaxation of the state space, allowing scaled rotation matrices R ∈ R^{3×3} such that RX^\top= uI where X = uR and u > 0 is a positive scalar, along with additional observer dynamics to force u → 1 asymptotically. With this simple augmentation of the observer state space, we propose a non-linear observer with a straightforward Lyapunov stability analysis that demonstrates almost global asymptotic convergence along with local exponential convergence. Simulations as well as experimental results are provided to demonstrate the performance of the proposed observer