A System of Autonomously Flying Helicopters for Load Transportation

Die Arbeit beschreibt Entwurf, Umsetzung und Validierung eines autonomen Lastentransportsystems, welches auf Basis mehrerer Modellhubschrauber realisiert wurde. Abhängig von den Anforderungen der zu transportierenden Last kann die Anzahl der verwendeten Hubschrauber individuell angepasst werden. Die präsentierten Modelle und Regler wurden durch Computersimulationen und reale Flugversuche verifiziert. Zwei nichtlineare Modelle werden präsentiert: Ein Model für Konfigurationen bestehend aus einem Helikopter und einer Last (single-lift) und eines für Konfigurationen bestehend aus zwei bzw. mehreren Helikoptern und einer Last (dual- und multi-lift). Neben diesen komplexen Modellen werden vereinfachte Modelle vorgestellt, die für den Reglerentwurf verwendet werden. Ein generischer Orientierungsregler wird entwickelt, der für die Regelung aller beschriebenen Transportkonfigurationen verwendet werden kann. Durch die Nutzung dieses Reglers vereinfacht sich der Entwurf der Translationsregler erheblich. Drei Translationsregler werden beschrieben: Ein Regler für single-lift Konfigurationen, der eine aktive Unterdrückung von Lastschwingungen erlaubt, und ein verteilter Regler für multi-lift Konfigurationen. Weiterhin wird ein dual-lift Regler präsentiert, der eine Kombination der anderen Regler darstellt. Die Regler für dual- und multi-lift Konfigurationen verwenden keine mechanischen Hilfskonstrukte wie Abstandshalter. Die Position der Last wird durch die Orientierung des Seils, gemessen nahe dem Helikopterrumpf, bestimmt. Externe Störungen wie Windstöße können eine Eigenschwingung des Seils anregen, welche die ermittelte Lastposition verfälscht. Die Eigenschwingung des Seils sowie der Einfluss der verwendeten Messeinrichtung werden analysiert. Auf Basis dieser Analyse wird ein Lastbeobachter entwickelt und in mehreren Experimenten verifiziert. Dieser Lastbeobachter ist von essentieller Wichtigkeit für den sicheren Betrieb des Lastentransportsystems, insbesondere bei schlechten Wetterbedingungen. Die entwickelten nichtlinearen Modelle des Systems wie auch die Regler der single- und multi-lift Konfigurationen wurden durch Flugversuche validiert. Dabei hat das System bewiesen, dass es auch bei sehr schlechten Wetterbedingungen einsetzbar iThis work covers the design, realization and validation of an autonomous load transportation system, utilizing several small size helicopters. The number of participating helicopters is configurable for the described system, depending on the requirements of the transported load. The presented models and controllers have been validated in computer simulation and flight experiments. Two non-linear models are presented: One model covers single-lift and one model covers dual- and multi-lift configurations. Simplified models are introduced beside the complex models, which are utilized for the translation controller design. A generic orientation controller is presented, which is applicable for the control of all presented slung load configurations. The utilization of this controller significantly simplifies the design of the translation controllers. The independence from the actual slung load configuration is achieved through measurement of the rope force vector in the rope attachment point, which is located on the helicopter fuselage. Three translation controllers are described: A controller for single-lift configurations, which allows the active compensation of load oscillations and a distributed controller for multi-lift configurations. A dual-lift translation controller is presented, which resembles a combination of single- and multi-lift translation controller. The presented controllers for dual- or multi-lift configurations do not utilize auxiliary constructs, like spreader-bars. The position of the load is estimated from the measured orientation of the rope, close to the helicopter fuselage. External disturbances, like wind gusts, are able to stimulate internal oscillations of the rope, which disturb the estimated load position. The internal motion of the rope as well as the influence of the used measurement device are analyzed and a flexible rope model is presented. Based on the results a load motion observer is developed and validated in several experiments. This load motion observer is essential for the safe operation of the slung load system, especially during bad weather conditions. The derived non-linear models of the system as well as the proposed controllers for single- and multi-lift configurations have been validated in flight experiments. The system has been proven to be operable even in presence of adverse weather conditions

Robust and Adaptive Control Methods for Small Aerial Vehicles

Recent advances in sensor and microcomputer technology and in control and aeroydynamics theories has made small unmanned aerial vehicles a reality. The small size, low cost and manoueverbility of these systems has positioned them to be potential solutions in a large class of applications. However, the small size of these vehicles pose significant challenges. The small sensors used on these systems are much noisier than their larger counterparts.The compact structure of these vehicles also makes them more vulnerable to environmental effects. This work develops several different control strategies for two sUAV platforms and provides the rationale for judging each of the controllers based on a derivation of the dynamics, simulation studies and experimental results where possible. First, the coaxial helicopter platform is considered. This sUAV’s dual rotor system (along with its stabilizer bar technology) provides the ideal platform for safe, stable flight in a compact form factor. However, the inherent stability of the vehicle is achieved at the cost of weaker control authority and therefore an inability to achieve aggressive trajectories especially when faced with heavy wind disturbances. Three different linear control strategies are derived for this platform. PID, LQR and H∞ methods are tested in simulation studies. While the PID method is simple and intuitive, the LQR method is better at handling the decoupling required in the system. However the frequency domain design of the H∞ control method is better at suppressing disturbances and tracking more aggressive trajectories. The dynamics of the quadrotor are much faster than those of the coaxial helicopter. In the quadrotor, four independent fixed pitch rotors provide the required thrust. Differences between each of the rotors creates moments in the roll, pitch and yaw directions. This system greatly simplifies the mechanical complexity of the UAV, making quadrotors cheaper to maintain and more accessible. The quadrotor dynamics are derived in this work. Due to the lack of any mechanical stabilization system, these quadrotor dynamics are not inherently damped around hover. As such, the focus of the controller development is on using nonlinear techniques. Linear quadratic regulation methods are derived and shown to be inadequate when used in zones moderately outside hover. Within nonlinear methods, feedback linearization techniques are developed for the quadrotor using an inner/outer loop decoupling structure that avoids more complex variants of the feedback linearization methodology. Most nonlinear control methods (including feedback linearization) assume perfect knowledge of vehicle parameters. In this regard, simulation studies show that when this assumption is violated the results of the flight significantly deteriorate for quadrotors flying using the feedback linearization method. With this in mind, an adaptation law is devised around the nonlinear control method that actively modifies the plant parameters in an effort to drive tracking errors to zero. In simple cases with sufficiently rich trajectory requirements the parameters are able to adapt to the correct values (as verified by simulation studies). It can also adapt to changing parameters in flight to ensure that vehicle stability and controller performance is not compromised. However, the direct adaptive control method devised in this work has the added benefit of being able to modify plant parameters to suppress the effects of external disturbances as well. This is clearly shown when wind disturbances are applied to the quadrotor simulations. Finally, the nonlinear quadrotor controllers devised above are tested on a custom built quadrotor and autopilot platform. While the custom quadrotor is able to fly using the standard control methods, the specific controllers devised here are tested on a test bench that constrains the movement of the vehicle. The results of the tests show that the controller is able to sufficiently change the necessary parameter to ensure effective tracking in the presence of unmodelled disturbances and measurement error

A Comparative Framework for Maneuverability and Gust Tolerance of Aerial Microsystems

Aerial microsystems have the potential of navigating low-altitude, cluttered environments such as urban corridors and building interiors. Reliable systems require both agility and tolerance to gusts. While many platform designs are under development, no framework currently exists to quantitatively assess these inherent bare airframe characteristics which are independent of closed loop controllers. This research develops a method to quantify the maneuverability and gust tolerance of vehicles using reachability and disturbance sensitivity sets. The method is applied to a stable flybar helicopter and an unstable flybarless helicopter, whose state space models were formed through system identification. Model-based static H-infinity controllers were also implemented on the vehicles and tested in the lab using fan-generated gusts. It is shown that the flybar restricts the bare airframe's ability to maneuver in translational velocity directions. As such, the flybarless helicopter proved more maneuverable and gust tolerant than the flybar helicopter. This approach was specifically applied here to compare stable and unstable helicopter platforms; however, the framework may be used to assess a broad range of aerial microsystems

Automatic Landing of a Rotary-Wing UAV in Rough Seas

Rotary-wing unmanned aerial vehicles (RUAVs) have created extensive interest in the past few decades due to their unique manoeuverability and because of their suitability in a variety of flight missions ranging from traffic inspection to surveillance and reconnaissance. The ability of a RUAV to operate from a ship in the presence of adverse winds and deck motion could greatly extend its applications in both military and civilian roles. This requires the design of a flight control system to achieve safe and reliable automatic landings. Although ground-based landings in various scenarios have been investigated and some satisfactory flight test results are obtained, automatic shipboard recovery is still a dangerous and challenging task. Also, the highly coupled and inherently unstable flight dynamics of the helicopter exacerbate the difficulty in designing a flight control system which would enable the RUAV to attenuate the gust effect. This thesis makes both theoretical and technical contributions to the shipboard recovery problem of the RUAV operating in rough seas. The first main contribution involves a novel automatic landing scheme which reduces time, cost and experimental resources in the design and testing of the RUAV/ship landing system. The novelty of the proposed landing system enables the RUAV to track slow-varying mean deck height instead of instantaneous deck motion to reduce vertical oscillations. This is achieved by estimating the mean deck height through extracting dominant modes from the estimated deck displacement using the recursive Prony Analysis procedure. The second main contribution is the design of a flight control system with gust-attenuation and rapid position tracking capabilities. A feedback-feedforward controller has been devised for height stabilization in a windy environment based on the construction of an effective gust estimator. Flight tests have been conducted to verify its performance, and they demonstrate improved gust-attenuation capability in the RUAV. The proposed feedback-feedforward controller can dynamically and synchronously compensate for the gust effect. In addition, a nonlinear H1 controller has been designed for horizontal position tracking which shows rapid position tracking performance and gust-attenuation capability when gusts occur. This thesis also contains a description of technical contributions necessary for a real-time evaluation of the landing system. A high-infedlity simulation framework has been developed with the goal of minimizing the number of iterations required for theoretical analysis, simulation verification and flight validation. The real-time performance of the landing system is assessed in simulations using the C-code, which can be easily transferred to the autopilot for flight tests. All the subsystems are parameterized and can be extended to different RUAV platforms. The integration of helicopter flight dynamics, flapping dynamics, ship motion, gust effect, the flight control system and servo dynamics justifies the reliability of the simulation results. Also, practical constraints are imposed on the simulation to check the robustness of the flight control system. The feasibility of the landing procedure is confimed for the Vario helicopter using real-time ship motion data

Aeronautical engineering: A special bibliography with indexes, supplement 49

The bibliography contains 368 abstract citations of reports, journal articles, and other documents concerned with the engineering and theoretical aspects of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines) and associated components, equipment, and systems. Research and development in aerodynamics, aeronautics, and ground support equipment are also treated. Subject, personal, and contract number indexes are included for ease of access

Aeronautical Engineering: A continuing bibliography with indexes, supplement 108

This bibliography lists 517 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1979

Low-order models and numerical techniques for the analysis of rotorcraft flight mechanics

The dissertation describes (i) a mathematically rigorous approach for the derivation and validation of low-order helicopter mathematical models from first principles and (ii) the development or improvement of a set of numerical techniques that provide computationally efficient and reliable tools for the analysis of rotorcraft flight mechanics, and in particular evaluation of maximum performance and assessment of handling qualities. Simplified models are expected to provide results at a fraction of the computational cost required for performing the same analysis on the basis of higher order models, but, at the same time, the reliability of these results needs to be carefully assessed, which is one of the objectives of the present work. The techniques developed are tested on various single main rotor rotorcraft configurations, with a focus on articulated, teetering, and two-bladed-gimballed rotor

Helicopter Handling Qualities

Helicopters are used by the military and civilian communities for a variety of tasks and must be capable of operating in poor weather conditions and at night. Accompanying extended helicopter operations is a significant increase in pilot workload and a need for better handling qualities. An overview of the status and problems in the development and specification of helicopter handling-qualities criteria is presented. Topics for future research efforts by government and industry are highlighted

Aeronautical Engineering: A special bibliography with indexes, supplement 48

This special bibliography lists 291 reports, articles, and other documents introduced into the NASA scientific and technical information system in August 1974

Modelling, estimation and control of a twin-helicopter slung load transportation system

The development of a control system to transport and assemble cargo using two helicopters is presented in this thesis. It is more economical to use multiple lower cost helicopters in a coordinated manner to carry cargo than to use a single high performance helicopter for the transportation task. The reason for the generally higher cost of hiring high performance helicopters, is because they are not required often, and so, remain idle for most of their lifetime. Thus, using less specialised, lower performing helicopters to share the load is cheaper. Beyond just sharing the load of the cargo, the objective in this investigation is to control the attitude such that precise placement of the cargo can be made. This objective cannot be achieved using a single helicopter, unless a sophisticated tethering mechanism is developed. The installation of wind-turbine blades, powerline towers and radio masts in remote locations, are examples of where the application of this technology may be useful. The investigation of this thesis is around modelling, estimation and control of the twinhelicopter slung load transportation system. The title reflects the investigation that was required to be done to determine whether a scheme could be realisable. To test the concept, an experimental platform was developed. A small, light-weight and high performance avionics system was designed and interfaced to the helicopters. The experimentation was done indoors, and hence, the flying volume was limited. For the purpose of feedback and analysis, a motion capture system was developed to track the position and attitude of the helicopters. A high-fidelity mathematical model of a small-scale helicopter was developed. Estimation algorithms were then developed to optimally fuse the data from the instrumentation designed. The data was then used in a system identification exercise to find the parameters that capture the dynamics of the helicopter. The full constrained model of the twin-helicopter slung load dynamics was then developed. The high-fidelity multivariable, interacting system was then linearised to generate a set of uncertain plants. Unexpected resonant modes were investigated using modal analysis to understand their source. Robust controllers were designed using Quantitative Feedback Theory (QFT) for the individual helicopter attitude and altitude loops. A solution was found for the twin-helicopter load transportation system by decoupling the plant with a static pre-compensator and then designing a decentralised QFT controller for the 6 × 6 plant. The effort of this thesis is towards the (practical) realisation of a twin-helicopter aerial crane capable of attitude control; the architecture for the industrialisation of the twin-helicopter load transportation system is proposed