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Control Allocation addresses the problem of the management of multiple, redundant control effectors. Generally speaking, control allocation is any method that is used to determine how the controls of a system should be positioned to achieve some desired effect. An infinite number of allocation methods exist, from the straight-forward direct allocation technique, to the daisy chaining approach, to the computationally simple generalized inverse method. Because different methods have advantages and disadvantages with respect to others, the determination of the "optimal" control allocation method is left to the system designer. The many tradeoffs that are addressed during control system design, of which control allocation is an integral part, dictate the need for a reliable, computer-based design tool. The Control Allocation Toolbox for MATLAB satisfies such a need by providing the designer with a means of testing/comparing the validity of certain allocation methods under prescribed conditions. The issues involved in the development and implementation of the Control Allocation Toolbox are discussed. / Master of Science

Methods for allocating redundant controls for systems with four or more objectives are studied. Previous research into aircraft control allocation has focused on allocating control effectors to provide commands for three rotational degrees of freedom. Redundant control systems have the capability to allocate commands for a larger number of objectives. For aircraft, direct force commands can be applied in addition to moment commands. When controls are limited, constraints must be placed on the objectives which can be achieved. Methods for meeting commands in the entire set of of achievable objectives have been developed. The Bisecting Edge Search Algorithm has been presented as a computationally efficient method for allocating controls in the three objective problem. Linear programming techniques are also frequently presented. This research focuses on an effort to extend the Bisecting Edge Search Algorithm to handle higher numbers of objectives. A recursive algorithm for allocating controls for four or more objectives is proposed. The recursive algorithm is designed to be similar to the three objective allocator and to require computational effort which scales linearly with the controls. The control allocation problem can be formulated as a linear program. Some background on linear programming is presented. Methods based on five formulations are presented. The recursive allocator and linear programming solutions are implemented. Numerical results illustrate how the average and worst case performance scales with the problem size. The recursive allocator is found to scale linearly with the number of controls. As the number of objectives increases, the computational time grows much faster. The linear programming solutions are also seen to scale linearly in the controls for problems with many more controls than objectives. In online applications, computational resources are limited. Even if an allocator performs well in the average case, there still may not be sufficient time to find the worst case solution. If the optimal solution cannot be guaranteed within the available time, some method for early termination should be provided. Estimation of solutions from current information in the allocators is discussed. For the recursive implementation, this estimation is seen to provide nearly optimal performance. / Ph. D.

This dissertation aims to apply non-zero attitude and position setpoint tracking to a quadrotor aircraft, achieved by solving the problem of a quadrotor’s inherent underactuation. The introduction of extra actuation aims to mechanically accommodate for stable tracking of non-zero state trajectories. The requirement of the project is to design, model, simulate and control a novel quadrotor platform which can articulate all six degrees of rotational and translational freedom (6-DOF) by redirecting and vectoring each propeller’s individually produced thrust. Considering the extended articulation, the proposal is to add an additional two axes (degrees) of actuation to each propeller on a traditional quadrotor frame. Each lift propeller can be independently pitched or rolled relative to the body frame. Such an adaptation, to what is an otherwise well understood aircraft, produces an over-actuated control problem. Being first and foremost a control engineering project, the focus of this work is plant model identification and control solution of the proposed aircraft design. A higher-level setpoint tracking control loop designs a generalized plant input (net forces and torques) to act on the vehicle. An allocation rule then distributes that virtual input in solving for explicit actuator servo positions and rotational propeller speeds. The dissertation is structured as follows: First a schedule of relevant existing works is reviewed in Ch:1 following an introduction to the project. Thereafter the prototype’s design is detailed in Ch:2, however only the final outcome of the design stage is presented. Following that, kinematics associated with generalized rigid body motion are derived in Ch:3 and subsequently expanded to incorporate any aerodynamic and multibody nonlinearities which may arise as a result of the aircraft’s configuration (changes). Higher-level state tracking control design is applied in Ch:4 whilst lower-level control allocation rules are then proposed in Ch:5. Next, a comprehensive simulation is constructed in Ch:6, based on the plant dynamics derived in order to test and compare the proposed controller techniques. Finally a conclusion on the design(s) proposed and results achieved is presented in Ch:7. Throughout the research, physical tests and simulations are used to corroborate proposed models or theorems. It was decided to omit flight tests of the platform due to time constraints, those aspects of the project remain open to further investigation. The subsequent embedded systems design stemming from the proposed control plant is outlined in the latter of Ch:2, Sec:2.4. Such implementations are not investigated here but design proposals are suggested. The primary outcome of the investigation is ascertaining the practicality and feasibility of such a design, most importantly whether or not the complexity of the mechanical design is an acceptable compromise for the additional degrees of control actuation introduced. Control derivations and the prototype design presented here are by no means optimal nor the most exhaustive solutions, focus is placed on the whole system and not just a single aspect of it.

No description available.

A comparison of two control allocation methods is performed utilizing the F-15 ACTIVE research vehicle. The control allocator currently implemented on the aircraft is replaced in the simulation with a control allocator that accounts for both control effector positions and rates. Validation of the performance of this Moment Rate Allocation scheme through real-time piloted simulations is desired for an aircraft with a high fidelity control law and a larger control effector suite. A more computationally efficient search algorithm that alleviates the timing concerns associated with the early work in Direct Allocation is presented. This new search algorithm, deemed the Bisecting, Edge-Search Algorithm, utilizes concepts derived from pure geometry to efficiently determine the intersection of a line with a convex faceted surface. Control restoring methods, designed to drive control effectors towards a ``desired" configuration with the control power that remains after the satisfaction of the desired moments, are discussed. Minimum-sideforce restoring is presented. In addition, the concept of variable step size restoring algorithms is introduced and shown to yield the best tradeoff between restoring convergence speed and control chatter reduction. Representative maneuvers are flown to evaluate the control allocator's ability to perform during realistic tasks. An investigation is performed into the capability of the control allocators to reconfigure the control effectors in the event of an identified control failure. More specifically, once the control allocator has been forced to reconfigure the controls, an investigation is undertaken into possible performance degradation to determine whether or not the aircraft will still demonstrate acceptable flying qualities. A direct comparison of the performance of each of the two control allocators in a reduced global position limits configuration is investigated. Due to the highly redundant control effector suite of the F-15 ACTIVE, the aircraft, utilizing Moment Rate Allocation, still exhibits satisfactory performance in this configuration. The ability of Moment Rate Allocation to utilize the full moment generating capabilities of a suite of controls is demonstrated. NOTE: (02/2011) An updated copy of this ETD was added after there were patron reports of problems with the file. / Master of Science

Control Allocation as it pertains to aerospace vehicles, describes the way in which control surfaces on the outside of an aircraft are deflected when the pilot moves the control stick inside the cockpit. Previously, control allocation was performed by a series of cables and push rods, which connected the 3 classical control surfaces (ailerons, elevators, and rudder), to the 3 cockpit controls (longitudinal stick, lateral stick, and rudder pedals). In modern tactical aircraft however, it is not uncommon to find as many as 10 or more control surfaces which, instead of being moved by mechanical linkages, are connected together by complex electrical and/or hydraulic circuits. Because of the large number of effectors, there can no longer be a one-to-one correspondence between surface deflections on the outside of the cockpit to pilot controls on the inside. In addition, these exterior control surfaces have limits which restrict the distance that they can move as well as the speed at at which they can move. The purpose of Constrained Control Allocation is to deflect the numerous control surfaces in response to pilot commands in the most efficient combinations, while keeping in mind that they can only move so far and so fast. The implementation issues of Constrained Control Allocation techniques are discussed, and an aerodynamic model of a highly modified F-15 aircraft is used to demonstrate the various aspects of Constrained Control Allocation. This work was conducted under NASA research grant NAG-1-1449 supervised by John Foster of the NASA Langley Research Center / Master of Science

The promise and potential of controllers that can reconfigure themselves in the case of control effector failures and uncertainties, and yet guarantee stability and provide satisfactory performance, has led to fault tolerant control being an active area of research. This thesis addresses this issue with the design of two fault tolerant nonlinear Structured Adaptive Model Inversion control schemes for systems with fixed magnitude discrete controls. Both methods can be used for proportional as well as discrete controls. However, discrete controls constitute a different class of problems than proportional controls as they can take only binary values, unlike proportional controls which can take many values. Two nonlinear control laws based on Structured Adaptive Model Inversion are developed to tackle the problem of control failure in the presence of plant and operating environment uncertainties. For the case of redundant actuators, these control laws can provide a unique solution. Stability proofs for both methods are derived and are presented in this thesis. Fault Tolerant Structured Adaptive Model Inversion that has already been developed for proportional controls is extended here to discrete controls using pulse width modulation. A second approach developed in this thesis is Fault Tolerant Control Allocation. Discrete control allocation coupled with adaptive control has not been addressed in the literature to date, so Fault Tolerant Control Allocation for discrete controls is integrated with SAMI to produce a system which not only handles discrete control failures, but also accounts for uncertainties in the plant and in the operating environment. Fault tolerant performance of both controllers is evaluated with non real-time nonlinear simulation for a complete Mars entry trajectory tracking scenario, using various combinations of control effector failures. If a fault is detected in the control effectors, the fault tolerant control schemes reconfigure the controls and minimize the impact of control failures or damage on trajectory tracking. The controller tracks the desired trajectory from entry interface to parachute deployment, and has an adaptation mechanism that reduces tracking errors in the presence of uncertainties in environment properties such as atmospheric density, and in vehicle properties such as aerodynamic coefficients and inertia. Results presented in the thesis demonstrate that both control schemes are capable of tracking pre-defined trajectories in the presence of control failures, and uncertainties in system and operating environment parameters, but with different levels of control effort.

To control the speed of an overactuated vehicle there may be many possible ways to use the actuators of the car achieving the same outcome. The actuators in an ordinary car is a combustion engine and a friction brake. In some cases it is trivial how to coordinate actuators for the optimal result, but in many cases it is not. The goal with the thesis is to investigate if it is possible to achieve the same or improved performance with a more sophisticated control structure than today's, using a model predictive controller. A model predictive controller combines the possibility to predict the outcome through an open-loop controller with the stability of a closed loop controller and gives the optimal solution for a finite horizon optimization problem. A simple model of the longitudinal dynamics of a car is developed and used in the model predictive controller framework. This is then used in simulations and in a real car. It is shown that it is possible to replace the current controller structure with a model predictive controller, but there are advantages and disadvantages with the new control structure.

The aim of this effort is to implement a nonlinear flight control architecture, specifically flight path control via command filtered backstepping, for use in AME UAS's Fury® 1500 unmanned flying wing aircraft. Backstepping is a recursive, control-effort minimizing, constructive design procedure that interlaces the choice of a Lyapunov function with the design of feedback control. It allows the use of certain plant states to act as intermediate, virtual controls, for others breaking complex high order systems into a sequence of simpler lower-order design tasks. Work herein is a simplified implementation based on publications by Farrell, Sharma, and Polycarpou. Online approximation is not applied, however command filtering along with two variants of control allocation is. This minimal approach was done to mitigate risk, as adaptation could be added in future work to this baseline. Command filtering assures that control inputs generated meet magnitude, rate, and bandwidth constraints for states and actuators as well as provides command derivatives that reduce computation. Two different forms of control allocation were implemented, the simplest a least-squares pseudo-inverse and the second an optimal quadratic programming method. Two Simulink based simulations successfully flew AME's Fury® 1500 UAS: a nominal case with fully operational actuators and a failure case with an actuator stuck at -10°. Coordinated flight for both cases with outer loop tracking was achieved for a demanding autopilot task of simultaneously varying heading and flight-path angle commands, ±60° and ±10° respectively, for a constant airspeed command of 135 ft/s. Command signals were generated were achievable due to the command filter implementation.