The emergence of complex and autonomous systems, such as modern aircraft, unmanned aerial vehicles (UAVs) and automated industrial processes is driving the development and implementation of new control technologies aimed at accommodating incipient failures to maintain system operation during an emergency. The motivation for this research began in the area of avionics and flight control systems for the purpose to improve aircraft safety. A prognostics health management (PHM) based fault-tolerant control architecture can increase safety and reliability by detecting and accommodating impending failures thereby minimizing the occurrence of unexpected, costly and possibly life-threatening mission failures; reduce unnecessary maintenance actions; and extend system availability / reliability. Recent developments in failure prognosis and fault tolerant control (FTC) provide a basis for a prognosis based reconfigurable control framework. Key work in this area considers: (1) long-term lifetime predictions as a design constraint using optimal control; (2) the use of model predictive control to retrofit existing controllers with real-time fault detection and diagnosis routines; (3) hybrid hierarchical approaches to FTC taking advantage of control reconfiguration at multiple levels, or layers, enabling the possibility of set-point reconfiguration, system restructuring and path / mission re-planning. Combining these control elements in a hierarchical structure allows for the development of a comprehensive framework for prognosis based FTC. First, the PHM-based reconfigurable controls framework presented in this thesis is given as one approach to a much larger hierarchical control scheme. This begins with a brief overview of a much broader three-tier hierarchical control architecture defined as having three layers: supervisory, intermediate, and low-level. The supervisory layer manages high-level objectives. The intermediate layer redistributes component loads among multiple sub-systems. The low-level layer reconfigures the set-points used by the local production controller thereby trading-off system performance for an increase in remaining useful life (RUL). Next, a low-level reconfigurable controller is defined as a time-varying multi-objective criterion function and appropriate constraints to determine optimal set-point reconfiguration. A set of necessary conditions are established to ensure the stability and boundedness of the composite system. In addition, the error bounds corresponding to long-term state-space prediction are examined. From these error bounds, the point estimate and corresponding uncertainty boundaries for the RUL estimate can be obtained. Also, the computational efficiency of the controller is examined by using the number of average floating point operations per iteration as a standard metric of comparison. Finally, results are obtained for an avionics grade triplex-redundant electro-mechanical actuator with a specific fault mode; insulation breakdown between winding turns in a brushless DC motor is used as a test case for the fault-mode. A prognostic model is developed relating motor operating conditions to RUL. Standard metrics for determining the feasibility of RUL reconfiguration are defined and used to study the performance of the reconfigured system; more specifically, the effects of the prediction horizon, model uncertainty, operating conditions and load disturbance on the RUL during reconfiguration are simulated using MATLAB and Simulink. Contributions of this work include defining a control architecture, proving stability and boundedness, deriving the control algorithm and demonstrating feasibility with an example.

As the technology scales, there is a need to develop design and optimization algorithms under various scenarios of uncertainties. These uncertainties are introduced by process variation and impact both delay and leakage. For future technologies at the end of CMOS scaling, not only process variation but the device defect density is projected to be very high. Thus realizing error tolerant implementation of Boolean functions with minimal redundancy overhead remains a challenging task. The dissertation is concerned with the challenges of low-power and area digital circuit design under high parametric variability and high defect density. The technology mapping provides an ideal starting point for leakage reduction because of higher structural freedom in the choices of implementations. We first describe an algorithm for technology mapping for yield enhancement that explicitly takes parameter variability into account. We then show how leakage can be reduced by accounting for its dependence on the signal state, and develop a fast gain-based technology mapping algorithm. In some scenarios the state probabilities can not be precise point values but are modeled as an interval. We extended the notion of mean leakage to the worst case mean leakage which is defined as the sum of maximal mean leakage of circuit gates over the feasible probability realizations. The gain-based algorithm has been generalized to optimize this proxy leakage metric by casting the problem within the framework of robust dynamic programming. The testing is performed by selecting various instance probabilities for the primary inputs that are deviations from the point probabilities with respect to which a point probability based gain based mapper has been run. We obtain leakage improvement for certain test probabilities with the interval probability based over the point probability based mapper. Next, we present techniques based on coding theory for implementing Boolean functions in highly defective fabrics that allow us to tolerate errors to a certain degree. The novelty of this work is that the structure of Boolean functions is exploited to minimize the redundancy overhead. Finally we have proposed an efficient analysis approach for statistical timing, which can correctly propagate the slope in the path-based statistical timing analysis. The proposed algorithm can be scaled up to one million paths.

Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Science, Department of Electrical and Computer Engineering, 2007. / Includes bibliographical references.

While variation analysis methods for compliant assemblies are not new, little has been done to include the effects of multi-step, fixtured assembly processes. This thesis introduces a new method to statistically analyze compliant part assembly processes using fixtures. This method, consistent with the FASTA method developed at BYU, yields both a mean and a variant solution. The method, called Piecewise-Linear Elastic Analysis, or PLEA, is developed for predicting the residual stress, deformation and springback variation in compliant assemblies. A comprehensive, step-by-step analysis map is provided. PLEA is validated on a simple, laboratory assembly and a more complex, production assembly. Significant modeling findings are reported as well as the comparison of the analytical to physical results.

This study presents Computer-Aided Fixture Design Verification (CAFDV) - the methods and implementations to define, measure and optimize the quality of fixture designs. CAFDV verifies a fixture for its locating performance, machining surface accuracy, stability, and surface accessibility. CAFDV also optimizes a fixture for its locator layout design, initial clamping forces, and tolerance specification. The demand for CAFDV came from both fixture design engineers and today's supply chain managers. They need such a tool to inform them the quality of a fixture design, and to find potential problems before it is actually manufactured. For supply chain managers, they will also be able to quantitatively measure and control the product quality from vendors, with even little fixture design knowledge. CAFDV uses two models - one geometric and one kinetic - to represent, verify and optimize fixture designs. The geometric model uses the Jacobian Matrix to establish the relationship between workpiece-fixture displacements, and the kinetic model uses the Fixture Stiffness Matrix to link external forces with fixture deformation and workpiece displacement. Computer software for CAFDV has also been developed and integrated with CAD package I-DEAS TM. CAD integration and a friendly graphic user interface allows the user to have easy interactions with 3D models and visual feedback from analysis results.

Cost effective high volume manufacture of solid oxide fuel cells (SOFCs) is a major challenge for commercial success of these devices. More than fifteen processing methods have been reported in the literature, many of which could be used in various combinations to create the desired product characteristics. For some of these processes, high volume manufacturing experience is very limited or non-existent making traditional costing approaches inappropriate. Additionally, currently available cost models are limited by a lack of incorporation of device performance requirements. Therefore, additional modeling tools are needed to aid in the selection of the appropriate processing techniques prior to making expensive investment decisions. This project describes the development of a SOFC device performance model and a manufacturing process tolerance model. These models are then linked to a preliminary cost model; creating a true multi-process, performance based cost model that permits the comparison of manufacturing cost for different combinations of three processing methods. The three processing methods that are investigated are tape casting, screen printing, and sputtering. . This model is capable of considering production volume, process tolerance and process yield, in addition to the materials and process details. Initial comparisons were performed against processes used extensively within the solid oxide fuel cell industry and the cost results show good agreement with this experience base. Sensitivity of manufacturing costs to SOFC performance requirements such as maximum power density and operation temperature are also investigated.

Tolerances play an important role in product fabrication. Tolerances impact the needs of the designer and the manufacturer. Engineering designers are concerned with the impact of tolerances on the variation of the output, while manufacturers are more concerned with the cost of fitting the parts. Traditional tolerance control methods do not take into account both these needs. In this thesis, the author proposes a framework that overcomes the drawbacks of the traditional tolerance control methods, and reduces subjectivity via fuzzy set theory and decision support systems (DSS). Those factors that affect the manufacturing cost (geometry, material etc) of a part are fuzzy (i.e. subjective) in nature with no numerical measure. Fuzzy comprehensive evaluation (FCE) is utilized in this thesis as a method of quantifying the fuzzy (i.e. subjective) factors. In the FCE process, the weighted importance of each factor affects the manufacturing cost of the part. There is no systematic method of calculating the importance weights. This brings about a need for decision support in the evaluation of the weighted importance of each factor. The combination of FCE and DSS, in the form of Conjoint Analysis (CA), is used to reduce subjectivity in calculation of machining cost. Taguchi's quality loss function is considered in this framework to reduce the variation in the output. The application of the framework is demonstrated with three practical engineering applications. Tolerances are allocated for three assemblies; a friction clutch, an accumulator O-ring seal and a Power Generating Shock Absorber (PGSA) using the proposed framework. The output performances of the PGSA and the clutch are affected by the allocated tolerances. On using the proposed framework, there is seen to be a reduction in variation of output performance for the clutch and the PGSA. The use of CA is also validated by checking efficiency of final tolerance calculation with and without use of CA.

With the current increased emphasis on the development of energy optimized vehicle systems architectures during the early phases in aircraft conceptual design, accurate predictions of these off-nominal requirements are needed to justify architecture concept selection. A process was developed for capturing architecture specific performance degradation strategies and optimally imposing their associated requirements. This process is enabled by analog extensions to traditional safety design and assessment tools and consists of six phases: Continuous Functional Hazard Assessment, Architecture Definition, Load Shedding Optimization, Analog System Safety Assessment, Architecture Optimization, and Architecture Augmentation. Systematic off-nominal analysis of requirements was performed for dissimilar architecture concepts. It was shown that traditional discrete application of safety and reliability requirements have adverse effects on the prediction of requirements. This design bias was illustrated by cumulative unit importance metrics. Low fidelity representations of the loss/hazard relationship place undue importance on some units and yield under or over-predictions of system performance.

The development and implementation challenges associated with a proposed load allocation paradigm for fault risk assessment and system health management based on uncertain fault diagnostic and failure prognostic information are investigated. Health management actions are formulated in terms of a value associated with improving system reliability, and a cost associated with inducing deviations from a system's nominal performance. Three simulated case study systems are considered to highlight some of the fundamental challenges of formulating and solving an optimization on the space of available supervisory control actions in the described health management architecture. Repeated simulation studies on the three case-study systems are used to illustrate an empirical approach for tuning the conservatism of health management policies by way of adjusting risk assessment metrics in the proposed health management paradigm. The implementation and testing of a real-world prognostic system is presented to illustrate model development challenges not directly addressed in the analysis of the simulated case study systems. Real-time battery charge depletion prediction for a small unmanned aerial vehicle is considered in the real-world case study. An architecture for offline testing of prognostics and decision making algorithms is explained to facilitate empirical tuning of risk assessment metrics and health management policies, as was demonstrated for the three simulated case study systems.

Thesis (MScEng)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: In this thesis the design, analysis, implementation, and verification of a fault-tolerant unmanned aerial vehicle (UAV) flight control system which is robust to structural damage causing the natural flight dynamics of the vehicle to become asymmetric, is presented. The main purpose of the robust control architecture is to maintain flight stability after damage has occurred. The control system must be able to handle an abrupt change from an undamaged to a damaged state, and must also not depend on explicit knowledge of the damage. A robust control approach is therefore preferred above an adaptive control approach. As a secondary objective, the system must provide robust flight performance to ensure adequate response times and acceptable transients’ behaviour, both in normal flight, and after damage has occurred. An asymmetric six degrees of freedom equations of motion model is derived. The model accounts for the changes in the aerodynamic model of the aircraft as well as changes in the centre of gravity location. Vortex lattice techniques are used to determine the aerodynamic coefficients of the aircraft for damage to the main wing resulting in 0% to 40% spanwise lifting surface loss. A sequential quadratic programming optimisation algorithm is applied to the force and moment equations to find the trim flight state and actuator deflections of the asymmetric aircraft for constant airspeed and altitude. The trim flight state can be further constrained to force zero bank angle, zero sideslip angle or a desired relative weighting of nonzero bank angle and nonzero sideslip angle. The calculated trim actuator deflections are compared to the physical deflection limits to determine the feasibility of maintaining trim flight for different percentages of wing loss. Assuming that a valid trim condition exists, the relative stability of the aircraft’s natural modes is analysed as a function of percentage wing loss by tracing the locus of the open-loop poles. An acceleration-based flight control architecture is designed and implemented, and the robustness of the flight control stability and performance is analysed as a function of percentage wing loss. The robustness and performance of the flight control system is verified with a nonlinear simulation for spanwise wing loss from 0 to 40%. Practical flight tests are performed to verify the robustness and performance of the flight control systems to in-flight damage. A detachable wing with release mechanism is designed and manufactured to simulate 20% wing loss. The flight control system is implemented on a practical UAV and a successful flight test shows that it performs fully autonomous flight control, and is able to accommodate an in-flight partial wing loss. / AFRIKAANSE OPSOMMING: In hierdie tesis word die ontwerp, analise, implementasie en verifikasie van ’n fout-verdraende onbemande vliegtuig beheerstelsel wat robuust is tot strukturele skade wat die natuurlike vlug dinamika van die voertuig asimmetries maak, voorgestel. Die hoofdoel van hierdie robuuste beheer argitektuur is om stabiliteit te verseker na die skade aangerig is. Die beheerstelsel moet die skielike verandering van normale na beskadigde vlug hanteer sonder enige eksplisiete kennis daarvan. Dus word ’n robuuste beheer aanslag verkies bo ’n aanpassende beheer struktuur. Tweedens moet die vlugbeheerstelsel robuust genoeg wees om steeds die gewenste reaksietyd en aanvaarbare oorgangsverskynsels te kan hanteer, tydens beide normale en beskadigde vlug. ’n Asimmetriese ses grade van vryheid beweginsvergelykings model word afgelei. Die model het die vermoë om veranderinge in die aerodinamiese model van die vliegtuig, sowel as massamiddelpunt verskuiwing, voor te stel. “Vortex Lattice” metodes is gebruik om die aerodinamiese koëffisiënte van die beskadigde vlerk voor te stel tussen 0% en 40% verlies. ’n Sekwensiële kwadratiese programmering optimiserings algorithme is aangewend op die krag en moment vergelykings om die ekwilibrium vlug toestand en aktueerder defleksies te vind vir ’n asimmetriese vliegtuig met konstante lugspoed en hoogte. Die ekwilibrium vlug toestand word verder beperk deur ’n nul rolhoek, ’n nul sygliphoek of ’n relatiewe weging van die twee. Die bepaalde ekwilibrium defleksies word dan vergelyk met die fisiese limiete om hulle geldigheid te bepaal vir ekwilibrium vlug. As ’n geldige ekwilibrium toestand bestaan, kan die relatiewe stabiliteit van die vliegtuig se natuurlike modusse ontleed word as ’n persentasie van vlerkverlies deur die wortellokusse van die ooplus pole na te gaan. ’n Versnellings-gebaseerde vlug beheerstelsel argitektuur is ontwerp en geïmplementeer. Daarna is die robuustheid ontleed as ’n funksie van die persentasie vlerkverlies. Die robuustheid en gedrag van hierdie vlugbeheerstelsel is geverifieer met ’n nie-linêre simulasie vir 0 tot 40% vlerkverlies. Praktiese vlugtoetse is onderneem om die robuustheid en gedrag tydens/na skade gedurende ’n vlug, te verifeer. ’n Vlerkverlies meganisme is ontwerp en vervaardig om 20% vlerkverlies te simuleer. Die vlugbeheerstelsel is geïmplementeer op ’n onbemande vliegtuig en die daaropvolgende suksesvolle vlug lewer bewys dat die vlugbeheerstelsel wel skade, in die vorm van gedeeltelike vlerkverlies, tydens vlug kan hanteer.