Unknown input observer approaches to robust fault diagnosis

This thesis focuses on the development of the model-based fault detection and isolation /fault detection and diagnosis (FDI/FDD) techniques using the unknown input observer (UIO) methodology. Using the UI de-coupling philosophy to tackle the robustness issue, a set of novel fault estimation (FE)-oriented UIO approaches are developed based on the classical residual generation-oriented UIO approach considering the time derivative characteristics of various faults. The main developments proposed are:- Implement the residual-based UIO design on a high fidelity commercial aircraft benchmark model to detect and isolate the elevator sensor runaway fault. The FDI design performance is validated using a functional engineering simulation (FES) system environment provided through the activity of an EU FP7 project Advanced Fault Diagnosis for Safer Flight Guidance and Control (ADDSAFE).- Propose a linear time-invariant (LTI) model-based robust fast adaptive fault estimator (RFAFE) with UI de-coupling to estimate the aircraft elevator oscillatory faults considered as actuator faults.- Propose a UI-proportional integral observer (UI-PIO) to estimate actuator multiplicative faults based on an LTI model with UI de-coupling and with added H∞ optimisation to reduce the effects of the sensor noise. This is applied to an example on a hydraulic leakage fault (multiplicative fault) in a wind turbine pitch actuator system, assuming that thefirst derivative of the fault is zero. - Develop an UI–proportional multiple integral observer (UI-PMIO) to estimate the system states and faults simultaneously with the UI acting on the system states. The UI-PMIO leads to a relaxed condition of requiring that the first time derivative of the fault is zero instead of requiring that the finite time fault derivative is zero or bounded. - Propose a novel actuator fault and state estimation methodology, the UI–proportional multiple integral and derivative observer (UI-PMIDO), inspired by both of the RFAFE and UI-PMIO designs. This leads to an observer with the comprehensive feature of estimating faults with bounded finite time derivatives and ensuring fast FE tracking response.- Extend the UI-PMIDO theory based on LTI modelling to a linear parameter varying (LPV) model approach for FE design. A nonlinear two-link manipulator example is used to illustrate the power of this method

Demystifying security and compatibility issues in Android Apps

Never before has any OS been so popular as Android. Existing mobile phones are not simply devices for making phone calls and receiving SMS messages, but powerful communication and entertainment platforms for web surfing, social networking, etc. Even though the Android OS offers powerful communication and application execution capabilities, it is riddled with defects (e.g., security risks, and compatibility issues), new vulnerabilities come to light daily, and bugs cost the economy tens of billions of dollars annually. For example, malicious apps (e.g., back-doors, fraud apps, ransomware, spyware, etc.) are reported [Google, 2022] to exhibit malicious behaviours, including privacy stealing, unwanted programs installed, etc. To counteract these threats, many works have been proposed that rely on static analysis techniques to detect such issues. However, static techniques are not sufficient on their own to detect such defects precisely. This will likely yield false positive results as static analysis has to make some trade-offs when handling complicated cases (e.g., object-sensitive vs. object-insensitive). In addition, static analysis techniques will also likely suffer from soundness issues because some complicated features (e.g., reflection, obfuscation, and hardening) are difficult to be handled [Sun et al., 2021b, Samhi et al., 2022].Comment: Thesi

INVESTIGATION OF DIAMOND COATINGS ON IRON BASED MATERIALS BY MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

Diamond thin films on Fe based materials (ferrous alloys) for the purpose of improving their surface properties such as wear and corrosion resistance have been investigated. There are main barriers restricting the quality and adhesion of diamond coatings on Fe based materials. Firstly, the incubation time of diamond nucleation is long due to the high solubility of carbon in Fe. Secondly, graphite soot forms before diamond nucleation due to the catalytic effect of Fe for formation of graphitic carbon. Thirdly, high internal stress remains at the interface which is induced by the large difference in the thermal expansion coefficients of diamond and most of the Fe based materials. Surface modification and interlayers are two important approaches to overcome these problems. In this work, the effect of Cr content in Fe-Cr alloys on diamond nucleation and growth is being studied in order to clarify the mechanisms of Cr in diamond deposition. Furthermore, in order to enhance the adhesion and quality of diamond coatings, Al based interlayers are being investigated on ferrous alloys. Fe-Cr alloys (with 20~80 wt.% Cr) were exposed to a CH4-H2 mixture in a microwave plasma enhanced chemical vapor deposition (MPCVD) reactor. Severe metal dusting and carburization were observed on the alloys with low Cr content and diamond did not nucleate on those alloys until a graphite intermediate layer had been formed, which takes a long incubation time. Increasing Cr concentration in the Fe-Cr alloys promotes the formation of a Cr carbide buffer layer, which inhibits metal dusting and the formation of graphite soot. Consequently, diamond nucleation and growth can be greatly enhanced, and continuous diamond films with enhanced adhesion have been deposited on the Fe-80Cr alloys. Al based interlayers including Al and Al/AlN interlayers were deposited on ferrous alloys (SS316 and Kovar: FeNiCo) to enhance diamond deposition. The deposition was carried out in a microwave plasma chemical vapor deposition (MPCVD) reactor using a CH4-H2 mixture. The obtained samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, synchrotron-based X-ray absorption spectroscopy (XAS) and indentation testing. The results show that a single Al layer can effectively suppress the formation of graphite at the interface and the inward diffusion of carbon into Fe based substrates, and thereby enhances diamond nucleation and growth. The dual layers of Al/AlN can further enhance the adhesion of diamond coatings comparing with the single Al interlayer

Flow Stability Model for Fan/Compressors with Annular Duct and Novel Casing Treatment

AbstractA three-dimensional compressible flow stability model is presented in this paper, which focuses on stall inception of multi-stage axial flow compressors with a finite large radius annular duct configuration for the first time. It is shown that under some assumptions, the stability equation can be obtained yielding from a group of homogeneous equations. The stability can be judged by the non-dimensional imaginary part of the resultant complex frequency eigenvalue. Further more, based on the analysis of the unsteady phenomenon caused by casing treatment, the function of casing treatment has been modeled by a wall impedance condition which is included in the stability model through the eigenvalues and the corresponding eigenfunctions of the system. Finally, some experimental investigation and two numerical evaluation cases are conducted to validate this model and emphasis is placed on numerically studying the sensitivity of the setup of different boundary conditions on the stall inception of axial flow fan/compressors. A novel casing treatment which consists of a backchamber and a perforated plate is suggested, and it is noted that the open area ratio of the casing treatment is less than 10%, and is far smaller than conventional casing treatment with open area ratio of over 50%, which could result in stall margin improvement without obvious efficiency loss of fan/compressors

On the Decision Tree Complexity of String Matching

String matching is one of the most fundamental problems in computer science. A natural problem is to determine the number of characters that need to be queried (i.e. the decision tree complexity) in a string in order to decide whether this string contains a certain pattern. Rivest showed that for every pattern p, in the worst case any deterministic algorithm needs to query at least n-|p|+1 characters, where n is the length of the string and |p| is the length of the pattern. He further conjectured that this bound is tight. By using the adversary method, Tuza disproved this conjecture and showed that more than one half of binary patterns are evasive, i.e. any algorithm needs to query all the characters (see Section 1.1 for more details). In this paper, we give a query algorithm which settles the decision tree complexity of string matching except for a negligible fraction of patterns. Our algorithm shows that Tuza\u27s criteria of evasive patterns are almost complete. Using the algebraic approach of Rivest and Vuillemin, we also give a new sufficient condition for the evasiveness of patterns, which is beyond Tuza\u27s criteria. In addition, our result reveals an interesting connection to Skolem\u27s Problem in mathematics