Thesis (MScIng)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Proefschrift Universiteit Maastricht. / Lit. opg.: p. 123-130. - Met samenvatting in het Nederlands.

Although much work has been done on assessing safety requirements in programmable systems, one very important aspect, the integration of hazard evaluation procedures and requirements engineering, has been somewhat neglected. This thesis describes the derivation and application of a methodology, HAZAPS (HAZard Assessment in Programmable Systems). The methodology assists at the requirements stage in the development of safety-critical embedded systems. The objectives are to identify hazards in programmable systems, construct and model the associated safety requirements, and, finally, to assess these requirements. HAZAPS integrates safety engineering and software modelling techniques. The analysis of more than 300 computer related incidents provided the criteria used to identify, select and modify safety engineering techniques.

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abstract: This thesis addresses control design for fixed-wing air-breathing aircraft. Four aircraft with distinct dynamical properties are considered: a scram-jet powered hypersonic (100foot long, X-43 like, wedge shaped) aircraft with flexible modes operating near Mach 8, 85k ft, a NASA HiMAT (Highly Maneuverable Aircraft Technology) F-18 aircraft, a McDonnell Douglas AV-8A Harrier aircraft, and a Vought F-8 Crusader aircraft. A two-input two-output (TITO) longitudinal LTI (linear time invariant) dynamical model is used for each aircraft. Control design trade studies are conducted for each of the aircraft. Emphasis is placed on the hypersonic vehicle because of its complex nonlinear (unstable, non-minimum phase, flexible) dynamics and uncertainty associated with hypersonic flight (Mach $>$ 5, shocks and high temperatures on leading edges). Two plume models are used for the hypersonic vehicle – an old plume model and a new plume model. The old plume model is simple and assumes a typical decaying pressure distribution for aft nozzle. The new plume model uses Newtonian impact theory and a nonlinear solver to compute the aft nozzle pressure distribution. Multivariable controllers were generated using standard weighted $H_{\inf}$ mixed-sensitivity optimization as well as a new input disturbance weighted mixed-sensitivity framework that attempts to achieve good multivariable properties at both the controls (plant inputs) as well as the errors (plant outputs). Classical inner-outer (PD-PI) structures (partially centralized and decentralized) were also used. It is shown that while these classical (sometimes partially centralized PD-PI) structures could be used to generate comparable results to the multivariable controllers (e.g. for the hypersonic vehicle, Harrier, F-8), considerable tuning (iterative optimization) is often essential. This is especially true for the highly coupled hypersonic vehicle – thus justifying the need for a good multivariable control design tool. Fundamental control design tradeoffs for each aircraft are presented – comprehensively for the hypersonic aircraft. In short, the thesis attempts to shed light on when complex controllers are essential and when simple structures are sufficient for achieving control designs with good multivariable loop properties at both the errors (plant outputs) and the controls (plant inputs). / Dissertation/Thesis / Masters Thesis Electrical Engineering 2015

abstract: The purpose of this dissertation is to develop a design technique for fractional PID controllers to achieve a closed loop sensitivity bandwidth approximately equal to a desired bandwidth using frequency loop shaping techniques. This dissertation analyzes the effect of the order of a fractional integrator which is used as a target on loop shaping, on stability and performance robustness. A comparison between classical PID controllers and fractional PID controllers is presented. Case studies where fractional PID controllers have an advantage over classical PID controllers are discussed. A frequency-domain loop shaping algorithm is developed, extending past results from classical PID’s that have been successful in tuning controllers for a variety of practical systems. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

This thesis is primarily concerned with motion control of robot manipulators with emphasis placed on adaptive impedance control and relevant computational issues. The general approach taken in our studies is a model-based approach, that is, the algorithms developed is based on the dynamic model of the robot(s) involved. Two computational formulations are derived for the evaluation of the so-called regressor dynamics of a robot manipulator, which has played a key role in the development of popular stable adaptive control algorithms for robot manipulators. The closed-form version of the formulations is based on the Lagrangian dynamics formulation while the recursive version is based on the Newton-Euler dynamics. As an application of the regressor dynamics formulation, the popular Slotine-Li adaptive control algorithm is modified and then implemented on a PUMA 560 robot. Satisfactory computational efficiency of the regressor formulas, especially the recursive formula, has been demonstrated in our experimental implementations. To extend adaptive position control algorithms to force control, the concept of target impedance reference trajectory is introduced which makes it possible to inject two stable adaptive position control algorithms into Hogan's conventional impedance control. These two adaptive impedance control algorithms have been shown stable. Simulation and real-time implementation of the algorithms on a PUMA 560 robot are reported. The last part of the thesis conducts a study on optimal load distribution and coordination of multiple robots. Optimal load distribution schemes using a p-norm type optimization approach are proposed. The algorithms are then adopted to dynamically link the two-level controllers in a proposed coordination framework. Simulation results are presented to show the performance of the proposed structure. / Graduate

This thesis presents one of the first applications of H∞-optimization to the design of controllers for industrial problems. The system considered was an unstable helicopter model, obtained from a large nonlinear simulation (provided by the Royal Aircraft Establishment, Bedford) configured to represent future high performance helicopters. The problem was to design a full authority flight control system, to stabilize the aircraft and decouple the controlled inputs, thus reducing pilot workload. Robustness was a primary issue because of model uncertainty, particularly due to the omission from the design model of higher order rotor dynamics. The optimization problem was based on the minimization of sensitivity (for performance) and control output (for robustness) transfer functions. Simple weighting functions were found to be useful for examining the fundamental performance-versus-robustness trade-off, and to be more effective at shaping the closed loop transfer functions than LQG/LTR techniques. A controller designed for a 4-input, 6-output, 8-state linearized plant model was successfully implemented in a non-linear simulation with rotor dynamics. This stabilized the system and enabled good control for small variations about the design operating point. The 'standard problem', consisting of the plant augmented with weights, had 20 states; the controller had 18, which was much smaller than researchers had been predicting, and it is conjectured that all H∞-optimal controllers will have at most the same number of states as the defining 'standard problem'. An important improvement to the H∞-optimization solution process was the development of a numerically reliable algorithm to perform minimal realization. This algorithm solves for a truncated balanced realization of stable state-space systems that are arbitrarily close to being either uncontrollable and/or unobservable. Depending on the choice of partitioning of the Hankel singular values, it can be used to perform minimal realization, or model reduction, with a guaranteed L∞ error bound.

Computer-based control systems are widely used in vehicle applications (e.g., aircraft, marine and automobile). The earlier forms of control systems were typified by a central computer connected to sub-systems using hard-wired point-to-point communication links. However, such systems suffered from several major drawbacks: (i) fault-tolerance problems, (ii) maintenance and cabling costs, and (iii) excessive cable weight. These problems were minimised by using master-slave networks with distributed control architecture. However, using such networks raises the question of fault-tolerance and integrity of the communication system. In view of this, efforts have been made to employ the architecture provided by the IEEE 802.4 Token Bus for real time control applications although the performance of the Token Bus is not adequate for applications where fast response times are required.