Browsing by Author "Braae, Martin"

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    A 3-phase Z-source inverter driven by a novel hybrid switching algorithm
    (2007) Malengret, Jean-Claude; Braae, Martin
    A 3-phase Z-source inverter has been researched, designed, simulated, builtand tested. The purpose of the inverter is to deliver 3-phase 400 VAC from aDC supply that can vary over a range of 20 to 70 Vdc. This is done with a Zsourceinverter topology which is a single conversion method with no additionalDC to DC boost converter. A novel DSP control algorithm allows the inverter toachieve the following:· Run Space Vector Pulse Width Modulation (SV-PWM) for maximum DCbus voltage utilization while boosting the DC bus during zero space vectorstates using shoot through.· Seamless transition between modulation control and modulation / shootthrough control.· Optimised efficiency and DC bus utilisation using Hybrid Space VectorBoost Pulse Width Modulation (HSVB PWM) which is unique to thisdissertation.Such a system is particularly suited to fuel cell and particularly wind turbineapplications where the DC bus voltage is varies over a wide range resulting inthe need for a DC to DC buck/boost to regulate the DC bus to maintain a steady3-phase sinusoidal output. A further application could be for general purpose 3-phase inverter capable of operating on different DC standard bus voltages ( e.g.24, 36, 48 VDC).The benefits of a Z-source topology for the above purposes are a reduction inhigh power semi-conductor components (e.g. power MOSFET). There is also areduction in switching losses and inherent shoot through protection.Furthermore, the inverter is more robust in the sense that it is not vulnerable to spurious shoot through, which could be disastrous in the case of a traditionalvoltage fed inverter.
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    A microcomputer controller for a nylon spinning machine
    (1985) Kirk, Terence Enfield; Braae, Martin
    This thesis will show how a new type of controller for a Nylon spinning machine was developed from an initial specification. The controller is a component in a loosely coupled feedback system which reads two tachometer pulse trains and various plant interlocks and produces two pulse trains which are used to control two solid state variable frequency variable voltage inverters and their AC motors. The specification calls for 24 controllers to be linked to a PDP 11/23 host computer which holds a library of operating parameters which can be downloaded into each control unit by ~n operator. After examining the requirements of the system, a microcomputer implementation was chosen as· best meeting the needs of the project. Elsewhere in the plant several earlier attempts at using micro-computers as dedicated controllers had been made, with rather poor results. Consideration of the future requirements of the company showed that there was a clear role for these controllers, and it was clear that there was a need to define standards for their development and implementation, and so a survey of the company's requirements was done, on the basis of which a standard was adopted. The thesis covers ali system related aspects of the project, from the initial selection of a microcomputer system and software development system to the design and implementation of the controller.
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    Chaos control using local polynomial approximation
    (2001) Yorke, Rory; Braae, Martin
    Chaotic systems may be defined as those whose behaviour is sensitively dependent on initial conditions. Such systems may be made periodic using small input perturbations, as proposed in [OGY90]; this is called Ott-Grebogi-Yorke (OGY) chaos control. The original method used a linear model for controller design; a later development of chaos control was [CCdF99], in which a polynomial model is used. This dissertation proposes using local Taylor polynomial models as a basis for chaos control.
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    A comparison of the Inverse Nyquist Array and Pole Assignment techniques
    (1989) Venzke, Rainer Horst Erich; Braae, Martin
    This dissertation compares the Inverse Nyquist Array (INA) and the Pole Assignment techniques in multivariable control system design. The representation of multivariable systems in both the frequency domain and state space is discussed. A laboratory flotation system, for which a model by step response analysis is derived, is used as a practical example for both methods. A detailed description of the theory of both techniques is given. Particular emphasis is given to how the theory can be applied with the use of a personal computer. Computer-aided control system design programs for the INA and Pole Assignment techniques are included. A complete feedback control scheme for the flotation rig is designed with the INA technique. Pole Assignment by state feedback is used to improve the speed of response of the rig. The implementation of a Kalman filter, which is required for the Pole Assigment technique, is also described.
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    Control system design using artificial intelligence
    (1991) Tebbutt, Colin Dean; Braae, Martin
    Successful multivariable control system design demands knowledge, skill and creativity of the designer. The goal of the research described in this dissertation was to investigate, implement, and evaluate methods by which artificial intelligence techniques, in a broad sense, may be used in a design system to assist the user. An intelligent, interactive, control system design tool has been developed to fulfil this aim. The design tool comprises two main components; an expert system on the upper level, and a powerful CACSD package on the lower level. The expert system has been constructed to assist and guide the designer in using the facilities provided by the underlying CACSD package. Unlike other expert systems, the user is also aided in formulating and refining a comprehensive and achievable design specification, and in dealing with conflicts which may arise within this specification. The assistance is aimed at both novice and experienced designers. The CACSD package includes a synthesis program which attempts to find a controller that satisfies the design specification. The synthesis program is based upon a recent factorization theory approach, where the linear multivariable control system design problem is translated into, and techniques efficiency solved as, a quadratic programming problem, which significantly improve the time and space of this method have been developed, making it practical to solve substantial multivariable design problems using only a microcomputer. The design system has been used by students at the University of Cape Town. Designs produced using the expert system tool are compared against those produced using classical design methods.
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    Controller design via H[infinity] optimal control, quantitative feedback theory and fuzzy logic control : an application to load frequency control
    (2000) Boesak, C D; Braae, Martin
    This thesis describes the application of robust controller design techniques namely H[infinity] optimal control, fuzzy logic control and quantitative feedback theory to the load frequency control problem of power systems. It thus forms a comparative study of these design methods. Load frequency control is the closed loop control of electrical generating units to regulate the system frequency at its nominal value in the presence of load disturbances. Load frequency control also includes the regulation of the tie-line interchange powers.
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    D.O. control and O.U.R. estimation in the activated sludge process
    (1987) Bocken, Stephan; Dold, Peter Lorimer; Braae, Martin
    The objective of this investigation was to evaluate Holmberg's method for simultaneous dissolved oxygen control and parameter estimation in a completely mixed diffused air activated sludge reactor.
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    Design of control systems based on Q(Youla)-parmetrization
    (1992) Jacobs, F C; Braae, Martin
    This thesis is concerned with the application of a method for control system design based on Youla(Q)-Parametrization. The fundamental concepts of the parametrization method have been used by Newton et al, Franklin and Raggazzini [Maciejewski, 1989] as early as 1957. The importance of its implications were not noticed during those years, but Youla and Kucera renewed it in the seventies [Maciejewski, 1989]. For the special case of an open loop stable plant, the implementation can be realized in the Internal Model Control(IMC)-Structure [Morari, 1987]. The IMC-Structure is essentially a open loop design method for closed loop controllers, thereby simplifying the design procedure. A Multivariable controller design method is proposed that unifies the advantages offered by three design approaches/techniques, namely, The Q-Parametrization, The IMC-Structure and The Characteristic Loci Technique. The proposed method is based on controller parametrization. In addition, it retains the engineering insight and simplicity of the IMC-Structure. This is particularly useful in solving complex multivariable problems. The design method was evaluated by its application to the design of a control law for a FLOTATION PROCESS SIMULATOR RIG(FLOTRIG) built by Mr Fisher[Fisher, 1988]. The design of the Multivariable controller was based on a Transfer Function Matrix, G(s), which is a Linear Time Invariant(LTI) model of the FLOTRIG.
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    The design of decentralised controllers for large scale systems
    (1988) Gear, A B J; Braae, Martin
    Decentralised control schemes are becoming more common in industry as the advantages of decentralised control become more apparent. These advantages include fewer tuning parameters than centralised controllers, the simplification and cost reduction of hardware requirements and greater reliability. In addition the application of decentralised controller design to large scale systems allows established CAD methods to be implemented easily and efficiently. When the control engineer designs a distributed controller the system is divided up into a number of subsystems and a controller designed for each subsystem. The controllers are designed independently for each subsystem ignoring any interaction that may occur between the different subsystems. In terms of the input-output representation of the system this means that the matrix representing the controller will be in a block diagonal form. In general the interactions between the different subsystems will not be negligible. In some cases the interactions will be such that stabilising the individual subsystems will not be sufficient to stabilise the system as a whole. Stability theorems are required to enable the designer to check if the decentralised controller that he has designed will in fact stabilise the system as a whole. Such stability theorems have been devised although at present they are too conservative. However even with such theorems available the designer must still select the subsystems to be controlled in such a way as to satisfy the conditions laid down for stability. The stability theories usually are based on a particular matrix structure. If the matrix representing the system possesses a structure detailed by the stability theorem in question then, subject to various conditions, the system as a whole will be stable under decentralised control. In this thesis a number of different matrix structures are considered that give information as to the stability of the closed loop system. Methods are developed that allow the designer to rearrange the matrix in such a way as to obtain a particular structure, if this is possible.
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    Electrification technology and processes to meet economic and social objectives in Southern Africa
    (2003) Gaunt, C T; Braae, Martin
    Bibliography: leaves 243-251.
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    Evaluation of optimal control-based deformable registration model
    (2014) Matjelo, Naleli Jubert; Braae, Martin; Nicolls, Fred
    The deformable image registration is central to many challenges in medical imaging applications. The basic idea of the deformable image registration problem is to find an approximation of a reasonable deformation which transforms one image to match another based on a chosen similarity measure. A reasonable deformation can be thought of as one that is physically realizable. A number of models, guaranteeing reasonable deformations, have been proposed and implemented with success under various similarity measures. One such model is based on the grid deformation method (GDM) and is the method of interest in this thesis. This work focuses on the evaluation of an optimal control-based model for solving the deformable image registration problem which is formulated using GDM. This model is compared with other four well-known variational-based deformable image registration models: elastic, fluid, diffusion and curvature models. Using similarity and deformation quality measures as performance indices, the non-dominated sorting genetic algorithm (NSGA-II) is applied to approximate the Pareto fronts for each model to facilitate proper evaluation. The Pareto fronts are also visualized using level diagrams analysis.
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    Evaluation of virtual reality technology for control engineering
    (1999) Kuc, Bernard; Braae, Martin
    Virtual Reality technology has over recent years become available for commercial use. Where initially it had only been available to research centres and the military, it is now accessible to the industrial and commercial sectors. What this dissertation covers is the suitability of the low cost end of the Virtual Reality hardware for use in Control Engineering. The use of Virtual Reality within Control Engineering could impart significant advantages over traditional control rooms currently in use in factories. The primary one, as rated by most commercial ventures, would be the cost saving of replacing all the physical hardware in a control room with virtual counter-parts in software. This is assuming that the Virtual Reality hardware will itself be of sufficiently low cost. The second is its ability to be used for operator training in instances where factors of safety and economics cannot allow for mistakes to be made on the real plant. A third advantage of a virtual control room, is its portability. As long as the factory can be accessed through some computer network, then the control room can be moved to any desired location. For example a copy of the control room for each factory can be maintained at head office where a chief engineer can occasionally check up on plant performance.
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    A feasibility study for an IMC application in the mining industry
    (1994) Hacker, R J; Braae, Martin
    This project is a feasibility study using Internal Model Control strategies to optimise the performance of a secondary and tertiary crusher stage at a mine. First, a mathematical model of the plant is_ extracted and simulated. The viability of using IMC on an unstable process is considered. Various general objectives are then explained, whereafter the manually controlled plant is evaluated. Three strategies are proposed that control the bin levels to optimise buffer capacity so that crusher throughput is increased and efficiency improved. These are tested on a simulator fed with real plant data to reveal their properties. Finally, an implementation scheme is then proposed.
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    A feasibility study for reset control of an industrial batch reactor
    (2010) Lahee, Roanne; Braae, Martin
    A feasibility study for the application of reset control to the temperature control loop of a pressurized exothermic batch leach reactor in the hydrometallurgical Precious Group Metals (PGM) industry is carried out. Keywords: Reset control; Clegg integrator; initial states; industrial batch reactor; temperature control; exothermic reactions; multiple reactions; dissolve; leach; hydrometallurgy; platinum; Precious Group Metals (PGMs).
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    Generalised predictive control : a study and application
    (1996) Prince, Karl John; Braae, Martin
    This dissertation investigates the Generalised Predictive Control (GPC) method derived by Clarke, Mohtadi and Tuffs in the mid eighties. GPC is an adaptive predictive control algorithm, of which there are number, and has proven to be one of the more popular variants having been applied to various control applications. The theory of the GPC method is studied in detail and a computer simulation program has been written for analyses. While the theory is perhaps not straightforward the actual use of GPC is quite easy. There are 'tuning knobs' available which provide a simple means of tuning the response to match specifications.The GPC method is extended to an overall Long-Range Predictive Control (LRPC) method using the Long-Range Predictive Identification (LRPI) developed by Shook, Mohtadi and Shah in the early nineties. The new identification algorithm replaces the recursive least squares (RLS) used in the original GPC and is essentially a duplicate of the control law. The effect of using LRPI as opposed to RLS is investigated theoretically by use of the simulator. In the simulations carried out comparison of the GPC and LRPC responses depended on the GPC parameters employed. On the whole the LRPI scheme improved the responses though, especially when disturbances and model changes were investigated.
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    H to infinity optimal control of a counter-current process
    (1991) Olivier, Brendan; Braae, Martin; Swartz, Chris
    The extraction of gold from ore that has been mined is the most important part of the process which eventually produces gold bullion. The process most commonly used today is that of carbon-in-pulp gold extraction (CIP). One of the main reasons for this is that it is the most economically efficient method of extracting gold from ore. The process uses activated carbon to absorb gold from a cyanide leach solution. Slurry containing the gold bearing ore and the activated carbon flow in a counter-current fashion. This counter-current flow enables a high percentage of the gold to be recovered. Gold can then be recovered through an elution process. Large amounts of activated carbon are used in the process and a formal multi variable control study of the adsorption section of the CIP process could provide further economic savings by extracting more gold with controlled amounts of carbon. A study was performed to identify the chemical mechanisms involved in the adsorption section of a CIP plant. It was felt that the workings of the process could best be established by designing a simple simulator of the process. The simulator was designed with four reactor tanks, in which the carbon absorbs gold from the leached slurry. The simulator uses a continuous transfer of carbon. In order to fully understand the operation of the HERIG, a simulation study was performed. This simulated model was a simplified version of the actual rig. The level changes of COLD water in the tanks were assumed to be instantaneous and the heat transfer coefficients were assumed to be the same for all four of the tanks. The calculation of the heat transfer coefficients was investigated thoroughly and care was taken to obtain accurate values. The simulator designed was a lumped parameter model. The pipes containing the HOT stream were divided into many small section, in each of which a constant temperature was assumed. A sum of the contributions of all the sections submerged under the COLD water was used to calculate the heat transferred into the COLD water. The COLD water in the tank is stirred continuously and is assumed to be at a constant temperature. The level of COLD water in each tank on the HERIG represents the mass of carbon in each of the tanks on a CIP plant. A change in the HOT water pipe temperature (concentration of Au in slurry) was examined as a function of a change in the level of COLD water in a tank (mass of carbon in a reactor). A steady state and dynamic analysis verified that trends observed from the CIP model were in fact mimicked by the trends observed on the HERIG. It was then decided to perform a formal control study of the HERIG, since the numerous similarities found between the CIP and HERIG enabled relevant conclusions to be drawn about the control of CIP from the control of the HERIG.
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    The importance of selecting the optimal number of principal components for fault detection using principal component analysis
    (2012) Khwambala, Patricia Helen; Braae, Martin
    Fault detection and isolation are the two fundamental building blocks of process monitoring. Accurate and efficient process monitoring increases plant availability and utilization. Principal component analysis is one of the statistical techniques that are used for fault detection. Determination of the number of PCs to be retained plays a big role in detecting a fault using the PCA technique. In this dissertation focus has been drawn on the methods of determining the number of PCs to be retained for accurate and effective fault detection in a laboratory thermal system. SNR method of determining number of PCs, which is a relatively recent method, has been compared to two commonly used methods for the same, the CPV and the scree test methods.
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    The internal performance of iterative feedback tuning
    (2008) Sikaundi, Jaston; Braae, Martin
    Under certain conditions Iterative Feedback Tuning (IFT) may produce a controller that cancels the poles of the process and as a result can give a closed loop that has poor internal performance. The disadvantage of this is that the closed loop will have poor input disturbance rejection. A solution for ensuring that IFT does not have poor internal performance is to make sure that the disturbance rejection is adequate. However an adequate input disturbance may lead to other undesirable dynamics in the closed loop performance. These are such as overshoot in the response for setpoint tracking and that for output disturbance rejection. On the other hand the advantage of pole shifting is that for a one degree of freedom control structure all the characteristic equations of the loop transfer functions will be the same. Four methods are proposed for avoiding pole-zero cancellation by concentrating on the input disturbance. These methods are using: a model for input disturbance rejection, time-weighted IFT for disturbance rejection, a setpoint-tracking model with overshoot and approximate pole placement IFT. Approximate pole placement IFT was chosen as the best method. The reason is that the dynamics of the closed loop can be specified with the choice of characteristic equation. This method was then investigated further to establish its feasibility on a physical system. After the evaluation of this method, it was applied on a DC motor for speed control to show that is viable in practice. Multiple experiments were done to show that this method does not produce a controller that cancels the process poles, confirming it as a good solution to prevent poor internal performance.
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    Investigation into iterative feedback control
    (2004) Machaba, Martin I; Braae, Martin
    The Iterative Feedback Tuning (IFT) method is investigated in this dissertation, starting with the history and origin of this algorithm. The objective of this thesis was to apply the IFT algorithm to a physical system and the system chosen was a DC motor configured for speed control. The inertia of the load for the DC motor is varied to ensure that the algorithm will make the necessary adjustments to compensate for changes in load. Since the IFT is closely related to the well known Model Reference Adaptive Control (MRAC) method application that uses the gradient approach to its adaption. The equations for the MRAC gradient approach method are presented and applied to a DC motor for a speed control. The purpose of this application was to investigate how the MRAC gradient approach will behave in practice in order to give a base case against which to compare the IFT method. The differences between the theoretical and experimental responses of the MRAC are explained by simulation study and modification of the basic MRAC equations. The one degree of freedom controller was chosen for IFT application to a DC motor system for its simplicity since it requires only two experiments to be carried out on the DC motor instead of three as in the case of the two degree of freedom controller. The appropriate signals were generated in experiment 1 and 2 of the IFT algorithm and the control parameters updated in the third step, the values of the controller parameters for which the quadratic criterion is a minimum were produced. The iterations were repeated for a different load.
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    Investigation into the applications of genetic algorithms to control engineering
    (1996) Thithi, Thabang Ignatious; Braae, Martin
    This thesis report presents the results of a study carried out to determine possible uses of genetic algorithms to problems in control engineering. This thesis reviewed the literature on the subject of genetics and genetic algorithms and applied the algorithms to the problems of systems parameter identification and Pl/D controller tuning. More specifically, the study had the following objectives: To investigate possible uses of genetic algorithms to the task of system identification and Pl/D controller tuning. To do an in depth comparison of the proposed uses with orthodox traditional engineering thinking which is based on mathematical optimisation and empirical studies. To draw conclusions and present the findings in the form of a thesis. Genetic algorithms are a class of artificial intelligence methods inspired by the Darwinian principles of natural selection and survival of the fittest. The algorithm encodes potential solutions into chromosome-like data structures that. are evolved using genetic ·operators to determine the optimal solution of the problem. Fundamentally, the evolutionary nature of the algorithm is introduced through the operators called crossover and mutation. Crossover fundamentally takes two strings, selects a crossing point randomly and swaps segments of the strings on either side of the crossover point to create two new individuals. There are three variations of crossover which were considered in this thesis: single point crossover, two point crossover and uniform crossover. It was important that these be given careful consideration since much of the outcome of the algorithm is influenced by both the choice and the amount with which they are applied.