Browsing Doctoral Degrees (Electrical and Electronic Engineering) by Title
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- ItemA computational architecture for real-time systems(Stellenbosch : Stellenbosch University, 2000-12) Mostert, Sias; Du Plessis, J. J.; Halang, W. A.; Stellenbosch University. Faculty of Engineering. Dept.of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The engineering of dependable real-time systems for mission critical applications is a resource intensive and error prone process. Achieving dependability requires a general consensus on the correctness of a system with regard to its intended function. For a consensus to be achieved, the properties of the system must be well understood which, in turn, requires consensus on a rigorously defined computational architecture. There is currently no single agreed upon computational architecture at the application level which can serve as a common denominator for the design and implementation of real-time systems. It is the thesis of this dissertation that a rigorous computational architecture, applicable from design to implementation, enables engineers to better understand software for real-time systems. To substantiate this claim, the real-time data flow architecture RDF with its notation allowing the description of complete systems from design to implementation will be explored. Four distinct research areas for improving the engineering process of real-time systems are dealt with in the dissertation: 1) the development of an architecture for real-time systems being suitable for design and implementation in software and hardware, 2) the consolidation of a number of graphical languages into a graphical notation for functional specification, design and construction of real-time systems, 3) the development of a simple processor architecture for the execution of real-time applications, and 4) and the evaluation of the architecture in the framework of a microsatellite case study. In particular, the following original contributions are made: 1) the firing semantics of data flow systems are expanded to include disjunctive firing semantics in a novel way in addition to the classical conjunctive firing semantics, 2) the inherent real-time data flow property, Le. that a receiving task must be ready to receive the next incoming message when it is sent, is extended to the synchronous data flow model, 3) a notation for describing all properties of real-time systems is defined 'with the real-time data flow language RDF as base language, 4) two hardware processor architectures are introduced that offer one-to-one correspondence between design and implementation and, thus, reduce the semantic gap between design language and program execution, and 5) the class of systems that can be modelled with data flow architectures is shown to include control systems and data flow systems. The language set and processor architecture were applied to certain aspects of the SUNSAT microsatellite project.
- ItemA multi-mode attitude determination and control system for small satellites(Stellenbosch : Stellenbosch University, 1995-12) Steyn, Willem Hermanus; Schoonwinkel, A.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: New advanced control techniques for attitude determination and control of small (micro) satellites are presented. The attitude sensors and actuators on small satellites are limited in accuracy and performance due to physical limitations, e.g. volume, mass and power. To enhance the application of sophisticated payloads such as high resolution imagers within these confinements, a multi-mode control approach is proposed, whereby various optimized controller functions are utilized during the orbital life of the satellite. To keep the satellite's imager and antennas earth pointing with the minimum amount of control effort, a passive gravity gradient boom, active magnetic torquers and a magnetometer are used. A "cross-product" detumbling controller and a robust Kalman filter angular rate estimator are presented for the preboom deployment phase. A fuzzy controller and magnetometer full state extended Kalman filter are presented for libration damping and Z-spin rate control during inactive imager periods. During imaging, when high performance is required, additional fine resolution earth horizon, sun and star sensors plus 3-axis reaction wheels are employed. Full state attitude, rate and disturbance estimation is obtained from a horizon/sun extended Kalman filter. A quaternion feedback reaction wheel controller is presented to point or track a reference attitude during imaging. A near-minimum time, eigenaxis rotational reaction wheel controller for large angular maneuvers. Optimal linear quadratic and minimum energy algorithms to do momentum dumping using magnetic torquers, are presented. A new recursive magnetometer calibration method is designed to enhance the magnetic in-flight measurements. Finally, a software structure is proposed for the future onboard implementation of the multi-mode attitude control system.
- ItemAccelerating deep reinforcement learning for autonomous racing(Stellenbosch : Stellenbosch University, 2023-03) Evans, Benjamin; Jordaan, Willem; Engelbrecht, Herman; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The F1/10th racing problem is to use the onboard LiDAR scan to calculate speed and steering references to move a 1/10th scale car around the track as quickly as possible. While planning has typically used perception, planning and control pipelines, recently, deep reinforcement learning (DRL) has grown in popularity due to its advantages of not requiring explicit state representation and environmental flexibility. Current approaches have suffered from poor performance at low speeds, safety concerns exacerbated by sim-toreal transfer, and few approaches have considered obstacle avoidance. The first contribution of this work is the development of high-speed learning formulations for autonomous racing. A comprehensive evaluation of previous approaches concludes that current learning formulations train agents to select infeasible speed profiles, resulting in the agents being unable to race using the vehicle’s full speed profile. This problem is overcome by using analytical vehicle models to develop learning formulations for improved speed selection. The performance evaluation shows that the novel formulations enable the vehicle to learn a feasible speed profile using the vehicle’s full speed range and achieve lower lap times than previous methods in the literature. This result indicates that using vehicle models improves high-performance racing behaviour. The second contribution of this work is to enable online learning by using a supervisory safety system (SSS). A safety system is designed that uses viability theory to ensure vehicle safety, irrespective of the planner used. The SSS is incorporated into the learning formulation and used to train DRL agents to race without them ever crashing. The novel learning formulation is extensively evaluated in simulation, demonstrating that online training can train agents to race without ever crashing, achieve a 10× improvement in sample efficiency and that the trained agents select conservative speed profiles. The proposed method is validated at constant speed on a physical vehicle, demonstrating that an agent can be trained from random to drive around a track without ever crashing. The final contribution of this work is to explore how DRL agents can be used to expand the ability of current classical planners to avoid unmapped obstacles. Three hybrid architectures that combine classical and learning components are presented and evaluated. The modification planner, which combines a path follower and DRL agent in parallel, demonstrates the ability to track a reference path while avoiding unmapped obstacles. The results indicate that combining classical and DRL components can improve the performance of DRL agents while enabling classical solutions to avoid obstacles.
- ItemAcceleration based manoeuvre flight control system for unmanned aerial vehicles(Stellenbosch : Stellenbosch University, 2008-12) Peddle, Iain K.; Jones, T.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: A strategy for the design of an effective, practically feasible, robust, computationally efficient autopilot for three dimensional manoeuvre flight control of Unmanned Aerial Vehicles is presented. The core feature of the strategy is the design of attitude independent inner loop acceleration controllers. With these controllers implemented, the aircraft is reduced to a point mass with a steerable acceleration vector when viewed from an outer loop guidance perspective. Trajectory generation is also simplified with reference trajectories only required to be kinematically feasible. Robustness is achieved through uncertainty encapsulation and disturbance rejection at an acceleration level. The detailed design and associated analysis of the inner loop acceleration controllers is carried out for the case where the airflow incidence angles are small. For this case it is shown that under mild practically feasible conditions the inner loop dynamics decouple and become linear, thereby allowing the derivation of closed form pole placement solutions. Dimensional and normalised non-dimensional time variants of the inner loop controllers are designed and their respective advantages highlighted. Pole placement constraints that arise due to the typically weak non-minimum phase nature of aircraft dynamics are developed. A generic, aircraft independent guidance control algorithm, well suited for use with the inner loop acceleration controllers, is also presented. The guidance algorithm regulates the aircraft about a kinematically feasible reference trajectory. A number of fundamental basis trajectories are presented which are easily linkable to form complex three dimensional manoeuvres. Results from simulations with a number of different aircraft and reference trajectories illustrate the versatility and functionality of the autopilot. Key words: Aircraft control, Autonomous vehicles, UAV flight control, Acceleration control, Aircraft guidance, Trajectory tracking, Manoeuvre flight control.
- ItemAcceleration based manoeuvre flight control system for unmanned aerial vehicles(Stellenbosch : Stellenbosch University, 2008-12) Peddle, Iain K.; Jones, T.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.A strategy for the design of an effective, practically feasible, robust, computationally efficient autopilot for three dimensional manoeuvre flight control of Unmanned Aerial Vehicles is presented. The core feature of the strategy is the design of attitude independent inner loop acceleration controllers. With these controllers implemented, the aircraft is reduced to a point mass with a steerable acceleration vector when viewed from an outer loop guidance perspective. Trajectory generation is also simplified with reference trajectories only required to be kinematically feasible. Robustness is achieved through uncertainty encapsulation and disturbance rejection at an acceleration level. The detailed design and associated analysis of the inner loop acceleration controllers is carried out for the case where the airflow incidence angles are small. For this case it is shown that under mild practically feasible conditions the inner loop dynamics decouple and become linear, thereby allowing the derivation of closed form pole placement solutions. Dimensional and normalised non-dimensional time variants of the inner loop controllers are designed and their respective advantages highlighted. Pole placement constraints that arise due to the typically weak non-minimum phase nature of aircraft dynamics are developed. A generic, aircraft independent guidance control algorithm, well suited for use with the inner loop acceleration controllers, is also presented. The guidance algorithm regulates the aircraft about a kinematically feasible reference trajectory. A number of fundamental basis trajectories are presented which are easily linkable to form complex three dimensional manoeuvres. Results from simulations with a number of different aircraft and reference trajectories illustrate the versatility and functionality of the autopilot. Key words: Aircraft control, Autonomous vehicles, UAV flight control, Acceleration control, Aircraft guidance, Trajectory tracking, Manoeuvre flight control.
- ItemAdaptive cross approximation methods for fast analysis of Antenna Arrays(Stellenbosch : Stellenbosch University, 2021-03) Sewraj, Keshav; Botha, Matthys M.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: This work is focused on developing efficient numerical electromagnetic algorithms forthe analysis of large antenna arrays, such as being considered as part of the internationalSquare Kilometre Array (SKA) radio astronomy project currently under development.Numerical electromagnetic simulation is a vital tool to evaluate performance during theantenna design process, and it is common to iterate through thousands of simulationsto fine-tune parameters. However, these simulations are often expensive and can be alimiting factor in the available design choices.The Method of Moments (MoM) is a numerical technique used to solve electromag-netic field problems, and is highly suitable for radiation problems such as the analysisof antenna arrays. However, the memory and runtime requirement of the MoM scale asO(N2)andO(N3), respectively, whereNis the number of Degrees of Freedom. As such,electromagnetic analysis performed by the MoM is limited by the electrical size of theproblem. For larger structures, fast MoM-based techniques tailored to specific problemsneed to be devised.In this work, a variety of techniques based on cross approximation is explored andimplemented, for array analysis. In this context, two Directional Cross Approximation(DCA)-based solvers are devised. The DCA is a nested multilevel algorithm which ef-ficiently compresses MoM sub-blocks due to far interactions as a product of low-rankfactors. During the computation of these factors, the far-field is segmented in angularsectors to ensure the numerical rank is limited irrespective of the cluster size.Firstly, the DCA is combined with the Macro Basis Function (MBF) method. In thistechnique, physics-based MBFs are defined over each antenna element, through linearcombinations of the low-level basis functions defined on that domain, in order to createa reduced matrix system, which can then be solved directly. However, one of the MBFsolvers’ bottlenecks is the high cost associated with the computation of reaction terms during the fill-in of the reduced matrix. As such, the DCA algorithm is used to efficientlyrepresent and compute the reaction terms in MBF solvers. The accuracy of using theMBF-DCA solver is validated, and a favorable memory scaling is obtained.Secondly, the single-level version of the DCA is formulated together with a sparsedirect solver scheme, based on the Inverse Fast Multipole Method (IFMM), to solve for antenna array MoM solution directly. The original IFMM formulation is extended forthe directional case, and a new procedure to eliminate and redirect compressible fill-insduring the Gaussian elimination of the sparse matrix is devised.Lastly, a hybrid single-level compression scheme is devised to accelerate the IterativeRadius-Based Domain Green’s Function Method (IRB-DGFM) solver, for array analysis.The compression algorithm combines the standard Adaptive Cross Approximation (ACA)to compress intermediate interactions, and the single-level Nested Cross Approximation(NCA) to represent far interactions efficiently.
- ItemAdvances in pedestal substrate integrated waveguide filters(Stellenbosch : Stellenbosch University, 2022-04) Johnson, Leanne; Meyer, Petrie; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: This dissertation presents a set of advances to the recently proposed Pedestal Substrate Integrated Waveguide (SIW) structure as a solution to various problem areas in the field of microwave filters. The structure consists of an evanescent-mode SIW cavity that is loaded with a square pedestal connected to the ground with a metal post. This structure is firstly investigated and compared to other common microwave resonators. It is then utilized in 3 different ways. The work includes the design of a coupled resonator filter with both positive and negative cross-coupling. Electric coupling is obtained with an I-shaped line between pedestal tops, while positive coupling is obtained in the traditional manner with inductive irises. Both types of couplings offer large ranges of coupling values and are easily implemented in this structure. The proposed structure can also introduce mixed coupling. A proof-of-concept sixth order cross-coupled bandpass filter, with a 5 % bandwidth at 5 GHz, and both real and imaginary axis transmission zeros, is designed and measured. Secondly, the dissertation presents a novel partially air filled pedestal resonator. The pedestal SIW topology is adapted to create partially air filled pedestal SIW resonators to reduce losses and increase Q factors while maintaining small sizes. A comparison of three different types of pedestal resonators is done between a totally filled pedestal SIW resonator, a partially air-filled pedestal resonator and an empty pedestal resonator, show ing the increase in Q factor for the same size. Then the use of micro-machining to create these novel resonators is illustrated and explained. A resonator and second order filter prototypes are manufactured, using the manufacturing process explained, and tested. It was shown that the Q factor can be increased from 186 for a fully filled dielectric resonator to 285 for a partially air filled resonator. This shows a 34.7 % increase. The last application of the pedestal resonator is in the use of tunable structures. The pedestal resonator is shown to be made easily tunable by the addition of PIN diodes to the pedestal post, seperated with an annular ring. Four separate stages of frequency are obtained in this manner. Two resonators and one third order filter are designed, manufac tured and tested. Good frequency tunability is obtained in simulation and measurement, but high losses were observed in the measured prototypes.
- ItemAdvances in spectral techniques for fruit quality evaluation: case of ULF-NMR and NIRS(Stellenbosch : Stellenbosch University, 2017-03) Isingizwe Nturambirwe, Jean Frederic; Perold, Willem; Opara, Lionel; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Industrial application of non-destructive analytical techniques still faces a challenge of lack of general and specialty models for quality evaluation. Current developments strive to alleviate this problem by the development of new cost effective equipment. In the food industry, and especially the horticultural industry, two spectroscopic methods seem to lead the way in terms of analytical variety, advancement in software for data handling and analysis, and relevance. The techniques in question are nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopy. This project used practical experimental studies of fruit quality, using both techniques, to further research towards their non-destructive and online application, especially for horticultural products. A SQUID-NMR (Bm = 1Gauss) system was used to study the ripening of banana and predict its ripening index. Measurements of the NMR spin-lattice (T1) and the spin-spin (T2) relaxation times were acquired prior to destructive measurements. Various physico-chemical attributes were monitored for changes during the ripening process. Four out of six measurements, taken over a period of 10 days of storage (at 15 oC and 85%RH), were significantly different. Average T2 gave less promising results than average T1, that was highly correlated to attributes that changed during ripening, namely,lightness, L* (r=0.61), chromaticity coefficient, b* (r=0.65), totoal soluble solids, TSS (r= 0.72), sugar:acid ratio, TSS/TA (r=0.82), chromaticity coefficient, a* (r=0.84) and hue angle, h (r=-0.85). Correlations with T2 were found for TSS (r=-0.53), TSS/TA (-0.54) b* (-0.58) and pH (r=0.70), all significant at p<0.05. The ripening index was defined subjectively, based on the visual standards of the ripening index in banana. Average T1 distinctively explained the variance in ripening index, together with TSS, TSS/TA color parameters a* and h and total color difference. Calculation of the multicomponent distribution of T1 resulted in two components, one slow and another fast. Improvements in consistency of the transform is still required before it can be used for further analysis and accurate peak assignment. The results above show that there are opportunities of using SQUID detected NMR spectral data in the T1 domain for further studies of banana quality, and very likely other fruits as well. It is apparent that issues of temperature dependence of T1 should be taken into account in building more robust models. Fourier transform NIR (FT-NIR) was used in studying internal quality and mechanical damage in apples. Using three cultivars from two sources and three spectrometer modes from two FT-NIR spectrometers, we were able to account for the need of variability in building robust models. Different levels of predictability for each attribute were obtained for different cultivars, using PLS regression methods. The predictive ability was different between distinct spectral acquisition modes as well, but also depended on attributes. It was noticed that, in all scenarios considered (single exclusive and all inclusive cultivar or source), the emission head (EH) of the Matrix-F spectrometer led to similar model performances as for the integrating sphere (IS) of the multipurpose analyzer (MPA) FT-NIR spectrometer, in models predicting TSS. Model optimization was done successfully using both pre-processing methods and genetic algorithms applied on PLS of non-processed spectra. The influence of either cultivar or instrument on models predicting TA was different than that obtained for TSS and for TSS/TA, and overall, with lower model performance. Results revealed aspects that are likely to favor calibration transfers between the EH and IS acquisition modes. Bruising in apples is very common and quite intricate to detect if it is internal or not showing on the outside yet. NIR, mostly multispectral, hyperspectral imaging and visible spectrum (VIS) combined with NIR (VIS/NIR), have been used frequently to distinguish between bruised and sound tissue of apple fruits. It has been customary, as seen from many reports, that bruise studies by NIR calls for variable selection. The study carried out on bruise damage in this project involved variable selection by genetic algorithm, influenced by cultivar, and validated by a variable importance in projection (VIP) method, that used a different approach to the filter method. Favored wavebands were brought to light. Both methods were compared to the literature. This may serve as a good basis for further development towards online applications in the horticulture industry. Overall, advanced prospects in applications of the two most highly developed spectroscopic techniques to non-destructive fruit quality evaluation were identified and recommendations were given in light of possibilities for future industrial application.
- ItemAgent-based modelling of paratransit as an intelligent complex adaptive system to improve efficiency(Stellenbosch : Stellenbosch University, 2021-03) Ndibatya, Innocent; Booysen, M. J.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Urban residents in Sub-Saharan Africa (SSA) face mobility challenges that limit theiraccess to jobs, services, markets, and socioeconomic opportunities. In most SSA cities,public transport is predominantly provided by the inefficient paratransit system – a flex-ible mode of passenger transport consisting of privately-owned, low-capacity unscheduledminibuses and motorcycle taxis. There is growing interest among city authorities andurban transport researchers in addressing the inefficiency problem associated with para-transit. Several approaches, such as complete overhaul to bus rapid transit (BRT), andphased banning of paratransit from the cities have previously been proposed and con-comitant implementation projects started. However, most of such projects have eitherfailed to take off, or they have stalled. This is likely because of the huge capital invest-ment required, the unique social and cultural dynamics associated with “third world”countries, and urban sprawl due to poor city planning. This study departs from the com-mon perspective held by several researchers and city authorities who view paratransit as“chaotic”, thus, the justification for its total overhaul and banning. Instead, this studyaims to leverage the beneficial aspects of existing paratransit – such as flexibility, demand-responsiveness and near-ubiquitous coverage – with the elusive objective of achieving amore efficient paratransit state as a result.Through theoretical modelling, field study and experimental approaches, this studyaimed to improve the efficiency of minibus taxis paratransit systems. The theoretical mod-elling work involved modelling paratransit systems as complex adaptive systems (CAS)and developing an agent-based model (ABM) for minibus taxi operations in an organically-evolved paratransit setting. The field study involved an in-depth investigation of minibustaxi operations in Kampala’s paratransit system, and collection and analysis of minibustaxi movement data that was used to validate the agent-based model. The experimen-tal approaches involved three separate simulation experiments, simulating the minibustaxi transportation dynamics with varying levels of agents’ intelligence and situational awareness. Machine learning methods, such as random forests and convolutional neuralnetworks were used to train agents in the subsequent simulation experiment to improvetheir intelligence during decision making. At each stage, several efficiency metrics’ valuessuch as passenger waiting time and minibus taxi occupancy were collected. The resultsfrom the experiments showed that there was an improvement in the overall efficiency ofthe minibus taxi paratransit system. For instance, the average passenger waiting time re-duced from 1.2 hours to 30 minutes, indicating a 55% improvement. Whereas the averageminibus taxi occupancy increased from 42% to 51%, indicating a 21% improvement. Ac-cordingly, we concluded that improving the micro-level agents’ intelligence and situationalawareness, results in an overall increase in the efficiency of the paratransit system.To the transportation researchers, we recommend further work on using ABM toinclude other modes of paratransit transport such as the three-wheeled rickshaws andmotorcycle taxis (boda bodas). To the city authorities, we recommend the integration ofsmart mobility and ICT applications into the paratransit ecosystem to support journeyplanning, booking, scheduling, and fare collection.
- ItemAlternative sphere decoding for finite control set model predictive control of power electronic converters(Stellenbosch : Stellenbosch University, 2021-03) Raath, Johannes Hendrik; Mouton, Toit; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The rapid advance in computational power over the past decade has enabled model predictive control (MPC) to emerge as a promising alternative control technique for fast reacting power electronic applications. Trending in the power electronic system eld is - nite control set MPC, whereby a single multiple-input multiple-output (MIMO) controller can be realised to replace complicated PI control loops and deliver control signals for direct application to the converter. As an extension of basic MPC, long-horizon MPC refers to the evaluation of the optimisation criterion over a prediction horizon of more than one. Long-horizon MPC is desirable for its enhanced closed-loop performance during steadystate operation. Unfortunately, long-horizon MPC requires more computations to nd the optimum control action, especially in FCS-MPC, as the computational burden increases exponentially with an extension of the prediction horizon. The computational burden associated with long-horizon MPC can be reduced through mathematical programming where the underlying MPC problem is re-formulated as an integer least-squares problem. With regard to power electronic systems in particular, a recent adaptation of the Sphere Decoding Algorithm (SDA) has emerged as a very popular choice for solving or decoding of the ILS problem. The main objective of this thesis is to devise an alternative decoding strategy that will solve the ILS problem relating to long-horizon FCS-MPC for multi-level inverters. Exponential space algorithms are investigated and matched to an appropriate power electronic application. From the investigation, a novel preconditioning algorithm or projection algorithm evolved. Characteristics that favour modern processing technology are captured and applied to the well-acclaimed SDA to propose a decoding approach that involves matrix calculations. This also permits the e cient precomputation and storage of system matrices o ine for online use. The proposed algorithms are incorporated into a predictive controller for a power electronic drive and are evaluated in real time.
- ItemAnalysis and evaluation of brush-DC equivalent controlled multiphase cage induction machine drive(Stellenbosch : University of Stellenbosch, 2011-03) Gule, Nkosinathi; Kamper, M. J.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The multiphase induction machine drive has been under investigation for the last half century. Although it offers several attractive advantages over the conventional three-phase induction machine drive, it is restricted to highly specialised applications. One aspect of the multiphase induction machine drive is the complexity of the control algorithm for decoupled flux and torque control. The complexity, arising from the required coordinate transformations, increases with increase in the number of phases of the machine. Recently, a method that allows the control of a six-phase induction machine drive without any coordinate transformations was developed and tested. This new control technique allows the control of the machine to be similar to that of dc machines through the use of special trapezoidal-shaped stator current waveforms. These stator phase current waveforms consist of field (flux) and torque current components, with flat-topped amplitudes allowing a stator phase to act alternately in time as either a flux or a torque producing phase. The idea is to have a number of stator phases acting as flux producing phases, whilst the remaining phases act as torque producing phases at each time instance. This dissertation takes a further step in the research on this particular control technique. As the control method relates directly to the brush-dc machine operation, in this dissertation, the control method is defined as a “brush-dc equivalent” (BDCE) control method. First, in this dissertation, a simple analytical method is developed to determine a defined optimal ratio of the number of field to the number of torque phases of a multiphase induction machine that utilises trapezoidal stator current waveforms. The method is applied to induction machines with up to fifteen stator phases. Finite element analysis is used to verify the validity of the developed criterion and to verify the square-like air gap flux density. Secondly, in this dissertation, an analytical method for predicting and evaluating the rotor bar current waveform of a cage multiphase induction machine is proposed. The method is based on the Fourier transform and the winding function theory under linear condition assumptions. The method also allows for the calculation of the electromagnetic torque and rotor bar losses. Skin effect is considered in the calculation of the rotor bar resistance of the machine. Again, finite element analysis is used to verify the analytically calculated results. The developed method can be expanded and used to evaluate the rotor current waveform of any multiphase induction machine supplied with any stator current waveforms. The BDCE control method is implemented on a prototype nine-phase cage-rotor induction machine drive. A nine-phase inverter and control system are developed for supplying the nine-phase induction machine with the trapezoidal stator current waveforms. Rotor current waveform measurements are taken on a specially designed rotor to verify the analytically predicted waveform. The linear relationship of the developed torque and torque current of the proposed BDCE control method is verified through measurements. Through the comparison of analytical calculated results with finite element calculated and measured results, it is shown in this dissertation that the developed analytical techniques can be used in the design and performance analysis of multiphase induction machines. Also, from the results, it is clear that the new control technique works remarkably well even in the flux weakening region. However, outstanding aspects, such as efficiency and generated torque quality of the proposed drive still need to be investigated further.
- ItemAnalysis and design of conical transmission line power combiners(Stellenbosch : University of Stellenbosch, 2007-12) De Villiers, Dirk I. L.; Meyer, P.; Van der Walt, P. W.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.This dissertation presents a technique for the design of N-way conical line power combiners, which offers significant advantages over existing axially symmetric combining techniques. A full analytical study of conical transmission lines is done, and evaluated results are presented. These include a proof of the cutoff frequency equation, and plots of the field patterns, of higher order modes which are unavailable in literature. A coaxial fed conical line combiner for 10 inputs is proposed, designed and evaluated. The design technique relies on the uniform transmission line characteristics of the conical lines to eliminate the need for complex full wave optimisation, typically needed in the design of the more commonly used radial line combiners. Circuit models are instead employed to achieve a wide matched bandwidth by using optimised stepped impedance coaxial lines to feed the combining structure. The prototype developed at X-band displays more than an octave bandwidth with a return loss of better than -14.5 dB. Using tapered line matching sections increases the power handling capability of the combiner by eliminating sharp edges, and allows for tolerance insensitive manufacture of the structure by widening conductor spacings. Such a 10-way prototype is developed at X-band which displays a -18.7 dB return loss bandwidth of 47% with very low losses. A study is done to determine the limitations on the design of general N-way combiners, and the results are incorporated into the design technique. The full process is demonstrated by the design and simulation of a 30-way combiner at Ku-band which displays a simulated -20 dB return loss bandwidth of 34%. The design technique is simple to execute and requires very little full wave analysis. Results obtained with the manufactured combiners are better than those of any previously published axially symmetric combiners.
- ItemAnalysis and design optimisation of grid-connected wound-rotor synchronous and induction motors(Stellenbosch : Stellenbosch University, 2019-12) Mabhula, Mkhululi; Kamper, M. J.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The dissertation presents new approaches in the analysis, design and optimisation of three-phase cylindrical wound-rotor synchronous (WRSM) and induction (WRIM) motor types. The new approaches are based on the analysis of the steady-state finite element method (FEM) in which the parameters and performances of the motor types are predicted. Absolute optimum designed WRSM and WRIM motor types can at best be obtained by the use of improved parameters and performance calculation methods in the design optimisation process. This dissertation proposes simple, fast and accurate parameter and performance calculation modern approaches for grid-connected WRSM and WRIM motor types using two-axes electromagnetic models. The modern approaches are based on non-classical models in which iterative processes are utilised using static FEM in conjunction with the motor types phasor diagrams. During the iterative processes, the electric circuit and magnetic field equations are coupled to access important parameters of the motor types. The access of parameters using iterative processes solves the excitation currents at different operating points of the motor. The parameters are accurately calculated using a FEM based method of freezing the motor core permeance. The dissertation investigates and expands the use of these parameters as an effective and powerful tool for explaining the motor behaviour under different operating points. A great deal of attention is also given to the design optimisation of the motor types during which a relatively efficient optimisation procedure is shown. In this study, the proposed model approaches are verified by employing a commercial FEM ANSYS-Maxwell and experimental tests in the laboratory. The study also gives the model approach and the design optimisation procedure of a 6 MW slip-ring motor used for milling purposes. In the analysis of this, the skin effects are perfectly fitted in the proposed model approaches The study presents a successful application of the proposed model approaches to the parameters and performance prediction and the design optimisation of the grid-connected WRSM and WRIM motor types. The proposed model approaches demonstrate accuracy, simplicity and fastness which are some of the key aspects in the electrical machine design.
- ItemAnalysis and performance evaluation of large-scale wound rotor synchronous wind generator with non-overlap winding technology(Stellenbosch : Stellenbosch University, 2021-12) Garner, Karen; Kamper, Maarten; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: This thesis covers the analysis, performance evaluation and design optimisation of a wound rotor synchronous generator (WRSG) for medium-speed wind generator drive train applications. Permanent magnet synchronous generators (PMSG) have been the preferred choice because of their high performance and low maintenance costs. The high market price and fluctuating availability of rare-earth permanent magnets have become drivers for alternative wind generator designs. WRSGs are rare-earth free, which lowers the material costs. WRSGs have a flux variation capability that allows for reactive power control without the need of a solid-state converter when direct electrical grid connection is considered. This research adopts the use of non-overlap coils in the stator winding of the WRSG and the use of a phase-shift technique between the coil currents to reduce certain harmful harmonics. Non-overlap windings are selected due to the associated lower manufacturing cost and lower copper losses compared to conventional overlap wind-ings. The study applies the phase-shifting technique to a 375 r/min wound rotor synchronous generator with a 16/18 pole/slot combination. The objective is to find an optimum design for possible direct grid connection, complying with key South African grid requirements, that is an attractive non-permanent magnet option. Various winding designs for synchronous generators are analysed theoretically and finite element analysis is used in the modelling and design optimisation of the generator. The focus is on (i) methods to minimise the losses in non-overlap windings of WRSGs and (ii) to compare the performance of these generators with that of conventional synchronous generators. The phase-shifting technique is shown to reduce sub- and higher-order harmonics in the MMF harmonic spectrum. The application also improves the working harmonic of the 16/18 pole/slot combination that is noted in the lowered torque ripple and improved machine performance of the 3 MW model. However, the rotor core losses are slightly increased after the application. The effect of the rotor yoke design on the rotor core losses is investigated. Placing flux barriers in the inner core of a large rotor yoke largely reduces the rotor core losses without affecting the machine performance negatively. The intent of the study is to test at a 3 kW power level but also to determine the design up to 3 MW. The conducted design optimisation results in a WRSG with a lower torque ripple and improved efficiency. The optimised 3 MW 16/18 WRSG with the phase-shifted non-overlap winding fairs competitively against the 3 MW 16/72 WRSG in terms of active mass, power factor, efficiency and torque ripple. However, the per unit synchronous reactance of the 3 MW 16/18 WRSG design of this study is considered too high for direct grid connection. This can be overcome by increasing the air gap length, subject to lowering the power density. To validate the 2D-FEA predictions, the 3 kW prototype is manufactured and tested. Based on the practical measurements, the theoretical results of design aspects are confirmed. A consolidated conclusion and further recommendations for the future of the design conclude the work in this study.
- ItemThe analysis and quantifiaction of uncertainty for least life-cost electrical low voltage distribution design(Stellenbosch : Stellenbosch University, 2000-12) Heunis, Schalk W. (Schalk Willem); Herman, R.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: The purpose of this dissertation is to provide methods for designing and managing low voltage residential feeders. These methods can be applied to the problem of planning residential networks under uncertainty while ensuring least life-cycle costs. By analysing collected load data from various communities in South Africa, a new probabilistic model for representing the load uncertainty of residential consumers was derived. This model uses the beta probability distribution to describe individual consumer loads over a period of time. Methods for combining the loads in linear combinations were used to derive a new probabilistic voltage regulation calculation procedure. This new method is different from previously developed voltage calculation methods in that it can be used to estimate the probable voltage performance of a feeder over a period of time. A simplification of the method is proposed which allows it to be implemented in any commercial spreadsheet program. The new probabilistic load model was also applied to the problem of calculating resistive losses in residential low voltage feeders. A new probabilistic method was formulated and this method can be used to estimate the probable range of resistive loss in a feeder for a period of time. This method is simple enough to implement in a commercial spreadsheet program. Probabilistic information about network and load parameter uncertainty is seldom available and these uncertainties are best modelled using fuzzy numbers. The probabilistic calculation methods cannot represent these uncertainties and only after applying a fuzzy-probabilistic approach can both types of uncertainties be used. This is a significant enhancement to the current methods and ensures that the uncertainty about the calculated results is realistically represented. The specification of load parameters for the methods was significantly simplified following a regression analysis of collected load data from South African communities. By specifying the distribution of the consumption of individual consumers in a community, the other load parameters can be estimated using a set of fitted linear regression equations. This greatly reduces the burden of specifying the load parameters and makes it possible for the proposed calculation methods to be applied to the design of new feeders in practice. The distribution of the consumption of individual consumers can be specified using the average and the standard deviation of the consumptions of individual consumers. Accurate estimates of these parameters can be obtained from sales information and can be used to manage existing networks effectively. Using the sales information with the proposed methods enables more cost-effective upgrades of existing feeders low voltage feeders. The identification of potential problems in existing low voltage networks is also possible if the layout of the feeders in a community is known. The use of the proposed methods is illustrated in step-by-step fashion. Typical input parameters are used and all the required calculations with intermediate results are presented.
- ItemAnalysis and synthesis algorithms for the electric screen Jauman electromagnetic wave absorber(Stellenbosch : Stellenbosch University, 1993-11) Du Toit, Leendert Johannes; Cloete, J. H.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic EngineeringENGLISH ABSTRACT: An extensive literature study revealed numerous Jauman absorber examples with reasonable absorption properties. Unfortunately, tractable and detailed design techniques were found to be scarce, and often only applicable to absorbers with two or three layers. The research described in this report was therefore aimed at, and culminated in, general design methods for multilayered electric screen J auman absorbers. As a starting point, the synthesis problem is formulated by idealizing the spacers (assumed lossless and commensurate) and resistive sheets (assumed to have zero thickness), and by considering the absorption of a normally incident plane wave. An equivalent circuit model is derived, using the analogy between plane waves in stratified media, and guided waves in TEM transmission lines. The network is analyzed using Richard's frequency surrogate, S = tanh(s = cr +jw), and concise equations and algorithms are presented for symbolic and numerical analysis. Maximum bandwidth synthesis of the classic one-layer absorber, or Salisbury screen, proved to be simple, clearly illustrates the analytic approach, and apparently has not been published before. The two-layer absorber was also found to be algebraically simple enough to be synthesized in closed form, is dealt with comprehensively, and the treatment consolidates and formalizes many of the design techniques available in the literature. Networks comprising commensurate transmission lines and conductances have been investigated by Richardsl , but unfortunately the topology-driven realizability constraints on the input impedance of the Jauman network is only dealt with briefly. Fruitless investigations by the author showed this to be a formidable problem, and as a result the research concentrated on tractable and iterative synthesis algorithms for multilayered absorbers, instead of formal filter synthesis techniques. These algorithms may be summarized as follows: • A key concept in the multilayer zero-placement synthesis methods that will be presented, is the ability to physically realize a given set of reflection coefficientzeros. This involves solving a set of highly non-linear equations, and a gradientmethod iterative algorithm has been developed to achieve this . • The first application of the aforementioned algorithm is to synthesize all reflection zeros at S →∞, thereby obtaining a maximally flat reflection coefficient magnitude response. Stable and rapid convergence was found for up to at least 20 layers, thereby extending the two- and three-layer algebraic solutions available in the literature. It was found that a stringent restriction exists on the maximum dielectric constant (Er) of the spacers, thereby limiting the practical implementation of these solutions . • Through judicious manipulation of reflection zeros at distinct physical frequencies, an equiripple absorption response may be obtained. An elegant algorithm is presented to facilitate this, and it was found that these solutions represent substantial improvements over examples available in the literature. Restrictions still apply to the spacer Er, but these are more relaxed and practical equiripple absorbers are possible. In addition, the spread in sheet resistivities is much smaller than in comparable maximally flat solutions. • Numerical searches indicated that the aforementioned equiripple responses are very close to, but not absolutely optimal, in the sense of maximum bandwidth. The small bandwidth and/or absorption improvements that were found were almost negligible from a practical viewpoint, but the optimal synthesis problem is academically very important. Through use of the general Chebyshev approximation method, an algorithm is developed which finds the local optimal response in the vicinity of such a parent equiripple solution. Although it might be tempting to classify the algorithm as a brute force method, it will be shown that this is not the case, and that its solutions provide the answer to the fundamental and unsolved optimal design problem. These algorithms have been implemented, and tables of resistive sheet values are presented for N up to 8, a range of Er values corresponding to low loss foams, and for various absorption levels.
- ItemAnalysis and synthesis of a 2 MVA series-stacked power-quality conditioner(Stellenbosch : Stellenbosch University, 1999-12) Mouton, Hendrik Du Toit; Enslin, J. H. R.; Akagi, H.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical & Electronic Engineering.ENGLISH SUMMARY: This thesis describes the development of a power electronic converter for a 2 MVA series-injection power-quality device. The converter is designed to interface directly with superconducting magnetic energy storage devices and operates at a nominal DC-bus voltage of 2.4 kV. In the first part of the thesis the viability of soft-switching for application to the 2 MVA converter is investigated. A new resonant turn-off snubber topology is introduced and a detailed theoretical study of the converter and snubber switching losses is carried out. An optimal snubber design procedure is derived. This is followed by a theoretical investigation of the effects of the different parasitic components on the snubber operation. In the final part of the investigation, a turn-on snubber is added to the turn-off snubber topology. An experimental evaluation of both the turn-off and combined turn-on and turn-off snubbers is carried out. In order to obtain a DC-bus voltage of 2.4 kV, a series-stacked converter topology, for use in the 2 MVA series-injection device, is investigated. A detailed theoretical analysis of the DC-bus balancing mechanisms is conducted. This theoretical analysis makes use of fundamental results from the theory of systems of linear differential equations; in particular of Floquet theory. In the final part of the thesis an experimental 700 kVA series-stacked phase-arm, operating at a 2.4 kV DC-bus voltage, is constructed. The operation of this converter is verified through a range of experiments and the measured results are compared with the theoretical predictions.
- ItemAnalysis and synthesis of an inductive storage millisecond pulse forming network(Stellenbosch : Stellenbosch University, 2001-12) Van der Merwe, Julian Barend; Mouton, H. du T.; Pretorius, J. H.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: Millisecond pulse forming networks (PFNs) as applicable to electro-thermal chemical (ETC) loads fall into the <1 MJ energy bracket. The energy requirements of these loads require specialised power sources involving staged energy conversion. For the <1MJ energy bracket, capacitive storage systems are usually employed. However, these systems exhibit low volume energy density and for volume sensitive applications; alternatives need to be considered. Inductive storage supplies form a sub-group of the static supplies that have theoretically superior volume energy density characteristics. This thesis documents the execution of a project concerned with the volumeoptimisation of an inductive storage supply. The system is composed of a three stage energy conversion chain. A prime power source (low power) charges an intermediate storage (IS) which is characterised by its medium power delivery capabilities. Energy is then transferred from the IS to the storage inductor which is characterised by its high power delivery capabilities. When sufficiently charged, the energy is then transferred to the load. Where pulse forming is required, the inductor storage must necessarily be modular. Switching elements to control the energy flow are also required. Work performed at Soreq, Israel, is used as the starting point. A topology variation of the XRAM topology presented by Soreq, original to this thesis, is presented and all its functioning modes are analysed. An existing volume model is analysed and expanded to incorporate heretofore unmodelled yet non-negligible considerations. The volume model generalises the effect of system modularity, subsystem technologies and allows for the incorporation of practical construction issues into the design process. The aim is to develop a 500 kJ, 80 kA, 20 kV system with a volume not exceeding lm3. This volume must include the IS, storage inductor and full switch volume. The optimisation algorithm and system topology developed in this thesis is validated through the construction and testing of a 1.2 kA, 2.5 kV 4 module prototype. A potential full ratings system, composed of contemporary device technologies and exhibiting a volume of just over 0.8m3, is proposed.
- ItemAnalysis of a radial flux-air-cored permanent magnet machine with a double-sided rotor and non overlapping windings(Stellenbosch : Stellenbosch University, 2012-03) Randewijk, Peter-Jan; Kamper, M. J.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.ENGLISH ABSTRACT: In this dissertation a new type of electrical machine, a Radial Flux Air-Cored Permanent Magnet machine with a Double-sided Rotor and utilising concentrated, non-overlapping windings, is proposed. The concept of the Double-sided Rotor Radial Flux Air-Cored Permanent Magnet machine, or RFAPM machine for short, was derived from the Double-sided Rotor Axial Flux Air-Cored Permanent Magnet (AFAPM) machine. One of the problems that AFAPM machines experience, is the deflection of the rotor discs due to the strong magnetic pull of the permanent magnets, especially with double-sided rotor machines. The main advantage of a RFAPM machine over a AFAPM machine is that the rotor back-iron is cylindrically shaped instead of disk shaped. Due to the structural integrity of a cylinder, the attraction force between the two rotors does not come into play any more. The focus of this dissertation is on a thorough analytical analysis of the Double-Sided Rotor RFAPM machine. With the RFAPM being an air-cored machine, the feasibility to develop a linear, analytical model, to accurately predict the radial flux-density and hence the induced EMF in the stator windings, as well as the accurate calculation of the developed torque of the machine, needed to be investigated. The need for a thorough analytical examination of the Double-Sided Rotor RFAPM machine stemmed from the need to reduce the blind reliance on Finite Element Modelling (FEM) software to calculate the back-EMF and torque produced by these machines. Another problem experienced with the FEM software was to obtain accurate torque results. Excessive ripple torque oscillations were sometimes experienced which took a considerable amount of time to minimise with constant refinement to the meshing of the machine parts. Reduction in the mesh element size unfortunately also added to the simulation time. The requirement for an accurate analytical model of the RFAPM machine was also necessary in order to reduce the amount of time spent on successive FEM simulation to obtain the optimum pole arc width of the permanent magnet in order to minimise the harmonic content of the radial flux-density distribution in the the stator windings. In this dissertation, the use of single-layer and double-layer, non-overlapping, concentrated winding for the RFAPM machine is also investigated. It was decided to include a comparison of these two non-overlapping winding configurations with a “hypothetical” concentrated, overlapping winding configuration. This would allow us to gauge the effectiveness of using nonoverlapping winding with respect to the reduction in copper losses as well as in the reduction in copper volume. It would also allow us to investigate the extent of how much the developed torque is affected by using non-overlapping windings instead of overlapping windings.
- ItemAnalysis of interior permanent magnet motors with non-overlapping windings(Stellenbosch : University of Stellenbosch, 2009-03) Germishuizen, Johannes Jacobus; Kamper, M. J.; Jockel, A.; University of Stellenbosch. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.