Browsing by Author "Mouton, Hennie"

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    Development of a fatigue tester and material model for flexible heart valve applications
    (2019) Van Breda, Braden; Mouton, Hennie; Bezuidenhout, Deon
    The leaflet material in heart valve prostheses is required to be both flexible and durable to eliminate the need for chronic anticoagulation medication and accommodate younger patients with longer life expectancies. This investigation aims to provide two of the necessary tools to design and test suitably flexible and durable materials for heart valve replacement. These tools address the question of how to model the stress-strain behaviour of polymer networks and thermoplastic polyurethanes in particular, as well as how to practically evaluate the durability of the proposed material. A model for polyurethane stress-strain behaviour is proposed, whereby the number of monomers between crosslinks is suggested to evolve with macroscopic strain. Following the polymer chain entanglement theory, the increase in the number of monomers between crosslinks is further extended to be a function of strain rate, incorporating the viscous effect observed in polyurethanes. A multistation, micro-tensile specimen fatigue tester was developed to evaluate material durability. The proposed equilibrium polyurethane model accurately predicts the experimental data across the full material strain range. The proposed model extension sufficiently captures the rate dependence of polyurethane, however, fails to account for the raised specimen temperatures at high strain rates. The developed fatigue tester is verified to successfully feature selectable variables including test frequency (1 - 20 Hz), amplitude (1 - 6 mm), waveform (Triangular, Sinusoidal, Square and Custom) and environmental temperature control (23 - 50 oC). Less than 10% error in measured force is observed when compared to a commercial tensile tester. The proposed model successfully provides a platform to aid the design of flexible materials suitable for heart valve leaflets. The developed fatigue tester enables the assessment of material durability across a range of test conditions, successfully providing a tool for leaflet material durability analysis and verification.
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    Guidance, navigation and control of a small, unmanned blended wing body aircraft
    (2020) van Wyk, David; Mouton, Hennie
    The purpose of this research is to document the design and optimisation of a full suite of guidance, navigation and control (GNC) algorithms for a small unmanned aerial vehicle (UAV), the Skywalker X8. This was performed so as to fill a void in the available literature on the selected airframe, which currently only focuses on aspects such as aerodynamic modelling, advanced controller design, or uses of the airframe to perform higher level tasks. All of these research areas make use of off-the-shelf flight controllers, but these are not always the most appropriate foundations for more advanced work as they are inherently sluggish so as to be broadly applicable to a variety of airframes. Subsequently, the Skywalker X8 airframe was modelled, using existing literature, and then characterised so as to establish what the goals might be for an optimal set of controllers. An autopilot was then designed which was optimised so as to be as close to the identified optimal performance characteristics as possible, with effort being put into ensuring that all non-linearities and disturbances were taken into account. This included advanced modelling of sensors, actuators, the environment, and the system itself. The autopilot design was then extended with a set of guidance and navigation algorithms, also developed as part of this research. This consisted of both path planning and path following algorithms which allowed for the synthesis of general classes of paths useful to the application. With both the autopilot and guidance laws developed, the system could be tested under several atmospheric flight conditions. These took the form of various wind directions and intensity levels being applied to the airframe whilst transitioning between a range of different waypoint configurations. The system was subsequently shown to be able to follow a set of waypoints very accurately, even with winds and turbulence with magnitudes of in excess of 60% of the aircraft's nominal airspeed. With a strong autopilot designed and illustrated in a high fidelity simulation environment, this work can now easily be extended into many fields. All of the tools used for this research are available and well documented, and the processes followed repeatable with all justification available in the text. As such, should a project which aims to extend this work wish to adjust the autopilot design or guidance laws, based on different requirements, this is easily accomplished and recommendations of starting points are provided. The system model and autopilot are also made available and are usable exactly as they are should one wish to undertake additional research which does not aim to modify, but to extend this work.
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    Temperature reconstruction and acoustic time of flight determination for boiler furnace exit temperature measurement
    (2018) Raikes, Geoff; Mouton, Hennie; Fuls, Wim
    The furnace exit gas temperature (FEGT) is one of the fundamental parameters necessary to determine the energy balance of the boiler in a coal-fired power plant, and is thus beneficial to the production of reliable thermo-fluid models of its operation and the operation of the systems down and upstream. The continuous measurement of the FEGT would also be a useful indicator to predict, prevent and diagnose faults, optimize boiler operation and aid the design of heat transfer surfaces. Acoustic pyrometry, a technique that measures temperature based on the travel time of an acoustic wave in a gas, is investigated as a viable solution for continuous direct measurement of the FEGT. This study focuses specifically on using acoustic pyrometry to reconstruct the temperature profile at the furnace exit including methods for accurately determining the time of flight (TOF) of acoustic waves. An improved reconstruction technique using radial basis functions (RBF) for interpolation and a least squares algorithm is simulated and its performance was compared to cubic spline interpolation, regression and Lagrange interpolation by evaluating its reconstruction accuracy in terms of mean and root-mean-squared (RMS) error when reconstructing set temperature profiles. Various parameters including transceiver positions, grid divisions and time of flight error, are investigated in terms of how they inform acoustic pyrometry implementation. The improved RBF interpolation function managed to reconstruct complex temperature profiles and had a greater reconstruction accuracy than compared interpolation methods, improving on the accuracy of previous work done. Random acoustic path error was found to not be additive with reconstruction error however repeating acoustic TOF readings improved reconstruction accuracy to mitigate this effect. In general, it was also found that symmetrical transmitter/receiver positions produced more accurate reconstructions as well as positioning receivers/transceivers and grid lines closer to the furnace walls, where the greatest temperature change occurs. In addition to testing reconstruction methods, a low-cost experimental set-up was constructed to measure the time of flight. The focus of this study was on using various signal processing methods to determine the time of flight and evaluating their accuracy in the presence of noise. Methods such as threshold detection with bandpass filtering, cross correlation, generalized cross-correlation (GCC) and a new method developed employing variable notch filters with locations and widths based on repetitive frequencies identified in the noise with cross correlation. The performance of methods was experimentally tested under varying signal to noise ratios (SNR) and noise conditions. These SNR tests showed that cross-correlation methods produced more reliable TOF readings under lower SNRs than threshold detection methods. Under white noise the smooth coherent transform (SCOT) GCC variation proved to produce the most accurate results producing an average TOF error of 0.84 % up until a SNR of 1.4 before reducing in accuracy. In coloured noise (generated based on previous boiler recordings) the variable notch filter method with crosscorrelation was able to identify repetitive noise frequencies filter them out and ultimately produced results with an average TOF error of 1.99 % up until a SNR of 0.67, where the noise level exceeds that of the signal.