A comparative study of artificial neural networks and physics models as simulators in evolutionary robotics
- Pretorius, Christiaan Johannes
- Authors: Pretorius, Christiaan Johannes
- Date: 2019
- Subjects: Neural networks (Computer science)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/30789 , vital:31131
- Description: The Evolutionary Robotics (ER) process is a technique that applies evolutionary optimization algorithms to the task of automatically developing, or evolving, robotic control programs. These control programs, or simply controllers, are evolved in order to allow a robot to perform a required task. During the ER process, use is often made of robotic simulators to evaluate the performance of candidate controllers that are produced in the course of the controller evolution process. Such simulators accelerate and otherwise simplify the controller evolution process, as opposed to the more arduous process of evaluating controllers in the real world without use of simulation. To date, the vast majority of simulators that have been applied in ER are physics- based models which are constructed by taking into account the underlying physics governing the operation of the robotic system in question. An alternative approach to simulator implementation in ER is the usage of Artificial Neural Networks (ANNs) as simulators in the ER process. Such simulators are referred to as Simulator Neural Networks (SNNs). Previous studies have indicated that SNNs can successfully be used as an alter- native to physics-based simulators in the ER process on various robotic platforms. At the commencement of the current study it was not, however, known how this relatively new method of simulation would compare to traditional physics-based simulation approaches in ER. The study presented in this thesis thus endeavoured to quantitatively compare SNNs and physics-based models as simulators in the ER process. In order to con- duct this comparative study, both SNNs and physics simulators were constructed for the modelling of three different robotic platforms: a differentially-steered robot, a wheeled inverted pendulum robot and a hexapod robot. Each of these two types of simulation was then used in simulation-based evolution processes to evolve con- trollers for each robotic platform. During these controller evolution processes, the SNNs and physics models were compared in terms of their accuracy in making pre- dictions of robotic behaviour, their computational efficiency in arriving at these predictions, the human effort required to construct each simulator and, most im- portantly, the real-world performance of controllers evolved by making use of each simulator. The results obtained in this study illustrated experimentally that SNNs were, in the majority of cases, able to make more accurate predictions than the physics- based models and these SNNs were arguably simpler to construct than the physics simulators. Additionally, SNNs were also shown to be a computationally efficient alternative to physics-based simulators in ER and, again in the majority of cases, these SNNs were able to produce controllers which outperformed those evolved in the physics-based simulators, when these controllers were uploaded to the real-world robots. The results of this thesis thus suggest that SNNs are a viable alternative to more commonly-used physics simulators in ER and further investigation of the potential of this simulation technique appears warranted.
- Full Text:
- Date Issued: 2019
- Authors: Pretorius, Christiaan Johannes
- Date: 2019
- Subjects: Neural networks (Computer science)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/30789 , vital:31131
- Description: The Evolutionary Robotics (ER) process is a technique that applies evolutionary optimization algorithms to the task of automatically developing, or evolving, robotic control programs. These control programs, or simply controllers, are evolved in order to allow a robot to perform a required task. During the ER process, use is often made of robotic simulators to evaluate the performance of candidate controllers that are produced in the course of the controller evolution process. Such simulators accelerate and otherwise simplify the controller evolution process, as opposed to the more arduous process of evaluating controllers in the real world without use of simulation. To date, the vast majority of simulators that have been applied in ER are physics- based models which are constructed by taking into account the underlying physics governing the operation of the robotic system in question. An alternative approach to simulator implementation in ER is the usage of Artificial Neural Networks (ANNs) as simulators in the ER process. Such simulators are referred to as Simulator Neural Networks (SNNs). Previous studies have indicated that SNNs can successfully be used as an alter- native to physics-based simulators in the ER process on various robotic platforms. At the commencement of the current study it was not, however, known how this relatively new method of simulation would compare to traditional physics-based simulation approaches in ER. The study presented in this thesis thus endeavoured to quantitatively compare SNNs and physics-based models as simulators in the ER process. In order to con- duct this comparative study, both SNNs and physics simulators were constructed for the modelling of three different robotic platforms: a differentially-steered robot, a wheeled inverted pendulum robot and a hexapod robot. Each of these two types of simulation was then used in simulation-based evolution processes to evolve con- trollers for each robotic platform. During these controller evolution processes, the SNNs and physics models were compared in terms of their accuracy in making pre- dictions of robotic behaviour, their computational efficiency in arriving at these predictions, the human effort required to construct each simulator and, most im- portantly, the real-world performance of controllers evolved by making use of each simulator. The results obtained in this study illustrated experimentally that SNNs were, in the majority of cases, able to make more accurate predictions than the physics- based models and these SNNs were arguably simpler to construct than the physics simulators. Additionally, SNNs were also shown to be a computationally efficient alternative to physics-based simulators in ER and, again in the majority of cases, these SNNs were able to produce controllers which outperformed those evolved in the physics-based simulators, when these controllers were uploaded to the real-world robots. The results of this thesis thus suggest that SNNs are a viable alternative to more commonly-used physics simulators in ER and further investigation of the potential of this simulation technique appears warranted.
- Full Text:
- Date Issued: 2019
A gatherer’s paradise? early humans and plant foraging on the Cape south coast, South Africa
- Authors: Botha, Maria Susan
- Date: 2019
- Subjects: Plant remains (Archaeology) -- South Africa , Plant physiology Plant ecology Botany -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/37139 , vital:34124
- Description: Humans were driven to refugia during the cold and dry glacial Marine Isotope Stage 6 (MIS6) (~195–125 ka); only a few places, including the Cape south coast, show archaeological evidence of continuous human occupation. It has been hypothesised that the Cape south coast provided the requisite shelter to ensure human’s earliest survival because it provided all the right ingredients. The shores offer abundant shellfish, the land a diverse array of plants, and the climate was ameliorated due to the proximity of the then exposed Palaeo-Agulhas Plain to the warm Agulhas current. The aim of this study is to determine whether the indigenous flora of this region could have provided a sufficient edible resource for early humans. Residents of the Cape south coast have genetic ancestry linking them to the Khoe-San, the original inhabitants of the area, and still have an extensive knowledge of the local edible plants. With their help, I set out to determine a) whether humans have been utilising the same plant species over time, b) what the foraging potential of the edible plants in the region is and c) how resilient these plants are to human foraging. If we know whether the plant species known and used today were the same as those that were utilised by past humans, we can then use the contemporary knowledge to make predictions about past utilisation. To answer this question, I collated two databases: archaeological (all plant species found in archaeological sites [dating 0 to 80,000 BP]) and contemporary (all plant species in the modern-day ethnographic literature [last 400 years]) that occur within the Greater Cape Floristic Region(GCFR). I found a significant number of plant species shared between the two databases, which suggests that at least some plant species have been used by humans over a long period of time. To determine the indigenous plant foraging potential of the region, I foraged for food in the Cape south coast (451 bouts) monthly over a two-year period with the help of local inhabitants. The findings show that edible plant resources are distributed patchily and focusing on specific vegetation types would greatly enhance chances of harvesting 2,000 kcal per day, which is considered the daily calorific requirements for a typical hunter-gatherer. I then sought to understand how resilient these plants [(with an emphasis on plants with an underground storage organ (USO)] would be to human foraging. To do this, I set out plots and harvested all edible foods for three consecutive years with the help of foragers. Results indicate that there was a significant reduction in edible weight only in the third year of consecutive harvesting. In conclusion, using various approaches, this study investigates the plant food potential of the Cape south coast from the perspective of early human consumers. The findings suggest that knowledge regarding useful plants dates back to at least 80,000 BP. Food resources are patchily distributed across the main vegetation types found within the Cape south coast and occur in hotspots, i.e. concentrated areas hosting high densities of edible plant foods, surrounded by areas with very low plant food densities. Foragers could have harvested their daily calorific quota more easily if they focused their harvesting efforts in specific vegetation types found in the Cape south coast. Furthermore, many USOs circumvent climatic fluctuations, herbivory or both by staggering their emergence over multiple years, which implies they have some resilience to human foraging.
- Full Text:
- Date Issued: 2019
- Authors: Botha, Maria Susan
- Date: 2019
- Subjects: Plant remains (Archaeology) -- South Africa , Plant physiology Plant ecology Botany -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/37139 , vital:34124
- Description: Humans were driven to refugia during the cold and dry glacial Marine Isotope Stage 6 (MIS6) (~195–125 ka); only a few places, including the Cape south coast, show archaeological evidence of continuous human occupation. It has been hypothesised that the Cape south coast provided the requisite shelter to ensure human’s earliest survival because it provided all the right ingredients. The shores offer abundant shellfish, the land a diverse array of plants, and the climate was ameliorated due to the proximity of the then exposed Palaeo-Agulhas Plain to the warm Agulhas current. The aim of this study is to determine whether the indigenous flora of this region could have provided a sufficient edible resource for early humans. Residents of the Cape south coast have genetic ancestry linking them to the Khoe-San, the original inhabitants of the area, and still have an extensive knowledge of the local edible plants. With their help, I set out to determine a) whether humans have been utilising the same plant species over time, b) what the foraging potential of the edible plants in the region is and c) how resilient these plants are to human foraging. If we know whether the plant species known and used today were the same as those that were utilised by past humans, we can then use the contemporary knowledge to make predictions about past utilisation. To answer this question, I collated two databases: archaeological (all plant species found in archaeological sites [dating 0 to 80,000 BP]) and contemporary (all plant species in the modern-day ethnographic literature [last 400 years]) that occur within the Greater Cape Floristic Region(GCFR). I found a significant number of plant species shared between the two databases, which suggests that at least some plant species have been used by humans over a long period of time. To determine the indigenous plant foraging potential of the region, I foraged for food in the Cape south coast (451 bouts) monthly over a two-year period with the help of local inhabitants. The findings show that edible plant resources are distributed patchily and focusing on specific vegetation types would greatly enhance chances of harvesting 2,000 kcal per day, which is considered the daily calorific requirements for a typical hunter-gatherer. I then sought to understand how resilient these plants [(with an emphasis on plants with an underground storage organ (USO)] would be to human foraging. To do this, I set out plots and harvested all edible foods for three consecutive years with the help of foragers. Results indicate that there was a significant reduction in edible weight only in the third year of consecutive harvesting. In conclusion, using various approaches, this study investigates the plant food potential of the Cape south coast from the perspective of early human consumers. The findings suggest that knowledge regarding useful plants dates back to at least 80,000 BP. Food resources are patchily distributed across the main vegetation types found within the Cape south coast and occur in hotspots, i.e. concentrated areas hosting high densities of edible plant foods, surrounded by areas with very low plant food densities. Foragers could have harvested their daily calorific quota more easily if they focused their harvesting efforts in specific vegetation types found in the Cape south coast. Furthermore, many USOs circumvent climatic fluctuations, herbivory or both by staggering their emergence over multiple years, which implies they have some resilience to human foraging.
- Full Text:
- Date Issued: 2019
An evaluation and comparison of the plasticiser effects of p -Menthane-3,8-diol-citronellal acetal plasticiser with triethyl citrate and bis(2-ethylhexyl) terephthalate on ethylcellulose films.
- Authors: Marx, Johannes Scheepers
- Date: 2019
- Subjects: Plasticisers
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44629 , vital:38164
- Description: In recent years, much attention has been focused on research to replace petroleumbased commodity plastics, in a cost-effective manner, with biodegradable materials offering competitive mechanical properties. The use of natural and/or biodegradable plasticisers, with low toxicity and good compatibility with several plastics, resins, rubber and elastomers in substitution of conventional plasticisers, such as phthalates and other synthetic plasticisers attract the market along with the increasing worldwide trend towards the use of bio-polymers. The main research objective entailed the behaviour of the novel bio-plasticiser PMDcitronellal acetal (PMD-CA) and its comparison with other commercial bio-plasticiser, triethyl citrate (TEC) and a non-phthalate plasticiser bis(2-ethylhexyl) terephthalate (DOTP), on ethylcellulose (EC) films to evaluate whether it would be feasible to replace them with PMD-CA. The films were produced by a casting technique and were characterised by mechanical properties, thermoanalysis, Scanning Electron Microscopy, disintegration and Fourier-Transform Infrared (FT-IR) spectroscopy. The addition of any of the three plasticisers to EC film increased the flexibility of the films. The plasticisers PMD-CA and DOTP added to EC formed tougher films as compared to EC films containing TEC shown by the high % elongation at break, strain and displacement. Statistically, it was proved that PMD-CA and DOTP had a more significant effect on % elongation of EC films than TEC, postulated to be due to their similar molecular size and structure. Thermoanalytical studies conducted revealed that the thermal degradation process of EC films proceeded in three basic stages, namely dehydration, depolymerisation and pyrolytic decomposition. Thermoanalytical analysis showed that unplasticised EC films had a higher glass transition value compared to the films containing any of the plasticisers and that PMD-CA and DOTP are more effective plasticisers compared to TEC due to their lower glass transition temperatures. The presence of plasticisers in the polymer matrix did change the surface morphology of polymer films. The addition of hydroxypropyl methylcellulose (HPMC) to EC PMD-CA and EC DOTP films caused more significant globule-like bubbles which were not uniform in size on the surface of these films. Observations were made that the type of plasticiser coating with or without pore former influenced the disintegration properties of films. HPMC, as pore former, played a significant role in the drug release system matrix. EC PMD-CA and EC DOTP polymeric films controlled the disintegration of the films in the acidic and weakly acidic medium. EC and EC TEC films had less significant control on the disintegration rate of the acidic and weakly acidic disintegration media. FT-IR characterisation of the films revealed comparable peaks observed in all the polymer-plasticiser combinations and therefore it was concluded that chemical interactions did not exist between the polymers and plasticisers. Added excipients (plasticisers and pore formers) were found to be compatible with EC films. It was concluded within the scope of this research that EC films plasticised with PMDCA, DOTP or TEC outperformed film properties of those compared to unplasticised EC film. The plasticising properties of the novel bio-plasticiser PMD-CA are similar compared to DOTP, and superior to triethyl citrate and could therefore be considered as an effective bioplasticiser for enteric coating formulations in the pharmaceutical industry.
- Full Text:
- Date Issued: 2019
- Authors: Marx, Johannes Scheepers
- Date: 2019
- Subjects: Plasticisers
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44629 , vital:38164
- Description: In recent years, much attention has been focused on research to replace petroleumbased commodity plastics, in a cost-effective manner, with biodegradable materials offering competitive mechanical properties. The use of natural and/or biodegradable plasticisers, with low toxicity and good compatibility with several plastics, resins, rubber and elastomers in substitution of conventional plasticisers, such as phthalates and other synthetic plasticisers attract the market along with the increasing worldwide trend towards the use of bio-polymers. The main research objective entailed the behaviour of the novel bio-plasticiser PMDcitronellal acetal (PMD-CA) and its comparison with other commercial bio-plasticiser, triethyl citrate (TEC) and a non-phthalate plasticiser bis(2-ethylhexyl) terephthalate (DOTP), on ethylcellulose (EC) films to evaluate whether it would be feasible to replace them with PMD-CA. The films were produced by a casting technique and were characterised by mechanical properties, thermoanalysis, Scanning Electron Microscopy, disintegration and Fourier-Transform Infrared (FT-IR) spectroscopy. The addition of any of the three plasticisers to EC film increased the flexibility of the films. The plasticisers PMD-CA and DOTP added to EC formed tougher films as compared to EC films containing TEC shown by the high % elongation at break, strain and displacement. Statistically, it was proved that PMD-CA and DOTP had a more significant effect on % elongation of EC films than TEC, postulated to be due to their similar molecular size and structure. Thermoanalytical studies conducted revealed that the thermal degradation process of EC films proceeded in three basic stages, namely dehydration, depolymerisation and pyrolytic decomposition. Thermoanalytical analysis showed that unplasticised EC films had a higher glass transition value compared to the films containing any of the plasticisers and that PMD-CA and DOTP are more effective plasticisers compared to TEC due to their lower glass transition temperatures. The presence of plasticisers in the polymer matrix did change the surface morphology of polymer films. The addition of hydroxypropyl methylcellulose (HPMC) to EC PMD-CA and EC DOTP films caused more significant globule-like bubbles which were not uniform in size on the surface of these films. Observations were made that the type of plasticiser coating with or without pore former influenced the disintegration properties of films. HPMC, as pore former, played a significant role in the drug release system matrix. EC PMD-CA and EC DOTP polymeric films controlled the disintegration of the films in the acidic and weakly acidic medium. EC and EC TEC films had less significant control on the disintegration rate of the acidic and weakly acidic disintegration media. FT-IR characterisation of the films revealed comparable peaks observed in all the polymer-plasticiser combinations and therefore it was concluded that chemical interactions did not exist between the polymers and plasticisers. Added excipients (plasticisers and pore formers) were found to be compatible with EC films. It was concluded within the scope of this research that EC films plasticised with PMDCA, DOTP or TEC outperformed film properties of those compared to unplasticised EC film. The plasticising properties of the novel bio-plasticiser PMD-CA are similar compared to DOTP, and superior to triethyl citrate and could therefore be considered as an effective bioplasticiser for enteric coating formulations in the pharmaceutical industry.
- Full Text:
- Date Issued: 2019
Cheetah Acinonyx jubatus behaviour and resource use in response to other African large carnivores
- Authors: Cornhill, Kristina
- Date: 2019
- Subjects: Cheetahs -- Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44735 , vital:38169
- Description: Competition occurs between species for shared resources. Subordinate species employ resource selection to shift their resource use away from that of dominant species in order to avoid the negative consequences of competition. Only in Africa is the post-Pleistocene large carnivore guild intact, consisting of lions Panthera leo, spotted hyaenas Crocuta crocuta, leopards Panthera pardus, cheetahs Acinonyx jubatus, and African wild dogs Lycaon pictus. Therefore, only in Africa can we explore how large carnivores co-exist with one another in a diverse community. Cheetahs are a subordinate member of the large carnivore guild due to their small stature and solitary nature. However, we still do not fully understand how competition shapes cheetah behaviour and resource use. I used cheetahs as a model subordinate predator to determine the behavioural responses and resource selection of cheetah in response to assumed competition from other large carnivores. I experimentally explored the behavioural response of cheetah to large carnivore scent sources (scats) and their presence at cheetah scent-marking sites to test for avoidance of such cues. Moreover, using unplanned experiments based on the resource use of cheetahs in the absence and then presence of lions (assessing space and time use by cheetah) and African wild dogs (assessing prey use by both species), I evaluated resource selection by cheetahs as a way to reduce competition with these large carnivores. In addition, I measured spatial and temporal partitioning between cheetahs and all four large carnivores at camera trap sites. Finally, I assessed whether cheetahs responded to competitors using a long term proactive response or a short term reactive response. I found that all four of the other large carnivores in the guild shaped cheetahs resource use, however, all did so on different axes in accordance with the niche complementarity hypothesis. Lions and spotted hyenas were avoided through time, African wild dogs through space and prey use, and lions and leopards on the spatiotemporal axis. Moreover, I show that cheetahs utilize a reactive response to competition that allows them to avoid risk while still obtaining necessary resources.
- Full Text:
- Date Issued: 2019
- Authors: Cornhill, Kristina
- Date: 2019
- Subjects: Cheetahs -- Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44735 , vital:38169
- Description: Competition occurs between species for shared resources. Subordinate species employ resource selection to shift their resource use away from that of dominant species in order to avoid the negative consequences of competition. Only in Africa is the post-Pleistocene large carnivore guild intact, consisting of lions Panthera leo, spotted hyaenas Crocuta crocuta, leopards Panthera pardus, cheetahs Acinonyx jubatus, and African wild dogs Lycaon pictus. Therefore, only in Africa can we explore how large carnivores co-exist with one another in a diverse community. Cheetahs are a subordinate member of the large carnivore guild due to their small stature and solitary nature. However, we still do not fully understand how competition shapes cheetah behaviour and resource use. I used cheetahs as a model subordinate predator to determine the behavioural responses and resource selection of cheetah in response to assumed competition from other large carnivores. I experimentally explored the behavioural response of cheetah to large carnivore scent sources (scats) and their presence at cheetah scent-marking sites to test for avoidance of such cues. Moreover, using unplanned experiments based on the resource use of cheetahs in the absence and then presence of lions (assessing space and time use by cheetah) and African wild dogs (assessing prey use by both species), I evaluated resource selection by cheetahs as a way to reduce competition with these large carnivores. In addition, I measured spatial and temporal partitioning between cheetahs and all four large carnivores at camera trap sites. Finally, I assessed whether cheetahs responded to competitors using a long term proactive response or a short term reactive response. I found that all four of the other large carnivores in the guild shaped cheetahs resource use, however, all did so on different axes in accordance with the niche complementarity hypothesis. Lions and spotted hyenas were avoided through time, African wild dogs through space and prey use, and lions and leopards on the spatiotemporal axis. Moreover, I show that cheetahs utilize a reactive response to competition that allows them to avoid risk while still obtaining necessary resources.
- Full Text:
- Date Issued: 2019
Development of titanium dioxide for photo-electrochemical hydrogen production
- Authors: Mbulanga, Crispin Munyelele
- Date: 2019
- Subjects: Titanium dioxide , Nanostructured materials Water chemistry Environmental chemistry
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40735 , vital:36231
- Description: TiO2 is an attractive material for photo electrochemical hydrogen production and in this work the synthesis of this compound by hydrothermal, gel-calcination and one-step templating methods is investigated. Rutile-phase TiO2 rods, which are known to offer direct electrical pathways for photo generated electrons when using TiO2 as a photo anode in a photo-electrochemical hydrogen cell, were prepared on both F:SnO2 (FTO) coated glass and Ti foil substrates. Anatase-phase TiO2 tubes and mixed rutile-anatase-phase TiO2 nanostructured films were developed on FTO-coated glass substrates. Rutile-phase TiO2 rod-like structures, were synthesized hydrothermally at 150 oC for different times (6 – 20 hours) on FTO-coated glass substrate, in an aqueous solution of hydrochloric acid and titanium butoxide. The rod-like structures were found to comprise bundles of crystalline prismatic TiO2 nanorods, each approximately 4 nm in width. Each bundle was tetragonal in shape and highly oriented with respect to the substrate surface. The average diameter of the bundles varied depending on the growth time, and reached a mean diameter of ~175 nm after 20 hours of growth. In terms of Raman scattering, these bundles of nanorods acted as single entities, appearing to act as “larger” crystals to the lattice phonons. Hence, phonon confinement effects could not be observed, because the translational symmetry is preserved at the boundaries between individual rods. Moreover, as the preferential perpendicular orientation of the bundles improved with growth time, an unusual increase in the room and low (77K) temperature Eg/A1g Raman band intensity ratios was observed. The low temperature Raman peak position and peak width data was interpreted as supporting the hypothesis that the bundles of nanorods acting as single entities from the point of view of the lattice phonons. The phonon symmetries and frequencies (in cm-1) of these bundles of rutile-phase TiO2 were found to be consistent with rutile-phase TiO2 phonon symmetries and frequencies. Rutile-phase TiO2 rod-like structures were prepared on Ti foil following a two-step gel-calcination method, which involves a gel-deposition process in 5 M NaOH solution at 76 oC for 24 hours to produce a Na- (and Ti-) based layer of gelatinous material, followed by calcination at 600 oC or 800 oC for 1 hour. It is shown that the use of an alkali-based solution such as NaOH and KOH during gel-deposition, leads to the formation of faceted nanorods of rutile-phase TiO2 upon calcination at high temperature. When a solution that does not contain any alkali element, such as H2O2, was used, the material formed upon calcination at 800 oC were clustered nanoparticles, rather than nanorods. From the experiments it was deduced that the high temperature calcination step converted the Na(or K)-based amorphous gel (formed on the Ti surface during a 24-h soak in NaOH (KOH) solution) into faceted Na-titanate rods, which converted into nanorods of rutile-phase TiO2 when Na(or K) evaporates in the form of an oxide. Anatase-phase TiO2 tube-like structures were produced by a one-step templating solution approach, on FTO-coated glass substrate. ZnO nanorod templates were prepared by chemical bath deposition on FTO-coated glass substrate, and then treated in an aqueous mixture of ammonium hexafluorotitanate and boric acid. To study the effect of the template morphology and deposition processes on the formation of anatase TiO2 tubes, different times (10 – 60 minutes) and concentrations of the precursors are used. Calcination at 550 oC converted the Ti-based material developed on the ZnO rods into TiO2 nanostructures. A 10 minute deposition yielded tubes with dimensions resembling those of the ZnO template. However, the tube walls still contained Zn traces. Based on experimental observations, it was concluded that the production of titanium hydroxide complexes on ZnO surfaces takes place through two competing processes: the development of a Ti-based material and partial dissolution of ZnO along the c-axis. Calcination at 550 oC in air finally yielded anatase TiO2. Mixed rutile-anatase-phase TiO2 nanostructured films on FTO-coated glass substrate were prepared by decorating bundles of crystalline prismatic TiO2 nanorods (prepared hydrothermally) with anatase-phase TiO2 particles, using the same precursors mentioned above. The effect of reaction time and precursor concentrations were investigated. It was found that the precursor concentration ratio and reaction time played key roles in controlling the decoration process. Optimal ratios and decoration times were established based on the density of anatase-phase TiO2 decorating particles on the surface of bundles of rutile-phase TiO2 nanorods. The optical properties of rutile-phase TiO2 rods were investigated. The thickness of the TiO2 layer was calculated from reflectance fringes, and agreed well with the length of rods observed using SEM. The room temperature absorption edge of Eg=2.90 eV extracted from the transmittance spectrum correlated with typical values reported for TiO2. The room temperature absorption edge of the conductive layer of F:SnO2 (Eg=3.56 eV) could also be extracted from the transmittance spectrum. Finally, the absorption of white light by rutile-phase TiO2 rods was confirmed to be enhanced by annealing the rods in either hydrogen or nitrogen at 600 oC. Defects (possibly oxygen vacancies) or disorder in the near surface layers of TiO2 induced during the reduction experiments, created new electronic states in the band gap, as reported in literature.
- Full Text:
- Date Issued: 2019
- Authors: Mbulanga, Crispin Munyelele
- Date: 2019
- Subjects: Titanium dioxide , Nanostructured materials Water chemistry Environmental chemistry
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40735 , vital:36231
- Description: TiO2 is an attractive material for photo electrochemical hydrogen production and in this work the synthesis of this compound by hydrothermal, gel-calcination and one-step templating methods is investigated. Rutile-phase TiO2 rods, which are known to offer direct electrical pathways for photo generated electrons when using TiO2 as a photo anode in a photo-electrochemical hydrogen cell, were prepared on both F:SnO2 (FTO) coated glass and Ti foil substrates. Anatase-phase TiO2 tubes and mixed rutile-anatase-phase TiO2 nanostructured films were developed on FTO-coated glass substrates. Rutile-phase TiO2 rod-like structures, were synthesized hydrothermally at 150 oC for different times (6 – 20 hours) on FTO-coated glass substrate, in an aqueous solution of hydrochloric acid and titanium butoxide. The rod-like structures were found to comprise bundles of crystalline prismatic TiO2 nanorods, each approximately 4 nm in width. Each bundle was tetragonal in shape and highly oriented with respect to the substrate surface. The average diameter of the bundles varied depending on the growth time, and reached a mean diameter of ~175 nm after 20 hours of growth. In terms of Raman scattering, these bundles of nanorods acted as single entities, appearing to act as “larger” crystals to the lattice phonons. Hence, phonon confinement effects could not be observed, because the translational symmetry is preserved at the boundaries between individual rods. Moreover, as the preferential perpendicular orientation of the bundles improved with growth time, an unusual increase in the room and low (77K) temperature Eg/A1g Raman band intensity ratios was observed. The low temperature Raman peak position and peak width data was interpreted as supporting the hypothesis that the bundles of nanorods acting as single entities from the point of view of the lattice phonons. The phonon symmetries and frequencies (in cm-1) of these bundles of rutile-phase TiO2 were found to be consistent with rutile-phase TiO2 phonon symmetries and frequencies. Rutile-phase TiO2 rod-like structures were prepared on Ti foil following a two-step gel-calcination method, which involves a gel-deposition process in 5 M NaOH solution at 76 oC for 24 hours to produce a Na- (and Ti-) based layer of gelatinous material, followed by calcination at 600 oC or 800 oC for 1 hour. It is shown that the use of an alkali-based solution such as NaOH and KOH during gel-deposition, leads to the formation of faceted nanorods of rutile-phase TiO2 upon calcination at high temperature. When a solution that does not contain any alkali element, such as H2O2, was used, the material formed upon calcination at 800 oC were clustered nanoparticles, rather than nanorods. From the experiments it was deduced that the high temperature calcination step converted the Na(or K)-based amorphous gel (formed on the Ti surface during a 24-h soak in NaOH (KOH) solution) into faceted Na-titanate rods, which converted into nanorods of rutile-phase TiO2 when Na(or K) evaporates in the form of an oxide. Anatase-phase TiO2 tube-like structures were produced by a one-step templating solution approach, on FTO-coated glass substrate. ZnO nanorod templates were prepared by chemical bath deposition on FTO-coated glass substrate, and then treated in an aqueous mixture of ammonium hexafluorotitanate and boric acid. To study the effect of the template morphology and deposition processes on the formation of anatase TiO2 tubes, different times (10 – 60 minutes) and concentrations of the precursors are used. Calcination at 550 oC converted the Ti-based material developed on the ZnO rods into TiO2 nanostructures. A 10 minute deposition yielded tubes with dimensions resembling those of the ZnO template. However, the tube walls still contained Zn traces. Based on experimental observations, it was concluded that the production of titanium hydroxide complexes on ZnO surfaces takes place through two competing processes: the development of a Ti-based material and partial dissolution of ZnO along the c-axis. Calcination at 550 oC in air finally yielded anatase TiO2. Mixed rutile-anatase-phase TiO2 nanostructured films on FTO-coated glass substrate were prepared by decorating bundles of crystalline prismatic TiO2 nanorods (prepared hydrothermally) with anatase-phase TiO2 particles, using the same precursors mentioned above. The effect of reaction time and precursor concentrations were investigated. It was found that the precursor concentration ratio and reaction time played key roles in controlling the decoration process. Optimal ratios and decoration times were established based on the density of anatase-phase TiO2 decorating particles on the surface of bundles of rutile-phase TiO2 nanorods. The optical properties of rutile-phase TiO2 rods were investigated. The thickness of the TiO2 layer was calculated from reflectance fringes, and agreed well with the length of rods observed using SEM. The room temperature absorption edge of Eg=2.90 eV extracted from the transmittance spectrum correlated with typical values reported for TiO2. The room temperature absorption edge of the conductive layer of F:SnO2 (Eg=3.56 eV) could also be extracted from the transmittance spectrum. Finally, the absorption of white light by rutile-phase TiO2 rods was confirmed to be enhanced by annealing the rods in either hydrogen or nitrogen at 600 oC. Defects (possibly oxygen vacancies) or disorder in the near surface layers of TiO2 induced during the reduction experiments, created new electronic states in the band gap, as reported in literature.
- Full Text:
- Date Issued: 2019
Early Jurassic dolerites of the Karoo Large Igneous Province ( KLIP): an analysis of their age and emplacement history from sea level to the Drakensberg Mountains in the Eastern Cape , South Africa Submitted
- Authors: Muedi, Thomas Tshifhiwa
- Date: 2019
- Subjects: Geological time , Geochemistry Geology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/41908 , vital:36608
- Description: South Africa hosts one of the largest Large Igneous Provinces (LIPs that is likely linked to the early separation of East and West Gondwana. However, despite many studies, the relationship between dolerites and volcanic basalts of this LIP (also known as the Karoo Large Igneous Province or KLIP) remains unsolved, because of poorly linked age dates (timing), geochemistry and emplacement mechanisms. This problem is in part because different unreliable dating techniques with large margin of errors have been applied This study aims to address these issues by performing new geo chemical and high resolution geochronological analyses on a number of dolerites (sills and dykes), volcanic s also referred to in the literature as the Drakensberg flood basalts) and samples from drill cores and field outcrops. This project is focused on dolerites in the Eastern Cape Province and provides results from field mapping of dolerites (sills and dykes) from the sea level to an elevation of circa 1350 metres above sea level (MASL) and their link to the volcanic in the Eastern Cape Province. The dolerite dykes observed trends from metres to hundreds of kilometres and cut across volcanic, which have similar geochemistry. The intrusive dolerites collected from the field and from d rill core samples were likely emplaced by magma infiltration through pre-existing sub vertical brittle fractures and fissures and then emplaced horizontally as sills circa 183 Ma Detailed fracture mapping across host rock to the dolerite was carried out to test if they acted as possible pathways for magmatic emplacements. The dolerite dykes and fractures mapped in the host sedimentary rocks have a dominant NW direction, especially towards volcanic basalts. The project provides tests based on the geochemical relationships of the dolerites and basalts from sixty-six (66) cores and field outcrop samples. The results confirm that the chemical analyses from the volcanic basalts an d dolerites are closely related and reveal that most samples are Ocean Island Basalt OIB), but some also reveal subduction related processes. This is consistent with models that suggest subduction along south west Gondwana may have influenced plumemagmatism derived from the lower mantle that initiated break up of this supercontinent e.g de Wit and Stern 1981 Storey et al , 1992 ; Torsvik et al., 2006 and Burke et al., 2008)
- Full Text:
- Date Issued: 2019
- Authors: Muedi, Thomas Tshifhiwa
- Date: 2019
- Subjects: Geological time , Geochemistry Geology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/41908 , vital:36608
- Description: South Africa hosts one of the largest Large Igneous Provinces (LIPs that is likely linked to the early separation of East and West Gondwana. However, despite many studies, the relationship between dolerites and volcanic basalts of this LIP (also known as the Karoo Large Igneous Province or KLIP) remains unsolved, because of poorly linked age dates (timing), geochemistry and emplacement mechanisms. This problem is in part because different unreliable dating techniques with large margin of errors have been applied This study aims to address these issues by performing new geo chemical and high resolution geochronological analyses on a number of dolerites (sills and dykes), volcanic s also referred to in the literature as the Drakensberg flood basalts) and samples from drill cores and field outcrops. This project is focused on dolerites in the Eastern Cape Province and provides results from field mapping of dolerites (sills and dykes) from the sea level to an elevation of circa 1350 metres above sea level (MASL) and their link to the volcanic in the Eastern Cape Province. The dolerite dykes observed trends from metres to hundreds of kilometres and cut across volcanic, which have similar geochemistry. The intrusive dolerites collected from the field and from d rill core samples were likely emplaced by magma infiltration through pre-existing sub vertical brittle fractures and fissures and then emplaced horizontally as sills circa 183 Ma Detailed fracture mapping across host rock to the dolerite was carried out to test if they acted as possible pathways for magmatic emplacements. The dolerite dykes and fractures mapped in the host sedimentary rocks have a dominant NW direction, especially towards volcanic basalts. The project provides tests based on the geochemical relationships of the dolerites and basalts from sixty-six (66) cores and field outcrop samples. The results confirm that the chemical analyses from the volcanic basalts an d dolerites are closely related and reveal that most samples are Ocean Island Basalt OIB), but some also reveal subduction related processes. This is consistent with models that suggest subduction along south west Gondwana may have influenced plumemagmatism derived from the lower mantle that initiated break up of this supercontinent e.g de Wit and Stern 1981 Storey et al , 1992 ; Torsvik et al., 2006 and Burke et al., 2008)
- Full Text:
- Date Issued: 2019
Ecological assessment and biogeography of coastal vegetation and flora in southern Mozambique
- Authors: Massingue, Alice Obed
- Date: 2019
- Subjects: Coastal plants -- Mozambique , Coastal plants Marine biodiversity -- Mozambique Coastal biodiversity -- Mozambique
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40988 , vital:36280
- Description: This study considered aspects of the vegetation of the coastal zone of southern Mozambique, an area with a rich biodiversity. The vegetation and flora were assessed in their current state including the associated anthropogenic pressures. The environmental conditions that determined the distribution of the vegetation were evaluated. A hierarchical classification, description and ecological interpretation of the vegetation and flora of the coastal zone in southern Mozambique are presented. Analysis was based on 242 sample quadrats that were distributed in a stratified manner throughout the study area. The vegetation could be grouped into six distinct types of vegetation: Dune Forest, Coastal Forest, Coastal Grassland, Coastal Miombo, Coastal Savanna and Coastal Woodland. Species diversity was high for the all the vegetation types, particularly in the woody elements. A total of 673 species in 410 genera was recorded. They represented 104 families with Fabaceae and Rubiaceae being the most common. Of these, 6.6% were endemic or near-endemic to the coastal zone of southern Mozambique with Coastal Forest being the habitat with the greatest endemism. All soils from different vegetation types had poor agricultural potential. Soil properties were the strongest defining environmental feature separating the vegetation types: e.g. pH was high in Dune Forest compared to the others. Results from Maxent modelling suggest that the distribution of endemic species is influenced by a combination of climatic and non-climatic variables. Soil type, temperature annual range and precipitation of the driest month were the most important predictor variables. Overlaying the potential distributions of the seven selected species indicated two areas of abundance of endemic species – these should be given attention for conservation. Endemic species are not well protected in southern Mozambique – their sampled and potential habitats are largely outside protected areas. Hence, additional reserves should be created to improve their protection. Most endemic and near-endemics species were found in the south, from Ponta de Ouro (Matutuine, south of Maputo Province) to Manhica district (north of the Maputo Province) forming part of the Maputaland Centre of Endemism. A second concentration of endemism was found in the Inhambane Province, specifically the Inhassoro and Vilanculos districts. This is proposed to be an Important Plants Area (IPA). Because most endemics and near-endemics are found in the Coastal Forest, their main threat is harvesting for charcoal production, although none of the endemic species are specifically targeted for charcoal production. In Inhambane they are also threatened by the tourism industry, agriculture and settlements. The impact of the habitat destruction on endemic species is expected to cause severe declines in the near future. The tourism industry and harvesting of trees for charcoal production and over-frequent fires are the main drivers of vegetation loss in this region. Shifting agriculture, harvesting for firewood and construction materials, cattle grazing (at a minor scale) are also impacts, but these were only observed in a small area of Maputo Province. Although the study was done in coastal zone of southern Mozambique, effective management of whole coastal zone of the country will be required to maintain a functioning and diverse ecosystem. Priorities for management are to ensure that forests are protected, in particular, Dune Forest. Actions are required to minimise degradation of coastal vegetation. Further research on Coastal Forest restoration should be planned, as field observations in this study confirm that forest in the coastal zone of southern Mozambique has resilience.
- Full Text:
- Date Issued: 2019
- Authors: Massingue, Alice Obed
- Date: 2019
- Subjects: Coastal plants -- Mozambique , Coastal plants Marine biodiversity -- Mozambique Coastal biodiversity -- Mozambique
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40988 , vital:36280
- Description: This study considered aspects of the vegetation of the coastal zone of southern Mozambique, an area with a rich biodiversity. The vegetation and flora were assessed in their current state including the associated anthropogenic pressures. The environmental conditions that determined the distribution of the vegetation were evaluated. A hierarchical classification, description and ecological interpretation of the vegetation and flora of the coastal zone in southern Mozambique are presented. Analysis was based on 242 sample quadrats that were distributed in a stratified manner throughout the study area. The vegetation could be grouped into six distinct types of vegetation: Dune Forest, Coastal Forest, Coastal Grassland, Coastal Miombo, Coastal Savanna and Coastal Woodland. Species diversity was high for the all the vegetation types, particularly in the woody elements. A total of 673 species in 410 genera was recorded. They represented 104 families with Fabaceae and Rubiaceae being the most common. Of these, 6.6% were endemic or near-endemic to the coastal zone of southern Mozambique with Coastal Forest being the habitat with the greatest endemism. All soils from different vegetation types had poor agricultural potential. Soil properties were the strongest defining environmental feature separating the vegetation types: e.g. pH was high in Dune Forest compared to the others. Results from Maxent modelling suggest that the distribution of endemic species is influenced by a combination of climatic and non-climatic variables. Soil type, temperature annual range and precipitation of the driest month were the most important predictor variables. Overlaying the potential distributions of the seven selected species indicated two areas of abundance of endemic species – these should be given attention for conservation. Endemic species are not well protected in southern Mozambique – their sampled and potential habitats are largely outside protected areas. Hence, additional reserves should be created to improve their protection. Most endemic and near-endemics species were found in the south, from Ponta de Ouro (Matutuine, south of Maputo Province) to Manhica district (north of the Maputo Province) forming part of the Maputaland Centre of Endemism. A second concentration of endemism was found in the Inhambane Province, specifically the Inhassoro and Vilanculos districts. This is proposed to be an Important Plants Area (IPA). Because most endemics and near-endemics are found in the Coastal Forest, their main threat is harvesting for charcoal production, although none of the endemic species are specifically targeted for charcoal production. In Inhambane they are also threatened by the tourism industry, agriculture and settlements. The impact of the habitat destruction on endemic species is expected to cause severe declines in the near future. The tourism industry and harvesting of trees for charcoal production and over-frequent fires are the main drivers of vegetation loss in this region. Shifting agriculture, harvesting for firewood and construction materials, cattle grazing (at a minor scale) are also impacts, but these were only observed in a small area of Maputo Province. Although the study was done in coastal zone of southern Mozambique, effective management of whole coastal zone of the country will be required to maintain a functioning and diverse ecosystem. Priorities for management are to ensure that forests are protected, in particular, Dune Forest. Actions are required to minimise degradation of coastal vegetation. Further research on Coastal Forest restoration should be planned, as field observations in this study confirm that forest in the coastal zone of southern Mozambique has resilience.
- Full Text:
- Date Issued: 2019
Elucidating the Molecular Basis of the Interaction between the β2-integrin, αXβ2, and the low-affinity IgE Receptor, CD23
- Authors: Clarke, Stephen
- Date: 2019
- Subjects: CD23 antigen , Immune response Cellular immunity Molecular immunology
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/37990 , vital:34277
- Description: The low affinity IgE receptor, CD23, is involved in a myriad of immune reactions. It is not only a receptor for IgE, but also functions in the regulation of IgE synthesis, isotype switching in B cells, and induction of the inflammatory response. These effector functions of CD23 arise through its interaction with another leukocyte-specific cell surface receptor – the β2 integrin subfamily. However, this interaction has not been fully described. It has been shown that CD23 is capable of interacting with the β3 and β5 integrin β-subunit of integrins via a basic RKC motif in a metal cation-independent fashion. The currently proposed mechanism for the interaction between CD23 and the integrin superfamily was applied to the β2 integrin subfamily. In this study the interaction was probed for whether or not the RKC motif governs the interaction as well. This was done by performing bioinformatics docking predictions between the CD23 and αXβ2 integrin proteins. This revealed that in the absence of cations, the RKC motif is involved in interaction with the integrin αI domain. However, since physiologically integrin activity has been shown to be regulated by metal cations, docking predictions were also performed in the presence of such cations. This showed the interaction to involve novel acidic motifs within the CD23 protein, GEF and LDL. This same pattern of interaction was seen in docking predictions between CD23 and the β2- and β3I-like domains. To further investigate, recombinant proteins of sCD23 and the αXI domain were produced using E. coli expression systems. The DNA sequence was mutated to produce mutant versions of the CD23 RKC and GEF motifs as well as a high-affinity locked αXI domain. These proteins were used in subsequent SPR spectroscopy analysis of the binding affinity between immobilised integrin and CD23 analyte. It was shown that the mutation within the RKC motif reduced the binding affinity under cation-independence, especially when the Arg172 residue was substituted. However binding was not completely lost. This result was supported by synthetic peptides containing the same RKC motif and substitutions. These showed complete loss in binding in the double RKΔAA substitution, suggesting the involvement of other residues in the RKC-dependent interaction. In contrast, under cation dependence, the RKC motif substitutions showed no effect on binding affinity, while the GEF motif substitution exhibited near complete loss in binding. This same effect on binding was validated by U937 cell-based ELISA using live cells. This showed decreased capture of differentiated U937 cells, expressing the αXβ2 integrin, by immobilised recombinant sCD23 protein. In this study it was noted that, 2 contrary to the SPR analysis, metal cations allowed for a higher titre of cells to be captured in comparison to the cation-free binding.
- Full Text:
- Date Issued: 2019
- Authors: Clarke, Stephen
- Date: 2019
- Subjects: CD23 antigen , Immune response Cellular immunity Molecular immunology
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/37990 , vital:34277
- Description: The low affinity IgE receptor, CD23, is involved in a myriad of immune reactions. It is not only a receptor for IgE, but also functions in the regulation of IgE synthesis, isotype switching in B cells, and induction of the inflammatory response. These effector functions of CD23 arise through its interaction with another leukocyte-specific cell surface receptor – the β2 integrin subfamily. However, this interaction has not been fully described. It has been shown that CD23 is capable of interacting with the β3 and β5 integrin β-subunit of integrins via a basic RKC motif in a metal cation-independent fashion. The currently proposed mechanism for the interaction between CD23 and the integrin superfamily was applied to the β2 integrin subfamily. In this study the interaction was probed for whether or not the RKC motif governs the interaction as well. This was done by performing bioinformatics docking predictions between the CD23 and αXβ2 integrin proteins. This revealed that in the absence of cations, the RKC motif is involved in interaction with the integrin αI domain. However, since physiologically integrin activity has been shown to be regulated by metal cations, docking predictions were also performed in the presence of such cations. This showed the interaction to involve novel acidic motifs within the CD23 protein, GEF and LDL. This same pattern of interaction was seen in docking predictions between CD23 and the β2- and β3I-like domains. To further investigate, recombinant proteins of sCD23 and the αXI domain were produced using E. coli expression systems. The DNA sequence was mutated to produce mutant versions of the CD23 RKC and GEF motifs as well as a high-affinity locked αXI domain. These proteins were used in subsequent SPR spectroscopy analysis of the binding affinity between immobilised integrin and CD23 analyte. It was shown that the mutation within the RKC motif reduced the binding affinity under cation-independence, especially when the Arg172 residue was substituted. However binding was not completely lost. This result was supported by synthetic peptides containing the same RKC motif and substitutions. These showed complete loss in binding in the double RKΔAA substitution, suggesting the involvement of other residues in the RKC-dependent interaction. In contrast, under cation dependence, the RKC motif substitutions showed no effect on binding affinity, while the GEF motif substitution exhibited near complete loss in binding. This same effect on binding was validated by U937 cell-based ELISA using live cells. This showed decreased capture of differentiated U937 cells, expressing the αXβ2 integrin, by immobilised recombinant sCD23 protein. In this study it was noted that, 2 contrary to the SPR analysis, metal cations allowed for a higher titre of cells to be captured in comparison to the cation-free binding.
- Full Text:
- Date Issued: 2019
Energy yield modelling and analysis of photovoltaic systems in Namibia
- Authors: Dobreva, Petja
- Date: 2019
- Subjects: Photovoltaic power systems , Photovoltaic power generation Photovoltaic power generation -- Mathematical models
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/39523 , vital:35287
- Description: Generation of electricity from solar energy by photovoltaic systems has the potential to reduce dependence on non-renewable sources for electrical energy production. It can also reduce the cost of electricity for consumers by moving away from the traditional models of centralised generation combined with extensive transmission. The prices of photovoltaic system components have decreased so dramatically in recent years that nowadays a large portion of the cost of electricity produced by photovoltaic systems is due to the cost of investment capital rather than the cost of the physical parts of the system. The cost of capital is determined by the perceived risk of the investment and is contingent on predictive models whose aim is to project outputs reasonably achievable by the system. The output of a photovoltaic system depends strongly on external factors, like solar radiation and temperature, as well as internal, system specific factors. Models that can accurately predict the output of a photovoltaic system have direct impact on investment cost. Several measures, like root-mean-square-error or coincident of determination, have been used in the past to assess accuracy of models but they appear to be inadequate for the task. I suggest a new method, comprising qualitative and quantitative measures, for model evaluation and introduce two new quantities to assess the predictive capabilities of the models. Models of several photovoltaic systems in Namibia were created, the new evaluation approach was applied to them and the results were compared to the conclusions that would have been drawn if the old measures were used. Outdoor current-voltage measurements are used to assess the power output and general state of the solar generator of a photovoltaic system, but the measured values strongly depend on irradiance and temperature. The prevailing approach has been to transform the current-voltage characteristics to standard-test-conditions with the so-called translation equations and then compare the translated power to the nominal value. The translated value of the power, however, depends on the translation equations used and is not necessarily an accurate reaction of the state of the solar generator. I have proposed a new method for assessment of the state of the solar generator that relies only on the measured quantities and does not require translations to standard-test-conditions. This alternative method has been used in the assessment of the condition of the solar generators in the systems included in this study.
- Full Text:
- Date Issued: 2019
- Authors: Dobreva, Petja
- Date: 2019
- Subjects: Photovoltaic power systems , Photovoltaic power generation Photovoltaic power generation -- Mathematical models
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/39523 , vital:35287
- Description: Generation of electricity from solar energy by photovoltaic systems has the potential to reduce dependence on non-renewable sources for electrical energy production. It can also reduce the cost of electricity for consumers by moving away from the traditional models of centralised generation combined with extensive transmission. The prices of photovoltaic system components have decreased so dramatically in recent years that nowadays a large portion of the cost of electricity produced by photovoltaic systems is due to the cost of investment capital rather than the cost of the physical parts of the system. The cost of capital is determined by the perceived risk of the investment and is contingent on predictive models whose aim is to project outputs reasonably achievable by the system. The output of a photovoltaic system depends strongly on external factors, like solar radiation and temperature, as well as internal, system specific factors. Models that can accurately predict the output of a photovoltaic system have direct impact on investment cost. Several measures, like root-mean-square-error or coincident of determination, have been used in the past to assess accuracy of models but they appear to be inadequate for the task. I suggest a new method, comprising qualitative and quantitative measures, for model evaluation and introduce two new quantities to assess the predictive capabilities of the models. Models of several photovoltaic systems in Namibia were created, the new evaluation approach was applied to them and the results were compared to the conclusions that would have been drawn if the old measures were used. Outdoor current-voltage measurements are used to assess the power output and general state of the solar generator of a photovoltaic system, but the measured values strongly depend on irradiance and temperature. The prevailing approach has been to transform the current-voltage characteristics to standard-test-conditions with the so-called translation equations and then compare the translated power to the nominal value. The translated value of the power, however, depends on the translation equations used and is not necessarily an accurate reaction of the state of the solar generator. I have proposed a new method for assessment of the state of the solar generator that relies only on the measured quantities and does not require translations to standard-test-conditions. This alternative method has been used in the assessment of the condition of the solar generators in the systems included in this study.
- Full Text:
- Date Issued: 2019
Static and bootstrapped neuro-simulation for complex robots in evolutionary robotics
- Authors: Woodford, Grant Warren
- Date: 2019
- Subjects: Robotics
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44656 , vital:38172
- Description: Evolutionary Robotics (ER) is a field of study focused on the automatic development of controllers and robot morphologies. Evolving controllers on real-world hardware is time-consuming and can damage hardware through wear. Robotic simulators can be used as an alternative to a real-world robot in order to speed up the ER process. Most simulation techniques in practice use physics-based models that rely on an understanding of the robotic system in question. Developing effective physics-based simulators is time consuming and requires a significant level of specialised knowledge. A lengthy simulator development and tuning process is typically required before the ER process can begin. Artificial Neural Networks simulators (SNNs) can be used as an alternative to a physics based simulation approach. SNNs are simple to construct, do not require significant levels of prior knowledge of the robotic system, are computationally efficient and can be highly accurate. Two types of ER approaches utilising SNNs exist. The Static Neuro-Simulation (SNS) approach involves developing SNNs before the ER process where these SNNs are used instead of a physics-based simulator. Alternatively, SNNs can be developed during the ER process, called the Bootstrapped Neuro-Simulation (BNS) approach. Prior work investigating SNNs has largely been limited to simple robots. A complex robot has many degrees of freedom and ifa low-level controller design is used, the solution search space is high-dimensional and difficult to traverse. Prior work investigating the SNS and BNS approaches have mostly relied on simplified controller designs which rely on built-in prior knowledge of intended robot behaviours. This research uses low-level controller designs which in turn rely on low level simulators. Most ER studies are conducted on a single type of robot morphology. This research investigates the SNS and BNS approaches on two significantly different classes of robots. A Hexapod and Snake robot are used to study the SNS and BNS approaches. The Hexapod robot exhibits limbed, walking behaviours. The Snake robot is limbless and generates crawling behaviours. Demonstrating the viability of the SNS and BNS approaches for two different classes of robots provides strong evidence that the tested approaches are likely viable on other classes of robots. Various proposed improvements to the SNS and BNS approaches are investigated. The Results demonstrate that the SNS and BNS approaches are viable when applied to Hexapod and Snake robots without restricting controller designs to those with significant levels of built-in prior knowledge of robot behaviours. SNNs configured in ensembles improve the likely performance outcomes of solutions. The expected benefit of adding simulator noise during the evolutionary process were not as pronounced for problems investigated in this work.
- Full Text:
- Date Issued: 2019
- Authors: Woodford, Grant Warren
- Date: 2019
- Subjects: Robotics
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/44656 , vital:38172
- Description: Evolutionary Robotics (ER) is a field of study focused on the automatic development of controllers and robot morphologies. Evolving controllers on real-world hardware is time-consuming and can damage hardware through wear. Robotic simulators can be used as an alternative to a real-world robot in order to speed up the ER process. Most simulation techniques in practice use physics-based models that rely on an understanding of the robotic system in question. Developing effective physics-based simulators is time consuming and requires a significant level of specialised knowledge. A lengthy simulator development and tuning process is typically required before the ER process can begin. Artificial Neural Networks simulators (SNNs) can be used as an alternative to a physics based simulation approach. SNNs are simple to construct, do not require significant levels of prior knowledge of the robotic system, are computationally efficient and can be highly accurate. Two types of ER approaches utilising SNNs exist. The Static Neuro-Simulation (SNS) approach involves developing SNNs before the ER process where these SNNs are used instead of a physics-based simulator. Alternatively, SNNs can be developed during the ER process, called the Bootstrapped Neuro-Simulation (BNS) approach. Prior work investigating SNNs has largely been limited to simple robots. A complex robot has many degrees of freedom and ifa low-level controller design is used, the solution search space is high-dimensional and difficult to traverse. Prior work investigating the SNS and BNS approaches have mostly relied on simplified controller designs which rely on built-in prior knowledge of intended robot behaviours. This research uses low-level controller designs which in turn rely on low level simulators. Most ER studies are conducted on a single type of robot morphology. This research investigates the SNS and BNS approaches on two significantly different classes of robots. A Hexapod and Snake robot are used to study the SNS and BNS approaches. The Hexapod robot exhibits limbed, walking behaviours. The Snake robot is limbless and generates crawling behaviours. Demonstrating the viability of the SNS and BNS approaches for two different classes of robots provides strong evidence that the tested approaches are likely viable on other classes of robots. Various proposed improvements to the SNS and BNS approaches are investigated. The Results demonstrate that the SNS and BNS approaches are viable when applied to Hexapod and Snake robots without restricting controller designs to those with significant levels of built-in prior knowledge of robot behaviours. SNNs configured in ensembles improve the likely performance outcomes of solutions. The expected benefit of adding simulator noise during the evolutionary process were not as pronounced for problems investigated in this work.
- Full Text:
- Date Issued: 2019
The evolution of the Lepilemuridae-Cheirogaleidae clade
- Authors: Andrews, Curswan Allan
- Date: 2019
- Subjects: Lemurs
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40216 , vital:35989
- Description: The Lepilemuridae and the Cheirogaleidae, according to recent molecular reconstructions, share a more recent common ancestor than previously thought. Further phylogenetic reconstructions have indicated that body size evolution in this clade was marked by repeated dwarfing events that coincided with changes in the environment. I aimed to investigate the morphological implications of changes in body size within the Lepilemur-cheirogaleid clade, testing four predictions. Together with Dr. Couette, I collected data on the overall palate shape and predicted that shape is likely to be influenced by several factors including phylogeny, body size and diet. Geometric morphometric analyses revealed that, although a strong phylogenetic signal was detected, diet had the major effect on palate shape. In a similar vein, when examining the arterial circulation patterns in these taxa, I predicted that changes in body size would result in changes and possible reductions in arterial size, particularly the internal carotid artery (ICA) and stapedial artery (SA). Analyses with micro-computed tomography (CT) and 3D imaging indicated that changes in body size led to reduction of a functional stapedial artery in Lepilemur, making it an intermediate stage between the daubentoniid, lemurid and indriid species with large stapedial arteries, and the smaller bodied cheirogaleids with an alternative blood supply in the form of an enlarged ascending pharyngeal artery. Lepilemur is the smallest living folivorous primate, and likely to be at the threshold body size to be able to subsist on such a poor diet. To investigate shifts in dietary patterns that accompanied changes in body size, I chose to explore the reported behaviour of caecotrophy as a possible means for the sportive lemurs to derive additional nutrient from their food sources. I predicted that, if caecotrophy is a way to assist folivory at small body size, the energy contained in “caecotrophic” and latrine faecal samples should be different. Analyses showed significant iv differences between the two types of faeces and, combined with an analysis of faecal bacterial diversity, support the occurrence of caecotrophy. Finally, I compared the digestive efficiency of two small, distantly related gummivorous primates that evolved their diets convergently. I studied the digestion of gum in Microcebus griseorufus and compared this with gum digestion in Galago moholi. I predicted that an evolutionary disposition to fermentation inherited from a folivorous ancestor would aid in the digestion of gum in mouse lemurs. Results indicated that retention time was prolonged by the presence of secondary compounds in Microcebus fed with Commiphora gum but relatively shorter (< 24 hrs) when fed Alantsilodenron gum, a preferred food. Despite the fact that G. moholi has an ansa coli, which is missing in M. griseorufus species, both are highly efficient at digesting gum. These data provide some of the first indicators of how dietary changes from a larger-bodied folivorous ancestor to partially gummivorous, small-bodied descendants may have occurred in evolutionary time.
- Full Text:
- Date Issued: 2019
- Authors: Andrews, Curswan Allan
- Date: 2019
- Subjects: Lemurs
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/40216 , vital:35989
- Description: The Lepilemuridae and the Cheirogaleidae, according to recent molecular reconstructions, share a more recent common ancestor than previously thought. Further phylogenetic reconstructions have indicated that body size evolution in this clade was marked by repeated dwarfing events that coincided with changes in the environment. I aimed to investigate the morphological implications of changes in body size within the Lepilemur-cheirogaleid clade, testing four predictions. Together with Dr. Couette, I collected data on the overall palate shape and predicted that shape is likely to be influenced by several factors including phylogeny, body size and diet. Geometric morphometric analyses revealed that, although a strong phylogenetic signal was detected, diet had the major effect on palate shape. In a similar vein, when examining the arterial circulation patterns in these taxa, I predicted that changes in body size would result in changes and possible reductions in arterial size, particularly the internal carotid artery (ICA) and stapedial artery (SA). Analyses with micro-computed tomography (CT) and 3D imaging indicated that changes in body size led to reduction of a functional stapedial artery in Lepilemur, making it an intermediate stage between the daubentoniid, lemurid and indriid species with large stapedial arteries, and the smaller bodied cheirogaleids with an alternative blood supply in the form of an enlarged ascending pharyngeal artery. Lepilemur is the smallest living folivorous primate, and likely to be at the threshold body size to be able to subsist on such a poor diet. To investigate shifts in dietary patterns that accompanied changes in body size, I chose to explore the reported behaviour of caecotrophy as a possible means for the sportive lemurs to derive additional nutrient from their food sources. I predicted that, if caecotrophy is a way to assist folivory at small body size, the energy contained in “caecotrophic” and latrine faecal samples should be different. Analyses showed significant iv differences between the two types of faeces and, combined with an analysis of faecal bacterial diversity, support the occurrence of caecotrophy. Finally, I compared the digestive efficiency of two small, distantly related gummivorous primates that evolved their diets convergently. I studied the digestion of gum in Microcebus griseorufus and compared this with gum digestion in Galago moholi. I predicted that an evolutionary disposition to fermentation inherited from a folivorous ancestor would aid in the digestion of gum in mouse lemurs. Results indicated that retention time was prolonged by the presence of secondary compounds in Microcebus fed with Commiphora gum but relatively shorter (< 24 hrs) when fed Alantsilodenron gum, a preferred food. Despite the fact that G. moholi has an ansa coli, which is missing in M. griseorufus species, both are highly efficient at digesting gum. These data provide some of the first indicators of how dietary changes from a larger-bodied folivorous ancestor to partially gummivorous, small-bodied descendants may have occurred in evolutionary time.
- Full Text:
- Date Issued: 2019
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