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  • The sustainability of stand-alone photovoltaic lighting systems in Tonga
    (2002) Tukunga, Tevita
    Thesis
    Stand-alone photovoltaic lighting systems have been used to provide electricity in unelectrified remote locations in Tonga since 1987. PV lighting systems power many remote households and community halls and this improves lifestyles and upgrades living standards. Donors have spent millions of dollars to fund the purchase and installation of PV lighting systems in Tonga but this development is yet to prove its sustainability. This research examines the problems that influence the sustainability of PV lighting systems in Tonga and identifies problems related to their environmental, economic, technical and institutional sustainability. Proposals are put forward to enhance the future sustainability of Tongan PV lighting systems and related PV applications. The examination of sustainability employs the Brundtland definition of sustainability, viewing sustainability in terms of its environmental, economic, technical and institutional aspects. The research employs and analyses household surveys, data extracted from PV stakeholder interviews, and a literature search. The exploitation of fossil fuel and fuelwood resources is still significant in Tonga because the remote economy is a subsistence one, and environmental protection is not a priority. Lack of a proper recycling method for PV equipment increases the solid waste materials in the environment. Economically, users are reluctant to pay monthly fees, compounding problems with maintenance and access to spare parts. Social and cultural concerns have generally been ignored during system design and project planning. Technically, users experience poor system performance and some systems no longer operate. Institutionally, stakeholders have different perceptions and interests towards PV development in the islands. As a result, the By-Laws implemented to administer the PV lighting systems have never been enforced and stakeholders generally experience a low level of participation in governance, undermining their satisfaction with project implementation. The sustainability of PV systems in Tonga would be enhanced through the utilization of mature PV technologies under appropriate technological and institutional frameworks that reduce environmental problems and meet the socioeconomic needs of target communities.
  • Single channel speech enhancement based on perceptual temporal masking model
    (2007) Wang, Yao
    Thesis
    In most speech communication systems, the presence of background noise causes the quality and intelligibility of speech to degrade, especially when the Signal-to-Noise Ratio (SNR) is low. Numerous speech enhancement techniques have been employed successfully in many applications. However, at low signal-to-noise ratios most of these speech enhancement techniques tend to introduce a perceptually annoying residual noise known as "musical noise". The research presented in this thesis aims to minimize this musical noise and maximize the noise reduction ability of speech enhancement algorithms to improve speech quality in low SNR environments. This thesis proposes two novel speech enhancement algorithms based on Weiner and Kalman filters, and exploit the masking properties of the human auditory system to reduce background noise. The perceptual Wiener filter method uses either temporal or simultaneous masking to adjust the Wiener gain in order to suppress noise below the masking thresholds. The second algorithm involves reshaping the corrupted signal according to the masking threshold in each critical band, followed by Kalman filtering. A comparison of the results from these proposed techniques with those obtained from traditional methods suggests that the proposed algorithms address the problem of noise reduction effectively while decreasing the level of the musical noise. In this thesis, many other existing competitive noise suppression methods have also been discussed and their performance evaluated under different types of noise environments. The performances were evaluated and compared to each other using both objective PESQ measures (ITU-T P.862) and subjective listening tests (ITU-T P.835). The proposed speech enhancement schemes based on the auditory masking model outperformed the other methods that were tested.
  • Identification and control of nonlinear laboratory processes
    (2007) Xi, Zhiyu
    Thesis
    In this thesis, a class of control and identification methods on a typical laboratory process - a ball and beam system - are discussed. The ball and beam is a common laboratory process which contains nonlinearity, a double integrator and time-delay. In our project, the hardware made by Wincon (Quanser SRV02 +BB01) is used. The main contribution of this work is the development of a variety of controller design methods, which together with suitable parameter identification techniques provide tools for rapid prototyping for real time control of processes within the laboratory, in preparation for industrial implementation of more complex schemes. The novelty of this work lies in the use of model predictive control (MPC) methods based on a non-minimal state space formulation, which permits the inclusion of process measurements and actuations in the state vector, leading to controller designs which are immediately ready for on-line implementation. A linear MPC controller based on a non-minimal state space model is based on an approximate linear model. The results from simulation and online experiment show that the linear MPC controller realizes a satisfying reference tracking in the face of nonlinearity and time-delay. In the following chapter, a nonlinear Hammerstein model is identified, which is a type of reliable structure for describing nonlinear plants. A nonlinear MPC scheme is developed based on the Hammerstein model. An inversion block is created to cancel the effect of the nonlinearity. The performance IS also tested in both simulation and experiment. Finally, MPC is combined with sliding mode control. The non-minimal state space model is also used here. In the first part of this chapter, the idea underlying sliding mode control contributes a method of modifying the definition of the cost function in MPC. In the second half, MPC is used to design the switching surface in sliding mode control. The performance of tests on the example (ball and beam system) illustrates that these are both valid methods for dealing with complex processes.