No description available.

No description available.

No description available.

A new general three-port systems model of the thickness-mode piezoelectric transducer is proposed. This model is considered to have several substantial advantages over existing modelling techniques. In particular, it may be readily used to explain the underlying physical operation of ultrasonic transducers. The model has been developed in such a manner that the effects of arbitrary electrical load and source elements and mechanical matching layers may be incorporated. The use of z-transforms in the calculation of the transducer transient response has been investigated. This has resulted in a fast, efficient and accurate method for calculating the transducer response to arbitrary transient excitation. In the course of this work, the model has been verified extensively by computer simulation and experimental measurement. Excellent agreement was obtained between the theoretical and practical results for a comprehensive range of electrical and mechanical configurations. Techniques for controlling piezoelectric transmitters electrically have been investigated. This involves the generation of a precisely defined force transient, by exciting the transducer system with a calculated voltage. These methods were verified by simulation and experiment, and it is concluded that they offer a significant improvement over existing transducer control strategies. Finally, it is considered that the techniques and models proposed have made a significant contribution to the investigation of thickness-mode piezoelectric devices.

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In the recently evolved fields of satellite and space communications as well as in a number of related areas, a vital requirement is an accurate knowledge of the radiating and receiving characteristics of the transmitting and receiving antennas as they appear at a large distance (in the so called far-field region). It is often impossible to obtain a direct measurement of the performance of an antenna and in such cases where it is possible, the accuracy obtainable is frequently limited by the many difficulties associated with the process. Over recent years, a number of techniques have begun to appear which allow measurement of data close to the test antenna (in the near-field region) and then by mathematical processing (the transformation) predict what the far-field performance will be. The earlier techniques while being basically simple from a mathematical viewpoint, were not completely general and tended to involve special, sophisticated, hardware. The later techniques use the most general spherical scanning system but involve much more complicated processing. A new approach to the problem is presented in which much of the computational burden is pre-processed so that the size and complexity of the ultimate prediction task is reduced. The various measurement systems are considered briefly and the spherical system is formulated in detail. Simulated and experimental predictions are carried out and studies are included of the various errors likely to be present and their effects. The important parameters, including the sampling criterion, are discussed in some detail. It is shown that this technique has the potential for producing rapid and accurate predictions of antenna far-field patterns including the facility of compensation for the characteristics of the measurement probe.

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No description available.

The Power Electronics Group at Aberdeen University is working towards the provision of a complete time domain model of an electrical drive for marine applications. The model has been developed using the SABER electrical simulation software package and includes a cycloconvertor which supplies an ac induction motor which in turn drives the shaft/propeller assembly. Effects such as vibration, noise and shaft oscillations due to the cycloconvertor will be studied. This thesis reports on the development and investigation of a mathematical model of the ac induction motor which can be incorporated into the existing system model to predict vibration levels at any point on the structure of the motor. The development of the mathematical model was achieved by combining a finite element (FE) representation of the motor structure with the modal parameters of the motor extracted using modal analysis techniques. The main experimental programme on the selected motor was based on a series of parallel tests on two "identical" machines. These machines were broken apart and subjected to a modal analysis at various stages. The vibration properties of a laminated structure, similar to the core of an ac motor, were also investigated. The experimental data was then used to direct the creation of the FE models. The models were created and analysed using SDRC I-DEAS integrated software. The modal analysis results were introduced into test software, where they were "curve-fitted" to extract the modal mass, modal stiffness and modal damping parameters. The FE model, created in the finite element modelling package, was then correlated with the test model and modifications implemented until the FE and test models compared well.

The electrical characteristics of a Metal-Semi-insulator-Semiconductor diode have been comprehensively studied at various temperatures. In particular the electrical, structural and compositional properties of the semi-insulator, silicon-rich-oxide (SRO), have been thoroughly investigated. The SRO films were all deposited by atmospheric pressure chemical vapour deposition, (APCVD), at 650 C with silane (SiH(_4)) and nitrous oxide (N(_2)O) reactant gases and a carrier gas of nitrogen. The reactant gas ratio, γ, was held at 0.22 and the deposition time varied between 0.5 and 8 minutes. The effects of film thickness, film annealing, forming, top contact metal (i.e. work function difference), top contact metal area, substrate material and temperature on the electrical characteristics in both forward and reverse bias have been reported. Various techniques have been employed to elucidate the physical and structural properties of the SRO film. These include: Auger Electron Spectroscopy; Secondary Ion Mass Spectroscopy; Glow Discharge Optical Emission Spectroscopy; Scanning Electron Microscopy; Transmission Electron Microscopy; Rutherford Backscattering Spectroscopy; Optical Ellipsometry and Alpha Step Analysis. A model for the structure of SRO films has been put forward. The films are thought to be extremely random in structure, containing many voids with a gradual variation in composition from substrate to top metal contact. Various models for conduction in the SRO film were investigated over a range of electric fields and temperatures to determine the predominant conduction mechanism for a particular set of conditions. Conduction in SRO is thought to be dominated by Schottky emission across the SRO-substrate interface. Once this Schottky barrier is conducting, at suitably high biases, conduction across the remainder of the device is thought to be by thermionic emission at high temperatures and by a Poole-Frenkel process at low temperatures.