In this thesis, surface vibration amplitudes and modes of piezoelectric transducers are studied. A system is developed to measure these amplitudes and observe the modes in different media. Piezoelectric transducers are used for underwater sound transmission. They are energized individually or in array form by continuous or pulsed electrical power and the acoustic energy is transmitted from the radiating front surface. This acoustic transmission and its beam pattern correspond to the vibration amplitudes and modes of transducer radiating surface when the electrical energy is applied. Optical techniques are the most common methods used to determine the small vibration amplitudes of objects and the Electronic Speckle Pattern Interferometry Technique (ESPI) has been used to detect the small movements of objects down to 10-20A in real time. Hogmoen and Lokberg have described a method for real time detection and have carried out measurements of small vibrations at frequencies up to 30 KHz . They claimed detection limits of 20A by visual observation and O.lA by photoelectric measurements using a lock in technique. In the thesis, the reference beam modulation is performed by an e1ectro-optic modulator in the path of the reference beam. This allowed ESPI to be used to measure high frequency vibration amplitudes into the MHz region semi-automatically. A video integrator gated by a digital scanner is used to sample the video information directly from a television screen. Various vibration modes of a PZT-4 transduter radiating surface were studied at its resonant frequencies in 3 different media. By using an automatic calibration procedure and a phase-sensitive detection technique, absolute vibration amplitude measurements were successfully carried out. Various experimental surface vibration modes are related to the theoretical values by using simple plate vibration theory which suggests that a PZT-4 transducer behaves like a simply-supported thin plate at low power levels. The relationship between transducer radiation beam patterns and the surface vibration amplitudes under water is presented in Chapter 8. An optical study of the Michelson Interferometer has been given alongside the speckle interferometer and the results of early experiments with the Miche1sori is presented. The reasons for abandoning the Miche1son Interferometer and using the speckle interferometer are given extensively in Chapter 1.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

In recent years, electromagnetic compatibility (EMC) problems associated with high frequency and high speed interconnects are becoming of increasing concern. Coaxial cables are a popular form of interconnect. In this thesis, the crosstalk coupling between two parallel coaxial cables in free space and above a ground plane is investigated. The degree of coupling is usually formulated analytically in the frequency domain. In this thesis, a method for time domain simulation is proposed using the TLM technique. Results are compared with frequency domain solutions and experimental results. Also; the standard model has been improved by including the skin depth effect in the coaxial cable braid. The crosstalk between the two coaxial cables is observed through the induced voltages on the loads of the adjacent cable, which is deemed to be the usual measureable form of cable coupling. The equivalent circuit developed for the coupling path of two coaxial cables in free space takes account of the differential mode (DM) current travelling in the braids of the cables. As for the coupling path of the cables via a ground plane, the equivalent circuit is developed based on the flow of differential mode (DM) and common mode (CM) currents in the braid, where the coaxial braid’s transfer impedance is modelled using Kley’s model. The radiated electric (E) field from the coaxial cable above a ground plane is also deduced from the predicted cable sheath current distribution and by the Hertzian dipoles’ approach. Results are validated against the radiated electric field of a single copper wire above ground. Both the simulated and experimental results are presented in the time and frequency domains and good agreement is observed thus validating the accuracy of the model.

Reliability and efficiency of power transmission has been at the forefront of research for some time and is currently being given critical consideration due to the increased dependence on electrical energy. With the increased demand for electricity, engineers are considering different methods of supply arrangement to improve the security of electricity supply. High Voltage Direct Current (HVDC) transmission is a technology that avails itself for distance power transmission, interconnection of asynchronous networks and cross sea or offshore power transmission. The main element of an HVDC system is the AC/DC or DC/AC power converter. Recently, a new breed of power converters suitable for HVDC transmission has been the subject of considerable research work. These converters are modular in structure with high efficiency and their operation results in higher power quality, with reduced filtering components when compared to the use of Line Commutated and two-level or three-level Voltage Source Converter (VSC) based transmission systems. One such modular circuit is the Parallel Hybrid Modular Multilevel Voltage Source Converter (PH-M2L-VSC). This research investigates the operation and control of the PH-M2L-VSC for HVDC applications. Control schemes supporting the operation of the converter as would be expected of an HVDC VSC are proposed, including operation with an unbalanced AC network. Simulation results from a medium voltage demonstrator and experimental results from a laboratory scale prototype are presented to validate the methods proposed and enable a performance comparison to be made with other topologies.

The transient temperature of solid state power switches is investigated using thermal resistance network modelling and experimental testing. The ability of a heat sink mounted to the top of the device to reduce the transient temperature is assessed. Transient temperatures for heat pulses of up to 100ms are of most interest. The transient temperature distribution inside a typical stack-up of a solid state power switch is characterised. The thermal effects of adding a heat sink to the top of the device are then assessed. A variety of heat sink thicknesses and materials are evaluated. Components of the device stack-up are varied in order to assess their affect on the effectiveness of the heat sink in reducing the device temperature. Thermal networks are successfully applied to model the transient heat conduction inside the stack-ups. This modelling technique allowed a good understanding of the thermal behaviour inside the stack-up and heat sink during the transient period. The concept of using a heat sink to suppress the transient temperature was validated experimentally on two types of solid state power switch.