No description available.

No description available.

No description available.

In recent years, orthogonal frequency division multiplexing (OFDM) has gained considerable attention in the development of underwater communication (UWC) systems for civilian and military applications. However, the wideband nature of the communication links necessitate robust algorithms to combat the consequences of severe channel conditions such as frequency selectivity, ambient noise, severe multipath and Doppler Effect due to velocity change between the transmitter and receiver. This velocity perturbation comprises two scenarios; the first induces constant time scale expansion/compression or zero acceleration during the transmitted packet time, and the second is time varying Doppler-shift. The latter is an increasingly important area in autonomous underwater vehicle (AUV) applications. The aim of this thesis is to design a low complexity OFDM-based receiver structure for underwater communication that tackles the inherent Doppler effect and is applicable for developing real-time systems on a digital signal processor (DSP). The proposed structure presents a paradigm in modem design from previous generations of single carrier receivers employing computationally expensive equalizers. The thesis demonstrates the issues related to designing a practical OFDM system, such as channel coding and peak-to-average power ratio (PAPR). In channel coding, the proposed algorithms employ convolutional bit-interleaved coded modulation with iterative decoding (BICM-ID) to obtain a higher degree of protection against power fading caused by the channel. A novel receiver structure that combines an adaptive Doppler-shift correction and BICM-ID for multi-carrier systems is presented. In addition, the selective mapping (SLM) technique has been utilized for PAPR. Due to their time varying and frequency selective channel nature, the proposed systems are investigated via both laboratory simulations and experiments conducted in the North Sea off the UK’s North East coast. The results of the study show that the proposed systems outperform block-based Doppler-shift compensation and are capable of tracking the Doppler-shift at acceleration up to 1m /s2.

The main purpose of this research is an investigation into the different strategies for the autoresonant control of an ultrasonic transducer. Numerical simulations were considered as the most appropriate method for analysis and a Matlab-Simulink computer model of a non-linear ultrasonic vibrating system with the possibility of autoresonant control was developed. The controlled system consists of two modules, the first of which is an electromechanical model of the ultrasonic transducer comprising a piezoelectric transducer and a step concentrator. The second module simulates influence from the machining process. The coefficients of the electromechanical model were calculated through an identification process based on the real measurement of the ultrasonic transducer's vibrations.

Ultrasonic Machining (USM) is the outstanding manufacturing process for producing complex cavities for all materials subjected to brittle fracture without the requirements of electrical/thermal conductivity or chemical reactivity. As a result, a wide range of materials can be fabricated efficiently. Even though the process benefits a variety of applications, the use of this method is not widespread because of the complexity of a large number of interacting parameters affecting the machining operation. Additionally, commercial machines available in the market tend to be rather large. In an age of micro. manufacturing and nanotechnology, as well as sustainable development, it is wasteful to machine hard and brittle materials in meso/micro scale geometries with the large equipment currently available. This research developed a miniaturised USM system. The effects of each of the different design parameters were considered and organised effectively using the Axiomatic Design approach to understanding the behaviour of a complex system. The system components were designed, modelled and then simulated with Finite Element Analysis software to characterise their responses, and were also optimised using a Design and Experiments method for a robust design. Prototypes were built and tested to validate the design. The dynamic characteristics based on prototype testing did not agree well with the computer simulation results. It was believed that this was the result of non-linearity behaviour caused by electrical impedance mismatching. Glass ceramic machining was investigated. A prediction equation of machining rates was proposed in a simple form. Through the novel miniature design, the material removal rates were comparatively superior compared with conventional systems while the size of the proposed transducer and horn assembly was reduced to half that of typical horn shapes.

Ultrasonic spectrometry is now recognised as an effective approach to monitoring chemical processes online, an important application being measurement of the particle size distribution (PSD) in suspensions. The ultrasonic method is based on adaptively fitting a model of wave propagation to measured attenuation. This requires seven physical properties for each of the dispersed and continuous phases, some of which may be unknown and difficult to measure. The sensitivity of the model to these physical parameters is established, with a view to using approximate values for them. This leads to novel approach to PSD estimation; it is based on a combination of "guessing" some parameters and fitting more than two when running the model adaptively. The current wave propagation model uses as its viscosity input the value for water; it breaks down at high solids concentration. Various viscosity models have been applied in the wave propagation model to better simulate local viscosity in the vicinity of suspended particles. The current particle sizing algorithm is relatively slow. Improvements in the algorithm speed have been achieved by the use of a two step fitting procedure which permits rapid adaption of the model and thereby much faster PSD estimation. Conventional ultrasonic spectrometers generate high excitation pulses of 50- 200 V. A novel ultrasonic spectrometer for highly attenuating media has been designed which operates at low transmitted voltage and over a wide frequency bandwidth whilst preserving workable signal to noise ratios (SNRs). The new spectrometer is based on pseudo random binary sequences (PRBSs).

No description available.

PZT films up to 35 μm thick were fabricated, using a composite sol gel route combining a PZT powder and a PZT sol. The maximum temperature for the process was 710°C. A demonstration of single layer and multilayer structures was given to show the flexibility of this technology. With Stoney’s Equation, studies of the in-situ film stress development as a function of the film thickness and density was effectuated. It helped to understand that the internal forces increase considerably with the film thickness and density. This study yields to set up experimental conditions in which a crack free surface finish of a 28μm thick film revealed the adaptability of the spin coating technique to fabricate thick films. The wet etching technology revealed the possibility of a great adaptability to pattern and shape innovative devices such as bars 10 μm wide of 21μm PZT thick film. The results open the way to a wide range of new industrial application requiring small features and/or multilayer PZT thick film with embedded electrodes. The single element and annular array devices have been shown to resonate at approximately 60MHz in air and 50 MHz in water. Three types of the composite thick film – 2C+4S, 2C+5S and 2C+6S – were used to fabricate the Tm-pMUT devices. In each case the most effective poling was obtained by maintaining the poling field of 8.4V/μm during cooling from the poling temperature (200ºC) to ‘freeze’ poled domains in place. This ‘freezing’ was required to prevent the tensile stresses within the film from reorienting the domains at high temperatures when the poling field is removed. Increasing values of thickness mode coupling coefficient (kt) were observed with increasing levels of sol infiltration (decreasing density). Such behaviour is thought to be due to non linear effects on the piezoelectric coefficient (e33) at high levels of porosity. For very dense thick film material a kt of 0.47 was observed which is comparable to that observed for the bulk material.

Les MEMS sont aujourd’hui une réalité économique et sont d’ores et déjà utilisés dans un grand nombre d’objets de notre quotidien. Ces composants peuvent utiliser un actionnement piézoélectrique, notamment à base de PZT déposé en couches minces, du fait de son fort coefficient piézoélectrique. Pour dimensionner au mieux et de manière prédictive ces MEMS à base de PZT, il est important de connaitre les propriétés mécaniques du PZT, matériau complexe. Nous avons utilisé l’acoustique picoseconde, technique qui permet de sonder la matière avec des ondes acoustiques générées par des impulsions laser ultra-courtes. Elle permet de transposer à l’échelle nanométrique le principe du sonar. Lors de cette thèse nous avons étudié le PZT en couches minces par acoustique picoseconde. Nous avons pu extraire le module d’Young et le coefficient de Poisson sans faire l’approximation de l’un ou de l’autre. Nous avons également étudié la relaxation des parois de domaines, en mettant en œuvre des mesures d’acoustique picoseconde en fréquence. A l’aide des propriétés mécaniques du PZT, issues des mesures d’acoustique picoseconde, nous avons pu extraire le coefficient piézoélectrique par la comparaison de modèles et de mesures sur une poutre encastrée-libre avec un actionneur à base de PZT. Enfin, nous avons appliqué ces données d’entrée au dimensionnement de dalles haptiques utilisant des actionneurs PZT. Le très bon accord entre la caractérisation de ces dispositifs et les modèles mis en place prouve l’apport de l’acoustique picoseconde pour le dimensionnement de MEMS. / MEMS components are today an economic reality and are already used in many mass market applications. These devices can use a piezoelectric actuation, in particular based on thin-film PZT due to its high piezoelectric coefficient. To perform predictive design of high performances components based on PZT actuators, mechanical properties of the PZT are required. We used the picosecond ultrasonic technique which probes thin layers with high frequency acoustics waves generated by ultra-short laser pulses. It allows the transposition of the sonar principle at nanometric scale. During this PhD, we studied thin-film PZT using picosecond ultrasonics. We extracted both Young’s modulus and Poisson ratio without an approximation of one or the other. We also studied wall domain relaxation using picosecond ultrasonic measurement at various frequencies. Using PZT mechanical properties obtained from picosecond ultrasonics, we extracted the PZT piezoelectric coefficient, from the comparison between PZT-based cantilever measurement and numerical modeling. Finally, we applied these data for the design of haptic plates using thin-film PZT actuators. The good agreement between haptic plate measurements and modelization proves all the benefit of picosecond ultrasonics for MEMS design.