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The retrieval of data from a sensor, enclosed by a metallic structure, such as a naval vessel, pipeline or nuclear flask is often very challenging. To maintain structural integrity it is not desirable to penetrate the wall of the structure, preventing any hard-wired solution. Furthermore, the conductive nature of the structure prevents the use of radio communications. Applications involving sealed containers also have a requirement for power delivery, as the periodic changing of batteries is not possible. Ultrasound has previously been identified as an attractive approach but there are two key challenges: efficient/reliable ultrasonic transduction and a method of overcoming the inherent multipath distortion resulting from boundary reflections. Previous studies have utilised piezoelectric contact transducers, however, they are impractical due to their reliance on coupling, i.e. the bond between the transducer and the metal surface, which leads to concerns over long term reliability. A non-contact transducer overcomes this key drawback, thus highlighting the electromagnetic acoustic transducer (EMAT) as a favourable alternative. This thesis presents the design and testing of an EMAT with appropriate characteristics for through-metal data communications. A low cost, low power data transmission scheme is presented for overcoming acoustic multipath based on pulse position modulation (PPM). Due to the necessary guard time, the data rate is limited to 50kbps. A second solution is presented employing continuous wave, Quadrature phase shift keying (QPSK) modulation, allowing data rates in excess of 1Mbps to be achieved. Equalisation is required to avoid intersymbol interference (ISI) and a decision feedback equaliser (DFE) is shown to be adept at mitigating this effect. The relatively low efficiency of an EMAT makes it unsuitable for power delivery, consequently, an alternative non-contact approach, utilising inductive coupling, is explored. Power transfer efficiency of ≈ 4% is shown to be achievable through 20mm thick stainless steel.

The design and function of receptors for molecular recognition is of vital importance for the construction of novel sensors. Initially the project has investigated the relationship between the shape of the active site of the receptor and its sensitivity and selectivity for analytes in solution. The techniques of electrochemistry, NMR and molecular modelling have been used to study materials interactions. An optimum receptor design was established for a range of analytes so that the surface of an electrode could be modified to create an analytical sensor. The specific adsorption of both the receptor and the analyte on the electrode surface were probed to produce an amperometric sensor for mandelic and lactic acids.;Subsequently, the project has involved the investigation of small molecule adsorption, recognition and permeation at polymer plant wax constituent layers that model leaf cuticles. Correlations have been sought between the responses to these surfactants/adjuvants and the wax compositions. At low levels of interaction, the quartz crystal microbalance was used as a gravimetric sensor of these processes. At higher levels of interaction, when permeation of the film is sufficient to plasticise it, the rheological effects were used to explore solvent/small molecule permeation. In each case, qualitative data was used to diagnose film rigidity vs. viscoelasticity and quantitative data to determine film mass (thickness) or shear moduli, as appropriate. The experimental measurements have been supported by a computer model derived from the processes occurring at the interface and the resultant changes in film viscoelasatic properties are discussed.

Structural Health Monitoring (SHM) ensures the structural health and safety of critical structures covering a wide range of application areas. This thesis presents novel, low-cost and good-performance fibre Bragg grating (FBG) based systems for detection of Acoustic Emission (AE) in aircraft structures, which is a part of SHM. Importantly a key aim, during the design of these systems, was to produce systems that were sufficiently small to install in an aircraft for lifetime monitoring. Two important techniques for monitoring high frequency AE that were developed as a part of this research were, Quadrature recombination technique and Active tracking technique. Active tracking technique was used extensively and was further developed to overcome the limitations that were observed while testing it at several test facilities and with different optical fibre sensors. This system was able to eliminate any low frequency spectrum shift due to environmental perturbation and keeps the sensor always working at optimum operation point. This is highly desirable in harsh industrial and operationally active environments. Experimental work carried out in the laboratory has proved that such systems can be used for high frequency detection and have capability to detect up to 600 kHz. However, the range of frequency depends upon the requirement and design of the interrogation system as the system can be altered accordingly for different applications. Several optical fibre configurations for wavelength detection were designed during the course of this work along with industrial partners. Fibre Bragg grating Fabry-Perot (FBG-FP) sensors have shown higher sensitivity and usability than the uniform FBGs to be used with such system. This was shown experimentally. The author is certain that further research will lead to development of a commercially marketable product and the use of active tracking systems can be extended in areas of healthcare, civil infrastructure monitoring etc. where it can be deployed. Finally, the AE detection system has been developed to aerospace requirements and was tested at NDT & Testing Technology test facility based at Airbus, Filton, UK on A350 testing panels.

The use of continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared (IR) region has great potential for the design of ultra-sensitive, compact, robust and cost-effective novel optical sensors capable of detecting trace species present in the atmosphere. The research work presented in this thesis focuses on the development and implementation of a fully automated, cryogen-free and compact cw-CRD spectrometer, combined with adsorbent filled dual-trap sample pre-concentration for monitoring trace C2H2 mixing ratios in ambient air.

The measurement of accurate three-dimensional (3D) shape information for an object is routinely required in many industrial processes. The size of the objects to be measured may vary over a very wide range, from the sub-centimetre to tens of meters. In the context of this work we will define 'small objects' as those that will fit within a one metre cubic volume and 'large objects' as those that are larger than this volume. Techniques for the measurement of relatively small objects are well developed at the moment. The most advanced of these are non-contact optical techniques. However, when applied to relatively large objects, existing techniques face a number of limiting factors, which results in a compromise between the field of view of the sensor and the spatial resolution of the resulting image.

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