A Testbench System for Structural Health Monitoring with Guided-Wave Ultrasound

This dissertation covers the design of the transducers and electronics of a structural health monitoring (SHM) testbench system targeted at plate-like structures. The health inspection principle behind the system is the transmission and reception of guided-wave ultrasound along the structure under test, using piezoelectric transducers made of poled polyvinylidene fluoride (PVDF) film. The aim of this work is the creation of a system with custom electronics that can serve as a versatile testbench for the research activities in the field of SHM of complex material, such as carbon and glass-fiber composites, and will eventually bridge the gap between research and the development of highly-integrated sensor networks to be used in industrial, automotive, and aerospace applications. While many guided-wave SHM techniques base their operation on monolithic elements, the proposed system moves in the direction of providing multichannel transmit/receive capabilities to each transducer, transforming them in small-scale phased arrays. Since different SHM applications require different topologies and number of transducers to effectively cover a structure, the system architecture is designed around the vision of a (wired) sensor network: each transducer is connected to its own dedicated electronics, emulating a sensor node. Multiple identical nodes can thus be placed on the target structure and interact to perform the required health monitoring functions in a distributed fashion. The transducers designed for this system are an improvement of the well-known interdigital transducer (IDT), where a few novelties are added: a circular sensor (intended for isotropic guided-wave reception) and a resistive temperature device. A different version of the IDT is also presented where every electrode (finger) has an independent connection that can be attached to different transmitters and receivers, thus creating an array. The electronics are designed to include multichannel transmission and data acquisition tailored to the proposed transducers. Guided-wave generation is performed by high-voltage, 5-level, differential class D amplifiers that can generate arbitrary signals up to 1MHz with inter-channel synchronization. The signal reception circuitry includes two swappable pre-amplifier stages (charge-mode and voltage-mode) in addition to a standard data acquisition chain. The electronics are completed by a system-on-chip (FPGA plus ARM processor) that operates the various components, performs signal analysis, and exchanges data with other nodes. The core contents of this dissertation include the development and testing of the transducers and a subset of the system electronics: the ultrasound transmission and reception modules. The remainder of the system is presented at the architectural level.