A Performance Evaluation Methodology for Reconfigurable Ultrasonic Sparse Arrays Used in Mobile Structural Health Monitoring

Xudong Niu, A. Croxford, B. Drinkwater, Michael D. Todd

Advances in robotics have led to autonomous platforms that convey sensor packages used for reconfigurable (mobile) monitoring solutions. In particular, ultrasound has rapidly evolved in this modality, where in-situ advanced autonomous manufacturing processes are outfitted with ultrasonic interrogation for near real-time part qualification. In the application motivating this current work, swarms of simple, low-cost robots are anticipated to monitor the interior of pipes with an ability to communicate to reconfigure the array for adaptive, optimal inspection. This work considers optimality to be the maximum probability of detection (POD) achievable by an array of robots, each outfitted with a single transmission/receive piezoelectric element capable of pulse-echo measurements only to minimize communication bandwidth among the mobile array robots. The fundamental objective is then to develop an initial framework for understanding and quantifying the performance of such reconfigurable arrays in terms of their ability to detect a target defect class characterized by its expected scattering behavior. A further assumption is to study array geometries that tesselate, since such geometries may be readily scaled to large structural areas while maintaining coverage. The work first derives a generalized POD model for a point defect characterized by its scattering matrix (orientation, size) and distance from a single transducer. The model is then tested and validated on an experiment with a manufactured defect in an aluminum plate. With the validated model, this work proceeds to build basic arrangements of transducers to investigate array-level POD performance by fusing the information provided by each individual transducer in the network. The work studies how topological array parameters (e.g., transducer pitch, overall array shape, and array depth) affect global POD performance. The overall framework, while quite generally applicable, will be discussed within the context of a highly directional scatterer (e.g., a crack) as a “worst case” defect example and may be used to inform reconfigurable array design. Some conclusions drawn from the study are: (i) highly specular defects (like a larger crack, with characteristic length larger than half the wavelength of the ultrasonic waves used, l/2) demand significant angular diversity, achieved by adding network layers, (ii) array pitch plays an important role, with smaller pitches consistently performing best as the distance dependence dominates angular dependence, but likely at the cost of much larger array sizes, and (iii) array shape (arrangement of transducers) plays a moderate role, e.g., minimal average distance between transducers and defect locations in triangular shapes maximizes POD for smaller defects, but hexagonal shape angular diversity becomes more important for larger defects. This study is fundamentally a design tool, and future work will look at different objective functions, including mapping POD performance to cost in order to include transducer/robot costs and path planning for a global cost optimization.