Dry-Coupled Airborne Ultrasonic Inspection Using Coded Excitation

Unmanned Aerial Vehicles (UAVs) offer significant potential benefits to the inspection of large-scale facilities due to their ability to access areas where manual inspection is not practical. Ultrasonic inspections typically utilise acoustic couplant, placed between the specimen and transducer surfaces, to eliminate any air gap and enable acoustic energy propagation. Conventional ultrasonic inspection UAVs contain a mechanical system to deliver a small quantity of liquid couplant between the transducer and inspection surface. Such mechanisms increase the system payload, resulting in the reduction of UAV flight endurance and inspection efficiency. Any couplant remaining on the surface may also increase the risk of corrosion. Instead of a liquid couplant layer, dry-coupled ultrasonic transducers utilise a thin layer of rubberised material. However, the acoustic characteristics of the conformable materials typically result in dry-coupled transducers with a lower Signal-to-Noise Ratio (SNR) than liquid-coupled sensors. Coded excitation, a pulse compression technology, improves SNR without sacrificing the measurement acquisition rate, as is the case with signal averaging. This paper explores the potential for application of coded excitation to maintain the SNR aboard a UAV deploying a dry-coupled transducer.