Unpowered Wireless Ultrasound Generation and Sensing for Structural Health Monitoring of Composites

Damage detection based on ultrasonic waves is one of the most popular inspection schemes employed by many structural health monitoring (SHM) systems. We propose a novel unpowered wireless ultrasound generation and sensing system for SHM. Since ultrasonic signals generally have a frequency of a few tens of kilohertz to a few megahertz, they cannot be easily transmitted using a wireless means because of high sampling and high bandwidth requirements. Our system uses a frequency mixer to up-convert the ultrasonic signal to microwave frequency so that it can be transmitted wirelessly using a small antenna and down-convert the ultrasonic signal back to its original frequency once the wireless signal is received. Because the mixing of the ultrasound and the microwave signal is performed using a passive microwave component, i.e. a frequency mixer, the wireless sensor nodes do not need any local power. For ultrasound generation, an ultrasound-modulated signal is transmitted to a remote ultrasound generator, the generator recovers the ultrasound excitation signal using down conversion and supply it to a piezoelectric actuator. Since the sensing and generating nodes have the same configuration, each node can either act as a generator or a sensor. Sensor array can also be implemented. Compared to SAW sensor, our approach requires smaller antenna and achieves longer distance between interrogation unit and the sensor. Another key difference is that SAW sensor requires special piezoelectric substrate whereas our approach can use any piezoelectric sensor. Finally, the proposed approach is compatible with previous works on ultrasound-based technology. Previously developed knowledge on data processing and feature extraction can be easily adopted.