The vast majority of the infrastructure in industrialized countries was constructed between the 1950s and 1970s. Since then, it deteriorated considerably, slowing down economic development and posing a threat to safety. Structural health monitoring enables more efficient maintenance of our aging and new infrastructure, such as bridges, pipelines, wind turbines, and similar structures. Unfortunately, the large number of sensors required for continuous and real-time monitoring drives the costs of deployment and supply. Batteryless sensors using backscatter communication are a promising way to cut down these costs immensely. However, existing radio-frequency-based technologies are infeasible in environments shielded by metal. Here, acoustic backscatter communication and power transfer are vital in realizing economically scalable wireless sensor networks embedded in such places.

The wireless transfer of energy through a metal barrier enables us to supply sensors in hard-to-reach places without the need for accessing it, e. g., for battery replacement—potentially over the whole lifetime of the structure. To realize this zero-maintenance vision, the sensor nodes embedded in the structure must be as energy-efficient as possible. Wireless communication, i. e., transferring data from the sensor to a central server, is typically a very power-hungry task. Therefore, the Institute for Autonomous Cyber-Physical Systems at Hamburg University of Technology is carrying out fundamental research on a novel communication paradigm: Passive acoustic backscatter communication. Instead of actively generating the communication signal, backscatter devices merely change how they reflect existing signals in the structure. This approach not only requires significantly less energy than conventional active communication, but it is also achieved with elementary circuits—reducing the cost and size of the sensors.

The research at our institute combines fundamental mathematical modeling of wave transmission through structures with simulation and real-world experiments with custom prototypes to test the opportunities and limitations of the technology. We demonstrated that communication and power transfer functions over distances of more than three meters, and it can be scaled up to work over several tens of meters. However, the metal structures pose significant challenges for practical applications: Metal structures reverberate strongly, as many will intuitively know from empty barrels, tanks or gongs. This reverberation significantly complicates the reception of acoustic backscatter messages, as many copies of the transmitted message superimpose at the receiver. We are working on novel methods to make acoustic backscatter communication more robust against the detrimental effects of reverberation. Furthermore, the current prototypes require a significant setup for every deployment.