# Ultrasonic Power and Data Transfer through Multiple Curved Layers Applied to Pipe Instrumentation
## 1 Introduction  
Ultrasonic waves can be used for wireless data communication and power transfer. Typical examples of well-developed practices include underwater acoustic communications [1; 2; 3], and biomedical applications where ultrasound is used to power and communicate with implantable devices [4; 5; 6]. The field of power and data transfer through metal layers using ultrasonic waves is, however, less well developed when compared to the aforementioned fields. Wireless transmission of power and data through metallic layers is relevant in applications where metallic walls cannot be penetrated, such as submarines [7] and pressure vessels [8]. Over the past 20 years, the use of an acoustic-electric channel on a metal barrier has been investigated [9], revealing a higher efficiency of power and data transfer when compared to electromagnetic-based solutions, either based on inductive [10; 11; 12] or capacitive [13] couplings. Several setups that rely on different types of transducers [14], channel configurations [8], and data modulations [10; 15; 16; 17] have been investigated. The most common setup consists of a pair of ceramic piezoelectric transducers encompassing a metal layer through which communication is intended. Adhesive epoxy is usually used to bond the transducers on the metal surfaces. The power and data communication through an acoustic channel typically contains three major components, namely the outside block, the inside block andthe acoustic channel. One possible topology is the so-called reflected-power [18]. In this setting, information is sent from the inside block to the outside block, whilst energy is sent from the outside to the inside block. Communication is possible by varying the impedance of the piezoelectric transducer located at the inside block, which consequently changes the amplitude of the reflected wave [15; 17; 18], and modulates the continuous wave signal sent from the outside block. Typical values of data rate and power transfer for this configuration are around hundreds of bps and hundreds of mW [9; 15; 18]. Usually the medium through which ultrasonic communication takes place consists only of a single metallic layer and bonding layers necessary to fix the transducers [9]. It is naturally expected that, if the medium is composed of several layers, then the communication efficiency would be reduced due to the impedance difference along the wave propagating path, which naturally introduces mismatches, and consequently, less power may be expected to cross the channel. There are, however, few published works on ultrasonic communication and power transfer across multilayered channels. [PERSON] et al. [19] analyzed the transmission of power and data in the case of multiple layers of different materials, in a steel-water-steel configuration. Two channels were characterized, the first with a steel thickness of 15.97 mm and a water layer of 88.3 mm, and the second with a steel thickness of 10.92 mm and a water layer of 55.62 mm. They experimentally showed a power transfer efficiency of 30% and a practical achievable data rate around 4 Mbps using orthogonal frequency-division multiplexing (OFDM) modulation. Another characteristic that directly impacts the efficiency of the ultrasonic wave transmission is the eventual curvature of a layer; a circular pipe or a metal curved shell could change the power and data transfer [17; 20]. Analysis and characterization of ultrasonic channels that have curved metal walls is gaining attention. [PERSON] [17] demonstrated the feasibility to transmit power and data through a curved metal wall made of aluminum. A pipe with an outer diameter of 76 mm and thickness of 3.2 mm was experimentally evaluated by attaching piezoelectric transducers on the pipe surface with a transition piece that matched the curvature of the tube. When the electric impedance of the system was matched, a power transfer efficiency close to 30% and a data transfer rate of 10 kbps were achieved. [PERSON] et al.