ACOUSTIC WAVES Through-metal data and energy transmission by means of piezo-acoustic ultrasonic technique
TDK has developed a process in which ultrasonic waves are used to transmit data and energy. These signals can even penetrate metal. This enables devices to be identified or sensor data to be read out, whereby the energy transmission is simultaneously possible in closed metal chambers or pipes, for example.
RFID has become a well-established technology in logistics. However, in some conditions, it is not possible to use electromagnetic waves. A metal enclosure, for example, offers protection against RFID radio waves, preventing the use of device identification and sensor data transmission by means of classical contactless communication. A technology developed by TDK, which uses acoustic waves in materials instead of electromagnetic waves, enables data transmission even in such challenging environments. By combining application-specific sensor mounts and the acoustic transmission channel, new application options may open up in system integration.
An acoustic channel may be formed by a structure consisting of (outside) piezo-element, glue layer, homogeneous metal plate, glue layer and (inside) piezo-element (Figure 1).
The piezo-element can change its thickness by +/- 3 nm, stimulated by an electric field applied on the material in the form of a voltage signal on the metallized material surface. Therefore, it can convert electrical signals, even in the 10 MHz range, into a mechanical vibration, which can excite acoustic waves on a material surface. A material geometry normally resonates in certain modes. A metal plate, for example, shows narrowband resonances at multiples of the acoustic wavelength in the material and deep notches in-between, which looks like a resonance comb. However, due to its geometry and material properties, the piezo element also resonates. Due to the elastic material properties of mainly the glue layers, a superposition of these modes forms an acoustic transmission channel with a relatively flat passband. In this channel, the deep notches of the resonance comb disappear (Figure 2), allowing data transmission according to the near-field communication (NFC) standard.
Very low variation with temperature offers a wide field of applications
The transmission channel’s dependency on temperature and the thickness of the homogeneous metal material is surprisingly low. This independence is due to the individual peaks and notches of the resonance comb in the communication window being less pronounced than outside of this area for lower frequencies (Figure 2). As the NFC protocol standard applies over this acoustic channel, engineers can use chipsets already available on the market. Data transmission can also be protected from faults without influencing the channel. This protection enables applications that cannot be implemented using classical NFC technology.
The trend towards wireless sensor interfaces, supported by NFC tag ICs featuring interfaces like I²C, Power Harvesting and qualification for increased temperature ranges, such as - 40 °C to +105 °C, also enables a swift introduction to this piezo-acoustic interface technique in a broad field of applications. This technique can transfer data through closed metal containers or even piping systems in processing plants. It is also suitable for batteries in e-mobility, for example, to check the charge level.
A demo illustrates the functional capability of this new technique, which performs data and energy transmission through the wall of a steel housing (Figure 3). Here, an environmental sensor inside the metal container is controlled, delivering temperature, humidity, and air pressure information. An air humidifier and fan-heater provide stimuli for the measurement. The 64 byte data frames provide updated information with 12 measurements per second, resulting in 6 kbps throughput. Besides that, 15 mW of harvested power is available to operate the sensor and an additional microcontroller.