Data and power going through metallic walls

In applications like process automation and food and beverage production, data often needs to be transmitted from sensors within closed metallic structures. So, how to get data into and out of these without drilling a hole? Acoustic Data Link is TDK’s answer.

Sealed metal containers, metal pipes, and alike closed metal structures form a Faraday cage which effectively blocks electromagnetic signals. This barrier prevents conventional wireless data transmission. Acoustic Data Link (ADL) is TDK’s answer to this challenge.

Initially introduced in 2021, the team has now developed ADL technology further for reliable and easy implementation for various applications such as sensor data monitoring. The technology can handle data rates of 11 kbps and power transfer of 30 mW.

The heart of the ADL system is the newly developed piezo transducer, called ADL link (Fig. 1). Two of these – one on either side of a homogeneous metal plate – form the acoustic channel and act as transducers (Fig. 2). In the transmit mode, they convert electrical signals into a directional mechanical vibration, while in the receive mode, the transducer converts the acoustic waves back into electrical signals. For power supply, a superimposed continuous sine wave from one ADL link is transferred into electrical voltage by the opposite transducer.

Figure 1:

Piezoelectric ADL links act as transducers and convert electrical signals into a directional mechanical vibration and vice versa.
 

Figure 2:

Two ADL links – one on either side of a homogeneous metal plate – form the acoustic channel that transmits data and power through metal walls.

ADL and NFC – a perfect match

Core development targets for ADL are effective and secure data transfer, application of the established NFC data protocol, reliable power supply, and components that allow process engineers or RF integrators an easy setup of the acoustic channel.

Solid materials resonate acoustic waves in certain modes. Metals show narrowband resonances at multiples of the acoustic wavelength. At frequencies between these resonances, the acoustic signals are largely attenuated.

The piezo element inside the ADL link also resonates, but due to the elastic material properties of intermediate layers and glue, a superposition of these modes forms an acoustic transmission channel with a relatively flat passband between 10 and 14 MHz. This enabled TDK engineers to create an acoustic passband wide enough for reliable data transmission and covering the frequency used by the Near Field Communication (NFC) standard – 13.56 MHz.

This made it possible to use the NFC protocol for data transmission in the ADL system. The usage of NFC enables the ADL system data transfer rates of about 11 kbps, along with the established data security of this protocol. The ADL system integration leverages the maturity of NFC technology to keep commercial costs low. For example, existing commercial NFC chipsets can be used.

ADL as a scalable solution

The power transfer with ADL is up to 10 mA at 3 V – roughly 30 mW. In prototypes, engineers demonstrated simultaneous power supply and data acquisition for more than ten different sensors monitoring temperature, pressure, humidity, and acceleration. The solution works over a wide temperature range from -40°C to +105°C.

Data rates of 11 kbps and power transfer of 30 mW are not maximums. Those parameters are tied to the use of NFC protocols. The ADL channel itself can support up to 2 MHz bandwidth, which in turn will support data transfer rates in the megabits-per-second range. Initial indications suggest that a power transfer of 100 mW should be possible.

Figure 3:

ADL demonstrator showing how data and power are sent back and forth to a pressurized and hermetically sealed tube.

This new ADL technology, though not commercialized yet, is being tested by commercial enterprises for applications like pipe installations for pressure and leakage detection, and applications where secure access to sensors or data storage devices in metallic structures is required (Fig. 3). For data storage applications, the NFC technology ensures that data access is limited to authorized persons by effective ID checks for which NFC is widely established.

Summary

Acoustic Data Link can work in any application where there is no option for establishing a wireline data connection or where a non-invasive sensor integration is mandatory.

TDK is interested in working with partners and adopters who would be interested to apply and test the technology in their specific applications. 

Volker Wischnat, TDK Customer Development Lead