Data Passes Through Metal Like Magic With a New Twist on ADL

Acoustic data link (ADL) technology translates electronic data into mechanical vibration and exciting acoustic waves that are transmittable through solid materials. The received acoustic signal can then be captured and converted back into a digital format. In parallel, ADL provides the power to operate the sensor without additional power supply.

Acoustic data transmission is not new, but recently engineers from TDK Electronics have devised an ADL technology that is easy to implement, easy to use, lends itself to affordable solutions, and reliably transmits energy and sensor data from inside sealed metal containers, metal pipes, and other metal structures.

Principle of the ultrasonic-based data transmission with two piezo discs. Image courtesy of TDK.

The initial implementation can sustain data rates up to 11 kilobits per second (kbps), although rates measured in megabits per second (Mbps) are eminently achievable. Power transfer is currently about 30 milliwatts, but 100 milliwatts or more is entirely possible.

The challenge of sending data through metal

Transmitting a wireless signal from inside a sealed metal container is a challenge because the Faraday cage of such structures is effectively blocking electromagnetic signals. Acoustic data transmission is one answer to the challenge but attempts to use the technology thus far have been difficult. A limiting factor has been that the amount of power that can be transferred has been quite low. In the low frequency range more energy can be transferred, but transmissible data rates are limited.

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And yet there are any number of applications in which phenomena or objects inside sealed metal containers should be monitored. The container might be anything from a box to a pipe, and the thing to be monitored might be an entirely physical process, such as the flow of a liquid, or it could be the status of a mechanical or electromechanical device, for example the power level of a rechargeable battery.

Researchers devised an acoustic channel structure that makes ADL more effective.

The new acoustic channel consists of two newly developed piezo transducers -the ADL links- each connected to either side of a homogeneous metal plate. These links convert electrical signals into a directed mechanical vibration which in turn excites acoustic waves on the material surface. The process in reverse converts the acoustic waves back into electrical signals.

Materials resonate in certain modes. Metal plates, for example, show narrowband resonances at multiples of the acoustic wavelength in the material. At frequencies between these resonances the acoustic signals are strongly dampened.

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At frequencies greater than 10.5 MHz, this results in an area with relatively low and constant attenuation, whereby signal transmission with acoustic waves is perfectly possible. Image courtesy of TDK.

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. The engineers were able to adopt that passband for ADL between 10 MHz and 14 MHz.

The ADL and NFC fit

This was what the engineers were aiming for – to enable an acoustic passband that is broad enough for reliable data transmission and encompasses the frequency used by the near-field communication (NFC) standard – 13.56 MHz. This made it possible for the engineers to use NFC protocols for data transmission in the ADL system they were developing. This implementation inherits from NFC a data transfer rate of about 11 kpbs, along with the data security features of the protocol.

The experimental ADL implementation will take advantage of the maturity of NFC technology in a manner that should keep commercial costs down. For example, existing commercial NFC chipsets can be used.

The power transfer with ADL is up to 10 milliamps at 3 volts – roughly 30 milliwatts. In their prototypes, engineers demonstrated the simultaneous power supply and data collection. The solution works over a wide temperature range, from as low as  -40 °C to 105 °C.

It is important to note that 11 kbps and 30 milliwatts are not maximums. Those performance parameters are tied to the use of NFC protocols. The ADL channel can support up to 2 MHz wideband, which in turn will support data transfer rates of megabits per second. Initial indications suggest that power transfer in excess of 100 milliwatts is entirely possible.

ADL + NFC in action

This new ADL technology, though not commercialized yet, is being tested by commercial  enterprises. The application involves pipe installations, for pressure and leakage detection, and applications where secure access to sensors or data storages in metallic structures is required.

For the latter applications, the NFC technology ensures that data access is limited to authorized persons by effective ID check for which NFC is widely established.

This technology could be recommended for any application that would benefit from sensor monitoring, but where there is no option for establishing a wireline data connection and otherwise no practical means of establishing a wireless connection.

In fact, TDK researchers are interested in working with partners and adopters who would be interested to apply and test the technology in their specific applications. ADL is a solution that should work with a wide variety of sensors, including those for pressure, temperature, and humidity.