Sensors use electrets to measure weak signals

Sensors use electrets to measure weak signals
A Kiel University research team, including Marleen Schweichel and Stefan Schröder, have developed a sensor concept for low frequencies.

Scientists at Kiel University have developed a new concept for cantilever sensors that can measure low frequency signals such as heart and brain activity. The small, energy-efficient sensors use electrets, permanently electrically charged material, that can be used in medical applications as well as in hearing aids or mobile phones.

The research team, led by Professor Rainer Adelung and Professor Franz Faupel, developed sensors comprising a thin silicon strip, which in the simplest case has two layers applied: the first responds to magnetic fields (magnetostrictive material), and the second can emit an electrical voltage (piezoelectric material). "If a magnetic field occurs, the first layer deforms and thus bends the whole strip—which vibrates like a diving board at a swimming pool," explained Faupel. Due to the deformation, the second layer emits a measurable voltage signal.

“With our new sensor concept, we searched for a way to make this conversion of mechanical energy into electrical energy even more effective, by giving the bending beam more impetus," explained doctoral researcher Marleen Schweichel. The more the bending beam vibrates, the stronger the emitted electrical signal.

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Adelung added, "With our approach, we were able to make a small bending beam made of hard material behave like a soft material, and vibrate at low frequencies—and what’s more, at an even greater amplitude.”

The key was the electret, which the researchers applied underneath the bending beam. Due to its self-equilibrating stress, the electret pulls the bending beam in the opposite direction, and thereby magnifies the vibration of the beam, and thus the electrical signal of the sensor.

To be able to read this signal as accurately as possible, the research team also integrated a new approach towards noise reduction into its alternative sensor concept. With an extremely fast measurement, the individual signals can be “picked up” in between the noise, according to first author Mona Mintken.   

Similar to permanent magnets, which create their own persistent magnetic field without a power supply, electrets also create their own permanent electric field. "The electret thereby gives the sensor a built-in electrical potential. The sensor itself then requires no external power supply, and can be used for mobile applications," explained doctoral researcher Stefan Schröder.

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