Guided acoustic waves in sensors and actuators
Since its foundation, ISAT has been conducting research in the field of non-invasive ultrasonic sensor technology with a focus on “guided acoustic waves”. Guided acoustic waves are ultrasonic waves that propagate on the surface of a substrate. The most commonly used types are Rayleigh and Lamb waves. Lamb waves are plate oscillations and occur when the wavelength of the excited sound wave is greater than or equal to the plate thickness. They therefore exhibit particle deflections on both sides of a substrate, i.e. the wave passes through the entire material. This property of lamb waves allows their excitation on the back of a material, while the actual sensory interaction can take place on the front. Rayleigh waves are when the wavelength of the excited sound wave is smaller than the thickness of the substrate. Particle deflections caused by Rayleigh waves are thus only generated on the side of the substrate on which the wave was excited. ISAT uses Lamb waves mainly for the development of non-invasive sensors and actuators due to the above mentioned properties. Rayleigh waves are used in the institute for material characterization or crack detection.
To generate guided acoustic waves at ISAT, comb-like structured metal electrode structures on piezo materials are primarily used. By the number of finger pairs and the finger distance of the combs, the most different frequency characteristics can be achieved, whereby an adaptation of the excitation is possible depending upon material type and material thickness. As carrier materials for guided acoustic waves the most different materials can function, e.g. glass, metals, ceramics or different plastics. It is also possible to excite guided acoustic waves contactlessly via optical methods (laser excitation) or magnetostrictively.
In both Lamb waves and Rayleigh waves, the vibrations occurring on the surface of the material are so small that they cannot be felt and no damage to the material occurs.
Guided acoustic waves can be used in a variety of sensor applications. They can also be used for actuator development. The basis for the use of guided acoustic waves in sensors and actuators is above all their specific interaction properties with liquids.
2. Evanescent wave field
If the speed of sound in a liquid is greater than the speed of sound of the guided acoustic waves on the substrate, the wave cannot be decoupled into the liquid in accordance with the law of refraction. The wave remains bound to the substrate. These waves that do not decouple are called Scholte waves. An important property of these Scholte waves is their evanescent field (“energy field”), which extends from the solid into the adjacent liquid.
Evanescent wave field in actuators
The evanescent field of Scholte waves generates eddy currents at the solid/liquid interface with sufficient excitation voltage. The dimension of the vortices corresponds to the order of magnitude of the wavelength of the acoustic wave, so that it can be controlled how far the evanescent field reaches into the liquid. The advantage of using Scholte waves is that processes can be specifically influenced at the interface between solid and liquid. ISAT uses this property in the field of actuator technology to influence electrochemical reactions, e.g. to accelerate the charging of accumulators.
Evanescent wave field in sensor technology
In the field of sensor technology, ISAT uses Scholte waves for level measurement, among other things.
Use of guided waves for material characterization
Guided waves, however, can also be used sensorically without the substrate having to be in contact with liquid. Guided waves are particularly suitable for material characterization. ISAT has developed a non-contact laser-based method for the excitation and detection of guided acoustic waves. A short laser pulse focused on the substrate surface leads to a locally sharp thermal expansion and contraction of the material, which allows broadband excitation of acoustic waves. The laser-based method can be used for detection and characterization of layer systems, stresses or crack detection. You can find out more about non-contact generation and detection of guided acoustic waves here.