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Ultrasonic Piezo Transducers for Smart Sensors
By Frank Moeller and Stefan Vorndran, PI (Physik Instrumente) P
iezoelectric transducers are based on electro-ceramic mate- rials that convert mechanical
power into electrical power and vice versa. There are two effects. First, when a force is applied, an electric charge is generated (direct piezo effect). Then, when an electric field is applied, a dimensional change occurs (inverse piezo effect). While the inverse piezo effect can be used for motion control applications, the direct piezo effect or a combination of both effects lends itself to the manu- facture of sensors.
No Wear The piezo effect is based exclu-
sively on displacements within the crystal lattice of the piezoelectric ele- ment and not subject to any mechani- cal friction or wear. It is also highly sensitive — the smallest deformations produce a measurable charge dis- placement, and potential changes as small as microvolts can produce sub- nanometric motion — after all, atomic force microscopes use the piezo effect. This opens up a multitude of potential applications, including ultrasonic,
Variety of standard piezo transducer elements, each one suited
for a specific application: tubes, disks, benders, shear transducers, or translators.
sound, for example, is a classic piezo application, because AC voltages make piezo elements oscillate. Oscillation frequencies of up to 20 MHz are feasible. Ultrasound is sound above the human hearing
industry, medical engineering and research. Ultrasonic piezo transduc- er applications include distance measurement, object recognition, fill- ing level or flow measurements, high-resolution material tests, and medical diagnosis and therapy.
Flexibility in Frequency and Geometry
In order to adapt the transduc- take off
ers to different applications, manufac- turers need to be able to modify the piezo formulation and mechanical design — starting with the piezo pow- der, and maintaining full control over all manufacturing steps, allowing for fully-customized components, assem- blies and systems. There are two basic classifications: sensor applications for frequencies of up to 20 MHz, and power ultrasound with much higher energy densities. A great variety of geome-
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tries and resonant frequencies can be realized, in addition to the material selection, for the respec- tive application. To work effi- ciently for an OEM, standard components, such as piezoceram- ic rings, piezo tubes and shear elements, need to be stocked or made available on very short notice on the basis of semi-fin- ished products. To measure flow, filling level, and force or accelera- tion, customized sensor compo- nents need to be integrated into the application.
Level Measurement A common application of
ultrasonic piezo transducers is the measurement of filling levels. Here, the ultrasonic delay measure- ment makes use of both the direct and inverse piezo effect. The piezo trans- ducer is placed outside of the medium to be detected and works as a trans- mitter and receiver. It emits an ultra- sonic pulse that is reflected by the fill- ing medium.
Propagation time is a measure To Be Precise.
of the distance traveled in the empty part of the container and inversely proportional to the filling level. This
Bending radii of up to 20 mm (0.8 in.) can be achieved with DuraAct patch transducers.
method makes the ceramic extremely bendable, which allows it to be attached to moving structures, where it gets deformed and thereby gener- ates charge transfers. In adaptive systems technology,
the bendable transducer is used as both sensor and actuator. These sys- tems measure interfering vibrations and compensate them at the same time. Another application — struc-
Continued on next page
force or acceleration sensors, and com- plex adaptive systems. Generating and detecting ultra-
range, typically starting with fre- quencies above 20 kHz. This frequen- cy range is used for many purposes in
principle allows non-contact meas- urements for both liquids and solids, for example, in food or material silos. The accuracy depends on the reflec- tivity of the respective surface. Piezo elements placed at differ-
ent heights in submersible transduc- ers or tuning-fork sensors can also be used as level switches in containers. The piezo transducers excite a tun- ing fork at its natural frequency. When it comes into contact with
the medium, the resulting shift in the resonant frequency is detected by an electronic circuit. This method is very reliable and completely inde- pendent of the type of filling medium.
Acceleration Sensors Piezoelectric acceleration sen-
sors operate at lower frequencies than most ultrasonic transducers. At their core is a piezo element that is connected to an inert seismic mass. When acceleration occurs, the inert mass amplifies the mechanical defor- mation of the piezo disc increasing the measurable electric voltage cre- ated by the piezo effect. These highly dynamic sensors
detect accelerations in a broad fre- quency range with a nearly linear characteristic over the complete measurement range. They provide high resolution and are available for tensile, pressure and shear forces.
Adaptive Systems Technology The DuraAct patch transducer,
developed by Physik Instrumente (PI), is a very versatile piezoelectric device commercially available for applications in industry and research. The thin, lightweight transducers can be integrated into a structure or or simply glued to a sur- face. A special manufacturing
August, 2017
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