SUCCESS STORY – Piezo Elements for a Multitude of Sensor Applications-- Frank Möller --Piezo Elements for a Multitude of Sensor Applications
From Level Measurement to Adaptive Systems Technology

PI Ceramic GmbH, Lindenstraße, 07589 Lederhose, Germany
Phone +49 36604 882-0, Fax +49 36604 882-4109,Email info@piceramic.com

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SUCCESS STORY – Piezo Elements for a Multitude of Sensor Applications-- Frank Möller --The Piezo EffectPiezoelectric materials can produce an electric voltage when aforce is applied to them (piezo effect), and they can change
their dimensions when an electric field is applied (inversepiezo effect). They convert mechanical power into electrical
power and vice versa; they are also referred to astransducers. While the inverse piezo effect can be used for
actuator applications, the direct piezo effect or a combinationof both effects lends itself to the manufacture of sensors. The
piezo effect is based exclusively on displacements within thecrystal lattice of the piezoelectric element. It is therefore notsubject to any mechanical friction or wear in the classicalsense and is highly sensitive. Even the smallest deformations
directly produce a measurable charge displacement, andconversely, a small electrical voltage produces an immediate
displacement of an actuator. This opens up a multitude ofpotential applications, starting with ultrasonic, force oracceleration sensors up to complex adaptive systems.
Generating and detecting ultrasound, for example, is a classic
piezo application because piezo elements start to oscillate
when an AC voltage is applied. The short response times andthe resulting high dynamics allow high oscillation frequenciesof up to 20 MHz. Ultrasound is sound with frequencies above
the range of human hearing frequencies, in other words
starting at around 16 kHz. This frequency range is used formany purposes in industry, medical engineering, andresearch. Applications range from distance measurement andobject recognition, filling level or flow measurements,
through high-resolution material tests, up to medical
diagnosis and therapy. Piezoceramics can be cost-effectively
manufactured in any shapes and thus offer tailor-madesolutions for different applications.

Flexible in Respect to Frequency
and DesignThe piezo elements from PI Ceramic are suitable for a
multitude of ultrasonic applications. These can be generallyclassified in mainly sensor applications for frequencies of up
to 20 MHz and power ultrasound, in which the energy
densities are higher.
Here, the piezo elements carry out considerable mechanicalwork; they crush kidney stones for example or remove
plaque, provide the mechanical energy for cleaning baths aswell as for industrial welding and bonding. The typicalfrequencies of power ultrasound are between 20 and3000 kHz.

PI Ceramic GmbH, Lindenstraße, 07589 Lederhose, Germany
Phone +49 36604 882-0, Fax +49 36604 882-4109,Email info@piceramic.com

Fig. 1

Many variants of piezo elements are possible, e. g. tubes, disks,benders, shear elements or translators, which makes it easy toadapt them to the respective application

With piezo components, different geometric variants andresonant frequencies can be realized in addition to thematerial selection for the respective application (Fig. 1);
components such as piezoceramic rings, piezo tubes andshear elements with standard dimensions can be delivered onvery short notice on the basis of semifinished products instock. Geometries beyond the standard dimensions are alsoavailable upon request.
PI Ceramic furthermore ensures integration in the final
product. This includes the contacting of the elements
according to customer specifications as well as mounting inprovided components, gluing or potting of the ultrasonictransducers. To measure the flow, filling level and force oracceleration,customized
sensorcomponents
aremanufactured that can be easily integrated in the respective
application.

Level Measurement
The application areas of piezoelectric components inindustrial sensors vary widely: ultrasonic delay measurement,
for example, uses the direct as well as the inverse piezoeffect. A typical application is the measurement of fillinglevels (Fig. 2). The piezo transducer is placed outside of themedium to be detected and works as a transmitter andreceiver. It emits an ultrasonic pulse that is reflected by thefilling medium. The propagation time required is a measure of
the distance travelled in the empty part of the container. This
makes non-contact measurements possible in which thefilling level of liquids as well as of solids, e.g. in silos, can bemeasured through distance measurement. The resolution oraccuracy depends on how good the ultrasonic pulse isreflected by the respective surface.

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SUCCESS STORY – Piezo Elements for a Multitude of Sensor Applications-- Frank Möller --Air Bubble Detection and Flow
Measurement
In monitoring dosing and filling systems, it is often necessary
to ensure an undisturbed flow without air or gas bubbles. Thisis done with so-called ultrasonic bubble detectors. The piezoelements in these sensors serve to generate and receive
ultrasonic waves. The sensors are mounted on the outside oftubes and operate without coming into contact with thetransported medium; they therefore do not interfere with theflow rate, nor is there any danger of contamination, and acontinuous quality monitoring is ensured.

Fig. 2A typical application for propagation time measurement is themeasurement of filling levels

Similar advantages are found with flow measurement. This isbased on the time difference during the alternate
transmission and receiving of ultrasonic pulses in and against
the flow direction (Fig. 4).

Piezo elements can be used as level switches in so-called
submersible transducers or tuning-fork sensors (Fig. 3). These
are then placed at different heights of the container. The
piezo transducers excite a tuning fork at its natural frequency.
When it comes into contact with the medium, the resonant
frequency shifts and the shift is electronically analyzed. Thismethod is very reliable and completely independent of thetype of filling medium.

Fig. 4

The flow measurement is based on the alternate transmission
and receiving of ultrasonic pulses in the direction of flow and in
the opposite direction

Here, two piezo transducers operating as both transmitterand receiver are arranged diagonally to the direction of flow.
When using the principles of the Doppler effect, the phaseand frequency shift of the ultrasonic waves which arescattered or reflected by particles of liquid are evaluated.

Fig. 3

Example of a tuning fork for filling level measurement

PI Ceramic GmbH, Lindenstraße, 07589 Lederhose, Germany
Phone +49 36604 882-0, Fax +49 36604 882-4109,Email info@piceramic.com

The frequency shift between the reflected wavefront emitted
and received by the same piezo transducer is proportional tothe flow speed. Many other tasks can be effectively solved ina similar way, such as object recognition or high-resolution
material tests.

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SUCCESS STORY – Piezo Elements for a Multitude of Sensor Applications-- Frank Möller --Acceleration Sensors and Force
Transducers
Ultrasound does not always play a role in sensor applicationsof piezo elements. A typical example of this is piezoelectric
acceleration sensors. At their heart is a piezo element that isconnected to a seismic mass. If the overall system isaccelerated, this inert mass amplifies the mechanicaldeformation of the piezo disc and thus increases themeasurable voltage created by the piezo effect.
The sensors detect accelerations in a broad range of
frequencies and dynamics with an almost linear characteristic
over the complete measurement range. They are suitable formeasuring dynamic tensile, pressure and shear forces. They
can be designed to be very stiff and can also measure highly
dynamic forces. The very high resolution is also typical.

Another current example are adaptive systems technology
solutions, in which the DuraAct transducer can be used asboth sensor and actuator. These systems can be used tomeasure interfering vibrations and compensate them at thesame time. In turn, Structural Health Monitoring involves
generating vibrations and measuring their propagation inmaterial structures. A changed vibrational pattern shows
failures in the structure even before fissures arise, e.g. inmachine parts, bridges, wings, etc.
At the heart of the transducer is a piezoceramic film with anelectrically conductive layer applied to each side. This
construction is then embedded in a flexible polymer strip. The
advantages of this design are multiple: the piezoceramic iselectrically insulated, mechanically preloaded, and, out of thebasically brittle ceramic, a unit is created which is so robustthat it can be affixed to surfaces with bending radii as low as
20 mm (Fig. 6).

A Combined Discipline: AdaptiveSystems Technology
A special type of sensor application is energy harvesting. With
the DuraAct patch transducer (Fig. 5), a very versatile
piezoelectric device is now commercially available that willfind wide application both in industry and in research. A
special manufacturing method makes the ceramic extremelybendable, which allows it to be attached to movingstructures, where it gets deformed and thereby generates
charge transfers that can be used as electric energy.

Fig. 6

Fig. 5The transducers can simply be glued to a surface, or they canbe integrated directly into a structure or structural material.
Custom transducer geometries are also possible for thisstructure, as is customization of the shape and thickness ofthe ceramic to perfectly fit the specific bending radiusrequired. The same is true for the electrical connections.
Once again, piezoelectric elements prove their versatility,
which will continue to open up further areas of application forthem in sensor technology.

Sensor, actuator or both: DuraAct piezoelectric elements

PI Ceramic GmbH, Lindenstraße, 07589 Lederhose, Germany
Phone +49 36604 882-0, Fax +49 36604 882-4109,Email info@piceramic.com

Bending radii of up to 20 mm can be easily realized with DuraAct
patch transducers

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SUCCESS STORY – Piezo Elements for a Multitude of Sensor Applications-- Frank Möller --Author

Dipl.-Phys. Frank Moeller, Sales at PI Ceramic

PI Ceramic in Brief
PI Ceramic is considered a global leading player in the field ofpiezo actuators and sensors. The broad range of expertise in
the complex development and manufacturing process offunctional ceramic components combined with state-of-theart production equipment ensure high quality, flexibility andadherence to supply deadlines.
Prototypes and small production runs of custom-engineered
piezo components are available after short processing times.
PI Ceramic also has the capacity to manufacture mediumsized to large series in automated lines. PI Ceramic, asubsidiary of Physik Instrumente (PI) GmbH & Co. KG, islocated in the city of Lederhose, Thuringia, Germany.

PI Ceramic GmbH in Lederhose, Thuringia, Germany

PI Ceramic GmbH, Lindenstraße, 07589 Lederhose, Germany
Phone +49 36604 882-0, Fax +49 36604 882-4109,Email info@piceramic.com

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