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Piezo effect: exploring its applications potential

14 September 2012

Piezoelectric materials produce a voltage when a force is applied to them (piezo effect), and can change their dimensions when an electric field is applied (inverse piezo effect).

They convert mechanical power into electrical power and vice versa, and while the inverse piezo effect is best suited to actuator applications, the direct piezo effect - or a combination of both effects - provides interesting opportunities for sensor designs. Frank Moller reports.

The piezo effect is based on displacements within the crystal lattice of a piezoelectric material. It is therefore not subject to any mechanical friction or wear in the classical sense and is inherently highly sensitive. Even the smallest of deformations produces a measurable charge displacement and, conversely, just a small potential difference across opposite faces of the material will produce a mechanical displacement. This opens up a multitude of potential applications, from ultrasonic, force or acceleration sensors to complex adaptive systems.

Generating and detecting ultrasound, for example, is a classic piezo application because piezo elements will oscillate when an ac voltage is applied to them. Indeed, the short response times and high dynamics of piezoelectric materials permit their oscillation at frequencies of up to 20MHz.

Ultrasound applications abound in industry, medical engineering, and research, ranging from distance measurement and object recognition, liquid and solid level or flow measurements, through to high-resolution materials testing, medical imaging and therapies. Piezoceramics, in particular, can be manufactured cost-effectively to any required shape, offering tailor-made products for a variety of applications. 

Piezo elements, such as those supplied by the author’s company, can be used in sensor applications (up to 20MHz) or as power ultrasound generators, providing high energy ultrasound pulses for applications such as lithotripsy (a minimally invasive medical procedure to disintegrate kidney stones in situ) or to remove dental plaque, provide the mechanical energy for ultrasound cleaning systems, as well as industrial welding and bonding machines. The typical frequencies of power ultrasound systems are between 20kHz and 3MHz.

With piezo components, there is freedom of choice with regard to geometry, resonant frequency and material type (see Figure 1). The author’s company, for example, can deliver components such as piezo-ceramic rings, piezo tubes and shear elements with standard dimensions on short lead times using semi-finished products from stock, while items of non-standard geometry are available on request. The company will also help with the integration of a piezo element into a customer’s product, including the location of contacts, its mounting within the final product, and the bonding or potting of transducers.

Level measurement 
Ultrasonic level measurement sensors exploit both the direct as well as the inverse piezo effect (see Figure 2). The piezoelectric transducer is placed on the outside of the vessel containing the medium whose level is to be detected, working as both transmitter and receiver. In transmitter mode, it emits an ultrasonic pulse that is reflected by the medium, returning to the device that will have switched to receiver mode. The propagation delay is a measure of the distance travelled in the empty part of the container to the surface of the medium (solid or liquid) and back to the sensor. The resolution or accuracy of this type of measurement will depend on the strength of reflection of the ultrasonic pulse from the surface of the medium.

Piezo elements can also be used as level switches in the form of submersible tuning-fork sensors (see Figure 3). These are located at specified heights within the container and are set to vibrate at their natural frequency. When the surface of the medium makes contact with a tuning fork sensor, there is a change in its resonant frequency, which is subsequently detected to signal that the level has reached that of the sensor. This method is very reliable and completely independent of medium type. 

Bubble detection and flow measurement 
In monitoring dosing and filling systems, it is often necessary to ensure an undisturbed flow with no entrained air or gas bubbles. Ultrasonic bubble detectors, containing piezo elements that both generate and receive ultrasonic waves, are mounted on the outside of the pipe carrying the fluid. They are thus non-contacting and therefore they do not affect flow rate or introduce any contamination into the system.

Similar advantages can be achieved using piezo elements for flow measurement. This is based on the time difference during the alternate transmission and receiving of ultrasonic pulses in and against the flow direction (see Figure 4). Here, two piezo transducers, operating as both transmitter and receiver, are arranged diagonally to the direction of flow.

Using the principles of the Doppler effect, the phase and frequency shift of the ultrasonic waves, which are scattered or reflected by particles entrained by the liquid, are evaluated. The frequency shift between the reflected wave front emitted and received by the same piezo transducer is proportional to the speed of flow of the medium. Many other tasks can be effectively solved in a similar way, such as object recognition or high-resolution materials testing.

Acceleration and force
Ultrasound does not always play a role in sensors containing piezoelectric elements. A typical example of this is the piezoelectric accelerometer. At its heart is a piezo element that is connected to a sprung or suspended mass. Under acceleration this mass moves, causing a mechanical deformation of the element and creating a potential difference across its faces according to the piezo effect. The force on the piezo element, and thus the voltage produced, is proportional to the mass acceleration.

Sensors of this type detect acceleration across a broad range of frequencies and dynamics with an almost linear characteristic over the complete measurement range. Suitable for measuring dynamic tensile, pressure and shear forces, they can be designed to be very stiff in order to measure very high dynamic forces. Very high resolution is also achievable with this type of sensor.

Adaptive systems
A special application of piezoelectric materials is energy harvesting. The DuraAct patch transducer (see Figure 5), is a particularly versatile device thanks to its easy mechanical flexibility. A special manufacturing method ensures the flexibility of the DuraAct ceramic; attached to moving structures, it can be deformed time and time again to generate electrical energy and thus power connected devices such as sensors

DuraAct can also be used as both sensor and actuator in so-called adaptive systems; an example may be a system that measures interfering vibrations while compensating for them at the same time. Structural health monitoring is another good example of an adaptive system in action. This type of device generates vibrations and measures their propagation through the material of the structure. Small changes in the vibration signature will indicate the onset of structural failure even before any fracture becomes apparent.

At the heart of the transducer is a piezoceramic film with an electrically 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 is electrically insulated, mechanically preloaded, and, from a basically brittle ceramic, a unit can be created which is so robust that it can be affixed to surfaces with bending radii as small as 20mm (see Figure 6). The transducers can simply be glued to a surface, or they can be integrated directly into a structure or structural material, their shape and electrical contact arrangements all easily customised according to the requirements of the application.

Frank Moller is with PI Ceramic GmbH, Germany

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