Piezoelectric sensor

Piezoelectric sensors use the piezoelectric effect and have been shown to be a universal instrument for measuring various processes. They are used for the determination of pressure, acceleration, voltage, power, or as a gas sensor in the quality as well as in process control.

Applications

Many beings use piezoelectricity in a very interesting way: bones act as a force sensor. If force is applied to produce bone electrical charges proportional to the internal stresses. This charge effect and stimulate the growth of new bone material, which leads to a strengthening of the bone structure at the positions at which the inner deformations are greatest. This leads to strain -specific minimum structures and thus an excellent ratio of weight to strength.

From the discovery of the piezoelectric effect by the Curie brothers to its industrial use in sensor applications, some time passed. Only since the forties of the last century, this measuring principle is used and is today a mature technology with an outstanding inherent reliability; as the piezoelectric effect is successfully used in many critical applications, such as medical, aerospace or nuclear technology.

The rise of piezoelectric technology is based on a number of inherent advantages. The high elastic moduli of many piezoelectric materials are comparable with the modulus of elasticity of many metals, ranging up to 105 N / mm ². Although piezoelectric sensors are electromechanical systems that respond to pressure, almost no deformation of the measurement show the elements (typically, the sensing elements are only a few micrometers compressed).

This is one reason for the robustness of the piezoelectric sensors that have very high natural frequency and the excellent linearity even under difficult conditions. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation. Some of the materials used - especially gallium phosphate and tourmaline - have excellent stability over wide temperature ranges, allowing a range of piezoelectric sensors to almost 1000 ° C. In addition to the piezoelectric effect exists, the pyroelectric effect in tourmaline. This effect also occurs in all piezoelectric ceramics (such as PZT).

A disadvantage of piezoelectric sensors is its unsuitability for use in purely static measurements. A static force leads to a defined amount of charge on the surface of the piezoelectric material. If this charge is not a charge amplifier, but - technically incorrect - measured with an impedance transformer, continuously lost charges, which ultimately leads to a continuous decrease in the signal. Elevated temperatures cause an additional decrease in the internal resistance, therefore, only materials with a high internal resistance can be used for such measurement conditions.

It would be wrong to assume that piezoelectric sensors can be used for very fast processes or under moderate conditions. There are a variety of applications, in which is measured under quasi-static conditions, as there are sensors for pressure measurements above 500 ° C.

Operation

In dependence on the section of the piezoelectric material and therefore three main effects modes of operation can be distinguished: transversal, longitudinal, and shear effects:

In contrast to longitudinal and shear can effect the amount of charge generated in the transverse effect (sensitivity) on the ratio of the width (a ) to height (b ) of the crystal element can be changed. For this reason, most pressure sensors are based almost exclusively on the transverse effect.

Sensor design

Based on the piezoelectric measurement numerous physical quantities such as pressure and acceleration can be measured. Pressure sensors for a thin membrane of known dimensions and a solid base is used to ensure that the pressure load, the elements in a specific direction. For accelerometers, a seismic mass to the crystal elements is connected. When the accelerometer experiences a motion charged the seismic mass according to Newton's second law of motion the elements.

The main difference in the operation of the two sensors is the way in which the force acting on the measuring elements. In a pressure sensor, a thin membrane is used to perform the power to the elements. In the acceleration force is applied by a seismic mass.

Sensors often tend to respond to more than a physical quantity. Pressure sensors show a signal at acceleration, as their membrane has a mass. Modern pressure sensors can be constructed acceleration compensation. This compensation is based on the fact that the actual measuring element measures both pressure and acceleration. A second measuring element is placed in the sensor so that only experiences acceleration. In order to obtain the "true" value of pressure, the acceleration signal of the additional elements of the combination of the actual signal (combined) pressure signal is subtracted.

Materials

For piezoelectric sensors two main groups of materials are used: piezoelectric ceramics and single crystal materials. Ceramics (such as PZT ceramics ) having a piezoelectric constant, which is the material of the crystal, two orders of magnitude and that can be produced in the sintering process. However, the high sensitivity is associated with a poor long-term stability. Piezoelectric ceramics are therefore mostly used when the demands on measurement accuracy and long term stability are not too high. Less sensitive single crystal materials (quartz, tourmaline and gallium phosphate ) have much higher and almost infinite - long-term stabilities.

Standards

• Integrated Electronics Piezo Electric ( IEPE )

Swell

• Piezoelectric sensors. In: Site of Piezocryst Advanced Sensorics GmbH. Accessed on September 10, 2007.

• Sensor
• Biomechanics