Piezoelectric sensor

Apiezoelectric sensoris a device that uses thepiezoelectric effectto measure changes inpressure,acceleration,temperature,strain,orforceby converting them to anelectrical charge.The prefixpiezo-is Greek for 'press' or 'squeeze'.

A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)

Applications

Piezoelectric sensors are versatile tools for the measurement of various processes.They are used forquality assurance,process control,and for research and development in many industries.JacquesandPierre Curiediscovered the piezoelectric effect in 1880,but only in the 1950s did manufacturers begin to use the piezoelectric effect in industrial sensing applications. Since then, this measuring principle has been increasingly used, and has become amature technologywith excellent inherent reliability.

They have been successfully used in various applications, such as inmedical,aerospace,nuclearinstrumentation, and as atilt sensorin consumer electronicsor a pressure sensor in the touch pads of mobile phones. In theautomotive industry,piezoelectric elements are used to monitor combustion when developinginternal combustion engines.The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built-in miniature piezoelectric sensor.

The rise of piezoelectric technology is directly related to a set of inherent advantages. The highmodulus of elasticityof many piezoelectric materials is comparable to that of many metals and goes up to 10⁶N/m².Even though piezoelectric sensors are electromechanical systems that react tocompression,the sensing elements show almost zero deflection. This gives piezoelectric sensors ruggedness, an extremely high natural frequency and an excellent linearity over a wideamplituderange. Additionally, piezoelectric technology is insensitive toelectromagnetic fieldsandradiation,enabling measurements under harsh conditions. Some materials used (especiallygallium phosphateortourmaline) are extremely stable at high temperatures, enabling sensors to have a working range of up to 1000 °C. Tourmaline showspyroelectricityin addition to the piezoelectric effect; this is the ability to generate an electrical signal when the temperature of the crystal changes. This effect is also common to piezoceramic materials. Gautschi inPiezoelectric Sensorics(2002) offers this comparison table of characteristics of piezo sensor materials vs other types:

Principle	Strain sensitivity [V/με]	Threshold [με]	Span to threshold ratio
Piezoelectric	5.0	0.00001	100,000,000
Piezoresistive	0.0001	0.0001	2,500,000
Inductive	0.001	0.0005	2,000,000
Capacitive	0.005	0.0001	750,000
Resistive	0.000005	0.01	50,000

One disadvantage of piezoelectric sensors is that they cannot be used for truly static measurements. Astatic forceresults in a fixed amount of charge on the piezoelectric material. In conventional readout electronics, imperfect insulating materials and reduction in internal sensorresistancecauses a constant loss ofelectronsand yields a decreasing signal. Elevated temperatures cause an additional drop ininternal resistanceand sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and temperature, the sensitivity reduces due totwin formation.Whilequartzsensors must be cooled during measurements at temperatures above300 °C,special types of crystals like GaPO4gallium phosphateshow no twin formation up to the melting point of the material itself.

However, it is not true that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, numerous piezoelectric applications produce quasi-static measurements, and other applications work in temperatures higher than500 °C.

Piezoelectric sensors can also be used to determine aromas in the air by simultaneously measuring resonance and capacitance. Computer controlled electronics vastly increase the range of potential applications for piezoelectric sensors.

Piezoelectric sensors are also seen in nature. The collagen inboneis piezoelectric, and is thought by some to act as a biological force sensor.Piezoelectricity has also been shown in the collagen of soft tissue such as theAchilles tendon,aortic walls, andheart valves.

Principle of operation

The way a piezoelectric material is cut defines one of its three main operational modes:

• Transverse
• Longitudinal
• Shear

Transverse effect

A force applied along a neutral axis (y) displaces charges along the (x) direction, perpendicular to the line of force. The amount of charge ($Q_{x}$) depends on the geometrical dimensions of the respective piezoelectric element. When dimensions$a,b,d$apply,

$Q_{x}=d_{xy}F_{y}b/a$,

where$a$is the dimension in line with the neutral axis,$b$is in line with the charge generating axis and$d$is the corresponding piezoelectric coefficient.[3]

Longitudinal effect

The amount of charge displaced is strictly proportional to the applied force and independent of the piezoelectric element size and shape. Putting several elements mechanically in series and electrically inparallelis the only way to increase the charge output. The resulting charge is

$Q_{x}=d_{xx}F_{x}n~$,

where$d_{xx}$is the piezoelectric coefficient for a charge in x-direction released by forces applied along x-direction (inpC/N).$F_{x}$is the applied Force in x-direction [N] and$n$corresponds to the number of stacked elements.

Shear effect

The charge produced is exactly proportional to the applied force and is generated at a right angle to the force. The charge is independent of the element size and shape. For$n$elements mechanically in series and electrically in parallel the charge is

$Q_{x}=2d_{xx}F_{x}n$.

In contrast to the longitudinal and shear effects, the transverse effect make it possible to fine-tune sensitivity on the applied force and element dimension.

Electrical properties

Figure 1. Frequency response of a piezoelectric sensor; output voltage over applied force versus frequency

A piezoelectrictransducercan be modeled as avoltage sourcewith afilter.The voltageVat the source is directly proportional to the applied force, pressure, or strain.The output signal is related to this mechanical force as if it had passed through the filter, which gives the transducer a very high and frequency-dependentoutput impedance,which results in afrequency responsesimilar to Figure 1.

Mechanical–electrical analogy

Figure 2. Schematic symbol and circuit incorporating themechanical–electrical analogyfor a piezoelectric sensor

Figure 2's detailed model includes the effects of the sensor's mechanical construction and other non-idealities.The inductanceL_mis due to the seismicmassandinertiaof the sensor itself.C_eis inversely proportional to the mechanicalelasticityof the sensor.C₀represents the static capacitance of the transducer, resulting from an inertial mass of infinite size.These inductances and capacitances are not real electrical elements of the transducer, but rather act as amechanical–electrical analogy.

R_ihowever is an actualelectric resistancerepresenting the insulationleakage resistanceof the transducer. If the sensor is connected to aload resistance,this also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency. Also not shown in this schematic is the actual capacitance of the sensor surface itself.

Simplified model

Figure 3. In the flat region, the sensor can be modeled as a voltage source in series with the sensor's capacitance or a charge source in parallel with the capacitance

Piezo sensors typically use the flat region of the frequency response (the "usable region" in Figure 1) between the high-pass cutoff and the resonant peak. The load and leakage resistance must be large enough that low frequencies of interest are not lost. A simplified equivalent circuit model (top of Figure 3) can be used in this region, in whichC_srepresents the capacitance of the sensor surface itself, determined by the standardformula for capacitance of parallel plates.This simplified model'sNorton equivalent(bottom of Figure 3) is acharge sourcein parallel with the source capacitance, with the charge directly proportional to the applied force.

Sensor design

Metal disks with piezo material, used in buzzers or ascontact microphones

Piezoelectric technology can measure various physical quantities, most commonly pressure and acceleration. Forpressure sensors,a thinmembraneand a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. Foraccelerometers,aseismic massis attached to the crystal elements. When the accelerometer experiences a motion, the invariant seismic mass loads the elements according to Newton's second law of motion$F=ma$.

The main difference in working principle between these two cases is the way they apply forces to the sensing elements. In a pressure sensor, a thin membrane transfers the force to the elements, while in accelerometers an attached seismic mass applies the forces. Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show false signal when they are exposed to vibrations. Sophisticated pressure sensors therefore use acceleration compensation elements in addition to the pressure sensing elements. By carefully matching those elements, the acceleration signal (released from the compensation element) is subtracted from the combined signal of pressure and acceleration to derive the true pressure information.

Vibration sensors can also harvest otherwise wasted energy from mechanical vibrations. This is accomplished by using piezoelectric materials to convert mechanical strain into usableelectrical energy.

Sensing materials

Three main groups of materials are used for piezoelectric sensors: piezoelectric ceramics, singlecrystal materialsand thin film piezoelectric materials. The ceramic materials (such asPZTceramic) have a piezoelectric constant/sensitivity that is roughly twoorders of magnitudehigher than those of the natural single crystal materials and can be produced by inexpensivesinteringprocesses. The piezoeffect in piezoceramics is "trained", so their high sensitivity degrades over time. This degradation is highly correlated with increased temperature.

The less-sensitive, natural, single-crystal materials (gallium phosphate,quartz,tourmaline) have a higher – when carefully handled, almost unlimited – long term stability. There are also new single-crystal materials commercially available such as Lead Magnesium Niobate-Lead Titanate (PMN-PT). These materials offer improved sensitivity overPZTbut have a lower maximumoperating temperatureand are currently more complicated to manufacture due to four compound vs. three compound material PZT.

Thin filmpiezoelectric materialscan be manufactured utilizingsputtering,CVD (chemical vapour deposition), ALD (atomic layer epitaxy) etc. methods. Thin film piezoelectric materials are used in applications where high frequency (> 100 MHz) is utilised in the measurement method and/or small size is favored in the application.

Self-sensing materials with an aluminum matrix and embedded piezoelectric phases, such as PZT (lead zirconate titanate)or barium titanate,can be produced through Friction Stir Processing (FSP).In this process, the piezoelectric particles are dispersed into the aluminum matrix, creating a composite material capable of both structural and sensing functions. The piezoelectric particles generate an electrical signal in response to mechanical stress or strain,enabling the material to monitor its own condition. FSP ensures a fine dispersion of the piezoelectric phase and enhances the bonding between particles and the matrix, leading to improved mechanical and sensing properties.

See also

• Charge amplifier
• List of sensors
• Piezoelectricity
• Piezoelectric speaker
• Piezoresistive effect
• Ultrasonic homogenizer
• Ultrasonic transducer
• Thin-film bulk acoustic resonator

External links

• Material constants ofgallium phosphate
• Thebasic functional principleof a piezoelectric accelerometer