Piezoelectric sensor

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Piezoelectric Sensors are devices that use piezoelectric effects, to measure changes in pressure, acceleration, temperature, strain, or force by converting them into electrical charges. The prefix piezo - is Greek for 'press' or 'blackmail'.

Video Piezoelectric sensor

Apps

The piezoelectric sensor is a multipurpose tool for the measurement of various processes. They are used for quality assurance, process control, and for research and development in many industries. Pierre Curie discovered the piezoelectric effect in 1880, but only in the 1950s manufacturers began to use piezoelectric effects in industrial sensing applications. Since then, this measurement principle is increasingly being used, and has become a mature technology with excellent inherent reliability.

They have been successfully used in various applications, such as in medical, aerospace, nuclear instrumentation, and as a tilt sensor in consumer electronics or pressure sensors in mobile touch pads. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensor either directly mounted to an additional hole to the cylinder head or plug spark/glow is equipped with a miniature piezoelectric sensor built-in.

The advent of piezoelectric technology is directly tied to a set of inherent advantages. The high elasticity modulus of many piezoelectric materials is proportional to many metals and rises to 10 ⁶ N/mÃ,² . Although the piezoelectric sensor is an electromechanical system that reacts to compression, the sensing element shows almost zero deflection. This provides the sharpness of piezoelectric sensors, extremely high natural frequencies and excellent linearity over a wide range of amplitude. In addition, piezoelectric technology is not sensitive to electromagnetic fields and radiation, enabling measurements in harsh conditions. Some materials used (especially gallium phosphate or tourmaline) are very stable at high temperatures, allowing the sensor to have a working range of up to 1000 Ã, Â ° C . Turmaline shows pyroelectricity in addition to the piezoelectric effect; this is the ability to generate electrical signals when the crystal temperature changes. This effect is also common in piezoceramic materials. Gautschi in Piezoelectric Sensorics (2002) offers a comparison table of the characteristics of piezo vs. other sensor materials:

One of the disadvantages of piezoelectric sensors is that they can not be used for truly static measurements. The static force produces a fixed charge on the piezoelectric material. In conventional electronic readings, incomplete insulation material and reduction of internal sensor resistance causes a constant loss of electrons and generates a decreasing signal. The increased temperature causes an additional decrease in internal resistance and sensitivity. The main effect on the piezoelectric effect is that with increasing pressure and temperature load, the sensitivity is reduced due to twinning. While quartz sensors should be cooled during measurements at temperatures above 300 Ã, Â ° C , special crystal types such as GaPO4 gallium phosphate do not show twin formations until the melting point of the material itself.

However, it is not true that piezoelectric sensors can only be used for very fast processing or at ambient conditions. In fact, many piezoelectric applications produce quasi-static measurements, and other applications work in temperatures higher than 500 Ã, Â ° C .

The piezoelectric sensor can also be used to determine the scent in air by simultaneously measuring the resonance and capacitance. Computer-controlled electronics greatly increase the range of potential applications for piezoelectric sensors.

The piezoelectric sensor is also seen in nature. Collagen in bone is piezoelectric, and is considered by some to be a biological force sensor.

Maps Piezoelectric sensor

Principle of operation

The way piezoelectric material is cut produces three main operational modes:

• Transverse
• Longitudinal
• Scroll.

Transverse effect

Gaya yang diterapkan sepanjang sumbu netral (y) menggantikan muatan sepanjang arah (x), tegak lurus dengan garis gaya. Jumlah muatan ( ${\ displaystyle C_ {x}}  ÂÂ$ ) bergantung pada dimensi geometri dari elemen piezoelektrik masing-masing. Ketika dimensi ${\ displaystyle a, b, c}  ÂÂ$ berlaku,

${\ displaystyle C_ {x} = d_ {xy} F_ {y} b/a}  ÂÂ$ ,

di mana ${\ displaystyle a}  ÂÂ$ adalah dimensi yang sejajar dengan sumbu netral, ${\ displaystyle b}  ÂÂ$ sejajar dengan sumbu penghasil muatan dan ${\ displaystyle d}  ÂÂ$ adalah koefisien piezoelektrik yang sesuai. [3]

Efek longitudinal

The amount of charge transferred is proportional to the applied force and does not depend on the size and shape of the piezoelectric element. Placing several elements mechanically in series and electrically in parallel is the only way to increase the load output. The result is

${\ displaystyle C_ {x} = d_ {xx} F_ {x} n ~}$ ,

where ${\ displaystyle d_ {xx}}$ is the piezoelectric coefficient for the charge in the x direction that is released by the force applied along the x direction (in pC/N). ${\ displaystyle F_ {x}}$ is the Style applied in the directions of x [N] and ${\ displaystyle n}$ according to the number of elements stacked.

Slide effect

The resulting allegations are very proportionate to the applied force and are independent of the size and shape of the element. For ${\ displaystyle n}$ elements mechanically in series and electrically in parallel the load is

${\ displaystyle C_ {x} = 2d_ {xx} F_ {x} n} Â Â$ .

In contrast to the longitudinal and shear effects, the transverse effects make it possible to refine the sensitivity to the force and dimensions of the applied elements.

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Electrical properties

A piezoelectric transducer has a very high output DC impedance and can be modeled as a proportional voltage source and filter network. The voltage V at the source is directly proportional to the force, pressure, or strain applied. The output signal is then associated with this mechanical strength as if it has passed the equivalent circuit.

Detailed models include the mechanical construction effects of sensors and other non-idealities. The inductance L _m is due to the seismic and inertia mass of the sensor itself. C _e is inversely proportional to the mechanical elasticity of the sensor. C ₀ represents the static transducer capacitance, which results from an inertia of infinite size. R _i is the isolation leakage resistance of the transducer element. If the sensor is connected to the load resistance, it also acts in parallel with the insulation resistance, both of which increase the high-pass cutoff frequency.

To be used as a sensor, a flat area of ​​the frequency response plot is usually used, between high-pass cutoff and peak resonance. Load and leakage resistance must be large enough so that the low interest frequency is not lost. A simplified equivalent circuit model can be used in this region, where C _s represents the capacitance of the sensor surface itself, determined by the standard formula for the parallel plate capacitance. It can also be modeled as a load source in parallel with the source capacitance, with a charge directly proportional to the applied force, as above.

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Sensor design

Based on piezoelectric technology, various physical quantities can be measured; The most common are pressure and acceleration. For pressure sensors, thin membranes and large bases are used, ensuring that applied pressure specially loads the elements in one direction. For the accelerometer, the seismic mass is attached to the crystal element. When the accelerometer is in motion, the invasive seismic mass will load the elements according to Newton's second law of motion ${\ displaystyle F = ma}$ .

The main difference in the working principle between these two cases is the way they apply power to the sensing element. In the pressure sensor, the thin membrane transfers the force to the element, while in the accelerometer the applied seismic mass applies the force.

Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show false signals when they are exposed to vibration. Therefore, advanced pressure sensors use accelerated compensation elements other than pressure sensing elements. By carefully matching those elements, the acceleration signal (released from the compensating element) is subtracted from the combined pressure and acceleration signals to obtain the actual pressure information.

The vibration sensor can also harvest the energy wasted from mechanical vibrations. This is done by using piezoelectric materials to convert mechanical strain into usable electrical energy.

src: www.researchgate.net

Sensing material

Two groups of main materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. Ceramic materials (such as PZT ceramics) have piezoelectric constants/sensitivity which are approximately two orders of magnitude higher than those made from a single natural crystal material and can be produced by inexpensive sintering processes. Piezoeffect in piezoceramics is "trained", so its high sensitivity decreases with time. This degradation is highly correlated with the increase in temperature.

A single, less sensitive, natural crystal material (gallium phosphate, quartz, tourmaline) has a higher stability - when handled with care, almost unlimited - long term. There is also a commercially available single crystal material such as Lead Magnesium Niobate-Lead Titanate (PMN-PT). These materials offer increased sensitivity to PZT but have a lower maximum operating temperature and are currently more expensive to produce.

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See also

• Cost booster
• Sensor list
• Piezoelectric
• piezoelectric loudspeakers
• Piezoresistive Effects
• Ultrasonic homogenizer
• Ultrasonic transducer

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References

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External links

• Material constants of gallium phosphate

Source of the article : Wikipedia