Piezoelectric sensor

From Wikipedia, the free encyclopedia

A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical signal.

1 Applications
2 Principle of operation
3 Sensor design
4 Sensing materials
5 See also
6 References
7 External links

[edit] Applications

Piezoelectric sensors have proven to be versatile tools for the measurement of various processes. They are used for quality assurance, process control and process development in many different industries.

Piezo electric sensors are also seen in nature. Bones act as force sensors. Once loaded, bones produce charges proportional to the resulting internal torsion or displacement. Those charges stimulate and drive the build up of new bone material. This leads to the strengthening of structures where the internal displacements are the greatest. With time, this causes weaker structures to increase their strength and stability as material is laid down proportional to the forces affecting the bone.

From the Curies’ initial discovery, it took until the 1950s before the piezoelectric effect was used for industrial sensing applications. Since then, the utilization of this measuring principle has experienced a constant growth and can be regarded as a mature technology with an outstanding inherent reliability. It has been successfully used in various critical applications as for example in medical, aerospace and nuclear instrumentation.

The rise of piezoelectric technology is directly related to a set of inherent advantages. The high modulus of elasticity of many piezoelectric materials is comparable to that of many metals and goes up to 105 N/mm². Even though piezoelectric sensors are electromechanical systems that react on compression, the sensing elements show almost zero deflection. This is the reason why piezoelectric sensors are so rugged, have an extremely high natural frequency and an excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. Some materials used (especially gallium phosphate or tourmaline) have an extreme stability over temperature enabling sensors to have a working range of up to 1000°C. Tourmaline shows pyroelectricity in 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.

Principle	Strain Sensitivity [V/µ*]	Threshold [µ*]	Span to threshold ratio
Piezoelectric	5.0	0.00001	100.000.00
Piezoresistive	0.0001	0.0001	2.500.000
Inductive	0.001	0.0005	2.000.000
Capacitive	0.005	0.0001	750.000

One disadvantage of piezoelectric sensors is that they cannot be used for true static measurements. A static force will result in a fixed amount of charges on the piezoelectric material. Working with conventional readout electronics, not perfect insulating materials, and reduction in internal sensor resistance will result in a constant loss of electrons, yielding a decreasing signal. Elevated temperatures cause an additional drop in internal resistance; therefore, at higher temperatures, only piezoelectric materials that maintain a high internal resistance can be used. Anyhow, it would be a misconception that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, there are numerous applications that show quasi-static measurements while there are other applications that go to temperatures far beyond 500°C.

[edit] Principle of operation

Depending on how a piezoelectric material is cut, three main modes of operation can be distinguished: transverse, longitudinal, and shear.

Transverse effect

A force is applied along a neutral axis (y) and the charges are generated along the (x) direction, perpendicular to the line of force. The amount of charge depends on the geometrical dimensions of the respective piezoelectric element. When dimensions

a

b

c

apply,

C x = d x y 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.

Longitudinal effect

The amount of charge produced is strictly proportional to the applied force and is independent of size and shape of the piezoelectric element. Using several elements that are mechanically in series and electrically in parallel is the only way to increase the charge output. The resulting charge is

C x = d x x F x n

where

d x x

is the piezoelectric coefficient for a charge in x-direction released by forces applied along x-direction (in pC/N).

F x

is the applied Force in x-direction [N] and

n

corresponds to the number of stacked elements .

Shear effect

Again, the charges produced are strictly proportional to the applied forces and are independent of the element’s size and shape. For

n

elements mechanically in series and electrically in parallel the charge is

C x = 2 d x x F x n

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

[edit] Sensor design

Based on piezoelectric technology various physical quantities can be measured; the most common are pressure and acceleration. For pressure sensors, a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. For accelerometers, a seismic mass is 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 $F = m a$ .

The main difference in the working principle between these two cases is the way forces are applied to the sensing elements. In a pressure sensor a thin membrane is used to transfer the force to the elements, while in accelerometers the forces are applied by an attached seismic mass.

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.

[edit] Sensing materials

Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. The ceramic materials (such as PZT ceramic) have a piezoelectric constant / sensitivity that is roughly two orders of magnitude higher than those of single crystal materials and can be produced by inexpensive sintering processes. The piezoeffect in piezoceramics is "trained", so unfortunately their high sensitivity degrades over time. The degradation is highly correlated with temperature. The less sensitive crystal materials (gallium phosphate, quartz, tourmaline) have a much higher – when carefully handled, almost infinite – long term stability.