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. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface In Physics, a force is whatever can cause an object with Mass to Accelerate.
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Applications
Piezoelectric sensors have proven to be versatile tools for the measurement of various processes. A pickup device acts as a Transducer that captures mechanical vibrations (usually from suitably equipped Stringed instruments such as the Electric guitar They are used for quality assurance, process control and process development in many different industries. Quality assurance, or QA for short refers to planned and systematic production processes that provide confidence in a product's suitability for its intended purpose Process control is a Statistics and Engineering discipline that deals with Architectures mechanisms and Algorithms for controlling
From the Curies’ initial discovery in 1880, it took until the 1950s before the piezoelectric effect was used for industrial sensing applications. Pierre Curie (15 May 1859 &ndash 19 April 1906 was a French physicist, a pioneer in Crystallography, Magnetism, Piezoelectricity 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. Medicine is the art and science of healing It encompasses a range of Health care practices evolved to maintain and restore Human Health by the This article is about the field of research and industry for the corporation see The Aerospace Corporation Aerospace comprises the Nuclear engineering is the application of the breakdown of atomic nuclei and/or other sub-atomic physics based on the principles of Nuclear physics.
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/m². An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i Even though piezoelectric sensors are electromechanical systems that react on compression, the sensing elements show almost zero deflection. Physical compression is the result of the subjection of a material to Compressive stress, resulting in reduction of Volume. 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. Amplitude is the magnitude of change in the oscillating variable with each Oscillation, within an oscillating system Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. The electromagnetic field is a physical field produced by electrically charged objects. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy 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. Modifications GaPO4 possesses in contrast to quartz no α-β Phase transition, thus the low temperature structure (structure like α-quartz of GaPO4 Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium 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. Pyroelectricity is the ability of certain materials to generate an Electrical potential when they are heated or cooled This effect is also common to piezoceramic materials.
| Principle | Strain Sensitivity [V/µ*] | Threshold [µ*] | Span to threshold ratio |
|---|---|---|---|
| Piezoelectric | 5. 0 | 0. 00001 | 100,000,000 |
| 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. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J 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. Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance. 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.
Piezoelectric sensors are also seen in nature. Dry bone is piezoelectric, and is thought by some to act as a biological force sensor. Bones are rigid organs that form part of the Endoskeleton of Vertebrates They function to move support and protect the various organs of the body produce [1][2]
Principle of operation
Depending on how a piezoelectric material is cut, three main modes of operation can be distinguished: transverse, longitudinal, and shear. [1]
  A diagram is needed in this article |
- 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,
- Cx = dxyFyb / 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 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. If two or more circuit components are connected end to end like a daisy chain it is said they are connected in series. The resulting charge is
- Cx = dxxFxn,
- where dxx is the piezoelectric coefficient for a charge in x-direction released by forces applied along x-direction (in pC/N). The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical Fx 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
- Cx = 2dxxFxn.
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.
Electrical properties
A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional voltage source and filter network. Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance. A voltage source is any device or system that produces an Electromotive force between its terminals OR derives a secondary voltage from a primary Electronic filters are Electronic circuits which perform Signal processing functions specifically intended to remove unwanted signal components and/or enhance wanted The voltage V at the source is directly proportional to the applied force, pressure, or strain. [2] The output signal is then related to this mechanical force as if it had passed through the equivalent circuit.
A detailed model includes the effects of the sensor's mechanical construction and other non-idealities. [3] The inductance Lm is due to the seismic mass and inertia of the sensor itself. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The vis insita or innate force of matter is a power of resisting by which every body as much as in it lies endeavors to preserve in its present state whether it be of rest or of moving Ce is inversely proportional to the mechanical elasticity of the sensor. C0 represents the static capacitance of the transducer, resulting from an inertial mass of infinite size. [3] Ri is the insulation leakage resistance of the transducer element. Leakage describes an unwanted loss or Leak, of something which escapes from its proper location If the sensor is connected to a load resistance, this also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency. The Input impedance, Load impedance, or external impedance of a circuit or electronic device is the Thévenin
For use as a sensor, the flat region of the frequency response plot is typically used, between the high-pass cutoff and the resonant peak. The load and leakage resistance need to be large enough that low frequencies of interest are not lost. A simplified equivalent circuit model can be used in this region, in which Cs represents the capacitance of the sensor surface itself, determined by the standard formula for capacitance of parallel plates. Capacitance is a measure of the amount of Electric charge stored (or separated for a given Electric potential. [3][4] It can also be modeled as a charge source in parallel with the source capacitance, with the charge directly proportional to the applied force, as above. [2]
Sensor design
Based on piezoelectric technology various physical quantities can be measured; the most common are pressure and acceleration. A contact microphone is a form of Microphone designed not for picking up airborne sound waves but rather to act as a Transducer which picks up vibrations For pressure sensors, a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. An artificial membrane, also called a synthetic membrane, is a membrane prepared for separation tasks in Laboratory and industry For accelerometers, a seismic mass is attached to the crystal elements. An accelerometer is a device for measuring Acceleration and gravity induced reaction forces 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 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.
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. Lead zirconate titanate ( 0 x Ceramic Perovskite material that shows a marked piezoelectric effect. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it Sintering is a method for making objects from powder, by heating the material (below its Melting point - solid state sintering until its particles adhere 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. Modifications GaPO4 possesses in contrast to quartz no α-β Phase transition, thus the low temperature structure (structure like α-quartz of GaPO4 Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Tourmaline is a Crystal Silicate mineral compounded with elements such as Aluminium, Iron, Magnesium, Sodium, Lithium
See also
References
- ^ Piezoelectric sensors. A charge amplifier is a circuit whose equivalent input Impedance is a Capacitance that provides a very high value of impedance at low frequencies Accelerometer Touch sensor Active pixel sensor Air flow meter Alarm sensor Piezocryst website. Retrieved on 2006-06-02. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 455 - The Vandals enter Rome, and plunder the city for two weeks
- ^ a b Interfacing Piezo Film to Electronics. Measurement Specialties (March 2006). Retrieved on 2007-12-02. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1409 - The University of Leipzig opens 1755 - The second Eddystone Lighthouse is destroyed by fire
- ^ a b c Alfredo Vázquez Carazo (2000). "Novel Piezoelectric Transducers for High Voltage Measurements". 242. Universitat Politècnica de Catalunya
- ^ Karki, James (September 2000). Signal Conditioning Piezoelectric Sensors. Texas Instruments. Retrieved on 2007-12-02. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1409 - The University of Leipzig opens 1755 - The second Eddystone Lighthouse is destroyed by fire
External links
- Material constants of gallium phosphate
- Langatate - new material for high-temperature piezoelectric sensors
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