Ignore everything past this point
PizeoElectric = Compression generates a current, and a current generates compression.
(Redirected from Piezoelectric effect)
Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. The word is derived from the Greek piezein, which means to squeeze or press. The effect is reversible; piezoelectric crystals, subject to an externally applied voltage, can change shape by a small amount. The effect is of the order of nanometres, but nevertheless finds useful applications - for example fine focusing of optical assemblies, etc.
Table of contents [showhide] 1 Mechanism 2 History 3 Materials 4 Applications 5 External links
Mechanism In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. When a stress is applied, this symmetry is destroyed, and the charge asymmetry generates a voltage. A 1 cm cube of quartz with 500 lb (2 kN) of correctly applied pressure upon it, can produce 12,500 V of electricity.
Piezoelectric materials also show the opposite effect, called converse piezoelectricity, where application of an electrical field creates mechanical stress (distortion) in the crystal. Because the charges inside the crystal are separated, the applied voltage affects different points within the crystal differently, resulting in the distortion.
The bending forces generated by converse piezoelectricity are extremely high, of the order of tens of millions of pounds (tens of meganewtons), and usually cannot be constrained. The only reason the force is usually not noticed is because it causes a displacement of the order of one billionth of an inch (a few nanometres).
History A related property known as pyroelectricity, the ability of certain mineral crystals to generate electrical charge when heated, was known of as early as the 18th century, and was named by Brewster in 1824. In 1880, the brothers Pierre Curie and Jacques Curie predicted and demonstrated piezoelectricity using tinfoil, glue, wire, magnets, and a jeweler's saw. They showed that crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt (sodium potassium tartrate tetrahydrate) generate electrical polarization from mechanical stress. Quartz and Rochelle salt exhibited the most piezoelectricity. Twenty natural crystal classes exhibit direct piezoelectricity.
Converse piezoelectricity was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. The Curies immediately confirmed the existence of the "converse effect," and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.
The first practical application for piezoelectric devices was sonar, first developed during World War I. In France in 1917, Paul Langevin (whose development now bears his name) and his coworkers developed an ultrasonic submarine detector. The detector consisted of a transducer, made of thin quartz crystals carefully glued between two steel plates, and a hydrophone to detect the returned echo. By emitting a high-frequency chirp from the transducer, and measuring the amount of time it takes to hear an echo from the sound waves bouncing off an object, one can calculate the distance to that object.
The use of piezoelectricity in sonar, and the success of that project, created intense development interest in piezoelectric devices. Over the next few decades, new piezoelectric materials and new applications for those materials were explored and developed.
Development of piezoelectric devices and materials in the United States was kept within the companies doing the development, mostly due to the wartime beginnings of the field, and in the interests of securing profitable patents. New materials were the first to be developed -- quartz crystals were the first commercially exploited piezoelectric material, but scientists searched for higher-performance materials.
Piezoelectric devices found homes in many fields. Ceramic phonograph cartridges simplified player design, were cheap and accurate, and made record players cheaper to maintain and easier to build. Ceramic electret microphones could be made small and sensitive. The development of the ultrasonic transducer allowed for easy measurement of viscosity and elasticity in fluids and solids, resulting in huge advances in materials research. Ultrasonic time-delay reflectometers (which send an ultrasonic pulse through a material and measure reflections from discontinuities) could find flaws inside cast metal and stone objects, improving structural safety. However, despite the advances in materials and the maturation of manufacturing processes, the United States market had not grown as quickly. Without many new applications, the growth of the United States' piezoelectric industry suffered.
In contrast, Japanese manufacturers shared their information, quickly overcoming technical and manufacturing challenges and creating new markets. Japanese efforts in materials research created piezoceramic materials competitive to the U.S. materials, but free of expensive patent restrictions. Major Japanese piezoelectric developments include new designs of piezoceramic filters, used in radios and televisions, piezo buzzers and audio transducers that could be connected directly into electronic circuits, and the piezoelectric igniter which generates sparks for small engine ignition systems (and gas-grill lighters) by compressing a ceramic disc. Ultrasonic transducers that could transmit sound waves through air had existed for quite some time, but first saw major commercial use in early television remote controls. These transducers now are mounted on several car models as an echolocation device, helping the driver determine the distance from the rear of the car to any objects that may be in its path.
Materials In addition to the materials listed above, many other materials exhibit the effect, including quartz analogue crystals like berlinite (AlPO4) and gallium orthophosphate (GaPO4), ceramics with perovskite or tungsten-bronze structures (BaTiO3, KNbO3, LiNbO3, LiTaO3, BiFeO3, NaxWO3, Ba2NaNb5O5, Pb2KNb5O15). Polymer materials like rubber, wool, hair, wood fiber, and silk exhibit piezoelectricity to some extent. The polymer polyvinlidene fluoride, (-CH2-CF2-)n, exhibits piezoelectricity several times larger than quartz.
Applications Devices that make use of piezoelectric effects are called piezoelectric devices. The converse piezoelectric effect can be used in devices like loudspeakers, where voltages are converted to mechanical movement of a piezoelectric polymer film. The opposite setup is used to make piezoelectric microphones (sound waves bend the piezoelectric material, creating a changing voltage) and piezoelectric pickups for electrically amplified guitars.
Direct piezoelectricity of some substances like quartz, as mentioned above, can generate thousands of volts. This property is exploited in the portable electrical sparkers used to light gas grills and cigarette lighters. The effect is being researched by DARPA in the USA in a project called Energy Harvesting, which includes an attempt to power battlefield equipment by piezoelectric generators embedded in soldiers' boots.
Digital watches (as well as most other electronic devices) employ a tuning fork made from quartz that uses a combination of both direct and converse piezoelectricity to generate a regularly timed series of electrical pulses that is used to keep track of the passage of time. The quartz crystal (like any material) has a precisely defined natural frequency (caused by its shape and size) at which it prefers to oscillate, and this is used to stabilize the frequency of a periodic voltage applied to the crystal.
The same principle is critical in all radio transmitters, and in computers where it creates a clock pulse. Both of these usually use a frequency multiplier to reach the megahertz and gigahertz ranges.
Piezoelectric elements can be used in laser mirror alignment, where their ability to move a large mass (the mirror mount) over microscopic distances is exploited to electronically align some laser mirrors. By precisely controlling the distance between mirrors, the laser operator can accurately maintain optical conditions inside the laser cavity to optimize the beam output.
Piezoelectric elements are also used in the generation of sonar waves. Piezoelectric microbalances are used as very sensitive chemical and biological sensors.
Atomic force microscopes and scanning tunneling microscopes employ converse piezoelectricity to keep the sensing needle close to the probe.
A piezoelectric transformer is a type of AC voltage multiplier. Unlike a conventional transformer, which uses magnetic coupling between input and output, the piezoelectric transformer uses acoustic coupling. An input voltage is applied across a short length of a bar of piezoceramic material such as PZT, creating an alternating stress in the bar by the inverse piezoelectric effect and causing the whole bar to vibrate. The vibration frequency is chosen to be the resonant frequency of the block, typically in the 100 kilohertz to 1 megahertz range. A higher output voltage is then generated across another section of the bar by the piezoelectric effect. Step-up ratios of more than 1000:1 have been demonstrated. An extra feature of this transformer is that, by operating it above its resonant frequency, it can be made to appear as an inductive load, which is useful in circuits that require a controlled soft start. A detailed analysis can be found here.
Quartz transducers find many uses in materials handling and chemical processing, as they are capable of measuring a wide range of pressures (such as those inside pipes) with great accuracy.
See also Wikipedia:Crystal radio receiver
* http://www.piezo.com/history.html * http://www.msiusa.com/piezo_documentation.htm * http://www.gapo4.com
From Wikipedia, the free encyclopedia.
A strain gauge is a device used to measure deformation (strain) of an object. It usually consists of a flexible, easily-stretched backing material with a wire glued to it in a zig-zag pattern. As the object is deformed, the wire is stretched, causing its resistance to change. This resistance change, usually measured using a Wheatstone bridge circuit, can be used to calculate the exact amount of deformation.
From Wikipedia, the free encyclopedia.
A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a 'bridge circuit', one leg of which includes the unknown component.
Here, Rx is the resistance we want to measure; R1, R2 and R3 are known resistors of known resistance; furthermore, the resistance of R2 is adjustable. If the ratio of the two resistances in the known leg (R2/R1) is equal to the ratio of the two in the unknown leg (Rx/R3), then the voltage between the two midpoints will be zero and no current will flow between the midpoints. R2 is varied until this condition is reached. The current direction indicates if R2 is too high or too low.
Detecting zero current can be done to extremely high accuracy (see Galvanometer). Therefore, if R1, R2 and R3 are known to high precision, then Rx can be measured to high precision. Very small changes in Rx disrupt the balance and are readily detected.
Alternatively, if R1, R2, and R3 are known, but R2 is not adjustable, the voltage or current flow through the meter can be used to calculate the value of Rx. This setup is frequently used in strain gauge measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage.
The Wheatstone bridge illustrates the concept of a difference measurement, which can be extremely accurate. Variations on the Wheatstone bridge can be used to measure capacitance, inductance, impedance and other quantities.
* Strain gauge * Potentiometer * Potential divider
(Redirected from Peltier cooler)
The Peltier-Seebeck effect, or thermoelectric effect, is the direct conversion of heat differentials to electric voltage and vice versa.
Table of contents [showhide] 1 Seebeck effect 2 Peltier effect 3 Thomson effect 4 See also 5 External links
Seebeck effect The Seebeck effect is the conversion of heat differences directly into electricity.
This effect was first discovered, accidentally, by the German physicist Thomas Seebeck in 1821.
He discovered that a voltage existed between two ends of a metal bar, when one end was at a different temperature than the other. This is simply due to a diffusion of electrons (which are relatively free to move in a metal) from the hot end to the cold end, since the electrons in the hot end have more thermal energy. This is called a heat current. As electrons are moving, it is also an electrical current. After reaching equilibrium, this creates a negative charge at the cooler end and a positive charge at the hotter end. The distance between the positive and negative charges produces a potential difference; an electrostatic voltage.
He also discovered that a compass needle would be deflected when a closed loop was formed of two metals, with a temperature difference between the junctions. This is because the metals respond differently to the heat difference, which creates a current loop, which produces a magnetic field.
A voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This usually causes a continuous current to flow in the conductors. The voltage created is on the order of several μV per degree Celsius of difference.
In the circuit:
the voltage developed can be derived from:
V12 = (QA - QB)(T1 - T2)
QA and QB are the Seebeck coefficients (also called thermoelectric power) of the metals A and B, and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear, and depend on the conductors' absolute temperature, material, and molecular structure.
Thus, a thermocouple works by measuring the difference in potential caused by the dissimilar wires. It can be used to measure a temperature difference directly, or to measure an absolute temperature, by setting one end to a known temperature. Several thermocouples in series are called a thermopile.
This is also the principle at work behind thermal diodes, thermoelectric generators which are used for creating power from heat differentials.
The Peltier effect is the reverse of the Seebeck effect; a creation of a heat difference from an electric voltage.
It occurs when a current is passed through two dissimilar metals or semiconductors (n-type and p-type) that are connected to each other at two junctions (Peltier junctions). The current drives a transfer of heat from one junction to the other: one junction cools off while the other heats up. This effect was observed 13 years after Seebeck's initial discovery in 1834 by Jean Peltier.
The conductors are attempting to return to the electron equilibrium that existed before the current was applied by absorbing energy at one connector and releasing it at the other. The individual couples can be connected in series to enhance the effect.
The direction of heat transfer is controlled by the polarity of the current, reversing the polarity will change the direction of transfer.
A Peltier cooler/heater or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other.
Peltier coolers are also called TECs (Thermo Electric Converter).
The Thomson effect, named for William Thomson (commonly known as Lord Kelvin), describes the heating or cooling of a current-carrying conductor with a temperature gradient.
Any current-carrying conductor, with a temperature difference between two points, will either absorb or emit heat, depending on the material.
The Seebeck effect is actually a combination of the Peltier and Thomson effects.
* Joule's law * Heat transfer
* General - Thermoelectricity, including equations and applications * General - Simple explanation of thermoelectric effects * General
* Semiconductors - a brief explanation * Semiconductors - An introduction to thermoelectric coolers * Semiconductors - The science and materials of thermoelectrics
* Metals - The origin of the thermoelectric potential * Metals - The "Seeback" and Peltier effects
* Directory of Peltier device information * A news article on the increases in thermal diode efficiency * A wristwatch powered by thermocouples, using the heat difference between the human body and its surroundings
From Wikipedia, the free encyclopedia.
The ignition system of an internal-combustion engine is an important part of the overall engine system. It provides for the timely burning of the fuel mixture within the engine. Not all engine types need an ignition system - for example, a diesel engine relies on compression-ignition, that is, the rise in temperature that accompanies the rise in pressure within the cylinder is sufficient to ignite the fuel spontaneously. All conventional petrol engines, by contrast, require an ignition system.
Table of contents [showhide] 1 Contact ignition 2 Glow plug ignition 3 Magneto system 4 Mechanical ignition 5 Electronic ignition 6 Engine management
The earliest petrol engines used a very crude ignition system. This often took the form of a copper or brass rod which protruded into the cylinder, which was heated using an external source. The fuel would ignite when it came into contact with the rod. Naturally this was very inefficient as the fuel would not be ignited in a controlled manner. This type of arrangement was quickly superseded by spark ignition, a system which is generally used to this day, albeit with sparks generated by more sophisticated circuitry.
Glow plug ignition
Glow plug ignition is used on some kinds of simple engines, such as those commonly used for model aircraft. A glow plug is a coil of wire (made from e.g. nichrome) that will glow red hot when an electric current is passed through it. This ignites the fuel on contact, once the temperature of the fuel is already raised due to compression. The coil is electrically activated for engine starting, but once running, the coil will retain sufficient residual heat on each stroke due to the heat generated on the previous stroke. Glow plugs are also used to aid starting of diesel engines.
The simplest form of spark ignition is that using a magneto. The engine spins a magnet inside a coil, and also operates a contact breaker, interrupting the current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are connected directly from the magneto output. Magnetos are not used in modern cars, but they are often found on 2-stroke engines and also in aircraft piston engines, where their simplicity and self-contained nature confers a generally greater reliability as well as lighter weight. Aircraft engines usually have multiple magnetos to provide redundancy in the event of a failure.
Most four-stroke engines have used a mechanical ignition system. Here, the power source is a battery, kept charged by the car's electrical system, which generates electricity using a dynamo or alternator. The engine operates contact breaker points, which interrupt the current flow to a coil - a form of autotransformer. This steps up the voltage, which is fed via a rotating switch called a distributor to the spark plugs. This system is not greatly different from a magneto system, except that more separate elements are involved. There are also advantages to this arrangement, for example the position of the contact breaker points relative to the engine angle can be changed a small amount dynamically, allowing the ignition timing to be automatically advanced with increasing RPM, giving better efficiency. This system was used almost universally until the late 1970s, when electronic ignition systems started to appear.
The disadvantage of the mechanical system is that it requires regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. Electronic ignition (EI) solves these problems. In an EI system, the contact breaker points are replaced by an angular sensor of some kind - either optical, where a vaned rotor breaks a light beam, or more commonly using a hall effect sensor, which responds to a rotating magnet mounted on a suitable shaft. The sensor output is shaped and processed by suitable circuitry, then used to trigger a switching device such as a thyristor, which switches a large flow of current through the coil. The rest of the system (distributor and spark plugs) remains as for the mechanical system. The lack of moving parts compared with the mechanical system leads to greater reliability and longer service intervals. For older cars, it is usually possible to retrofit an EI system in place of the mechanical one.
During the 1980s, EI systems were developed alongside other improvements such as fuel injection systems. After a while it became logical to combine the functions of fuel control and ignition into one electronic system known as an engine management system.
In an Engine Management System (EMS), electronics control both fuel delivery and ignition timing. Primary sensors on the system are engine angle (crank position), airflow into the engine and throttle demand position. The circuitry determines which cylinder needs fuel and how much, opens the requisite injector to deliver it, then causes a spark at the right moment to burn it. Early EMS systems used analogue computer circuit designs to accomplish this, but as embedded processors became fast enough to keep up with the changing inputs at high revolutions, digital systems started to appear.
Some designs using EMS retain the original coil, distributor and spark plugs found on cars throughout history. Other systems dispense with the distributor and coil and use special spark plugs which each contain their own coil (Direct Ignition). This means high voltages are not routed all over the engine, they are created at the point at which they are needed. Such designs offer potentially much greater reliability than conventional arrangements.
Modern EMS systems usually monitor other engine parameters such as temperature and the amount of pollution in the exhaust. This allows them to control the engine to minimise unburnt fuel and other noxious gases, leading to much cleaner and more effcient engines.