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Welding
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Processes
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Resistance welding Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1,000–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.
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Welding
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Processes
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Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process called shot welding, can be used to spot weld stainless steel.Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include butt welding, flash welding, projection welding, and upset welding.
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Welding
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Processes
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Energy beam welding Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties, laser cladding, and x-ray welding.
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Welding
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Processes
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Solid-state welding Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, including bonding aluminum to carbon steel in ship hulls and stainless steel or titanium to carbon steel in petrochemical pressure vessels.Other solid-state welding processes include friction welding (including friction stir welding and friction stir spot welding), magnetic pulse welding, co-extrusion welding, cold welding, diffusion bonding, exothermic welding, high frequency welding, hot pressure welding, induction welding, and roll bonding.
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Welding
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Geometry
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Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint (a variant of this last is the cruciform joint). Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly common—instead of having straight edges like the single-V and double-V preparation joints, they are curved, forming the shape of a U. Lap joints are also commonly more than two pieces thick—depending on the process used and the thickness of the material, many pieces can be welded together in a lap joint geometry.Many welding processes require the use of a particular joint design; for example, resistance spot welding, laser beam welding, and electron beam welding are most frequently performed on lap joints. Other welding methods, like shielded metal arc welding, are extremely versatile and can weld virtually any type of joint. Some processes can also be used to make multipass welds, in which one weld is allowed to cool, and then another weld is performed on top of it. This allows for the welding of thick sections arranged in a single-V preparation joint, for example.
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Welding
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Geometry
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After welding, a number of distinct regions can be identified in the weld area. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials. It is surrounded by the heat-affected zone, the area that had its microstructure and properties altered by the weld. These properties depend on the base material's behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.
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Welding
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Quality
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Many distinct factors influence the strength of welds and the material around them, including the welding method, the amount and concentration of energy input, the weldability of the base material, filler material, and flux material, the design of the joint, and the interactions between all these factors.For example, the factor of welding position influences weld quality, that welding codes & specifications may require testing—both welding procedures and welders—using specified welding positions: 1G (flat), 2G (horizontal), 3G (vertical), 4G (overhead), 5G (horizontal fixed pipe), or 6G (inclined fixed pipe).
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Welding
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Quality
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To test the quality of a weld, either destructive or nondestructive testing methods are commonly used to verify that welds are free of defects, have acceptable levels of residual stresses and distortion, and have acceptable heat-affected zone (HAZ) properties. Types of welding defects include cracks, distortion, gas inclusions (porosity), non-metallic inclusions, lack of fusion, incomplete penetration, lamellar tearing, and undercutting.
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Welding
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Quality
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The metalworking industry has instituted codes and specifications to guide welders, weld inspectors, engineers, managers, and property owners in proper welding technique, design of welds, how to judge the quality of welding procedure specification, how to judge the skill of the person performing the weld, and how to ensure the quality of a welding job. Methods such as visual inspection, radiography, ultrasonic testing, phased-array ultrasonics, dye penetrant inspection, magnetic particle inspection, or industrial computed tomography can help with detection and analysis of certain defects.
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Welding
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Quality
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Heat-affected zone The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alters the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Conversely, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat injected by the welding process plays an important role as well, as processes like oxyacetylene welding have an unconcentrated heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input. To calculate the heat input for arc welding procedures, the following formula can be used: 60 1000 )×Efficiency where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8. Methods of alleviating the stresses and brittleness created in the HAZ include stress relieving and tempering.One major defect concerning the HAZ would be cracking at the toes , due to the rapid expansion (heating) and contraction (cooling) the material may not have the ability to withstand the stress and could cause cracking, one method the control these stress would be to control the heating and cooling rate, such as pre-heating and post- heating Lifetime extension with after treatment methods The durability and life of dynamically loaded, welded steel structures is determined in many cases by the welds, in particular the weld transitions. Through selective treatment of the transitions by grinding (abrasive cutting), shot peening, High-frequency impact treatment, Ultrasonic impact treatment, etc. the durability of many designs increases significantly.
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Welding
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Metallurgy
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Most solids used are engineering materials consisting of crystalline solids in which the atoms or ions are arranged in a repetitive geometric pattern which is known as a lattice structure. The only exception is material that is made from glass which is a combination of a supercooled liquid and polymers which are aggregates of large organic molecules.Crystalline solids cohesion is obtained by a metallic or chemical bond that is formed between the constituent atoms. Chemical bonds can be grouped into two types consisting of ionic and covalent. To form an ionic bond, either a valence or bonding electron separates from one atom and becomes attached to another atom to form oppositely charged ions. The bonding in the static position is when the ions occupy an equilibrium position where the resulting force between them is zero. When the ions are exerted in tension force, the inter-ionic spacing increases creating an electrostatic attractive force, while a repulsing force under compressive force between the atomic nuclei is dominant.Covalent bonding takes place when one of the constituent atoms loses one or more electrons, with the other atom gaining the electrons, resulting in an electron cloud that is shared by the molecule as a whole. In both ionic and covalent bonding the location of the ions and electrons are constrained relative to each other, thereby resulting in the bond being characteristically brittle.Metallic bonding can be classified as a type of covalent bonding for which the constituent atoms are of the same type and do not combine with one another to form a chemical bond. Atoms will lose an electron(s) forming an array of positive ions. These electrons are shared by the lattice which makes the electron cluster mobile, as the electrons are free to move as well as the ions. For this, it gives metals their relatively high thermal and electrical conductivity as well as being characteristically ductile.Three of the most commonly used crystal lattice structures in metals are the body-centred cubic, face-centred cubic and close-packed hexagonal. Ferritic steel has a body-centred cubic structure and austenitic steel, non-ferrous metals like aluminium, copper and nickel have the face-centred cubic structure.Ductility is an important factor in ensuring the integrity of structures by enabling them to sustain local stress concentrations without fracture. In addition, structures are required to be of an acceptable strength, which is related to a material's yield strength. In general, as the yield strength of a material increases, there is a corresponding reduction in fracture toughness.A reduction in fracture toughness may also be attributed to the embrittlement effect of impurities, or for body-centred cubic metals, from a reduction in temperature. Metals and in particular steels have a transitional temperature range where above this range the metal has acceptable notch-ductility while below this range the material becomes brittle. Within the range, the materials behavior is unpredictable. The reduction in fracture toughness is accompanied by a change in the fracture appearance. When above the transition, the fracture is primarily due to micro-void coalescence, which results in the fracture appearing fibrous. When the temperatures falls the fracture will show signs of cleavage facets. These two appearances are visible by the naked eye. Brittle fracture in steel plates may appear as chevron markings under the microscope. These arrow-like ridges on the crack surface point towards the origin of the fracture.Fracture toughness is measured using a notched and pre-cracked rectangular specimen, of which the dimensions are specified in standards, for example ASTM E23. There are other means of estimating or measuring fracture toughness by the following: The Charpy impact test per ASTM A370; The crack-tip opening displacement (CTOD) test per BS 7448–1; The J integral test per ASTM E1820; The Pellini drop-weight test per ASTM E208.
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Welding
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Unusual conditions
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While many welding applications are done in controlled environments such as factories and repair shops, some welding processes are commonly used in a wide variety of conditions, such as open air, underwater, and vacuums (such as space). In open-air applications, such as construction and outdoors repair, shielded metal arc welding is the most common process. Processes that employ inert gases to protect the weld cannot be readily used in such situations, because unpredictable atmospheric movements can result in a faulty weld. Shielded metal arc welding is also often used in underwater welding in the construction and repair of ships, offshore platforms, and pipelines, but others, such as flux cored arc welding and gas tungsten arc welding, are also common. Welding in space is also possible—it was first attempted in 1969 by Russian cosmonauts during the Soyuz 6 mission, when they performed experiments to test shielded metal arc welding, plasma arc welding, and electron beam welding in a depressurized environment. Further testing of these methods was done in the following decades, and today researchers continue to develop methods for using other welding processes in space, such as laser beam welding, resistance welding, and friction welding. Advances in these areas may be useful for future endeavours similar to the construction of the International Space Station, which could rely on welding for joining in space the parts that were manufactured on Earth.
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Welding
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Safety issues
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Welding can be dangerous and unhealthy if the proper precautions are not taken. However, using new technology and proper protection greatly reduces risks of injury and death associated with welding.Since many common welding procedures involve an open electric arc or flame, the risk of burns and fire is significant; this is why it is classified as a hot work process. To prevent injury, welders wear personal protective equipment in the form of heavy leather gloves and protective long-sleeve jackets to avoid exposure to extreme heat and flames. Synthetic clothing such as polyester should not be worn since it may burn, causing injury. Additionally, the brightness of the weld area leads to a condition called arc eye or flash burns in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Goggles and welding helmets with dark UV-filtering face plates are worn to prevent this exposure. Since the 2000s, some helmets have included a face plate which instantly darkens upon exposure to the intense UV light. To protect bystanders, the welding area is often surrounded with translucent welding curtains. These curtains, made of a polyvinyl chloride plastic film, shield people outside the welding area from the UV light of the electric arc, but cannot replace the filter glass used in helmets.
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Welding
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Safety issues
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Welders are often exposed to dangerous gases and particulate matter. Processes like flux-cored arc welding and shielded metal arc welding produce smoke containing particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. This is because smaller particles have the ability to cross the blood–brain barrier. Fumes and gases, such as carbon dioxide, ozone, and fumes containing heavy metals, can be dangerous to welders lacking proper ventilation and training. Exposure to manganese welding fumes, for example, even at low levels (<0.2 mg/m3), may lead to neurological problems or to damage to the lungs, liver, kidneys, or central nervous system. Nano particles can become trapped in the alveolar macrophages of the lungs and induce pulmonary fibrosis. The use of compressed gases and flames in many welding processes poses an explosion and fire risk. Some common precautions include limiting the amount of oxygen in the air, and keeping combustible materials away from the workplace.
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Welding
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Costs and trends
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As an industrial process, the cost of welding plays a crucial role in manufacturing decisions. Many different variables affect the total cost, including equipment cost, labor cost, material cost, and energy cost. Depending on the process, equipment cost can vary, from inexpensive for methods like shielded metal arc welding and oxyfuel welding, to extremely expensive for methods like laser beam welding and electron beam welding. Because of their high cost, they are only used in high production operations. Similarly, because automation and robots increase equipment costs, they are only implemented when high production is necessary. Labor cost depends on the deposition rate (the rate of welding), the hourly wage, and the total operation time, including time spent fitting, welding, and handling the part. The cost of materials includes the cost of the base and filler material, and the cost of shielding gases. Finally, energy cost depends on arc time and welding power demand.For manual welding methods, labor costs generally make up the vast majority of the total cost. As a result, many cost-saving measures are focused on minimizing operation time. To do this, welding procedures with high deposition rates can be selected, and weld parameters can be fine-tuned to increase welding speed. Mechanization and automation are often implemented to reduce labor costs, but this frequently increases the cost of equipment and creates additional setup time. Material costs tend to increase when special properties are necessary, and energy costs normally do not amount to more than several percent of the total welding cost.In recent years, in order to minimize labor costs in high production manufacturing, industrial welding has become increasingly more automated, most notably with the use of robots in resistance spot welding (especially in the automotive industry) and in arc welding. In robot welding, mechanized devices both hold the material and perform the weld and at first, spot welding was its most common application, but robotic arc welding increases in popularity as technology advances. Other key areas of research and development include the welding of dissimilar materials (such as steel and aluminum, for example) and new welding processes, such as friction stir, magnetic pulse, conductive heat seam, and laser-hybrid welding. Furthermore, progress is desired in making more specialized methods like laser beam welding practical for more applications, such as in the aerospace and automotive industries. Researchers also hope to better understand the often unpredictable properties of welds, especially microstructure, residual stresses, and a weld's tendency to crack or deform.The trend of accelerating the speed at which welds are performed in the steel erection industry comes at a risk to the integrity of the connection. Without proper fusion to the base materials provided by sufficient arc time on the weld, a project inspector cannot ensure the effective diameter of the puddle weld therefore he or she cannot guarantee the published load capacities unless they witness the actual installation. This method of puddle welding is common in the United States and Canada for attaching steel sheets to bar joist and structural steel members. Regional agencies are responsible for ensuring the proper installation of puddle welding on steel construction sites. Currently there is no standard or weld procedure which can ensure the published holding capacity of any unwitnessed connection, but this is under review by the American Welding Society.
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Welding
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Glass and plastic welding
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Glasses and certain types of plastics are commonly welded materials. Unlike metals, which have a specific melting point, glasses and plastics have a melting range, called the glass transition. When heating the solid material past the glass-transition temperature (Tg) into this range, it will generally become softer and more pliable. When it crosses through the range, above the glass-melting temperature (Tm), it will become a very thick, sluggish, viscous liquid, slowly decreasing in viscosity as temperature increases. Typically, this viscous liquid will have very little surface tension compared to metals, becoming a sticky, taffy to honey-like consistency, so welding can usually take place by simply pressing two melted surfaces together. The two liquids will generally mix and join at first contact. Upon cooling through the glass transition, the welded piece will solidify as one solid piece of amorphous material.
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Welding
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Glass and plastic welding
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Glass welding Glass welding is a common practice during glassblowing. It is used very often in the construction of lighting, neon signs, flashtubes, scientific equipment, and the manufacture of dishes and other glassware. It is also used during glass casting for joining the halves of glass molds, making items such as bottles and jars. Welding glass is accomplished by heating the glass through the glass transition, turning it into a thick, formable, liquid mass. Heating is usually done with a gas or oxy-gas torch, or a furnace, because the temperatures for melting glass are often quite high. This temperature may vary, depending on the type of glass. For example, lead glass becomes a weldable liquid at around 1,600 °F (870 °C), and can be welded with a simple propane torch. On the other hand, quartz glass (fused silica) must be heated to over 3,000 °F (1,650 °C), but quickly loses its viscosity and formability if overheated, so an oxyhydrogen torch must be used. Sometimes a tube may be attached to the glass, allowing it to be blown into various shapes, such as bulbs, bottles, or tubes. When two pieces of liquid glass are pressed together, they will usually weld very readily. Welding a handle onto a pitcher can usually be done with relative ease. However, when welding a tube to another tube, a combination of blowing and suction, and pressing and pulling is used to ensure a good seal, to shape the glass, and to keep the surface tension from closing the tube in on itself. Sometimes a filler rod may be used, but usually not.
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Welding
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Glass and plastic welding
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Because glass is very brittle in its solid state, it is often prone to cracking upon heating and cooling, especially if the heating and cooling are uneven. This is because the brittleness of glass does not allow for uneven thermal expansion. Glass that has been welded will usually need to be cooled very slowly and evenly through the glass transition, in a process called annealing, to relieve any internal stresses created by a temperature gradient.
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Welding
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Glass and plastic welding
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There are many types of glass, and it is most common to weld using the same types. Different glasses often have different rates of thermal expansion, which can cause them to crack upon cooling when they contract differently. For instance, quartz has very low thermal expansion, while soda-lime glass has very high thermal expansion. When welding different glasses to each other, it is usually important to closely match their coefficients of thermal expansion, to ensure that cracking does not occur. Also, some glasses will simply not mix with others, so welding between certain types may not be possible.
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Welding
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Glass and plastic welding
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Glass can also be welded to metals and ceramics, although with metals the process is usually more adhesion to the surface of the metal rather than a commingling of the two materials. However, certain glasses will typically bond only to certain metals. For example, lead glass bonds readily to copper or molybdenum, but not to aluminum. Tungsten electrodes are often used in lighting but will not bond to quartz glass, so the tungsten is often wetted with molten borosilicate glass, which bonds to both tungsten and quartz. However, care must be taken to ensure that all materials have similar coefficients of thermal expansion to prevent cracking both when the object cools and when it is heated again. Special alloys are often used for this purpose, ensuring that the coefficients of expansion match, and sometimes thin, metallic coatings may be applied to a metal to create a good bond with the glass.
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Welding
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Glass and plastic welding
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Plastic welding Plastics are generally divided into two categories, which are "thermosets" and "thermoplastics." A thermoset is a plastic in which a chemical reaction sets the molecular bonds after first forming the plastic, and then the bonds cannot be broken again without degrading the plastic. Thermosets cannot be melted, therefore, once a thermoset has set it is impossible to weld it. Examples of thermosets include epoxies, silicone, vulcanized rubber, polyester, and polyurethane.
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Welding
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Glass and plastic welding
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Thermoplastics, by contrast, form long molecular chains, which are often coiled or intertwined, forming an amorphous structure without any long-range, crystalline order. Some thermoplastics may be fully amorphous, while others have a partially crystalline/partially amorphous structure. Both amorphous and semicrystalline thermoplastics have a glass transition, above which welding can occur, but semicrystallines also have a specific melting point which is above the glass transition. Above this melting point, the viscous liquid will become a free-flowing liquid (see rheological weldability for thermoplastics). Examples of thermoplastics include polyethylene, polypropylene, polystyrene, polyvinylchloride (PVC), and fluoroplastics like Teflon and Spectralon.
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Welding
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Glass and plastic welding
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Welding thermoplastic is very similar to welding glass. The plastic first must be cleaned and then heated through the glass transition, turning the weld-interface into a thick, viscous liquid. Two heated interfaces can then be pressed together, allowing the molecules to mix through intermolecular diffusion, joining them as one. Then the plastic is cooled through the glass transition, allowing the weld to solidify. A filler rod may often be used for certain types of joints. The main differences between welding glass and plastic are the types of heating methods, the much lower melting temperatures, and the fact that plastics will burn if overheated. Many different methods have been devised for heating plastic to a weldable temperature without burning it. Ovens or electric heating tools can be used to melt the plastic. Ultrasonic, laser, or friction heating are other methods. Resistive metals may be implanted in the plastic, which respond to induction heating. Some plastics will begin to burn at temperatures lower than their glass transition, so welding can be performed by blowing a heated, inert gas onto the plastic, melting it while, at the same time, shielding it from oxygen.Many thermoplastics can also be welded using chemical solvents. When placed in contact with the plastic, the solvent will begin to soften it, bringing the surface into a thick, liquid solution. When two melted surfaces are pressed together, the molecules in the solution mix, joining them as one. Because the solvent can permeate the plastic, the solvent evaporates out through the surface of the plastic, causing the weld to drop out of solution and solidify. A common use for solvent welding is for joining PVC or ABS (acrylonitrile butadiene styrene) pipes during plumbing, or for welding styrene and polystyrene plastics in the construction of models. Solvent welding is especially effective on plastics like PVC which burn at or below their glass transition, but may be ineffective on plastics like Teflon or polyethylene that are resistant to chemical decomposition.
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Correctness (computer science)
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Correctness (computer science)
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In theoretical computer science, an algorithm is correct with respect to a specification if it behaves as specified. Best explored is functional correctness, which refers to the input-output behavior of the algorithm (i.e., for each input it produces an output satisfying the specification).Within the latter notion, partial correctness, requiring that if an answer is returned it will be correct, is distinguished from total correctness, which additionally requires that an answer is eventually returned, i.e. the algorithm terminates. Correspondingly, to prove a program's total correctness, it is sufficient to prove its partial correctness, and its termination. The latter kind of proof (termination proof) can never be fully automated, since the halting problem is undecidable.
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Correctness (computer science)
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Correctness (computer science)
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For example, successively searching through integers 1, 2, 3, … to see if we can find an example of some phenomenon—say an odd perfect number—it is quite easy to write a partially correct program (see box). But to say this program is totally correct would be to assert something currently not known in number theory.
A proof would have to be a mathematical proof, assuming both the algorithm and specification are given formally. In particular it is not expected to be a correctness assertion for a given program implementing the algorithm on a given machine. That would involve such considerations as limitations on computer memory.
A deep result in proof theory, the Curry–Howard correspondence, states that a proof of functional correctness in constructive logic corresponds to a certain program in the lambda calculus. Converting a proof in this way is called program extraction.
Hoare logic is a specific formal system for reasoning rigorously about the correctness of computer programs. It uses axiomatic techniques to define programming language semantics and argue about the correctness of programs through assertions known as Hoare triples.
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Correctness (computer science)
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Correctness (computer science)
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Software testing is any activity aimed at evaluating an attribute or capability of a program or system and determining that it meets its required results. Although crucial to software quality and widely deployed by programmers and testers, software testing still remains an art, due to limited understanding of the principles of software. The difficulty in software testing stems from the complexity of software: we can not completely test a program with moderate complexity. Testing is more than just debugging. The purpose of testing can be quality assurance, verification and validation, or reliability estimation. Testing can be used as a generic metric as well. Correctness testing and reliability testing are two major areas of testing. Software testing is a trade-off between budget, time and quality.
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Bird feeder
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Bird feeder
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A birdfeeder, bird table, or tray feeder is a device placed outdoors to supply bird food to birds (bird feeding). The success of a bird feeder in attracting birds depends upon its placement and the kinds of foods offered, as different species have different preferences.
Most bird feeders supply seeds or bird food, such as millet, sunflower (oil and striped), safflower, nyjer seed, and rapeseed or canola seed to seed-eating birds.
Bird feeders often are used for birdwatching and many people keep webcams trained on feeders where birds often congregate, with some even living just near the bird feeder.
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Bird feeder
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Types of feeders
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Seed feeders Seed feeders are the most common type of feeders. They can vary in design from tubes to hoppers and trays. Sunflower seeds or mixed seeds are popular for use in these feeders and will attract many songbirds such as cardinals, finches, and chickadees. Black oil sunflower seeds are especially popular with bird enthusiasts. The outer shell of the black oil sunflower seeds are thinner and easier to crack than other types of sunflower seeds. In addition, the kernel is larger than the striped or white sunflower seeds. Black oil sunflower seeds also contain a large amount of fat; therefore they are especially good to use in the winter. Most bird feeders are designed to dispense sunflower-sized foods, but there are specialty "finch feeders" with smaller openings to dispense the tiny Guizotia abyssinica (Niger seed), which is a favorite of smaller finches.
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Bird feeder
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Types of feeders
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Seed feeders are mainly squirrel proof, tube-like or hopper. Due to the need of keeping squirrels away from the bird food, manufacturers have created different defense mechanisms that may deter squirrels from getting close to the seed. Some seed feeders come with weight sensitive technology which shuts off the access to the seed ports whenever a heavy weight is detected (as most squirrels are heavier than birds). Birds can still feed as they weigh less and the ports remain open under their weight. Other seed feeders are designed to be mounted on poles as it is believed that squirrels reach seed feeders more easily from trees than from poles. The simplest type of squirrel proof feeder is a tube-like feeder surrounded by a metal cage. These feeders also offer protection from larger and more aggressive birds. Tube seed feeders are primarily made of clear plastic tubes with plastic or metal caps, bases and perches. Hopper bird feeders look like a house and attract a wide range of birds such as finches, cardinals, blue jays, sparrows and titmice.
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Bird feeder
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Types of feeders
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Hummingbird feeders Hummingbird feeders, rather than dispensing seed, supply liquid nourishment to hummingbirds in the form of a sugar solution. The solution is normally 4 parts water to 1 part white sugar. Only pure refined white cane or beet sugar should be used, according to experts: Brown, turbinado, or raw sugar must not be used because they contain levels of iron that could be lethal.
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Bird feeder
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Types of feeders
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Honey must not be used, because it promotes dangerous fungal growth.
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Bird feeder
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Types of feeders
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The nectar should be changed every 3–5 days.Hummingbird feeders usually have red accents or red glass to help attract hummingbirds. The sugar mixture is sometimes colored with red food coloring to attract birds, though this is not necessary if the feeder itself is red, and may actually be harmful to the birds. Yeasts tend to grow in hummingbird feeders and spoil the solution, so they must be refreshed frequently and kept very clean to avoid harm to the birds. See the article on hummingbirds for more details. Ants and other insects are also attracted to hummingbird nectar. Smearing petroleum jelly on the stem or cap of the feeder (away from the perch or flower part where the bird may come into contact with it) may prevent the ants from crawling to the feeder. When placing a hummingbird feeder, the feeder is best suited 15 to 20 feet from windows; 10 to 15 feet from the nearest cover, like shrubs or bushes; and in an open area that receives partial sun, so that hummingbirds can move from nectar source to nectar source.Hummingbird top-fill feeders are popular among bird lovers because they are easy to fill and clean and also because they do not need to be turned upright which means that there are less chances that the nectar is spilled. The sports bottle top-fill hummingbird feeders have the design of a sports bottle, with a mechanism that works similarly to such a bottle. With this type of feeder, one has to push down the plastic container in order to close the nectar reservoir and then to unscrew the cap and pour the nectar. After the cap is replaced, the body of the nectar reservoir can be pulled up. This type of bird feeder has the advantage that the feeder does not need to be turned upside down to be refilled and which results in less nectar wasted by spilling. The traditional top-fill hummingbird feeders are one of the most popular types. There is also a plunger type of top-filling hummingbird feeder which comes with a small plunger in the container that creates the vacuum seal when the lid is tightened and the nectar will start flowing only when the lid is sealed correctly to the feeding ports.The bottom-fill hummingbird feeders include a traditional bottom-fill feeder and several variations of it. The traditional ones are filled from an opening at the bottom of the nectar container but many manufacturers have come up with improved variations of the traditional style of feeder, to make feeding birds easier and with less nectar wasted. Some bottom-fill feeders come with a funnel-like opening at the bottom of the container, through which the feeder is filled. Other bottom-filled hummingbird feeders can be attached to one's window to provide a close-up of the birds.
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Bird feeder
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Types of feeders
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In 1932, W. R. Sullivan invented a hummingbird feeder designed to prevent other birds or insects from drinking from it, which he produced and sold locally around Kerrville, Texas.
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Bird feeder
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Types of feeders
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Oriole feeders Oriole feeders, which are traditionally colored orange, also supply such artificial nectar and are designed to serve New World orioles, which have an unusually shaped beak and tongue. These orioles and some other birds also will come to fruit foods, such as grape jelly, or half an orange on a peg. Hummingbirds will also feed from Oriole feeders.
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Bird feeder
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Types of feeders
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Oriole feeders usually have nectar containers made of glass or plastic, which are designed to attract the orioles. Oriole feeders should be cleaned at least once a week and even more often when the temperatures are higher. Oriole feeders also come in top fill, bottom fill and dish-like designs.
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Bird feeder
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Types of feeders
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Suet feeders A suet feeder is typically a metal cage-like construction with a plastic coating that contains a cake or block of suet to feed woodpeckers, flickers, nuthatches, and many other species of insect eaters. Suet logs are also very common. These wooden logs have holes drilled out for suet to be inserted. Suet is high in fat which helps to keep birds warm and nourished during the cold winter. Suet cakes consist of sunflower seeds and wheat or oat flakes mixed with suet, pork fat, or coconut oil.
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Bird feeder
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Types of feeders
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Other Birds housed in wired or glass cages can be fed with electronic bird feeders. The electronic bird feeders are capable of storing bird food for days and even weeks, depending on the feeder type and automatically replenish the dish once it is empty.
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Bird feeder
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Types of feeders
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Providing a varied array of tastes and feeding venues will result in less competition for food and dining spots for birds, just as well-planned and maintained gardens provide many plants which supply different types of seeds and nectars. A shallow bird bath can attract as many birds as a feeder but it must be safe from cats, kept clean, and refreshed frequently with clean water to avoid mosquitoes. The birdbath should be placed where a frightened bird can fly up easily to an overhanging limb or resting place if disturbed or attacked.
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Bird feeder
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Squirrels
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Squirrels may also help themselves to the contents of bird feeders, often not merely feeding, but carrying away the food to their hoard. There are various anti-squirrel techniques and devices available to thwart attempts by squirrels to raid bird feeders. Several manufacturers produce feeders with perches that collapse under the weight of anything heavier than a bird, or that use battery power to shock an intruder lightly or spin the perching area to fling it off. Caged feeders are often designed so that squirrels cannot reach the seed inside, but birds can easily fly through the cage's holes. A UK company, The Nuttery, held the original patent on this cage-within-a-cage design. Caged feeders are best to keep out gray squirrels. Chipmunks and red squirrels can usually enter caged feeders. Hot pepper in bird seed and suet has also been shown to be effective against squirrels without harming birds, as birds are not sensitive to capsaicin oleoresin, but mammals experience a strong burning sensation when exposed to it.The placement of a bird feeder can also prevent squirrels from accessing the seed. In addition, baffles can be used that prevent squirrels from gaining their footing above feeders. Below feeders, baffles can prevent squirrels from climbing any further, however squirrels are very agile and acrobatic and often find a way to overcome devices of any nature.
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Bird feeder
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Negative impacts
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Feeding wild birds does carry potential risks. Birds may contract and spread diseases like salmonellosis by gathering at feeders; poorly maintained feeding and watering stations may also cause illness. Birds at feeders risk predation by cats and other animals, or may incur injury by flying into windows. Steps should be taken to reduce the risks to birds, such as: regular disinfecting of feeders and watering stations, ensuring feed has not become moldy or rancid, and proper positioning of feeders to reduce crowding and window collisions. Birds are less likely to fly into windows that have a wooden lattice. Collisions with windows can also be reduced by using window decals.Depending on the feeder design and the type of feed used, species such as the house sparrow can dominate the use of the feeder. As a result, the house sparrow population can become inflated locally where feeders are used. In North America, where the house sparrow is an invasive species, competition from house sparrows can exclude the indigenous bluebirds from available nest sites as well as attack indigenous birds.The use of bird feeders has been claimed to cause many other environmental problems; some of these were highlighted in a 2002 front-page article in The Wall Street Journal, which provoked responses nationwide from bird enthusiasts and scientists who refuted the article's arguments.Prior to the publication of the Wall Street Journal article, Canadian ornithologist Jason Rogers also wrote about the environmental problems associated with the use of bird feeders in the journal Alberta Naturalist. In this article, Rogers explains how the use of bird feeders is inherently fraught with negative impacts and risks such as fostering dependency, altering natural distribution, density, and migration patterns, interfering with ecological processes, causing malnutrition, facilitating the spread of disease, and increasing the risk of death from cats, pesticides, hitting windows, and other causes.
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Electro-pneumatic action
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Electro-pneumatic action
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The electro-pneumatic action is a control system by the mean of air pressure for pipe organs, whereby air pressure, controlled by an electric current and operated by the keys of an organ console, opens and closes valves within wind chests, allowing the pipes to speak. This system also allows the console to be physically detached from the organ itself. The only connection was via an electrical cable from the console to the relay, with some early organ consoles utilizing a separate wind supply to operate combination pistons.
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Electro-pneumatic action
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Invention
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Although early experiments with Barker lever, tubular-pneumatic and electro-pneumatic actions date as far back as the 1850s, credit for a feasible design is generally given to the English organist and inventor, Robert Hope-Jones. He overcame the difficulties inherent in earlier designs by including a rotating centrifugal air blower and replacing banks of batteries with a DC generator, which provided electrical power to the organ. This allowed the construction of new pipe organs without any physical linkages whatsoever. Previous organs used tracker action, which requires a mechanical linkage between the console and the organ windchests, or tubular-pneumatic action, which linked the console and windchests with a large bundle of lead tubing.
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Electro-pneumatic action
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Operation
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When an organ key is depressed, an electric circuit is completed by means of a switch connected to that key. This causes a low-voltage current to flow through a cable to the windchest, upon which a rank, or multiple ranks of pipes are set. Within the chest, a small electro-magnet associated with the key that is pressed becomes energized. This causes a very small valve to open. This, in turn, allows wind pressure to activate a bellows or "pneumatic" which operates a larger valve. This valve causes a change of air pressure within a channel that leads to all pipes of that note. A separate "stop action" system is used to control the admittance of air or "wind" into the pipes of the rank or ranks selected by the organist's selection of stops, while other ranks are "stopped" from playing. The stop action can also be an electro-pneumatic action, or may be another type of action This pneumatically assisted valve action is in contrast to a direct electric action in which each pipe's valve is opened directly by an electric solenoid which is attached to the valve.
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Electro-pneumatic action
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Advantages and disadvantages
|
The console of an organ which uses either type of electric action is connected to the other mechanisms by an electrical cable. This makes it possible for the console to be placed in any desirable location. It also permits the console to be movable, or to be installed on a "lift", as was the practice with theater organs.
While many consider tracker action organs to be more sensitive to the player's control, others find some tracker organs heavy to play and tubular-pneumatic organs to be sluggish, and so prefer electro-pneumatic or direct electric actions.
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Electro-pneumatic action
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Advantages and disadvantages
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An electro-pneumatic action requires less current to operate than a direct electric action. This causes less demand on switch contacts. An organ using electro-pneumatic action was more reliable in operation than early direct electric organs until improvements were made in direct electric components.A disadvantage of an electro-pneumatic organ is its use of large quantities of thin perishable leather, usually lambskin. This requires an extensive "re-leathering" of the windchests every twenty-five to forty years depending upon the quality of the material used, the atmospheric conditions and the use of the organ.Like tracker and tubular action, electro-pneumatic action—when employing the commonly used pitman-style windchests—is less flexible in operation than direct electric action. When electro-pneumatic action uses unit windchests (as does the electro-pneumatic action constructed by organ builder Schoenstein & Co.), then it works similarly to direct electric action, in which each rank operates independently, allowing "unification", where each individual rank on a windchest can be played at various octave ranges.
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Electro-pneumatic action
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Advantages and disadvantages
|
A drawback to older electric action organs was the large amount of wiring required for operation. With each stop tab and key being wired, the transmission cable could easily contain several hundred wires. The great number of wires required between the keyboards, the banks of relays and the organ itself, with each solenoid requiring its own signal wire, made the situation worse, especially if a wire was broken (this was particularly true with consoles located on lifts and/or turntables), which made tracing the break very difficult.
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Electro-pneumatic action
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Advantages and disadvantages
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These problems increased with the size of the instrument, and it would not be unusual for a particular organ to contain over a hundred miles of wiring. The largest pipe organ in the world, the Boardwalk Hall Auditorium Organ, is said to contain more than 137,500 miles (221,300 km) of wire. Modern electronic switching has largely overcome these physical problems.
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Electro-pneumatic action
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Modern methods
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In the years after the advent of the transistor, and later, integrated circuits and microprocessors, miles of wiring and electro-pneumatic relays have given way to electronic and computerized control and relay systems, which have made the control of pipe organs much more efficient. But for its time, the electro-pneumatic action was considered a great success, and even today modernized versions of this action are used in many new pipe organs, especially in the United States and the United Kingdom.
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SciShow
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SciShow
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SciShow is a collection of YouTube channels that focuses on science news. The program is hosted by Hank Green along with a rotating cast of co-hosts. SciShow was launched as an original channel.
The series has been consistently releasing new material since it was created in 2012.
Since its launch, three additional channels have been launched under the SciShow brand: SciShow Space, SciShow Psych, and SciShow Kids.
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SciShow
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History and funding
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The channel was launched as an "original channel", which meant that YouTube funded the channel. The show's initial grant was projected to expire in 2014, and in response, on September 12, 2013 SciShow joined the viewer-funding site Subbable, created in part by Green.In 2014, the channel landed a national advertisement deal with YouTube. The educational program was featured on platforms such as billboards and television commercials as a result. Green details that the advertisements had a positive effect on SciShow, stating, "My Twitter exploded, our followers and subscribers exploded."After Patreon acquired Subbable, the channel switched over to Patreon where it continues to receive support in exchange for various perks. SciShow currently has over four thousand patrons.
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SciShow
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Production and hosting
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Though Green hosts the majority of episodes, the show has alternate hosts; Michael Aranda has been with the show since its inception, and Olivia Gordon of the Missoula Insectarium joined in June 2016. Gordon left SciShow in August 2020, and was replaced by ethnobotanist Rose Bear Don't Walk. Prior to her move to Chicago, Emily Graslie of The Brain Scoop also occasionally hosted on the channel. There have also been guest appearances by Lindsey Doe, who hosts Sexplanations, another channel launched by Green; and by longtime SciShow staffer Stefan Chin, who since 2017 has been a regular host. SciShow has grown since its 2012 launch; it now employs a full editorial, production, and operations staff.SciShow Space has three rotating hosts: Hank Green, Reid Reimers, and Caitlin Hofmeister. Similarly, SciShow Psych rotates hosting between Hank Green, Brit Garner, and Anthony Brown. SciShow Kids is primarily hosted by Jessi Knudsen Castañeda, host of Animal Wonders Montana.
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SciShow
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Content
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Several different scientific fields are covered by SciShow, including chemistry, physics, biology, zoology, geology, geography, entomology, botany, meteorology, astronomy, medicine, psychology, anthropology, math and computer science. The videos on SciShow have a vast variety of different topics, such as nutrition, and "science superlatives". As of April 2020, SciShow has released over 2250 videos.A spin-off channel, SciShow Space, launched in April 2014 to specialize in space topics. A second spin-off, SciShow Kids, launched in March 2015 to specialize in delivering science topics to children. Kids went on hiatus in late 2018, returning in April 2020. A third spinoff channel was announced in February 2017, SciShow Psych, which debuted in March 2017, specializing in psychology and neuroscience. A podcast, SciShow Tangents, was launched in November 2018; it features entertaining exchanges of scientific facts among many of the shows' staffers, and is directed at a mature audience.
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SciShow
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Podcast
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In November 2018, a co-branded podcast titled SciShow Tangents was launched as a co-production with WNYC Studios. It consists of a panel format where Hank Green, Ceri Riley, Stefan Chin, and Sam Schultz share facts about science on a weekly theme; each episode has multiple segments, several of which are competitive. In late 2020, the podcast ceased its association with WNYC Studios, and continues as an independently produced entity. The podcast is a restructured and reimagined continuation of their previous podcast, Holy Fucking Science, which ran from January 2017 to March 2018.
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SciShow
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Reception
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As SciShow has amassed a large following, the channel has been featured on several media outlets.In October 2014 the channel surpassed two million subscribers, and over 210 million video views.
As of September 2021, the channel has over 6.7 million subscribers and over 1.4 billion total views.In 2017, SciShow won a Webby Award in the People's Voice category.
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Mexsana
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Mexsana
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Mexsana is an antiseptic medicated powder. It is used to relieve itching and chafing, to protect against perspiration odor and discomfort, while also keeping the skin's pH balanced. The product is also used to treat severely chapped skin, minor burns, and other minor skin irritations. Currently Mexsana medicated powder is produced by Merck Sharp & Dohme (MSD) Laboratories.
Not to be confused with "Mexana" medicated powder, sold in the Dominican Republic.
Active Ingredients: Zinc oxide, kaolin, benzethonium chloride.
Also contains: Camphor, eucalyptus oil, fragrance, lemon oil.
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Mexsana
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Process
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The product's elaboration process stands out for the strictness in its safety measures and also in the precise measures for weighting its components. The powder, mineral of less hardness in the Mohs scale, is the source that gives the name to the generic product used for personal hygienic purposes, and forms around 85% of the composition of the brand.
Initially, packages filled with huge amounts of this material, arranged in sacks of 25 kilograms each, are placed in warehouses that technically isolate the daily contamination, due to a sophisticated air flow mechanism.
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Mexsana
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In popular culture
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A can of Mexsana brand heat powder can be found on the back of the Greatest Hits album of Mötley Crüe.
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Mexsana
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Products available in Colombia
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Currently the brand has both powder and spray products available in the Colombian market (Made by Bayer Andina): Powder Talcos Mexsana Lady Mexsana Mexsana Avena Sprays Mexsana Pies Lady Mexsana Antibacterial Mexsana Pies Antiperspirant Mexsana Pies Ultra Mexsana Pies Avena
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Methyl dimethyldithiocarbamate
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Methyl dimethyldithiocarbamate
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Methyl dimethyldithiocarbamate is the organosulfur compound with the formula (CH3)2NC(S)SCH3. It is the one of simplest dithiocarbamic esters. It is a white volatile solid that is poorly soluble in water but soluble in many organic solvents. It was once used as a pesticide.
Methyl dimethyldithiocarbamate can be prepared by methylation of salts of dimethyldithiocarbamate: (CH3)2NCS2Na + (CH3O)2SO2 → (CH3)2NC(S)SCH3 + Na[CH3OSO3]It can also be prepared by the reaction of a tetramethylthiuram disulfide with methyl Grignard reagents: [(CH3)2NC(S)S]2 + CH3MgBr → (CH3)2NC(S)SCH3 + (CH3)2NCS2MgBr
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Model predictive control
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Model predictive control
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Model predictive control (MPC) is an advanced method of process control that is used to control a process while satisfying a set of constraints. It has been in use in the process industries in chemical plants and oil refineries since the 1980s. In recent years it has also been used in power system balancing models and in power electronics. Model predictive controllers rely on dynamic models of the process, most often linear empirical models obtained by system identification. The main advantage of MPC is the fact that it allows the current timeslot to be optimized, while keeping future timeslots in account. This is achieved by optimizing a finite time-horizon, but only implementing the current timeslot and then optimizing again, repeatedly, thus differing from a linear–quadratic regulator (LQR). Also MPC has the ability to anticipate future events and can take control actions accordingly. PID controllers do not have this predictive ability. MPC is nearly universally implemented as a digital control, although there is research into achieving faster response times with specially designed analog circuitry.Generalized predictive control (GPC) and dynamic matrix control (DMC) are classical examples of MPC.
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Model predictive control
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Overview
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The models used in MPC are generally intended to represent the behavior of complex and simple dynamical systems. The additional complexity of the MPC control algorithm is not generally needed to provide adequate control of simple systems, which are often controlled well by generic PID controllers. Common dynamic characteristics that are difficult for PID controllers include large time delays and high-order dynamics.
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Model predictive control
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Overview
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MPC models predict the change in the dependent variables of the modeled system that will be caused by changes in the independent variables. In a chemical process, independent variables that can be adjusted by the controller are often either the setpoints of regulatory PID controllers (pressure, flow, temperature, etc.) or the final control element (valves, dampers, etc.). Independent variables that cannot be adjusted by the controller are used as disturbances. Dependent variables in these processes are other measurements that represent either control objectives or process constraints.
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Model predictive control
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Overview
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MPC uses the current plant measurements, the current dynamic state of the process, the MPC models, and the process variable targets and limits to calculate future changes in the dependent variables. These changes are calculated to hold the dependent variables close to target while honoring constraints on both independent and dependent variables. The MPC typically sends out only the first change in each independent variable to be implemented, and repeats the calculation when the next change is required.
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Model predictive control
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Overview
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While many real processes are not linear, they can often be considered to be approximately linear over a small operating range. Linear MPC approaches are used in the majority of applications with the feedback mechanism of the MPC compensating for prediction errors due to structural mismatch between the model and the process. In model predictive controllers that consist only of linear models, the superposition principle of linear algebra enables the effect of changes in multiple independent variables to be added together to predict the response of the dependent variables. This simplifies the control problem to a series of direct matrix algebra calculations that are fast and robust.
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Model predictive control
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Overview
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When linear models are not sufficiently accurate to represent the real process nonlinearities, several approaches can be used. In some cases, the process variables can be transformed before and/or after the linear MPC model to reduce the nonlinearity. The process can be controlled with nonlinear MPC that uses a nonlinear model directly in the control application. The nonlinear model may be in the form of an empirical data fit (e.g. artificial neural networks) or a high-fidelity dynamic model based on fundamental mass and energy balances. The nonlinear model may be linearized to derive a Kalman filter or specify a model for linear MPC.
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Model predictive control
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Overview
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An algorithmic study by El-Gherwi, Budman, and El Kamel shows that utilizing a dual-mode approach can provide significant reduction in online computations while maintaining comparative performance to a non-altered implementation. The proposed algorithm solves N convex optimization problems in parallel based on exchange of information among controllers.
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Model predictive control
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Overview
|
Theory behind MPC MPC is based on iterative, finite-horizon optimization of a plant model. At time t the current plant state is sampled and a cost minimizing control strategy is computed (via a numerical minimization algorithm) for a relatively short time horizon in the future: [t,t+T] . Specifically, an online or on-the-fly calculation is used to explore state trajectories that emanate from the current state and find (via the solution of Euler–Lagrange equations) a cost-minimizing control strategy until time t+T . Only the first step of the control strategy is implemented, then the plant state is sampled again and the calculations are repeated starting from the new current state, yielding a new control and new predicted state path. The prediction horizon keeps being shifted forward and for this reason MPC is also called receding horizon control. Although this approach is not optimal, in practice it has given very good results. Much academic research has been done to find fast methods of solution of Euler–Lagrange type equations, to understand the global stability properties of MPC's local optimization, and in general to improve the MPC method.
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Model predictive control
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Overview
|
Principles of MPC Model predictive control is a multivariable control algorithm that uses: an internal dynamic model of the process a cost function J over the receding horizon an optimization algorithm minimizing the cost function J using the control input uAn example of a quadratic cost function for optimization is given by: J=∑i=1Nwxi(ri−xi)2+∑i=1MwuiΔui2 without violating constraints (low/high limits) with xi : i th controlled variable (e.g. measured temperature) ri : i th reference variable (e.g. required temperature) ui : i th manipulated variable (e.g. control valve) wxi : weighting coefficient reflecting the relative importance of xi wui : weighting coefficient penalizing relative big changes in ui etc.
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Model predictive control
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Nonlinear MPC
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Nonlinear model predictive control, or NMPC, is a variant of model predictive control that is characterized by the use of nonlinear system models in the prediction. As in linear MPC, NMPC requires the iterative solution of optimal control problems on a finite prediction horizon. While these problems are convex in linear MPC, in nonlinear MPC they are not necessarily convex anymore. This poses challenges for both NMPC stability theory and numerical solution.The numerical solution of the NMPC optimal control problems is typically based on direct optimal control methods using Newton-type optimization schemes, in one of the variants: direct single shooting, direct multiple shooting methods, or direct collocation. NMPC algorithms typically exploit the fact that consecutive optimal control problems are similar to each other. This allows to initialize the Newton-type solution procedure efficiently by a suitably shifted guess from the previously computed optimal solution, saving considerable amounts of computation time. The similarity of subsequent problems is even further exploited by path following algorithms (or "real-time iterations") that never attempt to iterate any optimization problem to convergence, but instead only take a few iterations towards the solution of the most current NMPC problem, before proceeding to the next one, which is suitably initialized; see, e.g.,.. Another promising candidate for the nonlinear optimization problem is to use a randomized optimization method. Optimum solutions are found by generating random samples that satisfy the constraints in the solution space and finding the optimum one based on cost function. While NMPC applications have in the past been mostly used in the process and chemical industries with comparatively slow sampling rates, NMPC is being increasingly applied, with advancements in controller hardware and computational algorithms, e.g., preconditioning, to applications with high sampling rates, e.g., in the automotive industry, or even when the states are distributed in space (Distributed parameter systems). As an application in aerospace, recently, NMPC has been used to track optimal terrain-following/avoidance trajectories in real-time.
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Model predictive control
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Explicit MPC
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Explicit MPC (eMPC) allows fast evaluation of the control law for some systems, in stark contrast to the online MPC. Explicit MPC is based on the parametric programming technique, where the solution to the MPC control problem formulated as optimization problem is pre-computed offline. This offline solution, i.e., the control law, is often in the form of a piecewise affine function (PWA), hence the eMPC controller stores the coefficients of the PWA for each a subset (control region) of the state space, where the PWA is constant, as well as coefficients of some parametric representations of all the regions. Every region turns out to geometrically be a convex polytope for linear MPC, commonly parameterized by coefficients for its faces, requiring quantization accuracy analysis. Obtaining the optimal control action is then reduced to first determining the region containing the current state and second a mere evaluation of PWA using the PWA coefficients stored for all regions. If the total number of the regions is small, the implementation of the eMPC does not require significant computational resources (compared to the online MPC) and is uniquely suited to control systems with fast dynamics. A serious drawback of eMPC is exponential growth of the total number of the control regions with respect to some key parameters of the controlled system, e.g., the number of states, thus dramatically increasing controller memory requirements and making the first step of PWA evaluation, i.e. searching for the current control region, computationally expensive.
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Model predictive control
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Robust MPC
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Robust variants of model predictive control are able to account for set bounded disturbance while still ensuring state constraints are met. Some of the main approaches to robust MPC are given below.
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Model predictive control
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Robust MPC
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Min-max MPC. In this formulation, the optimization is performed with respect to all possible evolutions of the disturbance. This is the optimal solution to linear robust control problems, however it carries a high computational cost. The basic idea behind the min/max MPC approach is to modify the on-line "min" optimization to a "min-max" problem, minimizing the worst case of the objective function, maximized over all possible plants from the uncertainty set.
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Model predictive control
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Robust MPC
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Constraint Tightening MPC. Here the state constraints are enlarged by a given margin so that a trajectory can be guaranteed to be found under any evolution of disturbance.
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Model predictive control
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Robust MPC
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Tube MPC. This uses an independent nominal model of the system, and uses a feedback controller to ensure the actual state converges to the nominal state. The amount of separation required from the state constraints is determined by the robust positively invariant (RPI) set, which is the set of all possible state deviations that may be introduced by disturbance with the feedback controller.
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Model predictive control
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Robust MPC
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Multi-stage MPC. This uses a scenario-tree formulation by approximating the uncertainty space with a set of samples and the approach is non-conservative because it takes into account that the measurement information is available at every time stage in the prediction and the decisions at every stage can be different and can act as recourse to counteract the effects of uncertainties. The drawback of the approach however is that the size of the problem grows exponentially with the number of uncertainties and the prediction horizon.
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Model predictive control
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Robust MPC
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Tube-enhanced multi-stage MPC. This approach synergizes multi-stage MPC and tube-based MPC. It provides high degrees of freedom to choose the desired trade-off between optimality and simplicity by the classification of uncertainties and the choice of control laws in the predictions.
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Model predictive control
|
Commercially available MPC software
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Commercial MPC packages are available and typically contain tools for model identification and analysis, controller design and tuning, as well as controller performance evaluation.
A survey of commercially available packages has been provided by S.J. Qin and T.A. Badgwell in Control Engineering Practice 11 (2003) 733–764.
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Model predictive control
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MPC vs. LQR
|
Model predictive control and linear-quadratic regulators are both expressions of optimal control, with different schemes of setting up optimisation costs.
While a model predictive controller often looks at fixed length, often graduatingly weighted sets of error functions, the linear-quadratic regulator looks at all linear system inputs and provides the transfer function that will reduce the total error across the frequency spectrum, trading off state error against input frequency.
Due to these fundamental differences, LQR has better global stability properties, but MPC often has more locally optimal[?] and complex performance.
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Model predictive control
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MPC vs. LQR
|
The main differences between MPC and LQR are that LQR optimizes across the entire time window (horizon) whereas MPC optimizes in a receding time window, and that with MPC a new solution is computed often whereas LQR uses the same single (optimal) solution for the whole time horizon. Therefore, MPC typically solves the optimization problem in a smaller time window than the whole horizon and hence may obtain a suboptimal solution. However, because MPC makes no assumptions about linearity, it can handle hard constraints as well as migration of a nonlinear system away from its linearized operating point, both of which are major drawbacks to LQR.
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Model predictive control
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MPC vs. LQR
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This means that LQR can become weak when operating away from stable fixed points. MPC can chart a path between these fixed points, but convergence of a solution is not guaranteed, especially if thought as to the convexity and complexity of the problem space has been neglected.
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Orthopedic oncologist
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Orthopedic oncologist
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An orthopedic (orthopaedic) oncologist is a physician and surgeon who specializes in the diagnoses and treatment of primary benign and malignant tumors of the bones.
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Orthopedic oncologist
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Education
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An orthopedic oncologist in the United States must complete 4 years of medical school. Following graduation from medical school, the completion of an orthopedic surgical residency (medicine) is required. This residency program is typically 5 years in length and focuses on general orthopedic surgical techniques for common orthopedic injuries. As the residency progresses, the level of injury, disease and trauma treated by the resident becomes increasingly complex. By completion of the residency program, the orthopedic surgeon should be able to competently diagnose and treat a variety of injury and trauma to the bony structures of the body. At this point, most orthopedic physicians become attending doctors specializing in general Orthopedic surgery. However, aspiring orthopedic surgeons who wish to sub-specialize in orthopedic oncology must complete an additional phase to their training known as a fellowship (medicine). A fellowship in orthopedic oncology general lasts an additional one to two years following the completion of the residency. During this time, the physician will learn in depth about the pathology and treatment of various forms of primary benign and malignant neoplasms of the bones and bony structures of the human body (any cancer which has originated from the bone, as opposed to cancers which originated from other organs and have secondarily spread, or metastasized, to the bones, which is much more common; these specialists deal mostly with primary bone tumors). The physician will study directly under an experienced attending orthopedic oncologist with one-on-one mentoring. The fellowship is designed to be an intense immersion into a complex medical topic.
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Orthopedic oncologist
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Specializations
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Due to the relative rarity of primary bone tumor in relation to other forms of cancer, there are fewer than two hundred orthopedic oncologists practicing around the United States, nearly all of whom work in major urban teaching hospitals. While general orthopedic surgeons may be qualified to perform surgical intervention on these tumors, it is advisable when confronted with primary malignancy of the bone to seek out the treatment of an orthopedic oncologist, due to their increased knowledge and experience dealing with these rare and very serious tumors.
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Cantharidic acid
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Cantharidic acid
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Cantharidic acid is a selective inhibitor of PP2A (protein phosphatase 2) and PP1 (protein phosphatase 1).It is the hydrolysis product of cantharidin.
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Glossary of rugby union terms
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Glossary of rugby union terms
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Rugby union is a team sport played between two teams of fifteen players.This is a general glossary of the terminology used in the sport of rugby union. Where words in a sentence are also defined elsewhere in this article, they appear in italics.
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Glossary of rugby union terms
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0-9
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22 The 22 m line, marking 22 metres (72 ft) from the tryline.89 An "89" or eight-nine move is a phase following a scrum, in which the number 8 picks up the ball and transfers it to number 9 (scrum-half).99 The "99" call was a policy of simultaneous retaliation by the 1974 British Lions tour to South Africa, (the 99 comes from the British emergency services telephone number which is 999). The tour was marred by on-pitch violence, which the match officials did not adequately control and the relative absence of cameras compared to the modern game made citing and punishment after the fact unlikely. The Lions' captain, Willie John McBride (Ireland), therefore instigated a policy of "one in, all in" - that is, when one Lions player retaliated, all other Lions were expected to join in the melee or hit the nearest Springbok. By doing so, the referee would be unable to identify any single instigator, and so would be left with the choice of sending off all or none of the team. In this respect, the "99" call was extremely successful - no Lions player was sent off during the tour.
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Glossary of rugby union terms
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A
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Accidental offside see OffsideAdvantage "Advantage" is the period of time after an infringement in which the non-offending side have the opportunity to gain sufficient territory or tactical opportunity to negate the need to stop the game due to the infringement. The referee will signal advantage with his arm out horizontally toward the non-infringing team. If no tactical or territorial advantage is gained, the referee will whistle and give the decision that had been delayed. If sufficient advantage is gained, the referee will call "advantage over", and play will continue. The Advantage Law allows the game to flow more freely, and not stop for every minor infringement, giving no incentive for a player to commit "intentional" fouls.An example of the application of advantage would be if Team A knocked the ball on (technical offence, conceding a scrum) but a Team B player picked the ball up and made a run forwards before being tackled.Advantage line Also called the gain line. It is an imaginary line drawn across the centre of the pitch when there is a breakdown in open play, such as a ruck, maul or scrum. Advancing across the gain line represents a gain in territory.Alickadoo A non-player associated with a rugby game or club, especially a committee member or administrative official. May perform various off-field roles, particularly on a match day.Ankle tap An ankle-tap or tap-tackle is a form of tackle. It is used when the player carrying the ball is running at speed and a defending player is approaching from behind. Even if the defender is not able to get close enough to the ball-carrier to wrap his arms around him in a conventional tackle, he may still be able to dive at the other player's feet and, with outstretched arm, deliver a tap or hook to the player's foot (or feet) causing the player to stumble.
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Glossary of rugby union terms
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B
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Ball back If the ball enters touch, then play is restarted by a line-out at the point where the ball left the field of play. The exception to this is if the ball is kicked into touch on the full. In this case, a line-out is taken at the point from where the ball was kicked, and not from where it entered touch.
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Glossary of rugby union terms
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B
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Ball back is waived in certain circumstances: If the kicking player is inside his own 22m line when he receives and then kicks the ball. If the player receives the ball outside the 22 then retreats back into the 22 and kicks into touch on the full it is a lineout at the nearest point on the touchline from where the ball was kicked.
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Glossary of rugby union terms
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B
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If a side elects to kick a penalty into touch.Bill Australian name given to the (William Webb Ellis) Webb Ellis Cup.Black dot A mark in the centre of the crossbar connecting the goal posts, usually black in colour, used as an aid to kickers with their aiming. A player scoring a try in the centre of the goal line, that is beneath the posts, can be said to have scored "under the black dot".Blindside The narrow side of the pitch in relation to a scrum or a breakdown in play; it is the opposite of openside. The blindside flanker is expected to cover the opposing team blindside at scrum and breakdown.The Blindside of the scrum is the side that the opposing scrum half is feeding the ball from. The Open side is the other side, 'Open' in this situation, means unimpeded.Blitz defence The blitz defence is a defensive technique similar to the defence used in rugby league. It relies on the whole defensive line moving forward towards their marked man as one, as soon as the ball leaves the base of a ruck or maul. The charge is usually led by the inside centre.The idea of this technique is to prevent the attacking team gaining any ground by tackling them behind the gain line and forcing interceptions and charged down kicks. However, the defending team can be vulnerable to chip kicks and any player breaking the defensive line will have much space to play because the defence are running the other way and must stop, turn and chase.Blood bin It is also called blood replacement or blood sub(stitution). A player who has a visible bleeding injury may be replaced for up to fifteen minutes (running time, not game time), during which he or she may receive first-aid treatment to stop the flow of blood and dress the wound. The player may then return to the pitch to continue playing.Bonus points Bonus points are a method of deciding table points from a rugby union match. It was implemented in order to encourage attacking play throughout a match, to discourage repetitive goal-kicking, and to reward teams for "coming close" in losing efforts.
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Glossary of rugby union terms
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B
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Under the standard system, 1 bonus point is awarded for scoring 4 (or more) tries and 1 bonus point for losing by 7 points (or fewer).
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Glossary of rugby union terms
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B
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The French professional league replaces the four-try bonus point with a point for a win in which the winning team scores at least 3 more tries than its opponent. In addition, the "losing" bonus point requires a margin of defeat of 5 or fewer points.Australia's National Rugby Championship uses the same system as France, except that a losing bonus point is awarded if the margin of defeat is 8 points or fewer.Box-kick This is a kick taken from behind a scrum, normally by the scrum-half, in which he turns away from the scrum facing the touchline, and kicks the ball back over the scrum into the clear "box" of space behind the opposition to allow his own team to chase through and regain the ball in undefended territory.Breakdown The breakdown is a colloquial term for the short period of open play immediately after a tackle and before and during the ensuing ruck. During this time teams compete for possession of the ball, initially with their hands and then using feet in the ruck. Most referees will call "ruck" or "hands away" as soon as the ruck is formed. Most infringements take place at the breakdown, owing to the greater variety of possible offences at a breakdown, for example handling in the ruck, killing the ball, offside at the ruck, and so on.
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Glossary of rugby union terms
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C
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Caution A player who deliberately or repeatedly infringes the laws is cautioned, and shown a yellow card. A cautioned player is suspended from playing for ten minutes.Cavalry Charge Typically during a penalty kick or free kick, the attacking players form a line behind their kicker. When signaled, they charge forward. The kicker then tap-kicks the ball and passes to one of the players behind. This move is explicitly forbidden under 10.4(p) and the penalty is a penalty kick.Centre They are the players wearing shirts numbers 12 & 13. They are divided into inside and outside centre. The inside centre is also known as the second five-eighths in New Zealand.Charge-down When a player makes a defensive clearance kick, but it hits an opponent who has run towards him in an attempt to block it, it is known as a charge-down. These can be good try-scoring opportunities.Choke tackle Tackle in which the tackler tries to keep the ball carrier on his feet and push him backwards before taking him to the ground. This is harder to execute but gives the tackler's side a gain in territory.Cibi The cibi is a Fijian war dance performed by the Fiji national team before each of their international matches.Colts Men's youth rugby players, generally with an upper age limit of 21 years, may be referred to as colts. In some instances the age limit may be as high as 23, but this is uncommon.A competition for players below the age of 19, for example, may be known as Under-19 Colts. The term is usually not applied to secondary school teams; most colts players have completed their high school years.Conversion If a team scores a try, they have an opportunity to "convert" it for two further points by kicking the ball through the goal - that is, between the posts and above the crossbar. The kick is taken at any point on the field of play in line with the point that the ball was grounded for the try, parallel to the touch-lines. So it is advantageous to score a try nearer to the posts as it is easier to convert it.The kick can be either a drop kick or a place kick in the 15-man game. However, in sevens, all conversions must be drop kicks.Counter rucking If a team (usually the team that took the ball into contact) has secured the ball at a ruck, and the other team manage to force them off the ball and secure possession themselves, the defending team are said to have "counter-rucked"Crash ball It is an attacking tactic where a player receives a pass at pace and runs directly at the opposition's defensive line. The crash ball runner attempts to commit two or more opposing players to the tackle, then attempts to make the ball available to teammates by off-loading in the tackle or recycling the ball quickly from the ruck.By committing players to the tackle, the crash ball runner creates holes in the opposition's defense, thereby creating attacking opportunities for teammates.Crash tackle Another name for the crash ball as mentioned above.Cross-Field Kick A kick which goes from one side of the field to the other and is kicked very high, usually resulting in an aerial battle between an attacker and defender to catch it. This is usually used near the defending team's try-line, often with the catch happening in the in-goal area itself. Most often used when the kicker knows the referee is playing advantage and his team will get a penalty if the kick fails - this is because the kick itself is very risky and likely to result in an interception.
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Glossary of rugby union terms
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D
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Drift defence The drift defence is a defensive technique that forces the attacking side into an ever-shrinking pocket near to the touchline. It operates by the defensive side moving forward and diagonally following the path of the attacking side's ball movements. If used successfully the ball will usually end up in the attacking winger's hands near the line of touch. This player would then find themselves surrounded on one side by a defending outside centre, with the opposing winger opposite and the touchline on his other side. This will prevent a cut-back and allows the Touchline to act as a 16th player. Its disadvantage is that if the attacking team are strong enough to break through the pocket tackle the defending team will have no players spare to cover a breakout.Drop goal A drop goal is scored when a player kicks the ball from hand through the opposition's goal, but the ball must touch the ground between being dropped and kicked. It is worth three points.The team awarded a free kick cannot score a dropped goal until the ball next becomes dead, or until an opponent has played or touched it, or has tackled the ball carrier. This restriction applies also to a scrum taken instead of a free kick.Drop kick A drop kick is when a player kicks the ball from hand and the ball touches the ground between being dropped and kicked. If a drop kick goes through a goal then it results in a drop goal.Dummy pass An offensive ruse, where the ball carrier moves as if to pass the ball to a teammate, but then continues to run with the ball himself; the objective is to trick defenders into marking the would-be pass receiver, creating a gap for the ball carrier to run into. If it is successful, the player is said to have "sold the dummy".Dummy runner Another offensive tactic; a player on the attacking team runs towards the opposition as if running onto a pass, only for the ball to be passed to another player, carried on by the ball carrier or kicked forwards. As with a dummy pass, this tactic draws defenders away from the ball and creates space for the attacking teamDump tackle It is a tackling technique. The tackler wraps his arms around the ball carrier's thighs and lifts him a short distance in the air before forcibly driving him to the ground. The tackler must go to ground with the ball carrier for the tackle to be legal. This technique is useful to completely stop the opponent in his tracks. A dump tackle that drops the ball carrier on his head or neck is known as a spear tackle, and will almost invariably concede a penalty and possibly result in a caution for the tackler. In rugby union, World Rugby has ruled that a dangerous tackle of this type, sometimes also called a tip tackle, should be punished with a straight red card.
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Glossary of rugby union terms
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E
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Eightman, Eighth-man Alternative name for the Number 8
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Glossary of rugby union terms
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F
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Five metre scrum, Scrum-five When a scrum offence is committed within 5 m of either try line, or in the in-goal area, the referee will award a scrum on the five metre line; this is to prevent all but the most brutal packs from driving the ball over the try line within the scrum.Fend or "hand off" Fending is the action by the ball carrier of repelling a tackler using his arm. For the action to be legal, the ball carrier's arm must be straight before contact is made; a shove or "straight-arm smash", where the arm is extended immediately before contact or on contact, is illegal and classed as dangerous play.First XV or First fifteen The preferred starting line-up of a team – more colloquially, the senior team of any club.Flanker Also known as breakaways or wing forwards. They are the players wearing shirts numbers 6 & 7. They are the players with the fewest set responsibilities. The player should have all round attributes: speed, strength, fitness, tackling and handling skills. Flankers are always involved in the game, as they are the real ball winners at the breakdown, especially the number 7. The two flankers do not usually bind to the scrum in a fixed position. Instead, the openside flanker will attach to the scrum on whichever side is further from the nearer touchline, while the blindside flanker attaches himself to the scrum on the side closer to the touchline.Fly half or five-eighth Referred to by a number of different names, including first five-eighth in New Zealand, this player wears shirt number 10. They are the back-line player most likely to be passed the ball from the scrum-half or half-back, and therefore make many key tactical decisions during a game. Often this player is also the goal kicker as the position requires excellent kicking skills.Forward pass It is called a throw-forward in the laws of the game.A forward pass occurs when the ball fails to travel backwards in a pass. If the ball is not thrown or passed forward but it bounces forward after hitting a player or the ground, it is not a throw-forward.If the referee deems it accidental, this results in a scrum to the opposing team; however deliberate forward passes result in the award of a penalty.Foul play Foul play is defined as the deliberate infringement of the laws of the game.Fourth official A fourth official is one who controls replacements and substitutes. He may also substitute for referee or touch judge in case of injury to either of them.Free-kick Also called short arm penalty. This is a lesser form of the penalty, usually awarded to a team for a technical offence committed by the opposing side such as numbers at the line-out or time wasting at a scrum. A free kick is also awarded for calling a mark.A team cannot kick for goal and the normal 22m rule applies for kicking for position from a free kick. A Free Kick is signalled by the referee with a bent arm raised in the air.Full-back They are the player wearing jersey number 15. They act as the last line of defence against running attacks by the opposing three-quarter backs. The full-back is expected to field high kicks from the opposition, and reply with a superior kick or a counter-attack. The full back is sometimes the specialist goal-kicker in a team, taking penalty and conversion kicks.Full house Scoring a try, conversion, penalty and drop goal in the same match.
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Glossary of rugby union terms
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G
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Gain line The gain line is an imaginary line drawn across the centre of the pitch when there is a breakdown in open play, such as a ruck, maul or scrum. Advancing across the gain line represents a gain in territory.Garryowen A Garryowen, or up-and-under kick, is a high short punt onto or behind the defending team.Goal A goal is scored when a player kicks the ball through the plane bounded by the two uprights and above the crossbar. A drop goal or penalty goal count for 3 points and conversions count for two.Goal from mark Goal from mark is an antiquated method of scoring. It occurred when a player "marked" and scored a goal from there. In the modern game, a goal cannot be scored from a free kick, but in the past the reward for scoring a "goal from mark" (which is a difficult kick to play) was three or four points. Occasionally referred to as a field goal.Goal line, Tryline Two solid, straight white lines (one at each end) stretching across the entire width of the pitch passing directly through the goal posts which defines the boundary between the "field of play" and the "in-goal". As the goal line is defined as part of the "in-goal", attacking players can score tries by placing the ball with downward pressure onto the goal line itself. The base of the goal posts and post protectors are also defined to be part of the goal line.The goal line is often referred to as the "try line" though that term does not appear in the Laws of the Game.
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Glossary of rugby union terms
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G
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Goose Step A goose step, made famous by Australian David Campese but now performed by many players, is a running technique where the player slows down and takes a small 'hop' into the air before sprinting off - sometimes in a different direction - upon landing. Its purpose is to confuse the defender, who is often unable to predict the sudden change of pace and direction.Group of death Is an informal sobriquet used to describe a situation that often occurs during the group stage of a tournament, where either (1) any team in the group could qualify and any team could be eliminated, or (2) more teams have a legitimate chance to advance to the next stage than allowed by the tournament structure.Typically, a group of death will see an unusual match-up of heavyweight sides, due to a quirk in the seeding system.Grubber kick It is a type of kick that makes the ball roll and tumble across the ground, producing irregular bounces making it hard for the defending team to pick up the ball without causing a knock-on. It gives the ball both high and low bounce and on occasions, the ball can sit up in a perfect catching position.
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Glossary of rugby union terms
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H
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Half-back Can refer to either the scrum-half or fly-half, but in New Zealand is exclusively used to describe the scrum-half.Haka The haka is a traditional Māori dance performed by New Zealand national teams, most famously the All Blacks, prior to international matches. It serves as a challenge to the opposing team.Hand-off Handing off (also called fend) is the action by the ball carrier of repelling a tackler using his arm. For the action to be legal, the ball carrier's arm must be straight before contact is made; a shove or "straight-arm smash", where the arm is extended immediately before contact or on contact, is illegal and classed as dangerous play.High tackle A high tackle (or head-high tackle) is a form of tackle where the tackler grasps the ball carrier above the line of the shoulders (most commonly around the neck or at the line of the chin and jaw). Executed violently or at speed, a high tackle is potentially dangerous, so are often not just sanctioned with a penalty, but also a yellow or red card.Hooker Hookers traditionally wear the number 2 shirt. The hooker is the player who is in the centre position of the front row of the scrum and who uses his/her feet to 'hook' the ball back. Due to the pressure put on the body by the scrum and the requirement to use both arms to bind to other players (and hence having no free arm to use to support or deflect bodyweight) it is considered to be one of the most dangerous positions to play.Hookers normally throw the ball in at line-outs, partly because they are normally the shortest of the forwards, but more often because they are the most skillful of the forwards.Hospital pass Any pass that is made which has the inevitable, unavoidable consequence of the receiver being tackled. This is because the receiver has already been marked and the opposing player is bearing down on the receiver so rapidly that, as soon as the ball is caught, the opposing player smashes into the receiver. Generally made in times of panic or when there is no one else available, it is called the hospital pass because of the inevitability of a hard tackle.
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Glossary of rugby union terms
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I
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Interception The gaining of possession by running forward from the defensive line and taking a pass meant for a member of the opposition. The result is similar to the result of a line break, and has a good chance of leading to a try.
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