Automation is the technology by which a process or procedure is performed with minimal human assistance. Automation or automatic control is the use of various control systems for operating equipment such as machinery, processes in factories and heat treating ovens, switching on telephone networks and stabilization of ships and other applications and vehicles with minimal or reduced human intervention; some processes have been automated, while others are semi-automated. Automation covers applications ranging from a household thermostat controlling a boiler, to a large industrial control system with tens of thousands of input measurements and output control signals. In control complexity it can range from simple on-off control to multi-variable high level algorithms. In the simplest type of an automatic control loop, a controller compares a measured value of a process with a desired set value, processes the resulting error signal to change some input to the process, in such a way that the process stays at its set point despite disturbances.
This closed-loop control is an application of negative feedback to a system. The mathematical basis of control theory was begun in the 18th century, advanced in the 20th. Automation has been achieved by various means including mechanical, pneumatic, electronic devices and computers in combination. Complicated systems, such as modern factories and ships use all these combined techniques; the benefit of automation include labor savings, savings in electricity costs, savings in material costs, improvements to quality and precision. The World Bank's World Development Report 2019 shows evidence that the new industries and jobs in the technological sector outweigh the economic effects of workers being displaced by automation; the term automation, inspired by the earlier word automatic, was not used before 1947, when Ford established an automation department. It was during this time that industry was adopting feedback controllers, which were introduced in the 1930s. Fundamentally, there are two types of control loop.
In open loop control the control action from the controller is independent of the "process output". A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building.. In closed loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy this would include a thermostat to monitor the building temperature, thereby feed back a signal to ensure the controller maintains the building at the temperature set on the thermostat. A closed loop controller therefore has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "Reference input" or "set point". For this reason, closed loop controllers are called feedback controllers; the definition of a closed loop control system according to the British Standard Institution is'a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero.'
A Feedback Control System is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. The advanced type of automation that revolutionized manufacturing, aircraft and other industries, is feedback control, continuous and involves taking measurements using a sensor and making calculated adjustments to keep the measured variable within a set range; the theoretical basis of closed loop automation is control theory. One of the simplest types of control is on-off control. An example is the thermostat used on household appliances which either opens or closes an electrical contact. Sequence control, in which a programmed sequence of discrete operations is performed based on system logic that involves system states. An elevator control system is an example of sequence control. A proportional–integral–derivative controller is a control loop feedback mechanism used in industrial control systems.
In a PID loop, the controller continuously calculates an error value e as the difference between a desired setpoint and a measured process variable and applies a correction based on proportional and derivative terms which give their name to the controller type. The theoretical understanding and application dates from the 1920s, they are implemented in nearly all analogue control systems. Sequential control may be either to a fixed sequence or to a logical one that will perform different actions depending on various system states. An example of an adjustable but otherwise fixed sequence is a timer on a lawn sprinkler. States refer to the various conditions that can occur in a sequence scenario of the system. An example is an elevator, which uses logic based on the system state to perform certain actions in response to its state and operator input. For example, if th
A loudspeaker is an electroacoustic transducer. The most used type of speaker in the 2010s is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice; the dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move back and forth due to Faraday's law of induction, which causes a diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound; the sound source must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker. Speakers are housed in a speaker enclosure or speaker cabinet, a rectangular or square box made of wood or sometimes plastic.
The enclosure's materials and design play an important role in the quality of the sound. Where high fidelity reproduction of sound is required, multiple loudspeaker transducers are mounted in the same enclosure, each reproducing a part of the audible frequency range. In this case the individual speakers are referred to as "drivers" and the entire unit is called a loudspeaker. Drivers made for reproducing high audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers, those for low frequencies are called woofers. Smaller loudspeakers are found in devices such as radios, portable audio players and electronic musical instruments. Larger loudspeaker systems are used for music, sound reinforcement in theatres and concerts, in public address systems; the term "loudspeaker" may refer to individual transducers or to complete speaker systems consisting of an enclosure including one or more drivers. To adequately reproduce a wide range of frequencies with coverage, most loudspeaker systems employ more than one driver for higher sound pressure level or maximum accuracy.
Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers; the terms for different speaker drivers differ, depending on the application. In two-way systems there is no mid-range driver, so the task of reproducing the mid-range sounds falls upon the woofer and tweeter. Home stereos use the designation "tweeter" for the high frequency driver, while professional concert systems may designate them as "HF" or "highs"; when multiple drivers are used in a system, a "filter network", called a crossover, separates the incoming signal into different frequency ranges and routes them to the appropriate driver. A loudspeaker system with n separate frequency bands is described as "n-way speakers": a two-way system will have a woofer and a tweeter. Loudspeaker driver of the type pictured are termed "dynamic" to distinguish them from earlier drivers, or speakers using piezoelectric or electrostatic systems, or any of several other sorts. Johann Philipp Reis installed an electric loudspeaker in his telephone in 1861.
Alexander Graham Bell patented his first electric loudspeaker as part of his telephone in 1876, followed in 1877 by an improved version from Ernst Siemens. During this time, Thomas Edison was issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he settled for the familiar metal horn driven by a membrane attached to the stylus. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air. A few companies, including the Victor Talking Machine Company and Pathé, produced record players using compressed-air loudspeakers. However, these designs were limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the system were used for public address applications, more other variations have been used to test space-equipment resistance to the loud sound and vibration levels that the launching of rockets produces; the first experimental moving-coil loudspeaker was invented by Oliver Lodge in 1898.
The first practical moving-coil loudspeakers were manufactured by Danish engineer Peter L. Jensen and Edwin Pridham in 1915, in Napa, California. Like previous loudspeakers these used horns to amplify the sound produced by a small diaphragm. Jensen was denied patents. Being unsuccessful in selling their product to telephone companies, in 1915 they changed their target market to radios and public address systems, named their product Magnavox. Jensen was, for years after the invention of a part owner of The Magnavox Company; the moving-coil principle used today in speakers was patented in 1924 by Chester W. Rice and Edward W. Kellogg; the key difference between previous attempts and the patent by Rice and Kell
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of rotation of a shaft. Electric motors can be powered by direct current sources, such as from batteries, motor vehicles or rectifiers, or by alternating current sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates in the reverse direction, converting mechanical energy into electrical energy. Electric motors may be classified by considerations such as power source type, internal construction and type of motion output. In addition to AC versus DC types, motors may be brushed or brushless, may be of various phase, may be either air-cooled or liquid-cooled. General-purpose motors with standard dimensions and characteristics provide convenient mechanical power for industrial use.
The largest electric motors are used for ship propulsion, pipeline compression and pumped-storage applications with ratings reaching 100 megawatts. Electric motors are found in industrial fans and pumps, machine tools, household appliances, power tools and disk drives. Small motors may be found in electric watches. In certain applications, such as in regenerative braking with traction motors, electric motors can be used in reverse as generators to recover energy that might otherwise be lost as heat and friction. Electric motors produce linear or rotary force and can be distinguished from devices such as magnetic solenoids and loudspeakers that convert electricity into motion but do not generate usable mechanical force, which are referred to as actuators and transducers; the first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the 1740s. The theoretical principle behind them, Coulomb's law, was discovered but not published, by Henry Cavendish in 1771.
This law was discovered independently by Charles-Augustin de Coulomb in 1785, who published it so that it is now known with his name. The invention of the electrochemical battery by Alessandro Volta in 1799 made possible the production of persistent electric currents. After the discovery of the interaction between such a current and a magnetic field, namely the electromagnetic interaction by Hans Christian Ørsted in 1820 much progress was soon made, it only took a few weeks for André-Marie Ampère to develop the first formulation of the electromagnetic interaction and present the Ampère's force law, that described the production of mechanical force by the interaction of an electric current and a magnetic field. The first demonstration of the effect with a rotary motion was given by Michael Faraday in 1821. A free-hanging wire was dipped into a pool of mercury; when a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire.
This motor is demonstrated in physics experiments, substituting brine for mercury. Barlow's wheel was an early refinement to this Faraday demonstration, although these and similar homopolar motors remained unsuited to practical application until late in the century. In 1827, Hungarian physicist Ányos Jedlik started experimenting with electromagnetic coils. After Jedlik solved the technical problems of continuous rotation with the invention of the commutator, he called his early devices "electromagnetic self-rotors". Although they were used only for teaching, in 1828 Jedlik demonstrated the first device to contain the three main components of practical DC motors: the stator and commutator; the device employed no permanent magnets, as the magnetic fields of both the stationary and revolving components were produced by the currents flowing through their windings. After many other more or less successful attempts with weak rotating and reciprocating apparatus Prussian Moritz von Jacobi created the first real rotating electric motor in May 1834.
It developed remarkable mechanical output power. His motor set a world record, which Jacobi improved four years in September 1838, his second motor was powerful enough to drive a boat with 14 people across a wide river. It was in 1839/40 that other developers managed to build motors with similar and higher performance; the first commutator DC electric motor capable of turning machinery was invented by British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor was built by American inventor Thomas Davenport, which he patented in 1837; the motors ran at up to 600 revolutions per minute, powered machine tools and a printing press. Due to the high cost of primary battery power, the motors were commercially unsuccessful and bankrupted Davenport. Several inventors followed Sturgeon in the development of DC motors, but all encountered the same battery cost issues; as no electricity distribution system was available at the time, no practical commercial market emerged for these motors.
In 1855, Jedlik built a device using similar principles to those used in his electromagnetic self-rotors, capable of useful work. He built a model electric vehicle that same year. A major turning point came in 1864; this featured symmetrically-grouped coils closed upon themselves and connected to the bars of a commutator, the brushes of which delivered non-fluctuating current. The first c
A microphone, colloquially nicknamed mic or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production and recorded audio engineering, sound recording, two-way radios, megaphones and television broadcasting, in computers for recording voice, speech recognition, VoIP, for non-acoustic purposes such as ultrasonic sensors or knock sensors. Several different types of microphone are in use, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal; the most common are the dynamic microphone. Microphones need to be connected to a preamplifier before the signal can be recorded or reproduced. In order to speak to larger groups of people, a need arose to increase the volume of the human voice; the earliest devices used to achieve this were acoustic megaphones. Some of the first examples, from fifth century BC Greece, were theater masks with horn-shaped mouth openings that acoustically amplified the voice of actors in amphitheatres.
In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the "lovers' telephone" made of stretched wire with a cup attached at each end. In 1861, German inventor Johann Philipp Reis built an early sound transmitter that used a metallic strip attached to a vibrating membrane that would produce intermittent current. Better results were achieved in 1876 with the "liquid transmitter" design in early telephones from Alexander Graham Bell and Elisha Gray – the diaphragm was attached to a conductive rod in an acid solution; these systems, gave a poor sound quality. The first microphone that enabled proper voice telephony was the carbon microphone; this was independently developed by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, most historians credit him with its invention.
The carbon microphone is the direct prototype of today's microphones and was critical in the development of telephony and the recording industries. Thomas Edison refined the carbon microphone into his carbon-button transmitter of 1886; this microphone was employed at the first radio broadcast, a performance at the New York Metropolitan Opera House in 1910. In 1916, E. C. Wente of Western Electric developed the next breakthrough with the first condenser microphone. In 1923, the first practical moving coil microphone was built; the Marconi-Sykes magnetophone, developed by Captain H. J. Round, became the standard for BBC studios in London; this was improved in 1930 by Alan Blumlein and Herbert Holman who released the HB1A and was the best standard of the day. In 1923, the ribbon microphone was introduced, another electromagnetic type, believed to have been developed by Harry F. Olson, who reverse-engineered a ribbon speaker. Over the years these microphones were developed by several companies, most notably RCA that made large advancements in pattern control, to give the microphone directionality.
With television and film technology booming there was demand for high fidelity microphones and greater directionality. Electro-Voice responded with their Academy Award-winning shotgun microphone in 1963. During the second half of 20th century development advanced with the Shure Brothers bringing out the SM58 and SM57; the latest research developments include the use of fibre optics and interferometers. The sensitive transducer element of a microphone is called its capsule. Sound is first converted to mechanical motion by means of a diaphragm, the motion of, converted to an electrical signal. A complete microphone includes a housing, some means of bringing the signal from the element to other equipment, an electronic circuit to adapt the output of the capsule to the equipment being driven. A wireless microphone contains a radio transmitter. Microphones are categorized by their transducer principle, such as condenser, etc. and by their directional characteristics. Sometimes other characteristics such as diaphragm size, intended use or orientation of the principal sound input to the principal axis of the microphone are used to describe the microphone.
The condenser microphone, invented at Western Electric in 1916 by E. C. Wente, is called a capacitor microphone or electrostatic microphone—capacitors were called condensers. Here, the diaphragm acts as one plate of a capacitor, the vibrations produce changes in the distance between the plates. There are two types, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, radio frequency or high frequency condenser microphones. With a DC-biased microphone, the plates are biased with a fixed charge; the voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation, where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor; the assembly of fixed and movable plates is called an "element" or "capsule". A nearly constant charge is maintained on the capa
In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the SI unit of energy is the joule, the energy transferred to an object by the work of moving it a distance of 1 metre against a force of 1 newton. Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field, the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, the thermal energy due to an object's temperature. Mass and energy are related. Due to mass–energy equivalence, any object that has mass when stationary has an equivalent amount of energy whose form is called rest energy, any additional energy acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.
Living organisms require exergy to stay alive, such as the energy. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy; the processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun and the geothermal energy contained within the earth. The total energy of a system can be subdivided and classified into potential energy, kinetic energy, or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or the composite motion of the components of an object – and potential energy reflects the potential of an object to have motion, is a function of the position of an object within a field or may be stored in the field itself. While these two categories are sufficient to describe all forms of energy, it is convenient to refer to particular combinations of potential and kinetic energy as its own form. For example, macroscopic mechanical energy is the sum of translational and rotational kinetic and potential energy in a system neglects the kinetic energy due to temperature, nuclear energy which combines utilize potentials from the nuclear force and the weak force), among others.
The word energy derives from the Ancient Greek: translit. Energeia, lit.'activity, operation', which appears for the first time in the work of Aristotle in the 4th century BC. In contrast to the modern definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness and pleasure. In the late 17th century, Gottfried Leibniz proposed the idea of the Latin: vis viva, or living force, which defined as the product of the mass of an object and its velocity squared. To account for slowing due to friction, Leibniz theorized that thermal energy consisted of the random motion of the constituent parts of matter, although it would be more than a century until this was accepted; the modern analog of this property, kinetic energy, differs from vis viva only by a factor of two. In 1807, Thomas Young was the first to use the term "energy" instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described "kinetic energy" in 1829 in its modern sense, in 1853, William Rankine coined the term "potential energy".
The law of conservation of energy was first postulated in the early 19th century, applies to any isolated system. It was argued for some years whether heat was a physical substance, dubbed the caloric, or a physical quantity, such as momentum. In 1845 James Prescott Joule discovered the generation of heat; these developments led to the theory of conservation of energy, formalized by William Thomson as the field of thermodynamics. Thermodynamics aided the rapid development of explanations of chemical processes by Rudolf Clausius, Josiah Willard Gibbs, Walther Nernst, it led to a mathematical formulation of the concept of entropy by Clausius and to the introduction of laws of radiant energy by Jožef Stefan. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time. Thus, since 1918, theorists have understood that the law of conservation of energy is the direct mathematical consequence of the translational symmetry of the quantity conjugate to energy, namely time.
In 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. The most famous of them used the "Joule apparatus": a descending weight, attached to a string, caused rotation of a paddle immersed in water insulated from heat transfer, it showed that the gravitational potential energy lost by the weight in descending was equal to the internal energy gained by the water through friction with the paddle. In the International System of Units, the unit of energy is the joule, named after James Prescott Joule, it is a derived unit. It is equal to the energy expended in applying a force of one newton through a distance of one metre; however energy is expressed in many other units not part of the SI, such as ergs, British Thermal Units, kilowatt-hours and kilocalories, which require a conversion factor when expressed in SI units. The SI unit of energy rate is the watt, a joule per second. Thus, one joule is one watt-second, 3600 joules equal one wa
A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa. They can be used to read credit/debit/gift cards because the strip of magnetic tape on the back of a credit card stores data the same way that other magnetic tapes do. Cassettes, reel-to-reel tapes, 8-tracks, VHS tapes, floppy disks and modern hard drive disks all use the same principle of physics to store and read back information; the medium is magnetized in a pattern. It moves at a constant speed over an electromagnet. Since the moving tape is carrying a changing magnetic field with it, it induces a varying voltage across the head; that voltage can be amplified and connected to speakers in the case of audio, or measured and sorted into'1's and zeroes in the case of digital data. The electromagnetic arrangement of a tape head is similar for all types, though the physical design varies depending on the application - for example videocassette recorders use rotating heads which implement a helical scan, whereas most audio recorders have fixed heads.
A head consists of a core of magnetic material arranged into a doughnut shape or toroid, into which a narrow gap has been let. This gap is filled with a diamagnetic material, such as gold; this forces the magnetic flux out of the gap into the magnetic tape medium more. The flux thus magnetises the tape at that point. A coil of wire wrapped around the core opposite the gap interfaces to the electrical side of the apparatus; the basic head design is reversible - a variable magnetic field at the gap will induce an electric current in the coil, an electric current in the coil will induce a magnetic field in the core and hence in the tape drawn across the gap. While a head is reversible in principle, often in practice, there are desirable characteristics that differ between the playback and recording phases. One of these is the impedance of the coil - playback preferring a high impedance, recording a low one. In the best tape recorders, separate heads are used to avoid compromising these desirable characteristics.
Having separate heads for recording and playback has other advantages, such as off-tape monitoring during recording, etc. The width of the head gap is critical - the narrower the gap, the better the head will be - a narrow gap gives much better transcription in the magnetic domain; the desirability for a narrow gap means that most practical heads are made by forming a narrow V-shaped groove in the back face of the core, grinding away the front face until the V-groove is just breached. In this way, gaps of the order of micrometres are achievable. A record head, on the other hand, has a gap six times larger than that of the replay head, this gives a larger flux to magnetise the tape; the ideal gap size in a cassette deck are. The larger gap does not affect frequency response because the'image' is made by the trailing edge of the gap. A combined record/replay head has a compromise size gap three times that of a replay only head. There are negative aspects of narrow head gaps for magnetic recording.
The narrower the head gap, the more bias signal must be used to maintain linearity of the signal on tape which in turn will reduce the high frequency headroom or SOL with slower tape speeds. Manufacturers must head gaps for this reason; the physical design of a head depends on whether it is rotating. In either case, the face of the head where the gap is must be made hard wearing and smooth to avoid excessive head wear, it can be seen that due to the construction method of the head gap, head wear will tend to widen the gap, reducing the head's performance over time. The vertical alignment of the heads must match between recording and playback for good fidelity, the gap should be as close to vertical as possible for highest frequency response. Most tape transport mechanisms will allow fine mechanical adjustment of the azimuth of the heads. Sometimes this can be achieved by automatic circuitry - the actual mechanical azimuth adjustment being carried out by taking advantage of the piezo effect of certain types of crystal material.
Rotating play heads, as used in video recorders, digital audio tape and other applications, are used to achieve a high relative head/tape speed while maintaining a low overall tape transport speed. One or more transducers are mounted on a rotating drum set at an angle to the tape; the drum spins compared to the speed that the tape moves past it, so that the transducers describe a path of stripes across the tape, rather than linearly along it as a fixed head does. The wear characteristics of such helical scan heads are more critical, polished heads and tapes are required; the electrical signals of rotating heads are coupled either inductively or capacitively - there is no direct connection to the head coils. An erase head is constructed in a similar manner to a record or replay head, but has a much larger gap, or more two large gaps; the erase head is powered during recording from a high frequency source. In some inexpensive cassette recorder designs, the erase head is a permanent magnet, mechanically moved into contact with the moving t
A linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque it produces a linear force along its length. However, linear motors are not straight. Characteristically, a linear motor's active section has ends, whereas more conventional motors are arranged as a continuous loop; the most common mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field. Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications. High-acceleration linear motors are rather short, are designed to accelerate an object to a high speed, for example see the coilgun. High-acceleration linear motors are used in studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion.
They are of the AC linear induction motor design with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. However, the direct current homopolar linear motor railgun is another high acceleration linear motor design; the low-acceleration, high speed and high power motors are of the linear synchronous motor design, with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent electromagnets; the Shanghai Transrapid motor is an LSM. In this design the rate of movement of the magnetic field is controlled electronically, to track the motion of the rotor. For cost reasons synchronous linear motors use commutators, so the rotor contains permanent magnets, or soft iron. Examples include coilguns and the motors used on some maglev systems, as well as many other linear motors. In this design, the force is produced by a moving linear magnetic field acting on conductors in the field.
Any conductor, be it a loop, a coil or a piece of plate metal, placed in this field will have eddy currents induced in it thus creating an opposing magnetic field, in accordance with Lenz's law. The two opposing fields will repel each other, thus creating motion as the magnetic field sweeps through the metal. In this design a large current is passed through a metal sabot across sliding contacts that are fed from two rails; the magnetic field this generates causes the metal to be projected along the rails. Piezoelectric drive is used to drive small linear motors; the history of linear electric motors can be traced back at least as far as the 1840s, to the work of Charles Wheatstone at King's College in London, but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described in the U. S. Patent 782,312, for driving lifts; the German engineer Hermann Kemper built a working model in 1935. In the late 1940s, Dr. Eric Laithwaite of Manchester University Professor of Heavy Electrical Engineering at Imperial College in London developed the first full-size working model.
In a single sided version the magnetic repulsion forces the conductor away from the stator, levitating it, carrying it along in the direction of the moving magnetic field. He called the versions of it magnetic river; because of these properties, linear motors are used in maglev propulsion, as in the Japanese Linimo magnetic levitation train line near Nagoya. However, linear motors have been used independently of magnetic levitation, as in Bombardier's Advanced Rapid Transit systems worldwide and a number of modern Japanese subways, including Tokyo's Toei Oedo Line. Similar technology is used in some roller coasters with modifications but, at present, is still impractical on street running trams, although this, in theory, could be done by burying it in a slotted conduit. Outside of public transportation, vertical linear motors have been proposed as lifting mechanisms in deep mines, the use of linear motors is growing in motion control applications, they are often used on sliding doors, such as those of low floor trams such as the Citadis and the Eurotram.
Dual axis linear motors exist. These specialized devices have been used to provide direct X-Y motion for precision laser cutting of cloth and sheet metal, automated drafting, cable forming. Most linear motors in use are LIM, or LSM. Linear DC motors are not used due to linear SRM suffers from poor thrust. So for long run in traction LIM is preferred and for short run LSM is preferred. High-acceleration linear motors have been suggested for a number of uses, they have been considered for use as weapons, since current armour-piercing ammunition tends to consist of small rounds with high kinetic energy, for which just such motors are suitable. Many amusement park launched roller coasters now use linear induction motors to propel the train at a high speed, as an alternative to using a lift hill; the United States Navy is using linear induction motors in the Electromagnetic Aircraft Launch System that will replace traditional steam catapults on future aircraft carriers. They have been suggested for use in spacecraft propulsion.
In this context they are called mass drivers. The simplest way to use mass drivers for spacecraft propulsion would be to build a large mass driver that can accelerate cargo up