In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system in the form of a backup or fail-safe, or to improve actual system performance, such as in the case of GNSS receivers, or multi-threaded computer processing. In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft, some parts of the control system may be triplicated, formally termed triple modular redundancy. An error in one component may be out-voted by the other two. In a triply redundant system, the system has three sub components, all three of which must fail before the system fails. Since each one fails, the sub components are expected to fail independently, the probability of all three failing is calculated to be extraordinarily small. Redundancy may be known by the terms "majority voting systems" or "voting logic". Redundancy sometimes produces less, instead of greater reliability – it creates a more complex system, prone to various issues, it may lead to human neglect of duty, may lead to higher production demands which by overstressing the system may make it less safe.
In computer science, there are four major forms of redundancy, these are: Hardware redundancy, such as dual modular redundancy and triple modular redundancy Information redundancy, such as error detection and correction methods Time redundancy, performing the same operation multiple times such as multiple executions of a program or multiple copies of data transmitted Software redundancy such as N-version programmingA modified form of software redundancy, applied to hardware may be: Distinct functional redundancy, such as both mechanical and hydraulic braking in a car. Applied in the case of software, code written independently and distinctly different but producing the same results for the same inputs. Structures are designed with redundant parts as well, ensuring that if one part fails, the entire structure will not collapse. A structure without redundancy is called fracture-critical, meaning that a single broken component can cause the collapse of the entire structure. Bridges that failed due to lack of redundancy include the Silver Bridge and the Interstate 5 bridge over the Skagit River.
Parallel and combined systems demonstrate different level of redundancy. The models are subject of studies in safety engineering; the two functions of redundancy are passive active redundancy. Both functions prevent performance decline from exceeding specification limits without human intervention using extra capacity. Passive redundancy uses excess capacity to reduce the impact of component failures. One common form of passive redundancy is the extra strength of cabling and struts used in bridges; this extra strength allows some structural components to fail without bridge collapse. The extra strength used in the design is called the margin of safety. Eyes and ears provide working examples of passive redundancy. Vision loss in one eye does not cause blindness but depth perception is impaired. Hearing loss in one ear does not cause deafness but directionality is impaired. Performance decline is associated with passive redundancy when a limited number of failures occur. Active redundancy eliminates performance declines by monitoring the performance of individual devices, this monitoring is used in voting logic.
The voting logic is linked to switching. Error detection and correction and the Global Positioning System are two examples of active redundancy. Electrical power distribution provides an example of active redundancy. Several power lines connect each generation facility with customers; each power line includes monitors. Each power line includes circuit breakers; the combination of power lines provides excess capacity. Circuit breakers disconnect a power line. Power is redistributed across the remaining lines. Charles Perrow, author of Normal Accidents, has said that sometimes redundancies backfire and produce less, not more reliability; this may happen in three ways: First, redundant safety devices result in a more complex system, more prone to errors and accidents. Second, redundancy may lead to shirking of responsibility among workers. Third, redundancy may lead to increased production pressures, resulting in a system that operates at higher speeds, but less safely. Voting logic uses performance monitoring to determine how to reconfigure individual components so that operation continues without violating specification limitations of the overall system.
Voting logic involves computers, but systems composed of items other than computers may be reconfigured using voting logic. Circuit breakers are an example of a form of non-computer voting logic. Electrical power systems use power scheduling to reconfigure active redundancy. Computing systems adjust the production output of each generating facility when other generating facilities are lost; this prevents blackout conditions during major events such as an earthquake. The simplest voting logic in computing systems involves two components: alternate, they both run similar software, but the output from the alternate remains inactive during normal operation. The primary monitors itself and periodically sends an activity message to the alternate as long as everything is OK. All outputs from the primary stop, including the activity message; the alternate activates its output and takes over from the primary after a brief delay when the activity message ceases. Errors in voting logic can cause both outputs to be active or inactive at the same time, or cause output
Green is the color between blue and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of 495–570 nm. In subtractive color systems, used in painting and color printing, it is created by a combination of yellow and blue, or yellow and cyan. By far the largest contributor to green in nature is chlorophyll, the chemical by which plants photosynthesize and convert sunlight into chemical energy. Many creatures have adapted to their green environments by taking on a green hue themselves as camouflage. Several minerals have a green color, including the emerald, colored green by its chromium content. During post-classical and early modern Europe, green was the color associated with wealth, merchants and the gentry, while red was reserved for the nobility. For this reason, the costume of the Mona Lisa by Leonardo da Vinci and the benches in the British House of Commons are green while those in the House of Lords are red, it has a long historical tradition as the color of Ireland and of Gaelic culture.
It is the historic color of Islam, representing the lush vegetation of Paradise. It was the color of the banner of Muhammad, is found in the flags of nearly all Islamic countries. In surveys made in American and Islamic countries, green is the color most associated with nature, health, spring and envy. In the European Union and the United States, green is sometimes associated with toxicity and poor health, but in China and most of Asia, its associations are positive, as the symbol of fertility and happiness; because of its association with nature, it is the color of the environmental movement. Political groups advocating environmental protection and social justice describe themselves as part of the Green movement, some naming themselves Green parties; this has led to similar campaigns in advertising, as companies have sold green, or environmentally friendly, products. Green is the traditional color of safety and permission; the word green comes from the Middle English and Old English word grene, like the German word grün, has the same root as the words grass and grow.
It is from a Common Germanic *gronja-, reflected in Old Norse grænn, Old High German gruoni from a PIE root *ghre- "to grow", root-cognate with grass and to grow. The first recorded use of the word as a color term in Old English dates to ca. AD 700. Latin with viridis has a genuine and used term for "green". Related to virere "to grow" and ver "spring", it gave rise to words in several Romance languages, French vert, Italian verde; the Slavic languages with zelenъ. Ancient Greek had a term for yellowish, pale green – χλωρός, cognate with χλοερός "verdant" and χλόη "chloe, the green of new growth". Thus, the languages mentioned above have old terms for "green" which are derived from words for fresh, sprouting vegetation. However, comparative linguistics makes clear that these terms were coined independently, over the past few millennia, there is no identifiable single Proto-Indo-European or word for "green". For example, the Slavic zelenъ is cognate with Sanskrit hari "yellow, golden"; the Turkic languages have jašɨl "green" or "yellowish green", compared to a Mongolian word for "meadow".
In some languages, including old Chinese, old Japanese, Vietnamese, the same word can mean either blue or green. The Chinese character 青 has a meaning that covers both green. In more contemporary terms, they are 綠 respectively. Japanese has two terms that refer to the color green, 緑 and グリーン. However, in Japan, although the traffic lights have the same colors as other countries have, the green light is described using the same word as for blue, because green is considered a shade of aoi. Vietnamese uses a single word for both blue and green, with variants such as xanh da trời, lục. "Green" in modern European languages corresponds to about 520–570 nm, but many historical and non-European languages make other choices, e.g. using a term for the range of ca. 450–530 nm and another for ca. 530–590 nm. In the comparative study of color terms in the world's languages, green is only found as a separate category in languages with the developed range of six colors, or more in systems with five colors; these languages have introduced supplementary vocabulary to denote "green", but these terms are recognizable as recent adoptions that are not in origin color terms.
Thus, the Thai word เขียว kheīyw, besides mean
In electrical engineering, a switch is an electrical component that can "make" or "break" an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch restores the conducting path in a circuit when it is operated, it may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. A switch will have one or more sets of contacts, which may operate sequentially, or alternately. Switches in high-powered circuits must operate to prevent destructive arcing, may include special features to assist in interrupting a heavy current. Multiple forms of actuators are used for operation by hand or to sense position, temperature or flow. Special types are used, for example, for control of machinery, to reverse electric motors, or to sense liquid level. Many specialized forms exist. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures.
By analogy with the devices that select one or more possible paths for electric currents, devices that route information in a computer network are called "switches" - these are more complicated than simple electromechanical toggles or pushbutton devices, operate without direct human interaction. The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts, which are connected to external circuits; each set of contacts can be in one of two states: either "closed" meaning the contacts are touching and electricity can flow between them, or "open", meaning the contacts are separated and the switch is nonconducting. The mechanism actuating the transition between these two states are either an "alternate action" or "momentary" type. A switch may be directly manipulated by a human as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch. Automatically operated switches can be used to control the motions of machines, for example, to indicate that a garage door has reached its full open position or that a machine tool is in a position to accept another workpiece.
Switches may be operated by process variables such as pressure, flow, current and force, acting as sensors in a process and used to automatically control a system. For example, a thermostat is a temperature-operated switch used to control a heating process. A switch, operated by another electrical circuit is called a relay. Large switches may be remotely operated by a motor drive mechanism; some switches are used to isolate electric power from a system, providing a visible point of isolation that can be padlocked if necessary to prevent accidental operation of a machine during maintenance, or to prevent electric shock. An ideal switch would have no voltage drop when closed, would have no limits on voltage or current rating, it would have zero rise time and fall time during state changes, would change state without "bouncing" between on and off positions. Practical switches fall short of this ideal; the ideal switch is used in circuit analysis as it simplifies the system of equations to be solved, but this can lead to a less accurate solution.
Theoretical treatment of the effects of non-ideal properties is required in the design of large networks of switches, as for example used in telephone exchanges. In the simplest case, a switch has two conductive pieces metal, called contacts, connected to an external circuit, that touch to complete the circuit, separate to open the circuit; the contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. Contact materials are chosen on the basis of electrical conductivity, mechanical strength, low cost and low toxicity. Sometimes the contacts are plated with noble metals, for their excellent conductivity and resistance to corrosion, they may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used. To prevent the formation of insulating oxides, a minimum wetting current may be specified for a given switch design. In electronics, switches are classified according to the arrangement of their contacts.
A pair of contacts is said to be "closed". When the contacts are separated by an insulating air gap, they are said to be "open", no current can flow between them at normal voltages; the terms "make" for closure of contacts and "break" for opening of contacts are widely used. The terms pole and throw are used to describe switch contact variations; the number of "poles" is the number of electrically separate switches which are controlled by a single physical actuator. For example, a "2-pole" switch has two separate, parallel sets of contacts that open and close in unison via the same mechanism; the number of "throws" is the number of separate wiring path choices other than "open" that the switch can adopt for each pole. A single-throw switch has one pair of contacts that can either be open. A double-throw switch has a contact that can be connected to either of two other contacts, a triple-throw has a contact which can be connected to one of three other contacts, etc. In a switch where the contacts remain i
Avionics are the electronic systems used on aircraft, artificial satellites, spacecraft. Avionic systems include communications, the display and management of multiple systems, the hundreds of systems that are fitted to aircraft to perform individual functions; these can be as simple as a searchlight for a police helicopter or as complicated as the tactical system for an airborne early warning platform. The term avionics is a portmanteau of electronics; the term "avionics" was coined by the journalist Philip J. Klass as a portmanteau of "aviation electronics". Many modern avionics have their origins in World War II wartime developments. For example, autopilot systems that are commonplace today began as specialized systems to help bomber planes fly enough to hit precision targets from high altitudes. Famously, radar was developed in the UK, the United States during the same period. Modern avionics is a substantial portion of military aircraft spending. Aircraft like the F‑15E and the now retired F‑14 have 20 percent of their budget spent on avionics.
Most modern helicopters now have budget splits of 60/40 in favour of avionics. The civilian market has seen a growth in cost of avionics. Flight control systems and new navigation needs brought on by tighter airspaces, have pushed up development costs; the major change has been the recent boom in consumer flying. As more people begin to use planes as their primary method of transportation, more elaborate methods of controlling aircraft safely in these high restrictive airspaces have been invented. Avionics plays a heavy role in modernization initiatives like the Federal Aviation Administration's Next Generation Air Transportation System project in the United States and the Single European Sky ATM Research initiative in Europe; the Joint Planning and Development Office put forth a roadmap for avionics in six areas: Published Routes and Procedures – Improved navigation and routing Negotiated Trajectories – Adding data communications to create preferred routes dynamically Delegated Separation – Enhanced situational awareness in the air and on the ground LowVisibility/CeilingApproach/Departure – Allowing operations with weather constraints with less ground infrastructure Surface Operations – To increase safety in approach and departure ATM Efficiencies – Improving the ATM process The Aircraft Electronics Association reports $1.73 billion avionics sales for the first three quarters of 2017 in business and general aviation, a 4.1% yearly improvement: 73.5% came from North America, forward-fit represented 42.3% while 57.7% were retrofits as the U.
S. deadline of Jan. 1, 2020 for mandatory ADS-B out approach. The cockpit of an aircraft is a typical location for avionic equipment, including control, communication, navigation and anti-collision systems; the majority of aircraft power their avionics using 14- or 28‑volt DC electrical systems. There are several major vendors of flight avionics, including Panasonic Avionics Corporation, Universal Avionics Systems Corporation, Rockwell Collins, Thales Group, GE Aviation Systems, Raytheon, Parker Hannifin, UTC Aerospace Systems and Avidyne Corporation. International standards for avionics equipment are prepared by the Airlines Electronic Engineering Committee and published by ARINC. Communications connect the flight deck to the flight deck to the passengers. On‑board communications are provided by public-address systems and aircraft intercoms; the VHF aviation communication system works on the airband of 118.000 MHz to 136.975 MHz. Each channel is spaced from the adjacent ones by 8.33 kHz in Europe, 25 kHz elsewhere.
VHF is used for line of sight communication such as aircraft-to-aircraft and aircraft-to-ATC. Amplitude modulation is used, the conversation is performed in simplex mode. Aircraft communication can take place using HF or satellite communication. Air navigation is the determination of position and direction above the surface of the Earth. Avionics can use satellite navigation systems, INS, ground-based radio navigation systems, or any combination thereof. Navigation systems calculate the position automatically and display it to the flight crew on moving map displays. Older avionics required a pilot or navigator to plot the intersection of signals on a paper map to determine an aircraft's location; the first hints of glass cockpits emerged in the 1970s when flight-worthy cathode ray tube screens began to replace electromechanical displays and instruments. A "glass" cockpit refers to the use of computer monitors instead of gauges and other analog displays. Aircraft were getting progressively more displays and information dashboards that competed for space and pilot attention.
In the 1970s, the average aircraft had more than 100 cockpit controls. Glass cockpits started to come into being with the Gulfstream G‑IV private jet in 1985. One of the key challenges in glass cockpits is to balance how much control is automated and how much the pilot should do manually, they try to automate flight operations while keeping the pilot informed. Aircraft have means of automatically controlling flight. Autopilot was first invented by Lawrence Sperry during World War I to fly bomber planes steady enough to hit accurate
The Apollo program known as Project Apollo, was the third United States human spaceflight program carried out by the National Aeronautics and Space Administration, which succeeded in landing the first humans on the Moon from 1969 to 1972. First conceived during Dwight D. Eisenhower's administration as a three-man spacecraft to follow the one-man Project Mercury which put the first Americans in space, Apollo was dedicated to President John F. Kennedy's national goal of "landing a man on the Moon and returning him safely to the Earth" by the end of the 1960s, which he proposed in an address to Congress on May 25, 1961, it was the third US human spaceflight program to fly, preceded by the two-man Project Gemini conceived in 1961 to extend spaceflight capability in support of Apollo. Kennedy's goal was accomplished on the Apollo 11 mission when astronauts Neil Armstrong and Buzz Aldrin landed their Apollo Lunar Module on July 20, 1969, walked on the lunar surface, while Michael Collins remained in lunar orbit in the command and service module, all three landed safely on Earth on July 24.
Five subsequent Apollo missions landed astronauts on the Moon, the last in December 1972. In these six spaceflights, twelve men walked on the Moon. Apollo ran from 1961 to 1972, with the first manned flight in 1968, it achieved its goal of manned lunar landing, despite the major setback of a 1967 Apollo 1 cabin fire that killed the entire crew during a prelaunch test. After the first landing, sufficient flight hardware remained for nine follow-on landings with a plan for extended lunar geological and astrophysical exploration. Budget cuts forced the cancellation of three of these. Five of the remaining six missions achieved successful landings, but the Apollo 13 landing was prevented by an oxygen tank explosion in transit to the Moon, which destroyed the service module's capability to provide electrical power, crippling the CSM's propulsion and life support systems; the crew returned to Earth safely by using the lunar module as a "lifeboat" for these functions. Apollo used Saturn family rockets as launch vehicles, which were used for an Apollo Applications Program, which consisted of Skylab, a space station that supported three manned missions in 1973–74, the Apollo–Soyuz Test Project, a joint US-Soviet Union Earth-orbit mission in 1975.
Apollo set several major human spaceflight milestones. It stands alone in sending manned missions beyond low Earth orbit. Apollo 8 was the first manned spacecraft to orbit another celestial body, while the final Apollo 17 mission marked the sixth Moon landing and the ninth manned mission beyond low Earth orbit; the program returned 842 pounds of lunar rocks and soil to Earth contributing to the understanding of the Moon's composition and geological history. The program laid the foundation for NASA's subsequent human spaceflight capability and funded construction of its Johnson Space Center and Kennedy Space Center. Apollo spurred advances in many areas of technology incidental to rocketry and manned spaceflight, including avionics, telecommunications, computers; the Apollo program was conceived during the Eisenhower administration in early 1960, as a follow-up to Project Mercury. While the Mercury capsule could only support one astronaut on a limited Earth orbital mission, Apollo would carry three astronauts.
Possible missions included ferrying crews to a space station, circumlunar flights, eventual manned lunar landings. The program was named after Apollo, the Greek god of light and the sun, by NASA manager Abe Silverstein, who said that "I was naming the spacecraft like I'd name my baby." Silverstein chose the name at home one evening, early in 1960, because he felt "Apollo riding his chariot across the Sun was appropriate to the grand scale of the proposed program."In July 1960, NASA Deputy Administrator Hugh L. Dryden announced the Apollo program to industry representatives at a series of Space Task Group conferences. Preliminary specifications were laid out for a spacecraft with a mission module cabin separate from the command module, a propulsion and equipment module. On August 30, a feasibility study competition was announced, on October 25, three study contracts were awarded to General Dynamics/Convair, General Electric, the Glenn L. Martin Company. Meanwhile, NASA performed its own in-house spacecraft design studies led by Maxime Faget, to serve as a gauge to judge and monitor the three industry designs.
In November 1960, John F. Kennedy was elected president after a campaign that promised American superiority over the Soviet Union in the fields of space exploration and missile defense. Up to the election of 1960, Kennedy had been speaking out against the "missile gap" that he and many other senators felt had developed between the Soviet Union and United States due to the inaction of President Eisenhower. Beyond military power, Kennedy used aerospace technology as a symbol of national prestige, pledging to make the US not "first but, first and, first if, but first period." Despite Kennedy's rhetoric, he did not come to a decision on the status of the Apollo program once he became president. He knew little about the technical details of the space program, was put off by the massive financial commitment required by a manned Moon landing; when Kennedy's newly appointed NASA Administrator James E. Webb requested a 30 percent budget increase for his agency, Kennedy supported an acceleration of NASA's large booster program but deferred a decision on the broader issue.
On April 12, 1961, Soviet cosmonaut Yuri Gagarin became the first person to fly in space, reinforcing American fears about being left behind in a technological competition with the Soviet Union. At a meeting of the US House Committee on Science and Astronaut
A voltage regulator is a system designed to automatically maintain a constant voltage level. A voltage regulator may include negative feedback, it may use electronic components. Depending on the design, it may be used to regulate one or more DC voltages. Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line. A simple voltage/current regulator can be made from a resistor in series with a diode. Due to the logarithmic shape of diode V-I curves, the voltage across the diode changes only due to changes in current drawn or changes in the input; when precise voltage control and efficiency are not important, this design may be fine.
Since the forward voltage of a diode is small, this kind of voltage regulator is only suitable for low voltage regulated output. When higher voltage output is needed, a zener diode or series of zener diodes may be employed. Zener diode regulators make use of the zener diode's fixed reverse voltage, which can be quite large. Feedback voltage regulators operate by comparing the actual output voltage to some fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error; this forms a negative feedback control loop. There will be a trade-off between stability and the speed of the response to changes. If the output voltage is too low, the regulation element is commanded, up to a point, to produce a higher output voltage–by dropping less of the input voltage, or to draw input current for longer periods. However, many regulators have over-current protection, so that they will stop sourcing current if the output current is too high, some regulators may shut down if the input voltage is outside a given range.
In electromechanical regulators, voltage regulation is accomplished by coiling the sensing wire to make an electromagnet. The magnetic field produced by the current attracts a moving ferrous core held back under spring tension or gravitational pull; as voltage increases, so does the current, strengthening the magnetic field produced by the coil and pulling the core towards the field. The magnet is physically connected to a mechanical power switch, which opens as the magnet moves into the field; as voltage decreases, so does the current, releasing spring tension or the weight of the core and causing it to retract. This allows the power to flow once more. If the mechanical regulator design is sensitive to small voltage fluctuations, the motion of the solenoid core can be used to move a selector switch across a range of resistances or transformer windings to step the output voltage up or down, or to rotate the position of a moving-coil AC regulator. Early automobile generators and alternators had a mechanical voltage regulator using one, two, or three relays and various resistors to stabilize the generator's output at more than 6 or 12 V, independent of the engine's rpm or the varying load on the vehicle's electrical system.
The relay employed pulse width modulation to regulate the output of the generator, controlling the field current reaching the generator and in this way controlling the output voltage producing back into the generator and attempting to run it as a motor. The rectifier diodes in an alternator automatically perform this function so that a specific relay is not required. More modern designs now use solid state technology to perform the same function that the relays perform in electromechanical regulators. Electromechanical regulators are used for mains voltage stabilisation — see AC voltage stabilizers below. Generators, as used in power stations or in standby power systems, will have automatic voltage regulators to stabilize their voltages as the load on the generators changes; the first automatic voltage regulators for generators were electromechanical systems, but a modern AVR uses solid-state devices. An AVR is a feedback control system that measures the output voltage of the generator, compares that output to a set point, generates an error signal, used to adjust the excitation of the generator.
As the excitation current in the field winding of the generator increases, its terminal voltage will increase. The AVR will control current by using power electronic devices. Where a generator is connected in parallel with other sources such as an electrical transmission grid, changing the excitation has more of an effect on the reactive power produced by the generator than on its terminal voltage, set by the connected power system. Where multiple generators are connected in parallel, the AVR system will have circuits to ensure all generat