A proportional–integral–derivative controller is a control loop feedback mechanism used in industrial control systems and a variety of other applications requiring continuously modulated control. A PID 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, hence the name. In practical terms it automatically applies accurate and responsive correction to a control function. An everyday example is the cruise control on a car, where ascending a hill would lower speed if only constant engine power is applied; the controller's PID algorithm restores the measured speed to the desired speed with minimal delay and overshoot, by increasing the power output of the engine. The first theoretical analysis and practical application was in the field of automatic steering systems for ships, developed from the early 1920s onwards, it was used for automatic process control in manufacturing industry, where it was implemented in pneumatic, electronic, controllers.
Today there is universal use of the PID concept in applications requiring accurate and optimised automatic control. The distinguishing feature of the PID controller is the ability to use the three control terms of proportional and derivative influence on the controller output to apply accurate and optimal control; the block diagram on the right shows the principles of how these terms are applied. It shows a PID controller, which continuously calculates an error value e as the difference between a desired setpoint SP = r and a measured process variable PV = y, applies a correction based on proportional and derivative terms; the controller attempts to minimize the error over time by adjustment of a control variable u, such as the opening of a control valve, to a new value determined by a weighted sum of the control terms. In this model: Term P is proportional to the current value of the SP − PV error e. For example, if the error is large and positive, the control output will be proportionately large and positive, taking into account the gain factor "K".
Using proportional control alone will result in an error between the setpoint and the actual process value, because it requires an error to generate the proportional response. If there is no error, there is no corrective response. Term I accounts for past values of the SP − PV error and integrates them over time to produce the I term. For example, if there is a residual SP − PV error after the application of proportional control, the integral term seeks to eliminate the residual error by adding a control effect due to the historic cumulative value of the error; when the error is eliminated, the integral term will cease to grow. This will result in the proportional effect diminishing as the error decreases, but this is compensated for by the growing integral effect. Term D is a best estimate of the future trend of the SP − PV error, based on its current rate of change, it is sometimes called "anticipatory control", as it is seeking to reduce the effect of the SP − PV error by exerting a control influence generated by the rate of error change.
The more rapid the change, the greater the controlling or dampening effect. Tuning – The balance of these effects is achieved by loop tuning to produce the optimal control function; the tuning constants are shown below as "K" and must be derived for each control application, as they depend on the response characteristics of the complete loop external to the controller. These are dependent on the behaviour of the measuring sensor, the final control element, any control signal delays and the process itself. Approximate values of constants can be entered knowing the type of application, but they are refined, or tuned, by "bumping" the process in practice by introducing a setpoint change and observing the system response. Control action – The mathematical model and practical loop above both use a "direct" control action for all the terms, which means an increasing positive error results in an increasing positive control output for the summed terms to apply correction. However, the output is called "reverse" acting if it is necessary to apply negative corrective action.
For instance, if the valve in the flow loop was 100–0% valve opening for 0–100% control output – meaning that the controller action has to be reversed. Some process control schemes and final control elements require this reverse action. An example would be a valve for cooling water, where the fail-safe mode, in the case of loss of signal, would be 100% opening of the valve; the overall control function can be expressed mathematically as u = K p e + K i ∫ 0 t e d t ′ + K d d e d t, where
A measuring instrument is a device for measuring a physical quantity. In the physical sciences, quality assurance, engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty. Scientists and other humans use a vast range of instruments to perform their measurements; these instruments may range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is used in the development of modern measuring instruments. In the past, a common time measuring instrument was the sundial.
Today, the usual measuring instruments for time are watches. For accurate measurement of time an atomic clock is used. Stop watches are used to measure time in some sports. Energy is measured by an energy meter. Examples of energy meters include: An electricity meter measures energy directly in kilowatt hours. A gas meter measures energy indirectly by recording the volume of gas used; this figure can be converted to a measure of energy by multiplying it by the calorific value of the gas. A physical system that exchanges energy may be described by the amount of energy exchanged per time-interval called power or flux of energy. For the ranges of power-values see: Orders of magnitude. Action describes, its dimension is the same as that of an angular momentum. A phototube provides a voltage measurement which permits the calculation of the quantized action of light. See photoelectric effect; this includes basic quantities found in classical- and continuum mechanics. Length, distance, or range meterFor the ranges of length-values see: Orders of magnitude PlanimeterFor the ranges of area-values see: Orders of magnitude Buoyant weight Overflow trough Measuring cup Flow measurement devices Graduated cylinder Pipette Eudiometer, pneumatic trough If the mass density of a solid is known, weighing allows to calculate the volume.
For the ranges of volume-values see: Orders of magnitude Gas meter Mass flow meter Metering pump Water meter Airspeed indicator Radar gun, a Doppler radar device, using the Doppler effect for indirect measurement of velocity. LIDAR speed gun Speedometer Tachometer Tachymeter VariometerFor the ranges of speed-values see: Orders of magnitude Accelerometer Balance Automatic checkweighing machines Katharometer Weighing scales Inertial balance Mass spectrometers measure the mass-to-charge ratio, not the mass. For the ranges of mass-values see: Orders of magnitude Ballistic pendulum Force gauge Spring scale Strain gauge Torsion balance Tribometer Anemometer Barometer used to measure the atmospheric pressure. Manometer see pressure measurement and pressure sensor Pitot tube Tire-pressure gauge in industry and mobilityFor the ranges of pressure-values see: Orders of magnitude Circumferentor Cross staff Goniometer Graphometer Protractor Quadrant Reflecting instruments Octant Reflecting circles Sextant Theodolite Stroboscope TachometerFor the value-ranges of angular velocity see: Orders of magnitude For the ranges of frequency see: Orders of magnitude Dynamometer de Prony brake Torque wrench See the section about navigation below.
Dumpy level Laser line level Spirit level Gyroscope Ballistic pendulum, indirectly by calculation and or gauging Considerations related to electric charge dominate electricity and electronics. Electrical charges interact via a field; that field is called electric field. If the charge doesn't move. If the charge moves, thus realizing an electric current in an electrically neutral conductor, that field is called magnetic. Electricity can be given a quality — a potential, and electricity has the electric charge. Energy in elementary electrodynamics is calculated by multiplying the potential by the amount of charge found at that potential: potential times charge. Electrometer is used to reconfirm the phenomenon of contact electricity leading to triboelectric sequences. Torsion balance force, see above. For the ranges of charge values see: Orders of magnitude Ammeter Clamp meter Galvanometer D'Arsonval galvanometer Oscilloscope allows quantifying time-dependent voltages Voltmeter Ohmmeter Time-domain reflectometer characterizes and locates faults in metallic cables by runtime measurements of electric signals.
Wheatstone bridge Capacitance meter Inductance meter Electric energy meter Electricity meter Wattmeter Field mill See the relevant section in the article about the magnetic field. Compass Hall effect sensor Magnetometer Proton magnetometer SQUIDFor the ranges of magnetic field see: Orders of magnitude Multimeter, combines the functions of ammeter and ohmmeter as a minimum. LCR meter, combines the functions of ohmmeter, capacitance meter and inductance meter
Temperature control is a process in which change of temperature of a space, or of a substance, is measured or otherwise detected, the passage of heat energy into or out of the space or substance is adjusted to achieve a desired temperature. Air-conditioners, space-heaters, water-heaters, etc. are examples of devices that perform temperature control. These are broadly classified as Thermostatically Controlled Loads. A home thermostat is an example of a closed control loop: It measures the current room temperature and compares this to a desired user-defined set point and controls a heater and/or air conditioner to increase or decrease the temperature to meet the desired set point. A simple thermostat switches the heater or air conditioner either on or off, temporary overshoot and undershoot of the desired average temperature must be expected. A more expensive thermostat varies the amount of heat or cooling provided by the heater or cooler, depending on the difference between the required temperature and the actual temperature.
This minimizes over/undershoot. This method is called Proportional control. Further enhancements using the accumulated error signal and the rate at which the error is changing are used to form more complex PID Controllers, the form seen in industry. An object's or space's temperature increases when heat energy moves into it, increasing the average kinetic energy of its atoms, e.g. of things and air in a room. Heat energy leaving an object or space lowers its temperature. Heat flows from one place to another by one or more of three processes: conduction and radiation. In conduction, energy is passed from one atom to another by direct contact. In convection, heat energy moves by conduction into some movable fluid and the fluid moves from one place to another, carrying the heat with it. At some point the heat energy in the fluid is transferred to some other object by means conduction again; the movement of the fluid can be driven by negative-buoyancy, as when cooler air drops and thus upwardly displaces warmer air, or by fans or pumps.
In radiation, the heated atoms make electromagnetic emissions absorbed by remote other atoms, whether nearby or at astronomical distance. For example, the Sun radiates heat as both visible electromagnetic energy. What we know as "light" is but a narrow region of the electromagnetic spectrum. If, in a place or thing, more energy is received than is lost, its temperature increases. If the amount of energy coming in and going out are the same, the temperature stays constant—there is thermal balance, or thermal equilibrium. Heat exchanger Moving bed heat exchanger Thermal Control System Thermodynamic equilibrium Industrial automation Spacecraft thermal control
A public company, publicly traded company, publicly held company, publicly listed company, or public limited company is a corporation whose ownership is dispersed among the general public in many shares of stock which are traded on a stock exchange or in over the counter markets. In some jurisdictions, public companies over a certain size must be listed on an exchange. A public company can be unlisted. Public companies are formed within the legal systems of particular nations, therefore have national associations and formal designations which are distinct and separate. For example one of the main public company forms in the United States is called a limited liability company, in France is called a "society of limited responsibility", in Britain a public limited company, in Germany a company with limited liability. While the general idea of a public company may be similar, differences are meaningful, are at the core of international law disputes with regard to industry and trade. In the early modern period, the Dutch developed several financial instruments and helped lay the foundations of modern financial system.
The Dutch East India Company became the first company in history to issue bonds and shares of stock to the general public. In other words, the VOC was the first publicly traded company, because it was the first company to be actually listed on an official stock exchange. While the Italian city-states produced the first transferable government bonds, they did not develop the other ingredient necessary to produce a fledged capital market: corporate shareholders; as Edward Stringham notes, "companies with transferable shares date back to classical Rome, but these were not enduring endeavors and no considerable secondary market existed." The securities of a publicly traded company are owned by many investors while the shares of a held company are owned by few shareholders. A company with many shareholders is not a publicly traded company. In the United States, in some instances, companies with over 500 shareholders may be required to report under the Securities Exchange Act of 1934. Public companies possess some advantages over held businesses.
Publicly traded companies are able to raise funds and capital through the sale of shares of stock. This is the reason publicly traded corporations are important; the profit on stock is gained in form of capital gain to the holders. The financial media and the public are able to access additional information about the business, since the business is legally bound, motivated, to publicly disseminate information regarding the financial status and future of the company to its many shareholders and the government; because many people have a vested interest in the company's success, the company may be more popular or recognizable than a private company. The initial shareholders of the company are able to share risk by selling shares to the public. If one were to hold a 100% share of the company, he or she would have to pay all of the business's debt; this increases asset liquidity and the company does not need to depend on funding from a bank. For example, in 2013 Facebook founder Mark Zuckerberg owned 29.3% of the company's class A shares, which gave him enough voting power to control the business, while allowing Facebook to raise capital from, distribute risk to, the remaining shareholders.
Facebook was a held company prior to its initial public offering in 2012. If some shares are given to managers or other employees, potential conflicts of interest between employees and shareholders will be remitted; as an example, in many tech companies, entry-level software engineers are given stock in the company upon being hired. Therefore, the engineers have a vested interest in the company succeeding financially, are incentivized to work harder and more diligently to ensure that success. Many stock exchanges require that publicly traded companies have their accounts audited by outside auditors, publish the accounts to their shareholders. Besides the cost, this may make useful information available to competitors. Various other annual and quarterly reports are required by law. In the United States, the Sarbanes–Oxley Act imposes additional requirements; the requirement for audited books is not imposed by the exchange known as OTC Pink. The shares may be maliciously held by outside shareholders and the original founders or owners may lose benefits and control.
The principal-agent problem, or the agency problem is a key weakness of public companies. The separation of a company's ownership and control is prevalent in such countries as U. K and U. S. In the United States, the Securities and Exchange Commission requires that firms whose stock is traded publicly report their major shareholders each year; the reports identify all institutional shareholders, all company officials who own shares in their firm, any individual or institution owning more than 5% of the firm's stock. For many years, newly created companies were held but held initial
Automatic process control in continuous production processes is a combination of control engineering and chemical engineering disciplines that uses industrial control systems to achieve a production level of consistency and safety which could not be achieved purely by human manual control. It is implemented in industries such as oil refining and paper manufacturing, chemical processing and power generating plants. There is a wide range of size and complexity, but it enables a small number of operators to manage complex processes to a high degree of consistency; the development of large automatic process control systems was instrumental in enabling the design of large high volume and complex processes, which could not be otherwise economically or safely operated. The applications can range from controlling the temperature and level of a single process vessel, to a complete chemical processing plant with several thousand control loops. Early process control breakthroughs came most in the form of water control devices.
Ktesibios of Alexandria is credited for inventing float valves to regulate water level of water clocks in the 3rd Century BC. In the 1st Century AD, Heron of Alexandria invented a water valve similar to the fill valve used in modern toilets. Process controls inventions involved basic physics principles. In 1620, Cornlis Drebbel invented a bimetallic thermostat for controlling the temperature in a furnace. In 1681, Denis Papin discovered the pressure inside a vessel could be regulated by placing weights on top of the vessel lid. In 1745, Edmund Lee created the fantail to improve windmill efficiency. With the dawn of the Industrial Revolution in the 1760s, process controls inventions were aimed to replace human operators with mechanized processes. In 1784, Oliver Evans created a water-powered flourmill which operated using buckets and screw conveyors. Henry Ford applied the same theory in 1910 when the assembly line was created to decrease human intervention in the automobile production process.
For continuously variable process control it was not until 1922 that a formal control law for what we now call PID control or three-term control was first developed using theoretical analysis, by Russian American engineer Nicolas Minorsky. Minorsky was researching and designing automatic ship steering for the US Navy and based his analysis on observations of a helmsman, he noted the helmsman steered the ship based not only on the current course error, but on past error, as well as the current rate of change. His goal was not general control, which simplified the problem significantly. While proportional control provided stability against small disturbances, it was insufficient for dealing with a steady disturbance, notably a stiff gale, which required adding the integral term; the derivative term was added to improve stability and control. Process control of large industrial plants has evolved through many stages. Control would be from panels local to the process plant; however this required a large manpower resource to attend to these dispersed panels, there was no overall view of the process.
The next logical development was the transmission of all plant measurements to a permanently-manned central control room. This was the centralisation of all the localised panels, with the advantages of lower manning levels and easier overview of the process; the controllers were behind the control room panels, all automatic and manual control outputs were transmitted back to plant. However, whilst providing a central control focus, this arrangement was inflexible as each control loop had its own controller hardware, continual operator movement within the control room was required to view different parts of the process. With the coming of electronic processors and graphic displays it became possible to replace these discrete controllers with computer-based algorithms, hosted on a network of input/output racks with their own control processors; these could be distributed around plant, communicate with the graphic display in the control room or rooms. The distributed control system was born; the introduction of DCSs allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, easy interfacing with other production computer systems.
It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, provided high level overviews of plant status and production levels. The accompanying diagram is a general model which shows functional manufacturing levels in a large process using processor and computer-based control. Referring to the diagram: Level 0 contains the field devices such as flow and temperature sensors, final control elements, such as control valves. To determine the fundamental model for any process, the inputs and outputs of the system are defined differently than for other chemical processes; the balance equations are defined by the c
West Sussex is a county in the south of England, bordering East Sussex to the east, Hampshire to the west and Surrey to the north, to the south the English Channel. West Sussex is the western part of the historic county of Sussex a medieval kingdom. With an area of 1,991 square kilometres and a population of over 800,000, West Sussex is a ceremonial county, with a Lord Lieutenant and a High Sheriff. Chichester in the south-west is the only city in West Sussex. West Sussex has a range of scenery, including wealden and coastal; the highest point of the county is at 280 metres. It has a number of stately homes including Goodwood, Petworth House and Uppark, castles such as Arundel Castle and Bramber Castle. Over half the county is protected countryside, offering walking and other recreational opportunities. Although the name Sussex, derived from the Old English'Sūþsēaxe', dates from the Saxon period between AD 477 to 1066, the history of human habitation in Sussex goes back to the Old Stone Age; the oldest hominin remains known in Britain were found at Boxgrove.
Sussex has been occupied since those times and has succumbed to various invasions and migrations throughout its long history. Prehistoric monuments include the Devil's Jumps, a group of Bronze Age burial mounds, the Iron Age Cissbury Ring and Chanctonbury Ring hill forts on the South Downs; the Roman period saw the building of Fishbourne Roman Palace and rural villas such as Bignor Roman Villa together with a network of roads including Stane Street, the Chichester to Silchester Way and the Sussex Greensand Way. The Romans used the Weald for iron production on an industrial scale; the foundation of the Kingdom of Sussex is recorded by the Anglo-Saxon Chronicle for the year AD 477. The foundation story is regarded as somewhat of a myth by most historians, although the archaeology suggests that Saxons did start to settle in the area in the late 5th century; the Kingdom of Sussex became the county of Sussex. With its origins in the kingdom of Sussex, the county of Sussex was traditionally divided into six units known as rapes.
By the 16th century, the three western rapes were grouped together informally, having their own separate Quarter Sessions. These were administered by a separate county council from 1888, the county of Sussex being divided for administrative purposes into the administrative counties of East and West Sussex. In 1974, West Sussex was made a single ceremonial county with the coming into force of the Local Government Act 1972. At the same time a large part of the eastern rape of Lewes was transferred into West Sussex; until 1834 provision for the poor and destitute in West Sussex was made at parish level. From 1835 until 1948 eleven Poor Law Unions, each catering for several parishes, took on the job. Most settlements in West Sussex are either along the south coast or in Mid Sussex, near the M23/A23 corridor; the town of Crawley is the largest in the county with an estimated population of 106,600. The coastal settlement of Worthing follows with a population of 104,600; the seaside resort of Bognor Regis and market town Horsham are both large towns.
Chichester, the county town, has a cathedral and city status, is situated not far from the border with Hampshire. Other conurbations of a similar size are Burgess Hill, East Grinstead and Haywards Heath in the Mid Sussex district, Littlehampton in the Arun district, Lancing and Shoreham in the Adur district. Much of the coastal town population is part of the Brighton/Worthing/Littlehampton conurbation. Rustington and Southwater are the next largest settlements in the county. There are several more towns in West Sussex; the smaller towns of the county are Arundel, Petworth and Steyning. The larger villages are Billingshurst, Crawley Down, Henfield, Hurstpierpoint, Lindfield and Storrington; the current total population of the county makes up 1.53% of England's population. West Sussex is bordered by Hampshire to Surrey to the north and East Sussex to the east; the English Channel lies to the south. The area has been formed from Upper Jurassic and Lower Cretaceous rock strata, part of the Weald–Artois Anticline.
The eastern part of this ridge, the Weald of Kent and Surrey has been eroded, with the chalk surface removed to expose older Lower Cretaceous rocks of the Wealden Group. In West Sussex the exposed rock becomes older towards the north of the county with Lower Greensand ridges along the border with Surrey including the highest point of the county at Blackdown. Erosion of softer sand and clay strata has hollowed out the basin of the Weald leaving a north facing scarp slope of the chalk which runs east and west across the whole county, broken only by the valleys of the River Arun and River Adur. In addition to these two rivers which drain most of the county a winterbourne, the River Lavant, flows intermittently from springs on the dip slope of the chalk downs north of Chichester; the county makes up 1.52% of the total land of England, making it the 30th largest county in the country. West Sussex is the sunniest county in the United Kingdom, according to Met Office records. Over the last 29 years it has averaged 1902 hours of sunshine per year.
Sunshine totals are highest near the coast wi