Editing Automation (section)

=== 20th century ===
Relay logic was introduced with factory [[electrification]], which underwent rapid adaption from 1900 through the 1920s. Central electric power stations were also undergoing rapid growth and the operation of new high-pressure boilers, steam turbines and electrical substations created a large demand for instruments and controls. Central control rooms became common in the 1920s, but as late as the early 1930s, most process controls were on-off. Operators typically monitored charts drawn by recorders that plotted data from instruments. To make corrections, operators manually opened or closed valves or turned switches on or off. Control rooms also used color-coded lights to send signals to workers in the plant to manually make certain changes.<ref>{{Harvnb|Bennett|1993|pp=31}}</ref>

The development of the electronic amplifier during the 1920s, which was important for long-distance telephony, required a higher signal-to-noise ratio, which was solved by negative feedback noise cancellation. This and other telephony applications contributed to the control theory. In the 1940s and 1950s, German mathematician [[Irmgard Flügge-Lotz]] developed the theory of discontinuous automatic controls, which found military applications during the [[Second World War]] to [[fire control system]]s and aircraft [[navigation system]]s.{{sfn|Bennett|1993}}

Controllers, which were able to make calculated changes in response to deviations from a set point rather than on-off control, began being introduced in the 1930s. Controllers allowed manufacturing to continue showing productivity gains to offset the declining influence of factory electrification.<ref name="Field_2011">{{cite book|title=A Great Leap Forward: 1930s Depression and U.S. Economic Growth |last=Field |first= Alexander J.|year= 2011 |publisher =Yale University Press|location= New Haven, London|isbn=978-0-300-15109-1 }}</ref>

Factory productivity was greatly increased by electrification in the 1920s. U.S. manufacturing productivity growth fell from 5.2%/yr 1919–29 to 2.76%/yr 1929–41. Alexander Field notes that spending on non-medical instruments increased significantly from 1929 to 1933 and remained strong thereafter.<ref name="Field_2011"/>

The First and Second World Wars saw major advancements in the field of [[mass communication]] and [[signal processing]]. Other key advances in automatic controls include [[differential equation]]s, [[stability theory]] and [[system theory]] (1938), [[Frequency response|frequency domain analysis]] (1940), [[Motion control|ship control]] (1950), and [[stochastic analysis]] (1941).

Starting in 1958, various systems based on [[solid-state (electronics)|solid-state]]<ref name="Wireless-World_1960"/><ref name="MBLE_1962_Norbit"/> [[digital electronics|digital logic]] modules for hard-wired programmed logic controllers (the predecessors of [[programmable logic controller]]s [PLC]) emerged to replace electro-mechanical relay logic in [[industrial control system]]s for [[process control]] and automation, including early [[Telefunken]]/[[AEG (German company)|AEG]] [[Logistat]], [[Siemens]] [[Simatic]], [[Philips]]/[[Mullard]]/{{interlanguage link|Valvo (company){{!}}Valvo|de|Valvo}} [[Norbit module|Norbit]], [[BBC]] [[Sigmatronic]], [[ACEC (company)|ACEC]] [[Logacec]], {{interlanguage link|Akkord-Radio{{!}}Akkord|de|Akkord-Radio}} [[Estacord]], Krone<!-- AG --> Mibakron, Bistat, Datapac, Norlog, SSR, or <!-- ABB -->Procontic systems.<ref name="Wireless-World_1960"/><ref name="Akkord_Estacord"/><ref name="Klingelnberg_1967"/><ref name="Parr_1993"/><ref name="Weissel_1995"/><ref name="Walker_2012"/>

In 1959 [[Texaco]]'s [[Port Arthur Refinery]] became the first chemical plant to use [[digital control]].<ref>{{Harvnb|Rifkin|1995|pp=}}</ref>
Conversion of factories to digital control began to spread rapidly in the 1970s as the price of [[computer hardware]] fell.