Editing Automation (section)

===Sequential control and logical sequence or system state control===
{{main|Programmable logic controller}}

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 use or 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 the operator presses the floor n button, the system will respond depending on whether the elevator is stopped or moving, going up or down, or if the door is open or closed, and other conditions.<ref>The elevator example is commonly used in programming texts, such as [[Unified Modeling Language]]</ref>

Early development of sequential control was [[relay logic]], by which [[electrical relay]]s engage electrical contacts which either start or interrupt power to a device. Relays were first used in telegraph networks before being developed for controlling other devices, such as when starting and stopping industrial-sized electric motors or opening and closing [[solenoid valve]]s. Using relays for control purposes allowed event-driven control, where actions could be triggered out of sequence, in response to external events. These were more flexible in their response than the rigid single-sequence [[cam timer]]s. More complicated examples involved maintaining safe sequences for devices such as swing bridge controls, where a lock bolt needed to be disengaged before the bridge could be moved, and the lock bolt could not be released until the safety gates had already been closed.

The total number of relays and cam timers can number into the hundreds or even thousands in some factories. Early [[computer programming|programming]] techniques and languages were needed to make such systems manageable, one of the first being [[ladder logic]], where diagrams of the interconnected relays resembled the rungs of a ladder. Special computers called [[programmable logic controllers]] were later designed to replace these collections of hardware with a single, more easily re-programmed unit.

In a typical hard-wired motor start and stop circuit (called a ''control circuit'') a motor is started by pushing a "Start" or "Run" button that activates a pair of electrical relays. The "lock-in" relay locks in contacts that keep the control circuit energized when the push-button is released. (The start button is a normally open contact and the stop button is a normally closed contact.) Another relay energizes a switch that powers the device that throws the motor starter switch (three sets of contacts for three-phase industrial power) in the main power circuit. Large motors use high voltage and experience high in-rush current, making speed important in making and breaking contact. This can be dangerous for personnel and property with manual switches. The "lock-in" contacts in the start circuit and the main power contacts for the motor are held engaged by their respective electromagnets until a "stop" or "off" button is pressed, which de-energizes the lock in relay.<ref>{{cite web|url=http://www.exman.com/mshoass.html|title=MOTOR STARTERS START STOPS HAND OFF AUTO|website=Exman.com|access-date=14 September 2013|archive-url=https://web.archive.org/web/20140413062641/http://www.exman.com/mshoass.html|archive-date=13 April 2014}}</ref>
[[Image:Finite state machine example with comments.svg|thumb|This state diagram shows how [[Unified modeling language|UML]] can be used for designing a door system that can only be opened and closed.|alt=]]
Commonly [[interlock (engineering)|interlock]]s are added to a control circuit. Suppose that the motor in the example is powering machinery that has a critical need for lubrication. In this case, an interlock could be added to ensure that the oil pump is running before the motor starts. Timers, limit switches, and electric eyes are other common elements in control circuits.

Solenoid valves are widely used on [[compressed air]] or [[hydraulic fluid]] for powering [[actuator]]s on [[Mechanical system|mechanical]] components. While [[motor]]s are used to supply continuous [[Rotary actuator|rotary motion]], actuators are typically a better choice for intermittently creating a limited range of movement for a mechanical component, such as moving various mechanical arms, opening or closing [[valve]]s, raising heavy press-rolls, applying pressure to presses.