Automatic control

Minimum human intervention is required to control many large facilities such as this electrical generating station.

Automatic control is the application of control theory for regulation of processes without direct human intervention. In the simplest type of an automatic control loop, a controller compares a measured value of a process with a desired set value, and 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, and advanced rapidly in the 20th.

Designing a system with features of automatic control generally requires the feeding of electrical or mechanical energy to enhance the dynamic features of an otherwise sluggish or variant, even errant system. The control is applied with a regulating the energy feed.

Examples

Automatic control can self-regulate a technical plant (such as a machine or a industrial process) operating condition or parameters by the controller with minimal human intervention. A regulator such as a thermostat is an example of a device studied in automatic control.

Components

A central concept with automatic control is that of the system to be controlled, such as a rudder and its engine, a propeller and its motor or a ballistic missile with its jet or rocket engine and the feedback of control information from the measured speed, direction and heading in a closed loop to enable proper feed forward of propelling energy.

• Sensor(s), which measure some physical state such as temperature or liquid level.
• Controller(s), which may be from simple physical components up to complex special purpose digital controllers or embedded computers.
• Actuator(s), which effect a response to the sensor(s) under the command of the responder, for example, by controlling an energy input, as e.g. a gas flow to a burner in a heating system or electricity to a motor in a refrigerator or pump.

Functions

• Control
• Sensing
• Metrics
• Measurement
• Comparison
• Computation
• Correction

History of automatic Control

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Ancient Greece

Ctesibius's clepsydra (3rd century BC).

It was a preoccupation of the Greeks and Arabs (in the period between about 300 BC and about 1200 AD) to keep accurate track of time. In about 270 BC the Greek Ctesibius invented a float regulator for a water clock, a device not unlike the ball and cock in a modern flush toilet. The invention of the mechanical clock in the 14th century made the water clock and its feedback control system obsolete. The float regulator does not appear again until its use in the Industrial Revolution. It is worth mentioning that a pseudo-feedback control system was developed in China in the 12th century for navigational purposes. The south pointing chariot had a statue which was turned by a gearing mechanism attached to the wheels of the chariot so that it continuously pointed south. Using the directional information provided by the statue, the charioteer could steer a straight course.

Industrial Revolution in Europe

The Industrial Revolution is generally agreed to have started in the third quarter of the 18th century (1769); however, its roots can be traced back into the 17th century. The introduction of prime movers, or self-driven machines advanced grain mills, furnaces, boilers, and the steam engine created a new requirement for automatic control systems including temperature regulators (invented in 1624 (see Cornelius Drebbel)), pressure regulators (1681), float regulators (1700) and speed control devices. James Watt invented the steam engine in 1769, and this date marks the accepted beginning of the Industrial Revolution. The design of feedback control systems up through the Industrial Revolution was by trial-and-error, together with a great deal of engineering intuition. Thus, it was more of an art than a science. In the mid-19th century mathematics was first used to analyze the stability of feedback control systems. Since mathematics is the formal language of automatic control theory, we could call the period before this time the prehistory of control theory.

First and Second World Wars

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 equations, stability theory and system theory (1938), frequency domain analysis (1940), ship control (1950), and stochastic analysis (1941).

Space/computer age

With the advent of the space age in 1957, controls design, particularly in the United States, turned away from the frequency-domain techniques of classical control theory and back to the differential equation techniques of the late 19th century, which were couched in the time domain. The modern era saw time-domain design for nonlinear systems (1961), navigation (1960), optimal control and estimation theory (1962), nonlinear control theory (1969), digital control and filtering theory (1974), and the personal computer (1983).

Further reading

• V. Gurevich "Electronic Devices on Discrete Components for Industrial and Power Engineering", CRC Press, New York, 2008, 418 p.
• Tang Bingxin "Fundamentals of Control Engineering", 37 p.
• http://www.tpub.com/content/doe/h1013v2/css/h1013v2_112.htm

See also

• Control theory
• Process control
• Controller (control theory)
• Control engineering
• PID loop
• VisSim
• EICASLAB
• Feedback
• Feedforward Control