Definition. A combination or arrangment of conponents to control a system.

Components can either be electrical (such as relays, contactors), mechanical (such as valves), chemical (such as elements or compounds), hydraulic (such as oil, fluids), pneumatic (such as air, gas) or thermal (such as thermocouple, RTDs).

Types. Open Loop is a control system characterized by the inability to compare the controlled variable (input) to the desired value (output) and to take action based on that comparison. The Open Loop type is very simple, thus inexpensive.

Closed Loop is a control system characterized by the ability to compare the actual value of the controlled variable (input) to the desired value (output) and to take immediate action based on that comparison. The Closed Loop type is very advantageous because 1) it reduces the error signal, 2) increases the speed of response, 3) stable and 4) takes account of disturbance.

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Feedback Control System

The Control System which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control.

General Closed Loop Block Diagram. Composed of five components: the Comparer, Controller, Amplifier or Final Correcting Device, Process and the Measuring Device.

Setpoint. also known as the set value, desired value, ideal value, command signal or reference signal.

Comparer. also known Comparator, error detector, difference detector, is a mechanical, electrical or pneumatic device that makes comparison between the measure value and the setpoint. It generates the Error signal = measured value minus the desired value (setpoint). When the measured value is greater than the setpoint, the error signal is positive; and when it is lesser than the setpoint, the error signal generated is negative.

Controller. is an electrical, mechanical, or pneumatic device which received the error signal and generates a corresponding output signal.

Amplifier. is a valve or valve-like device which varied the flow of fluid to the system. Fluid, in control systems' sense can be air, liquid, fluid, or electrical in nature such as current.

Process. is also known as the controlled variable unit, or the dynamic unit. Process Variables can either be temperature, pressure, fluid flow rate, chemical composition, concentration, humidity, viscosity, mechanical position, speed, acceleration, etc. The process is subject to two inputs, the amplified signal and disturbance. A disturbance is any external conditions that upset the process and affect the outcome (controlled variable signal). Examples of these are: pressure changes, viscosity changes, or electrical noise.

Measuring Device. is any electrical or mechanical detecting device. While, a Transducer is a measuring device which converts non-electrical signal (e.g., temperature, pressure) to an electrical signal (e.g., current, resistance, voltage).

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Modes of Control

Modes of Control. There are five modes of control, dependent on the speed and manner of response to the error signal.

On-Off Mode of Control. The final correcting device has only two positions or operating states, i.e., the ON and the OFF positions. Thus, this mode is better known as the two-position control or bang-bang control.

Proportional Mode of Control. The final correcting device has a continuous range of possible positions, which is (directly or inversely) proportional to the magnitude of the error signal.

Proportional-Derivative Mode of Control. The position of the final correcting device is determined by the magnitude (proportional part) of the error signal and the time rate of change of error (differential or derivative part).

Proportional-Integral Mode of Control. The position of the final correcting device is determined by the magnitude (proportional part) of the error signal and the time integral of error (integral part). The time integral is equal to the product of the magnitude and the time the error persisted.

Proportional-Integral-Derivate [P.I.D.] Mode of Control. The position of the final correcting device is determined by the magnitude (proportional part) of the error signal, initially by the time rate of change of error (differential or derivative part) and finally by the time integral of error (integral part).

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Solutions to Control Systems

Block Diagram. A graphical representation of the flow of information and the functions performed by each component of the system. This diagrams are characterized by rectangular blocks containing the transfer functions defining each component. The diagram represents the manner in which the functional components are connected the the mathematical equations (basically the transforms) that determine the response of each component. Block diagrams are used for single input, single output control systems.

Signal Flow Graph. A diagram which represents a set of simultaneous equations simplifying a complex network in which nodes are connected by directed branches. The graph basically consists of lines and arrows and the transforms of each component written near the line. Signal flow graphs are employed, especially and generally, for multiple input, multiple output control systems.