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Acontrol system manages, commands, directs, or regulates the behavior of other devices or systems usingcontrol loops. It can range from a single home heating controller using athermostat controlling a domestic boiler to largeindustrial control systems which are used for controllingprocesses or machines. The control systems are designed viacontrol engineering process.
For continuously modulated control, afeedback controller is used to automatically control a process or operation. The control system compares the value or status of theprocess variable (PV) being controlled with the desired value orsetpoint (SP), and applies the difference as a control signal to bring the process variable output of theplant to the same value as the setpoint.
Forsequential andcombinational logic,software logic, such as in aprogrammable logic controller, is used.[clarification needed]
Fundamentally, there are two types of control loop:open-loop control (feedforward), andclosed-loop control (feedback).
The definition of a closed loop control system according to theBritish Standards Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero."[2]
Likewise; "AFeedback Control System is a 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."[2]
Aclosed-loop controller or feedback controller is acontrol loop which incorporatesfeedback, in contrast to anopen-loop controller ornon-feedback controller.A closed-loop controller uses feedback to controlstates oroutputs of adynamical system. Its name comes from the information path in the system: process inputs (e.g.,voltage applied to anelectric motor) have an effect on the process outputs (e.g., speed or torque of the motor), which is measured withsensors and processed by the controller; the result (the control signal) is "fed back" as input to the process, closing the loop.[3]
In the case of linearfeedback systems, acontrol loop includingsensors, control algorithms, and actuators is arranged in an attempt to regulate a variable at asetpoint (SP). An everyday example is thecruise control on a road vehicle; where external influences such as hills would cause speed changes, and the driver has the ability to alter the desired set speed. ThePID algorithm in the controller restores the actual speed to the desired speed in an optimum way, with minimal delay orovershoot, by controlling the power output of the vehicle's engine.Control systems that include some sensing of the results they are trying to achieve are making use of feedback and can adapt to varying circumstances to some extent.Open-loop control systems do not make use of feedback, and run only in pre-arranged ways.
Closed-loop controllers have the following advantages over open-loop controllers:
In some systems, closed-loop and open-loop control are used simultaneously. In such systems, the open-loop control is termedfeedforward and serves to further improve reference tracking performance.
A common closed-loop controller architecture is thePID controller.
Logic control systems for industrial and commercial machinery were historically implemented by interconnected electricalrelays andcam timers usingladder logic. Today, most such systems are constructed withmicrocontrollers or more specializedprogrammable logic controllers (PLCs). The notation of ladder logic is still in use as a programming method for PLCs.[5]
Logic controllers may respond to switches and sensors and can cause the machinery to start and stop various operations through the use ofactuators. Logic controllers are used to sequence mechanical operations in many applications. Examples include elevators, washing machines and other systems with interrelated operations. An automatic sequential control system may trigger a series of mechanical actuators in the correct sequence to perform a task. For example, various electric and pneumatic transducers may fold and glue a cardboard box, fill it with the product and then seal it in an automatic packaging machine.
PLC software can be written in many different ways – ladder diagrams, SFC (sequential function charts) orstatement lists.[6]
On–off control uses a feedback controller that switches abruptly between two states. A simple bi-metallic domesticthermostat can be described as an on-off controller. When the temperature in the room (PV) goes below the user setting (SP), the heater is switched on. Another example is a pressure switch on an air compressor. When the pressure (PV) drops below the setpoint (SP) the compressor is powered. Refrigerators and vacuum pumps contain similar mechanisms. Simple on–off control systems like these can be cheap and effective.
Linear control are control systems andcontrol theory based onnegative feedback for producing acontrol signal to maintain the controlledprocess variable (PV) at the desiredsetpoint (SP). There are several types of linear control systems with different capabilities.
Fuzzy logic is an attempt to apply the easy design of logic controllers to the control of complex continuously varying systems. Basically, a measurement in a fuzzy logic system can be partly true.
The rules of the system are written in natural language and translated into fuzzy logic. For example, the design for a furnace would start with: "If the temperature is too high, reduce the fuel to the furnace. If the temperature is too low, increase the fuel to the furnace."
Measurements from the real world (such as the temperature of a furnace) arefuzzified and logic is calculated arithmetic, as opposed toBoolean logic, and the outputs arede-fuzzified to control equipment.
When a robust fuzzy design is reduced to a single, quick calculation, it begins to resemble a conventional feedback loop solution and it might appear that the fuzzy design was unnecessary. However, the fuzzy logic paradigm may provide scalability for large control systems where conventional methods become unwieldy or costly to derive.[citation needed]
Fuzzy electronics is an electronic technology that uses fuzzy logic instead of the two-value logic more commonly used indigital electronics.
The range of control system implementation is fromcompact controllers often with dedicated software for a particular machine or device, todistributed control systems for industrial process control for a largephysical plant.
Logic systems and feedback controllers are usually implemented withprogrammable logic controllers. The Broadly Reconfigurable and Expandable Automation Device (BREAD) is a recent framework that provides manyopen-source hardware devices which can be connected to create more complexdata acquisition and control systems.[7]
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