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A control loop is the fundamental building block of control systems in general and industrial control systems in particular. It consists of the process sensor, the controller function, and the final control element (FCE) which controls the process necessary to automatically adjust the value of a measured process variable (PV) to equal the value of a desired set-point (SP).
Note that a 4-wire instrument has a power-supply input separate from the current loop. Panel mount displays and chart recorders are commonly termed "indicator devices" or "process monitors". Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).
Instrumentation comprises sensor elements, signal transmitters, controllers, indicators and alarms, actuated valves, logic circuits and operator interfaces. An outline of key instrumentation is shown on Process Flow Diagrams (PFD) which indicate the principal equipment and the flow of fluids in the plant.
Loop performance in control engineering indicates the performance of control loops, such as a regulatory PID loop. [1] Performance refers to the accuracy of a control system's ability to track (output) the desired signals to regulate the plant process variables in the most beneficial and optimised way, without delay or overshoot.
The control action is the switching on/off of the boiler, but the controlled variable should be the building temperature, but is not because this is open-loop control of the boiler, which does not give closed-loop control of the temperature. In closed loop control, the control action from the controller is dependent on the process output.
PID controllers work best when the loop to be controlled is linear and symmetric. Thus, their performance in non-linear and asymmetric systems is degraded. A non-linear valve, for instance, in a flow control application, will result in variable loop sensitivity, requiring dampened action to prevent instability.
Each loop or circuit maybe identified using a unique code, with description about; process, material & degradation mode, material, cladding, C.A, specs. See system model comes under the general heading of system analysis the terms analysis and synthesis come from Greek where they mean respectively "to take apart" and "to put together".
Its name comes from the information path in the system: process inputs (e.g., voltage applied to an electric motor) have an effect on the process outputs (e.g., speed or torque of the motor), which is measured with sensors and processed by the controller; the result (the control signal) is "fed back" as input to the process, closing the loop.