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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).
Example of a single industrial control loop; showing continuously modulated control of process flow. A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an open-loop controller or non-feedback controller. A closed-loop controller uses feedback to control states or outputs of a dynamical ...
Many PID loops control a mechanical device (for example, a valve). Mechanical maintenance can be a major cost and wear leads to control degradation in the form of either stiction or backlash in the mechanical response to an input signal. The rate of mechanical wear is mainly a function of how often a device is activated to make a change.
Example of a single industrial control loop; showing continuously modulated control of process flow. Piping and instrumentation diagram of pump with storage tank. Symbols according to EN ISO 10628 and EN 62424. A more complex example of a P&ID. A piping and instrumentation diagram (P&ID) is defined as follows:
The system diagram for representing control loops is a Piping and instrumentation diagram. Commonly used control systems include programmable logic controller (PLC), Distributed Control System (DCS) or SCADA. Example of level control system of a continuous stirred-tank reactor. The flow control into the tank would be cascaded off the level control.
Example of a single industrial control loop; showing continuously modulated control of process flow. A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an open-loop controller or non-feedback controller. A closed-loop controller uses feedback to control states or outputs of a dynamical ...
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.
In open-loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building.