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control loop

Contributor(s): Matthew Haughn

A control loop is a system made up of all the hardware components and software control functions needed for the measurement and adjustment of a variable that controls an individual process.

Control loops are the elemental building block of industrial control systems (ICS) such as supervisory control and data acquisition (SCADA) or distributed control systems (DCS). Each control loop commands a given variable in an industrial process.

Found everywhere ICS are used, control loops are part of the systems used in several processes, including:

  • Agriculture
  • Chemical plants
  • Pulp and Paper Mills
  • Quality control
  • Boiler controls and power plant
  • Nuclear power plants
  • Environmental control
  • Water treatment plants
  • Sewage treatment plants
  • Food and food processing
  • Metal and mines
  • Pharmaceutical manufacturing
  • Sugar refining plants

A control loop is made of a number of components. Though the specifics vary, a control loop is generally made of a sensor, a controller and a final control element.  The sensor takes a reading of the process variable or a related measurement. The controller receives the signal from the sensor and then forwards it to instrumentation, to remote terminal units (RTU) and to final control elements where the process variable is adjusted, to be kept as desired at a set point.

There are two main types of control loops: open loops that take human input and closed loops that are fully autonomous. Some loops can be switched between closed and open. When open, a switchable loop is manually controllable and when closed it is fully automated.

This was last updated in September 2017

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