InstrumentationProcess Control

What are Open Loop & Closed Loop system


There are two types of control systems namely Open Loop & Closed Loop system                                                         Open loop control systems are non-feedback control systems
  Closed loop control systems are feedback control systems

In this session we are going to discuss about Open Loop & Closed Loop systems

Open Loop (Manual Control)

Figure  shows what is called OPEN LOOP or MANUAL control.  The process is temperature control.  The indicator is a thermometer.  The correcting unit is the gas control valve.  The controller is the operator who uses his own judgement to keep the water temperature constant.

Manual control has its uses as it is cheap to install and maintain, and simple to operate.  However, it is very seldom used in industry because:

  • The operator must remain in position at all times.
  • It cannot be used if the operator is placed in a dangerous area.
  • The process changes faster than the operator can react.
  • A mistake by the operator can have dangerous results.

These problems are avoided by using automatic control (closed loop).  The job of the instrument technician is to make sure that this type of control operates correctly.

Modern household appliances now use automatic control to make work easier.  For example:-

  • Refrigerators and water heaters use automatic temperature control.
  • Washing machines use automatic heating and water control


  • simple
  • stable
  • easy construction


  • Inaccurate
  • unreliable
  • Remove the disturbances occurring from external sources.

To know more about Open-loop controller

Closed Loop (Automatic Control)

Figure  shows a simple automatic controller.  The boiler now has the loop closed and no operator is required.  The following items are added.

  • The temperature transmitter (T.T) which measures (senses) the temperature of the hot water and changes it to a standard signal.
  • A signal line from the transmitter to the controller, the signal may be either pneumatic or electrical.
  • A controller which keeps the temperature of the hot water at a position set by the operator (set point)
  • The controller adjusts the correcting unit (automatic control valve) using an output signal line similar to the input line from the transmitter.
  • The controller may provide alarm signals to alert the operator if the system fails. It may also shut off the gas if the water starts to boil.


  • Accuracy
  • Noise reduction ability


  • complex  construction
  • reduces the overall gain of the system.
  • less Stable than open loop

To know more about closed loop

 Different types of process controls

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