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Process Engineering Feedback Loops

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Arguably the most ingenious tool of the control engineering profession is the feedback loop as shown in the Basic Feedback Loop graphic. It consists of five fundamental elements:

  • The PROCESS that is being controlled (to stay at a specific setpoint)
  • An instrument with a SENSOR that measures the condition of the process
  • A TRANSMITTER that converts that sensor measurement into an electronic signal
  • A CONTROLLER that reads the transmitter's signal and decides whether the current condition of the process is acceptable (i.e. if it meets the Process Setpoint), and
  • An ACTUATOR functioning as the final control element that applies a correction to the process per the controller's instructions.

                                                   CONTROLLER ----> ACTUATOR ----> PROCESS ----> SENSOR

                                                          ^                                                                                       /                                                                 

                                                           \                                                                                      /

                                                            \  <-----------------< TRANSMITTER <----------------------<

In a Closed-loop control system, Process information flows around a feedback loop from the PROCESS to the SENSOR to the TRANSMITTER to the CONTROLLER to the ACTUATOR and back to the PROCESS. This measure-decide-actuate sequence known as closed-loop control repeats as often as necessary until the desired process condition is achieved.  Typical examples include a thermostat controlling a furnace to maintain the temperature in a house, or setting cruise control to maintain the speed of a car.

But not all automatic control operations require feedback.  A much larger class of control commands can be executed in an open-loop configuration without confirmation or further adjustment. Open-loop control is sufficient for predictable operations such as opening a door, starting a motor, or turning off a pump. These are similar to manual on/off controls.

posted Apr 1 by Guy Andrus

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