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.