Feedback Control

The diagram shows the process of controlling the flow rate, which is the amount of fluid flowing through a pipe. The flow rate is adjusted by changing the opening of the control valve attached to the piping, and the flow meter measures the actual flow rate. The measured value is input into the controller, which calculates the manipulated variable (control valve opening) required to match the target flow rate (target value) preset in the controller based on the deviation from the measured value, and sends an opening command to the control valve.

• ① The actual flow rate of the fluid flowing through the pipe is measured by the flow meter, and the measured value is input into the controller.
• ② The controller calculates the manipulated variable from the deviation between the measured value and the target value and outputs an opening command to the control valve.
• ③ The control valve opens and closes according to the command from the controller to adjust the flow rate.
• ④ The adjusted flow rate is measured by the flow meter and is fed back to the controller.

In this way, feedback control is also called "closed-loop control" because it continuously repeats a series of actions: the actual flow rate is constantly fed back to the controller, the manipulated variable is reviewed based on that feedback, the opening of the control valve is adjusted, and the flow rate is controlled.

However, even with the same valve opening, if the pressure inside the pipe changes, the flow rate will also change, so it is not always possible to maintain control under constant conditions. Influencing factors other than the manipulated variable (valve opening) are called disturbances. The greatest advantage of feedback control is that the effects of disturbances are reflected in the control as measured values, allowing for appropriate adjustments to consistently match the target value and achieve high accuracy. For this reason, it is used in a wide range of fields, including control in food processing, which requires constant temperature control, and temperature control in air-conditioning systems.