The basic elements of a feedback control system are represented by the block diagram below. The functional relationships between these elements are also shown in the diagram.
The block diagram represents flow paths of control signals but do not represent the flow of energy through the system or process.
The plant is the system or process through which a particular quantity or condition is controlled. This is also called the controlled system.
The control elements are components needed to generate the appropriate control signal applied to the plant. These elements are also called the controller.
Related: Principles of Control Systems
The feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output.
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The reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. This signal represents the desired value of a controlled variable and is also called the ‘’setpoint’’.
The controlled output is the quantity or condition of the plant is the quantity or condition of the plant which is controlled. This signal represents the controlled variable.
The feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal.
The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and the feedback signal. This is called the ‘’error signal’’.
The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled output) at the desired value.
The disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.
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