CONTROL LOOP DIAGRAMS
Below are several terms associated with the closed-loop block diagram.
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."
The feedback elements are components needed to identify the functional relationship
between the feedback signal and the controlled output.
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 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 feedback signal. This is also called the
The manipulated variable is the variable of the process acted upon to maintain the plant
output (controlled variable) at the desired value.
The disturbance is an undesirable input signal that upsets the value of the controlled
output of the plant.
Figure 9 shows a typical application of a block diagram to identify the operation of a temperature
control system for lubricating oil. (A) in Figure 9 shows a schematic diagram of the lube oil
cooler and its associated temperature control system.