The general goal of control is to get some physical system to respond to a reference input in the way we would like.
Definition 1
The plant is the physical system which we would like to control.
In general, there are two different types of control.
Definition 2
Open-Loop control is where we pass a reference directly to the actuator to control the plant (see Figure 1).
Figure 1: Open-Loop Control
Open-Loop control is generally difficult because the disturbances make it difficult to copy the reference exactly.
Definition 3
Closed loop control is using the output of our system and comparing it to the reference in order to generate the control signal (see Figure 2).
Figure 2: Closed-Loop Control
Notice how the output signal is subtracted from a reference signal, and we use the difference (a.k.a the error) to determine what input we pass into the plant.
Definition 4
The control law K(e)is a function of error applied by the controller to determine the inputs to the plant.
