Introduction to Control
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
is a function of error applied by the controller to determine the inputs to the plant.
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