# 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.

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#### Definition 1

The plant is the physical system which we would like to control.
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In general, there are two different types of control.

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#### Definition 2

Open-Loop control is where we pass a reference directly to the actuator to control the plant (see Figure 1).
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![Figure 1: Open-Loop Control](https://3896527066-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MQYqMkpf2WPiP6MKWRe%2Fuploads%2Fgit-blob-91a01fd94da65b159966cf03555e7aac99a8888a%2F31c08937dc42492e6d975c5f59a6c131346cc29a.png?alt=media)

Open-Loop control is generally difficult because the disturbances make it difficult to copy the reference exactly.

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#### 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).
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![Figure 2: Closed-Loop Control](https://3896527066-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MQYqMkpf2WPiP6MKWRe%2Fuploads%2Fgit-blob-dc45805cbb3e818e56fcd59ac2d95a5afc414ca8%2F66fe2d6527f4672187b60442c735d00d783830bc.png?alt=media)

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.

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#### Definition 4

The control law $$K(e)$$is a function of error applied by the controller to determine the inputs to the plant.
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