Nyquist stability criterion

The Nyquist stability criterion is like a check or test to make sure a system will keep working smoothly as time passes. It's a way to make sure that things don't get all messed up and out of control. To do this, you picture a system as being like a car that needs to keep going in one direction, and you imagine a circle that sits in front of the car. The circle is used to represent the system's behavior, and you have to check that the car will always stay on the circle and not go off it. In order for the system to stay stable, it needs to make sure that the car won't ever go off the circle. That's the Nyquist stability criterion.