We can classify the systems based on stability as follows.

1. Absolutely stable system
2. Conditionally stable system
3. Marginally stable system

Absolutely Stable System

If the system is stable for all the range of system component values, then it is known as the absolutely stable system. The open loop control system is absolutely stable if all the poles of the open loop transfer function present in left half of ‘s’ plane. Similarly, the closed loop control system is absolutely stable if all the poles of the closed loop transfer function present in the left half of the ‘s’ plane.

Conditionally Stable System

If the system is stable for a certain range of system component values, then it is known as conditionally stable system.

Marginally Stable System

If the system is stable by producing an output signal with constant amplitude and constant frequency of oscillations for bounded input, then it is known as marginally stable system. The open loop control system is marginally stable if any two poles of the open loop transfer function is present on the imaginary axis. Similarly, the closed loop control system is marginally stable if any two poles of the closed loop transfer function is present on the imaginary axis.

A stable system is one that holds stability and is dependable. An unstable system is just the opposite. You do not know what to expect from an unstable system; it is not close to being dependable.

lollssxx

Marginally stable system is when the system control components are JUST right. So if they change slightly the system becomes unstable. I wouldn't fly on that airplain.

Hi from Panha at PUC.