Steady state error is the difference between the desired output and the actual output of a control system in the limit as time goes to infinity, when the response of the control system has reached steady-state. It is represented as $e_{ss}$ . Steady state error is a measure of the accuracy a control system has in tracking a command input. It is meaningless to find the steady state errors for unstable systems. Steady state error depends on the type of input (step, ramp, etc.) as well as the system type (0, I, or II) . The steady-state error can be calculated from the open- or closed-loop transfer function for unity feedback systems. The error constants, known as the position constant (Kp), the velocity constant (Kv), and the acceleration constant (Ka), are used to calculate steady-state error.