Sometimes, the principle of the system is not in steady state error action step function, known as non-heterodyne system: the system has a principle and steady-state error, called differential systems
Sometimes, a system withno severance error sits no effect under the step function is called a differential system: and a system with a principled steady-state error is called a differential system.
Sometimes, the system without principled steady-state error under step function is called error-free system, while the system with principled steady-state error is called error-free system.<br>