After analyzing the behavior of the control object based on linear theory, and calculating the necessary control laws, when implementing systems based on the calculations performed, you may encounter some discrepancies between the reactions received from the real object and the expected ones. The properties of the assembled system, built on real elements, can differ significantly from the calculated ones.So in a stable, according to a linear model, system, undamped oscillations can be observed. A small error in the processing of input actions in the linear model turns out to be much larger or even increases indefinitely. The transient process in a real system can be much longer than in a linear case. An astatic system in a linear approximation can in reality work out actions with a constant steady-state error. The reason for all these phenomena is the discrepancy between the properties of real elements and their linear model adopted in the calculation.