Performance margins in conventional and second order sliding mode controllers

A concept of the performance margins for systems controlled by conventional and second order sliding mode controllers is proposed. These margins are based on the system´s tolerance to self sustained oscillations with certain amplitude and frequency. The practical performance margins are introduced as the maximal gain increase or/and the maximal phase lag added to the frequency characteristic of the linear part of the open loop system that yield acceptable loss of system´s performance in terms of admissible parameters of self-sustaining oscillations. The method to compute the performance margins is presented. Examples and simulations which verify the utility of the concept are shown.