VSS identification approach is based on following concept, i.e. while in sliding motion, the switching of control inputs reflects system uncertainties (unknown parameters). Therefore, if there exist some operations that make the information from the switching control inputs be achievable, then the unknown parameters can be accuratlly identified as far as it is capable of selecting suitable identification mechanisms which can fully make use of the available information.
Two different types of VSS identifiers are taken into consideration. The first type uses adjustable model whose structure is similar to that of identified systems. From the viewpoint of control, this type of VSS identifiers may be regarded as direct identifier because the identified system is handled as an open loop. On the other hand, if the identified system is controllable in the sense of VSS (sliding mode can be generated through chosing control inputs), the second type VSS identifier, the indirect VSS identifier, can be constructed according to the linearized system structure while staying in sliding mode. Therefore it can be applied to some nonlinear systems which are not linear in parametric space by general identification algorithms, whereas linear in parametric space when sliding mode is existed.