The present thesis focused on the problem of observing the stiffness of Variable Stiffness Mechanical Stiffness, whose measure is impossible. Stiffness is in fact a derived quantity and no sensors exist for its direct measure. The main idea for solving such a problem, is designing a non linear observer with linear error dynamics. In the present we tackle the problem of extending such class of observer to locally unobsrvable systems, since our target systems succumb to unobservability conditions. We prove the strength of the method both through numerical and experimental analysis, the former on a simple single spring-damping-mass mechanical system, the latter on a Variable Stiffness Actuator (VSA).