Robotics manipulators may exhibit flexibility that adversely affects the dynamic and even static performance of controllers designed for rigid robots. Full state measurement is costly, so that part of the state must often be reconstructed. We present the design of two observers: 1) a variable structure observer requiring the measurement of link positions to estimate the full state of a flexible joint manipulator; and 2) a reduced adaptive observer that requires the measurement of link and motor positions. Stability and robustness properties are established and we propose a methodology to design the variable structure observer by solving simultaneously a Lyapunov equation and a matching condition. Simulation results are provided to evaluate their performance.
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