An observer for flexible joint robots

The problem of state observation of robots that have elastic joints is discussed. Outputs are assumed to be the global link coordinates and their time derivatives, and a nonlinear observer is proposed which asymptotically reconstructs all the robot state variables. The dynamic behaviour of the observation algorithm is illustrated by simulation tests referred to a manipulator with three revolute elastic joints. To verify the observer robustness, the previous simulation tests were repeated by using the same observer, designed for a nominal payload of 5 kg, and actual robot payloads of 0 and 10 kg. The differences with respect to the nominal case are not appreciable. However, the steady-state joint errors, which were about 10^-6 rad in the nominal case, became about 10^-4 rad