Comments on "Impedance control with adaptation for robotic manipulations

The commenters point out an error in the adaptive impedance-control approach for robotic manipulators given in the above-titled paper by W.-S. Lu and Q.-H. Meng (ibid., vol.7, no.3, p.408-15, June 1991). Lu and Meng assert that their adaptive impedance-control scheme can compensate for uncertainty in the force measurements; however, it is shown that it is possible for the uncertainty in the force measurements to cause the parameter estimates to go unbounded. To remedy this possible instability problem, an additional auxiliary controller is suggested. In their reply, Lu and Meng fail to find any error in the Lyapunov analysis of their algorithm. They are not convinced that one should seek a theoretically asymptotically stable solution with a controller that has a highly undesirable feature.>