Nonlinear proportional-integral-derivative (NPID) control is implemented by
varying the controller gains as a function of system state. NPID control h
as been previously described and implemented, and recently a constructive L
yapunov stability proof has been given. Here, NPID control analysis and des
ign methods are extended to tracking, and to systems with state feedback an
d integral control, Experimental results are presented showing improved tra
cking accuracy and friction compensation by NPID control.