Although friction is a complex nonlinear phenomenon,especially during motion reversal [14], [15], a frictionmodel consisting of only Coulomb and viscous friction,that is,τf ric = fCsign(˙q) + fv ˙q, is an acceptable simplification for many robotics applications[16]. Note that (3) is linear in the unknown Coulomband viscous friction parameters fC and fv and thus is consistentwith the linear model structure.