Closed-loop Reference Models for Output-Feedback Adaptive Systems
  Closed-loop reference models have recently been proposed for states accessible adaptive systems. They have been shown to have improved transient response over their open loop counter parts. The results in the states accessible case are extended to single input single output plants of arbitrary relative degree. 
Travis E. Gibson, Anuradha M. Annaswamy and Eugene Lavretsky
math.OC cs.SY nlin.AO