Guaranteed Transient Performance with $\mathcal{L}_1$ Adaptive   Controller for Systems with Unknown Time-varying Parameters: Part I
  This paper presents a novel adaptive control methodology for uncertain systems with time-varying unknown parameters and time-varying bounded disturbance. The adaptive controller ensures uniformly bounded transient and asymptotic tracking for system's both signals, input and output, simultaneously. The performance bounds can be systematically improved by increasing the adaptation gain. Simulations of a robotic arm with time-varying friction verify the theoretical findings. 
Chengyu Cao and Naira Hovakimyan