Control Systems/Adaptive Control
Adaptive Controllers 
What we've been studying up till this point are fixed systems, that is systems that do not change over time. However, real-world applications and experience tells us that the environment can change over time: New noise can be added to a signal, the signal quality can be degraded, or the specifications of the signals can be changed at the source.
There is a distinct need then for controllers which can modify themselves to produce the same output regardless of changes to the input. Herein lies the problem of adaptive control. We will introduce some of the concepts in this chapter, and discuss them in greater detail in future chapters.