What is Deviation variable? And what are the uses of these variables?

Ans: Deviation variable is defined as the difference between output signals when error is present and error is zero situations (steady state condition)

P = p - p_s

Where P = Deviation variable

p = output signal when error is present in the system

p_s = output signal when error is zero in the system.

Deviation variable are used to connect the initial or steady state condition of the system to state of the system at particular time.