Error coefficient and nonunity feedback revisited

A review of the concepts of type, error coefficient, and steady-state error is presented from the point of view of usefulness in applications and relevance in an introductory control systems course. The approach employed makes a clear distinction between system error and loop error. It is shown that categorizing feedback control systems as unity or nonunity is of limited practical value. Error constants are seen to be valid for both nonunity and unity feedback but are useful only in “simple” systems. A system classification scheme based on this definition of “simplicity” is suggested