Iterative Learning Control -- Deep Dive

The stability and convergence of an Iterative Learning Controller (ILC) may be assessed either by directly iterating the equations for a variety of inputs, or by finding the eigenvalues of the iterated system, or by forming the Z-transform and applying pole-zero or equivalent root locus. Two often-used criteria are (i) Asymptotic Convergence (AC) of the difference vectors, and (ii) mono-tonic convergence (MC) of the vector norm. The latter (MC) has a Z- domain counterpart. In this paper we apply all three methods and both convergence tests to a simple plant with an ILC wrapper. One, two and three-term learning functions are used. We can then ask the questions: do all the tests work, and do they agree on the stability?