Iterative Learning Control -- Gone Wild

TL;DR

This paper revisits iterative learning control (ILC), revealing long-lasting transients and introducing soliton-like solutions that provide new insights into the long-term behavior of ILC systems.

Contribution

It introduces soliton solutions to ILC equations, offering a novel perspective on the transient phenomena and long-term dynamics in both causal and noncausal learning.

Findings

Long transients can persist despite asymptotic stability.

Soliton-like solutions satisfy ILC recurrence equations.

New insights into causal and noncausal ILC behavior.

Abstract

Before AI and neural nets, the excitement was about iterative learning control (ILC): the idea to train robots to perform repetitive tasks or train a system to reject quasi-periodic disturbances. The excitement waned after the discovery of "learning transients" in systems which satisfy the ILC asymptotic convergence (AC) stability criteria. The transients may be of long duration, persisting long after eigenvalues imply they should have decayed, and span orders of magnitude. They occur both for causal and noncausal learning. The field recovered with the introduction of tests for "monotonic convergence of the vector norm"; but no deep and truly satisfying explanation was offered. Here we explore solutions of the ILC equations that couple the iteration index to the within-trial sample index. This sheds light on the causal learning - for which the AC test gives a repeated eigenvalue. Moreover, since 2016, this author has demonstrated that a new class of solutions, which are soliton-like, satisfy the recurrence equations of ILC and offer additional insight to long-term behaviour. A soliton is a wave-like object that emerges in a dis-persive medium that travels with little or no change of shape at an identifiable speed. This paper is the first public presentation of the soliton solutions, which may occur for both causal (i.e. look back) and noncausal (i.e. look ahead) learning func-tions that have diagonal band structure for their matrix representation.