Assessment of fractional order impact on performance of fractional ILC controller for upper limb exoskeleton

Abstract

In this research paper, the application of Iterative Learning Control (ILC), an intelligent control method, is suggested in the form of a fractional‐order PD‐type controller. The main task of the ILC controller is to reject process model uncertainties, which are often present in complex systems such as various multibody systems, and to sequentially reduce a trajectory tracking error. As a control plant, an exoskeleton support arm with three degrees of freedom is used herein. The control scheme consists of feedback linearization compensating for the known part of the dynamics model and the feedforward ILC controller of the PDα‐type. The feedback part of the control system is the classical PD controller. The feedforward control signal is filtered with a lowpass filter to avoid divergent behavior as iterations progress. Finally, simulation results are presented to demonstrate the proposed control system performance applied to the chosen control plant, as well as the achieved error convergence towards the steady‐state value for various values of the fractional order.