Performance of ILC applied to a flexible mechanical system

ILC is traditionally applied to systems where the controlled variable is the measured variable. However, in standard industrial robots the motor angles are measured, while the control objective is to follow a tool path. Assuming that the mechanical flexibilities of a modern robot are concentrated to the joints (elastic gearboxes), a flexible two-mass model can describe a single joint. A P-ILC algorithm is applied to the two-mass model, based on only measured angle of the first mass (motor angle) or estimated angle of the second mass (tool angle). Robustness of the algorithm and performance when model errors are introduced are discussed considering the tool-angle error. First, it can be concluded for a flexible system that the motor-angle reference characteristics are essential for the performance when the tool angle cannot be measured. Second, using a tool-angle estimate instead of the explicit motor angle in the ILC update reduces the tool-angle error significantly.