Modeling and Temperature Control of Retinal Laser Therapy

Abstract

The development of a noninvasive technique to measure tissue temperature during retinal laser treatment allows feedback control approaches to regulate the temperature rise to desired values. The main challenge is to provide fast and consistent good control performance regardless the uncertainty of the dynamics of the temperature increase at the different irradiated spots on the retina, which is due to the large variance of the retinal light absorption. In this paper, we demonstrate a successful experimental application in ex-vivo of robust H∞ PID control to handle such a control problem. The system input is the applied laser power and its output is the temperature increase. Based on measurements of input-output data, we employ system identification to model the range of the system dynamics at different retinal irradiation sites. Then, we use a loop shaping approach to express the performance specifications of the closed-loop system and we synthesize accordingly the controller using efficient robust H∞ synthesis tools for fixed structure controllers. The experimental implementation of the closed-loop system for tracking different reference temperatures demonstrates the achievement of the control objectives consistently at the different irradiation sites.