A time-varying fuzzy on-off control system with application to the control of tissue temperature during laser heating

Laser thermal therapies have shown great promise for minimally invasive treatment of benign and malignant tissue lesions. The paper presents a time-varying fuzzy control laser system to control tissue temperature in fresh pig liver samples for coagulation at 65°C and tissue welding at 85°C. The technical difficulties of our study lie in two facts: (1) laser tissue heating is a highly nonlinear time-varying dynamic process, and (2) the output power of the 810 nm diode laser that we used is not adjustable during operation. We have overcome the difficulties by regulating the laser via a variable on-off signal sequence, which is computed, at each sampling time, by our nonlinear time-varying fuzzy controller. The structure of the fuzzy controller is analytically analyzed and the result is related to the PD controller. Our experiments using the fuzzy control system show that the average of the absolute values of the maximum temperature derivations for three coagulation experiments and three welding experiments is 0.45°C and 2.90°C, respectively