21.4 A reduced-order sliding-mode controller with an auxiliary PLL frequency discriminator for ultrasonic electric scalpels

Piezoelectric transducer (PT) is an emerging energy-based technology for electrosurgery. With proper driving signals, the PT is utilized as an electric scalpel. It converts an electric signal into physical displacements for various electro-surgical functions, such as ultrasonic vessel sealing and dissecting. The major challenge is to precisely regulate and quickly build a mechanical power up when changing between various surgical functions. The pulse width modulation (PWM) mainly bases its design on a small signal model, thus, does not often fit for the large signal transient satisfactorily. This problem becomes even worse when changing load impedance due to the pressure upon the surgical tip. The sliding mode control (SMC) has reliable and fast performance for nonlinear large signal applications. However, the SMC cannot be simply applied since the electric scalpel is significantly different from generic DC-DC converters. In this paper, a SMC controller for an electric scalpel system is designed for the first time, featuring compact size, automatic resonance tracking, and fast response for electrosurgical operation. The architecture of the electric scalpel system has three nested loops: Loop I is a bandpass filter-based oscillator (BPO) for power generation. The conventional close-loop driving solution dynamically senses the PT's impedance, executes the power-factor correction (PFC), and tracks the target resonance . However, the complex sensing causes delay and does not fit for drastic load changing. The BPO method utilizes the PT's intrinsic nature as the bandpass filter, automatically captures the dominant resonance, and features a simpler architecture with less hardware cost. For power regulation, Loop II is a current-fed DC-AC converter with an enhanced SMC that achieves high regulation accuracy for steady-state and fast response for large load transition. Loop III utilizes a phase-locked loop (PLL) to discriminate the non-longitudinal modes in the time-domain, and prevents the scalpels from being stuck. This method features higher sensitivity than the conventional filter based approach. Compared with the discrete solutions, the monolithic integration reduces the delay of Loop I and enhances the Q and resonant efficiency. Moreover, the conventional bulky system with discrete mainframe box can be integrated into the scalpel's handler, improving the feasibility for the minimum invasive surgery.