Vessel sealing and dissection using ultrasonic transducers provides good performance over conventional electrosurgery. The purely mechanical action of the ultrasonic actuator eliminates the passage of electric current through the patient. A good power regulation ensures great precision and proper surgical jobs. To achieve this, precise amplitude control is needed.
5.1.1 Characteristics of the PT
Figure 97. Electromechanical model of the piezoelectric transducer at resonance.
A number of equivalent circuits have been developed over the years for PT [104]. In the vicinity of resonant frequency, the most commonly used model is the Butterworth-Van Dyke (BVD) model [105]. The PT in resonant mode can be modeled as
* Part of this section is reprinted with permission from X. Liu, A. Colli-Menchi, J. Gilbert, D. Friedrichs, K. Malang, and E. Sanchez-
Sinencio “An automatic resonance tracking scheme with maximum power transfer for piezoelectric transducers,” IEEE Transactions on Industrial Electronics (TIE), vol. 62, no. 11, pp. 7136-7145, Nov. 2015. Copyright [2015] by IEEE.
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a band-pass filter with a high quality factor Q. The electromechanical model used is shown in Figure 97, where Cm, Rm, and Lm in the mechanical motion branch represent
the compliance, loss, and mass of the PT. Cp represents the capacitance of the electrodes
upon PT. With sufficient cooling and regulated output power, its temperature coefficient can be neglected. In this application, Cp >> Cm. Rm also indicates the mechanical
loading. The impedance of the PT in resonance is expressed as,
𝑍𝑍𝑐𝑐𝑖𝑖(𝑠𝑠) =𝑠𝑠𝐶𝐶1 𝑐𝑐 �𝑠𝑠2+ 𝑠𝑠 𝐸𝐸𝑝𝑝 𝐿𝐿𝑝𝑝 + 1𝐿𝐿𝑝𝑝𝐶𝐶𝑝𝑝� �𝑠𝑠2+ 𝑠𝑠 𝐸𝐸𝑝𝑝 𝐿𝐿𝑝𝑝+ 𝐶𝐶𝑝𝑝+ 𝐶𝐶𝑐𝑐 𝐿𝐿𝑝𝑝𝐶𝐶𝑝𝑝𝐶𝐶𝑐𝑐� (51)
where two natural frequencies, resonance and anti-resonance, can be extracted as,
𝜔𝜔0,𝑅𝑅 = �𝐿𝐿 1
𝑝𝑝𝐶𝐶𝑝𝑝; 𝜔𝜔0,𝐸𝐸𝑅𝑅 = �
𝐶𝐶𝑝𝑝+ 𝐶𝐶𝑐𝑐
𝐿𝐿𝑝𝑝𝐶𝐶𝑝𝑝𝐶𝐶𝑐𝑐 (52)
where 𝜔𝜔0,𝑅𝑅 represents the correct longitudinal mechanical resonant frequency as shown in Figure 98. At this mechanical resonant frequency, the PT appears as a damping resistor Rm in parallel with the capacitor Cp, maximizing the amount of electrical power
converted to mechanical motion.
5.1.2 Design Challenges
Piezoelectric transducers are widely used as mechanical actuators to convert electrical signals into precisely controlled physical displacements for various purposes, such as vibrating air, moving material, and generating heat [106]-[110]. The main
challenge is to generate the mechanical power in the desired PT resonant mode with high electrical efficiency. Theoretically, the PT converts electrical real power into mechanical
motion; however, some energy can easily be dissipated due to the reactive elements of the transducer. Thus, for a high-efficiency system, the PT must be driven in the correct resonant mode to minimize its reactive power and realize maximum power transfer (MPT) [111]. There are multiple methods to drive the PT in resonance, including power factor correction (PFC)-based [106], [112]-[115], and phase-locked loop (PLL)-based [107]-[110], [116] solutions. PFC-based systems require additional reactive components and complicated compensation to minimize the reactive part of the PT impedance and thereby put the PT into resonance. PLL-based systems drive the PT in a closed-loop. However, they have a limited lock-in range and require a complex compensation to stabilize under large loading conditions [109].
The second challenge is that the PT has multiple resonant modes, which shift with load variation. For various loading conditions, the PT should be tracked in the designed resonant mode and not fall into other undesired resonant frequencies. Therefore, complex frequency or phase discriminators for driving signals are needed [115]. The third challenge is to precisely control the amplitude of the PT displacement and ensure proper mechanical functions. Thus, the electrical power delivered to the PT needs to be accurately regulated. Regulating schemes such as Burst-mode control have been proposed to achieve good efficiency at light load conditions. However, these regulating schemes did not sufficiently improve the PT wake-up time [117].
Different from the aforementioned solutions, this work proposes a band-pass filter (BPF) oscillator-based automatic resonance tracking scheme which is
conventionally used in an atomic force microscope (AFM) [118]-[120]. In this 140
electrosurgical scenario, the tracking scheme utilizes the intrinsic mechanical
characteristics of the PT as a BPF in the oscillator [121]-[122], providing automatic and accurate resonance tracking regardless of device variations and environmental
interferences. Therefore, maximum electrical power is converted into mechanical motion. In addition, the reuse of the PT as the BPF prevents any undesired resonant modes, eliminates the frequency discriminator, and features much less complexity. A switching power stage with high power efficiency is proposed to regulate the output mechanical power. Its amplitude control is implemented by a closed-loop architecture with negative feedback [123]. The controlled signal is the resonant current
corresponding to the mechanical motion of the PT.
Figure 98. Architecture of the ultrasonic vessel sealing and dissecting (UVSD) system. The proposed BPF oscillator-based scheme is illustrated for an ultrasonic vessel sealing and dissecting (UVSD) system as depicted in Figure 98, where accurate PT
displacement regulation over a wide range of loads is required. Unlike electrosurgery devices based on joule heating [124]-[126], the PT-based UVSD devices feature outstanding hemostasis and efficient dissections with minimal lateral thermal damage and low smoke generation. Furthermore, it has no risk of electrical current flowing through the patient [127]. The PT has both longitudinal and transverse resonance modes. Only the longitudinal ultrasonic wave can be transmitted by the waveguide and used for mechanical operation. Thus, the proposed controller automatically drives the PT in such mode and regulates the oscillating amplitude. The unwanted transverse mode is
prevented by the BPF oscillator detailed in Section 5.2.2.2. The surgical sealing and dissecting for blood vessels and tissue are achieved by the tip at the other end of the waveguide as illustrated in Figure 98.
5.2 Discrete Version: Automatic Resonance Tracking Technique