# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 2 Types, Applications, and Recent Advances of Piezoelectric Shunt Circuits
### Switching Circuits
In contrast to traditional SSDI, which only improves switching at resonance, this adaptive method matches resonance performance while improving off-resonance vibration control by using a gradient-based search to find the optimal switch timing depending on phase and system dynamics. [PERSON] et al. [76] suggested an interconnected SSDI technique that, under ideal phase conditions, reduces resonance peaks by more than 30% by establishing an energy transfer channel between piezoelectric patches. Using an improved SSDI technique with a switching algorithm based on displacement thresholds. [PERSON] et al. [77] proposed a different version of synchronized switching circuits. They developed a method called synchronized switch damping voltage (SSDV) to improve vibration damping in lightweight structures, especially when there is a weak electromechanical link. In contrast to SSDS and SSDI, which depend on the electromechanical coupling coefficient, SSDV enhances damping performance by compensating for poor coupling by raising the piezoelectric voltage [78; 79]. The SSDV can enhance control performance by amplifying the inverted voltage through the use of a voltage source connected in series with the inductor and connected to the shunting branch. However, using a constant voltage source caused a stability problem. To increase SSDV stability, [PERSON] et al. [80] suggested an improved SSDV technique where the voltage is proportional to the vibration amplitude. To overcome this limitation, further adaptive SSDV strategies are also proposed [81]. Also, to overcome the drawbacks of SSDI and SSDV, which necessitate frequent readjustments of control parameters, [PERSON] et al. [82] presented a self-powered SSDV (synchronized switch damping on voltage source) circuit for vibration control of motorized spindles by using two voltage sources. Synchronized switch damping on Voltage (SSDV) had been implemented to solve the issue of dependence on the quality factor of the SSDI; however, it introduced a new issue, which is the need for a constant voltage source. A technique known as synchronized switch damping on negative capacitance (SSD-NC) was introduced by [PERSON] et al. [83] to substitute a negative capacitor for the voltage source. This results in decreasing the inherent capacitance of the piezoelectric transducer, and thus increasing the electromechanical coupling factor, which in turn causes the damping to increase [84]. [PERSON] et al. [85] presented a self-sensing SSDNC circuit that eliminates the requirement for additional sensors and controllers by controlling vibration in cantilever beams with just a basic analog circuit. According to theoretical and experimental assessments, running the OP-AMP in saturation as opposed to the linear area greatly improves stability and damping performance, resulting in ideal attenuation that is less sensitive to changes, [PERSON] et al. [86] created a novel technique to improve vibration control performance by creating unsymmetrical bipolar voltage utilizing an SSDNC shunt circuit. This design simplifies the SSDNC set up in comparison to conventional unsymmetrical SSDV circuits by adding a diode and resistance to the negative capacitance circuit. The study offers a general equation for the switching voltage on the piezoelectric actuator and analyzes stability using offset resistance and negative capacitance. Simulation and experiment results are presented to validate the proposed technique. Figure 7 shows that it can achieve better vibration suppression performance compared to traditional control systems.