# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 2 Types, Applications, and Recent Advances of Piezoelectric Shunt Circuits
### Switching Circuits  
In order to control energy flow in piezoelectric shunt circuits, state switching involves alternating a piezoelectric actuator between open-circuit (high stiffness) and short-circuit (low stiffness). Up to four switching events are permitted every cycle by the switching sequence, which is controlled by a heuristic control law. The circuit switches, usually  
Figure 6: A schematic of the shunted piezo/beam system ([PERSON] and [PERSON], 2018). from open to short circuit by connecting an inductor or resistor, at maximum stretch (peak strain). The charge is redirected by this switch, oscillating and reversing its path. The circuit reopens after half a cycle, leaving the piezoelectric patch with an opposing charge that opposes its motion. This helps manage deflection, store energy, and release it when required. Piezoelectric actuators' ability to alter mechanical stiffness between open- and closed-circuit states, which improves electromechanical coupling, is a key feature. Because of its self-adaptive broadband behavior, it is less sensitive to changes in the excitation frequency and electric elements than passive resonant shunting, [PERSON] et al. [63]. State-switching was initially employed by [PERSON] [64] to develop an acoustic driver that operated across a wide frequency range. A similar method called synchronized switched damping (SSDS) was later used by [PERSON] et al. [65,66], who operated an actuator connected to a resistor in a high-stiffness state for the majority of the vibration cycle and then immediately switched to a low-stiffness state to release energy. Other researchers proposed connecting other electrical components in series with the state switch. [PERSON] [67,68] developed a semi-active control method for mechanical systems that makes use of state-switched piezoelectric actuators (SSD). To store and release energy as needed, the piezoelectric actuator is switched between an open-circuit, short-circuit, or resistive-circuit state. [PERSON] et al. [63] introduced a new semi-active piezoelectric damping technique using networking synchronized switch damping (SSD) to reduce vibration in integrally bladed disks (blisks). This method links piezoelectric materials in a network across blade sectors, allowing energy dissipation and redistribution through fewer SSD circuits, lowering system complexity in contrast to a typical SSD. [PERSON] et al. [69] presented a linearization technique for synchronized switch damping (SSD) that reduces the intricate nonlinear behavior of SSD to stiffness and viscous damping coefficients that vary with frequency. The utilization of SSD circuits in several applications was further investigated by [PERSON] et al. [70]. Another method for synchronized switch circuits that use an additional inductor (SSDI) was proposed in [71,72]. Rather than discharging the piezoelectric stored energy during a short circuit, the patches are coupled to an inductor, which allows the voltage to be inverted before being released to open circuit. And by that, the damping mechanism is improved by optimizing the voltage amplitude to be 90 degrees out of phase with the motion. A control method that controls the modal energy rate was proposed by [PERSON] and [PERSON] [73] for the (SSDI). This method implies that the energy is always dissipated during vibration by operating the switch at the appropriate times to keep the modal energy change rate below zero. In order to control blade vibration, [PERSON] et al. [74] introduced a semi-active multi-mode vibration control system. In multi-modal applications, this method reduces excessive switching, enhancing vibration control and energy conversion efficiency. The self-tuning (SSDI) technique, which was presented by [PERSON] and [PERSON] [75], adjusts the ideal switch time for vibration reduction without requiring system parameter knowledge.