# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 2 Types, Applications, and Recent Advances of Piezoelectric Shunt Circuits
### Single-Mode Resistive (R), Resonant (RL), and (RLC) Circuits  
[PERSON] and [PERSON] [16] proposed the concept of dissipating mechanical energy by a piezoelectric transducer that is shunted to a passive electrical circuit where the piezoelectric transducer is shunted to a resistor. The piezoelectric transducer behaves as a viscoelastic damper in this case. Where the mechanical energy that is converted to electrical energy will be dissipated as heat from the resistor, in most applications, the resistive shunt is the least considered due to its poor performance. In another case, they implemented a series resistor-inductor circuit (resonant) in the system. This configuration can create an electrical frequency and an impedance in the electric circuit, so it can be tuned to equal the structure's natural frequency for a single mode, thus damping the highest vibration amplitude by inducing antiresonance at the natural frequency and minimizing the capacitance of the piezoelectric patch. Moreover, [PERSON] and [PERSON]'s approach came with a limitation: damping a low-frequency resonance requires a low electrical resonance, which requires large inductors. The difference between the resistive and resonant circuits was further investigated by [PERSON] et al. [17]. They found out that while the RL-shunt is the most effective, the R-shunt works well for structures with minimal damping. It is simpler to set up and adjust, making it a viable option. And the most important parameter that controls the performance of the shunt, besides the tuning, is the electromechanical coupling factor that depends on the piezoelectric patch's ability to convert mechanical to electrical energy. Countering the large inductance problem, [PERSON] [18] demonstrated a passive technique using a multi-layered piezoelectric transducer for suppressing vibration in flexible structures. The purpose of utilizing a multi-layered piezoelectric element was to improve its capacitance, removing the necessity for a high inductance value in the circuit. Another solution for the large inductance problem was using a synthetic inductor, that by using operational amplifiers a large value of inductance can be emulated with lower energy consumption. The solutions for the large inductance were not confined solely to the piezoelectric patch aspect. Other researchers suggested improving the inductor's internal design (magnetic core) [19]. Different connections of RL circuits (in series and parallel) were investigated in [20], demonstrating a great damping performance by both configurations and especially by the proposed parallel connection that was invented by [PERSON] [21]. Other researchers tried to evaluate the performance of RL circuits under different circumstances and how the tuning is going to be affected. [PERSON] et al. [22] presented a resonant shunt that can maintain its tuning under different temperatures by using a variable inductor that has a magnetic core that is influenced by the variation in temperature. [PERSON] [23] presented a resonant circuit that damps the vibration of a cantilever beam under flow-induced vibrations in a water tunnel. The ability to work under different circumstances made the utilization of RL circuits in damping vibration presented in many applications, such as rotor and machine blades [24; 25], and aircraft structures [26; 27]. While RL shunt circuits have shown promise in improving vibration damping, their effectiveness is highly dependent on selecting the appropriate electrical parameters: resistance (R) and inductance (L). To maximize energy dissipation, aligning the electrical and mechanical resonance frequencies is a must. Many researchers worked on the optimum tuning of the shunt circuit [28]. The first two optimization techniques that were proposed for tuning the circuit were presented by [PERSON] and [PERSON] [16].