# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 2 Types, Applications, and Recent Advances of Piezoelectric Shunt Circuits
### Multi-Mode Resonant (RL) Circuits  
In most life applications, systems are going to have multiple modes; damping multiple modes of vibration was a concerning matter in the field of piezoelectric shunt circuits. [PERSON] [43] proposed an approach for suppressing multiple modes using a single piezoelectric element as shown in Figure 1. Additional L-R-C circuit branches are coupled in parallel to an RL branch, this work is considered to be the first in presenting a capacitor in a shunt circuit and also having multiple branches. The number of RLC branches equals the number of modes chosen to be damped. However, it initially lacked a controlled method for tuning each branch separately. Subsequently, researchers tackled this challenge by proposing other circuit configurations to control the branches.  
[PERSON] [42; 44] developed a multi-mode shunt that utilizes a current-blocking circuit. The electrical impedance of the LC anti-resonant circuits is designed to reach infinity and act as an open circuit at other branch frequencies, while the targeted branch that matches the resonance frequency of the structure operates and damps the vibration. On the other hand, [PERSON] et al. [45; 46] proposed using \"current-flowing\" branches in the shunt circuit, as shown in Figure 2. The function of the current-blocking shunt and the current-flowing shunt is similar. The current-flowing shunt permits the current to flow rather than stopping  
Figure 1: The electric circuit proposed by [PERSON] [43].  
it at the natural frequency. This method is better than \"current blocking\" because the latter requires an equal number of inductors for each mode. A distributed array of piezoelectric transducers connected with RL circuits on a cantilever beam is represented mathematically by [PERSON] et al. [47] each patch is adjusted to dampen a specific mode.  
In order to minimize vibrational energy, [PERSON] and [PERSON] [48] proposed a technique for optimizing piezoelectric transducer (PZT) multimode shunt circuits by minimizing the H2-norm of the damped system to obtain ideal resistance values. They achieved considerable mode reduction with reductions of 22 dB and 18 dB in the second and third structural modes, respectively. A neuro-fuzzy controller is proposed by [PERSON] [49] in order to improve the performance of multi-modal shunt damping using piezoelectric patches. The objective of the neuro-fuzzy control system was the activation of the suitable shunt circuit at the required frequency. [PERSON] et al. [50] proposed a tuning approach for a simplified current-blocking shunt circuit, and they introduced a two-port network with an electro-mechanical structure called a fictitious shunt branch. Its objective is to tune the electrical elements of the shunt circuit without involving any optimization algorithm. [PERSON] et al. [51] developed a novel sequential tuning method. Based on the effective characteristics of the piezoelectric system around resonance, the shunt impedance's characteristics, such as electrical damping ratios and resonance frequencies, were derived. [PERSON] and [PERSON] [52] proposed the use of passive control to improve aeroelastic stability in lightweight composite structures. The authors proposed using multimodal shunt piezoceramics in parallel topologies, as shown in Figure 3, to increase the supersonic flutter boundary of composite panels.  
Figure 3: (**a**) Composite plate FE model with PZT element and (**b**) resonant circuit in parallel topology for shunt damping of two modes [52].  
Figure 2: (**a**) Current-blocking circuit and (**b**) current-flowing circuit, [45; 46].