# Shunt Damping Vibration Control Technology: A Review
## 7 Challenges in Shunt Damping Vibration Control
### Dual-Functional Energy Harvesting and Shunt Damping  
In principle, shunt damping vibration control technology uses a smart transducer and a shunt circuit to realize vibration control performance. Smart transducers can transform the kinetic or vibrational energy into electricity, and the shunts (passive and active) regulate the electrical signal to obtain a considerable vibration attenuation performance. In view of energy conservation, this transformed electricity can be harvested [137]. Consequently, shunt damping technology is dual-functional as vibration control and energy harvesting [138]. Similarly, the dual-functional feature discussion of the shunt damping method can also be classified into passive and active.  
As to the traditional passive shunts, the RC and RL circuit can be used to construct a two-order resonant circuit to harvest vibration energy [139]. On the other hand, this kind of circuit is a single mode shunt for control of structural vibration. [PERSON] et al. [140] presented a multimode vibration control by using the controlled harvesting and storage of vibration energy as electrical charge. The stored energy can then be recycled enabling the system to achieve a vibration reduction superior to that of a traditional semi-active system and approaching that of a fully active system. An array of one or more pre-charged capacitors is employed to provide a selection of various voltages, which may be selected to approximate a desired control signal. The capacitors can apply a control voltage to the piezoelectric actuators and can also collect current generated by the actuators as the structure strains in vibration. [PERSON] et al. [141] discussed the vibration damping and energy harvesting of linear motion electromagnetic devices. [PERSON] et al. [142] utilized a piezoelectric film spring and a distributed mass layer suitable for the attenuation of surface vibrations and to convert a portion of the absorbed energy into electric power. [PERSON] and [PERSON] [143] investigated dual-functional energy-harvesting and robust vibration control by integrating tuned mass damper and electromagnetic shunted resonant damping.  
The mechanical energy transferred from the vibration source to the control system, and the electrical energy flowed from the smart transducer to the active shunt circuit. For the active shunt, there are two kinds of energy flowing and exchanging: the transformed energy from mechanical vibrations and that provided by the external source. [PERSON] at al. [144] designed an experimental system to determine the energy flows in the negative capacitance shunt. The result showed that the amplitudes of the electrical power were almost 100 times higher than the amplitudes of the input mechanical power. It was pointed out that such a high value of input mechanical and electrical power was caused by: (1) a relatively low electromechanical factor of the piezoelectric transducer, (2) the capacitance nature of the piezoelectric transducer, and (3) wall-plug efficiency. After that, [PERSON] et al. [145] established an experiment to analyze wall-plug efficiency by using linear and switching synthetic negative capacitance. The results demonstrated that more that 85% of electrical power was dissipated on the transistors of the linear amplifier. The switching amplifier can reduce the input electrical power by more than 90%, but it made the circuit complicated. The energy flowing discussion on active shunts can help researchers design energy-saving devices and make the active shunt more suitable for industrial applications.  
Finally, the investigation of energy harvesting of the piezoelectric and electromagnetic transducers and their vibration control effect have great potentials for future study. Furthermore, one should also pay attention to self-powered shunt damping by using energy harvesting technology [146; 147; 148; 149].