# Shunt Damping Vibration Control Technology: A Review
## 5 Nonlinear Shunt  
In some special conditions, the mass, stiffness or damping of a mechanical system may be state-switchable when a structural system suffers irrespective instantaneous changes. This state will result in a large response. [PERSON] et al. [116] presented a kind of switchable capacitor shunt that can change the stiffness of the actuator so as to change the natural frequency of the system. Thus, the vibration control can be achieved by avoiding the resonance shown in Figure 11a. [PERSON] et al. [117] proposed a switchable stiffness element that is obtained through control of the termination impedance of piezoelectric stiffness elements. The change of the stiffness results in the change of the resonant frequencies of the system by incorporating piezoelectric stiffness elements as part of the spring element of a vibration absorber [118]. Using two discrete stiffness states, \(k_{s1}\) and \(k_{s2}\), with \(k_{s1}<k_{s2}\), two resonance frequencies of the absorber \(\omega_{s1}\) and \(\omega_{s2}\) are obtained. Consequently, this switch impedance shunt can change the stiffness of the absorber to change the vibration control performance. [PERSON] et al. [119] developed a switching R-L shunted piezoelectric vibration control method, which works by briefly connecting a series resistor inductor shunt to the piezoelectric actuator to apply the necessary signed charge to allow energy dissipation (shown in Figure 11b). As with introducing the RL shunt, the magnitude of the charge applied to the piezoelectric actuator can be greater than the magnitude of the generated charge.  
The state switching shunt vibration control technique utilizes the concept of energy dissipation through electromechanical materials, which have the ability to change their equivalent effective stiffness. For example, as the piezoelectric transducer is in an open circuit state, it possesses a particular stiffness. Conversely, as the transducer is in short state, it possesses a different stiffness, particularly, a relatively low stiffness. Consequently, many other state switchable shunts were investigated to switch the stiffness or the damping of the system. Figure 11 shows some kinds of switchable shunt circuits that have been thoroughly discussed: synchronized switched damping on a resistance (SSDS), synchronized switched damping on an inductance (SSDI), synchronized switched damping on a voltage source (SSDV), synchronized switched damping on negative capacitance (SSDNC), and synchronized switched damping on negative capacitance and inductance (SSDNCI) [120]. The classical SSD remains in an open circuit for the majority of the vibration cycle and switches briefly to a shunt circuit at every extremum displacement. These researches pointed out that the switch timing was only optimal for excitation exactly at resonance. [PERSON] et al. [121] proposed a self-tuning approach that implemented the more general optimal switch timing for synchronized switch damping on an inductor without needing any knowledge of the system parameters. In order to improve the damping performance, [PERSON] et al. [122] connected SSDI circuit in parallel to a negative capacitance.  
The implementation of the nonlinear switchable shunts seem not to require external power for operation for the switches can be realized with MOSFETs that require almost no power to be switched. An additional piezoelectric transducer can supply the small amount of power required to switch the MOSFETs.