# Shunt Damping Vibration Control Technology: A Review
## 1 Introduction  
Some smart materials and structures that possess unique electromechanical characteristics have been widely used for active vibration control, such as magneto-rheological fluid damper [1], piezoelectric damper [2; 3; 4; 5], shape memory alloy [6; 7] and eddy current damper [8; 9; 10; 11]. The active vibration control system consists of actuators, controllers, and sensors. The controllers regulate the vibration response measured by sensors, and the output control signal is the feedback to the actuators after power amplifying. The active method can be adapted online, and the control efficiency and accuracy are higher than the passive one. The key work of the active control is the design of the control algorithm, such as, velocity feedback [12; 13], PID [14; 15], the feedforward [16; 17; 18], etc. With the development of the computer and analog integrated circuit technology, some advanced and intelligent algorithms have been widely used in vibration control engineering, such as, the adaptive [19; 20], fuzzy-logic algorithm [21], genetic algorithm [22; 23; 24; 25], ant colony optimization algorithm [26], etc. The hybrid intelligent algorithms can also be employed to optimize parameters of the controller, so that better vibration control performance and robust can be achieved. However, these intelligent algorithms inevitable increase the requirement of the processing speed and memories of the controller. Furthermore, the stability, reliability and robust of the control system should be taken into consideration when complicated and intelligent algorithms are used. The shunt damping technique involves the connection of an electrical impedance to the terminals of a transducer. Over the past several years the shunt damping vibration control technique has been widely studied [27], and it can be mainly classified into two categories shown in Figure 1. The first one is piezoelectric shunt damping (PSD) [28]. The piezoelectric transducer is bonded to a host structure. As the host structure vibrates, the transducer produces an electricity. An external shunt circuit dissipates the electrical energy thereby dissipating the mechanical vibration energy, and the vibration of the host structure is suppressed. Figure 1a shows a simplified model of the PSD. The property of the shunt circuit determines the vibration control principle of the shunt damping. A resistor connected to the transducer dissipates the kinetic energy like a viscoelastic damper [29]. If the shunt consists of an inductor and a resistor in series, the shunt combined with the inherent capacitance of the piezoelectric transducer creates a damped electrical resonance that can be tuned so that the PSD acts as a damped vibration absorber [28]. Electromagnetic transducers can be used as actuators, sensors, or both. When current is applied to terminals of the electromagnetic transducer, a force is generated. Conversely, when a transducer experiences a velocity, an electromotive force (emf) is induced. Electromagnetic transducers exhibit these similar electromechanical properties but have considerably different physical and electrical characteristics compared to piezoelectric transducers. They have a much greater stroke, typically in the millimeter range. Consequently, in applications where piezoelectric transducers cannot be used, electromagnetic transducers may serve as an alternative choice [30]. Similar to PSD, when a shunt is connected across the terminals of the electromagnetic transducer, the kinetic energy of the vibrating structure can be dissipated through the coupling of the mechanical and electrical fields, the method is electromagnetic shunt damping [31], and a simplified model is shown in Figure 1b. After years of exploration, the parallel RLC shunt [32; 33], the current flowing shunt [34], the synthetic shunt [35], the adaptive shunt [36] and the negative impedance shunt [37; 38] have been studied to improve vibration control performance.