# Semi-Active Vibration Control Based on a Smart Exciter with an Optimized Electrical Shunt Circuit
## 1 Introduction  
Flight comfort is greatly influenced by the noise level in the aircraft cabin. In this respect, it is always an important aim to reduce the level of noise in the cabin, which is produced by the propulsion system and aero-acoustic effects. However, as always in aviation, having a lightweight solution is crucial. Hence, the study of an effective vibration control system which offers a low weight is of significant interest for practical aircraft design. A smart exciter (SE) [1] attached to the cabin sidewall panels can function as a new type of loudspeaker for emergency announcements in the aircraft cabin [2]. Simultaneously, it may act as a semi-active system for the reduction of vibrations in aircraft structures. The advantages of using a SE as a vibration reduction device are: (i) it adds almost no additional weight to the cabin, (ii) it requires no additional power supply (if used for semi-active noise control) and (iii) several single systems can be used at the same time without cross-coupling effects (known from multiple-input-multiple-output (MIMO) systems applied for active control of sound) because noise reduction is obtained by dissipation of kinetic energy. In addition, it is easier to adapt to different problems than a purely active system, as described in [3, 4], due to the missing cross-coupling as well as its low energy and cost requirements [5, 6].  
Semi-active systems are based on a semi-active damper whose normal force is adjustable to velocity or displacement control. Applications are known from semi-active vibration isolation [7; 8]. Other examples of this concept can be found in the form of the Relaxation-Isolator (RI), Piezoelectric Shunt Damping (PSD) and Electromagnetic Shunt Damping (EMSD) [9; 10]. Figure 1a shows the mechanical model of a RI. In this system, the damper, \(c\), and the spring, \(k_{1}\), on the right-hand side are used to reduce the vibration of the object, \(M\). The effect of damping is concentrated at the resonance of the system. In Figure 1b,c, simplified models of a PSD and an EMSD are shown. Piezoelectric transducers [9; 11; 12; 13] and electromagnetic transducers [14; 15] function similarly. Both transform kinetic energy from the structure into electrical energy, which is then dissipated in an electrical shunt with a resistor that generates thermal energy. In this way, structural vibration as well as sound radiation are controlled. In recent years, the shunt damping has been widely studied because of its flexibility [16; 17; 18; 19; 20].  
In comparison with the PSD, the SE with electrical shunt (EMSD) can also be used as an emergency loudspeaker. Furthermore, it has better broad-range absorbing effectivity compared to the classical RI. For this reason, the vibration reduction potential of a SE acting in combination with an optimized damping shunt (a special case of EMSD) has been investigated.  
The results of this investigation will be presented in this paper, which is organized as follows. Section 2 describes the simplified SE with an electrical shunt. Results of numerical simulations of the SE with an electrical shunt attached to a lightweight panel (using MATLAB/Simulink and COMSOL) will be presented in Section 3. Section 4 reports on experiments that have been performed to experimentally verify the vibration reduction potential. Finally, the conclusion and an outlook on future research directions are summarized in Section 5.