Dynamics of an adaptive vibration absorber

Fraunhofer LBF, Darmstadt, Germany

The study presents the design and a theoretical analysis of a lightly damped adaptive vibration absorber. Vibration absorbers can efficiently absorb vibrations at their characteristic frequency. Therefore, they can be used to suppress vibrations generated by, for example, reciprocating machinery such as internal combustion engines or piston type compressors. This is because such machines generate vibrations at discrete frequencies: multiples of the machine RPM. However, as the RPM will typically vary during the operation, the absorber characteristic frequency should be tuneable so as to respond to the variation of the machine RPM.
In this study a low cost, beam-type vibration absorber is considered. The absorber consists of two masses symmetrically mounted at two ends of a flexible beam. In the middle, the beam is attached to the vibrating machine. The free length of the beam is used to accommodate piezoelectric strain actuators and the two masses at the end are equipped with inertial accelerometers. This arrangement can be used to generate a direct acceleration feedback. Such feedback control generates an active mass effect, which in turn shifts the absorbing frequency of the device. Three different sensor-actuator configurations were analysed theoretically, using a mobility-impedance approach in order to model the passive and active characteristics of the adaptive absorber. In particular, the stability of the feedback loops used is studied using the Nyquist criterion in order to estimate the limits on the tuneable range of frequencies. The analysis included estimation of the influence of small misplacement of the piezo actuators and also the effects due to the dynamical response of the sensors.
The study indicates that the sensors-actuator arrangements should be carefully considered in order to extend the range of tuneable absorbing frequencies. For example, it is shown that although unconditional stability is theoretically possible for ideal placement of the actuators and sensors, in practice the metrics of the feedback loop stability and thus the adaptability of the device decrease due to even small imperfections in the physical arrangement.

Laboratoire de Mécanique des Structures et des Systèmes Couplés - LMSSC