Passive vs. Active Isolation

Regarding the equations for the transmission it can be seen that passive isolation always exhibits amplification within the resonance and the isolation for higher frequencies decreases because of the viscous damping. The plot below shows the transmission calculated for passive and active isolation. The eigenfrequency has been set to 5Hz which is a very common value for passive isolation systems. Unfortunately the excitation from the building often occurs at the same frequency range. For this reason the remaining amplitude on top of the passive isolation system is higher than the amplitude on the floor of the building.

Another advantage of an active isolation system compared to a passive stage is the short settling time. The red curve of the plot below demonstrates the importance of a low eigenfrequency for passive systems since the isolation effect takes place for frequencies above the resonance. The low eigenfrequency combined with low damping leads to a long time constant. The passive system needs a long time to come to rest again. In the case of the active system the vibration decays much faster because the active system reacts with its actuators. The actuators are generating forces which counteract the movement of the isolated mass.