Passive vibration isolators versus active vibration isolators
Despite the fundamental differences in the internal design of passive and active isolators, the transmissibility curves are basically very similar. A typical curve is shown in Figure 2.
The transmissibility curve is characterized by a significant peak at the resonance frequency. At that frequency, the isolator actually amplifies the transmission of vibrations. For lower frequencies, the transmissibility approaches the value of 1 (0 dB), and above the resonance, the transmissibility is declined more or less.
The frequency range can therefore be divided into an amplification range and an isolation range, with the isolation range beginning above the resonance frequency (to be precise, at 1.41 times the resonance frequency). For this reason, the resonance frequency of isolators is an important design criterion, which should be as low as possible. Typical air spring isolators achieve resonances of approx. 2.5 Hz and isolate from 3.5 Hz. With simple rubber buffers, the isolation only occurs above 10 or 15 Hz.
The effect of the damping can be seen in Figure 3. While the resonance amplification is reduced by damping – but still remains above 1 and thus amplifying ground vibrations – the isolation is strongly diminished because the transmissibility is more flatly shaped. High isolation velues like -40 dB are therefore hardly reached. When choosing the damping, there is a conflict of goals that cannot be resolved in a passive manner.
Active vibration isolators such as the Seismion Reactio are also mechanically based on an elastic mount. However, there is also an active control system, which is set up as a classic feedback control. Especially developed acceleration sensors with piezoelectric ceramics monitor the amplitudes of the isolated platform with high precision. These measured signals are converted into the required actuation forces by an analog control system, which drive contact-less voice coil actuators. Multiple of these active control loops are in action in order to isolate all six degrees of freedom.
The basic idea of active control is to reproduce a so-called „sky-hook damper“ (Figure 4). Unlike in the passive system, this damper is not coupled to the (vibrating) floor, but to a virtual, stationary attachment point. Therefore, no further disturbances from the ground are introduced into the system via this virtual damper, and the sky-hook damper only has advantages for isolation and stabilization purpose. The corresponding transmissibility curves in Figure 5 show that the resonance can be completely suppressed by the sky-hook damping, and the isolation begins at significantly lower frequencies.
An ideal sky-hook damper is practically impossible to realize for several reasons. Active systems therefore also posses resonance amplifications, which, however, are well below 1 Hz in the case of Seismion isolators.