Active vs. Passive
Some applications require the performance that an active system can provide. Other applications that are not as sensitive can suffice easily with a passive system. So how do you decide which is more appropriate for a given application? The following is a brief examination of the advantages and disadvantages of active and passive isolation. Given the wide variability in technology and performance among active vibration control systems, we’ll constrain our discussion of active systems to Table Stable products.
Passive isolation systems, as they are simple mechanical structures, have natural resonances. So that they may begin isolating at lower frequencies, passive systems are designed to shift their natural resonance to as low a frequency as possible. This way the systems can begin isolating at lower frequencies and thus provide damping over a wider bandwidth of frequencies. But the resonance still exists at the low frequencies, usually one to three hertz. So if there is a source of low frequency noise, such as horizontal sway in a tall building, then the isolation system will amplify that noise rather than damp it. This can be especially troublesome for instruments that are sensitive to low frequency vibration, such as AFMs or SEMs.
The Table Stable active isolation systems, on the other hand, have an internal feedback loop which eliminates the systems’ low frequency resonance. This allows them to begin isolating at sub-hertz frequencies. The feedback loop also performs ‘housekeeping’ in higher frequencies ensuring predictably high levels of isolation at all frequencies. The lack of resonances is a definite benefit to those who have AFMs or SEMs or who have low frequency noise in their environment.
Due to the low frequency resonance of passive isolation systems, they do not provide damping at the lower frequencies. However, once they begin damping (usually in the four to five hertz range) the performance picks up quickly and can begin providing significant attenuation within a few hertz. For example, the air-based DT system begins isolation at four hertz and provides ninety percent attenuation by 6.5 Hz. Further along the spectrum, there can be other mechanical resonances in a passive system that decrease the level of damping. These are usually only significant in a few cases; air-based passive isolation systems provide a generally high level of performance at the higher frequencies.
As mentioned above, the Table Stable active isolation systems have a feedback loop which removes its resonances, so they have a smooth roll-off into the higher frequencies. Once the active systems achieve their peak performance at 10 Hz, this high level of reduction is maintained into higher frequencies. Excluding the issue of occasional resonances, the passive systems and active systems both provide 98 – 99% attenuation moving past 30 Hz.
The performance of passive isolation systems is dependent on the size, weight, and center of gravity of the instrumentation loaded on top. The instrumentation loaded on top of the passive system changes the mechanical structure of the system. Since passive systems rely on their mechanical structure for their isolation performance, addition of equipment shifts the resonances of the system and alters the performance of the isolation system. A passive system usually achieves peak performance with a load of approximately 80 – 90% of the system’s load capacity. Any departure from that ideal weight will degrade the performance.
Additionally, the size of the mounting platform needs to be proportional to the location of the instrumentation’s center of gravity. Namely, the center of gravity can not be too high in relation to the width of the support structure. The transmissibility graph given for a system is usually derived from data taken during peak performance conditions. So unless the instrumentation conforms to the ideal size, weight, and load distribution for the particular system, you can expect to get less performance than advertised.
The feedback loop on Table Stable active isolation systems compensates for mechanical variations. So, as long as the instrumentation is within the system’s load capacity and footprint, the system will deliver the high levels of performance that is expected from active systems.
WITH ACTIVE AND PASSIVE ISOLATION
Passive isolation systems are usually designed to isolate only Z axis vibrations. So they are unable to handle horizontal vibrations. In the cases that they can also isolate horizontal vibrations, a different isolation mechanism is usually employed. For example, an air table could use an air spring to isolate Z axis vibrations and use rubber damping for horizontal vibrations. When distinct isolation mechanisms are employed, the system is unable to adequately compensate for vibrations that are not purely X, Y, or Z. When looking at a passive system’s transmissibility graph, one should note which axis is being measured. Troublesome vibrations are rarely purely along one axis, so this is an important shortcoming of passive isolation systems.
Table Stable active isolation systems use the same sensing and damping mechanism to isolate in six degrees of freedom (X, Y, Z, roll, pitch, and yaw) and the sensors communicate with one another. This makes it capable of providing the same high level of isolation across all axes.
In designing passive isolation systems, engineers want to push the system’s low frequency resonance as low as possible. The most common way to achieve this is to make the system very soft. Soft systems take a longer time to settle after a major disturbance comes through the system, usually on the order of seconds but sometimes it can take minutes before the system is isolating again. This can slow the process of taking measurements and require the re-start of long scans. Soft systems are also more susceptible to being disturbed by load changes, user bumping, or air currents than stiffer systems.
Because the Table Stable systems have their low frequency resonance damped out, they are able to employ very stiff springs. These springs are roughly 500 times stiffer than standard air-based systems. This means that the systems are less likely to be unsettled and, when they are, there is a very short settling time, on the order of milliseconds. Instead of needing to wait seconds or minutes to re-start measurements, the settling time of the Table Stable system is almost imperceptible.
The chief advantage that passive systems offer over active systems is that they are less expensive. Rubber pads and mounts can provide a gross level of isolation that is sufficient for less sensitive applications for only a few dollars. Bungee systems, with proper tuning and geometry, can provide a very high level of isolation for under a hundred dollars. Air-based isolation systems can offer isolation that is adequate for a wide range of research instruments for a few thousand dollars. Active isolation systems usually cost over five thousand dollars. For some applications, such as instruments that only cost a thousand dollars themselves, active isolation is simply not a cost-effective solution.
Table Stable active isolation senses and damps vibrations at the level of the instrument. This allows the systems to damp some of the noise that occurs at the instrument, including acoustic noise coupled into the casing, air movements, parasitic noise from cabling, and even some of the noise created by the instrument itself. Passive isolation systems only damp ground-borne vibrations.
Passive systems often require a clean air supply or air compressor in the installation location. If it is a system that is inflated manually, it will require periodic re-inflation using a hand pump. After several years of use, air-based isolation systems usually require the replacement of their air diaphragms.
Table Stable active vibration control systems are designed to run continuously for years with no maintenance. These systems do not require air; their only requirement is 110 – 240 VAC power. If the system is being used in a confined space and heat dissipation is a concern, then ventilation should be used.