Vibration Isolation Systems
For many positioning systems of low to moderate resolution, no particular effort must be expended to isolate the positioning stage from environmental vibration.
As the system resolution increases, however, the need to provide isolation from external vibration increases. It is not a simple matter to determine if a system needs isolation, or the degree of sophistication of that isolation, since the problem has a number of contributing factors. Primary among these is the amplitude and spectral content of the background vibration itself. While in some cases, identifying and removing or abating the vibration source(s) can be accomplished, in other cases the sources are an unavoidable component of the immediate environment. The next issue is the set of natural frequencies (resonances) of both the positioning stage and the other structural members of the overall system. In general, we strive to make these as high and well damped as is practical. We are then left with the convolution of the external excitation, whose amplitude and spectral content vary with the various resonances of the stage and structural members. The resulting unwanted relative motion between the customer payload on the stage, and the customer process (optical head, SEM column, etc.) is then evaluated relative to the application requirements for stability. If system performance is degraded in an unsatisfactory manner, then some means of attenuating the external vibration sources is indicated.
Isolation techniques vary widely. In less demanding applications, simple rubber mounting devices provide adequate relief. More commonly, active pneumatic isolation modules are used as three or four supports for the system. Each of these includes a low frequency (1-3 Hz) horizontal and vertical isolation mechanism, together with a mechanical pneumatic servo valve to maintain a level condition. These systems provide substantially lower natural frequencies, and therefore greater attenuation at the frequencies of interest, than simple rubber isolators. Unlike leadscrew driven stages, which are relatively insensitive to platform tilts, linear motor systems see a tilt as a direct force on the servo loop, of magnitude: Mass x sine (q). Conflicts with traditional pneumatic isolators have arisen as a result of the recent trend towards the use of linear motors and higher speeds. More advanced isolation systems augment conventional pneumatic isolators with multiple axes of linear motors and velocity sensors. These can maintain a “rigid” yet isolated platform in response to background vibrations and stage movements. Anticipating accelerations, forces, and center of gravity shits, sophisticated DSP-based controls can also communicate with the stage motion controller and compensate accordingly. Our Design Engineers can help advise you on the appropriate isolation solution for your application, as well as provide complete systems consisting of an integrated and tested stage and isolation system.
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