Narrowband Damping with Tuned Vibration Absorbers

The primary reason why Newport RS Series passive damped optical tables are superior to all other products is their use of tuned vibration absorbers for narrowband damping. Commonly called "Tuned Dampers", these devices selectively eliminate a fundamental structural table mode and its entire harmonics. Multiple tuned dampers can be selected to eliminate multiple modes along with the resulting mode harmonics. Tuned damping is the most effective means of reducing relative motion across the table surface.

Narrowband "Tuned" Dampers vs. "Broadband" Dampers

Tuned damping techniques use individual mode selected vibration absorbers to eliminate a particular vibration mode of the table. On the other hand, "broadband" damping techniques use one design for all modes to indiscriminately absorb moderate amounts of vibration over the broadband. Using a single design damper is less expensive than specifically selected tuned dampers but not as effective in reducing the amplitude of resonance modes. A popular version of “broadband” damping is using several heavy steel plates positioned around the periphery of the table and supported at the edges by small rubber support pieces, hoping that several resonance frequencies of these tables will somehow interact with the resonance frequencies of the table. This “hit or miss” strategy is seldom effective. This is especially true for the lower frequency resonances found on large tables. In contrast, Newport stocks tuned dampers for large tables down to 10 Hz. For smaller tables, dampers up to 500 Hz are also stocked. Figure 1 compares the typical effects of narrow-band vs. broadband damping on an individual mode.

Newport Tuned Damping - Broadband Damping

Figure 1: Newport tuned dampers (left) concentrate damping where it's needed most, at the frequencies of dominant resonance modes. Since broad-band dampers (right) are designed to provide moderate damping over a wide range of frequencies, they are not as effective at damping the dominant modes of table vibration.

Another problem with broadband dampers is their weight. Since such dampers are not selective, they must be quite heavy to be effective. Unfortunately, this additional weight is added to the edges of the table and can significantly degrade the bending mode performance. The heavy edges and poor bending mode performance greatly increase unwanted relative motion in the center of the table where most experiments are built. Newport tuned dampers are much lighter and produce minimal mass loading impact on the fundamental table modes. Finally, "broadband" techniques use rubber support element that can change over time under load resulting in a degradation of their damping effects and change in stiffness; besides, their properties are strongly temperature-dependent.

Newport "Tuned Dampers" use a rugged, bolted construction that will not degrade over time. They feature a compact inertial mass supported on two sides by steel leaf springs; damping is provided by thin layers of specially engineered highly damped polyurethane elastomer molded with the plate and working in the conditions guaranteeing longevity and stability of properties. More information on design and application of tuned mass dampers can be found in the recent paper^[i]

Figure 2: Schematic of Newport vibration absorber used in tuned damping table systems US patents 8,857,585 et al.

Unlike competitive offerings that can only offer several table tops with obscure names such as "nominal" damping, Newport tables can be tuned damped into thousands of configurations depending on the table geometry and individual experimental need. Most of these configurations fall into two product offerings: RS4000™ and RS2000™. However, custom tuned tables are available to account for special geometries and heavily loaded surfaces.

How an RS Series table is "Tuned"

A vibration engineer trained in structural analysis tunes each RS Series table design. The table is built with provisions for adding the dampers. After construction, the table is tested using modal analysis techniques to experimentally determine the actual resonance frequencies or modes. Then the tuning frequencies of dampers are determined using known optimization techniques. Different table sizes will exhibit different natural frequencies and modes. Even tables that are close in size can have dramatically different modal signatures. This is a primary reason why the "one size fits all" approach to damping is a poor choice.

For example: Newport's RPR Series Industrial and Educational Grade Optical Tables are the mechanical equivalent of the RS Series Passive Damped Optical Tables but without the tuned dampers. The RPR-38-8 (3 ft wide, 8 ft long, 8 in. thick) exhibits a fundamental "bending" mode at 223 Hz. The most effective location for the damper to absorb the bending mode is in each end of the table. In contrast, the RPR-48-8 (4 ft wide) exhibits a fundamental "torsion" mode at 217 Hz. The most effective location for the damper to absorb the torsion mode is in the corners of the table. The plots in Figure 3 show the FEAs for these tables.

Figure 3

First bending mode

The example above shows the basic process used for determining the proper damper and location to eliminate the fundamental mode. Once the dampers are built into the table to attenuate this mode, the table is transformed from an RPR to an RS2000 with two dampers, or to RS4000 with six dampers.

Specialized Tuning

Newport's tuned damping techniques allow the table construction to be tuned for specific applications.

Doubled Tables: Modular Doubled Tables are multiple tables that are doubled together into complex monolithic structures that can exhibit very low frequency fundamental modes. When building these large structures, Newport assembles the doubled tables into their final configuration and tunes them as a monolithic system. Multiple dampers placed in selected locations (usually up to six dampers per section) can be used. Broadband techniques are very ineffective for such complex low frequency structures.

Large Table Loads: Tuned dampers are heavily damped, and their tuning is not “sharp” so that they stay effective in a frequency range centered around the tuning frequency. The performance is not significantly affected unless the equipment load exceeds 30% of the table weight. For example, an RS4000-48-12 weighs close to 1200 lbs - if the table load starts to exceed 360 lbs, the tuned dampers will be slightly less effective. In these cases, Newport will simulate the positional load and specially damp the table to the application requirement. Fortunately, most optical table experiments rarely exceed 30% of the table mass.

Special Shapes: Table structures with special shapes, cutouts, multiple levels and other custom features are modeled and tuned using the same process described above.

On-site tuning: Newport tuned mass dampers can be tuned or adjusted on-site. All it takes is to provide special removable corner pieces of the table.

Advantages of Tuned Dampers

Eliminate fundamental mode resonances
Offer much better vibration absorption at low frequencies
Lighter weight does not mass load table construction
Rugged bolted design with the damper mass supported by leaf steel springs is thermally stable and does not deteriorate in time
Can be adjusted or re-tuned in the field (with special table corner covers providing accessibility)

Problems with "Broadband Dampers"

Less effective in reducing vibration amplitudes
Does not work well for large tables with low frequency modes
Heavy weight reduces table stiffness-to-weight ratio
Rubber support pieces can degrade with time and change properties with temperature

^[i] V.M. Ryaboy, Practical aspects of design, tuning and application of dynamic vibration absorbers, Proceedings of Meetings on Acoustics, 26, 065006 (2016); https://doi.org/10.1121/2.0000231 .