Damping Performance Comparison Between Grades
Research Grade: corner compliance data measures the displacement of the table in response to impact by a calibrated hammer. The lack of response below 300 Hz is indicative of extremely high damping and excellent overall structural performance. Compliance was measured on a 48 x 96 x 12 in. table.
Scientific Grade: corner compliance data shows higher peak compliance value than the Research Grade. Compliance was measured on a 48 x 96 x 12 in. table.
Laboratory Grade: Corner compliance data shows higher amplification at the table's resonant frequency. Compliance was measured on a 48 x 96 x 12 in. table.
Structural Damping
TMC has long adhered to the philosophy that dry damping of an optical top is preferable to oil-based dampers. Oil's characteristics can change over time and hidden oil reservoirs are always in danger of being pierced by an end-user customizing his system.
Our approach to damping of structural resonances has consistently been based on a "broadband damping" approach. "Tuned damping," or using a tuned mass-damper to resonate out-of-phase with a top's bending mode, is a risky approach. First, it assumes the damper can be set to exactly coincide with the resonant frequency of the top. An optical top's resonant frequency will vary based on load, distribution of load, temperature, and even the presence of the dampers themselves. Therefore, in practice, it is difficult to tune the dampers to the top's resonance. Furthermore, it assumes that only the lowest resonant frequency requires damping when many secondary bending and twisting modes require attention.
More importantly, the notion of incorporating a tuned mass-damper to suppress a structural resonance is a flawed one. Tuned damping is only effective in damping discrete resonances and is misapplied when used to damp a broadband structural resonance. In simple terms, a tuned damper "splits" a structural resonance into two resonances by creating a coupled mass system.
TMC's proprietary broadband damping techniques are the most effective way to damp an optical top. This approach works over the entire frequency range of interest, dissipating energy at the top's primary, secondary, and higher resonant frequencies. In addition, performance will not be compromised by adding weight to the top.
TMC's CleanTops are engineered using the most advanced methods for structural analysis and design. The Operational Deflection Shape shown above was measured using a technique called Laser Scanning Vibrometry (LSV). LSV is among the most sensitive and most accurate non-contact vibration measurement techniques commercially available. It uses the laser doppler effect to measure the behavior of the entire table rather than the behavior one discrete point.
Structural Damping Performance Summary
TMC optical tops have guaranteed performance levels which are unsurpassed. In addition, with three levels of broadband damping and three environmental choices, TMC offers the most flexibility in choosing a performance level. Guaranteed maximum compliance levels for the maximum damping level are tabulated in the plots below. The standard damping level offers compliance levels a factor of four times higher than those tabulated. The minimum damping level is only recommended for non-sensitive applications. The curves summarize the guaranteed performance levels of TMC optical tops. In addition, table top corner compliance data are presented for the three damping levels available. Data were acquired by impact testing, using a one-pound calibrated hammer, accelerometer, and dual-channel spectrum analyzer. As these examples demonstrate, actual measured performance is often considerably better than our guaranteed performance.
