The Effects of Removing Vibration with Optical Tables

When beginning a precision experiment or manufacturing process in optoelectronics, one of the key issues to consider is that of vibration. The primary methods for controlling this are through the various sizes and shapes of optical tables and breadboards. They control the vibrations using the soft isolators they rest on that filter out floor vibrations.

However, other sources of disturbance – acoustic noise, atmospheric turbulence and onboard sources – remain a problem and may become prevalent vibration sources at higher frequencies. If the spectral contents of these disturbances are close to the resonance frequencies of the platform are, they can have a significant negative impact on the optomechanical performance.

For this reason, internal damping, or dissipation of mechanical energy, that reduces the effect of resonance is crucial for high-quality vibration-isolated platforms. The most effective methods for suppressing resonance vibrations involve electromechanical sensors, actuators and control systems, known collectively as active vibration damping1,2.

SmartTable ADD Active Dampers

Recently, Newport has announced an extension of this technology with their add-on SmartTable ADD active dampers, this new technology expands the limits of applications beyond standard optical tables to include virtually any vibration-isolated platform that may experience unwanted resonance vibrations.

Active vibration control works on a theory that predicts that several active dampers strategically placed near antinodes of the main vibration modes is sufficient to suppress resonance vibration across the entire structure. For a typical rectangular platform, two of these active dampers placed at adjacent corners should be enough.

This example is agreed by theoretical analysis and by multiple measurements of the dynamic reaction at remote locations on the table by highly sensitive seismic accelerometers.3 In order to provide a more striking demonstration of this phenomenon, researchers at Newport Corporation turned to the scanning laser Doppler vibrometer from Polytec Inc. and the experimental power it can make use of. The experiment also promised valuable information about the proper tuning of the active control feedback loops.

Optical breadboard with add-on active dampers prepared for laser Doppler vibrometer test.

Figure 1. Optical breadboard with add-on active dampers prepared for laser Doppler vibrometer test.

Figure 1 shows the custom-made all-steel optical breadboard that was used to test the effect of add-on active dampers (Figure 1). The 4 ft × 5 ft × 4.5 in. breadboard was supported by four spring-based vibration isolators placed at each corner, so that the “rigid body” natural frequency of the suspension equaled 5.25 Hz.

In order to test the active vibration control, two SmartTable ADD dampers were fixed at the two front corners of the breadboard. In order to obtain a consistent stationary vibration signal, the breadboard was excited by a stationary random force provided by a miniature electromagnetic shaker (seen in the far-left corner of the breadboard in Figure 1).

The resulting vibration experienced by the breadboard were at a level below those of a typical laboratory environment, fitting to the VC-A vibration category (1 ⁄3-octave rms velocity below 50 µm/s).4 The force sensor, inserted between the shaker and the breadboard at the driving point, provided a reference signal for data processing by the laser vibrometer. The final addition was a lightweight low-noise accelerometer installed close to the shaker to conduct a traditional dynamic compliance measurement alongside the laser vibrometer test.

Scanning Laser Doppler Vibrometry

With scanning laser Doppler vibrometry the researcher can estimate the kinetic energy of the whole structure due to the rapid vibration measurements at various points of the structure. The widely recognized criterion of vibration damping is the reduction of the total kinetic energy of the system.5

Polytec Inc. is a specialist in noncontact vibration measurements. It provided its PSV scanning laser vibrometer chart the vibrations across the breadboard. Scanning vibrometry is used for rapid, accurate analysis and visualization of structural vibration for a large range of applications. This experiment was ideal for laser vibrometry and allowed measurements down to subnanometer excitation levels.

In the test configuration, the laser head was attached to the gantry above the measured surface.

Figure 2. In the test configuration, the laser head was attached to the gantry above the measured surface.

The measured surface as seen by the Polytec software through the laser-head camera.

Figure 3. The measured surface as seen by the Polytec software through the laser-head camera.

Figure 2 shows how the laser scanning head was mounted on a gantry system for movement over the breadboard area; Figure 3 is the view of the measured area as seen through the camera of the laser scanning head by the Polytec software. Each one of the twenty evenly distributed measurement points was equipped with a reflector for identification. The full scan took only a few minutes.

Operational modes of the breadboard with damping turned off – first torsion mode at 194 Hz.

Figure 4a. Operational modes of the breadboard with damping turned off – first torsion mode at 194 Hz.

Operational modes of the breadboard with damping turned off – first bending mode at 278 Hz.

Figure 4b. Operational modes of the breadboard with damping turned off – first bending mode at 278 Hz.

Operational modes of the breadboard with damping turned on – first torsion mode at 177 Hz.

Figure 4c. Operational modes of the breadboard with damping turned on – first torsion mode at 177 Hz.

Operational modes of the breadboard with damping turned on – first bending mode at 284 Hz.

Figure 4d. Operational modes of the breadboard with damping turned on – first bending mode at 284 Hz.

The results of the scan enabled a complete picture of how the damping took place over the whole surface of the breadboard. Figures 4a-d visualize the deflection of the breadboard with active damping on and off.

Reduction of the total kinetic energy of the breadboard by the active damping. The contour charts show the rms vibration velocity over the surface of the breadboard over the entire bandwidth of excitation. The graphs show the square root of the sum total of the velocities squared over all the measurement points. Left side: damping off; right side: damping on.

Figure 5. Reduction of the total kinetic energy of the breadboard by the active damping. The contour charts show the rms vibration velocity over the surface of the breadboard over the entire bandwidth of excitation. The graphs show the square root of the sum total of the velocities squared over all the measurement points. Left side: damping off; right side: damping on.

Active Vibration Damping Systems

The ultimate performance criterion for active vibration damping systems is usually considered to be total kinetic energy is often considered, and the use of scanning laser vibrometry enables this criterion to be estimated. Figure 5 shows the contour charts of the square root of the mean velocity squared over the surface of the breadboard, with the sum total of these mean squares being proportional to an estimate of the total kinetic energy.

The graphs below show the plotted values of the spectral composition, in rms units. By comparing the graphs, it is clear that the active damping practically eliminates both resonance peaks at 194 and 278 Hz, which correspond to the breadboards primary twisting and bending vibration modes.

Figures 4a and 4b show the contour graphs of the undamped vibration, and demonstrate the characteristic property of vibration-isolated platforms: peak vibration occurs at the corners of the platform. This is the reason active dampers are installed at the corners of the breadboard. When placed at two corners, the dampers provide an effective means of reducing vibration over the whole surface.

As well as the proper placement of the dampers, another key consideration in active vibration damping is the choice of control functions and feedback gains. Decentralized velocity feedback has garnered a broad following due to its simple setup and (theoretical) unconditional stability.4

The practical range of feedback gains is limited by two factors: first, the “pinning” phenomenon, and second, the instability caused by frequency responses of the actuators and other elements of the control loops.6

Vibration Measurement Using Scanning Laser Vibrometer

To investigate the effects of increasing the feedback gain on the global vibration control, vibration measurement with the scanning laser vibrometer was performed using the feedback gains below and above the gains recommended by the Newport multi step autotuning algorithm.3

Figure 6. Feedback gain optimization. The sum total of mean velocities squared over all measurement points is plotted against the ratio of the feedback gains in both channels to the “autotune” values found by the SmartTable® algorithm.

With each Increase in gains level, the sum of mean square velocities at all scanned points, which is proportional to the total kinetic energy, was calculated. Figure 6 is a summary of these results. Through the autotuning process, the kinetic energy of the system was reduced to almost the absolute minimum with a reasonable safety margin.

The experiment confirmed that using active dampers is an effective means of reducing the vibration in a custom isolated platform, when used in “bolt-on” configuration as well as when they are embedded in standard tables.

A fifteenfold reduction of the main resonance peak and tenfold reduction in the total kinetic energy was observed. Scanning laser Doppler vibrometry gave a visible demonstration of the active damping performance.

Meet the Authors

Dr. Vyacheslav M. “Salva” Ryaboy is principal mechanical engineer at Newport Corporation in Irvine, Calif.; email: [email protected] Jerome Eichenberger is senior application engineer at Polytec Inc. in Irvine, Calif.; email: [email protected]

References

  1. W. Booth (Decemeber 2006). Table improves optical systems from the ground up. Laser Focus World.
  2. J. Fisher and V.M. Ryaboy (August 2008). Effective vibration reduction stabilizes laser beams. Laser Focus World.
  3. V. M. Ryaboy et al (June 21, 2005). Optical table with embedded active vibration dampers (SmartTable). Proc SPIE, Vol. 5762.
  4. H. Amick et al (September 2005). Evolving criteria for research facilities: I – vibration. Proc SPIE, Vol. 5933.
  5. S.J. Elliot (2007). Global vibration control through local feedback. Adaptive Structures: Engineering Applications. David Wagg, ed. John Wiley & Sons, p. 59.
  6. S.J. Elliot et al (2010). Self-tuning of local velocity feedback controllers to maximise power absorption. Recent Advances in Structural Dynamics: Proceedings of the X International Conference.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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