Keeping Optical Tables Steady Tech
lenge for precision experiments and manufacturing processes in optoelectronics. Optical tables and breadboards in various shapes and forms are the de facto standard of vibration control. They rest on soft isola-tors that effectively filter out vibration from the floor. However, other sources of disturbance – acoustic noise, atmospheric turbulence and onboard sources – still exist and may become prevalent vibration sources at higher frequencies. These disturbances can have a significant negative impact on the optomechanical performance if their spectral contents are close to the resonance frequencies of the platform. For this reason, internal damping, or dissipation of mechanical energy, that mitigates the effect of resonance is very important for high-quality vibration-isolated platforms. Active vibration damping that involves electromechanical sensors, actua-tors and control systems has proved most effective for suppressing resonance vibrations. Recent extension of this technology by Newport with add-on SmartTable ADD active dampers expands the limits of applications beyond standard optical tables to virtually any vibration-isolated platform that can experience unwanted resonance vibrations. The theory of active vibration control predicts that a small number of active dampers placed near antinodes of the main vibration modes suffice to suppress resonance vibration over the whole structure. In the case of a typical rectangular platform, two dampers placed at adjacent corners do the job. This has been confirmed by theoretical analysis and by multiple measurements of the dynamic reaction at remote locations on the table by highly sensitive seismic ac-celerometers. 3 To illustrate this in a more vivid demonstration, researchers at New-port Corporation turned to the powerful experimental means provided by the scanning laser Doppler vibrometer from Polytec Inc. The experiment also promised valuable information about the proper tuning of the active control feedback loops. A custom all-steel optical breadboard was chosen for testing the effect of add-on active dampers (Figure 1). The breadboard, whose size was 4 ft × 5 ft × 4.5 in., was supported by four spring-based vibration isolators, so that the " rigid body " natural frequency of the suspension equaled 5.25 Hz. Two SmartTable ADD dampers were bolted onto the two front corners of the breadboard. To obtain a consistent stationary vibration signal, the breadboard was excited by a stationary random force. A miniature electromagnetic shaker was used for this purpose; it is seen at the far left corner of the breadboard in Figure 1. The resulting vibration of the bread-board did not exceed the levels routinely …
Effective vibration reduction stabilizes laser beams
The impact of vibration isolation and opticaltable damping on laser beam stabilization for both short- and long-duration applications cannot be overemphasized.
ACTIVE ISOLATION OF STRUCTURAL VIBRATION ON A MULTIPLE-DEGREE-OF-FREEDOM SYSTEM, PART II: EFFECTIVENESS OF ACTIVE CONTROL STRATEGIES
Abstract This is the second of two companion papers concerned with the active control of structural vibration transmission. Five different active control strategies have been studies for the reduction of structural power transmission from a source to a receiver via a number of active mounts. The effects of transducer errors and the problems created by the presence of an uncontrolled flanking excitation acting on the receiver have been analyzed. Minimization of the total power transmitted to the receiver through the mounts has been compared with more practical control strategies at the junctions connecting the mounts to the plate: the cancellation of out-of-plane velocities, the cancellation of out-of-plane forces, the cancellation of the power due only to the out-of-plane velocities and forces and the minimization of the sum of squared out-of-plane velocities and weighted square forces. The control of total power gives the best results under ideal conditions but, for realistic cases, characterized by measurement errors and flanking paths, the cancellation of velocity or force is more effective than the active control of measured power. The minimization of the sum of squared velocities and weighted squared forces gives a particularly interesting result since the performance for the active control system is then almost the same as that of minimizing total power and this performance is not sensitive to measurement errors or flanking paths.