In an environment where both industry and research are working in a micron realm, vibration can play havoc with the outcome. This article explains how to identify and control vibration in the workplace.
Vibration control plays a critical role in research and industry. Mechanical noise, for example, can obscure high-resolution images and affect the quality of nanotechnology processes. Fortunately, vibration can be controlled through proper experimentation, process design and the use of specialized equipment. In general, there are three approaches to reducing mechanical excitation of photonic systems.
Figure 1. Mechanical noise comes from a variety of sources.
The first is lowering the ambient noise level at the source; this includes machinery noise and external vibrations. Both can be reduced through proper design, maintenance and mechanical isolation techniques. Where possible, vibration-sensitive systems should be installed on a solid, ground-level floor away from large machinery.
Common machinery problems include pump cavitation, unbalanced fans and flow excitation of air-handling systems. Machinery faults can be eliminated through redesign or maintenance. Machines also should be installed on soft mounts that attenuate vibration before it is transmitted to the floor. In some cases, machinery can be contained in acoustic housings to filter airborne noise as well.
Location makes a difference
Building location and construction also will affect ambient vibration. Sites near highways and traffic can exhibit very high levels of low-frequency noise. Equipment located on upper floors can be exposed to building sway. Proper building design can reduce structural excitation and isolate the work environment from external seismic noise.
Figure 2. Vibration control is essential for sensitive optical systems.
A second approach is directly isolating the system from local ambient noise. Floor vibrations can be attenuated by supporting the work surface on a variety of soft coupled systems. These include passive rubber mounts, air springs and regulated pneumatic isolators. In more advanced systems, electronically controlled isolators actively cancel vibration. The choice of isolation system depends on the resolution requirements of the application, the amount of mass to be supported and ambient noise in the work environment.
The third approach is maximizing the rigidity and damping characteristics of any optical support structures. Work surfaces and optomechanical assemblies must be designed to reduce their response to any vibration transmitted through the isolation system.
Honeycomb platforms optimize rigidity with respect to weight, which pushes structural resonance modes to higher, less detrimental frequencies. The composite nature of these structures also allows them to be designed with excellent vibration- damping characteristics.
Figure 3. Optical tables are a composite of vertical honeycomb and horizontal laminates.
Granite is sometimes used to provide inertial resistance to vibration-induced motion and can be ground extremely flat — a great advantage in precision motion and optical systems. However, as an optical table, granite exhibits very poor damping characteristics.
Finally, the system designer must maximize the rigidity of any other mounting or support structures controlling the electro-optical beam path. A primary goal in any system is the elimination of undesired relative motion among components in the beam path. A highly rigid support structure can tolerate a greater amount of ambient vibration.
In most systems a combination of methods will minimize vibration. Semiconductor manufacturers use advanced construction techniques to isolate fabrication cleanrooms from surrounding building structures. Acoustic isolation frequently is added to high-resolution metrology equipment such as atomic force microscopes to supplement vibration control and improve image quality. Interest in active vibration cancellation is growing as manufacturers push the technology beyond the capabilities of concrete and air mounts. However, the vast majority of electro-optical experiments and processes are built on honeycomb optical tables supported by regulated pneumatic isolators.
Honeycomb optical tables exhibit the optimal combination of high rigidity, low weight, good modal response and vibration damping. These composite structures usually consist of a steel honeycomb core laminated with steel top and bottom skins. The design greatly reduces weight when compared with solid steel or granite, yet it remains extremely rigid.
Rigidity or stiffness defines the structural deformation response to external forces. Highly rigid structures allow less relative motion between optical components and are crucial for dimensional stability in optical applications. External forces may be static because of weight on the table or dynamic as a result of floor vibration or motion systems on the work surface. High static rigidity is needed to prevent both sag under the table’s own weight and deflection caused by the mass of any equipment on the table. Deflection due to static loads is determined by adding mass to the table and measuring deflection with a strain gauge or interferometer.
Dynamic rigidity describes how the table changes shape when excited by a time-varying force. High dynamic rigidity is required to eliminate relative motion among different points of the optical work surface when subjected to vibration. Dynamic rigidity can be modally tested by exciting the table with a shaker or impact hammer and measuring the response with an accelerometer. A spectrum analyzer can be used to produce a compliance curve. Compliance curves describe the table’s deformation response to broadband force excitation.
Figure 4 shows the compliance curve for a typical honeycomb optical table. The peaks show the table’s natural resonance frequencies; the antiresonance troughs are caused by phase cancellation of differing bending modes. Maximizing the frequency of the natural bending modes is critical because the amount of displacement for a constant vibration force input decreases as frequency increases.
Figure 4. The compliance curve of an optical table shows natural bending modes superimposed on the ideal rigid body line.
Rigidity and damping
If the table were perfectly rigid, the slope of the compliance curve would decrease at a theoretical rate of 40 dB per decade. In other words, as the frequency of excitation is increased by a factor of 10, the resultant displacement for a constant force will decrease by a factor of 100. Because the table is not perfectly rigid, structural resonances are superimposed on the curve and the table departs from the ideal rigid body line
Optical tables are designed with damping to reduce the amplitude of the resonance peaks. Damping also eliminates ringing
caused by impacts such as dropping a tool on the table or periodic excitation from rotating machinery. The two basic techniques used in optical table design are broadband and tuned damping.
Broadband, as the name implies, provides some damping over a wide range of frequencies. On the other hand, tuned damping provides much higher levels of damping at selectively tuned frequencies. Figure 5 shows the compliance curve from a table identical to that in Figure 4, but with tuned damping applied. Tuned mass dampers selectively phase cancel the first several natural modes. This technique effectively splits the single high-amplitude peaks to two low-amplitude peaks.
Figure 5. Tuned mass dampers selectively attenuate table resonance modes.
Regulated pneumatic isolators are commonly used to support optical tables. They are in effect a mechanical low-pass filter system. Pneumatic isolators consist primarily of an elastomer diaphragm supported on a rigid cylinder of air. As floor vibrations drive the isolator, air is allowed to compress or expand. They exhibit a low spring constant and therefore a very low (commonly 1 to 2 Hz) natural frequency. Vibration is attenuated sharply above the isolator’s natural frequency. Figure 6 shows the vibration transmissibility of a common optical table isolator.
A primary advantage of regulated pneumatic isolators is that they have a very low spring constant compared with their small range of travel. Equivalent mechanical springs require much greater travel and are less stable.
Another advantage is that the stiffness of an air spring is dependent upon the height of the air column but relatively independent of load. This means that if the air pressure can be adjusted to maintain constant height, the natural frequency, and therefore the transmissibility of the isolator, will remain constant over a wide range of loads. The constant height is maintained and controlled by an air regulator valve actuated by table motion. This system also provides self-leveling of the table when loads are moved across the work surface.
Figure 6. Transmissibility plot shows a pneumatic isolator’s natural frequency and vibration attenuation above 1.4 Hz.
Lowering the natural frequency
Most pneumatic isolators are effective above several hertz. However, at their natural frequency they do not isolate and even provide some amplification of vibration. Therefore one of the primary goals in pneumatic isolator design is to lower the natural frequency in both the horizontal and vertical directions. Damping is provided to reduce the amplification at the natural frequency and to prevent the system from oscillating in response to transient vibrations. In applications where low-frequency vibrations cannot be damped effectively, active vibration cancellation will be required.
Vibrations are coupled to the experimental or process work surface directly through the support structure or acoustically through the atmosphere. In most cases, structure-borne vibration is the greatest concern. However, as control of structure-borne vibration improves, acoustic noise is becoming more of an issue.
Figure 7. Optical tables and breadboards provide a work surface for many components.
The first approach to vibration control is to reduce ambient vibration and acoustic noise from external sources. This includes isolating both building and machinery noise sources. Some common sense in where to locate vibration-sensitive equipment also can do much to minimize vibration problems. However, it is not always possible to find that quiet location. This leads to the need for specially designed vibration-control equipment. Vibration isolators are available to attenuate the transmission of floor vibration passively or to cancel vibrations actively before they reach the work surface. Regulated pneumatic isolators are favored for floating optical tables and many other work-surface structures.
Work-surface platforms may be made of granite, aluminum, steel or composite honeycomb materials. Honeycomb technology is usually selected for optical table design. The composite structure increases work-surface rigidity and also provides vibration damping. For acoustic problems, honeycomb technology can be used to attenuate airborne vibration as well. As a last defense, lens towers and other beam path assemblies should be extremely rigid.
The ultimate goal is to eliminate any relative motion among components in the beam path. Close cooperation among structural and facilities engineers, optical system manufacturers and vibration control technology companies makes this possible.