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SOURCES OF VIBRATION

Introduction
2.1 Sources of Vibration

Vibration, which is commonly referred to as noise, can be segregated into three main categories: seismic (ground) vibrations, acoustic vibrations, and forces applied directly to the load on the working surface. Seismic vibrations include all sources that make the floor under the experimental setup vibrate. Common seismic vibration sources are foot traffic, vehicular traffic, wind blowing the building, and building ventilation fans, to name a few. Many of the sources that generate seismic vibrations also generate acoustic vibrations. The difference is that acoustic vibrations are a measure of the effects of air pressure variations on the experiment. The final contributor to vibration is forces applied directly to the load on the working surface; these are vibration sources that are directly coupled mechanically to the experimental setup but not transmitted through the table supports. Examples include vibrations resulting from a moving positioning stage with a sample on top of it or the vibrations transmitted to the working surface via vacuum system tubing.

2.2 Vibration Characteristics

Vibrations can be classified as either random or periodic. Periodic noise obviously includes the constant vibrations caused by a continuously running vacuum system, but it also includes the vibrations caused by the fans of an air handling system that turn on and off based on the temperature of the room. Random vibrations are classified as vibrations from unpredictable sources like wind blowing a building or a jack hammer crew digging up a water main in the street. In addition, it is important to know the frequency and amplitude of the vibrations. Typically, the frequency of the vibrations will range from 4-100 Hz.

Many sources of noise contribute via more than one mechanism to the overall vibration of the experimental setup. For example, a vacuum pump located on the floor beside the experimental setup creates seismic vibrations in the floor as well as acoustic vibrations. Both of these channels of vibration should be considered when analyzing noise sources. However, since mechanical coupling efficiency is typically higher than coupling from acoustic sources, the largest contributions to overall noise are generally due to seismic vibrations and forces directly applied to the load. Hence, placing the vacuum pump on a vibration absorbing pad may provide the necessary reduction in vibration to make its contribution to the overall noise insignificant when compared to other sources.

2.3 Identifying Vibration Sources

Figure 3 and the Table 1 below identify common vibration sources, some of which will be found in almost any research laboratory.


Figure 3. Common Vibration Sources in the Laboratory

Therefore, it is common in laboratories to find an ambient noise spectrum where the inputs are most likely to be structural and acoustic. Table 2 lists some of the most common sources of noise (acoustic and structural energy).

Type

Frequency(Hz)

Amplitude

Air Compressors

4 – 20

10-2 in

Handling Equipment

5 – 40

10-3 in

Pumps (Vacuum, comp or non-comp fluids)

5 – 25

10-3 in

Building Services

7 – 40

10-4 in

Foot Traffic

0.5 – 6

10-5 in

Acoustics (B)

100 – 10000

10-2 to 10-4 in

Air currents

Labs can vary depending on class

Not applicable

Punch Presses

Up to 20

10-2 to 10-5 in

Transformers

50 – 400

10-4 to 10-5 in

Elevators

Up to 40

10-3 to 10-5 in

Building Motion

46/Height in meters, Horizontal

0.1 in

Building Pressure Waves

1 – 5

10-5 in

Railroad*

5 - 20

±0.15g

Highway Traffic*

5 - 100

±0.001g

*Amplitude is reported in dB using the acceleration due to gravity as the reference acceleration.

Table 1. Frequency and Amplitude of Common Vibration Sources

Before choosing a vibration isolation system, it is beneficial to consider the sources of noise present and to remove them if possible. By removing sources of noise from the working environment, it may be possible to reduce the noise, thereby reducing the isolation requirements for and hence the cost of the needed vibration isolation system. For instance, in Fig. 3, an oscilloscope was placed directly on the experimental working surface. As a result, vibrations from the fan in the oscilloscope will directly apply a force to the experiment. By placing the oscilloscope on an overhead shelf that is not in direct contact with the experimental working surface, this source of vibration can be eliminated. When considering which sources of noise to eliminate, remember that seismic vibrations and forces applied directly to the load on the working surface are typically the most intense due to the high mechanical coupling efficiency.

Thermal disturbances from air conditioning systems and cooling fans can also cause relative motion between components due to material expansion and contraction resulting from temperature fluctuations. Most air conditioning systems will typically only maintain temperatures to within 1 degree/hour. Additionally, some experimental techniques will be sensitive to virtual movements caused by changes in the refractive index of air and its density, both of which have a temperature dependency. If this is the case, it is often beneficial to build an enclosure system around the sensitive components in order to limit temperature change and air flow.

2.4 Vibration Criteria

Although sometimes the specific vibration criteria are known because the manufacturer of a device might provide the required environmental specifications required for the proper use of that device, this is not always the case. In these cases, generic criteria have been developed by consultants like Colin Gordon & Associates. Table 2 below has a description of what types of applications can be implemented successfully given the severity of vibrations present in that environment.

Criterion Curve

RMS Vibration Velocity Amplitude
(One-Third Octave Bands Range of Measurement)

Detail Size (line width)

Description of Use

Workshop(ISO)

800 µm/s (8-80 Hz)

N/A

Vibrations can be distinctly felt. Appropriate to workshops and non-sensitive areas

Office(ISO)

400 µm/s (8-80 Hz)

N/A

Vibration can be felt. Appropriate to offices and non-sensitive areas

Residential Day(ISO)

200 µm/s (8-80 Hz)

75 µm

Vibrations can be barely felt. Probably adequate for computer equipment, probe test equipment, and low-power to 20X microscopes

Operating Theatre(ISO)

100 µm/s (8-80 Hz)

25 µm

Vibration cannot be felt. Suitable in most instances for up to 100X microscopes.

VC-A

50 µm/s (8-80 Hz)

8 µm

Adequate for most optical microscopes up to 400X, microbalances, optical balances, and proximity and projection aligners.

VC-B

25 µm/s (8-80 Hz)

3 µm

Appropriate for optical microscopes up to 1000X and inspection and lithography equipment (including steppers) down to 3 micron line widths.

VC-C

12.5 µm/s (1-80 Hz)

1 µm

A good standard for lithography and inspection equipment down to 1 micron detail size.

VC-D

6 µm/s (1-80 Hz)

0.3 µm

Suitable for the most demanding equipment, including electron microscopes (TEMs and SEMs) and E-beam systems.

VC-E

3 µm/s (1-80 Hz)

0.1 µm

A difficult criterion to achieve in most instances. Assumed to be adequate for long path laser based interferometers and other systems requiring extraordinary dynamic stability.

Table 2. Application and interpretation of criterion curves.
Courtesy of Colin Gordon Associates


Figure 4. RMS velocity versus one-third octave band center frequency for various vibration criteria.
Generic Vibration Criterion

With a clear understanding of the potential noise sources and vibration criteria that need to be attained for high quality results, it is now possible to construct a vibration isolation system. The system should be able to attenuate all dynamic inputs in the range of frequencies for which the experiment is sensitive (4-100 Hz) if it is not possible to remove those noise sources from the laboratory and local environment. In addition, the system must minimize the duration of any disturbance produced on the working surface by damping those impulses.

2.5 Selecting a Vibration Isolation System

Before choosing a vibration isolation system it is important to determine two factors:

  • The severity of the environment - where the table is going to be placed (e.g., in a basement or on the upper floor of a steel frame building). This is the primary factor in determining the level of isolation required in an optical table support.
  • The sensitivity of the application - what is the intended application to be conducted on the table (i.e., how sensitive is the experiment). This is the primary factor in determining the stiffness and internal damping features required in the optical table
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