| 1.0 General Introduction | ||||
For over 30
years, TMC has specialized in providing precision working surfaces
and vibration isolation systems for precision measurement laboratories
and industry. To provide optimal performance, both precision “tops” and
their supporting isolators must be
designed to address the central issue: control of environmental
noise. |
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| 1.1 Sources of Vibration (Noise) | ||||
| There are three primary sources of vibration (noise) which can disturb a payload: ground vibration, acoustic noise, and “direct force” disturbances. Ground or seismic vibration exists in all environments throughout the world. This noise has various sources, from waves crashing on coastal shorelines, the constant grind of tectonic plates, wind blowing trees and buildings, to manmade sources like machinery, HVAC systems, street traffic, and even people walking. TMC vibration isolation systems are designed to minimize the influence of these vibration sources.Acoustic noise comes from many of the same sources but is transmitted to the payload through air pressure waves. These generate forces directly on the payload. Even subsonic acoustic waves can disturb a payload by acting as a differential pressure on the diaphragms of pneumatic isolators. Air currents generated by nearby HVAC vents can also be a source of “acoustic” noise. TMC manufactures Acoustic Enclosures for OEM applications which protect payloads from this type of disturbance by providing a nearly airtight, heavy, energy-absorbing enclosure over the entire payload. | ||||
| Acoustic noise can be measured, but its influence on a payload depends on many factors which are difficult to estimate (such as a payload’s acoustic cross-section). The analysis of this type of noise source goes beyond the scope of this discussion.* In general, acoustic noise is the dominant noise source of vibration above 50Hz. | ||||
The third source of vibration is forces applied directly to the payload. These can be in the form of a direct mechanical coupling, such as vibration being transmitted to the payload through a hose, or a laser water cooling line. They can also come from the payload itself. This is the case in semiconductor inspection equipment, where moving stages are used to position silicon wafers. The force used to accelerate the stage is also applied to the “static” portion of the payload in the form of a reaction force. Moving stages also shift the payload’s overall center-of-mass (COM). |
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| Reducing these sources of vibration can be done passively, with TMC’s MaxDamp® line of isolators or actively using feedback or feedforward techniques (for more information on active systems, click here). Payload-generated noise sources are usually of a well-known nature and do not require any measurements to characterize. | ||||
| The influence
of vibration transmitted to the payload can be minimized through
good payload design. TMC offers a wide range of honeycomb
optical tables, breadboards,
and platform
laminations. These are available in standard and custom shapes
and sizes. All reduce the influence of environmental noise by having
high resonant frequencies and exceptional damping characteristics
(for more information on Practical
Pneumatic Isolators click here. Section 3). Back to Technical Background Index |
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| 1.2 Measuring Noise | ||||
| Seismic (floor) noise
is not usually known in advance and must be measured. There are two
types of seismic noise sources: periodic or coherent noise and random
or incoherent noise. The first requires the use of an amplitude
spectrum while the second is analyzed using an amplitude
spectral density. To determine the expected levels of vibration
on a payload, these must be combined with the vibration
transfer function for the isolation system supporting it. Back to Technical Background Index |
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| 1.2.1 Periodic Noise | ||||
| Periodic noise usually comes from rotating machinery. By far the most common example is the large fans used in HVAC systems. These fans spin at a constant rate and can generate a continuous, single-frequency vibration (and sometimes several harmonic frequencies as well). Another common source is air compressors. Unlike building fans, these cycle on and off according to demand. Compressors should be considered periodic, coherent noise sources, though they are nonstationary, meaning a measurement will change depending on whether the source is active or not. All periodic noise sources should be measured using an amplitude spectrum measurement, whether they are stationary or not. | ||||
| An amplitude
spectrum measurement is produced by
taking the Fourier transform of
data collected from a sensor measuring the noise. The most common
sensor is an accelerometer, which will produce a spectrum with units
of acceleration as
a function of frequency. Accelerometers are popular because they
have a “flat” frequency response, and random ground
noise is usually fairly “flat” in acceleration (see
section 1.2.2 below). Amplitude spectrums can also be expressed
as velocity or position amplitudes as a function of frequency. Most
spectrum analyzers use the Fast Fourier Transform, or FFT. An FFT
analyzer finds the amplitude of each frequency in the input data
and plots it. This includes the amplitudes and frequencies of any
periodic noise sources. The amplitudes of periodic noise sources
measured using an amplitude spectrum are independent of the length
of the data record. Back to Technical Background Index |
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| 1.2.2 Random Noise | ||||
Random,
or incoherent noise, is measured using an amplitude
spectral density. The difference is that the amplitude spectrum
(above) is multiplied
by the square root of the data record’s length before being
displayed by the analyzer. The result is a curve which measures
the random noise with units of [units] / |
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Figure 1. Velocity, Position, and Acceleration for Different Environments  |
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| 1.2.3 Measuring RMS Values | ||||
| Since most locations have a combination of both random and periodic noise sources, it is often desirable to come up with a single number which characterizes noise levels. This is usually done by quoting an RMS (Root-Mean-Squared) noise level within a specified range of frequencies. | ||||
| Fortunately, this is easily done by integrating the power spectral density or PSD over the frequency range of interest. The PSD is the square of the amplitude spectral density. This gives the following expression for the RMS motion between the frequencies f1 and f2: | ||||
 [1] |
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This formula correctly
calculates the RMS value of the measurement taking into account both
periodic and random noise sources. Most spectrum analyzers are capable
of performing this integration as a built-in function. The contribution
to this RMS value from any single periodic source can be measured
using the amplitude spectrum (not the amplitude density)
and dividing the peak value by .
The contribution from several peaks can be combined by adding them
in quadrature. RMS values are also sometimes expressed in “1/3
octave plots” in which a histogram of the RMS values calculated
in 1/3 octave frequency bins is displayed as a function of frequency.
An octave is a factor of two in frequency.Back to Technical Background Index |
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| 1.2.4 Characterizing Isolators | ||||
| The noise level on a payload can be predicted by measuring the ground noise as described above, then multiplying those spectra by the transfer function for the isolation system. The transfer function is a dimensionless multiplier specified as a function of frequency and is often referred to as the isolator’s transmissibility. It is typically plotted as the ratio of table motion to ground motion as a function of frequency. It is common to express transmissibility in terms of decibels, or dB: | ||||
 [2] |
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In practice, measuring
the transfer function for an isolation system
can be corrupted by other noise sources acting
on the payload (such as acoustic noise). This is the primary reason
why many measured transfer functions are noisy. To improve the quality
of a transmissibility measurement, a “shake table” can
be used. This is dangerous, however, as it can misrepresent the
system’s performance at
low levels of vibration. |
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| ** Other normalizations often apply such as corrections for “data-windowing” which is beyond the scope of this text. See “The Fundamentals of Signal Analysis,” Application Note Number 243. Hewlett Packard Corporation. Back | ||||
***
Reprinted with permission from Colin Gordon Associates. VCA–VCE
refer to accepted standards for vibration sensitive tools and instruments.The
levels displayed are RMS values measured in 1/3 octave band center
frequencies. Back |
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where [units] may be acceleration, velocity, or position. This 
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