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Vibration lsolation Concepts
| Source of Vibration |
There are three primary sources of vibration which can disturb a payload, such as ground
vibration, acoustic noises, and direct force disturbances.
At one extreme, the ground
vibration environment may consist of low level seismic
disturbances present
everywhere on earth and the disturbances, imperceptible under ordinary
circumstances, present operating problems for highly sensitive equipment. When
cultural vibration effects are added, even wider range of sensitive equipment is
affected.
For example, even Low - amplitude vibration can affect the performance and yield
of lithography equipment such as stepper, the resolution of electron microscopes,
the accuracy of measuring machines, and the performance of many types of
precision equipments and instruments for electro-optical research.
The cultural disturbances affecting the sensitive equipments are man-made and
caused by phenomena such as vehicle and foot traffic, human activity, air handling
systems, elevators, machinery and numerous other sources. |
| 3 Factors In Dynamic System |
In discussing vibration isolation, it is useful to identify three elements of a dynamic system.
1. The equipment need to be isolated.
2. The support structure (floor).
3. The isolation system between the equipment and the support structure. |
| Vibration |
In a passive isolation system, two factors affecting isolation efficiency are the
natural frequency and damping of the isolator. The natural frequency is the rate of
free oscillation per unit time and damping is the characteristic which dissipates
energy in a dynamic system.
The ratio of forcing frequency (the disturbing
frequency) to natural frequency (f/fn) is used to determine the isolation efficiency
of any isolation systems.
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Transmissibility Tr = I 1-(f/fn)² /1 I × 100%
where
f/fn = the ratio of forcing frequency to natural frequency
Graph shows typical plots
of isolation efficiency Notice
that when f/fn is less than
f2=1.414, the curves show that
the vibration is magnified,
when the forcing frequency is
equal to the natural frequency
(f/fn=1 ), maximum magnification
occurs. At ratios above 1.414,
the curves are in the isolation
range. Typically isolators which
exhibit the greatest magnification
at resonance have the best isolation
efficiency (undamped coil spring).
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Generally speaking, low amplification
at resonance as shown for the
plot of a damped coil spring
is desirable; however, notice
that this is accomplished at
the expense of isolation efficiency.
Pnumatic isolators with an air
spring and damping chamber on
the other hand, combines the
desirable characteristics of
low magnification at resonance
and high isolation efficiency
as shown the graph. |
The equation for determining the natural frequency of
a pneumatic isolators is
|
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fn = 2∏ / 1 √rAG/V
where
r = Ratio of specific heat, 1.4 for air
A = Effective area of air piston, ㎠
G = Gravity acceleration
V = Volume of air chamber,㎤ |
As seen from the equation,
the natural frequency of the
pneumatic isolator depends on
the ratio of the piston area
to the volume of the air isolator |
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| Unites of Vibration |
1.Displacement(㎛)
D = {a ÷ (2∏f)²} ×10⁴
2.Velocity(㎜/sec)
V = D×2∏f ÷10³
3.Acceleration(㎝/sec²)
Agal = D × (2∏f)² ÷10⁴ |
D=amplitude
1G=980 gal
1gal=1cm/sec² |
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