Mechanical vibration and shock exist just about everywhere. The problems caused by these disturbances range from negligible to catastrophic depending on the severity of the disturbance and the sensitivity of the equipment.
At one extreme, the vibration environment may consist of low level seismic disturbances present everywhere on earth. These disturbances, imperceptible under ordinary circumstances, present operating problems for highly sensitive equipment. When cultural vibration and shock effects are added, an even wider range of sensitive equipment is affected. The cultural disturbances are man-made and are caused by phenomena such as vehicle and foot traffic, air handling systems, machinery and numerous other sources.
Excessive vibration and shock can affect the performance and yield of lithography equipment such as steppers, the resolution of electron microscopes, the accuracy of measuring machines, the finish of machine tools and the performance of many types of equipment and instruments.
In discussing vibration isolation, it is useful to identify the three elements of a dynamic system:
- The equipment to be isolated
- The support structure (floor)
- The isolation system between the equipment and the support structure
The effectiveness of an isolator in reducing the transmitted vibration is known as ISOLATION EFFICIENCY, defined as:
For example, at an isolation efficiency of 90%, only 10% of the vibration is transmitted between the equipment and the support structure by the isolator.
Passive Vibration Isolation Systems
In a passive isolation system, two factors affecting isolation efficiency are the NATURAL FREQUENCY and DAMPING of the isolator. 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 (fd/fn) of FORCING FREQUENCY (the driving frequency) to natural frequency is used to determine the isolation efficiency of any isolation system.
Graph 1 shows typical plots of isolation efficiency. Notice that when (fd/fn) is less than
, the curves show that the vibration is magnified. When the forcing frequency is equal to the natural frequency (fd/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). 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.
Graph 1 - click to enlarge |
Pneumatic isolators with a spring and damping chamber (Serva-Levl) on the other hand, combines the desirable characteristics of low magnification at resonance and high isolation efficiency as shown on Graph 1.
The equation for determining the undamped natural frequency of a mass supported on an air column is:
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=
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Ratio of specific heat, 1.4 for air |
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A
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=
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Effective area of air piston..in2 |
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g
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=
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386 in./sec2 |
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v
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=
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Volume of air chamber.. in3 |
As can be seen from the equation, the natural frequency of the pneumatic isolator is not affected by load and depends only on the ratio of the piston area to the volume of the air isolator.
