Fig. E. A conventional pneumatic isolator with damping.

Pneumatic isolators are one of the best methods of vibration isolation for critical applications. When properly designed and carefully constructed, their performance combines the "fast roll-off" of the Simple Harmonic Oscillator at vibration frequencies above resonance with the "low amplification at resonance" of the damped harmonic oscillator near resonance.

The conventional design for a pneumatic isolator with damping is shown in Figure E. The isolated mass M (for example, an optical table, or precision instrument such as a microscope) is supported by a piston which rests on a flexible rolling diaphragm. The diaphragm separates the piston from the top section of the air chamber called the "spring chamber." In damped systems, air can flow between the spring chamber and a secondary chamber, called the "damping chamber," through a flow restrictor, usually a small orifice. As air flows through the orifice, energy is dissipated, reducing the amplification of the isolator at resonance. While this two-chamber isolator design has been used for many applications, there is a growing need to provide better protection in areas ranging from laser-based experimentation to atomic scale resolution of many devices. Newport -- realizing the need for better vibration isolation -- designed a new isolator that has gained much attention and use of our customers by combining elegant and ergonomic design -- with much improved performance. (See Figures F and G.)

Fig F. Damping efficiency is proportional to the air flow and pressure drop through the damper. Because the hybrid-chamber design minimizes air volume between the piston and damper, there is a better linkage between piston motion and damper air flow. This generates a higher damping force for a given piston displacement for faster, more efficient damping of vertical motion.

The all-new design of this hybrid chamber isolator represents a significant advance in vibration control technology and offers improved performance when compared to two-chamber isolator designs. A pneumatic, rolling-diaphragm design and exclusive laminar-flow damper provide superior vertical motion isolation. Instead of the single damping orifice found in conventional designs, this unique damper is comprised of thousands of tiny orifices. Simultaneous air movement through the orifices creates a laminar flow which improves damping efficiency for both large and small displacements. See Figure F.

The major technical advance introduced by the isolator is a novel hybrid-chamber design. The hybrid-chamber configuration also enhances damping efficiency by minimizing the air volume between the piston and damper. Better linkage between piston motion and damper air flow significantly improves damping performance.

The result of these improvements is less motion amplification at the isolator's natural frequency and faster settling time, two key measures of isolator performance. Here are some of the major benefits of the isolator's laminar-flow damper and hybrid-chamber design:

1.  Faster Settling Time

Rapid settling time is an important factor in most applications, from laboratory experimentation to production-line systems. The higher efficiency of Newport's laminar-flow damper reduces settling time after both large- and small-magnitude table disturbances. What's more, the smaller total isolator volume improves isolator responsiveness because less air is required to re-establish equilibrium.

2.  Better High-Center-of-Mass Stability

A smaller total isolator volume generates a larger restoring force for very low-frequency disturbances, resulting in improved high-center-of-mass stability -- a critical consideration in applications with heavy equipment.

3.  Lower Natural Frequency

The lowest possible natural frequency provides the best protection against difficult-to-control low-frequency vibrations below about 5 Hz. In a conventional two-chamber isolator, the size of the spring chamber determines natural frequency: the larger the chamber, the lower the natural frequency. Since nearly all of the hybrid-chamber's internal volume is devoted to a single, large hybrid chamber (versus two smaller chambers), it has a designed lower natural frequency.

4.  Less Amplification at Resonance

Higher damping efficiency results from the laminar-flow damper design and the smaller volume of air between the piston and the damper. This reduces motion amplification at the isolator's natural frequency for better stability.

5.  Patented Self-Centering Piston Provides Fast, and Easy Installation(U.S. Patent 5,071,108)

Even under "normal" operating conditions, piston movement of ordinary isolators can be inadvertently restricted if care is not taken to ensure exact alignment. The result is that isolation performance can be severely compromised. This exclusive, patented design automatically self-centers the piston at the top and bottom of its vertical stroke, guaranteeing unrestricted piston movement for best
performance.

6.   Newport's SafeLock^TM mounting bracket

The SafeLock^TM mounting bracket eliminates the need to precisely align isolators and table mounting holes. The new mounts fit the same hole pattern as Newport's older XL and XK Series isolators for modification-free replacement of earlier isolator sets. Overall height adjustment of 1.3 inches (33 mm) accommodates uneven flooring without compromising isolator travel. For added safety, mechanical stops limit isolator movement in all directions to protect the isolator's precision mechanism.

7.  An Isolator for a Wider Range of Loads and Table Heights

For greater versatility and long-term value, a single isolator design meets the needs of virtually all applications. Due to the advanced design, a set of four isolators offers superior performance with loads from 660 to 8,000 lb (300 to 3,600 kg) without modification of any kind. Interchangeable spacer bases (see Figure H) allow quick, inexpensive modification of isolator height or the addition of a caster system at any time in the field. The isolator is also suitable for cleanroom applications.

Fig. H. Seven economical, interchangeable spacer bases of different heights permit convenient cost-effective adjustment of working surface height or retrofitting of caster systems.

The theoretical analysis of this system with damping is significantly different from that of a simple mass-spring system, and the resulting ratio of the displacement of the isolated mass to the displacement of the floor is given by:

where:

*0 is the natural frequency of the undamped system

* is the damping coefficient determined by the details of the damping mechanism

* is the frequency of the vibration

B, D are constants which depend on the details of the isolator design

A is the cross sectional area of the piston

Vs is the spring chamber volume

P _g is the gauge pressure, that is the pressure in the isolator (above the pressure outside)

which is dependent on the mass supported by the isolator.

P _a is the atmospheric pressure

Note that the transmissibility of a damped, pneumatic isolator is very different from that of the damped simple harmonic oscillator, and illustrates the main features of pneumatic isolators:

High performance over a wide range of loads.

The resonance frequency w₀ of the isolator is only weakly dependent upon the load M.

Low natural frequency when the system is operated at several times atmospheric pressure.

In practice Newport pneumatic isolators are operated so that the air pressure in each leg is between about 10 and 80 psi.

1/w² "roll-off" even with high damping and low resonance peak transmissibility.

The steep decrease in transmissibility T as the frequency increases is much faster in pneumatic isolators than the damped simple harmonic oscillator, for which the transmissibility decreases as 1/w at high frequencies.Newport has found that the transmissibility T and resonant frequency *0 can differ considerably from theory due primarily to effects in the design of the diaphragm and in the engineering of the piston and chamber design. The actual measurement of the vertical transmissibility of Newport's Stabilizer^TM I-2000 series isolator system, is shown in Figure I.

Designing Effective Horizontal Isolation

The pneumatic isolator described above provides isolation mainly from vertical vibrations only. For improved, high performance horizontal vibration isolation, another technique must be used. Newport uses a patented damped pendulum design which is shown in Figure J. As the floor moves relative to the isolated object, the table behaves similar to a pendulum with the pivot point moving back and forth. The equations of motion thus would be the same as those of a simple harmonic oscillator, and the natural frequency of this system is:

where g is the acceleration of gravity, and L is the length of the pendulum.

The actual system dynamics are more complex, and the measured natural frequency of Newport's horizontal isolation is about 1.5 Hz, substantially lower than predicted by the simple production equation. For frequencies near resonance, there is amplification. However, the amount of amplification is determined by the amount of damping in the system. (Newport's Stabilizer^TM I-2000 Series isolators have been optimally damped both vertically and horizontally.)

An important feature of Newport's patented horizontal isolation technique is that horizontal vibrations are not coupled into vertical vibrations to achieve damping, as is the case with "gimballed pistons". Actual measured transmissibility data (Figure K) shows the horizontal isolation of the pendulum system.

Fig. I.(left) Measured vertical transmissibility of StabilizerTMI-2000 Series pneumatic isolator.	Fig. K.(right) Measured horizontal transmissibility of StabilizerTM I-2000 Series pneumatic isolator.