3.0 Practical Pneumatic Isolators
|Figure 6 shows a cutaway view of TMC’s Gimbal
Piston™ isolator. It uses two air chambers instead of
one. These are connected by a small orifice. As the piston moves
up and down, air is forced to move through this orifice, producing
a damping force on the payload. This type of damping is very strong
for large displacements of the piston and less for small displacements.
This allows for fast settling of the payload, without compromising
small amplitude vibration isolation performance. Damping of this
type usually produces a Q 3
for displacements on the order of a few millimeters.
|The damping provided
by an orifice is limited by several factors. TMC’s MaxDamp® isolators
use a different method: multi-axis viscous fluid damping (Patent
No. 5,918,862). These isolators can extend the damping to near
critical levels for those applications which require it. For example,
semiconductor inspection equipment often uses very fast moving stages
to transport wafers. MaxDamp® isolators
allow the payload to settle very quickly after a stage motion, while
still providing significant levels of vibration isolation. The isolator
uses a very low outgassing, high-viscosity synthetic oil which is
hermetically sealed within the isolator’s single air chamber.
A special geometry ensures that the isolator damps both vertical
and horizontal motions (in both X and Y directions) with equal efficiency.
|Both the Gimbal
Piston™ and MaxDamp® isolators
incorporate a simple and robust pendulum isolator to provide horizontal
isolation. Like air springs, pendulums also produce an 0,
which is payload-independent, and equal to /l
, where l is the length of the pendulum. In the Gimbal
Piston™, the pendulum is actually the piston itself: The
payload is supported by a load disk,
which transfers its burden to the bottom of the piston well through
the load pin. The load pin contacts
the bottom of the well with a pivoting thrust bearing. As the payload
moves sideways, the piston well pivots like a gimbal in the plane
of the diaphragm. Thus a pendulum is formed, whose length is equal
to the vertical distance from the roll in the diaphragm to the bottom
of the load pin.
|TMC’s CSP® (Compact Sub-Hertz Pendulum System) Patent
No. 5,779,010) uses a different type of pendulum concept to
extend horizontal resonant frequencies as low
as 0.3 Hz. This isolator uses a geometrical
lever effect to “fold” a
0.3 Hz pendulum into a package less than 16
in. (400 mm) high. An equivalent simple pendulum
would have to be 110 in. (almost 3 m) tall.
For more information see the Pneumatic
Vibration Isolators for OEM Applications page of this
|Horizontal damping in
most isolators comes from horizontal-to-tilt coupling: As a payload
moves sideways, it also exercises the isolators in the vertical direction
(through tilt), thereby providing damping. Some systems, like TMC’s MaxDamp® isolators,
damp horizontal motions directly with fluidic damping.
|At small amplitudes,
small amounts of friction in the rolling diaphragm and the small
resistance to flow presented by the damping orifice have an impact
on the isolator’s performance. For this reason it is important
to use as small an excitation level as possible when measuring their
Number and Placement of Isolators
|Three or more isolators
are required to support a payload, the most common number being four.
Since there can only be three valves in a system (see Section
3.3), two legs in a 4-post
system must be connected as a master/slave combination. Although
a master/slave combination forms an effective support point, the
damping it produces is much different than a single (larger) isolator
at that point would provide. TMC always recommends using at least
four isolators (except for “round” payloads like NMR
spectrometers). Placement of these isolators under a payload has
a dramatic effect on the performance of systems.
|For small rigid payloads,
like the granite structures in semiconductor manufacturing equipment,
it is best to place the isolators as close to the corners of the
payload as possible. This dramatically improves the tilt stability
of the system, reduces the motions of the payload caused by onboard
disturbances, and improves both the leveling and settling
times for the system. Leveling time is the time for the valving
system to bring the payload to the correct height and tilt. Settling
time is the time for a payload to come to rest after an impulse
|For extended surfaces,
such as large optical tables, the isolators should be placed under the surface’s
nodal lines. This minimizes the influence of forces transmitted to
the table through the isolators. This is discussed in Section
4.3. For either type of payload, it is always better to position
the payload’s center-of-mass in the same plane as the isolator’s
effective support points. This improves the stability of the system
(see Section 3.4)
and decouples the horizontal and tilt motions of the payload.
Uneven floors can
be accommodated in several ways. Most TMC isolators have a ±0.5
inch travel range, and this provides enough flexibility for almost
all applications. Some systems also provide leveling feet. If a
floor is extremely uneven, providing piers for the isolators may
be required. Some free-standing isolators or other types of supports
(like rigid tripods) must be grouted to the floor if the floor’s surface
has a poor surface quality. Quick-setting “ready-mix” concretes
or epoxies are well suited for this purpose.
|3.2 Safety Features
|The ease with which
pneumatic isolators can lift payloads weighing several thousand
pounds belies the severity of their burden. By tying isolators together
with “tiebars,” the
risk of toppling such massive loads through accident or events like
earthquakes is dramatically reduced. TMC’s
tiebars are heavy-gauge, formed channels which use constrained-layer
damping to prevent them from resonating. Such damping is hardly
required, however, since the isolation efficiency of the isolators
at those frequencies is extremely high. Systems can also be provided
with earthquake restraint brackets which prevent the payload from
shaking off the isolators in an extreme event.
|Of great importance
to safety are the travel limits built into all TMC’s isolators. Figure
6 shows an internal “key” (yellow) which prevents
the system from overextending even when pressurized to 120 psi (830
k Pa ) under “no load” conditions. Since there can be
several thousand pounds of force behind the isolator’s piston,
an isolator without such a travel limit can quickly become a cannon
if suddenly unloaded. Protection, such as chain-linked pressure
reliefs, does not provide the intrinsically high level of safety
a mechanical travel limit does.
|3.3 Leveling Valves
|All rigid payloads,
even those with ten isolators, use only three height control valves.
Because three points define a plane, using a greater number of valves
would mechanically overconstrain the
system and result in poor position stability (like a four-legged
restaurant table) and a continuous consumption of air. Proper placement
and plumbing of these three valves is crucial to optimizing the performance
of a system.
Figure 7a and
show the typical plumbing for a 4-post and 6-post system.
A system contains three valves, a pressure regulator/filter (optional),
some quick-connect tees and an orifice “pigtail” on
each isolator. The pigtail is a short section of tubing with an
orifice inserted inside. This section is marked with a red ring,
and has a union on one end to connect to the height control valves’ air
lines. A mechanical valving system is a type of servo, and these
orifices limit the “gain” of the servo to prevent oscillation.
Some very high center-of-gravity systems may require smaller orifices
to prevent instabilities. TMC uses fixed orifices rather than adjustable
needle valves because of their long-term stability and ease of
|In a system with four
or more isolators, two or more of those isolators need to be tied
together. Usually the valve is mounted near an isolator (for convenience)
and that isolator is called the “master.” The remote
isolators(S) using that valve are called “slaves.” Choosing
which legs are “master” and “slave” affects
the stability of the system (see Section 3.4) and has a large impact on a system’s dynamic behavior.
Dynamic performance is particularly important in semiconductor inspection
machines which have fast moving stages. There are several “rules
of thumb” which can be applied to make the correct choice.
These can conflict with each other on some systems. Some experimentation
may be required to determine the optimal choice.
|These rules, in approximate
order of importance, are:
1. The effective support point for a master and its slaves is at their geometric center. For a master with a single slave, this point is midway between the mounts. There are always only three “effective” support points for any system. Connecting these points forms a “load triangle.” The closer the payload’s center-of-mass (COM) is to the center of this triangle, the more stable the system will be. For example, on a 4-post system, the master/slave combination should support the lighter end of the payload .
2. A corollary to rule
#1 is that the system should be plumbed so that the pressure difference
between all isolators is minimized.
|In addition to valve
location, there are several different types of
valves which are available. TMC offers standard and precision mechanical valves. The standard valve
is less expensive and has a positioning accuracy
(dead band) of around 0.1 in. (2.5 mm). It has the property that
the valve is tightly sealed for motions smaller than this. This makes
it ideal for systems which must use pressurized gas bottles for an
air supply. Precision valves offer a 0.01 in. (0.3 mm) or better
positioning accuracy but leak a very small amount of air (they use
all-metal valve seats internally). This makes them less suitable
for gas bottle operation. Finally, TMC offers electronic valving
systems such as the PEPS® (Precision
Electronic Positioning System, U.S.
Patent No. 5,832,806), which has a 0.0001in.
( 2 m)
position stability. For more information visit
the PEPS and PEPS-VX pages.
|For cleanroom applications,
TMC offers versions of the mechanical
valves made from stainless steel and/or supplied with a vented
|Like a pen balanced
on its tip, payloads supported below their center of mass are inherently
unstable: As the payload tilts, its center-of-mass moves horizontally
in a way that wants to further increase the tilt. Fighting this
is the stiffness of the pneumatic isolators, which try to restore
the payload to level.
|The balance of these two forces determines whether the system is gravitationally stable or not. Figure 8 shows a payload supported by two idealized pneumatic isolators. The width between the isolators’ centers is W, the height of the payload’s COM is H above the effective support point for the isolators, and the horizontal position of the COM from the centerline between the isolators is X. It can be shown that there is a region of stability given by the condition:|
|or, for X = 0,
where n is the gas constant
and is equal to 1.4.
|This relationship is
shown in Figure 8 as an inverted parabola which defines the stable and unstable regions for the COM location. The second equation clearly
shows that the stability improves with the square of
the isolator separation. This is important as it demonstrates that
it is not the aspect ratio H/W that determines the stability of a
system (as some references claim) and that the stable region is not
a “triangle” or “pyramid.” Unfortunately,
real systems are not as simple as the one in Figure 8.
|The ratio A/V in Equations
10 and 11 represents the stiffness of the isolators (see Equation 9). In a two-chamber isolator, however, what is the proper V? Unlike the isolators in Figure
8, which have a fixed spring constant, real isolators have
a spring constant which is frequency dependent. At high
frequencies, the orifice between the two chambers effectively blocks
air flow, and V may be considered the top air volume alone. At
the system’s resonance, the “effective” air volume
is somewhere between the top and total (top plus bottom) volumes.
At low frequencies, the action of the height control valves gives
the isolators an extremely high stiffness (corresponding to a very
small V). Moreover, the action of the height control valves also
tries to force the payload back towards level. These are only a
few reasons why Equation
10 can’t be applied to two chamber isolators. Instead,
we assign three regions: stable, unstable, and borderline, the
first two being based on the “total” and “top
only” air volumes, respectively. The stability region is
also different for the axes parallel and perpendicular to the master/slave
9 defines the two different axes for a four-leg system. The
pitch axis is less stable because the master/slave legs on the left
of the figure offer no resistance to pitch at low frequencies (though
they do resist pitch at frequencies above 1 Hz). To compensate for this, the master/slave combination is chosen such that Wp is greater than Wr (rule 3 from Section 3.3). The region of stability is the volume defined by the inverted parabolas along the two axes.
|The condition for absolute stability is:
|and the formula for absolute
|with the volume between
being “possibly” or “marginally” stable.
The ratios A/V are not universal and should be confirmed for different
capacities and models of isolators but are approximately 0.1 in–1 for
(A/V)Top and 0.05 in–1 for (A/V)Tot .
Figure 10 illustrates what the marginally stable region looks like
for two chamber isolators. Unfortunately, the COM of many systems
ends up in this indeterminate region. These rules do not account
for the actions of the height control valves, which will always
improve a system’s stability. If the payload has a mass which
can shift (a liquid bath or a pendulum) these rules can also change.
14 and 15 give “rules
of thumb” for calculating the stability of a system. As with
all such rules, it is only an approximation based on an “average” isolation
system. It is always best to use as low a COM as possible.
|Because MaxDamp® isolators use a single air chamber, they are more stable, and the rule becomes:
Note that the effective
support point for TMC’s Gimbal
Piston™ isolators is approximately 7 in. below the top
of the isolator. For lightly loaded isolators, these rules underestimate
system stability. If your system violates these equations, or is
borderline, the stability can be improved using counterweights,
special volume isolators, different isolator valving, etc. Contact
a TMC Sales
Engineer for advice on the best approach.