Spring devices – Vehicle – Comprising compressible fluid
Reexamination Certificate
2001-04-17
2003-04-15
Lavinder, Jack (Department: 3683)
Spring devices
Vehicle
Comprising compressible fluid
C267S064280, C248S631000
Reexamination Certificate
active
06547225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
An improved pneumatic vibration isolation system which has strong immunity from fluctuations in barometric pressure.
2. Description of the Relevant Art
In many sensitive instrumentation applications it is desirable to isolate a payload from ground vibrations. It is well known to integrate a pneumatic spring with a simple pendulum to isolate from vertical and horizontal ground noise respectively. Pneumatic isolators have an advantage over conventional springs in that they can maintain a payload at a given operating height independent of changes in the payload's weight. The vibration isolation characteristic of such isolators is also largely independent of the payload's weight. Payloads are generally supported by at least three isolators, with four being the most common number. More isolators can be used to support additional weight, with little change on the isolation system's performance. The height of the payload is maintained in such systems by a mechanical or electronic valving system which monitors the payload's height and adjusts the amount of air in each isolator. In this way the isolators can return to the same height with changing or shifting payload weights. Pneumatic isolators are typically of a two-chamber design, where motion of the payload forces air to move through a small orifice or flow restrictor. The resistance to this flow provides vertical damping in the isolators.
In normal pneumatic isolators, changes in barometric pressure generate noise forces on the payload. While the pressure in the isolator acts to supply a force in the upward direction, supporting the payload, barometric pressure acts on the top of a piston and supplies a downward force. This downward force is equal to the piston area times the barometric pressure. An increase or decrease in barometric pressure increases or decreases this downward-acting force on the piston. Since the pressure inside each isolator is constant (they are sealed air chambers), these pressure fluctuations are not seen on the bottom surface of the piston, and the resultant force acts as a source of noise on the payload.
Sources of barometric pressure fluctuations are common. They can be caused by the on-off cycling of HVAC (Heating, Ventilation, and Air Conditioning) systems, opening and closing of doors (which can change the loading on HVAC systems), wind, and changes in atmospheric conditions. These noise sources are distinct from acoustic noise because their low frequency means their acoustic wavelength is much longer than the typical dimensions of a building's room.
The degree of sensitivity is illustrated in the following example: Consider an 800 pound payload supported by isolators which have a 1.5 Hz natural resonant frequency. This means the combined vertical spring constant for the isolators is approximately 180 pounds/inch. Ground noise can vary by a factor of 100 or more, but a ‘typical’ value at 1 Hz of 0.1 micron or approximately 4×10
−6
inches is assumed. This results in a force on the example payload of 7.1×10
−4
pounds at 1 Hz. If the system consists of four isolators with 4 square inches of area each, then the downward force on the payload due to atmospheric pressure (which we assume is 15 psi) is 240 pounds of force. Thus a 3 ppm (part per million) change in barometric pressure causes a disturbing force to the payload equal to the contribution from ground noise. This pressure change is equivalent to the barometric pressure drop due to a 1.2 inch change in elevation, a very small number.
One environment where noise generated by HVAC systems is particularly severe is in semiconductor manufacturing facilities, where the use of pneumatic isolators is common. In these ‘cleanrooms’ air is aggressively cycled through ceiling-mounted HEPA filters down through grated floors (also known as sub-floors). Part of cleanroom design is to use positive pressure, so any leaks in the clean room only causes clean air to escape, rather than allow particulate-contaminated air to enter. As a result, whenever a door is opened to the cleanroom, air escapes and the room pressure drops. In such environments, barometric pressure fluctuations can become the dominant source of noise for payloads supported by pneumatic isolators.
The present invention eliminates this source of noise by adding a downward-facing piston to the isolators.
BRIEF SUMMARY OF THE INVENTION
Broadly the invention comprises an isolator which includes a vertical pneumatic isolator assembly and means for grounding the isolator assembly to earth. The assembly has a first pressurized air volume and a piston which vertically supports and isolates a payload. A second pressurized air volume is sealed with a second piston facing in the opposite (downward) direction. The second piston is also coupled to the payload and generates a force which compensates for fluctuations in barometric pressure. A minimum of three such isolators are typically used to support a payload.
Each pneumatic isolator assembly is grounded to earth directly or by a set of supporting wires. When wires are used they prevent the assembly from tilting with horizontal displacements of the payload relative to the earth, while providing a soft suspension for horizontal vibration isolation. There are at least three wires, each being grounded at its top (fastened to earth via a supporting structure) and connected to the pneumatic isolator assembly at its lower end.
The first pressurized air volume in the assembly is contained in an upper pressure vessel which comprises a pressure vessel wall, which wall is common to both the upper pressure vessel and a lower pressure vessel, an upwardly facing (upper) piston which supports the payload, a diaphragm which flexibly secures and seals the piston to the vessel wall, and a sealing bulkhead between the 1
st
and 2
nd
(lower) air chambers. Within this first (upper) pressure vessel are a first pneumatic chamber, which with the piston supports the payload, and a second pneumatic chamber connected to the first pneumatic chamber through a small orifice. As the payload moves in the vertical direction, air is forced to flow between the two chambers through the orifice. This provides a means for damping vertical oscillations of the payload. Fluidic damping (a bob fastened to the piston which moves through a viscous fluid) can also be used to damp motions.
The lower pressure vessel comprises a downward-facing (lower) piston flexibly secured to the vessel wall by a diaphragm. A pressure is applied to the lower pressure vessel such that the pressure differential across the lower diaphragm is enough to shape the diaphragm. This ensures the effective area of the lower piston matches the upper piston. As barometric pressure changes, the change in force on the upper piston is canceled by the (equal and opposite) barometric force acting on the downward-facing lower piston. Here, ‘effective piston area’ is the area which satisfies the equation (force)=(pressure differential)×(effective area) where the force is the result of an applied pressure differential across the isolator piston.
In general, the downward-facing piston(s) can be located anywhere between the payload and earth, as long as the total area of up-facing and down-facing piston areas are equal. Optimal cancellation occurs when the lower piston acts on the same point as the upper piston. The downward-facing piston preferably acts co-linearly with the upward facing piston, and the pistons are coupled one to the other with either flexible cables or rigid rods.
In the preferred embodiment, the lower pressure vessel is evacuated. This has several advantages over pressurizing the lower pressure vessel. A vacuum generates approximately 15 psi of pressure difference across the diaphragm, which is more than enough to form the diaphragm to shape. Because the pistons work on the compressibility of air, the air-spring constant of the evacuated vessel is zero (there is no air to compress). This allows one to use an ab
Carpenter, Esq. Brian A.
Lavinder Jack
LeBoeuf, Lamb, Greene & Macrae, L.L.P.
Pezzlo Benjamin A
Technical Manufacturing Corporation
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