Pneumatic isolator with barometric insensitivity

[0002] 1. Field of Invention

[0003] An improved pneumatic vibration isolation system which has strong immunity from fluctuations in barometric pressure.

[0004] 2. Description of the Relevant Art

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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 4×10⁻⁶inches is assumed. This results in a force on the example payload of 7.1×10⁻⁴pounds 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.

[0009] 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.

[0010] The present invention eliminates this source of noise by adding a downward-facing piston to the isolators.

[0011] 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.

[0012] 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.

[0013] 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^stand 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.

[0014] 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.

[0015] 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.

[0016] 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 absolute minimum volume on the downward-facing piston's air chamber, improving the compactness of the isolator. In addition, the lower pressure vessel also supplies an upward force, and aids in the support of the payload. This reduces the pressure required in the upper upward-facing piston's chamber, and thus reduces the stiffness of its air spring. This pressure compensation is done automatically by the height control valving system. These systems are well known in the art.

[0017] In this embodiment, the upper and lower pistons are secured to one another by rigid rods external to the pendulum. To prevent the connecting rods from bending when the payload tilts, a thrust bearing is added to the top supporting surface of the isolator.

[0018] In principle, the preferred embodiment has a lower spring constant than a normal pneumatic isolator. In practice, there is a stiffness associated with the rolling rubber diaphragms, and the addition of the second lower diaphragm increases the isolator's stiffness.

[0019] To minimize the impact of additional lower diaphragms on a system's stiffness, these diaphragms are made very thin and flexible. This is possible because they only need to support 15 psi of pressure difference, unlike the upper piston which must sustain high pressures (up to 120 psi or more) to support heavy payloads. Typically, an upper diaphragm can have a thickness of 0.6 mm to 1 mm and a lower diaphragm, in this embodiment, a thickness of 0.4 mm or less.

[0020] The required vacuum does not have to be extremely good, and can be provided by a simple venturi-type vacuum generator which can run off the same air line which supplies pressure to the height control system. All isolators in a system can share the same vacuum generator, since the isolators are nominally air-tight, and there is no load on the generator.

[0021] In an alternate embodiment, the lower pressure vessel is pressurized with enough air to shape the diaphragm. The pressure is set with a simple pressure regulator, which can be shared with all isolators in a system. The pressure must be set low enough so the lower pressure vessel does not significantly increase the overall vertical stiffness of the isolator. To compensate for the downward force introduced by the lower piston, the pressure in the upper pressure vessel must be increased by an amount equal to the pressure in the lower pressure vessel. This compensation is done automatically by the height control valving system.

[0022] It is also possible to use downward-facing pistons with different surface areas (evacuated or pressurized). For example, a system consisting of several conventional isolators can have a single downward-facing piston if its area is equal to the sum of the upward-facing piston areas. Likewise, there may be more downward-facing pistons than upward-facing as long as the total area of upward and downward facing pistons is equal. In addition, they must have the same effective center-of-area to prevent pressure fluctuations from inducing tilts on the payload.

[0023] Another embodiment uses piston areas of different sizes, but has mechanical linkages (levers) to affect the same force. For example, the downward-facing piston could have one-half the area of the upward-facing piston if it was connected to the payload via a 2:1 lever mechanism. This would produce a force equal to that which would be generated by a full-sized diaphragm.

[0024] FIG. 1 is a front sectional view of the preferred embodiment of the invention; and

[0025] FIG. 2 is a front sectional view of an alternative embodiment of the invention; and

[0026] FIG. 3 is a front sectional view of another alternative embodiment of the invention.

[0027] Referring to FIG. 1, an isolator is shown generally at 10 and comprises three or more wires 12 (only two shown) each having an upper end 14 and a lower end 16. The lower ends 16 of the wires 12 are flexibly secured to and support a pneumatic isolator assembly 18. The upper ends 14 of the wires 12 are flexibly secured to a structure 15 supported by earth.

[0028] The assembly 18 comprises an upper pressure vessel 20 and a lower pressure vessel 50. The pressure vessels 20 and 50 share a common wall 22 and are partitioned by a sealing bulkhead 24.

[0029] Inside the upper pressure vessel 20 is a first upper pneumatic chamber 26 and a second lower pneumatic chamber 28 which are connected by a. flow-restricting orifice 30. The flow of air through 30 provides damping in the isolator 10. The control of the pressure is done by a valving system (not shown) connected at 32.

[0030] The top of the chamber 26 is sealed by a fabric-reinforced rubber diaphragm 34 which supports an upper piston 36. The diaphragm 34 is secured to the piston 36 by a clamping disk 38, and to the wall 22 by a clamping ring 40. The piston 36 is secured to a load disk 42. A payload 44 is supported by a semispherical bearing 46 which is fastened to the load disk 42 which allows the payload to tilt. When the chamber 26 is pressurized, the payload 44 is lifted, and the assembly 18 functions as a vertical pneumatic vibration isolator.

[0031] The lower pressure vessel 50 is separated from the upper pressure vessel 20 by the bulkhead 24. The bottom of the pressure vessel 50 is sealed by a diaphragm 52 which forms around a piston 54. The diaphragm 52 is clamped to the piston 54 by a clamping disk 56. The disk 56 is secured to a lower plate 58 which has rigid rods 60 connecting the plate 58 to the load disk 42. The outer edge of the diaphragm 52 is fastened to the wall 22 by a clamping ring 62, and is kept in proper shape by a ring 64. The wall 22, the bulkhead 24, the diaphragm 52 and the piston 54 define an air chamber 66. The rods 60 transfer the upward force generated by barometric pressure acting on the piston 54 to the payload 44 through the disk 42. The equal and opposite-facing areas of pistons 36 and 54 effectively cancel any fluctuations in barometric pressure. The air chamber 66 is evacuated by a vacuum generator (not shown) connected to the port 68.

[0032] FIG. 2 shows an alternate embodiment of the invention and an isolator is generally shown at 100 and comprises three or more wires 112 (only two shown) each having an upper end 114 and a lower end 116. The lower ends 116 of the wires 112 are flexibly secured to and support an pneumatic isolator assembly 118 which comprises an upper pressure vessel 120 and a lower pressure vessel 150. The pressure vessels 120 and 150 share a common wall 122 and are partitioned by a sealing bulkhead 124.

[0033] The upper ends 114 of the wires 112 are flexibly secured to a structure 115 supported by earth. Inside of the pressure vessel 120 is a first pneumatic chamber 126 and a second pneumatic chamber 128 which are connected by a flow-restricting orifice 130. The flow of air through 130 provides vertical damping in the isolator. The control of the pressure is done by a valving system (not shown) connected at 133.

[0034] The top of the upper chamber 126 is sealed by a fabric-reinforced rubber diaphragm 132 which supports an upper piston 134. It is secured to the piston 134 by a clamping disk 136, and to the wall 122 by a clamping ring 138. The piston 134 is secured to a load disk 140. A payload 142 is supported by the load disk 140. When the chamber 126 is pressurized, the payload 142 is lifted, and the assembly 118 functions as a vertical pneumatic vibration isolator.

[0035] The lower pressure vessel 150 comprises an air chamber 152 which is separated from the upper pressure vessel 120 by the sealing bulkhead 124. The bottom of the chamber 152 is sealed by a diaphragm 154 which forms around a lower piston 156. The diaphragm 154 is clamped to the piston 156 by a disk 158. The outer edge of the diaphragm 154 is fastened to the wall 122 by a clamping ring 160. The piston 156 is secured to a lower plate 162 which has flexible cables 164 connecting the plate 162 to the load disk 140. The flexible nature of the cables 164 allows the payload 142 to tilt without restriction. The cables 164 transfer the downward force generated by the pressure in the lower chamber 152 acting on the piston 156 to the payload 142. The equal and opposite-facing areas of pistons 134 and 156 effectively cancel any fluctuations in barometric pressure. The air chamber 152 is pressurized by a pressure regulator (not shown) connected to a port 166.

[0036] FIG. 3 shows another alternative embodiment of the invention and a vertical pneumatic isolator assembly is shown generally at 200. The assembly 200 has a wall 202 which is rigidly secured to earth. The assembly 200 comprises an upper pressure vessel 206 and a lower pressure vessel 240. The pressure vessels 206 and 240 have in common the wall 202 and are partitioned by a sealing bulkhead 210.

[0037] The upper pressure vessel 206 has a pneumatic chamber 212. A source air chamber 214 (not to scale) is connected to the chamber 212 through an orifice 216. The flow of air through orifice 216 provides vertical damping in the isolator assembly 200. The control of the pressure is done by a valving system (not shown) connected to 219. When the chamber 212 is pressurized, a load plate 234 lifts a payload 236 and the assembly functions as a vertical pneumatic vibration isolator.

[0038] A gimbal piston assembly 218 is received in the chamber 212 and the top of the chamber 212 is sealed by a fabric-reinforced rubber diaphragm 222 which supports an upper ring-like piston 224. The piston 224 is secured to the diaphragm 222 by a ring-like clamp 226 and to the pressure vessel 206 by a clamping ring 228. A piston well 230 depends from the piston 224. A support rod 232 is supported on and pivots on the floor of the well 230. The rod 232 supports a load plate 234. The well 230 acts like a pendulum, and provides horizontal vibration isolation when the system is inflated.

[0039] The pressure vessel 240 is dimensionally the same as the upper pressure vessel 206 and has a pneumatic chamber 260. A gimbal piston assembly 242 is received in the chamber 260 and the bottom of the chamber is sealed by a diaphragm 244 which supports a lower ring-like piston 250. The piston 250 is secured to the diaphragm 244 by a ring-like clamp 252 and to the pressure vessel 240 by a clamping ring 254. The equal and opposite-facing areas of the piston assemblies 218 and 242 effectively cancel any fluctuations in barometric pressure. The chamber 218 is pressurized through the port 246 by a pressure regulator set with enough pressure to form the diaphragm 244. Rods 248 transfer the downward force generated by the piston assembly 242 to the load disk 234.

[0040] The foregoing description has been limited to a specific embodiment of the invention. It will be apparent, however, that variations and modifications can be made to the invention, with the attainment of some or all of the advantages of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.