Optical table active leveling and vibration cancellation system

An optical table active leveling and vibration cancellation system is disclosed. The system includes an optical table with three active isolation devices connected to the optical table, and one sole pneumatic device, such that the table is supported on all four corners. Each of the isolation devices includes a pneumatic suspension device and an electromagnetic suspension device. The pneumatic suspension device includes a pressurized load chamber, a gimballed piston received in the pressurized load chamber and coupled to the walls of the pressurized load chamber by a thin walled diaphragm, and a support rod extending from the optical table to the floor level. The electromagnetic bearing suspension device includes a magnetic bearing rotor having two planar surfaces and a peripheral surface, and two magnetic bearing stators, the stators each defining a central aperture and each stator being in a spaced apart relationship from each of the rotor planar surfaces. The rotor is secured to the support rod. Each of the stators includes an annular coil of wire. The electromagnetic suspension system may also include four radial electromagnets spaced around the peripheral surface of the rotor. The radial electromagnets also include an annular coil of wire. The system also includes three proximity sensors mounted under the optical table to detect motion of the table in relation to the floor or another fixed reference, and generate a signal in response to the motion. The system also includes three accelerometers mounted underneath the optical table in a triangle. The accelerometers measure vibration of the table in relation to inertial space and generate a signal in response to the movement. A controller receives the signals from the sensors and accelerometers, and generates a second signal, used to activate the pneumatic and electromagnetic bearing suspension devices to provide active leveling and vibration cancellation of the optical table.

FIELD OF THE INVENTION 
This invention relates to an active leveling and vibration cancellation 
system, and more particularly to an active leveling and vibration 
cancellation system utilizing pneumatic suspension elements in connection 
with electromagnetic bearings for improving the response time and 
performance of the system. 
BACKGROUND OF THE INVENTION 
Optical tables are used to support sensitive instrumentation, and thus 
require precise leveling and freedom from vibration. In a typical optical 
table, the optical table top is supported by a suspension system or table 
mount. Three types of suspension systems are known in the art, namely 
rigid mounts, pneumatic self-leveling mounts, and elastomeric or pneumatic 
mounts augmented by very expensive electromagnetic suspension systems. 
The first mentioned suspension systems, rigid mounts, although the least 
expensive, are only suitable for applications where leveling is required, 
but not vibration isolation. The third mentioned suspension systems, 
elastomeric or pneumatic suspension systems augmented by expensive 
electromagnetic suspension systems, are very expensive, and are tailored 
to the specific needs of a small segment of the active vibration 
cancellation market. Therefore, the most commonly used suspension systems 
are the pneumatic self-leveling suspension system. 
The pneumatic self-leveling suspension systems typically comprise sensors, 
pneumatic controls, and structural supports. Each of the vibration 
isolators in the system act as air springs in that they utilize the 
compressibility of air contained in a chamber, a sealing element, and a 
piston to produce the characteristics of a low frequency spring. Thus the 
pneumatic self-leveling system can accommodate varying loads without major 
deflections by varying the air pressure in the air chamber. 
However, one problem with the pneumatic self-leveling suspension systems is 
that the systems are incapable of isolating the table from disturbances 
either generated directly on the table, or otherwise directly impacting 
the table. For example, rapid load shifts on the table may cause large 
direct disturbances, such that the table is no longer level. As another 
example, rotating or moving components on the optical table top will cause 
the table to respond more than a comparable table placed on rigid mounts. 
More specifically, the small disturbances on the table may cause the table 
to vibrate in unison with the disturbances, while the table appears to 
remain level. Moreover, if not properly designed, the system itself may 
generate a significant amount of vibration as a result of the contacting 
forces on the table by mechanical lever arms, or allow the passage of 
vibrations by the hysteresis of the thin rubber skins used as rolling 
diaphragms in the system. 
Attempts have been made to improve the pneumatic suspension systems. For 
example, efforts have been made to reduce the effect of changes in the 
static load on the systems, thus creating zero deflection pneumatic 
suspensions. The zero deflection pneumatic suspensions make small 
adjustments to the pneumatic flexibility of the suspensions. The changes 
in flexibility are made by adjusting the pressure in the load bearing. The 
pressure adjustments are continued until the suspended entity to the 
position and/or orientation it occupied prior to the change in the static 
suspension load. The problem however, with such known types of the zero 
deflection devices is caused by their slow response. The slowness of the 
response to changes in static load renders the systems ineffective for 
many applications. 
Efforts have also been made to reduce the effect of the dynamic forces on 
the suspended entities. Systems have thus been developed using transducers 
to ascertain the characteristics of the disturbance forces, wide bandwidth 
analog or high-speed digital techniques to determine cancellation forces 
proportional to the additive inverses of the dynamic forces, and 
transducers again to impart the cancellation forces on the suspended 
entities. However, several problems existed with the transducers used in 
these known systems. For one, the transducers impart the cancellation 
forces on the suspended entities at points different than the points of 
suspension, reducing the effectiveness of the system. Secondly, the 
cancellation forces created by the system are too small in comparison to 
the forces needed to minimize the suspended entity's response to sudden 
dynamic forces created by movement of components connected to the 
suspended entity. 
Therefore, a need exists for an optical table vibration cancellation and 
active leveling system that provides a quick response to changes in static 
and dynamic load. Also, a need exists for an optical table vibration 
cancellation and active leveling system that provides sufficient forces at 
the points of suspension to minimize the effect of sudden forces exerted 
on the optical table. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an optical table active 
leveling and vibration cancellation system that provides a quick response 
time for active leveling and vibration cancellation. 
Another object of the present invention is to provide an optical table 
active leveling and vibration cancellation system that imparts 
cancellation forces at the suspended entity's various points of 
suspension. 
A further object of the present invention is to provide an optical table 
active leveling and vibration cancellation system that is inexpensive to 
manufacture and install. 
Another object of the present invention is to provide an optical table 
active leveling and vibration cancellation system that is easily installed 
on existing optical tables. 
A still further object of the present invention is to provide an optical 
table active leveling and vibration cancellation system that includes the 
advantages provided by pneumatic isolators. 
Another object of the present invention is to provide an optical table 
active leveling and vibration cancellation system that provides for 
control along six degrees of freedom. 
Yet another object of the present invention is to provide an optical table 
active leveling and vibration cancellation system that maintains the 
optical table level during rapid load shifts. 
A further object of the present invention is to provide a optical table 
active leveling system that eliminates the resonant peak characteristics 
of lightly damped pneumatic isolators. 
These and other objects of the present invention are achieved through an 
optical table active leveling and vibration cancellation system comprising 
an optical table having two surfaces, a sensor for detecting motion of the 
table and generating a first signal in response to the motion, at least 
one active isolation device connected to one of the the optical table 
surfaces, and a controller for receiving the first signal from the sensor 
and generating a second signal in response to the first signal, wherein 
the second signal is used to activate the active isolation device to 
provide active leveling of the optical table. 
Each of the isolation devices comprises a pneumatic suspension device and 
an electromagnetic suspension device. The pneumatic suspension device 
further comprises a pressurized load chamber, a gimballed piston received 
in the pressurized load chamber and coupled to the walls of the 
pressurized load chamber by a thin walled diaphragm, and a support rod 
extending from the optical table to a pivot point in the pressurized 
chamber. The electromagnetic bearing suspension device further comprises a 
magnetic bearing rotor having two substantially planar surfaces and a 
periphery, and two magnetic bearing stators each being in a spaced apart 
relationship from each of the rotor surfaces. The rotor is attached to the 
support rod, and each of the stators defines a central aperture. Each of 
the stators also includes an annular wire coil. The electromagnetic 
suspension device is preferably coupled to the pneumatic suspension device 
in parallel. 
The electromagnetic suspension system may also include a magnetic radial 
bearing comprising four radial electromagnets substantially spaced apart 
around the peripheral surface of the rotor, and at least one accelerometer 
for detecting horizontal vibration of the table and providing a horizontal 
vibration signal to the controller in response to horizontal vibration of 
the table. The second signal is generated by the controller also in 
response to the horizontal vibration signal. 
These and other objects of the present invention will now become apparent 
from a review of the drawings and the following description of the 
preferred embodiment.

DETAILED DESCRIPTION 
Referring now to FIG. 1, one embodiment of an optical table active leveling 
and vibration cancellation system 12 of the present invention is shown. In 
the embodiment shown, the system 12 includes an optical table 14, three 
sensor pairs 16, four pneumatic suspension devices 18, four 
electromagnetic suspension devices 20, and a digital controller 22. In the 
embodiment shown, the optical table 14 has an upper surface 24 and a lower 
surface 26. The sensor pairs 16, pneumatic suspension devices 18, and 
electromagnetic suspension devices 20 are all mounted on the lower surface 
26 of the optical table 14. One of the electromagnetic suspension devices 
20 and a corresponding one of the pneumatic suspension devices 18 combine 
to form an isolator assembly 28. It is to be noted that the pneumatic and 
electromagnetic suspension devices are not required to be mechanically 
interconnected to form an isolator assembly. Instead a pneumatic 
suspension device and an electromagnetic suspension device may be coupled 
in that both directly affect the optical table or other supported entity, 
as best shown in FIG. 7. 
In FIG. 1, one of the isolator assemblies 28 is located at each of three 
corners of the optical table 14, and the fourth corner is solely a 
pneumatic device 18. Therefore, the table 14 is supported at all four 
corners. Each of the isolator assemblies 28 is also coupled to the sensor 
pairs 16 and the digital controller 22, as described later in this 
specification. However, the system may utilize various combinations of 
pneumatic suspension devices and electromagnetic devices. For example, 
four pneumatic and four electromagnetic suspension devices may be 
utilized. 
Each of the sensor pairs 16 may be comprised of at least one proximity 
sensor 30 and an accelerometer 32. The proximity sensor 30 detects 
movement of the table in relation to the floor surface 34 or another fixed 
reference. The accelerometer 32 detects movement of the table in relation 
to inertial space. The sensor pairs 16 are preferably mounted on the lower 
surface 26 of the optical table 14. A second proximity sensor may be 
mounted to the pneumatic suspension device outer wall, which extends 
orthogonal to the floor surface, in order to sense movement of the table 
along an axis parallel to the floor. The sensor pairs, may comprise other 
types of sensing devices that detect motion of the table in relation to a 
fixed reference or inertial space. 
Referring now to FIG. 2, a first embodiment of one of the isolator 
assemblies 28 used in the optical table active leveling and vibration 
cancellation system 12 is shown in detail. The isolator assembly 28 shown 
comprises a support plate 36, the electromagnetic suspension device 20, 
and the corresponding pneumatic suspension device 18. In the embodiment 
shown, the support plate 36 is attached to an upper end 38 of a support 
rod 40 which extends through the pneumatic and electromagnetic suspension 
devices 18, 20. The support plate 36 is attached to the optical table 
lower surface 26 and used to support the optical table 14. The 
electromagnetic suspension device 20 is located below the support plate 
36, and the pneumatic suspension device 18 below the electromagnetic 
suspension device 20. Thus, in the embodiment shown, the electromagnetic 
device is in direct physical contact with both the pneumatic suspension 
device and the optical table. It should be noted however, that this 
orientation is not required for use of the invention. For example, the 
electromagnetic suspension device 20 may be oriented vertically below the 
pneumatic suspension device 18 and the support plate 36, within the 
pneumatic suspension device 18, or horizontally spaced from and 
substantially parallel to the pneumatic suspension device 18. 
Still referring to FIG. 2, the pneumatic suspension device 18 is similar to 
several suspension systems known in the art. By way of example, a 
Technical Manufacturing Company's Gimbal Piston isolator may be used. The 
pneumatic suspension device 18 shown comprises a the support rod 40, a 
piston 42, a pressurized load chamber 44, a soft flexure 46, and a 
thin-walled rolling diaphragm 48,. The support rod 40 is received within 
the gimballed piston 42, such that a lower end 50 of the support rod 40 is 
fixed in the soft flexure 46. The upper end 38 of the support rod 40 
includes the support plate 36. The pressurized load chamber 44 surrounds 
the gimballed piston 42, with the thin walled diaphragm 48 connecting the 
piston 42 to the wall of the pressurized chamber 44. 
The pneumatic suspension device 18 operates such that horizontally directed 
forces activate the piston 42 in a gimbal-like fashion, as known in the 
art. The piston 42 and diaphragm 48 respond to the horizontal forces by 
translating the horizontal motion into vertical motion. The piston 42 is 
also protected against overtravel in the vertical direction. 
Still referring to FIG. 2, the electromagnetic suspension device 20 is 
described. The electromagnetic suspension device 20 shown in FIG. 2 
comprises a magnetic bearing rotor 54, and two magnetic bearing stators 
56. The magnetic bearing rotor 54 is securely attached to the support rod 
40, and is substantially planar with an upper surface 58 and a lower 
surface 60. The magnetic bearing stators 56 each define a central aperture 
62. Each of the stators 56 is disposed in a spaced apart relationship from 
each of the surfaces 58, 60 of the rotor 54. An annular coil of wire 64 is 
shown in each of the stators 56. It should be noted that instead of the 
magnetic stators, a voice coil, or another type of device controlled by 
Lorentz' law may be used to provide the thrust electromagnetic force. 
The integration of the electromagnetic bearing suspension devices 20 with 
the pneumatic suspension devices 18 allows the isolator assembly 28 to 
have substantially similar dimensions to the previously used isolation 
device which only the incorporated the pneumatic suspension device. As a 
result, the new isolator assemblies 28 may easily replace the previously 
used pneumatic suspension isolators without requiring alterations to the 
optical table 14. 
Referring now to FIGS. 3 and 4, the first and a second embodiment of the 
electromagnetic suspension device 20 for the isolator assembly 28 are 
shown. The first embodiment, shown in FIG. 3 is similar to the embodiment 
shown in FIG. 2. As explained in more detail below, the first embodiment 
provides an electromagnetic force in a thrust direction, indicated by 
arrow A in FIG. 3, that extends in the same direction as the support rod 
40. The second embodiment, shown in FIG. 4, is comprised of an 
electromagnetic rotor 66, and four radial electromagnets 68. The 
electromagnetic rotor 66 is securely attached to the support rod 40. As 
shown in FIG. 4, the rotor 66 is preferably circular in cross-section, 
however, the rotor may also be substantially square having rounded 
corners. One of each of the four radial electromagnets 68 is disposed in a 
spaced relationship from the peripheral surface of the rotor 66. In the 
embodiment shown, the radial electromagnets 68 are spaced around the 
peripheral surface of the rotor 66. As an alternative embodiment, two or 
three radial electromagnets 68 may be used, with the two electromagnets 68 
disposed in a spaced apart relationship from the periphery of the rotor 
66. As yet another alternative embodiment of the invention, three 
electromagnets may be used. An annular wire coil 78 is shown in each of 
the radial electromagnets 68. It should be noted that instead of the 
radial electromagnets, a voice coil or other type of device controlled by 
Lorentz law may be used to provide the radial electromagnetic force. The 
attractive force of the radial electromagnets 68 extends radially from the 
center of the rotor 66, as shown by the arrows B in FIG. 4. 
The first and second embodiments of the electromagnetic suspension device 
20 may be combined to form a third embodiment of the invention. In the 
third embodiment, the rotors 54 and 66 are incorporated into a single 
rotor, however, both the stators 56 and the radial electromagnets 68 are 
included in the device. The third embodiment provides electromagnetic 
force in both the thrust and radial direction. 
With reference now to FIGS. 5 and 6, the results of using the three 
different embodiments of the electromagnetic suspension devices 20 is 
described. As shown in FIG. 5, the thrust electromagnetic configuration 
shown in FIG. 3 responds to static load changes and dynamic disturbances 
along three separate degrees-of-freedom in the optical table 14. Point X 
on FIG. 5 may represent a corner of the optical table 14. The three 
degrees-of-freedom to which the thrust electromagnetic bearing responds 
are (a) vertical translation up-down, represented by arrow C, (b) 
horizontal tilt roll, represented by arrow D, and (c) horizontal tilt 
pitch, represented by arrow E. As shown in FIG. 6, the radial 
electromagnetic configuration, shown in FIG. 4, also responds to static 
load changes and dynamic disturbances along three separate 
degrees-of-freedom to which the radial electromagnetic configuration 
responds are (a) yaw about the vertical axis, represented by arrow F, (b) 
front-to-back translation, represented by arrow G, and (c) side to side 
translation, represented by arrow H. The third embodiment, by utilizing 
both the thrust and radial electromagnet bearings, allows for control 
along all six degrees-of-freedom, or three lines of action. Therefore, 
different embodiments of the invention are available depending on which 
degrees-of-freedom require control. 
Referring now to FIG. 7, a block diagram of the electronics schematics for 
each of the isolators assemblies 28 is shown. The optical table 14 is 
coupled to both the proximity sensor 30 and the accelerometer 32. The 
proximity sensors 30 and accelerometers 32 are also coupled to the digital 
controller 22. The digital controller 22 comprises an analog to digital 
converter 70, a controller card 72, a digital to analog converter 74, and 
a current amplifier 76. The analog signals received from both the sensors 
30 and the accelerometers 32 are converted to digital signals by the 
analog to digital converter 70. The digital signals are read by the 
controller card 72, which produces a second digital signal, which is then 
converted back to a second analog signal by the digital to analog 
converter 74. The second analog signal is sent to the pneumatic suspension 
device 18, and, via the current amplifier 76, to the electromagnetic 
suspension device 20 in order to provide the active leveling and vibration 
cancellation of the suspended entity. 
The digital controller 22 may also contain power supplies. Depending on the 
embodiment of the invention used, the digital controller box 22 may 
further include sensor signal conditioners, and PWM drivers. The digital 
controller box 22 is preferably mounted under the optical table 14, and in 
one embodiment is preferably approximately 19.times.15.times.6 inches in 
dimension. 
As previously mentioned, the accelerometers 32 detect vibration of the 
table 14 in relation to inertial space. The accelerometers 32 serve the 
function of providing vibration feedback signals to the vibration 
cancellation system. Preferably, the system 12 utilizes high-sensitivity 
accelerometers, followed by a low-noise monolithic instrumentation 
amplifier, in order to keep the cost of the system low. The accelerometers 
32 also preferably have a noise spectra of approximately 16 to 30 decibels 
less than floor vibration spectra in a typical environment. As a result, 
the typical vibration spectra will drop by at least approximately 13 to 27 
decibels when the vibration cancellation loop is activated. The resonant 
frequency of the accelerometers 32 when mounted is approximately 550 
hertz. Alternatively, the system may use geophones or seismometers to 
detect the vibrations in the table. 
The proximity sensors 30 are mounted in a spaced apart relationship from 
the optical table lower surface 26 and are used to detect movement of the 
table 14 in relation to the floor or another fixed reference, for example, 
fixed reference that is perpendicular to floor level. The sensors 30 serve 
the functions of providing DC low frequency system reference, and relative 
table position feedback signals to the pneumatic suspension device and 
active leveling system. The sensors 30 preferably have a dynamic range of 
approximately 100 mils to 25 uinch (72 dB). The sensors 30 are also 
preferably low noise with a high selectivity filter circuit, and high 
bandwidth. As an alternative, the system may incorporate interferometers 
for sensing movement of the table in relation to a fixed reference. 
The digital to analog converter 74 and current amplifier 76 are utilized as 
the last two components to drive the electromagnetic suspension devices 
20. The converter 74 converts the analog signals from the sensors 30, 32 
to digital signals for use by the digital controller card 72. The 
converter 74 preferably has a 12 bit D/A range of 72 dB. The amplifier 76 
is preferably a low noise linear current amplifier with approximately a 
110 dB range. 
The table acceleration dynamic range is approximately 325 mg to 1 ug. The 
accelerometers 32 preferably operate in the lower portion of approximately 
1 ug to 4 mg. The proximity sensors 30 preferably operate in the higher 
range. Therefore, the full dynamic range is covered by two A/D channels 
driven by the accelerometers 32 and the proximity sensors 30. The digital 
controller 22 may also include a mode switch (not shown) to switch between 
the active leveling function and the vibration cancellation function. 
Alternatively, the system may be designed such that the controller 
continues from one function into the second function without operator 
intervention. 
In order to demonstrate the effectiveness of the isolators 28 of the 
present invention, the performance of one embodiment of the invention in 
response to a 100 pound force dropped at the optical table corner is 
described. In this example, the peak electromagnetic force of 60 LBF, 
provided by the electromagnetic suspension devices 20, is achieved almost 
instantaneously, at less than 0.02 seconds. The electromagnetic force then 
drops off such that it is approximately 25 LBF at 0.75 seconds, and 
approximately 1 LBF at 4 seconds. In comparison, at less than 0.02 
seconds, the pneumatic force, provided by the pneumatic suspension devices 
18, is approximately 1 LBF. The pneumatic force then rises such that at 
approximately 0.75 seconds, the pneumatic force equals the electromagnetic 
force of 25 LBF, and at 4 seconds, the pneumatic force is approximately 49 
LBF. This example demonstrates the advantages of the use of the 
electromagnetic suspension system in conjunction with the pneumatic 
suspension system. It is shown that the electromagnetic force responds 
almost instantaneously to the force drop. The pneumatic system is capable 
of taking over the reducing steady state forces on produced by the 
electromagnets so as to reduce power dissipation on the electromagnets. 
Therefore, the electromagnetic suspension system provides for a very quick 
response time, while the pneumatic suspension system provides for the 
extended force requirements of the system. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations, and 
modifications may be made within the scope of the invention. Thus, by way 
of example, but not of limitation, the optical table may actually comprise 
a suspended vertical surface as compared to a horizontal surface. 
Moreover, the isolators devices or active leveling and vibration 
cancellation system may be used to softly suspended or supported entities 
other than optical tables. Also, the system may be modified so as to 
include a laser system which is utilized to align tables in different 
locations, creating a net effect of electronically, as opposed to 
mechanically joining the tables, with each of the tables having its own 
active suspension system. Accordingly, it is to be understood that the 
present invention is not limited to the precise construction as shown in 
the drawings and described hereinabove.