This paper presents experimental results in active vibration isolation using a new version of the adaptive Least Mean Squares control algorithm. The algorithm is implemented digitally as a tracking notch filter in an experiment where the actuator and error sensor are collocated. A pair of all-pass filters are utilized to generate the sine and cosine signals needed by the adaptive filter and troublesome modes of the plant are identified off-line and cancelled by direct inversion. The closed loop system shows two orders of magnitude reduction in force transmissibility over open loop and over a decade of frequencies.

A two stage, position stabilized vibration isolation system has been constructed and is now in operation at the Cold Neutron Research Facility of the National Institute of Standards and Technology, Gaithersburg, MD. The system employs pneumatic isolators with a multiple input/multiple output pneumatic servo system based upon pulse width modulation control loops. The first stage consists of a 40,000 kg reinforced concrete table supported by pneumatic isolators. A large environmentally isolated laboratory enclosure rests on the concrete table. The second stage consists of a 3000 kg granite optical table located within the enclosure and supported by another set of pneumatic isolators. The position of the two stages is monitored by proximity sensors and inclinometers with 12 degrees of freedom. The system controls 12 independent pneumatic airsprings. The signals from these sensors are fed into a personal computer based control system. The control system has maintained the position of the two stages to better than 1 micrometers in translation and 5 (mu) rad in orientation for a period of a few months. A description of the system and its characteristics is given.

We are developing an active vibration isolation system for reducing the disturbing effects of ground motion about six orders of magnitude between 1 and 30 Hz. The system consists of three cascaded stages, each comprising a suspended platform on which are located seismometers and actuators for six degrees of freedom. Our goal is to reduce the vibration noise to [1 X 10^-13 X (1 Hz/f)^2.5 + 3 X 10^-15] m/(root)Hz on the third stage. We have built and tested the initial stage, which is intended to reduce our laboratory noise level to that of a quiet site. The design, control system models, measured performance and current limitations for this stage are presented. The two main stages require a vacuum environment and interferometric readout. The design and models for the main stages are also presented.

A hybrid active-passive vibration isolation system is considered for isolating sensitive payloads from seismic level base disturbances and payload induced force disturbances. Stability robustness requirements justify a detailed investigation of system dynamics, highlighting physical consequences of conventional actively enhanced soft mounting. The proposed `hard- mount' strategy provides desired performance and stability robustness, confirmed by theoretical and experimental results.

The relative merits and shortcomings of three current state-of-the-art concepts for defining the damping characteristics of optical tables are presented and discussed. The methods presented are: (1) The traditional method using impulse vibration decay rates which are measured directly; (2) an experience derived comparative standard based on Corner compliance values which are measured by impact tests with a force gage and accelerometer analyzed on a two channel FFT computer, and (3) a recent hypothesis which imputes a table's maximum relative deflection based on Compliance damping data and an idealized analysis of random vibration response.

A series of Corner Compliance Measurements have been made with increasing static loads on a highly damped Research Grade honeycomb Optical Table with tuned absorbers. Test results are presented for static loads up to 50% of the table weight. Contrary to expectations, there is no significant degradation of maximum Compliance at table resonance as a result of the added loads.

The end of the Cold War has made large-aperture telescope technologies from the U.S. Strategic Defense Initiative program available for non-defense missions. Now, a four-meter aperture space telescope, a seventy percent larger aperture than that of the Hubble space telescope, has been proposed for a dual military and astronomical mission. An important part of the preliminary design work was to determine how to meet the telescope's pointing and jitter criteria. The telescope will be required to maintain an rms pointing accuracy of 24 nrad, preferably over periods of several hours. Vibration was a critical issue in the study because of the stringent pointing requirement, the relatively light structures desirable for spacecraft, thermal transients, the presence of disturbances from many spacecraft mechanisms (solar array drives, momentum wheels, thrusters, antenna steering mechanisms, etc.), and the many external appendages. The four-meter telescope design uses an inertial optical reference system combined with an actively controlled `fast steering mirror' in the target beam path to actively counteract vibration.

The performance of optical tools used by the microelectronics industry is sensitive to their vibration and noise environments. Such environments result in internal tool vibrations that degrade tool performance. Relative vibrations between a tool and the workpiece on the order of only a few microinches per second can prevent the tool from successfully performing its task. Despite this, vendors often provide no specifications for allowable noise and vibration environments for their tools. In other cases, vendors provide allowable specifications that are incomplete, difficult to use, and/or hard to interpret for design-evaluation purposes. As a result, microelectronics facilities usually have to be designed very conservatively (and thus expensively) to assure acceptable tool performance. In this paper, we recommend simple test procedures that both vendors and users can implement to obtain useful specifications for allowable environments.

TMC has developed a practical method of rigidly supporting vibration-sensitive and heavy equipment in raised-grid cleanroom floors. The direction application has been in semiconductor fabrication and testing areas. The primary interest is to provide stable support for such equipment where elastomeric, or `spring', suspension systems are not required for actual attenuation of vibration. A system of modular support stands and rigid platform tops is described. Accelerometers and a spectrum analyzer are used to make adjustments to the stands so as to optimize uniformity of support. Dynamic analysis is therefore an intrinsic part of the `system', and allows immediate evaluation of the overall vibration condition.

The National Physical Laboratory is planning to house the Division of Mechanical and Optical Metrology and the Division of Material Metrology in a new purpose built laboratory building on its site at Teddington, London, England. The scientific staff were involved in identifying and agreeing the vibration performance requirements of the conceptual design. This was complemented by an extensive surgery of vibration levels within the existing facilities and ambient vibration studies at the proposed site. At one end of the site there is significant vibration input from road traffic. Some of the test equipment is also in itself a source of vibration input. These factors, together with normal occupancy inputs, footfalls and door slams, and a highly serviced building led to vibration being dominant in influencing the structural form. The resulting structural concept comprises three separate structural elements for vibration and geotechnical reasons. The laboratories most sensitive to disturbance by vibration are located at the end of the site farthest from local roads on a massive ground bearing slab. Less sensitive laboratories and those containing vibration sources are located on a massive slab in deep, piled foundations. A common central plant area is located alongside on its own massive slab. Medium sensitivity laboratories and offices are located at first floor level on a reinforced concrete suspended floor of maximum stiffness per unit mass. The whole design has been such as to permit upgrading of areas, eg office to laboratory; laboratory to `high sensitivity' laboratory, to cater for changes in future use of the building.

In recent years, vibration has become increasingly important as a factor to consider in the design of microelectronics fabrication facilities. As the demand for higher precision tools increases, the importance of vibration control also increases. In fact, at the present time, many aspects of structure/foundation design are controlled by vibration considerations rather than the traditional needs for load-carrying capacity. Vibration design involves geotechnical considerations such as site soils characteristics, soil type, depth of soil medium to bedrock; foundation considerations such as native soil preparation, fill material selection and preparation, types of foundation; and structural considerations such as the type of floor/column system, location of isolation breaks, etc. This paper examines soil characteristics and foundation design considerations as they apply to vibration transmission and performance in a typical fabrication facility. Dynamic soil-structure interaction analysis is employed to perform a parametric study of the important geotechnical factors affecting vibration performance of such a facility. Different foundation types are investigated in an attempt to arrive at optimum foundation design systems for various areas of the facility. Guidelines are provided in the areas of site selection, soil preparation, and foundation design.

One instrument used in clinical medicine to create a high-contrast image of the body's soft tissues involves the principle of nuclear magnetic resonance. The machine is called a magnetic resonance imager (MRI). The MRI scanning process is analogous to optical devices in that it is somewhat sensitive to vibration. This paper describes some field tests intended to quantify the effect of vibration upon image quality as heavy vehicles passed near two on-grade MRI installations. The measured field data are compared to recently published criteria and also are correlated with test images created by the MRI during the vibration events. The paper includes recommendations for improving the format of the manufacturer's vibration specification.

The use of fiber Bragg grating sensors for distributed vibration sensing is described. This technology has the capability of allowing high resolution, multi-point dynamic strain sensing at various points along a single fiber cable. Results obtained with a four grating time-division multiplexed system are described.

Experiments have shown that the magnitude of the displacement of structural surfaces excited by periodic vibrations can be simply and very accurately measured employing Speckle Field Interferometry. A Speckle Field Displacement Interferometer can be used to quantitatively characterize the motion of vibrating structures to monitor, analyze, and control the induced behavior. This type of analytical approach is well suited to objects or devices which undergo small scale periodic vibration requiring measurement without contact. This includes active piezoelectric devices and transducers of all types.

We have developed a number of instruments that are capable of measuring vibrations at very low frequencies (below 1 Hz). Two of these instruments are a laser strainmeter, which can respond to strains with frequencies ranging from 0 to about 100 Hz and a borehole tiltmeter which can respond to deflections of the vertical or horizontal accelerations in the frequency range from 0 to about 1 Hz. We have also modified commercial gravity meters (vertical accelerometers) and have used them to make measurements at comparable frequencies. Most of our work has been motivated by geophysical considerations including the dynamics of the earth in seismically active regions, but the instrumentation might also be useful in vibration monitoring for other purposes.

Vibration monitoring is a valuable tool in the operation of facilities which house vibration- sensitive equipment. Monitoring can detect trends in the vibration environment that may signal the need for remedial action before vibration-related problems are encountered. Monitoring is invaluable in situations where construction work is carried out within, or close to, the facility. The monitor can alert the construction team to particular events that exceed a set vibration threshold. In this paper we describe our experience with the long-term use of a multi-channel monitoring system. This experience has provided insights to the problems that can be encountered and the ways in which these can be remedied.

We present results of detailed experimental study of threshold sensitivity of GaAs adaptive photodetector at 632.8 nm. The minimal experimentally detected sinusoidal phase modulation was approximately equals 1.6 X 10^-6 rad(root)mW/Hz that approached the theoretical limit for these devices (approximately equals 2 X 10^-7 rad(root)mW/Hz) and allowed detection of approximately equals 1.6 X 10^-7 micrometers vibration amplitudes with 1 mW laser power in 1 Hz bandwidth.

A new method for measuring vibrations using a quadrant position sensor has been developed. It allows non-contact measurement of natural frequencies of objects which are small or light and transparent. The natural frequencies in two dimensions can be received at the same time. The system consists of a laser light source, a quadrant position sensor and spectrum analyzer. The object is illuminated by divergence laser rays coming from a microscope objective. The vibration amplitudes are magnified and detected by the position sensor. By adjusting the distance between object and microscope objective or detector, the sensitivity and linearity of the system can be changed. The natural frequencies of a plastic plate were measured by this method. The experimental results agreed well with the calculation results according to the theory of material mechanics. This method is very useful in the research field of millibioflight. The natural frequencies of the dragonfly wing, which is lighter than 0.1 mg, has been measured by this method. We can determine the natural frequencies of bending and torsional deformation by using the information of two dimensions.

The 3-meter stroke `LAROCA' Delay Line (DL) was designed, built and tested in France in 1993, with full success in terms of performance and of length and loading scalability. The highly stringent stability requirements in delay line motion led to devise an original guiding and motorizing concept that affords outstanding displacement accuracy, minimizing lateral and, more importantly, longitudinal vibrations. LAROCA's architecture hinges on a two-stage concept: a high-precision upper carriage, for precision positioning, controlled for longitudinal vibration compensation by a high-bandwidth, hybrid analog/digital controller, and a less accurate lower carriage for global DL translation on a two-cylindrical-rail track using a classical position control system. Excellent interstage decoupling, using a contactless suspension, yields long-stroke capability together with the required stability of the high- precision stage. After a presentation of the LAROCA project requirements and an overview of the general architecture, this paper describes the overall servo-control subsystem. Finally some important results are presented.