Choose Appropriate Vibration Equipment
April 30, 2010
Vibration caused by traffic, machinery, HVAC systems, weather or structural natural frequency can damage sensitive equipment and reduce productivity. Vibration-isolation workstations and related equipment can control vibrations and improve equipment performance and productivity. Most building vibrations are vertical, with horizontal vibrations averaging 25-30% of the vertical-and, vibrations don’t have to be obvious for performance to suffer. Problems include signal noise, low-frequency jitter and image blur.
Choosing a SystemWhen you consider the cost of vibration isolation is minimal compared with producing products that are flawed or production that is stalled due to vibration problems, vibration control makes sense.
The two most important factors when choosing vibration-isolation equipment are natural frequency and isolation efficiency. The best systems achieve very low natural frequencies and attenuate most potentially damaging vibration amplitudes in the 5 Hz to 200 Hz broadband random-vibration spectrum.
The increased sensitivity of the latest research and testing equipment usually makes sophisticated vibration isolation a must. Modern vibration-control solutions range from simple breadboards to highly efficient systems constructed with technologies and materials that take them far beyond conventional vibration isolators such as rubber blocks and metal springs.
Breadboards are available in larger variety than ever before, in many sizes and thicknesses, with or without mounting holes. They can be used for certain inspection and assembly tasks that require little more than a flat, stable surface.
Passive Air Systems
A “passive” air system relies on manually inflatable air bladders (“air springs”) replenished with a hand pump or a tank of compressed air. Passive air workstations and benchtop platforms offer high-frequency vibration isolation for less critical applications or wherever low-frequency vibrations below 7 Hz are not expected.
Active Air Systems
Although an “active” air system also relies on a pressurized air source, it is self-leveling, automatically feeding or bleeding air from each isolator as needed to maintain the platform at a pre-set zero-deflection level as the load increases or decreases. Vibration-isolation efficiencies can approach 99% for vertical and 95% for horizontal.
Active air workstations provide stability for confocal, tunneling, atomic-force or optical microscopes; roundness checkers; balances; surface profilometers; and other equipment used in tasks including testing, inspections, wafer probin, and mask aligning in industries such as medical, semiconductor and aerospace engineering.
There are a variety of products that use active air systems. Personal workstations provide economical vibration isolation in tight spaces. High-performance workstations can be easily customized with accessories. High-capacity workstations often can handle heavy, tall or moving loads. Variable-height workstations reduce user fatigue and back stress by allowing the user to raise or lower the tabletop. Modern workstation tabletops may be aluminum, steel, or composite with plastic, anti-static or stainless-steel laminate and may have honeycomb-type cores of hexagonal cells for lighter weight without sacrificing rigidity. Granite tabletops are also available.
Workstations for use in cleanrooms are typically constructed of welded, stainless-steel, cylindrical braces to facilitate cleaning. Completely enclosed isolation modules, stainless-steel valves, and vented exhaust keep these workstations in compliance with cleanroom standards.
Active air benchtop platforms are smaller and enhance the performance of certain microscopes, microhardness testers, profilometers, balances, audio components and other devices. They are available with a variety of tops (stainless steel, plastic laminate). Some models offer horizontal as well as vertical vibration control.
Also available are modular mounts and platforms for large equipment weighing up to 20,000 pounds. These make use of passive or active air technology. For very large equipment, a raised-floor “island” can provide vibration isolation for a section of a room.
Active Electronic Equipment
Active electronic systems have microprocessors that use “active feedback” to sense and promptly eliminate vibrations. Rapidly and automatically adjusting to different loads, this type of system dynamically isolates all six translational and rotational modes of vibration.
Up to 500 times stiffer than an air table, an active electronic workstation can accommodate high-centers-of-gravity or moving loads. The best ones have ultra-low natural frequencies (<1 Hz) and produce no low-frequency resonance.
Active electronic workstations are ideal for check-weigher, atomic force microscopes, micro-hardness testers, profilometers and other sensitive equipment. Available in a variety of load capacities and tabletops, they can actively damp vibrations from 1.0 to 1000 Hz. Isolation efficiency is especially efficient over 2 Hz, typically reaching 99% (40 dB) at 10 Hz. For frequencies above 1000 Hz, these workstations provide passive vibration control.
Active electronic benchtop platforms provide portable vibration-isolation for interferometers, imaging microscopes, and other equipment. Their tops, with or without mounting holes, can be constructed of aluminum plate, ferromagnetic stainless steel, plastic laminate, or anti-static laminate.
Another type of vibration-isolation workstation makes use of a stiff spring and a “negative-stiffness” mechanism to provide ultra-low natural frequencies, high internal structural frequencies, and excellent vertical and horizontal isolation efficiencies for static loads. Horizontal isolation is provided by beam columns connected in series with a vertical-motion isolator. A recently introduced model for lighter loads uses trifilar pendulum mounts and offers automatic leveling. For supporting microscopes in semiconductor processing, aerospace engineering, medical research and other fields, these workstations can achieve superb vibration isolation: 93% isolation efficiency at 2 Hz, 99% at 5 Hz, and 99.7% at 10 Hz.
ConclusionContrary to some claims, vibration-isolation equipment performs at different efficiencies depending on the load. For the best performance, the weight of a typical load should be no more than 80% of the equipment’s rated load capacity.
Ergonomics are also important. There is no point in eliminating blur and jitter from microscope work, for example, if the operator cannot stay comfortable and alert. Good design can make vibration-isolation equipment user-friendly.
Many accessories are available to add convenience to workstations or optical tables, including shelves, instrument racks, electrical outlets, monitor supports, lighting, guard rails, padded armrests, keyboard shelves, auxiliary work surfaces, HEPA filters/blowers, retractable casters, cleanroom enclosure, Faraday cages to protect sensitive operations from electromagnetic interference and other tabletop enclosures to protect against harsh manufacturing environments. It is best to consult an expert to find the most appropriate equipment for a particular application.