Bad Vibrations: How to Keep the Effects of Environmental Bounce Out of Your Data

(2007 legacy article) - Whether it's an NMR or a two-photon microscope, scientists love toys - at least when they work. Sometimes the most mundane things bungle technology: environmental vibrations from cars driving by, central air conditioning, the voices of the operators, and even the ocean. As instruments become more sensitive, subsonic rumblings become more insidious, particularly for nano-technology applications. With many instruments, such as atomic-force and electron microscopes, cutting down on vibration is essential to collecting good data. "You could spend a million or two million on a microscope and have it rendered useless because of vibration," says Kurt Alberline, an anatomist at the University of Utah School of Medicine, who runs an electron microscopy lab.

When researchers suspect vibration is wreaking havoc on their data, they should identify the origin of the noise or get an environmental engineer to find it, say scientists who regularly deal with vibration. For example, Vicki Colvin, a chemist at Rice University in Houston, noticed images moving around in a circle on her transitional electron microscope. "It was like a ghost," she says. Colvin discovered that an air duct was causing the problem and spent $1.20 on a shield to divert air away from the scope. "The easiest way to get rid of vibrational noise is to stop it at its source." says Larry Cohen, a neuroscientist at Yale University.

The design of a building is critical to the vibration that reaches an instrument, says Ahmad Soueid, senior vice president at HDR Architecture in Omaha, Neb., which has designed more than a dozen nanotech laboratories. Isolating air-handling equipment from laboratories and using special joints that redirect vibration to the ground are some of the fixes his firm uses. Recently, concerns over vibration plagued a $250 million NIH facility under construction in Baltimore. Initial reports indicated the building's quivers could render confocal microscopes useless, although later measurements suggested most instruments will work with proper dampening

There's no universal fix, says David Platus, president of Minus K, a company that makes high-end vibration-isolation tables. Solutions vary, from cheap rubber pads that rest under instruments, to the air-cushioned tables that have been around for 50 years, to tables that sense vibration and cancel it out. "The more sensitive the instrument, the better isolation you need." he says.

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Example Causes of Bad Vibrations:
Automotive
Buildings
Columns
Computers
Elevators
Engines/Motors
Floors
Freeway & Road Traffic
Generators
HVAC (Heating & Air Conditioning Issues)
Machinery
Mechanical Entities
Plumbing, Piping
Pneumatic factors
Seismic Waves (including from ocean waves)
Trains & Subways
Transformers
Winds Against Buildings
(Examples with Hz...)

Featured Product: BM-1 Bench Top Vibration Isolation Platform

• Vertical Load Adjustment Crank - Simple manual adjustment compensates for changes in vertical load.
  
• Vertical Load Adjustment Indicator - Easily determine optimum setting using this simple visual indicator.
  
• Vertical Stiffness Adjustment Screw - Dial in Guaranteed 1/2 Hz or less Vertical natural frequency using this simple adjustment.
  
• Isolator Dimensions: 24" W x 22.5" D x 9" H (610mm W x 572mm D x 216mm H).
  
• Weight: Approximately 90 lb (41 kg) - Dimensions - an additional 0.675" taller than the above units
  

Vertical natural frequency of 1/2 Hz or less can be achieved over the entire load range.

Horizontal natural frequency is load dependent. 1/2 Hz or less can be achieved at or near the nominal load.

Specifications (pdf)

Pricing

Minus K vibration isolation systems can be designed for very heavy payloads. The following are some typical isolator dimensions. The 10,000 lb and 25,000 lb isolator dimensions are approximate and are based on preliminary designs.

XM-1: 10,000 lb capacity: 20"W x 20"D x 22"H

The James Webb Space Telescope is the largest cryogenic instrument telescope to be developed for space flight. It is a large-aperture infrared space telescope and the scientific successor to NASA's Hubble Space Telescope and used a set of six custom heavy capacity Minus K vibration isolators for ground testing.

The ground testing confirmed the telescope and science instrument systems will perform properly together in the cold temperatures of space. Additional test support equipment including mass spectrometers, infrared cameras and television cameras were also supported by Minus Ks heavy capacity vibration isolators which allowed engineers to observe the testing.

Each of the isolators was designed for 10,000 lbs. and the total payload supported from the top of the Johnson Space Center vacuum Chamber A was 60,000 lbs.

The isolators allowed NASA to simulate the telescopes performance in space while preventing all the ground-based disturbances, such as the pumps and motors, and even traffic driving by from interfering with the ground testing. Case study: NASA James Webb Space Telescope (JWST).

Vibration Criterion (VC) Curves-Lab Analysis
Codes and curve descriptions for different vibration environments and solutions.

The VC (Vibration Criteria) curves were developed in the early 1980s by Eric Ungar and Colin Gordon. They were originally developed as a generic vibration criteria for vibration-sensitive equipment for use in the semiconductor, medical and biopharmaceutical industries, but have found application in a wide variety of technological applications.

The criteria takes the form of a set of one-third octave band velocity spectra, together with the International Standards Organization (ISO) guidelines for the effects of vibration on people in buildings. The criteria apply to vibration as measured in the vertical and two horizontal directions.

The NIST-A criterion was developed for metrology, but has gained popularity within the nanotechnology community. The NIST-A criterion is a very difficult criterion to meet at some sites with significant low-frequency vibrations.

The VC curves are now widely accepted throughout the world as a basis for designing a facility to meet the requirements of a group of highly vibration sensitive equipment used close together.

University of Michigans Ultra-Low Vibration Lab (ULVL) was completed in 2014. After the construction, a vibration survey was done on the Ultra-Low Vibration Lab chambers. The measurements demonstrated that even when a single vehicle was driving on a nearby street, the vibrations exceeded the NIST-A specifications necessary for the ULVL.

The University of Michigan ordered seven customized tabletops and 31 custom Minus K Negative-Stiffness vibration isolators with pedestals provided for the eight Ultra-Low Vibration Lab chambers.

Customized Minus K Technology Negative-Stiffness vibration isolation table installed in one of the Ultra-Low Vibration Lab chambers

The final vibration survey by Colin Gordon Associates (CGA), after installation of the customized Minus K Negative-Stiffness isolators and tables, showed the measured vibration levels in all ULVL chambers from VC-K to VC-M at frequencies above 2.5 Hz, well below the NIST-A Vibration Criterion required.

"VC-M is the lowest we have ever measured, though we werent able to measure below 2.5 Hz because our most sensitive sensor wont go lower, due to sensor noise floor," said Hal Amick, Vice President of Colin Gordon Associates.

The updated VC Curve on Minus K's website shows these lower curve levels that were measured by CGA and have already assisted University of Michigans ULVL with two major scientific milestones.

Vibration site surveys can tell you a lot about how to
specify equipment for vibration isolation in your laboratory.

Updated VC Curve on Minus K's website...

How They Work>>Negative-Stiffness Vibration Isolators

Minus K® vibration isolators employ a revolutionary concept in low-frequency vibration isolation. Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with a negative-stiffness mechanism (NSM). The net vertical stiffness is made very low without affecting the static load-supporting capability of the spring. Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation. The horizontal stiffness of the beam-columns is reduced by the "beam-column" effect. (A beam-column behaves as a spring combined with an NSM.) The result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and very high internal structural frequencies.

Figure 1

Minus K® isolators typically use three isolators stacked in series: A tilt-motion isolator on top of a horizontal-motion isolator on top of a vertical-motion isolator. A vertical-motion isolator is shown in Figure 1. It uses a conventional spring connected to an NSM consisting of two flexures connected at their inner ends to the spring and supported at their outer ends, and loaded in compression by forces P. The spring is compressed by weight W to the operating position of the isolator, as shown in Figure 1. The stiffness of the isolator is K=KS-KN where KS is the spring stiffness and KN is the magnitude of a negative stiffness which is a function of the design of the flexures and the load P. The isolator stiffness can be made to approach zero while the spring supports the weight W.

A horizontal-motion isolation system consisting of two beam-column isolators is shown in Figure 2. Each isolator behaves like two fixed-free beam columns loaded axially by a weight load W. Without the weight load the beam-columns have horizontal stiffness KS. With the weight load the lateral bending stiffness is reduced by the "beam-column" effect. This behavior is equivalent to a horizontal spring combined with an NSM so that the horizontal stiffness is K=KS-KN, and KN is the magnitude of the beam-column effect. Horizontal stiffness can be made to approach zero by loading the beam-columns to approach their critical buckling load.

Figure 2

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Performance...

Minus K engineers can design a variety of custom vibration isolation systems. Frequently these are modifications of other designs such as changing the load capacity of an SM-1 isolator, or the capacity, size and isolator locations on a FP-1.