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Custom Strain Sensors

Close up image of sensor on bell crack -- Click to enlarge

To ensure that the Liberty Bell remains all it's cracked up to be—but not a micron more—engineers have attached wireless sensors to monitor the slightest changes in the Bell's famous fissure as the Liberty Bell moved into its new home on October 9, 2003.

While miniscule movements were detected during the move, none caused further damage to the bell.

Scientists fix sensors to inside of bell -- Click to enlargeSensing a delicate technical challenge and an opportunity to help protect a national treasure, a Vermont company, MicroStrain, provided the gauges and monitoring system for free. The technology was developed in part with support from the National Science Foundation's Small Business Innovation Research program.

The challenge was to recognize miniscule changes--or micromotions--in the Liberty Bell's crack as the 250-year-old, 2,080-pound icon was moved to a new museum space about 200 yards away. Specifically, conservators wanted to guard against two basic forces: any widening (or narrowing) of the crack's gap, and-perpendicular to that-any shearing along the opening.

To do so, they used tiny "differential variable reluctance transducers," also known as DVRTs. The sensors were originally designed for applications ranging from control of the robotics used in semi-conductor production to the measurement of strain in structures. The variety MicroStrain attached to the Liberty Bell, their NanoDVRT, measures the smallest motions, down to one hundredth the width of a human hair.

At the heart of each NanoDVRT is what resembles a tiny mechanical pencil: a tubular stainless steel shaft, three-sixteenths of an inch in diameter and less than half an inch long, within which is a thinner nickel-titanium core that protrudes from it and can move linearly, ever so slightly.

Bonded within the core is a tiny cylinder of an iron-rich compound called ferrite, about one-sixteenth of an inch in diameter. If the space that the DVRT spans changes, the core, with its ferrite cylinder, slides almost imperceptibly past two magnetically-shielded electrical coils imbedded in the tubular body.

The slightest movement of the core past the coils—as little as one-fourth of a micron or less—causes a proportional change in reluctance, a magnetic property of the current conducted by the coils. (A micron is a millionth of a meter.) The change in reluctance creates an imbalance in a sensitive, alternating-current, bridge circuit.

Before the move, technicians also attached a wireless transmitter under the Bell to detect any imbalance and then amplify, filter, digitize and transmit the signal to a nearby computer.

Test sensor on wooden crack -- Click to enlargeAfter checking the fit of the sensors on a wooden model of the Bell's crack, technicians tested them on the Liberty Bell last spring. As the Bell was hoisted gently off its base during the test lift, attached wireless accelerometers indicated that the overall maximum G-force did not exceed 1.02 Gs—very close to the force of gravity at rest. With the bell weighing about a ton, that increased load equates to an additional 20 to 40 pounds of weight.

As the bell rose a few inches, the two sensors along the crack detected minimal motions, roughly 1 to 2 micrometers of shear and no significant change in width, tiny movements that do not seem to stress the Bell.

MicroStrain used the test data to create upper and lower limits for vibration that the researchers monitored to keep riggers informed of potential danger to the Bell. Fortunately, the researchers did not detect any significant motion that could have permanently widened the crack.

It may not seem like much—less than a millionth of a meter--but being able to sense a change that small is a big deal to movers who don't want to be shakers.

Video: DVRT interior Video: DVRT interior
Video: DVRT horizontal motion Video: DVRT horizontal motion
Video: DVRT vertical motion Video: DVRT vertical motion

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** Photo Credits:
-- Curt Suplee, National Science Foundation
-- MicroStrain, Inc.
** Video Credits: Stephen Pendo, MicroStrain, Inc.

Close-up of MicroStrain’s NanoDVRT strain sensor -- Click to enlarge Close-up of a MicroStrain G-Link wireless accelerometer -- Click to enlarge A screen capture of the data collected by MicroStrain sensors from the Liberty Bell test lift on March 14, 2003 -- Click to enlarge Steve Arms conducts initial measurements on the Liberty Bell -- Click to enlarge


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