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.
Sensing
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.
After
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.
Videos
require the free RealPlayer which
is available at real.com.
** Photo
Credits:
-- Curt Suplee, National Science Foundation
-- MicroStrain, Inc.
** Video Credits: Stephen Pendo, MicroStrain,
Inc.
|