01 December 200301 December 2003In its time, the Liberty Bell has faced a number of technical challenges.
First cast in London, England, it traveled directly to Independence Hall in Philadel-phia, Pa., in 1753. The clapper cracked the bell on its first use.
Two pot and pan makers then recast the bell twice, adding more copper to make it less brittle the first time, which made the bell’s tone dull sounding, and then adding more silver the second time to sweeten the bell’s tone.
Recently the 2,080-pound bell moved 963 feet to a new home in Liberty Bell Center. It was fitted with wireless motion sensors and put in a pneumatic cart to make the journey.
Wired magazine reported the engineers who worked out the proper way to move the 250-year-old Liberty Bell were particularly concerned about a hairline fracture that extends from the main crack on the bell to the rear of the bell.
If that fracture splits, so would the Liberty Bell.
Wireless motion sensors monitored the slightest changes in the bell’s celebrated crack. The sensors, connected to wireless transmitters, could issue an alert if the crack widened or narrowed by even a micrometer during the move.
Vermont-based MicroStrain developed the sensor technology with support from the National Science Foundation’s Small Business Innovation Research program.
The sensors that monitored the bell were differential variable reluctance transducers (DVRTs). Their first and intended use was to measure structural strains in buildings, but engineers have adapted them for other uses.
Researchers at the University of Arizona are currently using the sensors to develop more durable artificial joints. Embedded within an orthopedic device, the sensor monitors how much stress the joint experiences and relays the information to doctors.
The sensors that attached to the bell are the smallest ones MicroStrain makes, its NanoDVRT model. The sensors can measure the most minute motion changes, down to one-hundredth the width of a human hair.
Reluctance is the resistance of a substance to the passage of magnetic lines of force. Reluctance is the reciprocal of magnetic permeability. Some refer to reluctance as magnetic resistance.
Sensorland.com explains that a variable reluctance sensor is strain based, wherein a magnetic circuit is formed, and the parameter input causes mechanical deflection of the spring member as a function of pressure, force, or acceleration.
To provide a static output capability, variable reluctance sensors require an oscillator and demodulator system internally limiting operational temperatures from –40°C to +120°C. The spring member is comprised of magnetic, high-permeability material and is centrally located between two coils.
The coils seal off from the measure by nonmagnetic welded stainless-steel barriers. In the case of the differential pressure transducer, the difference in pressure between the two sides of the spring member will cause distortion of the spring member towards the magnetic pole piece on the low-pressure side of the spring member. This results in modulation of the inductance of the two coils.
Each NanoDVRT contains a stainless-steel shaft, three-sixteenths of an inch in diameter and less than one-half inch long. A thinner nickel-titanium core protrudes from the shaft and can move linearly.
Technicians have fastened the sensors over the bell’s crack. If the space that the sensors span changes, the core will slide out past two magnetically shielded electrical coils imbedded in the tube’s body.
The slightest movement of the core past the coils disrupts the magnetic current conducted by the coils. A wireless transmitter attached under the bell would then transmit a distress signal and information about the possible problem to a nearby engineer’s computer.
MicroStrain has an ultramicro-miniature DVRT (UM-DVRT) for measuring strain in small joints and tight spaces. The device is available in a two-coil configuration.
However, the single-coil UM-DVRT design, with its diameter of less than 1 millimeter, may prove superior when an application demands extremely small size, such as monitoring strain in knee ligaments. IT
Nicholas Sheble writes and edits the Sensors and Technology Advances department. Write him at nsheble@isa.org.