Case Studies: Ultra-precise Calibration Leads the Way
To be sure load cells remain accurate while controlling costs, most aerospace centers run substantial in-house metrology operations. Reliable load cell readings are the heart of structural testing of aerospace components and are indispensable to their integrity, performance and safety when in flight.
To improve the practice of load
cell calibration, Lockheed Martin (Bethesda, MD) has set up a new 40,000 square-foot metrology center of excellence at the John C. Stennis Space Center in south Mississippi and equipped its force measurement department more like a National Standards Lab than a corporate testing station. "Load cells are the validators of airframe integrity, and here we validate the validators," says Brian King, metrology engineer at Lockheed Martin Integrated Metrology Center (LMIMC) at Stennis.
Lockheed Martin opened the laboratory to expand its metrology capacity and improve service in the East and Southeast. Its other metrology labs are in Denver and Sunnyvale, CA. "Here at Stennis we've concentrated the most skilled metrology people, state-of-the-art equipment and best practices under ISO and other international standards," King says. Soon, the laboratory will be accredited by the American Association for Laboratory Accreditation.
Among the key equipment in the new metrology center are the first TOP Transfer force transducers from HBM Inc. (Marlborough, MA) to be used in the United States and HBM's DMP 40 precision measuring amplifier with a
1-part-per-2-million resolution. Already in use at National Standards Institutes in Europe, the TOP Transfer transducers are 10 times more accurate than the previous best ISO 376 class 00 transducers. The force transducers have repeatability up to 10 times the requirements of ISO 376, their accuracy class is better than 00 (EN 10002-3). Long-term stability is leading edge as well-variance is less than 0.002% per year.
They are designed for applications at the top of the calibration pyramid: intercomparisons among National Standards Institutes. Those are the standards at LMIMC. "Clearly we're moving U.S. company-level metrology several steps forward," King says.
The metrology center, which began operation in October 2003, calibrates load cells sent in from more than 100 Lockheed Martin facilities around the globe and is beginning to service other companies as well.
LMIMC runs the calibrations on either of two Morehouse force presses -100,000 or 500,000 pound-depending on the load range of the test article. The larger force press is the company's largest in the United States, enabling the company to calibrate larger load cells than before. The two presses and all the HBM reference transducers handle both tensile and compressive tests. For each calibration run, technicians load both the test article and the corresponding TOP transducer for simultaneous and identical testing over the entire rated load range, with the TOP transducer serving as the reference standard. Typically, the lab makes at least three test runs on each test article to ensure reliability. Their respective analog response signals feed to the HBM model DMP 40 precision measuring amplifier, which measures stress and strain within 0.0005% accuracy and 2,000,000-digit resolution.
While the standard for load cell calibration is normally four times the nominal accuracy of the load cell being tested, King estimates that they are working closer to 10 times that accuracy. "And that accuracy standard extends over the entire load range, especially at the low end. The low-end accuracy and repeatability we're seeing with the HBM package is something we haven't seen in reference-
standard load cells before," he says.
Some load cells to be calibrated fit in the palm or your hand; others weigh up to 500 pounds. Most have a 30 millivolts/
volts characteristic and 10 volt excitation range. Load ratings vary from a few ounces to several tons. "The TOP transducers are especially good at very low loads," King says. "That has been problem with virtually all other load cells available for this type of work."
Presently, the laboratory makes two calibration runs a day, checking cells one at a time. Tests take about two hours, including multiple runs, setup and teardown. As volume builds and the queue includes several of the same load rating, King expects to make up to six to 10 calibration runs a day and to test several similarly-rated load cells at a time. "That will increase our capacity and provide some economies of scale," he says.
The ability to "gang" the load cells will depend on their size relative to bed size on the force presses, not on amplifier channel capacity. He added that repeatability of the HBM measurement package also saves time by reducing the number of repeat runs because of outliers.
Switching among test-article capacities is quick and easy because the test amplifier is programmable for up to eight load ranges and loaded with HBM's CATMAN Express measurement software. The test engineer simply punches in the appropriate program. "Programmability of the measuring amplifier has been a big time saver already," King says.
• TOP Transfer transducers from HBM Inc. are up to 10 times more accurate than the previous best ISO 376 class 00 transducers.
• The transducers can measure very low loads.
• The test amplifier is programmable for up to eight load ranges, and has measurement software, making it easy to switch among test-article capacities.
• Variance of the force transducer is less than 0.002% per year.