Prototype KDFR device mounted on laboratory bench for testing.
Credit: NIST
Static force, such as the weight of a person standing motionless on a bathroom scale or the force that an office full of equipment exerts on a high-rise floor, can be easily determined using scales, balances, load cells, and the like because it does not change over time. It is straightforward to calibrate such devices with an unvarying force such as gravity acting on a known mass.
Dynamic force, however, which changes continuously as it is applied – as in automobile crash testing, fatigue testing of materials, and the changing forces applied during machining -- has traditionally been difficult to measure. In many applications, a time-varying force causes large errors in instruments calibrated to measure static force. It is thus necessary to calibrate the response of these instruments to dynamic forces.
To meet that need, NIST scientists have devised the Kibble Dynamic Force Reference (KDFR), a traceable dynamic force source based on the same principle used in a Kibble balance to generate an exactly known static force to counterbalance the weight of an unknown mass.
By contrast, the patented KDFR generates waves or pulses of repeated dynamic force. “There is noise – incidental measured output that is not related to the force applied -- in any kind of dynamic measurement,” co-inventor Ako Chijioke said. “As a result, many repeated pulses are generally required to obtain a sufficient signal-to-noise ratio.”
In addition, measurements focused on behavior at specific frequencies can be challenging. These require the ability to excite the structure at the specific frequency of interest, together with an accurate measurement of the excit ..
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