Following the 2019 redefinition of the kilogram in the International System of Units (SI), realization of torque no longer needs to be traceable to physical mass and length artifacts using a calibrated weight suspended from a known lever arm. Instead, the Kibble principle can be adapted from its primary use of realizing the kilogram to realizing torque with electromechanical force measurable in electrical units.
NIST, in conjunction with the U.S. Air Force, has designed and developed a tabletop-sized, self-calibrating proof-of-concept device for realizing torque traceable to the revised SI. The device, known as the Electronic NIST Torque Realizer (ENTR), is analogous to a Kibble balance that realizes mass vertically by balancing it against the force generated by an electromagnetic coil inside a cylindrical permanent magnet. ENTR realizes torque in the horizontal plane. Two disk-shaped permanent magnets are mounted on a central rotor. Between the disks is a plate electromagnetic coil. Current passing through the coil generates a magnetic field that causes the rotor to twist.
ENTR utilizes a commercial voltmeter, current source and data acquisition system along with custom electronic and mechanical components to generate torque traceable to the quantum-electrical SI to calibrate torque tools, among other uses. It’s designed for realizing torques ranging from 0.25 to 3 inch ounce-force (0.0018 to 0.02 Newton meter) with plans to expand the operational range to 142 inch ounce-force (one Newton meter).
Present torque calibration processes require regular calibration of both mass and length artifacts. This process often necessitates shipment of heavy mass sets from the end user to calibration labs and then back again. By contrast, the ENTR approach does not require mass and length standards to realize torque, and instead utilizes standards of voltage and resistance traceable to the present definitions of the SI.
ENTR is intended for use in calibrations laboratories that already have voltage and resistance standards, lightening logistical burdens and increasing operational efficiency.
Furthermore, ENTR can outperform most low-range commercial torque transducers currently on the market. Traditional strain-gauge torque transducers are transfer standards with uncertainties of approximately 0.25% of torque measured, while ENTR has 0.1% uncertainty or less.
Many threaded fasteners such as screws, bolts and nuts require a specific amount of torque to be applied to ensure they are tight enough, but not too tight. One proposed application of ENTR’s abilities would be screws in electrical components such as night-vision goggles. The screws must be tight enough to hold the electrical components together, but not too tight as to damage them. The Goldilocks zone of tightness is ensured using tools such as torque wrenches. Torque screwdrivers are often used in cases where one needs to tighten a smaller fastener. These tools need to be tested and calibrated regularly to ensure they are correctly measuring the amount of torque applied to a screw, bolt or nut.
ENTR is primarily intended for use as a torque tool calibration device; but the technology can theoretically be adapted for any application where low-uncertainty torque generation is needed. It is possible that this better-performance torque realization device will open the door for advances in precision design, manufacturing and nanotechnology.
Zane Comden, Stephan Schlamminger, Charles Waduwarage Perera, Frank Seifert, David Newell, Jay Hendricks, Barbara Goldstein and Leon Chao. A new spin on Kibble: A self calibrating torque realization device at NIST. euspen’s 22nd International Conference & Exhibition. May/June 2022.
Zane Comden et al. The design and performance of an electronic torque standard directly traceable to the revised SI. IEEE Transactions on Instrumentation and Measurement. Published online May 25, 2023. DOI: 10.1109/TIM.2023.3279911