NASA’s Langley Research Center has developed three techniques and systems to calibrate and validate wind-tunnel force balances and other multi-component force transducers. The first is the Single Vector Calibration System (SVS), which uses a single deadweight for calibration and has been in active use at NASA for over 15 years. The second system is the In-Situ Load System (ILS). The ILS is based on the same fundamental concept as the SVS, but is designed for in-situ verification just prior to testing. Building off of the SVS and ILS, the third system is the Variable Acceleration Force Calibration System (VACS), which shares the single-vector force application concept, but generates those forces differently by keeping the mass constant and varying the acceleration. These techniques and systems provide for less complex and less labor-intensive calibration and verification of multi-component force transducers.
This portfolio of technologies has been developed for calibrating and verifying wind-tunnel force balances, although the technologies also have other multi-component force transducer applications. A force balance is a complex structural spring element instrumented with strain gauges for measuring three orthogonal components of aerodynamic force (normal, axial, and side force), and three orthogonal components of aerodynamic torque (rolling, pitching, and yawing moments). In order to set the independent variables of applied load to calibrate a force balance, a high-precision mechanical system is required. Over the past decade, advancements in calibration and verification systems have been developed at NASA to increase efficiency, enhance availability, and improve accuracy.
The SVS exploits a single deadweight loading to create variable, multi-component loads through rotation and offset of the point of load application with respect to the balance moment center. The primary components include a multiple-degree-of-freedom load-positioning system, a three-axis orthogonal accelerometer system, and calibrated weights. The SVS enables the efficient execution of a formal experimental design, is relatively inexpensive to manufacture, requires minimal time to operate, and provides a high level of accuracy.
The ILS is based on the SVS’s force-vector concept where a single deadweight load is used to apply up to six loads simultaneously. The ILS provides for complex, multiple-component load verification prior to testing that is not typically done with current methods. The ILS also provides axial force (aerodynamic drag) verification in wind-tunnel applications, which is often the aerodynamic component of highest interest that is rarely verified during the checkout process.
The VACS employs a novel loading technique using a minimal number of weights and varying the centripetal force to obtain the complete set of forces necessary for the calibration. A proof-of-concept version of the new variable acceleration system has been shown to be capable of performing force balance calibration.
This technology can be used in wind-tunnel force balance applications, robotics applications such as rovers or prosthetic shoulder joints, and other multi-component force transducer calibration and verification applications.