Calibration and Uncertainty Analysis Method for a Pyramidal External Balance

Abstract

The Oran W. Nicks Low Speed Wind Tunnel (LSWT) uses a large-scale pyramidal-type balance to measure aerodynamic loads. The balance was built before computers were widely available and, consequently, it was designed to have a fixed, linear calibration without interactions between components. The physical system would be periodically adjusted to match the mathematical model. However, modern data acquisition and data processing make it much easier to leave the physical system fixed and modify the calibration to match the physical system. This thesis details a new calibration and uncertainty analysis method based on this principle. The method of calibration involves using pneumatic cylinders to apply a load to the external balance and using a strain gauge to measure the applied load, then comparing this measurement to the raw output from the balance in the form of rotary encoder counts. The relationship between these encoder counts and the forces and moments constitutes the calibration. Results show that the calibration coefficients have changed less than 1% from their original nominal values. The relationship between the forces and encoder counts has also remained very linear with very little evidence of interactions between components. It is recommended that the LSWT retain the linear model with independent components, but allow the coefficients to vary independently of each other. This 6-degree of freedom calibration allows a reduction in uncertainty from the current 1-degree of freedom calibration, but remains simpler than the more general 36-degree of freedom calibration.