Development and Implementation of Improved External Balance Load Measurements

The Texas A&M Oran W. Nicks Low Speed Wind Tunnel (LSWT) uses a large pyramidal-type external force balance to measure aerodynamic loads. This balance was designed and constructed in the 1940s and measures the six wind frame aerodynamic forces and moments through a system of levers and pushrods that terminate in six movable poise weights and individual analog controllers. These systems were updated in the mid 1970s to use vacuum tubes, double wound motors, rotary encoders, and analog linear potentiometers. Although antiquated, these analog components provide outstanding sensitivity and linearity. However, the fragility, tight tuning margins, and overall age of the system has proven troublesome for the LSWT. The objective of this thesis is to design, implement, and provide a preliminary calibration of new measurement electronics that improve sensor reaction time, robustness, and ease of use while taking advantage of existing mechanical structure. This updated system must meet certain minimum requirements, namely, it must be at least as accurate as the original system, it must react more quickly to disturbances, and it must enable straightforward future upgrades. To achieve these objectives, two different approaches were investigated. These were: first, a PID digital controller to act as a modernized version of the existing analog control system, and second, commercially available load cells placed in line within the pushrods directly connected to the balances. The commercial load cell approach was selected for implementation. This upgrade scheme resulted in an improvement on the accuracy, settling time, reliability, and serviceability of the existing system while eliminating one of its largest sources of hysteresis. Additionally, total reversion to the original analog system is still readily possible on short notice, should a problem with the new system arise