A minicomputer-based system for the calibration of strain gage balances

Strain gage balances and angular position indicators are used to measure the aerodynamic loads acting on aircraft, missiles, and space vehicles during wind tunnel testing. A calibration of these balances and indicators furnishes data which represent the response of the devices to certain loads and angles. From these data, equations are derived which may be used to determine the load applied to a balance or the angle of a position indicator during a test. This study describes the development and design of a minicomputer-controlled system for the calibration of these devices.

During this investigation, all of the hardware required to calibrate balances and angular position indicators was developed, as well as the operating specifications for the software. This system uses an analog-to-digital converter to digitize all calibration data and uses analog instrumentation to condition and amplify strain gage signals. The minicomputer controls the entire balance calibration process including system setup and calibration. The previous calibration system, which was not computer-based, was used to determine effective methods for improving balance calibrations.

The new system provides a significant improvement in speed and accuracy. The analog-to-digital converter used in this system is far more accurate than the voltage-to-frequency conversion technique used previously. Unlike the old system, this system provides digital and analog filtering to improve the signal-to-noise ratio. The automation of system calibration and setup was the major factor in reducing calibration time. The resulting system is quite flexible, allowing the operator to satisfy the specific calibration requirements of various balances and angular position indicators. Such things as full-scale signal input, bridge excitation, and filter cutoff frequency can be changed easily.