Effect of Sensor Failure on Dynamometry Calibration

Dynamometers are used to measure integrated fluid dynamic loads such as thrust, torque or side forces. To resolve all of three force and three moment components, multiple embedded force gages are often used. Due to arrangement, static loads, and redundancy, the number of sensor channels can exceed the six degrees of freedom needed to resolve the generalized rigid body forces. This paper considers modeling of the force gages as simple springs to develop an elastic model of the dynamometer. The method was applied to a dynamometer consisting of six three-component force gages arranged in an axisymmetric ring. A calibration matrix based on the elastic model with individual force gage sensitivities was shown to match a full calibration matrix where properly summed force gage voltages were obtained under global load application. The elastic model was then extended to consider calibration matrices where sensors were assumed to fail. In this scenario, several virtual loads were applied to the dynamometer and the calibration matrix was obtained by minimizing the least square error. It was found that nearly half of the sensors could be lost and still a virtual calibration could be applied to the measurements. Extending the least square idea, an actual in-situ calibration matrix was formed by striking the dynamometer with a diverse set of instrumented hammer strikes. This calibration matrix also agreed with the other calibrations at frequencies below where system dynamics become important.