880. Use of Standards for Center of Gravity Measurements

Paper

Abstract

A scientifically accepted axiom is that no two objects are alike. The science of engineering measurements agrees with this axiom in that all measured quantities are compared, in one way or another, to an accepted standard. It is common practice to consider a measurement acceptable if it does not differ in excess of a specified value from the accepted standard. This allowable difference is labeled tolerance, and includes uncertainties associated with the ability to measure the standard, the ability to measure the component being compared to the standard, and the median through which the comparison is made. The real significance of this practice is that a measurement is
considered acceptable if it can be proved that the measurement is within the prescribed tolerance.
This paper presents a unique method of determining the mass centroid of a test object by comparing the measurement to a proven standard. The technique involves three pieces of hardware: (1) a test unit, (2) a suitable holding fixture with a sensitive balancing pivot, and (3) a calibrated standard of comparison or mass simulator. In a specific application, the accuracy of the method proved to be within +/- 0.0023-inch of the true center of gravity. A by-product of the system provides the capability of measuring moments of inertia to within 0.3 percent.
The method involves balancing the test unit in a fixture, and then replacing the test unit with a calibrated mass simulator. The simulator is then manipulated in such a manner that the true centroid is determined independent of the fixture mass, mass centroid, or pivot location. By the principle of comparison, the test unit centroid must then be the same distance from a common reference as the centroid of the mass simulator.
Discussions herein include the formulation of methods, derivations of pertinent equations, and system advantages and limitations. Discussions on techniques that can be utilized to minimize errors in the test also are included.