Abstract:

This paper describes a new low speed vertical axis aerospace spin balance machine which takes advantage of recent advances in technology. This machine measures moment of inertia (MOI) in addition to product of inertia (POI) and center of gravity (CG) offset. Spin speeds as low as 15 RPM yield useful results. This machine has a number of unique features. The operation is totally automatic; even the conversion from spin balance to moment of inertia measurement can be accomplished without the operator touching the machine. Gas bearing technology is used throughout, resulting in unrivaled sensitivity and accuracy.

Balancing machines used to require a tedious procedure to adjust the plane separation controls. No matter how many times you did this procedure, it was so complex that you had to have the instruction manual handy as you worked your way through the steps. With the new Space Electronics POI Series Spin Balance Machine, this process is done automatically by the on-line digital computer which is supplied with the machine. This same computer also prompts the operator with user-friendly menus, so that it is rarely necessary to refer to the instruction manual while balancing the payload.

In addition to providing 2-plane correction weight locations, the on-line digital computer can store the location of a variety of ballast points for a specific vehicle and determine the best solution to the balancing problem. This system has the ability to map the available locations for unbalance correction weights (so that the machine will not specify a disallowed location). An optional removable hard disc ensures the safeguarding of secret data. This computer can also calculate the tilt of the principal axis.

This machine first rotates the payload at a slow speed and predicts the unbalance forces at the desired speed. If these forces are in excess of the ratings of the machine or the payload, then the operator is warned. He can then balance at a slower speed, or stop the machine and determine why the unbalance is so great. Digital filtering techniques are used to reject any forces which do not vary sinusoidally with the rotation speed. If the force transducer outputs include random variations due to air turbulence on the surface of the payload, then the filtering is automatically increased to smooth out this variation.

This paper includes a mathematical analysis of the errors of measurement as a function of the relative magnitudes of POI and CG unbalance, the moment equations which relate the transducer forces to payload POI and CG offset, and a practical discussion of fixturing and accessory equipment needed to properly balance an aerospace payload.

This instrument is probably the most accurate slow speed mass properties machine in the world. Although originally designed to have unbalance reduction ratios of 97%, ratios better than 99% are generally achievable, so that the object under test can usually be balanced in a single run. Center of gravity measurement is typically better than 0.001 inch. The machine produces an illustrated report of test data.