A New Spin Balance Machine

Two Plane Spin Balance Machines with MOI Capability POI7000 Dynamic Balance
NASA JPL’s Mars Curiosity Rover mounted on a Space Electronics Model POI12000 Spin Balance Machine.

This page describes a new low-speed vertical axis aerospace balance machine that 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 mass property calculations converting 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 separate 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 online digital computer that 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.

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 that 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 variety.

spin balance machine design
Basic design of the machine. The test object weight is supported on an upper spherical gas bearing. A lover gimbal mounted cylindrical gas bearing resists the overturning moment due to unbalance.

This paper includes a mathematical mass properties analysis of the errors of measurement as a function of the relative magnitudes of POI and CG unbalance, the moment equations that 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 the most accurate slow speed mass properties machine in the world. Unbalance reduction 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 content of this page is derived from a paper presented to the Society of Allied Weight Engineers [SAWE] at Biloxi, Mississippi, by Richard Boynton and Robert Bell of Space Electronics.  Click here to download the complete paper: A New Spin Balance Machine .

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