The Seven Secrets of Accurate Mass Properties Measurement

by

Richard Boynton, President
Space Electronics, LLC, Berlin, CT 06037

Abstract:

If you have a mass properties instrument which has an accuracy of 0.1%, how accurately can you measure the mass properties of your payload? Some people think that the answer is 0.1%. In fact, the instrument is usually the least important factor in determining measurement accuracy. There are a number of fatal mistakes which you can make that will produce errors that are 10 or 100 times as large as the inaccuracy of the instrument. Over the years I have concluded that there are seven secrets for accurate mass properties measurement. These are:

  1. The payload must have precisely defined measurement axes. Both your calculated and your measured data are only as good as your ability to define the axes of the payload.
  2. The fixture must hold the payload so its measurement axes are precisely located relative to the instrument. Fixturing error is the number one source of measurement error in most mass properties measurements.
  3. You must follow the correct measurement procedure. The four most common procedural errors are:

    a. Tare measurement is made with mounting bolts left out or clamp set in a different position than when the measurement was made.

    b. Payload is not in flight configuration (arming switch must be set, shipping clamps must be removed, protective covers must be removed, tanks must be filled with fuel).

    c. Fixture unbalance is too large. This must be smaller than the desired balance. Otherwise you will be subtracting two large numbers. A small error in either will result in a large error in the difference. Also, the machine will have to be set on a low sensitivity range.

    d. Payload is fixtured so its CG is too far from balancing machine axis. (This is similar to the problem described above). This also results in overturning moment stiffness errors. Since no balancing machine is infinitely stiff to an overturning moment due to CG offset, all payloads lean toward their CG. To get the most accurate measurements, reposition the payload so its CG is on center and re-measure.

  4. You must eliminate external influences (drafts, temperature changes, vibration, air mass).
  5. The payload must weigh more than 2% of the capacity of the mass properties machine. Instrument accuracy is generally a function of payload weight or moment of inertia. If you try to measure a small object on a large machine, then it is like trying to weigh yourself on a 20,000 pound truck scale; the answer gets lost in the background noise.
  6. You must use an accurate instrument. Sensitivity and repeatability are not the same as accuracy. If your calibration standard is in error, then all of your measurements will also be in error. If the payload leans away from the centerline of the instrument, this will result in a large measurement error.
  7. You must define your symbols and polarities. Define which axis is X, which axis is Y, etc. Your X may be someone else's Y. Even within one company, one department may call the roll axis X and another department may call it Y. If you submit the data without defining the axes, each group will use its own set of coordinates in interpreting the data.

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