Space Electronics - SE8913 Center of Gravity Instruments

The SE8913 instruments measure CG location of objects weighing up to 33 pounds. The instrument measures unbalance moment due to the displacement of test object center of gravity from the center of the rotary table. On the manual version, some math is required to convert this moment readout to CG location. The automatic version includes an interface for a Windows-based computer which acquires the raw data and converts it to 2 axis CG location. This type of instrument is more accurate than a 3-point weight and CG machine. This instrument does not measure moment of inertia or weight.

Concept: Moment reading (measures CG only). A separate scale must be used to measure weight.

Calibration Standard A precision weight is supplied to calibrate the instrument.

Test Fixture A fixture is required to support the test object at a known location relative to the instrument. Since there is no motion during measurement, this can be a simple light-weight bracket. Contact our sales department for more details. We offer a number of standard fixtures.

Measurement Concept This greatly simplified drawing illustrates the basic theory of operation. A rotary table is mounted on a precision spindle bearing. Crossed web flexures provide a horizontal pivot axis which is essentially frictionless. A torsion rod in line with this pivot axis creates a linear torsion spring about this axis. When the test part CG is displaced from the measurement axis, an unbalance moment is created, resulting in a displacement of the LVDT mounted at the end of the moment arm. Rotating the test part 180^o on the rotary table eliminates systematic errors such as incorrect leveling, zero drift in the readout, displacement of the rotary table axis from the pivot axis, etc. Since the weight of the test part is supported by the crossed web flexure pivot and only the unbalance moment is sensed by the LVDT, extremely high sensitivity is possible. The output of the LVDT is digitally filtered in the moment readout, which eliminates vibratory and other high frequency components. To measure CG offset, the test operator rotates the table to 0^o and records the value of unbalance moment. He then rotates the table to 180^o and records a second moment. CG is computed by subtracting the two moment readings and dividing by the test part weight. By measuring moments at 90^o and 270^o, the second CG coordinate can be calculated

	Model SE8913A	Model SE8913B	Model SE8913C
Max weight (test part and fixture):	2.2 lb	11 lb	33 lb
Max CG height:	3 inches	5 inches	5 inches
Maximum moment without damage to instrument:	5 lb-in	10 lb-in	20 lb-in
Full scale moment:	0.25 lb-in	2 lb-in	10 lb-in
Moment sensitivity:	0.0005 lb-in	0.002 lb-in	0.015 lb-in
Resolution:	0.0001 lb-in	0.0002 lb-in	0.001 lb-in

Model SE9707 Computer Readout Option To use the basic manual version of the instrument, the operator must record four moment readings. CG is calculated by subtracting two moment readings, multiplying this number by a calibration constant, subtracting a tare offset, and dividing by the test object weight. This calculation must be done for each axis. We offer a computer option which reads the data automatically through an RS232 interface, performs the calculations to yield CG offset in inches, and prints a report. This program runs on an IBM compatible computer.

Available Online SE8913 Literature:
   SE8913 Product Sheet (Imperial Units) (pdf, 103 KB)
   SE8913 Product Sheet (Metric Units) (pdf, 103 KB)
   SE8913 Specifications (pdf, 126 KB)