Mass (Weight) Instruments: WCG High-Accuracy

Measurement Concept

The object is lowered onto the Mass (or Weight) and CG Table and positioned relative to the machine zero (fixturing may be required). The center of gravity location and weight of the object are then determined by the computer which reads the force transducers and performs the necessary algebraic calculations. Mass (weight) is calculated by summing the output of the force transducers. CG is calculated using an equation involving the spacing of the transducers, and the distribution of force. For example, if the weight of the test item is applied equally to all transducers, then the CG of the test item is at the midpoint between the transducers. These instruments measure two axis CG. The third CG coordinate may be measured after rolling the object 90 degrees.

Key Features

Software for these instruments is provided so that any Windows compatible PC with an available RS-232 port may be used. All instructions for use are given on screen. The measurement software supplied prompts the operator, reads the transducers, calculates weight and CG location, and prints a report. There is a provision for keying in the description or serial number of the object under test, so that the data report can be used to document a series of tests on different objects. Optional custom software calculates the ballast weights required to shift CG location to meet certain specifications.

Safety Features – The test table is electrically grounded to the instrument frame for static protection. Optional Class I, Group D or Class II, Div 2, Groups E, F, G explosion proofing can be provided.

Test Fixtures – The instrument mounting surface is a precision grid plate (see photo to the left). Rectangular objects with at least one flat surface may be mounted directly on the table, eliminating the need for a custom fixture in many instances. This mounting surface is a flat rectangular plate with numerous holes in a grid pattern with known relationship to the table origin. The holes are equally spaced and allow for mounting test parts so they have a known location with respect to the machine’s reference axes. For test parts which are basically rectangular in shape, a fence and one pin (or 3 pins) which are supplied with the instrument, are installed on the grid plate to make contact with the test part and establish a datum corner. The measurement software provided automatically calculates CG results relative to this defined datum corner. For test parts that are cylindrical or irregularly shaped, a precision fixture is needed to locate the test item so its nominal CG is close to the machine zero.

The machine accuracy is only as good as the accuracy and repeatability of this fixture. Fixture position error can be minimized by calibrating the machine with a precision test mass that contacts the fixture at the same location as the part. Space Electronics manufactures a number of vee block fixtures which are designed to hold cylindrical test items. Custom fixturing may be required for other shapes (contact Space Electronics for more information). Whenever possible, test items should be fixtured so the longest dimension is horizontal, to minimize center of gravity height.

Note: When measuring radial CG offset of cylindrical parts, accuracy can be greatly improved by providing a fixture in which the test object can be rolled 180o about its longitudinal axis. This technique gives two CG offset measurements which can be averaged to eliminate fixturing and leveling error.

Center of Gravity Accuracy

CG accuracy is a function of test object weight, center of gravity height, fixture accuracy, and the care with which the instrument is leveled. These instruments are less accurate than our KSR series gas bearing pivot type instruments.

Our Standard Series WCG series instruments can detect changes in CG as small as a few thousandths of an inch for test items which are near the weight limit of the instrument. To calculate the sensitivity to CG change, divide the instrument moment sensitivity (lb-inch) by test item weight (lb). The accuracy limitation with this type of instrument is due to a lack of definite zero reference.

On our KSR instruments, zero is the center of rotation of the gas bearing and can easily be related to the position of the test item by a dial indicator. On this type of instrument, the zero reference is affected by the dimensional accuracy of the fixture and the leveling accuracy (on rotary table pivot type instruments such as our KSR series, leveling error is eliminated automatically). These errors can be minimized by calibrating the instrument with a test mass whose CG location is precisely known. This test mass must interface with the fixture in precisely the same manner as the item being measured. A different test mass is required for each fixture.