SPIN BALANCE MACHINES

Two-Plane Spin Balance Machines with Moment of Inertia Capability.
Measure product of inertia , dynamic unbalance, center of gravity and moment of inertia.

• True static CG measurement eliminates errors due to air turbulence on irregularly shaped objects such as satellites
• Slow spin speed minimizes centrifugal forces on payload
• Two plane concept allows simultaneous measurement of CG offset and product of inertia
• No hydraulic pump; no danger of explosion due to mist from high pressure oil line; no risk of contamination of specimen or cleanroom facility

With the Space Electronics POI series Spin Balance Machines, only one machine is needed to make mass properties measurements. Our unique gas bearing design allows measurement of dynamic unbalance, product of inertia (POI), center of gravity (CG), and moment of inertia (MOI) with a single setup on a single machine. By eliminating the need for multiple machines, the time required to make these measurements is greatly reduced. Alignment errors that occur during setup on different machines are eliminated. In addition, since the test object is not moved between measurements, the potential for handling damage is minimized.

Separating CG Offset and Product of Inertia These slow speed spin balance machines are designed to measure satellites and other fragile payloads which cannot withstand high centrifugal forces. Since there are two rigid transducers on our machine, this machine can be classified as a "two-plane hard bearing permanently calibrated" vertical spin balance machine. It accurately measures CG offset and POI simultaneously, even if the forces due to CG offset are larger than those due to product of inertia. Only one run at a single speed is necessary to measure both quantities. 

Basic Concept  The payload mounting plate is attached to a spherical air bearing which is suspended flexurally from the machine base. A tube extends from this bearing to a flexibly mounted lower air bearing. Two independent force cells measure the reaction forces on both the upper and lower bearings due to an unbalance in the object being measured. These cells are extremely rigid, so that the resonant frequency of the machine is higher than the maximum spin speed.

POI Series Basic Concept

Static CG vs. Dynamic CG Most spin balance machines do not accurately measure the CG of irregularly shaped objects such as satellites. The turbulent forces created by air flow over the surface of a large irregular object prevents accurate measurement while spinning. Often the operator is not aware of this error, since the spin balance machine is capable of accurately measuring the smooth cylindrical proving rotor which is used to test the accuracy of the machine. If you are measuring the CG of a partially filled fuel tank, then centrifugal force will cause the fuel to ride up the sides of the tank when you spin it, resulting in an erroneous CG measurement. If the test object has extended solar panels, then centrifugal or windage forces may damage or deflect them. One solution to some of these problems is to make the measurement in a vacuum chamber. However, this is very expensive and time consuming. A much better solution is to measure CG without spinning the object under test.

The Space Electronics mass properties instrument has the capability to measure CG in either a static or a dynamic mode. In the static mode, the test object is stationary during unbalance measurement, so that the force of gravity through the test object CG is the only factor contributing to the unbalance moment detected by the machine. In the dynamic mode, the object is rotated at a fixed speed, and the unbalance forces on both the upper and lower bearing assemblies are measured as the object rotates. These forces are due to the combination of the force of gravity acting downward through the CG of the test object and the centrifugal force acting outward as a result of the distribution of mass in the object. The computer makes use of these dynamically measured forces to calculate CG offset and product of inertia.

CG Measurement technology The CG measurements made on the Space Electronics machine use force restoration technology in combination with a gas bearing pivot. Machines made by other manufacturers use a strain gage load cell in combination with an ordinary knife edge pivot. The Space Electronics method is 10 to 100 times more accurate and sensitive than these methods. Static CG is measured by slowly rotating the mounting table to each of the four quadrants and measuring static overturning moment (the product of the CG offset distance times the weight of the payload). The computer then calculates the X and Y moment and divides by the payload weight to yield the two coordinates of CG offset. 

Advantages of the Space Electronics machines when measuring moment of inertia Space Electronics spin balance machines use a gas bearing spindle. This means that the same bearing can be used for both product of inertia and moment of inertia, eliminating the need for a second bearing. Since the Space Electronics spindle bearing consists of a spherical upper bearing and a cylindrical lower bearing, our machines have enormous stiffness to overturning moment, and do not become unstable when tall test items are mounted on the machine. Our method is superior to the lower cost type of flat-plate gas bearing machine. The gas bearing of these machines do not have sufficient resistance to an overturning moment. For tall test items, this gas bearing can become unstable and rock from side to side. A CG offset in the test item will cause this bearing to tilt. Our machines do not have this problem. 

On our machines, MOI is measured by engaging a torsion rod on the mounting plate to create an inverted torsion pendulum. This conversion is automatic. The mounting plate is automatically twisted and released by remote control, causing the table to oscillate at a frequency proportional to MOI. A sensor measures the time period of oscillation and calculates the payload MOI.  Space Electronics machines do not initiate oscillation by pushing the gas bearing table with an pneumatic actuator. Instead, we position the oscillating table at a precise starting amplitude and then release it, resulting in identical starting amplitude for all types of test items.

Continue to "Satellite Mass Properties Machines" Page

SAWE Paper:            A New Spin Balance Machine