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POI Spin Balance Series

Product Details

Product of Inertia (POI) models are vertical two-plane spin balance machines. They are the only instruments in the world that measure all mass properties: center of gravity (CG), moment of inertia (MOI), product of inertia (POI), and dynamic unbalance.

With the Raptor Scientific POI series Spin Balance Machines, only one machine is needed to measure all mass properties. Our unique Spherical air 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.

Available in 19 standard models for payloads from 1 to 23,000 lb / 0.5 to 10,500 kg – from determining product of inertia of a spacecraft to dynamically balancing a rocket or missile, our spin balance machines offer the highest accuracy available.

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.

Typical Applications of the POI Instruments

POI instruments are mainly used for the testing of spacecraft (especially spin-stabilized satellites), re-entry vehicles, projectiles (munitions and bombs), or missiles and rockets.

Highlighted Benefits

True static CG measurement eliminates errors due to air turbulence on irregularly shaped objects such as satellites. It also allows static measurement of CG and MOI on payloads that cannot be spun.

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. Clean dry air or nitrogen is used to power the spherical air bearings in our instruments.

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 Raptor Scientific 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 Raptor Scientific machine use force restoration technology in combination with a air 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 Raptor Scientific machines when measuring moment of inertia: Raptor Scientific spin balance machines use a air 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 Raptor Scientific 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 a flat-plate air bearing machine. The spherical air bearing of these machines do not have sufficient resistance to an overturning moment. For tall test items, this 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 air bearing table with a 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.

Hydraulic vs. Gas Bearings: Space Electronics machines use an air bearing in place of a hydrostatic oil bearing. This has the advantage that only one bearing is necessary for both MOI and POI measurement. Furthermore, gas bearings are inherently superior to hydraulic bearings. A spherical air bearing has less bearing noise than any other kind of bearing, is less susceptible to damage from a dirty environment (since the bearing is continuously purged with clean air), and can be made with smaller runout than any other type of bearing.

There have been explosions which resulted from a leak in a high pressure hydraulic line. Under the right conditions, the oil mist ignites spontaneously. Recently, the US Government made the decision to replace a number of test machines which use hydrostatic oil bearings because of this danger. Our machines do not use high pressure oil and do not have this safety problem. Both oil and gas bearings can be damaged if the pressure is lost while the bearing is spinning. For our air bearing, we solve this problem by providing a reserve air tank which supplies lubricating air to the bearing during emergency shutdown. A pressure sensor automatically switches to this backup supply if the normal air supply pressure is lost, so there is ample time to smoothly decelerate the payload. Since oil is incompressible, it is not possible to provide this type of protection with a hydrostatic oil bearing.

Calibration hardware provided: A proving rotor and a set of precision weights are supplied to calibrate and verify the performance of the spin balance measurement. A certified calibration beam and weights are supplied to calibrate the moment of inertia measurement feature of the machine. All values are certified traceable to NIST.

On-line computer: A computer is supplied with each instrument. The software guides the operator through the procedure for each measurement or calibration sequence, so that the operator rarely needs to refer to the instruction manual.

  • Options & Accessories
    • Custom interface plate
    • Explosion proofing
    • Integrated CMM arm
    • Dial indicator stand
    • Air supply
    • Integrated weight platform
    • Hydraulic load positioner
    • Other accessories
  • Fixtures
    • Custom Fixtures
    • L Adapter fixture for measuring payloads in a horizontal orientation


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