Custom Fixtures & Adapters

Roller Vees: Roller Vee fixtures are used to position cylindrical objects on a mass properties measurement instrument. It allows you to measure CG in two axes and MOI in one axis, then rotate the payload by 90 degrees to measure the third CG coordinate and a second MOI.

Tilt / translation tables: These fixtures allow translation of the payload in two horizontal axes and rotation around the center of gravity of the payload. They are used to precisely adjust the verticality of the payload and the position of its center of gravity over the centerline of our KSR and POI Series of instruments.

All fixtures are custom made to fit your payload or specific range of payloads. If you would like to request an estimate for a fixture, please contact us with your fixturing requirements by clicking the “request a quote” link in the top right corner of this web page or by completing the form below.

Spacecraft Positioner: A New Fixture for Determining All Mass Properties of a Spacecraft Without Spinning

Spacecraft are frequently moved and reoriented during assembly and ground testing, bringing with each move the risk of physical damage. During mass properties testing, a spacecraft will typically be oriented vertically and horizontally as well as rolled horizontally to obtain moment of inertia about the three axes. When product of inertia has to be determined, spacecraft are oriented at additional interim angles in order to obtain all the required inertia measurements.

A new Spacecraft Positioner is developed to securely support the spacecraft on a mass properties measurement instrument and automatically drive the spacecraft to position. This new fixturing and measurement system has been proven to minimize risk by eliminating the handling steps previously required during the measurement of spacecraft mass properties.

Mass properties measurement is one of the environmental tests (along with thermal vacuum tests and RF tests) that require tilting the spacecraft to a horizontal position and lifting the spacecraft in that orientation to mount it to an L-shaped fixture.

For mass properties tilting the spacecraft to a horizontal orientation was the only way to measure center of gravity height and two lateral inertias directly. Rotation of the interface plate of an L-shaped fixture also allows measurement of moment of inertia at several angles and calculation of one product of inertia.

Each spacecraft handling is a difficult task; and much more so when it is in a horizontal position. Some spacecraft manufacturers have built-in pick up points to pick up horizontal spacecraft with a crane, but most manufacturers can only pick up spacecraft through their clamp bands.

Picking up the spacecraft through this one plane is a complex task. An inverted L-shaped fixture with a movable lifting point must be used to keep the spacecraft horizontal. Mating the spacecraft to any other device requires absolute parallelism between the two surfaces and is very difficult to achieve. Each handling takes a significant amount of time and presents a significant risk of damage.

Another technique exists which consists of mounting the L-shaped fixture onto a tilting trolley and rotating the L-fixture by 90 degrees so that its mounting plane with the spacecraft is in a horizontal plane. Then the spacecraft is picked up in a vertical position and mated to the L fixture. The tilting trolley rotates the combined L fixture and spacecraft 90 degrees so that the spacecraft is now horizontal. The combination of L fixture and spacecraft must now be picked up and placed onto the mass properties instrument. This is done with an inverted L-shaped fixture again, which presents two disadvantages:

• The entire load is cantilevered so the inverted L fixture is working in shear. It must be heavy enough to resist the stress.
• Adjusting horizontality is difficult. The system must be picked up, and if it is not horizontal it must be put down again, the lifting point must be adjusted (note: the lifting point can be 30 feet above ground at that point), and the system must be picked up again… Repeat this until the level is good.
• Attaching and detaching the inverted L is problematic. The lifting point must be repositioned so that the inverted L does not collide with the spacecraft when it is lifted.

What if there was a device that could automatically maneuver a spacecraft to the various required positions? The spacecraft would be handled only to load and unload. Tilt and rotation to various attitudes would be handled automatically.

Traditional Mass Properties Measurement on a Spacecraft

Mass properties measurement of a spacecraft is performed using a combined center of gravity and moment of inertia instrument such as Space Electronics’s KSR series. For each orientation of the payload this instrument measures center of gravity in two axes (in a horizontal plane) and moment of inertia around the vertical axis.

A spacecraft is first measured in vertical orientation. This allows measurement of:

• Two lateral components of center of gravity: CGx and CGy (with Z the vertical axis or longitudinal axis of the spacecraft).
• Inertia around the longitudinal axis of the spacecraft (Izz)

Then the spacecraft is fitted onto an L fixture and mounted horizontally to measure center of gravity height (CGz) and the other two moments of inertia (Ixx and Iyy).

Spacecraft in vertical orientation (left) and horizontal orientation (right) on a Space Electronics KSR6000 mass properties instrument

This method presents the major drawback of requiring complex handling of the spacecraft. The satellite must be tilted from vertical to horizontal position, then lifted in horizontal position and mated to the L-shaped fixture.

The other problem with this method is the impossibility of getting the two cross-products Ixz and Iyz. The only solution up to now for getting these products of inertia was to spin the satellite at a minimum rate of 30 rpm, which can only be done on a few spacecraft.

Spacecraft Positioner

The spacecraft positioner is a complex fixture that allows measurement of all mass properties (CG in 3 axes, MOI in 3 axes, and POI in 3 planes) with precision compatible with or exceeding space industry standards.

The spacecraft is loaded and unloaded vertically, which reduces the handling complexity and risk to a minimum.

The positioner tilts and rotates the spacecraft to various orientations. Tilt is limited to 40 degrees from vertical. Rotation around the spacecraft longitudinal axis is continuous.

Positioner design criteria included:

• Light weight
• Low MOI
• Minimum fixture deflection under load
• Reliable mounting reference
• Hold the spacecraft securely while orienting it to all the positions necessary to solve for the full inertia tensor.
• Hold the estimated spacecraft CG center over the mass properties measurement machine to insure greatest accuracy.
• Maintain a minimum static unbalance, the positioner must include a counter weighting system to ensure that as the upper trolley assembly for the fixture moves, a counter weight move in reverse to maintain the fixture balance on the measurement system.
• Repeatable positioning performance; measurement positions shall be repeated accurately enough that its own mass properties can be extract from the measurements with uncertainties that are a fraction portion of the measurement accuracy.
• Must be onboard powered to insure no power cabling connections from measurement structure to the ground causes mass properties errors will exist.
• Must have non-contacting communications to insure no communications cabling from measurement structure to the ground causes mass properties errors will exist.

Spacecraft positioner loaded with a mass mockup and tilted to various orientations Left: spacecraft loading / unloading position – 0 degree tilt Center: 20 degree tilt and 45 degree rotation Right: 40 degree tilt and 270 degree rotation

The repeatability and accuracies of the measurements of payloads weighing from 400 to 3,500 kg are shown below.

Final accuracy in the determination of POI (Ixy, Iyz, and Ixz) depends on the number of measurements used. These results are based on 41 tests. Repeatability goes to about ± 2 kg-m2 with nine measurements.