Testing Satellite Thruster Orientation Mechanisms (TOM) with Inclinometers

Sherborne Sensors’ precision servo inclinometers are customized to withstand extreme changes of temperature under hard vacuum and offered precision performance throughout three-months of life test qualification. This helped RUAG Space successfully complete rigorous testing of its innovative satellite Thruster Orientation Mechanism (TOM).

RUAG Space is the largest independent supplier of space technology in Europe and is well known for its precision mechanisms for deployment, pointing and high-performance separation in spacecraft applications. The majority of European Space Agency (ESA) satellites use RUAG’s structures and well-known examples include: the separation system of the Huygens Probe from the Cassini Spacecraft; the electrical propulsion pointing (EP) mechanism for the SMART-1 and Artemis satellites; and the primary deployment mechanism of the solar array for the Hubble Space Telescope.

Commercial satellites use pointing mechanisms and EP thrusters for performing micro-positioning maneuvers and moving from launch orbit into their real orbit. A new type of thruster orientation mechanism (TOM), developed by RUAG, has simplified the overall design of a satellite by replacing the conventional eight stationary thruster units with two TOMs.

Each TOM is powered by actuators and features one or two thrusters mounted on a gimbal structure. RAUG’s TOM can support the largest range of thruster combinations and thruster mass in the current market. This means that only a quarter of the usual amount of Xenon tubing is needed to supply fuel to the EP thrusters.

Due to the nature of RAUG’s TOM design, it must accommodate the environmental loads induced during launch and spacecraft separation from the launch vehicle and must also endure the extreme of temperature experienced in space.

As a result, it has been subjected to a design qualification test programme designed to verify its performance against everything that it can reasonably expect to experience from manufacture through mission to end-of-life, which could be ten years or more. This programme entails a series of rigorous performance and functional tests.

Rigorous Qualification Testing

A special variant of Sherborne Sensors’ LSI Servo Inclinometer was developed in order to meet the rigors of RUAG’s lifetime qualification tests. This is a self-contained, precision gravity-referenced servo inclinometer and was mounted on the OTM qualification model in order to perform three important tests: potentiometer verification, motor margin and mechanical pointing accuracy. The tests were conducted in an extremely low pressure (10-7 mbar) vacuum chamber known as a ‘hard vacuum’ which stimulates the in-orbit environment.

Finding measurement devices capable of operating at this very low pressure is not easy to do.

But having discussed our design requirements with Sherborne Sensors, we were able to ensure that their inclinometers fulfilled out requirements. The most important aspect was that they were able to operate between -40oC and +40oC under hard vacuum conditions. Only Sherborne gave us the range that we wanted, together with the accuracy.

Andrew Skulicz, AIT Engineer, RUAG Space.

The outgassing of organic compounds such as adhesives and rubber can destroy the vacuum and potentially ruin the tests in a high vacuum environment. Therefore, Sherborne Sensors was careful to make sure that the inclinometer did not contain compounds that would suffer this deficiency.

Additionally, the case of the inclinometer was provided with a vent to allow the internal volume to assume the same pressure as the external conditions. This countered the effect of differential pressure between the sealed case of the inclinometer and the vacuum conditions it was being used in.

These customizations ensured that there was no danger of any minor leaks destroying the high vacuum conditions over time, as well as relieving any mechanical stresses that could occur during de-pressurization.

The LSI was also characterized for performance over the applications operable temperature range to give a high degree of accuracy. Because RUAG had the ability to correct for thermal errors within its data acquisition algorithms, we also provided them with a ‘look-up’ chart listing the individual temperature errors over the complete range of environmental temperatures expected to be met in the application. This enabled RUAG to correct in real time for the effects of temperature and deliver more accurate results.

Mike Baker, Director, Sherborne Sensors.

Customized Inclinometer Solution Delivers Qualified Success

In order to achieve mechanical pointing accuracy, the inclinometers were used to measure the pointing vector of the TOM with respect to a reference time frame. Accuracy to a higher degree than 0.05o was essential.

The inclinometers were used to measure and characterize how the pointing vector of the mechanism varied in different thermal conditions,

Andrew Skulicz, AIT Engineer, RUAG Space.

The performance of the potentiometers was also checked in different thermal conditions in order to make sure that they could return accurate telemetry back to the spacecraft. Motor margin tests were performed to ensure that the performance of the on-board stepper motors did not degrade.

The performance of the potentiometers was tested over the full angular range of -14o/+34o, with a required accuracy of more than +/-0.05o, using the inclinometers. However, the inclinometers were removed during shock and vibration testing as they would have been damaged.

Such tests were arduous for both the mechanism and the inclinometers, given that is was necessary to detect if the motor losses steps with an accuracy of at least 0.01o.

Additionally, tests were carried out at extreme positions (+34o) to further test the performance of the inclinometers at their full range. The inclinometers on the TOM not only successfully operated throughout a sequence of thermal vacuum cycles, but also sustained that operation for nearly three months while the mechanism was undergoing its life test.

Andrew Skulicz, AIT Engineer, RUAG Space.

The fact that the pointing performance of the mechanism did not change throughout the programme while the variation in motor margin at different temperatures was clearly visible showed, according to Andrew, that the inclinometers were sensitive and able to perform well under thermal vacuum conditions and extreme temperature.

“I could also be confident the inclinometers performed all the way through the test programme as expected, because the inclinometers measure pointing accuracy, which is based on gearbox geometry and should remain constant. It’s a bit of a circular reference, but this substantiates the fact the inclinometers didn’t degrade during the test.”

The test results have been approved by ESA and RUAG’s TOM programme represents the cutting edge of the European scientific community. “This is not easy to obtain and requires that we are able to substantiate that the results are valid. The team at Sherborne Sensors has been very cooperative, providing strong technical support and we worked together really well to ensure that this part of the programme ran smoothly,” concludes Andrew.

This information has been sourced, reviewed and adapted from materials provided by Sherborne Sensors.

For more information on this source, please visit Sherborne Sensors.

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