Inclinometers help test satellite thrusters

Paul Boughton

Customised to withstand extreme changes of temperature under hard vacuum, precision servo inclinometers offered precision performance throughout three-months of life test qualification to complete rigorous testing of its satellite thrusters.

As the largest independent supplier of space technology in Europe, RUAG Space is known for its precision mechanisms for pointing, deployment and high-performance separation in spacecraft applications.

Most European Space Agency (ESA) satellites employ RUAG's structures, with well-known examples being the primary deployment mechanism of the solar array for the Hubble Space Telescope, the separation system of the Huygens Probe from the Cassini Spacecraft and the electrical propulsion pointing (EP) mechanism for the SMART-1 and Artemis satellites.

Pointing mechanisms and EP thrusters are used by commercial satellites for moving from launch orbit into their real orbit and to perform micro-positioning manoeuvres.

RUAG has developed a new type of thruster orientation mechanism (TOM) that simplifies the overall design of a satellite by having two TOMs instead of the eight stationary thrusters units employed in conventional designs. Each TOM features one or two thrusters mounted on a gimbal structure and is powered by actuators. Able to support the largest range of thruster combinations and thruster mass in the market today, RUAG's TOM means only a quarter of the normal amount of Xenon tubing is required to supply fuel to the EP thrusters.

The nature of RUAG's TOM design means it has to accommodate the environmental loads induced during launch and spacecraft separation from the launch vehicle, as well as the extreme of temperature experienced in space. It has therefore been subjected to a design qualification test programme that entailed a series of rigorous functional and performance tests in order to demonstrate and verify its performance against everything it can reasonably expect to experience from manufacture through mission to end-of-life, which could be ten years or more.[Page Break]

Rigorous qualification testing

In order to meet the rigours of RUAG's lifetime qualification tests, a special variant of Sherborne Sensors' LSI Servo Inclinometer was developed. The LSI Servo Inclinometer is a self-contained, precision gravity-referenced servo inclinometer and was mounted on the TOM qualification model in order to perform three key tests - mechanical pointing accuracy, potentiometer verification and motor margin. Tests were conducted in a large vacuum chamber, where an extremely low pressure of 10-7mbar is achieved. Known as a 'hard vacuum', this simulates the in-orbit environment.

"Finding measurement devices capable of operating at this very low pressure is not easy to do," says Andrew Skulicz, AIT Engineer at RUAG Space. "But having discussed our design requirements with Sherborne Sensors, we were able to ensure that their inclinometers fulfilled our requirements. The most important aspect was that they were able to operate between -40°C and 40°C under hard vacuum conditions. Only Sherborne gave us the range that we wanted, together with the accuracy."

In a high vacuum environment, the outgassing of organic compounds such as adhesives and rubber can destroy the vacuum conditions and potentially ruin the tests. Sherborne Sensors was therefore careful to ensure that the inclinometer did not contain any compounds that would suffer this deficiency. In addition, to counter the effect of differential pressure between the sealed case of the inclinometer and the vacuum conditions it was being used in, the case of the inclinometer was provided with a vent to allow the internal volume to assume the same pressure as the external conditions.

"These customisations 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-pressurisation," says Mike Baker, Director at Sherborne Sensors. "The LSI was also characterised 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."[Page Break]

For mechanical pointing accuracy, the inclinometers were used to measure the pointing vector of the TOM with respect to a reference frame, with accuracy to higher than 0.05° being essential. "The inclinometers were used to measure and characterise how the pointing vector of the mechanism varied in different thermal conditions," continues Andrew Skulicz at RUAG Space.

The performance of the potentiometers was also checked under different thermal conditions to ensure they could return accurate telemetry back to the spacecraft, while motor margin tests were conducted to verify that the performance of the on-board stepper motors did not degrade. The inclinometers were used to verify the performance of the potentiometers over the full angular range of -14°/+34°, with the required accuracy being better than +/-0.05°. The inclinometers were removed during vibration and shock testing however, as they would have been damaged.

"Such tests were arduous for both the mechanism and the inclinometers, given that it was necessary to detect if the motor looses steps with an accuracy of at least 0.01°," says Andrew. "Additionally, tests were carried out at extreme positions (+34°) to further test the performance of the inclinometers over 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."[Page Break]

According to Andrew, 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 that the inclinometers were sensitive and able to perform well under extreme temperature and thermal vacuum conditions. "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."

RUAG's TOM programme represents the cutting edge of the European scientific community, with the test results having been approved by ESA. "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 co-operative, providing strong technical support and we worked together really well to ensure that this part of the programme ran smoothly," Andrew concludes.

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