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Inclinometers support life test in vacuo

14 September 2012

Customised to withstand extreme changes of temperature under hard vacuum, precision servo inclinometers maintain precision and reliability throughout a three-month life test qualification, helping RUAG Space to complete a programme of tests on its innovative satellite thruster orientation mechanism.

As the largest independent supplier of space technology in Europe, RUAG Space ( develops, manufactures and tests subsystems and equipment for satellites and launch vehicles. The company is particularly well 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 extremes of temperature experienced in space. It was therefore 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.

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. This 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 10-7 mbar 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,” says Andrew Skulicz, the 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. Indeed, 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 those from adhesives and rubbers, can destroy the vacuum conditions and potentially ruin the tests. Care was taken to ensure that the inclinometer did not contain any such compounds, and to counter the effect of differential pressure between the sealed case of the inclinometer and the vacuum chamber, its case 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 Sherborne Sensors director, Mike Baker. “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.”

For mechanical pointing accuracy, the inclinometers were used to measure the pointing vector of the TOM in relation to a reference frame, with an accuracy better than 0.05 degrees deemed essential. “The inclinometers were used to measure and characterise how the pointing vector of the mechanism varied in different thermal conditions,” Andrew Skulicz adds.

The performance of the potentiometers was also checked under different thermal conditions to verify their ability to return accurate telemetry back to the spacecraft, while motor margin tests were conducted to check 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 degrees to +34 degrees, with the required accuracy being better than +/-0.05 degrees. 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 lost steps with an accuracy of at least 0.01 degrees,” says Andrew. “Additionally, tests were carried out at extreme positions [+34 degrees] 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.”

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 they 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 that 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 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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