Reliability Case Notes No. 4: Autosub6000 and Autosub3 actuator potentiometer failure analysis and testing report

Abstract

During the James Cook Cruise 27 Autosub6000 aborted mission 12 due to a failure in the position feedback potentiometer of the stern plane actuator. The same actuator is used on Autosub3 which is heading to the Pine Island glacier in Antarctica in January 2009. A similar failure of the Autosub3 actuator while under the ice would result in the loss of the AUV.
This report initially describes the investigation into the failure of the feedback potentiometer and shows that the potentiometer’s conductive plastic track became detached from its ceramic substrate and broke up. The report then describes the testing performed to evaluate the reliability of the potentiometer. This involved an accelerated aging test to simulate the worst case conditions seen by the potentiometer in the actuator. This was achieved by oscillating the potentiometer at 4Hz to simulate the actuator movements whilst cycling the pressure of the Morlina 10 oil surrounding the potentiometers.
During the testing the 10k? potentiometers used in the actuator were not available, and so 5k? potentiometers from the same range were tested as a substitute. It was assumed that these 5k? potentiometers would produce similar results, however it was found during the testing that the formulation of the 5k? potentiometer track was different from the 10k?; whether this affects the reliability is not known.
Due to the large amount of time required to perform each test only 16 5k? potentiometers were tested. Although no failures occurred, the sample was too small to give a high statistical confidence that the potentiometers would survive the cruise. To further reduce the risk four 5k? potentiometers that were to be used on Autosub3 were tested for approximately 72 hours in a ‘burnt in’ process. As an early failure similar to that of Autosub6000 potentiometer would have been detected during this process, the chance of the potentiometers failing was significantly reduced. Thus the burnt in potentiometers were considered acceptable for use on Autosub 3 during the Pine Island campaign.

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Identifiers

ORCID for M.P. Brito: orcid.org/0000-0002-1779-4535

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