291 Lessons Learned 

Parasitic Torque Model Shortcomings  – The brush pivot drag torque model clearly came up short  based on the test results and observations presented above. The lesson learned is to do a more thorough  job validating models earlier in the development process (i.e., prior to CDR). The model used in the design process incorporated test data for seal drag from a past program – this data may or may not be valid as it was seal drag for higher operating speeds as opposed to the near static situation measured during the torque margin tests discussed above. One source of drag that may have been overlooked is the friction between the spring and arbor. It is difficult to predict how torsion springs end constraints will behave. Often torsions springs (especially those with few turns and short legs) will cant or cock to one side causing part of the spring to rub against its housing or arbor in an unanticipated manner. This can introduce drag and strange behavior that is difficult to model.   Test Program Inadequacy  – In designing a test program that saved time by directly measuring brush  contact force over the pivot range of motion, the team overlooked the fact that the test method was not  going to collect all the data needed to verify the pivot spring torque margin requirement. Force measurements were taken in only one direction as opposed to both directions which is required to quantify losses due to internal drag. Additionally, while there are benefits to testing in a manner consistent with how the tool would ultimately be used, it was an error to omit a test that isolated the pivot spring and drag torque at temperature. Instead the bristle compliance was included as an unobservable variable in the same test. The lesson learned is to test pivot or hinge torque margin at the pivot/hinge level and to be sure to make a torque measurement in both directions so as to isolate the drag torque from the spring torque.   High-Reduction (10.4:1) Planetary Stage  – While not ideal, it was demonstrated that given careful tooth  profile selection and analysis, a relatively high-reduction (10.4:1) planetary stage can be an efficient  torque amplifier.   Image Processing Provided Easy, Objective Measurement Method  – Surface cleanliness can be a  very subjective quality that is hard to define. Using image processing algorithms to measure cleanliness  and brush performance turned out to be very easy. Its objectivity and ease allowed personnel to focus on collecting more data rather than fiddle with complicated alternative measurement techniques. This method could be used for other more general purposes like quantifying the amount of wear particulate generated in a mechanism over time.   Conclusion   MSL is scheduled to land at Gale Crater on Mars in August 2012. Using the rover’s Dust Removal Tool, Earth-bound scientists will sweep away the blinding dust that eventually coats every object on the Martian surface. Spectrometers and cameras will take aim at these freshly exposed surfaces and will gather information that will allow scientists to decide whether those rocks may harbor evidence of organic materials or not. The DRT promises to be as instrumental to MSL’s success as the RAT was and is still to the success for MER.   The DRT development process had many challenges. From engineering a brush design that utilizes a single actuator and can deal with 20-mm surface height variations to determining proper methods for testing such a novel device, there were many things learned over the course of the project.  There was a significant discrepancy between a key mechanism analytical model and the tested performance of that mechanism. The root cause of this large discrepancy has not been identified yet. But important lessons were drawn from the experience. In the future, greater consideration will be given to validating such analytical models earlier in the development process and performing component level verification tests on the flight hardware to isolate key parameters.