52   Figure 11. SmCo magnets damaged during AIT  

Breadboard Testing  

The breadboard model of the BepiColombo Antenna De- Spin Mechanism was used to demonstrate the  feasibility of  the chosen control loop design and to obtain an understanding of the required motor and  encoder performance  at an early stage of the project. As its design comprised  a single bearing pair in back - to-back layout, it was also used to assess the suitability of a back -to-back bearing layout for  the Derotator.   The Antenna De- Spin Mechanism breadboard was hardware- in-the-loop during controller testing with an  xPC and Simulink models of the controller. The outcome of the test s was:  • Chosen cascaded control loop (motor current, velocity, position) suitable   • Motor stator to be skewed in order to reduce cogging torque disturbances   • Encoder spikes at high speeds during acceleration phase to be filtered   • High-frequency error of encoder has significant impact on measured PDE of mechanism   Consequently, a skewed motor design was selected (also refer to section Performance Requirements ), a  median filter was implemented into t he controller design, and the high- frequency error specification of the  encoder was narrowed.  

Lubricant Testing and Re-lifing  

An outgassing test of Nye 2001a oil was performed in order to assess the suitability of this oil and the  Maplub SH051- a grease for the project. As pointed out  in section Cleanliness and Contamination, the oil  outgassing behavior was significantly  better than expected from literature.  

The grease Maplub SH051- a has been discontinued.  This type of grease was  used for most heritage  scanning mechanisms  at Airbus Friedrichshafen. To date, no successor showing similar performance has  been identified. Maplub SH type b lifetimes are expected to be approx imately  one order of magn itude lower  than that of  the type a greases [2]. In addition, torque peaks at low speeds and de- mixing at temperatures  higher than 40°C  were reported for the Maplub type b greases in an information note distributed by supplier  MAP. The torque peaks were later confirmed by analysis and bench testing [7] . Based on these findings,  the project decided to keep using the discontinued Maplub SH051- a grease. The batch of grease available  for the MWI, ICI, MWS and METimage projects  was manufactured in 1999  and its shelf life has formally  expired. It was tested in a spiral orbit tribometer at the European Space Tribology Laboratory (ESTL) and  compared against test data of a freshly manufactured sample of the same grease.  The tribological  performance (friction and lifetime) of the 20-year-old Maplub SH051- a grease is presented in Table 3; it  was demonstrated to be no worse than that of a  fresh ly manufactured grease batch.  The variance of spiral  orbit tribometer test data was higher for the re- lifed grease.  Therefore, mixing of the re- lifed grease was  recommended as a precaution , although i mprovements gained by physical re- mixing of the gr ease were   anticipated to be marginal at best at bearing  level.  This is due to the fact that the amount of grease in a  bearing is several orders of magnitude higher than the amount  applied to the ball in a spiral orbit tribometer  test. [5]