15 had been seen in the previous brush wear tests and was caused by worn brush/collector material sticking  between the collector bars and partially shorting the windings. No other major problems were seen during the strip down.   Although the EC22 completed all the life tests, including the vacuum part of the test sequence, two major  problems were encountered. Most serious was the failure of two hall sensors after the “operating cycles” test listed in Table 11. In order to allow the sequence to proceed, the rest of the testing (including the  main lifetime in CO 2 test) was performed using a sensorless controller (i.e., one that relies on back EMF  to commutate). All the remaining hall sensors were still operational at the end of the test sequence. Upon  strip down it was also noticed that the rear flange had worked loose (by about 0.5 mm) and had resulted in the partial failure of both bearings (due to extra forces on them since the rotor was no longer perfectly aligned to the stator. It is unlikely that the motor would have run for much longer in this condition.   

Planned Future Work   Current work is concentrating on understanding the cause of the rear flange movement (for which a fairly simple mechanical design improvement is expected to provide a solution) and the more complex issue of why the hall sensors failed. So far, considerably more extreme temperature cycling on several additional PCBs has failed to reproduce the problem.  The next stage of the development program, which is expected to be performed during 2012, is to develop motor gearbox combinations using the motors described in this paper as well as the missing types listed in Table 1 that have not yet been qualified. This will also be an opportunity to confirm the effectiveness of the design changes to the EC22 to remove the two identified weaknesses.   Summary   The work presented here has shown that, with appropriate modifications, the COTS motors RE13 and EC22 are capable of standing up to the launch vibration and shock and a landing on Mars and then functioning for the full planned ExoMars mission in the environmental conditions found on Mars.  The necessity of a qualification campaign as described here is made clear by the failures that were encountered in the hall sensors and the weakness in the rear flange design of the EC22. With suitable modifications in these areas, we expect a trouble-free qualification of these motors when attached to the gearboxes for the next stage of development. 

Acknowledgements 

This work was performed in collaboration between maxon motor, RUAG Space and Selex-Galileo and  was funded by an ESA development contract.   References 

Braun, David and Noon, Don. "Long Life" DC brush motor for use on the Mars Surveyor Program.  32 nd  Aerospace Mechanisms Symposium, 1998.  Ezell, Edward C., and Linda N. Ezell. On Mars: Exploration of the Red Planet 1958-1978.  Vols. SP-4212.  NASA, 1984.  Kminek, G. Planetary Protection Requirements.  ESA Doc. EXM-MS-RS-ESA-00005 Iss.3 Rev.1, 2007.  Kminek, G. Planetary Protection Requirements.  ESA Doc. EXM-MS-RS-ESA-00005 Iss.3 Rev.1, 2007.  Mutch, Thomas A. The Martian Landscape.  Vols. SP-425. NASA, 1978.  Noon, Don E. "Motor Brush Testing for Mars and Vacuum" 33rd Aerospace Mechanisms Symposium  1999  Noon, Don. Motor Brush Development Testing Results.  Unpublished, 2001.  Reid, Lisa K., David F. Braun, and Don E. Noon. "Robotic Arm and Rover Actuator Systems for Mars  Exploration." New Perspectives in Mechatronics Systems.  Winterthur, 1999.