174 Conventional Actuator and Avionics   Another distinction between the MSL percussion and Mars 2020 is the use of a conventional brushless DC  motor/gearbox actuator as compared to a voice coil . This allows the Mars 2020 percussion to be driven at  a range of speeds using very conventional avioni cs, software, and control system techniques . Essentially,  the mechanism could leverage the whole system up to an actuator output shaft that was already in place  for the numerous other actuators on board the rover . This more common actuator and avionics arc hitecture  might also make the Mars 2020  style percussion mechanism more accessible to other projects.  

Testing Results  

For the Mars 2020 Verification & Vallidation  campaign,  a dedicated Life Test Unit percussion mechanism  was built and successfully passed a 3x life test at the mechanism level . In the mechanism- only  arrangement, the mechanism is placed on the percussion dyno and is tested across the qualification  temperature range ( -70°C to +70°C) and percussion life frequency distribution . In addition, a percussion  mechanism in a complete Qualification M odel (QM) corer  has passed a 1x accelerated life test and is now  in the process of completing a comprehensive life test performing sampling operations on rocks  in the  qualification environment .  

All units undergo a mechanism- level test for approximately one hour in the dyno configuration to ensure  workmanship. At the end of the test, force data is collected across the operational frequency range for  characterization purposes . Figure 6 show s example data from this type of dyno test, taken on the Life Test  Unit. The mechanism is driven at speeds from 28 Hz to 40 Hz in 1 Hz increments and held at each speed  for 3 seconds, thereby gathering approximately 80 to 120 consecutive impacts at each speed . This process  is repeated for  both actuator rotation directions . Each impact produces a complex force- time profile starting  when the init ial stress wave arrives and continuing for several impact durations  as the load cell and  surrogate anvil, which are preloaded to a rigid base plate,  ring down. The peak value of force from the initial  blow is taken as the “peak force” and statistics for each operational speed/frequency are based on those  values.  

Challenges and Lessons Learned  

The requirements of a long -life percussion mechanism for use on Mars present major design challenges .  High wear sliding interfaces have the extra complication that the precise loads experienced are difficult to  define and understand and although conservatism of design can solve the problem to a degree, gaining  insight into the actual capabilities of the design through extensive prototype testing proved invaluable.  Fatigue failures were also a challenge for the hammer assembly parts, owing to the large number of  harsh  shock load cycles the hammer assembly must cope with. The long time it takes to complete life tests in  relevant environments underscore the general lesson – take great care in designing features such a sliding  wear interfaces or points susceptible t o fatigue failure that can only be proven out by lengthy life tests .  Design of these areas calls for extra conservatism, extra time spent reducing uncertainty in the loads, and  early and environmentally relevant testing that is possible only with hardware rich development.