172 simulation. For example, when a change to the core sample tube dimensions rippled through the bit and  then the entire corer, the hammer needed to grow in diameter to a degree that maintaining its mass was  not possible . The entire system was successfully rescaled with some other parameters changed to reduce  the sensitivity of first impact frequency to anvil position using the referenced theory.  

With basic design parameters such as hammer mass, spring stiffness, driver amplitude and frequency, and  the static gap to the stop determined, three dimensional FEA was now needed to determine the level of  force that impacts of a given velocity would produce. LS-Dyna is an FEA tool with an explicit solver that is  capable of accurately simulating dynamic stresses and large- scale motions and it was used extensively in  the design of this system [2]. Parts in the impact chain (hammer, anvil, bit) have many required features  that are detrimental to efficient transfer of force to the rock . In the Mars 2020  Corer, peak force under the  hammer is reduced by 50% by the time it transits the percussive chain and reaches the end of the bit . This  knockdown would be far worse without design changes and compromises made possible by LS -Dyna  simulations . LS-Dyna provided tremendous general insight that primar ily facilitated general changes in  direction (i.e. , this area thicker or thinner, this par heavier or lighter) but in many cases it was also used as  for stress analysis of parts that were driven primarily by operational percussion loads –  although there was  an expectation that these parts had large margins (e.g. unlimited fatigue life).  

Beginning early in the project, hardware testing was used to improve mechanism performance and  durability , and to validate the simulations . Impacts from the percussion mechanism have a peak force  magnitude exceeding 40 kN and a typical impulse duration is 50 microseconds . A “percussion dyno”  was  built using a piezoelectric load washer  (Kistler 9041A) and charge amplifier  that had the necessary stiffness  and fr equency bandwidth to measure these impacts . A sampling rate of about 200 kHz was used to ensure  an accurate measurement of peak impact force could be made. Extensive functional testing was also done  using the percussion mechanism in a complete coring drill . Multiple generations of percussion mechanisms  cored and abraded hundreds of rocks. The data gathered from this testing was used to correlate percussion  output level to the rate of penetration in various rocks . That test data, combined with requirements f or drill  bit life, was used to create the force output and life duration margin story . A combination of testing in rocks,  dyno measurements, and LS -Dyna analysis were used to verify and validate the design.  

Mechanism Function  

Mechanism operation consists of simply driving  the actuator to the required speed to achieve the intended  percussion frequency and force level output . The hammer will first begin to impact the anvil around 25 Hz,  depending on the precise anvil position†. Impact peak force will increase as the actuator speeds up and the  hammer impact frequency is increased. The maximum output frequency is 40 Hz and it is primarily intended  for special use. The mechanism functions the same with the actuator rotating in eit her direction,  and  nominal operation calls for the life to be accumulated approximately equally in both directions for the  purposes of wear leveling, particularly the flanks of the gear teeth .  

Vibration produced by hammer and shock loads due to hammer/anvil impacts have a few uses for Mars  2020 beyond coring and abrading. Vibrations from the percussion mechanism can be used to help transport  powder that is collected by special regolith bits and to free the root piece of a core that is improperly broken  off from the parent rock (so called “mushrooms” that extend below the mouth of the sample tube) . The most  extreme special use of percussion is to “self -clean” the turret . Because the drill is designed to safely allow  the hammer to impact the anvil with no dr ill bit present (or with a bit in the chuck but no weight -on-bit  applied) the percussion will be used at the 40 Hz max output setting to apply an enveloping dynamic   environment for the turret post -landing that should either remove any Earth spores still pr esent on the turret,  or ensure that they will not be able to be dislodged during future sampling activities . This guarantees these  spores would not contaminate a Martian surface sample with Earth life.  

† The anvil is separate from the drill body by a spring that is intended to isolate the corer (particularly nearby spindle  bearings) from the rebound shock loads from impacts – therefore the anvil to hammer static gap will depend slightly  on weight -on-bit preload.