165 Percussion Mechanism  for the Mars  2020 Coring Drill  

Kyle Chrystal* 

Abstract  

The Mars  2020 rover includes a Sampling & Caching Subsystem that is required to acquire core samples  of hard rocks and to abrade rocky surfaces flat and smooth in order to facilitate surface science . Hard, high  strength rocks exhibit brittle failure modes, and can be drilled and chiseled efficiently using percussive  impacts . The coring drill percussion mechanism was designed to provide those impacts with variable output  force and to be used with various drill bits . A unique hammering mechanism, using a base- driven hammer,  powered by a conventional rotary actuator driving a Scotch yoke, was designed to robustly provide the  required function in a Martian environment for a life exceeding 2 million impacts.  

Introduction  

The coring drill, also called the “corer ,” is the sampling tool on the turret at the end of the rover’s robotic  arm. The primary functions of the corer are to drill into the surface with a coring bit to generate a rock core  sample and to abrade the surface with an abrasion bit . The abrasion bit creat es a spot-faced surface that  allows other turret -mounted instruments to perform contact surface science. Hard rocks are not generally  worked by cutting. They  are usually either fractured apart (a brittle failure of the rock) by some form of  chiseling or spalling or they are ground down when abraded by a harder material than the rock . A practical   method for fracturing  chips from a rocky  surface is to use a percussive impact  to chisel chips from the  parent rock . Such impact s would deliver high force, short duration impulses to the rock . The force  magnitude of the impulse must be large enough to cause a large enough stress to break the rock apart and  the short duration of the impulse precludes the entire drill from having to react  large  forces . By breaking the  rock with many short duration impulses, large forces can be reacted by only a very small subset of hardware  and the time- average reaction required to hold the drill bit against the rock can be on the order of 100 N or  less. This method is also practical for spa ce flight applications because it enables low powered actuators  (order 50 W to 10 0 W) to produce loads sufficient to fracture hard rocks . 

The percussion mechanism uses an oscillating hammer to produce the impacts required for the corer to  drill and abrade  hard rocks effectively . The mechanism is situated near the center of the drill, above an  anvil that combines  rotational motion and percussive impact forces and transfers them both to an  interchangeable drill bit . The bi t is held into the front of the drill by a chuck  and it is free to spin and translate  along the drill axis to do rotary -percussive drilling. When the hammer contacts the anvil , typically  at  velocities up to 3 m/s , the mutual deformation of the contacting surfaces creates a local compress ive stress  in each part . The stress gradient in the anvil causes a wave to travel at the speed of sound down the axis  of the anvil, into the bit, and finally from the carbide bit teeth into the rock, causing portions of the rock to  be chipped off from the parent rock . The subject of this paper is the mechanism that creates this hammer  motion and performs the percussion function.  

*Jet Propulsion Laboratory, California Institute of Technology ; kyle.chrystal@jpl.nasa.gov  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020