171 Requi rements Development  

The percussion mechanism functional requirements are derived from the planned set of sampling  operations which specify operation type and number  (i.e., 43 cores, 74 abrasions ) and the distribution of  those operations across  a range of representative rocks (the “rock suite”)  of varying hardness . The  percussion mechanism must provide enough impact force to allow the drill bits to penetrate these materials  and must have enough life to complete the mission and both output force and life must be sufficiently  margined . At the same time, excessive conservatism is not possible due to tight constraints on available  actuator power and the volume and mass that can be accommodated by a largely heritage robotic arm.  

There is an interesti ng force- life entanglement that emerges when the additional requirement of  drill bit  lifetime is brought into consideration . Impact force, Rate-Of-Penetration (ROP)  into the rock (i.e. , “speed”  of drilling), and drill bit lifetime are all closely related b ecause insufficient impact force will not allow the drill  bit teeth to chisel the rock effectively . This in turn causes the bit to do too much rotating in place without  much rock being chipped away (low ROP) and with weight -on-bit applied during this rotation, the bit teeth  will be continually  ground against the rock  and will wear down quickly . Therefore, in order to allow drill bits  to meet their required lifetime,  the percussion mechanism must output a  sufficient impact force to k eep the  bits effectively chiseling at all times (when drilling hard rocks ). This is measured by proxy using ROP . The  result was that the percussion mechanism nominal required output force level  (“1x” force ) was set by finding  the minimum percussion output level (frequency)  that would allow a drill bit to successfully complete a 2x   life test without falling below a minimum ROP . Then percussion output forces were measured across the  operational frequenc y range using a “percussion dyno”  (see next section) . Force margins could now be  computed  by comparing the output force levels  measured on the dyno  at the max operating frequency and  at the frequency previously established by the bit life test as  “1x” force  (i.e., the lowest percussion frequency  that could pass a 2x  bit life test ). 

Finally, a  life distribution across percussion output levels was derived based on taking all the planned  sampling operations  and assigning each of them a percussion level that should ac hieve a reasonable ROP,  and therefore an associated duration for each activity .  The result of this was a total 1x  percussion  mechanism life of 2.3 million impacts distributed across five percussion frequencies/impact force levels .  This nominal lifetime is  equivalent to 2 1.5 hours of continuous percussing. Due to the mechanism containing  a dry -sliding interface (hammer bushing on hammer guide shaft ) and JPL Design principles, a 3X life test  was conducted and completed successfully with over 7.2 million impacts performed in the test .  

The approach to deriving force and life requirements described above was designed to be appropriately  conservative and philosophically in- line with normal NASA and JPL mechanism design practices, but  several non- linear affects complicate the true interpretation of the margins . Drill bit life and drilling efficiency  are highly non- linear responses to varying impact force, due to the nature of brittle failure and rock material  properties . Simply stated, doubling impact forces will  not produce a doubling of drill bit lifetime, and in fact  probably results in a greater than linear increase in bit life to a point at which effective chiseling is no longer  the dominant factor in bit wear.  

Development Program – Combining Theory , Analysis and Testing  

A combination of tools and techniques were used to respond to the unusual mechanism requirements and  difficulty in sizing the output . Several generations of prototypes were built in the course of the development  and these proved to be invaluable for developing the requirements, the design of the mechanism, validating  analyses, and for interacting with other teams and specialists who needed particular test data.  

The basic parameters for the mechanism were initially based on iterative use of a numerical simulation  (Simulink) to achieve a target impact velocity and impact energy . Subsequent research found mathematical  models of vibro- impact systems that provided tremendous insight into the non -linear dynamics of the  mechanism [1]. These models allowed the design to be altered in response to other changes to the  Sampling & Caching Subsystem in a predictable way without relying exclusively on guess and check