8 the cam to reduce the contact stress. The new cam design with an improved dry lubricant process was  tested again to 5X life (Figure 6c) with the same di rt tolerance conclusion and no detriment to the cam  surface.    Lesson learned:   High-fidelity development test hardware is worth its weight in unobtainium. Although the initial  development test results show that there was su fficient torque margin at end of life, it was  deemed inappropriate to implement a design that would degrade under expected operational conditions especially with the likely unknown unkn owns associated with planetary exploration.  

Figure 6. (a) Development Test DCM Dirty Testing   (b) Post Test Flat Cam Surface  (c) Post Test Grooved Cam Surface 

 Drill Percussion Mechanism

The Drill Percussion Mechanism (DPM) generates the impact needed to break the rock and the dynamic  (vibration) environment required to move powdered sample through the DBA (Figure 7). The DPM is a functionally simple device consisting primarily of a hammer assembly, a spring, and a housing assembly.  The DPM is actuated by a long-stroke voice coil developed by BEI Kimko Magnetics. It is wound with bifilar magnet wire providing graceful degradation in the case of an open winding.     The 0.4-kg hammer assembly strikes the end of a sp ring-suspended anvil rod that remains stationary and  in contact with the end of the bit. The anvil spring rate is one half of the axial spring rate of the DBA such that it supports one third of the weight on bit. The pe rcussion action is a transfer of momentum from the  moving hammer, through the anvil in the form of a stress wave, then through the bit and ultimately to the rock. The DPM operates at 1800 blows-per-minute with va riable impact energies from 0.05 to 0.8 Joules.    There are three integral linear bearing rails with polished hard anodized surfaces in the aluminum housing of the DPM. Spring elements are used across most of the interfaces within and external to the  mechanism to accommodate the various coefficients of thermal expansion.  The percussion housing  features grooves that contain internal and external radial expansion spring pairs to accommodate relative  radial dimensional variations to the steel voice coil  magnet field assembly mounted within it and the  titanium tube it is mounted within, respectively. In th e axial direction, dedicated wave springs ensure that  preloads are maintained and thermally induced stresses are minimized between the following: the percussion housing and the rest of the Drill, the voice co il field assembly and field cable guide, and the  percussion housing and the voice coil field assembly. The last spring also functions as the retraction hardstop absorbing the residual kinetic energy if the hammer exceeds its nominal range of motion.     The hammer assembly is comprised of a maraging steel hammer head fastened to an aluminum flexure  body which is adjoined to the voice coil bobbin (windings) via a threaded joint and structural adhesive.  External loads  applied at this interface   Unacceptable surface damage  and lubricant removal on flat  cam surface at 4X life After 5X life performance   with grooved cam and cosputtered Ni / MoS 2 lube NASA/CP-2010-216272