238 disassembly of the entire Turret. Modularity was abandoned in favor of saving the mass and volume of an  intermediate mounting structure. Turret mass and volu me are both highly constrained resources that had  to be committed to early in the design process. Tu rret mass drives the Arm loads and sizing while Turret  volume affects the overall configuration of the subsystem since clearances between the Turret and other  hardware had to be considered in both the stowed condi tion and during operations which put the Turret in  close proximity to the Rover.  The Arm exerts large forces between the Drill contac t sensor/stabilizer mechanism and the rock surface,  stabilizing the Drill against the rock. This keeps the Drill from walking across the rock surface when the  cutting bit engages the rock surface. The Arm creates this preload force by placing the Drill against the  rock surface using the Drill contact sensor/stabilizer (Figure 5a) and t hen overdriving the Arm actuators  so that the entire system winds up against the overall stiffness of t he Arm. The Arm produces >240 N of  preload force at its tip in many drilling configurations. The Arm uses t he contact sensors on each  instrument for placements of APXS and MAHLI. Instru ment placements occur with much smaller tip  forces (<3.5 N) since neither instrument requires preloading to perform its function. 

In SA/SPaH, sample is acquired through either rotary  percussive drilling with the Drill on rock targets or  by scooping loose regolith with the CHIMRA. The Drill acquires powdered rock samples from up to 50  mm below the rock surface using 3 of its 4 actuat ed degrees-of-freedom. While the Drill bit is translated  into the rock surface, percussion and rotation of the Drill bit also occur to cut and powder the rock material and convey it into the sample collecti on chamber of the bit assembly. The collected powder  travels through the bit using the pathway shown in Figure 6. The fourth degree-of-freedom is used to  release the Drill bit assembly from the Drill and to acqui re a replacement bit from the front of the Rover.  For scooping, the Arm positions the CHIMRA scoop abov e the loose regolith in an open position. Once  positioned, the scoop is closed using its singl e actuated degree-of-freedom, acquiring material. 

Figure 6.  The Drill sample pathway is contained in the Drill Bit. [Reference 1] 

Powdered sample from the Drill moves into the CHIMR A for processing using Arm motions to bias the  sample flow and Drill percussion to create material motion. All processing of the powdered or scooped  samples, which includes sieving and portioning, ta kes place in the CHIMRA. Various chambers,  labyrinths, and sieves within CHIMRA are used to sieve and portion the material. These functions are  carried out by rotating the Arm Turret Actuator to align CHIMRA with respect to the gravity vector to bias  the sample flow into the desired chamber while pr oducing material motion with a vibration mechanism.  Figure 7 depicts the sample pathways used in the CHIMRA to perform its processing functions. The  CHIMRA sieves are mounted to mechanisms that impart shock into the sieves to clear and clean them, preventing cross-contamination and clogging throughout the mission life. This is termed thwacking. The  intentional dynamic environments created to move sample around are a key feature of the subsystem and device designs. These are percussion in the Drill and vibration in the CHIMRA. Orientation with  Sample chamber  inside the Bit  Powder exits here and then enters the Sample Transfer Tube on CHIMRA  Powder from rock enters here  NASA/CP-2010-216272