242 Vibe was a likely candidate for hardware redundancy,  however there was no place to package a  redundant mechanism. Unlike the Arm joints, Vibe requires a high-speed, high-life mechanism so  electrical circuit redundancy without mechanical redundancy is not as strong a solution and was not  pursued. Instead, Drill percussion may be a viable back up for CHIMRA vibration during these activities.  Initial indications from dynamic characterization testi ng show promise for this possibility but the result will  not be definitive until it is demonstrated with sample.   Another failure scenario considered along the sample pat hs is clogging. This is prevented most effectively  through good design practices: large physical paths where particles flow and large margin in the vibration  and percussion used to move the particles. The redundant sample paths in CHIMRA allow for some functionality if one path is clogged. CHIMRA can also be opened up and viewed by the Rover cameras  which may assist in diagnosing the clog and using dynamic  inputs to clear it. However, if the failure to  acquire sample with the Drill or to move sample th rough the Drill is due to clogging of the bit with sample,  the bit can be exchanged for a spare bit. Bit release c an also be used if the bit is stuck in the ground and  cannot be extracted using the large retraction fo rce capability of the Drill translation mechanism.   To enable the effective use of the functional redundancy approach for the Drill and CHIMRA, both the  Turret configuration and the subsyste m hardware configuration must ensur e that the failure of one tool  does not preclude the use of the others. More specifically, a CHIMRA scoop or thwack actuator failed in any position cannot prevent the us e of the Drill and a Drill feed actuat or failed in any position cannot  prevent the use of the CHIMRA. In addition these failures cannot prevent the Arm and Turret from  returning to its restraint for driving.  Lessons

In a sampling system of this complexity, ther e is an inherent tradeoff between redundancy of capability  (function) and breadth of function when choosing the r equired sample pathways. The specific choices  have consequences for both the kind of scientific investigation t hat can be conducted and for the  engineering implementation so the appropriate balance needs to be achieved through iterative  conversations between engineering and science. Moreover , the fault tolerance approach is intimately  connected to these choices and needs to be considered at this point in time.    The required sample paths must be determined early in the design phase and specified completely by  defining the volume of sample, particle size, processing steps, and sequence of processing. The  processing of sample and movement of material are volume intensive activities. Sample paths need to be  configured and volume and mass resources allocated to  the functions so the detailed mechanical design  can begin. Once sample paths are selected, the system rigidizes around t hem and significant changes  are no longer possible. The process here is  analogous to configuring a spacecraft.  

Dynamic Environments   Both the Drill and the CHIMRA create intentional operational dynamic environm ents to perform their  functions on the rocks and regolith they operate on. Although this appr oach is a robust way to move  sample, there are difficulties and challenges with creating an intentional dy namic environment on the  coupled dynamic system of the Arm and Turret. One of  the main challenges is to create the dynamic  environment in the areas where it is needed (the sample processing and flow areas) while keeping it away from the places it is unwanted (the sens itive Turret-mounted instruments, APXS and MAHLI).    The basic design concept for the Turret regarding oper ational dynamic environments is to separate the  operational frequencies of the Turret- mounted tools, CHIMRA and the Drill,  and to provide isolation for  the Turret-mounted instruments, APXS and MAHLI. Drill percussion operates at about 32 Hz. The  translating components inside the Drill are mounted on springs that act to reduce the kickback force disturbance to the rest of the Turret devices from percussion. CHIMRA vibration is created by rotating an  eccentric mass at a constant speed where the s peed is chosen to be at the frequency of a CHIMRA  mode of vibration. Some adjustm ent of the CHIMRA frequency can be m ade by changing the thickness of  NASA/CP-2010-216272