253 The key element of the STM that allows for portioning functionality is the float between the motion of the  secondary thwack arm and the scoop. The secondar y thwacker arm is preloaded closed against the  scoop. When the scoop is opened the secondary thwack arm will follow the scoop for ten degrees until the STM latch is engaged and the thwack arm restrained.  This provides the opportunity to discard all  excess sample in the 1-millimeter  chamber as well as sample greate r than 1 millimeter contained in the  scoop. The only sample retained will be what is contai ned in the portion tube integral  to the thwack arm. A  bypass valve allows this generated portion to be bro ught back into the scoop chamber where it can be  distributed to the instruments by pouring it out the opened scoop. The sacrifice made by not dedicating an  actuator to 1-millimeter portioning is that the ge nerated portion size has th e possibility of containing  particles that did not pass through the 1-mm screen. Spec ifically, this is any sample that did not fall out of  the scoop at the previous step. This  is because the sample distributed to the instruments does not exit  CHIMRA from a chamber that can only be accessed by first passing through a screen as is the case with  the 150-micron portion design. This shortcoming is mitigated by including a visual inspection step in  which the scoop is opened and the sample is visually examined prior to delivery.                          Figure 7. 150-micron Portioning Method                     Figure 8. 1-millimeter Portioning Method 

Sample Processing Robustness via Design Configuration   With few examples of design heritage to learn fr om and base the CHIMRA design on, a set of sample  handling guidelines was established prior to stru cturing the CHIMRA layout. This allowed design  iterations to be evaluated against predetermined principles that were chosen to maximize the design robustness independent of the implementation constrai nts. These established guidelines can be broken  into two categories: features that minimize the lik elihood of internal particle clogging and elements that  allow assessment of problems or clogs as they arise. The CHIMRA also required configuration  adaptability for sample processing in the case of an  actuator failure. These elements established the  driving principles behind CHIMRA’s sample processing robustness.    While the design features utilized to mitigate clogging are straightfo rward, the design difficulty for  CHIMRA arose when external volume or mass constraints forced compromise between sample processing guidelines and subsystem device requirement s. For example, the aspect ratio of a chamber’s  size to size of a particle passing through that chamber was carefully monitored; the final CHIMRA design strived to maintain a 10:1 aspect ratio. However, this was a compromise from the original 20:1 design  philosophy that was eroded as wall thicknesses were increased to meet structural requirements, passageways were reduced to meet subsystem volume allocations, and overall design was changed to  Bypass Valve 1 millimeter  chamber One way valve  Portion tube Portion tube  NASA/CP-2010-216272