133 SHA reachability and Z -stage travel constraints were a significant driver of the ACA configuration. From  initial flight concept to final configuration, the ACA went through well over 50 different configuration layouts .  The ACA had to fit within the defined volume in the Rover, while preventing ACA stations from interfering  with each other, particularly during robotic operations . SHA geometry was dictated by the ACA volume in  the Rover, with link lengths maximized to allow for motion within the Rover  with clearances to structure of  approximately 10 mm. Placement of the SHA on the CCMD was the result of ensuring the SHA could  successfully reach all ACA stations and operate with sufficient position accuracy and force margin . This  placement required the SHA to flip (invert) in the Rover, meaning the SHA would fully extend and rotate  back on itself in the opposite direction at the elbow joint , extending the reachable workspace in the available  volume. While the SHA Z -stage extends below the Rover bellypan during operations, it must fit within the  Rover volume for launch through landing  which limits overall available travel . The resulting SHA Z -stage  travel drives ACA station and component heights to ensure sufficient clearances exist during robotic  operations . A hermetically sealed tube on a glove on the EE results in the largest stack -up on the SHA  requiring the largest Z -travel, while maintaining approximately 10 mm of clearance between the top of the  seal and bottom of the ACA stations . In a few  locations, clearance is reduced to as low as 4 mm, which is  the result of design maturity  of ACA hardware within exist ing constraints . Analysis and test verified  acceptability of these close clearances.  

Because of SHA -driven hardware height limitations, sample tube, glove, and cover ball lock mechanisms  were constrained such that their required spring forces could not be achieved with a single spring solution  in the available volume. As a result, nested compression springs were implemented that are counter wound  to avoid unwanted interaction (i.e., intertwining) . In addition, the sample tube ball lock mechanism could not  support a sufficient length- over-diameter (L/D) ratio for close clearance sliding components, which  increases the chances of wedging/jamming. The glove and cover mechanism designs achieved adequate  L/D ratios, minimizing their risk of issues . During hardware testing, sample tube ball lock mechanisms  exhibited jamming in the ball lock mechanisms, which was mitigated by the addition of sputtered moly - disulfide dry film lubrication to hardware that was previously bare within the assembly to reduce friction tha t  was attributed to a number of failures . Follow -on testing showed no issues across environments, providing  confidence in the corrective action implemented.  

SHA operations were conducted from the start  using flight software to ensure the hardware and software  worked in sync , as well as ensured fault protection and positioning required to operate the SHA within the  ACA volume limits were in place to protect the hardware. While this is beneficial for proving out  hardware/softw are behavior, t his introduced a number of challenges since the ACA was operated using  Actuator Electrical Ground Support Equipment ( AEGSE)  versus the flight avionics Rover Motor Control  Assembly ( RMCA ) during ACA -level testing. Behavior differences between controllers existed that  impacted position error during operation. For example, if a motor gets bogged down and needs more current  to reach a position, the RMCA will allow a temporary increase (i.e. , boost) in current that allows a motor to  reach position  within about ±1 hall count . While AEGSE attempt ed to match the RMCA boost current  behavior , significant position errors occurred during  hard stop calibration activities  affecting functionality .  Therefore, two operational modes were developed for the AEGSE to achieve similar position accuracy as  the RMCA for all functional activities:  boost current was disabled to accommodate hard stop calibration,  while boost current was  enabled for nominal functions.  

Force control response was impacted when using flight software for SHA operations . Response times for  force control were unexpectedly high as a result of filters on the force torque sensor signal  causing force  overshoots . Additional force overshoots occurred as the motor controller accelerated t he motor through the  speed range where the motor detent brake interacts with the motor to avoid erratic behavior . The combined  overshoot s reduce force limits , which  results in a reduction of the SHA -to-station interaction operational  margin by approximatel y 10% , as well as more occurrences of force overshoot nuisance faults . Software  updates were made prior to launch to correct these overshoots , regaining operational margin required for  surface operations.