14   Figure 12. Two of the DTM Qualification Test s: (left) Margin under Drilling Loads and   (right) Retraction Force Capability Charac terization (Lifting 10,000 N at -70°C) 

Drill Contact Sensor / Stabilizer   The Drill Contact Sensor / Stabilizer (DCSS) is a passive mechanism that  indicates the placement of the  Drill on a target rock (Figure 13). The articulati ng two point-contact design of the DCSS accommodates a  combination of target surface height variations be tween the contact points and Drill axis misalignment to  the surface normal. The DCSS will indicate contact if and only if both prong tips are in contact with the target. Once preloaded to the target by the Robotic  Arm, the DCSS stabilizes the Drill by locking out 4  degrees-of-freedom (3 translational DOFs and a rota tional DOF about the bit axis) between the Drill and  the target. This device is conceptually similar to the contact sensor implemented  on the Mars Exploration  Rover for the Rock Abrasion Tool [3]. The DCSS consists of two counter articulating spring-loaded 4-bar linkages interconnected by a coupler assembly. The coupler assembly consists of a piston with cam  surfaces that actuate two micro switches. The two pron gs of the DCSS and the Drill forward housing  structure make up the coupler links and ground link, respectively. The total linkage is centered by  counter-acting clock springs mounted in the aft links’ lower joint assembly. These springs also reset the coupler piston thus eliminating the need for a return spring within the coupler sub-assembly. The springs are machined (rather than hand wound) to ensure there is no coil rubbing which would add undesired friction. They also have spline features on the inner and outer diameter that allow fine adjustment of the  spring preload.  A low and consistent switch trigger force will yield more reliable preloading by the robotic arm. This  presents a conflicting requirement for the DCSS: the return spring force should be high enough to ensure the release of the switches but not so large as to drive the trigger force above 40 N. The solution was to seal the moving elements: including all rotary joints and the piston of the coupler with felt seals. This  would ensure that the Martian dirt  would not foul up the device and keep its performance consistent. The  return springs were then preloaded to ensure sufficient margin to un-trigger the contact switches against the total measured friction in the assembly.  Another design challenge for DCSS was structural ca pacity. One of the drivers was, once again, the  worst-case load scenario: the rover can hang off one prong on a 20 degrees Mart ian zero-friction slope.  However, the biggest driver was the Translation Mec hanism retraction force which is reacted back to the  rock by the DCSS. The loads in the links of the four -bar can get quite severe especially when the DCSS  is in its fully articulated configuration. This is comp ounded with the tight packaging  constraints of the turret  (Figure 1b); the DCSS is the closest neighbor to the DRT and CHIMRA. Through the use of high strength  materials, spherical bearings at each joint, and clever link geometry th e device meets the load  NASA/CP-2010-216272