143 line down to where it intersects the grey line. The hardstop acts as a spring, pushing the geartrain  backwards giving it momentum which can cause it to move past the first orange and grey intersection. The  system may come to rest at any of the green dots or it may completely unload. Figure 8 left shows  a system  with a soft hardstop relative to the detent step size, where ensuring preload when motor power is removed  is easier than where. Conversely, Figure 8 right shows  a system with a stiff hardstop. The Ram Mechanism  more closely resembles the left plot, while the Gripper more closely resembles the right. During testing of  the Gripper,  it was observed to back drive for certain commanded preload positions, which are now avoided.  

Figure 9. Convention for stable and un- stable nodes for a rotational system with magnetic detent brake.  Circle markers indicate un- stable nodes, where,  while the system torque is zero, any small perturbation  will result in significant motion to the stable node indicated by the square markers. The motor hall sensor  locations are centered on the stable nodes.  

Component Development Testing  

Roller Screw Component Development Testing   At the start of mechanism design,  the PRS was selected because it offers  a higher load capacity densi ty  than ball screws or lead screws  with a high efficiency . Packaging of the screw and nut and supporting  elements for each proved difficult , as the manufacturer s of these components recommend that cross - moment loading and radial loads  be prevented between them . In the pursuit  of a smaller mechanism  package a prototype mechanism was built and tested to determine  if omitting screw support was viable for  this application. A view of the PRS nut is visible in Figure 14. 

First, a characterization test was performed to measure the loads during a 2X worst case aligned seal  activation using the test equipment shown in Figure  10. With Hermetic S eals being a limited resource the  test setup was reconfigured to replicate these observed loads without them with the test equipment shown  in Figure 12. More cycles were executed on this prototype than is expected to ever be used on the flight  unit, to an axial load near to that of the flight units max capability, and to a cross -moment ranging from  matching the flight expected load to 4X that , see Figure 11 and Figure 13. Both lubricated and nonlubricated ram t ips were used, and each was tested pas t 1X life. No degradation in the performance of the  mechanism  PRS was observed. The need for additional support of the s crew is not required for this  application, and the PRS is shown to have a non- zero cross moment load capacity , contrary to the limit  specified in the manufacturers catalogs [ 3][4][5]. These tests were not intended to extensively characterize  the cross -moment capacity, only to evaluate compatibility  for this specific application. Critical to these  results and this application is the high axial load relative to the cross- moment  levels tested, and the large  diameter of the supporting bearings , since the axial force acts as a restorative force, balancing the cross  moment. This application of the PRS has a very low total life cycle count of just a few hundred, at most,  load cycles where ec centric loading can occur. A higher cycle count application may not be compatible with  supporting the PRS screw in this  fashion and would  warrant further development testing.