and h andles RAT control and d ata acquisition ta sks. The p roblem with the E GSE was the motor current  that it measu red a nd stored was not the a ctual current consumed by the m otors. In some cases, the  EGSE showed more cu rrent than wa s actually co nsumed  and  in othe r cases, the EGSE showed l ess  curre nt than was con sumed. It appea red that the P ulse Width M odulato r (PWM) current ou tput wa s not a  linear rel ationship with respect to t he comman ded percentag e issued to th e PWM. As a result, the d ata  collected could not be  used to det ermine the a mount of cu rrent need ed fo r each motor gi ven variou s  loadin g and tempe rature condition s. Fu rtherm ore, torque  margins coul d not be asse ssed b ased on the   informatio n at hand. Si nce the t wo flight units ha d alrea dy been delivered, we could  not use them  to  perfo rm a se cond roun d of tests with a proper me asurement of the motor current. Conseque ntly, we  decided to dy no-te st the  EM2 unit using the same  erroneo us curre nt coll ectin g EGSE. This allowe d us to  comp are the  EM2 unit to  the two flight units and make a determi nation a s to whether the E M2 unit was “in  family”, mea ning th e diff erence in behavior is in significant, wit h regard to the two flight  units . After  compl eting th e same no lo ads test s, usi ng the er roneous EGSE equipme nt that were cond ucted on both   flight units, and co mparing the data, we con cluded that all three units were in deed within “family”. With  this un derstanding, we could then  test the EM2 u nit unde r Qualif ication Test condition s using a correct  method of measurin g each motor’ s current a nd assess toque margins that we co uld a pply to both flight  units. The foll owing is a de tailed accou nt of the meth ods we used  in determini ng if the assembled u nits  behave d the same  and the techniqu e of determinin g motor torqu e margins for each of th e three moto rs  within both fli ght units. 

Dynamo meter Test s  The purpo se of the EM 2 RAT actuator dynamom eter testing was to characterize the spe ed-torquetemperature relation ships of the RAT actuators in cluding the d rive trai ns, and to verify the ability of the   actuato r compone nts to  survive ex cursio ns to extreme te mperatu res. The a ddition al purpose of  dynamom eter testing wa s (1) to det ermine the clo sed loop relatio nship bet ween motor cu rrents requi red  for ro ck gri nding and th e actual fo rces and torque s appli ed, (2) to verify that the perfo rmance of EM2,  FM1 an d FM 2 RAT s are “in family” a nd explain a ny disparities that exist, (3) to determi ne the torq ue  margi n on each actuator at key tem peratu re set points, (4) to determi ne a set of F M1 and FM2  parameters t hat are safe for the RAT, but ro bust en ough to successfully grin d, and (5) to determin e the  maximum turn-on  time for actuato rs that  exhibit low margin at certain tempe ratures, using analytical a nd  experim ental method s. 

Closed Loop Motor Curren t / Torque Rel ationship   All EM2 RAT  motors h ave been  dynam ometer te sted in order to characterize their current ve rsus torque/  force  profiles at seve ral different  tempe rature s. Seve ral grindin g tests have  also  been performed with th e  EM2 RAT at  0°C. The se grindi ng te sts represent a worst-ca se scenario in terms  of rock compress ive  stren gth and topology and the data wa s use d to deter mine the wo rst-ca se torq ue and force requireme nts  for all three RAT motors. The ro ck used for tests wa s a stro ng basalt, and th e surfa ce top ology wa s flat  (no peaks or valleys on the rock) in  order to demonstrate the full capabilit y of the RAT. Four grinding test s  were pe rform ed on basalt, with the RAT at 0°C. Th ese te sts are numbe red #197 - #20 0, and will be  referred to as s uch. 

Rotate (Grind) Motor Analy sis  Using the erroneo us EGS E data obtain ed from gri nding tests #1 97 - #200, the  amplifier cu rrent vs. time   was plotted f or the grind motor th roughout the d uration of  the te st. Occa sional “spike s” were ob serve d in  the data wh ere the amplifi er current would jump m uch hi gher than the typica l range of va lues. The se  spikes occu rred over a very short time perio d and d o not negatively affect the perform ance of the RAT.  The p eaks in the amplifie r current data  do not represent the  worst-ca se grinding current b ecause they   only o ccur fo r very short amounts of tim e, and  only a few tim es during a typical grind. Therefore, in  order  to determin e the wo rst-case torque requireme nt, a statistical ap proach was take n for the data  analysi s. 

Figure 4  shows a histo gram of the  grind moto r amplifier current  during grinding test #197. The ve rtical  dotted line s show where 95 and 9 9 percent of the am plifier current fall and the resulting value. This  approa ch wa s take n in order to help filter out some of the anoma lous current spikes in the data, and to  have a  math ematical  basis from which to define  the current / torque  relation ship. The 99th percentile value   of 415.16 mA  was u sed from this test to determi ne the  current / torq ue relatio nship for the grin d motor.   283