from rotating unde r worst ca se loadin g conditions. Howe ver, whe n motion  is desired, the  brake  is small  enou gh to be overcome by the motor. 

Using a p redetermin ed a mount of cu rrent, the Z-a xis positio n is kn own thro ugh a “Hom e” routin e that   drives th e lea d scre w up against a linea r jammin g hard sto p. Afterwards, the l ead screw i s backed a way  from the hom e positio n by 0.25 mm re sulting in the  true home p osition fro m whe re all op eration s start.  This hard stop metho d of positio ning th e Z-axis was cho sen over a switch b ecause of the  unde sired  adde d real e state and wire cou nt need ed for a swit ch to be used.  

Brush es 

After grindi ng, the abra ded su rface must be fre e of debri s for the microscopi c ima ger, an d oth er  instru ment s on the IDD to accurately view the rock’s grain structure and geological p roperties. Fo r this  purpose two f ine stai nless steel wire brushes are mounted on th e RAT, the rotate  brush mentio ned e arlier  and a revolve bru sh fixe d to the sid e of the revo lve hou sing. These two  brushes wo rk tog ether i n  removin g dust and cutting s from the  hole. The  rotate  brush remov es cutting s from the  hole  as th ey are  gene rated  and deposits th e cuttings at op the  perimeter of  the e xcavation site. As th e depth of the  hole  increa ses, the rotate brush ca n no longer deposit ma terial ato p the increa singly high p erimeter of cuttings.  This i s where the revolv e brush ai ds in the cleaning op eration . With eve ry revolutio n of the revolve   housing ab out the center o f the RAT, th e revolve bru sh pushes the building pe rimeter of ma terial away  from the e dge of the hole  so that the rotate b rush can eject the cutting s ont o a rel atively debri s free  perim eter. Th is du al brush action keeps the hole clean and free of material th at woul d othe rwise inte rfere  with the qualit y of data collected  by the other i nstrum ents. Alternati vely, the dual  brush actio n can be used  to cle an th e surface of  a rock with out g rinding. T hrough thi s approach any of the scie nce in strume nts can  view a ro ck’s surface dust free. 

Butterfly Unit 

The lower section of the RAT is mad e up of a bu tterfly mount and two but terfly wings t hat act as a   prelo ading structure bet ween the IDD a nd the rock. The butte rfly assembly i s required b ecause th e IDD is  not stiff enou gh to su spend the RAT a bove a ro ck in a free-floati ng state a nd provide reaction force s to  counter the grinding process. When the RAT is placed onto a feature of i nterest, wh ether it is a rock,  duricrust, or b locky soil, the  IDD prel oads the knurl ed balls (Fi gure 3, label 4) of the butterfly wings (Figu re  1, label 2) o nto the ro ck. As the RAT  engag es a surfac e feature, two switch es (Fi gure 1, label 5) that   reside withi n the butterfly assembly will close when a pr eload of 5 N is reached.  After the switches close,   the arm will move furth er into the  rock to establish the commande d preload for grinding. Should  the  prelo ad drop below 5 N at  any point du ring the g rinding operation , possi bly as a result of ro ver or ro ck  movement d ue to settling, one o r both of the switch es will op en and the softwa re will comm and the Z -axis  to ba ck away from the  rock a nd the n re-engage the rock. Shoul d repeate d switch openi ngs contin ue, the   result is that the penet ration rate will e ither sto p or slow to the p oint whe re a revolve time out flag will   appe ar that will abort the grinding alg orithm. This eve nt will be sent  to Mission Control alo ng with all other  RAT grin ding data. 

The de sign of  the butterfly element s that engage a nd become prelo aded ag ainst a rock was a formida ble  task. It is  necessary to ac tually pr eload the ground structure of the RAT ag ainst the rock, effectively  closing the lo op, whi ch will  dramati cally increa se the overall stiffne ss of the IDD for force reaction. Th e  need to reside the groun d structure outside of the grinding a rea account s for t he de sign of the butterfly  wings. The RAT engag es the ro ck at t he bottom su rface of each  knurled b all mounted to t he butterfly  wings. The se balls h ave this hemisp herical de sign to allow th e wing s to freely slid e over the ro ck  topography d uring the app roach vector without getting caught on a ledg e or vesicle until th e preload is  establi shed. The pair of butterfly win gs is free to  rotate +/-16 degrees about the  pivot point on the butterfly  mount. Additionally, each butterfly wing  can move rel ative to each other by abo ut one degre e (for co ntact   switch motio n) and have comp ression springs mo unted bet ween the two b utterfly wings to provide a  spring loa ded bias of 5 N applie d at the knu rled b all area on the  wing s. In this way, wh en the butterfly  wings are loaded a gainst a rock surface, they will rotate relative to each othe r by about one degree u ntil  the gol d switches are closed which al so act a s hard stops. The wings can accommodate  engaging  a rock  279