The RAT i s a stand -alone device in t hat it does n ot requi re an y input or m ovement of the rove r’s IDD  (Instrument Deployment Device) o r “arm” on ce pl aced onto a ro ck’s surfa ce. The IDD m ust preload the   RAT ag ainst the ro ck withi n a rang e of 10 to 10 0 N. It is desi rable for the p reload to be at least 50 N for  stability. The device’ s three motors each serve a di stinct function and operat e in different  modes. The  RAT g rinds a hole in  the rock by rotating the g rinding wheel (Figure 3, lab el 2) at a pproximately 3000   RPM wh ose cente r is offset from the center of the RAT by 11.1 1 mm.  The grinding wheel i s 23.37 mm i n  diamete r, 6.35-mm wi de, and looks like a “pad dle wh eel” but is ref erred to as a grindi ng “whe el”. Mount ed  only alo ng th e oute r region is a diam ond-im pregnated resin. Th e grinding  whe el also revolv es about the  cente r of the RAT at  a ve ry slo w speed of 0 - 1 RPM  under closed  loop  control.  The com bination of high - speed rotate and revolve create s a circular gri nding  area 45 mm i n diam eter. T he lo wer half of the RAT  housing th e grindin g wheel is subsequently advance d into the  rock in small incremental steps of 0.05 m m  per revolution  until the desi red grinding depth is a chieved. 

Motor s 

The g rind m otor rotate s the two shafts at the bottom  of the unit where th e grinding wheel and the rotate   brush, used to remove cuttings from the excavated ho le, are attach ed. These sh afts are equ ally displaced   from the  center of the  housing and centerline  of th e RAT by  11.1 1 mm and are the refore separated from  each othe r by a dista nce of 22.22 m m. This moto r operates at a  set voltag e parameter, ty pically 2 7 V,  which will rotate the gri nding wheel a nd rotate b rush at app roximately 3000 RPM. Simultaneo usly the  curre nt provided to  the m otor is monitored and used a s feedback for the grind motor a s well as the   revolve m otor. Durin g mid-day activitie s, it is typically expecte d that the  bus voltage will be between  28  and 34 V. 

The revolve motor causes the revolve  housi ng tha t suppo rts th e two rotate  shafts to ro tate about a  comm on center point so th at the g rindin g wheel and rotate b rush sweep th rough a full circle  45 mm  in  diamete r. Usi ng a sin gle 45-mm dia meter wheel that  rotates a bout the center of the RAT would not be   suitabl e because the velo city of the gri nding m aterial  at the cente r of the wheel  woul d effectively be ze ro,  preventin g grinding at the  center, an d therefo re hal ting any adva ncement of the wh eel into the rock.   Additionally, with a sin gle wheel app roach, rem oving cutting s would be pro blematic. 

The spee d at which the revolve hou sing rotate s is inversely p roportional to the  grind motor current draw.  For exampl e, if a set point of 400 mA o f current is comman ded for the grin d motor, the re volve housin g  will rotate at its maximum  velocity whe n there is no load on the grind m otor as in the case whe n the  grindi ng wheel is not to uching th e rock.  The maximu m ve locity of the revolve h ousing is also a set point   that is sel ectable for e ach grin ding ta sk. Spe ed c onsta nts exist for the rev olve motor a pplied to the   grindi ng algorithm eq uation s. As the  current d raw from the grind motor increases, the revolve m otor slows  down to  prevent ove r aggressive cutting  into th e rock that can lead to stability probl ems. When th e grind  motor dra ws the same am ount of cu rrent as defin ed by the current set point, t he revolve m otor will stop  advan cing th e grindi ng wheel acro ss the ro ck until the grind current dra w drops when ro ck mate rial is  worn away.  

The Z-axi s moves the lower se ction of the RAT in a linea r fashion. Exten ding th e axis eng ages the  grindi ng whe el with the ta rget ro ck to g rind to a u ser sele ctable d epth. A very high resol ution of motion   from the Z-a xis permit s a very small amount of mate rial to be re moved with each full revolution of the   grindi ng wheel about the cente r of the  RAT. Attach ed to a pla netary gea r head is th e motor with an  18.777:1 reduction. A set of spu r gears furthe r reduces the rotational output.  A 5 mm x 0.5 mm lea d  screw, cou pled to the outpu t of the dr iven spur g ear, provides lin ear motion. 

The Z -axis h as a ma gneti c detent b rake built within the up per se ction of the motor next to  the e ncode r.  During lau nch, landing, o r rover trave rses the dete nt brake will prevent the Z-axis from migratin g. Th e  detent b rake contai ns four small  mag nets (Figu re 3, label 5) attach ed to th e motor housing that su rroun ds  a steel  cross-sha ped plate fixed to the m otor’s roto r. The attra ction betwe en the  ends of the  cross an d the   magnet s act s like a rotatio n brake that has p referred alignment ev ery 90 deg rees. Governed  by the gap   betwe en the end s of the cross an d the magnet s, the strength of the bra ke is designed to h old the shaf t  278