grind motor i s 25.1 3 mNm , the worst-case to rque require d by the  revolve mot or is 166.6 m Nm, and th e  worst-ca se fo rce fo r the Z-axis motor i s 5.907 N.  

Using no -load revolve and  Z-axis moto r data, it was d etermin ed tha t the action of grindin g a rock does not  contri bute g reatly to the overall torqu e/force on th e motors. Most of this to rque/fo rce is abso rbed by  friction i n the drive trai ns of  these  two motors. The re volve motor g ear train absorbs approximately 156.8  mNm, whil e the Z-axi s gear train ab sorbs app roximat ely 4.04 N.  

Using moto r dynamom eter data an d the  torqu e required to grind a worst-ca se scen ario ro ck, the therm al  margi n was determi ned f or the grin d motor, the motor mo st likely to overhe at. It was fou nd that with a  temperature l imit of 110ºC on the roto r, and 85º C on  the ca se, the RE25 m otor doe s ove rheat at +55 °C  ambient tem peratu re but doe s not ove rheat at an a mbient temp eratu re at 35 ºC after fou r full hours of   grindi ng. The maximum turn on tempe rature (MT O) will therefore be  specified a s 35°C. 

The the rmal margi n was also calculate d for the revol ve and Z -axis motors in th e wo rst-case scena rio that  the motors operate  contin uously, rathe r than inte rmittently. It was found that th e revolve motor do es not  overhe at at any of the temperatu re set points with a safety factor of 2.0.  It was also fou nd th at the Z-axis  motor sta rts to overhe at when ru n in voltage mode (a t 8 V) at temperatu res below –55 ºC, at a load of 30  N, an d with  a safety fa ctor of 2.0. Sin ce the Z-ax is motor typically operates bet ween 4 V and 6 V with a  load of less than 6 N a nd a very low du ty cycle, this wa s not se en as a co ncern . 

The revolve and Z -axis motor m argins we re determined using the no-load current at sev eral different   temperature set point s (+55ºC, +23º C, 0ºC, -55º C), since intern al losses in th e gear train s outwei gh the  force s and to rques due to grindi ng. To rque a nd force loads were applie d to the  revolve an d Z-axis motors  until the mot or curre nt increa sed to cl ose to double the no -load current. Thi s shows that th e motors can  overcome the  internal lo sses in the driv e train. 

Although the Rock Ab rasion Tool was specifi cally de veloped for t he Mars Expl oration Rove r Missi on, the   lesson s learned du ring development provide cru cial insig ht for f uture endeavors. Undoubtedly future   missi ons targ eting Ma rs and othe r celestial bo dies will re quire drilling a nd g rinding i nstruments. When  specifying de sign criteria one mu st be more tole rant of the unce rtainties of the unkno wn targ ets. System  develop ers m ust reali ze the philo sophy of desi gning mech anisms with force and torq ue ma rgins fo r a truly  unkno wn targ et environm ent shoul d not be overlo oked. 

Referen ces 

1. Hitchi ner, M. P., Wilks, E. M., and Wil ks, J. “The P olishin g of Di amond a nd Diamond Composite   Material s.” Wear , (1984), pp. 103-1 19. 

2. Gorevan, S. P., Myrick, T ., David, K., Chau, J.  J., Bartlett, P., Mukherj ee, S ., Anderson, R.,  Squyre s, S. W., Arvidson,  R. E., Ma dsen, M. B., B ertelsen, P., Goetz, W., B inau, C. S., and  Richter, L. “Ro ck Abrasi on Tool: Mars Exploratio n Rover Mi ssion.” Journal of Geophysical   Research, 108(E12 ), 8068,  doi:10.102 9/2003 JE00 2061, in pre ss, 2003.   290