Table 1.  Closed Loop E M2 Motor Current to Forc e/Torque Relationship Su mmar y  Motor Qualific ation  Test # 99th Perce ntile Amplifier  Current 99th Perce ntile  Motor Current 99th Perce ntile Grindin g  Torque / F orce  Rotate (Grind) 197 415.1 6 mA 459.3 2 mA 25.13 mNm  Revolv e 199 28.29 mA 41.92 mA 166.6 mNm  Z-axis 197 23.8 mA 29.07 mA 5.907 N 

EM2 RAT Thermal Margin Anal ysis 

Grind M otor  The gri nd mo tor perfo rmance limits the RAT sin ce it is the moto r most likely to overhe at. The heating of  the grin d mo tor is gove rned prim arily by two varia bles, (1) the  motor curre nt and (2 ) the ambie nt  temperature. Using the g rinding to rque found by closing th e loop  betwe en grinding current and torque   (25.13  mNm), the moto r current can be foun d at all tempe ratures tested. Thi s current, along with the  corre spondin g ambient te mperature, was ente red into a RE25 m otor therm al model in o rder to determin e  the time at which the m otor will ove rheat. The temp eratu re limits on the grin d motor are +110º C for the   rotor and +85ºC fo r the case. At nomi nal ope rating torque (25.1 3 mNm ), the motor will o verheat in 28  minutes at +55ºC. The motor d oes not overh eat at the next lowes t temperature tes ted, +35ºC. For this   reason, +35º C is specified as the  maxi mum tu rn-on temperature  (MTO). The thermal  time li mitation was  found at a safety factor (SF) of 1.5 and 2.0 (ma rgin of 0.5 and  1.0 respe ctively). This was do ne using  motor current  vs. torque d ata to find the motor curr ent co rresponding to a to rque of 3 7.69 mNm (SF o f  1.5) an d 50. 26 mNm (SF  of 2.0). The se value s for motor curre nt were then  applied to t he tran sient  thermal mo del in orde r to arrive at a time  limitation for the motor at margi n. 

Table 2.  Grind Motor Th ermal Margins  Temp.  (ºC) Grind Curr ent  Margin of 0 (mA) Grind Curr ent  Margin of 1 (mA)Time to Overheat  (min) for Margin of 0Time to Overheat (min)  for Margin of 0. 5 Time to Overheat (min)  for Margin of 1  +55 378.7 696.6 28 23.67 9.33  +35 396.3 710.7 7 Infinite 43.67 14.67  +23 400.83 704.2 7 Infinite 140.6 7 20  0 459.32 773.9 6 Infinite Infinite 22  -20 471.37 779.3 Infinite Infinite 31.67  -40 510.65 782.7 9 Infinite Infinite 48  -55 539.5 844.4 5 Infinite Infinite 41.67  -70 567.98 868.0 8 Infinite Infinite 47.67 

Revolve Moto r  Due to the  internal  losses in the drive train  of the revolve  motor, the to rque produ ced by a g rinding  operation i s very small compa red to t he torq ue a bsorbed by the gea r redu ction. Duri ng a grin d, the  revolve motor has a much lowe r duty cycle than the gr ind moto r, and for this reason there is no ch ance  for the  revolv e moto r to overheat. T his ne cessitate s a diffe rent way of determining th e torque margin.  There are two potential failure mo des in  the revolve mo tor’s ope ration, (1) abn ormally high in ternal lo sses  in the drive t rain, and (2) unexpe cted areas of highly dense rock. Due to th e relatively low contributio n to  overall to rque of the gri nding op eratio n, the mo st pro bable fail ure point is  within  the drive t rain. Since th e  torque due to  the drive trai n cannot be measured di rectly, the torq ue ma rgin a nalysis will be based on a  no external lo ad motor current, rather th an mea sured torque. 

For e ach the rmal te st poi nt (+5 5ºC, +23ºC, 0 ºC and –5 5ºC), the moto r wa s run in velo city mode to   simulate the requi red spe ed that it needs to ope rate at during a grind (0.35  radian s/sec). This wa s  perfo rmed while de-cou pled from the dynamom eter. This  no-load motor current  was u sed as the basis fo r  the torqu e margin since the revolve m otor ne eds to dr aw at le ast this a mount of cu rrent at a certain   temperature i n orde r to overcome inte rnal losse s in the gear trai n. To determin e the torque margi n, the  286