revolve moto r was coupled with the dynamomete r and again run i n velocity mo de at 0.3 5 radians/sec at  each tem perature  set poi nt. The  dyna mometer was used  to a pply a torque  load on the motor until th e  motor cu rrent was a s close to twice the no-lo ad cu rrent as po ssible. For so me temperatu re set points  (0ºC and +23ºC) it wa s not possi ble to prod uce eno ugh torq ue to  double the n o-load current due to the   limitation of the dynamo meter (m aximu m applied to rque of 350 m Nm). 

To determine thermal margin, the  peak motor curre nt was calculated while  running th e revolve motor in  voltage mo de. This current  was doubled and used in the RE020  motor th ermal model  to d etermine  the  transi ent he ating of the mo tor at all tem peratu re set points. It wa s found that th e revolve mot or does not  overhe at at any of the temper ature set points o r voltage s. 

Table 3.  Rev olve Motor Thermal Marg in (RE20 Mo tor)  Temp  (ºC). Maximum Mot or  Current (mA) at  3V, 150 mNm 2X Maximum Motor  Current (mA) at 3V,  150 mNm Overheat  (min) Maximum Mot or  Current (mA) at  6V, 150 mNm 2X Maximum Motor  Current (mA) at 6V,   150 mNm Overheat  (min)  +55 34 68 Infinite 35.8 71.6 Infinite  +35 34.3 68.6 Infinite 36.9 73.8 Infinite  +23 36.1 72.2 Infinite 37.5 75 Infinite  0 35.8 71.6 Infinite 40.6 81.2 Infinite  -20 41.3 82.6 Infinite 53.1 106.2 Infinite  -40 67.7 135.4 Infinite 71 142 Infinite  -55 89.4 178.8 Infinite 74.6 149.2 Infinite  -70 142.1 284.2 Infinite 152.9 305.8 Infinite 

Z-Axis  Motor   As with the revolve motor,  the force caused by a gr inding o peration is very small com pared  to the force   absorb ed by the ge ar reduction  and friction due to  internal losses in th e drive train of the Z-axis moto r.  During a gri nd, the duty cycle of the Z-a xis motor is an orde r of ma gnitude lo wer than that of the grin d or  revolve m otors. Fo r this reason, there is no chance for the Z-axis motor to  overheat. Th e revolve an d Zaxis moto rs share the same pote ntial failure modes during operation. Ag ain, due  to the  relatively low  contri bution  to overall fo rce of the  grinding operation,  the m ost probable  failure point i s in the drive  train .  The exact ap proach used to determin e the revolve mo tor torq ue margin was also u sed for the Z-axis  motor. 

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 required speed that it n eeds to operate at duri ng a grind (0.05 mm/sec). Thi s was perfo rmed  while d e-coupled from th e force controll ed air cylinde r. The moto r was run for close to a full stroke of the   output and th e resulting m otor cu rrent wa s avera ged. To determi ne the torque  margin, the Z-axis moto r  was coupl ed with the ai r cylinder a nd run in velo city mode at 0.05 mm/se c at e ach te mperature set p oint.  The air cylind er was then u sed to ap ply a load on the  motor until the motor current wa s as close to twi ce  the no-l oad current as possible. Th e ma ximum force that  the load cell can me asure a ccurate ly is slightly  over 50 N (the load cell spe cification has 50 N as the maximu m mea surable load; however, it wa s  observed th rough te sting that this is  not the absolut e limit). For one tempe rature set point (-55ºC) it was  not po ssible to pro duce sufficient loa d to dou ble th e current b ecause it w ould have well e xceed ed th e  limitation of the load cell. 

To dete rmine  thermal m argin, the pea k motor current  was cal culated from dyn amometer te sting of the Zaxis moto r in voltage mod e. This current wa s doubled an d used in the RE0 20 motor th ermal mod el to  determi ne the transi ent heating of the motor at all te mperature set points. It was a ssumed t hat the motor  was run constantly rathe r than inte rmittently. It was found that the Z-axis m otor only overheats at and   below –55º C when operating at a voltag e of 8 V, with a load of 30 N or m ore on the motor, an d at a safety  factor of 2.0. This is well a bove the exp ected 4 V operating  voltag e, and fa r above the exp ected 6 N loa d  on the motor durin g grinding.   287