Table 4.  Z-Axis Motor Thermal Margin (RE20 Motor)  Temp  (ºC). Maximum Mot or  Current (mA) at 4V,  30N 2X Maximum Motor  Current (mA) at 4V,  30 N Overheat  (min) Maximum Mot or  Current (mA) at 8V,  30N 2X Maximum Motor  Current (mA) at 8V,  30 N Overheat  (min)  +55 46.6 93.2 Infinite 43 86 Infinite  +35 41.4 82.8 Infinite 39.1 78.2 Infinite  +23 40.2 80.4 Infinite 46.3 92.6 Infinite  0 46.1 92.2 Infinite 52.2 104.4 Infinite  -20 56.7 113.4 Infinite 73.7 147.4 Infinite  -40 100.9 201.8 Infinite 137.8 275.6 Infinite  -55 127.8 255.6 Infini te 172.7 345.4 24.33  -70 167.4 334.8 Infini te 285.1 570.2 2.75 

“In Family ” RAT M odels  

During the FM1 and FM2 dynamom eter test, the  methods for measuring moto r curre nt were inaccu rate.  The m otor cu rrent data  recorde d during those  tests was amplifie r current. The true (direct) motor cu rrent  was me asure d thro ughout the EM2 dyn amomete r tests and it i s unde rstood that the m otor cu rrent is a   nonlin ear fun ction of am plifier current  and P WM duty cycle  among othe r thin gs. Sufficie nt data d oes not  exist to allow the FM1 and FM2 erron eous motor current s to be  transfo rmed  to actual motor cu rrents.   Therefore, in  orde r to determin e that the EM2 is “i n family” with  the FM1 an d FM2, the amplified moto r  curre nt had to be co rrelat ed to the direct motor current. The dat a sho ws that all of the actuators in the  three units behave si milarly  at most tem peratu re set points; h owever, there  are a few dispari ties bet ween  the motors, which mo stly occur at lowe r temperature s, -70° C in particular. 

Most of the dispa rities can be attributed to (1) me chanical vari ations in the mot or drive trai ns (the current  obtaine d from the dynam ometer te sts include the cu rrent due to the intern al losses in the d rive train, and   althoug h all parts are ma de to same print, there are fitting differen ces and the  lubricated parts can behave  differently at cold temp eratures), (2) set limits in the control software, and (3) an impro per b ake out durin g  one test. 

Disparitie s at –70° C: 

 The F M1 revolve no -load curre nt at 3  V is m uch lo wer than  the FM2 a nd EM 2. The FM1 revolve  did not move  at –70° C be cause the current rea ched its limit. When FM1 was tested, in creasing  curre nt limits at cold er te mperature s was not considere d due to lack of test experie nce. These   limits we re increased befo re FM2 and (subsequ ently) EM2 were tested. 

 The EM2  revolve no -load  curre nt at 6V is mu ch higher tha n the other t wo units. This is due to the   revolve cu rrent reaching it s upp er limit and not movi ng. 

 The Z -axis n o-load motor curre nt comp arison at 8 V  showed that t he FM 2 curre nt is m uch hi gher  than the FM 1 cu rrent an d the EM 2 current is m uch hi gher than the FM 2. For FM 1 again,  increa sing the current limit s at colder temperatur es was not con sidered, th erefore the FM 1 Z-axis  did n ot move  at -70 C because the  current limit was reached. Thi s current limit was increased for  EM2 testing, but the unit was not prope rly baked out d uring the te st, so  the Z-axi s did not move at  -70ºC be cause it rea ched its cu rrent limit. The effects of the  improp er bake o ut we re also  appa rent at –55° C, wh ere the EM2 Z-axis moto r curre nt was runni ng hig h. However, it wa s  determi ned th at the EM2 Z-axis no-l oad curre nt is clo se to that of FM2. 

Overall, the  differen ces betwee n the t hree units at nomin al operation te mperature s are minor and it is  concluded th at all three u nits are “in fa mily”, and so  the re sults from the EM2 Margi n Analy sis based o n  the EM2 grin ding an d dyn amomete r test data can be applie d to FM1 and FM2.  

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