160 Test Proceedings and Results   Tests were conducted to combine three various motor sizes with two lead screws with varying pitch . The  goal was to produce a balance of speed, size, and torque margin. Navigation and control designers required  that the AMT move slowly to increase precision and reduce dynamics imparted into the vehicle. The design  volume, driven greatly by motor diameter,  limited the available height of the AMT . Lastly, the design  requirements levied certain torque margins onto the system, which when compounded by conservatism in  the preliminary thermal, mechanical and dynamic analysis, yielded  driving torques far greater than  expected. Nonetheless, the system had to be sized accordingly . Changes to the motor diameter directly  affected the AMT vertical size constraint, while combinations of the motor and threads adjust ed the speed  and torque margins simultaneously . For example, a motor -thread combination could yield acceptable  torque margins and size , but produce  too rapid of motion. Adjusting to a higher  pitched thread would  increase the torque margi ns and reduce the driving speed -  a single change benefiting two figures of merit.  

A few of the combinations were ruled out by hand calculations . The largest motors (Configurations E and  F from Figure 5 ) were too large for the design volume and had calculated torque margin at beginning of life  magnitudes higher than required. Furthermore, when the final two motors were paired with both threaded  rods, hand calculations  again ruled out two more options . The smallest motor paired with a high pitch thread  (Configuration A)  and the middle- sized motor paired with the lower pitched thread (Configuration D)   produced comparable speeds and margins, but different form factors . The test fixture was used to compare   the final two motor and thread combinations  and close the trade. The results were  as follows:  

16.4 8.2 6 A B 8 C D 10 E FConfiguration Title Motor Size [mm]Thread Pitch [threads/cm]

Vertical Height Required Translation Speed Torque Required Torque Available A 13 mm 0.032 cm/s 1.5 mNm 35 mNm B 13 mm 0.064 cm/s 2.5 mNm 35 mNm C 17 mm 0.032 cm/s 1.5 mNm 120 mNm D 17 mm 0.064 cm/s 2.5 mNm 120 mNm E 21 mm 0.080 cm/s 1.8 mNm 200 mNm F 21 mm 0.160 cm/s 4.2 mNm 200 mNmConfigurationFigures of Merit

Figure 5.  Drive system configuration trade  

At the conclusion of the trade, the AMT design er chose Configuration A . This motor -thread combination  minimized the vertical profile of the AMT while also minimizing translation speed and retaining an  acceptable torque margin. Despite the small motor size, the designer  was surprised by the force produced  by the system (an estimated 150 N). Originally, the 8 -mm diameter motors served as a baseline design, yet  this test led to a design change that will reduced  the AMT’s volume requirement.  

Moving forward, there are options to modify the torque output and speeds pending engineering development unit  test results . This will be done by modifying the gearboxes and control inputs. According  to the tests, all configurations had large torque margins, with configurations E and F clearly oversized for  the given application.    As expected, the test setup proved beneficial to the AMT design process . The designer  gained confidence  in the system’s design, particularly with the pairing of the stepper motors, lead screws and rail guides . The  motors, despite their small size, were capable to produce driving torques far greater than demanded, even