New Motor Design  Based on the results from the corrected stability model, a new set of motor requirements was devised.  Instead of increasing the maximum allowed current at the expense of the BEMF constant, a higher BEMF  constant was specified, and higher resistance/lower current was allowed. This resulted in slightly higher  powered torque with the advantage of higher BEMF voltage at lower rotor velocities, maximizing the  damping provided by the drive electronics. This change was shown via the stability model to drastically  improve motor stability. In addition to the increased BEMF constant, the detent cogging torque requirement  was slightly increased with the allowable range significantly narrowed, and the hysteresis torque  requirement was decreased substantially. These changes showed significant stability margin in the  simulation, i.e. the motor performance characteristics had to be lowered substantially to cause a loss of  stability, and predicted torque margin was very high. In order to meet these requirements, a completely  different motor stator design was devised, with a different tooth count and winding layout. This configuration  had fewer teeth and allowed larger coils so that the required BEMF constant could be met without  excessively fine magnet wire. With the new design completed, a different motor supplier was selected to  build the frameless rotor and stator. Prototype testing of the new motor design showed better than expected  performance, further improving predicted stability margin.  Stability Verification  Since the stability model did not catch the loss of stability observed with the original motor, a stability  demonstration program was desired to ensure that the new motor design was not at the “ragged edge” from  a stability standpoint. Since it was observed that the stability of the original motor was very sensitive to  pulse-width variation, a “pulse-width margin” test was conceived. In this test, the motor pulse-width was  varied to find the minimum and/or maximum pulse-width that results in stable stepping. This threshold  pulse-width can then be compared to the flight pulse-width range to compare stability margin.  To close on the motor failure investigation, the CSADA, with the original motor was first subjected to the  pulse-width margin test. A thermal chamber was used to test the CSADA in the same thermal conditions  as the original TVAC failure. The loss of stability did repeat, and stability margin was measured at ambient  and cold. Next, the prototype of the new motor design was installed in the CSADA qual unit and the stability  test was repeated again. This resulted in stability at hot (Figure 15), ambient, and cold, at the maximum  and minimum input voltage. Stability margin was significantly higher than with the original motor. The new  motor prototype was then left in the CSADA qual unit for the duration of the life test, and stability margin  was measured during the periodic and final functional tests performed as part of the life test. Stability margin  remained high over the entirety of the life test.  Figure 15. Trending of Stability Threshold Pulse-Width Over Life Test at Hot NASA/CP—2018-219887 304