of the system, but all still resulted in insufficient difference between the pulse and overshoot signals of the  RA, and we still had a concern about mechanism fatigue.  The final step of testing was to run the drive with the load inertia removed. When this was done, there  was no change in the performance of the system. This was the “Ah-ha!” moment of our testing. Our initial  most probable root cause was incorrect, for if the system had an excessive inertia factor, removing the  load inertia would have solved the problem. We now realized that we had an underdamped stepper motor  system and believed our wave drive method was the root cause. To validate our new root cause we  shorted the redundant winding of the EDU by tying the three redundant lead wires together. With the  shorted windings we now had the critically damped system that we had originally expected based on  component level testing of the stepper motor by the supplier. While we had known that the supplier had  performed their component level testing of the stepper motor using a bi-polar driver, we had not  understood the implications of our different drive methods. (Wave and bi-polar drive methods are  described in the later section Method of Three-Phase Control .)  Homing in on the Solution Three methods of damping the system were evaluated. The first was modifying the electronics to short  the redundant side windings when driving with the primary side and to short the primary side windings  when driving with the redundant side. The system requirement that the mechanism be one fault tolerant  eliminated this solution. If one of the primary windings failed open, causing the system to switch to the  redundant side, the damaged primary windings would be unable to provide the required damping.  The second was adding delta configuration load resistors across the bridge to provide damping. This  method was ruled out because it required a new layout of the printed wire assembly to accommodate the  large load resistors. The added power loss on the board was also a disadvantage. However, empirical  testing of various load resistors across the redundant winding allowed us to determine the magnitude of  damping desired to achieve adequate damping and robust step counting.  This left us with our final option, developing a new motor with additional shorted coils that would act as an  integral eddy current damper. This solution had the advantage of being transparent to the electronics, but  it did have significant cost and schedule ramifications for the mechanism. Schedule issues were  alleviated by changing the assembly sequence of the mechanism. Communication provided by the ERB process kept program management and effected disciplines informed and participating in the decision  making process. Cost impacts were understood and accepted.  The Solution  Calculated Solution Once we decided to incorporate the internal damping via redundant shorted coils, we needed to revise  the motor torque constant and determine the requirements for the shorted damping coils. From EDU  actuator torque margin testing with the RA, we determined we could reduce the motor torque by 50% and  still be well above the 100% torque margin required for flight hardware by MIL-A-83577. From the shorted  load resistor testing conducted, we determined that the mechanism required at least 15% of a fully  shorted winding to provide enough damping for reliable stepping and robust RA signal processing.  The EDU motor had the following performance criteria for primary and redundant:  • Motor Torque Constant (Kt) = 0.205 Nm/Amp  • Winding Resistance ( /g1) = 40 Ohms  • Motor Constant (Km) = 0.0324 Nm/ /g1watt  • Holding Torque at Minimum Voltage (Th) = 0.092 Nm  • Damping Rate with fully shorted redundant windings (Bm) = 1.048 X 10-3 Nm-sec/rad  One of the many advantages of working in SI units is the simple determination of the damping rate of a  motor with shorted coils. The damping rate is equal to the motor constant, squared, if units are Nm/ /g1watt  (Bm = Km2) [2]. This first order relationship is applicable for shorted alternator analysis. The equation  36