CSADA Design Overview  The CSADA consists of two main sub-assemblies: the drive module and the slip ring assembly, as shown  in Figure 1. The slip ring assembly has 118 circuits. Each circuit consists of a slip ring with redundant  brushes. There are 96 power circuits (5.25-A capacity) and 22 signal circuits (1-A or 2-A capacity). The slip  ring assembly is self-contained with its own bearings. The drive module consists of a 1.8-degree, 4-phase  stepper motor, a 200:1 pancake-style harmonic drive, a potentiometer, and bearings. The drive module and  slip ring housing are bolted together. An Oldham coupling mechanically links the two shafts.   At the time the author took responsibility for the CSADA qualification effort, manufacturability issues had  been found in the drive module and a review of the design tolerances was performed. It was found that  some critical CDR analyses had been inadequate, and a significant number of tolerance issues were found  via a detailed critical clearance analysis. Additional review of the design led to the discovery of a potential  failure of the potentiometer during vibration testing that was averted via design change. Concerns were  raised about the inconsistent detent performance of the stepper motor design, but the budget and schedule  were a big concern for the program, so the decision was made to proceed with risk in hopes of preserving  schedule and cost rather than incur further delays by re-designing the motor. Mechanism engineering  recommended deferring shock testing to the vehicle level to avoid the potential over-conservatism of  traditional shock testing. Because shock was considered a risk, the program opted to perform shock testing  during CSADA qualification, so that risk mitigation would occur sooner, but with a higher risk level, instead  of mitigating the risk later at a lower risk level.  Shock Test Failure  Initially, shock testing was to be performed by an outside test house, however they were not able to tune  their Mechanical Impulse Pyrotechnic Simulator (MIPS) table to an acceptable input shock level. Their MIPS  table resulted in significant over-test over most of the frequency range. The large MIPs table in Lockheed  Martin’s Sunnyvale test facility was able to reduce the level of over-test, but not completely. Great effort  was taken to eliminate the out-of-tolerance shock input, but it could not be completely eliminated. The  Lockheed Martin Dynamics group was consulted and determined that risk was low moving forward since  the out of tolerance (high) shock input was in the lower frequency range, which was believed to pose a  lesser risk than a high input at higher frequencies. This was based on a review of the predicted modes for  the slip ring, which did not predict significant brush motion at lower frequencies.  During the initial shock test, a short was found between adjacent slip ring circuits that was isolated to the  unit itself, as illustrated in Figure 2. Removal of the slip ring inspection covers revealed that the short was  caused by a brush having jumped over the raised barrier between the slip rings, making electrical contact  with the adjacent ring.  In order to understand the failure mechanism, an old development unit was subjected to shock pulses with  its cover removed, and high-speed video was taken. The high-speed video revealed that the pumping mode  of the shaft was causing the “bouncing” motion of the brushes between the plastic barriers to resonate, and  the impact of the brushes with the barriers was causing out of plane (lifting) motion on the brushes. The  effective resonant frequency of the brushes was lower than predicted because the shaft displacement was  large enough to impact the brushes. This created a non-linear resonance at a much lower frequency than  the simple cantilevered brush motion expected for a linear analysis. The pumping mode of the shaft was  lower in frequency (~200 Hz) and had a large displacement amplitude, which would not be excited to such  an amplitude with a flight-like shock pulse consisting of a short duration and small displacement. It was also  recognized that the “frequency” of the brushes bouncing between the barriers is a non-linear behavior, such  that a reduced pumping amplitude of the shaft would increase the frequency of the brush motion. A higher  frequency for the brush motions would reduce their shock susceptibility at the lower frequency range,  assuming the contact with the barriers was reduced or eliminated.  NASA/CP—2018-219887 294