407 the drive electronics were not providing the correct pulses to the motor. The fact that the motor had  provided full rotation and a pull in torque value of at least 75% of the specified requirement, at room  temperature, seemed to indicate that all was well with the drive electronics. 

The first review of the drive electronics test data showed that the curr ent, voltage and power being  delivered to the motor at 35K were indeed correct. We expected that these parameters would in some  way, be out of specification.    A more detailed and thorough review of the drive electronic motor pulses revealed a problem in the  stepping sequence. The stepper motor operates using a si x step sequence of closed and open circuits for  the three windings. The sequence is provided to the motor by virtue of progra mming of an FPGA in the  drive electronics. It was found that the sequence in the FPGA was actually making the motor step from  position 1 to position 3 to position 5 and then to position 1 again. This did cause the motor to rotate, but with diminished torque and half the power. This caus ed the anomalous behavior at cryogenic temperature  and the below specification pull in torque at room temperature. When the sequence was corrected and the motor was tested again at 35K, motor performance was excellent.   Several new technologies had been put into one test, dominated by a new motor design, and its new drive electronics; all operating at the cryogenic temperat ure environment. This fact made it very difficult to  determine the root cause of the issue. The anomalou s behavior was incorrectly attributed to the geared  motor because the assumption that the drive electronics were correct had been established at room temperature. A month later the problem had been tr aced back to the stepping sequence in the drive  electronics. Also, all of the motor test data gathered at ambient and cryogenic temperature had to be repeated. The lesson learned was that a test should not try to combine too many new technologies in the first test. A check of all items that could affect motor performance should be completed before assigning a  cause to a problem.       Thanks go to our vendor, CDA Intercorp, for their ti mely assistance in trouble shooting this problem. In  this case, it was very beneficial to have a “reference ” set of drive electronics to expedite testing at the  cryogenic temperatures.        

Ball Bearing Testing 

The PW and the FW wheel bearings are critical  items for the dual wheel mechanism. For each  mechanism, the torque ratio would in large part be de termined by the torque exhibited by these bearings  at cryogenic temperatures. An ext ensive test program was devised to  determine bearing performance at  cryogenic temperatures.    The design parameters of each bearing are shown in Table 1. A single bearing for the PW and the duplex  pair bearing for the PW installed in the PW machining is shown in Figure 6.   It was decided early in the program that the initial bearing tests would be carried out with the bearings already installed in the wheel and hub assembly. It was originally thought to be highly desirable to test the bearings in their actual flight configuration and that “bearing only” tests would be un-necessary. This philosophy would later prove to be overly optimistic . 

NASA/CP-2010-216272