144 of 320 mA and a Z-axis current of 220 mA. For the HG AS, the nominal micro-step resolution is 16 microsteps per cardinal step with a motor current of 240 mA on both axes.   Single Step Errors

Although no position errors were encountered, a few si ngle step errors were detected on both systems at  the beginning of tracking motions immediately after the initialization sequence. A step error is an inherent  feature of the actuator harmonic drive which, occasiona lly during the first few steps of motion, results in  the motor encoder not detecting a step within the time  the GCE expects to see a transition. The step  errors did not result in any position errors. These single step errors during the gimbal operation are a good indication of the balance between the motor current, encoder LED current and encoder alignment.  SAS Actuator Power Dissipation Reduction

The SAS Y-axis actuator operations also underwent on-orbit testing on July 1, 2009. Prior to testing,  analyses were conducted for the worst-case inertia of 321 N-m-s2 (2841 in-lb-s2). By reducing the torque  factor to 1.25, it was determined that a 240-mA operational current would provide a positive torque margin for the Y-axis actuator. Analyses also showed  that slightly increasing the parameter “Number of  steps to accelerate inertia to full speed” conservatively set to 15 (with approximately 2:1 margin,) would  result in stable operation with both 200-mA and 220-mA currents.    On-orbit testing wa s executed at both 200 mA and 200 mA for bo th SAS gimbals, exercised over the full  range of motion, and demonstrated reliable, error fr ee operation. To add additional margin, the decision  was made to set the Y-axis operational current to 240 mA, reducing actuator power dissipation from approximately 5.5 W to 3.0 W.    The on-orbit testing, and the implementation of the te st and analysis findings, was in response to Thermal  team requests to minimize motor power dissipat ions effects on SAS Y-axis actuator temperature.   Delta Angle Violation Anomaly

A single type anomaly, initially detected by the Gu idance, Navigation and Control (GN&C) team, was  occasionally observed on the HGAS gi mbals during slewing operations at 67 pps in the CW direction,  referred to as a Delta Angle Violation (DAV). The first HGAS DAV happened at 2009-17001:07:30.43345. DAV flags are generated by the FSW if the difference between two consecutive samples of position telemetry data, sampled at 200-ms inte rvals, exceeds 14 counts, which corresponds to the  fastest allowable rate of 67 steps per second. The source of this error was traced to the flight  actuator’s  encoder operation associated with misalignment and amplified by the operation in the micro stepping  mode. This also explains why the error was never seen during ground tests or simulations, during which  the commercial encoders were used.   The anomaly mechanism can easily be seen by analyzing the plots shown in Figure 11. When the GCE reads a proper track transition (level change exceeds the conditioning circuitry threshold) it updates the  position counter by increasing the count for CW co rresponding transition or decreases it for the CCW  corresponding transition. When the “wiggle” occurs, it causes the GCE to increase the counter on the  lowering transition prior to the “wiggle.” It then decreases  it on the “wiggle-up” transition (since this is the  proper CCW corresponding transition). This results in the net zero count change with the following “wiggle-down” transition producing a legitimate step count. If the position telemetry counter is sampled prior to the “offsetting wiggle” it would produce an ex tra step count, which should happen at a later time. A  similar scenario could occur on either the A or B track. Since the position counter is sampled asynchronously with the DAV flag, o ccasional violations are produced.   This anomaly can be easily resolved by switching from micro-stepping (which is not required for the  HGAS operation) to cardinal stepping, but since this  anomaly does not affect gimbal operation or the  control algorithm, the HGAS operational mode was not changed.       NASA/CP-2010-216272