Computer Science
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7927w&link_type=abstract
HST Proposal ID #7927
Computer Science
Hst Proposal Id #7927
Scientific paper
The bearings and/or lubricants in the bearings of the Star Selector Servo A {SSA} in FGS2 have degraded to the point that the compensated error signal in FGS2 SSA during small angle maneuvers following a guide star acquisition generally reaches 6 to 8 volts and 7 to 9 volts during small angle maneuvers following the subsequent reacquisition. Further reacquisitions frequently lead to saturation of the SSA compenstated error signal at 10.4 volts. These saturation events can cause loss of science due, primarily, to loss of lock on the guide star during the maneuvers. This problem should be significantly reduced by commanding the A servo to a position that will result in the crossing of any Coarse Track {CT} bumps set up by the guide star acquisition or reacquisition. This is supported by observed on-orbit behavior {e.g. the effect of CT drift on the primary guide star} and by ground testing of the returned FGS1. A change has been made to the SSM FSW as a part of the FSW 10.0 build to allow the two star selector servos of the FGSs to be controled independently. This capability is used by a new PLCP group, PCPCL2A, that executes a clearing slew of the FGS2 SSA bearing. To assess the efficacy of the clearing slews in FGS2 SSA it is necessary to execute the clearing slew PLCP group on-orbit. This execution will provide the compensated error signals {related to torque in the bearings} that will demonstrate that the clearing slews do effectively reduce the CT bumps. The test will contain crossings of the CT bump following two sets of guide star acquisitions and reacquisitions. The first set will not contain the clearing slew and the second set will contain the clearing slew. Direct comparison of the magnitude of the CT bumps encountered will demonstrate how well the clearing slew works. This test will also provide precise timing information on the duration of the clearing slews. Unlike the B servo, which is commanded to Default at the end of each orbit, the A servo is not slewed between acquisitions and reacquisitions {or subsequent reacquisitions} if the vehicle pointing is not changed. FSW 10.0 will change the #58 and #63 commands to allow the two servos to be slewed independently. PCS has developed a new PLCP group that will be called by Planning and Scheduling before each reacquisition. This group commands the FGS out of Default into SSM control, slews the A servo by a #63 command to a specified servo angle {ThetaA = 128 deg}, slews the A servo back to the original position by a #58 command to a specified quaternion, and then returns the FGS to Default. The position to slew the A servo to was chosen based on the facts that the FGS can be slewed from anywhere in the unvignetted field of view to the Default position and the A servo then slewed to ThetaA = 128 deg and the FGS instantaneous field of view will remain at least 9 arcsec from the edge of the vignetted FGS field of view. This is sufficient to prevent the PMTs from being illuminated in all cases.
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