GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

2.4.3.6 Spinning Up the GyrosSpinning up the Gravity Probe B gyroscopes is a delicate and complex process that required over half the IOCperiod to complete. Each of the science gyros underwent a series of incremental spin-up and testing sequences,gradually increasing its speed to the final spin rate for the science phase of the mission. Following is a summaryof the whole process, as it actually happened during IOC.Within the first three days of the mission, all four Gyro Suspension Systems (GSS) were activated, and each ofthe gyros underwent a “lift check” to ensure that the suspension systems were working properly. These were thefirst suspensions of the GP-B gyros in orbit, and all were successful. Within the first two weeks in orbit, thegyros were first suspended in analog mode, and then, one by one, they were digitally suspended. The analogsuspension mode is used primarily as a backup suspension mode. With digital suspension, the GSS uses acomputer to provide the primary gyro suspension.Approximately one month into the mission, the team began spinning up the gyros. In preparation for spin-up,the digital suspension system for each gyro was first tested. This was accomplished by suspending each gyro inthe center of its housing, electrically “nudging” it slightly off center in one of eight directions (the corners of acube), and monitoring its automatic re-centering. This checkout was first performed under low-voltageconditions (fine control) and then under high-voltage conditions (secure hold).To begin the actual spin-up process, ultra-pure helium gas was flowed over gyro #1 and gyro #4 for 15 seconds,which started them spinning at approximately 0.125 Hz (7.5 rpm). While these gyros were slowly spinning, thesuspension test was repeated under high voltage conditions on gyros #2 and #3. During this high-voltagesuspension test on gyro #3, the team discovered an error in its command template, which turned off the highvoltageamplifier to gyro #1 and caused it to lose suspension. There was no damage to gyro#1.At this point in the mission, more than 1,000 commands had been sent to the Gyro Suspension System (GSS),and this was the first error found. Discovering an error in these numerous, intricate command templates wasexactly the kind of situation that the painstaking gyro spin-up process was designed to identify; it enabled theteam to correct the command template for gyro #3 without serious consequences. Also, as a further precaution,the team thoroughly reviewed the command templates for the remaining three gyros. By the end of May 2004,gyros #2 and #3 were spinning at 0.26 Hz (15 rpm) and 0.125 Hz (7.5 rpm) respectively, and gyro #1 was in theprocess of being spun-up.The first week in June, the team started performing a highly methodical and painstaking series of calibrationtests on the four science gyroscopes. These tests began at very low gyro spin rates of 0.333 Hz (20 rpm) or lessand involved briefly applying voltages asymmetrically to the suspension electrodes on a given gyro, causing thatgyro rotor (sphere) to move off center by a few micrometers -or less, and then re-centering it.Carrying out these tests at very slow spin rates enabled the team to check out the software command templatesthat control the tests, without risk of damage to the gyro rotors or housings, which could occur at higher speeds.In fact, during these initial calibration tests, the team discovered that the performance characteristics of thegyros on orbit are slightly different from the simulator predictions on which the original command templateswere based. As a result of these differences, during one of the initial tests, gyro #4 touched one of its electrodesand stopped spinning. A simple modification to the spin-up command templates corrected the problem.Calibration testing of the gyros at spin rates less than 1 Hz (60 rpm) continued throughout the month of June.By the third week in June, the performance characteristics of each of the gyros was understood, and the resultsof this testing were used to fine-tune the GSS for each gyro. This significantly improved the suspensionperformance of all the gyros, especially gyro #2.Gravity Probe B — Post Flight Analysis • Final Report March 2007 53