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

9.1 GSS Hardware DescriptionFigure 9-1 shows the four GSS units and their relationship to the rest of the payload electronics package. Eachgyroscope has a dedicated GSS system, and each GSS system comprises two electronics enclosures: forward andaft (see Figure 9-2).Figure 9-2. Photograph of GSS units. Aft processor and power supply (left), forward quiet analogelectronics (right). One pair is required for each gyroscope. During the mission, thetemperature of the aft assembly ranges between 300 K and 320 K; the forward assembly range is 265 K to 300 K.The forward electronics unit (FSU) resides in the spacecraft’s forward equipment enclosure (FEE), that ispassively thermally controlled to a range of 300-320K over the mission. The FSU contains the sensitive analogelectronics that are used to sense the position of the rotor and apply suspension voltages. A block diagram ofthis system is shown in Figure 9-3. The main components of this system are described in the following subsections:9.1.1 Low voltage drive amplifierDuring nominal science operation, voltages are applied to the electrodes through six ±50V low noise amplifiers;the bandwidth of these amplifiers was set by design to 100 Hz. Each amplifier has a input filter set that allowstwo modes of operation: 1) constant gain to DC, and 2) a band pass mode that rolls off the gain below 1 Hz to10% of the nominal peak. The DC mode is provided to simplify the design and operation of the backup analogcontrol loops. The band pass mode is used to limit the electrostatic force noise on the gyroscope by allowingonly signals in the neighborhood of the 30 Hz AC science drive carrier to make their way to the gyroscope.9.1.2 High voltage drive amplifierA ±1600 V set of amplifiers is used during ground test and gyroscope spin-up to provide enough electrostaticforce on the gyroscopes to suspend them in a 1g field or while spin-up gas is flowing in the channel. 1g groundsuspension requires approximately 650V to maintain suspension once centered, and roughly twice that to pickup the rotor from the housing wall. Gas spin up requires ~150V per axis to hold the gyroscope in the spin-upchannel. Nominal bandwidth of these amplifier is 2 kHz and power draw during spin-up is 0.8 W per channel.The output of the low and high voltage amplifiers is switched to the gyroscope by a 3 HV RF vacuum relay, oneper electrode. Switch time is approximately 1 ms, during which the rotor is not under active suspension control.During the mission, many checks are made on vehicle activity prior to making the switch to ensure the gyro willnot move significantly during this uncontrolled period.258 March 2007 Chapter 9 — Gyro Suspension Subsystem (GSS) Analysis