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

Precise temperature control is achieved through a several control loops that link heaters to temperature sensorsand increase or decrease the heater voltage according to the temperature readout. The QBS, SQUID Brackets,and SRE boards (FLL, DAC, DAS and a board controlling the SQUID Bracket temperature) all have their ownheater, temperature sensor and control loop. The temperature control on these parts of the spacecraft limits thethermal bias of the gyroscope readout to less than 12 μarcs—an extraordinary achievement.8.2.2.1 QBS Temperature ControlThe SQUID Bracket, which is thermally coupled to the QBS, requires the QBS to have a peak-to-peak thermalstability of 1 mK so that the SQUID Bracket will be able to meet it’s own thermal stability requirements.Thermal stability of the QBS is achieved through two PI (Proportional Band, Integral) control circuits thatoperate the QBS heaters: A-side controller (QBS heater A in conjunction with an A-side GRT located on theQBS) and B-side controller (QBS heater B in conjunction with a B-side GRT located on the QBS). Eachcontroller has three modes of operation:1. Analog closed-loop, 80 Hz sinusoidal AC excitation;2. Open-loop, 80Hz AC excitation;3. Open-loop, DC voltage excitation.However, under normal conditions only the analog closed-loop is used to achieve temperature control. It worksas follows:The desired QBS temperature (the heater set point) has a corresponding GRT resistance.GRT resistance X current source level X amplifier gain = GRT voltageA D/A Converter with 8-bit resolution is programmed to produce the desired GRT voltage. The analog PIcontroller heats the QBS to maintain the GRT voltage equal to the DAC voltage and thus control the QBStemperature.It is important to note that because the controller can only add heat to the system, the controller must bias theQBS to a small delta above its sink temperature in order to achieve temperature control.8.2.2.2 SQUID Bracket and SRE Temperature ControlSQUID Bracket and SRE temperature control is important because it is necessary to keep the electronic boardsat a stable temperature to ensure precise SQUID measurements with little error.As stated earlier, the SQUID Bracket requires a 5 μK thermal stability with a 1mK disturbance. This is achievedthrough a control circuit which is located on the SRE boards (outside the Probe) and is itself temperaturecontrolled.The SRE boards are located outside the Probe and have to overcome much greater thermal variations than theQBS and SQUID bracket do inside the Probe. The temperature controls for the SRE boards were built to handlethe following expected thermal environment variations (although they are capable of handling highervariations):1. Roll temperature variation: less than 30 mK (peak-to-peak).2. Orbital Temperature Variation: less than 0.5 K (peak-to-peak).3. SRE Annual Temperature Variation: less than 15 K (peak-to-peak).There is one DAS, FLL and DAC for each of the four gyros. Each of these has its own temperature controlcircuit that links its temperature sensor to a heater in order to provide temperature control.Gravity Probe B — Post Flight Analysis • Final Report March 2007 219