Henry Spencer wrote: > ... it's quite routine in big rockets for the *control* gyros and > the *guidance* gyros to be different hardware. The high-bandwidth control > gyros are used to keep the thing flying in the desired direction, and the > low-bandwidth guidance gyros are used to decide just which direction that > is. For example, the shuttle has rate gyros separate from its IMU.
Done because they're gimbaled (stable platform) IMU's with a comparatively slow digital interface, and no good way to get low-latency rate data. A lightweight IMU for a small rocket is almost certainly going to be a "strapdown" system, which inherently generates body-axis rate data. The accuracy requirements for flight-control rate gyros are at least an order of magnitude easier than for inertial-grade gyros, so it makes sense to share the hardware if possible. > ... one could perhaps do what's done for high-precision timing > crystals: keep them at a constant temperature, by putting them in an > insulated enclosure with a temperature-controlled electric heater. Not > actually all that hard to do, although the power requirement is annoying. AD has an app note on how to do that with their (first-generation) MEMS accelerometers. And of course, that's how high-precision IMU's work anyway. Inertial-grade instruments make great thermometers (unfortunately). > Centrifuge calibration of *gyros* may be a bit tricky! That's how AD's data sheet says they got the G-sensitivity numbers in the first place. Presumably you could use an arm with a constant rotation rate, and place the gyro at various points along the arm to get varying axial g-levels (including zero) with the same rotation rate. And it's pretty easy to get constant 1-G acceleration along any given axis, with (nearly) zero rotation. :) With some care, I think you could combine that data to give useful calibrations. [Probably not good enough for ICBM-level navigation, though: the rocket sled at Holloman AFB was built to calibrate ICBM IMU parts, and they wouldn't have built it if they thought centrifuges were good enough.] Cheers! --Stu _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
