The crossbow products are broken down like this:The non-fog crossbow 6DOF IMUs were $3995 for the 400 series, and $2995 for the 300 series.My limited understanding of them suggests that the accuracy is largely just clever filtering, and that a bit better than 1 degree per second is all they can really achieve currently. The crossbow units appear to be correlating multiple sensors (gravity and magnetic, and the attitude sensors) to zero the drift out, which would work great for aviation, most of the time, although even then, they seem to lack FAA approval, which is suspicious. If I'm right, you'd want to steer clear for your application- since there's no direct way to measure gravity once a rocket has left the ground.
"solid state" gyros should perform much better than the 1 deg/sec that tuning-fork style gyros like the gyration parts get. I am certainly interested in hearing results from work with them.
IMU = inertial measurement unit. Gyros and accelerometers only, outputs corrected for temperature and (as much as possible) acceleration / cross axis coupling on non laser gyros.
VG = vertical gyro. The exact same guts, but with the addition of software to attempt to correct the drift in the two tilt axis by using the accelerometer as an inclinometer and assuming that "much" of the flight will be level. The exact parameter for this is tunable.
AHRS = attitude heading reference system. Adds a magnetometer and more software so drift in the yaw (for planes, roll for rockets) axis can be accurately compensated for.
They do have FAA certification for one of their models now.
The leveling algorithm would not be useful on a rocket, although the magnetometer output could still be used. I use the base IMU units.
They offer two quality levels of solid state gyros, both of which are quite a bit better (really, not just filtering) than the murata / gyration level of units. They used to offer an even lower end unit that used parts like that, but it was discontinued some time ago. The solid state gyros only offer 10hz bandwidth, against 100hz and no acceleration effects at all on the fiber optic gyros.
GPS doesn't return acceleration, just position and velocity. A stable vehicle inside the atmosphere can have the tilt axis checked against GPS velocity, but that doesn't help with either exoatmospheric flight or unstable vehicles.In any case, GPS by itself doesn't let you re-sync your gyros, you need a multi-antenna GPS attitude sensing system, not just a position output GPS.During a burn the GPS track acceleration is going to be the same as the attitude, pretty much; atleast when outside the atmosphere. Within the atmosphere it's not quite as simple, but it's earlier in the flight, so the error is smaller, and my vehicle may be nose heavy which helps keep the track and the attitude aligned.
John Carmack
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