Posted by serge petiot on December 3, 2009 at 12:46am
Dear All,Since I'm new here, allow me to introduce myself: Name is Serge and I am a Embedded Hardware/Software engineer. As I am into experimental rocketry, I decided to built an IMU specific to Rockets. I have built a board based around a dsPIC33FJ128MC506 (runs at 40MIPS), and it has a 3D magnetometer (MicroMag3), one IDG500, one IXZ500 gyro along with a MMA7340 accelerometer. Of course, it also measures T°, Atmospheric pressure, and is able to interface to a Pitot daughter board, and GPS. It has 6 low-side 12V outputs for igniters ans 6 PWM servo outputs. Now, the time has come to write the Software, so I have read as much as I could on the subject of DCM (thanks Mr Premerlani and Bizard, great paper). Now, a point that is really bugging me is: I need 2 reference vectors to correct the gyros drift. I cannot use the accelerometer since it is absolutely useless in a rocket (at least during boost and coast phase) and the GPS is of no use as well but at least I got reliable Magnetic North. My question is: can I get by using only one vector to correct the gyros?
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You cannot maintain a converged solution with only one outside vector. You can (I think) keep the solution converged with only one vector for two axes - for example in a plane you could keep pitch and roll but not yaw, so long as you kept pitch and bank angles shallow. But in the rocket scenario I don't think this would work out as your reference vector might get too close to one of the axes that is important for control and then the solution would diverge.
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