Discussions

INS on new IMU-projects

Posted by Sami Finnila on March 18, 2010 at 4:38am

Practically every open source autopilot is going for an IMU right now (Paparazzi v3, ArduPilot (mega), OpenPilot..) and they rely heavily on GPS to be able to correct errors caused by centripetal forces. But due to the fact that GPS is an outside resource it might not be available 100% of the time and even when available it's possible to get rather large errors momentarily which would make an IMU attitude estimate go all over the place.

Ok. The probability of anything happening is small but security first, right? So what I thought of is that has anyone tried to program an INS based on these cheaper IMU-units, like UAVDevBoard? The problem isn't programming but rather the accuracy of the sensors.

Besides security the INS could be used to reduce the error and lag in position between GPS updates which would make it possible to write navigation algorithms with much higher coefficients and, thus, tighter control.

Thoughts?

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Replies

Developer

Doug Weibel March 18, 2010 at 9:13am

Actually Chris' answer is a little bit wrong. We also use gps info to correct for centrepital accel. You can also use airspeed for that function rather than gps. In a quad that is restricted to slower speeds you can probably get by without but in an airplane you cannot.
Tom Yochum March 18, 2010 at 8:20am

Sami,

We (my neighbor and myself) are working on a tightly-coupled GPS/INS implementation using a UBlox GPS and the CHR6d. We started with this IMU from Sparkfun and just coded an INS just to see how bad it would perform. We were getting pitch and roll errors of > 10 degrees after about 20 seconds of gentle rotations. Fast movements and rotations made it even worse.

This is why people here are implementing an AHRS instead of an INS. An AHRS has essentially zero drift when in steady-state and has some sort of heading/course information.
3D Robotics

Chris Anderson March 18, 2010 at 4:50am

Sami, with the DCM, the only thing that happens if you lose GPS is that your yaw becomes unreliable. The rest of the attitude estimation should be okay...

Regardless, ArduPilot Mega has pins for you to connect an optional external (to avoid interference) magnetometer if you want to use it in a GPS-denied environment or just have the extra data for safety.