IMU

IMU attitude estimation is often summarized as: fuse the accelerometer and gyroscope.

That is true, but too vague. The real problem is: the gyroscope is fast but drifts, the accelerometer gives a long-term gravity reference but is polluted by motion, and the magnetometer can correct heading but is easily polluted by local magnetic fields.

Attitude fusion is not simple averaging. It decides which sensor to trust at which time scale and under which conditions.

Tilt sensors, accelerometers, gyroscopes, IMUs, and attitude sensors are often mixed together as if they all measure “angle”.

That is too rough.

Many tilt sensors are based on accelerometers, but a tilt sensor does not necessarily contain a gyroscope. A 6-axis IMU usually means a three-axis accelerometer plus a three-axis gyroscope. That is not the same thing as every tilt sensor.

The first model is: in static or low-dynamic scenes, tilt can be calculated from the gravity vector projected onto the sensor axes. In dynamic scenes, linear acceleration and vibration pollute the accelerometer, so gyroscopes, filtering, and fusion become necessary.