Discussions

Hi,

Just to check if I have understood anyhing :)

I am trying to get to the precise meainings of the pitch, roll, yaw angles.

Isn't it like:

To get the airframe (positive Y forward, positive X to the local right, positive Z towards local ceiling) to an attitude given by pitch, roll, yaw angles, do:

1. Orient it identical to the global system (local X, Y, Z vectors = global x, y, z). That is level, nose to the north (or east, depending on definition)
2. Rotate around the z = Z axis by the yaw angle
3. Rotate about the (already yawed) X' axis by the pitch angle.
4. Rotate about the (already twice rotated) Y'' axis by the roll angle

Is this (or at least the order of rotations and the axes) about right?

According to Wikipedia, this is not really Euler angles then, as those use the same axis twice, but it is often called Euler angles anyway.

And another question - in ArduPilot there are the terms IMU (Inertial Maneuvering Unit), INS (Inertial Navigation System) and AHRS (Attitude Heading Reference System), and InertialSensor.

Do all these 3 terms mean something different? Which is more upper vs. lower level?

It looks to me that:

• "Inertial Sensor" describes just a collection of sensors that will measure rotation rates about local axis, and acceleration.
• "INS" and "IMU" are used synonomously for "inertial sensors" some places. Like "    // tell the IMU to grab some data
      _ins->update();
  " in AP_AHRS_DCM.cpp.
• If this is the meaning of INS, then I am confused about the new "inertial navigation" feature, which is about improving position estimates byt using dead reckoning,.
• The AHRS is the thing that computes the pitch, roll, yaw angles from the sensor data (and some past history).
• So what is the component called that controls (maneuvers?) the aircraft? All the PIDs etc. in the case of ArduPlane/ArduCopter.

Hope I can get a little bit less confused :)

Regards

Soren