Are Simscape's Principal Axes in a particular order from least to greatest moment of inertia?

When using Simscape's "Inertia Sensor" block in a multi-link rigid body with varying principal axes, how are the X-Y-Z axes of the principal axes determined? Are they in order from smallest to largest moment of inertia (x smallest, y intermediate, z largest)? Or some other method?

I notice that there are discontinuities where the principal axes directions suddenly change by 90 degrees (the previous Y-axis becomes a new X-axis, for example), and I'm trying to figure out how to deal with this. Ideally I'd like to get rid of these discontinuities and view how the principal axis reference frame changes over time.

[Paul]

Hi Michael,

What are you seeing that shows the principal axes directions changing by 90 degrees? Not doubting you, just curious how you're making that observation.

The 2019b release notes that introduced the Inertia Sensor has a visualization that seems to show the x-axis bouncing around, but it's hard (for me) to follow.

I'm surprised that the Simscape Multibody doc doesn't define the convention it uses for principal axes insofar as principal axes are used in a few different places and so would help to know what that convention is. At least I couldn't find it.

Does using the R output of the Inertia Sensor help? Based on your description I don't see how it could, but thought I'd ask.

Maybe @Steve Miller or @MathWorks Simscape Team can comment.

Answer 1

[J Chen]

The axes of the Inertia Sensor are the same as the axes of the frame that the sensor is attached to.

[Michael P]

I don't see how it is possible for the principal axes of a multilink robot (for example) to be aligned with the axes of the base frame that the sensor is attached to. The principal axes will vary as the geometry of the multibody system changes, and therefore the axes will generally be pointing in different directions from the base reference frame. So, how does Simscape "decide" which principal axes will be denoted as X, Y and Z?