Projected gravity: how the robot knows it fell

Projected gravity is the gravity vector expressed in the robot’s own frame. It is the only IMU quantity on robotctl monitor’s state stream.

• Upright is about [0, 0, -1]
• Face-down, face-up and lying on one side give distinctly different directions

The fallen state is decided from it.

Why policies depend on it too

The StandUp task has to rise from face-down, face-up and sitting, and those need completely different motions. The policy tells them apart from projected gravity — so the quantity serves both diagnosis and control.

A failure that never errors

One failure mode is worth remembering on its own:

When sensor fusion stops, the board keeps answering the bus. Nothing errors, and the gravity vector holds a plausible attitude indefinitely.

The numbers look fine. The fall verdict looks fine. Neither reflects reality any more.

Exactly one place says so — the condition row at the bottom of robotctl monitor:

orientation frozen — 25 stale reads

The stale-read counters and the ratios they mean anything against are in robotctl health. This is where to look when the robot behaves strangely and every indicator reads normal.