What a pilot is asking the robot to do.

Why this is a throttle and not a velocity

A BB.Message.Geometry.Twist2D would be the conventional thing to send, and it would be a lie. Its vx is a linear velocity, and this robot cannot be asked to travel at a speed: with no encoders it has no idea how fast it is going, so there is nothing to close a velocity loop with.

Trying anyway is how this message came to exist. The balance controller's trim regulates the commanded wheel velocity, which looked like a velocity loop with the target hardcoded to zero — so pointing it at a non-zero target seemed free. It isn't: the commanded wheel velocity is kp times the lean error, so holding it at a value means holding a lean error, and a constant lean error is a constant acceleration. Asked for 4 rad/s the robot obediently accelerated through 6, 9, 13, 18 and fell over.

The trim works at zero because zero sustained command genuinely does mean not accelerating. That is a regulator. Any other target is an instruction to accelerate until something stops it.

So throttle is what it says: a fraction of the lean the controller is willing to spend on going somewhere. Push it and the robot accelerates; let go and it leans back and stops. The pilot regulates speed by watching, which is the same loop a Segway rider closes and is the whole of what this sensor set supports.

turn is a genuine rate, because yaw is measured — the gyroscope reports it directly and the heading loop tracks it properly. The asymmetry between the two fields is the asymmetry in what the robot can sense.

Fields

• throttle - lean to spend on driving, as a fraction of the controller's drive_limit. Positive is forwards.
• turn - yaw rate to hold, in radians per second. Positive is to the robot's left.