Physical dimensions must be reflected in the type system. Meaning that there must be a dedicated message (or nested) type for kilogram, radian per second, and so on. Putting this all into a union or a large array is hardly sensible because it basically brings us back to the point where it’s no better than just using a raw array of dimensionless numbers. Also, on timestamping, see this: On timestamping
I think what we defined here so far may be hugely inefficient, but it’s effective. Exposure of clearly separated physical quantities allows us to build very modular and robust interfaces. Perhaps a few simple case studies would help us here; these may not be perfect because I made them up on the spot, but they should clarify the objectives.
- Imagine that you have a robotic arm, and there is a joint motor controlled by a UAVCAN controller. The joint motor controller needs to know the position and the angular velocity of the joint so that it can close the position control loop. You connect the motor controller and a joint position sensor to the bus, and you see that the joint motor controller has two inputs: joint angle, of type
uavcan.si.angular_position.Scalar, and joint velocity, of typeuavcan.si.angular_velocity.Scalar. The joint position sensor has two outputs of same types. You assign them proper matching subject identifiers, and you’re done. Network configuration tools can check that there aren’t any type conflicts (some nodes can choose to check that for themselves automatically, too). - Imagine that you have an aircraft with a camera gimbal. You want the gimbal to point the camera towards a particular geo coordinate. You supply the coordinate to the controller; the controller picks up the orientation, angular velocities, and the geo location of the craft from the bus using
uavcan.si.angular_position.RollPitchYaw,uavcan.si.angular_velocity.RollPitchYaw, anduavcan.si.angular_position.Vector2(longitude, latitude; this choice of name is questionable). The gimbal computes the required angle relative to the body from the supplied information (difference in geo position and the orientation of the craft). Again, network management tools or nodes themselves can ensure correct type matching. - Suppose there is a generic air data computer provided with the inputs from total air temperature sensor, static pressure sensor, and a pitot probe. Like in the examples above, correct setup can be verified through the type system.
- Have a look at the ESC example in the Stockholm summit recap post.