Converting post-allocation actuator commands to SI body wrench setpoint (N, N·m)

PX4 Autopilot
1

I need a denormalized body wrench setpoint (force in N, torque in N·m) in FRD, computed from post-mixer actuator commands.

Current approach:

  1. Read actuator_motors (normalized [-1, 1] per rotor)
  2. Use rotor geometry (CA_ROTOR*_PX/PY/PZ, CA_ROTOR*_AX/AY/AZ) and effectiveness matrix to compute net body wrench
  3. Result is still normalized

The problem:
To get SI units, I need to convert normalized actuator commands to physical thrust/torque. I see CA_ROTOR*_CT (thrust coefficient) with Thrust = CT * u^2, but the units of CT aren’t clear, and I’m not sure if it’s intended for SI conversion.

Questions:

  1. Has anyone converted post-allocation actuator commands to SI body wrench (N, N·m)?
  2. What’s the recommended way to denormalize? Should I:
    • Add new parameters like CA_ROTOR*_THRUST_MAX_N?
    • Use CA_ROTOR*_CT with documented SI units?
    • Use another existing mechanism?
  3. Are there vehicle-specific considerations (ESC protocol, thrust curves, etc.) I should account for?

Context: This is for an EKF that needs the allocated actuator wrench setpoint (post-mixer, pre-motor dynamics) as a process input. The signal should reflect saturation/limiting from the allocator.

Any guidance or examples would be appreciated.