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Typically, PID exists as a class which is then implemented per-axis. This means that each axis has a singular output, so it’s necessary to determine Positive and Negative orientations, as certain actuators may need to reverse the PID output depending on their orientation. See the ZP3.0 architecture for our drone reference frame ZeroPilot 3.0 Architecture - Embedded Flight Software - WARG (atlassian.net)
Although the inputs may be angles or rates, the output of a PID should be a motor % that can be understood by each actuator.