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Betaflight 3.0's new 2 degree-of-freedom PID controller is one of its most exciting features. Let's learn what the new "setpoint weight" sliders do! I also discuss the new RC interpolation feature.
Error in a PID controller can be separated into externally-induced error and error caused by moving the set-point. The 2DOF PID controller allows you to distinguish between these two types of error. In other words, it allows you to control how aggressively the PID controller responds to your stick movements.
P term setpoint weight basically controls overshoot. Higher P term setpoint weight results in sharper stick response, lower P gains, and more overshoot and oscillation. Lower P term setpoint weight results in softer stick response, higher P gains, and less overshoot and oscillation.
D term setpoint weight is harder to describe. Higher D term setpoint weight results in an overall much sharper and more precise flight feel. But it also makes the copter fly less smoothly, since it is responding exactly to every little wiggle and jiggle of the stick. Lower D term setpoint weight results in a smoother, more "organic feeling" flight experience, but also a softer and less precise one.