Fixed-wing PID Tuning Guide

Fixed-wing PID Tuning Guide

This guide explains how to tune the fixed_wing PID loop.

Warning This guide is for advanced users / experts only. Incorrect PID tuning may crash your aircraft.

Note

Incorrectly set gains during tuning can make attitude control unstable. A pilot tuning gains should therefore be able to fly and land the plane in manual (override) control.
Excessive gains (and rapid servo motion) can violate the maximum forces of your airframe - increase gains carefully.
Roll and pitch tuning follow the same sequence. The only difference is that pitch is more sensitive to trim offsets, so trimming has to be done carefully and integrator gains need more attention to compensate this.

Tip All parameters are documented in the Parameter Reference. The most important parameters are covered in this guide.

Establishing the Airframe Baseline

If a pilot capable of manual flight is available, its best to establish a few core system properties on a manual trial. To do this, fly these maneuvers. Even if you can’t note all the quantities immediately on paper, the log file will be very useful for later tuning.

Success All these quantities will be automatically logged. You only need to take notes if you want to directly move on to tuning without looking at the log files.

Fly level with a convenient airspeed. Note throttle stick position and airspeed (example: 70% → 0.7 throttle, 15 m/s airspeed).
Climb with maximum throttle and sufficient airspeed for 10-30 seconds (example: 12 m/s airspeed, climbed 100 m in 30 seconds).
Descend with zero throttle and reasonable airspeed for 10-30 seconds (example: 18 m/s airspeed, descended 80 m in 30 seconds).
Bank hard right with full roll stick until 60 degrees roll, then bank hard left with full roll stick until 60 degrees in the opposite side.
Pitch up hard 45 degrees, pitch down hard 45 degrees.

This guide will use these quantities to set some of the controller gains later on.

Tune Roll

Tune first the roll axis, then pitch. The roll axis is safer as an incorrect tuning leads only to motion, but not a loss of altitude.

Tuning the Feedforward Gain

To tune this gain, set the other gains to zero.

Gains to set to zero

FW_RR_I = 0
FW_RR_P = 0
FW_RSP_OFF = 0

Gains to tune

FW_RR_FF - start with a value of 0.4. Increase this value (doubling each time) until the plane rolls satisfactorily and reaches the setpoint. Back down the gain 20% at the end of the process.

Tuning the Rate Gain

FW_RR_P - start with a value of 0.06. Increase this value (doubling each time) until the system starts to wobble / twitch. Then reduce gain by 50%.

Tuning the Trim Offsets with the Integrator Gain

FW_RR_I - start with a value of 0.01. Increase this value (doubling each time) until there is no offset between commanded and actual roll value (this will most likely require looking at a log file).

Tune Pitch

The pitch axis might need more integrator gain and a correctly set pitch offset.