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Autotune: Update README with fixed-wing rate tuning instructions (#42)
* autotune: update README with fixed-wing rate tuning instructions * Apply suggestions from code review Co-authored-by: Mathieu Bresciani <brescianimathieu@gmail.com> * Add new line delim in nested list * Update autotune/README.md * Apply suggestions from code review --------- Co-authored-by: Mathieu Bresciani <brescianimathieu@gmail.com>
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‎autotune/README.md‎

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![image](https://github.com/user-attachments/assets/fcdf5c25-d92d-4487-9736-e77f6576d180)
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# Tuning Worflow
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## Fixed-Wing Rate Tuning
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**Prerequisites**:
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- Requires valid airspeed measurement on vehicle
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- Correctly set FW_AIRSPD_TRIM
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### Flight Maneuvers
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Using default PX4 gains. Perform these maneuvers separately for each axis: roll, pitch, and yaw.
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1. Requires valid airspeed measurement, and a correctly set FW_AIRSPD_TRIM.
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2. Fly in stabilized mode and start from level flight.
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3. Apply a manual sine chirp input (sine wave with increasing frequency) on the selected axis. Avoid any other control inputs simultaneously.
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4. For pitch and yaw: The amplitude of the input should not be too high, but should be enough to overcome noise.
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5. Repeat the maneuver a few times if necessary to capture good data.
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### Using the Tool
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#### Load the flight log and select the relevant tuning loop (roll, pitch, or yaw):
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![image](https://github.com/user-attachments/assets/f962a004-eae8-433f-9b7e-adc618dcbeae)
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#### Window selection:
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1. Select the portion of the log where the maneuver occurred.
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2. Ensure the window starts from level flight and covers the full maneuver.
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3. (Experimental) Use the coherence plot to check whether your input-output data have a strong enough linear relationship at the desired frequencies. For fixed-wings: \
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i. We are interested in frequencies between 0.5-10Hz. \
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ii. Values > 0.6 are sufficient. \
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iii. If the coherence function is oscillating dramatically, the identified model may be unreliable at those frequencies.
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4. Once you have identified the window, load the selection into the tool.
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<table>
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<tr>
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<td align="center">
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<b>Good window selection</b><br>
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<img src="https://github.com/user-attachments/assets/25c30295-6825-4cb4-828e-47eaa17e1557" width="400"/><br>
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<i>Starts from level flight and covers the full maneuver. Coherence is sufficient.</i>
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</td>
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<td align="center">
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<b>Bad window selection</b><br>
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<img src="https://github.com/user-attachments/assets/1e05d411-37cf-42ab-8d6f-0cbbbb00921c" width="400"/><br>
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<i>Does not isolate the maneuver. Coherence is insufficient. </i>
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</td>
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</tr>
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</table>
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#### Model Verification
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1. Check that the estimated dynamic model correctly tracks the actual aircraft output.
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2. If not, adjust the system order (number of poles and zeros) and/or delay parameters.
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3. Verify that the system is minimum-phase (i.e., zeroes lie inside the unit circle).
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<img src="https://github.com/user-attachments/assets/87168b77-1b52-4d6f-bb4d-ace3f76fe4ee" width="700"/>
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#### Tuning Gains
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1. Set all gains to zero
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2. Increase the P-gain until the step response shows oscillations. Then reduce the P-gain by approximately 50%:
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<b>Increase P-gain</b><br>
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<img src="https://github.com/user-attachments/assets/db05a018-f9fb-4b3c-9f55-a4d7a577251f"/>
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<b>Decrease by 50%</b><br>
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<img src="https://github.com/user-attachments/assets/77dc38aa-f829-4878-a58e-32d775fa542c"/>
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3. Adjust the I-gain to achieve a good step response with acceptable steady-state error.
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![image](https://github.com/user-attachments/assets/78f3bca5-2642-44e2-b3f2-7444fc4ffa40)
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4. Fine-tune both gains as needed. At the 1 second mark, a disturbance is injected into the system. The tuned response to this disturbance is visible in the step response plot.
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5. Use the Bode plot to confirm that the resonant frequency peak remains below 0dB.
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![image](https://github.com/user-attachments/assets/86d99295-43a8-47a9-b052-324ab5ac3082)
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### Applying the Gains
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1. The gains shown in the parallel form correspond to the PX4 rate controller parameters.
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![image](https://github.com/user-attachments/assets/2db77238-bd33-4454-af1f-a38901571e88)
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2. If the identified parameters are strongly different to the currently set ones (more than 30% for multiple axes), an incremental application while flying is recommended (not prior to takeoff, during takeoff, or during landing).
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3. For all loops tuned with the strategy outlined in this guide: increase the respective integrator limit parameters to 1 (FW_RR_IMAX, FW_PR_IMAX, FW_YR_IMAX).

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