How to use the servo auto-calibration feature in ArduPilot

1. Principle

In fixed-wing flight, if the aircraft's center of gravity shifts or there are minor installation errors in the control surfaces, the flight attitude may continuously deviate from level flight. The SERVO_AUTO_TRIM (Servo Auto Trim) feature provided by ArduPilot automatically adjusts the servo output neutral value during flight, enabling fine-tuning of the aircraft's attitude to enhance flight stability and handling feel.

This function works by recording the flight controller's control outputs in the current flight mode and iteratively updating the neutral value. It is suitable for auto-flight or attitude stabilization modes. This adjustment does not alter the flight controller's electronic horizon; it only affects the servo neutral value, making it easy to quickly compensate for hardware errors or CG shifts.

2. Equipment Preparation

1) Flight Controller: Any of the CoreWing F405 WING V2, SpeedyBee F405 WING APP, or SpeedyBee F405 WING MINI flight controllers are suitable for this tutorial.
2) RC Transmitter: Taking the RadioMaster Boxer /RadioMaster TX16S transmitter as an example.
3) Receiver: Taking the SpeedyBee ELRS Nano 2.4G RX receiver as an example.
4) GPS Module: Taking the Beizheng BZ-251 GPS Module as an example.
5) A fully equipped aircraft: including a VTX.

3. Specific Operations

Step 1: Preparation and Configuration
1. Set Parameters

• Enter Configuration/Debug
• Enter All Parameters Table
• Search for SERVO_AUTO_TRIM
• Set the value of SERVO_AUTO_TRIM to 1
• Click Write Parameters
  

2. It is recommended to take a screenshot of the current SERVOx_TRIM (servo neutral value) parameters for later comparison;
   

Step 2: Flight Test

Important

This function only works in FBWA, FBWB, and AUTO modes. Modes such as ARCO and Manual do not support automatic adjustment.

Mode comparison:

Mode	Trim Effect	Characteristics
FBWA	Effective, but significantly affected by wind	Handling is acceptable, but data fluctuates slightly
FBWB	Recommended, most stable effect	Constant speed cruise, more consistent control surface movement
AUTO	Good effect	Requires uploading a mission plan

1. Set the aircraft to ARCO mode for takeoff;
2. After the aircraft ascends to a safe altitude, switch to FBWB (recommended) mode (for AUTO mode, the mission plan must be uploaded in advance);
3. Maintain stable cruise flight for about 1 to 2 minutes in the above mode; the system will record and update the servo neutral value every 10 seconds;

Important

Note that if the flight speed does not reach the minimum trigger threshold of this function (≥8 m/s), the servo neutral value cannot be modified.

4. After completing the test, switch back to ARCO mode for landing; the parameters will be automatically saved upon locking.
5. View and compare whether the new SERVOx_TRIM values have changed.

Important

What is the adjustment range of auto trim?
The servo neutral value adjustment range is ±10% of the servo output travel. For example: default output range 1100–1900, then the neutral value adjustment range is 1420–1580.
1900-1100=800 800*10%=80 Default neutral 1500 1500-80=1420 1500+80=1580

Step 3: Disable the Feature and Fly Again

1. Set SERVO_AUTO_TRIM = 0 to disable auto trim;
   
2. Take off again and feel the flight handling.
3. Compare the flight performance (attitude stability, handling feel) with the previous time to see if there are any differences.

4. Q&A

Q: What is the adjustment range of auto trim?
A: The servo neutral value adjustment range is ±10% of the servo output travel. For example: default output range 1100–1900, then the neutral value adjustment range is 1420–1580.

Q2: Why are there no changes in the servo neutral value after flying?
A: Flight time was too short;
Flight attitude was not kept stable;
Flight speed did not reach the minimum trigger threshold (≥8 m/s);
Unsupported flight modes were used;
The actual adjustment amount of the servo has exceeded the allowed range.

Q: Is it suitable for attitude deviations caused by flaps or speed brakes?
Yes. You can enable the trim function during flight to mitigate pitch changes caused by speed brakes/flaps by adjusting the trim neutral value.

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