Abnormal Neutral Position Changes in Servo Motors: Causes and Troubleshooting

I. Symptom

After flight, if you find that the servo midpoint values on the ground control station's "Output" page have changed abnormally, it may cause inconsistent flight feel or control issues. This article will help you quickly locate the problem and conduct effective troubleshooting.

II. Cause Analysis

The midpoint value of servos (default 1500us) may change in certain situations, with main causes including:
1. Flight controller parameter changes: Servo autotrim (SERVO AUTOTRIM) mode may automatically adjust the midpoint value.
2. RC transmitter output error: The RC transmitter itself has not been calibrated, causing stick deviation and resulting in servo output signal offset.

III. Solutions

Troubleshooting 1: Check Servo Midpoint Value
1. Connect the flight controller to the ground control station and enter the "Output" page.
2. Check if the servo midpoint value is 1500us (default value). If not 1500, manually change it to 1500us.
3. Then click Save and Restart.

Troubleshooting 2: Check SERVO AUTOTRIM Settings

Important

SERVO AUTOTRIM Principle This mode automatically adjusts the servo midpoint in self-stabilization modes like ANGLE to maintain a more stable aircraft attitude. However, when switching to MANUAL 模式, it may cause the midpoint value to change, affecting flight feel.

1. Enter the "Configuration" page and find the SERVO AUTOTRIM (Servo Autotrim Mode) option.
2. Check the switch status:

• On (blue): The flight controller will automatically adjust the servo midpoint during flight.
• Off (gray): The flight controller will not modify the servo midpoint value.
  3. If you don't want the flight controller to modify the midpoint value, turn this option off.
  
  Troubleshooting 3: Check RC Transmitter Stick Midpoint Output
  1. Connect the flight controller to the ground control station and ensure the RC transmitter has been bound to the receiver.
  2. Enter the "Receiver" page and check the stick's midpoint value:
• Normal range: 1495 ~ 1505us.
• If beyond this range, adjust the stick's midpoint PWM output in the RC transmitter.
  
  Troubleshooting 4: Calibrate RC Transmitter
  Check the RC transmitter's midpoint output when sticks are released:
  Navigate to MDL→OUTPUT interface:
• Normal range: 1495 ~ 1505us.
• If beyond this range, perform RC transmitter calibration.
  
  Calibration Steps:
  Navigate to SYS→HARDWARE:
• Click Calibration and complete the calibration according to the prompts.
• Re-test the output value after the sticks return to center, ensuring it's within the 1495 ~ 1505us range.
  
  Scenario Extension:
  Case 1: Flight Abnormalities Caused by Improper Use of SERVO AUTOTRIM
  User A enabled the "Continuously adjust servos on fixed-wing (SERVO AUTOTRIM)" function on a P51 fixed-wing aircraft, then:

1. Flew in ANGLE mode for a few minutes.
2. Switched to MANUAL 模式 and found the aircraft's flight feel was different, with a nose-up tendency.
3. Mistakenly thought it was a flight controller hardware issue and began checking hardware.
   Troubleshooting Process
4. Connected to the ground control station and checked stick midpoint:

• RC transmitter PITCH axis output: 1496us (normal).
  
• Receiver page PITCH axis output: 1495us (normal).
  

2. Used a PWM tester to test the flight controller's PITCH control PWM output and found the value was 1573us (abnormal).

Important

User A mistakenly determined a flight controller hardware issue, believing that in pure Manual mode, the flight controller theoretically should not modify the servo midpoint value of 1500us, but during actual testing found the PWM output was 1573us, thus suspecting a problem with the flight controller.

3. Until User A discovered in the ground control station's "Output" page that the servo midpoint value had been modified to 1570us, and through detecting the flight controller's PWM output signal found the real cause of the abnormal flight feel in Manual mode.
It turned out that after the first flight, the flight controller automatically adjusted the servo midpoint based on the "Continuously adjust servos on fixed-wing" function (SERVO AUTOTRIM), but User A ignored the effect of this function and mistakenly thought it was a flight controller hardware failure.

IV. Result Verification

1. After completing the above troubleshooting and adjustments:

• Enter the ground control station's "Output" page and ensure the midpoint signal of servo channels is within the normal range of 1495 ~ 1505us.
  

2. Flight test:

• Observe the flight status in ANGLE mode to ensure stable flight.
• Switch to MANUAL 模式 and confirm that the servo response meets expectations.
• If there are issues, please follow the above tutorial for troubleshooting.

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