Fix servo throttle mix outputting wrong position when disarmed

[sensei-hacker]

User description

Summary

Fix a safety issue where reversed throttle servos and servos with negative mixer weights go to the wrong position when disarmed.

When disarmed, the servo mixer was unconditionally overwriting servo outputs with motorConfig()->mincommand (typically 1000µs) for any servo with a throttle mixer rule. This ignored servo reversal (negative servoParams rate) and negative mixer weights, causing certain servo configurations to output full throttle instead of minimum on disarm.

Changes

• Force throttle inputs (INPUT_STABILIZED_THROTTLE and INPUT_RC_THROTTLE) to -500 (minimum) before the mixer loop when disarmed, so the existing mixer pipeline correctly computes safe positions accounting for servo reversal and negative weights
• Remove the post-hoc override at the end of servoMixer() that was setting servo[target] = motorConfig()->mincommand

Testing

Tested with SITL using automated MSP test script covering 5 scenarios:

Scenario	Before (Bug)	After (Fix)	Expected
Normal servo + normal weight	1000	1000	1000
Reversed servo + normal weight	1000	2000	2000
Normal servo + negative weight	1000	2000	2000
Reversed + negative (double inversion)	1000	1000	1000
Non-throttle servo (pitch only)	1500	1500	1500

All 5 tests pass after the fix. The two previously failing cases (reversed servo, negative weight) now correctly compute the safe position.

Built successfully for SITL, MATEKF405, and SPEEDYBEEF405WING.

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PR Type

Bug fix

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Description

• Fixes safety issue where reversed throttle servos output wrong position when disarmed

• Forces throttle inputs to minimum before mixer pipeline instead of post-hoc override

• Ensures servo reversal and negative mixer weights are correctly accounted for

• Removes broken unconditional mincommand override that ignored servo configuration

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Diagram Walkthrough

flowchart LR
  A["Disarmed State"] --> B["Force Throttle Inputs to -500"]
  B --> C["Mixer Pipeline Processes Inputs"]
  C --> D["Applies Servo Reversal & Weights"]
  D --> E["Correct Safe Position Output"]
  F["Old Approach: Post-hoc Override"] -.->|removed| G["Ignored Reversal & Weights"]

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File Walkthrough

Relevant files

Bug fix

servos.c Replace post-hoc override with input forcing                          src/main/flight/servos.c Added logic to force throttle inputs (INPUT_STABILIZED_THROTTLE and INPUT_RC_THROTTLE) to -500 when disarmed, before the mixer loop executes Removed the broken post-hoc override that unconditionally set servo outputs to motorConfig()->mincommand for throttle-mixed servos This allows the existing mixer pipeline to correctly compute safe positions accounting for servo reversal and negative mixer weights +10/-13

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[qodo-code-review]

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