feat(failure_detector): unite motorfailure offsets by ttechnick · Pull Request #26990 · PX4/PX4-Autopilot

ttechnick · 2026-04-07T14:15:16Z

Solved Problem

This PR simplifies the motor over/under-current detection introduced in #26262.

Currently, overcurrent and undercurrent thresholds must be configured separately. In practice, estimating even one of these reliably is already difficult, making the feature unnecessarily complex to use.

Solution

Instead of defining separate offsets for overcurrent and undercurrent, this PR introduces a single symmetric offset.

The new parameter MOTFAIL_OFF is applied symmetrically around the expected current-to-throttle relationship (MOTFAIL_C2T). This simplifies configuration while preserving the ability to detect motor failures.

Using the original image from @MaEtUgR:

Purple: Ideal current-to-throttle relationship (as defined by MOTFAIL_C2T)
Green: Symmetric offset band (defined by MOTFAIL_OFF)

Changelog Entry

For release notes:

Feature: Use Symmetric motorfailure offset limit
Removed param `MOTFAIL_LOW_OFF` & `MOTFAIL_HIGH_OFF` replaced by `MOTFAIL_OFF`

Issue

Both the current and proposed implementations assume:

0 current at 0 throttle
a linear increase in current with throttle

However, as shown in the original analysis by @MaEtUgR, this assumption does not hold in practice. There is typically a dead zone, where motor command increases before significant current is drawn.

This behavior cannot be represented in the current model—neither before nor after this change. Despite this limitation, the simplification introduced by this PR is considered a worthwhile trade-off.

Alternatives

To address the limitation mentioned above, the following options come to my mind:

Introduce a parameter to better describe the current curve (e.g., current at 50% throttle or a similar reference point).
Estimate the throttle offset at which current starts flowing and use it as a fixed approximation.

Open to hear your thoughts 👐

ttechnick · 2026-04-07T14:19:30Z

Fun fact:

The image shown above—originally used to advocate for separate low and high limits—actually supports using a symmetric limit.

The key issue is that the yellow and red “lines” should be parallel to the purple line, not the measured blue “ideal” line. This follows from the assumption of 0 current at 0 command, which defines the reference slope.

As a result:

The overcurrent deviation is largest at high motor commands
The undercurrent deviation is largest at low motor commands

In this specific example, both deviations appear to be very similar in magnitude, based on the image provided by @MaEtUgR.

😀

github-actions · 2026-04-07T14:23:08Z

No broken links found in changed files.

github-actions · 2026-04-07T14:23:21Z

🔎 FLASH Analysis

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Updated: 2026-04-07T14:23:20

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

github-actions · 2026-04-30T06:57:24Z

💡 Commit messages could be improved

Not blocking, but these commit messages could use some cleanup.

Commit	Message	Suggestion
`7c37755d2a`	Update docs/en/config/safety.md	Missing conventional commit format (e.g. "feat(ekf2): add something")
`587a936a1e`	docs: update docs/en/config/safety.md	Missing conventional commit format (e.g. "feat(ekf2): add something")
`32eea20833`	docs: Update src/modules/commander/HealthAndArmingChecks/esc_check_params.yaml	Missing conventional commit format (e.g. "feat(ekf2): add something")

See the commit message convention for details.

This comment will be automatically removed once the issues are resolved.

github-actions · 2026-04-30T07:04:10Z

🔎 FLASH Analysis

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Updated: 2026-05-07T12:24:00

github-actions · 2026-04-30T07:30:10Z

No broken links found in changed files.

sfuhrer

Agree with simplifying the setup.

Co-authored-by: Silvan Fuhrer <silvan@auterion.com>

…rams.yaml Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

* feat(failure_detector): unite motorfailure offsets * docs: update docs/en/config/safety.md * docs: Update src/modules/commander/HealthAndArmingChecks/esc_check_params.yaml --------- Co-authored-by: Hamish Willee <hamishwillee@gmail.com> Co-authored-by: Silvan Fuhrer <silvan@auterion.com>

feat(failure_detector): unite motorfailure offsets

b6c78ca

ttechnick requested a review from MaEtUgR April 7, 2026 14:15

ttechnick self-assigned this Apr 7, 2026

github-actions Bot added the Documentation 📑 label Apr 7, 2026

ttechnick requested a review from dakejahl April 7, 2026 14:20

hamishwillee reviewed Apr 9, 2026

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Comment thread docs/en/config/safety.md Outdated

dakejahl previously approved these changes Apr 13, 2026

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ttechnick marked this pull request as draft April 14, 2026 08:05

hamishwillee added scope:docs and removed Documentation 📑 labels Apr 26, 2026

ttechnick marked this pull request as ready for review April 30, 2026 06:56

Update docs/en/config/safety.md

7c37755

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

ttechnick dismissed dakejahl’s stale review via 7c37755 April 30, 2026 06:57

ttechnick requested a review from dakejahl April 30, 2026 06:57

Merge branch 'main' into pr-unite-motorfailure-offset

dee1eec

ttechnick requested a review from sfuhrer April 30, 2026 07:06

sfuhrer reviewed May 5, 2026

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Comment thread docs/en/config/safety.md Outdated

Comment thread src/modules/commander/HealthAndArmingChecks/esc_check_params.yaml Outdated

ttechnick and others added 3 commits May 7, 2026 14:16

docs: update docs/en/config/safety.md

587a936

Co-authored-by: Silvan Fuhrer <silvan@auterion.com>

docs: Update src/modules/commander/HealthAndArmingChecks/esc_check_pa…

32eea20

…rams.yaml Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

Merge branch 'main' into pr-unite-motorfailure-offset

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sfuhrer approved these changes May 7, 2026

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ttechnick merged commit 6d0de73 into main May 7, 2026
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ttechnick deleted the pr-unite-motorfailure-offset branch May 7, 2026 15:20

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feat(failure_detector): unite motorfailure offsets#26990

feat(failure_detector): unite motorfailure offsets#26990
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ttechnick commented Apr 7, 2026

Solved Problem

Solution

Changelog Entry

Issue

Alternatives

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