PWM: Servo center & asymetric deflection by ttechnick · Pull Request #25897 · PX4/PX4-Autopilot

ttechnick · 2025-11-11T15:05:20Z

Solved Problem

Setting up servos is very tedious, because when you change the PWM_MIN and PWM_MAX, the neutral point also changes. Additionally, it was not possible to have different deflections for the positive and negative direction.

Solution

Add PWM_*_CENTERx for each servo.
Use a bilinear transform to map actuator_servos to PWM signals.

This solution only works for PWM based servos. Other types of servos are not affected.

Changelog Entry

For release notes:

Feature PWM CENTER
New parameter: `PWM_MAIN_CENTx` and PWM_AUX_CENTx`
Documentation: Need to clarify page [Actuator Configuration and Testing](https://docs.px4.io/main/en/config/actuators#output-assignment-and-configuration)

Alternatives

We could also implement this feature for other types of servos.

Context

Illustration of the mapping form actuator_servos to pwm using bilinear transform:

Example Result:

Log (Bench):
https://review.px4.io/plot_app?log=5fed3401-4849-4417-a807-212a4ce540ab
Log (Flight):
https://review.px4.io/plot_app?log=899e5f98-3985-401c-8513-0f324339e625

ttechnick · 2025-11-11T15:18:50Z

For review: @mahima-yoga, @sfuhrer

mahima-yoga

Didn't get through everything yet but since I am going on holiday for a few days I will post my partial review in case I don't manage to get through the rest today/tomorrow 😄

sfuhrer

We should start thinking about how to handle the transition from the current "trim on the control surface function" to "trim on the servo output".

remove the function trim (but then how is trimming done for UAVCAN servos? And can we translate the settings on existing airframes to the PWM_TRIM?)
display warning when the legacy fields are being altered ("consider setting PWM_TRIM instead of surface trim")
set all surface trims to 0 when any PWM_TRIM is set to non-1500PWM

ttechnick · 2025-11-17T12:57:53Z

We should start thinking about how to handle the transition from the current "trim on the control surface function" to "trim on the servo output".
* remove the function trim (but then how is trimming done for UAVCAN servos? And can we translate the settings on existing airframes to the PWM_TRIM?)

* display warning when the legacy fields are being altered ("consider setting PWM_TRIM instead of surface trim")

* set all surface trims to 0 when any PWM_TRIM is set to non-1500PWM

I included a formula in the docs that will translate the current trim settings to the new parameter.
We cannot (yet) remove the surface trims, as they are needed for other types of servos, such as UAVCAN or Dshot. If these drivers are updated, we should certainly do that.
I added a Note to the CA_SV_CSx_TRIM field, that it should not be used with PWM servos.
All surface trims are set to 0 when any PWM_TRIM is set to non-1500PWM automatically, and a note has been added about this.

sfuhrer

It's in a very good state. What I would still consider:

default the _CENT params to -1, so keep it disabled and do the PWM range linearily from MIN to MAX
hide the CENT params for motors in the UI (if complexity is reasonable)

ttechnick · 2025-11-20T13:10:51Z

It's in a very good state. What I would still consider:

* default the _CENT params to -1, so keep it disabled and do the PWM range linearily from MIN to MAX

* hide the CENT params for motors in the UI (if complexity is reasonable)

The default is now -1.
Old legacy trims can thus still be used the exact same way, as long as the *_CENTx values are kept at -1.

Unfortunately, hiding these parameters is quite complex and would required large changes in mavlink, and maybe the groundstations, that I do not think are worth the effort.
However, I added clearer warnings, that this is only to be used for pwm servos (for now).

Signed-off-by: Silvan <silvan@auterion.com>

github-actions · 2025-11-26T16:20:09Z

No flaws found

sfuhrer · 2025-11-26T16:20:11Z

@ttechnick I've added the event for the warning when TRIM settings are present and you add CENTER.

sfuhrer

Ready to go now from my side. The transition (from TRIM to CENTER) is handled diligently, and docs are clean.
Let's squash-merge.

Add PWM_*_CENTERx for each servo. Use a bilinear transform to map actuator_servos to PWM signals. This solution only works for PWM based servos. Other types of servos are not affected. * PWM: Add servo trim option * PWM: Improve documentation of PWM trim feature * PWM: cleaner clamping and docs typo * update documentation & safety * add migration formula * rename param from trim to center * docs with center instead of trim * move clamping and reorder values * improve documentation * adress failing range check * improve documentation * CA: add event for setting CENTER with TRIM Signed-off-by: Silvan <silvan@auterion.com> --------- Signed-off-by: Silvan <silvan@auterion.com> Co-authored-by: Silvan <silvan@auterion.com>

PWM: Add servo trim option

f1acf76

github-actions bot added the Documentation 📑 label Nov 11, 2025

PWM: Improve documentation of PWM trim feature

cf09318

ttechnick marked this pull request as ready for review November 11, 2025 15:16

ttechnick marked this pull request as draft November 11, 2025 15:16

mahima-yoga requested review from mahima-yoga and sfuhrer November 11, 2025 19:58

mahima-yoga reviewed Nov 11, 2025

View reviewed changes

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

Comment thread src/lib/mixer_module/mixer_module.cpp Outdated

ttechnick and others added 2 commits November 12, 2025 09:12

PWM: cleaner clamping and docs typo

d69a215

Merge branch 'main' into pr-servo-pwm-trim

7599b2c

sfuhrer reviewed Nov 14, 2025

View reviewed changes

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

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

update documentation & safety

9a5109a

add migration formula

2b53193

ttechnick force-pushed the pr-servo-pwm-trim branch from 269797d to 2b53193 Compare November 17, 2025 12:59

Merge branch 'main' into pr-servo-pwm-trim

d5fe247

ttechnick marked this pull request as ready for review November 17, 2025 13:00

ttechnick requested a review from sfuhrer November 17, 2025 13:00

ttechnick added 2 commits November 17, 2025 17:20

rename param from trim to center

6cb79e0

docs with center instead of trim

00ab877

ttechnick force-pushed the pr-servo-pwm-trim branch from 6d9b229 to 00ab877 Compare November 17, 2025 16:53

Merge branch 'main' into pr-servo-pwm-trim

a4d19d0

sfuhrer changed the title ~~PWM: Servo Trim & asymetric deflection~~ PWM: Servo center & asymetric deflection Nov 18, 2025

Merge branch 'main' into pr-servo-pwm-trim

b4eae77

sfuhrer reviewed Nov 20, 2025

View reviewed changes

move clamping and reorder values

264d53e

ttechnick force-pushed the pr-servo-pwm-trim branch from 89072cd to 264d53e Compare November 20, 2025 12:44

improve documentation

c435b7d

Merge branch 'main' into pr-servo-pwm-trim

2545ec3

ttechnick requested a review from sfuhrer November 20, 2025 13:14

ttechnick and others added 6 commits November 24, 2025 09:07

adress failing range check

9227050

Merge branch 'main' into pr-servo-pwm-trim

67c7f06

Merge branch 'main' into pr-servo-pwm-trim

97db616

improve documentation

7b9ce15

Merge branch 'main' into pr-servo-pwm-trim

9577f20

CA: add event for setting CENTER with TRIM

f92937a

Signed-off-by: Silvan <silvan@auterion.com>

sfuhrer approved these changes Nov 26, 2025

View reviewed changes

sfuhrer merged commit 575fa08 into PX4:main Nov 26, 2025
71 of 73 checks passed

hamishwillee mentioned this pull request Nov 27, 2025

Actuator page tweaks #26004

Merged

ttechnick mentioned this pull request Jan 5, 2026

[Bug] Servo center Feature only works for Servos #26210

Closed

ttechnick deleted the pr-servo-pwm-trim branch January 5, 2026 14:33

ttechnick mentioned this pull request Jan 21, 2026

[BACKPORT 1.17] Include PWM Center PR #26317

Merged

dakejahl mentioned this pull request Jan 24, 2026

actuators: add center to Actuators::parseJson mavlink/qgroundcontrol#13897

Closed

dakejahl mentioned this pull request Feb 19, 2026

actuators: remove function from center param #26517

Merged

sfuhrer mentioned this pull request Feb 20, 2026

PCA9685: add PWM_CENTER to auto generated params #26543

Merged

ttechnick mentioned this pull request Mar 11, 2026

feat(px4io): Enable pwm center for PWM MAIN by IO processor #26711

Merged

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Solved Problem

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ttechnick commented Nov 11, 2025 •

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