ArduPilot/ardupilot

Rover: servo output range not set after SERVOx_FUNCTION changed

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#9,478 opened on Sep 26, 2018

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BUGRovergood first issue

Description

Servo output ranges are set as part of the AP_MotorsUGV::init function. This is only called once soon after startup meaning that if a user changes a SERVOx_FUNCTION value the servo's output range will not be correct. This could lead to odd values being sent out to the servo until the board is rebooted.

We should first verify that the issue is real by setting a second SERVOx_FUNCTION for throttle and then compare it's output to the the servo3 outputs and see if they're different.

The solution might to one of these:

  • call the motors init regularly (1hz?) while the vehicle is disarmed
  • add boolean values of whether various frame types have been enabled (i..e the return value of have_skid_steering()) and then regularly check if the true/false value has changed and if it has call the init() function again. Note that this might not work in the edge case where the user is setting up a second channel for throttleLeft or throttleRight because setting up this second channel won't affect the return value of the has_skid_steering() function.

Contributor guide

Tech stack
cpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ experienceFamiliarity with ArduPilot architectureUnderstanding of servo output parameter system
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Start by examining the AP MotorsUGV::init function in AP MotorsUGV.cpp to see how servo output ranges are set. Then, check if changing SERVOx FUNCTION triggers any update mechanism. The bug report suggests two possible solutions: either call init() periodically while disarmed, or add boolean flags to track frame type changes and check them. Review the existing callbacks for parameter changes and consider implementing a periodic check or a flag based approach. Test on a real or simulated Rover to confirm the bug and verify the fix.