Moving a servo depending on the flight mode

ArduPilot Lua Scripting
1

Hello, I built a VTOL tailsitter and am trying to use Q_WVANE with side into wind vtol modes. The problem is that I cannot get a heading since the gps is looking towards the sky so the heading is wrong or doesn’t really change.
What I want to do to correct this is to put the gps on a servo so that when the tailsitter transitions to VTOL the gps is still parallel to the ground thus giving a heading.
This is the code that I’ve written for this action:

local SERVO_CHANNEL = 9 – Canal utilisé par le servo
local PWM_VTOL = 1700 – Valeur PWM pour le mode VTOL (weathervaning actif)
local PWM_AVION = 1300 – Valeur PWM pour le mode avion

– Initialise le servo à la position avion
SRV_Channels:set_output_pwm(SERVO_CHANNEL, PWM_AVION)

function update_gps_mount()
local mode = vehicle:get_mode() – Récupère le mode de vol

if mode == nil then
    gcs:send_text(6, "Erreur: mode de vol non défini")
    return
end

-- Affiche le mode actuel
gcs:send_text(6, "Mode actuel: " .. tostring(mode))

if mode == 17 or mode == 19 then  -- QSTABILIZE = 17, QRTL = 19
    gcs:send_text(6, "Passage en VTOL, servo en position VTOL")
    -- Envoi PWM pour le mode VTOL
    SRV_Channels:set_output_pwm(SERVO_CHANNEL, PWM_VTOL)
elseif mode == 5 then  -- FBWA = 5
    gcs:send_text(6, "Passage en avion, servo en position avion")
    -- Envoi PWM pour le mode avion
    SRV_Channels:set_output_pwm(SERVO_CHANNEL, PWM_AVION)
else
    gcs:send_text(6, "Mode non pris en charge: " .. tostring(mode))
end

end

function update()
update_gps_mount()
return update, 100 – Réexécute toutes les 100 ms
end

gcs:send_text(6, “gps_mount.lua est en cours d’exécution”)
return update()

Note: the servo is not a control surface, it is not linked to any RC channel and SERVO_FUNCTION9 is set to 0. I tried to set it to script1 or other but without change. It’s like the servo isn’t getting any power, but when I toggle SERVO9 manually it moves just fine.

As I can see in the messages window the lua script runs just fine, it detects the flight mode correctly.

What I think is happening is happening is that the command SRV_Channels:set_output_pwm doesn’t work. I tried using the servo.set_output function which is what is recommended on the ardupilot documentation, but when I do it gives me this error:

x a nil value (global ‘servo’)
11/03/2025 00:51:24 : Lua: /APM/scripts/gps_mount.lua:6: attempt to inde

I am using a Matek H743 V3 running Arduplane 4.5.1 and I don’t understand why the ‘servo’ isn’t recognised.

Thank you for any help.

2

You might want to test this manually before you get too far because if you move the GPS relative to the flight controller you will get EKF errors.

1 Like
3

Heed @Allister’s advice.

But if you want to continue with this project, try SRV_Channels:set_output_pwm_chan (or SRV_Channels:set_output_pwm_chan_timeout if you have an RC channel assigned to the servo).

4

you would probably be better using 2 GPS receivers with one in each orientation, I think there is a blend mode that will smooth the transition between them.

5

This was a topic of discussion with the Aerobatic plane guy flying inverted. That was one suggestion.

6

Thank you for all your responses, I think that I will go with the 2 gps option and write a lua script to help prioritise everything.