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How does Mavros RC override work?


I've been trying to work with the mavros, but the wiki doesn't have a lot of detail about it. I'm using an Iris and connecting with roslaunch mavros apm2_radio.launch.

First I'm setting SYSID_MYGCS to 1, as specified at the bottom of this github issue. Then I'm publishing messages on the command line

$ rosrun mavros mavparam set SYSIS_MYGCS 1
$ rostopic pub -1 /mavros/rc/override mavros/OverrideRCIn '[1500, 1500, 1500, 1500, 1325, 1560, 2000, 1500]'

I'm testing RC by leaving the quadcopter in an unarmed state, connecting a gimbal to RC Channel 6 using APM Mission Planner, then publishing various values on Channel 6 (1560 in the example code) and trying to get the gimbal to move. What happens now is that

  1. When the quadcopter is first powered I have control of the gimbal using our RC controller, channel 6
  2. After setting the SYSID_MYGCS value I still have control with the RC controller
  3. Publishing a mavros/rc/override message with a small deviation in the Channel 6 value does not change the gimbal position, but it does make it so that the RC controller can no longer affect the gimbal position
  4. Publishing messages with a variety of Channel 6 values has no effect, except when there is a large change of >150 in the Channel 6 measurement. This makes the gimbal travel fully in one direction and stay stuck there until you send a largely different Channel 6 value that makes the gimbal rail in the other direction.

I feel like I just don't understand what mavros/rc/override is sending to the quadcopter. The gimbal is perfectly controllable using the RC controller before we publish anything to mavros/rc/override, so we know the gimbal is commandable with Channel 6.

I apologize if this is a drone-discuss (or other quadcopter forum) question, not a mavros question, but I don't have a good enough understanding of what mavros is calling under the surface in order to usefully search the drone-discuss forums. If someone has a link to useful non-ROS information I'd welcome it...