We are using an APM 2.6 to trigger a custom built vaccine dispenser. It is triggering based on distance. It is mounted on an ATV and soon to be on a plane or multirotor. Switching to Pixhawk for those adventures.
It is easy to see if the pellets are not dispensing while on the ATV but we are planning on running hour long missions with the plane and it would suck to have the plane come back after an hour flight and still have the pellet hopper full.
We would like to mount an IR sensor on the output chute and it would detect if pellets are moving at the time of the trigger. I image there may be some instances where it may jam intermittently, so it would also have to have a counter for >X amount of fails before it triggers a RTL.