Connect a Gimbal via DroneCAN

[nexton-winjeel]

Feature request

We'd like to be able to connect a gimbal to ArduPilot using DroneCAN. The primary use cases for this are to enable the use of gimbals on:

• large vehicles where it is unreliable to run long connections to a gimbal; and
• vehicles/boards that have a limited number of ports.

As discussed with @tridge and @rmackay9, the initial way to do this is to connect a gimbal to an AP_Periph node, and connect the AP_Periph node to the Autopilot via CAN. In order to control the gimbal from the AP_Periph node, the existing AP_Mount drivers need to be made available in AP_Periph. To implement this, the following will need to be done:

• Remove the AP_AHRS singleton reference from AP_Mount, and instead pass a smaller AHRS interface as a reference into AP_Mount (see #22143);
• In AP_Periph, create a class (e.g. AP_PeriphAHRS) that implements the new AHRS interface, and:
  • pass this new class into an AP_Mount instance in AP_Periph;
  • populate the AP_PeriphAHRS state from DroneCAN messages.
• Create an AP_Mount backend (e.g. AP_Mount_DroneCAN) that:
  • publishes the required AHRS data from the Autopilot to the AP_Periph node;
  • converts Mount MAVLink commands to equivalent DroneCAN messages and publishes to the AP_Periph node;
  • receives DroneCAN messages from the AP_Periph node and updates the Mount status.
• In AP_Periph, create a class (or extend the existing CAN handler) to:
  • convert DroneCAN messages to equivalent Mount MAVLink commands and pass them into AP_Mount;
  • publish Mount status via equivalent DroneCAN messages.

There are a number of open questions that will need to be worked through, including:

• How do we manage Mount parameters between the Autopilot and the AP_Periph node?
• What DroneCAN messages do we need to send/receive? As discussed with @rmackay9:
  • We should map the ArduPilot MAVLink Mount messages to DroneCAN messages. This will likely require the creation of new DroneCAN messages.
  • Longer term, we should map the full MAVLink Gimbal v2 protocol to DroneCAN messages.

Platform [ ] All [ ] AntennaTracker [ X ] Copter [ X ] Plane [ X ] Rover [ X ] Submarine

[nexton-winjeel]

I have some time now to get the framework for this implemented, i.e. get the AHRS data and Mount drivers into AP_Periph.

The full implementation of this, particularly mapping the existing MAVLink messages to DroneCAN messages, will require further input from the community,

[joshanne]

Just to be more explicit here, the end goal is support "smart gimbals" on the Peripheral - one that requires a backend driver to be able to transmit the required data (eg. AHRS, ROI, etc) and calculates where to drive the camera to internally to the gimbal.

Standard "servo gimbals" can already be configured with AP_Periph and a FlightController/AP_Mount driving PWM outputs over CAN.

[rmackay9]

I've started on a DroneCAN gimbal driver here https://github.com/rmackay9/rmackay9-ardupilot/commits/mount-dronecan