AP_Gripper: support stop command option

[ES-Alexander]

Feature request

Is your feature request related to a problem? Please describe.
Explicit gripper control is currently limited to active releasing and grabbing, which leaves out stopped/neutral operation. The EPM gripper driver seemingly has an option to automatically return to neutral some fixed time after a grab or release, but that is privately scoped and has no direct control action.

Describe the solution you'd like

1. MAVLink now supports a STOP command for grippers, which I think should be used to trigger stopped / neutral functionality of the gripper.
2. With drivers that expose that functionality, it could also be integrated into AUX functions / joystick button function support, as the return point when a grab or release button stops being held.

Describe alternatives you've considered
We could just not implement this, but it's a missing functionality that seems useful for certain gripper types and applications.

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

Additional context
The lack of this functionality is what has thus far stopped ArduSub from regularly using ArduPilot's gripper functionality (opting to instead use momentary servo commands via joystick buttons), so this would reduce ArduSub's reliance on joystick button functions, while also making gripper configuration more intuitive.

[neo-0007]

Hello, I would like to take on this feature request.

I see that the necessary MAVLink change (GRIPPER_ACTION_STOP) has already been merged in mavlink/mavlink PR #2342.

Looking at the code in libraries/AP_Gripper/, I found that a STATE_NEUTRAL already exists in the gripper_state enum but lacks command interface:

    typedef enum {
        STATE_NEUTRAL,
        STATE_GRABBING,
        STATE_RELEASING,
        STATE_GRABBED,
        STATE_RELEASED,
    }  gripper_state;

Current Implementation Details:

• In Servo backend it is used once during initialization to set the servo's starting position. After that, it only moves between the grab and release positions.
• In EPM gripper there is a private neutral() method . After a grab() or release() command, a timer starts, and the update_gripper() function automatically calls neutral() to de-energize the magnet

// neutral - return the epm pwm output to the neutral position
void AP_Gripper_EPM::neutral()
{
    if (!should_use_uavcan()) {
        // move the servo to the off position
        SRV_Channels::set_output_pwm(SRV_Channel::k_gripper, config.neutral_pwm);
    }
}

The missing functionality is the ability to explicitly command the gripper to this neutral state via MAVLink.

Proposed Implementation Plan:

MAVLink Command Handling

• Update command processors in GCS_Common.cpp and vehicle-specific MAVLink handlers
• Add GRIPPER_ACTION_STOP recognition to existing MAV_CMD_DO_GRIPPER handling

AP_Gripper Core Changes

• Add public set_neutral() method to main gripper class
• Ensure state machine properly handles neutral transitions
• Update status reporting for neutral state

Driver Backend Updates

• Make EPM neutral() method accessible via base class
• Ensure servo gripper can return to neutral position during operation
• Verify all backends implement neutral behavior consistently

AUX Function Integration

• Extend RC_Channel options to include gripper neutral command
• Enable momentary button control where button release triggers neutral
• Update documentation for new RC function

Does this approach seem correct? If so, please feel free to assign this issue to me. I'm looking forward to contributing!