# Nav2 Recovery Behaviors Configuration

**Issue #479**: Configure conservative recovery behaviors for SaltyBot autonomous navigation.

## Overview

Recovery behaviors are triggered when Nav2 encounters navigation failures (path following failures, stuck detection, etc.). The recovery system attempts multiple strategies before giving up:

1. **Clear costmaps** — Reset perception obstacles
2. **Spin recovery** — In-place 90° rotation to detect surroundings
3. **Wait recovery** — 5-second pause for dynamic obstacles to move
4. **Backup recovery** — Reverse 0.3m to escape deadlocks

## Configuration Details

### Backup Recovery (Issue #479)
- **Distance**: 0.3 meters reverse
- **Speed**: 0.1 m/s (very conservative for FC + Hoverboard ESC)
- **Max velocity**: 0.15 m/s (absolute limit)
- **Time limit**: 5 seconds maximum

### Spin Recovery
- **Angle**: 1.57 radians (90°)
- **Max angular velocity**: 0.5 rad/s (conservative for self-balancing robot)
- **Min angular velocity**: 0.25 rad/s
- **Angular acceleration**: 1.6 rad/s² (half of normal to ensure smooth motion)
- **Time limit**: 10 seconds

### Wait Recovery
- **Duration**: 5 seconds
- **Purpose**: Allow dynamic obstacles (people, other robots) to clear the path

### Progress Checker (Issue #479)
- **Minimum movement threshold**: 0.2 meters (20 cm)
- **Time window**: 10 seconds
- **Behavior**: If the robot doesn't move 20cm in 10 seconds, trigger recovery sequence

## Safety: E-Stop Priority (Issue #459)

The emergency stop system (Issue #459, `saltybot_emergency` package) runs independently of Nav2 and takes absolute priority:

- **Trigger**: Obstacles within 0.3m (configurable)
- **Action**: Immediate zero velocity command, overrides all recovery behaviors
- **Integration**: Publishes directly to `/cmd_vel`, pre-empting Nav2 output

Recovery behaviors cannot interfere with E-stop because the emergency system operates at the motor driver level on the STM32 firmware.

## Behavior Tree Sequence

Recovery runs in a round-robin fashion with up to 6 retry cycles:

```
NavigateRecovery (6 retries)
├── Normal navigation (compute path → follow path)
└── Recovery fallback:
    ├── Clear costmaps
    ├── Spin 90°
    ├── Wait 5s
    └── Back up 0.3m @ 0.1 m/s
```

Each cycle, the next recovery action is attempted (round-robin). If navigation succeeds, recovery stops. If all retries exhaust, goal fails.

## Constraints for FC + Hoverboard ESC

This configuration is specifically tuned for:
- **Drivetrain**: Flux Capacitor (FC) balancing controller + Hoverboard brushless ESC
- **Max linear velocity**: 1.0 m/s
- **Max angular velocity**: 1.5 rad/s
- **Recovery velocity constraints**: 50% of normal for stability

The conservative recovery speeds ensure smooth deceleration profiles on the self-balancing drivetrain without tipping or oscillation.

## Configuration Files

- **nav2_params.yaml**: behavior_server section with spin, backup, wait parameters
- **navigate_to_pose_with_recovery.xml**: Behavior tree defining recovery sequence
- **emergency.launch.py**: E-stop system (independent, higher priority)

## Testing

To test recovery behaviors:

```bash
# Run Nav2 with recovery enabled
ros2 launch saltybot_bringup nav2.launch.py

# Trigger recovery by creating obstacles in the path
# or setting unreachable goals

# Monitor recovery with:
ros2 topic echo /cmd_vel  # Check velocity commands during recovery
ros2 topic echo /diagnostics  # Monitor Nav2 status
```

## References

- Issue #479: Nav2 recovery behaviors
- Issue #459: Emergency stop cascade (E-stop takes priority)
- Nav2 recovery behavior documentation: https://docs.nav2.org/
