📦 Drop Node
See the source code for implementation details.
The Drop Node is responsible for releasing the object at a designated location (drop zone) with precision and safety.
It works closely with the Mission Manager to complete the final stage of an object-handling task.
🔄 Drop Operation Workflow
- Receive trigger from Mission Manager (
/drop_triggeror action call). - Navigate robot to drop zone coordinates (predefined or dynamic).
- Move arm to pre-drop pose (safe above drop location).
- Open gripper to release object.
- Move arm back to home position.
- Confirm drop completion to Mission Manager.
🎯 Positioning & Accuracy
-
Base Pose:
The robot returns to a fixed base pose before dropping. This ensures consistent accuracy by aligning the arm with the drop zone. -
Pose Calibration:
- Align the arm’s coordinate frame with the robot’s base_link.
- Use measured offsets to ensure the gripper is centered over the drop area.
Pro Tip
Always perform a one-time calibration for the drop pose after hardware changes.
🛡 Safety Mechanisms
| Safety Feature | Purpose |
|---|---|
| Pre-drop safe height | Prevents collisions during approach |
| Collision limits | Stops arm if unexpected resistance is felt |
| Timed gripper control | Avoids overdriving servo motors |
| Home position reset | Ensures arm is safely stowed after drop |
🧪 Testing Procedures
Step-by-step:
- Dry Run:
Run drop sequence without holding an object — verify arm paths. - Low-Speed Test:
Hold a lightweight object and run the sequence slowly. - Load Test:
Drop real objects at operational speed. - Repeatability Test:
Drop 10+ times and measure deviation from target.
Pass Criteria:
- Drop point within ±2 cm of target.
- Arm returns to home position safely.
- No collisions or grip failures.
🚀 Performance Optimization
- Reduce approach time:
Move arm in a single smooth motion from transport pose to pre-drop pose. - Use fixed drop offsets:
Avoid re-calculating positions each time. - Tune joint speeds:
Increase joint velocity for faster cycles (while staying safe). - Parallelize:
Prepare next navigation goal while arm returns to home.
🛠 Troubleshooting
| Issue | Possible Cause | Solution |
|---|---|---|
| Object misses drop area | Incorrect calibration or offsets | Recalibrate drop pose |
| Arm collides with environment | Pre-drop height too low | Increase Z offset |
| Gripper fails to open | Servo issue or wrong topic/service name | Test gripper manually |
| Arm stuck after drop | Missing return-to-home command | Add home pose command |