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The pick-and-place demo runs an autonomous loop that finds an AprilTag-marked object, picks it up using the vacuum gripper, and deposits it at a fixed drop-off point. The arm repeats the cycle indefinitely until you trigger the /stop service or kill the process.

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How it works

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Pick-and-place cycle

1

Move to LOOK_ONE

The arm moves to the LOOK_ONE joint pose — a position above the workspace that gives the wrist camera a clear view of the table.
2

Detect tag_1

TagLookup.get_tag_pose() waits 1 second then takes two TF2 readings of tag_1 relative to base_link. If the readings are more than 3 mm apart in any axis the result is discarded and the arm returns to LOOK_ONE.
3

Approach above the object

The arm moves to the detected tag position with a +5 cm Z offset (APPROACH_OFFSET = 0.05) to clear the object before descending.
4

Descend and pick

The arm moves straight down to the exact tag pose, then activates the vacuum gripper (digital output pin 0, ON).
5

Lift

The arm rises back to the approach height (+5 cm above the pick point) while the vacuum holds the object.
6

Move to drop-off approach

The arm moves to the fixed drop-off position with a +5 cm Z offset:
DROP_OFF = (0.2, -0.2, 0.0381)  # x, y, z in base_link metres
# Approach height = DROP_OFF[2] + APPROACH_OFFSET = 0.0881 m
7

Lower and release

The arm descends to DROP_OFF exactly, then deactivates the vacuum (digital output pin 0, OFF) to release the object.
8

Lift and repeat

The arm rises to the drop-off approach height and the cycle restarts from step 1.

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Key constants

APPROACH_OFFSET = 0.05          # 5 cm safety clearance above pick and drop poses
DROP_OFF        = (0.2, -0.2, 0.0381)   # fixed drop position in base_link (metres)
EE_DOWN         = (1.0, 0.0, 0.0, 0.0) # end-effector quaternion (pointing straight down)

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Named joint poses

These joint configurations are used for safe transit between workspace regions: 
UP = {
    "elbow_joint": 0,
    "shoulder_lift_joint": -1.57,
    "shoulder_pan_joint": 0,
    "wrist_1_joint": 0,
    "wrist_2_joint": 1.59,
    "wrist_3_joint": 0,
}

LOOK_ONE = {
    "elbow_joint": -0.6769,
    "shoulder_lift_joint": -1.4927,
    "shoulder_pan_joint": 0,
    "wrist_1_joint": -2.3952,
    "wrist_2_joint": 1.6315,
    "wrist_3_joint": 0,
}

LOOK_FOUR = {
    "elbow_joint": -0.6769,
    "shoulder_lift_joint": -1.4927,
    "shoulder_pan_joint": -1.5708,
    "wrist_1_joint": -2.3952,
    "wrist_2_joint": 1.6315,
    "wrist_3_joint": 0,
}

COMPACT = {
    "elbow_joint": -2.2043,
    "shoulder_lift_joint": -0.5901,
    "shoulder_pan_joint": 0,
    "wrist_1_joint": -0.3818,
    "wrist_2_joint": 0,
    "wrist_3_joint": 0,
}

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Speed profiles

All motion uses the OMPL planner. Two speed profiles are available:
ProfileVelocity scalingAcceleration scalingUsed for
slow0.10.1All pick-and-place motions
fast0.30.3Available for transit (not used by default)

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Tag stability check

TagLookup rejects unstable detections before the arm commits to a pick: 
error_m = 0.003  # 3 mm
wait_s  = 1.0

first  = self.lookup(tag_frame, base_frame)
time.sleep(wait_s)
second = self.lookup(tag_frame, base_frame)

if max(dx, dy, dz) >= error_m:
    return None   # discard — tag is moving or detection is noisy

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Vacuum control

VacuumControl calls the /io_and_status_controller/set_io service to toggle a digital output: 
# Activate vacuum (pick)
vacuum.set_digital_out(pin=0, state=True)

# Deactivate vacuum (release)
vacuum.set_digital_out(pin=0, state=False)
The vacuum is always deactivated in the finally block to prevent the gripper from holding an object if the demo is interrupted.

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Stop control

The StopControl node exposes a /stop Trigger service. Calling it sets stop_event, which causes the main loop to exit cleanly at the next opportunity, and immediately cancels any in-progress trajectory execution.

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Prerequisites

ColmanBringup must be running before you start this demo. It brings up the UR robot driver, ros2_control, and the AprilTag detection node that publishes TF frames for tag_1.
Place an AprilTag 36h11 marker with ID tag_1 on the object you want the arm to pick up, and ensure it is visible to the wrist camera when the arm is in the LOOK_ONE pose.

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Running the demo

pixi run demo-gripper

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Launch arguments

ArgumentDefaultDescription
gripper_typevacuumGripper model to load in the URDF (vacuum is the only supported type)

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Stopping the demo

Call the /stop service from another terminal: 
ros2 service call /stop std_srvs/srv/Trigger
The arm will finish its current motion segment, then the vacuum will be deactivated and the process will shut down cleanly.

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