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Introduction

Pinkrab (Pink+Crab) is a bimanual robot fixed to an IKEA SKÅDIS pegboard on a wall. Each arm has 6 degrees of freedom, not counting the gripper. It adds a wrist yaw joint to the 5-DoF arm SO-ARM 101 and uses a newly designed base so the arm can be mounted on a wall.

Hermit, which goes with it, is a UMI gripper built so the arm’s claw can be held in the hand. When you move Hermit, the arms follow the same motion (teleoperation), and the demonstration data for imitation learning is also collected with Hermit.

Pinkrab’s parts are printed in hot pink filament. The Pink in the name comes from this color, and the Crab from the way the body stays fixed to the wall while only the pincers move to do the work.

Pinkrab

Section titled “Pinkrab”

The added wrist yaw joint sits between wrist flex and wrist roll, and the bracket that supports it is also a printed part. Each arm holds seven Feetech STS3215 servos, which drive six joints and the gripper. Six joints let you specify both the position and the orientation of the tool, so all arm motion is computed with URDF-based inverse kinematics.

Two arms hung on a pegboard above a desk

The arm is fixed to the board with an L-shaped base. The base holds the arm upright, so even when it hangs on a wall the shoulder pan axis is vertical, just like an arm lying on a desk.

The arm seen obliquely from the front. The claw, the wrist camera, and the L-shaped base on the pegboard

A global shutter fisheye camera (ELP AR0234) on the wrist captures the claw and the work area in front of it in a single view.

The arm seen from the opposite side. The wrist camera above the claw

Specifications

Section titled “Specifications”
Item Value
Degrees of freedom 6 joints + gripper per arm
Servos Feetech STS3215 (12 V), 7 per arm
Reach Up to about 570 mm from the board surface to the tool tip (440 mm from the shoulder lift axis)
Payload About 0.45 kg (calculated for the arm fully extended horizontally, from the shoulder lift servo’s 30 kg·cm stall torque)
Arm weight About 0.7 kg (including the base, URDF value)
Gripper Jaw opens up to 80°
Pose accuracy Joint error up to 0.8°, RMS around 0.2°, tool tip mean 1 mm (measured)
Max joint speed 300°/s (limited by the safety gate)
Wrist camera ELP AR0234, global shutter fisheye, 1920×1200
Mounting Default spacing between the two arms 480 mm, base top surface 180 mm above the desk (change it in mounting dimensions)
Power and communication One 12 V 6 A adapter drives both arms; each arm connects over USB through one Seeed XIAO bus servo board

Reach

Section titled “Reach”

The painted area on the desk is where the tool can reach in grasping poses. With the tool tip 25 mm above the desk, a 2 cm cell is painted if an inverse kinematics solution exists for at least one of these directions: pointing down at 30°, 45°, or 60°, and 0° or ±45° left or right from the front of the board. It is the same calculation as the reach display in studio.

Drag to rotate

  • Left arm
  • Right arm

Joint range

Section titled “Joint range”

Choose a joint and the left arm moves it from one limit to the other. The right arm’s range is mirror-symmetric to the left arm’s.

Drag to rotate

Angles are measured from the joint origin (dotted line). The fan is the range the joint moves through.

Hermit

Section titled “Hermit”

Left-hand and right-hand Hermit with UMI markers, and the table marker on the desk

Hermit is a gripper made by attaching a handle to the arm’s claw. The shapes of the claw and jaw, and the camera and where it is mounted, are the same as on the arm. The one servo inside does not drive the jaw and is used only to read how far the jaw is open.

During a demonstration, a top camera (global shutter AR0234, 120° lens) worn on the demonstrator’s head films the UMI marker on the Hermit handle together with the table marker fixed to the desk. The pose of the UMI marker is computed relative to the table marker, so the tool pose is obtained in desk coordinates for every frame.

Hermit held in the hand. The UMI marker on the handle and the camera above the claw

You hold Hermit with the thumb in the claw’s two rings and the index finger in the jaw’s two rings. Curling the index finger closes the jaw and extending it opens the jaw; on the arm, a servo opens and closes the jaw the same way.

The thumb and index finger in Hermit’s rings, the jaw closed

Hermit and the arm have the same camera and claw, so a policy trained on video filmed with Hermit receives video that looks the same as in training when it runs on the arm. That is why collecting demonstrations does not require Pinkrab.

Software components

Section titled “Software components”
Component Role
pinkrab CLI Naming servo boards, checking servos, arm calibration, camera lens fit
pinkrab-studio Browser workspace. Pinkrab rendered with MuJoCo, teleoperation, demonstration collection, training, policy execution
pinkrab-umi Hermit-side programs. Camera recording, marker tracking, converting episodes to LeRobot datasets
ROS 2 nodes The servo driver, the safety gate that checks the joint range, scene collisions, and speed of every command, and the bridge connecting studio to ROS
LeRobot plugin Registers both arms as a single LeRobot robot type, bi_pinkrab_follower

LeRobot is used for policy training. The policy is LeRobot’s ACT (Training).

Built on

Section titled “Built on”

Pinkrab is based on the ideas, designs, and code of the following projects.