Robotics

Tokyo Lab Built Arms That Unplug From Their Motors

Robb Harlan 4 min read

Takahiro Hattori, Kento Kawaharazuka, and Kei Okada at the University of Tokyo’s JSK lab have an IROS 2026 paper on a problem every reconfigurable robot hits: if every module carries its own motors, the pile gets heavy and expensive. Their answer is Detachable Wire Drive. Park the actuators in one base. Snap different arms onto the same tendons.

The project page puts the usual waste at 30–50% of system weight and cost. The paper’s claim is simpler: one motor set, many shapes.

Detachable Wire Drive base with interchangeable arms and grippers
One motor base, two arms, two grippers, and the Wire Detach Units that join them. Source: Hattori et al., project page / arXiv:2608.15461.

A coupler for tendons, not just a latch

The Wire Detach Unit lives in a sector-shaped shell: winding pulley, alignment pulley, and convex-concave faces that take torque. Up to six units pack in a ring. The tendons are 1 mm Vectran rated at 1000 N.

The motor base has four quasi-direct-drive motors, 15 Nm peak, and 312 N continuous wire tension. Attach is push-to-click. Detach is rotate-and-pull.

They built four payloads for that one base:

  • 2-DoF rigid arm with a claimed 10 kg payload (three routed wires plus a Bowden cable for the tool)
  • Continuum arm of seven spring-plate modules
  • Parallel-jaw gripper
  • 3-finger adaptive gripper

Arm and gripper swaps on the shared motor base. Source: JSK / YouTube.

Ten seconds on, one second off

There are no joint encoders in the limbs. A state machine runs FREE → ATTACH → CALIBRATE → CONTROL. Joint angles are estimated from motor encoders and the geometric tendon map.

Bench numbers from the project page:

  • Tensile cycling to 450 N with no structural damage; friction scatter about ±16 N
  • Arm attach about 10 s; detach about 1 s
  • End-effector swap about 1 s
  • Across 225 postures, AprilTag RMSE of 0.087 rad (actual vs estimated) and 0.177 rad (actual vs reference)
Sequential attach, control, and detach of the rigid and continuum arms
Same motor unit: attach the rigid arm, then the continuum arm. Source: Hattori et al., project page.
Rigid arm swapping a parallel gripper and a three-finger gripper
Gripper swap on the rigid arm, then box and glue-stick picks. Source: Hattori et al., project page.

A Human’s Take

Sharing motors is an old factory idea. Doing it with tendons you can unplug is the part that made me look twice.

I care less about the 10 kg slide than about whether the 1-second detach still works after a few hundred hot swaps and a frayed Vectran line. The state machine is the real product. If ATTACH and CALIBRATE stay boring, this architecture is how a small lab owns one expensive actuator pack and a drawer of arms.

Sources

Tags: Robotics