Source note
MM-Hand: A 21-DOF Multi-modal Modular Dexterous Robotic Hand with Remote Actuation
Summary
MM-Hand is an open-source 21-DOF dexterous robot hand that moves its fingers through long tendon-sheath cables from a remote motor hub. The design trades some delay and friction for lower hand mass, more internal space, easier maintenance, and more sensing in the hand.
Problem
- High-DOF robot hands need many actuators and sensors, but putting motors inside the hand adds mass, uses up space, and creates heat near the end effector.
- Remote tendon actuation can move motors away from the hand, but long tendon-sheath paths add friction, hysteresis, and arm-motion-dependent length changes that hurt control accuracy.
- A practical research hand also needs to be easy to build, repair, and extend with sensing hardware.
Approach
- The paper builds a 21-DOF, 5-finger hand with remote tendon-driven actuation, mostly using single-tendon spring-return joints and an antagonistic dual-tendon design for thumb base rotation.
- It analyzes two main transmission effects: tendon length change from total sheath bending angle and friction loss that grows exponentially with bending angle under a capstan-style model.
- The mechanical design is modular and 3D-printed, with split phalanges, internal routing space, and quick tendon connectors in the palm so fingers or sheaths can be replaced without rerouting the full system.
- The sensing stack includes joint encoders, tactile sensors, motor-side feedback, and in-palm stereo cameras. Control uses joint-mounted absolute encoders with PID closed-loop control instead of relying on an accurate feedforward tendon model.
- The system uses software pretensioning for spring-return joints, reeling tendons back when encoder readings indicate slack.
Results
- The hand reaches 25 N fingertip force with a 1 m remote sheath and about 33 N with a 0.1 m sheath, so the longer routing costs about 8 N of peak force in this test.
- In step-response joint control, the reported steady-state error is below 0.1°, with about 0.2 s delay between motor actuation onset and joint motion onset because of tendon-sheath friction.
- Dynamic tracking was tested with a 0.5 Hz sinusoidal command on one joint for over 20 s, both with the arm fixed and while the arm moved through the workspace. Tracking got worse during arm motion, but the paper states that friction-caused delay had a larger effect than arm-motion disturbance.
- Friction experiments tested 4 sheath types, 0° to 180° wrap angles, and 10 to 100 mm disk diameters. The measured kinetic friction followed the paper's predicted exponential trend with bending angle, and the authors selected a metal spring tube for later tests based on balanced friction and simplicity.
- The hand also demonstrated in-palm stereo depth sensing using RAFT-Stereo, but the excerpt gives no quantitative depth accuracy numbers.