REWW-ARM: Remote Wire-Driven Mobile Robot

• REWW-ARM is a novel robotics platform employing a Remote Wire Transmission Mechanism for electronics-free distal actuation in exposed environments.
• It integrates a separated motor base with a distal mobile robot via high-performance Vectran wires, achieving high transmission efficiency and reliable control.
• The system demonstrates robust locomotion, posture control, and object manipulation even under extreme conditions such as radiation, underwater, or high temperatures.

The Remote Wire-Driven Mobile Robot (REWW-ARM) is a novel robotics platform that achieves electronics-free distal actuation for mobile robots via the “Remote Wire Drive” system. REWW-ARM integrates a new power transmission mechanism, the Remote Wire Transmission Mechanism (RWTM), enabling advanced locomotion, posture control, and object manipulation in environments inaccessible to conventional electrically actuated robots. Fully electronics-free at the mobile tip, REWW-ARM consolidates sensing, control, and actuation in a motor-base located remotely from the harsh environment, while transmitting mechanical power and feedback through high-efficiency wire mechanisms over meter-scale distances (Hattori et al., 5 Dec 2025).

1. System Composition and Architecture

REWW-ARM comprises three functionally separated subsystems connected via a set of six Vectran wires traversing a 4 m long RWTM:

• Motor-Unit (Base): Houses all electronic hardware, including six CAN-controlled Steadywin GIM8108-8 winch modules with a winding pulley diameter $d_p = 0.019\,\mathrm{m}$, computer, and power electronics. The motors generate and regulate all wire tensions.
• Remote Wire Transmission Mechanism (RWTM): Alternating sequence of $N=4$ decoupled dual-axis joints and PTFE-lined tendon-sheath segments, yielding high flexibility and near-constant transmission efficiency. The Vectran fibers (1 mm diameter, $\epsilon=0.000185\,\mathrm{m/N}$) provide high mechanical robustness.
• Electronics-Free Distal Mobile Robot: Features three gear-coupled dual-axis joints (GCD-Joints, $\pm130^\circ$ range), two variable-stiffness contract links (VSC-Links, $0.185\!-\!0.302$ m), and an anchor-gripper integrated end-effector (AGI-EE). All actuation is wire-driven and coordinated exclusively from the base.

This separation of electronics and mobility enables operation in environments with moisture, radiation, extreme temperature, or particulates that preclude local electronic control.

2. Remote Wire Transmission Mechanism: Design, Materials, and Modeling

2.1 Mechanical Design

Each decoupled joint in the RWTM utilizes two coaxial axes geared in opposite senses, ensuring constant pulley wrap with diameter $\phi\, 20\,\mathrm{mm}$, coupled to a $30\,\mathrm{mm}$ pitch radius gear. The friction efficiency per decoupled joint is $\eta_{dj} \approx 0.98-0.99$ and the range is $\pm3\pi/4$ rad. Tendon-sheath segments consist of PTFE-lined tubes facilitating low-friction, flexible power transmission.

The wire path alternates as [TSM]–[decoupled joint]–[TSM]–... through four dual-axis joints, providing high compliance without sacrificing transmission quality.

2.2 Materials and Mass Parameters

• Wire: Vectran fiber, offering high durability ($>4\times10^4$ cycles), resistance to radiation, chemicals, and temperature range $N=4$0 to $N=4$1.
• Decoupled Joint: Mass $N=4$2, length $N=4$3.
• TSM Bundle: Mass per length $N=4$4.

2.3 Transmission Efficiency and Mathematical Modeling

• TSM Capstan Efficiency: $N=4$5, $N=4$6, $N=4$7 total bend angle in radians.
• Total Efficiency: For $N=4$8 decoupled joints and sheath bending,

$N=4$9

• Force and Elongation:

$\epsilon=0.000185\,\mathrm{m/N}$0

This enables translation of remote torque inputs to well-characterized distal tip forces, with high overall efficiency ($\epsilon=0.000185\,\mathrm{m/N}$1 in the full system).

3. Distal Mobile Robot: Actuation and Structural Features

3.1 Joint and Link Design

• GCD-Joints: Dual-axis, coupled via gear trains, wire tracks designed for constant moment arms, $\epsilon=0.000185\,\mathrm{m/N}$2 constant to within 4%.
• VSC-Links: Four articulated modules with internal springs ($\epsilon=0.000185\,\mathrm{m/N}$3), allowing transition between compliant and locked-rigid mode through differential wire tension.
• AGI-EE (End-Effector): Two-finger gripper actuated by a linear wire path (close at 450 N tension, open via spring at 149 N), mechanically latches in the open state to withstand up to $\epsilon=0.000185\,\mathrm{m/N}$4 external torque.

3.2 Actuation Morphology

• Wire-Joint Kinematics: For joints 1–3 (actuated by 4 wires):

$\epsilon=0.000185\,\mathrm{m/N}$5

with $\epsilon=0.000185\,\mathrm{m/N}$6 (joint angles), $\epsilon=0.000185\,\mathrm{m/N}$7 (tensions), $\epsilon=0.000185\,\mathrm{m/N}$8 constant ($\epsilon=0.000185\,\mathrm{m/N}$9).

• Special Functions: Distal VSC-Link contracts at 300 N tension; AGI-EE closes at 450 N tension.

All distal actuation avoids onboard electronics, enabling robust operation in previously inaccessible domains.

4. Motor-Unit Control, State Estimation, and Control Policy

4.1 Sensing and Measurements

• Base Sensing: Motor shaft angle $\pm130^\circ$0 and current $\pm130^\circ$1 (for tension detection via $\pm130^\circ$2).
• Distal Estimation: No electronics or direct sensors at the distal end; joint angles and states are estimated solely by wire extension and current feedback.

4.2 Wire State Estimation

Estimation proceeds in three steps:

1. Wire Extension: $\pm130^\circ$3
2. Centering: $\pm130^\circ$4 to correct for VSC-Link drift.
3. Weighted Least Squares Joint Estimation:

$\pm130^\circ$5

with $\pm130^\circ$6 weighted by tension.

4.3 Control Law

• Desired Joint Torque via PID:

$\pm130^\circ$7

• Wire Tension Optimization:

$\pm130^\circ$8

with $\pm130^\circ$9, $0.185\!-\!0.302$0, $0.185\!-\!0.302$1.

• Torque Quantization: (Optional) Discrete steps of $0.185\!-\!0.302$2m for hysteresis suppression.

5. Experimental Results and Performance Evaluation

5.1 Transmission Measurements

• Pure TSM Efficiency: Mean $0.185\!-\!0.302$3, std $0.185\!-\!0.302$4 (for bend $0.185\!-\!0.302$5 up to $0.185\!-\!0.302$6).
• Four Decoupled Joints: $0.185\!-\!0.302$7 (across $0.185\!-\!0.302$8 up to $0.185\!-\!0.302$9).
• Full RWTM: $\phi\, 20\,\mathrm{mm}$0, exceeding the efficiency of TSM alone in $\phi\, 20\,\mathrm{mm}$1 of tested bending configurations.

5.2 Controller and Locomotion Performance

• Controller Tracking: For random joint commands ($\phi\, 20\,\mathrm{mm}$2 rad, 10 Hz), integrated NCC = 0.596, MSE = $\phi\, 20\,\mathrm{mm}$3. End-effector (EE) mean errors: $\phi\, 20\,\mathrm{mm}$4 (position), $\phi\, 20\,\mathrm{mm}$5 (orientation).
• Ground Locomotion: Peristaltic crawl (10 cycles in 110 s) covers $\phi\, 20\,\mathrm{mm}$6, with on-the-spot turns of $\phi\, 20\,\mathrm{mm}$7 via selective joint actuation.
• Underwater Operation: Full distal robot submerged to $\phi\, 20\,\mathrm{mm}$8 demonstrated grasping a $\phi\, 20\,\mathrm{mm}$9 baseball, ~45$30\,\mathrm{mm}$0 yawing, small translational and ~90$30\,\mathrm{mm}$1 roll adjustments, all without performance degradation.

6. Applications, Comparative Advantages, and Limitations

6.1 Advantages Over Preceding Methods

• Environment Robustness: Electronics-free at the distal tip, enabling use in nuclear, deep-sea, high-temperature, or particulate-laden environments.
• No Hydraulic Fluid: Eliminates leakage and temperature limitations inherent to remote hydraulics and simplifies maintenance compared to fluid-filled systems.
• Transmission Range and Efficiency: Demonstrated effective power transmission at 4 m reach, overall transmission efficiency $30\,\mathrm{mm}$2, exceeding that of classic Bowden cable (typically $30\,\mathrm{mm}$3 under bend).

6.2 Principal Limitations

• Friction and Hysteresis: Compliance in the RWTM introduces joint estimation and actuation errors, with mean EE position error $30\,\mathrm{mm}$4.
• Bandwidth Constraints: Mechanical compliance and transmission delay limit the dynamic response.
• System Mass: RWTM mass (cumulatively $30\,\mathrm{mm}$5) imposes requirements for sufficient distal traction and support.

6.3 Prospects for Future Development

• Friction Compensation: Advanced modeling and feedforward/adaptive control for millimeter-scale precision.
• Transmission Optimization: Placement and number of joints/sheath segments guided by efficiency trade-off curves (typically maintaining $30\,\mathrm{mm}$6).
• Sensing Integration: Lightweight distal feedback (optical fiducials, fiber Bragg gratings).
• Application Domains: Plausible targets include continuum manipulators, walking robots, and pipeline crawlers deployed in environments with high radiation, undersea depths, or high temperatures.

The REWW-ARM system constitutes the first demonstration of the Remote Wire Drive paradigm, combining advantages of remote hydraulics and wire-driven robotics, with high reach, environmental resistance, and base-localized control electronics, substantiated by experimental validation in ground and underwater scenarios (Hattori et al., 5 Dec 2025).