REWW-ARM -- Remote Wire-Driven Mobile Robot:
Design, Control, and Experimental Validation

Modern robots rely on electronic components, which makes them vulnerable in harsh environments like disaster sites or areas with high radiation, moisture, or extreme temperatures. A novel solution to this problem is the "Remote Wire Drive" system, which separates the sensitive electronics from the mobile robot. This paper introduces REWW-ARM, a proof-of-concept robot that embodies this principle. It combines the high environmental resistance of wire-driven systems with the high reachability of mobile robots, all while being controlled by sophisticated electronics kept at a safe distance.

Mechanical Design

REWW-ARM consists of three core components: a motor-unit, a Remote Wire Transmission Mechanism (RWTM), and an electronics-free distal mobile robot.

1. Motor-Unit: This unit houses all the electronic components—motors, a computer, and power systems. It generates and controls the power transmitted to the mobile robot.

2. Remote Wire Transmission Mechanism (RWTM): This is the key innovation. The RWTM is a 4-meter-long, flexible component that transmits power and control signals mechanically via six wires made of high-performance Vectran fiber. Its crucial challenge is to transmit high tension through the wires without impeding the robot's free movement. The design cleverly achieves this by alternating two types of components:

   • Tendon-Sheath Mechanisms (TSMs): Similar to bicycle brake cables, these are flexible tubes that allow wires to pass through them. They provide flexibility, especially for twisting movements, but suffer from high friction, which causes significant power loss as they bend.
   • Decoupled Joints: These special gear-based joints allow bending without affecting wire tension or length. They have extremely high transmission efficiency (over 98%) but are rigid and only bend on a single axis.

   By combining these, the RWTM preferentially uses the highly efficient decoupled joints for bending, resorting to the TSMs for torsion and more complex contortions. This hybrid approach ensures high and stable power transmission efficiency while remaining flexible enough to follow the robot's movements.

   

3. Distal Mobile Robot: This electronics-free, snake-like robot is the part that enters the harsh environment. It is composed of several specialized modules that enable both locomotion and manipulation:

   • Gear-Coupled Dual-Axis Joints (GCD-Joints): Three compact, high-range-of-motion joints that allow the robot to bend and orient itself.
   • Variable-Stiffness Contract Links (VSC-Links): Two links that can change their length and rigidity. When rigid, they transfer motion from the joints; when flexible, they help the robot conform to surfaces. This is key for its locomotion.
   • Anchor-Gripper Integrated End-Effector (AGI-EE): A unique end-effector that serves as both a gripper for manipulating objects and an anchor to brace against the environment during its peristaltic locomotion.

   

Control Architecture

Controlling REWW-ARM is a unique challenge since the mobile robot has no sensors. The state of the robot (its joint angles) must be estimated remotely. The control system, running on the motor-unit's computer, uses an "Estimator-Follower" model.

• The Estimator continuously calculates the robot's joint angles by reading the motor angles and wire tensions. It uses a weighted least-squares method to solve for the most likely posture based on a model of how wire lengths correspond to joint angles.
• The Follower then compares these estimated angles to the desired target angles and calculates the precise wire tensions needed to move the robot into the target joint angle. This is achieved via a PID controller that outputs a target joint torque, which is then linearly transformed into tension commands for the six motors.

For locomotion, the robot uses a peristaltic motion sequence, coordinating the anchoring of the AGI-EE with the expansion and contraction of the VSC-Links to inch its way forward.

Experiments and Results

We conducted several experiments to validate REWW-ARM's design and capabilities.

• Transmission Efficiency: A detailed evaluation confirmed the superiority of the hybrid RWTM design. It maintained a high and stable average efficiency of 88.4%, far outperforming a pure TSM system (66.9%), whose efficiency dropped sharply with bending.

• Locomotion and Manipulation: The robot successfully demonstrated both straight-line and rotational locomotion, moving 0.45 meters and turning 15 degrees. It also proved capable of grasping a variety of objects, from balls and bottles to flat boards.

• Underwater Operation: In a key demonstration of its environmental resistance, the entire distal robot and part of the RWTM were submerged in water. The robot successfully performed complex maneuvers—including grasping, rolling, and translating—unaffected by the water, proving the viability of the electronics-free concept for underwater applications.

While the controller was successful, experiments revealed positioning errors (average 0.42 m) primarily due to unmodeled friction and hysteresis in the long transmission system, a target for future improvement. The research successfully demonstrates that the Remote Wire Drive system is a promising approach for creating robust, capable robots for the hazardous environments.

Bibtex

                
@article{hattori2024rewwarm,
  title={{REWW-ARM -- Remote Wire-Driven Mobile Robot: Design, Control, and Experimental Validation}},
  author={Takahiro Hattori and Kento Kawaharazuka and Temma Suzuki and Keita Yoneda and Kei Okada},
  journal = {Advanced Intelligent Systems},
  volume = {n/a},
  number = {n/a},
  pages = {e202501034},
  keywords = {nuclear robot, remote drive, snake robot, transmission mechanism, underwater robot, wire-driven},
  doi = {https://doi.org/10.1002/aisy.202501034},
  url = {https://advanced.onlinelibrary.wiley.com/doi/abs/10.1002/aisy.202501034},
  eprint = {https://advanced.onlinelibrary.wiley.com/doi/pdf/10.1002/aisy.202501034},
}

              

Contact

If you have any questions, please feel free to contact Takahiro Hattori (gmail: t-hattori @ jsk.imi.i.u-tokyo.ac.jp).