Research Article Summary: An Eel-like Robot Based on a Dielectric Elastomer

Objective

To design, model, manufacture, and analyze the performance of an eel-inspired underwater robot using dielectric elastomer (DE) actuators for propulsion. The robot aims to achieve high efficiency, maneuverability, and stability, inspired by the swimming behavior of real eels.

Key Points

1. Background and Significance

• Eels as Inspiration: Noted for their long-distance migrations and endurance; their motion is efficient for underwater robots (lowest energy consumption per unit distance).
• Dielectric Elastomers (DEs): Soft, smart materials with large strain, fast response, light mass; capable of significant deformation under electric fields.
• Applications: DEs are used in soft robots, loudspeakers, sensors, artificial muscles, and bionic robots due to their excellent actuation properties.

2. Previous Work

• Existing Eel-like Robots: Various implementations globally (Norway, Switzerland, USA), using different actuation technologies (shape memory alloys, DE actuators), and modular body designs.

3. Cylindrical Dynamics Model of DE

• Model Development: Established a cylindrical dynamics model for DE to analyze deformation under electric fields and radial forces.
• Assumptions:
  • Constant temperature environment.
  • Incompressible material (Poisson’s ratio).
  • Gent function for strain energy.
  • Radial force perturbations modeled as Gaussian white noise.

4. Mathematical & Kinematic Modeling

Eel Robot Motion Equation

• Traveling Wave Principle: Eel motion modeled as a traveling wave, increasing in amplitude from head to tail, with each joint actuated by a gait equation (sinusoidal pattern).
• Equation Parameters: Amplitude, angular frequency, phase shift, joint offset control the robot’s movement and shape.

Kinematic Model

• Assumptions: One-dimensional transverse wave along body; body can elongate during swimming.
• Motion Equation: Propagation of a sine wave along the body to simulate realistic eel-like swimming.

5. Design and Structure

Overall Design

• Robot Specifications: Length: 273 mm, Height: 35.3 mm, Weight: 26.83g, Volume: 84000 mm³.
• Composition: Five connected drive modules, head, tail, and tail fin.

Tube Actuator and Connection Mechanism

• Actuator Size: Circular frame of 30 mm diameter, 3 mm thickness.
• Installation: DE is attached to top/bottom of frame; lightweight materials used for underwater operation.
• Design Feature: Removable/replaceable tandem actuator modules for ease of repair.

Head and Tail Fin Design

• Shape: Smooth to reduce fluid resistance.
• Materials: CFRP (carbon fiber reinforced plastic) for tail fin.

6. Robot Performance Analysis

Experimental Method

• Setup: Robot actuated in Fluorinert insulating liquid using 6.0 kV sinewave at different frequencies (0.1–10 Hz).
• Measurement: Speed recorded via camera, displacement tracked for kinematic analysis.

Propulsion Speed

• Maximum Speed: 43.7 mm/s at 1.5 Hz (0.16 body lengths/s).
• Comparative Performance: Faster than previously reported DE-based fish robots.
• Influence Factors:
  • Increased internal mass (fluid intrusion) reduces speed at higher frequencies.
  • Maximum body swing increases force, resistance, and energy consumption.

Swim Number (Efficiency)

• Definition: Ratio of body length traveled to tail fin’s push distance (efficiency metric).
• Result: Max value at 0.5 Hz: 0.15 (compared to ~0.6 for real fish).
• Cause of Low Efficiency: Excess lateral motion, inefficient usage of body length.

Swimming Shape Analysis

• Findings: Head amplitude (vibration) is higher than in real fish; tail fin amplitude should be higher for optimal propulsion.
• Improvement Needed: Adjust mass/rigidity distribution to better mimic fish movement.

7. Conclusions

• Soft Robotics Advantage: Reduces risk to environment and living beings.
• Robot Achievements:
  • Novel tube joint actuator for DE.
  • Detachable series-connect mechanism.
  • Demonstrated swimming in insulating fluid; practical underwater performance.
  • Max speed: 43.7 mm/s.

8. Discussion & Future Directions

• Challenges:
  • Weight increase from fluid intrusion.
  • Excessive head vibration impacts efficiency.
• Future Improvements:
  • Optimize actuator frequency response.
  • Implement sealed robot structure.
  • Adjust mass/rigidity for more realistic swimming shape.

References

A comprehensive list of related works on DE actuators, soft robotics, bionic fish robots, and swimming kinematics for context and validation.

Author Contributions

Complete design, analysis, execution, and writing by Chenghong Zhang.

For further details, refer to:
PLoS One | https://doi.org/10.1371/journal.pone.0324738