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Switchable Adhesion Actuator for Amphibious Climbing Soft Robot
Yichao Tang, Qiuting Zhang, Gaojian Lin, Jie Yin
TL;DR
Soft climbing robots need fast, robust, switchable adhesion for attachment and detachment on vertical surfaces. This paper introduces a pneumatic bilayer actuator that domes under positive pressure to create cavity suction, then combines it with a bending actuator in an amphibious climbing robot. The robot climbs and walks across varied dry, wet, slippery, smooth, and semi-smooth surfaces on ground and underwater while carrying loads.
Problem
Fast, robust, switchable adhesion for soft robots remains challenging, especially for amphibious climbing across vertical surfaces.
Method
The paper combines a bilayer Ecoflex actuator with an embedded spiral channel and cavity with a soft bending actuator to build an amphibious climbing robot.
Results
The robot walks horizontally and climbs vertically on smooth, semi-smooth, dry, wet, and slippery substrates on ground and underwater with load-carrying capability.
Takeaways & Limitations
The actuator supports a proof-of-concept amphibious soft robot for climbing and walking across varied substrates on ground and underwater.
Takeaways & Limitations
The idealized Equation (3) does not account for the internal cavity pressure drop caused by volume change.
Abstract
from arXiv · showhide
Climbing soft robots are of tremendous interest in both science and engineering due to their potential applications in intelligent surveillance, inspection, maintenance, and detection under environments away from the ground. The challenge lies in the design of a fast, robust, switchable adhesion actuator to easily attach and detach the vertical surfaces. Here, we propose a new design of pneumatic-actuated bioinspired soft adhesion actuator working both on ground and under water. It is composed of extremely soft bilayer structures with an embedded spiral pneumatic channel resting on top of a base layer with a cavity. Rather than the traditional way of directly pumping air out of the cavity for suction in hard polymer-based adhesion actuator, we inflate air into the top spiral channel to deform into a stable 3D domed shape for achieving negative pressure in the cavity. The characterization of the maximum shear adhesion force of the proposed soft adhesion actuator shows strong and rapid reversible adhesion on multiple types of smooth and semi-smooth surfaces. Based on the switchable adhesion actuator, we design and fabricate a novel load-carrying amphibious climbing soft robot (ACSR) by combining with a soft bending actuator. We demonstrate that it can operate on a wide range of foreign horizontal and vertical surfaces including dry, wet, slippery, smooth, and semi-smooth ones on ground and also under water with certain load-carrying capability. We show that the vertical climbing speed can reach about 286 mm/min (1.6 body length/min) while carrying over 200g object (over 5 times the weight of ACSR itself) during climbing on ground and under water. This research could largely push the boundaries of soft robot capabilities and multifunctionality in window cleaning and underwater inspection under harsh environment.
Introduction
The paper addresses amphibious soft climbing on ground and underwater surfaces, where fast, strong, switchable adhesion and load carrying remain challenging. It proposes a pneumatic soft adhesion actuator and demonstrates an amphibious robot operating across varied surfaces.
- Motivation: Amphibious soft climbing robots remain underexplored on ground and underwater surfaces.
- Motivation: Climbing robots could support surveillance, inspection, maintenance, and detection in environments away from the ground.
- Challenge: Fast, strong, switchable adhesion must enable attachment, detachment, and load carrying across complex working environments.
- Limitations of existing approaches: Existing gecko-inspired adhesives are challenging for wet adhesion, while directly transferring air-pumping suction to soft robots risks structural collapse.
- Proposed approach: The proposed Ecoflex bilayer actuator inflates a top spiral channel to form a stable 3D dome and generate switchable dry/wet adhesion without collapse.
- Demonstration: An inchworm-inspired amphibious robot demonstrates vertical climbing and underwater walking across smooth, semi-smooth, dry, wet, and slippery surfaces with load carrying.
Working mechanism of soft adhesion actuators
The actuator generates adhesion by inflating an embedded spiral channel, which domes the bilayer and lowers pressure in the underlying cavity. Pressurizing and depressurizing the channel rapidly switch adhesion on and off while avoiding soft-structure collapse.
- Structure: The actuator uses a bilayer structure with a top spiral pneumatic channel and an underlying cylindrical cavity.
- Doming mechanism: Inflating the top channel creates mismatched layer expansion that transforms the planar actuator into a 3D dome.
- Pressure generation: The domed cavity expands from V_o to V_o + ΔV, producing a pressure drop that generates adhesion against the external surface.
- Tunable adhesion: Adhesion depends mainly on the internal-external pressure difference and can be tuned through channel size, layer thicknesses, and cavity volume.
- Switching: Pressurizing and depressurizing the spiral channel rapidly and reversibly switch adhesion on and off.
- Structural stability: Unlike direct cavity evacuation, positive-pressure actuation is more stable and controllable because inflation stiffens the soft structure.
Modeling of adhesion actuator as simplified bilayer doming system
The actuator is modeled as a simplified bilayer doming system in which pressurization-induced mismatched expansion produces dome deformation and cavity volume change. The model predicts cavity pressure change, with radial expansion mismatch as the dominant determinant.
- Bilayer model: A continuum bilayer model represents the actuator as a circular thin film on a substrate with nonuniform, axisymmetric radial misfit strain.The model neglects the detailed channeled structure while retaining pressurization-induced mismatched expansion.
- Bilayer model: The dome height is obtained from the radial displacement field under the boundary condition u_z(R) = 0.The displacement formulation uses Young’s modulus, Poisson’s ratio, radial position, and film/substrate subscripts.
- Volume and pressure prediction: Cavity volume change is calculated by integrating the normal displacement over the actuator radius.The idealized expression ignores radial displacement because the cavity wall is assumed sufficiently thick to limit whole-structure radial expansion.
- Volume and pressure prediction: The model predicts cavity pressure change from the calculated volume change and the pressure difference between cavity air and atmosphere.The radial expansion mismatch ε_m(r) between the layers plays the dominant role in determining the pressure change.
Effect of the geometric parameters on adhesion
Adhesion depends on actuator geometry and inflation. Increasing inflation produces dome formation and rising shear adhesion until a plateau, while geometric parameters govern the trade-off between adhesion strength and conformability.
- Inflation response: Maximum shear adhesion rises monotonically with inflation and approaches a plateau at an actuation pressure of 62 kPa.The actuator progressively deforms into a dome as input air volume increases.
- Geometric design: A moderate h1/h0 and h2, together with a relatively high spiral-channel revolution count, is recommended for high and robust adhesion.These parameters govern the doming deformation and the resulting maximum shear adhesion force S_max.
- Spiral-channel density: Increasing the spiral-channel revolution count increases maximum shear adhesion until it approaches a plateau.The actuator uses 4 revolutions in the spiral channel for subsequent adhesion testing.
Amphibious Climbing Soft Robot (ACSR)
The ACSR combines two switchable adhesion actuators with a central pneumatic bending actuator and a slider-based support structure. Sequential pressurization and depressurization produce inchworm-like locomotion with substantial translation and load-carrying capability.
- Robot architecture: The ACSR uses adhesion actuators at both ends and a central pneumatic bending actuator to attach, detach, and locomote.The adhesion actuators provide switchable attachment, while the bending actuator mimics the inchworm body.
- Robot architecture: The bending actuator uses embedded rectangular wave-like channels and a strain-limiting layer to bend and unbend the soft body.A hard PLA slider connects the adhesion actuators, permits in-plane translation, restricts rotation, and adds support and stability.
- Locomotion performance: One cycle moves the robot about 38 mm, with a locomotion speed of about 286 mm/min.The motion can be tuned by varying pneumatic pressurization and geometric parameters.
Climbing soft robots on multiple types of surfaces
The adhesion actuator supports robot operation across dry, wet, slippery, smooth, and semi-smooth surfaces. Surface-material changes have limited effect on dry smooth substrates, while semi-smooth climbing requires bottom-surface flattening to improve contact.
- Wet and slippery surfaces: Wet acrylic produces approximately 6.96 N adhesion versus 7.97 N on dry acrylic, while lubricated acrylic produces 6.23 N under the tested condition.The actuator therefore retains adhesion on wet and slippery surfaces, although lubrication amount contributes to measurement variation.
- Semi-smooth surfaces: Semi-smooth sandpaper with Sa = 17.43 μm produces 8.95 N maximum shear adhesion, exceeding the measured values on smooth surfaces.The enhanced adhesion is attributed to greater friction on the semi-smooth surface.
- Climbing demonstrations: The robot climbs sandpaper and an indoor painted wall while carrying a 200 g load.The reported demonstrations include dry, wet, slippery, and semi-rough surfaces.
- Surface preparation: Flattening the adhesion-actuator bottoms is necessary for semi-smooth climbing because it increases direct contact, reduces air leakage, and improves conformability and friction.This treatment promotes adhesion to foreign surfaces.
Climbing and walking under water
The soft adhesion actuator enabled amphibious walking and climbing, including underwater operation on smooth glass with load-carrying capability. Underwater adhesion was measured at ~10.62N, exceeding the dry value on glass, although the mechanism for this enhancement remains unresolved.
- Underwater demonstrations: The actuator was applied to an amphibious soft robot for underwater walking and climbing demonstrations.The robot operated smoothly on a glass surface underwater while carrying a certain load.
- Underwater adhesion: ~10.62N maximum shear adhesion force was measured underwater on glass at 62kPa pressurization.This underwater value was obtained using the characterization method shown in Fig. S7.
- Underwater adhesion: ~7.51N dry adhesion on glass was lower than the measured underwater adhesion.The passage directly compares the underwater and air measurements for the same actuator and glass surface.
- Proposed mechanism: Increased cavity volume may lower internal water pressure and enlarge the ambient-to-cavity pressure difference underwater.The authors propose that water pulled into tension during cavity expansion contributes to firmer attachment.
- Scope: The enhanced underwater adhesion mechanism remains scheduled for more detailed future study.The authors identify this as an unresolved aspect of the current work.
Conclusion
The study presents a proof-of-concept amphibious soft robot that walks horizontally and climbs vertically across varied surfaces on ground and underwater. Its embedded spiral-channel adhesion actuator supports switchable, strong adhesion, while the current robot remains limited by single-degree-of-freedom motion.
- Conclusion: The proof-of-concept amphibious soft robot walks horizontally and climbs vertically on varied surfaces both on ground and underwater.Demonstrated substrates include smooth, semi-smooth, dry, wet, and slippery surfaces, with load-carrying capability.
- Conclusion: Embedded spiral pneumatic channels provide switchable, strong, and mechanically robust adhesion across different surfaces upon pressurization.The actuator is presented as the central design basis for the robot’s demonstrated capabilities.
- Implications: The soft adhesion actuator provides a platform for soft robots operating on vertical surfaces and underwater.The authors identify potential applications including object transportation, wall-cleaning, camouflage machines, and underwater soft machines.
- Limitations: The current robot has only one degree of freedom because it uses a classic pneumatic bending actuator.The study prioritizes adhesion behavior over developing a high-degree-of-freedom driving actuator.
- Future work: The driving actuator determines gait and locomoting efficiency, motivating future development of faster speed and 3D mobility.Planned capabilities include turning, ceiling locomotion, and switching between dimensions.