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Leveraging Elastic instabilities for Amplified Performance: spine-inspired high-speed and high-force soft robots
Yichao Tang, Yinding Chi, Jiefeng Sun, Tzu-Hao Huang, Omid H. Maghsoudi, Andrew Spence, Jianguo Zhao, Hao Su, Jie Yin
TL;DR
Soft robots face limited speed and manipulation strength because of their soft bodies. This paper uses tunable snap-through bistability for rapid energy storage and release, demonstrating faster terrestrial and underwater locomotion and stronger, tunable manipulation.
Problem
Soft robots remain challenged in achieving high-speed locomotion and high-strength manipulation because of intrinsic limitations of their soft bodies.
Method
The paper introduces a spine-inspired soft actuator with tunable bistability that rapidly stores and releases energy for versatile soft machines.
Results
Bistable machines demonstrate 174.4 mm/s terrestrial locomotion, 0.78 BL/s underwater swimming, and 20 times faster response with over 3 times higher exertion force than conventional stable actuators.
Takeaways & Limitations
The bistability strategy provides a generic design paradigm for multifunctional soft robots across different actuation methods and materials.
Takeaways & Limitations
Inducing bistability can make bending motion harder to control, so this work avoids controlling the bending angle after snap-through begins.
Abstract
from arXiv · showhide
Soft machines typically exhibit slow locomotion speed and low manipulation strength because of intrinsic limitations of soft materials. Here, we present a generic design principle that harnesses mechanical instability for a variety of spine-inspired fast and strong soft machines. Unlike most current soft robots that are designed as inherently and unimodally stable, our design leverages tunable snap-through bistability to fully explore the ability of soft robots to rapidly store and release energy within tens of milliseconds. We demonstrate this generic design principle with three high-performance soft machines: High-speed cheetah-like galloping crawlers with locomotion speeds of 2.68 body length/s, high-speed underwater swimmers (0.78 body length/s), and tunable low-to-high-force soft grippers with over 1 to 103 stiffness modulation (maximum load capacity is 11.4 kg). Our study establishes a new generic design paradigm of next-generation high-performance soft robots that are applicable for multifunctionality, different actuation methods, and materials at multiscales.
INTRODUCTION
Soft robots offer safe, adaptive interaction and capabilities such as delicate manipulation and confined-space navigation, but their soft bodies make high-speed locomotion and high-strength manipulation difficult. The paper addresses this challenge with a spine-inspired hybrid actuator that combines bistable linkages, pneumatic bending actuators, and tunable energy storage.
- Soft robots enable safe interaction, delicate-object manipulation, confined-space navigation, and multiple degrees of freedom.
- High-speed locomotion and high-strength manipulation remain challenging because soft bodies intrinsically limit performance.High-speed locomotion additionally requires fast response, large force output, high strain-energy storage, and precise motion.
- Galloping cheetahs motivate spine-inspired designs because spine flexion and extension support self-stabilization and elastic energy storage during fast locomotion.Galloping is used at the highest running speeds among quadrupedal gaits and has been suggested as energy efficient.
- Relaxing the usual unimodal-stability constraint allows soft robots to exploit elastic instability for rapid energy storage and release.Prior bistable soft-robot applications included directional propulsion, airflow control, amplified fluidic actuation, and high-power-density hydrodynamic interaction.
- The proposed spine-inspired actuator combines spring-based bistable linkages with soft pneumatic bending actuators to produce versatile fast and strong soft machines.The linkage acts as a skeletal spine, the pneumatic actuator as skeletal muscle, and spring pretension or stiffness tunes energy-storage capacity.
1. Design Principles of a Bistable Hybrid Soft Actuator
The actuator combines a spring-based bistable spine with pneumatic bending actuators to store and rapidly release energy through tunable snap-through. Spring stiffness and pretension shape equilibrium states, energy barriers, actuation pressure, stiffness, force, and response time.
- Actuator architecture: The BH-SBA combines a pre-tensioned spring and hinged spine linkages with pneumatic bending actuators to enable reversible, switchable two-way bending.Dual pneumatic channels actuate the bistable spine mechanism, while the spring stores mechanical energy for snap-through.
- Energy landscape: Bistability arises from competition between the soft actuator’s strain energy and the spring’s stretching energy, producing two stable states around an unstable central state.The total potential energy is the sum of actuator and spring energies; stable states occur at θ = ±θeq, while θ = 0° is unstable.
- Tunable bistability: Increasing spring stiffness or pretension increases the equilibrium angle and energy barrier nonlinearly, provided the stored energy exceeds the threshold needed to trigger instability.Bistability requires a positive energy barrier, ΔE > 0; below the threshold, the actuator does not bend after pretension release.
- Tunable bistability: 10 N/m to over 500 N/m stiffness modulation is achieved by tuning pretension length from about 9 to 17 mm, enabling low-to-high-force manipulation.The stiffness is defined from the change in blocking force with deflection and is presented as a tunable actuator property.
- Actuation control: A 1.5-fold pretension increase from approximately 5.6 to 8.6 mm raises critical actuation pressure from about 10 to 34 kPa, exceeding a 3.4-fold increase.The pressure increase is nonlinear and reflects the larger energy barrier associated with greater pretension.
- Performance amplification: The BH-SBA produces the highest dynamic blocking force among compared actuators, reaching 2.5–3.4 N at 8 mm pretension, about 30% above the 6 mm condition.Greater spring potential energy increases blocking force, but higher pretension also requires higher critical pneumatic pressure.
- Performance amplification: Less than 1 s response time enables BH-SBAs to switch between negative and positive resting angles, whereas the H-SBA requires 2.60 s for its tested bend.At 30 kPa and approximately 3 L/min, snap-through divides the response into pre- and post-transition phases.
2. Bistability for High-Speed Crawler
A cheetah-inspired bistable spine crawler converts snap-through energy release into fast terrestrial locomotion. Spring pretension tunes its force output and speed, while passive friction switching drives forward motion.
- Crawler design and performance: Passive friction switching between forefeet and rear feet generates forward motion as the crawler alternates between downward- and upward-bent shapes.The friction asymmetry reverses between the two body configurations, supporting directional locomotion.
- Crawler design and performance: 174.4 mm/s, or 2.49 BL/s, is achieved by the bistable hybrid crawler at 20 kPa on a wooden surface.The crawler operates at an average actuation frequency of 3.2 Hz and reaches 2.1 times the speed of its SBA counterpart.
- Tunable locomotion: Spring pretension increases the energy barrier, dynamic blocking force, ground reaction force, and locomotion speed.Among crawlers with ∆xI = 6 mm, 7 mm, and 8 mm, the largest pretension produces the highest locomotion velocity at 30 kPa.
- Performance comparison: The bistable crawler exceeds 2.5 times the speed of a high-energy-density dielectric crawler while operating at only 20 kPa.The comparison includes representative soft and hybrid soft robots categorized by body-length speed and actuation frequency.
3. Bistability for High-Speed Underwater Swimmer
A fin-equipped, encapsulated bistable actuator extends the spine-inspired design to underwater swimming. Snap-through bistability increases both actuator force and swimming speed relative to springless counterparts.
- Swimmer design: The swimmer combines an encapsulated bistable hybrid actuator with a 0.25 mm-thick plastic-sheet fin to enhance propulsion force.
- Actuator performance: Over 4 N maximum dynamic blocking force and a 45° maximum bending angle are produced by the bistable hybrid actuator at 160 kPa.Both springless hybrid and soft counterparts produce smaller force and bending angles.
- Swimming performance: 0.78 BL/s is achieved by the bistable hybrid swimmer at 160 kPa and 1.3 Hz.This speed is 32% and 122% faster than the soft and hybrid springless swimmers, respectively.
- Performance comparison: 0.78 BL/s at 1.3 Hz outperforms most reported soft swimmers despite their different actuation methods and frequencies.The proposed swimmer remains slower than biological fishes, typically reported at 2–10 BL/s.
4. High-Force Bistable Soft Gripper with Tunable Stiffness
The actuator’s monostable regime enables a strength-adjustable gripper with programmable stiffness and dual actuation. The paper demonstrates a broad application range while identifying control and energy-storage boundaries.
- Gripper principle: The monostable regime enables a high-force soft gripper with wide-range stiffness modulation.The gripper uses the same bistable hybrid actuator architecture in a monostable operating regime.
- Motivation: Variable stiffness supports shape maintenance and adaptable force exertion across different working environments.The paper contrasts this strategy with granular jamming, phase changes, and tendon-driven stiffening approaches.
- Gripper design: Independent pneumatic and tendon actuation supports delicate-object handling and stiff, heavy-object manipulation.A DC motor drives the tendon attached to the extension spring.
- Gripper performance: Spring stiffness primarily determines grasping capacity, so higher-stiffness springs can further improve load handling.
- Limitations and future improvements: Snap-through bistability makes bending-angle control more difficult because the pneumatic actuator has low bandwidth.The demonstrated system intentionally avoids controlling bending angle after snap-through onset.
- Limitations and future improvements: 1.2 J/kg stored energy density and 8 W peak power remain below combustion-based jumping soft actuators.Increasing energy storage density and peak power is identified as a future improvement.
MATERIALS AND METHODS Motion characterization
Motion characterization combines high-speed imaging with measurements of actuator kinematics, pressure, flow rate, force, torque, and stiffness. Supplementary experiments and simulations document actuator behavior and robot-testing setups.
- Motion characterization: High-speed camera recordings captured actuator motions at 60 Hz for 10 seconds, producing 600 frames.Tracking used SLIC super-pixels followed by a 2D Kalman filter to predict actuator positions.
- Motion characterization: Supplementary setups measured bending angle, pressure, flow rate, static blocking force, joint torque, and dynamic blocking force.The supplementary figures separately document these measurement configurations and force-related characterizations.
- Motion characterization: Quasi-static indentation characterized force-displacement curves for the bistable linkages alone.These measurements complement actuator-level blocking-force and torque measurements.
- Motion characterization: Simulations evaluated bending angle versus response time for BH-SBAs with different spring pretensions.The broader supplementary materials also include actuation timing controls and simulated timing data.
- Motion characterization: Experimental setups covered soft robotic crawlers, soft robotic swimmers, and bending-stiffness measurements.Supplementary tables and movies provide associated geometrical, material, timing, and motion records.