Soft Fluidic Actuators Research Articles

Fluidic Multichambered Actuator and Multiaxis Intrinsic Force Sensing.

Soft robots have morphological characteristics that make them preferred candidates, over their traditionally rigid counterparts, for executing physical interaction tasks with the environment. Therefore, equipping them with force sensing is essential for ensuring safety, enhancing their controllability, and adding autonomy. At the same time, it is necessary to preserve their inherent flexibility when integrating sensory units. Soft-fluidic actuators (SFAs) with hydraulic actuation address some of the challenges posed by the compressibility of pneumatic actuation while maintaining system compliance. This research further investigates the feasibility of utilizing the incompressible actuation fluid as the means of actuation and of multiaxial sensing. We have developed a hyperelastic model for the actuation pressure, acting as a baseline pressure. Any disparities from the baseline have been mapped to external forces, using the principle of pressure-based fluidic soft sensor. Computed tomography imaging has been used to examine inner deformation and validate the analytically derived actuation-pressure model. The induced stresses within the SFA are examined using COMSOL simulations, contributing to the development of a calibration algorithm, which accounts for geometric and cross-sectional nonlinearities and maps pressure variations with tip forces. Two force types (concentrated and distributed) acting on our SFA under different configurations are examined, using two experimental setups described as "Point Load" and "Distributed Force." The force sensing algorithm achieves high accuracy with a maximum absolute error of 0.32N for forces with a magnitude of up to 6N.

Low‐Volume Cores for Fabrication of Compact, Versatile, and Intelligent Soft Systems

AbstractThis study introduces the low‐volume core (LVC) fabrication method, which enables the monolithic molding of compact, complex, versatile, and intelligent soft robotic systems. This method uses thin and flexible thermoplastic sheets to mold internal chambers in soft fluidic actuators, valves, and circuits. The LVC fabrication method creates low‐volume networks in soft actuators (LV‐net actuators) that can be made with compact and complex geometries, enabling both low actuation volume input and multi‐degree‐of‐freedom actuators. LVC fabrication can also be used for compact, completely soft, and monolithic logic components (valves with low‐volume core, also called as LV valves) to provide directional resistance as well as a switching mechanism that enables fluidic logic in soft systems. The compatibility of the fabrication methods for both soft actuators and valves facilitates the creation of compact, integrated, and versatile soft robotic systems with embodied intelligence. This study introduces two examples of such intelligent soft robotic systems that integrate both LV‐net actuators and LV valves to demonstrate capability for complex system fabrication.

A Soft Robotic Morphing Wing for Unmanned Underwater Vehicles

Actuators based on soft elastomers offer significant advantages to the field of robotics, providing greater adaptability, improving collision resilience, and enabling shape‐morphing. Thus, soft fluidic actuators have seen an expansion in their fields of application. Closed‐cycle hydraulic systems are pressure agnostic, enabling their deployment in extremely high‐pressure conditions, such as deep‐sea environments. However, soft actuators have not been widely adopted on unmanned underwater vehicle control surfaces for deep‐sea exploration due to their unpredictable hydrodynamic behavior when camber‐morphing is applied. This study presents the design and characterization of a soft wing and investigates its feasibility for integration into an underwater glider. It is found that the morphing wing enables the glider to adjust the lift‐to‐drag ratio to adapt to different flow conditions. At the operational angle of attack of 12.5°, the lift‐to‐drag ratio ranges from −70% to +10% compared to a rigid version. Furthermore, it reduces the need for internal moving parts and increases maneuverability. The findings lay the groundwork for the real‐world deployment of soft robotic principles capable of outperforming existing rigid systems. With the herein‐described methods, soft morphing capabilities can be enabled on other vehicles.

Quasi-Static Modeling Framework for Soft Bellow-Based Biomimetic Actuators.

Soft robots that incorporate elastomeric matrices and flexible materials have gained attention for their unique capabilities, surpassing those of rigid robots, with increased degrees of freedom and movement. Research has highlighted the adaptability, agility, and sensitivity of soft robotic actuators in various applications, including industrial grippers, locomotive robots, wearable assistive devices, and more. It has been demonstrated that bellow-shaped actuators exhibit greater efficiency compared to uniformly shaped fiber-reinforced actuators as they require less input pressure to achieve a comparable range of motion (ROM). Nevertheless, the mathematical quantification of the performance of bellow-based soft fluidic actuators is not well established due to their inherent non-uniform and complex structure, particularly when compared to fiber-reinforced actuators. Furthermore, the design of bellow dimensions is mostly based on intuition without standardized guidance and criteria. This article presents a comprehensive description of the quasi-static analytical modeling process used to analyze bellow-based soft actuators with linear extension. The results of the models are validated through finite element method (FEM) simulations and experimental testing, considering elongation in free space under fluidic pressurization. This study facilitates the determination of optimal geometrical parameters for bellow-based actuators, allowing for effective biomimetic robot design optimization and performance prediction.

A retrofit sensing strategy for soft fluidic robots

Soft robots are intrinsically capable of adapting to different environments by changing their shape in response to interaction forces. However, sensory feedback is still required for higher level decisions. Most sensing technologies integrate separate sensing elements in soft actuators, which presents a considerable challenge for both the fabrication and robustness of soft robots. Here we present a versatile sensing strategy that can be retrofitted to existing soft fluidic devices without the need for design changes. We achieve this by measuring the fluidic input that is required to activate a soft actuator during interaction with the environment, and relating this input to its deformed state. We demonstrate the versatility of our strategy by tactile sensing of the size, shape, surface roughness and stiffness of objects. We furthermore retrofit sensing to a range of existing pneumatic soft actuators and grippers. Finally, we show the robustness of our fluidic sensing strategy in closed-loop control of a soft gripper for sorting, fruit picking and ripeness detection. We conclude that as long as the interaction of the actuator with the environment results in a shape change of the interval volume, soft fluidic actuators require no embedded sensors and design modifications to implement useful sensing.

Dynamic Modeling of a Fluidic Soft Actuator: First Results Within a Fractional Approach

Dynamic modeling of soft actuators is a challenging field of research mainly due to the nonlinear properties of the materials they are made of, as well as the infinite degrees of freedom that they exhibit. In this paper, the defection of an eccentric soft fluidic actuator is characterized in terms of the dynamic displacement using both integer and fractional order (FO) models. The actuator, which is cylindrical in shape, is made of a silicone polymer called polydimethylsiloxane (PDMS) from a water-soluble mold and a rod that, when placed eccentrically, allows to create the internal chamber of the actuator. Experimental results are presented to show that the best fitting is obtained with FO models. In fact, the dynamics of the actuator can be fully described with a two-parameter model, namely a fractional integrator of order of about 1.2 with a gain.

Electronics-Free Soft Robotic Knee Brace for Dynamic Unloading During Gait for Knee Osteoarthritis: A Proof-of-Concept Study

Abstract We present a novel electronics-free soft robotic knee brace which employs a closed-loop fluidic regenerative (CLFR) system for dynamic unloading in unicompartmental tibiofemoral osteoarthritis (OA). The existing dynamic unloaders are bulky, large, and heavy, and have low compliance likely due to the use of an electrical control box, which is eliminated in the CLFR system. The system consists of a commercial unloading knee brace, a spring-loaded bellow inserted under the heel inside a shoe, a soft-fluidic actuator (bladder), and tubing for fluid transfer. The novelty lies in the fact that the user's body weight (self-powered) compresses the bellow to provide energy to inflate the air bladder placed at the knee. As a result, the yielded pressure unloads the undesirable forces due to knee OA during the stance phase of gait while strategically applying no forces during the swing phase. The knee bladder contact pressure/force, the system response time, and the durability were evaluated via contact pressure measurements for six systems with varying bellow volumes and either pneumatic or hydraulic configurations. All systems produced safe pressure outputs for human skin within a tested bodyweight range of 60–90 kg. Pneumatic and hydraulic systems achieved 250 ms and 400 ms pressurization response times, respectively. During cyclic loading, pneumatic and hydraulic systems demonstrated less than 1% and ∼10% pressure loss, respectively. Overall, the CLFR system created a promising electronics-free solution for dynamically unloading the knee during gait, indicating a potential new paradigm for knee braces.

A compact DEA-based soft peristaltic pump for power and control of fluidic robots.

Fluid-driven robotic systems typically use bulky and rigid power supplies, considerably limiting their mobility and flexibility. Although various forms of low-profile soft pumps have been demonstrated, they either are limited to specific working fluids or generate limited flow rates or pressures, making them ill-suited for widespread robotics applications. In this work, we introduce a class of centimeter-scale soft peristaltic pumps for power and control of fluidic robots. An array of high power density robust dielectric elastomer actuators (DEAs) (each weighing 1.7 grams) were adopted as soft motors, operated in a programmed pattern to produce pressure waves in a fluidic channel. We investigated and optimized the dynamic performance of the pump by analyzing the interaction between the DEAs and the fluidic channel with a fluid-structure interaction finite element model. Our soft pump achieved a maximum blocked pressure of 12.5 kilopascals and a run-out flow rate of 39 milliliters per minute with a response time of less than 0.1 second. The pump can generate bidirectional flow and adjustable pressure through control of drive parameters such as voltage and phase shift. Furthermore, the use of peristalsis makes the pump compatible with various liquids. To illustrate the versatility of the pump, we demonstrate mixing a cocktail, powering custom actuators for haptic devices, and performing closed-loop control of a soft fluidic actuator. This compact soft peristaltic pump opens up possibilities for future on-board power sources for fluid-driven robots in a variety of applications, including food handling, manufacturing, and biomedical therapeutics.

Human-Powered Master Controllers for Reconfigurable Fluidic Soft Robots.

Fluidic soft robots have the advantages of inherent compliance and adaptability, but they are significantly restricted by complex control systems and bulky power devices, including fluidic valves, fluidic pumps, electrical motors, as well as batteries, which make it challenging to operate in narrow space, energy shortage, or electromagnetic sensitive situations. To overcome the shortcomings, we develop portable human-powered master controllers to provide an alternative solution for the master-slave control of the fluidic soft robots. Each controller can supply multiple fluidic pressures to the multiple chambers of the soft robots simultaneously. We use modular fluidic soft actuators to reconfigure soft robots with various functions as control objects. Experimental results show that flexible manipulation and bionic locomotion can be simply realized using the human-powered master controllers. The developed controllers which eliminate energy storage and electronic components can provide a promising candidate of soft robot control in surgical, industrial, and entertainment applications.

Modeling of soft fluidic actuators using fluid–structure interaction simulations with underwater applications

Soft robots have been developed for a variety of applications including gripping, locomotion, wearables and medical devices. For the majority of soft robots, actuation is performed using pneumatics or hydraulics. Many previous works have addressed the modeling of these fluid-driven soft robots using static finite element simulations where the pressure inside the actuator is assumed to be constant and uniform. The assumption of constant internal pressure is a useful simplification but introduces significant errors during events such as pressurization, depressurization, and transient loads from a liquid environment. Applications that use soft actuators for locomotion or propulsion operate using a sequence of transient events, so accurate simulation of these events is critical to optimizing performance. To improve the simulation of soft fluidic actuators and enable the modeling of both internal and external fluid flow in underwater applications, this work describes a fully-coupled, three-dimensional fluid–structure interaction simulation approach, where the pressure and flow dynamics are explicitly solved. This approach provides a realistic simulation of soft actuators in fluid environments, and permits the optimization of transient responses, which may be due to a combination of environmental fluid loads and non-uniform pressurization. The proposed methods are demonstrated in a number of case studies and experiments for a range of actuation and both internal and external inlet flow configurations, including bending actuators, a soft robotic fish fin for propulsion, and experimental results of a bending actuator in a high-speed fluid, which correlate closely with simulations. The proposed approach is expected to assist in the design, modeling, and optimization of bioinspired soft robots in underwater applications.

An efficient technique in dynamic modeling and analysis of soft fluidic actuator

Soft actuators with embedded fluidic channels are promising for soft robotics, but their continuous nature presents challenges in accurate and efficient modeling. Existing studies on dynamic behavior have shown that both approximate and exact methods have limitations in accurately predicting the deflection of soft actuators. Additionally, conventional methods require experimental parameter extraction for each individual actuator. To address these challenges, we propose a modified analytical method and use finite element analysis as the defined correction functions to improve accuracy and computational efficiency, providing a promising tool for the design and control of soft robotic systems. In this work, we propose a solution methodology for simulating the dynamic behavior of fluidic soft actuators, which are widely used in soft robotics applications. Our approach takes into account the cross-sectional geometry and the number of channels in the actuator’s configuration. By scrutinizing various fluidic actuation designs, we can deduce the most suitable cross-sectional configuration for optimal performance. Compared to existing analytical methods, our approach is more accurate and does not require additional calculation time. As the method is adaptable with experimental models, it can be used to design and optimize soft actuators while can be refined and improved through experimental model updating.

Soft Mini Fuse Valve for Resilient Fluidically-Actuated Robots

Lately, soft fluidic actuation has gained widespread interest in all fields where compliance and adaptability are the main keywords. Despite their well-known advantages, soft fluidic actuators frequently present problems related to the elastomeric chambers' durability, affecting the overall system robustness and safety. Indeed, if a robot relies on the parallel pressurisation of multiple actuators, the burst of a single chamber leads to the failure of the entire fluidic circuit, with consequent potentially hazardous leaks. Here, we present the development of a Soft Mini-Fuse (SMIF) valve able to secure and maintain the system functionality even in case of burst failure of single components without affecting their overall bulkiness. By modelling the valve through both analytical and finite element tools, we defined the correlation between main geometrical features, material properties and a selected range of blocking pressures (0.1-1.0 bar). Finally, after validating the modelling tools, we characterised the device behaviour in a range of commonly employed actuation flows (0-15 l/min). The compact dimensions, the ease of integration and the demonstrated performances underline that the SMIF valve represents a novel valuable ally that guarantees stable actuation, limits human intervention and paves the way towards more resilient and autonomous soft fluidic robotic systems.

Energy in Robotics: An Interdisciplinary Challenge

Energy in Robotics: An Interdisciplinary Challenge

Toward Intrinsic Force Sensing and Control in Parallel Soft Robots

With soft robotics being increasingly employed in settings demanding high and controlled contact forces, recent research has demonstrated the use of soft robots to estimate or intrinsically sense forces without requiring external sensing mechanisms. While this has mainly been shown in tendon-based continuum manipulators or deformable robots comprising of push–pull rod actuation, fluid drives still pose great challenges due to high actuation variability and nonlinear mechanical system responses. In this work, we investigate the capabilities of a hydraulic, parallel soft robot to intrinsically sense and subsequently control contact forces. A comprehensive algorithm is derived for static, quasi-static, and dynamic force sensing, which relies on fluid volume and pressure information of the system. The algorithm is validated for a single degree-of-freedom soft fluidic actuator. Results indicate that axial forces acting on a single actuator can be estimated with mean error of 0.56 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.66 N within the validated range of 0–6 N in a quasi-static configuration. The force sensing methodology is applied to force control in a single actuator as well as the coupled parallel robot. It can be seen that forces are controllable for both systems, with the capability of controlling directional contact forces in case of the multidegree-of-freedom parallel soft robot.

Bioinspired and Hierarchically Textile‐Structured Soft Actuators for Healthcare Wearables

AbstractSoft pneumatic actuators possess the increasing potential for various healthcare applications, such as smart wearable devices, safe human‐robot interaction, and flexible manipulators. However, it is difficult to translate the existing technologies to commercial applications due to their inefficient volumetric power, sophisticated control with high operation pressure, slow production, and high cost. To overcome these issues, herein, a caterpillar‐inspired actuator using hierarchical textile architectures based on simple fabrication and low‐cost strategy is designed. Unlike the existing textile‐based pneumatic actuators, the designed actuators are constructed by combining boucle fancy yarns with a novel trilayer‐knit architecture. The as‐prepared actuators concurrently possess fast response (1100° s−1), large bending actuation strain (1080° m−1), high‐power density (272 W m−3), mechanical robustness, easy‐programmable motions, and human‐tactile comfort, which outperforms currently reported textile‐based pneumatic actuators. Furthermore, due to the geometrical transition of the engineered hierarchical structure, the developed actuators exhibit superior dual‐stiffness effect with stress evolution, providing a facile approach to addressing the conflict of flexibility and force output in soft fluidic actuators. This concept as a paradigm provides new insights to develop soft actuators with outstanding design flexibility, adaptability, and multifunctionality using engineered textile‐structure, which has great potential for real‐world applications in medical rehabilitation, physiotherapy, and soft robotics.

Soft Gripper Design and Fabrication for Underwater Grasping

Underwater manipulation with current robotics technology is a challenging task with significant limits in versatility and robustness terms. Such functionality has tremendous potential covering a broad spectrum of applications, mainly replacing divers performing hazardous jobs. Soft robotics provides an efficient solution for operating in these scenarios and adapting to uncertain environmental conditions. This paper presents the design and fabrication of a simple, low-cost, and easily deployable soft gripper for underwater manipulation. We use modelling and simulation techniques for designing the soft fluidic elastomer actuators that compose the soft gripper and additive manufacturing techniques for rapid test cycles and validation. These techniques allow for a fast redesign depending on the application requirements. The proposal combines materials and fabrication techniques to take advantage of their strengths. We validate the feasibility and ability of the proposed soft gripper in a challenging underwater scenario using a subaquatic vehicle.

Energy-saving trajectory optimization of a fluidic soft robotic arm

Soft robots have attracted increasing attention due to inherent environmental adaptability and reliable human–machine interaction. However, there is relatively few research about their energy efficiency which acts as an important indicator. This paper firstly derives the time-domain model of energy consumption for a fluidic soft robotic arm. With the introduction of forward kinematics, the trajectory of the soft robotic arm is optimized for energy saving under motion constraints and solved using interior point method. A series of experiments are implemented to evaluate the performance of the proposed model and the optimized trajectory. The results show that the time-based model can capture the dynamical energy behaviors of the fluidic soft actuators under various motions. It is also found that the energy consumption of the soft robotic arm is effectively reduced when the trajectory optimization is applied. This work can provide further reference to the energy-based optimization of the soft robots.

Controlling Soft Fluidic Actuators Using Soft DEA-Based Valves

Fluidic soft actuators have been widely used for applications where compliance is desirable – such as in delicate robotic manipulation. As their operation is based on pressurization, fluidic actuators typically rely on bulky, rigid pumps, valves, and pressure regulators for control. This dependence on rigid components hinders the development of compact and fully-soft robots. Soft regulation systems designed for control of these actuators have been recently developed based on soft valves using dielectric elastomer actuators, but precise control has not been achieved. In this work, we leverage these valves to introduce a soft regulation system capable of precise closed-loop position control of a soft hydraulic actuator with multiple controllers. We also achieve open-loop trajectory tracking based on a data-driven model of the fluidic system. Finally, we combine the valve system with wearable strain sensors to create the first teleoperated fluidic circuit where the sensor, actuator, and regulation system are all soft. This work presents control strategies for fluid-driven actuators with soft sensors and regulation systems, showing the potential for future all-soft motion control of soft hydraulic robots.

Soft Fluidic Actuator for Locomotion in Multi-Phase Environments

This letter presents the design, development, and experimental assessment of a soft fluidic actuator that can enable locomotion in a variety of aquatic and terrestrial environments. Most actuation strategies for amphibious locomotion rely on rigid, fast moving components to generate thrust and tractive forces. Our prototype, comprising soft materials, and relying on simple motion planning and control strategies, demonstrates two gaits, that we employ for locomotion in two vastly different scenarios, underwater swimming and moving on granular terrain with varying levels of water content. By adjusting its internal pressure, the actuator dynamically varies its stiffness and shape, and transitions between wheel and soft paddle form. Experimental results of locomotion in controlled laboratory conditions serve as proof-of-concept for the proposed actuator's efficacy. Using two different motion patterns and control schemes, we show that this prototype achieves both thrust and tractive forces.

High‐Gain Microfluidic Amplifiers: The Bridge between Microfluidic Controllers and Fluidic Soft Actuators

Soft fluidic robots are typically controlled using manifolds containing large and rigid electromechanical valves. These bulky controllers limit scalability and hinder motion, in particular for untethered soft robots. There has been recent interest in using fluidic controllers analogous to electrical logic gates and microcontrollers to replace rigid valve systems. However, these microfluidic networks typically operate with small volumes, low flow rates, and low pressures relative to what is needed to power fluidic soft actuators. This article presents the design, fabrication, and analysis of a soft, fluidic amplifier as the “missing link” between microfluidic analogies of microcontrollers and the high fluidic power loads representative of soft actuators. The article demonstrates amplification gains of pressure signals up to a factor of four. The amplifier is a step toward fully autonomous soft robots by allowing designers to develop control strategies from soft materials with minimal additional rigid components or tethering.