Pneumatic Bending Actuator Research Articles

A Compliant Hinge Joint Driven by the PneuNets Bending Actuator

Abstract While soft robots enjoy the benefits of high adaptability and safety, their inherent flexibility makes them suffer from low load-carrying capacity and motion precision, which limits their applications to a broader range of fields. To address this problem, we propose a novel compliant hinge joint with a stiff backbone for load-carrying coupled with soft pneumatic networks (PneuNets) bending actuators. We derive a pseudo-rigid-body model of the joint design and validate it through experiments and simulations. The results show that the joint can achieve a large range of bending angles. The off-axis stiffness is from 16.74 to 627.63 times the in-axis stiffness. This design can carry a heavy load off-axis while maintaining the in-axis flexibility. This work lays out the foundation for designing high-performance soft robots by combining various flexure mechanisms and pneumatic bending actuators.

A pneumatic particle-blocking variable-stiffness bending actuator

This study introduces a variable-stiffness pneumatic bending actuator to enhance the stiffness of flexible robots. The bending actuator combines the working principles of a pneumatic drive, a wedge structure, and particle blockage. It enables one-dimensional, bidirectional bending motion and offers pose-maintenance capabilities. The approach to variable-stiffness is both driven and antagonistic. Stiffness in both bending and opposite directions is nonlinearly correlated with air pressure values. Specifically, at 0.5 MPa of air pressure, the stiffness in the bending direction increases to 6.1 times the initial stiffness. At 0.15 MPa, the stiffness in the opposite direction is 2.1 times the initial value. When air pressure is greater than 0.15 MPa, the stiffness incrementally increases due to the wedge impedance force. A driven variable-stiffness control is simple and suitable for applications with a constant load direction, while the antagonistic approach is more suitable for occasions where the load direction changes during movement.

A Hierarchical Design Framework for the Design of Soft Robots

This paper demonstrates the effectiveness of a hierarchical design framework in developing environment-specific behaviour for fluid-actuated soft robots. Our proposed framework employs multi-step optimisation and reduced-order modelling to reduce the computational expense associated with simulating non-linear materials used in the design process. Specifically, our framework requires the designer to make high-level decisions to simplify the optimisations, targeting simple objectives in earlier steps and more complex objectives in later steps. We present a case study, where our proposed framework is compared to a conventional direct design approach for a simple 2D design. A soft pneumatic bending actuator was designed that is able to perform asymmetrical motion when actuated cyclically. Our results show that the hierarchical framework can find almost 2.5 times better solutions in less than 3% of the time when compared to a direct design approach.

Continuous deformation analysis and contact force estimation for pneumatic bending actuators interacting with environment

Soft bending actuators show high adaptability for applications such as rehabilitation or grasping. Although constant curvature assumption has been extensively used for free motion modeling, these actuators do not bend circularly when interacting with the environment. In such situation, conventional bending sensors cannot provide useful information on their shape. In this paper, the Finite Rigid Elements approach is utilized to model the behavior of a soft pneumatic bending actuator in free motion and contact. With this method, the variable curvature configuration under different external loads can be modeled. Then, the contact force between the actuator and an object located in a specific position is estimated utilizing the Gradient Descent optimization method. Experimental results verified the combinatorial proposed approach for both force estimation and structure deformation.

Effect of Segment Types on Characterization of Soft Sensing Textile Actuators for Soft Wearable Robots.

The use of textiles in soft robotics is gaining popularity because of the advantages textiles offer over other materials in terms of weight, conformability, and ease of manufacture. The purpose of this research is to examine the stitching process used to construct fabric-based pneumatic bending actuators as well as the effect of segment types on the actuators' properties when used in soft robotic glove applications. To impart bending motion to actuators, two techniques have been used: asymmetry between weave and weft knit fabric layers and mechanical anisotropy between these two textiles. The impacts of various segment types on the actuators' grip force and bending angle were investigated further. According to experiments, segmenting the actuator with a sewing technique increases the bending angle. It was discovered that actuators with high anisotropy differences in their fabric combinations have high gripping forces. Textile-based capacitive strain sensors are also added to selected segmented actuator types, which possess desirable properties such as increased grip force, increased bending angle, and reduced radial expansion. The sensors were used to demonstrate the controllability of a soft robotic glove using a closed-loop system. Finally, we demonstrated that actuators integrated into a soft wearable glove are capable of grasping a variety of items and performing various grasp types.

Modeling and identification of rate-dependent and asymmetric hysteresis of soft bending pneumatic actuator based on evolutionary firefly algorithm

Soft-bending pneumatic actuators (SBPA) have shown great potential in various applications owing to their intrinsic compliance. However, motion control is still challenging because of the complicated hysteresis of the elastic material and pneumatic system, which unlike the general hysteresis found in the rigid body mechanism, is rate-dependent and asymmetric. A precise hysteresis model is crucial for improving the performance of SBPA. This study proposed a rate-dependent modified generalized Prandtl–Ishlinskii (RMGPI) model for a vulcanized silicone rubber-based fast pneu-net soft bending pneumatic actuator (fp-SBPA) to describe complicated hysteresis characteristics. A visual feedback system obtained the bending angle in real time. With the increase in the number of operators, the identification of the proposed model parameters became more complicated and time-consuming; thus, an evolutionary firefly algorithm (EFA) was developed to improve identification efficiency. A series of experiments and comparison studies were conducted to verify the effectiveness of the proposed model and identification method.

Optimizing Out-of-Plane Stiffness for Soft Grippers

In this letter, we presented a data-driven framework to optimize the out-of-plane stiffness for soft grippers to achieve mechanical properties as hard-to-twist and easy-to-bend. The effectiveness of this method is demonstrated in the design of a soft pneumatic bending actuator (SPBA). First, a new objective function is defined to quantitatively evaluate the out-of-plane stiffness as well as the bending performance. Then, sensitivity analysis is conducted on the parametric model of an SPBA design to determine the optimized design parameters with the help of Finite Element Analysis (FEA). To enable the computation of numerical optimization, a data-driven approach is employed to learn a cost function that directly represents the out-of-plane stiffness as a differentiable function of the design variables. A gradient-based method is used to maximize the out-of-plane stiffness of the SPBA while ensuring specific bending performance. The effectiveness of our method has been demonstrated in physical experiments taken on 3D-printed grippers.

Segmented Soft Pneumatic Bending Actuator with Artificial Neural Network for Parameter Prediction

Segmented Soft Pneumatic Bending Actuator with Artificial Neural Network for Parameter Prediction

Eccentric High-Force Soft Pneumatic Bending Actuator for Finger-Type Soft Grippers

Abstract Soft pneumatic actuators (SPAs) play an important role in leading the development of soft robotics. However, due to the inherent characteristics of soft materials, the low driving force limits the application of SPAs. This study presents a high-force soft pneumatic bending actuator (SPBA) that consists of a spring, an eccentric silicone cylinder, and a limiting fiber. Based on the Neo-Hookean model, a theoretical model is established to predict the relationship between the bending angle and the pressure of SPBA. Furthermore, we characterize the performance of SPBA in terms of the bending capability, tip force, as well as response time. The results demonstrate the effectiveness of the theoretical model, as well as the high tip force (10.2 N) and fast response capability of SPBA. Finally, SPBAs are used to construct a three-finger soft gripper. The load capacity of the gripper is proofed, which indicates that the gripping force of the gripper increases with the pressure of the fingers and the diameter of the object. The gripping test of the gripper is performed. The result shows that the gripper with the pinching mode can grip objects of various sizes and shapes in the air and underwater, and the gripper with enveloping mode can grip objects with weight up to 1.25 kg.

Topology Optimization Design and Experiment of a Soft Pneumatic Bending Actuator for Grasping Applications

Soft pneumatic bending actuators are commonly used in robotic grasping applications and can be applied for handling both delicate and irregularlyshaped objects. Because these soft pneumatic actuators are typically embedded with multiple air chambers inside soft, thin structures, their maximum payloads are usually low compared to rigid designs of similar size. This article presents a soft pneumatic bending actuator design using a topology optimization procedure that can maximize the bending capability and thus increase the allowable payload of soft grippers that consist of multiple pneumatic bending actuators. After an optimum design is obtained, multiple identical actuators are prototyped through a molding process using a silicone rubber material. Both two-fingered and three-fingered soft grippers are developed using the topology-optimized pneumatic bending actuators. Experiments, including a bending angle test, an output force test, and a payload test, are conducted in this study,and the results are compared against the test results of the commercial SRT soft pneumatic actuator. The experimental results show that the bending angle for the developed bending actuator is 111 degrees on average of 10 tests at an input pressure of 50 kPa. The output force is 9.45 N at an input pressure of 80 kPa, while the maximum payloads of the two-fingered and three-fingered grippers are 2.66 kg and 5.12 kg, respectively.

Numerical investigation of a bioinspired multi-segment soft pneumatic actuator for grasping applications

Soft robotic gripper is useful for grasping applications, such as pick-and-place tasks, post-disaster rescue and health monitoring of civil infrastructure. Inspired by the versatility of the human hand and bird claws, this paper designed a bioinspired actuator made of multi-segment fibre-reinforced pneumatic bending actuator and bellow-shape pneumatic rotational actuator joints. A numerical model was established to investigate the bending performance and stress distribution of the designed actuator under different input pressures. The influences of bottom layer thickness, chamber number at the actuator joints, and bending segment length on the overall bending performance of the designed soft actuator have been comprehensively investigated. In addition, the stress distribution of the actuator joints has been studied for understanding the mechanical performance of the actuator joints. Finally, a soft gripper made of two designed actuators has been demonstrated for grasping objects with different shapes and weights. The findings found in this investigation are helpful for the design and fabrication of robotic grasper, which can be further integrated on an industrial robotic arm and flying robotic platform for grasping and prosthetic hand applications.

Worm‐Inspired Soft Robots Enable Adaptable Pipeline and Tunnel Inspection

The inspection, maintenance, and repair of pipeline and tunnel infrastructure have motivated the increasing research in the development of suitable robots with high flexibility, good adaptability, and large load capacity. Herein, a worm‐inspired soft robot that is capable of operating and performing a variety of tasks in a complicated pipeline/tunnel environment is reported. The soft tubular robot consists of elongation pneumatic actuators (EPAs), radial expansion pneumatic actuators (REPAs), and a spatial bending pneumatic actuator (SBPA). A series of experiments are performed to demonstrate the capability of the soft robot to crawl robustly under different pipeline conditions, including varying diameter, different cross‐sectional shapes, and wet or oil‐covered internal surfaces, and underwater environment. The soft robot can carry a load of more than 11 times of its own weight in a vertical pipeline. Equipped with a visualization unit, the soft robot can detect the internal conditions of the pipeline and cross the multibranched pipeline as needed. The tubular soft robot provides a useful tool for the inspection, cleaning, and maintenance of pipelines and tunnels.

Machine-Knitted Seamless Pneumatic Actuators for Soft Robotics: Design, Fabrication, and Characterization

Computerized machine knitting offers an attractive fabrication technology for incorporating wearable assistive devices into garments. In this work, we utilized, for the first time, whole-garment knitting techniques to manufacture a seamless fully knitted pneumatic bending actuator, which represents an advancement to existing cut-and-sew manufacturing techniques. Various machine knitting parameters were investigated to create anisotropic actuator structures, which exhibited a range of bending and extension motions when pressurized with air. The functionality of the actuator was demonstrated through integration into an assistive glove for hand grip action. The achieved curvature range when pressurizing the actuators up to 150 kPa was sufficient to grasp objects down to 3 cm in diameter and up to 125 g in weight. This manufacturing technique is rapid and scalable, paving the way for mass-production of customizable soft robotics wearables.

Flexible Pneumatic Bending Actuator for a Robotic Tongue

There are not many physical models of oral and laryngeal systems for human speech movement in both computer simulators and mechanical simulators. In particular, there is no robot tongue mechanism that completely reproduces the deformation motion of the human tongue. The human tongue is an aggregate of muscles devoid of a skeleton. It only possesses a small hyoid. The purpose of this study is to develop a flexible actuator without a rigid link, aiming at the development of a tongue mechanism for a mechanical speech robot. We propose a flexible pneumatic bending actuator using thin McKibben muscles and a soft body formed by a silicone resin. We have verified its mechanical characteristics and described a control method for displacement and curvature. The elasticity/compliance of the silicone resin forming the soft body of this actuator was quantified by tensile tests. The oscillation parameters were identified, and it is suggested that the dynamic model can be described by a spring-mass-damper system. Assuming an arc-shaped deformation model, a simultaneous control system for the arc length and curvature was constructed and its effectiveness was confirmed.

Parallel Helix Actuators for Soft Robotic Applications.

Fabrication of soft pneumatic bending actuators typically involves multiple steps to accommodate the formation of complex internal geometry and the alignment and bonding between soft and inextensible materials. The complexity of these processes intensifies when applied to multi-chamber and small-scale (~10 mm diameter) designs, resulting in poor repeatability. Designs regularly rely on combining multiple prefabricated single chamber actuators or are limited to simple (fixed cross-section) internal chamber geometry, which can result in excessive ballooning and reduced bending efficiency, compelling the addition of constraining materials. In this work, we address existing limitations by presenting a single material molding technique that uses parallel cores with helical features. We demonstrate that through specific orientation and alignment of these internal structures, small diameter actuators may be fabricated with complex internal geometry in a single material—without- additional design-critical steps. The helix design produces wall profiles that restrict radial expansion while allowing compact designs through chamber interlocking, and simplified demolding. We present and evaluate three-chambered designs with varied helical features, demonstrating appreciable bending angles (>180°), three-dimensional workspace coverage, and three-times bodyweight carrying capability. Through application and validation of the constant curvature assumption, forward kinematic models are presented for the actuator and calibrated to account for chamber-specific bending characteristics, resulting in a mean model tip error of 4.1 mm. This simple and inexpensive fabrication technique has potential to be scaled in size and chamber numbers, allowing for application-specific designs for soft, high-mobility actuators especially for surgical, or locomotion applications.

Customization Methodology for Conformable Grasping Posture of Soft Grippers by Stiffness Patterning

Soft grippers with soft and flexible materials have been widely researched to improve the functionality of grasping. Although grippers that can grasp various objects with different shapes are important, a large number of industrial applications require a gripper that is targeted for a specified object. In this paper, we propose a design methodology for soft grippers that are customized to grasp single dedicated objects. A customized soft gripper can safely and efficiently grasp a dedicated target object with lowered surface contact forces while maintaining a higher lifting force, compared to its non-customized counterpart. A simplified analytical model and a fabrication method that can rapidly customize and fabricate soft grippers are proposed. Stiffness patterns were implemented onto the constraint layers of pneumatic bending actuators to establish actuated postures with irregular bending curvatures in the longitudinal direction. Soft grippers with customized stiffness patterns yielded higher shape conformability to target objects than non-patterned regular soft grippers. The simplified analytical model represents the pneumatically actuated soft finger as a summation of interactions between its air chambers. Geometric approximations and pseudo-rigid-body modeling theory were employed to build the analytical model. The customized soft grippers were compared with non-patterned soft grippers by measuring their lifting forces and contact forces while they grasped objects. Under the identical actuating pressure, the conformable grasping postures enabled customized soft grippers to have almost three times the lifting force than that of non-patterned soft grippers, while the maximum contact force was reduced to two thirds.

Hybrid System Analysis and Control of a Soft Robotic Gripper with Embedded Proprioceptive Sensing for Enhanced Gripping Performance

Soft robots are considered to have infinite degrees of freedom based on their structural compliance, providing high adaptability to the environments, and recent study has focused mostly on advancement of their physical designs for increasing the adaptability. However, interaction itself with the environment has not been taken into serious account in previous studies despite the importance in applications. A soft robot as a hybrid system described by both discrete and continuous states is considered and a method of analysis for enhanced manipulation is proposed. The method is tested with a soft gripper composed of a pneumatic bending actuator and an embedded soft sensor for a task of object gripping. The optimum sensor location on the actuator based on the calibration map obtained from the actuator characterization is first determined. Using the sensor information, the interaction with the environment (i.e., object) classified into four discrete states is understood. In addition, a control strategy to find the best position to grip the object based on the estimated states is developed. The gripper is able to successfully complete the task using the proposed method for three test scenarios with different initial conditions and control parameters. Finally, the results are demonstrated with supporting videos.

A Compact McKibben Muscle Based Bending Actuator for Close-to-Body Application in Assistive Wearable Robots

In this letter we demonstrate a pneumatic bending actuator for upper-limb assistive wearable robots which uses thin McKibben muscles in combination with a flexure strip. The actuator features both active soft actuation and passive gravity support, and in terms of force transmission bridges the gap between the classic rigid type actuators and the emerging soft actuator technologies. Its flexure strip leverages the high-force low-displacement properties of McKibben muscles towards a large rotational range of motion and reduces localized forces at the attachments. We explain the synthesis method by which these actuators can be obtained and optimized for high specific moment output. Physical specimens of three optimized actuator designs are built and tested on a dedicated experimental setup, verifying the computational models. Furthermore, a proof-of-concept upper-limb assistive wearable robot is presented to illustrate a practical application of this actuator and its potential for close-to-body alignment. We found that based on our currently available components actuators can be built which, given a width of 80 mm, are able to produce a moment exceeding 4 Nm at an arm elevation of 90 deg.

Optimal Design of Soft Pneumatic Bending Actuators Subjected to Design-Dependent Pressure Loads

Soft actuators, mainly composed of soft materials, have made a great impact on applications in unstructured or unknown environments due to their high flexibility and customizability. The design methods of soft actuators can be divided into two groups: the bionic design method and the topology optimization method. Compared to the bionic design method that requires numerous experiments, the topology optimization method can generate innovative structures according to the design requirements. However, the existing topology optimization method cannot be applied to soft pneumatic bending actuator (SPBA) designs because SPBAs are usually subjected to design-dependent pressure loads of which the position depends on the structure. In this paper, we propose an optimal design framework of SPBAs to resolve the design-dependent load problem using an adaptive bi-directional evolutionary structural optimization method. Herein, SPBAs are considered as compliant mechanisms and our goal is to achieve maximum bending deformation as well as structural stiffness. In finite element analysis, each element in the design domain is set to solid or void according to sensitivity number, which is approximated by the objective function derivative with respect to the design variables. During the iterative optimization procedure, we explicitly define the movable solid-void boundary surfaces on which the pressure will act. A precision prototype actuator is fabricated, and its performance is evaluated in terms of free travel experiment. Some extensions are supplied to validate the optimality and reliability of the proposed method. This framework paves a way for the diversity of soft actuators.

Inflatable L-shaped prisms as soft actuators for soft exogloves

Soft pneumatic bending actuators generally produce a bending deformation through pressurization, but the force they produce diminishes significantly throughout the deformation for a given pressure level. This is useful to produce a controlled motion, but sensors are required to enable force control of the actuator throughout this motion. This paper introduces the use of an inflatable L-shaped prism as a soft actuator that has a constant equilibrium angle and produces a fairly constant force throughout most of its actuation range. This combination of characteristics makes the actuator ideal for applications where an easily controllable force without the use of sensors is required or preferable. However, it is unsuitable for applications where motion control is necessary due to its constant equilibrium angle. The effect of different geometric parameters and materials were tested, and it was found that this actuator could produce a blocked force of up to 13 N at 80 kPa and could maintain this force throughout most of this range of motion. This actuator concept was then used to build a three-fingered soft exoglove capable of grasping a wide range of objects.