Soft Pneumatic Actuators Research Articles

Active tactile sensing of small insect force by a soft microfinger toward microfinger-insect interactions

Human–robot interaction technology has contributed to improving sociality for humanoid robots. At scales far from human scales, a microrobot can interact with an environment in a small world. Microsensors have been applied to measurement of forces by flying or walking insects. Meanwhile, most previous works focused on the measurement of the behavior of insects. Here, we propose microrobot-insect interactions by soft microfingers integrated with artificial muscle actuators and tactile sensors, which has been developed for a haptic teleoperation robot system. A soft pneumatic balloon actuator acts as the artificial muscle, and a flexible strain sensor using a liquid metal provides tactile sensing. Force interaction between a pill bug and the microfinger could be accomplished. The microfinger (12 mm × 3 mm × 490 μm) can move and touch an insect, and it can detect reaction force from an insect. The measured reaction force from the legs of a pill bug as a representative insect was less than 10 mN. This paper presents a microfinger as an end effector for the active sensing of reaction force from a small insect. We anticipate that our results will lead to further evaluation of small living things as well as technology development for human–environment interaction.

Develop Control Architectures to Enhance Soft Actuator Motion and Force

Study: Soft robots can achieve the desired range of motion for finger movement to match their axis of rotation with the axis of rotation of the human hand. The iterative design has been used to achieve data that makes the movement smooth and the range of movement wider, and the validity of the design has been confirmed through practical experiments. Limitation: The challenges facing this research are to reach the most significant inclined angle and increase the force generated by the actuator, which is the most complicated matter while maintaining the desired control accuracy. The motion capture system verifies the actual movement of the soft pneumatic actuator (SPA). A tracking system has been developed for SPA in action by having sensors to know the position and strength of the SPA. Results: The novelty of this research is that it gave better control of soft robots by selecting the proportional, integral, and derivative (PID) controller. The parameters were tuned using three different methods: ZN (Ziegler Nichols Method), GA (Genetic Algorism), and PSO (Particle Swarm Optimization). The optimization techniques were used in Methods 2 and 3 in order to reach the nominal error rate (0.6) and minimum overshoot (0.1%) in the shortest time (2.5 s). Impact: The effect of the proposed system in this study is to provide precise control of the actuator, which helps in medical and industrial applications, the most important of which are the transfer of things from one place to another and the process of medical rehabilitation for patients with muscular dystrophy. A doctor who treats finger muscle insufficiency can monitor a patient’s ability to reach a greater angle of flexion or increase strength by developing three treatment modalities to boost strength: Full Assisted Movement (FAM), Half Assisted Movement (HAM), and Resistance Movement (RM).

A Review on the Development of Pneumatic Artificial Muscle Actuators: Force Model and Application

Pneumatic artificial muscles (PAMs) are soft and flexible linear pneumatic actuators which produce human muscle like actuation. Due to these properties, the muscle actuators have an adaptable compliance for various robotic platforms as well as medical applications. While a variety of possible actuation schemes are present, there is still a need for the development of a soft actuator that is very light-weight, compact, and flexible with high power-to-weight ratio. To achieve this, the development of the PAM actuators has become an interesting topic for many researchers. In this review, the development of the different kinds of PAM available to date are presented along with manufacturing process and the operating principle. The various force models for artificial muscle presented in the literature are broadly reviewed with the constraints. Furthermore, the applications of PAM are included and classified based on the fields of biorobotics, medicine, and industry, along with advanced medical instrumentation. Finally, the needful improvements in terms of the dynamics of the muscle are discussed for the precise control of the PAMs as per the requirements for the applications. This review will be helpful for researchers working in the field of robotics and for designers to develop new type of artificial muscle depending on the applications.

Effects of Flexural Rigidity on Soft Actuators via Adhering to Large Cylinders

This study proposes a soft pneumatic actuator with adhesion (SPAA) consisting of a top fluidic-driven elastic actuator and four bottom adhesive pads for adhering to large cylinders. Finite element models were developed to investigate the bending properties under positive air pressure and the effect of “rib” height on the flexural rigidity of the SPAA. A synchronous testing platform for the adhesive contact state and mechanics was developed, and the bending curvature and flexural rigidity of the SPAA were experimentally measured relative to the pressure and “rib” height, respectively, including the adhesion performance of the SPAA with different rigidities on large cylinders. The obtained results indicate that the SPAA can continuously bend with controllable curvature under positive air pressure and can actively envelop a wide range of cylinders of different curvatures. The increase in the “rib” height from 4 to 8 mm increases the flexural rigidity of the SPAA by approximately 230%, contributing to an average increase of 54% in the adhesion performance of the SPAA adhering to large cylinders. The adhesion performance increases more significantly with an increase in the flexural rigidity at a smaller peeling angle. SPAA has a better adhesion performance on large cylinders than most existing soft adhesive actuators, implying that is more stable and less affected by the curvature of cylinders. To address the low contact ratio of the SPAA during adhesion, the optimization designs of the rigid–flexible coupling hierarchical and differentiated AP structures were proposed to increase the contact ratio to more than 80% in the simulation. In conclusion, this study improved the adhesion performance of soft adhesive actuators on large cylinders and extended the application scope of adhesion technology. SPAA is a basic adhesive unit with a universal structure and large aspect ratio similar to that of the human finger. According to working conditions requirements, SPAAs can be assembled to a multi-finger flexible adhesive gripper with excellent maneuverability.

Transferable Shape Estimation of Soft Pneumatic Actuators Based on Active Vibroacoustic Sensing

A soft pneumatic actuator (SPA) is one of the most prominent components in a soft robotic system. The sensing of SPAs is challenging owing to their elasticity and deformability. Sensors for specific factors are required to successfully sense an SPA. A flexible sensor is an important component for sensing the conditions of SPAs, such as the shape and deformation due to contact events during applications. Developing versatile sensors with high flexibility and tolerability for SPAs is challenging. Data-driven sensing approaches involve individual machine learning models for different actuators. In contrast, it is an enormous advantage to have versatile sensors as hardware and machine learning models as software employed on various actuators. Therefore, we propose a transferable shape estimation method based on active vibro-acoustic sensing to achieve tolerability and increase versatility. We created easily transferable sensing devices for SPAs. In addition, we employed a data-driven approach that utilizes a simple transfer learning technique on a two-dimensional convolutional neural network model. We confirm the feasibility and versatility of the proposed method through evaluation experiments. A transferable estimation method was used on SPAs to estimate the bending angle and length under various sensing and environmental conditions, and the average errors were less than 3.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> and 2.1 mm, respectively.

Electro-Hydraulic Rolling Soft Wheel: Design, Hybrid Dynamic Modeling, and Model Predictive Control

Locomotion through rolling is attractive compared to other forms of locomotion thanks to uniform designs, high degree of mobility, dynamic stability, and self-recovery from collision. Despite previous efforts to design rolling soft systems, pneumatic and other soft actuators are often limited in terms of high-speed dynamics, system integration, and/or functionalities. Furthermore, mathematical description of the rolling dynamics for this type of robot and how the models can be used for speed control are often not mentioned. This article introduces a cylindrical-shaped shell-bulging rolling soft wheel that employs an array of 16 folded-HASEL actuators as a mean for improved rolling performance. The actuators represent the soft components with discrete forces that propel the wheel, whereas the wheel's frame is rigid but allows for smooth, continuous change in position and speed. We discuss the interplay between the electrical and mechanical design choices, the modeling of the wheel's hybrid (continuous and discrete) dynamic behavior, and the implementation of a model predictive controller (MPC) for the robot's speed. With the balance of several design factors, we show the wheel's ability to carry integrated hardware with a maximum rolling speed at 0.7 m/s (or 2.2 body lengths per second), despite its total weight of 979 g, allowing the wheel to outperform the existing rolling soft wheels with comparable weights and sizes. We also show that the MPC enables the wheel to accelerate and leverage its inherent braking capability to reach desired speeds—a critical function that did not exist in previous rolling soft systems.

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.

Identification and Control of a Soft-Robotic Bladder Towards Impedance-Style Haptic Terrain Display

This letter evaluates the capabilities of a soft robotic pneumatic actuator derived from the terrain display haptic device, the “Smart Shoe.” The bladder design of the Smart Shoe is upgraded to include a pressure supply and greater output flow capabilities. Forcing characteristics are identified and incorporated as feedforward terms in a sliding-mode tracking controller to provide impedance control. A bench top setup is created to rigorously test this new type of actuator. The bandwidth and stiffness capability are evaluated relative to forces and displacements encountered during human gait. Four force vs. displacement profiles relevant to haptic terrain display are proposed and evaluated. It was found that the actuator could sustain a stiffness similar to a soft-soled shoe on concrete, as well as other terrain (sand, dirt, etc.). Compressions of the bladder done at 20 mm/s, which is similar to the speed of human gait, showed promising results in tracking a desired force trajectory. The results in this letter show this actuator can display haptic terrain trajectories, providing a basis for future wearable haptic terrain display devices.

Experimental Study on the Closed-Loop Control of a Soft Ring-Shaped Actuator for Gastric Simulator

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In-vitro</i> gastric simulators are soft robotic systems that reproduce the change of the food or drug by emulating the stomach's motility functions. The control of gastric simulators traditionally lies in tuning the actuators via open-loop approaches. However, the lack of precise position control, external disturbances, sensitivity to model uncertainties, and continues retuning are solid motivations for employing closed-loop methods to control gastric simulators. A bellows-driven viscoelastic soft ring-shaped pneumatic actuator (RiSPA), including five spring-like bellows, was developed for stomach simulators’ primary actuation system. This article studies the implementation of closed-loop methods equipped with the online feedback system on the RiSPA for the first time. All technical aspects for implementing the closed-loop control and the sensor-actuator integration are discussed in detail. A model-based closed-loop method has been implemented and compared with a classic model-free method to show the closed-loop system's ability to reject model uncertainties. The experimental results in both the tracking trajectory and the regulation performances are thoroughly compared to the open-loop approach, where the superiority of closed-loop techniques is demonstrated. A mechanical setup is designed to show the ability of the closed-loop approach in dealing with the disturbances and external loads as well.

Active Modeling and Control of the Ring-Shaped Pneumatic Actuator: An Experimental Study

A soft ring-shaped pneumatic actuator (RPA) that contracts radially plays a key role in biomimicking the contractions of the digestive system. However, the modeling and control of the RPA are still challenging due to the nonlinearity, hysteresis, and unique structure. In this article, we present an active-model-enhanced control (AMEC) scheme for the precise contraction control of the RPA. First, a generalized mass–spring–damper model is proposed to describe the dynamics of the RPA focusing on the midpoint of the inner wall. Based on the proposed model, the AMEC scheme is developed to enhance a proportional–integral–derivative (PID) controller in terms of tracking a reference trajectory. In this scheme, the modeling errors of the generalized mass–spring–damper model are estimated by an active modeling algorithm, which is integrated into the PID controller to eliminate the effect of the modeling errors. Finally, experiments are conducted to validate the proposed modeling and control scheme. The results indicate that the modeling errors are estimated by the active modeling algorithm with an acceptable estimation error. The tracking error of the AMEC is 52.7% smaller than that of the PID controller. The proposed modeling and control methods for the RPA lay a solid foundation for its biomimetic application.

Locomotion via Active Suction in a Sea Star-Inspired Soft Robot

Some marine animals, such as sea stars, have developed versatile adhesive appendages that can be used for a variety of behaviors including sticking to surfaces, manipulating objects, and locomoting. These appendages couple reversible adhesion capabilities with muscular structures that can do work on the world while still being soft enough to conform to various surfaces and to squeeze through tight spaces. We took inspiration from the hydraulic tube feet of sea star to create modules consisting of soft active suction discs and soft pneumatic linear actuators. Tube feet convert fluid motion into linear actuation using soft tubes that are constrained radially. In this work, we study the tradeoff between the stiffness of such radial constraints, and the ability of the linear actuators to apply axial forces. The adhesive force of the suction disc is dependent on both the pressure differential between the suction cavity and the environment and the deformation within the body of the disc while it is being loaded. We show that the tube foot modules are capable of creating locomotion over flat surfaces, up small steps, and in a confined space due to the redundancy caused by an array of actuators. We also show that active suction not only enables the use of softer tube foot actuators, but that sensing this active suction line provides feedback to increase the efficiency of locomotion over obstacles.

Analytical Modeling of a Membrane-Based Pneumatic Soft Gripper

Finite deformation is the principal actuation basis of elastomer-based pneumatic soft actuators. Desired deformation behavior is the key design requirement for such actuators. The objective of current study is to optimize the design of a flat shell gripper and to investigate its interaction with a cylindrical object. Herein, we propose an analytical model for a membrane-based flat shell gripper. The model is based on finite strain membrane theory and neo-Hookean material. The proposed model considers the contact interaction of the actuator with flat and cylindrical rigid substrates. The model is developed for three different states of the actuator: 1) free-space; 2) contact with a flat substrate; and 3) contact with a cylindrical substrate. In application, the model was used to predict the relative position and air pressure required to grasp a cylindrical object by a parallel two-fingered shell gripper. Additionally, the frictional behavior of the actuator in contact with a cylindrical substrate is investigated. The model involves only solving nonlinear algebraic equations and is computationally efficient. The theoretically predicted deformation behavior of the actuator is experimentally validated via free-space deformation, force measurement, and grasping tests.

A self-sensing soft pneumatic actuator with closed-Loop control for haptic feedback wearable devices

The intuitive and effective haptic feedback interaction is essential for human–machine interfaces. However, most current commercial haptic feedback solutions are rigid, simplex, and insufficient to fully immerse users in virtual and teleoperated environments. Here, we report the design, fabrication and performance of a low-cost and lightweight self-sensing actuator (SenAct) that provides accurate force and vibration feedback to recreate the tactile perception. The SenAct shows good performance such as fast response (10 ms), high robustness (>10,000 cycles), and large output force (up to 1.55 N) with high controllable resolution (up to 0.02 N) based on the real-time closed-loop control. It is capable of providing the corresponding haptic feedback to the wearer through external force or vibration signal input. With this prototype, we presented a haptic feedback glove that supports precise operation by enhancing immersion and comprehensive sensation. Furthermore, we demonstrated it could shape appropriate interaction in different scenarios, including touching or grasping objects in virtual reality and teleoperation, illustrating its potential applications in human-in-loop interaction system and metaverse.

Modeling, Characterization, and Application of Soft Bellows-Type Pneumatic Actuators for Bionic Locomotion

Modeling, Characterization, and Application of Soft Bellows-Type Pneumatic Actuators for Bionic Locomotion

FEA-Based Inverse Kinematic Control: Hyperelastic Material Characterization of Self-Healing Soft Robots

Recent advances in soft continuum robots revealed the need for accurate models, required to develop advanced control strategies. In this article, a general methodology is presented that allows to create an accurate inverse kinematic control based on hyperelastic models, fitted on mechanical material properties. This methodology is based on finite element analysis (FEA) and links the mechanical properties of the material to the simulated behavior of a Pneunet actuator. This procedure is valid for any sort of hyperelastic material used in soft robotics, however, as a case study, a specific material with a self-healing ability is considered. This elastomeric polymer can recover from macroscopic damages without losing its mechanical performances, after undergoing a heat treatment. In this article, an accurate characterization of the mechanical behavior of this material is provided, involving mechanical testing, both in tensile and compression. On this experimental characterization data constitutive laws are fitted, using a FEA simulator. The robustness of this material modeling is shown, refitting the curve for material samples that were exposed to multiple damage-healing cycles. With this obtained constitutive model, a FEA simulation of a bending soft pneumatic actuator is developed. The simulation results are experimentally validated with a dedicated test bench consisting of a pressure control unit and a motion tracking camera. Using this validated model, an inverse kinematic control is developed, including the FEA simulation in the control scheme.

A Non-Invasive, Soft Robotic Wearable Glove to Attenuate Hand Tremors

Hand tremors are a widespread medical condition that decreases a patient’s quality of life. Due to uncontrollable hand shaking, patients have difficulty completing essential daily tasks. Many also experience embarrassment and anxiety in social settings, which negatively affects many areas of their lives. Current treatments are often invasive, ineffective, and have unwanted side effects. Some non-invasive solutions excessively limit hand motion, making them ineffective in helping the wearer complete tasks, while others are highly visible and bulky. This paper presents a minimalistic, non-invasive, wearable glove that uses active feedback control and miniature, soft robotic pneumatic actuators to attenuate hand tremors while allowing freedom of movement in other directions. We 3D-modelled the hand and wrist and designed our glove and actuators using Fusion 360. These models were imported into Matlab and Simulink, where we simulated the hand tremor, actuation, and control system using spatial contact forces. To actively reduce the tremor amplitude, we used an accelerometer to detect the onset of tremor motion and create a real-time estimate of the hand’s position. This estimate drove a closed-loop PI controller that produced force commands for the pneumatic actuators, which applied a counterforce opposing the direction of hand motion. Based on our simulation results, our active glove control system design successfully reduced the hand tremor amplitude by 75%, from a baseline of ±8.0mm to a significantly improved amplitude of ±2.0mm. These results indicate that our proposed solution could effectively attenuate hand tremors to a level that would increase the wearer’s quality of life.

Three-Dimensional Printable Ball Joints with Variable Stiffness for Robotic Applications Based on Soft Pneumatic Elastomer Actuators.

This paper contributes to a new design of the three-dimensional printable robotic ball joints capable of creating the controllable stiffness linkage between two robot links through pneumatic actuation. The variable stiffness ball joint consists of a soft pneumatic elastomer actuator, a support platform, an inner ball and a socket. The ball joint structure, including the inner ball and the socket, is three-dimensionally printed using polyamide−12 (PA12) by selective laser sintering (SLS) technology as an integral mechanism without the requirement of assembly. The SLS technology can make the ball joint have the advantages of low weight, simple structure, easy to miniaturize and good MRI compatibility. The support platform is designed as a friction-based braking component to increase the stiffness of the ball joint while withstanding the external loads. The soft pneumatic elastomer actuator is responsible for providing the pushing force for the support platform, thereby modulating the frictional force between the inner ball, the socket and the support platform. The most remarkable feature of the proposed variable stiffness design is that the ball joint has ‘zero’ stiffness when no pressurized air is supplied. In the natural state, the inner ball can be freely rotated and twist inside the socket. The proposed ball joint can be quickly stiffened to lock the current position and orientation of the inner ball relative to the socket when the pressurized air is supplied to the soft pneumatic elastomer actuator. The relationship between the stiffness of the ball joint and the input air pressure is investigated in both rotating and twisting directions. The finite element analysis is conducted to optimize the design of the support platform. The stiffness tests are conducted, demonstrating that a significant stiffness enhancement, up to approximately 508.11 N·mm reaction torque in the rotational direction and 571.93 N·mm reaction torque in the twisting direction at the pressure of 400 kPa, can be obtained. Multiple ball joints can be easily assembled to form a variable stiffness structure, in which each ball joint has a relative position and an independent stiffness. Additionally, the degrees of freedom (DOF) of the ball joint can be readily restricted to build the single-DOF or two-DOFs variable stiffness joints for different robotic applications.

A Comprehensive Comparison of Different Control Strategies to Adjust the Length of the Soft Contractor Pneumatic Muscle Actuator

According to the growing interest in the soft robotics research field, where various industrial and medical applications have been developed by employing soft robots. Our focus in this paper will be the Pneumatic Muscle Actuator (PMA), which is the heart of the soft robot. Achieving an accurate control method to adjust the actuator length to a predefined set point is a very difficult problem because of the hysteresis and nonlinearity behaviors of the PMA. So the construction and control of a 30 cm soft contractor pneumatic muscle actuator (SCPMA) were done here, and by using different strategies such as the PID controller, Bang-Bang controller, Neural network controller, and Fuzzy controller, to adjust the length of the (SCPMA) between 30 cm and 24 cm by utilizing the amount of air coming from the air compressor. All of these strategies will be theoretically implemented using the MATLAB/Simulink package. Also, the performance of these control systems will be compared with respect to the time-domain characteristics and the root mean square error (RMSE). As a result, the controller performance accuracy and robustness ranged from one controller to another, and we found that the fuzzy logic controller was one of the best strategies used here according to the simplicity of the implementation and the very accurate response obtained from this method.

Compact Multilayer Extension Actuators for Reconfigurable Soft Robots.

Soft robotic pneumatic actuators generally excel in the specific application they were designed for but lack the versatility to be redeployed to other applications. This study presents a novel and versatile soft compact multilayer extension actuator (MEA) to overcome this limitation. We use the MEA linear output in different hybrid configurations to achieve this versatility. The unique design and fabrication of the MEA allow for a compact elastomeric actuator with innate tension, capable of reverting to its initial state without the need for external stimulus. The MEA is made from alternating elastomers with different Young's modulus, bestowing the MEA with high durability, force, and extension capabilities. In addition, the MEA is lightweight at 4 g, capable of a high force-to-weight ratio of 1000 and an extension ratio of 525%. We also explored varying the MEA parameters, such as its material and dimension, which further enhance its properties. Subsequently, we showed four different design configurations encompassing the MEA to produce four basic motions, that is, push, pull, bend, and twist. Finally, we demonstrated three possible hybrid configurations for manipulation, locomotion, and assistive applications that highlight the versatility, manipulability, and modularity of the MEA.

Bioinspired untethered soft robot with pumpless phase change soft actuators by bidirectional thermoelectrics

Soft pneumatic actuators play an important role in the technological advancement of soft robotics, but they face intrinsic limitations due to the presence of complex and bulky air compressors and tubes, that make it difficult to design completely soft robotic systems. In this study, we develop a first demonstration of the soft thermo-pneumatic actuating module that operates based on the thermally controlled gas-liquid phase transition with the soft thermoelectric device. The bi-directional thermal managing ability (heating and cooling) of the thermoelectric device is capable of controlling the inflation and deflation of the pneumatic chamber with the aid of the thermally conductive silicone rubber that enhances the heat transfer. For the first time to the authors’ best knowledge, we demonstrate the active cooling of the thermoelectric device that accelerates the deflation rate by the active cooling functionality that significantly reduces the whole operating time. Additionally, to validate its practical usage in soft robotics, the soft actuating module shows different motions, such as bending and rectilinear stretching, based on the assembly design that is employed to develop the soft gripper and the entirely untethered soft earthworm robot without a complicated and bulky compressor system.