Soft Pneumatic Actuators Research Articles

A Comparison of Pneumatic Actuators for Soft Growing Vine Robots.

Soft pneumatic actuators are used to steer soft growing "vine" robots while being flexible enough to undergo the tip eversion required for growth. In this study, we compared the performance of three types of pneumatic actuators in terms of their ability to perform eversion, quasi-static bending, dynamic motion, and force output: the pouch motor, the cylindrical pneumatic artificial muscle (cPAM), and the fabric pneumatic artificial muscle (fPAM). The pouch motor is advantageous for prototyping owing to its simple manufacturing process. The cPAM exhibits superior bending behavior and produces the highest forces, whereas the fPAM actuates fastest and everts at the lowest pressure. We evaluated a range of dimensions for each actuator type. Larger actuators can produce more significant deformations and forces, but smaller actuators inflate faster and can evert at a lower pressure. Because vine robots are lightweight, the effect of gravity on the functionality of different actuators is minimal. We developed a new analytical model that predicts the pressure-to-bending behavior of vine robot actuators. Using the actuator results, we designed and demonstrated a 4.8 m long vine robot equipped with highly maneuverable 60 × 60 mm cPAMs in a three-dimensional obstacle course. The vine robot was able to move around sharp turns, travel through a passage smaller than its diameter, and lift itself against gravity.

Design of an actuator with bionic claw hook–suction cup hybrid structure for soft robot

To improve the adaptability of soft robots to the environment and achieve reliable attachment on various surfaces such as smooth and rough, this study draws inspiration from the collaborative attachment strategy of insects, cats, and other biological claw hooks and foot pads, and designs an actuator with a bionic claw hook–suction cup hybrid structure. The rigid biomimetic pop-up claw hook linkage mechanism is combined with a flexible suction cup of a ‘foot pad’ to achieve a synergistic adhesion effect between claw hook locking and suction cup adhesion through the deformation control of a soft pneumatic actuator. A pop-up claw hook linkage mechanism based on the principle of cat claw movement was designed, and the attachment mechanism of the biological claw hooks and footpads was analysed. An artificial muscle-spring-reinforced flexible pneumatic actuator (SRFPA) was developed and a kinematic model of the SRFPA was established and analysed using Abaqus. Finally, a prototype of the hybrid actuator was fabricated. The kinematic and mechanical performances of the SRFPA and entire actuator were characterised, and the attachment performance of the hybrid actuator to smooth and rough surfaces was tested. The results indicate that the proposed biomimetic claw hook–suction cup hybrid structure actuator is effective for various types of surface adhesion, object grasping, and robot walking. This study provides new insights for the design of highly adaptable robots and biomimetic attachment devices.

Seahorse-Tail-Inspired Soft Pneumatic Actuator: Development and Experimental Characterization.

The study of bio-inspired structures and their reproduction has always fascinated humans. The advent of soft robotics, thanks to soft materials, has enabled considerable progress in this field. Over the years, polyps, worms, cockroaches, jellyfish, and multiple anthropomorphic structures such as hands or limbs have been reproduced. These structures have often been used for gripping and handling delicate objects or those with complex unknown a priori shapes. Several studies have also been conducted on grippers inspired by the seahorse tail. In this paper, a novel biomimetic soft pneumatic actuator inspired by the tail of the seahorse Hippocampus reidi is presented. The actuator has been developed to make a leg to sustain a multi-legged robot. The prototyping of the actuator was possible by combining a 3D-printed reinforcement in thermoplastic polyurethane, mimicking the skeletal apparatus, within a silicone rubber structure, replicating the functions of the external epithelial tissue. The latter has an internal channel for pneumatic actuation that acts as the inner muscle. The study on the anatomy and kinematic behaviour of the seahorse tail suggested the mechanical design of the actuator. Through a test campaign, the actuator prototype was characterized by isotonic tests with an external null load, isometric tests, and activation/deactivation times. Specifically, the full actuator distension of 154.5 mm occurs at 1.8 bar, exerting a maximum force of 11.9 N, with an activation and deactivation time of 74.9 and 94.5 ms, respectively.

Learning control for body caudal undulation with soft sensory feedback

Soft bio-mimetic robotics is a growing field of research that seeks to close the gap with animal robustness and adaptability where conventional robots fall short. The embedding of sensors with the capability to discriminate between different body deformation modes is a key technological challenge in soft robotics to enhance robot control–a difficult task for this type of systems with high degrees of freedom. The recently conceived Linear Repetitive Learning Estimation Scheme (LRLES)–to be included in the traditional Proportional–integral–derivative (PID) control–is proposed here as a way to compensate for uncertain dynamics on a soft swimming robot, which is actuated with soft pneumatic actuators and equipped with soft sensors providing proprioceptive information pertaining to lateral body caudal bending akin to a goniometer. The proposed controller is derived in detail and experimentally validated, with the experiment consisting of tracking a desired trajectory for the bending angle envelope while continuously oscillating with a constant frequency. The results are compared vis a vis those achieved with the traditional PID controller, finding that the PID endowed with the LRLES outperforms the PID controller (though the latter has been separately tuned) and experimentally validating the novel controller’s effectiveness, accuracy, and matching speed.

Shape estimation for a TPU-based multi-material 3D printed soft pneumatic actuator using deep learning models

Shape estimation for a TPU-based multi-material 3D printed soft pneumatic actuator using deep learning models

A Versatile 3D-Printable Soft Pneumatic Actuator Design for Multi-Functional Applications in Soft Robotics.

Soft pneumatic actuators (SPAs) play a crucial role in generating movements and forces in soft robotic systems. However, existing SPA designs require significant structural modifications to be used in applications other than their original design. The present article proposes an omni-purpose fully 3D-printable SPA design inspired by membrane type mold and cast SPAs. The design features a spring-like zig-zag structure 3D-printed using an affordable 3D printer with thermoplastic polyurethane and a minimum wall thickness between 0.4 and 0.6 mm. The new SPA can perform unidirectional extension (30% extension) and bidirectional (rotation around same axis) bending (100°), with the ability to exert 10 N blocking force for 350 kPa pressure input. In addition, the design exhibits the capability to be scaled down for the purpose of accommodating limited spaces, while simultaneously enabling the reconfigurable interconnection of multiple SPAs to adapt to larger areas and navigate intricate trajectories that were not originally intended. The SPA's ability to be used in multiple applications without structural modification was validated through testing as a robot end-effector (gripper), artificial muscles in a soft tendon-driven prosthetic hand, a tube/tunnel navigator, and a robot crawler.

Toward Onboard Proportional Control of Multi-Chamber Soft Pneumatic Robots: A Magnetorheological Elastomer Valve Array.

Soft pneumatic actuators (SPAs) are commonly used in various applications because of their structural compliance, low cost, ease of manufacture, high adaptability, and safe human-robot interaction. The traditional approach for achieving proportional control of soft pneumatic robots requires the use of industrial proportional valves or syringe drivers, which are not only rigid and bulky but also hard to be integrated into the body of soft robots. In our previous research, we developed a Magnetorheological elastomer (MRE)-based soft valve that showed advantages for controlling SPAs due to its compliance, compactness, robustness, and compatibility for continuous pressure modulation. Modern soft robots with multiple chambers require more MRE valves onboard for their control. However, merely packing more MRE valves for soft robots can cause problems like magnetic interference, flow rate deviation, and overheating. Therefore, in this study, we proposed a two-dimensional MRE valve array design to solve issues of magnetic interference and overheating when expanding from a single MRE proportional valve into an integrated array. The magnetic interference and the overheating problem were investigated through multiphysics simulation, bringing the optimal choice of valve spacing (1.2 times the single valve diameter), magnetic coil pole arrangement (same pole), and the cooling system design (internal cooling chamber with flowing water). Physical experiments showed that our MRE valve array maintained its original flowrate performance with low magnetic interference (0.89 mT) and low coil temperature (under 73.9°C for 5 min).

Proprioception and Control of a Soft Pneumatic Actuator Made of a Self-Healable Hydrogel.

The current evolutionary trends in soft robotics try to exploit the capacities of smart materials to achieve compact robotics designs with embodied intelligence. In this way, the number of elements that compose the soft robot can be reduced, as the smart material can cover different aspects (e.g., structure and sensorization) all in one. This work follows this tendency and presents a custom-designed hydrogel that exhibits two smart features, self-healing and ionic conductivity, used to build a pneumatic actuator. The self-healing capability provides the actuator's structure with the ability to self-repair from damages (e.g., punctures or cuts), an important quality to prolong the life cycle of the actuator. The ionic conductivity enables the actuator's proprioception: the structure itself serves as a curvature sensor. The behavior of this proprioceptive curvature sensor is analyzed in this work, studying its linearity, stability, and performance after a self-healing process. This sensor is also proposed as feedback in a closed-loop scheme to automatically control the actuator's curvature. A proportional-integral-derivative controller is designed based on an empirical model of the actuator's dynamics, and then validated in experimental tests, proving the proprioceptive sensor as proper feedback. These control tests are performed over undamaged and self-healed actuators, thus demonstrating all the capabilities of our soft material.

Soft Pneumatic Haptic Wearable to Create the Illusion of Human Touch.

The ability to deliver sensations of human-like touch within virtual reality remains an important challenge to immersive, realistic experiences. Since conventional haptic actuators impart distinctively unnatural effects, we instead tackle this challenge through the design of a rendering mechanism using soft pneumatic actuators (SPA), embedded within a wearable jacket. The resulting system is then evaluated for its ability to mimic realistic touch gesture sensations of grab, touch, tap, and tickle as performed by human fingertips. The results of our experiments indicate that the stimuli produced by our design were reasonably effective in presenting realistic human-generated sensations.

Multidirectional Bending Soft Pneumatic Actuator With Fishbone-Like Strain-Limiting Layer for Dexterous Manipulation

Multidirectional Bending Soft Pneumatic Actuator With Fishbone-Like Strain-Limiting Layer for Dexterous Manipulation

Soft alchemy: a comprehensive guide to chemical reactions for pneumatic soft actuation

Soft robotics has emerged as a transformative field, leveraging bio-inspired novel actuation mechanisms to enable more adaptable, compliant, and sophisticated robotic systems. However, the portability of soft pneumatic actuators is typically constrained by the tethering to bulky power sources. This review offers a thorough analysis of autonomous power alternatives facilitated by chemical reactions for gas generation and absorption, a concept analogous to biological energy conversion processes. These bio-inspired strategies propel soft pneumatic actuators towards new horizons of autonomy and portability, essential for real-world applications. This comprehensive review explores the critical intersection of gas evolution reactions (GERs) and gas consumption reactions (GCRs) as a power source for pneumatic actuation in soft robotics. We here emphasize the importance and impact of bio-inspired design, control, efficiency, safety, and sustainability within soft robotics to not only mimic biological motions but to enhance them. This review explores the fundamentals of both pneumatic and chemically powered actuation, highlighting the need for careful consideration of reaction kinetics. Additionally, this work highlights key aspects of smart materials that draw from biological structures and response mechanisms, along with state-of-the-art techniques for precise pressure modulation. Finally, we chart prospective development pathways and provide a future outlook for bio-inspired soft robotics, emphasizing the transformative impact of integrating chemical actuation methods. This exploration underlines the quest for further autonomy in soft robotic systems and points towards the future opportunities in this exciting and fast-developing field.

Comparison between tendon-driven and soft pneumatic actuators in soft robotic hands

Bionic robotic hands have huge potential for applications. Traditional rigid robotic hands have the disadvantages of complex control and poor compliance, while soft robotic hands can well overcome these difficult drawbacks. Tendon-driven actuators and soft pneumatic actuators are two main actuating methods in soft robotic hands which are increasingly used due to their good compliance. This article first reviews the basic principles and structures of these two actuating methods in order to develop a basic understanding of different actuating methods. Then through comparing the dexterity and grasp performance of two specific designs, this article analyzes the characteristics of both methods in terms of their structures. Through discussion of structural differences, the systematic impact of the two actuating methods on function is analyzed and a general conclusion about differences in applicability is drawn. This article aims to help researchers have a better understanding of the two main actuating methods for soft robotic hands before they choose a specific actuating method, which can contribute to better completion of expected functions.

Design and performance analysis of soft pneumatic manipulator‐based linear cutter and stem holder for sweet pepper harvesting

AbstractEfficient automation of stem‐cutting processes in agricultural practices is crucial but little‐studied for improving productivity and reducing labor‐intensive tasks. To address this need, we present a customized stem cutter and stem holder designed explicitly for sweet pepper harvesting. The stem cutter comprises a fixed plate and a moving plate with a blade, which moves in a single plane at an upward angle of 30° toward the fixed plate. The movement of the blade is facilitated by a soft bellows pneumatic actuator constructed using silicon rubber material. By applying air pressure from a DC air pump, the actuator extends forward, causing the blade part to make contact with the fixed plate and effectively cut the stem. The stem holder holds the stem securely before cutting. It contains two inflatable manipulators made of soft materials that expand inwards when air‐filled, ensuring that the stem is securely held and readily collected after harvest. Both simulation and experimental results are presented, in which the average cutting time was 2.356 s. We believe that these innovations represent an efficient method for automating the stem‐cutting process in sweet pepper cultivation and thus significantly contribute to the field of agricultural robotics.

A hybrid ankle-foot orthosis with soft pneumatic actuation

This paper presents the design, development, and analysis of a powered ankle-foot orthosis for dorsiflexion assistance which aims to improve gait restoration by addressing issues related to orthosis misalignment, limited degrees of freedom, restricted range of motion, and muscular disuse. The proposed orthosis utilizes a novel hybrid design with a combination of both traditional and soft robotics for compliant and unrestrictive ankle dorsiflexion assistance for sufferers of footdrop. This article describes the complete design of the orthosis including analytical modeling and experimental testing of the soft pneumatic actuator and the development of gait phase detection and ankle angular feedback systems using wearable sensors for accurate and responsive control. Preliminary analysis was completed which validates the orthosis as a lightweight, unrestrictive, and compliant device that is capable of dorsiflexing the ankle of a person up to 100 kg, and at walking speeds appropriate for safe and effective community ambulation of up to 1.04 m/s. The novel design of the device demonstrates the potential for improved rehabilitative outcomes for patients with footdrop, due to the ability to adjust the assistive force of the device, throughout the progression of rehabilitation, which encourages muscular participation of the user and therefore reduces issues caused by muscular disuse.

Conceptualization and Topology Optimization of Ampheel: An Integration of Rolling Wheel and Turtle-Inspired Mechanism for Amphibious Mobile Robot

The primary distinguishing feature of mobile robots is the ability to traverse various environments, setting them apart in the realm of robotics. The mobility of a robot hinges primarily on its locomotion mechanism, which dictates how it moves. The existing unimodal mobile robots are limited to work within the environment for which they are designed for and hence lack a scope to adapt the change in the terrain especially when they put to work in a mixed environment like land and water. Many applications like land and underwater search/rescue, shore infrastructure inspection, coastal area defence and security, offshore energy harvesting, space exploration, etc. demand a mobile robot that can traverse in both terrestrial and aquatic environments with the help of dual or multimodal locomotion mechanism, something like an amphibious animal. Most of the available amphibious robotic solutions have different appendages for both the environment, need human intervention to changeover the mechanism for transition, require different driving system for land and water locomotion and have fragile structures that limit the manoeuvrability. The proposed conceptual design called “Ampheel” is a novel amphibious locomotion mechanism inspired by the biomechanics of freshwater turtles. Ampheel incorporates a rigid wheel, enabling the robot to move on land, integrating soft actuators within it which emulate the turtle's leg-like extensions and enable the aquatic locomotion. Unlike the existing amphibious robots, the Ampheel utilizes the rotational motion of itself as a common driving system for both the environments. This reduces the need of multiple driving systems and also simplifies the control system. Ampheel is designed for safe travelling on land considering the maximum payload of robot as 20 kg including self-weight. Topology optimization of Ampheel is also carried out using ANSYS software for reduction of weight. Additionally, a unique interfacing shaft is designed that transmits the required torque to Ampheel for rotation and also channelise the compressed air to soft pneumatic actuators for inflation during rotation of Ampheel in aquatic setting. The fabricated Ampheel assembly is experimentally checked for failure under the applicable loading condition and found safe.

Model-based linear control of nonlinear pneumatic soft bending actuators

Advanced model-based control techniques hold great promise for the precise control of pneumatic soft bending actuators (PSBAs) with strong nonlinearities. However, most previous controllers were designed in a cumbersome nonlinear form. Considering the simplicity of linear system theory, this paper presents a novel perspective on using model-based linear control to handle nonlinear PSBAs, and for the first time, summarizes two methodologies, global linearization and pseudo-linear construction. Derived from them, Koopman-based and hysteresis-based linear control approaches are proposed, respectively. For the former, a novel fusion prediction equation is developed to build a high-fidelity Koopman model, realizing global linearization, and then the linear model predictive control (MPC) is deployed. For the latter, the inverse of the generalized Prandtl–Ishlinskii (GPI) model cascades with the PSBA to construct a pseudo-linear system, eliminating the asymmetric hysteresis, which activates the linear proportional-integral-derivative (PID) control. It is worth noting that the above two are based on data-driven models adapted to various PSBAs with material and structural customization. Finally, the two model-based linear control approaches are verified and compared through a series of experiments. The results show that the proposed linear controls, with more concise designs, achieve comparable or even superior performance than nonlinear controls.

Experimental investigation on adaptive grasping of a novel 3D-MSSPA gripper in complex space

Many achievements for soft grippers are made in the structure design and grasping of target objects. However, there are limited studies on grasping objects with different attributes by soft grippers with omnidirectional bending capabilities within environments containing obstacles. In this paper, drawing inspiration from the multi-segment structure of the biological finger, a 3D-multi-segment soft pneumatic actuator (3D-MSSPA) with a strong envelope and omnidirectional independent bending is designed and manufactured. Additionally, we developed a novel prototype for a soft gripper prototype. A quasi-static model is proposed to effectively describe the bending deformation of 3D-MSSPA. The experimental results show that, for an applied pressure of 40kPa, the single segment SPA achieves a maximum bending angle of 175.2°, aligning closely with the theoretical prediction. Furthermore, the omnidirectional bending ability of multi-segments is analyzed through the finite element method, and a position compensation method for gravity deviation is presented. Through experiments, the pressure fore and lifting force of the soft gripper are investigated, yielding maximum values of 11N and 4.08N, respectively. These results serve as a benchmark for the mass grasping range of the target objects. Finally, the experiments are conducted to demonstrate the soft gripper’s adaptive and flexible grasping capabilities for objects with varying attributes, in both free space and complex space with obstacles. These experiments showcase their ability to avoid obstacles, adaptively grasp objects, and their good envelope ability. The proposed 3D-MSSPA can provide good inspiration and reference for flexible applications, particularly in fields such as post-disaster rescue and rehabilitation medicine.

Combined Soft Grasping and Crawling Locomotor Robot for Exterior Navigation of Tubular Structures

This paper presents the design, development, and testing of a robot that combines soft-body grasping and crawling locomotion to navigate tubular objects. Inspired by the natural snakes’ climbing locomotion of tubular objects, the soft robot includes proximal and distal modules with radial expansion/contraction for grasping around the objects and a longitudinal contractile–expandable driving module in-between for providing a bi-directional crawling movement along the length of the object. The robot’s grasping modules are made of fabrics, and the crawling module is made of an extensible pneumatic soft actuator (ePSA). Conceptual designs and CAD models of the robot parts, textile-based inflatable structures, and pneumatic driving mechanisms were developed. The mechanical parts were fabricated using advanced and conventional manufacturing techniques. An Arduino-based electro-pneumatic control board was developed for generating cyclic patterns of grasping and locomotion. Different reinforcing patterns and materials characterize the locomotor actuators’ dynamical responses to the varying input pressures. The robot was tested in a laboratory setting to navigate a cable, and the collected data were used to modify the designs and control software and hardware. The capability of the soft robot for navigating cables in vertical, horizontal, and curved path scenarios was successfully demonstrated. Compared to the initial design, the forward speed is improved three-fold.

BiLSTM-MLP based hysteresis modeling for soft pneumatic joint actuator

Soft pneumatic joint actuators show great potential in applications such as medical machinery, wearable devices, and soft grippers due to their inherent compliance. However, due to the internal friction and periodic relaxation of the elastic materials, the output force of soft pneumatic joint actuators has complex hysteresis characteristics, which seriously affects their control accuracy. To address the asymmetry hysteresis of actuator output force, a neural network model combining a bidirectional long-short-term memory network (BiLSTM) and a multilayer perceptron (MLP) is proposed. The MLP serves as the output layer of the BiLSTM, which is utilized to capture the time dependency of the actuator output force. By applying further nonlinear transformations, the feature representation of the BiLSTM output is retrieved and merged, improving the prediction and generalization capabilities of the model. The experimental results show that the BiLSTM-MLP model has a maximum output force error of only 0.306 N, an average error of less than 0.08 N, and a goodness of fit more than 0.999. Compared with the MLP, LSTM, BiLSTM, and improved PI models, the BiLSTM-MLP model can have higher prediction accuracy, better characterize the hysteresis properties of soft joint actuators, and provides a promising approach for hysteresis modeling of various elastomer actuators.

Analysis and Comparison of 3D Printed Soft Pneumatic Actuator Configurations for Swaying Lateral Motion of Planar Objects

Soft pneumatic actuators (SPAs) offer unique possibilities in robotics and automation due to their compliance and versatility. Unlike conventional actuators that can move either revolute or prismatic, SPAs can produce complex motions, especially when configured in combinations. This paper systematically investigated the performance of four distinct SPA configurations (Types 1-4) concerning the task of moving a planar square object and swaying it laterally to the right and left. These configurations are analyzed for their ability to achieve desired movements and orientations. Type 4, involving two SPAs, emerges as the most effective configuration, successfully navigating the challenges posed by the desired task. In contrast, Type 2, consisting of four SPAs, encounters difficulties in executing the task attributed to increased inertia, which was similar as type 3. Types 1, while not precisely meeting the primary task, reveal distinctive movements - left-right motion without orientation change. This study contributes valuable insights into the complex dynamics of SPAs, offering considerations for optimal design and performance. The outcomes could be developed in the future in soft actuator applications, especially those involving configurations based on specific needs.