Inflatable Actuators Research Articles

Effect of Filler Aspect Ratio on Stiffness and Conductivity in Phase‐Changing Particulate Composites

AbstractVariable stiffness in elastomers can be achieved through the introduction of low melting point alloy particles, such as Field's metal (FM), enabling on‐demand switchable elasticity and anisotropy in response to thermal stimulus. Because the FM particles are thermally transitioned between solid and liquid phases, it is beneficial for the composite to be electrically conductive so the stiffness may be controlled via direct Joule heating. While FM is highly conductive, spherical particles contribute to a high percolation threshold. In this paper, it is shown that the percolation threshold of FM particulate composites can be reduced with increasing particles aspect ratio. Increasing the aspect ratio of phase‐changing fillers also increases the rigid‐to‐soft modulus ratio of the composite by raising the elastic modulus in the rigid state while preserving the low modulus in the soft state. The results indicate that lower quantities of high aspect ratio FM particles can be used to achieve both electrical conductivity and stiffness‐switching via a single solution and without introducing additional conductive fillers. This technique is applied to enable a highly stretchable, variable stiffness, and electrically conductive composite, which, when patterned around an inflatable actuator, allows for adaptable trajectories via selective softening of the surface materials.

Paper-Based Robotics with Stackable Pneumatic Actuators.

This work presents a unique approach to the design, fabrication, and characterization of paper-based origami robotic systems consisting of stackable pneumatic actuators. These paper-based actuators (PBAs) use materials with high elastic modulus-to-mass ratios, accordion-like structures, and direct coupling with pneumatic pressure for extension and bending. The study contributes to the scientific and engineering understanding of foldable components under applied pneumatic pressure by constructing stretchable and flexible structures with intrinsically nonstretchable materials. Experiments showed that a PBA possesses a power-to-mass ratio greater than 80 W/kg, which is more than four times that of human muscle. This work also illustrates the stackability and functionality of PBAs by two prototypes: a parallel manipulator and a legged locomotor. The manipulator consisting of an array of PBAs can bend in a specific direction with the corresponding actuator inflated. In addition, the stacked actuators in the manipulator can rotate in opposite directions to compensate for relative rotation at the ends of each actuator to work in parallel and manipulate the platform. The locomotor rotates the PBAs to apply and release contact between the feet and the ground. Furthermore, a numerical model developed in this work predicts the mechanical performance of these inflatable actuators as a function of dimensional specifications and folding patterns. Overall, we use stacked origami actuators to implement functionalities of manipulation, gripping, and locomotion as conventional robotic systems. Future origami robots made of paper-like materials may be suitable for single use in contaminated or unstructured environments or low-cost educational materials.

Passive Flow Control for Series Inflatable Actuators: Application on a Wearable Soft-Robot for Posture Assistance

This paper presents a passive control method for multiple degrees of freedom in a soft pneumatic robot through the combination of flow resistor tubes with series inflatable actuators. We designed and developed these 3D printed resistors based on the pressure drop principle of multiple capillary orifices, which allows a passive control of its sequential activation from a single source of pressure. Our design fits in standard tube connectors, making it easy to adopt it on any other type of actuator with pneumatic inlets. We present its characterization of pressure drop and evaluation of the activation sequence for series and parallel circuits of actuators. Moreover, we present an application for the assistance of postural transition from lying to sitting. We embedded it in a wearable garment robot-suit designed for infants with cerebral palsy. Then, we performed the test with a dummy baby for emulating the upper-body motion control. The results show a sequential motion control of the sitting and lying transitions validating the proposed system for flow control and its application on the robot-suit.

A Novel Inflatable Actuator Based on Simultaneous Eversion Retraction.

Inflatable robotics is a promising area for the deployment of low-cost structures that are easy to transport and deploy while allowing safe interactions with humans and the environment. One of the key elements in the development of inflatable robots is their actuation system. In this work, we introduce the original concept of a Simultaneous Eversion-Retraction Inflatable Actuator, used in the actuation of an inflatable joint of a long-range manipulator. Through an analytical study, simple relations about the total stroke and the effective area are obtained. These relations are compared to finite elements simulations and contrasted with experimental data obtained from tensile tests of an actuator prototype. The results show that the proposed design outperforms existing concepts in terms of total stroke and force distribution through the entire stroke, which makes it suitable for the actuation of long reach inflatable arms.

Design of an Inflatable Wrinkle Actuator With Fast Inflation/Deflation Responses for Wearable Suits

In recent years, inflatable actuators have been widely used in wearable suits to assist humans who need help in moving their joints. Despite their lightweight and simple structure, they have long inflation and deflation times, which make their quick use difficult. To resolve this issue, we propose an inflatable wrinkle actuator with fast inflation and deflation responses. First, a theoretical model is proposed to develop an actuator that satisfies the design requirements: the desired assistive torque and the foam factor based on the wearability. Second, we reduce the inflation and deflation times by partially controlling the actuator layers and by designing pneumatic circuits using a vacuum ejector. To validate the usability of the actuator in wearable suits, we applied it to a wearable knee suit, and the inflation and deflation times were 0.40 s and 0.16 s, respectively. As a result, we ensured that the actuator did not interfere with human knee joint movement during walking by creating any residual resistance.

Inflatable Soft Wearable Robot for Reducing Therapist Fatigue During Upper Extremity Rehabilitation in Severe Stroke

Intense therapy is a key factor to improve rehabilitation outcomes. However, when performing rehabilitative stretching with the upper limb of stroke survivors, therapist fatigue is often the limiting factor for the number of repetitions per session. In this work we present an inflatable soft wearable robot aimed at improving severe stroke rehabilitation by reducing therapist fatigue during upper extremity stretching. The device consists of a textile-based inflatable actuator anchored to the torso and arm via functional apparel. Upon inflation, the device creates a moment of force about the glenohumeral joint to counteract effects of gravity and assist in elevating the arm. During a device-assisted (i.e. inflated) standard stretching protocol with a therapist, we showed increased range of motion across five stroke survivors, and reduced muscular activity and cardiac effort by the therapist, when comparing to a vented device condition. Our results demonstrate the potential for this technology to assist a therapist during upper extremity rehabilitation exercises and future studies will explore its impact on increasing dose and intensity of therapy delivered in a given session, with the goal of improving rehabilitation outcomes.

Towards Untethered Soft Pneumatic Exosuits Using Low-Volume Inflatable Actuator Composites and a Portable Pneumatic Source

The application of pneumatic soft robots is limited by factors such as operational pressure and air flow rates. Pneumatic soft robots are typically tethered in nature due to the high energy costs for actuation as well as the lack of portable pneumatic sources capable of providing high pressures and air flow rates. This work presents a low-volume inflatable actuator composite (IAC) designed to reduce energy costs of actuation and a portable pneumatic source to overcome the aforementioned issues towards untethered applications. The pressure-deflection characteristics of the fabricated IAC are compared with those of a completely fabric-based beam using experimental results and finite element analysis. FEM models of IACs with varying volumes are generated and actuation speeds are measured using a pressure step response test. The force output and hysteresis of the actuator are studied. The developed portable pneumatic source is capable of generating a pressure and flow rates of 0.131 MPa and 21.45 standard litres per minute (SLPM), respectively. The IACs and portable pneumatic source are integrated with a soft exosuit to assist knee extension, and the integrated system is evaluated with three healthy participants for incline walking. A reduction of muscle activities in the Vastus Lateralis muscle group is observed for all the three participants when the exosuit is active.

Soft pneumatic actuator-driven origami-inspired modular robotic “pneumagami”

This article presents a new modular robotic platform for enabling reconfigurable, actively controlled, high-degree-of-freedom (high-DoF) systems with compact form factor. The robotic modules exploit the advantages of origami-inspired construction methods and materials, and soft pneumatic actuators (SPAs) to achieve an actuator embedded, parallel kinematic mechanism with three independently controlled “waterbomb” base legs. The multi-material, layer-fabricated body of the modules features selectively compliant flexure hinge elements between rigid panels that define the module as a kinematic 6R spherical joint. The precision layer-fabrication technique is also used to form embedded distribution channels within the module base to connect actuators to onboard control hardware. A decentralized control architecture is applied by integrating each module with small-scale solenoid valves, communication electronics, and sensors. This design approach enables a single pneumatic supply line to be shared between modules, while still allowing independent control of each leg joint, driven by soft, inflatable pouch actuators. A passive pneumatic relay is also designed and incorporated in each module to leverage the coupled, inverted inflation, and exhaust states between antagonistic actuator pairs allowing both to be controlled by a single solenoid valve. A prototype module is presented as the first demonstration of integrated modular origami and SPA design, or pneumagami, which allows predefined kinematic structural mechanisms to locally prescribe specific motions by active effect, not just through passive compliance, to dictate task space and motion. The design strategy facilitates the composition of lightweight, high-strength robotic structures with many DoFs that will benefit various fields such as wearable robotics.

Soft fluidic actuator based on nylon artificial muscles

Soft robots have become increasingly prevalent due to their distinct advantages over traditional rigid robots such as high deformability and good impact resistance. However, soft robotics are currently limited by bulky, non-portable methods of actuation. In this study, we propose an inflatable soft actuator driven by nylon artificial muscles. By utilizing nylon artificial muscles, the system does not require sizable pumps or compressors for actuation. The paper investigates the properties of the proposed soft actuation system both analytically and experimentally.

Shape Programming Using Triangular and Rectangular Soft Robot Primitives.

This paper presents fabric-based soft robotic modules with primitive morphologies, which are analogous to basic geometrical polygons—trilateral and quadrilateral. The two modules are the inflatable beam (IB) and fabric-based rotary actuator (FRA). The FRA module is designed with origami-inspired V-shaped pleats, which creates a trilateral outline. Upon pressurization, the pleats unfold, which enables propagation of angular displacement of the FRA module. This allows the FRA module to be implemented as a mobility unit in the larger assembly of pneumatic structures. In the following, we examine various ways by which FRA modules can be connected to IB modules. We studied how different ranges of motion can be achieved by varying the design of the rotary joint of the assemblies. Using a state transition-based position control system, movement of the assembled modules could be controlled by regulating the pneumatic pressurization of the FRA module at the joint. These basic modules allow us to build different types of pneumatic structures. In this paper, using IB and FRA modules of various dimensions, we constructed a soft robotic limb with an end effector, which can be attached to wheelchairs to provide assistive grasping functions for users with disabilities.

Marionette-based programming of a soft textile inflatable actuator

Soft inflatable actuators show several advantages in flexible and versatile behaviours. A wide range of motions such as bending, expanding and twisting motions has been obtained by introducing oblique structures in silicon-based actuators. However, this mechanical programming allows an actuator to generate one single behaviour while inflated. In this paper we propose an online programming of behaviour of a soft textile inflatable actuator. Four strings are attached to the tip of the actuator and travelling through its longitudinal axis to the fixed side where they are pulled with different tension forces. The applied forces on all strings define the actuator behaviour when inflated. Bending in different directions, twisting, and elongation behaviours were generated online by varying the strings pulling forces and the air pressure of the actuator.

Design and Evaluation of a Novel Hybrid Soft Surgical Gripper for Safe Digital Nerve Manipulation.

Forceps are essential tools for digital nerve manipulation during digital nerve repair surgery. However, surgeons have to operate forceps with extreme caution to prevent detrimental post-operative complications caused by over-gripping force. Their intrinsically safe characteristics have led to the increasing adoption of soft robotics in various biomedical applications. In this paper, a miniaturized hybrid soft surgical gripper is proposed for safe nerve manipulation in digital nerve repair surgery. This new surgical gripper includes a soft inflatable actuator and a gripper shell with a hook-shaped structure. The ability to achieve a compliant grip and safe interaction with digital nerves is provided by the inflated soft pneumatic actuator, while the rigid hook retractor still allows surgeons to scoop up the nerve from its surrounding tissues during surgery. The performance of the proposed surgical gripper was evaluated by the contact/pulling force sensing experiments and deformation measurement experiments. In the cadaver experiments, this new surgical gripper was able to complete the required nerve manipulation within the limited working space. The average deformation of the digital nerve with an average diameter of 1.45 mm gripped by the proposed surgical gripper is less than 0.22 mm. The average deformity is less than 15% of its original diameter.

Sit-to-Stand Support for Elderly People using Wearable Inflatable Actuator

Sit-to-Stand Support for Elderly People using Wearable Inflatable Actuator

Hardware Sequencing of Inflatable Nonlinear Actuators for Autonomous Soft Robots.

Soft robots are an interesting alternative for classic rigid robots in applications requiring interaction with organisms or delicate objects. Elastic inflatable actuators are one of the preferred actuation mechanisms for soft robots since they are intrinsically safe and soft. However, these pneumatic actuators each require a dedicated pressure supply and valve to drive and control their actuation sequence. Because of the relatively large size of pressure supplies and valves compared to electrical leads and electronic controllers, tethering pneumatic soft robots with multiple degrees of freedom is bulky and unpractical. Here, a new approach is described to embed hardware intelligence in soft robots where multiple actuators are attached to the same pressure supply, and their actuation sequence is programmed by the interaction between nonlinear actuators and passive flow restrictions. How to model this hardware sequencing is discussed, and it is demonstrated on an 8-degree-of-freedom walking robot where each limb comprises two actuators with a sequence embedded in their hardware. The robot is able to carry pay loads of 800 g in addition to its own weight and is able to walk at travel speeds of 3 body lengths per minute, without the need for complex on-board valves or bulky tethers.

Fiber Embroidery of Self-Sensing Soft Actuators.

Natural organisms use a combination of contracting muscles and inextensible fibers to transform into controllable shapes, camouflage into their surrounding environment, and catch prey. Replicating these capabilities with engineered materials is challenging because of the difficulty in manufacturing and controlling soft material actuators with embedded fibers. In addition, while linear and bending motions are common in soft actuators, rotary motions require three-dimensional fiber wrapping or multiple bending or linear elements working in coordination that are challenging to design and fabricate. In this work, an automatic embroidery machine patterned Kevlar™ fibers and stretchable optical fibers into inflatable silicone membranes to control their inflated shape and enable sensing. This embroidery-based fabrication technique is simple, low cost, and allows for precise and custom patterning of fibers in elastomers. Using this technique, we developed inflatable elastomeric actuators embedded with a planar spiral pattern of high-strength Kevlar™ fibers that inflate into radially symmetric shapes and achieve nearly 180° angular rotation and 10 cm linear displacement.

Chip-on-tip endoscope incorporating a soft robotic pneumatic bending microactuator.

In the ever advancing field of minimally invasive surgery, flexible instruments with local degrees of freedom are needed to navigate through the intricate topologies of the human body. Although cable or concentric tube driven solutions have proven their merits in this field, they are inadequate for realizing small bending radii and suffer from friction, which is detrimental when automation is envisioned. Soft robotic actuators with locally actuated degrees of freedom are foreseen to fill in this void, where elastic inflatable actuators are very promising due to their S3-principle, being Small, Soft and Safe. This paper reports on the characterization of a chip-on-tip endoscope, consisting out of a soft robotic pneumatic bending microactuator equipped with a 1.1 × 1.1mm2 CMOS camera. As such, the total diameter of the endoscope measures 1.66mm. To show the feasibility of using this system in a surgical environment, a preliminary test on an eye mock-up is conducted.

A New Spiral-Type Inflatable Pure Torsional Soft Actuator.

Soft robot has become a hot topic recently due to its distinct advantages over traditional rigid robots such as high deformability and good impact resistance. However, the coupled deflections of flexile materials bring challenges to soft robotic research in many aspects such as kinematic modeling, dynamic analysis, and control. Besides, unwanted deformations might enlarge external dimensions of soft robots, causing a reduction in the efficiency and bringing about unexpected or harmful contacts with surrounding environments that will significantly affect the robots' performance. In this study, we propose a new inflatable soft actuator driven by two spiral chambers twined with fibers for the first time. A key feature of this actuator is that it possesses a pure and high-efficient torsional motion with no bending and extension movements when works without load, which reduces the difficulties of theoretical analysis and control to some extent. Kinematic model is established by combining virtual work principle and elastic strain energy function for nonlinear flexible materials. The new soft torsional actuator module is carefully designed and fabricated, of which both the kinematic property and output torque are investigated experimentally. Results show that the module exhibits good linearity with air pressure ranging from 35 to 100 kPa, and can provide a torsion angle of up to 110° with an angular displacement accuracy of ±2° in empty loaded conditions; the maximum output torque reaches 0.026 N·m with the corresponding air pressure of 100 kPa. Finally, three soft robots are assembled by utilizing this new, inflatable, pure, soft torsional actuator, and successfully carry out different manipulating tasks. This work might provide some insights into the design of linear soft actuators without coupled deformations in future.

Manufacturing 2DOF Inflatable Joint Actuator by Pneumatic Control

Manufacturing 2DOF Inflatable Joint Actuator by Pneumatic Control

A Soft-Inflatable Exosuit for Knee Rehabilitation: Assisting Swing Phase During Walking.

In this paper, we present a soft-inflatable exosuit to assist knee extension during gait training for stroke rehabilitation. The soft exosuit is designed to provide 25% of the knee moment required during the swing phase of the gait cycle and is integrated with inertial measurement units (IMUs) and smart shoe insole sensors to improve gait phase detection and controller design. The stiffness of the knee joint during level walking is computed using inverse dynamics. The soft-inflatable actuators, with an I cross-section, are mechanically characterized at varying angles to enable generation of the required stiffness outputs. A linear relation between the inflatable actuator stiffness and internal pressure as a function of the knee angle is obtained, and a two-layer stiffness controller is implemented to assist the knee joint by providing appropriate stiffness during the swing phase. Finally, to evaluate the ability of the exosuit to assist in swing motion, surface-electromyography (sEMG) sensors are placed on the three muscle groups of the quadriceps and two groups of the hamstrings, on three healthy participants. A reduction in muscle activity of the rectus femoris, vastus lateralis, and vastus medialis is observed, which demonstrates feasibility of operation and potential future usage of the soft inflatable exosuit by impaired users.

Inflatable actuators: an attempt for a common approach based on Treloar’s rubber elasticity theory

Inflatable actuators are defined as pressure hyperelastic vessels whose expansion is constrained for generating either movements in extension, or typical contractile movements of artificial muscles. By using Treloar’s theory of rubber elasticity, applied to thin-walled pressure vessels, we propose to determine in which conditions they can be considered as stable open-loop positioning actuators. Antagonism can be viewed as an extension of this open-loop stability principle applicable to artificial muscles as to extensible actuators. We especially show its relevance for multiple chambers pressurized cylinders and how Treloar’s theory can help to model their bending in a readable and relevant formal way. We also try to justify why we think that antagonism applied to extensible actuators can actually appear as the best way for designing miniaturized multiple degrees of freedom rubber made microactuators if, however, only a limited power is required.