McKibben Actuators Research Articles

Soft skeletons transmit force with variable gearing.

A hydrostatic skeleton allows a soft body to transmit muscular force via internal pressure. A human's tongue, an octopus' arm and a nematode's body illustrate the pervasive presence of hydrostatic skeletons among animals, which has inspired the design of soft engineered actuators. However, there is a need for a theoretical basis for understanding how hydrostatic skeletons apply mechanical work. We therefore modeled the shape change and mechanics of natural and engineered hydrostatic skeletons to determine their mechanical advantage (MA) and displacement advantage (DA). These models apply to a variety of biological structures, but we explicitly consider the tube feet of a sea star and the body segments of an earthworm, and contrast them with a hydraulic press and a McKibben actuator. A helical winding of stiff, elastic fibers around these soft actuators plays a critical role in their mechanics by maintaining a cylindrical shape, distributing forces throughout the structure and storing elastic energy. In contrast to a single-joint lever system, soft hydrostats exhibit variable gearing with changes in MA generated by deformation in the skeleton. We found that this gearing is affected by the transmission efficiency of mechanical work (MA×DA) or, equivalently, the ratio of output to input work. The transmission efficiency changes with the capacity to store elastic energy within helically wrapped fibers or associated musculature. This modeling offers a conceptual basis for understanding the relationship between the morphology of hydrostatic skeletons and their mechanical performance.

Architectural gear ratio depends on actuator spacing in a physical model of pennate muscle.

Pennate muscles are defined by the architectural arrangement of their muscle fibers, which run at an angle to the primary axis of muscle shortening. Pennation angles can vary dynamically over the course of individual contractions, influencing the speed and distance of muscle shortening. Despite their relevance to muscle performance, the physical mechanisms that drive dynamic changes in pennation angle remain poorly understood. Muscle fibers bulge radially as they shorten, a consequence of maintaining a constant internal fluid volume, and we hypothesized that radial interactions between tightly packed muscle fibers are essential to dynamic pennation angle changes. To explore this, we built physical models of pennate muscles in which the radial distance between fiber-like actuators could be experimentally altered. Models were built from pennate arrays of McKibben actuators, a type of pneumatic actuator that forcefully shortens and bulges radially when inflated with compressed air. Consistent with past studies of biological muscle and engineered pennate actuators, we found that the magnitude of pennation angle change during contraction varied with load. Importantly, however, we found that pennation angle changes were also strongly influenced by the radial distance between neighboring McKibben actuators. Increasing the radial distance between neighboring actuators reduced pennation angle change during contraction and effectively eliminated variable responses to load. Radial interactions between muscle fibers are rarely considered in theoretical and experimental analyses of pennate muscle; however, these findings suggest that radial interactions between fibers drive pennation angle changes and influence pennate muscle performance. Our results provide insight into the fundamental mechanism underlying dynamic pennation angle changes in biological muscle and highlight design considerations that can inform the development of engineered pennate arrays.

Pressure Stimuli and Spatiotemporal Illusions on the Forearm.

To design complex wearable haptic interfaces using pressure, we have to explore how we can use pressure stimuli to their full potential. Haptic illusions, such as apparent motion and apparent location, can be a part of this. If these illusions can be evoked with pressure, haptic patterns can increase in complexity without increasing the number of actuators or combining different types of actuators. We did two psychophysical experiments with pressure stimuli on the forearm using a pneumatic sleeve with multiple, individually controlled McKibben actuators. In Experiment 1, we found that spatial integration of two simultaneously presented stimuli occurred for distances up to 61 mm. In Experiment 2, we found that apparent motion can be elicited with distinct pressure stimuli over a range of temporal parameters. These results clearly show spatio-temporal integration in the somatosensory system for pressure stimuli. We discuss these findings in relation to effects found for vibration and the mechanoreceptors in the glabrous skin.

Active textile: woven-cloth-like mechanisms consist of thin McKibben actuators

ABSTRACT Pneumatic artificial muscles have some advantages. Especially, the flexibility of artificial muscles contributes to compose novel mechanisms. Authors succeeded in manufacturing novel soft mechanisms that are composed of only woven artificial muscles and strings. We called this mechanism active textile. The structure is uniformed with woven artificial muscles; nevertheless, the forms and movements of mechanisms are diverse. We aim to establish a design method for these active textiles. We reported that deformations of active textiles are changed by woven structures. In addition to that, wefts' characteristics such as cross-sectional shape, bending stiffness and elasticity determine the characteristic of a textile. Deformations of active textiles are caused by the contraction and the expansion of thin McKibben muscles. In this paper, authors focus on the contraction and curvature deformation of the active textile due to the drive of the artificial muscles.We fabricated active textiles with different wefts and different weave structures.Contraction and bending deformations of active textiles in the warp and weft directions are assumed geometrically. In addition, we verified the validity by experiments.

Performance Evaluation for Braided McKibben Pneumatic Actuators in Telescopic Nested Structure

This study presents the usage of Braided Actuators with Nested Structures to improve the stroke characteristics of the McKibben actuators. Thin McKibben actuators can contract 23% of their original length when pressurized. Usage of braiding and nested structures to improve contraction ratio has proven successful. In this study, we use a combined telescopic Nested Structure and Braided Actuators to increase the contraction ratio of the actuators. The Nested Braided Actuator (NBA) performance was compared to Single Actuator (SA), Braided Actuator (BA), and Nested Actuator (NA). The results at 350 kPa indicate that, NBA had the highest contraction ratio of 45.5%, followed by NA (39.38%), BA (29.57%), and SA (23.41%). At 350 kPa, the contraction force per actuator NBA was 36.29 N, NA exerted 41.33 N, BA exerted 30.41 N and SA exerted 43.8 N. The actuator's performance showed that it was capable of high stroke with only a 20–30% loss in contraction force compared to the SA.

Development of soft actuators for stroke rehabilitation using deep learning

Automation has created a mind-blowing impact in diversified fields all over the world. Not only in business but also in various domains like health care sectors, manufacturing, etc. a faultless execution is a prime concern. Robotic Process Automation has paved the way for research in the mechanical and mechatronics field. Software robots are trained well to complete repetitive tasks in an efficient manner. A design of such a soft robot can be greatly helpful in the arena of healing. Automation of Rehabilitation therapy has gained attention in recent years. The main aspiration towards the conduct of this research work is to accomplish a soft exoskeleton robot using a thin McKibben actuator applying Deep Learning approaches to aid automatic therapy to the paralyzed patients and help them carry out the hand movement-based exercises. Convolutional Neural Network (CNN) algorithm will be used to support the training of the AI-enabled automated device. The proposed methodology will support stroke survivors to perform exercises independently to enhance their hand motor recovery. For this purpose, it involves pneumatic soft actuator technology using thin McKibben artificial muscles to create a cognitive potential to induce rehabilitation. A soft actuator is proposed so as to confirm the safety purposes of stroke patients.

Experimentally Identified Models of McKibben Soft Actuators as Primary Movers and Passive Structures

Abstract Soft robots join body and actuation, forming their structure from the same elements that induce motion. Soft actuators are commonly modeled or characterized as primary movers, but their second role as support structure introduces strain–pressure combinations outside of normal actuation. This article examines a more complete set of possible strain–pressure combinations for McKibben actuators, including passive or unpressurized, deformation, pressurized extension and compression of a pressurized actuator beyond the maximum actuation strain. Each region is investigated experimentally, and empirical force–displacement–pressure relationships are identified. Particular focus is placed on ensuring that empirical relationships are consistent at boundaries between an actuator’s strain–pressure regions. The presented methodology is applied to seven McKibben actuator designs, which span variations in wall thickness, enclosure material, and actuator diameter. Empirical results demonstrate a trade-off between maximum contraction strain and force required to passively extend. The results also show that stiffer elastomers require an extreme increase in pressure to contract without a compensatory increase in maximum achieved force. Empirical force–displacement–pressure models were developed for each variant across all the studied strain–pressure regions, enabling future design variation studies for soft robots that use actuators as structures.

Modeling of Resistive Forces and Buckling Behavior in Variable Recruitment Fluidic Artificial Muscle Bundles

Fluidic artificial muscles (FAMs), also known as McKibben actuators, are a class of fiber-reinforced soft actuators that can be pneumatically or hydraulically pressurized to produce muscle-like contraction and force generation. When multiple FAMs are bundled together in parallel and selectively pressurized, they can act as a multi-chambered actuator with bioinspired variable recruitment capability. The variable recruitment bundle consists of motor units (MUs)—groups of one of more FAMs—that are independently pressurized depending on the force demand, similar to how groups of muscle fibers are sequentially recruited in biological muscles. As the active FAMs contract, the inactive/low-pressure units are compressed, causing them to buckle outward, which increases the spatial envelope of the actuator. Additionally, a FAM compressed past its individual free strain applies a force that opposes the overall force output of active FAMs. In this paper, we propose a model to quantify this resistive force observed in inactive and low-pressure FAMs and study its implications on the performance of a variable recruitment bundle. The resistive force behavior is divided into post-buckling and post-collapse regions and a piecewise model is devised. An empirically-based correction method is proposed to improve the model to fit experimental data. Analysis of a bundle with resistive effects reveals a phenomenon, unique to variable recruitment bundles, defined as free strain gradient reversal.

Influence of hydraulic versus pneumatic working fluids on quasi-static force response of fluidic artificial muscles

There is currently an increased interest in extending the application of McKibben actuators beyond gaseous environments and working fluids. Extensive efforts have already established force response models for pneumatically driven McKibben actuators that are independent of working fluid material properties, however, the model’s independence to working fluid choice has yet to be validated with empirical evidence. This paper experimentally investigates the effect of working fluid type on the quasi-static pressure dependent force-contraction response of McKibben actuators. Using either air or water as the working fluid, characteristic isobaric force-contraction response curves are compared for both large and small-scale McKibben actuators. To ensure truly isobaric force-contraction characterizations, hydraulic and pneumatic pressure systems were developed to provide precise and accurate control of pressure. Experimental testing proved using air or water as the working fluid resulted in nearly identical isobaric force-contraction response curves, demonstrating that McKibben actuator’s quasi-static force response is independent of working fluid choice. This study establishes that the applicability of existing force response models for pneumatic McKibben actuators can be extended to any practical liquids or gases.

Design and Modeling of Soft Pneumatic Helical Actuator with High Contraction Ratio

Soft contraction actuators are becoming important elements particularly for human-friendly robotic applications. However, it is challenging to achieve both a large operating distance while generating practical force. Hence, we present a new soft contraction actuator capable of realizing a high ratio contraction by pneumatic power. It can be easily fabricated using soft materials, including rubber tubes, one-way extensible cloth, and inextensible wire. Its initial shape is tubular but it can curve and coil to a helix shape owing to its different extensibilities on two sides when pressurized. A maximum contraction ratio of 78% and a 23 N contraction force can be achieved with an 11.6 mm initial outer diameter tube under 0.3 MPa. The effect of the tilt angle of a one-way extensible cloth on the helical shape is investigated, and a mathematical model illustrating the relationship between the contraction ratio and force is derived. Our experimental results suggest that this helical actuator has a much higher contraction ratio than a McKibben actuator under the same conditions. Finally, we discuss the potential application of the proposed actuator to a wearable device, i.e., for assisting the dorsiflexion of an ankle joint requiring a wide range of motion.

Realistic and Highly Functional Pediatric Externally Powered Prosthetic Hand Using Pneumatic Soft Actuators

It is known that introducing a pediatric externally powered prosthetic hand from an early age has certain merits such as the recovery of body image. However, this process is not popular in Japan. The high cost and technological problems of the hand have resulted in difficulty in its popularization. The pediatric prosthetic hand must be lighter and smaller than the adult one. Furthermore, parents of users prefer a prosthetic hand, such as a human arm and hand. We developed a prosthetic hand that demonstrates certain functionalities and appearances similar to a real human hand. The prosthetic hand consists of miniature McKibben actuators and is manufactured from acrylonitrile-butadiene-styrene resin and covered by a silicon glove. It has flexible joint structures and can grasp objects of various shapes. In this paper, we present a prototype of the pediatric prosthetic hand and the results of gripping experiments, bending and extension of finger experiments, and user tests.

An Euler–Bernoulli beam model for soft robot arms bent through self-stress and external loads

Soft robot arms remain challenging to model effectively. The arm’s primary deformation is bending, coupled with extension or compression, but the strains experienced can be high, the materials are generally nonlinear, and the deformations are large. Existing work has focused on models intended to improve control, which rely on post-manufacturing empirical characterization of each arm design. This article presents a quasi-static model based on Euler–Bernoulli beam theory that generalizes across a broad set of arm designs. The model is implemented for fluid-driven soft arms constructed with McKibben actuators. Actuators are treated as active materials, and their force is characterized as a nonlinear function of pressure and uniaxial strain. The model is validated for multiple soft arms under external loads, and further use is demonstrated through an investigation of the soft arms’ loaded workspace. Higher load capacities are shown to be concentrated at the arms’ midlines. Distal taper is examined in an example arm design, and is shown to improve range of motion and load resistance when compared to a constant width arm. The model can be used to evaluate variations on the number and arrangement of actuators in an arm, and it is proposed as a first order design and analysis method for soft robot arms.

Developing a hybrid soft mechanism for assisting individualized flexion and extension of finger joints.

Various robotic devices have been developed for home rehabilitation and support of therapists. Special attention has been focused on soft actuators due to their high viscoelasticity and flexibility, which can contribute to the safety and affinity with the users. However, most of them aimed at the assist of finger flexion, and few have been designed to support extension actively. Moreover, for most soft actuator based mechanisms, the individual-adaptability have not been considered nor appropriately evaluated. Consequently, the effect of individual difference on the assistance using soft robotic devices is unclear, and for the purpose of dealing with the individual difference, the whole mechanism should be designed and fabricated for each individual user, which is ineffective for the rehabilitation support. In this study, we proposed a hybrid soft mechanism with modularized fiber-reinforced elastomer actuators for joint-dependent flexion support and McKibben actuators for finger extension support. Without further changing the design of the elastomer actuator, the hybrid mechanism could be adapted to individual hand difference: proportions of hand segments, range of motion (ROM) and torque characteristics of the joints. A prototype of the mechanism was fabricated and evaluated. The results showed that the mechanism could meet the requirement of finger function assist. Moreover, the mechanism could be fine-tuned towards the individual hand by changing the fiber-reinforcement and adjusting the fittings of the actuators.

Comparing model-based control methods for simultaneous stiffness and position control of inflatable soft robots

Inflatable robots are naturally lightweight and compliant, which may make them well suited for operating in unstructured environments or in close proximity to people. The inflatable joints used in this article consist of a strong fabric exterior that constrains two opposing compliant air bladders that generate torque (unlike McKibben actuators where pressure changes cause translation). This antagonistic structure allows the simultaneous control of position and stiffness. However, dynamic models of soft robots that allow variable stiffness control have not been well developed. In this work, a model that includes stiffness as a state variable is developed and validated. Using the stiffness model, a sliding mode controller and model predictive controller are developed to control stiffness and position simultaneously. For sliding mode control (SMC), the joint stiffness was controlled to within 0.07 Nm/rad of a 45 Nm/rad command. For model predictive control (MPC) the joint stiffness was controlled to within 0.045 Nm/rad of the same stiffness command. Both SMC and MPC were able to control to within 0.5° of a desired position at steady state. Stiffness control was extended to a multiple-degree-of-freedom soft robot using MPC. Controlling stiffness of a 4-DOF arm reduced the end-effector deflection by approximately 50% (from 17.9 to 12.2cm) with a 4 lb (1.8 kg) step input applied at the end effector when higher joint stiffness (40 Nm/rad) was used compared with low stiffness (30 Nm/rad). This work shows that the derived stiffness model can enable effective position and stiffness control.

Segmentation of a Soft Body and its Bending Performance using Thin McKibben Muscle

In recent years, soft actuator has been extensively developed in robotic research. This type of robot is expected to work with human with its flexible and adaptable advantage. The actuator material is soft, light, safe and high compliant. Due to these factors, soft McKibben is of interest as an actuator for this research for bending application. This paper introduces a variant bending analysis of a soft body manipulated using soft McKibben actuators. A series of 1.80 mm width with the length of 120.0 mm McKibben actuator is used to control the bending motion. The design consists of four McKibben actuators arranged in parallel and compacted in a soft body. The bending behavior was evaluated using an experimental test with a variety of pneumatic input pressure and length section on the actuator. The experiment showed that the bending angle was influenced by the segmentation length of the actuator, where the segmentation length and increased input pressure also allow more bending on the actuator. The actuator with lot of section gave more bending response compared to the actuator with lesser section. With the performance exhibited from this study, McKibben actuator can be applied in a wider use for continuum manipulator.

Continuous Progressive Actuator Robot for Hand Rehabilitation

This paper presents the development of a soft rehabilitation robot to conduct Continuous Passive Motion (CPM) for hand rehabilitation. The main contribution of this work is the implementation of a McKibben actuator as an artificial muscle due to its proven advantages: simple structure, light weight, and high power-to-weight ratio. The development worked successfully when tested on a healthy subject, where the flexion and extension of the finger were controlled with an antagonistic pair of actuators. However, there is a limitation of the McKibben actuator regarding its length-dependency. In this research, the concept of a pulley system was proposed to overcome this limitation. Although there is a friction factor that reduces the contracting displacement by at least 15% of the original displacement, a pulley is still a potential solution as it can reduce the installation space of the actuator from 40 to 15cm while still producing sufficient force for the finger motion. Throughout this research, it was found that the pattern of the flexor pulley system is affecting the system’s efficiency in terms of motion assistance.

Electrically-Driven Soft Fluidic Actuators Combining Stretchable Pumps With Thin McKibben Muscles.

Soft wearable robots could provide support for lower and upper limbs, increase weight lifting ability, decrease energy required for walking and running, and even provide haptic feedback. However, to date most of wearable robots are based on electromagnetic motors or fluidic actuators, the former being rigid and bulky, the latter requiring external pumps or compressors, greatly limiting integration and portability. Here we describe a new class of electrically-driven soft fluidic muscles combining thin, fiber-like McKibben actuators with fully Stretchable Pumps. These pumps rely on ElectroHydroDynamics, a solid-state pumping mechanism that directly accelerates liquid molecules by means of an electric field. Requiring no moving parts, these pumps are silent and can be bent and stretched while operating. Each electrically-driven fluidic muscle consists of one Stretchable Pump and one thin McKibben actuator, resulting in a slender soft device weighing 2 g. We characterized the response of these devices, obtaining a blocked force of 0.84 N and a maximum stroke of 4 mm. Future work will focus on decreasing the response time and increasing the energy efficiency. Modular and straightforward to integrate in textiles, these electrically-driven fluidic muscles will enable soft smart clothing with multi-functional capabilities for human assistance and augmentation.

Finite Element Driven Design Domain Identification of a Beating Left Ventricular Simulator.

Almost ten percent of the American population have heart diseases. Since the number of available heart donors is not promising, left ventricular assist devices are implemented as bridge therapies. Development of the assist devices benefits from both in-vivo animal and in-vitro mock circulation studies. Representation of the heart is a crucial part of the mock circulation setups. Recently, a beating left ventricular simulator with latex rubber and helically oriented McKibben actuators has been proposed. The simulator was able to mimic heart wall motion, however, flow rate was reported to be limited to 2 liters per minute. This study offers a finite element driven design domain identification to identify the combination of wall thickness, number of actuators, and the orientation angle that results in better deformation. A nonlinear finite element model of the simulator was developed and validated. Design domain was constructed with 150 finite element models, each with varying wall thickness and number of actuators with varying orientation angles. Results showed that the combination of 4 mm wall thickness and 8 actuators with 90 degrees orientation performed best in the design domain.

Modeling and Analysis of a High-Displacement Pneumatic Artificial Muscle With Integrated Sensing.

We present a high-displacement pneumatic artificial muscle made of textiles or plastics that can include integrated electronics to sense its pressure and displacement. Compared to traditional pneumatic muscle actuators such as the McKibben actuator and other more recent soft actuators, the actuator described in this paper can produce a much higher (40~65%) contraction ratio. In this paper, we describe the design, fabrication, and evaluation of the actuator, as well as the manufacturing process used to create it. We demonstrate the actuator design with several examples that produce 120 and 300 N at pressures of 35 and 105 kPa, respectively, and have contraction ratios of 40–65%.

Modeling and Implementation of the McKibben Actuator in Hydraulic Systems

McKibben actuators, comprised of an elastomeric tube wrapped in inextensible fibers, were developed in the 1950s and have been used primarily in pneumatic applications. Mc-Kibben actuators provide superior force and power density at a lower cost compared to many conventional actuators due to their simple design. This paper explores driving McKibben actuators with high-pressure hydraulics and reflects on some challenges associated with modeling the actuators at hydraulic pressures. Twelve actuators were designed, fabricated, and tested experimentally over a range of contraction ratios. Four modeling approaches were evaluated against experimental data including a new model that eliminates some of the limitations of existing models by predicting output force as a function of initial actuator geometry only and capturing the nonlinear variation in elastomer wall thickness as the actuator is strained. The experimental results suggest that the existing McKibben force models presented in this paper are not as accurate as the new model for tracking total axial force generation. The absolute value of the error from experimental values of each of the four models ranged from 9.1% for the new model developed in this paper to 9.9%, 10.0%, and 10.5% for the three other existing approaches, but the new model has unique advantages in modeling hydraulic McKibben actuators that require thicker tube walls. This study lays the ground work for future research in modeling soft robots comprised of compliant actuators powered hydraulically at high pressures.