Experimentally identified models of McKibben soft actuators as primary movers and passive structures

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 manuscript 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 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.

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.

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.

Inflatable Octet-Truss Structures

The development of variable-stiffness systems is key to the advance of compact engineering solutions in a number of fields. Rigidizable structures exhibit variable-stiffness based on external stimuli. This function is necessary for deployable structures, such as inflatable space antennas, where the deployed structure is semi-permanent. Rigidization is also useful for a wide range of applications, such as prosthetics and exoskeletons, to help support external loads. In general, variable-stiffness designs suffer from a tradeoff between the magnitude of stiffness change and the ability of the structure to resist mechanical failure at any stiffness state. This paper presents the design, analysis, and fabrication of a rigidizable structure based on inflatable octet-truss cells. An octet-truss is a lattice-like configuration of elements, traditionally beams, arranged in a geometry reminiscent of that of the FCC lattice found in many metals; namely, the truss elements are arranged to form a single interior octahedral cell surrounded by eight tetrahedral cells. The interior octahedral cell is the core of the octet-truss unit cell, and is used as the main structure for examining the mechanics of the unit as a whole. In this work, the elements of the inflatable octet truss are pneumatic air muscles, also called McKibben actuators. Generalized McKibben actuators are a type of tubular pneumatic actuator that possess the ability to either contract or expand axially due to an applied pressure. Their unique kinematics are achieved by using a fiber wrap around an isotropic elastomeric shell. Under normal conditions, pressurizing the isotropic shell causes expansion in all directions, like a balloon. The fiber wrap constrains the ability of the shell to freely expand, due to the fiber stiffness. The wrap geometry thus guides the extensile/contractile motion of the actuator by controlling its kinematics. It is their ability to contract under pressure that makes McKibben actuators unique, and consequently they are of great interest presently to the robotics community due to their similitude to organic muscles. Kinematic analysis from constrained maximization of the shell volume during pressurization is used to obtain relations between the input work due to applied pressure and the resulting shape change due to strain energy. Analytical results are presented to describe the truss stiffness as a function of the McKibben geometry at varying pressures.