skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: 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.  more » « less
Award ID(s):
1734627
PAR ID:
10291981
Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Journal of Mechanisms and Robotics
Volume:
14
Issue:
1
ISSN:
1942-4302
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; (, Actuators)
    null (Ed.)
    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. 
    more » « less
  2. ; ; (, Proceedings of the 38th IMAC, A Conference and Exposition on Structural Dynamics 2020)
    This paper experimentally investigates the mechanical behavior of inactive and low-pressure fluidic artificial muscle (FAM) actuators under applied axial load. In most cases, the active characteristics of an actuator are of interest because they provide valuable information about its force-strain relationship. However, a system of actuators requires attention to the interaction between individual units. One such configuration is a bundle of McKibben artificial muscle actuators arranged in parallel and used for load-adaptive variable recruitment. This bio-inspired actuator bundle sequentially increases the number of actuators activated depending on the load required, which is analogous to how motor units are recruited in a mammalian muscle tissue. While using the minimum number of actuators allows the bundle to operate efficiently, the resistive force of inactive elements acts against total bundle contraction due to their inherent stiffness. In addition, when the bundle transitions between recruitment levels, motor units for a given recruitment level may be gradually pressurized; these low-pressure motor units can also cause resistive forces. Experiments were conducted to characterize the complex interaction between the bladder and braided mesh that cause the resistive force and deflection of inactive and low-pressure elements. Based on observations made from experiments, the paper proposes the initial criteria for developing a model of the resistive forces of a McKibben actuator, both individually, and within the context of a variable recruitment bundle. 
    more » « less
  3. ; ; ; (, Springer, Cham.)
    Sensing and actuation are intricately connected in soft robotics, where contact may change actuator mechanics and robot behavior. To improve soft robotic control and performance, proprioception and contact sensors are needed to report robot state without altering actuation mechanics or introducing bulky, rigid components. For bioinspired McKibben-style fluidic actuators, prior work in sensing has focused on sensing the strain of the actuator by embedding sensors in the actuator bladder during fabrication, or by adhering sensors to the actuator surface after fabrication. However, material property mismatches between sensors and actuators can impede actuator performance, and many soft sensors available for use with fluidic actuators rely on costly or labor-intensive fabrication methods. Here, we demonstrate a low-cost and easy-to manufacture-tubular liquid metal strain sensor for use with soft actuators that can be used to detect actuator strain and contact between the actuator and external objects. The sensor is flexible, can be fabricated with commercial-off-the-shelf components, and can be easily integrated with existing soft actuators to supplement sensing, regardless of actuator shape or size. Furthermore, the soft tubular strain sensor exhibits low hysteresis and high sensitivity. The approach presented in this work provides a low-cost, soft sensing solution for broad application in soft robotics. 
    more » « less
  4. ; (, Proceedings of the ASME 2022 Conference on Smart Materials, Adaptive Structures and Intelligent Systems)
    This paper will investigate the effects of pennate angle on fluidic artificial muscle (FAM) bundles for a robot arm motion. Rising interest in soft fluidic actuators exists due to their prospective inherent compliance and safe human-robot interaction. The high force-to-weight ratio, innate flexibility, inexpensive construction, and muscle-like force-contraction behavior of McKibben FAMs make them an attractive type of soft fluidic actuator. Multi-unit architectures found in biological muscles tissues and geometric fiber arrangements have inspired the development of hierarchical actuators to enhance the total actuator performance and increase actuator functionality. Parallel, asymmetric unipennate, and symmetric bipennate are three muscle fiber arrangement types found in human skeletal muscle tissues. Unique characteristics of the pennate muscle tissue, with muscle fibers arranged obliquely from the line of muscle motion, enable passive regulation of effective transmission between the fibers and muscle. Prior studies developed an analytical model based on idealized assumptions to leverage this pennate topology in optimal fiber parameter design for FAM bundles under spatial bounds. The findings showed FAMs in the bipennate topology can be designed to amplify the muscle output force, contraction, and stiffness as compared to that of a parallel topology under equivalent spatial and operating constraints. This work seeks to extend upon previous studies by investigating the effects of pennate angle on actuation and system hydraulic efficiency for a robot arm with a more realistic FAM model. The results will progress toward tailoring actuator topology designs for custom compliant actuation applications. 
    more » « less
  5. ; ; (, 2022 IEEE 5th International Conference on Soft Robotics (RoboSoft))
    Recent work in fluid-driven soft robots has demonstrated the potential to achieve high power-to-weight ratios, low fabrication costs, and improved safety, making them well suited for interactive tasks. However, the low speed of pneumatic actuation prevents use of these robots in more dynamic tasks. This paper aims to design, characterize, and model a bistable elastomeric actuator for swift tasks (BEAST). This actuator enables both fast actuation and mechanical compliance, and is designed by integrating silicone and polyethylene terephthalate (PET) in a bendy straw structure. The BEAST contains three states - compressed, natural, and stretched states. Two operation modes - compressed and stretched modes, are defined to model the continuous elongation dynamics before and after the quickly switching around the natural state. A set of design rules and a novel fabrication method are presented to develop the BEAST. The actuator characterization shows that the maximum extension ratio, snapping speed, and output force of the BEAST to be 0.58, 1.5m/s, and 48N, respectively. A hybrid linear parameter varying (HLPV) model is developed to describe the pressure-dependent dynamics of the actuator. The actuators are evaluated in an object sorting task where both fast and gentle behaviors are demonstrated. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email