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Title: Investigation of Resistive Forces in Variable Recruitment Fluidic Artificial Muscle Bundles
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 more » for developing a model of the resistive forces of a McKibben actuator, both individually, and within the context of a variable recruitment bundle. « less
Authors:
; ;
Award ID(s):
1845203
Publication Date:
NSF-PAR ID:
10146739
Journal Name:
Proceedings of the 38th IMAC, A Conference and Exposition on Structural Dynamics 2020
Sponsoring Org:
National Science Foundation
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  1. 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 effectsmore »reveals a phenomenon, unique to variable recruitment bundles, defined as free strain gradient reversal.« less
  2. Lakhtakia, Akhlesh ; Martín-Palma, Raúl J. ; Knez, Mato (Ed.)
    This paper investigates the effect of resistive forces that arise in compressed fluidic artificial muscles (FAMs) within a variable recruitment bundle. Much like our skeletal muscle organs that selectively recruit different number of motor fibers depending on the load demand, a variable recruitment FAM bundle adaptively activates the minimum number of motor units (MUs) to increase its overall efficiency. A variable recruitment bundle may operate in different recruitment states (RSs) during which only a subset of the FAMs within a bundle are activated. In such cases, a difference in strain occurs between active FAMs and inactive/low-pressure FAMs. This strain difference results in the compression of inactive/lowpressure FAMs causing them to exert a resistive force opposing the force output of active FAMs. This paper presents experimental measurements for a FAM for both tensile and compressive regions. The data is used to simulate the overall force-strain space of a variable recruitment bundle for when resistive force effects are neglected and when they are included. Counterintuitively, an initial decrease in bundle free strain is observed when a transition to a higher RS is made due to the presence of resistive forces. We call this phenomenon the free strain gradient reversal of a variablemore »recruitment bundle. The paper is concluded with a discussion of the implications of this phenomenon.« less
  3. Abstract Fluidic artificial muscles (FAMs) are a popular actuation choice due to their compliant nature and high force-to-weight ratio. Variable recruitment is a bio-inspired actuation strategy in which multiple FAMs are combined into motor units that can be pressurized sequentially according to load demand. In a traditional ‘fixed-end’ variable recruitment FAM bundle, inactive units and activated units that are past free strain will compress and buckle outward, resulting in resistive forces that reduce overall bundle force output, increase spatial envelope, and reduce operational life. This paper investigates the use of inextensible tendons as a mitigation strategy for preventing resistive forces and outward buckling of inactive and submaximally activated motor units in a variable recruitment FAM bundle. A traditional analytical fixed-end variable recruitment FAM bundle model is modified to account for tendons, and the force–strain spaces of the two configurations are compared while keeping the overall bundle length constant. Actuation efficiency for the two configurations is compared for two different cases: one case in which the radii of all FAMs within the bundle are equivalent, and one case in which the bundles are sized to consume the same amount of working fluid volume at maximum contraction. Efficiency benefits can be foundmore »for either configuration for different locations within their shared force–strain space, so depending on the loading requirements, one configuration may be more efficient than the other. Additionally, a study is performed to quantify the increase in spatial envelope caused by the outward buckling of inactive or low-pressure motor units. It was found that at full activation of recruitment states 1, 2, and 3, the tendoned configuration has a significantly higher volumetric energy density than the fixed-end configuration, indicating that the tendoned configuration has more actuation potential for a given spatial envelope. Overall, the results show that using a resistive force mitigation strategy such as tendons can completely eliminate resistive forces, increase volumetric energy density, and increase system efficiency for certain loading cases. Thus, there is a compelling case to be made for the use of tendoned FAMs in variable recruitment bundles.« less
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