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1. Effects of Pennate Angle on FAM Bundle Hydraulic Efficiency for Robot Arm Motion

   https://doi.org/10.1115/SMASIS2022-92022  

   Duan, Emily ; Bryant, Matthew ( September 2022 , 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. 

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2. Design of Pennate Topology Fluidic Artificial Muscle Bundles Under Spatial Constraints

   https://doi.org/10.1115/SMASIS2021-68183  

   Duan, Emily ; Bryant, Matthew ( September 2021 , Proceedings of the ASME 2021 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS 2021)

   In this paper, we investigate the design of pennate topology fluidic artificial muscle bundles under spatial and operating constraints. Soft fluidic actuators are of great interest to roboticists and engineers due to their potential for inherent compliance and safe human-robot interaction. McKibben fluidic artificial muscles (FAMs) are soft fluidic actuators that are especially attractive due to their high force-to-weight ratio, inherent flexibility, relatively inexpensive construction, and muscle-like force-contraction behavior. Observations of natural muscles of equivalent cross-sectional area have indicated that muscles with a pennate fiber configuration can achieve higher output forces as compared to the parallel configuration due to larger physiological cross-sectional area (PCSA). However, this is not universally true because the contraction and rotation behavior of individual actuator units (fibers) are both key factors contributing to situations where bipennate muscle configurations are advantageous as compared to parallel muscle configurations. This paper analytically explores a design case for pennate topology artificial muscle bundles that maximize fiber radius. The findings can provide insights on optimizing artificial muscle topologies under spatial constraints. Furthermore, the study can be extended to evaluate muscle topology implications on work capacity and efficiency for tracking a desired dynamic motion. 

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3. Implications of Spatially Constrained Bipennate Topology on Fluidic Artificial Muscle Bundle Actuation

   https://doi.org/10.3390/act11030082  

   Duan, Emily ; Bryant, Matthew ( March 2022 , Actuators)

   In this paper, we investigate the design of pennate topology fluidic artificial muscle bundles under spatial constraints. Soft fluidic actuators are of great interest to roboticists and engineers, due to their potential for inherent compliance and safe human–robot interaction. McKibben fluidic artificial muscles are an especially attractive type of soft fluidic actuator, due to their high force-to-weight ratio, inherent flexibility, inexpensive construction, and muscle-like force-contraction behavior. The examination of natural muscles has shown that those with pennate fiber topology can achieve higher output force per geometric cross-sectional area. Yet, this is not universally true for fluidic artificial muscle bundles, because the contraction and rotation behavior of individual actuator units (fibers) are both key factors contributing to situations where bipennate muscle topologies are advantageous, as compared to parallel muscle topologies. This paper analytically explores the implications of pennation angle on pennate fluidic artificial muscle bundle performance with spatial bounds. A method for muscle bundle parameterization as a function of desired bundle spatial envelope dimensions has been developed. An analysis of actuation performance metrics for bipennate and parallel topologies shows that bipennate artificial muscle bundles can be designed to amplify the muscle contraction, output force, stiffness, or work output capacity, as compared to a parallel bundle with the same envelope dimensions. In addition to quantifying the performance trade space associated with different pennate topologies, analyzing bundles with different fiber boundary conditions reveals how bipennate fluidic artificial muscle bundles can be designed for extensile motion and negative stiffness behaviors. This study, therefore, enables tailoring the muscle bundle parameters for custom compliant actuation applications. 

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

   https://doi.org/10.3390/act10030042  

   Kim, Jeong Yong ; Mazzoleni, Nicholas ; Bryant, Matthew ( March 2021 , 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. 

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5. Investigation of Resistive Forces in Variable Recruitment Fluidic Artificial Muscle Bundles

   Kim, Jeong Yong ; Mazzoleni, Nicholas ; Bryant, Matthew ( January 2020 , 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. 

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