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Lakhtakia, Akhlesh ; Martín-Palma, Raúl J. ; Knez, Mato (Ed.)In this study, the implementation and performance of bipennate topology fluidic artificial muscle (FAM) bundles operating under varying boundary conditions is investigated and quantified experimentally. Soft actuators are of great interest to design engineers due to their inherent flexibility and potential to improve safety in human robot interactions. McKibben fluidic artificial muscles are soft actuators which exhibit high force to weight ratios and dynamically replicate natural muscle movement. These features, in addition to their low fabrication cost, set McKibben FAMs apart as attractive components for an actuation system. Previous studies have shown that there are significant advantages in force and contraction outputs when using bipennate topology FAM bundles as compared to the conventional parallel topology1 . In this study, we will experimentally explore the effects of two possible boundary conditions imposed on FAMs within a bipennate topology. One boundary condition is to pin the muscle fiber ends with fixed pin spacings while the other is biologically inspired and constrains the muscle fibers to remain in contact. This paper will outline design considerations for building a test platform for bipennate fluidic artificial muscle bundles with varying boundary conditions and present experimental results quantifying muscle displacement and force output. These metrics aremore »Free, publicly-accessible full text available April 20, 2023
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Lakhtakia, Akhlesh ; Martín-Palma, Raúl J. ; Knez, Mato (Ed.)Fluidic artificial muscles (FAMs) have emerged as a viable and popular robotic actuation technique due to their low cost, compliant nature, and high force-to-weight-ratio. In recent years, the concept of variable recruitment has emerged as a way to improve the efficiency of conventional hydraulic robotic systems. In variable recruitment, groups of FAMs are bundled together and divided into individual motor units. Each motor unit can be activated independently, which is similar to the sequential activation pattern observed in mammalian muscle. Previous researchers have performed quasistatic characterizations of variable recruitment bundles and some simple dynamic analyses and experiments with a simple 1- DOF robot arm. We have developed a linear hydraulic characterization testing platform that will allow for the testing of different types of variable recruitment bundle configurations under different loading conditions. The platform consists of a hydraulic drive cylinder that acts as a cyber-physical hardware-in-the-loop dynamic loading emulator and interfaces with the variable recruitment bundle. The desired inertial, damping and stiffness properties of the emulator can be prescribed and achieved through an admittance controller. In this paper, we test the ability of this admittance controller to emulate different inertial, stiffness, and damping properties in simulation and demonstrate that it canmore »Free, publicly-accessible full text available April 20, 2023
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In this paper, we present an approach to obtain a desired leading-edge vortex (LEV) shedding pattern from unsteady airfoils through the execution of suitable motion kinematics. Previous research revealed that LEV shedding is associated with the leading-edge suction parameter (LESP) exceeding a maximum threshold. A low-order method called LESP-modulated discrete vortex method (LDVM) was also developed to predict the onset and termination of LEV shedding from an airfoil undergoing prescribed motion kinematics. In the current work, we present an inverse-aerodynamic formulation based on the LDVM to generate the appropriate motion kinematics to achieve a prescribed LESP variation, and thus, the desired LEV shedding characteristics from the airfoil. The algorithm identifies the kinematic state of the airfoil required to attain the target LESP value through an iterative procedure performed inside the LDVM simulation at each time step. Several case studies are presented to demonstrate design scenarios such as tailoring the duration and intensity of LEV shedding, inducing LEV shedding from the chosen surface of the airfoil, promoting or suppressing LEV shedding during an unsteady motion on demand, and achieving similar LEV shedding patterns using different maneuvers. The kinematic profiles generated by the low-order formulation are also simulated using a high-fidelity unsteadymore »Free, publicly-accessible full text available May 1, 2023
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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 comparedmore »
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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 »
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Abstract Biological musculature employs variable recruitment of muscle fibers from smaller to larger units as the load increases. This orderly recruitment strategy has certain physiological advantages like minimizing fatigue and providing finer motor control. Recently fluidic artificial muscles (FAM) are gaining popularity as actuators due to their increased efficiency by employing bio-inspired recruitment strategies such as active variable recruitment (AVR). AVR systems use a multi-valve system (MVS) configuration to selectively recruit individual FAMs depending on the load. However, when using an MVS configuration, an increase in the number of motor units in a bundle corresponds to an increase in the number of valves in the system. This introduces greater complexity and weight. The objective of this paper is to propose, analyze, and demonstrate an orderly recruitment valve (ORV) concept that enables orderly recruitment of multiple FAMs in the system using a single valve. A mathematical model of an ORV-controlled FAM bundle is presented and validated by experiments performed on a proof-of-concept ORV experiment. The modeling is extended to explore a case study of a 1-DOF robot arm system consisting of an electrohydraulic pressurization system, ORV, and a FAM-actuated rotating arm plant and its dynamics are simulated to further demonstrate themore »
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This paper presents a state-variable formulation to model and simulate the 2D unsteady aerodynamics of an airfoil undergoing arbitrary motion kinematics. The model builds upon a large-angle unsteady aerodynamic formulation in which the airfoil is represented using a lumped vortex element (LVE) model. The airfoil is divided into several panels, with a bound vortex placed on each panel. At any time instant, the bound-vortex strengths are determined by employing zero-normal-flow conditions at the control points located on each panel. The vorticity shed from the trailing edge of the airfoil is modeled using discrete vortices that move freely in the flow field. The required state variables are first identified, and all the time derivative terms of the state variables are then derived to form the final state-variable representation. Trailing-edge vortex shedding is incorporated using the Kelvin condition. The final state variable equation can be solved as an ordinary differential equation using any standard ODE-solving algorithm. Three case studies are presented here to evaluate the predictions of the model. In the cases considered here, the airfoil undergoes various unsteady plunge motions. The aerodynamic load history and the wake patterns are compared against the results from the low-order model developed by Narsipur etmore »
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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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The interaction between upstream flow disturbance generators and downstream aeroelastic structures has been the focus of several recent studies at North Carolina State University. Building on this work, which observed the modulation of limit cycle oscillations (LCOs) in the presence of vortex wakes, this study examines the design and validation of a novel disturbance generator consisting of an oscillating cylinder with an attached splitter plate. Analytical design of the bluff body was performed based on specific flow conditions which produced LCO annihilation in previous studies. Computational fluid dynamics simulations and experimental wind tunnel tests were used to validate the ability of the new disturbance generator to produce the desired wake region. Future work will see the implementation of this novel design in conjunction with aeroelastic structures in an effort to modulate and control LCOs, including the excitation and annihilation thereof.
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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 »