Free strain gradient reversal of a variable recruitment fluidic artificial muscle bundle

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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Power Savings of a Variable Recruitment Hydraulic Artificial Muscle Actuation Scheme

Volume 2: Mechanics and Behavior of Active Materials; Integrated System Design and Implementation; Bioinspired Smart Materials and Systems; Energy Harvesting ◽

10.1115/smasis2014-7718 ◽

2014 ◽

Cited By ~ 1

Author(s):

Michael Meller ◽

Ephrahim Garcia

Keyword(s):

Pressure Drop ◽

Motor Neurons ◽

Muscle Fibers ◽

Artificial Muscle ◽

Motor Units ◽

Hydraulic Control ◽

Hydraulic Systems ◽

Muscle Bundle ◽

Variable Recruitment ◽

Actuation Scheme

We investigate utilizing inelastic bladder hydraulic artificial muscle actuators as muscle fibers. These muscle fibers are then grouped together to form a variable recruitment artificial muscle bundle. This muscle bundle configuration is biologically inspired, where in skeletal muscle, different numbers of motor units are recruited to match the load by increasing the number of motor neurons firing. This results in extremely efficient locomotion in nature. It is desired to use a similar methodology to increase the actuation efficiency of valve-controlled hydraulic systems. Such hydraulic control systems induce a pressure drop in the valves to throttle the flow to the cylinder actuators. Using the valves in this manner is simple but very inefficient. Hence, this paper presents selectively recruiting different numbers of the hydraulic artificial muscle fibers to match a required loading scenario similar to our bipedal robot. By using fewer of the muscle fibers to match a smaller load, less power is consumed from the hydraulic power unit because instead of inducing a pressure drop, the volume of fluid delivered is decreased. The potential efficiency improvements associated with this actuation scheme is compared to a traditional hydraulic system with differential cylinders.

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The effect of resistive forces in variable recruitment fluidic artificial muscle bundles: a configuration study

Bioinspiration, Biomimetics, and Bioreplication X ◽

10.1117/12.2557907 ◽

2020 ◽

Author(s):

Nicholas Mazzoleni ◽

Jeong Yong Kim ◽

Matthew Bryant

Keyword(s):

Artificial Muscle ◽

Resistive Forces ◽

Variable Recruitment

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Investigation of Resistive Forces in Variable Recruitment Fluidic Artificial Muscle Bundles

Topics in Modal Analysis & Testing, Volume 8 - Conference Proceedings of the Society for Experimental Mechanics Series ◽

10.1007/978-3-030-47717-2_32 ◽

2020 ◽

pp. 305-313

Author(s):

Jeong Yong Kim ◽

Nicholas Mazzoleni ◽

Matthew Bryant

Keyword(s):

Artificial Muscle ◽

Resistive Forces ◽

Variable Recruitment

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Bioinspired passive variable recruitment of fluidic artificial muscles

Journal of Intelligent Material Systems and Structures ◽

10.1177/1045389x18783070 ◽

2018 ◽

Vol 29 (15) ◽

pp. 3067-3081 ◽

Cited By ~ 3

Author(s):

Edward M Chapman ◽

Matthew Bryant

Keyword(s):

Control Method ◽

Applied Pressure ◽

Artificial Muscle ◽

High Threshold ◽

Artificial Muscles ◽

Control Approach ◽

Muscle Tissues ◽

Wide Range ◽

Variable Recruitment ◽

Analogous Behavior

This article presents a novel, passive approach to creating variable actuator recruitment in bundles of fluidic artificial muscles. The passive recruitment control approach is inspired by the functionality of mammalian muscle tissues, in which a single activation signal from the nervous system sequentially triggers contraction of progressively larger actuation elements until the required force is generated. Biologically, this behavior is encoded by differences in electrical resistance properties between smaller and larger muscle-fiber groups. The approach presented here produces analogous behavior using a uniform applied pressure to all fluidic artificial muscles while creating differential pressure responses and threshold pressures among the fluidic artificial muscles via tailored bladder elasticity parameters. A model for using elastic bladder stiffness to control an artificial muscle bundle with a single valve is explored and used to compare a bundle of fluidic artificial muscles with both low and high threshold pressure units to a single fluidic artificial muscle of equivalent displacement and force capability. The results of this analysis indicate the efficacy of using this control method; it is advantageous in cases where a wide range of displacements and forces are necessary and can increase efficiency when the system primarily operates in a low-force regime but requires occasional bursts of high-force capability.

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Toward Variable Recruitment Fluidic Artificial Muscles

Volume 2: Mechanics and Behavior of Active Materials; Structural Health Monitoring; Bioinspired Smart Materials and Systems; Energy Harvesting ◽

10.1115/smasis2013-3136 ◽

2013 ◽

Cited By ~ 5

Author(s):

Matthew Bryant ◽

Michael A. Meller ◽

Ephrahim Garcia

Keyword(s):

Traditional Method ◽

Artificial Muscle ◽

Artificial Muscles ◽

Experimental Characterization ◽

Force Output ◽

Preliminary Results ◽

Fluidic Control ◽

Variable Recruitment ◽

Save Energy

We investigate taking advantage of the lightweight, compliant nature of fluidic artificial muscles to create variable recruitment actuators in the form of artificial muscle bundles. Several actuator elements at different diameter scales are packaged to act as a single actuator device. The actuator elements of the bundle can be connected to the fluidic control circuit so that different groups of actuator elements, much like individual muscle fibers, can be activated independently depending on the required force output and motion. This novel actuation concept allows us to save energy by effectively selecting the size of the actuators on the fly based on the instantaneous required load, versus the traditional method wherein actuators are sized for the maximum required load, and energy is wasted by oversized actuators most of the time. This design also allows a single bundled actuator to operate in substantially different force regimes, which could be valuable for robots that need to perform a wide variety of tasks and interact safely with humans. This paper will propose this actuator concept and show preliminary results of the design, fabrication, and experimental characterization of three such bioinspired variable recruitment actuator prototypes.

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Development and Demonstration of an Orderly Recruitment Valve for Fluidic Artificial Muscles

ASME 2020 Conference on Smart Materials, Adaptive Structures and Intelligent Systems ◽

10.1115/smasis2020-2330 ◽

2020 ◽

Author(s):

Jeong Yong Kim ◽

Nicholas Mazzoleni ◽

Dheeraj Vemula ◽

Matthew Bryant

Keyword(s):

Motor Unit ◽

Artificial Muscle ◽

Motor Units ◽

Artificial Muscles ◽

Proof Of Concept ◽

Experimental Proof ◽

Load Demand ◽

Variable Recruitment ◽

And Performance ◽

Spool Valve

Abstract Variable recruitment fluidic artificial muscle (FAM) bundles consist of multiple FAMs arranged in motor units that are sequentially activated as load demand increases. The conventional configuration of a variable recruitment FAM bundle requires a valve for each motor unit, which is referred to as a multi-valve system (MVS). As each motor unit within the bundle is selectively recruited, this configuration is highly adaptable and flexible in performance. However, as the number of motor units increases, the valve network can become complex and heavy in its design. To decrease complexity and weight, the concept of an orderly recruitment valve (ORV) has been proposed and analyzed. The ORV allows multiple motor units to be controlled using a single valve that recruits and pressurizes all motor units. The ORV concept consists of a spool valve with multiple outlet ports and a motor unit connected to each port. A linear actuator controls the position of the spool, allowing fluid flow into each port in succession. Naturally, de-recruitment happens in reverse order. The objective of the ORV is to strike a balance between performance and compactness of design. The purpose of this paper is to present analytical modeling that can be used to understand the behavior and performance of an ORV system and develop an experimental proof-of-concept that illustrates the ORV operation in hardware. A pneumatic ORV prototype was constructed and used to actuate two FAMs sequentially, each representing a motor unit. The results demonstrate the ORV as a compact system with which a variable recruitment bundle with multiple recruitment states can be controlled.

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Motor unit buckling in variable recruitment fluidic artificial muscle bundles: implications and mitigations

Smart Materials and Structures ◽

10.1088/1361-665x/ac49d9 ◽

2022 ◽

Author(s):

Nicholas Mazzoleni ◽

Jeong Yong Kim ◽

Matthew Bryant

Keyword(s):

Energy Density ◽

Weight Ratio ◽

Artificial Muscle ◽

Motor Units ◽

Mitigation Strategy ◽

Force Output ◽

Operational Life ◽

Resistive Forces ◽

Variable Recruitment ◽

Fixed End

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 fluidvolume at maximum contraction. Efficiency benefits can be found 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.

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