skip to main content

Attention:

The NSF Public Access Repository (PAR) system and access will be unavailable from 8:00 PM ET on Friday, March 21 until 8:00 AM ET on Saturday, March 22 due to maintenance. We apologize for the inconvenience.

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


Title: Motor unit buckling in variable recruitment fluidic artificial muscle bundles: implications and mitigations
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 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.  more » « less
Award ID(s):
1845203
PAR ID:
10323000
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Smart Materials and Structures
Volume:
31
Issue:
3
ISSN:
0964-1726
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 ASME 2022 Conference on Smart Materials, Adaptive Structures and Intelligent Systems)
    This paper investigates the effect of resistive forces within a variable recruitment (VR) bundle actuators during recruitment state transition. Due to their versatility in design, ease of manufacturing, high force-to-weight ratio, and inherent compliance, FAMs have become a favorable actuation method for the robotics research community. Recently, researchers have adapted mammalian muscle topology to construct a multi-chamber FAM bundle actuator, consisting of separate units of actuation called motor units (MUs). These bundle actuators have VR functionality in which one or more MUs are sequentially activated according to the load demand. This activation scheme has been shown to have higher actuator efficiencies as compared to a single equivalent cross-sectional area FAM actuator. A characteristic behavior of VR bundles is the interaction between FAM elements in the bundle. Distinctively during recruitment state transition, inactive/low-pressure FAMs buckle outward and are compressed past its free strain due to the higher strain of fully active FAMs. There exists an onset pressure above which such FAMs need to contribute positively to the overall force output of the bundle. This paper presents a realistic scenario in which MU pressure is controlled by a hydraulic servo valve. As a result, the overall bundle force exhibits a sharp decrease during recruitment state transition while the MU being recruited is below the onset pressure. 
    more » « less
  3. ; ; ( , Proc. SPIE 11586, Bioinspiration, Biomimetics, and Bioreplication XI)
    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 variable recruitment bundle. The paper is concluded with a discussion of the implications of this phenomenon. 
    more » « less
  4. ; ; ( , 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
  5. ; ; ; ( , Proceedings of the ASME 2020 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2020)
    null (Ed.)
    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. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email