scholarly journals Modeling of Resistive Forces and Buckling Behavior in Variable Recruitment Fluidic Artificial Muscle Bundles

Actuators ◽  
2021 ◽  
Vol 10 (3) ◽  
pp. 42
Author(s):  
Jeong Yong Kim ◽  
Nicholas Mazzoleni ◽  
Matthew Bryant
Keyword(s):  
Artificial Muscle ◽  
Motor Units ◽  
Low Pressure ◽  
Correction Method ◽  
Force Output ◽  
Resistive Force ◽  
Resistive Forces ◽  
Mckibben Actuators ◽  
Variable Recruitment ◽  
Free Strain

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.

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

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.

Power Savings of a Variable Recruitment Hydraulic Artificial Muscle Actuation Scheme

2014 ◽  
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.

Toward Variable Recruitment Fluidic Artificial Muscles

2013 ◽  
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.

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

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.

Considerations on Designing Artificial Muscle Working at High Pressure

2012 ◽  
Vol 268-270 ◽  
pp. 1457-1463
Author(s):  
Li Chao Wang ◽  
Xiao Dong Wang
Keyword(s):  
High Pressure ◽  
Material Properties ◽  
Weight Ratio ◽  
Artificial Muscle ◽  
Force Output ◽  
Compact Structure ◽  
Small Power ◽  
Small Force ◽  
Working Principle ◽  
Contraction Angle

Artificial muscle is a new style of actuator with novel working principle, which owns the advantages of compact structure, high power-to-weight ratio, compliance and easy application. Pneumatic artificial muscle (PAM) is usually used in robotics, medical auxiliaries and other small force output occasions nowadays. However, it suffers problems of small power, hysteresis and poor repeatability. A kind of artificial muscle working at high pressure was researched. Different muscle styles are compared and MicKibben structure is selected while fluid media is determined. Furthermore, factors of geometry and material properties, which limit the ultimate pressure, are analyzed. Formulas and simulations verify the influence of limitation and help to calculate key parameters of 18MPa artificial muscle. Data show that it is possible in theory to design high pressure artificial muscle by overall consideration of initial diameter, initial contraction angle and material properties, initial length only influent the stroke.

Neuronal mechanisms underlying physiological tremor

1978 ◽  
Vol 41 (3) ◽  
pp. 557-571 ◽  
Author(s):  
J. H. Allum ◽  
V. Dietz ◽  
H. J. Freund
Keyword(s):  
Power Spectrum ◽  
Pathological Condition ◽  
Power Spectra ◽  
Motor Units ◽  
Physiological Tremor ◽  
Force Level ◽  
Force Output ◽  
Experimental Conditions ◽  
Single Motor Unit ◽  
Total Fusion

1. Tremor force was recorded during stationary isometric contractions of intrinsic hand muscles of normal subjects. Subjects maintained a steady force level between their thumb and forefinger for 30 s. The force level varied from weak (0.2 kg) to strong contractions (7 kg). These experimental conditions were the same as those in two preceding studies, where single motor-unit activity (14) and the correlation between the discharges of two simultaneously recorded motor units and physiological tremor (11) have been investigated. 2. Two alterations of the power spectra were observed at successively stronger contractions: increase of tremor amplitude and changes in the shape of the power spectrum. At all force levels, the power spectra of tremor force show the well-known decay of tremor amplitude from the lower to the higher frequencies with a local peak at 6--10 Hz. This peak does not show a significant change with respect to frequency when the force level is varied. It is shifted toward lower frequencies in a pathological condition (Parkinsonism) where the recruitment firing rates of the motor units are significantly lower than in the normal. 3. Higher frequencies (greater than 20 Hz) are barely present in the power spectrum during the very weak contractions. They become significant as the contractions become stronger. 4. The steep decay of the power spectrum toward higher frequencies has a similar slope (--43 dB/decade) as the reduction in amplitude of the unfused part of the muscle contractions with increasing stimulus rates (--38 dB/decade). The cutoff of the power spectrum above 25 Hz parallels the achievement of total fusion of muscle twitches above this rate. 5. The results are consistent with the hypothesis that the power spectrum over the range of 6--25 Hz is mainly caused by the unfused parts of the twitch contractions of motor units firing between recruitment (6--8/s) and total fusion of the twitches (25--30/s). The decline of the power spectrum toward higher frequencies can be explained by mechanical damping, which results from increasing fusion of the twitch contractions. The low-frequency part of the power spectrum is assumed to be the result of the slow force deviations produced by changes in the net output of the motoneuron pool. 6. These assumptions were supported by additional animal experiments where the number and rate of force-producing elements could be controlled. Bundles of ventral root filaments innervating cat soleus and gastrocnemius muscles were stimulated synchronously and asynchronously at a number of different rates. The force output of the strain gauge was recorded, filtered, and analyzed in the same way as the human force records. 7. Stimualtion of one nerve bundle at one fixed frequency led to a sharp peak in the power spectrum at that frequency plus peaks of decreasing height representing the harmonics of the stimulation frequency. The height of the peaks decreased at --37 dB/decade. 8...

Physiological responses of rat plantaris motor units to overload induced by surgical removal of its synergists

1988 ◽  
Vol 60 (6) ◽  
pp. 2138-2151 ◽  
Author(s):  
A. E. Olha ◽  
B. J. Jasmin ◽  
R. N. Michel ◽  
P. F. Gardiner
Keyword(s):  
Motor Unit ◽  
Fatigue Resistance ◽  
Motor Units ◽  
High Threshold ◽  
Surgical Removal ◽  
Type I ◽  
Myofibrillar Atpase ◽  
Force Output ◽  
Cross Sectional ◽  
Single Motor Units

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

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