Steady-state Force Research Articles

Design of a Fuzzy Fractional Order Adaptive Impedance Controller with Integer Order Approximation for Stable Robotic Contact Force Tracking in Uncertain Environment

Abstract Current research in robot compliance control is unable to take both transient contact force overshoots and steady-state force tracking problems into account. To address this problem, we propose a fuzzy fractional order (FO) adaptive impedance controller to avoid the force overshoots in the contact stage while keeping force error in the dynamic tracking stage, where traditional control algorithms are not competent. A percentage gain is adopted to map FO parameters to integer order (IO) parameters by their natural properties, and a fuzzy logical controller is introduced to improve the system stability. The simulation results indicate that the proposed controller can be made more stable than and superior to the general impedance controller, and the force tracking results also have been compared with the previous control methods.

Study on the Influence of Triangular Groove Structure on Steady-State Flow Force Compensation Characteristics

In this study, a structurally improved spool was designed. The diameter of one side of the spool stem was reduced, making the spool stem into a rounded table shape. A triangular groove was circumscribed on the step and on the same side. After liquid flow was guided through the triangular groove, the flow direction changed. A flow component in the negative direction was generated, which reversely impacted the liquid flow in the positive direction. The liquid flow angle at the outlet increased; that is, jet angle increased and flow force decreased. The simulation results show that, increasing the depth, H, of the triangular groove has a positive effect on flow-force compensation and was conducive to the stability of the valve core. Properly increasing the groove’s bottom diameter, D1, of the triangular groove was conducive to the stability of the spool, but when D1 was too large, the flow force increased. The experimental results are consistent with the simulation results, which proves that the improved structure can effectively reduce the flow force of the spool.

Effect of Fascicle Length Range on Force Generation of Model-Based Biomimetic Controller for Tendon-Driven Prosthetic Hand.

Model-based biomimetic control with neuro-muscular reflex requires accurate representation of muscle fascicle length, which affects both force generation capability of muscle and dynamics of muscle spindle. However, physiological data are insufficient to guide the selection of range of fascicle length for task control. Here a reverse engineering approach was used to investigate the effects of different fascicle length range on controller's force control ability, so as to justify the selection of operating range of muscle length for a grasp force task. We compared 3 different ranges of fascicle length for their effects on force generation, i.e. R1: 0.5 - 1.0 Lo, R2: 0.5 - 1.3 Lo and R3: 0.5 - 1.6 Lo. The rationale to test these range selections was based on both physiological realism and engineering considerations. The steady state force output and transient force responses were evaluated with a range of step inputs as controller input. Results show that the prosthetic finger can produce a linear steady state force response with all 3 ranges of fascicle length. Peak force was the largest with R3. Fascicle length range had no significant effect on the rise time in force generation tasks. Results suggest that a wider range of fascicle length may be more favorable for force capacity, since the contact point of force control may well fall near the optimal length (Lo) region.

A Nonsmooth Quasi-Static Modeling Approach for Hydraulic Actuators

Abstract This paper presents an analytical approach for modeling the quasi-static characteristics of hydraulic actuators driven by four-valve independent metering circuits. The presented model is described as a nonsmooth, set-valued function from the velocity to the set of forces with which the equilibrium is achieved at the velocity. It is derived from algebraic relations among the velocity of the actuator, the steady-state force generated by the actuator, and the flowrate and the steady-state pressure at all valves in the circuit. This approach is also applied to more involving circuits including a regeneration pipeline and those with multiple actuators. The contribution of the paper can be seen as an example case study of the fact that these complicated circuit structures are analytically tractable through an extension of the conventional hydraulic–electric analogy. The obtained analytical expressions of the steady-state velocity–force relations allow for concise visualization of the actuators' characteristics.

Crashworthiness performance and multiobjective optimization of a combined splitting circular tube energy absorber under eccentric impact for subway vehicles

This paper proposes a new type of combined splitting circular tube energy absorber for subway vehicles. An impact experiment is conducted to investigate the dynamic crushing performance of this combined splitting circular tube energy absorber. The results show that the combined tubes experience steady dynamic splitting deformation. The crush force efficiency (CFE) and effective stroke ratio (ESR) of the combined splitting circular tube energy absorber are 85.7% and 85.9%, respectively. A theoretical solution for the dynamic splitting crushing load is derived. Finite element (FE) models of the energy absorber are then established, and the dynamic crushing forces agree well with those of the impact experiment. The cumulative error and validation metrics of the crushing force obtained from the numerical simulation and impact experiment are 0.0938 and 0.9122, respectively. The effect of the strain rate of AISI 1020 steel on dynamic steady-state forces is analytically studied, and the results are compared with those of FE simulations and impact experiments. The numerical results show that the dynamic steady-state force of the combined splitting circular tubes under dynamic loading increases by 31.69% compared to that of the tubes under quasistatic loading. The effects of various structural parameters are discussed using the validated FE models. The results show that the combined splitting tube energy absorber has excellent crashworthiness under eccentric loading. The eccentric impact of the two absorbers has a significant influence on the crashworthiness performance of the structure. The specific energy absorption (SEA) increases by 15.2% as the eccentric distance increases from 20 to 80 mm. Compared with the influence of the wall thickness t of the combined splitting circular tubes, the tube radius and die angle have a more significant influence on the SEA. To improve the crashworthiness of combined splitting circular tubes under eccentric loading, a Pareto front of the SEA and average axial crushing force (AACF) under an eccentric distance of H = 40 mm was obtained after optimization by multiobjective particle swarm optimization (MOPSO). The results show that the SEA and AACF are positively correlated and that a balance between the SEA and AACF was obtained at optimal point B (SEA = 14.6 kJ/kg and AACF = 938 kN).

Lateral thermokarst patterns in permafrost peat plateaus in northern Norway

Abstract. Subarctic peatlands underlain by permafrost contain significant amounts of organic carbon. Our ability to quantify the evolution of such permafrost landscapes in numerical models is critical for providing robust predictions of the environmental and climatic changes to come. Yet, the accuracy of large-scale predictions has so far been hampered by small-scale physical processes that create a high spatial variability of thermal surface conditions, affecting the ground thermal regime and thus permafrost degradation patterns. In this regard, a better understanding of the small-scale interplay between microtopography and lateral fluxes of heat, water and snow can be achieved by field monitoring and process-based numerical modeling. Here, we quantify the topographic changes of the Šuoššjávri peat plateau (northern Norway) over a three-year period using drone-based repeat high-resolution photogrammetry. Our results show thermokarst degradation is concentrated on the edges of the plateau, representing 77 % of observed subsidence, while most of the inner plateau surface exhibits no detectable subsidence. Based on detailed investigation of eight zones of the plateau edge, we show that this edge degradation corresponds to an annual volume change of 0.13±0.07 m3 yr−1 per meter of retreating edge (orthogonal to the retreat direction). Using the CryoGrid3 land surface model, we show that these degradation patterns can be reproduced in a modeling framework that implements lateral redistribution of snow, subsurface water and heat, as well as ground subsidence due to melting of excess ice. By performing a sensitivity test for snow depths on the plateau under steady-state climate forcing, we obtain a threshold behavior for the start of edge degradation. Small snow depth variations (from 0 to 30 cm) result in highly different degradation behavior, from stability to fast degradation. For plateau snow depths in the range of field measurements, the simulated annual volume changes are broadly in agreement with the results of the drone survey. As snow depths are clearly correlated with ground surface temperatures, our results indicate that the approach can potentially be used to simulate climate-driven dynamics of edge degradation observed at our study site and other peat plateaus worldwide. Thus, the model approach represents a first step towards simulating climate-driven landscape development through thermokarst in permafrost peatlands.

INFLUENCE AND REGULATION OF INTERFACIAL ADHESION PROPERTIES OF A MAGNETIC SENSITIVE FILM/SUBSTRATE BY MAGNETIC FORCE AND FILM'S CURVATURE

The controllable interface adhesion of attachment and detachment has important application requirements in climbing devices, adhesion switches and mechanical grippers. In present paper, the influence mechanism of external magnetic field and film's initial curvature on the interfacial adhesion of a magnetic sensitive film/substrate is studied. The peel-test of the magnetic sensitive thin film with initial curvature on a substrate as well as the corresponding theoretical study are respectively carried out. Both the experimental and theoretical results indicate that the interfacial adhesion force of the magnetic sensitive thin film/substrate increases with increasing the initial curvature of the film, and the external magnetic field can enhance the interfacial adhesion force. Compared with the steady-state peel-off force of a flat thin film that is independent on the bending stiffness, the bending stiffness would decrease the steady-state peel-off force of the film with initial curvature. The interface effective adhesion energy is further considered from the energy point of view, which can disclose the comparing mechanisms of the film's bending energy, the potential energy of external magnetic field and the adhesion energy. Lastly, based on the experimental and theoretical results, a simply mechanical gripper controlled by both the magnetic field and film's initial curvature is proposed, which can continuously realize the gripping, transport and release of an object. The results obtained in the present paper can not only be helpful for understanding the interface reversible adhesion mechanism actuated by multi-field, but also provide a novel approach to design functional devices with controllable interface adhesion.

Multiscale multivariate transfer entropy and application to functional corticocortical coupling

Objective. Complex biological systems consist of multi-level mechanism in terms of within- and cross-subsystems correlations, and they are primarily manifested in terms of connectivity, multiscale properties, and nonlinearity. Existing studies have each only explored one aspect of the functional corticocortical coupling (FCCC), which has some limitations in portraying the complexity of multivariable systems. The present study investigated the direct interactions of brain networks at multiple time scales. Approach. We extended the multivariate transfer entropy (MuTE) method and proposed a novel method, named multiscale multivariate transfer entropy (MSMVTE), to explore the direct interactions of brain networks across multiple time scale. To verify this aim, we introduced three simulation models and compared them with multiscale transfer entropy (MSTE) and MuTE methods. We then applied MSMVTE method to analyze FCCC during a unilateral right-hand steady-state force task. Main results. Simulation results showed that the MSMVTE method, compared with MSTE and MuTE methods, better detected direct interactions and avoid the spurious effects of indirect relationships. Further analysis of experimental data showed that the connectivity from left premotor/sensorimotor cortex to right premotor/sensorimotor cortex was significantly higher than that of opposite directionality. Furthermore, the connectivities from central motor areas to both sides of premotor/sensorimotor areas were higher than those of opposite directionalities. Additionally, the maximum coupling strength was found to occur at a specific scale (3–10). Significance. Simulation results confirmed the effectiveness of the MSMVTE method to describe direct relationships and multiscale characteristics in complex systems. The enhancement of FCCC reflects the interaction of more extended activation in cortical motor regions. Additionally, the neurodynamic process of brain depends not only on emergent behavior at small scales, but also on the constraining effects of the activity at large scales. Taken together, our findings provide a basis for better understanding dynamics in brain networks.

Residual force enhancement is reduced in permeabilized fiber bundles from mdm muscles

Residual Force enhancement (RFE) is defined as the increase in steady state force after active stretch compared with force after isometric contraction at the same final length. Recent studies have demonstrated a pivotal role for titin in RFE. In myofibrils stretched beyond overlap of thick and thin filaments, Ca2+ activated myofibrils had a higher titin-based force and a steeper increase with force with stretch than passively stretched myofibrils at the same average sarcomere length. These results suggest higher titin-based stiffness in active myofibrils, perhaps due to N2A titin-actin interactions in active wild type (WT) muscle. Mdm mice carry a small deletion in titin that includes 53 amino acids in Ig83 in the N2A region. It has been proposed that the mdm mutation prevents N2A titin-actin interactions so that active mdm muscles are more compliant than WT muscles. This decrease in active muscle stiffness should be associated with a reduction in RFE. Based on this rationale, we investigated RFE in permeabilized soleus (SOL) and extensor digitorum longus (EDL) fiber bundles from 20-30 day old mdm mice (n=5), harvested at a young age to diminish the effects of degeneration and regeneration, and 30-60 day old WT mice (n=5). Contractile stress and RFE of EDL (mdm n=19 and WT n=20) and SOL fibers (WT n=19 and mdm n=16) was analyzed using JMP and Graph pad (PRISM) software. Statistics included Shapiro-Wilk (normality), Levene's and Bartlett (homogeneity of variances), Welch's one-way ANOVA (unequal variances), Dunnett's T3 (multiple comparisons), and T-tests with Welch's correction (unpaired). To investigate differences in RFE between genotypes, we performed isometric contractions, as well as active and passive stretch from an average sarcomere length of 2.6 - 3.0 µm on each fiber. This stretch amplitude, within the physiological range, resulted in no observable breakage or slipping of fibers during experiments. We predicted that mdm fiber bundles would show reduced RFE compared to WT fibers. Contractile stress when activated at 2.6 µm was significantly greater in WT EDL (Welch's one-way ANOVA; P<0.05) bundle and SOL (Welch's one-way ANOVA; P<0.05) compared to fiber bundles. Our results showed that mdm muscles exhibit significantly lower RFE than WT for both SOL (Welch's one-way ANOVA; P<0.0001) and EDL (Welch's one-way ANOVA; P<0.0001). This result is consistent with previous observations in single myofibrils and intact muscles. The data support the hypothesis that titin contributes to RFE. We suggest that RFE is reduced in mdm muscles due to impaired Ca2+ dependent titin-actin interactions, resulting from the small deletion in titin that includes 53 amino acids in Ig83 in the N2A region.

CFD Prediction of Ship-Bank Interaction

Ships operating in different shallow-water areas are at risk of grounding due to squat, heel and wave-induced motions. The paper aims not only at assessing the ship squat in any bathymetry, but also to reveal the waves influence on the environment. A 3D numerical model for predicting ship resistance in confined waters by taking the ship sinkage into account is proposed. A time-domain model is used to predict unsteady squat and dynamic acceleration effects for a vessel travelling in non-uniform water depth. A quasi-steady approach where the prediction at each time step is based on steady-state heave force and pitch moment in uniform water depth is used. Resistance, sinkage and trim, the free-surface elevation as well as the wake flow structure are considered in the ship performance analysis. The results show the wave pattern in the subcritical, critical and supercritical regions. The total resistance/drag is calculated at various ship speeds in shallow water using CFD and compared with the calculated deep water resistance. Attention is paid to the verification and validation based on a grid convergence study, as to the exploration of the modelling error in RANS computations to enable more accurate and reliable predictions of the bank effects.

Fast skeletal myosin-binding protein-C regulates fast skeletal muscle contraction

Fast skeletal myosin-binding protein-C (fMyBP-C) is one of three MyBP-C paralogs and is predominantly expressed in fast skeletal muscle. Mutations in the gene that encodes fMyBP-C, MYBPC2, are associated with distal arthrogryposis, while loss of fMyBP-C protein is associated with diseased muscle. However, the functional and structural roles of fMyBP-C in skeletal muscle remain unclear. To address this gap, we generated a homozygous fMyBP-C knockout mouse (C2-/-) and characterized it both in vivo and in vitro compared to wild-type mice. Ablation of fMyBP-C was benign in terms of muscle weight, fiber type, cross-sectional area, and sarcomere ultrastructure. However, grip strength and plantar flexor muscle strength were significantly decreased in C2-/- mice. Peak isometric tetanic force and isotonic speed of contraction were significantly reduced in isolated extensor digitorum longus (EDL) from C2-/- mice. Small-angle X-ray diffraction of C2-/- EDL muscle showed significantly increased equatorial intensity ratio during contraction, indicating a greater shift of myosin heads toward actin, while MLL4 layer line intensity was decreased at rest, indicating less ordered myosin heads. Interfilament lattice spacing increased significantly in C2-/- EDL muscle. Consistent with these findings, we observed a significant reduction of steady-state isometric force during Ca2+-activation, decreased myofilament calcium sensitivity, and sinusoidal stiffness in skinned EDL muscle fibers from C2-/- mice. Finally, C2-/- muscles displayed disruption of inflammatory and regenerative pathways, along with increased muscle damage upon mechanical overload. Together, our data suggest that fMyBP-C is essential for maximal speed and force of contraction, sarcomere integrity, and calcium sensitivity in fast-twitch muscle.

A theoretical model of the intermittent contact of piezoelectric actuator based on Greenwood-Williamson theory

A theoretical model of the intermittent contact mechanism of linear piezoelectric actuator was proposed, in which the microscopic characteristics of the contact surfaces were taken into account. Greenwood-Williamson (GM) theory was involved for the first time to describe the microscopic characteristics of the contact surfaces in which the contact surfaces were considered as random rough surfaces that were composed of large amounts of asperities and the heights of the asperities conformed to a Gauss distribution. The developed theoretical model gives an insight into the influence of the microscopic properties, as well as the macroscopic and material properties on the output performance of the actuator, which may provide guidance for the design of such type of piezoelectric actuator. During the modeling, normal kinetic contact force, kinetic friction coefficient, friction force and steady-state output force of were obtained in sequence. And some simulations were carried out to analyze how factors such as the type of material, the roughness of the contact surface, the initial distance between the stator and the rotor, and the preload, et.al, affect the contact performance. A set of experiments were carried out to reveal the influence of surface roughness, material type and preload on the steady-state output thrust. The simulated results were found in good agreement with the experiment ones at most of the tested points. Finally, surface roughness of the stator and rotor were tested before and after the operation, which indicated that the rotor and stator had similar roughness after operation.

Salvesen’s Method for Added Resistance Revisited

Abstract Almost 4 years after the appearance of Salvesen–Tuck–Faltinsen (STF) strip theory (Salvesen et al., 1970, “Ship Motions and Sea Loads,” Annual Meeting of the Society of Naval Architecture and Marine Engineers (SNAME), New York, Nov. 12–13), Salvesen in 1974 published his popular method for calculation of added resistance (Salvesen, 1974, “Second-Order Steady State Forces and Moments on Surface Ships in Oblique Regular Waves,” Vol. 22; Salvesen, 1978, “Added Resistance of Ships in Waves,” J. Hydronautics, 12(1), pp. 24–34). His method is based on an exact near-field formulation; however, he applied the long-wave and the weak-scatterer assumptions to present his approximate method using the integrated quantities (hydrodynamic and geometrical coefficients). Considering the available computational powers in the 1970s, both of these assumptions were absolutely justifiable. The intention of this paper is to disseminate the results of a recent study at the Technical University of Denmark, whereby the Salvesen’s formulation has been revisited and the added resistance is computed from the original exact equation without invoking the weak-scatterer or the long-wave assumptions. This is performed using the solutions of the radiation and the scattering problems, obtained by a low-order boundary element method and the two-dimensional free-surface Green function inside our in-house STF theory implementation (Bingham and Amini-Afshar, 2020, DTU_Strip Theory Solver). The weak-scatterer assumption is then removed through a direct calculation of the x-derivatives of the velocity potentials and the normal vectors along the body. Knowing the velocity potentials over each panel, the long-wave assumption is also avoided by a piece-wise analytical integration of sectional Kochin Function (Kochin, 1936, “On the Wave Resistance and Lift of Bodies Submerged in Fluid,” Transactions of the Conference on the Theory of Wave Resistance, Moscow.). The presented results for five ship geometries testify that the correct treatment of the original equation is achieved only after both of the above-mentioned assumptions are removed. Implemented in this manner, Salvesen’s method proves to be relatively more accurate and robust than has been generally perceived during all these years.

Force-velocity and tension transient measurements from Drosophila jump muscle reveal the necessity of both weakly-bound cross-bridges and series elasticity in models of muscle contraction

Muscle contraction is a fundamental biological process where molecular interactions between the myosin molecular motor and actin filaments result in contraction of a whole muscle, a process spanning size scales differing in eight orders of magnitude. Since unique behavior is observed at every scale in between these two extremes, to fully understand muscle function it is vital to develop multi-scale models. Based on simulations of classic measurements of muscle heat generation as a function of work, and shortening rate as a function of applied force, we hypothesize that a model based on molecular measurements must be modified to include a weakly-bound interaction between myosin and actin in order to fit measurements at the muscle fiber or whole muscle scales. This hypothesis is further supported by the model's need for a weakly-bound state in order to qualitatively reproduce the force response that occurs when a muscle fiber is rapidly stretched a small distance. We tested this hypothesis by measuring steady-state force as a function of shortening velocity, and the force transient caused by a rapid length step in Drosophila jump muscle fibers. Then, by performing global parameter optimization, we quantitatively compared the predictions of two mathematical models, one lacking a weakly-bound state and one with a weakly-bound state, to these measurements. Both models could reproduce our force-velocity measurements, but only the model with a weakly-bound state could reproduce our force transient measurements. However, neither model could concurrently fit both measurements. We find that only a model that includes weakly-bound cross-bridges with force-dependent detachment and an elastic element in series with the cross-bridges is able to fit both of our measurements. This result suggests that the force response after stretch is not a reflection of distinct steps in the cross-bridge cycle, but rather arises from the interaction of cross-bridges with a series elastic element. Additionally, the model suggests that the curvature of the force-velocity relationship arises from a combination of the force-dependence of weakly- and strongly-bound cross-bridges. Overall, this work presents a minimal cross-bridge model that has predictive power at the fiber level.

On the effect of anisotropy on the performance and simulation of shrinking tubes used as energy absorbers for railway vehicles

The standard BS EN 15227 requires accurate numerical modelling of railway vehicle energy absorbers that must be correlated against experimental data. Although thin-walled tubes can exhibit anisotropy, such numerical models have traditionally included isotropic material properties. Thus, this work investigates whether anisotropic material models may increase the accuracy of numerical models of shrinking tube energy absorbers.Tensile testing of extruded AW-6082 aluminium alloy shrinking tubes showed the yield strength of the tubes was 10% lower in the hoop direction than in the longitudinal direction. Further, to assess the effect of incorporating anisotropic material behaviour in the numerical model, the tubes were compressed under quasi-static conditions.Numerical models of the shrinking tubes, including isotropic (von Mises yield function) and anisotropic (Hill’s quadratic yield function) material properties, were compared to the experimental data. The isotropic numerical models overestimated the steady-state reaction force, even without the inclusion of friction, indicating that such models do not fulfil the requirements of the standard. Conversely, incorporating anisotropic material models predicted a lower reaction force and enabled the inclusion of energy dissipation by friction, by means of a coefficient of friction μ ​= ​0.03. Although these results demonstrate the need to include anisotropy in the numerical simulation, the friction value was lower than expected due to the methodology of the material characterization and the accuracy of the anisotropic model implemented.

The ForceLAB simulator: Application to the comparison of current models of cardiomyocyte contraction

Mathematical models are useful tools in the study of physiological phenomena. However, due to differences in assumptions and formulations, discrepancy in simulations may occur. Among the models for cardiomyocyte contraction based on Huxley's cross-bridge cycling, those proposed by Negroni and Lascano (NL) and Rice et al. (RWH) are the most frequently used. This study was aimed at developing a computational tool, ForceLAB, which allows implementing different contraction models and modifying several functional parameters. As an application, electrically-stimulated twitches triggered by an equal Ca2+ input and steady-state force x pCa relationship (pCa = -log of the molar free Ca2+ concentration) simulated with the NL and RWH models were compared. The equilibrium Ca2+-troponin C (TnC) dissociation constant (Kd) was modified by changing either the association (kon) or the dissociation (koff) rate constant. With the NL model, raising Kd by either maneuver decreased monotonically twitch amplitude and duration, as expected. With the RWH model, in contrast, the same Kd variation caused increase or decrease of peak force depending on which rate constant was modified. Additionally, force x pCa curves simulated using Ca2+ binding constants estimated in cardiomyocytes bearing wild-type and mutated TnC were compared to curves previously determined in permeabilized fibers. Mutations increased kon and koff, and decreased Kd. Both models produced curves fairly comparable to the experimental ones, although sensitivity to Ca2+ was greater, especially with RWH model. The NL model reproduced slightly better the qualitative changes associated with the mutations. It is expected that this tool can be useful for teaching and investigation.

Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Muscle force is enhanced during shortening when shortening is preceded by an active stretch. This phenomenon is known as the stretch-shortening cycle (SSC) effect. For some stretch-shortening conditions this increase in force during shortening is maintained following SSCs when compared to the force following a pure shortening contraction. It has been suggested that the residual force enhancement property of muscles, which comes into play during the stretch phase of SSCs may contribute to the force increase after SSCs. Knowing that residual force enhancement is associated with a substantial reduction in metabolic energy per unit of force, it seems reasonable to assume that the metabolic energy cost per unit of force is also reduced following a SSC. The purpose of this study was to determine the energy cost per unit of force at steady-state following SSCs and compare it to the corresponding energy cost following pure shortening contractions of identical speed and magnitude. We hypothesized that the energy cost per unit of muscle force is reduced following SSCs compared to the pure shortening contractions. For the SSC tests, rabbit psoas fibers (n = 12) were set at an average sarcomere length (SL) of 2.4 μm, activated, actively stretched to a SL of 3.2 μm, and shortened to a SL of 2.6 or 3.0 μm. For the pure shortening contractions, the same fibers were activated at a SL of 3.2 μm and actively shortened to a SL of 2.6 or 3.0 μm. The amount of ATP consumed was measured over a 40 s steady-state total isometric force following either the SSCs or the pure active shortening contractions. Fiber stiffness was determined in an additional set of 12 fibers, at steady-state for both experimental conditions. Total force, ATP consumption, and stiffness were greater following SSCs compared to the pure shortening contractions, but ATP consumption per unit of force was the same between conditions. These results suggest that the increase in total force observed following SSCs was achieved with an increase in the proportion of attached cross-bridges and titin stiffness. We conclude that muscle efficiency is not enhanced at steady-state following SSCs.

Fuzzy Adaptive Sliding Mode Impedance Control of Fracture Reduction Robot

With the development of robotics, it will be possible for fracture reduction robots to replace doctors in performing surgery operations. The surgery accuracy and safety by the reduction robot is important. In order to compensate various uncertainties and nonlinear problems in the reduction robot impedance control system, this paper proposes a fuzzy adaptive sliding mode impedance control of the reduction robot, which includes force residual observer (FRO) and fuzzy adaptive sliding mode controller (FASMC). The FRO is used to estimate the external force between the reduction robot and the fractured musculoskeletal tissue in real-time, no force sensor is installed at the end-effector of reduction robot. The FASMC mainly includes three parts: linearizing the system by inverse dynamics and PD control, improving the system's robustness by the sliding mode control and fuzzy adaptive switching gain to reduce chattering, eliminating the nonlinear disturbance term of the system, and the normalized factor linear combination fuzzy system is used to compensate. The co-simulation of the fracture reduction robot shows that the FASMC impedance control method has a better steady-state position and force tracking accuracy, the steady-state position accuracy is -0.733 mm, and the steady-state force accuracy is -2.12 N. This paper provides an effective method for impedance control of the fracture reduction robot.

Modifiability of residual force depression in single muscle fibers following uphill and downhill training in rats.

Following active muscle shortening, steady‐state isometric force is less than a purely isometric contraction at the same muscle length and level of activation; this is known as residual force depression (rFD). It is unknown whether rFD at the single muscle fiber level can be modified via training. Here we investigated whether rFD in single muscle fibers is modifiable through downhill and uphill running in the extensor digitorum longus (EDL) and soleus (SOL) muscles in rats. Rats were run uphill or downhill 5 days/week for 4 weeks. After muscles were dissected and chemically permeabilized, single fibers were tied between a length controller and force transducer, transferred to an activating solution, with ATP and pCa of 4.2 for mechanical testing. rFD was quantified after active fiber shortening from an average sarcomere length (SL) of 3.1–2.5 µm at a relative speed of 0.15 fiber lengths/s (slow) and 0.6 fiber lengths/s (fast). rFD was calculated as the difference in force (normalized to cross‐sectional area) during the isometric steady‐state phase following active shortening and the purely isometric contraction. In addition to rFD, mechanical work of shortening and stiffness depression were also calculated. rFD was present for both the EDL (6–15%) and SOL (1–2%) muscles, with no effect of training. rFD was greater for the EDL than SOL which closely corresponded to the greater stiffness depression in the EDL, indicating a greater inhibition of cross‐bridge attachments. These results indicate that while rFD was observed, training did not appear to alter this intrinsic history‐dependent property of single muscle fibers.

Influence and Improvement of Hydraulic Power on Spool Valve Reversing

This paper analysed the effect of steady-state flow force and transient flow force to sliding direction valve, and two examples were given to illustrate adverse consequences caused by excessive fluid power, put forward the compensation measures. The effect of flow force should be considered when designing the hydraulic system in order to make the hydraulic system work more stable.