Explicit Dynamic Model Research Articles

Trigger thresholds and their dynamics of vegetation production loss under different atmospheric and soil drought conditions

Many evidences have shown that both atmospheric and soil droughts can constrain vegetation growth and further threaten its ability to sequester carbon. However, the trigger thresholds of vegetation production loss under different atmospheric and soil drought conditions are still unknown. In this study, we proposed a Copula and Bayesian equations-based framework to investigate trigger thresholds of various vegetation production losses under different atmospheric and soil drought conditions. The trigger thresholds dynamics and their possible causes were also investigated. To achieve this goal, we first simulated the gross primary production, soil moisture, and vapor pressure deficit over China during 1961–2018 using an individual-based, spatially explicit dynamic global vegetation model. The main drivers of the dynamic change in trigger thresholds were then explored by Random Forest model. We found that soil drought caused greater stress on gross primary production loss than atmospheric drought, with a larger impact area and higher probability of damage. In terms of spatial distribution, the risk probability of gross primary production loss was higher in eastern China than in western China, and the drought trigger threshold was also smaller in eastern China. In addition, the trigger thresholds for atmospheric and soil drought in most regions exhibited a decreasing trend from 1961 to 2018, while the CO2 fertilization enhanced the drought tolerance of vegetation. The reduction in CO2 fertilization effect slowed down the downward trend of trigger threshold for soil drought, while the increase in temperature exacerbated the downward trend of trigger threshold for atmospheric drought. This study highlighted the larger effect of soil drought on vegetation production loss than atmospheric drought and implied that climate change can modulate the trigger threshold of vegetation production losses under drought conditions. These findings provide scientific guidance for managing the increasing risk of drought on vegetation and optimizing watershed water allocation.

A Novel Explicit Canonical Dynamic Modeling Method for Multi-Rigid-Body Mechanisms Considering Joint Friction

Friction is an inevitable phenomenon in mechanical systems that affects the dynamic characteristics of systems. To reduce the modeling complexity of complex multi-rigid-body mechanisms, a novel explicit canonical dynamic modeling method considering joint friction is proposed. Based on the explicit dynamic modeling theory that we have proposed, the solution of the constraint force required by the joint friction modeling of multi-rigid-body mechanisms is derived and improved, which greatly simplifies the solution of the constraint force. According to the obtained explicit expression of the constraint force equations, two joint friction models of the Coulomb–viscous effect and Stribeck effect are derived in analytical form. Moreover, the Stribeck effect of the joint is experimentally analyzed. A five-axis tree-chain mechanism and a three-loop closed-chain mechanism are chosen to demonstrate the method and compared with ADAMS software. Moreover, the proposed model is analyzed and compared with other methods.

Dynamic Modeling and Improved Nonlinear Model Predictive Control of a Free-Floating Dual-Arm Space Robot

With the increasing demand for space missions, space robots have become the focus of research and attention. As a typical representative, the free-floating dual-arm space robot has the characteristics of multiple degrees of freedom, a floating base, and dynamic coupling between the manipulator and the base, so its modeling and control are very challenging. To address these challenges, a novel dynamic modeling and control method is proposed for a free-floating dual-arm space robot. First, an explicit dynamic model of a free-floating dual-arm space robot is established based on the explicit canonical multi-rigid-body dynamic modeling theory and combined with the concept of a dynamic equivalent manipulator. The establishment process of this model is not only simple and canonical to avoid the definition and calculation of many intermediate variables, but the symbolic result expression of the model also has the characteristics of iteration, which is convenient for computer automatic modeling. Next, aiming at addressing the problem of trajectory tracking and the base attitude stability of a free-floating dual-arm space robot with parameter perturbation and external disturbance, an improved nonlinear model predictive control method introducing the idea of sliding mode variable structure is proposed. Theoretical analysis shows that the proposed controller has better robustness than the traditional nonlinear model predictive controller. Then, an in-orbit service task is designed to verify the effectiveness of the proposed dynamic modeling and control strategy of the free-floating dual-arm space robot. Finally, the dynamic modeling and control methods proposed are discussed and summarized. The proposed methods can not only realize the tracking of the desired trajectory of the arms of the free-floating space robot, but can also realize the stable control of the base of the free-floating space robot. This paper provides new insights into the difficult problems regarding the dynamics and control of free-floating dual-arm space robots.

Adaptive backstepping controller based on a novel framework for dynamic solution of an ankle rehabilitation spherical parallel robot

AbstractThis research offers an adaptive model-based methodology for autonomous control of 3-RRR spherical parallel manipulator (RSPM) based on a novel modeling framework. RSPM is an overconstrained parallel mechanism that has a variety of applications in medical procedures such as ankle rehabilitation because of its precision and accuracy. However, obtaining a complete explicit dynamic model of these mechanisms for tracking purposes has been a problematic challenge due to their inherent singularities, coupling effects of the limbs, and redundant constraints imposed by the intermediate joints. This paper presents a novel algorithm to obtain the analytical kinematic solutions of RSPMs based on the closed-loop vector method, which includes constraint analysis. By incorporating constrained kinematics into the dynamic model, a comprehensive explicit dynamic solution of the non-overconstrained version 3-RCC of RSPM is developed in task space, based on screw theory and the linear homogeneous property of algebraic equations on the manipulator twist. Based on the proposed computational framework, a robust self-tuning backstepping control (STBC) strategy is applied to the robot to overcome the effect of external disturbances and time-varying uncertainties. Furthermore, an observer-based compensation (OBC) method is presented for dealing with the nonlinear hysteresis loops of the ankle during trajectory tracking purposes. The closed-loop stability of the whole system including STBC and OBC is theoretically performed by Lyapunov methods. The proposed methodologies are validated by realistic co-simulations in different scenarios. For instant, in the presence of external disturbances, the maximum tracking error norm of STBC is 37.5% less than the sliding mode approach.

Equilibrium population dynamics of site-dependent species

Adults of site-dependent species require a discrete structure, e.g., a cavity, for breeding, which they are unable to construct and must locate and occupy. The environment provides only a limited number of such sites, which may vary in overall quality due to their environmental context. Heterogeneity of site quality can result in population equilibrium, often construed as source-sink dynamics. Rodenhouse et al. (Ecology 78:2025-2042, 1997) proposed a mechanism of site-dependent equilibrium that they claimed was more general than source-sink dynamics. After defining notions of source and sink, I use explicit dynamical models for a site-dependent population, based on the life history of golden eagles (Aquila chrysaetos), with two levels of site quality, to investigate the existence of population equilibria under several scenarios: source-source, source-sink, and source-floater. The life history traits I employ are not overly restrictive and serve the purpose only of providing models explicit enough to be treated analytically. I use a generalized notion of “golden eagle” since site dependency is often discussed in the literature on raptors, and I have exploited details from Hunt et al. (PLoS ONE 12:e0172232, 2017) for numerical simulations. The crucial features of the modeling, however, are those of site dependency. The modeling emphasizes that equilibrium results from the limited supply of source sites and that vital rates averaged across site qualities do not provide a compelling explanation of equilibria, contra Rodenhouse et al. Counterintuitively, equilibria are theoretically possible, even when both site qualities are intrinsically source sites.

Numerical and data-driven uncertainty quantification of process parameters on clinching joint geometries

Clinching is a prevalent mechanical joining method involving clamping and locking two sheet work pieces using a punch and die. Numerical simulation offers a cost-effective alternative to time-consuming experiments for design. Our study employs an ABAQUS explicit dynamic model with remeshing to accurately replicate clinching joint geometry. However, the challenge of hard-to-measure parameters and the investigation of these parameters is limited leading to different simulation results compared to the millimeters-scale experiments. To address this, we apply PyMC3-based uncertainty quantification (UQ) to explore the material parameter effects on clinching dimensions. Our data-driven Bayesian inference models highlight friction and high-plastic-strain flow stress as significant geometry influencers. We propose estimating challenging-to-measure parameters from experiments, leveraging UQ for confident parameter intervals. Through PyMC3, our research offers insights into parameter impact on clinching dimensions, enhancing numerical simulations for process design optimization.

A complete mathematical modelling of robots actuated by epicyclic-gear

The industrial robots often use planetary gear system to have high joint torques; therefore, the influence of the rotary inertia of the number of the equally spaced planet-gears on the dynamical behavior of the robot is very important. The main objective of this paper is to develop the dynamic modeling of robot actuated by (n) equally spaced planet-gears in the case where the planet-carrier is fixed, no closed solution has been reported for this dynamic modeling, and to compare between the dynamic behavior of robot actuated by (n+1) and (n) equally spaced planet-gears for a same trajectory planning. The authors derive the explicit dynamic model for an elbow down of 2-R manipulator holding an external mass. Finally, the obtained simulation results of the dynamic modeling are verified by modeling the same robot and using an advanced simulation via SolidWorks.

Water supply following drought: Effects on drougt legacy and resilience in a tropical forest - A case study in Xishuangbanna, China

Climate change has made it essential to understand the effects of drought on carbon dynamics in tropical forest ecosystems. However, research on the postdrought (PD) period is relatively limited, particularly with regard to the resilience of the carbon cycle affected by available water. To determine the effects of water availability during PD on resilience and legacy effects in tropical forests, we used a spatially explicit individual-based dynamic global vegetation model (SEIB-DGVM) to simulate gross primary productivity (GPP) to proxy the process. We established multiple water supply scenarios based on ecological water demands and regional climatic standard precipitation through the composite of gradients in quantity and duration to characterize variations in environmental water availability after droughts end. Under different PD water supply patterns, the degree of GPP resilience and its legacy effects varied significantly. Specifically, ecosystem resilience to drought was positively correlated with the percentage of PD water supply and negatively correlated with the duration until water conditions returned to normal, while the negative legacy effect of drought on GPP exhibited the opposite relationship. After a drought, a rapid high-level water supply promoted stronger GPP resiliency and moderated the drought legacy effects. Although legacy effects varied with water supply patterns, the negative legacy effect on GPP appeared only in the first year after drought. Observing biomass changes in roots, leaves, woody plants, and herbaceous plants after droughts, it is clear that tropical forest ecosystems can adjust their carbon allocation strategies in response to differences in water availability. To more accurately predict the fate of tropical forests in response to climate change, it is necessary to assess the impact of PD environmental climate condition changes on their dynamic resilience processes.

Ultrasonic-Vibration-Assisted Waterjet Drilling of [0/45/-45/90]2s Carbon-Fiber-Reinforced Polymer Laminates.

The pure waterjet (WJ) drilling process of carbon-fiber-reinforced polymer (CFRP) laminates causes damage, such as tears and delamination, leading to poor-quality hole-wall. Ultrasonic-vibration-assisted technology can improve the quality of hole walls and repair such damage, particularly the delamination of CFRP laminates. In this study, we conducted a numerical and experimental investigation of a high-pressure pure WJ drilling process of CFRP laminates performed using ultrasonic vibration to improve the delamination phenomena of the pure WJ drilling process. An explicit dynamic model using the smoothed particle hydrodynamics method was employed to simulate the ultrasonic-vibration-assisted WJ drilling of CFRP laminates and ascertain the optimal drilling performance. Thereafter, WJ drilling experiments were conducted to verify the numerical simulation. The results illustrate that the employment of ultrasonic vibration significantly increased the material removal rate by approximately 20%. Moreover, the water-wedging action that induces the propagation of delamination was weakened with an increase in the amplitude of the ultrasonic vibration. The hole-wall quality was optimal with the following drilling parameters: amplitude, 10 μm; frequency, 20 kHz; and WJ velocity, 900 m/s. The delamination zone length was only 0.19 mm and was reduced by 85.6% compared with the values obtained using non-assisted WJ drilling.

Experimental and numerical investigation of cold-formed steel telescopic studs under compression

This paper presents a thorough experimental and numerical investigation into the structural behaviour of a novel cold-formed steel (CFS) telescopic stud system under axial compression. A total of 40 specimens were tested, considering variations in screw configurations, steel grades, and section thicknesses. The test results revealed that tilting and bearing led to failure in all screw connections. In comparison to the single-screw connection with a gap at the middle of the web, the telescopic studs with 2 screws in the web, 2 screws in the flanges, and 4 screws in both the web and flanges showed approximately 35–37% higher strengths. A nonlinear explicit dynamic finite element model was then developed and validated against the test results in terms of its connection strengths and failure modes. The experimental results were compared with the design strengths determined by the current design standards (AISI S100 (2016), AS/NZS 4600 (2018), Eurocode 3 (2006), BS5950–5 (1998) and GB50018 (2002).). The comparison results revealed that the AISI S100 (2016) and AS/NZS 4600 (2018) are conservative, underestimating the connection strengths of telescopic studs with different screw layouts by 1–13%.

Instability mechanism and failure criteria of large-span flexible PV support arrays under severe wind

Compared with independent flexible PV support, the entire structure force performance and transfer mechanism of inter-row cables and inter-span rods of flexible PV support arrays are more complex, it is easy to have large vibration or even instability failure under strong wind. In this study, the three-span and five-row flexible PV support array of a 66 MW Fishery-PV Complementary demonstration site in the eastern coastal region of China is used as the research object. The rigid body pressure measurement wind tunnel test was designed and carried out, and the wind pressure distribution characteristics of the PV panels surface were analyzed. A three-dimensional explicit dynamics model of the flexible PV support array considering inter-row cables and inter-span rods is established, and the wind-induced dynamic response characteristics and instability processes of the large-span flexible PV support array are effectively simulated. Finally, the instability mechanism of the large-span flexible PV support array is revealed, and the dual failure criteria based on structural deformation and energy increment are proposed. Results demonstrated that the maximum values of displacement and von Mises stress of the large-span flexible PV support array are concentrated in the 1st row of the span in the windward side. The array under 0° and 180° wind direction angles induces local instability and failure at wind speeds of 51 m/s and 46 m/s due to the failure of the 1st row of triangular rods and crossed diagonal rods on the windward side, respectively, and the rods instability mechanisms are Eulerian buckling mechanism and Limit point buckling mechanism, respectively. When the axial compression ratio [Δ] > 0.1 or the strain energy increment [ΔU] > 1.0 × 105J of the instability rod, the structure will be locally destabilized. This paper presents a systematic work around the wind-induced response and instability characteristics of the large-span flexible PV support array, the results are of significance for the engineering application of solar energy generation structures.

Modeling of non-structural carbohydrate dynamics by the spatially explicit individual-based dynamic global vegetation model SEIB-DGVM (SEIB-DGVM-NSC version 1.0)

Abstract. Forest dynamics need to be considered when estimating the global carbon budget. The alteration of forest structure and function under a changing climate and expanding human activity could lead to a reduction of forest canopy cover and a spread of lower-biomass ecosystems in warm and dry regions. A non-structural carbohydrate (NSC) acts as a storage buffer between carbon supplied by assimilation and carbon consumed by, inter alia, respiration, reproduction, and pests. Estimation of NSC concentrations in a tree is very important for accurate projection of future forest dynamics. We developed a new NSC module for incorporation into a spatially explicit individual-based dynamic global vegetation model (SEIB-DGVM) to validate the simulated NSC dynamics with observations. NSC pools were simulated in three plant organs: leaves, trunk, and roots. The seasonal dynamics of the NSCs varied among plant species, and the sizes of the NSC pools inferred from observations differed between the boreal, temperate, and tropical climates. The NSC models were therefore validated for each of the three climatic regions at both point and global scales to assess the performance of the models. The modeled NSCs showed good agreement in seasonality with the observed NSCs at four sites – Canada (boreal), Austria and Switzerland (temperate), and Panama (tropical) – and in mean values for three climate zones derived from the global NSC dataset. The SEIB-DGVM-NSC version 1.0 is expected to enable simulation of biome shifts caused by the changes in NSC dynamics worldwide. These dynamics will contribute to changes in not only the global carbon cycle but also in forest structure and demography at a global scale.

Simulation Research on Cotton Stalk Cutting and Crushing Based on ANSYS/LS-DYNA and Field Experiments

In order to solve the problem of high straw content in recovered residual film and the low rate of qualified straw crushing in combination with a front-mounted cotton-straw-crushing device, the cutting and crushing mechanisms of cotton stalks were studied based on ANSYS/LS-DYNA. The height h and dip angle α of the fixed blade were determined to be 30 mm and 75° through a finite element analysis. On the basis of the device design, explicit dynamic models of the cutting and crushing of a single cotton stalk were established based on ANSYS/LS-DYNA. The results of the dynamic analysis revealed the cutting mechanism of the cotton stalk, and the influences of the cutting edge angle γ and front baffle height h1 on cotton stalk cutting were studied by using single-factor simulation tests. An edge angle of γ = 45° and a height of h1 = 265 mm were determined. Meanwhile, the mechanism of cotton straw crushing was revealed, and the motion states of the straw were studied at different times. The results of the simulation experiments on the influence of the cutter shaft’s rotational speed showed that with an increase in the cutter shaft’s speed, the rate of qualified crushing and the removal rate were both increased. At the design speed of n = 1800 RPM, the rate of qualified crushing was 84.6%, and the removal rate was 95.1%. Then, field experiments were carried out. The test results were as follows: the stubble height was 8.0 cm, the rate of qualified straw crushing was 91.8%, the clearance rate of film-surface impurities was 92.3%, and the film content was 3.6%, which met the working quality requirements (not less than 85%) of NYT 500-2015: “Operating quality for straw-smashing machines”.

Balanced Functional Herbivore Composition Stabilizes Tree-Grass Coexistence and Productivity in a Simulated Savanna Rangeland Ecosystem

Savanna ecosystems provide resources supporting the livelihoods of millions of people globally. Ongoing exploitation through high livestock densities, among other factors, however, threatens these ecosystems. Shrub encroachment, a consequence of this exploitation, causes severe declines in fodder biomass for cattle. For example, in Namibia, this leads to losses of land productivity of up to 100%. Consequently, we must rethink land-use options to ensure sustainable utilization of savanna resources. In southern Africa, many landowners and custodians have switched to or included wildlife in their management systems, making it an excellent model region to study how changing herbivory affects ecosystems. The diversity of indigenous wildlife species is expected to promote efficient herbaceous layer utilization with browsing herbivores counteracting nondesirable woody plant encroachment. Nevertheless, surprisingly little research has been conducted on the ecological consequences of altering grazer-to-browser ratios.We developed a large-scale spatially explicit dynamic vegetation model including key hydrological and ecological processes, fire, and an herbivore algorithm that accounts for aggregated animal herds in a heterogeneous savanna landscape. We find that different ratios of wild herbivore feeding types greatly influence vegetation composition and dynamics. By dynamically computing plant biomass as a resource, our model estimates the occurrence of fodder biomass shortages, providing a valuable tool for land users. The model also reports the number of patches with low to medium tree functional cover (10–25%), indicating structural diversity with its implications for species richness, and is thus essential for nature conservation. We find that a herbivore species composition of approximately 40% browsers and 60% grazers is beneficial for plant structural and species diversity. Theoretically, therefore, the browse-graze ratio achieved through wildlife-based land-use can lead to more sustainability and resilience. The interplay between diverse herbivory, vegetation, rain, and fire complicates but also stabilizes the savanna rangeland ecosystem and may contribute to its long-term persistence.

Numerical analysis of initial cloud of powder dynamic dispersion

Numerical simulations were performed to investigate the explosion dispersion characteristics of powder materials with respect to the implicated velocity, and a fluid–solid coupling simulation model of the explosion dispersion process of the initial cloud was established. Explicit dynamics models of Finite-element method and smooth particle hydrodynamics for the detonation driving stage of central charge and the cloud isentropic expansion stages were developed, the pressure loads of the explosive-driven were generated using the user-defined function program, and computational fluid dynamics models for the isentropic expansion and free expansion stages of the initial cloud were developed. Additionally, the maximum expansion radius of the cloud in explosion dispersion tests and theoretical calculations were compared with the radius in the numerical simulation results to verify the accuracy of the simulation model. Analyses of the morphology, concentration, and turbulence field in the numerical simulation revealed that vortex stratification occurred in the “two wings” of the initial cloud in its development process and that a particle-free “hollow” area is subsequently formed. Analyses also revealed that the vortex stratification occurs earlier as the implicated velocity of powder material increases. Additionally, an increase in the implicated velocity can effectively weaken the concentration of cloud and make the distribution of cloud particles more uniform.

Deployment analysis of composite thin-walled lenticular tubes with effect of storage time and temperature

Composite Thin-walled Lenticular Tube (CTLT) is increasingly utilized in small satellites missions as a lightweight, foldable, and rollable structural material that facilitates the construction of large deployable systems. The CTLT is initially flattened and coiled around a central hub for storage before launch, during which elastic energy is stored as deformation energy, allowing it to be self-deployed on demand for use in orbit. This work presents a comprehensive investigation into the coiling, storage and deployment behaviors of CTLT that wraps around a central hub. A nonlinear explicit dynamic finite element model was developed with both deformable CTLT and rigid-bodies mechanisms including the central hub and guide rollers, as well as the complex interactions among them. The coiling mechanics characteristics such as stored strain energy and rotational moment were presented and validated against experimental data in the literature. Then, the dynamic deployment behaviors were analyzed in terms of two different deployment methods, namely, controlled deployment and free deployment. The effect of material property change during storage was also discussed through numerical experiments.

Multiscale analysis of wheel-rail rolling contact wear and damage mechanisms using molecular dynamics and explicit finite elements

As traffic density and axle loads increase, the macroscopic wheel-rail contact can lead to severe rail wear. This paper constructs a multi-scale analysis framework from macroscopic force characteristics to microscopic wear mechanisms based on explicit finite element methods and molecular dynamics models. The study shows that the mutual proximity and annihilation of reverse dislocations in the rail material under the action of wheel-rail forces is an important cause of grain boundary fragmentation, while the proliferation of dislocations on the Subsurface leads to work hardening of the rail. The analytical framework proposed in this paper achieves a multiscale correlation between wheel-rail macroscopic contact and material microscopic response, and the conclusions obtained reveal the microscopic wear mechanism of rail materials.

A real-time dynamic simulation platform for 6R manipulators based on an explicit modeling method

Dynamic simulation is critical for robotic design and control. In this study, a modular simulation platform is developed using a novel explicit dynamic modeling method. Combined with a PD controller and a globally linearized compensator, the dynamic modeling method presents an explicit and canonical form for tree-chain and closed-chain multi-body systems. Paired with the new approach, the platform makes it convenient for modeling, and it performs well in real-time with a high update frequency. The platform also offers a thorough parameter setting and simulation scene presentation interface through the construction of several modules. A simulation experiment using a 6-DOF manipulator demonstrates the platform’s capacity for real-time, stable, and practical simulations, which is also applicable for high-DOF multi-body systems of future uses.

Self-inhibition and optimal ramet mortality in clonal plants

We used a spatially explicit dynamic model for studying the relationship between stochastic ramet and genet mortality in clonal plants. Since genets consist of ramets, a naive hypothesis would suggest that genet mortality increases monotonically with ramet mortality. Our simulations show that the relationship is non-monotonous. There is an optimum in ramet mortality at which the genet's mortality is minimal. The reason is that clonal growth is directional, and thus the existing ramets may block some growth directions. This blockage can lead to the formation of rings and other characteristic growth patterns. Ramet death can remove the blockage. Our results suggest that terminal vs. internal ramets along the branches have different effects. The genet's survival probability is strongly influenced by the terminal ramets but is almost unaffected by the internal ones in a broad parameter range. However, in a narrow range in which both mortality rates are low, and thus the frontline of spreading is densely populated, internal ramets become decisive. They strongly influence the genet's survival and almost exclusively determine its size (i.e., the number of ramets) in the steady state. We discuss the implications of within-genet regulation of death for the occupation of space and species coexistence in plant communities.

Koopman-Operator-Based Robust Data-Driven Control for Wheeled Mobile Robots

The Koopman operator theory offers a way to construct explicit control-oriented high-dimensional linear dynamical models for the original nonlinear systems, solely using the input–output data of the dynamical system. The modeling accuracy of the Koopman model largely depends on the basis functions (lifting functions), dimensionality, and data quality. However, there has not been a systematic way to solve the problems mentioned above. In this article, a Koopman-operator-based robust data-driven control framework is proposed for wheeled mobile robots, via incorporating tools from control theory, to solve the problem of modeling errors of the Koopman model. By employing an extended state observer, the modeling errors of the Koopman model, including unknown external disturbances, are online estimated and compensated in the control signal in real time. Then, sliding-mode control is used to synthesize the controller. Importantly, the method of virtual control input is proposed, to cope with the model errors arising from the rotational motion of all the mobile robots. Besides, stability analysis is conducted, and the optimal dimensionality of the Koopman model is experimentally selected. Finally, experimental tests on an omnidirectional mobile robot are carried out to verify the effectiveness of the proposed control scheme, in terms of tracking performance and robustness.