High Degree Of Freedom Research Articles

Removal of Stair-Step Effects in Binder Jetting Additive Manufacturing Using Grayscale and Dithering-Based Droplet Distribution.

Binder jetting is a layer-based additive manufacturing process for three-dimensional parts in which a print head selectively deposits binder onto a thin layer of powder. After the deposition of the binder, a new layer of powder is applied. This process repeats to create three-dimensional parts. The binder jetting principle can be adapted to many different materials. Its advantages are the high productivity and the high degree of freedom of design without the need for support structures. In this work, the combination of binder jetting and casting is utilized to fabricate metal parts. However, the achieved properties of binder jetting parts limit the potential of this technology, specifically regarding surface quality. The most apparent surface phenomenon is the so-called stair-step effect. It is considered an inherent feature of the process and only treatable by post-processing. This paper presents a method to remove the stair-step effect entirely in a binder jetting process. The result is achieved by controlling the binder saturation of the individual voxel volumes by either over or underfilling them. The saturation is controlled by droplet size variation as well as dithering, creating a controlled migration of the binder between powder particles. This work applies the approach to silica sand particle material with an organic binder for casting molds and cores. The results prove the effectiveness of this approach and outline a field of research not identified previously.

Spatiotemporal mode-locked multimode fiber laser with dissipative four-wave mixing effect

The high degree of freedom and novel nonlinear phenomena of multimode fiber are attracting attention. In this work, we demonstrate a spatiotemporal mode-locked multimode fiber laser, which relies on microfiber knot resonance (MKR) via dissipative four-wave-mixing (DFMW) to achieve high-repetition-rate pulses. Apart from that, DFMW mode locking with switchable central wavelengths can also be obtained. It was further found that high pulse energy induced nonlinear effect of the dominant mode-locking mechanism transforming from DFMW to nonlinear Kerr beam cleaning effect (NL-KBC). The experimental results are valuable for further comprehending the dynamic characteristics of spatiotemporal mode-locked multimode fiber lasers, facilitating them much more accessible for applications.

Nonlinear integrated control with friction estimation for automatic lane change on the highways

A vehicle is a highly nonlinear dynamical system with high degrees of freedom. There are also very strong couplings between the longitudinal and lateral dynamics. Therefore, the simultaneous longitudinal and lateral motion control in different road conditions is a challenging problem. This paper presents an integrated adaptive control for longitudinal and lateral vehicle guidance on the highways. The contribution of this work is the design of a robust integrated controller that can handle tyre-road friction changes with low computational cost The proposed controller can adapt to the variations of tyre and road specifications. The strength of this method is that without the use of complex models, the nonlinear tyre dynamic is considered in the design of the controller. Therefore, despite the consideration of a comprehensive nonlinear vehicle dynamic model and the ability to adapt to changes in tyre and road specifications, the computational cost of the algorithm is very small. Besides, due to the controller robustness, there are no concerns about uncertainties. To validate the proposed algorithm, a full vehicle model in CarSim software is used. In obtained results, it is assumed that the yaw rate, longitudinal and lateral acceleration signals are available and have noises. The estimation of longitudinal and lateral velocities and other vehicle states has been done using these signals. Results for different road conditions as well as various vehicle movement modes confirm that tracking of longitudinal and lateral positions has been carried out with high precision. Besides, the results show that the proposed integrated control guarantees the stability of the vehicle, it will manage the tyre-road friction changes well, and also it is resistant to unmodeled uncertainties.

Layered disturbance rejection path-following control with geometry-based feedforward for unmanned rollers

As a widely used engineering vehicle, drum rollers have a higher degree of freedom in motion than conventional passenger vehicles. The uncertainties, caused by road and vehicle condition variations, introduce severe disturbances in the path-following control of unmanned rollers. In this work, a single-drum roller is considered, for which a composite disturbance rejection controller (CDRC) is proposed for path following. The CDRC comprises a disturbance rejection controller (DRC) and a modified pure pursuit controller (MPPC). The DRC lumps all the discrepancies of the models from the roller as total disturbance, which is estimated by the extended state observer (ESO) and rejected in the feedback control loop. For enhanced performance, MPPC is added as a feedforward controller, which calculates the target articulation angle based on the roller geometry model. Compared with the DRC, the settling time of the CDRC is reduced by 12.3%, and the lateral errors are reduced by 43.1% and 39.9% in the presence of uncertainties in the positioning system and steering motor, respectively, while it still maintains a low computational cost. The proposed controller have been used in 15 unmanned rollers for 2 years, using which over 5 million square meters area has been compacted.

Doherty Power Amplifier With Extended High-Efficiency Range Based on the Utilization of Multiple Output Power Back-Off Parameters

This article presents a novel method of extending the high-efficiency range of Doherty power amplifiers (DPAs) by utilizing multiple output power back-off (OBO) parameters. The OBO extension techniques of the DPAs, such as complex combining load (CCL), virtual stub (VS), and out-phased current combining (OCC) methods, were defined, and a general expression for these OBO extension techniques was derived. Using the general expression, three OBO extension methods were comparatively and comprehensively analyzed. Based on the analysis, the DPA with an extended OBO can be designed by simultaneously employing multiple back-off techniques with a high degree of freedom. To verify the general expression and its analysis, a DPA design for the frequency band of 3.45–3.75 GHz with GaN HEMTs was conducted using multiple OBO extension techniques of VS and OCC methods for a large OBO of 8.5 dB. Using a long-term evolution (LTE) signal with a signal bandwidth of 20 MHz and a peak-to-average power ratio (PAPR) of 8 dB, the drain efficiency (DE) of 38.9%–56.8% was achieved at the average output power levels of 34.4–36.8 dBm while satisfying ACLR of no larger than −45 dBc with digital predistortion (DPD) linearization. Under excitation of a 5G new radio (NR) signal with a signal bandwidth of 100 MHz and a PAPR of 8 dB, the DE of 38.5%–50.2% was achieved at the average output power levels of 34.6–36.8 dBm with an ACLR of under −24.6 dBc without DPD linearization.

Analysis of the phase transition of patch colloidal particles and the molecular dynamics simulation

Anisotropic particle system means that the interaction between particles in the system depends not only on their spatial coordinates, but also on the orientation of each particle due to the influence of many factors such as structure, shape, electronic properties and environment. The introduction of orientation coordinates makes the phase space describing this kind of system have higher degrees of freedom and more complex symmetry. Therefore, the study of its general properties has more basic significance in physics. Patch colloidal particles, which have become one of the research hotspots in the field of soft matter physics in recent years, are a typical mesoscopic physical system with anisotropic interaction. In this paper, the two-dimensional patch colloidal particle system is studied as follows: (1) the phase structure of the system is preliminarily studied and analysed. It is found that the phase behaviour of the system at high temperature is similar to that of isotropic Lennard - Jones system is similar. Under low temperature, the system with low patch coverage has strip structure in the condensed phase and complex self-assembly structure in the gas phase; (2) It is found that the long-term diffusion belongs to isotropic normal diffusion, while the short-term diffusion is anisotropic ballistic diffusion, and the particle motion direction is always perpendicular to the patch orientation; (3) The melting behavior of the system is studied. It is found that the melting is a two-step phase transition, which is the same as that of the two-dimensional isotropic colloidal system;

A\xa0review of critical issues in the design of lightweight flywheel rotors with composite materials

Composite materials are widely used to build high-performance flywheels due to their high material strength and low mass density. The high degrees of freedom in material selection, design, and manufacturing techniques lead to a variety of rotor structures. This paper presents the characteristics of different composite rotors and the critical considerations in terms of designing, manufacturing, and testing them. The introduction starts with the limitations of a single filament-wound composite rim. Then, various rotor structures are presented as well as the critical issues regarding the composite rim design, rim-shaft connection, and rotor failure in order to make safe design recommendations. The aim is to summarize the current techniques and provide references for further developments.

RBF approximation of three dimensional PDEs using tensor Krylov subspace methods

In this paper, we propose different algorithms for the solution of a tensor linear discrete ill-posed problem arising in the application of the meshless method for solving PDEs in three-dimensional space using multiquadric radial basis functions. It is well known that the truncated singular value decomposition (TSVD) is the most common effective solver for ill-conditioned systems, but unfortunately the operation count for solving a linear system with the TSVD is computationally expensive for large-scale matrices. In the present work, we propose algorithms based on the use of the well known Einstein product for two tensors to define the tensor global Arnoldi and the tensor Gloub Kahan bidiagonalization algorithms. Using the so-called Tikhonov regularization technique, we will be able to provide computable approximate regularized solutions in a few iterations. The tensor formulation will allow us to develop an RBF approximation for high dimensional PDEs based on Hierarchical tensors and Adaptive Cross Approximation, which in turn reduces significantly the storage and computational costs, while the accuracy of the method is preserved. The performance of the proposed methods is illustrated with a variety of benchmark examples and large-scale industrial applications with high degrees of freedom.

Self-winding liquid crystal elastomer fiber actuators with high degree of freedom and tunable actuation

Liquid-crystal-elastomer (LCE) fiber actuators capable of reversible, large-scale, programmable deformation in response to external stimuli have great potential in many applications, including artificial muscles, robotics, and wearable devices. Despite their exciting prospects, limitations -such as few modes of shape transformation in a single actuator due to limited degree of freedom (DOF), difficulty to concurrently gain large length change and powerful stress, lack of scalable manufacturing method - seriously restrict their engineering applicability. Here we present bioinspired self-winding LCE fiber actuators that possess diverse controllable shape transformations (bending, twisting, coiling, and shortening), a combination of high contraction ratio (1750%) and high stress (∼3.4 MPa), long term photomechanical robustness (over 1000 photodeformation cycles without obvious fatigue), and readily, scalable manufacture. Our fiber actuators can simultaneously conduct two or three kinds of deformation and thus enables complex morphing behaviors to manipulate objects (grabbing, dragging, lifting, and winding), and even drive gear set. We envision that these self-winding fiber actuators combined with high DOF, tunable actuation, photomechanical robustness, and mass production could be developed as high-performance artificial muscles for broad engineering applications.

BOLD Monitoring in the Neural Simulator ANNarchy.

Multi-scale network models that simultaneously simulate different measurable signals at different spatial and temporal scales, such as membrane potentials of single neurons, population firing rates, local field potentials, and blood-oxygen-level-dependent (BOLD) signals, are becoming increasingly popular in computational neuroscience. The transformation of the underlying simulated neuronal activity of these models to simulated non-invasive measurements, such as BOLD signals, is particularly relevant. The present work describes the implementation of a BOLD monitor within the neural simulator ANNarchy to allow an on-line computation of simulated BOLD signals from neural network models. An active research topic regarding the simulation of BOLD signals is the coupling of neural processes to cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2). The flexibility of ANNarchy allows users to define this coupling with a high degree of freedom and thus, not only allows to relate mesoscopic network models of populations of spiking neurons to experimental BOLD data, but also to investigate different hypotheses regarding the coupling between neural processes, CBF and CMRO2 with these models. In this study, we demonstrate how simulated BOLD signals can be obtained from a network model consisting of multiple spiking neuron populations. We first demonstrate the use of the Balloon model, the predominant model for simulating BOLD signals, as well as the possibility of using novel user-defined models, such as a variant of the Balloon model with separately driven CBF and CMRO2 signals. We emphasize how different hypotheses about the coupling between neural processes, CBF and CMRO2 can be implemented and how these different couplings affect the simulated BOLD signals. With the BOLD monitor presented here, ANNarchy provides a tool for modelers who want to relate their network models to experimental MRI data and for scientists who want to extend their studies of the coupling between neural processes and the BOLD signal by using modeling approaches. This facilitates the investigation and model-based analysis of experimental BOLD data and thus improves multi-scale understanding of neural processes in humans.

Extended topological valley-locked surface acoustic waves

Stable and efficient guided waves are essential for information transmission and processing. Recently, topological valley-contrasting materials in condensed matter systems have been revealed as promising infrastructures for guiding classical waves, for they can provide broadband, non-dispersive and reflection-free electromagnetic/mechanical wave transport with a high degree of freedom. In this work, by designing and manufacturing miniaturized phononic crystals on a semi-infinite substrate, we experimentally realized a valley-locked edge transport for surface acoustic waves (SAWs). Critically, original one-dimensional edge transports could be extended to quasi-two-dimensional ones by doping SAW Dirac “semimetal” layers at the boundaries. We demonstrate that SAWs in the extended topological valley-locked edges are robust against bending and wavelength-scaled defects. Also, this mechanism is configurable and robust depending on the doping, offering various on-chip acoustic manipulation, e.g., SAW routing, focusing, splitting, and converging, all flexible and high-flow. This work may promote future hybrid phononic circuits for acoustic information processing, sensing, and manipulation.

Research on the phase structure of Styrene-Butadiene-Styrene modified asphalt based on molecular dynamics

To study the phase structure of the SBS modified asphalt by considering the polymer characteristic of liner SBS molecular with different block ratios and concentrations on the molecular scale. A total of 18 molecular dynamics(MD) models were constructed by Gromacs. The phase phenomenon was analyzed by number density and density distribution map. The adsorption behavior of SARA components and SBS were analyzed by considering the SARA molecules' contact characteristics and the adsorption number of SARA molecules on the SBS surface. The result shows that SBS molecular will form the dispersed phase in low concentration (0–4%). With the concentration increasing, the phase will transfer from the dispersed phase to the continuous phase. After the critical concentration, the state of SBS molecule changes from the stable state to the dispersed phase. the SBS with S: B block ratio closer to the middle point is easier to form a continuous phase in different concentrations. The asphaltenes aggregation would also be attracted by SBS, and interact with SBS by branch chains. The saturates molecular is wrapped around the SBS molecules. Resins and aromatics molecules with higher degrees of freedom in space and easy to adjust their spatial structure are also more likely to adsorb around SBS molecules. The highest adsorbed molecules number and molecular weight of different SBS molecules are the Resins, followed by the aromatics. The amount of the adsorbed asphaltenes is nearly the same as that of the saturates. Due to the highest weight of asphaltene molecules in the SARA component, the asphaltene molecular weight absorbed by SBS is close to that of aromatics. The phase structure change may cause a dramatic change in the number of adsorbed molecules.

The enhanced beam sweeping algorithm for DOA estimation in the hybrid analog-digital structure with nested array

As an emerging technology, the hybrid analog-digital structure has been considered for use in future millimeter-wave communications. Although this structure can reduce the hardware cost and power consumption considerably, the spatial covariance matrix (SCM), as the core of subspace-based direction of arrival (DOA) estimation, cannot be obtained directly. Previously, the beam sweeping algorithm (BSA) has been found effective for reconstructing the spatial covariance matrix and realizing DOA estimation by forming the beams to difference directions. However, it is computationally intractable owing to the high-dimensional matrix operation. To address this problem and improve the DOA estimation performance, this paper applies the nested array to the hybrid analog-digital structure and proposes the enhanced BSA (EBSA) for DOA estimation. By deleting a large number of redundant elements exist in the SCM to be reconstructed, the computational cost can be considerably reduced. Also, the nested array can offer high degrees of freedom. Finally, simulation experiments are conducted to verify the performance of EBSA. The results indicate that the proposed EBSA is better than the state-of-the-art method in terms of estimation accuracy and computational cost.

Structural similitude for a scaled rotor system considering stiffness characteristics of bolted joints

Rotor system with bolted joints is widely used in aero-engine, and has the features of large axial span, high-degrees of freedom, and nonlinearity. There exist several researches on similitude of rotor systems, but these studies mainly focus on the simple and linear systems. The similitude investigation for the rotor system considering bolted joints has not been conducted. To fill this gap, a method to obtain the scaling laws for low-pressure (LP) compressor are presented based on the generalized and fundamental equations of substructures (bolted joint, shaft, and disk), which considers the nonlinearity of bolted joints and different materials. The dynamic model of the LP compressor considering the nonlinearity produced by bolted joints is established. Moreover, the effect of bolted joints on the LP compressor is investigated to illustrate the necessity for considering bolted joints in similitude procedure. In order to ensure the similitude of nonlinear dynamic characteristics, the scaling factors are divided into the linear and nonlinear scaling factors, where the nonlinear scaling factors are used to put the bolted joint into similitude. The results show that the natural frequencies, vibration responses, frequency components, and stability can be predicted accurately by scaled models, even the models are made of relatively cheap materials.

Distributed event-triggered average consensus control strategy with fractional-order local controllers for DC microgrids

Recently, direct current (DC) microgrids have become prominent due to their less complicated architecture and better integration of renewable energy sources (RES). Energy storage systems (ESSs) and RES, such as photovoltaic (PV) panels and fuel cells (FCs), are internally DC. Hence, it is easier for them to be integrated into DC microgrids compared to AC systems. However, DC microgrids have low inertia, and voltage stability is harder to achieve due to the inexistence of reactive power compensators. Therefore, this paper suggests a novel distributed control system for DC microgrids to achieve voltage stabilization and accurate energy balancing of ESSs based on event-triggered average consensus protocol and fractional-order proportional-integral (FOPI) local controllers. The fractional-order controller has many advantages over the integer-order classical controller, such as achieving better performance, flexibility, and a higher degree of freedom in the controller design. In this regard, the FOPI local controllers provide accurate load sharing for the droop mechanism. The distributed secondary layer controller compensates for the voltage offset, meanwhile balancing the energy level of ESSs. In the conventional distributed control strategies, the communication between the agents is periodically at discrete sampling time, resulting in network traffic increase and excessive energy consumption in the devices. Therefore, in this paper, the average consensus protocol is event-based, saving considerable network capacity for other microgrid communication needs. The effectiveness of the proposed control strategy is confirmed by the simulation of a 380V DC microgrid in MATLAB/Simulink that shows the achievement of the control objectives. Simulation results show the proper voltage stabilization and accurate energy balancing of ESSs in both islanded and grid-connected modes and a significant reduction in the transmitted information between agents.

Interlayer Reinforcement in Shotcrete-3D-Printing

Additive manufacturing with cement-based materials has recently become increasingly common on construction site. The high degree of freedom in individual geometric shapes, the associated potential for resource-efficient designs, and the high degree of automation could make this technology a milestone in the history of construction industry. Many of the existing manufacturing techniques are initially based on unreinforced concrete. However, for many structural elements, the use of reinforcement is indispensable and therefore the reinforcement integration represents a prerequisite. One promising reinforcement strategy is the use of interlayer reinforcement. This method specifically uses the layered characteristic of the additive manufacturing process by integrating reinforcement between the applied layers. In combination with an adaptive path planning, it is therefore possible to manufacture force-flow-compliant reinforced elements with a minimal increase in process complexity compared to an unreinforced production. However, besides the integration process itself, material-process interactions represent an important research topic. Especially for Shotcrete-3D-Printing, the use of accelerators can significantly change the structural build-up of the applied material and thus effect the bonding ability of the sprayed concrete to the integrated reinforcement element. The present study investigates the effect of accelerator dosage on the bond properties of integrated rebars. The resulting bond is analyzed non-destructively via computed tomography and mechanically by pull-out tests according to RILEM RC6. The results show that the material compaction caused by the sprayed application leads to excellent bond properties. However, by using high accelerator dosages, bond deteriorations can be observed.

An adaptive isogeometric phase-field method for brittle fracture in rock-like materials

In recent years, the phase-field method (PFM) has gained considerable attention to model fracture process. This is because, in a single framework, the method allows to model crack nucleation, growth, coalescence and branching. In addition, existing FE codes can be used with minimal modifications. However, one of the major limitations of the PFM is that, it requires extremely fine mesh to accurately capture the fracture process. An adaptive phase-field method within the framework of isogeometric analysis is presented in this work to model tensile and compressive-shear crack propagation in rock-like materials. Locally refined NURBS are adopted for describing both the geometry and the field variables. The local refinement is performed by structured mesh refinement strategy based on an error indicator that depends on the user-defined threshold value for the phase-field variable. Two phase-field models are adopted for simulating tensile and compressive-shear fractures in rock-like materials. For numerical implementation, a hybrid formulation combined with a staggered solution scheme is adopted. Several two-dimensional benchmark problems are investigated to demonstrated the robustness of the proposed framework. From the numerical study, it can be found that the developed adaptive framework achieves results comparable to a uniformly refined mesh with higher degrees of freedom.

Langevin based turbulence model and its relationship with Kappa distributions

Kappa distributions (or kappa -like distributions) represent a robust framework to characterize and understand complex phenomena with high degrees of freedom, as turbulent systems, using non-extensive statistical mechanics. Here we consider a coupled map lattice Langevin based model to analyze the relation of a turbulent flow, with its spatial scale dynamic, and kappa -like distributions. We generate the steady-state velocity distribution of the fluid at each scale, and show that the generated distributions are well fitted by kappa -like distributions. We observe a robust relation between the kappa parameter, the scale, and the Reynolds number of the system, Re. In particular, our results show that there is a closed scaling relation between the level of turbulence and the kappa parameter; namely kappa sim text {Re},k^{-5/3}. We expect these results to be useful to characterize turbulence in different contexts, and our numerical predictions to be tested by observations and experimental setups.

Surface quality parameters for structural components manufactured by DED-arc processes

Additive manufacturing of structural metallic components is an emerging technology. The DED-arc (direct energy deposition) process combines efficiency with high degrees of freedom and predictable quality. Compared to powder-based processes, the surface is relatively coarse as a result of high heat input and larger molten zones. In industrial applications, the post-treatment of surfaces of DED-arc components cannot be guaranteed for economic reasons or due to geometric constraints. Hence, the surface topography must be evaluated and quality levels need to be defined. Unfortunately, established surface parameters and their determination cannot be directly transferred to DED-arc components and further research on this topic is needed. This article demonstrates surface topography features of representative DED-arc components. Furthermore, waviness profiles were established by applying an alternative cutoff wavelength to better represent the DED-arc surface characteristics. Based on these waviness profiles, 2-dimensional height parameters were calculated. The waviness parameters were determined for four DED-arc specimens made from high strength wire electrodes. The manufacturing parameters were varied in terms of different energy input and interpass temperature, resulting in varying deposition rates and surface qualities. The waviness parameters correlated with manufacturing parameters. A mixed deterministic and random nature of the surface topography was verified.

The use of models and modelling in design projects in three different technology classrooms

In this study, we aim to investigate activities using models in a design project in three technology classrooms. Activities that use models are important for students’ development of knowledge and skills connected to the design process. Nevertheless, few empirical studies have thus far examined how models and modelling are used in a classroom environment when students and teachers are involved in a design project. In order to meet our aim, we video-recorded eight lessons from three different technology classrooms (students aged 13–15), where the students were involved in different problem-solving activities using models and modelling. The three projects had different specifications, and the students’ degrees of freedom thereby varied. The video recordings were analysed using a qualitative content analysis. The analysis resulted in seven activities being identified where the teachers and students talked about models and modelling in order to solve the problem. The results also revealed three different dimensions of models: material, structure and function. These dimensions are present in almost all activities that use models. In a project with a high degree of freedom, all three dimensions of models are present. On the contrary, in a project with a lower freedom, only one of the dimensions is present, resulting in a lower degree of complexity for the students. The study emphasizes that the presumptions and openness of a design project in technology education can provide different possibilities for students learning in relation to models and modelling.