Number Of DOF Research Articles

Articulating Resilience: Adaptive Locomotion of Wheeled Tensegrity Robot

Resilience plays an important role in improving robustness for robots in harsh environments such as planetary exploration and unstructured terrains. As a naturally compliant structure, tensegrity presents advantageous flexibility for enhancing resilience in robotic applications according to existing research. However, tensegrity robots to date are normally based on monolithic morphologies and are slow in locomotion. In this paper, we demonstrate how we adopt such flexibility to improve the robustness of wheeled robots by articulating modules with tensegrity mechanisms. The test results reveal the robot is resistant and resilient to external hazards in a fully passive manner owing to the continuous elasticity in the structure network. It possesses a good number of DoFs and can adapt to various terrains easily either with actuation or not. The robot is also capable of crawling locomotion aside from wheeled locomotion to traverse uneven surfaces and provide self-recovery from rollovers. It demonstrates good robustness and mobility at the same time compared with existing tensegrity robots and shows the competitiveness with conventional rigid robots in harsh scenarios. The proposed robot presents the capability of tensegrity robots with resilience, robustness, agility, and mobility without compromise. In a broader perspective, it widens the potential of tensegrity robots in practical applications.

Nonlinear optimal control for a five-link parallel robotic manipulator

Control and stabilization of parallel robotic manipulators is a non-trivial problem because of nonlinearities and the multi-variable structure. In this article, a nonlinear optimal control approach is proposed for the dynamic model of such robotic systems, using as a case-study the model of a five-link parallel robot. The dynamic model of the parallel robotic manipulator undergoes approximate linearization around a temporary operating point that is recomputed at each time-step of the control method. The linearization relies on Taylor series expansion and on the associated Jacobian matrices. For the linearized state-space model of the system, a stabilizing optimal (H-infinity) feedback controller is designed. This controller stands as a solution to the nonlinear optimal control problem under model uncertainty and external perturbations. To compute the controller’s feedback gains, an algebraic Riccati equation is repetitively solved at each iteration of the control algorithm. The stability properties of the control method are proven through Lyapunov analysis. Finally, to implement state estimation-based control without the need to measure the entire state vector of the parallel robotic manipulator, the H-infinity Kalman filter is used as a robust state estimator. Among the advantages of this control method, one can note that (i) unlike the popular computed-torque method for robotic manipulators, the new control approach is characterized by optimality and is also applicable when the number of control inputs is not equal to the robot’s number of DOFs and (ii) it achieves fast and accurate tracking of reference set points under minimal energy consumption by the robot’s actuators.

Trajectory Control–An Effective Strategy for Controlling Multi-DOF Upper Limb Prosthetic Devices

Despite great innovations in upper-extremity prosthetic hardware in recent decades, controlling a multiple joint upper limb prosthesis such as an elbow/wrist/hand system is still an open clinical challenge, in large part due to an insufficient number of control inputs available to users. While simultaneous control is in its early stages, the common control approach is sequential control, in which joints and grasps are driven one at a time. In this paper, we introduce and evaluate a concept we call <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">trajectory control</i> , that builds upon this approach, in which motions of the wrist, elbow, and shoulder DOFs (and subsets of them) are coupled into predefined sets of coordinated trajectories; to be selected by the user and driven with a single input variable. These trajectories were designed based on an earlier motion study of activities of daily life obtained from human demonstrations. We experimentally evaluate the efficacy of our approach through a human subjects study in which tasks are performed in a virtual environment. The results show that as device complexity increased (i.e. greater number of DOFs corresponding to more proximal amputations), participants were able to complete tasks faster with trajectory control while exhibiting similar levels of body compensation when compared to sequential and simultaneous control. Additionally, participants found trajectory control to be more intuitive and displayed more natural movement.

Fourth-order hybrid phase field analysis with non-equal order elements and dual meshes for simulating crack propagation

High computational cost handicaps the practical application of the phase field approach for rock-like materials. The aim of this study was to investigate the feasibility of saving computing resources, especially computing time, by reducing the number and order of displacement elements. Specifically, we developed a novel scheme (i.e., TMNE) based on dual meshes and non-equal order elements using NURBS-based IGA, and then combined it with the hybrid formulation of the fourth-order model to tackle the high computing cost. The scheme was shown to save more than 50% of the computing time and the number of DOFs compared with the traditional scheme in most cases. This indicates that there is a possibility of wasting computing resources in traditional discretization of displacement field. Additionally, the scheme simplifies the data transfer between meshes and only one case need to be considered, even when combined with adaptive remeshing. Last but not least, the scheme can also be extended to a variety of NURBS elements (C0-C4) as required.

Can Deep Synthesis of EMG Overcome the Geometric Growth of Training Data Required to Recognize Multiarticulate Motions?

By being predicated on supervised machine learning, pattern recognition approaches to myoelectric prosthesis control require electromyography (EMG) training data collected concurrently with every detectable motion. Within this framework, calibration protocols for simultaneous control of multifunctional prosthetic hands rapidly become prohibitively long-the number of unique motions grows geometrically with the number of controllable degrees of freedom (DoFs). This paper proposes a technique intended to circumvent this combinatorial explosion. Using EMG windows from 1-DoF motions as input and EMG windows from 2-DoF motions as targets, we train generative deep learning models to synthesize EMG windows appertaining to multi-DoF motions. Once trained, such models can be used to complete datasets consisting of only 1-DoF motions, enabling simple calibration protocols with durations that scale linearly with the number of DoFs. We evaluated synthetic EMG produced in this way via a classification task using a database of forearm surface EMG collected during 1-DoF and 2-DoF motions. Multi-output classifiers were trained on either (I) real data from 1-DoF and 2-DoF motions, (II) real data from only 1-DoF motions, or (III) real data from 1-DoF motions appended with synthetic EMG from 2-DoF motions. When tested on data containing all possible motions, classifiers trained on synthetic-appended data (III) significantly outperformed classifiers trained on 1-DoF real data (II), although significantly underperformed classifiers trained on both 1- and 2-DoF real data (I) (I < 0.05). These findings suggest that it is feasible to model EMG concurrent with multiarticulate motions as nonlinear combinations of EMG from constituent 1-DoF motions, and that such modelling can be harnessed to synthesize realistic training data.

Intrinsic extended isogeometric analysis with emphasis on capturing high gradients or singularities

Formulations of an extra-degree-of-freedom (DOF) free extended isogeometric analysis (IGA) are presented in this study. The idea is achieved by reconstruction of the coefficients through a mesh-free-based local approximation, based on the framework of a generalized finite-element method. The enrichment functions are embedded in the mesh-free basis implicitly, resulting in an identical number of DOFs. Moreover, the system condition number is of the same level between the enriched IGA and non-enriched version. The approach to handling the blending issues is discussed. Numerical examples with a solution containing sharp features/singularities are designed and studied in terms of accuracy and convergence.

Plane wave finite element model for the 2-D phononic crystal under force loadings

Direct numerical simulations of the dynamic response of the phononic crystal to point or distributed forces via conventional numerical methods are still formidable currently due to large number of DOFs involved in the simulations. In this study, based on the plane wave expansion (PWE) method, a plane wave finite element (PWFE) model for the 2-D phononic crystal which is infinite in the horizontal direction and finite in the vertical direction and subjected to a point harmonic force is proposed. To this end, the point harmonic force is decomposed into the wavenumber domain (WND) components by the Fourier transform first. The variables of the 2-D phononic crystal under the WND force component are expanded into a series of plane waves along the horizontal direction, and the variables associated with each plane wave are discretized by the FE method in the vertical direction. By using the virtual work principle, FE equations for the plane waves of the phononic crystal are established. With the FE equations for the plane waves, the eigenvalue equation for the displacement of the plane waves and wavenumber is formulated. Superposing the eigenvectors of the above eigenvalue equation yields the displacement for the plane waves due to the WND force component. Synthetization of the contributions from all the plane waves and inversion of the Fourier transform with respect to the wavenumber via the contour integration method generate the response of the phononic crystal to the point force. Comparison of present results with those due to the transfer matrix method validates the proposed model. Also, presented numerical results suggest that the dynamic response of the phononic crystal to a point harmonic force exhibits remarkable resonance phenomenon.

Genome-wide identification of DCL, AGO and RDR gene families and their associated functional regulatory elements analyses in banana (Musa acuminata).

RNA silencing is mediated through RNA interference (RNAi) pathway gene families, i.e., Dicer-Like (DCL), Argonaute (AGO), and RNA-dependent RNA polymerase (RDR) and their cis-acting regulatory elements. The RNAi pathway is also directly connected with the post-transcriptional gene silencing (PTGS) mechanism, and the pathway controls eukaryotic gene regulation during growth, development, and stress response. Nevertheless, genome-wide identification of RNAi pathway gene families such as DCL, AGO, and RDR and their regulatory network analyses related to transcription factors have not been studied in many fruit crop species, including banana (Musa acuminata). In this study, we studied in silico genome-wide identification and characterization of DCL, AGO, and RDR genes in bananas thoroughly via integrated bioinformatics approaches. A genome-wide analysis identified 3 MaDCL, 13 MaAGO, and 5 MaRDR candidate genes based on multiple sequence alignment and phylogenetic tree related to the RNAi pathway in banana genomes. These genes correspond to the Arabidopsis thaliana RNAi silencing genes. The analysis of the conserved domain, motif, and gene structure (exon-intron numbers) for MaDCL, MaAGO, and MaRDR genes showed higher homogeneity within the same gene family. The Gene Ontology (GO) enrichment analysis exhibited that the identified RNAi genes could be involved in RNA silencing and associated metabolic pathways. A number of important transcription factors (TFs), e.g., ERF, Dof, C2H2, TCP, GATA and MIKC_MADS families, were identified by network and sub-network analyses between TFs and candidate RNAi gene families. Furthermore, the cis-acting regulatory elements related to light-responsive (LR), stress-responsive (SR), hormone-responsive (HR), and other activities (OT) functions were identified in candidate MaDCL, MaAGO, and MaRDR genes. These genome-wide analyses of these RNAi gene families provide valuable information related to RNA silencing, which would shed light on further characterization of RNAi genes, their regulatory elements, and functional roles, which might be helpful for banana improvement in the breeding program.

Wave dispersion analysis of three-dimensional vibroacoustic waveguides with semi-analytical isogeometric method

Material and structural non-destructive evaluations using guided-wave (GW) testing techniques rely on the knowledge of wave dispersion characteristics. When studying coupled fluid–solid waveguides having complex geometries using the semi-analytical finite element (SAFE) method, an excessive computational effort may be required, especially at high-frequency ranges. In this paper, we show the robustness of an efficient computational approach so-called the semi-analytical isogeometric analysis (SAIGA) for computing the wave dispersion in 3D anisotropic elastic waveguides coupled with acoustic fluids. This approach is based on the use of Non-Uniform Rational B-splines (NURBS) as the basis functions for the geometry representation as well as for the approximation of pressure/displacement fields. The obtained results are compared with the ones derived from using the conventional SAFE method which uses Lagrange polynomials. It is shown that for computing the dispersion of GWs, using SAIGA leads to a much faster convergence rate than using the conventional SAFE with the same shape function’s order. For hollow prismatic structures immersed in fluids, using high-order NURBS (e.g, p=8) is particularly efficient as it only requires a few elements to achieve solutions having the same precision as the ones obtained by SAFE which requires up to five times of number of DOFs. Moreover, the continuity of normal displacement at fluid–solid interfaces could be significantly improved thanks to the smoothness feature of NURBS, showing the advantage of SAIGA over SAFE in the evaluation of the shape modes of GWs in coupled fluid–solid systems.

Combining shell and GBT-based finite elements: Vibration and dynamic analysis

This paper extends the approach previously proposed by the authors Manta et al. (2020, 2021) to the dynamic case, more precisely to the calculation of natural frequencies (including pre-loaded members) and the linear time-history response of thin-walled members and frames with complex geometry, undergoing global–local–distortional deformation. This approach relies on a combination of standard shell and GBT-based (beam) finite elements, where the prismatic beam parts are modelled with standard GBT-based elements with a minimum number of deformation modes (hence a minimum number of DOFs), while the geometrically complex zones (perforations, tapering, joints) are handled using shell elements. Three numerical examples are presented to demonstrate the capabilities and potential of the proposed approach. These examples concern (i) a lipped channel cantilever with two long holes, (ii) a lipped channel cantilever with a tapered segment, subjected to pre-loading, and (iii) an L-shaped frame with I-section members and a tapered joint. For comparison and validation purposes, full shell finite element model solutions are provided. In all examples, it is concluded that the proposed approach leads to very accurate results and a significant DOF economy with respect to full shell finite element models.

Multibody Modeling Method for UHV Porcelain Arresters Equipped with Lead Alloy Isolation Device

This paper presents a multibody modeling method for seismic analysis of UHV porcelain surge arresters equipped with a kind of seismic isolation device. An UHV arrester is modeled as a planar multibody system whose number of DOF is equal to the number of the arrester units. Joint coordinate method is adopted to construct the governing equations of motion. The seismic isolation device utilizing a number of lead alloy dampers as its core energy dissipation components is also investigated. An analytical model of this device is given by modeling each lead alloy damper as a hysteretic spring and reducing the entire device to a planar system consisting of a range of hysteretic springs. Its mechanical characteristic is derived theoretically, and the obtained moment-angle relationship is expressed as a system of differential algebraic equations. The initial rotational stiffness of the device is formulated in terms of the structural and mechanical parameters of the device. This analytic expression is used in estimating the fundamental frequency of the isolated equipment. By this modeling method, it is easy to construct the governing equations of motion for the isolated system. An UHV arrester specimen is analyzed by this proposed method. The effectiveness of the isolation device in terms of reducing the internal base moment is significant and the influence of system parameters on the effectiveness is also discussed. The proposed method shows its potential usefulness in optimal design of the isolation device.

3-D nonlinear magnetic field analysis with a novel adaptive finite element method

In this paper, an adaptive degrees-of-freedom finite element method is extended to three-dimensional (3-D) nonlinear magnetic field analysis. The error distribution of the discrete solution changes along with the iterative process, and mesh coarsening or other operations with the same effect is needed to keep the scale of the problem small. In this proposed adaptive method, dispensable degrees of freedom (DoFs) are eliminated from the unknown list by constraining them with supplementary interpolation functions, which are formulated with master DoFs. Compared with mesh coarsening, the administration of DoFs and geometric data are no longer required, while the topology of the mesh is maintained. To extend to 3-D problems, a novel constraint, which produces accurate coefficients for an alterable number of master DoFs, is presented. The constraint is integrated into the element algebraic equation, followed by a conventional assembly. Other techniques for the adaptive algorithm are also included in this method. Several numerical examples are tested to showcase the effectiveness of this method in 3-D problems.

Thin-Wire Integral Equation Formulation With Quasistatic Darwin Approximation

Darwin's model is a reduced form of Maxwell's equations, where radiations are neglected, while inductive, resistive, and capacitive effects are still taken into account. In this article, we propose a method for the efficient modeling of electrically long, thin wires using a Darwin-based integral equation approach, thus keeping the number of DoFs and runtime low. This model provides further numerical techniques to efficiently take into account special inhomogeneities via the modification of the Green's functions unlike in full-wave models. The benefits are discussed through the example of the automotive CISPR 25 measurement setup.

A shell superelement for mechanical analysis of cylindrical structures

This paper aims at developing a new cylindrical shell element, called shell superelement. The element is defined based on the classical shell theory, and it consists of eight nodes each with six degree-of-freedoms (dofs). In this element, the trigonometric shape functions are incorporated along the angular direction of element while polynomials were used in other two directions. Therefore, there is no need for meshing a shell structure with cylindrical geometry through the angular direction. This property helps an engineer to deal with complicated analyses on cylindrical shell structures with less number of dofs. At the end, the defined element is used in the stress analysis of two different classical shell problems and the results are compared with the ones reported in the literature, and obtained by means of shell elements in a commercial software package.

User training for machine learning controlled upper limb prostheses: a serious game approach

BackgroundUpper limb prosthetics with multiple degrees of freedom (DoFs) are still mostly operated through the clinical standard Direct Control scheme. Machine learning control, on the other hand, allows controlling multiple DoFs although it requires separable and consistent electromyogram (EMG) patterns. Whereas user training can improve EMG pattern quality, conventional training methods might limit user potential. Training with serious games might lead to higher quality EMG patterns and better functional outcomes. In this explorative study we compare outcomes of serious game training with conventional training, and machine learning control with the users’ own one DoF prosthesis.MethodsParticipants with upper limb absence participated in 7 training sessions where they learned to control a 3 DoF prosthesis with two grips which was fitted. Participants received either game training or conventional training. Conventional training was based on coaching, as described in the literature. Game-based training was conducted using two games that trained EMG pattern separability and functional use. Both groups also trained functional use with the prosthesis donned. The prosthesis system was controlled using a neural network regressor. Outcome measures were EMG metrics, number of DoFs used, the spherical subset of the Southampton Hand Assessment Procedure and the Clothespin Relocation Test.ResultsEight participants were recruited and four completed the study. Training did not lead to consistent improvements in EMG pattern quality or functional use, but some participants improved in some metrics. No differences were observed between the groups. Participants achieved consistently better results using their own prosthesis than the machine-learning controlled prosthesis used in this study.ConclusionOur explorative study showed in a small group of participants that serious game training seems to achieve similar results as conventional training. No consistent improvements were found in either group in terms of EMG metrics or functional use, which might be due to insufficient training. This study highlights the need for more research in user training for machine learning controlled prosthetics. In addition, this study contributes with more data comparing machine learning controlled prosthetics with Direct Controlled prosthetics.

A robust optimal control approach in the weighted Sobolev space for underactuated mechanical systems

This paper proposes new robust nonlinear optimal control formulations in the weighted Sobolev space, here called nonlinear W2 and W∞ controllers, to handle with two classes of underactuated mechanical systems: the reduced ones with a reduced number of DOF; and the entire underactuated mechanical systems with input coupling. The optimal control problems are formulated via dynamic programming and particular solutions are presented for the resulting Hamilton–Jacobi equations with the corresponding stability analysis. Also, the concepts of Wm,p,σ-stability and Wm,p,σ-gain for a general class of systems are established, with the demonstration for the particular case of study. The novel W2 and W∞ controllers are synthesized for an unmanned aerial vehicle benchmark. The results demonstrate that these controllers provide better transient performance with faster response against external disturbances in comparison with a classic nonlinear H∞ controller, in addition to have a simple design.

A reduced model for nonlinear analysis and design of thin-walled structures prone to multi-modal buckling

A reduced model for nonlinear analysis and design of thin-walled structures prone to multi-modal buckling

Cramér-Rao Bound Analysis of Underdetermined Wideband DOA Estimation Under the Subband Model via Frequency Decomposition

A class of Cramér-Rao bounds (CRBs) for wideband direction-of-arrival (DOA) estimation under the subband (or frequency bin) model is studied for the underdetermined case, where the number of sources is no less than that of physical sensors. A unified framework is proposed to encompass the closed-form CRB expressions for DOAs in four cases where the sources are known a priori to 1) have flat spectra/cross spectra, 2) be spatially uncorrelated, 3) be spatially uncorrelated and have proportional spectra up to unknown factors, 4) be spatially uncorrelated and have flat spectra. The relationship between the wideband CRBs and the subband ones is investigated, and the order relationship among the derived CRBs are provided. The asymptotic behavior of the CRBs with respect to the number of snapshots and the signal-to-noise ratio (SNR) is discussed. Two asymptotic expressions for sufficiently large SNR are derived in both overdetermined and underdetermined cases. Existence of the derived CRBs is examined through rank conditions of the introduced matrices, which yields upper bounds on the resolution capacities. Different from the narrowband scenario, underdetermined wideband DOA estimation is feasible even if a sparse array is not used given different a priori knowledge about the source spectra. It is possible to resolve more wideband Gaussian sources than the number of DOFs offered by the difference co-array. Finally, further interpretations of the subband model are provided, revealing the underlying connections with the multi-frequency co-array augmentation concept and the non-coherent subarray system.

Robust and Efficient Estimation of Relative Pose for Cameras on Selfie Sticks.

Taking selfies has become one of the major photographic trends of our time. In this study, we focus on the selfie stick, on which a camera is mounted to take selfies. We observe that a camera on a selfie stick typically travels through a particular type of trajectory around a sphere. Based on this finding, we propose a robust, efficient, and optimal estimation method for relative camera pose between two images captured by a camera mounted on a selfie stick. We exploit the special geometric structure of camera motion constrained by a selfie stick and define this motion as spherical joint motion. Utilizing a novel parametrization and calibration scheme, we demonstrate that the pose estimation problem can be reduced to a 3-degrees of freedom (DoF) search problem, instead of a generic 6-DoF problem. This facilitates the derivation of an efficient branch-and-bound optimization method that guarantees a global optimal solution, even in the presence of outliers. Furthermore, as a simplified case of spherical joint motion, we introduce selfie motion, which has a fewer number of DoF than spherical joint motion. We validate the performance and guaranteed optimality of our method on both synthetic and real-world data. Additionally, we demonstrate the applicability of the proposed method for two applications: refocusing and stylization.

Combining shell and GBT-based finite elements: Plastic analysis with adaptive mesh refinement

In a recent work [1], the authors proposed a versatile and computationally efficient method to model thin-walled members with complex geometries, which combines standard shell and GBT-based (beam) finite elements. In the present paper, the approach is extended to the physically non-linear case (in particular, J2 plasticity), by using an adaptive mesh refinement strategy that updates the finite element model such that the plastic zones are handled by the shell elements, whereas the elastic prismatic beam parts are dealt with using standard GBT-based elements with a minimum number of deformation modes (hence a minimum number of DOFs). This proposed approach offers two advantages: (i) versatility, in the sense that non-prismatic zones can be easily modelled, using shell elements, and (ii) computational efficiency, since the adaptive plastic zones are confined to the shell substructures, which require a much lower computational cost than GBT elements in handling physically non-linearity, whereas the elastic zones are most efficiently dealt with by GBT elements. For illustrative purposes, three examples are presented to demonstrate the capabilities of the proposed approach. These examples concern (i) a simply supported hat section beam, (ii) a lipped channel cantilever with two long holes in the web and (iii) a plane frame with I-section members. For comparison and validation purposes, full shell finite element model solutions are provided. It is concluded that, in all cases, the proposed approach leads to excellent results throughout the load-displacement range considered.