Tuning mechanical behaviors of highly entangled hydrogels with the random distribution of mobile entanglements

Highly entangled hydrogels exhibit excellent mechanical properties,including high toughness,high stretchability,and low hysteresis.By considering the evolution of randomly distributed entanglements within the polymer network upon mechanical stretches,we develop a constitutive theory to describe the large stretch behaviors of these hydrogels.In the theory,we utilize a representative volume element(RVE)in the shape of a cube,within which there exists an averaged chain segment along each edge and a mobile entanglement at each corner.By employing an explicit method,we decouple the elasticity of the hydrogels from the sliding motion of their entanglements,and derive the stress-stretch relations for these hydrogels.The present theoretical analysis is in agreement with experiment,and highlights the significant influence of the entanglement distribution within the hydrogels on their elasticity.We also implement the present developed constitutive theory into a commercial finite element software,and the subsequent simulations demonstrate that the exact distribution of entanglements strongly affects the mechanical behaviors of the structures of these hydrogels.Overall,the present theory provides valuable insights into the deformation mechanism of highly entangled hydrogels,and can aid in the design of these hydrogels with enhanced performance.

A Comparative Study on Kinematic Calibration for a 3-DOF Parallel Manipulator Using the Complete-Minimal,Inverse-Kinematic and Geometric-Constraint Error Models

Kinematic calibration is a reliable way to improve the accuracy of parallel manipulators, while the error model dramatically afects the accuracy, reliability, and stability of identifcation results. In this paper, a comparison study on kinematic calibration for a 3-DOF parallel manipulator with three error models is presented to investigate the relative merits of diferent error modeling methods. The study takes into consideration the inverse-kinematic error model, which ignores all passive joint errors, the geometric-constraint error model, which is derived by special geometric constraints of the studied RPR-equivalent parallel manipulator, and the complete-minimal error model, which meets the complete, minimal, and continuous criteria. This comparison focuses on aspects such as modeling complexity, identifcation accuracy, the impact of noise uncertainty, and parameter identifability. To facilitate a more intuitive comparison, simulations are conducted to draw conclusions in certain aspects, including accuracy, the infuence of the S joint, identifcation with noises, and sensitivity indices. The simulations indicate that the complete-minimal error model exhibits the lowest residual values, and all error models demonstrate stability considering noises. Hereafter, an experiment is conducted on a prototype using a laser tracker, providing further insights into the diferences among the three error models. The results show that the residual errors of this machine tool are signifcantly improved according to the identifed parameters, and the complete-minimal error model can approach the measurements by nearly 90% compared to the inverse-kinematic error model. The fndings pertaining to the model process, complexity, and limitations are also instructive for other parallel manipulators.

A Client Selection Method Based on Loss Function Optimization for Federated Learning

Federated learning is a distributedmachine learningmethod that can solve the increasingly serious problemof data islands and user data privacy,as it allows training data to be kept locally and not shared with other users.It trains a globalmodel by aggregating locally-computedmodels of clients rather than their rawdata.However,the divergence of local models caused by data heterogeneity of different clients may lead to slow convergence of the global model.For this problem,we focus on the client selection with federated learning,which can affect the convergence performance of the global model with the selected local models.We propose FedChoice,a client selection method based on loss function optimization,to select appropriate local models to improve the convergence of the global model.It firstly sets selected probability for clients with the value of loss function,and the client with high loss will be set higher selected probability,which can make them more likely to participate in training.Then,it introduces a local control vector and a global control vector to predict the local gradient direction and global gradient direction,respectively,and calculates the gradient correction vector to correct the gradient direction to reduce the cumulative deviationof the local gradient causedby theNon-IIDdata.Wemake experiments to verify the validity of FedChoice on CIFAR-10,CINIC-10,MNIST,EMNITS,and FEMNIST datasets,and the results show that the convergence of FedChoice is significantly improved,compared with FedAvg,FedProx,and FedNova.

Regression Method for Rail Fastener Tightness Based on Center-Line Projection Distance Feature and Neural Network

In the railway system,fasteners have the functions of damping,maintaining the track distance,and adjusting the track level.Therefore,routine maintenance and inspection of fasteners are important to ensure the safe operation of track lines.Currently,assessment methods for fastener tightness include manual observation,acoustic wave detection,and image detection.There are limitations such as low accuracy and efficiency,easy interference and misjudgment,and a lack of accurate,stable,and fast detection methods.Aiming at the small deformation characteristics and large elastic change of fasteners from full loosening to full tightening,this study proposes high-precision surface-structured light technology for fastener detection and fastener deformation feature extraction based on the center-line projection distance and a fastener tightness regression method based on neural networks.First,the method uses a 3D camera to obtain a fastener point cloud and then segments the elastic rod area based on the iterative closest point algorithm registration.Principal component analysis is used to calculate the normal vector of the segmented elastic rod surface and extract the point on the centerline of the elastic rod.The point is projected onto the upper surface of the bolt to calculate the projection distance.Subsequently,the mapping relationship between the projection distance sequence and fastener tightness is established,and the influence of each parameter on the fastener tightness prediction is analyzed.Finally,by setting up a fastener detection scene in the track experimental base,collecting data,and completing the algorithm verification,the results showed that the deviation between the fastener tightness regression value obtained after the algorithm processing and the actual measured value RMSE was 0.2196 mm,which significantly improved the effect compared with other tightness detection methods,and realized an effective fastener tightness regression.

General Order Principle for Multi-Bennett Linkages

Order analysis for multi-Bennett linkages is a difficult topic in kinematics. Traditional methods fail to obtain the order of multi-Bennett linkages due to considering the special geometric distributions among joint axes. An order principle for multi-Bennett linkages is presented. For a summated multi-Bennett linkage, three procedures are included in the order principle. Firstly, a homogeneous screw equation is obtained by taking linear superposition operations and then the maximum order is determined according to linear dependency of all screws. Secondly, two theorems are employed to determine the maximum order, where the first is used to judge the linear independency of four-system screws and the second is fit for identifying the linear independency of five-system screws. Lastly, all possible cases in the order range are considered until the valid order is screened out. For a syncopated multi-Bennett linkage, an equivalent summated model is built and then the order analysis is the same as that of summated linkages. In order to verify the effectiveness of the presented order principle, the orders of summated 5R and 6R linkages as well as a syncopated 6R linkage are analyzed. The computed orders of the former two summated linkages are both 4 and the computed order of the last syncopated 6R linkage is 5. The results coincide with the prototype data. The advantage of the proposed principle is that it can get the correct order of a multi-Bennett linkage without solving the geometric conditions of joint axes and has wide application in variety of multi-Bennett linkages.

Mobility Analysis of Altmann Overconstrained Linkages by Modified Grübler-Kutzbach Criterion

Developing a general mobility method/formula is a hot topic lasting for more than 150 years in kinematics. It is necessary to apply any mobility method to puzzling overconstrained mechanisms for verification of its generality. Altmann linkages are such recognized puzzling mechanisms that their mobility analysis is of important significance. A necessary condition for judging a general mobility method is that the method can be fit for Altmann linkages. Firstly, this study classes Altmann linkages into 17 types in terms of the numbers and types of kinematic pairs, and then Altmann overconstrained linkages are further classified into 4 types. Secondly, the mobility of Altmann overconstrained linkages is systematically analyzed by the Modified Grübler-Kutzbach criterion based on screw theory, where passive freedoms are defined as limb passive freedoms and mechanism passive freedoms. In addition, the full-cycle mobility is judged, which overcomes the shortcoming of instantaneous property of screw theory. It is shown that Modified Grübler-Kutzbach criterion not only obtains the correct numerical mobility, but also gives the mobility character by resolving reciprocal screws for the constraint system. This study lays the foundation of verification for the generality of Modified Grübler-Kutzbach criterion. Besides, Altmann overconstrained linkages almost comprise all kinds of modern parallel mechanisms and some classical mechanisms, which provides an important reference for mechanism mobility calculation.

Mobility of the Myard 5R Linkage Involved in “Gogu Problem”

Since the traditional Griibler-Kutzbach criterion fails in many overconstrained mechanisms, developing a general mobility formula is a hot topic lasting for more than 150 years in mechanisms. GOGU systematically investigated various mobility methods, and pointed that the methods were not fit for two kinds of paradoxical overconstrained mechanisms. The mobility on the two kinds of mechanisms is regarded as ＂Gogu problem＂. The Modified Griibler-Kutzbach criterion has solved the mobility of the second kind of mechanisms in ＂Gogu problem＂, and has developed into a systematic mobility methodology. Myard 5R linkage is one of the single-loop mechanisms involved in ＂Gogu problem＂, its joint axes are distributed in space with special geometric conditions, which increases the difficulty of mobility analysis. The study is to calculate the global mobility of the Myard 5R linkage using the mobility methodology. Firstly, the mobility methodology based on screw theory is briefly introduced. Secondly, some homogeneous transforms are performed according to the D-H parameters and the invariance of the linkage plane symmetry is revealed, which provides an idea to judge a plane-symmetric loop. The special geometric features of the axes distribution are discussed as well. Finally, the global mobility of the Myard 5R linkage is determined by the Modified Grubler-Kutzbach criterion. The results show that the methodology can be applied to more paradoxical mechanisms.

A Stiffness Variable Passive Compliance Device with Reconfigurable Elastic Inner Skeleton and Origami Shell

Devices with variable stiffness are drawing more and more attention with the growing interests of human-robot interaction,wearable robotics,rehabilitation robotics,etc.In this paper,the authors report on the design,analysis and experiments of a stiffness variable passive compliant device whose structure is a combination of a reconfigurable elastic inner skeleton and an origami shell.The main concept of the reconfigurable skeleton is to have two elastic trapezoid four-bar linkages arranged in orthogonal.The stiffness variation generates from the passive deflection of the elastic limbs and is realized by actively switching the arrangement of the leaf springs and the passive joints in a fast,simple and straightforward manner.The kinetostatics and the compliance of the device are analyzed based on an efficient approach to the large deflection problem of the elastic links.A prototype is fabricated to conduct experiments for the assessment of the proposed concept.The results show that the prototype possesses relatively low stiffness under the compliant status and high stiffness under the stiff status with a status switching speed around 80 ms.

Singular Assembly Configurations and Configuration Bifurcation Characteristics of the Semi-regular Hexagons 6-6 Gough-Stewart Manipulator

It is well known that singular configurations are inherent to parallel manipulators and have serious influences on their properties. Therefore, these singular configurations should be avoided in the design and application of mechanisms. The researches on the singularity identification and distribution have revealed the relations among the six configuration parameters at singular points. Few works have dealt with the relation between the singularity and the input parameters, as wcll as the properties of the manipulator nearby the singularity. In this paper, taking the semi-regular hexagons 6-6 Gough-Stewart manipulator （SRHGSMP） as an example, the configuration bifurcation characteristics going with the input parameters, the assembly configurations at singular points, and the reasons to cause the singularity are analyzed. The research reveals that the number and the combination of the input parameters have great influences on the complexity of the singularity and the curvature radiuses of the configuration curves. Under different number of input parameters, the dimensional-utmost singularity, line vectors correlation singularity and Jacobian matrix correlation singularity can occur individually or jointly. Choosing the adjacent input parameters, the simple singularity and the large singularity-free input parameters zones can be obtained. And selecting multiple input parameters, the self-motion regions and the singularity avoidance errors can be reduced. These new discoveries are valuable and of significance for the trajectory design, the singularity avoidance, and the self-motion control of the parallel manipulator.

Design and Analysis for a Three-Rotational-DOF Flight Simulator of Fighter-Aircraft

Most of researchers focused on traditional six degrees of freedom(DOF) Stewart flight simulator,which can not be adaptive in fighter?aircraft flight simulator. A three rotational DOF flight simulator of fighter?aircraft based on dou?ble parallel manipulator and hybrid structure is presented. The flight simulator is composed of two identical 3?RRS(revolute?revolute?spherical) spherical parallel manipulators and one cabin,called Twins. The cabin has an additional independent DOF for 360° continuous rotation,so it can be applied as a flight simulator for a fighter?aircraft to achieve spin maneuvering. Because of the introduction of the hybrid structure and double parallel manipulator of themechanism,the redundancy exists with respect to both kinematics and actuation. Kinematics is carried out and Jaco?bian matrix is established by means of screw theory. The inverse kinematics is given out by the analytical method. 64 groups inverse solutions are showed in a table by permutation. Forward kinematics is solved by an e ectively numeri?cal method. The forward numerical method is realized based on the analytically inverse kinematics and Jacobian matrix. The numerical examples show that the forward numerical method can be used in real?time control. The rollingmotion is considered in forward kinematics and a numerical example is given out. The proposed flight simulator can spin and there are three rotational DOF with a hybrid structure so that the novel flight simulator can be used in the field of the fighter?aircraft for pilots to train.

PROPERTY IDENTIFICATION OF SINGULARITY LOCI OF GOUGHSTEWART MANIPULATOR

The problem of identifying the property of singularity loci of Gough-Stewart manipulators is addressed. After constructing the Jacobian matrix of the Gough-Stewart manipulator, a cubic polynomial expression in the mobile platform position parameters, which represents the constantorientation singularity locus of the manipulator, is derived. Graphical representations of the singularity locus of the manipulator for different orientations are illustrated with examples. Further, the singularity locus of the manipulator in the principal-section, where the mobile platform lies, is analyzed. It shows that singularity loci of the ,manipulator in parallel principal-sections are all quadratic expressions including a parabola, four pairs of intersecting straight lines and infinite hwerbolas. Their geometric and kinematic properties are also researched as well.

A new algorithm for wireless sensor network based on NS-2

Considering wireless sensor network characteristics,this paper uses network simulator,version2(NS-2)algorithm to improve Ad hoc on-demand distance vector(AODV)routing algorithm,so that it can be applied to wireless sensor networks.After studying AODV routing protocol,a new algorithm called Must is brought up.This paper introduces the background and algorithm theory of Must,and discusses the details about how to implement Must algorithm.At last,using network simulator(NS-2),the performance of Must is evaluated and compared with that of AODV.Simulation results show that the network using Must algorithm has perfect performance.

AltBOC navigation signal quality assessment and measurement

In order to ensure that Chinese BeiDou satellite navigation system runs smoothly,the assessment of signal quality has become a significant task.Alternative binary offset carrier(AltBOC)is BeiDou B2 frequency signal.The acquisition of BeiDou signal is processed in off-line mode and the evaluation is performed by taking signal power spectrum,eye diagram,constellation,correlation,loss and s-curve deviation on AltBOC as signal quality evaluation parameters.The results illustrate that the new system signal,namely AltBOC signal,has the best performance in code tracking precision,anti-jamming and anti-multipath.

Property Identification of the Singularity Loci of the 6/6-Stewart Manipulator

This paper addressed the problem of identifying the property of the singularity loci of the 6/6-Stewart manipulators. The singularity locus equation of the manipulator can be obtained by setting the determinant of the Jacobian matrix be zero. The singularity loci for different orientations are illustrated with examples. The singularity locus equation of the manipulator with respect to the oblique plane is derived either which is a polynomial expression of two degree. It shows that the singularity loci of the manipulator in the parallel oblique planes are hyperbolas accompanied with four pairs of intersecting straight lines and a parabola. Their geometric interpretations and the properties of the singularity loci are further analyzed.

Kinematics Analysis and Singularity Avoidance of a Parallel Mechanism with Kinematic Redundancy

The kinematic redundancy is considered as a way to improve the performance of the parallel mechanism.In this paper,the kinematics performance of a three degree-of-freedom parallel mechanism with kinematic redundancy(3-DOF PM-KR)and the influence of redundant parts on the PM-KR are analyzed.Firstly,the kinematics model of the PM-KR is established.The inverse solutions,the Jacobian matrix,and the workspace of the PM-KR are solved.Secondly,the influence of redundancy on the PM-KR is analyzed.Since there exists kinematic redundancy,the PM-KR possesses fault-tolerant performance.By locking one actuating joint or two actuating joints simultaneously,the fault-tolerant workspace is obtained.When the position of the redundant part is changed,the workspace and singularity will be changed.The results show that kinematic redundancy can be used to avoid singularity.Finally,the simulations are performed to prove the theoretical analysis.

Mobileman,Construction Agile Goods Delivery Robot

The traditional automated guided vehicle(AGV) on goods delivery faces the challenges when task space expands beyond 2 D plans. 3 D environments such as uneven terrain, ramps, and staircase are typical in construction site. Thus, the key to introducing this technology into construction industry is to improve AGV’s stability and autonomous navigation ability in more complex three-dimensional environments. In this paper, mobileman, a novel tracked autonomous guide vehicle, is introduced. Compared with other construction robots, mobileman maximizes its load capacity on the basis of assuring accessibility. Furthermore, its modular designs and self-balancing platform enable it to cope with more complex challenging scenarios, such as staircase with 35-degree sloped staircase, while another modular design featured automated loading and unloading functionality. The mobile base specifications were presented in section two, and modular designs and exploration of the navigation system on construction site were illustrated in the rest of sections.

A Wireless Communication Method with Dynamic Adding Nodes for the Underground Search and Rescue Robot

有为地下的搜索和营救机器人的动态增加的节点的一个无线通讯方法被建议：修理控制器的地址，动态地增加 repeater 节点进网络，并且转移活动终端的地址。与这个方法，搜索和营救机器人能到达一个矿的更深的地方帮助在一条单个隧道通过无线通讯与控制器救并且保持联系，甚至在收音机波浪不能通过厚墙的一个复杂巷道。在在单个隧道的双向多跳跃通讯的过程的碰撞将被通讯方向优先级和反应信号机制也解决，到提高通讯的可靠性。最后，一件样品被设计，一个实验被进行验证方法的效率。

Robustifying Dynamic Positioning of Crane Vessels for Heavy Lifting Operation

Construction crane vessels make use of dynamic positioning(DP)systems during the installation and removal of offshore structures to maintain the vessel’s position.Studies have reported cases of instability of DP systems during offshore operation caused by uncertainties,such as mooring forces.DP"robustification"for heavy lift operations,i.e.,handling such uncertainties systematically and with stability guarantees,is a long-standing challenge in DP design.A new DP method,composed by an observer and a controller,is proposed to address this challenge,with stability guarantees in the presence of uncertainties.We test the proposed method on an integrated cranevessel simulation environment,where the integration of several subsystems(winch dynamics,crane forces,thruster dynamics,fuel injection system etc.)allow a realistic validation under a wide set of uncertainties.

A general methodology for mobility analysis of mechanisms based on constraint screw theory

It is well known that the traditional Grübler-Kutzbach formula fails to calculate the mobility of some classical mechanisms or many modern parallel robots,and this situation seriously hampers mechani-cal innovation.To seek an efficient and universal method for mobility calculation has been a heated topic in the sphere of mechanism.The modified Grübler-Kutzbach criterion proposed by us achieved success in calculating the mobility of a lot of highly complicated mechanisms,especially the mobility of all recent parallel mechanisms listed by Gogu,and the Bennett mechanism known for its particular difficulty.With wide applications of the criterion,a systematic methodology has recently formed.This paper systematically presents the methodology based on the screw theory for the first time and ana-lyzes six representative puzzling mechanisms.In addition,the methodology is convenient for judgment of the instantaneous or full-cycle mobility,and has become an effective and general method of great scientific value and practical significance.In the first half,this paper introduces the basic screw theory,then it presents the effective methodology formed within this decade.The second half of this paper presents how to apply the methodology by analyzing the mobility of several puzzling mechanisms.Finally,this paper contrasts and analyzes some different methods and interprets the essential reason for validity of our methodology.

Displacement manifold method for type synthesis of lower-mobility parallel mechanisms

Type synthesis of lower-mobility parallel mechanisms is a hot and frontier topic in international academic and industrial field. Based on the Lie group theory, a displacement manifold synthesis method is proposed. For all the nine kinds of lower-mobility parallel mechanisms, the mechanism displacement manifold, limb displacement manifold and the geometrical conditions which guarantee that the intersection of the limb displacement manifold is the desired mechanism displacement manifold are enumerated. Various limb kinematic chains can be obtained using the product closure of displacement subgroup. Parallel mechanisms can be constructed with these limbs while obeying the geometrical conditions. Hence, all the nine kinds of lower-mobility parallel mechanisms can be synthesized using this method. Since displacement manifold deals with finite motion, the result mechanism of synthesis have full-cycle mobility. Novel architectures of lower-mobility parallel mechanisms can be obtained using this method.