Interlimb coordination plays an important role in adaptive locomotion of humans and animals. This has been investigated using a split-belt treadmill, which imposes different speeds on the two sides of the body. Two types of adaptation have been identified, namely fast and slow adaptations. Fast adaptation induces asymmetric interlimb coordination soon after a change of the treadmill speed condition from same speed for both belts to different speeds. In contrast, slow adaptation slowly reduces the asymmetry after fast adaptation. It has been suggested that these adaptations are primarily achieved by the spinal reflex and cerebellar learning. However, these adaptation mechanisms remain unclear due to the complicated dynamics of locomotion. In our previous work, we developed a locomotion control system for a biped robot based on the spinal reflex and cerebellar learning. We reproduced the fast and slow adaptations observed in humans during split-belt treadmill walking of the biped robot and clarified the adaptation mechanisms from a dynamic viewpoint by focusing on the changes in the relative positions between the center of mass and foot stance induced by reflex and learning. In this study, we modified the control system for application to a quadruped robot. We demonstrate that even though the basic gait pattern of our robot is different from that of general quadrupeds (due to limitations of the robot experiment), fast and slow adaptations that are similar to those of quadrupeds appear during split-belt treadmill walking of the quadruped robot. Furthermore, we clarify these adaptation mechanisms from a dynamic viewpoint, as done in our previous work. These results will increase the understanding of how fast and slow adaptations are generated in quadrupedal locomotion on a split-belt treadmill through body dynamics and sensorimotor integration via the spinal reflex and cerebellar learning and help the development of control strategies for adaptive locomotion of quadruped robots. Humans and animals change their locomotor behaviors depending on the environment and situation. Interlimb coordination plays an important role in such adaptive locomotion. For example, to walk along a curved path, the outer legs have a longer stride and higher speed than those of the inner legs . Split-bAlthough these adaptive behaviors are observed in walking on a split-belt treadmill, locomotion is a complicated dynamical phenomenon generated through interactions between the central nervous system, the body’s musculoskeletal system, and the environment, and thus it is difficult to fully understand the locomotion mechanism based on only observations and measurements of the locomotor system. To overcome this limitation, mathematical models and legged robots have been applied to study locomotion . In our Quadrupeds such as cats and mice also exhibit fast and slow changes in interlimb coordination during split-belt treadmill walking . InterliIn this study, we improve our locomotion control system for a biped robot and apply it to a quadruped robot. We demonstrate that although the basic gait pattern of our robot is different from that of general quadrupeds (due to limitations of the robot experiment), fast and slow adaptations similar to those of quadrupeds appear during split-belt treadmill walking of the quadruped robot. Furthermore, we clarify the adaptation mechanisms from a dynamic viewpoint, as done in our previous work . These rIn this study, we used the quadruped robot  developeElectric power is externally supplied and the robot is controlled by an external host computer , which calculates the desired joint motions and solves the oscillator phase dynamics in the control system see . The robThe robot walked on the split-belt treadmill  developeIn our previous work , we deveOur spinal CPG model is based on a physiological two-layer network model composed of rhythm generator (RG) and pattern formation (PF) networks . The RG ω is the basic oscillator frequency, i occurs at time i oscillator For the RG model, we used four simple phase oscillators (Leg 1–4 oscillators), whose phases are denoted by by a CPG , we moduD and the gait cycle as T  touchdown timing α is the learning rate. Because D remains unchanged). In addition, the TDP gravitates to alignment with the AEP  with the spinal model but without the cerebellar model, that is, the desired (predicted) touchdown timing π, which gives For the quadruped robot, we used the following control parameters: ous work ]. The saFor the split-belt treadmill, we used the tied configuration at the beginning of the robot walk with low side . When weWe performed these robot trials five times for each speed condition and investigated the robot’s behavior from the averages of the results for six steps in each configuration period. When we incorporated the cerebellar model, we separated the periods of the split-belt configuration and the second tied configuration into two halves to clarify early and late stages of adaptation in each period. We used one-way repeated-measures analysis of variance (ANOVA) to compare the differences between the periods and to clarify the significance of the locomotor behavior changes. When the ANOVA results showed a significant difference, we conducted post hoc analysis using Tukey’s honestly significant difference test, where we considered that We first used only the spinal model for the robot experiment. At the beginning, the robot walked on the treadmill in the tied configuration with the fore and hind legs in contact with the ipsilateral belt. It continued walking after the treadmill speed condition changed to the 3x, 4x, and 5x conditions of the split-belt configuration. Note that when we did not use phase resetting in π and ijdt see . Δ12 andWe next used both the spinal and cerebellar models for the robot experiment. At the beginning, the robot walked on the treadmill in the tied configuration. It continued walking when the treadmill speed condition changed to the 5x condition of the split-belt configuration. Furthermore, the robot continued walking after the treadmill speed condition returned to the tied configuration.π in the first tied configuration. It quickly decreased at the early stage  of the split-belt configuration and slowly returned to π at the late stage . In addition, it quickly decreased at the early stage of the second tied configuration, which suggests the after-effect of learning. Finally, it slowly returned to π at the late stage of the second tied configuration. The relative phase In this experiment, the duty factors for Legs 1 and 2 clearly changed depending on the treadmill speed condition, whereas the other duty factors did not, similar to previous results. π, When we used only the spinal model, the relative phase guration . The dutguration . The asyguration . Such asguration . Our resguration , becauseIn the tied configuration, the stance legs on both the fast and slow sides are pulled at the same speed. The fore and hind legs on the ipsilateral side contact the belt simultaneously . In contπ at the early stage, it slowly returned to π at the late stage; that is, the asymmetry in the interlimb coordination was slowly reduced  is advanced at the early stage , the swiπ; that is, the asymmetry in interlimb coordination appeared again  are delayed relative to the slow side due to the learning effect, which reduces the pitching moment to tilt the body forward by the strong pulling . When thAlthough the relative phase learning . That islearning . Our resBecause the touchdown of the fore leg on the slow side (Leg 2) is delayed at the early stage , the swiA split-belt treadmill imposes different speeds on the two sides of the body and highlights the functional role of interlimb coordination in adaptive locomotion. In particular, the adaptive behavior in interlimb coordination can be classified into two types, namely fast and slow adaptations. That is, the locomotion control system has two different time scales. These adaptations are primarily achieved by the contributions of different layers in the neural system, namely the spinal cord and cerebellum. The spinal cord provides motor commands through the RG and PF networks  and moduAnimals make predictions by evaluating various parameters to enhance their movements through learning in motor control. The cerebellum contributes to this prediction and learning. However, because it remains unclear what is predicted and how to use it in learning, modeling studies have attracted attention. In particular, learning models of human arm movements have been proposed to minimize jerk and torque change . AlthougIn this study, we used a robotic platform to investigate the gait adaptation mechanism during quadrupedal locomotion on a split-belt treadmill. The robot mechanical system is much simpler than an animal musculoskeletal system. Furthermore, the robot body is rigid and the joints are strictly controlled by motors, whereas an animal body and joints are flexible due to control by muscles. In addition, we used a much simpler locomotion control system than the neural system used by animals. These simplifications in the robot mechanical and locomotion control systems facilitated the capture of the essential aspects of adaptive locomotion. However, they caused quantitative differences in locomotor behavior. In particular, these simplifications forced our robot to use a pace pattern, unlike the walk and trot patterns of general quadrupeds. For intact cats walking on a split-belt treadmill using a walk pattern, when the left and right belt speeds are changed, the relative phases are altered on both the contralateral and ipsilateral sides to induce asymmetric interlimb coordination, where the contralateral sides for the fore and hind legs change most significantly . These rAlthough this study focused on split-belt treadmill walking to investigate the contribution of interlimb coordination to adaptive quadrupedal locomotion, interlimb coordination plays an important role in numerous other locomotor tasks. For example, the gait transition between walk, trot, and gallop changes the phase relationship between the movements of four legs while creating and breaking the synchronization between the leg movements . When cr


With increase in the use of contactless deliveries during the times such as COVID-19 pandemic, the emphasis was to minimize the human presence to reduce the spread of virus. In this regard, drones are one promising alternative to be used as delivery agents. However, security and Quality of Service (QoS) are major concerns while making use of drones for deliveries. In order to secure the drone communication system, we propose, CovaDel: a blockchain-based scheme to secure data transactions for the drone delivery use case that works in a phased manner. The proposed scheme make use of decoupled blockchain architecture to overcome the limited resources capabilities of drones to perform blockchain-based computations. Further, to ensure the QoS adherence, we also propose a QoS-aware communication approach that handles collisions and congestion on the basis of firefly algorithm’s attractiveness parameter (light intensity) received by the drones. Results obtained using a simulated environment verify the efficacy of the proposed scheme on the basis of gas consumed, transaction time, average network throughput, and delay. The transportation sector has helped the common life during the pandemic as it brought essential services and goods to the people. With the widespread emphasis of contactless deliveries of goods to the consumers, drones have become one potential solution for providing essential goods and services to the people. Many successful drone-based delivery services and systems are already in work such as, F-Drones, 7-Eleven, Swoop Aero , Ziplineblocks in the blockchain are indestructible. It enables addition of data in the form of transactions and periodically update the transactions from various participating entities into blocks [In this regard, different researchers have tried to address these security concerns  in-part for different scenarios involving drones. For example,  proposedo blocks . Overallo blocks . The traEven though blockchain is an effective solution to deal with the security related issues, the computations involved in blockchain-based solutions can be computationally expensive for the (computation limited) drones. Blockchain has been found as a suitable solution for the drone delivery or other drone operations as pointed by many studies , 9, but When we have large number of resources, optimal utilization becomes essential to have the desired output of the system. So, if a drone-based delivery system is to be scaled commercially it must be designed keeping the optimal resource allocation and utilization in mind. Hence, to use drones for the purpose of delivery a optimized drone scheduler is needed , 12. SpeMoreover, the foundation of drone-based system relies primarily on the prompt communication between the involved entities and in this regard, some of the related communication concerns like, network congestion and collisions can have damaging impact on the overall QoS and QoE at the large. The density of the drones in a given area have big impact on the fluctuations of the bandwidth availability. This also impacts the on board resource utilization attributed to handling the network congestion . FurtherCovaDel is focused on the delivery of essential goods (or medicines) using drones in pandemic or disaster-like scenarios when the physical moment is restricted (or limited). This article is the extension of our previous work  where weA systematic architecture for drone delivery use case has been presented wherein blockchain has been used for secure data transactions. A decoupled blockchain has been used to overcome the challenge of limited resources capabilities of drones concerning the heavy computational perspective of blockchain process.An end-to-end drone delivery mechanism using blockchain and multi-level queuing is designed considering different categories of drones based on their capabilities .A QoS-aware communication approach that handles collisions and congestion is proposed on the basis of light intensity received by the drones.In CovaDel, the block structure used for drone-to-drone, drone-to-ground station communication, and data storage differs from the traditional blockchain approach Sect. . The appThe existing literature in this field is presented in this section driven in two research directions focusing on (1) research aspects, and (2) drone delivery use cases.In , the autIn , the autKeeping in view of these facts, the authors in  presenteEssentials and medicine Contactless deliveries are booming during the pandemic time and drones are playing an important role in connecting consumer and the service providers by catering to such demands. Drones have become the first choice for delivering the essential goods such as medicine [medicine , first amedicine . Drones medicine .e-commerce The delivery of parcels and goods with the help drone is popular these days. The logistics companies such as DHL [h as DHL , Anavia h as DHL  and Amazh as DHL , utilizeh as DHL  exploitsGrocery and food delivery 7 Eleven utilizes the drones for the delivery of grocery items to the customers [ustomers . To avaiustomers . Moreoveustomers .Table Drone delivery is a massive technology which has proven itself to be an essential part of futuristic transportation systems. Drones helps to redefine the conventional logistic industry, which results in saving of crucial operational time, resources and the cost. This section provides information about the various drone delivery applications categorized into three domains discussed below.In this section, the various components of the drone delivery system are discussed. In the system model, blockchain is used to ensure a secure communication environment and smart contracts ensure the trust of various entities . Any change in the system state during the drone delivery process is referenced as a transaction on the blockchain. Figure In the proposed model, the products are sold by an entity called seller  drones are pushed into the pool  so they can communicate among themselves and to the This approach uses the properties of fire-fly optimization algorithm  and propInspired form the above concept, a communication approach has been designed wherein we have used the time slot mechanism proposed in  to form irmth is denoted by In this approach, the average-light intensity received by the drones positioned at location as below .8\documeTHR represents a threshold and D represents a set of drones.The work pattern of the original firefly algorithm tends to increase the light intensity (attractiveness or brightness) that doesn’t fit with the requirement of the proposed model. The requirement of this approach is concerned with the lower light intensity as higher intensity (brightness) depicts the possibility of a collision. Thus, we used the modified firefly algorithm from  that opeOnce the collision possibilities are identified, the next task relates to the congestion control in the network. For this purpose, for the incoming traffic, the intensity mechanism  is used as the base to decrease (or increase) the congestion window by using lower (or higher) streaks for any given connection as proposed in . Let us THR represents the threshold that is based on the average rate that is sustainable over a defined number of channels.In the above defined congestion-control evaluation, if at an instance there is an increase in the value of Based on the above evaluations, the Algorithm 3 represents the workflow of the QoS-aware drone communications. Here, the first step involves satisfying the collision avoidance criteria. The mechanism continuously checks the possibilities of collisions over the defined way-points thereby averting any possible termination in the transmission. The mechanism ensures that a drone must satisfy the defined light-intensity constraints keeping in view of the neighbouring drone. If in a case, these conditions are not satisfied or doesn’t hold TRUE, then an instant feedback is triggered so that the timing control can consider it while providing a time slot for next drone communication.Once the collision-avoidance possibilities are satisfied, the next step includes the fulfilment of congestion control evaluations. The algorithm checks the possible collisions over the way-points in a continuous manner by managing the size of the congestion window. This helps to prevent any unintentional breakage in the transmissions and traffic overloading scenario. Once both the criterion are satisfied, we proceed to check for route while maintaining conditions related to the QoS requirements. The overall algorithmic complexity of this algorithm is The proposed scheme is evaluated on the basis of three directions, (1) blockchain validation for smart contracts, (2) numerical validation, and (3) QoS validation based on simulation experiments.Environment setup The logic used in the proposed scheme is translated in the form of smart contacts, which are executed through remix IDE [emix IDE  in combiemix IDE . Metamasemix IDE  account The smart contacts are used in the proposed framework to ensure the integrity of the system resulting in trust of the involved entities. The experimental setup for validation is elaborated below.The effectiveness of the smart contracts, which is regarded as mileage according to Ethereum terminology, is evaluated through the price paid for the gas for deployment of the smart contracts. The first tested smart contact is for the interaction between the buyer and the seller. The results obtained are depicted in Fig. The proposed model witness number of interactions between different parties. So, we compute the computational and communication costs related to these parties.Initial agreement and smart contract phase In the initial agreement phase, four entities Delivery phase In this phase, Payment phase The final phase of payment processing requires the This brings the total computational cost of all the three phases to 20.25 ms.The computational cost associated for each phase is calculated as below.Initial agreement and smart contract phase The first phase requires the communication between Delivery phase Here, Payment phase This phase comprises of The communication cost of the proposed scheme is 640 bits per item.The communication cost for different phases is calculated as below:To understand the performance of the proposed scheme, the validation is performed through a simulation study performed using Network Simulator (NS-2). The length of RTS, CTS and ACK packets considered for the evaluations is 170, 120, and 120 bits, respectively. The physical layer header and default packet lengths are same as 802.11b (pause time 2s). The performance results are evaluated on the basis of network throughput and end-to-end delay. To guarantee the QoS satisfaction, a maximum resource utilization must be achieved with an increase in the average transmission rate. The network throughput increases when the achieved utilization rate nears the permissible rate. However, the average network throughput decreases when we increase the number of users. The proposed scheme aims to handle the above challenge by sustaining the average network throughput to its best, even when the number of drones are increased. Fig. The end-to-end transmission delay is dependant on many factors and it is not possible to control all these factors. But, the major contributors to the overall delay can be considered as processing and queuing tasks. If both of these tasks are controlled effectively, then it is possible to reduce the overall end-to-end delay. The proposed approach provides a very simple processing model supported by a multi-level queuing scheme for drone scheduling and product delivery. The proposed congestion and collision-aware transmission scheme helps to reduce the overall end-to-end delay in the network. Fig. Various existing proposals have explored the problems related to drones but most of them have not considered the drone delivery case. Here, the proposed work is compared with the existing works in terms of the complexities of the underlying approaches. The findings are summarized in the Table The proposed framework leverages the advantages of blockchain and smart contracts resulting into more transparent delivery operations COVID-19 like situations. Here, a derived blockchain is proposed for the part of the model where the normal blockchain will not be very suitable candidate still preserving the heritage of immutability of blockchain. In this framework, a virtual currency-based transactions are presented eliminating the reliance on third parties for payment processing hence eliminating the extra burden in terms of monetary cost. One of the key contribution comes from the drone scheduling algorithm which is backed by multi level queuing model ensuring optimal allocation of drones for various delivery jobs with minimal overhead. Incorporation of the QoS aware drone to drone communication approach has brought forward advantage of meeting the most challenging service levels in such scenarios. The proposed framework have been verified and validated for its reliability and performance in a simulated environment. The results indicate the performance of the system is effective. Further, the model is evaluated theoretically for the communication and computation costs which also favours the model.


As a result of long-term pressure from train operations and direct exposure to the natural environment, rails, fasteners, and other components of railway track lines inevitably produce defects, which have a direct impact on the safety of train operations. In this study, a multiobject detection method based on deep convolutional neural network that can achieve nondestructive detection of rail surface and fastener defects is proposed. First, rails and fasteners on the railway track image are localized by the improved YOLOv5 framework. Then, the defect detection model based on Mask R-CNN is utilized to detect the surface defects of the rail and segment the defect area. Finally, the model based on ResNet framework is used to classify the state of the fasteners. To verify the robustness and effectiveness of our proposed method, we conduct experimental tests using the ballast and ballastless railway track images collected from Shijiazhuang-Taiyuan high-speed railway line. Through a variety of evaluation indexes to compare with other methods using deep learning algorithms, experimental results show that our method outperforms others in all stages and enables effective detection of rail surface and fasteners. In recent years, rail transportation has become one of the most important modes of travel. As the total mileage of rail transit continues to increase, how to ensure safe railway operation has become a dominant issue that has attracted public attention. As shown in Using a localization algorithm is necessary to improve the accuracy of defect detection and localize the track components to be inspected, so that the influence of redundant information such as background can be reduced. Commonly used localization methods are template matching , pixel sThe detection method of rail fasteners based on traditional vision mainly uses artificially designed features to extract the features within the fastener area and then inputs the extracted features into a classification model based on shallow learning to classify the state of the fasteners. The shallow features used in the research articles on fastener detection mainly include Haar-like feature , 14, DenIn the last decade, many scholars have conducted research on the detection methods of rail surface defects. These methods mainly solve three problems, namely, the classification of rail surface defects , 29, locThe aforementioned methods are mainly aimed at detecting a single railway track component. However, the track images collected in the railway line usually contain both rails and fasteners. If both are detected at the same time, the detection efficiency can be greatly improved. To the best of our knowledge, only one article considers the defect detection problem of rail surface and fasteners simultaneously. Wei et al.  used theA railway line key component multiobject detection method is proposed based on a series of deep convolutional neural networks, which can achieve the detection of rail surface defects and fastener state.An improved YOLOv5s framework is proposed to localize the rail and fastener in the railway track line image at the same time, and the Ghost bottleneck is used to optimize the backbone network of the original YOLOv5s to effectively reduce the number of parameters and the computational cost. This method can be used for both ballast and ballastless track line image detection. Compared with the original YOLOv5s and other advanced object detection models, the detection speed is significantly improved while maintaining high accuracy.The two-stage object detection algorithm, Mask R-CNN, is used in the detection of rail surface defects, which effectively improves the recognition and segmentation accuracy.A set of state classification criteria for slab fast clip (SFC) type fastener are proposed.The contributions of this study are summarized as follows:The rest of this article is organized as follows: In this study, we use the improved YOLOv5s object detection neural network to localize the rail and fasteners in the original track images collected from the railway site. The network framework is shown in The You Only Look Once (YOLO) series network is a one-stage object detection algorithm for object localization and recognition in the image. This algorithm extracts image features by CNN and directly calculates the classification score and object localization . CompareThe original YOLOv5 network used cross stage partial (CSP) bottleneck  to increX is the input data, ∗ is the convolution operation, and Y= is the output data, which means that the m channel feature map is obtained after the convolution operation, 1 ≤ i ≤ m, φi, j)((yi) in the aforementioned formula is the j-th linear transformation of the i-th feature map, and  Y′=[y′, y′,…, yi, j) is added after the convolutional layer of each module, and the hard-Swish [The Ghost module can flexibly define the number of convolution kernels and enlarge the number of channels of the input feature map by rd-Swish  activatiIn this paper, the Mask R-CNN model is used to localize and segment the defects in the rail surface image.Mask R-CNN is an improved two-stage object detection network based on the Faster R-CNN framework . On the First, the rail surface image is input to the feature extraction network to generate a multiscale feature map. Second, the obtained feature map is input to the region proposal network (RPN) network to generate a region of interest (RoI). Then, the RoI of different dimensions generated by the RPN network is transformed to features of the same dimension by the RoI Align operation. Finally, the obtained features are, respectively, input to the fully connected layer and FCN for rail surface defect classification, bounding box regression, and segmentation mask prediction.k0=4, w and h are the width and height of RoI, and k is the number of layers of the feature map in FPN. To input RoIs of different dimensions to the fully connected layer for classification score calculation and bounding box regression, transforming RoIs of different dimensions to the same dimension is necessary. Mask R-CNN utilizes RoI Align instead of RoI Pooling in Faster R-CNN. RoI Align uses a bilinear interpolation to obtain the values of multiple sampling points and then uses the maximum pooling of the values of multiple sampling points to obtain the final value of the point. This method effectively solves the position mismatch problem caused by two quantization operations in RoI Pooling and can effectively improve the accuracy of detection or segmentation. Finally, the loss function of Mask R-CNN isLbox and Lcls are the same as in Faster R-CNN [Lmask is the mask loss. The mask branch in the network uses the Sigmoid function for each pixel on the mask, then feeds it into the cross-entropy loss, and defines the average of all pixel losses as the mask loss.The rail surface defect detection model designed in this study uses Resnet50  +FPN as er R-CNN , represeThe track fasteners used in this experiment are Pandol fast clip. The fasteners in the track images collected on the railway line are in three states, namely, normal, loosening, and broken, as shown in x and y are the input and output vectors of the residual block. F represents the feature vector obtained after the input vector passes through i convolutional layers. If the residual block has the same structure as that shown in F is as follows:f1 is ReLU function.ResNet  is a claDifferent depth ResNet models can be obtained by setting various channel numbers and residual blocks in the module. In this study, the ResNet101 model is used to detect the state of the Pandrol clip fasteners.The images are collected from the Shijiazhuang-Taiyuan high-speed railway line, as shown in In the localization experiment on rails and fasteners, 2,572 collected original images were selected as the data set. The data set of the rail surface defect detection experiment is composed of two parts: one is derived from the rail subimage obtained from the rail and fastener localization experiment results, and the other is derived from the public rail surface discrete defect (RSDD) data set . We obtaThe experimental environment of this study is based on Windows 10, NVIDIA RTX 2080TI 11 GB GPU, Intel Xeon Silver 4214 2.2 GHz dual CPU and 64 GB RAM. The algorithm based on deep learning was developed using PyTorch framework.  Step 1: use LabelImg to mark the rail and fastener area in the images of original data set for the training of the improved YOLOv5s to obtain the rail and fastener localization model.  Step 2: use the images of original data set as the input to the rail and localization model to obtain the rail subimage and the fastener subimage.  Step 3: combine the rail subimage obtained in Step 2 with the public RSDD dataset as the rail dataset, and use LabelMe to mark the rail surface defect contours in the dataset for Mask R-CNN training to obtain rail surface defect detection model.  Step 4: use the fastener subimage obtained in Step 2 as the fastener data set for the training of the ResNet101 model to obtain the fastener state classification model.The overall training process of our method is shown in In the process of training the rail and fastener localization model, 2572 images were randomly assigned 1543 images as the training set, 2 257 images as the verification set, and the remaining 772 images as the test set. Due to the limitation of the performance of the GPU, the input image is resized to 1024 × 512 pixels during the training. The specific parameter settings of the model are shown in During the training process of the rail surface defect detection model, 526 images were randomly assigned 368 as the training set, 52 as the verification set, and the remaining 106 images as the verification set. In this experiment, the size of the image input to the training model is resized to 160 × 650 pixels. The threshold value of the intersection over union (IoU) in the RPN network was set as 0.6; that is, the IoU between the proposal and ground truth was greater than 0.6, which was retained as the positive sample. Other parameters of the model are shown in D represents the number of categories detected. D=2 was used in this experiment.To further verify the effectiveness of the proposed model, five object detection methods, namely, SSD , Faster The results are shown in pij represents the total number of pixels that belong to the i class but are predicted to be in jclass, and c represents the number of categories. Two categories are used in this experiment, namely, defects and background.To obtain quantitative experimental results, pixel accuracy (PA), mean pixel accuracy (MPA), mean intersection over union (MIoU), and frequency weighted intersection over union (FWIoU) were used as evaluation indexes in this experiment. Their specific expressions are as follows:VGG16: a classic deep learning framework is widely used in object classification and feature extraction networks.HOG + SVM: HOG feature extraction is performed on the coupler image, and then the extracted HOG feature is input to SVM for coupler status classification.Canny + HOG + SVM: Canny operator  first exWe selected some classification models based on deep learning algorithms and some classification models based on traditional shallow learning algorithms to compare our method:The results of different classification models are shown in This study proposed a nondestructive detection method based on deep learning algorithms to implement rail surface and fasteners defect detection. At the object localization stage, part of the structure of the backbone based on the YOLOv5 framework is improved to achieve the localization of the rail and fastener rapidly. Compared with other object detection methods, our method has the highest detection accuracy and fastest detection speed, and the model size is only 12.6 M. At the defect detection stage, Mask R-CNN is used as the defect detection model of the rail surface. Experiments show that our method is more suitable for defect detection of rail surface compared with other advanced semantic segmentation methods. In the state detection of fasteners, a set of criteria for judging the state of SFC-type fasteners is given to judge whether the fasteners are in a normal or loosening state. A comparison between the classification models based on deep learning or traditional machine learning theory can show that ResNet is the most suitable classification method for the fasteners in this data set. In general, the proposed method can effectively detect rail surface defects and fastener states.In the future, we intend to gain more advanced knowledge of deep learning and optimize the rail surface defects detection model to improve the accuracy of defect segmentation. In our data set, few samples of rail surface defects and fastener defects are available, so we will try to use more data augmentation methods to expand the defect samples and can further improve the robustness of our method.


An analytical solution to the anti-plane dynamics problem of semi-space rare earth giant magnetostrictive media with circular cavity defects near the horizontal boundary under the action of SH wave is studied. Based on the Helmholtz theorem and the theory of complex function, the elastic-magnetic dynamic equation of magnetostrictive medium is established, and the semi-space incident wave field is written. In addition, based on the theory of complex function and the method of wave function expansion, the expression of the wave function of the scattered displacement field and the corresponding magnetic potential of the scattered wave under the condition of no stress and magnetic insulation of the horizontal boundary are obtained. Then, based on the conditions of free boundary stress, continuous magnetic induction intensity and continuous magnetic potential around the circular cavity, the infinite linear algebraic equations are established. Finally, the analytical expressions of dynamic stress concentration factor and magnetic field intensity concentration factor around circular cavity in semi-space rare earth giant magnetostrictive medium are obtained. Numerical examples show that the analysis results depend on the following parameters: permeability, dimensional-piezomagnetic coefficient, frequency of the incident wave, incident angle, distance between the circular cavity and horizontal boundary. These results have certain reference value for the study of non-destructive testing and failure analysis of rare earth giant magnetostrictive materials. Rare earth giant magnetostrictive materials, as a new type of intelligent material with strategic characteristics in the twenty-first century, are widely regarded in the world. Rare earth giant magnetostrictive material has a big magnetostrictive strain ratio, which has been proved to be superior to piezoelectric material. So it is a high-tech product for both military and civil use. It is expected that the main applications of the materials in the future will be as follows: defence industry and aerospace, marine science and offshore engineering, machinery and automotive manufacturing, high-power ultrasound and medical services. China has basically reached the international advanced level in the manufacture of the rare earth giant magnetostrictive materials, but the theoretical research of mechanical properties is still in a backward stage. As a large country of rare earth resources, China's resource advantages have not been fully brought into play and applied. Due to the problem of magnetoelastic coupling, natural factors, or the formation of many different defects in the processing and manufacturing process, such as irregular circular cavity in shape, inclusions of different medium parameters, small cracks and defects separated from the medium are widely found in various giant magnetostrictive elements. When the magnetostrictive material enters the working state, it will produce mechanical vibration inside the material, and the scattering phenomenon will occur when the mechanical wave encounters defects or interfaces in the process of internal propagation of the material, which will cause displacement mutation or stress concentration. It will have an unnegligible influence on the strength of giant magnetostrictive materials. Engineering practice has also proved that the destruction of giant magnetostrictive materials began from these tiny defects and interfaces. In recent years, great attention has been paid to the problems of internal defects, boundary and interface damage of magnetostrictive materials at home and abroad, but the research results are relatively few. Therefore, the research of this paper is extremely urgent. When the circular cavity defect exists near the boundary, the anti-plane dynamic problems will be more complex than that of the elastic materials.1 studied the scattering problem of SH wave propagation in two-material semi-space with cylindrical inclusions near the interface by using Green's function and complex function method. Zhao2 studied the scattering problem of SH wave propagation in the middle of a two-phase space with circular hole and cylindrical inclusions on the horizontal interface. Zhang4 studied the scattering of SH waves by cylindrical inclusions and semi-cylindrical holes in the semi-space of bi-phase media by using Green's function method and complex function method, and obtained the steady-state solution of the problem and then analyzed the anti-plane dynamics of waves caused by interface cracks in bi-phase piezoelectric media. Shindo5 studied the dynamics of an elliptical cylindrical piezoelectric inclusion in an infinite piezoelectric medium by using the wave function expansion method. Feng6 studied partial elliptic cylindrical piezoelectric inclusions in piezoelectric medium by using wave function expansion method and singular integral equation technique. Sahu7 used the elastic wave theory to study the transference of Love-type waves in functionally graded piezoelectric material layer bonded between viscous liquid and pre-stressed piezoelectric half-space. Chaudhary8 adopted the analytical approach to investigate the SH waves in a composite structure consisting of initially stressed rotating piezoelectric layer and initially stressed substrate with rotation.In recent years, many valuable achievements have been made in the study of SH wave propagation in elastic and piezoelectric media. Qi10 computed the normal and shear stresses, dielectric, and electric pontential in an irregular initially stressed piezoelectric substrate with a irregularity depression under the moving load. Then proposed an analytical technique to study the shear horizontal wave propagation in a layered pre-stressed rotating cylindrical tube structure consisting of a piezoelectric material layer which is wrapped on piezomagnetic material having an imperfect interface. Singhal12 studied the propagation of Love-type wave in functionally graded piezoelectric material layer bonded between piezomagnetic plate and pre-stressed piezoelectric half-space and then used liouville-green approximation to study the surface waves in functionally graded piezoelectric material clubbed between two dissimilar piezomagnetic media. Jiang13 used the decoupling technique and wave function method to study the dynamic behavior around the circular hole in exponential gradient piezomagnetic material. Pang14 used liner magneto-electro-elastic spring model to study the scattering problem of SH wave propagation in the layered piezoelectric and piezomagnetic plates with nonideal magneto-elastic interface. Ray15 established the magneto-elastic dynamic equation, and by using the Green’s function method, the characteristics of the wave propagation behavior of the piezomagnetic material layer on the functional gradient piezomagnetic material under the action of the interface pulse point source had been revealed. Guo16 studied the scattering problem of SH wave propagation in layered piezoelectric/piezomagnetic medium with cylinders when SH wave meets the functionally graded interlayers. Kong17 studied the scattering behavior of SH wave in the coupled structures of piezomagnetic substrates and orthogonal piezoelectric layers under different shear directions. Ezzin18 used the ordinary differential equations and stiffness matrix method to study the scattering problem of SH wave propagation in piezomagnetic/piezoelectric laminates. Wei19 presented a numerical solution to the problem of SH wave scattering in the piezoelectric/piezomagnetic plate with a non-ideal interface. Liu20 studied the propagation of SH wave in periodic layered piezomagnetic structure, then concluded that the band gap characteristics of magnetic closure and opening were the same. Ebrahimi21 used the nonlocal elasticity theory to study the bending of magneto-electric-elastic nanobeams.Recently, scholars at home and abroad have carried out some numerical, experimental and simulation analysis on giant magnetostrictive materials to study the propagation of coupled magnetoelastic waves. In the piezomagnetic fracture mechanics problem, the problem of magnetoelastic coupling in the rare earth giant magnetostrictive media needs to be paid more attention. The rare earth giant magnetostrictive materials exhibit significant magnetostrictive effects. So, it is necessary to consider the effect of magnetostrictive effect on the deformation of such materials. ChaudharyIn the previous research work, the influence of boundary or defect on the propagation of elastic wave in magnetostrictive materials has been discussed respectively in relevant studies. However, when the internal defect and external boundary of the material exist simultaneously and interact, the anti-plane dynamics problem of magnetostrictive material needs further theoretical analysis. So, in this article, the physical model is simplified into a semi-space rare earth giant magnetostrictive medium with a circular cavity. We established the elastic-magnetic dynamic equation of the rare earth giant giant magnetostrictive medium. Then the scattered wave field inside the material can be constructed by using the complex function and Green's function method. According to the boundary conditions of circular cavity, the dynamic stress concentration factor and magnetic field intensity concentration factor of the rare earth giant magnetostrictive material are obtained by numerical examples. The purpose of this paper is to provide an effective theoretical analysis method for the damage and failure of rare earth giant magnetostrictive materials in engineering practice.Figure Then, according to the physical model diagram, we can determine the boundary conditions of the semi-space rare earth giant magnetostrictive medium, which can be expressed as:In this paper, let the Z-axis be the magnetic polarization direction of the rare earth giant magnetostrictive material, then the constitutive equations describing its linear magnetoelastic coupling are Eq.  and the Its constitutive equations are expressed as:By introducing polar coordinates, the governing Eq.  can be eThe constitutive Eq.  can be eBy using the complex function method, the complex plane ning Eq.  can be eThen, in the complex plane, the constitutive equations shown in Eq.  can be tBy introducing the polar coordinate transformation relation, let In the isotropic and uniform semi-space rare earth giant magnetostrictive medium, after removing the time factor, the incident plane displacement field Then, in order to satisfy the stress free boundary condition on the horizontal boundary, the constructed reflected displacement field can be expressed as:In order to discuss the magnetic potential corresponding to the reflected wave, according to the governing Eq. , the magAccording to the Laplace equation, we obtain the magnetic potential additional function in the corresponding magnetic potential of the reflected wave as follows:Since the magnetic potential at infinity can not be infinite, the Eq.  can be eIn order to obtain the unknown coefficients, we take the semi-space rare earth giant magnetostrictive medium without defects as an example to obtain the unknown coefficients in the magnetic potential additional function. The conditions of stress freedom and magnetic insulation should be satisfied at the interface of semi-space, then there are the following:Let's first discuss the case of stress freedom. In the complex plane  the Eq.  and the  the Eq. .17\documThen the total stress of the semi-space rare earth giant magnetostrictive medium can be expressed as:Substitute the coordinates Then we discuss the case of magnetic insulation, namely Substitute the coordinates It is not difficult to find that the solutions of the two equations are equivalent by combining Thus, we find that Eq.  is true Therefore, we represent the total magnetic potential in the semi-space rare earth giant magnetostrictive medium, and the total magnetic potential is expressed as:Since when  formula  can be eAccording to the condition that the magnetic potential cannot be infinite at the horizontal boundary, we put In Eq. , when \dAccording to the derivation process of the magnetic potential corresponding to the reflected wave in the semi-space rare earth giant magnetostrictive medium, the corresponding magnetic potential of the reflected wave in the semi-space rare earth giant magnetostrictive medium containing circular cavity can be expressed as Eq. .In complex plane In order to verify that Eqs.  and 30)30) satisWe know that there is no displacement inside the circular cavity, but it can produce a magnetic field, and the magnetic potential can not be infinite, then the expression of the magnetic potential function can be expressed as follow:In Eq. , \documen in Eq. , namely:Then the magnetic potential In the semi-space rare earth giant magnetostrictive medium, the total displacement field and the total magnetic potential can be expressed as follows:By substituting Eqs.  and 36)36) into In the above equations, By using Fourier expansion method, multiply both sides of Eq.  by \docuAmong them raic Eq. .The total stress and the total magnetic field strength around the circular cavity in the semi-space rare earth giant magnetostrictive medium can be expressed as follows:Then the dynamic stress concentration factor of the circular cavity edge can be expressed as follow:The magnetic field intensity concentration factor of the circular cavity edge can be expressed as follow:Since we need to truncate finite terms to solve the infinite algebraic equations, the selection of truncated terms will affect the final calculation results. Because there is no displacement field inside the circular cavity, the radial stress at the boundary of the circular cavity should be 0. The accuracy of the analysis method can be tested by the residual dimensionless stress at the boundary of the circular cavity. The results show that when the maximum residual stress is less than 5%, the accuracy of the calculated results can be guaranteed. In this paper, truncation of Fourier series is taken 22 have been used to solve the distribution of In this paper, the calculation result diagrams of dynamic stress concentration factor (DSCF) and magnetic field intensity concentration factor (MFICF) around the circular cavity are presented, which vary with the dimensionless wave number entclass1pt{minimaFigures Figures Figures Figure Figure mentclasspt{minimamentclasspt{minimaFigures Figures Figures Figure Figure The factors such as incident wave frequency, incident angle, material geometrical parameter, permeability and the distance between the horizontal boundary and the circular cavity have influence on the DSCF and MFICF around the circular cavity.When the SH wave is horizontally incident, in the case of low frequency, the DSCF and MFICF will get the maximum value. With the increase of frequency, the DSCF and MFICF tend to be stable.When the wave incident vertically, the DSCF and MFICF around the circular cavity change more strongly as the case of the increase of frequency.When the distance between the horizontal boundary and the circular cavity reaches a certain distance, the influence of the boundary on DSCF and the MFICF around the circular cavity will tend to be fixed.The magnetic field intensity concentration factor increases at first and then decreases with respect to the piezomagnetic coefficient and permeability.In this paper, the problem of scattered stress and magnetic field intensity concentration of the SH wave caused by the circular cavity near the horizontal boundary of the rare earth giant magnetostrictive medium are studied by using the elastic wave theory and complex variable function. Through a specific example, the following conclusions can be obtained:The present study is carried out under specific conditions, but the exact solution of the scattering of SH wave by circular cavity in semi-space rare earth giant magnetostrictive medium is presented, which can provide a reference for the approximate method of this problem. In addition, with reference to the excellent performance of the research method in this paper in the damage analysis of elastic materials and piezoelectric materials, the damage analysis of rare earth giant magnetostrictive materials in the future can be done by adding the repeated image method to deal with the strip plate model, or by adding the Green’s function method to study the two-phase magnetostrictive material model.


In this study, we propose a scaled-up, immersive, science fiction Human-Robot Interaction (HRI) scenario for intrinsic motivation on human-robot collaboration, built upon the investment game and aimed at adapting the investment game for human-robot trust. For this purpose, we utilize two Neuro-Inspired COmpanion (NICO) - robots and a projected scenery. We investigate the applicability of our space mission experiment design to measure trust and the impact of non-verbal communication. We observe a correlation of 0.43 (As robots become more advanced and capable, developing trust is an important factor of human-robot interaction and cooperation. However, as multiple environmental and social factors can influence trust, it is important to develop more elaborate scenarios and methods to measure human-robot trust. A widely used measurement of trust in social science is the  As robot capabilities become more and more sophisticated, we not only want them to solve increasingly complex tasks independently but ultimately aid humans in their day-to-day life. Moreover, such social robots should act in a way that is reliable, transparent, and builds trust in their capabilities as well as their intentions . As soonhuman-human collaboration. In the investment game, trust is measured as the amount of money a person is willing to give to an anonymous counterpart, in the prospect of a future profit. While others have used it in an HRI setting, some report limitations and differences when applying it to human-robot collaboration  humanoid robots by robot interaction, as the implicit, robotic, non-verbal communication improves the efficiency and transparency of the interaction, leading to a better cooperation between human subjects and robots  affect human-robot trust positively?As non-verbal communication is essential to both human-human and human-robot trust , we striAfter surveying the latest research on measuring trust in human-robot interaction and its shortcomings (Chapter 2) we describe our approach (Chapter 3) and introduce an empirical study to evaluate our hypotheses (Chapter 4). We discuss the results as well as the limitations of this study (Chapter 5) and conclude our findings (Chapter 6) with an outlook on further research.One of the biggest challenges in human-robot interaction is to develop a more natural relationship with robots. Previous research shows that people refrain from accepting, tolerating, and using robotic agents in everyday tasks, mainly because robots still appear like intruders . A survewith one . A consiwith one .cognitive trust - the willingness to rely on another person’s competence and reliability - from affective trust - the confidence that the other person’s actions are intrinsically motivated.This begs the question of what could aid in easing humans into collaboration with a robot. As robots become more advanced and take greater responsibility in social jobs such as in the education sector  or healtperformance and affective trust by assessing a robot’s motives. Prominent factors that influence cognitive trust in a robot are its task performance and characteristics  (r. They can then anonymously decide which fraction p of their monetary resource r they want to give to the trustee. This fraction is then multiplied by a predetermined factor to incentivize investment. The receiving person (trustee) is free to keep the whole of the increased amount or can opt to send a fraction q of the received sum back to the trustor, thereby reciprocating. Trust then is quantitatively measured as the amount of money invested by the trustor in the trustee.To quantitatively measure human-human trust, previous work relies heavily on the st game) , an econGive-Some Game slightly change the rules toward an economic analogue of the prisoner’s dilemma  plays a major role : various machine .via the investment game in human-agent interaction  making the participant reliant on the robots’ expertize to foster cooperation, and ii) ensuring that the invested currency and investment outcome have an inherent value to both the participant and the robots. We achieve the former by designing a challenging scenario setting of a space journey: all participants will have negligible expertize regarding space travel. Thus, the robotic officers that are introduced as specifically designed to advise in interstellar travel will be perceived as more knowledgeable in the subject matter. In combination, this should prevent participants from making decisions based on their previous experiences, leaving the participant primarily reliant on the robots’ advice.energy cells, represented by cubes. From the perspective of game design, the value of items is often determined by their aesthetics and functionality , effectively forcing them to voice a preference.These three aspects - i) the inability of the participants to make choices based on prior existing domain knowledge, ii) a shared currency between human trustor and robot trustee, and iii) the weighted choice between two agents - allow us to go beyond an anonymous exchange of money while maintaining the structure of the investment game, and meet the requirements for a suitable human-robot interaction scenario.One of the main goals of our scenario design is to achieve an immersive and enjoyable experience for the participants. Besides concealing our research question, our scenario needs to establish enough involvement to allow trust-building toward the robots. For this purpose, we developed a fully autonomous system that only requires intervention in case of larger technical failures or misunderstandings, which most likely would then result in a cancellation of the experiment run. A schematic of our experimental setup can be seen in Knowledge Technology group at the Universität Hamburg  (We utilize NICO (Neuro-Inspired COmpanion) , 2017, aem (ROS) , charactvia an Embodied Dialogue System, integrated with loudspeakers in the robotic torsos to produce enhanced speech.NICO is also capable of displaying a range of programmable facial expressions through LED matrices in its eyebrows and mouth. The utterance of spoken messages is enabled As elaborated in via head movements toward the participant and the other robot, four different facial expressions , as well as gestures toward the participant such as pointing, saluting or beat gestures. These facial expressions and body movements show evidence to improve the transparency of the interaction and reinforce the spoken word  by This questionnaire asks for standard demographic questions such as age, sex, former experience with robots and computers, and general attitude toward robots. We include the 30-item Big Five Inventory-2 Short Form questionnaire  to assesAfter completing the pre-experiment questionnaire, the experimenter then guides the participant toward the experiment room with the half-spherical canvas, depicted as the spaceship cockpit. By entering the cockpit, the experiment context is set and immersion is fostered by the screen depicting the outside view of a space cargo hangar, dimmed lights, dry ice, as well as the experimenter from now on addressing the participants as “commander”. Following the scripted introductory narrative, the experimenter instructs the participants to the space mission task, their goal as the commander to deliver important cargo safe and fast, and makes them aware of the two robotic officers who accompany them on their journey. The participants are encouraged to familiarize themselves with the cockpit environment, the energy cells, the allocation compartments, and the list of possible questions that can be asked to the robots during the game. The experimenter also elaborates on the meaning and impact of the energy cubes, and demonstrates how they can be distributed by way of example. The experimenter asks for any remaining questions, then steps back out of the experiment room behind a curtain before the trial scene 0 begins.Wendigo and the robotic officers introduce themselves, then conduct an introductory round of the cube allocation, which is concealed as a system check. This trial round serves to acquaint the participants with the experiment procedure and reveal possible misunderstandings. It familiarizes them with the ship’s visual and auditory feedback mechanics and accustoms them to the delay between voice input and feedback response. The trial round furthermore allows the operator behind the curtain to possibly re-calibrate the microphone sensitivity without breaking immersion.In this scene, the voice-controlled artificial intelligence system After the trial scene 0, the experimenter briefly enters the cockpit again to answer any remaining questions before the start of the actual experiment. At this point, we consider participants to be informed about the game mechanics, prepared for the upcoming tasks, and motivated to achieve the game’s objective, following their mental model they have formed about the game.Wendigo draws attention to the challenge at hand .1. 2. Both robotic officers advertise their solution for which they require energy cells.3. The participant can ask a question from the list of predefined options, to which the robotic officers reply one after another and in a randomized order.Wendigo, I am done! ‘when they are done.4. The participant is asked to distribute the energy cells as they see fit, and say ‘Wendigo provides feedback on the decision outcome .5. 6. After the participant places all energy cells back into their own compartment, the state machine autonomously transitions to the next scene.Note that after each cube allocation, we employ rich visual and auditory feedback (see step 5) in terms of ambient light and spoken response lines disguised as status reports, such as “Unsuccessful. Ship damaged. The breach has been closed but the life support system is damaged.” as an example for negative feedback. By design, in the second scene, the feedback for the investment decision (regardless of how the energy cubes were distributed) will be portrayed as unsuccessful, while on each of the other investments the participant receives positive feedback instead. This control of the narrative, regardless of the participant’s concrete decision, enables us to reproducibly observe the effects of building and destroying trust.During the experiment, the experimenter behind the curtain, observing on the extra camera view  on trust.The study was conducted over two consecutive weeks at the end of February 2020 on the campus of the computer science department of Universität Hamburg. It was advertised Our study was conducted with 53 participants, of whom 45 finished the experiment successfully. For 8 participants the experiment was started but had to be aborted because of technical issues such as robot actuator overloading, language barriers or a misunderstanding of the game rules. All following statistics, therefore, apply to the 45 participants who completed the experiment without complications. Our participants’ mean age  between robots on how trustworthy the human perceives them. We anticipate that this serves as a positive example of extending socioeconomic experiments to a human-robot social interaction setting.Our experiment revolves around three main characteristics: the weighted choice between two agents, the participants’ inability to make choices based on prior domain knowledge, and the additional incentive for interaction between the trustor and the trustee. Maintaining these characteristics, we believe our game design can be adapted to various situations and environments where trust and NVCs play a role. Such environments comprise, but are not limited to, a work environment or a public service environment.Overall, our results show that our variant of the investment game provides a reliable measure for human-robot trust and that non-verbal communication positively affects human-robot trust. However, there are some points of discussion which we address further in the following section.In our study, we chose a futuristic environment since most people know robots from media and science fiction stories . While wWe found two different gameplay behaviors that identify the two groups on which results were compared: the main group  against minimal non-verbal communication (MNVC). There is currently no widely established baseline or notion of We provided an elaborate HRI scenario to model the building of trust more closely to human relationships than in the original investment game. Our experimental setup includes social interaction, non-verbal communication, a shared goal, and intrinsic motivation, thereby allowing participants to collaborate with robots more realistically than in the original investment game, and measuring trust reliably. The environmental variables that our scenario (and its life-like agents) adds to the data are a natural reflection of the many factors, internal and external, that influence human trust and how different levels of trust affect human behavior in different contexts, modeling aspects of human-robot trust that the original trust game does not cover.We found a correlation between the self-assessed trust and the trust measured from the game for the majority of participants (main group). These same participants allocated more energy cells to the robot with non-verbal communication (NVC) in the first scene of the game. We were therefore able to replicate the positive effect of non-verbal communication on trust and robot perception. The Godspeed  values fFuture research should comprise an investigation of the gameplay behaviors observed and could explore the effects of the use of different robots in this setup. Moreover, a similar setup can be used in future studies as a platform for studying trust and other potential factors that influence trust, in a real-world scenario and without losing the complex dynamics of building, breaking and maintaining trust given life-like agents and complex real-world situations. We can use it to formulate an in-depth trust analysis without losing the complex dynamics between internal and external factors that influence the human ability to trust others - be they humans or robots.


We describe the collective behavior of isotropic droplets dispersed over a spherical smectic bubble, observed under microgravity conditions on the International Space Station (ISS). We find that droplets can form two-dimensional hexagonal structures changing with time. Our analysis indicates the possibility of spatial and temporal periodicity of such structures of droplets. Quantitative analysis of the hexagonal structure including the first three coordination circles was performed. A peculiar periodic-in-time ordering of the droplets, related to one-dimensional motion of droplets with non-uniform velocity, was found. C* (SmC*), nonpolar Smectic-C (SmC) and Smectic-A (SmA) films10 and is associated with deformations of the orientational ordering of molecules in smectic layers. Topological defects of the director field localized near the particles or on their boundary play an essential role in self-organization11. At large distances, the embedded particles interact as topological dipoles or quadrupoles. The distance between the surfaces of the particles in the observed structures (linear and branched chains) is typically less than the particle diameter in the case of both dipole–dipole and quadrupole–quadrupole interactions. At present, the mechanism of self-organization involving topological defects is well understood, with the collective behavior of particles described by continuum theory using analytical and numerical calculations. However, for a large number of particles in SmC free standing films, qualitatively different structures are observed with hexagonal ordering and with interparticle distances considerably larger than for the case of quadrupole–quadrupole and dipole–dipole interaction12. The mechanism of formation of two-dimensional hexagonal structures is not yet fully understood.Self-organization of embedded particles is one of the richest and most intriguing phenomena in the physics of free-standing films of liquid crystals. Self-organization was discovered and has been widely investigated in polar Smectic-A (SmA) films. In contrast to the smectic C phase, the molecules are oriented preferentially normal to the smectic layers, so that there are no topological dipoles or quadrupoles and elastic interactions via in-plane orientational field (c-director) can be excluded. A spherical film geometry was chosen in order to minimize the influence of the film meniscus on objects in the film, and microgravity was needed to prevent the inclusions from sedimenting in the gravitation field. The experimental data analyzed in our paper were obtained in microgravity conditions (< 10–3 g) on the International Space Station (ISS)2 as a part of the OASIS mission. The data were transported from the ISS and transferred to Boulder (USA), Magdeburg (Germany), and Chernogolovka (Russia).Various types of collective droplet behavior were found. Results published by the Boulder and Magdeburg groups2 motivated us to analyze the positional and orientational ordering of droplets in the hexagonal structure. The widths of the distribution functions for radial and azimuthal droplet positions increase with distance. In the presence of velocity gradients in the film, a periodic appearance and disappearance of the hexagonal ordering of the droplets was observed.In this work, we analyze the static and dynamic properties of the self-organized particles (droplets of isotropic liquid) leading to the formation of hexagonal structures in smectic 14 and coming back into fashion with a number of high-impact publications. Although the original idea of F. Wilczek has been proven as impossible15, time crystals are still the subject of vigorous scientific debate . Wilczek’s theory has been replaced by a less fanciful definition of a time crystal, for a driven system with time-translation broken by integer multiples of the driving period (as evidenced by observation of a subharmonic response). There exist many examples of systems displaying various types of spatio-temporal order. To name a few, we mention classical pattern formation dynamic instabilities . Very recently, another dynamic analogue of time crystalline order has been studied19. As is often the case, the abundance of terminology seen in the literature  reflects the complexity of the phenomenon. The possibility of forming a time crystal is closely connected with the fundamental concept of spontaneous symmetry breaking used in different areas of physics, ranging from conventional condensed matter phase transitions to the cosmological Kibble mechanism of topological defect formation20. It has been shown that discrete time crystals can exist only in non-equilibrium systems23 and they have been realized in several quantum systems25. In this paper we analyze the spatial and temporal distribution of inclusions in smectic films as observed in the OASIS experiment.Such non-trivial dynamic behavior suggests to look for some analogy to the concept of time crystals proposed by F. Wilczek in 2012A liquid crystal (Displaytech MX12160) with a direct phase transition from the SmA to the isotropic (Iso) phase (SmA– 51.1 °C—Iso) was employed2. The method used to prepare the smectic bubbles with diameter about 1.5 cm is described in1. Two video cameras were used, one to capture images of the whole bubble , droplets of isotropic phase formed in the film. Droplet sizes and their number density can differ in different regions of the bubble, which are separated by dislocation lines. The registration of micro-images of groups of droplets was performed with droplets of about 15 μm in diameter. In some regions of the film where the number density of droplets was greater than around 500/mm2, a quasi-hexagonal structure of droplets could be observed . Two-dimensional hexatic ordering is characterized by a quasi-long-range bond-orientational order, while the positional order is only short range. Phases with hexatic order have been found in several systems, such as electrons at the surface of helium, dust plasmas, charged polymer colloids, and smectic liquid crystals31.The distribution of droplets in the three coordination circles after correction is shown in Fig. Similarly to equilibrium systems, it is possible to introduce an order parameter describing the two-dimensional, quasi-hexagonal ordering of droplets in smectic films. This would involve finding the dynamic action derived from the non-linear dynamic equations. This challenging theoretical task is beyond the scope of our study.2 focused on lattices that were either stationary or moved within a uniformly flowing film region without spatial velocity gradients. Here, we focus on such parts of the lattice that are exposed to shear flow. In our case, close to the temperature of the SmA-Iso transition, regions of different thickness . In such conditions, disordered colloidal particles can be dynamically forced to order in space  or, conversely, initially ordered particles can be melted under the influence of shear stresses. These investigations enable the measurement of the shear moduli and the mean-squared displacements of the colloidal particles. Then one can apply the well-known Lindemann criterion ) to analyze melting and freezing phenomena and non-equilibrium phase coexistence . Applying shear stresses to relatively soft  colloidal structures is one of the most successful methods employed to deform and orient anisotropic phases. When applying a shear field, the system is transferred from equilibrium into a non-equilibrium state. Initiated by the shear, orientation arises from the complex interplay between structure and flow. Little is known about the nature of the non-equilibrium state that depends on the viscoelastic properties of the material. Depending on the strain amplitude (and frequency in the case of oscillatory shear experiments), applied shear can drive a system far from its original equilibrium state, where the measurable rheological parameters show strongly non-linear behavior. The behavior strongly depends on the ratio of relaxation times of positional and orientational fluctuations and applied non-stationary shear rates. If the characteristic relaxation time is shorter than the shear rate, the flow merely translates fluctuations in space. In the opposite limit, when the relaxation time for those fluctuations exceeds the characteristic time of the shear flow, fluctuations live in an averaged environment, similar to the equilibrium one. For a stationary shear, one should not expect occurrence of new (not observed in equilibrium) structures. In principle, many ideas from this field might be applied to other systems although their practical implementation requires much additional work beyond the scope of this paper. Although Fourier transforms are useful in analyzing the dynamics of ideal periodic structures which change in time , in the case of our experiments, where there is only quasi-periodicity along the rows and the number of rows is limited, this technique is not as reliable as directly using particle tracking data to extract information about spatial correlations in the observed structures.In our analysis of the OASIS data, the appearance and disappearance of a quasi-long-range hexagonal lattice, formed by isotropic droplets on a spherical bubble, was observed. The driving force for such “time crystalline” behavior is hydrodynamic flow with non-uniform velocity (not reducing to a simple planar shear flow) within the SmMicrogravity is essential for our experiments. Under normal gravity, a number of difficulties have to be overcome to realize a similar experiment even in flat free-standing film geometry. A major problem is that the film has to be heated to a well-defined temperature and kept at that temperature. Even small gradients of the temperature profile lead to thermally driven convection in the film that advects the liquid crystal material and affects the subtle interactions stabilizing the lattices. Moreover, the meniscus of a flat film has a strong influence on liquid inclusions in the film. The effects of the meniscus are minimized in tethered bubbles but performing observations of inclusions on spherical smectic bubbles in normal gravity is practically impossible since a fast sedimentation of the droplets occurs.In summary, two-dimensional structures of droplets in smectic films were observed under microgravity conditions. A stationary hexagonal arrangement of droplets was observed and analyzed quantitatively. When the droplets in the film move with a velocity gradient, the appearance and disappearance of the hexagonal structure was observed. Such behavior can be considered as a dynamic analogue of crystalline translation order.The observed behavior in the OASIS experiments resembles a kind of time crystal. The structures are time dependent, appearing at certain moments of time in a non-equilibrium dynamical system. This is the characteristic feature of the phenomenon called “time crystals” in modern literature. One of the aims of our work is to show that liquid crystals can serve to model or simulate such non-trivial behavior. At this time, a unified theory explaining the stability of the hexagonal structure in the absence of in-plane orientational interactions is still not available. The necessary conditions for creating structures of droplets with different types of spatio-temporal ordering will be explored in future experiments.1. The complete setup is enclosed in a thermostated chamber for the control of the sample temperature. Smectic bubbles are inflated automatically on the tip of two coaxial capillaries. The gap between inner and outer capillaries is filled with smectic material. By pumping a small amount of the mesogen from a reservoir to the tip, we first create a smectic cap above the inner capillary. Then, by slowly pumping air through the inner capillary, an air bubble topped by a freely suspended smectic film forms. As soon as this film has formed, more air is pumped in so that a smectic bubble of the desired size (approximately 1.5 cm diameter) forms. Limited control of the film thickness of this bubble can be achieved by varying the initial inflation speed. When a small bubble with fairly uniform film thickness has formed, further inflation usually preserves this thickness. Smectic film material is constantly supplied from the meniscus at the capillary. Film thicknesses in this experiment were of the order of a few dozen to a few hundred nanometers. After the desired bubble size was reached, we heated the chamber at the highest achievable heating rate to close to the smectic A to isotropic transition. A slight overshoot of a few hundred mK, or perhaps up to about 1 K, led to partial melting of the inner layers of the smectic film. The temperature was then kept in the vicinity of the bulk clearing temperature (which is a few degrees below the film melting point). The molten smectic material organizes in droplets with a fairly monodisperse size distribution. As long as the temperature is kept constant, the number of droplets and their sizes remains unchanged at least within the experiment time of several minutes. Airflow was generated near the bubble surface using four airjet needles. These needles were multifunctional and were also used for local heating and for applying electric fields. Airflow was used following creation of the bubble in order to generate island emulsions and in order to rotate the bubble so that the micro-view camera could observe different regions of the bubble surface. In the experiments analyzed here, airflow was employed to induce shear flow in the bubble.The experiment was performed in microgravity conditions of the International Space Station (ISS) as a part of OASIS mission


In this paper, we theoretically designed and numerically analyzed an ultrabroadband meta-absorber with near unity absorptivity that works in terahertz spectrum. A wideband meta-absorber composed of bilayer patterned graphene and dielectric cylinder array with high symmetry was proposed. The wideband absorption mechanism benefited from two aspects. The first one was enhanced surface plasmons based on bilayer patterned graphene. And the second one was the coupling of continuous resonant modes within Fabry-Perot cavities to the enhanced surface plasmons in the graphene. An ultrawide bandwidth with absorptivity over  Electromagnetic wave absorber is a device that converts electromagnetic wave into ohmic heat or other forms of energy by interacting with the electric field or magnetic field of the incident electromagnetic wave . ElectroTraditionally, the absorption was realized using absorbing materials, by absorbing electromagnetic energy efficiently and reducing the reflection. Since Landy first proposed the metamaterial perfect absorber in 2008 ,4,5,6. FMany tunable absorbers with materials consisting graphene ,14, blacHowever, the absorption bandwidth relied on resonances of surface plasmon polaritons of graphene is too narrow to satisfy certain potential applications ,25. MuchIn this letter, we propose a polarization insensitive broadband meta-absorber based on patterned graphene in terahertz spectrum. We utilize the enhanced surface plasmons with dielectric cylinders loaded patterned bilayer graphene structure and merge varied resonances to realize the increased absorption within an ultra-wide bandwidth. Due to simultaneously generated electric and magnetic resonances of both plasmon resonance and Fabry-Perot (FP) resonance, the proposed broadband absorber can have tunable absorption spectrum from The schematic unit cell of the proposed absorption structure is depicted in model as : ϵ(ω)=ϵ∞T is the temperature in Kelvin, e is the electron charge, and ℏ is the reduced Planck’s constant, In the above configuration, the periodic silicon cylinder arrays loaded with patterned graphene can also supply local resonant reflection, forming an asymmetric FP cavity. When the incoming electromagnetic wave propagates in the multilayer medium, the electric field and magnetic field will be modulated, and the propagation state of the electromagnetic wave will change complex. Different from the traditional medium, graphene is a monolayer of carbon with a specific conductivity governed by the Kubo formula . In the ector as ,38: (2)kThe surface plasmons of graphene can be achieved when the wave vector matching condition is met as:Then we can conclude that our structure can produce multiple discrete plasmon resonances, what’s more, for each mode, a continuous resonances exist for the reason of continuously variable y. Then, the multi-band continuous resonances can be adjusted through parameter scanning to locate within the spectrum of two absorption peaks caused by the patterned graphene. In the silicon cylinders, we define R, the plasmon resonance can be adjusted within the expected spectrum. Then, we will have a wideband property with near-unity absorption under certain conditions.Equation  implies When the uniform electromagnetic wave is incident into the multilayer medium, part of the energy is reflected and part is transmitted. The final absorption rate in this paper can be expressed as Theoretical research is developed using the commercially available finite element simulator HFSS. To asure the computing accuracy and high efficiency, the term Maximum Number is set to 15 and the term Maximum Delta S is set to Before analyzing the absorption mechanism of the whole structure, the adoption of bilayer patterned graphene is clarified below. The comparison of normalized absorption spectra of three kinds of absorber with monolayer graphene, bilayer graphene and tri-layer graphene are plotted respectively, as shown in Then, no matter how to tune the loading voltage, the absorption in a wide band can not be improved only based on the bilayer structure. Once the periodic silicon cylinders with high permitivity are attached to the top, as mentioned above, the simulation result shows that we indeed have improved the absorption rate within the wide spectrum, as shown in To further validate the absorption mechanism, simulated electric field distribution, surface current density of the first graphene layer, the second graphene layer and the backed Au ground and the profile magnetic field distribution of structure at In For the second absorption peak of 6 THz, strong electric field appears in the edge of the graphene and the junction of graphene and cylindrical dielectric, as shown in Then in order to give the verification of the mechanism derived above further, the main parameters that affect the main absorption peaks are analyzed and presented in Even if the structural parameters are fixed, the tunability of the absorption is still achievable benefited from the controllable eletromagnetic characteristics of graphene. Through adjusting the biasing voltage from 0.1 eV to 0.97 eV, as shown in x-axis and electric vector of plane wave. We have investigated the absorption consistency of variational trend under the changes of the polarization angle ranging from Next, we will investigate the property of polarization independence under different incident angles. In the above mentioned analysis, we discussed the absorption characteristics only under normal incidence scenario with angle Besides, the absorption spectrum under varied oblique incidence in both the TE and TM modes are studied, as depicted in Finally, we obtain a polarization insensitive and wide angle independence broadband meta-absorber with absorption larger than In summary, a bilayer graphene-based absorption structure with Au cylinder arrays on parylene substrate is proposed and numerically simulated at THz wavelengths. We use the patterned bilayer graphene to produce localized surface plasmon polaritons and delocalized surface plasmon polaritons to obtain dual frequency absorption, then additional silicon cylinder arrays on the top of the patterned graphene produce a series of discrete plasmon resonances located within the dual frequency interval, at the same time, enhancing the absorption rate through improving the matching condition. Ultra-wide bandwidth is realized when the guided surface plasmon resonance is enhanced by tuning the parameter of silicon cylinders, promoting field confinement on the surface of the structure. The absorption rate and bandwidth of the structure can be independently tuned by varying the chemical potential of biasing voltage on both of the graphene layers. Simulation results demonstrate that the absorption of the proposed structure can be as high as more than 90% from