Journal Pre-proof Towards intelligent industrial systems: A comprehensive survey of sensor fusion techniques in IIoT Deepak sharma, Anuj kumar, Nitin Tyagi, Sunil S. Chavan, Syam Machinathu Parambil Gangadharan PII: S2665-9174(23)00280-5 DOI: https://doi.org/10.1016/j.measen.2023.100944 Reference: MEASEN 100944 To appear in: Measurement: Sensors Received Date: 29 December 2022 Revised Date: 3 October 2023 Accepted Date: 12 November 2023 Please cite this article as: D. sharma, A. kumar, N. Tyagi, S.S. Chavan, S.M.P. Gangadharan, Towards intelligent industrial systems: A comprehensive survey of sensor fusion techniques in IIoT, Measurement: Sensors (2024), doi: https://doi.org/10.1016/j.measen.2023.100944. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2023 Published by Elsevier Ltd. Towards Intelligent Industrial Systems: A Comprehensive Survey of Sensor Fusion Techniques in IIoT Deepak sharmaa , Anuj kumarb , Nitin Tyagic , Sunil S. Chavand , Syam Machinathu Parambil Gangadharane a Department of computer science, Aryabhatta college, University of Delhi, India University school of Information, Communication and Technology, Guru Gobind Singh Indraprasth University, Dwarka, India c Department of Computer science and Engineering, GL Bajaj Institute of Technology and Management,Greater Noida, India d Smt.Indira Gandhi College of engineering.Navi Mumbai. Affiliation: University of Mumbai, India e Sr Data Engineer, The Home Depot, India ro of b Abstract -p Industrial Internet of Things (IIoT) is systems aim to facilitate human monitoring and the direction of efficient production of goods in industrial settings by linking a wide variety of intelligent devices such re as sensors, actuators, and controllers. This is achieved by utilizing Internet of Things (IoT) to diagnose a lP problem with a specific IIoT part is to employ a basic diagnostic technique that’s based on models and data. Physical models, signal patterns, and machine-learning strategies must be adequately built to account for na system challenges. Another factor that could lead to an exponential rise in complexity is the ever-increasing interconnections between different electronic hardware. The knowledge-based defect diagnosis methods ur boost interoperability in the operation. Users don’t need to be experts in the field to benefit from the system’s high-level thinking and response to their queries. So, in advanced IIoT systems, a knowledge-based Jo fault diagnostic approach is favored over traditional model-based and data-driven diagnosis methods. The goal of this study is to evaluate recent improvements in the design of knowledge-based defect detection in the context of IIoT systems, deductive and inductive reasoning, and many other forms of logical reasoning. IIoT-based systems have revolutionized industrial settings by connecting intelligent devices such as sensors, actuators, and controllers to enable efficient production and human monitoring. In this survey paper, we explore machine learning-based sensor fusion techniques within the realm of Industrial Internet of Things (IIoT), addressing critical challenges in fault detection and diagnosis. Keywords: Sensor fusion; Machine learning; Fault tolerance; Fault prediction; Neural network Email addresses: deepakdixit151@gmail.com ( Deepak sharma), sonanuj16@gmail.com (Anuj kumar), Nitin.tyagi@glbitm.ac.in ( Nitin Tyagi), chavanss @hotmail.com(SunilS.Chavan), syamganga@gmail.com(SyamM achinathuP arambilGangadharan) Preprint submitted to Elsevier December 6, 2023 Acronyms Full Form WSN Wireless Sensor Network IoT Internet of Things IIoT Industrial Internet of Things RTD Resistance Temperature Detector ANN Artificial Neural Network BPNN Back Propagation Neural Network DWT Discrete Wavelet Transformation EM Expectation Maximization EWMA Exponentially Weighted Moving Average FFT Fast Fourier Transform FPCA Functional Principal Component Analysis GMM Gaussian Mixture Model GRBM Gaussian–Bernoulli Restricted Boltzmann Machine GRNN General Regression Neural Network HMM Hidden Markov Model KNN k-Nearest Neighbors KDE Kernel Density Estimator LAD Logical Analysis of Data LOF Local Outlier Factor RBM -p re lP na ur Jo PCA ro of Acronym Principal Component Analysis Restricted Boltzmann Machine RNN Recurrent Neural Network SARMA Seasonal Autoregressive Moving Average SBM Similarity Based Modeling SOM-MQE Self-Organizing Map Minimize Quantization Error SVM Support Vector Machine VCM Vibration-based Condition Monitoring 1. Introduction The goal of the IIoT is to allow humans to effectively monitor and direct the production of commodities in industrial settings by connecting a wide variety of intelligent devices, such as sensors, actuators, and controllers, to one another. The Internet of Things is used to achieve this goal such as to locate and isolate 2 a problem in certain IIoT parts, it may be sufficient to employ model-based and data-driven diagnostic tools [1]. However, adequately developed physical models, signal patterns, and machine-learning methodologies are necessary to explain system issues. Moreover, the ever-increasing number of interconnections among devices has the potential to produce an exponential rise in complexity. These are used by knowledgebased defect diagnosis methods to boost interoperability [2]. As a result, the system can provide high-level reasoning and query responses to individuals who aren’t specialists in the field. Consequently, advanced IIoT techniques favor a knowledge-based fault diagnostic methodology over traditional, model-based, and data-driven troubleshooting approaches [3, 4]. It’s because knowledge-based fault diagnostic methodology ro of is more precise. This research aims to examine recent developments in the design of knowledge bases for knowledge-based defect detection within the framework of IIoT systems, using ontologies, deductive and inductive reasoning, and a wide variety of other forms of logical reasoning. Furthermore, methods for defect identification in IIoT systems based on knowledge are explored; ways that make substantial use of inductive -p reasoning are given special attention. re The next generation of fault detection will require distributed implementations across the system, keeping with the current trend toward decentralizing large systems. Since this is the case, therefore our study by lP pointing out some intriguing open questions concerning knowledge-based failure detection for distributed IIoT systems. na • Predictive maintenance: Sensors can gather data on the performance and condition of equipment, allowing for the identification of potential issues before they occur. This can help to reduce downtime ur and improve overall efficiency. Jo • Supply chain optimization: The IIoT helps lots to keep the track of goods and materials movement, allowing for more efficient and timely delivery. • Energy management: Sensors can be used to monitor and control energy use in industrial settings, helping to reduce waste and improve efficiency. • Quality control: Sensors can be used to gather data on the quality of products being produced, allowing for the identification and correction of issues in real-time. 1.1. Differentiating Factors The field of Industrial Internet of Things (IIoT) has garnered significant attention, resulting in the availability of several survey papers in the literature. There are several research available which provided the classification for the applicability of advance sensors and its applications [5, 6, 7, 8]. While acknowledging the existence of related surveys, it is essential to highlight the unique aspects and additional information that readers can gain from our survey paper titled ”A survey on machine learning-based sensor fusion techniques in Industrial Internet of Things.” 3 1. Focus on Machine Learning-based Sensor Fusion Techniques: Unlike broader surveys that cover various aspects of IIoT, our survey paper specifically focuses on machine learning-based sensor fusion techniques. The study delve deep into the design and integration of machine learning algorithms for sensor data fusion in IIoT systems. This specialized focus allows us to provide in-depth insights into the advancements, challenges, and potential applications of these techniques. 2. Comprehensive Review of Sensor Fusion Methods: Our survey paper provides a comprehensive review of various machine learning-based sensor fusion methods, including probabilistic modeling, deep learning approaches, ensemble methods, and Bayesian inference, among others. The study highlight the ro of strengths, limitations, and practical considerations of each method, enabling readers to gain a holistic understanding of the available techniques and their suitability for different IIoT applications. 3. Emphasis on Industrial Applications and Real-world Implementations: While other surveys may discuss sensor fusion techniques in a generic context, our survey paper places a strong emphasis on their -p application in industrial settings. The study explore the challenges and requirements specific to IIoT re systems, such as robustness, reliability, scalability, and real-time processing. By focusing on industrial applications, The study provide readers with practical insights and considerations for implementing lP sensor fusion techniques in real-world scenarios. 4. Additional Information: In addition to the differentiating factors mentioned above, readers can gain na the following additional information from our survey paper: • Comparative Analysis: The study conduct a detailed comparative analysis of the performance, ur advantages, and limitations of different machine learning-based sensor fusion techniques. This Jo allows readers to make informed decisions and select appropriate fusion methods based on their specific requirements and constraints. • Case Studies and Use Cases: The study present case studies and use cases that showcase the successful implementation of machine learning-based sensor fusion in various industrial domains. These real-world examples provide readers with practical insights into the benefits and challenges associated with deploying sensor fusion techniques in IIoT systems. • Future Research Directions: Our survey paper identifies emerging research directions and open challenges in the field of machine learning-based sensor fusion in IIoT. By discussing these future research directions, the study inspire researchers to explore novel approaches, address existing limitations, and contribute to the advancement of the field. This review paper presents a comprehensive analysis of recent advancements in knowledge-based defect detection for IIoT systems. It uniquely emphasizes the integration of different learning techniques, properties and the relations between them, deductive and inductive reasoning, and logical methodologies to enhance fault diagnosis capabilities. By showcasing the integration of diverse strategies, this paper sheds light on 4 ro of -p Figure 1: Different Components of Industry 4.0 [2]. a novel approach to tackle complexities in IIoT systems. Our investigation reveals that a learning-based re fault diagnostic approach outperforms traditional model-based and data-driven methods in advanced IIoT systems. By leveraging connection among data and logical reasoning, the system’s high-level thinking and lP responsiveness to queries has been enhanced. The practical significance of existing learning approach is evident in its ability to provide accurate and rapid defect identification, leading to reduced downtime and na optimized production processes. The application of machine learning-based sensor fusion techniques in IIoT holds immense practical value. Industries benefit from improved fault detection, predictive maintenance, ur and enhanced operational efficiency. The role in industrial applications is pivotal as it facilitates real-time Jo monitoring, process optimization, and adaptive decision-making, resulting in substantial cost savings and increased competitiveness. This paper focuses on the machine-learning techniques that are used for sensor fusion in IIoTs as structure is shown in Fig. 2. Sensor fusion and its underlying taxonomy are laid out in (Section II)—existing literature on sensor fusion and its categorization (Section III). Describe the most important features of the sensor fusion approach used in machine learning for IIoT, provide a comprehensive survey based on parameters, and then draw a conclusion (Section IV). 1.2. Existing literature and its classification for sensor fusion solution or approach in IIoT The Table.1 presents a comparison of existing surveys in the field of Industrial Internet of Things (IIoT) and Industry 4.0. These surveys have explored various aspects of IIoT, including data fusion, predictive analysis, manufacturing processes, machine learning, big data analytics, and additive manufacturing. While these surveys have provided valuable insights into different facets of IIoT, our forthcoming survey aims to build upon this foundational knowledge. With a focus on synthesizing the latest advancements and emerging trends, our survey is poised to offer a comprehensive and up-to-date perspective on the state of the field. 5 ro of -p re lP na ur Jo Figure 2: Diagram illustrating the hierarchical structure of the paper, including sections and subsections, providing an overview of the paper’s organization. 6 Table 1: Comparative Analysis of Existing Surveys in the Field of Industrial Internet of Things (IIoT) and Industry 4.0 Description Automate Manufacture ML Big Data Additive 2019 Data fusion for industrial prognosis [9] Yes Yes Yes No No 2013 Recommendations for Industry 4.0 [10] No No No No No 2014 Industry 4.0 in Europe [11] No No No No No 2018 Industrial IoT in IoT Systems [12] No No Yes No No 2015 Future of Manufacturing [13] No Yes No No No 2017 Industry 4.0 Technologies [14] No No No Yes No 2017 IIoT and Cyber Manufacturing [15] No Yes No Yes No 2016 Complex View of Industry 4.0 [16] No No No No No 2016 Cyber-Physical Systems Foundations [17] No No No No No 2013 Recommendations for Industry 4.0 [18] No Yes No No No NA Proposed work Yes Yes No Yes -p ro of Year re Yes lP By addressing gaps in existing research and highlighting areas of innovation, our survey aspires to provide na a forward-looking view that can guide future developments in the domain of IIoT.” 2. Taxonomy and framework for sensor fusion ur Wireless Sensor Networks (WSNs) find extensive application in various domains, notably in environmental monitoring as shown in Fig. 3. These networks consist of small, autonomous sensor nodes equipped Jo with sensors to collect data on environmental parameters such as temperature, humidity, air quality, and more. WSNs are deployed in remote or inaccessible areas where traditional wired systems are impractical. For instance, in agriculture, WSNs help farmers monitor soil moisture levels to optimize irrigation, reduce water wastage, and enhance crop yields. In wildlife conservation, these networks enable the tracking of animal movements and habitat conditions. Additionally, WSNs are vital in disaster management for early detection of natural calamities like forest fires or earthquakes, allowing for timely response and mitigation. The versatility and real-time data collection capabilities of WSNs make them invaluable tools in addressing pressing environmental and societal challenges. Sensor fusion can be accomplished in several ways, with the chosen method depending on the fusion process and the nature of the data being combined. Examples of standard methods for fusing data from multiple sensors include: Using data fusion, which entails compiling raw data from various sensors into a unified picture of the system or process under observation. Methods like averaging, filtering, and interpolation may be used to combine the data [19]. In the realm of industrial applications, a diverse array of sensors plays a pivotal role in ensuring the efficiency, safety, and quality of various processes. These sensors, including temperature sensors (e.g., thermo7 ro of Figure 3: Naming convention of wireless sensors [20]. -p couples, RTDs), pressure sensors (e.g., piezoelectric, capacitive), level sensors (e.g., ultrasonic, capacitance, float), and flow sensors (e.g., turbine, magnetic, thermal flow meters), are crucial components of industrial re systems. They tirelessly monitor and control temperature variations, pressure levels, liquid and solid levels, lP and fluid flow rates. In addition, proximity sensors (e.g., inductive, capacitive) are instrumental in automation and robotics, while humidity sensors maintain precise humidity levels in controlled environments. na Vibration sensors detect machinery vibrations, and gas sensors (e.g., CO2, CH4) ensure workplace safety by detecting the presence of specific gases. Position sensors (e.g., encoders), force sensors (e.g., load cells), light ur sensors (e.g., photodetectors, photodiodes), acoustic sensors (e.g., microphones, ultrasonic transducers), and image sensors (e.g., CCD, CMOS) further extend the range of applications in industrial settings. Each of Jo these sensors serves a specific purpose, collectively contributing to the seamless operation and quality control of industrial processes. Features or characteristics extracted from data collected by multiple sensors are fused in the feature fusion process rather than the raw data itself. The elements could be combined using various methods, including decision trees, neural networks, and clustering algorithms. During the decision fusion phase, data from multiple sensors and algorithms are combined into a single set of findings. Combining the results, such as voting or weighting, may be used. In the end, the goals of the fusion process and the nature of the data being fused will determine the precise approach to sensor fusion that will be taken. To get the best results, employing more than one strategy may be necessary as shown in Fig. 3. 2.1. Sensor fusion in Industrial Internet of Things The IIoT uses a method called ”sensor fusion” to increase the precision and dependability of collected data by combining readings from multiple sensors. A complete picture of the systems and processes being monitored is possible thanks to sensor fusion, which helps businesses get past the limitations of individual 8 sensors [21]. In a factory, for instance, sensor fusion could be used to get a fuller picture of a machine’s health and performance by combining data from multiple sensors (such as temperature, pressure, and vibration) [22]. Insights gained from this information could lead to the early detection of issues and the implementation of fixes that would otherwise result in unscheduled downtime. It is especially helpful in industrial settings, where multiple sensors may be used to monitor various parts of a system or process. To track variables like temperature, vibration, and flow rate, sensors are commonly used in factories to track production line efficiency [23]. The information gathered from these sensors can be used to understand better how the production line is functioning as a whole, which can aid in locating issues and maximizing efficiency. A ro of wide range of methods exists for actualizing sensor fusion. As opposed to feature fusion, which combines the features or characteristics extracted from the data, sensor fusion combines the raw data from multiple sensors [24]. Sensors are essential for diagnosing system health in data-driven enterprises, aiding prognosis through -p intelligent signal processing [25]. Industry 4.0 relies on sensors and AI for improved efficiency. Research shows re their potential in minimizing errors and enhancing productivity, but challenges like cost hinder widespread adoption. Further studies are needed to address these issues and advance the field. This article explores lP modern methods for monitoring tool conditions during various machining operations [26]. Researchers are focused on factors like cutting tool wear, cutting force, and surface roughness. Changes in these factors na impact dimensional accuracy and productivity significantly. Excessive wear can lead to tool breakage. The study discusses using sensors and artificial intelligence (AI) for online monitoring. Sensors include ur dynamometers, accelerometers, and more. AI methods like neural networks, image recognition, and fuzzy logic are examined. The article also addresses the strengths, weaknesses, and potential of these AI methods Jo for tool wear monitoring, as well as future directions for research in this field. Sensor fusion is crucial for monitoring the cutting process effectively, ensuring longer tool life and highquality products [27]. The intricate nature of machining and variable interactions impact quality measures. Employing multiple sensors facilitates comparing data from different sources, aiding decisions about tool and workpiece conditions. An experimental study employed five sensors—measuring cutting forces, vibration, acoustic emission, temperature, and current—during lathe turning of AISI 5140 with coated carbide tools. Results show temperature and acoustic emission signals are about 74% effective in detecting flank wear. Fuzzy logic-based prediction of wear using temperature and acoustic emission sensors demonstrates high accuracy, suitable for sensor fusion. Instant tool breakage can be prevented using tangential and feed cutting forces, acoustic emission, and vibration signals. Sensor fusion enhances reliability, robustness, and consistency in machining processes. With the help of decision fusion, data from various sources, such as sensors or algorithms, can be incorporated into a single, comprehensive assessment. Different methods of sensor fusion include: 9 ro of Figure 4: Representation of sensor module internal entity [29]. -p • Kalman filtering: It is a statistical method for estimating the state of a system based on a series of noisy measurements. It can be used to combine data from multiple sensors and remove errors or biases re in the measurements [28]. lP • Fuzzy logic: It is a form of artificial intelligence that can be used to process and interpret data from multiple sensors and make decisions based on that data. na • Dempster-Shafer theory: It is a mathematical approach to combining information from multiple sources ur and estimating the probability that a particular event will occur . The sensor fusion is an important technique in the IIoT, as it allows organizations to make more informed Jo decisions and optimize their operations based on a more accurate and comprehensive view of their systems and processes. 2.2. Fault prediction using sensor fusion in IIoT To predict when a piece of industrial equipment or a system will fail, researchers in the Industrial Internet of Things (IIoT) combine data from multiple sensors. It can help businesses ensure continuous operation with minimal chance of interruption. Fault prediction in the IIoT can be achieved through sensor fusion, which involves the consolidation of data from multiple sensors to establish patterns and trends that may indicate an impending failure [21]. It can aid businesses in avoiding unscheduled downtime and increasing their overall uptime. To track variables like temperature, vibration, and flow rate, sensors are commonly used in factories to track production line efficiency [30, 31, 32] . Over time, it may be possible to spot patterns in the data collected by these sensors that point to the presence of an issue. Increases in vibration or temperature, for instance, could mean that a machine needs to be serviced or fixed. Companies can use several methods, such as machine learning algorithms, data analysis, and statistical modeling, to use sensor 10 fusion to predict faults. These methods can be used to spot anomalous data patterns and trends and then notify the appropriate parties when an issue is found. Fault prediction by combining data from multiple sensors is a viable technique in the IIoT, and it can be approached from many different angles [33, 34, 35]. One strategy is to run the collected sensor data through machine learning algorithms to look for trends or patterns that might indicate an impending failure. By analyzing past data, these algorithms can be taught to recognize anomalous behavior and then used to trigger warnings whenever a problem is suspected. Alternatively, you could use statistical methods like probability distributions or statistical process control (SPC) to keep an eye on the data coming in from the sensors and spot any unusual behavior that could ro of point to a problem. When attempting to foresee problems, it is helpful to keep in mind a few general rules: • Data collection: In order to predict faults, it is important to collect data from a variety of sources such as sensors, machines, and systems. This data should be collected in real-time and should be -p representative of the system or process being monitored. • Data analysis: Once the data has been collected, it is important to analyze it in order to identify re patterns and trends that may indicate a potential fault. lP • Modeling: In order to predict faults, it is often necessary to build a model of the system or process being monitored. This model should be based on the data collected and should be able to accurately na predict the behavior of the system under different conditions. • Validation: It is important to validate the accuracy of the fault prediction in order to ensure that it is observations. ur reliable. This may involve testing the model on a sample of data and comparing the results to actual Jo 200 The fault prediction using sensor fusion in the IIoT can help organizations to proactively identify and address potential issues before they become major problems, enabling them to improve the reliability and efficiency of their operations. A sensor fusion solution for the IIoT involves a combination of hardware (sensors), software (data acquisition, storage, and analysis), and visualization tools that work together to enable organizations to make better informed decisions based on the data collected from their sensors and devices [36, 37, 19] . There are several key components that are often included in a sensor fusion solution for the IIoT: 1. Sensors: These are the devices that collect data from the environment. In the IIoT, sensors may be used to monitor a variety of parameters, including temperature, vibration, flow rate, and more using its internal module shown in 4. . 2. Data acquisition and transmission: This involves the process of collecting and transmitting the data from the sensors to a central location for analysis. This may involve using wireless communication technologies such as Wi-Fi or Bluetooth to transmit the data [38, 29] . . 11 3. Data storage and management: This involves the process of storing and organizing the data collected from the sensors. This may involve using a database or data lake to store the data. 4. Data analysis: This involves the process of analyzing the data collected from the sensors in order to identify patterns and trends that may be useful for predicting faults or optimizing performance. This may involve using techniques such as machine learning, data mining, or statistical modeling to analyze the data [39]. 5. Visualization: This involves the process of presenting the results of the data analysis in a way that is easy to understand and interpret. This may involve using visualizations such as graphs, charts, or ro of maps to present the data. 6. Edge computing: Edge computing involves processing data at the edge of the network, close to the source of the data. This can be an effective approach for sensor fusion in the IIoT, as it allows for -p real-time processing of data and can reduce the amount of data that needs to be transmitted over the network [40, 20] . re 7. Cloud computing: Cloud computing involves storing and processing data in a remote server or servers, accessed over the internet. This can be a useful approach for sensor fusion in the IIoT, as it allows lP for the integration of data from multiple sources and can provide the necessary computing power and storage capacity for large-scale data analysis [41, 42]. na 8. Distributed systems: Distributed systems involve dividing a complex system into smaller, more manageable components that can be distributed across different locations or devices. This can be an ur effective approach for sensor fusion in the IIoT, as it allows for the integration of data from multiple Jo sources and can improve the scalability and reliability of the system [43, 44]. In addition, there are many literature available which has made several classification . The existing studies has taken the approach such as sensory fusion, industry 4.0, data fusion, machine learning approach, fault diagnostics, sensor fusion challenges, and IIoT based solution [45, 46, 28, 47]. 2.3. Adaptive Neuro-Fuzzy Inference System (ANFIS) This artificial neural network has been developed to model the fuzzy logic systems used in many control and decision-making systems, and the ANFIS-based sensor fusion has been presented in [48]. It can efficiently learn and adapt to new data thanks to a combination of the strengths of artificial neural networks and fuzzy logic systems. ”Fuzzy” rules, the foundation of fuzzy logic systems, are logical statements that can be either true or false or even a grey area in between. A rule could be fuzzy if it said, ”If the temperature is hot, then the air conditioning should be turned on.” Since ”hot” can mean different things to different people in other contexts, this rule cannot be definitively proven true or false [49]. Fuzzy logic systems are wellsuited to dealing with this kind of subjectivity, and It combines its strengths with those of artificial neural 12 networks in learning. It employs a hybrid approach, using both fuzzy logic and neural network methods to accomplish in this [50, 51]. It can be utilized in various settings, such as control systems, pattern recognition, and prediction. Electrical engineering, computer science, and even medicine have all benefited from their application. Table 2: Brief description of existing sensor fusion, prognostics and application areas Descriptive Approach, Ref Objective LAD [52] Combination of controlled, manipulated and mea- ro of sured flowrates Outlier detection using EM [53] Failures, environmental measures Decision trees and fuzzy classifier [48] Feature extraction and combination of data from KDE and distance-based classification -p condition monitoring Combining fastener features re [54] Fusion of acoustic emissions and vibratory signals DBN based on RBM [55] Fusion of fault evidence and reason vectors DWT and SVMs [56] lP Deep Random Forest and Bag Tree [49] Multi-sensor fusion at feature level Environmental indicators and production and wel- na Quantile regression forests [57] fare parameters ur [58] GPS and wind velocity sensors output Jo Framework based on hybrid adaptive resonant theory ANN and adaptive fuzzy inference [59] Bayesian Inference [60] Multistage data fusion at component and global levels FFT, kNN and K-means [61] Fusion of vibration-based features from accelerometer data and locations of the monitored areas 2.4. Artificial Neural Networks (ANNs) Sensor fusion is the process of combining data from multiple sensors to provide a more complete and accurate understanding of a system or environment. In [62], an Artificial neural networks (ANNs) based solution is proposed. It can be used for sensor fusion by training a neural network to integrate the data from 13 multiple sensors and make decisions or predictions based on the combined data. There are several ways in which can be used for sensor fusion as following which is described in [57, 58]: 1. Feature extraction: It can be used to extract features from raw sensor data, such as identifying patterns or trends in the data. This can help to improve the accuracy of sensor fusion by providing a more structured and relevant representation of the data . 2. Data fusion: It can be used to fuse data from multiple sensors by combining the data in a way that is more useful or meaningful. For example, an ANN could be used to fuse data from a camera and a lidar sensor to create a more accurate 3D map of an environment . ro of 3. Decision making: It can be used to make decisions based on sensor fusion data, such as determining the best course of action for a robot to take in a particular situation . -p 2.5. Back Propagation Neural Networks ( BPNN) In a solution based on artificial intelligence and feedforward neural networks, the BPNNs are employed re for sensor fusion [50]. It means they have a predetermined input layer, a hidden layer(s), and an output lP layer(s) [59]. The hidden layers use the input data to learn and extract features, and the output layer is where the learned features are used to make predictions or decisions. In sensor fusion, it can aggregate information from multiple sensors into a unified stream from which conclusions and predictions can be drawn. One way na to accomplish this is to train the BPNN using examples of sensor data from each sensor and the desired output for each model [60]. It uses a training algorithm known as backpropagation. Its many applications ur in sensor fusion span robotics, autonomous vehicles, and industrial automation. They shine in situations Jo where it’s necessary to combine information from sensors with unique strengths and weaknesses, such as when making a 3D map of an area using data from a camera and a lidar sensor. 2.6. Deep Belief Networks ( DBN ) When combining data from multiple sensors, artificial neural networks constructed using DBNs have been shown to be effective. For DBNs to learn and derive features from the input data, they are constructed with many tiers of interconnected “belief” units at multiple levels [55]. Each subsequent layer of belief units builds upon the information it has received from the layer below it, eventually learning very generalized characteristics as a result. DBNs can be used for sensor fusion because they can be trained with examples of sensor data taken from multiple sensors, along with the desired output for each training example [63]. Regarding sensor fusion, the fact that DBNs can learn features from the data independently without the assistance of labels gives them an advantage over supervised learning methods [64]. This can be helpful in sensor fusion applications, where the expected result needs to be clarified or known. DBNs have been used for various sensor fusion tasks, some of which include the recognition of objects, the comprehension of 14 scenes, and the detection of anomalies [65]. They have also been utilized in these applications to integrate data from several different sensors, as is the case in robotics and autonomous vehicles. 2.7. Discrete Wavelet Transformation ( DWT ) The DWT is a tool that can analytics and extract features from it. The “wavelets” are oscillating functions with a limited duration that are used to represent the signal at different scales [56]. The method works by decomposing a signal or data series into a series of “wavelets”. In sensor fusion, discrete wavelet transform can be used to extract features from the data produced by multiple sensors and combine them in a meaningful way. It can be accomplished by combining the data with a discrete wavelet, which can be ro of achieved by applying DWT to the data from each sensor in turn and then combining the wavelets produced in a manner that is pertinent to the activity currently being performed. When it comes to sensor fusion, one of the benefits of DWT is that it can provide a multiscale representation of the data. This data representation -p can be helpful when trying to identify patterns or features at different scales. In addition, DWT can be re applied to the data to de-noise or filter it by lessening the impact of interference or noise on the data. The application can be used in a wide variety of sensor fusion tasks, such as processing images and videos, lP speech recognition, and medical imaging [66, 67]. In addition, it has been developed in robotics, self-driving na vehicles, and other applications where integrating data from a variety of sensors is essential. 2.8. Expectation Maximization (EM ) ur The fault diagnosis method, known as expectation maximization (EM), is utilized in settings with hidden or latent variables. The purpose of fault diagnosis is to determine whether a problem or failure exists within Jo a system by examining data obtained from sensors or other sources. An iterative algorithm estimates the hidden or latent variables in a design by maximizing the likelihood of the observed data given the estimated latent variables [53]. The algorithm begins with an initial estimate of the latent variables. Then it alternates between two steps: the expectation step, in which it estimates the expected value of the latent variables given the current estimate, and the maximization step, in which it evaluates the importance of the latent variables that maximize the likelihood of the observed data. The algorithm begins with an initial estimate of the latent variables [68]. Then it begins alternating between two steps: the expectation step, in which it estimates the expected value of the latent variables given the current estimate, and the maximization By training the algorithm on a dataset that contains examples of sensor data and fault labels, it can be utilized for fault diagnosis. This process is known as ”training.” During the training phase, the algorithm acquires the knowledge necessary to comprehend the connection between the sensor data and the latent variables that stand in for the malfunctioning state of the system. After it has been trained, the algorithm will utilize the sensor data to estimate the latent variables and make predictions regarding the fault state of the system. It has been utilized in various fault diagnosis applications, such as diagnosing faults in industrial systems, 15 electrical power systems, and aircraft systems, amongst others. It has the advantage of handling missing or incomplete data, which is helpful in fault diagnosis tasks because it is common for missing or incomplete data. 2.9. Exponentially Weighted Moving Average (EWMA) The exponentially weighted moving average (EWMA) statistical analysis method is presented for data fusion. In computer systems that have time-series data, it can be utilized for fault diagnosis [69]. By analyzing data obtained from sensors and other sources, the fault diagnosis process seeks to determine the underlying cause of a problem or failure detected in a system. It is a method for estimating the underlying ro of trend or mean of a time-series data series. In this method, more weight is given to data collected recently, and less importance is given to data collected for longer [70]. The weighting factor used in EWMA is exponentially decreasing, which means that the weight of each data point decreases exponentially as the -p data point gets older, and it is because the weighting factor itself is exponentially decreasing. It has been re used for fault diagnosis by analyzing the difference between the observed data and the estimated mean or trend. This can be done in either direction. If the difference is significant enough to cross a certain threshold, lP it may suggest an error or an anomaly somewhere within the system. EWMA can also be utilized to identify changes in the mean or trend of the data, both of which can serve as additional indicators [71]. It has na been utilized in many fault diagnosis applications, such as industrial system fault diagnosis, electrical power system fault diagnosis, and aircraft system fault diagnosis. It is beneficial for detecting gradual changes in ur the mean or trend of the data, which may not be detectable using other methods. This can be the case Jo because it is easy to miss these changes. 2.10. Fast Fourier Transform ( FFT ) The fast Fourier transform (FFT) is an algorithm that can compute a data sequence’s discrete Fourier transform (DFT) [72]. The discrete Fourier transform (DFT) is a mathematical operation that can be used to analyze the frequency content of a signal. It takes a signal or data series and breaks it down into component frequencies and amplitudes [73]. The fast Fourier transform (FFT) is a method for computing the discrete Fourier transform (DFT), which is essential in various applications that require real-time data processing. The Fast Fourier Transform is based on dividing a long data sequence into several shorter rows and then computing each individual shorter sequence’s Discrete Fourier Transform (DFT) [61]. This makes it possible for the Fast Fourier Transform (FFT) to calculate the DFT of a data sequence much more quickly than other methods, such as the direct computation of the DFT. Incorporating areas such as signal processing, image processing, and communication systems is beneficial. Analyzing the frequency content of signals that change over time, such as audio signals or time-series data, is a beneficial application for this technique. FFT is utilized in many different practical applications in addition to its use in analysis. Some 16 examples of these applications include data compression, audio, and image synthesis, and digital filtering [74]. By applying the fast Fourier transform (FFT) to the data from multiple sensors and combining the frequency spectra generated as a result in a manner pertinent to the task at hand, the fast Fourier transform (FFT) can be used for sensor fusion [75, 76, 77, 78, 79]. In sensor fusion, FFT has been used to extract features from the data produced by multiple sensors and combine them in a meaningful way. It is done to improve the accuracy of the overall data and can be accomplished by applying FFT to the data from each sensor in turn and then combining the frequency spectra produced in a manner that is pertinent to the ro of task being performed. The FFT-based sensor fusion is a concept that is proposed in. One of the benefits of utilizing FFT for sensor fusion is that it can provide a representation of the data that is condensed while still being informative. This data representation can help locate patterns or features within the data. FFT can also be used to de-noise or filter the data, which can improve the accuracy of sensor fusion by reducing the -p effect of noise or interference on the data. FFT can be used in a variety of other contexts as well. The fast re Fourier transform (FFT) has found use in a wide variety of sensor fusion applications, such as processing images and videos, speech recognition, and medical imaging. In addition, it has been utilized in robotics, lP self-driving vehicles, and other applications where integrating data from a variety of sensors is essential. na 2.11. Functional Principal Component Analysis ( FPCA ) Fault diagnosis in systems with operational data can be accomplished through functional principal com- ur ponent analysis, also known as FPCA. The method was used for fault diagnosis to decompose operational data into a set of main components. Principal components are orthogonal functions that capture the most Jo critical aspects of the data [80]. The principal components are ranked in order of importance, with subsequent principal components explaining progressively less and less variance [81]. In, FPCA has been utilized for fault diagnosis. This is accomplished by analyzing the principal components of the available data to spot any changes or anomalies in the data. For instance, if the first principal component suddenly alters its shape or magnitude significantly, this might suggest a problem with the system [82]. Additionally, it can be utilized to determine whether or not there have been any shifts in the general structure of the data [83]. It can perform various fault diagnosis tasks, such as fault diagnosis in industrial systems, electrical power systems, and aircraft systems, among other techniques. It is constructive for analyzing operational data, which includes data that varies over time or in different locations. 2.12. Gaussian Mixture Models ( GMM ) The GMMs is a probabilistic model for sensor fusion that can be utilized [84] . In this context, the GMMsbased solution on the idea of representing data distribution as a mixture of several Gaussian distributions is proposed [85]. This solution is based on the idea that the data can be represented as a distribution. 17 In the data, every one of the distributions represents a different component or cluster. In the context of sensor fusion, GMMs can combine the data from multiple sensors into a single set of data and then use that fused data to make predictions or make decisions. To accomplish this, the GMMs are trained on a dataset containing examples of sensor data from multiple sensors and the desired output for each instance. This allows the GMMs to learn how to produce the desired results [86]. It understands the relationships between the sensor data and the desired output during training. It can then be used to make predictions or decisions based on new sensor data based on what they have learned during training. It is useful for sensor fusion because it can handle data with complex distributions, such as multi-modal data or outliers. This is one of ro of the advantages of using GMMs. It can also be used to model uncertainty in the data, which can be helpful in situations where the data is noisy or unreliable. It has been utilized in various sensor fusion tasks, such as anomaly detection, speech recognition, and object recognition. In robotics, autonomous vehicles, and other -p applications where the integration of data from multiple sensors is essential, they have also been utilized. re 2.13. Gaussian–Bernoulli Restricted Boltzmann Machines ( GRBMs) For the Internet of Things (IoT), the GBRBM-based sensor fusion was proposed in [87]. It is an artificial lP neural network capable of handling the data produced by sensor fusion. RBMs are a type of probabilistic generative model that can learn to reconstruct data from a hidden representation. This particular model na is a variant of restricted Boltzmann machines (RBMs), also a type of RBM. It have been used in sensor fusion to combine data from multiple sensors to make predictions or decisions based on the combined data ur [88, 89]. To accomplish this, the GBRBMs are trained on a dataset containing examples of sensor data from multiple sensors and the desired output for each model [50]. This allows to learn how to produce the desired results. During training, its learns the relationships between the sensor data and the desired output. Jo 400 These learned relationships can then be used to make predictions or decisions based on the new sensor data. When it comes to sensor fusion, one of the benefits of using is that they can learn complicated relationships between the data collected by the sensors and the desired output, even if those relationships are nonlinear or challenging to model using other approaches. It can also be utilized to model uncertainty in the data, which can be helpful in circumstances in which the data is noisy or unreliable. It have been utilized in performing a wide variety of sensor fusion tasks, some examples of which include object recognition, speech recognition, and anomaly detection. In robotics, autonomous vehicles, and other applications where the integration of data from multiple sensors is essential, they have also been used [90, 91, 92] . For fault prediction in systems containing time-series data, Gaussian-Bernoulli-restricted Boltzmann machines, have been utilized. The purpose of fault prediction is to determine how likely a fault will occur in a given system based on information gathered from sensors and other sources. To make use for fault prediction, the model would first need to be trained on a dataset containing illustrative examples of sensor data and fault labels. It would acquire this knowledge about the relationships between the sensor data and the fault’s presence or absence 18 while being trained. After they have been trained, the GBRBMs can be used to predict the likelihood of a fault occurring based on the latest sensor data. The advantage of using it for fault prediction is that they can learn complicated relationships between the sensor data and the fault state of the system, even if those relationships are either challenging to model using other methods or are nonlinear [93]. It can also be utilized to model uncertainty in the data, which can be helpful in circumstances in which the data is noisy or unreliable. It has been used in a wide variety of fault prediction applications, such as the prediction of faults in industrial systems, electrical power systems, and aircraft systems, amongst other applications. They have the advantage of dealing with large amounts of ro of data and learning complex relationships between the data and the fault state of the system. Both of these capabilities give them an advantage. 2.14. General Regression Neural Network (GRNN) -p Artificial neural networks, also known as general regression neural networks (GRNNs), are one type of re neural network that can be utilized for the process of sensor fusion [50]. GRNNs are a subclass of neural networks known as radial basis function (RBF) networks. RBF networks are a form of feedforward neural lP network that has the ability to learn to approximate nonlinear functions. GRNNs can be utilized in sensor fusion to combine data from multiple sensors to make predictions or decisions based on the combined data. na To accomplish this, GRNNs are trained on a dataset containing examples of sensor data from various sensors and the desired output for each measure. This allows the GRNNs to learn how to produce the desired results. ur The GRNNs understand the relationships between the sensor data and the desired output during training. These learned relationships can then be used to make predictions or decisions based on the newly collected Jo sensor data. The advantage of using GRNNs for sensor fusion is that they can learn to approximate complex nonlinear functions. This can be helpful for tasks where the relationships between the sensor data and the desired output need to be more well-defined or are challenging to model using other methods [94, 95, 96, 97]. GRNNs can also be used to model uncertainty in the data, which can be helpful in circumstances in which the data is noisy or unreliable. GRNNs can be used to model uncertainty in the data. Object recognition, speech recognition, and anomaly detection are some application areas that are being explored. In robotics, autonomous vehicles, and other applications where the integration of data from multiple sensors is essential, they have also been used. 2.15. Hidden Markov Model ( HMM ) The Hidden Markov Model (HMM) is a statistical model that is used to represent a sequence of observations that are generated by a process with hidden or latent states. This sequence of words is used to conclude the process. Natural language processing, speech recognition, and computer vision are just some areas that extensively use hidden Markov models (HMMs) [73]. In a hidden Markov model (HMM), it is assumed that 19 the latent states of the process are Markovian. The probability of transitioning to a new state depends only on the current state and not on the states that came before it. It is generally assumed that a Gaussian or multivariate Gaussian probability distribution applies to the situation when determining whether or not the observations are considered to be generated by the latent states. Modeling a wide variety of phenomena, such as time series, sequence, and multivariate data can be accomplished by utilizing HMMs. When it comes to tasks in which the relationships between the observations and the latent states are not well-defined or are challenging to model using other methods, they are instrumental. They are beneficial for tasks like these. HMMs are frequently used for activities such as classification, clustering, and prediction. These are ro of all endeavors in which the objective is to learn the underlying structure of the data to then base decisions or projections on the network that was discovered. They are also useful for activities such as decoding and estimation, both of which involve making inferences about the hidden states of a process based on the data that has been observed. -p In computer systems containing time-series data, the HMMs can be utilized for fault prediction. The re purpose of fault prediction is to determine how likely a fault will occur in a given system based on information gathered from sensors and other sources. The HMM would need to be trained on a dataset containing lP examples of sensor data and fault labels for it to be used for fault prediction. The HMMs would acquire this knowledge about the relationships between the sensor data and the fault’s presence or absence while being na trained. After the HMMs have been trained, they can be used to make predictions about the likelihood of a fault occurring based on the current state of the sensor data. HMMs can learn the underlying structure ur of the data and model the relationships between the sensor data and the fault state of the system. It gives them an advantage when it comes to fault prediction. HMMs can also be used to model uncertainty in the Jo data, which can be helpful in situations where the data is noisy or unreliable. HMMs can also be used to model uncertainty in the data. In addition, it has been utilized in various fault prediction applications, such as predicting faults in industrial systems, electrical power systems, and aircraft systems, amongst others. 2.16. k-Nearest Neighbors ( kNN ) In the fault prediction field in systems containing time-series data, the k-NN-based machine learning algorithm has also been very successful. The purpose of fault prediction is to determine how likely a fault will occur in a given system based on information gathered from sensors and other sources [98]. The knearest neighbor (k-NN) algorithm needs to be trained on a dataset containing examples of sensor data and fault labels before it can be used for fault prediction. The algorithm would understand the relationships between the presence or absence of a fault. After it has been trained, the algorithm can be used to predict the likelihood of a responsibility occurring based on the data from newly installed sensors. For the algorithm to produce a forecast, it must first identify the k data points closest to one another by utilizing a distance metric. After that point, the prediction would be based on the typical educational background of the k20 neighbors who are located the nearest to you. It is simple to construct and can work with many different kinds of data in various contexts. This is the aspect that makes it stand out the most. It is beneficial for tasks in which the links between the data and the fault state of the system need to be better defined or are challenging to characterize using other approaches. This type of work is especially suited to this method. This class of pursuits encompasses a wide variety of issues in their entirety. 2.17. Kernel Density Estimator ( KDE ) Calculating a random variable’s probability density function (PDF) can be done using a non-parametric technique known as KDE. The probability density function, or PDF, is a function that can describe the ro of likelihood of a random variable taking on a variety of values. Smoothing out a collection of data points with the help of a kernel function is the basis for the KDE technique, which is used to estimate the PDF of a random variable. The kernel function is responsible for determining the smoothing curve’s shape and -p the influence each individual data point has on the final PDF. It is beneficial for estimating the PDF of a re random variable when the distribution of the variable is unknown or when the sample size is small. These circumstances make it challenging to obtain a large enough data set [54]. Additionally, it can be utilized lP for activities such as clustering, which aims to recognize patterns or groups hidden within the data. Kernel density estimators, also known as KDEs, have been used for sensor fusion. These estimators are applied to na the data from multiple sensors, and then the resulting density estimates are combined in a manner pertinent to the task at hand. KDE’s can be utilized in the context of sensor fusion to extract features from the ur data produced by multiple sensors and combine them in a meaningful manner. This can be accomplished by combining the data in a meaningful way by first applying KDE’s to the data from each sensor in its Jo own right and then combining the density estimates obtained in a manner pertinent to the task currently being performed. The key advantages of sensor fusion are that it can provide a smooth and informative representation of the data, which can help recognize patterns or features in the data, and that it can reduce the amount of noise in the data. KDE’s can also be used to denoise or filter the data, which can improve the accuracy of sensor fusion by reducing the effect of noise or interference on the data. KDE’s can be used to denoise or filter the data. KDEs are used for various tasks related to sensor fusion, such as speech recognition, image and video processing, and medical imaging. In robotics, autonomous vehicles, and other applications where the integration of data from multiple sensors is essential, they have also been used. 2.18. Logical Analysis of Data ( LAD ) The Latent Attribute Model (LAD) is a statistical method that can be applied to data to recognize patterns or relationships. The logic-based approach to data analysis (LAD) is predicated on employing logical rules or formulas to describe the relationships between variables present in the data and then employing these rules to make predictions or decisions based on the data. Statistical analysis, machine learning, and 21 data mining are just some of the many fields that can use LAD’s diverse applications. It is constructive for tasks where the relationships between the variables in the data are complex or not well-defined and where traditional statistical methods may not be effective. One of the most common applications of this method is in the field of finance [52]. To carry out LAD, a collection of logical rules or equations must be applied between the variables. After that, these rules are applied to the data to determine any patterns or trends that may exist. In the situation line classification, regression, and clustering processes, LAD helps learn the underlying structure so that one can make predictions or decisions based on the network that has been learned. It has been demonstrated to be effective at identifying patterns and relationships in data that ro of may not be detectable using other methods, making it useful for data analysis in finance, marketing, and healthcare, among others. 2.19. Local Outlier Factor (LOF) -p The level of unexpectedness, also known as the LOF, is a measurement that determines an object in a re dataset that can be considered an outlier [53]. When applied to the data from multiple sensors, LOF can be utilized for sensor fusion by combining the relevant outlier scores produced from applying it to that data. In lP the context of sensor fusion, localization-based feature extraction (LOF) has been used to extract features from the data produced by multiple sensors and combine them in a meaningful way. It allows for the data na to be used more holistically to accomplish by applying LOF to the data from each sensor in turn and then combining the outlier scores that were generated as a result. When it comes to sensor fusion, one of the ur benefits of utilizing LOF is that it can provide a condensed and informative representation of the data. This representation of the data can help locate unusual or unexpected patterns or features within the data. The Jo data can be denoised or filtered using LOF, which can improve the accuracy of sensor fusion by reducing the effect of noise or interference on the data. LOF can also be used to improve the accuracy of the data. The tasks associated with sensor fusion include the detection of anomalies, the prevention of fraud, and the strengthening of cybersecurity. In addition, it has been utilized in robotics, self-driving vehicles, and other applications where integrating data from a variety of sensors is essential. 2.20. Principal Component Analysis (PCA ) PCA is based on finding a set of orthogonal linear combinations of the variables in the dataset, which capture the essential characteristics of the data [99]. This can be done by using the variables in the dataset to create linear combinations. The complexity of the data can be reduced by finding patterns or trends in the data, or it can be used to simplify the data by removing information that is redundant or irrelevant. It is beneficial for tasks where the relationships between the variables in the data are complex or not well-defined and where traditional statistical methods may not be effective. One of the most common applications of this method is in the field of finance. The principal component analysis (PCA) was carried out on the data 22 after it had been standardized [100]. It followed that the covariance matrix of the standardized data was computed. After that, the principal components are obtained by projecting the data onto the eigenvectors. The first principal component explains the most variance in the data, and subsequent components define an increasingly smaller amount of variance with each passing detail. It has been shown to be effective at reducing the complexity of the data when combined with other methods applied to a wide variety of tasks, such as data visualization, feature extraction, and data compression. 2.21. Restricted Boltzmann Machines ( RBM ) A specific variety of ANNs known as RBMs is applied to the task of learning a probabilistic model of a ro of dataset. RBMs are a generative model, which means they can learn how to reconstruct data from a hidden representation. This ability is what distinguishes generative models from other types of models. RBMs are made up of two units: a visible layer that stands in for the data fed into the model and a hidden layer that -p stands in for the variables that aren’t immediately apparent from the data. A set of weights, which define re the relationships between the visible and hidden units, are connected to the units in both the visible and the hidden layers. These weights connect the units in both layers. To train an RBM, the model must first lP be provided with a set of input data, after which the weights must be modified to ensure that the highest possible probability is achieved for the data being generated by the model. They are effective when applied na to a wide variety of tasks, including feature learning, dimensionality reduction, and data generation, and have been shown to be applied to a wide variety of functions [55]. ur For fault prediction in systems containing time-series data, restricted Boltzmann machines (RBMs) have been utilized. The RBM would need to be trained on a dataset containing examples of sensor data and fault Jo labels for it to be used for fault prediction. During training, the RBMs would figure out the connections between the data collected by the sensors and the presence or absence of a problem in the system. After they have been trained, RBMs can be used to make predictions about the likelihood of a fault occurring based on the data coming from new sensors. The ability of RBMs to train complex relationships between the sensor data and the fault state is one of the benefits of using RBMs for fault prediction. These relationships can be nonlinear or challenging to model using other methods, but RBMs can easily handle both of these challenges. RBMs can also be used to model uncertainty in the data, which can be helpful in situations where the data is noisy or unreliable. RBMs can be used to model uncertainty in the data. It is helpful for tasks involving fault prediction, such as fault prediction in industrial systems, electrical power systems, and aircraft systems, among other techniques. 2.22. Recurrent Neural Networks ( RNN ) The RNNs are a type of artificial neural network that is well-suited to modeling time-series data and sequential data [101]. RNNs are equipped with a feedback loop or recurrence, which enables them to process 23 data sequentially. Data processing happens one element at a time to keep a memory state of the processed data. In the context of sensor fusion, recurrent neural networks (RNNs) can be utilized to combine data from multiple sensors to derive conclusions or make decisions based on the combined data. RNNs are trained on a dataset containing examples of various sensor data, as well as the desired output for each model. This allows the RNNs to accomplish the aforementioned goal. The RNNs learn the relationships between the input data and the expected output during training. These learned relationships can later be used to make predictions or decisions based on new sensor data. RNNs can learn complex relationships between the sensor data and the desired output, even if those relationships are nonlinear or challenging to model using other methods, ro of which is one advantage of using RNNs for sensor fusion. Another advantage is that RNNs can learn complex relationships between the sensor data and the desired output. RNNs can also be used to model uncertainty in the data, which can be helpful in circumstances in which the data is noisy or unreliable. RNNs can be used to model uncertainty in the data. RNNs are used for various tasks in sensor fusion, including speech -p recognition and time series prediction. It has been put to use in robotics, autonomous vehicles, and other re applications where it is necessary to integrate data from several different sensors. lP 2.23. Seasonal Autoregressive Moving Average ( SARMA ) A statistical model called SARIMA is utilized to make projections regarding time-series data that demon- na strates seasonality [69]. This model is a generalization of the moving average (MA) model and the autoregressive model (AR). The models are defined by three parameters: the autoregressive (AR) order, the moving ur average (MA) order, and the seasonal (S) order. These three orders are listed in the following order: • The number of lags of the time-series data that are used in the model is determined by the AR order. Jo 600 • The MA order determines the number of lags of the forecast error that are used in the model. • The S order determines the number of lags of the time-series data that are used to model the seasonal component of the data. These three orders are listed in the following order: AR, MA, and S. In most cases, this is accomplished through the utilization of an optimization algorithm such as gradient descent, which works to reduce the gap between the actual data and the data that was predicted. The SARIMA modeling approach is useful for various applications, including time-series prediction, trend analysis, and forecasting. They have been shown to be effective at predicting the future values of time-series data, which is particularly useful for modeling time-series data that exhibit seasonality. They are particularly useful for modeling time-series data that exhibit seasonality. For fault prediction in systems with time-series data that demonstrates seasonality, the SARIMA models have been put to use. The purpose of fault prediction is to determine the likelihood that a fault will occur in a given system based on information gathered from sensors and other sources. To make use 24 of SARIMA models for fault prediction, the model would first need to be trained on a dataset containing illustrative examples of sensor data and fault labels. The SARIMA model would learn the relationships between the sensor data and the presence or absence of a fault, taking into account the seasonality of the data while it was being trained. This would take place during the training process. After it has been trained, the SARIMA model can be used to make predictions about the likelihood of a fault occurring based on the data coming from new sensors. One of the benefits of using SARIMA models for fault prediction is that they can model the seasonality of the data. It can be helpful in situations involving systems where the probability of a fault occurring may change over time due to factors such as the season. It is also possible ro of to use SARIMA models to model uncertainty in the data, which can be helpful in circumstances where the data are noisy or unreliable. 2.24. Similarity Based Modeling ( SBM ) -p Modeling that is based on similarities between data points and uses similarity measures to compare the re data points and draw conclusions or make predictions as a result of those similarities is called similaritybased modeling [69]. A similarity measure is used in similarity-based modeling to compute the degree of lP similarity between two sets of data points based on the features or characteristics that each group of data points possesses. Any function that can quantify the degree to which the data points are similar can be na used as the similarity measure. It is helpful for tasks in which the relationships between the variables in the data are complex or not well-defined, and the use of traditional statistical methods may not be effective. ur For sensor fusion, similarity-based modeling has been carried out here. It models a set of data points as the first data points collected, then extracts the features or characteristics of the data points. After that, the Jo similarity measure is used to compute the similarity between each pair of data points, and the similarities that are computed are then used to make forecasts or judgments based on the data. When the goal is to learn the underlying structure and then make predictions or decisions based on the learned configuration, similarity-based modeling can be used to accomplish both goals. 2.25. Self-Organizing Map Minimize Quantization Error (SOM-MQE) Unsupervised learning and data visualization are two of the most common applications for self-organizing maps (SOMs), a type of artificial neural network [99]. SOMs are trained by adjusting the weights of the connections between the units to minimize the quantization error that exists between the data that is input and the information that is output. SOMs are composed of a set of units or nodes that are arranged in a grid. SOMs can be utilized in the context of fault prediction to minimize the quantization error between the sensor data and the fault labels present in a dataset [102]. SOMs would need to be trained on a dataset containing examples of sensor data and fault labels to accomplish this goal. During the training process, the SOMs would make any necessary adjustments to the weights of the connections between the units to reduce 25 the quantization error between the sensor data and the fault labels [103]. After they have been trained, the SOMs can be used to make predictions about the likelihood of a fault occurring based on the data coming from new sensors. SOMs can learn to represent the relationships between the sensor data and the fault labels in a compact and informative manner, which can help identify patterns or features in the data relevant to the task. This is one of the advantages of using SOMs for fault prediction. Another advantage is that SOMs can learn to represent the relationships between the sensor data and the fault labels. SOMs can also be used to visualize the data in a low-dimensional space, which can help understand the data’s structure and identify patterns or trends in the data. SOMs can also be used to visualize the data in a high-dimensional ro of space [104, 105, 106]. 2.26. Support Vector Machines (SVMs) SVMs are especially beneficial for use in situations where the relationships between the variables in -p the data are difficult to understand or poorly defined, and traditional statistical methods may need to re be more productive. As a result of their resistance to noise and ability to process high-dimensional data, they are ideally suited for a wide variety of machine-learning applications [107, 108]. To train a support lP vector machine (SVM), one must first collect a dataset and then extract the features or characteristics of the individual data points. The SVM algorithm is then applied to locate the hyperplane that separates the na various classes of data points to the greatest extent possible. It has been demonstrated that the applications, which include classification, regression, and the detection of outliers, are practical for learning complex ur relationships in the data and making accurate predictions or decisions based on the data [109, 110]. Support vector machines, also known as SVMs, are machine learning algorithms that can be utilized for Jo fault prediction in settings where the data can be separated into distinct classes or categories. The purpose of fault prediction is to determine how likely a fault will occur in a given system based on information gathered from sensors and other sources. The SVM model would need to be trained on a dataset containing examples of sensor data and fault labels for it to be used for fault prediction. During training, the SVM algorithm would locate the hyperplane in the high-dimensional space defined by the sensor data that maximally separates the different classes of data points (fault and no-fault). After the SVM model has been trained, it can be used to predict the likelihood of a fault occurring based on new sensor data by determining which class the data belongs to and which category it belongs to. The fault prediction is that they can learn complicated relationships between the data and the state, even if the connections are nonlinear or challenging to model using other methods. This is a significant advantage over other methods [111]. In addition, SVMs are resistant to noise and can process high-dimensional data, both of which are essential characteristics for many fault prediction tasks. 26 2.27. Vibration-based Condition Monitoring ( VCM ) Vibration-based condition monitoring offers several distinct benefits compared to more conventional condition monitoring methods. One of the benefits is that it is possible to carry it out continuously, which makes it possible to monitor the system’s state [112, 113, 114]. It is conducive for systems that are essential or operate continuously because it enables the early detection of problems and timely implementation of corrective action [61]. It is particularly useful for systems that are essential or run continuously [115]. Condition monitoring based on vibrations has several benefits, one of which is that it is non-invasive and only moderately expensive. Compared to other condition monitoring methods, it does not require the ro of system to be disassembled or shut down to perform the analysis, which can result in significant time and financial savings. Because vibration-based condition monitoring is relatively simple to implement and can be automated, it is ideally suited for various industrial and commercial uses [116, 117]. Manufacturing, aerospace, and energy are the application areas, and some of the tasks that can be performed with this -p technology include fault diagnosis, performance optimization, and maintenance planning. re The fault diagnosis method, known as vibration-based condition monitoring, is utilized frequently in various industries, including the manufacturing, aerospace, and energy industries. The purpose of fault lP diagnosis is to determine the nature of the problem or malfunction within a system by analyzing data obtained from sensors and other sources [97, 118, 119]. In performing vibration-based condition monitoring na on a system to diagnose problems, sensors are typically installed to measure the vibrations produced by the system. The vibration data is then analyzed using methods such as spectral analysis or time-frequency ur analysis to identify patterns or trends in the data that may indicate changes in the system’s condition. These patterns or trends can be used to determine whether or not there has been a change in the system’s Jo state. After that, you can use the patterns or trends you found to diagnose the issue and figure out what the primary source of the issue is. Vibration-based condition monitoring is an excellent choice for fault diagnosis because it can be carried out continuously, enabling real-time monitoring of the system’s condition. It can make vibration-based state monitoring an ideal choice. To carry out the analysis, the system does not need to be taken apart or powered down, which means that it is relatively inexpensive and non-intrusive [120, 121]. Vibration-based condition monitoring is an essential component of Industry 4.0, the fourth industrial revolution. It is used to monitor the health and performance of machinery and equipment in real-time by utilizing sensors and advanced analytics to detect changes in the vibrations produced by the system. Industry 4.0 is intended to monitor the health and performance of machinery and equipment. This enables the early detection of problems and timely corrective action, improving the system’s efficiency and reliability. Moreover, early detection of the issues allows for timely disciplinary action. The optimization of maintenance schedules and the planning of maintenance activities are both areas in which vibration-based condition monitoring plays a role in Industry 4.0. It is possible to determine when maintenance will likely be required by analyzing the data on vibration and then planning the maintenance activities following that 27 prediction. It could improve the system’s overall efficiency and reduce the time it is down. Overall, vibration-based condition monitoring is an essential component of Industry 4.0 because it enables the continuous monitoring of the health and performance of machinery and equipment, which can improve the efficiency and reliability of the system. In addition, this type of monitoring reduces the risk of human error and enhances safety. In addition, it is an essential tool for fault diagnosis in many industries. It is because it enables the early detection of problems and the timely adoption of corrective action, which can improve the system’s efficiency and reliability. ro of Table 3: Brief description of existing sensor fusion, prognostics and application areas Descriptive Approach, Ref Objective GRBMs [87] Deep statistical feature learning from vibration -p measurements Operational data GMM, SOM-MQE and PCA [99] CMS data and SCADA variables Model-based reasoning [122] Sensor fusion of real-time monitoring of system sta- re KDE and SVMs [111] lP tus ANNs [62] Operational parameters, environmental conditions na and the electric energy consumption of the alternator ur SBM [69] Operational parameters and environmental condi- tions Vibration and current signals GRNN, BPNN and ANFIS [50] Operating parameters (static data) fused with as- Jo Rule-based fuzzy semantic inference [63] set (i.e. current, load and temperature) and fault conditions Interval-valued fuzzy reasoning [65] Combination of the system performance decay PCA and kNN [100] Multiple sensors fusion at accelerometer and load cell data 3. Implementation challenges in sensor fusion Sensor fusion involves combining data from multiple sensors to provide a more accurate and comprehensive understanding of the environment or system being monitored. There are several challenges that can 28 arise when implementing sensor fusion: 1. Data alignment: Sensors may not be perfectly synchronized, which can make it difficult to accurately align the data from different sensors. 2. Data uncertainty: Sensors have varying levels of accuracy and precision, which can make it difficult to combine the data in a meaningful way. 3. Sensor failure: Sensors can fail or malfunction, which can impact the accuracy of the sensor fusion system. 4. Complexity: Sensor fusion systems can be complex, especially when sensors are different in nature ro of and have multiple sources of data. This can make it difficult to design and implement the system effectively. 5. Real-time performance: In many applications, sensor fusion systems must operate in real-time, which -p can be challenging due to the computational demands of processing and combining data from multiple sensors. re 6. Limited resources: Sensor fusion systems often require significant computational resources, which can lP be a challenge in resource-constrained environments. 7. Data privacy: In some cases, sensor fusion systems may involve processing sensitive data, which can na present challenges in terms of data privacy and security. 3.1. Future Scope and Research Directions ur In this section, we elaborate on the future scope and potential research directions in the field of machine Jo learning-based sensor fusion techniques in the Industrial Internet of Things (IIoT). We aim to provide extensive discussions and insights into the emerging trends and opportunities for further advancements in this domain. 1. Emerging Trends in Machine Learning-based Sensor Fusion: The field of machine learning-based sensor fusion in the IIoT is witnessing rapid advancements and evolving trends. To gain a comprehensive understanding of the recent developments, The study conducted a statistical analysis of the literature in this area. We collected relevant data from reputable academic databases, conference proceedings, and scholarly publications to identify key trends and patterns. Our analysis revealed a significant increase in the number of publications and research activities related to machine learning-based sensor fusion techniques in recent years. This growth signifies the growing interest and recognition of the potential of these techniques in addressing the challenges of data integration and advanced analytics in IIoT systems. 2. Research Focus Areas: Based on our analysis, the study identified several research focus areas that are gaining prominence in the field: 29 • Enhanced Sensor Integration: Researchers are exploring novel approaches to effectively integrate data from diverse sensors in IIoT systems. This includes techniques such as deep learning-based fusion, probabilistic modeling, and information fusion algorithms to improve the accuracy and reliability of sensor data fusion. • Edge Computing and Distributed Fusion: With the proliferation of edge computing in IIoT, there is a growing interest in developing distributed sensor fusion algorithms that can leverage edge devices’ computational capabilities. This enables real-time processing, reduces latency, and enhances the scalability of sensor fusion in large-scale IIoT deployments. ro of • Security and Privacy: As IIoT systems handle sensitive data, ensuring robust security and privacy measures is crucial. Researchers are focusing on developing secure and privacy-preserving sensor fusion techniques that protect sensitive information while maintaining the accuracy and integrity -p of fused data. 3. Emerging Topics: Our analysis also revealed several emerging topics that show promising potential for re future research in machine learning-based sensor fusion in IIoT: lP • Federated Learning: The application of federated learning techniques in sensor fusion allows collaborative model training across distributed sensors while preserving data privacy. This emerging ments. na area holds great promise for improving sensor fusion performance in decentralized IIoT environ- ur • Explainable AI: With the increasing complexity of machine learning models, there is a growing Jo need for explainable AI techniques in sensor fusion. Researchers are exploring methods to provide interpretable fusion results, enabling stakeholders to understand the decision-making process and build trust in the system. • Adaptive Fusion Algorithms: As IIoT systems operate in dynamic environments, adaptive fusion algorithms that can dynamically adjust their parameters and fusion strategies based on changing conditions are gaining attention. This area presents opportunities for developing self-learning and self-adapting fusion algorithms that can optimize performance under varying scenarios. 4. Open Questions and Future Directions: While significant progress has been made in machine learningbased sensor fusion techniques in the IIoT, several open questions and research directions remain. This 800 study discuss some intriguing open questions that require further investigation: • Standardization and Interoperability: Developing standardized frameworks and protocols for sensor fusion in IIoT systems is essential to ensure interoperability and seamless integration across different platforms and devices. 30 • Resource-Constrained Environments: Exploring efficient fusion techniques that can operate in resource-constrained environments, such as low-power sensors or networks with limited computational capabilities, is an important area for future research. • Real-time Adaptive Fusion: Investigating real-time adaptive fusion algorithms that can dynamically adjust fusion strategies and parameters based on changing environmental conditions and system requirements is crucial for achieving optimal performance in 4. Conclusion ro of In this survey paper, we have provided an extensive overview of machine learning-based sensor fusion techniques in the context of the Industrial Internet of Things (IIoT). The IIoT has emerged as a transformative force in industrial settings, connecting a diverse range of intelligent devices and enabling more efficient -p production processes. Sensor fusion plays a crucial role in harnessing the potential of IIoT by integrating re data from various sensors, enabling predictive analysis, fault detection, and other critical functions. Our survey has explored a wide spectrum of sensor fusion techniques, including machine learning models such lP as Artificial Neural Networks (ANNs), Back Propagation Neural Networks (BPNN), and Gaussian Mixture Models (GMM), among others. We have discussed the application of these techniques in fault prediction, na adaptive monitoring, and quality control in industrial processes. Furthermore, we have highlighted the challenges and complexities associated with implementing sensor fusion in IIoT systems, emphasizing the need ur for robust methodologies and scalable solutions. This study present a set of research directions and trends in data-driven industrial prediction. As IIoT continues to evolve, addressing these challenges will be crucial Jo for realizing its full potential in industrial applications. In conclusion, this survey paper provides valuable insights into the state-of-the-art sensor fusion techniques in IIoT and their applications. We hope that this comprehensive overview will serve as a valuable resource for researchers, practitioners, and decision-makers in the field of industrial automation and contribute to the continued advancement of IIoT technologies. 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Wang, Research on software architecture of prognostics and health management system for civil aircraft, in: International Conference on Sensing, Diagnostics, Prognostics, and Control (SDPC, p. 510–513. 37 CONFLICT OF INTEREST Date: 29.12.2022 Attention: ManagingEditor journal - Measurement: Sensors – Elsevier 2665-9174 Dear editor/manager, In conjunction with the submission of the unpublished original manuscript Titled -p ro o f “Paper Title: A survey on machine learning based sensor fusion techniques in Industrial Internet of Things “. sent via the online system of Evise for your consideration, peer-review and possible publication in the journal - Measurement: Sensors – Elsevier ISSN: 2665-9174 lP Sincerely re I clearly state that I have the consent from all contributing authors to submit the manuscript and all of them accept complete responsibility for its contents. None of the authors declare any conflicts of interest. List of all contributing authors (full names in regular order): Jo ur na Dr Deepak sharmaa, Anuj kumarb, Dr. Nitin Tyagic, Dr. Sunil S. Chavand, Syam Machinathu Parambil Gangadharane “I hereby confirm and electronically sign this agreement form on behalf of all above listed contributing authors.” Authors Signature Dr. Sunil S. Chavan Smt.Indira Gandhi College of engineering.Navi Mumbai. Affiliation: University of Mumbai Email id: chavan_s_s@hotmail.com Orcid id:0000-0002-5913-6960