Abstract
Introduction
With rapid advancements in technology, sensor nodes (SNs) are becoming smaller while consuming less energy. They are capable of connecting to one another in an ad hoc manner forming a single unit referred to as wireless sensor network (WSN). 1 Often the nodes are dispersed either randomly or uniformly over an area under observation for the purpose of monitoring and analysis. Notably, the network is cost-effective with minimal human intervention for many environmental and habitat monitoring applications, for instance, animal and bird habitat tracking,2,3 agriculture and industrial land monitoring,4,5 smart irrigation,6,7 flood control and monitoring, 8 underwater and space exploration exploration,9,10 environmental pollution, 11 and health monitoring.12,13
In many of these aforementioned applications, information coming from within the WSN is only useful when the referred location is known, for instance, during search and rescue, target localization, monitoring, and so on. More specifically, target tracking in WSN estimates the location and direction of the target using the information received by SNs. That is, the SNs gather the target signatures and forward it to the cluster head (CH).14,15 Once the information is received at the CH, it is processed to predict target location and forwarded to the base station (BS). In all, the steps involved are target detection and tracking, tracking information forwarding, and target location prediction. The resulting system providing an accurate location of the target (indoor and outdoor) opens up new applications for crisis and commercial services, for example, rescue operation, 16 forest fire monitoring, 17 location known vehicle, 18 shipment tracking, 19 navigation,20,21 health monitoring, 22 home automation,23,24 and so on. In the end, making daily life easier in terms of safety and efficiency.
One important concept emerging from these advancements is the industrial Internet of things (IIoT) that refer to a network of connected devices, sensors, and actuators forwarding data in the network without human interaction.25,26 This real-time data forwarding for analysis and decision making is considered the main facet of efficient industrial automation. But it all starts with target tracking, a vital step in any IIoT application. Over the years, researchers have proposed multiple tracking frameworks for efficient scheduling of industrial equipment, effective data gathering, real-time data processing, target localization, and so on (Table 1).
Comparison of different techniques.
H: highway path; M: Manhattan; R: random; RFID: radio-frequency identification; RW: random waypoint; S: spiral.
Problem statement
Evidently, target tracking is inaccurate when done using a fewer number of SNs. But it becomes even more challenging when tracked frequently for maximum accuracy with scarce resources. Traditional schemes are centralized, compute-intensive, and complex; thus, inapplicable to WSN scenario due to limited processing capability and power constraint of SNs. The motivation of this study is to develop a fast and precise tracking algorithm that is self-sustained in terms of communication and processing. The main contributions of this article are as follows:
Propose a cooperative approach integrating cross-grid and region-overlapping scheme for indoor industrial environments with the goal to improve target localization and tracking accuracy.
Design a technique to select reliable nodes based on the distance between nodes within-cluster and to the target for reduced positioning error. Furthermore, a cluster node is dynamically selected based on distance from the BS.
Evaluate the proposed approach in terms of positioning and tracking error. Three most commonly used mobility models are used to benchmark the proposed work. Furthermore, the proposed work is compared with the conventional trilateration technique.
The rest of this article is organized as follows. Related work is presented in section “Related work.” System model is formulated in section “System model.” Section “Design and implementation” details the proposed algorithm design and implementation. The framework evaluation is covered in section “Performance evaluation” followed by discussion in section “Discussion.” Finally, the conclusion is presented in section “Conclusion.”
Related work
This section covers the existing contribution for target tracking. In this section, we have categorized the existing techniques in terms of clustering and non-clustering. However, a work on IIoT is added under separate headings.
Clustering-based target tracking
Over the years, researchers have proposed multiple techniques to improve the efficiency of WSNs. Among these techniques network clustering is widely used. It involves the initiation of cluster topology to divide the network into clusters and responsibilities involving the definition of nodes per cluster, coverage area, CH selection, and so on. The techniques are further categorized into static and dynamic clustering. In static clustering, the whole network is divided into clusters as soon as network become active, for instance, a trilateration-based static clustering scheme proposed in Khan et al. 27 Here, each SN calculates its distance to the target using a received signal strength indicator (RSSI) and forwards it to the CH. Using this information, the CH predicts the target location. The mechanism fails to cater to target path changes with no recovery mechanism. To overcome this, in Rouhani and Haghighat, 31 a prediction-based target tracking scheme comprising three steps, clustering, tracking, and prediction, are proposed. In the first step, the network is divided into static clusters with CHs elected based on residual energy or distance to the BS. Next, a trilateration-based mechanism is used to track the target followed by a linear prediction (LP) model to predict the target’s next location. Only the SNs close to the predicted location remain active for the next sensing iteration.
In contrast, dynamic clustering forms clusters only when the necessity arises or on the onset of some triggered event. A relevant work in Zhang et al. 28 proposed an adaptive consensus-based distributed filter (ACDF) scheme to track linearly moving targets. The scheme dynamically performs hierarchical clustering to calculate target location using the odometry motion model. Here, the CH is selected based on residual energy followed by cluster formation. The SNs within the cluster forward target location to the CH, which uses Kalman filtering (KF) to calculate the target location. This is followed by the addition and removal of SNs from the cluster based on the moving target. Since only SNs within the cluster gets activated, this results in prolonged WSN lifetime. However, the scheme fails to cater to random target path changes with no recovery mechanisms. Generally, in such scenarios, a more centralized approach 29 is used. That is, in addition to the CH responsible for the aggregation of target location information from the SNs, it forwards this aggregated information to the BS. The BS maintains a target failure probability (TFP) against each CH, in case one is lost, the node with the smallest TFP is selected as the new CH.
A dynamic clustering-based energy efficient target tracking, called Improved Prediction-based Adaptive-Head (IPAH), algorithm has been proposed in Darabkh et al. 30 They couple prediction method with the proposed scheme to get better results. IPAH efficiently selects CH and efficiently tracks the target.
In summary, clustering makes the network flexible and scalable. It provides the topology for accurate target tracking while ensuring efficient bandwidth utilization and data transmission rate.
Non-clustering-based target tracking
In non-clustering-based network topology, each SN contributes equally in network development and deployment. The topology is set to achieve best effort delivery augmented with probability-based target tracking techniques to reduce duplicate packet flow and data flooding. A relevant study in Luo et al. 32 proposes a cooperative localization and tracking algorithm (CLTA) for an indoor environment. The algorithm is divided into two phases: offline and online. In the offline phase, the network is divided into grid-based known and/or reliable SNs. These nodes subsequently take part in target localization. During the online phase, the target location is calculated using a centroid weighting algorithm for finding overlapping regions. However, the technique fails at grid borders. Moreover, the overall network energy depletes much faster because the SNs are active all the time irrespective of the target location.
In Bhowmik and Giri, 33 a tree-based localization scheme is proposed, referred to as convoy tree-based fuzzy target tracking (CTFTT). The scheme incorporates fuzzy sensing model (FSM) with RSSI to track the target. Once an SN detects the target in its vicinity, it gets connected to the tree and starts following the target within a monitoring area. The scheme results in maximal coverage with farther SNs remaining asleep and, hence, saving energy. Nonetheless, in dense networks, the overall energy dissipation is higher with SNs joining and leaving the tree frequently. Moreover, in the case of a fast-moving target, the scheme fails to locate the target accurately. There exist more robust techniques similar to one proposed in Yu 35 that integrates the concept of data association to determine the most suitable set of SNs for target localization.
Even though, non-clustering-based schemes require no topology maintenance, they are easy to scale out with low packet overhead. On the contrary, clustering-based schemes incur more packet overhead during the CH selection. Thereafter, the schemes provide accurate target localization and better network lifetime.
Target tracking in IIoT
With the advancement of IIoT, the communication between operators and equipment referred to as cobots has improved. With robust techniques for tracking, real-time data forwarding makes it a cornerstone for industrial automation. Traditionally, the communication between operators is directly linked to the assembly line, a slow process. Moreover, the lack of tracking affects production, especially in assembly lines manufacturing multiple goods.
Over the years, many solutions are developed for the purpose of tracking on the factory floor. One such attempt is OmniTrack, 36 an orientation-aware radio-frequency identification (RFID) tracking solution. The solution is based on signal phase changes to predict the target location and direction. The specialized and standalone hardware has its limitations; for instance, it only works unless the perpendicular distance from the antennas to the target location is known. Similar RFID-based localization system 37 locates and tracks the target in a complex environment. It adopts a double tag for RFID localization to minimize the negative effect of propagation and eradicate the phase ambiguity. The result is greater accuracy when localizing the target.
More sophisticated mechanisms, like one in Chiani et al., 38 implement a non-cooperative target tracking framework for an indoor environment using impulse radio (IR) and ultra-wideband (UWB) technology. A radar sensor network (RSN) coupled with signal processing steps is used to achieve high localization accuracy. Though the RSN with no prior knowledge of target location and/or its trajectory generates a high number of false alarms. This can be improved by distinguishing between true and false alarms, that is, using a ghost mitigation algorithm or a naive sample wise detector with a constant threshold to improve network performance. Moreover, the framework incorporates cardinalized probability hypothesis density (CPHD) filter to further improve target detection and tracking accuracy. However, this adds up to additional computational complexity, affecting the overall network lifetime.
More recent cooperative techniques 39 are developed to work with heterogeneous networks. Here, some of the nodes (RFIDs, cameras, and proximity sensors) are used to achieve high accuracy of target tracking, while other passive nodes are used to process, aggregate, and forward the information. In addition, some prediction mechanisms can be incorporated to predict the target next location. Undoubtedly, modern industrial services heavily rely on IIoT to locate and track workers and industrial equipment on the factory floor. There is a need to develop frameworks capable of accurate localization and tracking to ensure improved industrial productivity and safety. All this while ensuring a favorable balance between target localization accuracy and network energy depletion.
System model
Consider a set of SNs
It is worthwhile to mention that tracking a moving target using deployed sensors can become inefficient due to the limited power available. In many of the existing techniques either the SN energy depletes rapidly or it compromises the tracking accuracy. There is a need for a robust tracking scheme that maintains a balance between efficiency and accuracy.
Design and implementation
CH selection for IIoT environment
The CH selection is important for efficient management of data flow in IIoT environment. Here, we have proposed an adaptive CH selection mechanism. The method selects the CH at runtime as all the sensors are not active, that is to save energy. Once the SNs
The CH selection is a two-phase process. During the initialization phase, the SN detects the target in its range. All the SNs

Flowchart of the proposed dynamic clustering-based target tracking scheme.
Real-time Target Tracking in IIoT
In IIoT, sensors are deployed in a specific region to track the movement of cobots and equipment as shown in Figure 2. Due to limited battery power, it is unfeasible to keep the SNs in surveillance mode all the time. This results in rapid energy depletion. Thus, to optimize the use of energy, we propose a dynamic clustering-based multilateral target tracking framework. The framework allows the SNs to change their states from active to sleep and vice versa. This change in state is managed dynamically through context sensing. Each node at a random point in time becomes active to sense its surrounding, in the case of no anomalies, it goes back to sleep state. This mechanism is used to initially detect the target of

Typical sensors network topology.
CH processing
The active nodes
Performance evaluation
The proposed algorithm is designed to improve the accuracy and reliability of a mobile target in the IIoT environment. The simulation environment consists of the uniformly distributed SNs in an industrial environment within the area of 100 × 100 m2. To verify the efficiency and accuracy of our proposed scheme, we simulate the target tracking using two approaches: regular interval and complete path. First, we evaluate the tracking accuracy of our proposed scheme under a different number of rounds (time steps). Second, we simulate the proposed scheme for complete path tracking of a cobot or equipment in an industrial environment. To do this, we simulate three different tracks separately and compare the results. The target tracks are randomly generated using normal distribution represented as
where
Furthermore, the probability density function (PDF) of the target
where
The target distance
where
For evaluation, we compare both schemes in terms of average error per meter denoted as
where
Regular interval
In this scenario, the target has been tracked after regular interval of time
We evaluate the proposed algorithm with varying number of rounds to identify the target locations after every round. For all the experiments, the round interval is set to 5 s. The comparison is performed with the trilateration technique proposed in Khan et al. 27 We plot the output of target tracking simulation time against root mean square error (RMSE) generated in tracking to observe the behavior of the proposed scheme under regular interval as well as complete path tracking. Furthermore, we plot the output as a cumulative distribution function (CDF) against tracking RMSE, to get the probability of tracking error in the proposed and the trilateration technique.
Figure 3 shows the tracking RMSE error overtime with varying round intervals when tracking a target. Note that the proposed scheme localizes the target after regular intervals referred to as rounds. We observe that the proposed scheme performs better localization of the target overtime compared to trilateration. This is due to the fact that trilateration is limited to positioning information from only the first three sensing nodes, not the case in the proposed dynamic clustering scheme with more information for reliable target tracking. Moreover, the average target localization errors at different rounds are summarized in Table 2. The results show that the proposed scheme predicts the location of the target node with better accuracy. Moreover, with the increasing number of moving steps, the proposed scheme outperforms the trilateration technique in terms of positioning accuracy of tracking.

Localization positioning error overtime in terms of RMSE (in meters) for the proposed technique compared to trilateration with varying round intervals (5, 10, and 15 s).
Comparison of average localization positioning error in terms of RMSE (in meters).
RMSE: root mean square error.
In Figure 4, we plot the CDF of the tracking error measured as RMSE to evaluate the performance of the proposed scheme against the trilateration scheme. We observe that the overall performance of the proposed scheme is better than trilateration. Moreover, we test this observation using the Kolmogorov–Smirnov test with the hypothesis that CDF of trilateration lies below that of CDF (proposed) in the case of round interval 5 s. The probability that CDF of trilateration is worse than CDF of proposed is 33% (

Cumulative distribution function plot for localization positioning error (RMSE in meters) of the proposed technique compared to trilateration with varying round intervals (5, 10, and 15 s) from left to right.
Localization positioning error CDF in terms of RMSE in meters.
RMSE: root mean square error; CDF: cumulative distribution function.
The behavior of the proposed technique is further tested in different sensor density areas like

Box plots for localization positioning error in terms of RMSE (in meters) of the proposed technique compared to trilateration with varying sensor densities (8 × 8, 16 × 16, and 32 × 32) for different round intervals (5, 10, and 15 s for subplots from top to bottom).
Complete path
In an industrial environment, with the lack of prior information, unpredictable movement of the target, and inefficient tracking systems can affect the overall automation system. Therefore, three different target mobility models are selected based on their path complexity level and commonly used in WSN. The selected models can give a better comparison between the proposed and conventional trilateration models. In this scenario, each SN senses the target in its range. If the target is within the range they form a cluster
The results of target tracking under different mobility models are shown in Figure 6. The simulation results show that the proposed scheme provided 22% performance improvement under the spiral model, whereas 17% and 18% performance is achieved in random walk and Manhattan models, respectively. The performance gain is achieved in comparison with the base trilateration scheme. Table 4 summarizes the results of the proposed scheme against the base scheme.

Target tracking for three different target mobility models.
Comparison of average tracking error in terms of RMSE (in meters) for different mobility models.
RMSE: root mean square error.
Discussion
IIoT has a strategic value for smart factories soon. It improves the overall efficiency of internal operations while reducing the overall operational cost and also reflects in the production of low cost and affordable products. Therefore, the use of smart equipment, especially cobots on factory floors, informing about their status has become an important selling point for smart factories. It not only enables the different equipment on the factory floor to communicate in real-time with one another but also improves the overall manufacturing process with more control. For instance, failure of equipment may not get reported on time, affecting the overall performance of other systems. Such failures are important to detect and track at the earliest.
No doubt tracking of such faulty equipment in a real-time environment is challenging for industrial applications where accuracy and run-time performance are important factors. In this article, we presented a robust target/fault tracking scheme that is capable of overcoming some of the problems faced in industrial applications. The solution can be deployed in areas where human access is difficult like rotatory machinery, unstrapped vehicles, steel melting units, and radiation units. Therefore, we proposed this solution to support real-time monitoring and fault detection.
Nevertheless, the proposed scheme can also be used for rescue operations. In such situations, the trustworthiness of the received information makes the difference between life and death. For instance, when workers are trapped in underground units due to earthquake, flood, explosion, or any other natural disaster. The proposed scheme can pinpoint the precise location of the workers through effective deployment of Internet of things (IoT) resources.
Conclusion
In this article, we propose a cooperative multilateral sensing scheme for target tracking in an industrial environment. We demonstrate that the proposed scheme improves the accuracy and reliability of target tracking. A simulation with regular interval tracking and complete path tracking has been carried out to analyze the proposed scheme. Our results show that the proposed scheme improves the accuracy and reliability of target tracking, especially in an indoor industrial environment. In the future, we would like to implement and investigate our scheme for an outdoor environment. Such environments span over a vast area with challenges of coverage and network lifetime. Moreover, an extension for seamless integration of indoor and outdoor environments will help to predict and track the target more accurately and precisely. In IIoT environment, CH selection should be based on the distance between SNs and the BS. This becomes challenging when different factors like energy, reliability of link, and distance between competing SNs affect the performance. Nevertheless, in the future, we would also like to implement and extend the scheme while maintaining a balance between these factors.
