The Role of AI, Machine Learning, and Big Data in Digital Twinning: A Systematic Literature Review, Challenges, and Opportunities

Abstract

Digital twinning is one of the top ten technology trends in the last couple of years, due to its high applicability in the industrial sector. The integration of big data analytics and artificial intelligence/machine learning (AI-ML) techniques with digital twinning, further enriches its significance and research potential with new opportunities and unique challenges. To date, a number of scientific models have been designed and implemented related to this evolving topic. However, there is no systematic review of digital twinning, particularly focusing on the role of AI-ML and big data, to guide the academia and industry towards future developments. Therefore, this article emphasizes the role of big data and AI-ML in the creation of digital twins (DTs) or DT-based systems for various industrial applications, by highlighting the current state-of-the-art deployments. We performed a systematic review on top of multidisciplinary electronic bibliographic databases, in addition to existing patents in the field. Also, we identified development-tools that can facilitate various levels of the digital twinning. Further, we designed a big data driven and AI-enriched reference architecture that leads developers to a complete DT-enabled system. Finally, we highlighted the research potential of AI-ML for digital twinning by unveiling challenges and current opportunities.

Full text

Received January 19, 2021, accepted February 1, 2021, date of publication February 22, 2021, date of current version March 2, 2021.
Digital Object Identifier 10.1109/ACCESS.2021.3060863
The Role of AI, Machine Learning, and Big Data in
Digital Twinning: A Systematic Literature Review,
Challenges, and Opportunities
M. MAZHAR RATHORE 1, (Member, IEEE), SYED ATTIQUE SHAH2, DHIRENDRA SHUKLA3,
ELMAHDI BENTAFAT1, AND SPIRIDON BAKIRAS 1, (Member, IEEE)
1Division of Information and Computing Technology, College of Science and Engineering, Hamad Bin Khalifa University, Doha, Qatar
2Data Systems Group, Institute of Computer Science, University of Tartu, 51009 Tartu, Estonia
3Dr. J. Herbert Smith Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
Corresponding author: Spiridon Bakiras (sbakiras@hbku.edu.qa)
ABSTRACT Digital twinning is one of the top ten technology trends in the last couple of years, due to its high
applicability in the industrial sector. The integration of big data analytics and artificial intelligence/machine
learning (AI-ML) techniques with digital twinning, further enriches its significance and research potential
with new opportunities and unique challenges. To date, a number of scientific models have been designed
and implemented related to this evolving topic. However, there is no systematic review of digital twinning,
particularly focusing on the role of AI-ML and big data, to guide the academia and industry towards future
developments. Therefore, this article emphasizes the role of big data and AI-ML in the creation of digital
twins (DTs) or DT-based systems for various industrial applications, by highlighting the current state-of-
the-art deployments. We performed a systematic review on top of multidisciplinary electronic bibliographic
databases, in addition to existing patents in the field. Also, we identified development-tools that can facilitate
various levels of the digital twinning. Further, we designed a big data driven and AI-enriched reference
architecture that leads developers to a complete DT-enabled system. Finally, we highlighted the research
potential of AI-ML for digital twinning by unveiling challenges and current opportunities.
INDEX TERMS Digital twin, artificial intelligence, machine learning, big data, industry 4.0.
I. INTRODUCTION
Digital twinning is a process that involves the creation of a
virtual model (i.e., a twin) of any physical object, in order
to streamline, optimize, and maintain the underlying physi-
cal process. Theoretically, the digital twin concept was first
presented in 2002 by Grieves et al. [1] during a special
meeting on product life-cycle management at the University
of Michigan Lurie Engineering Center. In his subsequent
article [2], he further defined digital twinning as a combi-
nation of three primary components: 1) a virtual twin; 2) a
corresponding physical twin (a physical object that can be
a product, a system, a model, or any other component such
as, a robot, a car, a power turbine, a human, a hospital, etc.);
and 3) a data flow cycle that feeds data from a physical twin
to its virtual twin and takes back the information and pro-
cesses from the virtual twin to the physical twin. The virtual
The associate editor coordinating the review of this manuscript and
approving it for publication was Claudio Zunino .
twin is nothing but an algorithm that replicates the behavior
(fully or partially) of the corresponding physical counterpart,
by generating the same output as does the physical object on
given input values. Mostly, it is considered as part of the smart
manufacturing process, but it can be used in any domain, such
as construction, education, business, transport, power and
electronics, human and healthcare, sports, and networking
and communications.
Digital twinning was first adopted by Tuegel et al. [3]
in 2011 to digitally reproduce the structural behavior of an
aircraft. Initially, digital twinning was used as a mainte-
nance tool to continuously monitor the craft’s structure. Then,
it was replicated as a complete twin in order to simulate
its entire life-cycle and predict its performance [3]. Later,
digital twinning started gaining popularity in several indus-
tries that aimed at making their processes smarter, intelligent,
and optimally dynamic, based on the operating conditions.
The technology raises its global demand, as it facilitates in
finding the product flaws, reducing production cost, real-time
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M. M. Rathore et al.: Role of AI, Machine Learning, and Big Data in Digital Twinning: A SLR, Challenges, and Opportunities
monitoring of resources, and increasing the life of the product
by predicting product failure. On this account, digital twin-
ning became one of the top-ten technology trends [4].
Several surveys have been published, highlighting the cur-
rent research trends of digital twinning in various fields.
For instance, Wanasinghe et al. [5] pointed out the state-of-
the-art works of digital twinning in the oil and gas indus-
tries. Lu et al. [6] and Cimino et al. [7] reviewed the
current reference models, applications, and research issues
in manufacturing. Qi and Tao [8] emphasized on the role of
data and digital twinning in achieving smart manufacturing.
DT-related patents are discussed by Tao et al. [9] in different
industries. And, the modeling perspective of digital twinning
is explored by Rasheed et al. [10].
Recently, the use of IoT, big data, and AI-ML technologies
have brought new potentials in digital twinning. The adoption
of these techniques ensures a perfect digital twin and intro-
duces new research challenges and opportunities. Since 2015,
several digital twins have been developed in various indus-
tries using AI-ML and big data analytics, and the number of
related research articles is growing rapidly. Despite the grow-
ing popularity, adaptability, and applicability of AI-enabled
digital twinning in the industrial sector, exploited by IoT
and big data technologies, no systematic review has been
performed that explicitly focuses on the role of these tech-
nologies in digital twinning. The above-mentioned surveys
do not fully cover the importance of these technologies in the
DT domain. Therefore, there is an exigency of a systematic
approach towards the thorough review of the current develop-
ments in AI-enabled digital twinning using IoT technology
and big data. This can drive both academia and industry
towards further research, by highlighting the current findings,
future potentials, challenges, and applications of AI-enabled
digital twinning in the industrial sector.
In this article, we carried out a systematic literature review
that incorporates all the research work in the form of articles,
patents, and web-reports, covering digital twinning and its
integration with state-of-the-art AI-ML and big data analytics
techniques. We highlighted the role of big data, AI, machine
learning, and IoT technologies in the process of digital twin
creation, by listing examples from current deployments in
various industrial domains. We introduced the digital twin
paradigm, by explaining its basic concepts and highlighting
its applications in several industrial areas. After a thorough
literature survey, we identified 1) tools that can be used for
digital twin creation; 2) the criteria for successful digital twin-
ning; and 3) research opportunities and challenges in digital
twinning for diverse industrial sectors. Finally, we designed
a reference model for digital twinning that exploits IoT, big
data, and AI-ML approaches.
The rest of the paper is organized as follows. Section II
briefly presents the survey methodology. Section III formally
defines digital twinning, its creation method, and other basic
concepts. Section IV summarizes the application of digital
twinning in various industries. Section Vbriefly describes
big data and AI, while Section VI discusses the relationship
between IoT, big data, AI, and digital twinning. Section VII
summarizes the role of AI in digital twinning with state-
of-the-art research developments. Section VIII outlines the
important data-driven patents in digital twinning. Section IX
presents the evaluation criteria for an ideal digital twin-
ning, and Section Xlists the tools that may be required in
the process of digital twinning. The design details of the
reference architecture for AI-enabled DT creation is pre-
sented in Section XI, while the current research opportunities
and research challenges in digital twinning are described in
Section XII. The article is concluded in Section XIII.
II. METHODOLOGY
To the best of our knowledge, the survey at hand is the
first of its kind in terms of reviewing AI-ML and big data
analytics techniques for digital twinning. The systematic lit-
erature review (SLR) carried out in this study is based on
the guidelines recommended by [11], [12], with the aim of
summarizing the current literature and establishing the basis
for qualitative synthesis and information extraction. SLR is
an organized, efficient, and widely recognized method that
is comparatively better than the traditional literature review
process [13].
We identified the following six research questions that
directed our entire review process:
1) What is digital twinning, how does it work, and what
are the standards and technologies to create a digital
twin (DT)?
2) What is the relationship between AI-ML, big data, IoT,
and digital twinning?
3) What is the role of AI-ML and big data analytics in
digital twinning, its related applications, and current
deployments in different industrial sectors?
4) What are the tools required for the creation of
AI-enabled DT?
5) What is the criteria for a successful DT or DT-based
system?
6) What are the main challenges, market opportunities,
and future directions in digital twinning?
To capture the wide range of digital twinning applica-
tions, we searched eight multidisciplinary electronic biblio-
graphic databases, including 1) IEEE Xplore (IEEE, IET);
2) ACM digital library; 3) Scopus (ScienceDirect, Else-
vier); 4) SpringerLink (Springer); 5) Hindawi; 6) IGI-Global;
7) Taylor & Francis online; and 8) Wiley online library.
We also searched the US patents database. Using suitable
search strings is crucial to extracting the appropriate liter-
ature from the electronic bibliographic databases. Due to
the diverse nature of this study, we used a set of appro-
priate keywords that assures the inclusion of AI-ML and
big data analytics in industrial digital twinning. Specifically,
as shown in Table 1, we defined various keywords, combined
with logical operators, to search the electronic bibliographic
databases.
The search was carried out just before August 2020. Prior
to 2015, we found very few papers on digital twinning.
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TABLE 1. Search strings.
FIGURE 1. Number of journal papers published by different libraries.
In 2017, the topic gained popularity and became one of the
top 10 trends in strategic technology [14]–[16]. In the period
2015–2020, more than 2000 Scopus-indexed journal articles,
more than 1000 patents, 250 book chapters, and 20 books
have been published, discussing digital twinning technology.
However, we identified over 850 articles that match the search
criteria defined in Table 1. Fig. 1and 2show the total
number of journal and conference papers published on the
topic of digital twinning by the different libraries. Among
other publishers, IGI-Global published seven articles, Hin-
dawi published three articles, and ACM published only two
articles in their journals. Additionally, Fig. 3illustrates the pie
chart of published articles related to various applications of
DT (it includes both conference and journal papers). Clearly,
manufacturing is the dominant application area for digital
twinning.
Considering the aforementioned research questions,
we defined a set of inclusion and exclusion criteria for an
article as follows:
1) The study is written in English.
2) The study is published in a scientific journal, magazine,
book, book chapter, conference, or workshop.
3) The journal article is included only if the journal’s
impact factor is >1.0.
4) The conference article is included only if the confer-
ence is mature enough (it has already published at least
fifteen versions of its proceedings).
5) Publications such as dissertations, in-progress research,
guest editorials, poster sessions, and blogs are
excluded.
6) Duplicate papers that appear in several electronic
databases will only be considered once.
7) The study is excluded if not fully focusing on the digital
twinning concept or any of its specified applications.
FIGURE 2. Number of conference papers published by different libraries.
FIGURE 3. AI-ML driven digital twinning research statistics in different
fields.
Among the 850 articles that matched the designated key-
words, a total of 213 papers were selected after applying
the above inclusion and exclusion criteria. IEEE ACCESS
and Elsevier Journal of Manufacturing Systems are the top
two journals that have published the most articles within the
set criteria. The selected publications were first evaluated on
the basis of their titles and abstracts. The concept of digital
twinning in relation to the research questions was critically
examined, and a total of 63 papers were excluded in this
phase. Some paper-abstracts were not clear enough to be
directly evaluated, hence a full-text screening was performed
on 150 papers, resulting in the exclusion of 52 additional
papers. Snowball sampling was performed on the remaining
set of 98 papers. Then, we used the references and citations of
the selected papers to perform backward and forward search,
respectively, for identifying new potential papers.
Finally, a total of 117 papers concerning digital twin-
ning, its applications, and related technologies, were selected
for data extraction and synthesis of this study. Among the
117 articles, 61 articles discussed AI-ML based digital twins.
For each selected article, metadata forms were maintained
to categorize the information about the articles and to note
the observations assessed. The extracted metadata was then
coded for analysis, according to the year of publication,
authors’ names, affiliated universities or organizations, key-
words, name of journal or conference, research model, area
of focus, data source, and opportunities/issues highlighted.
The categories were derived according to the data needed to
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answer the research questions and for identifying the paper’s
main research areas. In addition to journal and conference
articles, we included 20 US patents, 15 technical web-reports,
and 5 standards, focusing on digital twinning. Some other
articles that indirectly relate to digital twinning, such as sup-
porting tools, technologies, and survey methodologies, are
also referred in our study.
III. DIGITAL TWIN: INTRODUCTION AND BACKGROUND
A. DIGITAL TWIN: DEFI NITIONS AND CONCEPT
Researchers define digital twins in several ways. The pioneers
of digital twinning, Grieves and Vickers [17], define a digital
twin as ‘‘a set of virtual information constructs that fully
describes a potential or actual physical manufactured product
from the micro atomic level to the macro geometrical level.
At its optimum, any information that could be obtained from
inspecting a physical manufactured product can be obtained
from its Digital Twin.’’ In their opinion, the digital twin can
be any of the following three types: 1) digital twin prototype
(DTP); 2) digital twin instance (DTI); and 3) digital twin
aggregate (DTA). A DTP is a constructed digital model of
an object that has not yet been created in the physical world,
e.g., 3D modeling of a component. The primary purpose of a
DTP is to build an ideal product, covering all the important
requirements of the physical world. On the other hand, a DTI
is a virtual twin of an already existing object, focusing on only
one of its aspects. Finally, a DTA is an aggregate of multiple
DTIs that may be an exact digital copy of the physical twin.
For example, the digital twins of a spacecraft structure and a
spacecraft engine are considered DTIs that may be aggregated
into a DTA.
In this article, we assume the concepts of DTI and DTA
when referring to a DT. Note that, the majority of academic
scholars and industries follow similar definitions for a digital
twin. For instance, Glaessgen and Stargel [18] defined it from
the perspective of vehicles as ‘‘A digital twin is an inte-
grated multiphysics, multiscale, probabilistic simulation of
an as-built vehicle or system that uses the best available physi-
cal models, sensor updates, fleet history, etc., to mirror the life
of its corresponding flying twin.’’ Similarly, Tao et al. [19]
considered the aspect of product life cycle and interpreted
the digital twin as ‘‘a real mapping of all components in the
product life cycle using physical data, virtual data and inter-
action data between them.’’ Söderberg et al. [20] focused on
the application of optimization while defining a digital twin.
According to them, digital twinning is an approach to perform
a real-time optimization to a physical system using its digital
copy. Finally, Bacchiega [21] made it simpler by defining it
as ‘‘a real-time digital replica of a physical device.’’
With our understanding, shown in Fig. 4, digital twinning
is a process that involves the construction of 1) a cyber
twin that digitally projects a living or non-living physical
entity or a process (a system); and 2) a physical connection
between cyber and physical twins to share data (and informa-
tion) between them aimed at dynamic optimization, real-time
monitoring, fault diagnostics and early prediction, or health
FIGURE 4. Digital twinning concept.
monitoring of the physical counterpart. A physical twin can
be a process, a human, a place, a device, or any other object
with a special purpose, and which is able to be replicated in
the digital world as either a partial twin with limited function-
alities, or a complete twin that incorporates the full behavior
of its physical peer. Digital twinning is mostly employed
in industries for physical objects in their units. However,
there exist some digital twins that are mirrors of processes
in the physical world, such as digital twins of a mobile-edge
computing (MEC) system [22], human protein–protein inter-
action (PPI) [23], supply chain [24], components-assembly at
a manufacturing unit, and job scheduling [25].
B. A BR IEF HISTORY OF DIGITAL TWINS
The idea of creating a digital copy of a physical entity was
introduced in the early 2000s. However, the term ‘‘digital
twin’’ originated around ten years ago. Michael Grieves,
in one of his articles [2], claimed that the concept of dig-
ital twins was first presented during a lecture on product
life-cycle management (PLM) in 2003. Whereas, in his other
book chapter [1], he stated that the concept was originally
proposed, without a name, in 2002 while presenting a paper
in a special meeting at the University of Michigan Lurie
Engineering Center. Grieves mentioned in this book chapter,
‘‘While the name has changed over time, the concept and
model has remained the same.’’ He added that it was given
the name ‘‘mirrored spaces model (MSM)’’ in 2005 and
changed to ‘‘information mirroring model’’ in 2006. NASA
started using this concept of virtual and physical models in
their technology roadmaps [26] and proposals for sustain-
able space exploration [27] since 2010. However, the name
‘‘digital twin’’ was first coined in 2011 by John Vickers
of NASA. Practically, the first digital twin was developed
by Tuegel et al. [3] for the next-generation fighter aircraft,
in order to predict its structural life.
C. OPER ATIONAL MECHANISM
Although the digital twin concept was introduced in 2002,
it became a popular trend due to the advancement in sensor
technology and IoT, which play a vital role in digital twinning
by collecting real-time data from the physical world and
sharing it with the digital world. The twinning can be viewed
as a bridge between a physical twin and the corresponding
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virtual twin. The physical-to-virtual connection is established
with a technology that allows the transfer of information from
the physical environment to its virtual twin, including web
services, cellular technology, WiFi, etc. The virtual twin is
adjusted gradually with the functioning of the physical twin
by continuously collecting the differences between the two
environments. These connections allow the monitoring of
responses to both conditions and interventions. The condi-
tions mainly occur in the physical environment, whereas the
interventions take place within the virtual twin. Thus, a digital
twin holds a real-time status of the physical counterpart.
The virtual-to-physical connections represent the
information circulating from the virtual to the physical envi-
ronment. This information may change the state of the physi-
cal twin by displaying some data or changing the system’s
parameters (for optimization, diagnostics, or prognostics).
Although virtual-to-physical connections are very helpful in
DT modeling, they are not always included in the description.
Instead, it is common to consider a one-way connection,
i.e., physical-to-virtual. Finally, the data and the information
from both physical and virtual worlds are stored and analyzed
at a centralized server—or a cloud computing platform—
where the final decisions related to optimization, diagnostics,
or prognostics, are made.
D. DIGITAL TWIN STANDARDS
Currently, there is no particular standard that solely focuses
on the technical aspects of digital twinning. Standardiza-
tion efforts are under-development by the joint advisory
group (JAG) of ISO and IEC on emerging technologies [28].
However, the ISO standard ISO/DIS 23247-1 [29] is the
only standard that offers limited information on digital twins.
In addition, there are other related standards that may facil-
itate DT creation. For example, the ISO 10303 STEP stan-
dard [30], the ISO 13399 standard [31], and the OPC unified
architecture (OPC UA) [32] technically describe ways to
share data between systems in a manufacturing environment.
IV. DIGITAL TWINNING IN INDUSTRIES: APPLICATIONS
Digital twinning is becoming apparent in various industries,
including manufacturing, medical, transportation, business,
education, and many more. In this section, we present the role
of digital twinning and the current research followed in these
areas.
A. MANUFACTURING
Digital twinning is conceived as a major tool in the man-
ufacturing industry to carry out smart manufacturing, fault
diagnosis, robotic assembly, quality monitoring, job shop
scheduling, and meticulousness management. In this way,
Rosen et al. [33] emphasizes the use of digital twinning in
manufacturing. Modules in a computerized system communi-
cate with each other during every step of the production, thus
depicting a realistic model of its physical counterpart. Simi-
larly, the work by Qi and Tao [8] explains the benefits of big
data-driven DT in smart manufacturing. The DT combines
all the manufacturing processes, starting from product design
to maintenance and repair. The virtual model is capable of
identifying the constraints of the virtual design in the physical
world, which are iteratively improved by the designers. Data
produced by sensors and IoT devices are then analyzed and
processed using big data analytics and AI applications to
enable the manufacturers to select a satisfactory plan.
On the other hand, DT is also used to monitor a component
or a product, considering its usage, health, and performance
during the life-cycle of manufacturing. Real-time data pro-
vided to the virtual model allows it to self-update and predict
any abnormal behaviors. Optimal solutions are developed for
problems found in the virtual models, and the actual manufac-
turing model is adjusted accordingly. Maintenance and repair
of the physical system can also be scheduled timely, based on
the predictions of the virtual models. One of such digital twin
projects is originated by Slovak University of Technology in
Bratislava [34] for a physical production line of pneumatic
cylinders, where they defined the continuous optimization of
production processes and performed proactive maintenance,
based on the real-time monitoring data. Similarly, a digital
twin of manufacturing execution system (MES) was devel-
oped by Negri et al. [35] that enables the supervisory con-
trol over the physical MES system using sensor technology,
by allowing the multi-directional communication between
digital and physical sides of manufacturing assets.
Several state-of-the-art works highlight that DTs should
be capable of self-healing and predictions. These predictions
play a vital role in an important aspect of smart manufactur-
ing, i.e., fault diagnosis, since a minor issue during production
can cause irreparable damages. A variety of technologies
used in fault diagnosis like Support Vector Machines [36],
Bayesian Networks [37], Deep Learning [38]–[40], and many
others [41]–[44] are capable of enhanced fault diagnosis.
However, Xu et al. [45] highlight that, in production sys-
tems, conditions are constantly changing. Therefore, the same
training model cannot be applied throughout the process, but
creating a new model requires a lot of time and resources.
As such, they proposed a digital twin-assisted fault diagno-
sis using deep transfer learning (DFDD) approach. DFDD
has been applied to fault diagnosis in smart and complex
manufacturing. The framework involves two phases. In the
first phase, the virtual model of the system is constructed.
Repeated designs of the model are tested and evaluated in the
virtual space until all anomalies are discovered. Simulation
data during design testing is provided to an embedded fault
diagnosis model in the virtual space. The diagnosis model
keeps learning from the simulation data using Deep Neu-
ral Networks, in order to increase its efficiency for failure
prediction during the start of the production phase when
there is insufficient training data. The second phase starts
when the virtual model achieves acceptable performance. The
physical entity is constructed and linked to its corresponding
virtual model. Data is transferred from a physical entity to the
virtual model through sensors during production. A diagnosis
model is formed and updated using the current data from
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the physical entity and the knowledge learned from the
previous phase, which is transferred using deep transfer
learning (DTL).
Robotic assembly, in industrial manufacturing, is responsi-
ble for handling a notable amount of work [46]. It is involved
in packaging, labeling, painting, welding, and many others.
With the advancements in the complexity of manufactur-
ing, these robotic assemblies have become more error-prone.
The concept of DT is being utilized in this area to monitor
and optimize the assembly process. In [47], a multisource
model-driven digital twin system (MSDTS) is designed for
robotic assembly. The MSDTS model consists of three parts.
The physical space consists of sensors, its associated data,
and the robotic arm for moving and gripping objects. The
virtual space consists of a server, a multisource model, and a
virtual reality display and control (VRDC). A communication
interface offers the exchange of data between two spaces in
real-time. Initially, a 3D model of the entire physical space
is constructed using a depth sensor that is mounted on the
robot arm. During operation, the VRDC provides a complete
view of the physical system by receiving a video stream from
an RGB camera. When the robot arm moves, angular data is
sent to the virtual twin through the communication interface
in real-time, and the graphical model in the virtual system
follows the same trajectory. The physical contact of the robot
arm with the surrounding object is simulated in the virtual
system using the Kelvin-Voigt model (KVM), where param-
eters of the model are estimated through the data of contact
force and relative motion of contact point. A surface-based
deformation algorithm is used to simulate the deformation
of an object using the data generated by KVM. The results
of the models are rendered in the VRDC. A complete view
of the system is provided to the operator via a head mount.
Interaction with the physical space is done using a control
handle.
Another important element in manufacturing is job shop
scheduling, which makes efficient use of resources to
reduce production time and maximize production effi-
ciency. In real-life situations, due to errors and anomalies,
the scheduling process can be rendered inefficient. With the
introduction of smart manufacturing and digital twins, new
DT-based job shop scheduling methods are introduced to
overcome scheduling plan deviation and provide a timely
response. One such model is proposed in [48]. A DT-based
job shop consists of a physical and a virtual space, which
communicate through a CPS. Scheduling data from the phys-
ical space is sent to the virtual space, and multiple scheduling
strategies are simulated and retrieved from the virtual models.
The finalized scheduling plan is fed into the physical space.
Since a physical system has many modules, the plan is divided
and categorized based on the respective modules. Continuous
communication between the physical and virtual space results
in achieving precise scheduling parameters, as well as pre-
diction of any disturbances in the schedule. The scheduling
plan can hence be updated and fed to the physical system for
increased efficiency and timely response.
Digital twin and big data are playing an important role
in smart manufacturing starting from product life-cycle to
maintenance and repair. Some of the stated research articles
highlighted the importance of digital twinning in the areas
of smart manufacturing. The concept of utilizing a variety of
data and integrating it with IoT, virtual reality, and data ana-
lytics, results in high fidelity monitoring, timely prediction
and diagnosis of faults in assembly or production, and overall
optimization and improvement of the manufacturing process.
B. MEDICAL
Applications of DT in medical include the maintenance of
medical devices and their performance optimization. DT,
along with AI applications, are also used to optimize the
life-cycle of hospitals by transforming a large amount of
patient data into useful information. The ultimate aim of the
digital twinning in healthcare is to help authorities in man-
aging and coordinating patients. Mater private hospitals in
Dublin (for cardiology and radiology) were facing problems
regarding increased services, patient demand, deteriorating
equipment, deficiency of beds, increased waiting time, and
queues. These problems indicated the call for the improve-
ment in the current infrastructure to cater to increasing
needs. Mater private hospitals (MPH) partnered with Siemens
Healthineers to develop an AI-based virtual model of their
radiology department and its operations [49]. As a result,
the simulations of the model provided insights towards the
optimization of workflows and layouts. The realistic 3D mod-
els of the radiology department, provided by DT techniques,
allowed for the prediction of operational scenarios and the
evaluation of the best possible alternatives to transform care
delivery.
In recent years, with the introduction of ‘‘precision
medicine,’’ the focus of DT technology is shifted towards a
human DT. Precision medicine is the branch of healthcare
that promotes tailored treatments on an individual level. The
human DT would be linked to its physical twin and would
display the processes inside the human body. It can result
in an easier and accurate prediction of illness with proper
context, and bring a paradigm shift in the way patients are
treated. Virtual physiological human (VPH) was the earliest
human DT that was developed [50]. VPHs would act as a
‘‘Virtual Human Laboratory’’ where each VPH was modified
based on the specific patient, and different treatments would
be tested on the modified VPH platform.
Apart from human DTs, organs or human body parts digital
twins have also been developed. Data from Fitbit devices,
smartphones, and IoT devices are sent in real-time to such
DTs, in order to provide constant feedback regarding human
organ activity. Some organs’ DTs have been used by experts
to perform clinical analysis, whereas many others are under
development. In a study, a 3D digital twin of a heart was
developed by Siemens Healthineers [51], after performing
a comprehensive research on approximately 250 million
images, functional reports, and data. The model exhibited
the physical and electrical structure of a human heart. This
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DT is currently under research at the Heidelberg university
hospital (HUH), Germany, where DTs of 100 patients have
been created, who had a history of heart diseases within a
period of six years. Simulations of these DTs were compared
with the ground truth, which provided promising results.
Another DT of the heart has been developed by researchers
at the Multimedia Communications Research Lab in Ottawa,
Canada. It is called a Cardio Twin and targets the detection
of ischemic heart disease (IHD) [52]. IHD is a condition
characterized by reduced blood flow to the heart, which can
lead to chest pain or mortality in case of delayed treatment.
The researchers developed the DT on the concept of edge
computing/analytics, where the time is considered very criti-
cal. Data is collected from social networks, sensors, and med-
ical records. The accumulated data is fed to an AI-inference
engine, where data fusion, formatting, and analytics are per-
formed using TensorFlow Lite to discover new information.
The Cardio Twin can communicate with the physical twin in
the real world, using a multimodal interaction component that
employs WiFi/4G or Bluetooth communication. Cardio Twin
performed a sample classification of 13420 ECG segments
with an accuracy of 85.77%, in a short span of 4.84 seconds.
However, no method to evaluate Cardio Twin in the real world
has been introduced.
Sim&Cure, a company based in Montpellier, France,
developed a simulation model for the treatment of aneurysm.
Aneurysm is an outward bulging of blood vessels, typically
caused by an abnormally weakened vessel wall. A serious
case of aneurysm can result in clotting, strokes, or death.
The last option for treating aneurysm is surgery. However,
endovascular repair (EVAR) is generally used, since it is
less invasive and low-risk. In EVAR, a stent-graft/catheter
is placed into the affected area to minimize the pressure.
In many cases, choosing the stent-graft/catheter is difficult
and depends on the size of the blood vessels. The Sim&Cure’s
DT helps surgeons in selecting an ideal implant to cater to the
size of the aneurysm as well as the blood vessels. A 3D model
of the affected area and surrounding vessels is created, and
multiple simulations are run on the personalized DT, which
allows surgeons to have a better picture. Promising results
have been presented in preliminary trials [53], [54].
Researchers at the Oklahoma State University developed a
human airway DT—named ‘‘virtual human’’—in their com-
putational biofluidics and biomechanics laboratory (CBBL).
They tracked the flow of air particles in aerosol-delivered
chemotherapy and found out that, the aerosol-based drug
hit the cancerous cells with less than 25% accuracy [55].
This caused more harm than benefits to patients, as the
remaining drug would fall on healthy tissue. The version
1.0 of ‘‘virtual human’’ was based on a 47-year-old standing
male, containing the entire respiratory system. V1.0 also
allowed patient-specific structural modifications, e.g., creat-
ing a respiratory system of a standing/seated female or a kid
with/without respiratory conditions. Following the success of
V1.0, CBBL researchers developed its successor version 2.0.
The V2.0 was patient-specific, and was created by performing
an MRI/CT scan of the patient. The scanned data was used
to construct a 3D model of the lungs. The researchers at
CBBL then created a virtual population group (VPG), which
was a large group of human DTs. The VPG exhibited trends
within different groups/sub-groups. Simulations to analyze
the trends of aerosol particle movement were conducted on
the VPG, by varying the particle sizes, inhalation rate, and
initial position of the medication. These simulations indicated
that the drug’s effectiveness would increase to 90% if the drug
delivery method was personalized to each patient, rather than
distributing the drug evenly for every patient [55].
In another study, Liu et al. [56] proposed a cloud-based
DT healthcare solution (CloudDTH) for elderly people. The
cloud-based solution provides a fusion of physical and virtual
systems to address real-time interaction between patients
and medical institutions, and personalized healthcare for the
entire life-cycle of the elderly. CloudDTH has a layered archi-
tecture, providing health resources, identification of medical
personnel, user interface, virtualization, and security services
to users. CloudDTH obtains real-time data from sensors for
ECG, BP, pulse rate, and body temperature. These sensors
are already implemented in the CloudDTH framework. The
sensor data are then transmitted to the cloud server, using
TCP. In case of an incident, such as patient falling, heart
attack, stroke, etc., the monitoring model, after performing
analysis on the received data, sends a high-frequency and
multi-attribute monitoring order of the patient to medical
personnel. A case study was performed by researchers, where
data from two patients with normal and abnormal heart rates
was input to the system. The simulation results indicated
symptoms of arrhythmia in one patient, and recommended
the dosage of medication based on their physical conditions.
The CloudDTH framework simulations also provided a fea-
sible scheduling mechanism for elderly patients in hospitals,
in order to avoid long queues.
C. TRANSPORTATION
Numerous innovative technologies have been brought for-
ward with the development of IoT, including digital twins,
autonomous things, immersive technology, etc. Various types
of digital twins are developed in transportation sector, includ-
ing DTs for automobile components, vehicles, vehicular
networks, and road infrastructures. However, the purpose
remains the same i.e., monitoring, optimization, and prognos-
tics and health management. For example, Wang et al. [57]
developed a framework for connected vehicles based on
digital twins. The framework used vehicle-to-cloud (V2C)
communication to provide advisory speed assistance (ADSA)
to the driver. Real-time data from sensors was obtained in
the physical system, which was sent to the cloud through
the V2C module. All processing of the data from V2C was
performed on the cloud server. The computed results were
sent back to the physical system and served as a guidance sys-
tem for components within the physical world. The authors
demonstrated the effectiveness of their framework with a case
study of cooperative ramp merging involving three passenger
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vehicles, and the results showed that the digital twin can
indeed assist transportation systems.
Cioroaica et al. [58] worked on the context of connected
vehicles in smart ecosystems. The establishment and achieve-
ment of goals in smart ecosystems are possible when smart
entities within the ecosystem co-operate with each other.
This is achieved when the systems have a level of trust.
The authors developed a virtual hardware-in-the-loop (vHiL)
testbed model to evaluate the trust-building capability of
smart systems within an ecosystem. A smart agent, capa-
ble of interacting with the vehicle’s electronic control unit
(ECU), is installed at the vehicle along with its corresponding
DT. In Phase 1, the trustworthiness of the smart agent is
evaluated by simulation in its corresponding virtual twin.
Phase 2 involves trust-building, where the smart agent is
executed on the ECU. Evaluation of simulated and actual
results identifies the obstacles. These obstacles are overcome
by the collaboration of virtual and physical entities to achieve
trustworthiness in a smart ecosystem.
Chen et al. [59] studied the use of unmanned aerial vehi-
cles (UAVs) as complementary computation resources in a
mobile edge computational (MEC) environment for mobile
users (MU). MEC provides computational capabilities to
MUs within a radio access network (RAN). Mobile users send
computational tasks to UAVs by creating the corresponding
VMs. The tasks arriving at the UAVs are stored in queues and,
due to limited resources, the MUs have to compete for them.
The authors proposed deep reinforcement learning (DRL)
techniques for the scheduling of tasks on the UAV, and for
minimizing the response delay from the UAV to the MUs.
The training of the DRL network in an offline manner is
achieved by creating a digital twin of the entire MEC system.
Simulations with varying parameters were conducted and the
best results were selected. The results of the DRL scheme
trained on digital twins ensured significant performance gains
when compared to other baseline approaches.
Digital twins have also been utilized in transportation sys-
tems for traffic congestion management, congestion predic-
tion, and avoidance. Kumar et al. [60] worked on intelligent
transport systems, leveraging technologies such as fog/edge
analytics, digital twins, machine learning, data lakes, and
blockchain. The authors captured situational information
from cameras, and performed edge analytics on the acquired
data. An entire virtual vehicle model was created via a dig-
ital twin to replicate the real-world scenario. Driver inten-
tions were predicted using machine and deep learning algo-
rithms to avoid traffic congestion. This virtual vehicle model
allowed autonomous vehicles to make decisions regarding
optimal paths, but also helped drivers of non-autonomous
vehicles to make better decisions based on the traffic situation
and the mined driver intentions.
Digital twins have also been used for the maintenance
of different systems. The work implemented by Venkate-
san et al. [61] monitored and projected the health conditions
of electric motor vehicles using an intelligent digital twin
(i-DT). The framework tracked the health of the electric
motor in an electric vehicle using fuzzy logic and artificial
neural networks (ANNs). The average speed of the vehicle
and the duration of travel was fed into the ANN i-DT and
fuzzy logic i-DT for training purposes. In addition, simula-
tions carried out on a digital twin tested the performance of
the entire framework. Parameters such as winding and casing
temperature, deterioration in magnetic flux, and lubricant
refill time were set for the digital twin. The comparison of
theoretical and i-DT computations indicated that an i-DT can
effectively be used in electric vehicles to foresee their health.
D. EDUCATION
Another important area where digital twins can play a crucial
role, is education. Digital twins of physical entities such
as labs, construction, mechanical equipment, can be created
and provided to students for online learning. However, there
has not been a lot of research effort on the use of DT in
the education domain. One such work was performed by
Sepasgozar [62] that used digital twins and virtual gaming
for online education. The authors created a digital twin of
an excavator along with a virtual game for the course of
construction management and engineering. The project con-
tained four modules named 1) group wiki project and role
play (GWiP); 2) interactive construction tour 360 (ICRT 360);
3) virtual tunnel boring machine (VTBM); and 4) piling
augmented reality and digital twin (PAR-DT). GWiP was
used for doing group projects online. ICRT 360 consisted of
recorded videos to provide details on construction sites and
machinery. VTBM was a virtual game-based environment
that helped students to learn about the working of a tunnel
boring machine. Virtual equipment was introduced in the
game, where a student or a group of students could explore
their interests. PAR was developed for smartphones and Ocu-
lus headsets to provide students an augmented environment
to collaborate and understand the importance of piling in
construction. The final module involved a digital twin of
an excavator, which was also linked to a physical instance.
The DT provided hands-on learning about the functions and
movements of an excavator. The students’ feedback empha-
sized the importance of an immersive environment in online
education.
E. BUSINESS
Business is also one of the areas where DT is playing an
important role. According to PropTechNL [63], the real estate
sector is fragmented in terms of architects, installation, con-
struction, transport, and management. This fragmentation
results in an inefficient system that has a negative impact
on people living in a society. Digital twins can provide huge
opportunities in real state, and facilitate the creation of smart
societies. For example, a wide range of sensors can collect
data, and the performance of a building can be measured and
improved. Digital twins in real estate may add significant
value by re-positioning buildings to the needs and require-
ments of customers, hence improving the customer experi-
ence. The design of buildings, the usage, effectiveness, and
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strength of raw materials, as well as maintenance and running
costs, can be managed through digital twins. Thus, it provides
a cost-effective, fast, and smart way of developing a real
estate. For instance, an American multinational company,
GE Healthcare, has incorporated the use of DT to redesign
its systems, in order to run new hospitals more efficiently.
Kampker et al. [64] introduced a framework for the devel-
opment of successful business models in smart services. The
scenario of crop (potato) harvesting was taken into consider-
ation during their research. In traditional harvesting mecha-
nisms, the harvesting machines are set up based on historical
data and the experiences of individual operators. However,
the lack of standard procedures may cause damage to the
crop. Therefore, the authors developed a framework, based
on a digital twin, to reduce the damage to the crop during
harvesting. Specifically, a digital twin is set up near the phys-
ical field. The virtual model then passes through the same
stages as the real crop. During the simulation, the condition
of the neighboring crop is analyzed for potential damage.
The results of the analysis lead to adjusting the parameters,
and repeated simulations continue until the optimal settings
are found. Tests carried out by the authors indicated that
more damage to the crop is caused by its impact on multiple
conveyor belts during the transition. Hence, adjustment to
the height and position of conveyor belts can reduce the
risk of damage. This framework can also tweak the settings
of autonomous harvesting machines, apart from providing
recommendations to operators.
F. OTHER INDUSTRIES
Digital twinning can be a part of smart construction, where
a DT may be designed for buildings, roads, or any other
infrastructure development. For example, a virtual twin was
developed for office buildings [65] that manages the build-
ing’s life-cycle, by collecting data through sensors. Further-
more, DT technology may advance the disaster management
approaches in smart cities [66]. Possibly, the technology also
has a potential to protect industrial control systems and data
from cyber attacks. On this account, Dietz and Pernul [67]
proposed the use of digital twinning technology to identify
security threats that target industrial control systems (ICSs),
and rectify their effects. Theoretically, they focused on the
Stuxnet worm [68] that compromised the speed of centrifuge,
and Triton [69] that digitally invaded a petrochemical plant
in Saudi Arabia. Deitz et al., indicated in the Stuxnet exam-
ple that the outliers in the historical network traffic would
have detected a threat. Similarly, in the case of simulations,
the deviation of network traffic between the virtual and phys-
ical systems would have identified the attack.
V. AI-ML AND BIG DATA: AN INTRODUCTION
Big data remains one of the top research trends from last
few years. It is different from an ordinary data because of
its high volume, high velocity, and heterogeneous variety,
as interpreted in Fig. 5. Researchers named these character-
istics as ‘‘the 3Vs of big data,’’ i.e., volume, velocity, and
FIGURE 5. Big data definition.
variety. Later, two more Vs—value and veracity—were added
to the list. Thus, we refer to any data as big data, if it is
of significant size (volume), it is being produced at very
high-speed (velocity), and it is heterogeneous with structured,
semi-structured, or unstructured nature (variety). The worth
of big data analytics brings the fourth V (i.e., value) into
its characteristics, thus making it an asset to the organiza-
tion. Big data analytics is a process that analyzes big data
and converts it to valuable information, using state-of-the-art
mathematical, statistical, probabilistic, or artificial intelli-
gence models. However, the 3Vs of big data lead us to a new
world of challenges, including capturing, storing, sharing,
managing, processing, analyzing, and visualizing such high-
volume, high-velocity, and diverse variety of data. To this end,
various frameworks [70]–[73] have been designed to handle
big data for effective analytics in different applications.
Artificial intelligence (AI) is the digital replication of
three human cognitive skills: learning, reasoning, and self-
correction. Digital learning is a collection of rules, imple-
mented as a computer algorithm, which converts the real
historical data into actionable information. Digital reasoning
focuses on choosing the right rules to reach a desired goal.
Whereas, digital self-correction is the iterative process of
adopting the outcomes of learning and reasoning. Every AI
model follows this process to build a smart system, which
performs a task that normally requires human intelligence.
Most of the AI systems are driven by machine learning, deep
learning, data mining, or rule-based algorithms, where others
follow logic-based and knowledge-based methods. Nowa-
days, machine learning and deep learning are widely used AI
approaches.
It is often confusing to differentiate between artificial
intelligence, machine learning, and deep learning techniques.
Machine learning is an AI method, which searches for partic-
ular patterns in historical data to facilitate decision-making.
The more data we collect, the more accurate is the learning
process (reflects the value of big data). Machine learning
can be 1) supervised learning, which accepts data sets with
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labeled outputs in order to train a model for classification or
future predictions; 2) unsupervised learning, which works on
unlabeled data sets and is used for clustering or grouping; and
3) reinforcement learning, which accepts data records with no
labels but, after performing certain actions, it provides feed-
back to the AI system. Examples of supervised learning tech-
niques are regression, decision trees, support vector machines
(SVMs), naive Bayes classifiers, and random forests. Sim-
ilarly, K-means and hierarchical clustering, as well as mix-
ture models, are examples of unsupervised learning. Finally,
Monte Carlo learning and Q-learning fall under the reinforce-
ment learning category. On the other hand, deep learning is a
machine learning technique that is motivated by biological
neural networks with one or more hidden layers of digital
neurons. During the learning process, the historical data are
processed iteratively by different layers, making connections,
and constantly weighing the neuron inputs for optimal results.
In this article, we mainly focus on digital twin systems based
on machine learning.
VI. RELATIONSHIP BETWEEN IoT, BIG DATA, AI-ML,
AND DIGITAL TWINS
The emerging sensor technologies and IoT deployments in
industrial environments have paved the way for several inter-
esting applications, such as real-time monitoring of phys-
ical devices [74], indoor asset tracking [75], and outdoor
asset tracking [76]. IoT devices facilitate the real-time data
collection—that is necessary for the creation of a digital twin
of the physical component [77], [78]—and enable the opti-
mization [79] and maintenance [80] of the physical compo-
nent by linking the physical environment to its virtual image
(using sensors and actuators). Note that, the above-mentioned
IoT data is big in nature [81] (as explained in Section V),
so the big data analytics can play a key role in the develop-
ment of a successful digital twin. The reason is that indus-
trial processes are very complex, and identifying potential
issues in early stages is cumbersome, if we use traditional
techniques. On the other hand, such issues can easily be
extracted from the collected data, which brings efficiency
and intelligence into the industrial processes. However, han-
dling this enormous amount of data in the industrial and DT
domains requires advanced techniques, architectures, frame-
works, tools, and algorithms. For instance, Zhang et al. [82],
[83] proposed a big data processing framework for smart
manufacturing and maintenance in a DT environment.
Oftentimes, cloud computing is the best platform for pro-
cessing and analyzing big data [84]. Additionally, an intelli-
gent DT system can only be developed by applying advanced
AI techniques on the collected data. To this end, intelligence
is achieved by allowing the DT to detect (e.g., best pro-
cess strategy, best resource allocation, safety detection, fault
detection) [85], predict (e.g., health status and early main-
tenance) [80], [86], optimize (e.g., planning, process con-
trol, scheduler, assembly line) [87], [88], and take decisions
dynamically based on physical sensor data and/or virtual twin
data. In short, IoT is used to harvest big data from the physical
FIGURE 6. Relationship between IoT, big data, AI-ML, and digital twins.
environment. Later, the data is fed to an AI model for the
creation of a digital twin. Then, the developed DT can be
employed to optimize other processes in the industry. The
overall relationship among IoT, big data, AI, and digital twins
is presented in Fig. 6.
VII. CURRENT DEPLOYMENTS OF DIGITAL TWINS USING
BIG DATA AND MACHINE LEARNING
We have identified the primary sectors where DT-based sys-
tems are developed with the help of AI-ML techniques. In the
following sections, we discuss the current deployments in
these sectors, including smart manufacturing, prognostics
and health management (PHM), power and energy, automo-
tive and transport, healthcare, communication and networks,
smart cities, and others.
A. SMART MANUFACTURING
Smart manufacturing involves 1) the acquisition of data from
manufacturing cells through a variety of sensors; 2) the
management of the acquired data; and 3) the data exchange
between different devices and servers. In a DT environment,
the data is collected from a physical manufacturing cell and/or
its corresponding virtual cell. Such data can be further utilized
for manufacturing process optimization, efficient assembly
line, fault diagnosis, etc., using AI approaches. The AI-ML
based digital twinning process for smart manufacturing is
depicted in Fig. 7.
Manufacturing is the top industry where most digital twins
are being developed. Xia et al. [91] proposed a manufacturing
cell digital twin to optimize the dynamic scheduler for smart
manufacturing. An intelligent scheduler agent, called digital
engine, was developed and trained for optimization using
deep reinforcement learning algorithms (DRLs), such as nat-
ural deep Q-learning [101], double deep Q-learning [102],
and prioritized experience replay (PER) [103]. The underly-
ing features were captured from both the physical and virtual
environments of the cell by an open platform communica-
tions (OPC) server. The training of the DRL network was
done through a gradient descent process, which requires finite
learning iterations and is sufficiently intelligent, reliable, and
robust. The developed DT-based dynamic scheduler opti-
mizes the manufacturing process by accelerating the training,
testing, and validation of smart control systems. The system
was tested on a robot cell to optimally select the strategy
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FIGURE 7. DT-based smart manufacturing using big data analytics and
AI-ML.
for performing the lower level tasks that are necessary to
accomplish the higher level manufacturing goal.
Zhou et al. [79] performed a geometric optimization
of centrifugal impeller (CI) by collecting features, such
as meridional section (MS), straight generatrix vectors
(SGV), and set of streamlines (SSL), from both the physi-
cal and CAD-based digital model of the CI. However, with
the improvement in machinability, the DT-based geometric
optimization reduces the aerodynamic performance. Thus,
the best model for the CI is selected by training the deep
deterministic policy gradient (DDPG) reinforcement learn-
ing model [104] to iteratively select the fair geometry of
the CI-design with optimum values of machinability and
aerodynamic performance. For the DDPG algorithm, they
used two actor networks (online and target network) as the
strategy function πto control the agent-actions, and two critic
networks (online and target network) to evaluate these actions
and give rewards. The proposed DT-based optimization sped
up significantly the design and manufacturing of the impeller.
Similarly, Zhang et al. [95] also developed an impeller DT,
but for the purpose of manufacturing process planning. They
employed a knowledge reuse deep learning network (PKR-
Net) [105], which takes data from dynamic knowledge base,
views from 3D computer-aided impeller design (CAD), 2D
drawings, and process knowledge. The objective is to opti-
mize the theoretical processes and generate the best process
plan for product manufacturing, by considering both manu-
facturing time and monetary cost.
Furthermore, Lee et al. [106] designed a deep learning and
cyber-physical system based digital twinning (DTDL-CPS)
architecture for smart manufacturing, that can be used in shop
floor optimization, fault diagnosis, product design optimiza-
tion, and predictive maintenance. BDHDTPREMfg [84] is
a similar CPS-based big data-driven model for DT-enabled
re-manufacturing. Several other digital twins have been
developed in the manufacturing industry using AI approaches
that could not be fully discussed in this article. Rather,
Table 2summarizes these digital twins with respect to the
problem they solved (i.e., the application), the ML-approach
they used to solve the problem, and the DT use-case they
developed.
B. PROGNOSTICS AND H EALTH MANAGEMENT
The persistent use of a product degrades its performance over
time, which may lead to malfunctioning. Thus, prognostics
and health management (PHM) is very crucial in all indus-
tries. PHM process involves the prediction of the remaining
useful life of a product and the consistent monitoring of its
health. This is the second most important application of DT,
following smart manufacturing. Note that, several alternative
terms, such as ‘‘predictive modeling’’ [86], ‘‘structural life
prediction’’ [3], ‘‘remaining useful life’’, and ‘‘predict and
act’’ [107] have also been used in place of PHM. DT-based
PHM regularly monitors the physical equipment based on
the data generated by the equipment-sensors, performs diag-
nosis and prognosis operations on the data using big data
analytics and AI, and recommends design rules for immediate
maintenance. The process of DT-enabled PHM is depicted
in Fig. 7.
Tao et al. [108] developed a digital twin for a wind turbine
in a power plant, in order to monitor its health by perform-
ing gearbox prognosis and fault detection. The proposed
DT-driven PHM can be applied to any complex equipment in
harsh environments, such as aircraft, ships, and wind turbines.
The wind turbine DT is built based on various geometry
levels, physics, behavior, and rules. The DT can detect the
disturbances in the turbine environment, as well as potential
faults in itself and its designed model. The data is collected
from the DT model (both physical and digital) and is matched
against the thresholds for degradation detection. In addition,
past DT-data is used to train a single hidden layer neural
network for better prediction of gradual faults and detection
of its causes, using extreme learning machine (ELM) [109].
The abrupt fault in the turbine is detected by comparing the
data from the physical and virtual environments. Similarly,
to improve ship efficiency and avoid unnecessary mainte-
nance operations, a data-driven ship digital twin was devel-
oped by Coraddu et al. [110]. Their goal was to determine the
speed loss due to marine fouling. Multilayered-deep extreme
learning (DELM) [111] predicts the ship’s speed, based on the
features collected from on-board sensors, such as designed
and ground speed, draft, engine and shaft generator power,
wind speed, temperature, fuel consumption, etc. The expected
ship speed is compared with the measured speed to compute
the speed loss. Finally, robust linear regression is applied to
the speed loss information to determine whether the speed
loss is due to marine fouling.
Numerous other digital twins have been developed
for PHM of industrial components, including pho-
tovoltaic energy conversion unit [112], battery sys-
tem [113], vehicle motor [61], UAV [115], spacecraft [116],
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TABLE 2. State-of-the-art AI-ML developments in digital twinning for smart manufacturing.
aircraft [3], [118]–[120], gillnet [122], gearbox, aircraft-
turbofan engine, rotating shaft-bearing [121], etc. All these
systems are summarized in Table 3.
C. POWER AN D ENERGY
In the power and energy sector, most of the DTs are developed
in electronic systems, wind-power farms, cooling systems,
and fuel-related systems. The digital twin of an inverter
model [125] was developed by imitating the voltage con-
troller, the current control loop, and the controlled plant,
based on three distinct neural networks (NNs). Each of
the three NNs is trained on real data collected from the
physical model, where the back propagation (BP) algorithm
is deployed to tune, in real-time, the proportional–integral
(PI) controller. Also, Andryushkevich et al. [126] introduced
the digital twin of power-system using ontological model-
ing. The developed DT selects the optimal configuration
of the hybrid power supply system, by utilizing genetic
algorithms [127]. Likewise, a digital twin framework for
power grids was designed by Zhou et al. [128] to per-
form real-time analysis. Specifically, NN-based learning was
applied to predict the grid operational behavior for fast secu-
rity assessment, based on the voltage stability and oscillation
damping.
In addition, a DT for a dew-point cooler was devel-
oped [99] to improve its cooling performance, by optimizing
operational and design parameters, including cooling capac-
ity, coefficient of performance (COP), dew point efficiency,
wet-bulb efficiency, supply air temperature, and surface area.
The DT of the cooler is developed with feed-forward neural
networks (FFNNs), and digitally mimics the cooler’s behav-
ior by utilizing the air characteristics (i.e., temperature, rel-
ative humidity) as well as the main operational and design
parameters (i.e., air velocity, air fraction, HMX height, chan-
nel gap) as inputs to the FFNN. Later, the DT-collected data
are supplied to a genetic algorithm (GA) for multi-objective
evolutionary optimization, in order to maximize cooling,
COP, and wet-bulb efficiency, and minimize the surface area
within four diverse climates (i.e., tropical rainforest, arid,
Mediterranean hot summer, and hot summer continental cli-
mates).
Apart from design and performance optimizations,
ML-based PHM is accomplished for power and energy
related components with the use of DTs, such as wind-
turbine, [108], electric vehicle motor [61], photovoltaic sys-
tems [112], battery systems [113], plasma radiation detection
in metal absorber–metal resistor bolometer [114], as dis-
cussed in Section VII-B.
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TABLE 3. State-of-the-art AI-ML research in industrial digital twinning for PHM.
D. VEH ICLES AND TRANSPORTATION
A vehicle digital twin was developed by Alam and El
Saddik [85] in a vehicular cyber-physical system (VCPS),
by mimicking its speed behavior, fuel consumption, and
airbag status. The system utilized fuzzy rule base with a
Bayesian network [129], in order to build a reconfiguration
model for driving assistance. Similarly, Kumar et al. [60]
built virtual models of running vehicles in the cloud, which
obtained real-time road and vehicular data through fog or
edge devices, in order to avoid traffic congestion. The driver
behavior and intention are predicted using machine learning
on historical data. LSTM-based recurrent neural networks
(RNNs) [130] are applied on the data to obtain the best
route for a particular vehicle. Besides, digital twins have
also been developed for vehicular network system, itself.
For instance, the digital twin of a mobile edge comput-
ing (MEC) system was developed [59] for resource alloca-
tion in unmanned aerial vehicle (UAV) networks, using deep
recurrent Q-networks (DRQNs) [131]. Likewise, the digital
twin of software-defined vehicular networks (SDVNs) [132]
allows for the predictive verification and maintenance diag-
nosis of running vehicles network, using machine learning.
Furthermore, prognostics and health management is con-
ducted by developing digital twin of aircraft [118] and space-
craft [116], ship [110], and electric vehicle motor [61]. All of
these PHM approaches employ machine learning techniques.
E. HEALTHCAR E
In healthcare, the majority of AI-ML enabled DTs are human
digital twins [23], [56], [133]–[136]. Mimicking the full
functionalities of a human body is not currently possible,
thus, a human digital twin can only focus on limited aspects
of human biology. For example, the digital twin by Barri-
celli et al. [133] focuses on fitness-related measurements of
athletes. Specifically, their virtual patient classified physi-
cal athletes and predicted their behavior using KNN classi-
fiers [137] and support vector networks [138], by training
models on physical patient data collected by IoT devices.
Björnsson et al. [23] concentrated on protein–protein inter-
action (PPI) networks to diagnose and treat patients of a
particular disease. Their model is implemented as an AI
system that monitors the effect of drugs on the human body,
using machine learning tools, such as Bayesian networks,
deep learning, and decision trees.
Furthermore, Chakshu et al. [135] mimicked the patient’s
head behavior for detecting the severity of carotid stenosis.
Their model selects components from a patient video and
applies principal component analysis (PCA) to identify the
severity of carotid stenosis, by comparing it with the virtual
model components. The authors also recommended the use of
deep learning, machine learning, and other AI techniques for
better detection accuracy. Similarly, Mazumder et al. [134]
digitally replicated the process of generating synthetic PPG
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signals to create the digital twin of a cardiovascular sys-
tem. In the virtual model, parameters are optimized using a
particle-swarm-optimization (PSO) algorithm. The algorithm
minimizes the integral-squared-error (ISE) in the feature set,
in order to generate the synthetic PPG signal. On the other
hand, Laamarti et al. [136] and Liu et al.’s [56] models are
generic ML-enabled frameworks for providing health ser-
vices to elderly people.
F. COMMUNICATIONS AND NETWORKS
In the networking and communications domain, the digi-
tal twin of an indoor space environment [139] is imple-
mented to model, predict, and control the terahertz (THz)
signal propagation characteristics in an indoor space. The
DT selects the best THz signal path from the base station
to the mobile target, by avoiding obstacles. The DT iden-
tifies the obstacle, its position, and dimensions, by apply-
ing a you only look once (YOLO) machine learning algo-
rithm [140] on the monochromatic image of the obstacle.
Furthermore, deep learning algorithms are used for material
texture recognition and classification. On the other hand,
a new network architecture, equipped with ML-based virtual
twin of a software-defined vehicular network (SDVN) [132],
is designed to benefit from intelligent networking and adap-
tive routing. Dong et al. [22] developed a similar digital twin
of a real network for mobile edge computing. The virtual
model of the MEC is equipped with a deep neural network
that is frequently updated based on the variation of the real
network. The model then selects the optimal resource alloca-
tion and offloading policy at each access point.
G. SMART CITIES
In the smart city sector, a Zurich city digital twin [141]
was developed by transforming 3D spatial data and city
models, including buildings, bridges, vegetation, etc., to a
virtual world. The authors discussed the effects of urban cli-
mate, which can be predicted by machine learning techniques
based on the current weather and air-quality data. Similarly,
a Vienna city digital geoTwin [142] can be linked with city
data, such as socioeconomic, energy consumption, and main-
tenance management data, in order to make it a living digital
twin with the aim of AI technologies. Furthermore, a vision
for integrating artificial and human intelligence for a disaster
city digital twin is introduced by Fan et al. [66]. Finally,
a geospatial digital twin [143] is the digital replica of a spatial
entity, where machine learning and deep learning techniques
are used for interpretation, analysis, and organization of 3D
point clouds.
H. OTHER INDUSTRI ES
DT systems that utilize AI-ML techniques have been
deployed in other industries as well. For instance, the supply
chain DT by Marmolejo-Saucedo [24] was developed for a
pharmaceutical company, using machine learning and pattern
recognition algorithms. The objective was to identify the
behavior, dynamics, and changing trends in the supply chain.
Data management for DT environments is another area of
active research. Specifically, a DT-enabled collaborative data
management framework was proposed, using edge and cloud
computing [100]. The goal was to perform advanced data
analytics in additive manufacturing (AM) systems, in order to
reduce the development time and cost, and improve the prod-
uct quality and production efficiency. To this end, the authors
introduced cloud-DTs and edge-DTs, developed at different
product life-cycle stages, which communicate with each other
in order to support intelligent process monitoring, control,
and optimization. As a use case, the framework was imple-
mented within the MANUELA project, where layer defect
analysis was performed by a deep learning model on product
life-cycle data. Moreover, Tong et al. [144] introduced an
intelligent machine tool (IMT) digital twin model for machin-
ing data acquisition and processing, using data fusion and ML
approaches.
VIII. DATA-DRIVEN DIGITAL TWINNING PATENTS
The importance of DT technologies can be verified by the
number of patents in this field. In particular, more than one
thousand patents have been awarded on AI-enabled digi-
tal twinning in all around the world. A wind-power farm
digital twin was filed as a U.S. Patent in 2016 by Gen-
eral Electric (GE) [145], where the DT is composed of two
communication networks: 1) a farm-based communication
network, which enables the coupling of control systems from
individual wind turbines with the main wind farm control
system and with other wind turbines; and 2) a cloud-based
communication network that is composed of an infrastruc-
ture of digital wind-turbine models, where the plurality of
the models are continuously changing during farm opera-
tion, by investigating and analyzing data generated by the
farm-based communication network using machine learning.
Furthermore, they provided a fully functional graphical user
interface (GUI) of the digital wind-turbines, where the user
can control the input features of the DT model to optimize
the performance of the wind farm using machine learning
algorithms. In another patent, Shah et al. [146] developed the
digital twin of a vehicle cooling system, by using status data
(such as health scores) to predict cooling system failures and
optimize its performance. Similar data-driven digital twin-
ning systems have been designed in the energy and power
sector [147].
In predictive analytics for machine maintenance, GE’s Her-
shey et al. [148] developed a system to predict the lifetime of
a component in the electromechanical industry (such as an
aircraft engine), by developing a digital twin of the physical
system. The component is monitored by IoT-based sensors
and its remaining life is assessed based on the monitoring
conditions. In this process, they developed a stress analysis
model, a fluid dynamics model, a structural dynamic model,
a thermodynamic model, and a fatigue cracking model. Then,
they utilized probabilistic models, such as a Kalman filter,
to predict the lifetime and detect component faults. Sim-
ilarly, the Siemens corporation designed a generic digital
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twin model [149] for a variety of machines, including heat-
ing, ventilation, and air conditioning (HVAC). They uti-
lized data-driven approaches for energy-efficient machine
maintenance, utilizing sensor data and model-based analyt-
ics. Several other patents focus on predictive analytics with
AI-enabled digital twins [150]–[152].
A few digital twin patents have also been developed in the
healthcare sector. GE researchers designed a patient DT [153]
to diagnose diseases, treat, and prescribe medicines. The
digital representation of the patient (i.e., the DT) consists of
medical record data structures, medical images, and historical
patient information. The DT is equipped with healthcare soft-
ware applications (such as expert systems), patient medical
data, and AI models (neural networks, machine learning) that
can diagnose, identify health issues, and prescribe treatments
(e.g., medication, surgery, etc.). Also, Nagesh [154] build
an X-ray tube DT to predict tube-liquid bearing failures.
He used X-ray tube housing vibration data, collected by a
sensor in a free run mode of an X-ray tube, and applied
AI-based prediction. There are also patents in DT-based
surgery for the healthcare industry that utilize data-driven
approaches [155], [156].
Finally, there are hundreds of additional patents that
emphasize AI-enabled data-driven digital twinning, which
could not be covered here. These digital twinning systems
belong to a variety of industrial sectors, including manufac-
turing [157], [158], run-time environment [159], transport
and automotive industry [160]–[162], building and construc-
tion systems [163], etc.
IX. EVALUATING A SUCCESSFUL DIGITAL TWIN
A successful digital twin can only be justified when its virtual
twin closely matches the functionality of its physical coun-
terpart. This justification can be presented by comparing the
outputs of the physical and virtual models, and computing
the loss. On this account, accuracy is the main factor to
consider when evaluating digital twins. On the other hand,
the purpose of building a digital twin also matters in eval-
uating its success. This can be justified by the performance
improvement of the corresponding physical system that is
attributed to its digital twin. For example, for a DT whose
purpose is to optimize the assembly line, the improvement
can be measured by computing the number of actions (or sub-
tasks) and the time taken to manufacture a full component
(or to complete a main task/goal) with the DT and without
DT. This is also the case with other applications, includ-
ing product design optimization, product performance opti-
mization, process optimization, control optimization, sched-
uler optimization, resource management, component PHM,
etc. In addition, the processing time and efficiency of the
digital twinning system can also be one of the success
criteria.
In addition, when using AI or machine learning
approaches, the accuracy of the selected model affects the
efficacy of the DT. Specifically, the accuracy of the underly-
ing ML model, as well as the feature selection process and
the amount of training data, may greatly affect the outcome
of the DT. Therefore, when designing a DT-based system
that employs ML techniques, we have to select the model
with the higher accuracy and efficiency. The same approach
should be taken with the selection of other technologies for
DT-development, such as IoT, edge computing, and cloud
computing.
To this end, only a few state-of-the-art digital twinning sys-
tems have been fully evaluated in the literature. For instance,
Zhang et al. [87] assessed their job-floor digital twin by
comparing the performance of the job-floor with and without
digital twinning. They selected job scheduling time, utility
rate, and job tardiness as performance parameters. Similarly,
Zhang et al. [93] highlighted the importance of digital twin-
ning by showing the performance improvement in process
time, fault time, and maintenance time of blisk machining due
to its digital twin. Likewise, Min et al. [164] conveyed a rise
in the oil yield ratio due to a petrochemical industry DT. Fur-
thermore, Xu et al. [45] used the accuracy of fault diagnosis
as a metric to assess the performance of the developed virtual
twin. Finally, Akhlaghi et al. [99] verified the accuracy of the
developed twin by comparing the outputs of the digital and
physical twins. They also showed the effectiveness of their
digital twinning mechanism, by pointing out the optimization
achieved for the dew point cooler. All the aforementioned
DTs were developed using various machine learning models
and, in each case, the authors selected the model that provided
the best accuracy.
X. DIGITAL TWIN DEVELOPMENT TOOLS
There is no standalone technology for DT implementa-
tion, rather, there is an integration of multiple technologies,
including big data, AI-ML, IoT, CPS, edge computing, cloud
computing, communication technologies, etc. Every tech-
nological component can be implemented with a variety
of tools. Here, we only focus on the tools that facilitate
components integration, digital twin simulation, twins bridg-
ing, physical twin control, data storage and processing, and
machine learning. Table 4presents the summary of widely
used tools that may provide support in different stages of
digital twinning.
Integrating physical components for data collection and
then digitally mimicking them in a virtual environment are
two important stages of digital twinning. There are various
tools available to accomplish these tasks in an industrial unit.
Siemens MindSphere is one of the widely used tools to inte-
grate components in a manufacturing industry. Siemens also
developed an object-oriented-based Tecnomatix API to sim-
ulate physical components in a virtual environment, as used
by [91]. The Open Simulation Platform (OSP) is another one,
which is jointly developed by the Det Norske Veritas Ger-
manischer Lloyd group (DNV GL), the Norwegian Univer-
sity of Science and Technology (NTNU), Rolls-Royce, and
SINTEF Ocean. OSP can digitally mimic any component of
the maritime industry. Other popular integration and simula-
tion tools are FIWARE, Predix (a cloud-based platform from
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TABLE 4. Digital twinning supporting tools.
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GE digital), CNC machine tools control platform IndraMo-
tion MTX, Beacon, Thingworx, and others.
Next, bridging physical and virtual twins is another pri-
mary aspect of digital twinning. This bridge is used by a vir-
tual twin to harvest the real-time data from the corresponding
physical peer using sensors. On the other side, the physical
peer is controlled (optimized) based on the output of the
virtual twin. Popular tools in the market to facilitate the bridg-
ing between physical and virtual twins are TwinCat, SAP,
Codesys, CNC tools, Aspera, and RaySync. Similarly, there
are few applications that are used in initial modeling and twin
design, such as ANSYS Twin Builder, MWorks, Siemens NX
software, SolidWorks, Autodesk tools, and FreeCAD.
In the machine learning domain, there are hundreds of
models available for tasks such as optimization, prediction,
classification, and clustering. However, there is no single
platform that offers APIs for all existing ML models. The
most widely used and well-known libraries for implement-
ing, training, and testing supervised ML-models are Tensor-
flow, CNTK and Caffe. Keras and Weka provide easier and
user-friendly interfaces for developing basic machine learn-
ing models. There are also commercial tools available, such
as Mathworks Matlab, which is equipped with vast libraries
of neural networks and Microsoft-Azure implemented ML
models. Reinforcement learning is one of the most popular
techniques that is widely used for dynamic optimization and
process planning in digital twinning. To this end, OpenAI’s
Gym and rllab are tools with standardized interfaces for
reinforcement learning.
Industrial components produce large amounts of data,
termed as big data, which are hard to process with standard
data management tools in a digital twin environment. Hadoop
is one of the most popular ecosystems for big data processing
that offers parallel processing capabilities with multiple com-
pute nodes. Apache has also developed several tools for big
data processing and effective analysis, including Cassandra,
Spark, Storm, S4, Hive, Mahout, Flink, and HBase. Most
of the Apache tools are open-source and support machine
learning APIs. Similar tools include HPCC by LexisNexis
Risk Solution, Qubole, Statwing, Pentaho, and VoltDB.
XI. DATA-DRIVEN REFERENCE ARCHITECTURE FOR
DIGITAL TWINNING
To effectively exploit the value-added capabilities offered by
the integration of big data analytics and AI-ML within the
scope of digital twinning, we present a novel reference model
derived from the conducted systematic literature review.
Fig. 8shows the designed reference layered-architecture for
the efficient handling of big data analytics in DT-based indus-
trial environments. The process starts with the collection of
data from the physical environment (using sensors and actu-
ators) or from the virtual environment (using computer-aided
software and/or simulations). The data is fed to the data anal-
ysis and decision-making layer, where AI models, statistical
and probabilistic approaches, or mathematical models are
employed to create the DT-based system or the digital twin
itself. During the entire process, various big data processing
tools may be utilized, such as Hadoop, Storm, S4, Spark,
etc., that allow for parallel processing on multiple compute
nodes. Fig. 9depicts the overall data flow for creating an
ML-enabled digital twin, and then using it for optimization,
PHM, or other purposes. First, the virtual model is created
by deploying one of the AI models on the data generated by
the physical twin. Once the digital twin is produced, the data
from both the physical and virtual twins are given to other
AI models to achieve the given industrial goals, such as
design optimization, dynamic process planning, healthcare,
or PHM. Moreover, the results can be further used to update
and improve both the physical and virtual twins.
XII. MARKET OPPORTUNITIES AND RESEARCH
CHALLENGES
A. MARKET OPP ORTUNITIES AND RESEARCH AREAS
Based on the detailed literature survey, we have summarized
the following major application areas where DT research can
play a vital role.
1) OPTIMIZATION
Optimization is required in almost every industrial
process, including product design, product performance, pro-
cess planning, assembly line, task-scheduling, and resource-
allocation. Digital twinning is an emerging technology that
provides a direct pathway to optimization with little effort.
However, careful consideration of the optimization algorithm
(i.e., ML model) and the underlying feature set (for the
optimization algorithm) is desired for better results.
2) PROCESS MONITORING, DIAGNOSTICS, AND
PREDICTION
Digital twins can be developed for industrial process mon-
itoring, defect diagnosis (i.e., product quality assurance),
dynamic process or product design updating for time and
cost savings, industrial process surveillance (e.g., robot DT
for obstacle avoidance), product time-to-complete prediction,
and damage detection.
3) PREDICTIVE ANALYTICS FOR MANUFACTURED
PRODUCTS
The quality of every physical entity degrades over time, thus
affecting its performance. Early detection of failures may
promote on-time maintenance, fatigue avoidance, as well as
time and cost savings. Such failures can be attributed to faults
and cracks in the product, performance degradation due to
aging, and other minor or major complications. Moreover,
health monitoring is crucial for certain components that may
potentially cause human casualties, e.g., brake systems in
cars, vehicles, aircraft, and ship engines, fueling systems,
gearboxes, etc. Digital twinning is the most powerful technol-
ogy for predictive analytics and health monitoring of physical
components. This is also an area where AI-ML techniques
can have a significant impact.
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FIGURE 8. Data-driven reference architecture for digital twinning.
FIGURE 9. Overall data-flow framework for digital twinning using big
data analytics and AI-ML.
4) HEALTHCARE
Digital twinning has a wider scope in the healthcare sector
where human-DTs assist in day-to-day human fitness and
health monitoring, early disease diagnosis, and the over-
all well-being of individuals, especially for the elderly and
infants. In addition, it can be used for the treatment or
surgery of patients, by developing a patient-DT. Developing
digital twins for human organs or biological systems will
bring a revolution in the healthcare sector, such as DTs for
lungs, liver, pregnant female womb or uterus, cardiac system,
digestion system, neural system, reproductive system, etc.
Other than biological digital twins, the healthcare sector can
benefit by developing DTs for hospitals, medical and surgical
instruments, remote surgery, surgical processes, etc.
5) SMART CITIES
In the context of smart cities, DT technologies can be imple-
mented for traffic systems, smart homes and devices, park-
ing, buildings, livestock, lighting systems, and renewable
energy. Furthermore, 3D virtual city models may facilitate
urban planning and monitoring in various smart city areas,
including road monitoring and construction, city garbage
management, bridge and housing constructions, etc.
6) OTHER APPLICATIONS
Research opportunities are not limited to the above-mentioned
sectors, but the potential is there in every field, including edu-
cation, construction, mining, communications and networks,
food and agriculture, sports, and so on.
B. RESEARC H CHALLENGES AND ISSUES
The rapidly increasing DT popularity and scope, as well as the
involvement of IoT, big data, and AI technologies, broaden
the research challenges of digital twinning. These challenges
are categorized in the following five areas.
1) DATA COLLECTION
IoT facilitates data harvesting from a physical twin (using
sensors), data integration, and data sharing with the corre-
sponding virtual twins. This process can amount to a consid-
erable cost. Sometimes, the digital twin may be more costly
than the asset itself, in which case it does not make sense to
create the DT. On the other hand, the collected data is large
(big data), heterogeneous in nature, unstructured, and noisy.
Thus, further processing on the data is required to ensure
its effective use. Specifically, we need to apply data clean-
ing techniques, and also organize, restructure, and make the
data homogeneous. Furthermore, controlling the flow of such
large amount of data is also a significant challenge. Finally,
to improve the accuracy of the DT model, the underlying
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machine learning algorithms require a certain amount of data
for training purposes.
2) BIG DATA CHALLENGES
The explosive growth of IoT technologies in the industrial
sector has led to the generation of large amounts of moni-
toring (sensor) data. To this end, big data analytics requires
advanced architectures, frameworks, technologies, tools, and
algorithms to capture, store, share, process, and analyze the
underlying data. There is also a potential for edge and cloud
computing platforms to handle DT-related data. Specifically,
edge computing enables the distributed processing at the net-
work’s edge, while the aggregate processing is accomplished
in the cloud. However, the aggregation of data in the cloud
may cause an increase in response time.
3) DATA ANALYSIS
AI-algorithms for data analytics played a major role in DT
for decision-making, as discussed in the literature. How-
ever, the selection of a particular model among hundreds of
ML-models with customized configuration is challenging.
Every AI-approach has diverse accuracy and efficiency levels
with different applications and datasets (feature set). On the
other hand, accuracy may affect the efficiency on the other
side. Hence, depending on the motive and application of
a DT, the selection of the best ML-algorithm and features
is challenging. Besides, fewer practical implementations of
AI-techniques for digital twinning in the literature raises
more challenges.
4) DT STANDARDIZATION CHALLENGES
Even though many digital twins have been developed in var-
ious industries, the creation of a complex and reliable digital
twin demands standardization. Currently, there is no single
standard that solely focuses on digital twinning. The ISO/DIS
23247-1 standard [29] has only limited information on digital
twinning and, therefore, DT deployment challenges grow due
to the lack of standardization. Standardization efforts are
underway by the joint advisory group (JAG) of ISO and IEC
on emerging technologies [28].
5) SECURITY AND PRIVACY ISSUES
Some DT systems, such as human-DTs, product PHM,
or defense-related DTs, are considered critical and may
require stringent security and privacy guarantees. First, due
to the involvement of IoT devices in digital twinning, a lot
of emphasis has to be placed on the security of the under-
lying communication protocols. Additionally, the large col-
lection of asset-related data needs to be stored securely,
in order to prevent data breaches from insider and outsider
threats.
XIII. CONCLUSION
We performed a systematic literature review of the state-of-
the-art DT systems that employ machine learning and AI
technologies. In particular, we focused on papers published
in top multidisciplinary electronic bibliographic and patent
libraries, and summarized the current DT deployments in
a variety of industries. With the immersion of AI-ML and
big data, digital twinning is evolving at a rapid rate and,
with it, a lot of unique challenges and new opportunities are
emerging. This article highlighted the research challenges
and potentials in many diverse areas, for both academia and
industry. Furthermore, we identified the DT criteria and tools
that aid its successful development. Finally, we designed a
reference model for an AI-ML and big data-enabled digital
twinning system to further guide industrial developers in
establishing DTs that can make their systems smarter, intelli-
gent, and dynamically adaptable to changing conditions.
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M. MAZHAR RATHORE (Member, IEEE)
received the master’s degree in computer and com-
munication security from the National University
of Sciences and Technology, Pakistan, in 2012,
and the Ph.D. degree in computer science and
engineering from Kyungpook National University,
South Korea, in 2018. He is currently working
as a Postdoctoral Researcher with the College of
Science and Engineering, Hamad Bin Khalifa Uni-
versity, Qatar. His research interests include big
data analytics, the Internet of Things, smart systems, network traffic analysis
and monitoring, remote sensing, smart city, urban planning, intrusion
detection, and information security and privacy. He is a professional member
of ACM. He received the Best Project/Paper Award in the 2016 Qual-
comm Innovation Award at Kyungpook National University, for his paper
‘‘IoT-Based Smart City Development Using Big Data Analytical Approach.’’
He was also a nominee for the Best Project Award in the 2015 IEEE
Communications Society Student Competition, for his Project ‘‘IoT-Based
Smart City.’’ He is serving frequently as a Reviewer for various IEEE, ACM,
Springer, and Elsevier journals.
SYED ATTIQUE SHAH received the Ph.D. degree
from the Institute of Informatics, Istanbul Techni-
cal University, Istanbul, Turkey. During his Ph.D.
degree, he was a Visiting Scholar with National
Chiao Tung University, Taiwan, The University of
Tokyo, Japan, and the Tallinn University of Tech-
nology, Estonia, where he completed the major
content of his thesis. He was an Assistant Professor
and the Chairperson of the Department of Com-
puter Science, BUITEMS, Quetta, Pakistan. He is
currently a Lecturer with the Data Systems Group, Institute of Computer
Science, University of Tartu, Estonia. His research interests include big data
analytics, cloud computing, information management, and the Internet of
Things.
DHIRENDRA SHUKLA is currently a Professor
and the Dr. J Herbert Smith ACOA Chair in tech-
nology management and entrepreneurship of the
University of New Brunswick (UNB), Canada.
He utilizes his expertise from the telecom sec-
tor and extensive academic background in the
areas of entrepreneurial finance, masters of busi-
ness administration, and engineering, to promote
a bright future for New Brunswick. Recogni-
tion of his tireless efforts and vision are demon-
strated through the UNB’s 2014 Award from Startup Canada as the ‘‘Most
Entrepreneurial Post-Secondary Institution of the Year,’’ his nomination as a
Finalist for the Industry Champion by KIRA, and his nomination as a Finalist
for the Progress Media’s Innovationin Practice Award. He was nominated for
the RBC Top25 Canadian Immigrant Award and selected by a panel of judges
as a Top 75 finalist. Most recently, he received the Entrepreneur Promotion
Award by Startup Canada in 2017, as well as the Outstanding Educator
Award by the Association of Professional Engineers and Geoscientists of
New Brunswick in 2018.
ELMAHDI BENTAFAT received the bachelor’s
and M.Sc. degrees in computer science from the
Ecole Nationale Supérieure d’Informatique, Alge-
ria, in 2012 and 2016, respectively. He is currently
pursuing the Ph.D. degree with the College of Sci-
ence and Engineering, Hamad Bin Khalifa Univer-
sity, Qatar. His research interests include applied
cryptography, privacy, information security, and
network security.
SPIRIDON BAKIRAS (Member, IEEE) received
the B.S. degree in electrical and computer engi-
neering from the National Technical University of
Athens, in 1993, the M.S. degree in telematics
from the University of Surrey, in 1994, and the
Ph.D. degree in electrical engineering from the
University of Southern California, in 2000. He is
currently an Associate Professor with the College
of Science and Engineering, Hamad Bin Khalifa
University, Qatar. Before that, he held teaching
and research positions at Michigan Technological University, The City Uni-
versity of New York, The University of Hong Kong, and The Hong Kong
University of Science and Technology. His current research interests include
security and privacy, applied cryptography, mobile computing, and spa-
tiotemporal databases. He is a member of ACM. He was a recipient of the
U.S. National Science Foundation (NSF) CAREER Award.
32052 VOLUME 9, 2021