The first cloud manufacturing platform was developed and presented in [
39], and was more oriented towards business activities than engineering-manufacturing activities. The emphasis was on data security. The example given in [
40] refers to a cloud manufacturing model that supports engineering designs (CAD/CAM) and manufacturing management (ERP), as one of the first examples. The architecture of a networked enterprise supported by the cloud computing (CC) is presented in [
41], with special emphasis on interoperability, security, and standardization model and flows data. Example of additive manufacturing, with 3 D model are shown in [
42], for real industrial parts, from the aerospace industry, and also the manufacturing of medical devices and parts. The digital eco-factory is simulated as a virtual factory, from eco-performance aspects, prior to the real manufacturing process [
43]. Software agents are used for simulation, thus creating an e-library of machines and devices (CPS) in the digital factory. The paper discusses various aspects of the emergence and development of a digital enterprise [
44], where the focus is on smart sensors and sustainable development. The reference model - open distributed processing (ODM) is proposed. The development of the smart manufacturing (SM) model at the national level, based on the Industry 4.0 model, is discussed on the examples of Germany, USA and South Korea in [
45]. This model is set as a new manufacturing paradigm with the following elements: a model for SMEs, standardization of reference architecture (IoT and IIoT), real-time data flow management, and integrated performance metrics of smart manufacturing. The paper [
46] presents a model of a collaborative robot in smart manufacturing, based on the application of a hierarchical planning method, with case example. The IT architecture for a data-driven factory, as a research project, is presented in [
47]. The focus of the entire model is on big data analysis (BDA) from manufacturing, as a key factor for raising the quality of processes and products and reducing manufacturing costs (scrap), with example from the automotive industry. The NIST model of smart manufacturing [
48] is based on the information interoperability of data in smart manufacturing about products, manufacturing and business. It is based on the integration within and between the three dimensions of the life cycle manufacturing: product, manufacturing system and business, thus observing a smart manufacturing, as a new paradigm. Smart the factory performs integration between physical and virtual technologies [
49], through the three layers: physical resources, networks resources and application data, connected via CPS, IoT, BDA and cloud. An overview of definitions, models and characteristics of smart manufacturing based on Industry 4.0 is given in [
50]. It defines a semantic set of twenty-seven characteristics (digital infrastructure, modularity, ..., vulnerability) and thirty-eight technologies (Industry 4.0 elements), that support the model of smart manufacturing (intelligence, intelligent management, ..., SPC). They concluded that basic concept in design and development of the smart manufacturing considers interoperability, virtualization, decentralization, work in real time, orientation on the services and modularity. Agility, productivity and quality are complemented with different networked entities [
51]. An example of a successful model from industrial practice is given, based on the ISA-95 model, observes a smart factory in three dimensions (digitalization, IoT and CPS). One of the recent trends in the development of smart manufacturing is the definition and implementation of a reference architecture. In [
52] an analysis of four different models was performed : (a) RAMI 4.0 model, (b) IIRA (Industrial Internet Reference Architecture), (c) IBM Industry 4.0 model, and (d) NIST smart manufacturing model [
48]. It was concluded that from the aspect of modeling and management of micro-services in the enterprise and optimization topology interaction between micro - services, smart objects and people, RAMI 4.0 model has the best characteristics. The core of the Industry 4.0 model is smart manufacturing, and the attention, as mentioned above, is focused on the development and application of the reference model. However, in the broader context of this model, many versions of the international and national standards are being developed and already implemented, especially in the field of IC technologies [
53]. They relate to the following areas: (a) business operations and management standards (QMS and others), (b) manufacturing standards, (c) design standards, (d) system integration standards, (e) supporting technology standards, and (f) environmental standards. In this way, the overall model of smart manufacturing is rounded off. Today, it is considered that Industry 4.0 and smart manufacturing are the future that has arrived, based on digitization. Also, in this context, resource virtualization and cloud computing are considered to be the basis of the digitization. In [
54], some models of these approaches with a focus on data security are presented. The Digital Manufacturing Platform Reference Model (I3RM) is presented in [
55]. It represents a multi-dimensional eco-system, which integrates different stakeholders, including supply chains, supported by IT models. A special dimension of these studies is the integration of this model with a zero defect manufacturing (ZDM). Custom model of smart manufacturing based on RAMI 5.0 model is presented in [
56]. The structure is based on the product life cycle model, and through the global cloud, it is connected to the local cloud, that manages business processes, connected by the ERP and MES model. Logistics, technological and manufacturing processes at the CPS level are defined and managed by means of digital twins. The role and functions of the CPS within the smart manufacturing concept is discussed in [
57]. They defined a new CPS architecture, based on the wide range of different IoT models. The machine tool in the smart manufacturing model is defined and monitored over four levels: physical, network, server and client levels [
58]. During the processing, large amounts of data are generated that are monitored online and collected through formatting, filtering, correlation, memorization, passing through intelligent algorithms and decision making. Data from different sources are used in BDA models and further used for the monitoring of the machining process, quality management, smart planning and maintenance of the machine. The study [
59] considers a service-related value-added smart manufacturing model, thus developing a smart service manufacturing (SMS) model. Such SMS model includes: business digital model, service evaluation, decision management and knowledge management in the product/service life cycle. The application of the R-CNN AI model for the simulation of smart manufacturing is shown in [
60]. The whole concept is based on the digital twins (DT) and includes a manufacturing cell with a robot. In the study [
61], the reference model of smart manufacturing - JWG21 meta-model (ISO TC 184 / IEC TC 65) was presented, created by the work of this joint working group. In total eight models were analyzed, and a meta (joint) model of smart manufacturing was defined in two dimensions: a generic product life cycle model, and information integration based on digital twins (DT). In [
62], a special dimension of smart manufacturing, its efficiency, was investigated, at the factory and/or supply chain level. The results of the study showed that the efficiency of the smart factory at the plant level is the highest, and decreases at higher country levels due to its depndence on digital infrastructure that varies across countries. Foreign direct investment has made a huge contribution to the development and application of the Industry 4.0 model in China's metal industry [
63]. This created the conditions for the wide application of this concept in all industrial and economic branches. Smart manufacturing based on digitization has shown high resistance to disruptions caused by the COVID 19 pandemic [
64]. This was shown by research on the digital readiness and maturity of the manufacturing industry in the developed countries of Europe, North America and the Far East. The study [
65] presents a comparative analysis of smart manufacturing models in two dimensions: categories (cyber, lifetime, physical, human and architecture) and reference models (twelve models included). These studies show that the reference models for smart manufacturing have reached the appropriate maturity and that the positive experiences in their development and application should be applied to the design of smart city models, especially in the field of IoT. The study [
66] shows a special aspect of smart manufacturing, which refers to the simulation of the manufacturing process, prior to the real manufacturing. They showed two examples from pharmaceutical manufacturing, where using the meta digital model of the process, an effectiveness of 0.95 was achieved between real manufacturing and the simulation model. Case example of a digital twin shows another proven advantage of smart manufacturing. Digitization has enabled the development of the new collaborative networks between organizations, thus creating added value for products within such digital ecosystems [
67]. The research showed that it is necessary to work on the development and implementation of a digital platform for a collaborative network considering the following: (i) define the technology leader of the network, (ii) by establishing digital twins, the technology leaders must move to a higher level, the manufacturing and delivery of smart machines and devices, (iii) encourage members of the network to innovate, (iv) these facts are especially related to companies with added value products (cars, mobile phones, etc.), and (v) develop parallel new markets for new products from the network. Digitalization of the eco-system enables all previosly listed. In the study [
68], a model of hybrid digital manufacturing is presented, that includes the following elements: (i) digitization of physical objects and processes (CPS, IoT), (ii) management of manufacturing resources (ERP/MES), (iii) relations between products and processes (PDM/PLM), and (iv) information system (CRM, CAD/CAM, SCM). This is illustrated by an example of the agricultureal product (special plows for soil cultivation). Robot-human cooperation in a smart manufacturing model with robot path optimization is presented as a research result in [
69]. Additionally, the reconfiguration of the robot's manufacturing task is performed, thus generating a dynamic configuration of the entire system for the online mode of operation. During the last decade, digitization has established new rules of consumption and purchase and created new consumer habits, that has led to the manufacturing adaptation. Application of the Industry 4.0 model, with elements: CPS, IoT, additive manufacturing, BDA and AI/ML is shown in [
70]. Research presented in [
71], shows that innovation is a key factor for the spread of the Industry 4.0 model in manufacturing from the following aspects: (i) sustainability and digital manufacturing, (ii) development of knowledge for digital manufacturing, (iii) effects of digital manufacturing on customer's experience, (iv) apperance of digital twins, and (v) AI in digital manufacturing. Reference architecture research for digital manufacturing is presented in [
72]. A total of fifteen models are presented for: (i) smart manufacturing, (ii) IoT, (iii) vertical integration, and (iv) CPS.
It can be concluded that: (i) decade of developing and applications of Industry 4.0 model, clearly defined frame and structure of smart manufacturing model, and (ii) furthermore the most important elements of that framework are determined: CPS, IoT, BDA, digital twins, AI / ML, vertical integration and cloud computing.