## 1 Introduction
### Definitions
#### 1.2.2 CPS and CPPS  
The fourth industrial revolution is only possible with the advantage of the so-called CPS. CPS can be described as the integration between the digital and physical worlds. It consists of a control unit, sensors, and actuators. The control unit, usually implemented with one or several microcontrollers, receives data from the sensors, which can get information about the surrounding environment and controls actuators interacting with the real world. CPS opens the possibility for manufacturing systems to monitor physical processes in real time and make intelligent decisions through cooperation with humans, machines, sensors, etc. During the past years, the focus of attention was directed to this subject due to the evolution of the following [10]: (a) sensing systems that allow precise monitoring of the system; (b) communication technologies that provide high speed and low latency on data exchange, both on wired and wireless communications; (c) computer processing power that enables the process and analyses of large amounts of data at an affording cost; (d) cognition systems, which are systems that bring together machines and humans, giving more human-like behaviors to machines; and (e) autonomous control that emphasizes the reaction of the system without direct human intervention. According to [PERSON]. [PERSON] and [PERSON] [11], a suitable CPS architecture for shop-floor intelligent manufacturing should include three layers, as demonstrated in Figure 1. Each CPS can autonomously collect and send data as well as send and receive information as needed. The three layers identified are a physical connection layer, a middleware layer, and a computation layer. Figure 1: CPS architecture on shop floor, based on [11]. Physical connection layer: This layer deals with collecting and transmitting data from operations. Sensors and other measurement devices gather the information at a low level and distribute it through the production environment. These components should be chosen considering the operational constraints such as protocols, processing, location, distance, and storage. On the shop floor, groups of machines are typically connected through fieldbus technology and/or industrial Ethernet. Middleware layer: This layer is intended to transfer the collected data from the embedded components to a more powerful device, to be analyzed, and to send commands to the production from the computation layer or other external applications. This layer deals with the reception of data and information in variable data formats, depending on the brand and type of the embedded component, and translates the production commands from the computational layer (or external applications) into commands that can be triggered by the shop-floor machines. Computational layer: In this layer, models, algorithms, and tools are developed to analyze and extract patterns from the raw data received from the embedded devices and sensors. Also, data can be received from other software tools, namely ERP, MES, and Supply Chain Management (SCM). With all these data, it is possible to have a better insight into machine working conditions, product and sub-product quality, manufacturing processes, etc. In this layer, two kinds of big data computing need to be provided: stream computing, to process near-real-time raw shop-floor data, and batch computing, to process large volumes of historical data. With this combination, and with human inputs, it is possible to create a unified view of information and knowledge that supports intelligent decision making to control processes and operations, and execute maintenance on the shop floor. CPSSs are, like CPSs, recognized as one of the keys to unlocking the potential benefits of Industry 4.0 as they enable enterprises to evolve into this industrial revolution. CPPS is performed when all CPSs are connected to a production system.