Dong Yu-hua, Zou Ben-hao, Fu Yun-bin
1 Ninth People’s Hospital of Qingdao, Qingdao 266000, Shandong Province, China
2 Linqing People’s Hospital, Linqing 252600, Shandong Province, China
AbstractThis article mainly introduces the technical architecture of the Internet of Things communication system and the functions and implementations of various communication methods in the four layers, namely peripheral interface layer, management layer, dispatching control layer,and job execution layer. Additionally the development trend of communication technology is predeterminated.
Key words: Internet of Things; communication
The Internet of Things connects people, computers, objects and animals to communicate with each other, and achieves real-time sharing of the environment and information status. Due to its high automation, high reliability, and high flexibility, Internet of Things has become an effective means to improve management level, management efficiency and the quality of life. It is the combination of all current communication technologies with computers and the Internet. The core and key of the Internet of Things technology is the communication technology between various objects and the intelligent collection, processing,transmission and execution of various information[1].What are the communication methods that connect computers, objects, and animals in the Internet of Things?
In order to better adapt to various application scenarios, a variety of Internet of Things communication technologies have been developed and used at present, and there are also multiple classification methods, for example it is classified into wired transmission, wireless transmission,short-distance transmission, long-distance transmission,industrial transmission, Internet transmission, etc. From the perspective of the entire Internet of Things architecture,according to the four functional modules of the Internet of Things system: interface, management, control, and execution, the Internet of Things communication technology can be divided into the corresponding four layers: peripheral interface layer, management layer, dispatching control layer, and job execution layer[2]. So does each layer of the Internet of Things system has its own communication characteristics? Which technologies are included? How do they realize their control process?
Based on the above system functions and control hierarchy division, the Internet of Things communication system model is shown in figure below.
The job execution layer is composed of a variety of materials, terminals, and equipment. This is the lowestlevel equipment execution layer. These include stackers,conveyors, spreader, AGV automatic guided vehicles,robots, sorting lines and other bottom-level execution equipment. Between the dispatching layer and the job execution layer, between different job execution layers, the communication function is achieved through industrial Internet of Things communication technology, mainly using serial port communication, field bus, NDC8000 and other special-purpose software.
The dispatching control layer responsible for equipment dispatching and path dispatching, the management layer responsible for material management and report management, and the interface layer responsible for information interaction and communication with peripheral MES, ERP and other management systems, are all placed above the job execution layer.
Upon the receipt of the job plan or man-machine instruction generated by the management module, the dispatching control layer may decompose the job plan according to the job dispatching strategy, the status of each equipment in the path and the instruction execution report, and then generate the job instructions and distribute to the corresponding execution equipment.
The job control module is a part of the internal control function of the dispatching module. In this figure, the job control process of the dispatching module is clearly demonstrated. The execution equipment generates corresponding execution actions according to the received job instructions and the read barcode information.
At the same time, these three functional modules interact in real time manner through field bus, Ethernet, and shared database.
The interface module is a system that can operate independently, and can realize information interaction with the upper system ERP and MES. The majority of information exchange at the interface layer is the basic information and the warehouse receipts of the system.At the same time the actual inventory situation is feed backed. The interface service adopts a middleware-based architecture to realize three basic functions:
Actively obtain business data in external systems, such as basic data (materials and personnel).
Actively transmit business data to external systems, such as inbound and on-shelf data, outbound and off-shelf data,inventory data, etc.
The external system actively transfers business data:inbound receipt data such as orders, and outbound issue data such as production plans.
The management module acquires the production plan from the MES system through the interface function,and obtains the data such as warehousing receipt document, issue document, delivery document, personnel information, material information from the ERP system.According to the established access strategy (such as firstin first-out) and equipment operation, job execution report and other information, the module generates the job plan and distributes to the dispatching module.
The Internet of Things management system can be developed based on the Microsoft.net platform, etc.,and realize system functions through database servers,application servers, and intelligent clients. The intelligent client implements the user interface and the definition of appearance attributes. The application server implements business logic and access control to data.
The intelligent client provides a consistent access interface for the Web layer, which is mainly used for Web users to access applications. For the input data, a preliminary judgment is made. For example, when the password is registered, if the two passwords are inconsistent, the registration is unsuccessful. In the business appearance category, a method handling such errors is designed.Wrong judgments involving business rules are not included in the intelligent client.
Combining the operation of data entities and the control of the database, these two parts realize the interaction of application layer data.
As a main function of the management layer, the user interface mainly realizes the functions of humancomputer interaction, equipment monitoring, subsystem monitoring, path monitoring, task execution monitoring,etc. The normal operation of the system does not require the deployment of relevant personnel to maintain the system. In the event of a failure, it can quickly complete troubleshooting through monitoring prompts.
So what data does each module has to obtain and what results are required to be output, according to the realization of these functions and the division of module levels? What are the information exchanged between each functional module?
Interface layer communication method
The way of interface adopts interface table, WebService,Remote, DCOM, etc.
The interface table method is an option for integration with a system that uses database tables to exchange data.This integration option is applicable only to enterprise services and publishing channels, and is always processed asynchronously by using JMS queues. The interface queue list identifies the order in which the receiving system processes the records in the respective interface table.There are two queue lists, one for inbound transactions and the other for outbound transactions. The receiving system must process these records in the same order in which the records are created by the sending system. All inbound and outbound transactions must have a record inserted into the corresponding inbound or outbound queue list. The record contains a TRANSID value, which is a unique identifier for identifying the interface table to which the transaction data is written. The corresponding interface table uses the TRANSID value to identify one or more records associated with the transaction. The content of the transaction can be identified by looking up all the records with a given TRANSID value in the corresponding interface table.
WebService is a remote calling technology using crossprogramming language and cross-operating system platform. The so-called cross-programming language and cross-operating platform means that the server program is written in java, and the client program may be written in other programming languages, and vice versa! Crossoperating system platform means that the server program and client program can run on different operating systems.
The Remote interface method is used to identify the interface whose method can be called from a non-local virtual machine. Any remote object must directly or indirectly implement this interface. Only these methods specified in the “remote interface” (the interface that extends java.rmi.Remote) can be used remotely. The implementation class can implement any number of remote interfaces and can extend other remote implementation classes. RMI provides useful classes that can be extended by remote object implementations. These classes facilitate the creation of remote objects.
The DCOM interface method is called Distributed Component Object Model, which is a series of Microsoftconcepts and program interfaces. Using this interface,client program objects can request server program objects from another computer on the network.
DCOM is based on the Component Object Model(COM). COM provides a set of interfaces that allow communications between a client and a server on the same computer. It supports communications between components on two different machines, regardless of they are running on a local area network, a wide area network,or the Internet. With the help of DCOM applications, it will be possible to arbitrarily distribute the space.
Management layer communication method
The .Net management function is the processing of all business logic and user interaction, independent of the equipment.
The operation of the Net management system is taskdriven, and the main management service generates and processes tasks based on request information. As a business-oriented back-end management service, only the source and target points need to be considered when generating tasks, instead of the intermediate handling process, and all intermediate handling processes and equipment information interaction are completed by the dispatching system.
The management layer uses a communication method called .NET Remoting, which is a distributed processing method. It provides a framework that allows an object to interact with another object through the application domain. In the Windows operating system, the application is separated into separate processes. This process forms a boundary around application code and data. If the inter-process communication (IPC) mechanism is not used, the code executed in one process cannot access another process. This is an operating system’s protection mechanism for applications. However, in some cases, we need to cross the application domain to communicate with another application domain, that is, across the boundary,then we need to use Remoting communication technology.
In Remoting, the communication of objects between two applications and domains is achieved through channels.First, the client accesses the channel to obtain the server object through Remoting, and then resolves it to the client object through the proxy. This provides a possibility to publish server objects as a service. The remote object code can run on the server (such as server-activated objects and client-activated objects), and then the client connects to the server through Remoting, obtains the service object and runs on the client through serialization.
In Remoting, for the object to be transferred, the designer does not need to know the format of the data packet,except for the channel type and port number. This not only ensures the loose coupling of related objects between the client and the server, but also optimizes the communication performance. It mainly includes: remote object, channel, formatter, formatter provider, message receiver, etc.
Dispatching layer communication
Dispatching is the link that connects the information management layer and the equipment control layer. It is responsible for decomposing tasks to each transportation section, and issuing corresponding control instructions according to the definition of the transportation section controller, while collecting controller’s real-time information, then transforming into a material receipt or issue request according to the function definition of the control equipment and the completion of the instruction.
The dispatching module is designed with an objectoriented method, and the control subsystem is integrated downward through the interface plug-in mode. This structure is convenient for maintenance and upgrading.Each control unit is a separate thread. The system exchanges information through the control bus and each control unit. When the sub-control system failure occurs, it will not affect the operation of the entire system.When the sub-control system failure recovers, the system automatically maintains the connection.
The dispatching module mainly realizes the real-time scheduling of various execution equipment, the humancomputer interaction of the scheduling service, the scheduling service of the internal business logic, and the access to the database. It not only needs to access the database stored data, also controls the process, and realizes the business logic of managing the main service and event service.
Different communication protocols are used between the dispatching system and other system interfaces. As shown in the figure below, the dispatching module has a total of 4 communication methods, which are the OPC field bus method of PLC, the .NET REMOTING method of management main service, the event main service and the dispatching interface, the TCP/IP method of the AGV host computer, and the serial communication method of the stacker.
OPC, referring to OLE for Process Control, establishes a bridge between Windows-based applications and field in-process control applications. The OPC standard is considered the communication standard based on Microsoft’s OLE technology and uses OLE/COM mechanism for applications. OLE/COM is a client/server model, which has the advantages of language independence, code reusability, and easy integration. OPC standardizes the interface function, no matter in what form the field equipment exists, the client will access it in a unified way, thus ensuring the transparency of the software to the clients, so that the user is completely separated from the low-level development. There must be an OPC interface between the application and the OPC server.
Serial communication refers to a communication method that transmits data bit by bit between peripheral equipment and computers through data signal lines,ground wires, control wires, etc. This communication method uses fewer data lines, which can reduce communication costs in long-distance communication, but its transmission speed is lower than parallel transmission.For the serial port, the communication distance can reach 1200 meters. The communication is completed using 3 wires, namely ground wire, sending and receiving. Since serial communication is asynchronous, the port can send data on one wire while receiving data on the other wire.The most important parameters of serial communication are baud rate, data bits, stop bits and parity[3]. For two ports to communicate, these parameters must match.
Understanding the communication protocol of each interface is the basic knowledge that must be understood for failure diagnosis, because different protocols will involve different diagnostic methods. For example:
● For TCP/IP type interfaces, you need to know the IP address of the equipment.
● For OPC type interface, you need to know the IP address of the equipment and the configuration in SIMATIC NET.
● For serial ports, you need to know the settings of serial port parameters.
● For the .NET REMOTING type interface, you need to know the IP address of the equipment and the correct configuration file.
Execution layer communication
The job execution level deals with a variety of equipment,materials, computers, and even people, animals. Because of the different recipients of information, the communication methods are also various, including Bluetooth, wifi, RFID,optical fiber, even 2G and 3G signals in the field of mobile phone communication. The communication methods of several typical underlying execution devices are described as follows.
Shuttle communication
The dispatching system generally does not communicate directly with the shuttle, but communicates through the electronic control system PLC. The dispatching system can monitor the operating status and failure information of the equipment, also view and rewrite the information carried by the conveyor.
AGV automatic guided vehicle communication
AGV has a specialized communication software, such as NDC8000 (Sweden) and communication software independently developed by Shenyang SIASUN Robot &Automation CO., Ltd (China). The software can realize the dispatch of various communication instructions such as job scheduling, path planning, charging management and the status of the vehicle conditions for the AGV.
Sorting control
In most of the underlying electrical equipment control systems, the PLC field bus communication method is generally adopted, such as Siemens simtic NET adopts simtic as the communication configuration tool. During the configuration process, it is necessary to confirm the PLC IP address and communication site. Sorting control generally has a separate communication system, which can realize the communication function independently. It communicates by means of field bus through PLC. We can query and set the CPU status of the PLC.
Stacker control
There are two ways of stacker communication, one is 485 serial communication, the other is LAN to serial port. In the process of communicating with the upper dispatching system, MOXA (Mossaka) is generally used, the communication port number, communication speed and parity bit need to be confirmed during the configuration process.
In the near future, the Internet of Things communication technology will enable various objects to be related to each other with the assistance of sensing devices. The transmission of information can be distributed to all kinds of objects through the cloud computing platform,so that all the objects are combined into a series of intelligent networks with identification, positioning,tracking, controlling, and supervision functions. Such an Internet of Things requires a coordination from all kinds of information communication technologies, such as 5G,wired gigabit broadband and so on. The transmission speed and transmission quality of 5G can support people’s interactive experience in various sensory technologies.Moreover, due to the wireless nature of 5G, the user’s place of use is also liberated. The joint assistance of multiple networks promotes the development of the Internet ofThings communication technology, so that the Internet ofThings communication technology is not only technologycentric, but also serves the user experience. It integrates a variety of network and various types of access network,providing users with unprecedented experience. The Internet of Things cooperated with the Internet can greatly satisfy and service users[4].
