Abstract: This paper proposes a data communication protocol for smart factory workshops based on Manufacturing Execution System (MES). This protocol enables data communication and interaction between the MES and intelligent manufacturing terminal controllers within the smart factory. It covers a series of data interaction aspects, including data interaction methods and formats. The proposed data communication protocol connects the MES in the production control center with the intelligent manufacturing terminal controllers in the smart factory. It serves as an agreement to ensure normal data communication between the MES system and the intelligent manufacturing terminal controllers in the smart factory. It is modular, customizable, scalable, and portable, enabling all automated equipment within the smart factory to communicate with the MES in a unified manner.
1 Introduction
Smart factories represent a new stage in the informatization of modern factories and a new direction for the development of intelligent industries. The intelligent operation of smart factories is based on the communication and docking between the Manufacturing Execution System (MES) and the intelligent manufacturing equipment controller. It aims to realize an integrated management model of factory processes, including production scheduling, quantity statistics, process data monitoring, alarm and fault management, and intelligent equipment management, so that managers can clearly grasp the production process [1]. At the same time, the intelligent manufacturing equipment controller can receive digital work instructions from the MES system, including important parameters such as machine number, production scheduling, template number, and process parameter issuance to ensure the normal operation of intelligent manufacturing equipment. The MES system can also read the production line data in real time and accurately, improve the controllability of the production process, reduce human intervention on the production line, and thus realize intelligent manufacturing [2-3].
Therefore, to realize this smart factory management model based on digital manufacturing and intelligent manufacturing, it is necessary to customize relevant stable and reliable communication protocols to unify the communication methods and content between the MES system control center and multiple machine equipment terminals after the industrial fieldbus layout, MES control center and intelligent manufacturing equipment controllers are in place.
Figure 1 Data transmission diagram of a smart factory
Figure 1 shows the data communication transmission diagram of a smart factory. The data communication protocol serves as the link between the MES system and the intelligent device terminals, enabling normal data communication between the MES system control center and the intelligent manufacturing equipment controllers within the smart factory. On one hand, the MES system can issue task orders and production scheduling instructions to the intelligent manufacturing equipment controllers within the smart factory and verify the responses of the intelligent manufacturing equipment. The intelligent manufacturing equipment controllers respond promptly after receiving the task orders and production scheduling instructions and begin operations. On the other hand, during their operation, the intelligent manufacturing equipment controllers can upload real-time data to the MES system and verify the MES system's responses. If the intelligent manufacturing equipment encounters alarm or fault information, the controller will promptly send an alarm notification to the MES system. Upon receiving the alarm information, the MES system will arrange relevant personnel to handle the situation based on the alarm number uploaded by the intelligent manufacturing equipment.
2. Smart Factory Production Data Interaction
The normal operation of a smart factory requires the MES system and intelligent manufacturing equipment to work in an orderly manner. As an important component connecting the MES system and intelligent manufacturing equipment for data interaction, the communication protocol module strives for a clear hierarchical structure, clear interaction between modules, easy expansion, easy portability, and easy maintenance, so as to ensure stable and reliable performance. At the same time, it should have good integrability and robustness at the application layer, and have stronger real-time performance and intelligence.
2.1 Smart Factory Production Workshop Management Process
Smart factory workshop management refers to a series of management tasks involving the unified planning, organization, command, coordination, and control of all production and operation activities carried out in the workshop.
Figure 2. Production data flow diagram of the smart factory workshop
All operations in the smart factory workshop are carried out according to the instructions sent by the MES system. The MES system arranges operations for the smart manufacturing equipment based on order requirements and the real-time operating status of the smart manufacturing equipment. After receiving a new order, the MES control system will work with various warehouses to analyze and summarize whether the raw materials, packaging materials, consumables and other materials required for the order are sufficient. If there is a shortage, the purchasing department will be notified to purchase them within the specified period. During the raw material procurement and quality inspection period, the production department will be notified to prepare for production, check the actual situation of manufacturing equipment and production facilities, and reasonably arrange work tasks for the corresponding equipment based on the production schedule of the equipment in the smart factory and the remaining orders. After the production schedule is issued to the smart factory, the first step of the smart manufacturing equipment to start production requires human intervention to import the corresponding process parameters according to the production schedule of the work instructions, debug and confirm the first piece, and then upload the fine-tuned process parameters to the MES control system database before automatic operation can begin [4-5].
All data interactions require a stable and reliable data communication layer to ensure stable communication between the MES system and the smart manufacturing equipment. The data interaction required for the normal operation of a smart factory can be seen in Figure 2. Figure 2 illustrates the data interaction process between the MES system and the smart equipment controller during production. Before first-piece confirmation, manual confirmation by operators is still required. After first-piece confirmation, automated operations can be performed under fixed parameter mode.
2.2 Data Communication Module Division
The production data communication protocol designed in this paper follows the design concept of modularity and hierarchy, striving for high cohesion and low coupling, laying a solid foundation for the standardization promotion of its application in actual production in the later stage. The data communication protocol uniformly adopts XML format to package and transmit data, and the interactive data flow structure of its modules is shown in Figure 3.
Figure 3 Data communication protocol interaction data flow
As can be seen from the interactive data flow structure diagram in Figure 3, the data communication protocol layer is an independent module, which is the content of the data communication layer between the MES and the PLC controller. The content involved in the communication layer 1 needs to be developed independently, which is the application layer protocol defined when the MES system interacts with the data communication layer. The communication layer 2 supports the current mainstream PLC communication protocols, such as PLC Handler, OPC-UA and other international mainstream communication protocols. For different PLC devices, it supports different communication protocols for data interaction [6]. The production data communication protocol proposed in this paper is the data interaction protocol between the communication layer 1 and the communication layer 2. Its data format is the content that is standardized by the protocol.
3. State Machine of Intelligent Manufacturing Equipment
This paper proposes a production data communication protocol that defines the state machine portion of the intelligent device. This protocol accurately transmits the working status of intelligent manufacturing equipment to the MES system, enabling the MES system to precisely control the intelligent manufacturing equipment and simultaneously allowing the MES system to monitor the status of the controlled intelligent manufacturing equipment, thereby truly realizing intelligent production. In all workshops of a smart factory, the states of intelligent manufacturing equipment can be mainly divided into seven categories: powered on, ready, running, suspended, alarm, sleep, and stopped. The communication protocol proposed in this paper defines each device state and describes in detail the conditions for state transitions. Only after the MES system reads the working status of the intelligent manufacturing equipment's state machine in real time can it gain a detailed understanding of the status of all intelligent equipment in the entire smart factory.
4. Communication Protocol Operation Flow
The communication protocol proposed in this paper is defined to enable normal communication between the MES system and the intelligent manufacturing equipment controller. Its data flow and communication protocol should run synchronously on both the MES system and the intelligent manufacturing equipment [7]. The following describes the protocol specifications for the processes and data run on the MES system and the intelligent manufacturing equipment. This protocol is defined at the application layer and does not limit the scope of the physical layer and the data link layer.
4.1 Communication Protocol and MES System Data Interaction Protocol
When the MES system interacts with the data communication layer, the main data content processed by the main loop is shown in Figure 4. The specific data content of the interaction includes six parts: instruction parsing, production scheduling information issuance, process parameter processing, real-time data processing, alarm and fault handling, and post-production scheduling processing.
Figure 4. Main loop of MES system data interaction protocol
4.1.1 Instruction Parsing
The MES system first needs to parse the instructions transmitted from the intelligent manufacturing equipment before it can perform further operations based on the instructions. The received instructions include two types: data instructions sent by the intelligent manufacturing equipment and responses from the intelligent manufacturing equipment to instructions sent by the MES system earlier. The specific instruction parsing process is shown in Figure 5.
Figure 5 MES System Command Receiver Analysis
Both the data commands sent by the intelligent manufacturing equipment and the commands sent by the MES system in response to the intelligent manufacturing equipment require exception handling. If a sent command is abnormal, an exception message is sent back to the intelligent manufacturing equipment, and the corresponding memory of the received command is cleared. If an abnormal response command is received, the relevant sent command is identified and resent. The command parsing module checks for anomalies in data connections and communication commands to ensure the normal operation of subsequent modules.
4.1.2 Issuance of Production Scheduling Information
Figure 6 illustrates the workflow in the communication protocol layer proposed in this paper where the MES system sends production scheduling information to the intelligent manufacturing equipment in the workshop. Here, the MES system has already assigned tasks to the corresponding intelligent manufacturing equipment based on existing data in the database, forming a corresponding production schedule. Following the specifications defined in the communication protocol, the generated production schedule is sent to the specific intelligent manufacturing equipment. The equipment can only correctly read the production schedule information and begin production when it is in a ready state.
Figure 6. Production scheduling information issued by the MES system.
4.1.3 Process Parameter Processing
After receiving a request for a process parameter table from a smart manufacturing device, the MES system first determines whether the smart manufacturing device is in operation. Only when the device is in operation will the system retrieve the corresponding process parameter table from the database based on the instruction and send it to the requested smart manufacturing device. If the smart manufacturing device is not in operation, the MES system will issue an alarm and send it to the production workshop for investigation and verification.
4.1.4 Real-time Data Processing
During production, intelligent manufacturing equipment needs to upload key production data to the MES system database in real time. This enables the MES system to monitor the intelligent manufacturing equipment and facilitates later data retrieval, thereby improving the intelligent management of the smart factory. When intelligent manufacturing equipment is running, it requests real-time data updates. After verifying the equipment status and the content of the transmitted data, the MES system connects to the database to create a new information list and stores the received data sequentially according to time. If the received data is incorrect or the equipment status is abnormal, the corresponding alarm information will be submitted to the production workshop for investigation.
4.1.5 Alarm Fault Handling
In the production process of intelligent manufacturing equipment, alarm handling and fault handling are two different methods. Since many alarms in intelligent manufacturing equipment can be triggered and cleared on-site in the workshop without requiring a repair request to the MES system, the MES system only stores the alarm record without triggering an alarm. If an alarm cannot be cleared on-site after it occurs and a repair request is needed, the operator must press the "Request Repair" button to submit a fault request, notifying the MES system to arrange for repair personnel. At this point, the intelligent manufacturing equipment enters an alarm state.
4.1.6 Processing after production scheduling is completed
After completing the production scheduling task, the intelligent manufacturing equipment needs to notify the MES system to make the next arrangement. At this time, the staff can press the "Apply to join the scheduling" button to notify the MES system to make a new round of production scheduling. If the equipment has a new production scheduling arrangement, it will enter the production scheduling information delivery module. If the equipment does not have a new production scheduling arrangement, it will be manually operated to put the equipment into a stopped state, sleep state, or keep ready state according to the arrangement of the MES system.
4.2 Communication Protocol and Data Interaction Protocol for Intelligent Manufacturing Equipment
Generally, when a new order is assigned to an idle intelligent manufacturing device, the MES system first notifies the staff via the workshop dashboard to start the device from a stopped or sleep state. The intelligent manufacturing device first enters the start-up state, and after the staff confirms that the device is in normal condition, they press the "Request to Add to Scheduling" button. Upon receiving the instruction, the MES system begins to send the scheduling task to the intelligent manufacturing device. When the intelligent manufacturing device interacts with the data communication layer, the data content processed in the main loop is shown in Figure 7. The specific data content of the interaction includes six parts: instruction parsing, scheduling task processing, process parameter processing, real-time data uploading, alarm and fault handling, and post-scheduling processing.
Figure 7. Main loop of device-side communication interaction
4.2.1 Instruction Parsing
When the intelligent manufacturing equipment interacts with the data communication layer, it also needs to parse the received MES system commands in order to perform relevant operations according to the command requirements. The commands received by the equipment include two types: one is the data command sent by the MES system, and the other is the command sent by the intelligent manufacturing equipment in response to the MES system.
Just like the instructions sent by the MES control system, the data sent by the intelligent manufacturing equipment also needs to be checked for abnormal instructions and verified for responses before it can be used normally.
The specific process of parsing instructions from the intelligent manufacturing equipment is shown in Figure 8.
Figure 8. Analysis of Command Received by Intelligent Manufacturing Equipment
4.2.2 Issuance of Production Scheduling Information
Before scheduling production tasks for intelligent manufacturing equipment, the MES system needs to understand the equipment status. It determines whether scheduling can proceed based on the equipment status bits. Scheduling can only be performed when the equipment is in a ready state; otherwise, the MES control system will prompt appropriate actions. After the MES system sends the scheduling information to the intelligent manufacturing equipment, and the instructions are verified as valid, the scheduling information can be manually imported into the equipment controller PLC.
4.2.3 Process Parameter Request
After importing production scheduling data into the PLC, the intelligent manufacturing equipment needs to continue importing production-related process parameters into the PLC in order to work normally. At this time, the equipment sends a process parameter request instruction to the MES system. After receiving and verifying the request instruction, the MES system extracts the requested process parameters from the database and sends them to the intelligent manufacturing equipment.
4.2.4 Real-time data upload
During normal production, each time a product is manufactured using intelligent manufacturing equipment, the relevant process parameters need to be packaged and uploaded according to a specific data format. This allows the MES control system to monitor the equipment's operation data in real time and achieve real-time monitoring of the intelligent manufacturing equipment. 4.2.4 Alarm and Fault Handling
During production, if an alarm signal is encountered, the intelligent manufacturing equipment will automatically trigger an alarm, enter an alarm state, and transmit the alarm sequence number to the MES control center for storage in real time. Workshop staff will handle the equipment based on the on-site alarm prompts. If a malfunction is detected requiring repair, staff will manually press the "Request Repair" button on the equipment panel to notify the MES system to arrange for repair personnel.
4.2.5 Processing after production scheduling is completed
After the intelligent manufacturing equipment completes all production according to the production schedule, it will trigger an alarm to notify the staff that production has ended. At this time, the production schedule completion signal will also be sent to the MES system simultaneously. The staff needs to verify whether the production schedule has ended. If the production schedule has indeed ended, they need to determine whether the machine is in normal condition. If it is normal, they can manually press the request production schedule button to confirm with the MES system that the production schedule has indeed ended and can schedule the equipment for the next production. The MES system will also arrange the equipment status according to the order requirements. If there are no order requirements, it will issue an instruction to the equipment to notify the staff to disconnect the power to the electrical parts of the equipment and enter sleep mode or disconnect the power to the entire equipment and enter shutdown mode.
5 communication instructions
Since the named communication commands are based on application layer protocols, the physical layer runtime environment of the commands is not given here. The data sending and receiving patterns are as follows:
#Data0，Data1:Data2:…Datan#
Data0 is the header instruction used to define the specific content of the Data1:Data2:…Datan data packets.
This article takes the sending and receiving of instructions by the MES system as an example to briefly describe the content of the header instruction Data0, as shown in Tables 1 and 2.
Table 1. Commands sent by the MES system
Table 2. Commands Received by the MES System
6 Conclusion
The data communication protocol proposed in this paper, applicable to smart factory production workshops, is an intermediate layer data communication protocol that connects the MES system with intelligent devices. Its implementation mainly takes place at the application layer of the MES system and intelligent device controllers. It has good scalability and portability, and can communicate with Beckhoff PLCs and Siemens injection molding machine controller PLCs. This communication protocol has also been successfully applied in the corrugated paper workshop renovation project, and its practical data interaction application is good.
