Research on Large Equipment Monitoring System Based on MODBUS Fieldbus

Abstract: This paper briefly introduces the fieldbus technology of Modbus communication protocol, and focuses on the software and hardware technology for remote monitoring and monitoring of large equipment in fully mechanized mining face.

Fieldbus technology is a revolution in the field of automation. Because the field bus is simple, reliable, economical and practical, it has become a hot spot in the development of automation field. At present, the new continuous casting machine has begun to use fieldbus technology.

Fieldbus is a digital, two-way transmission, multi-branch communication network that connects intelligent field devices and automation systems. It mainly solves the digital communication between field devices such as intelligent instrumentation, controllers and actuators in industrial fields and these sites. The problem of information transfer between the control device and the advanced control system. It is mainly used in industrial sites to realize bidirectional serial multi-node digital communication between microcomputer centralized control devices.

In order to improve mine safety to a greater extent and cater to the future development trend of unmanned working face, the realization of remote monitoring and monitoring of large equipment in coal mine underground fully mechanized mining face is very important. At present, large-scale equipment such as coal mining machines, scraper conveyors, crushers, and transfer machines in fully mechanized mining faces are mostly controlled by intelligent combination appliances, but lack remote monitoring and monitoring functions. This paper takes HT6L1-400Z/1140 intelligent combination electric appliance as the research object. According to the network characteristics of the existing safety production monitoring and control system of coal mine, the communication software is compiled by Modbus bus technology, which realizes the seamless link between the combined electric appliance and the safety monitoring and control system. , to achieve the purpose of remote monitoring and monitoring of large equipment.

1 Introduction to Modbus Protocol

The Modbus communication protocol was developed by Modican and is one of the mainstream fieldbus protocols today. Through it, the centralized control devices and the centralized control devices can communicate with each other via the network and other devices; through it, the control devices produced by different manufacturers can be connected into an industrial control network for centralized monitoring and the like.

1.1 main features of Modbus protocol

The Modbus protocol has the following features:

1) The physical interface conforms to the EIA-485 specification; can form a single master control network for master-slave access; completes the read and write operations on the slave node through simple communication messages;

2) When the primary node polls, that is, accesses the secondary nodes one by one, it is required to return a response message from the node; the primary node can also perform broadcast communication on all the secondary nodes on the network segment.

1.2 Modbus protocol message frame format

The Modbus communication protocol has two message frame formats:

1) ASCII and RTU frame formats. The ASCII message begins with a colon character and ends with a carriage return line feed. The transmission characters used by other fields are hexadecimal;

2) The RTU message starts and ends with a period of idle time, which is not less than 3.5 times the character transmission time T, which is generally 4T.

The Modbus communication protocol defines the format of a message frame that is continuously transmitted over these networks, and determines the method of packing data into message frames and decoding. The frame format of the Modbus communication protocol packet using the RTU format is shown in Table 1.

Table 1. RTU message frame format of Modbus communication protocol

1.3 function of each function block in the RTU frame

The function blocks in the RTU frame function as follows:

1) Address field: The address field is the slave node address to be queried. The slave node's effective address range is 0~247, where 0 is the broadcast address.

2) Functional domain: The code range of the functional domain is 1~255. Some codes are applicable to all controllers, some are used for specific controllers, and some are reserved for user function extensions. When a message is sent from the master to the slave, the function code field tells the slave which actions to perform. For example, function code 03H is a read register (internal word variable) for reading the register value of the slave node.

3) Data field: The data field includes the number of bytes of the data field of the frame, and the data bytes l~n. These data may be I/O values, status data or other measurement control information.

4) CRC check field: RTU frame check in Modbus communication protocol usually adopts CRC cyclic redundancy check. The entire information frame is transmitted in a continuous stream of data bits, and the CRC check verifies all data of the message frame from the address field.

The slave identifies the message sent by the primary station by address and decides what action to perform. The master device can communicate with the slave device separately, and the slave device returns a message as a response; the master device can also communicate with all slave devices in a broadcast manner, at which time the slave device does not respond. The normal response of the slave is to send the same message frame for the master to acknowledge. If there is an error, the function code will be modified to indicate that the response message is incorrect, and the data segment contains the code describing the error message. The error check field allows the master to confirm that the message content is available. When configuring each controller, all devices on a Modbus network must select the same transfer mode and serial parameters.

2 system design

2.1 Centralized control of fully mechanized mining face

At present, most of the large-scale equipment in China's fully mechanized mining face is controlled by intelligent combination appliances. The HT6L1-400Z/1140 intelligent combination switch is a widely used one. It can mine coal in fully mechanized mining face. Machines, scraper conveyors, transfer machines, etc. for centralized control and protection. Although the S7-200 PLC in the combination switch has an RS485 interface, it does not have a communication function, and cannot be remotely controlled by the existing safety production monitoring and monitoring backbone network. The safety production monitoring and monitoring system with the Modbus bus technology as the monitoring and monitoring backbone network has a relatively high proportion in the coal mine. In order to be compatible with the Modbus field bus, the PLC (monitoring substation) in the combined electrical appliance performs centralized monitoring on the electrical equipment on the working surface. At the same time, remote monitoring and control must be supported. That is, the monitoring substation can monitor, control and protect each motor circuit in real time, and can also upload the voltage, current, running status and fault information of each load in response to the polling of the main station. In addition, the monitoring substation also supports the control commands of the master station, such as forcing multi-coil commands (function code is hexadecimal number 0F), writing multiple register commands (function code is hexadecimal number 10), and so on.

2.2 Hardware Design

The measurement and control system of HT6L1-400Z/1140 intelligent combination electric appliance is composed of signal detection, signal acquisition, signal processing, control mode conversion, Chinese character display and PLC, which can independently control and protect the six-way load. The measurement and control system can complete protection functions such as leakage lockout, overload, short circuit, phase failure, undervoltage and overvoltage, and the pilot circuit is an intrinsically safe circuit. It has single-loop independent control, multi-loop program control, single-machine two-speed control and two-machine two-speed control. It has the advantages of high intelligence, stable performance and reliable operation.

In order to improve the versatility of the monitoring software, the operator can manually set the communication parameters of the monitoring substation according to the actual situation of the network, to avoid having to modify the software due to network parameter changes, and design communication parameters (baud rate, verification mode, The dial input circuit of the slave address, etc.). The input of communication parameters requires five dials to be completed. The first three are used to set the station address from 1 to 247. The fourth is set to the baud rate (0~7 stands for 1200, 2400, 4800, 9600, 19200, 38400 respectively). Eight kinds of baud rates such as 57600 and 115200 (bps); the last one is used to set the parity mode (0~2 means no parity, odd parity, even parity). In addition, in order to prohibit the operation of large-scale equipment in the fully mechanized mining face at the same time and avoid accidents, a conversion joint is added to the measurement and control system of the intelligent combination switch, and the two mutually exclusive states provided by it are used as the field control and The enable flag for remote control thus avoids the possibility of both field control and remote control being effective at the same time.

The central processing unit of the HT6L1-400Z/1140 intelligent combination unit is Siemens S7-200 PLC (CPU 226) [3]. As long as the corresponding communication software is compiled, the Modbus protocol can be well supported, and communication with the host computer is realized. In this case, the CPU 226 only has the port Port0 supporting the Modbus remote communication protocol [4], and the port Port1 is used for communication with the liquid crystal display TP7 in the measurement and control system.

2.3 Software Design

The communication software is designed in the STEP7-Micro/WIN programming software environment. The instruction library contains subroutines and interrupt service programs designed for Modbus communication [4], which makes the preparation of the communication software of the measurement and control system simple and easy [5].

2.3.1 Software Compilation

The HT6L1-400Z/1140 intelligent combination switch measurement and control system itself is mature in both hardware and software, and its own intelligence is relatively high. It also has the hardware foundation to support the remote monitoring and monitoring of the host computer, so how to monitor the program well The software integrated in the measurement and control system is the key and difficult point in the software compilation of this substation.

The software flow chart of the monitoring substation is shown in Figure 1. Initialization includes the original system hardware and software initialization and Modbus communication initialization. Communication initialization only needs to be performed once in the program. After the initialization is completed, the operator can check the correctness of the communication parameters and the current setting value through the setting screen of the liquid crystal display TP7. The Modbus communication program detects and responds to the request of the primary station in real time, and uploads working condition parameters of each controlled motor to the primary station, including operation mode, opening and closing state, current, voltage, fault status and fault parameters; The monitoring substation also supports control commands of the primary station, such as forcing single or multiple coils, writing single registers or multiple register commands. The master station commands supported by this monitoring substation are shown in Table 2.

Table 2. Master commands supported by this substation

2.3.2 Software Configuration

After programming with STEP7-Micro/WIN, you need to configure the communication block, including symbol table configuration, communication data area configuration and command parameter configuration, otherwise it is not a unified whole.

The symbol table configuration allocates a 780-byte V-region address space to the symbol table of the communication program, and it cannot overlap with the communication data area.

The communication data area configuration is the configuration of the parameter storage space such as voltage, current, system status and fault information that need to be uploaded. The substation needs to allocate 20 words space to store these parameters in real time, including six-way setting current, system state, system voltage, six-way operating state and six working currents. The specific storage format is shown in Table 3. Considering the expansion of the system in the future, the communication data area has a certain margin, and 32 word spaces are actually allocated here.

Table 3. Data storage format for communication data area

The configuration of the command parameters is the setting of some key parameters in the communication command, including Modbus slave address, communication baud rate, parity selection, additional inter-character delay, maximum I/Q, maximum AI, maximum data area, etc. setting. Among them, the slave address, communication baud rate and parity mode can be input by the dialing disk in the form of fixed parameters. Other parameters must be configured in the program. These parameters must be consistent with the actual monitoring network parameters and actual requirements, so that the monitoring substation can operate normally.

3 on-site commissioning

In order to verify whether the communication program of the monitoring substation can run normally and reliably, after the program is programmed, the communication test can be performed by using software tools (such as Modscan 32, Commix) that support Modbus RTU serial port debugging, so that the program can be determined in the laboratory. Can run reliably.

Through the field communication test of the serial debugging software Commix, it can be determined that the monitoring software of the substation is reliable, and it can correctly respond to the common master commands in Table 2. For example, when the system initialization is completed, the display screen of the TP7 of each loop current setting value and system voltage value of the monitoring substation is as shown in FIG. 2, and the current setting value and the system voltage value monitored by the Commix software at this time (16) The display is exactly the same as the display value of the TP7 screen, as shown in Figure 3. The hexadecimal numbers 0032 and 0096 in the figure correspond to the current setting values ​​of 50A and 150A, and 047C is the system voltage value.

The above shows that the monitoring function of the monitoring substation is correct and reliable, and the field monitoring function test of the Commix serial port debugging software shows that the remote monitoring function of the monitoring substation is also real-time and reliable. For example, after starting six motors by using the write multi-coil command (0FH), stop (0FH) the second, third, fourth, and sixth motors, and stop the monitoring screens of the four motors as shown in Figure 4. At this time, the monitoring The TP7 display screen of the substation is shown in Figure 5. The lower byte of the hexadecimal number 0011 in Figure 4 is 11, and the lower six bits of the binary form correspond to the start and stop control bits of six motors (1: start or hold operation; 0: stop). The display content of Figure 5 illustrates that the monitoring function of the monitoring substation is also real-time, accurate and reliable.

The system has been successfully applied to the monitoring and monitoring of large-scale equipment in the fully mechanized mining face of the Hongjingta Mine, and the network is the KJ95 system produced by Changzhou Automation. The on-site debugging results show that the system parameters are transmitted accurately and in real time, which not only can realize the monitoring function of the running equipment status and parameters, but also realize the remote control of the equipment through the backbone network.

4 Conclusion

Based on the existing centralized control system for large-scale equipment in coal mine underground mining face, according to the characteristics of coal mine safety production monitoring and control system, the research on large-scale equipment monitoring and monitoring technology of Modbus bus was made, and the system hardware was re-engineered. Configuration, communication procedures have been compiled.

The field operation results show that it has a single distributed controlled device as the network node, with the field bus as the link, connecting all the controlled devices into a network system that can communicate information with each other and jointly complete the automatic control tasks, with decentralized control, The system has the advantages of simple structure, saving hardware equipment, easy installation and maintenance, reliable hardware, reasonable software writing, real-time and accurate response to the request of the main station, and seamless link between the combined electrical appliance and the safety monitoring and monitoring system. The purpose of equipment remote monitoring and monitoring meets the industrial control requirements of coal mines.