Fieldbus technology emerged internationally in the mid-1980s. Applied in production environments, it enables bidirectional, serial, multi-node digital communication between microcomputer-based measuring devices. Adapting to the trend of industrial control systems towards decentralization, networking, and intelligence, it quickly became a global hot topic in industrial automation technology, attracting widespread attention worldwide. Since the late 1980s, several fieldbus technologies, such as FF, Lonworks, CANbus, and Profibus, have matured and influenced the progress of industrial automation. Profibus, short for Process FieldBus, is a fieldbus technology used for workshop-level monitoring and data communication and control of field devices in factory automation. It enables distributed digital control and field communication from the field device level to the workshop level, thus providing a feasible solution for achieving comprehensive factory automation and intelligent field devices.
Conveyor belt transportation is a crucial part of coal mine production, and the implementation of monitoring systems in this环节 (link/stage) is a trend in modern coal mine production. This article takes the underground conveyor belt monitoring system of Qidong Coal Mine under the Wanbei Mining Bureau as an example to briefly introduce the application of Profibus fieldbus technology in monitoring systems.
I. Profibus Fieldbus Technology (I) Overview of Profibus
Profibus is an international, open, and manufacturer-independent fieldbus standard widely used in industrial automation. Based on application characteristics, Profibus is divided into three compatible versions: Profibus-DP, Profibus-FMS, and Profibus-PA. Profibus-DP is a high-speed (data transmission rate 9.6 kbit/s to 12 Mbit/s) and economical device-level network, primarily used for communication between field controllers and distributed I/O, meeting the fast response time requirements of AC/DC speed control systems. Profibus-PA adopts the IEC II 58-2 standard, with a transmission rate of 31.25 kbit/s and provides intrinsic safety features, suitable for applications with high safety requirements and bus-powered applications. Profibus-FMS mainly solves shop floor communication problems, completing cyclic or non-cyclic data exchange tasks at medium transmission speeds.
(ii) Bus topology
Depending on the connection method from field devices to the controller, fieldbus topologies can take various forms, typically including linear, tree, and ring topologies. Profibus uses a linear structure, characterized by its simplicity. A single trunk line connects the controller to the mechanical device (controlled object), with bus cables branching from the trunk to the field devices. The controller scans all inputs at I/O stations and can send information to output channels when necessary. This bus structure enables multi-master and peer-to-peer communication, allowing two controllers to share information and I/O stations within the same system. Furthermore, an I/O device can be removed from the bus without shutting down the system, greatly simplifying maintenance.
(iii) Profibus-DP device types
Based on actual design requirements, this system adopts Profibus-DP. Each Profibus-DP system includes the following three different types of devices:
1. DP Master Type 1: This is the central component of a Profibus-DP application. Within a defined, recurring information cycle, the central controller or PC exchanges information with distributed slaves (DP slaves). Non-cyclically transmitted data does not change frequently compared to cyclically transmitted measurements; therefore, this data is transmitted alongside fast-cyclically transmitted useful data, but with lower priority. Interrupt acknowledgments in the master station ensure reliable transmission of interrupts from DP slaves.
2. DP Master Type 2 This type of device (such as a programmer, configuration device, or operating device) is used for the startup, configuration, or operation of the DP system during normal operation (such as diagnostics). This type of master station can read input, output, diagnostic, and configuration data from the device.
3. DP Slave A DP slave is an I/O device that reads input information and provides output information to I/O. The number of input and output information depends on the device type, with a maximum of 244 bytes.
(iv) Profibus-DP communication protocol
The Profibus fieldbus adopts the physical layer and data link layer of the OSI model, as shown in Figure 1. Its transmission rate ranges from 9.6 kbps to 12 Mbps, with a maximum transmission distance of 100 m at 12 Mbps and 400 m at 1.5 Mbps, which can be extended to 10 km using repeaters. Its transmission medium can be either twisted-pair cable or optical fiber, and it can connect up to 127 stations.
The Profibus-DP physical layer is the same as the first layer of the ISO/OSI reference model, using the EIA-RS485 protocol. Depending on the data transmission rate, it can use either twisted-pair cable or optical fiber as the transmission medium.
The PROFIBUS-DP data link layer protocol's Media Access Control (MAL) portion employs a controlled access token bus and a master-slave architecture. The token bus conforms to the IEEE 8024 local area network protocol; tokens are passed between masters on the bus, and the master holding the token gains control of the bus. This master communicates with slaves or other masters according to a relational table. The master-slave data link protocol differs from the LAN standard, conforming to the unbalanced normal response mode (NRM) in HDLC. The characteristics of this mode are: one master controls multiple slaves on the bus, establishing a logical link between the master and each slave; the master issues commands, and the slaves respond; slaves can continuously send multiple frames until no more information is sent, the required number of frames is reached, or the master stops them. The frame transmission process in the data link consists of three stages: data link establishment, frame transmission, and link release. The frame format transmitted between the master and slave in normal response mode is shown in Figure 2.
F is the frame flag field (8 bits). A is the slave address field. The control field C indicates the frame type, number, command, and control information, classifying HDLC frames into three types: information frames, monitoring frames, and unnumbered frames. Information frames are used for transmitting application data and piggybacking on responses; monitoring frames are used to monitor normal operation on the link and respond to link status (e.g., acknowledgment frames, requests for retransmission, or pauses); unnumbered frames (without an information field) are used to transmit various unnumbered commands and responses, such as establishing link operating modes, releasing the link, and reporting special situations. The information field consists of application data in PKW + PZD. PKW is used to read and write parameter values, such as writing control words or reading status words, and is generally 4 bytes long. PZD is used to store specific control values of the controller and set parameters for stations or status words, and is generally 2 to 10 bytes long. For example, the second byte of PZD can be set as the start/stop control bit for devices 0# to 7#. FCS is the frame check field, which performs cyclic redundancy check (CRC) on the entire frame content. HDLC frames can be up to 24 bytes long.
Instead of adopting the ISO/OSI application layer, Profibus-DP sets up its own user layer. This layer defines the functionality, specifications, and extension requirements of DP.
In summary, Profibus-DP offers significantly better real-time performance than other local area networks, making it particularly suitable for industrial environments.
II. Hardware Structure of the Downhole Conveyor Belt Monitoring System
Profibus-DP is used in the underground conveyor belt monitoring system of Qidong Coal Mine, Wanbei Mining Bureau. The hardware system is shown in Figure 3. The entire system consists of a host computer, a Profibus-DP master station, Profibus-DP slave stations, and their field devices. The Profibus-DP bus connects all the devices. Both the Profibus-DP master station and the Profibus-DP slave stations use SIMATIC S7-300 module series. The master station uses a CPU315-2DP series module, and the slave stations use corresponding I/O modules.
1. Distributed I/O System. This system uses the ET200 communication module connected to PROFIBUS-DP. The ET200 fully utilizes the SIMATIC S7-300 module series, connecting all S7-300 I/O modules to the fieldbus via the IM153 interface template. Actuators and sensors under the I/O modules are connected to field devices. The I/O modules provide output data to field devices and feed input data to the CPU or host computer in master/slave mode. The I/O modules are DP slave stations.
2. The CPU acts as a DP Type 1 master station, located in the control center. This system uses the CPU315-2DP modular medium-sized PLC, which has powerful processing capabilities and integrates a PROFIBUS-DP fieldbus interface. It also boasts a processing speed of 1024 statements in 0.3ms. After the PLC program is compiled using the STEP7 programming tool on the host computer, it is downloaded to the CPU315 and stored there. The CPU315 can automatically run the program, reading the status words of all I/O modules on the bus according to the program content to control the hardware devices.
3. The host computer is a DP type 2 master station. This system uses an Advantech industrial PC as the host computer, which is connected to the fieldbus via a CP5611 fieldbus interface card. This connects the industrial PC to the fieldbus network segment, forming a complete control network system capable of configuration, operation, and other functions. To ensure system stability, dual-machine redundancy is employed, with another industrial PC connected to the fieldbus via the same CP5611 fieldbus interface card. If one industrial PC fails, the other can continue operating.
III. Software Structure of the Downhole Conveyor Belt Monitoring System
The software architecture includes the Windows NT operating system, lower-level programming software, and upper-level monitoring software.
(a) Lower-level computer programming software
This system uses the STEP7 programming tool, the companion programming tool for SIMATIC S7-300, to complete hardware configuration, parameter setting, PLC program development, testing, debugging, and documentation. Typically, a user program consists of organization blocks (OBs), function blocks (FBs, FCs), and data blocks (DBs). OBs serve as the interface between the system operating program and the application program under various conditions, controlling program execution. FBs and FCs are user subroutines. DBs are user-defined storage areas for data access; in this system, they are the data interface points between the host computer monitoring software and the STEP7 program. Configuring the corresponding DB block in MPI enables the data interface between the host computer monitoring software (FIX) and the STEP7 program.
(ii) Host computer monitoring software
FIX industrial control configuration software is a large-scale application software developed by Intellution, Inc. of the United States, based on Windows 9X & NT. It integrates control technology, human-machine interface technology, graphics technology, database technology, and network technology. It includes components such as dynamic display, alarm, trend, control strategy, and control network communication, and provides a user-friendly interface that allows users to generate corresponding application software according to actual production needs.
1. Interface with PROFIBUS fieldbus (1) Data flow FIX uses I/O drivers to read and write data from the device. Each I/O driver supports its specific hardware. For the PROFIBUS network of this system, MPI drivers are used to obtain data from it. The FIX configuration software first obtains data from the process hardware in the field through the MPI driver software interface and stores it in the DIT driver image table (the driver image table is actually a memory area when the system is running). The FIX internal database (PDB) obtains the data it needs from the DIT table through the SAC program. The application software (such as the FIX screen running program, report generation program, etc.) obtains information from the process hardware from the FIX internal database through the internal database access software. In this way, the operating status of each process hardware in the field can be dynamically displayed on the industrial process screen. The data can also be written back to the process hardware in the field in reverse order to execute control operations. The corresponding data acquisition process is shown in Figure 4.
(2) MPI Configuration A crucial issue in MPI driver applications is the address translation between STEP7 and FIX. The DB blocks set in STEP7 should be translated into MPI DB blocks, which needs to be implemented in the MPI configuration. MPI configuration includes channel, device, start address, and other parameters to ensure that the MPI DB blocks correspond to the DB blocks set in STEP7, allowing the FIX application to obtain field data.
2. User Interface Development The human-machine interface developed for this control system includes the following types:
(1) Information display screen The information display screen mainly displays the current operating status information values of each conveyor belt, such as the current conveyor belt speed, the position of the coal storage bin, and some fault information, such as belt deviation, blockage, slippage, etc. Different colors can be used to indicate whether the current status is normal or abnormal.
(2) Although the lower computer program can realize data acquisition and control signal output on the fieldbus and implement some simple control algorithms such as PID control, complex control functions still need to be manually controlled on the upper computer. Clicking the corresponding device button on the screen can control the device individually.
(3) Real-time alarm processing: The system judges the data collected in real time, issues alarm signals, processes them according to technical requirements, and automatically performs corresponding equipment control, such as unlocking and restoring tape fault signals.
(4) Report printing: Real-time reports are developed using FIX’s DDE function and have the function of printing at any time.
(5) Real-time data curves display the trend curves of important parameters of the monitoring equipment, so as to understand the operating status of the equipment over a period of time.
(6) The historical trend screen function is similar to the real-time data curve, except that it displays the operating parameter values of the device over a period of time.
The underground conveyor belt monitoring system at Qidong Coal Mine of Wanbei Mining Bureau is now in operation. The equipment is functioning well, generating significant economic benefits and receiving high praise from users.
