Abstract: This paper introduces a four-motor coordinated control system composed of an S7-300 series PLC and a Schneider ATV71 series frequency converter. This system has been successfully applied in the control system of a four-motor roving frame. The paper explains the hardware structure and working principle of the system, and focuses on the communication programming method between the PLC and the frequency converter in the Profibus-based control system.
Keywords: Profibus, PLC, frequency converter, communication technology
1. Introduction
Programmable logic controllers (PLCs) and AC frequency converters are widely used in industrial control. To meet the control requirements of coordinated operation of multiple frequency converters in a control system, most frequency converters on the market have network communication capabilities and open their communication protocols to users for ease of use. Among the various bus types used in numerous control systems, Profibus has gained widespread application due to its unique technical characteristics, open standard, and stable transmission performance.
In a PROFIBUS-based control system, the PLC acts as the master station, responsible for bus management and slave communication. The frequency converter, as the actuator, is a slave station. The PLC's powerful logic processing and communication capabilities can be fully utilized through PROFIBUS. This paper describes a control system that uses a Siemens S7-300 series PLC (315-2DP) and a Schneider Electric ATV71 series frequency converter, and applies it to a four-motor roving frame control system in the textile industry.
2. System Hardware Structure
The system hardware structure diagram is shown in Figure 1. The four motors in the control system are controlled by corresponding frequency converters. The speed control of the four motors must strictly follow the requirements of the roving process. The PLC calculates the speed changes according to the process curve at different stages to ensure good product quality.
In the control system, the PLC acts as the master station. At different stages of the spinning process, the PLC calculates the speed of each motor according to the program and sends the speed command signal to the frequency converter according to the PROFIBUS communication protocol. After receiving the command signal, the frequency converter executes the action and sends back the operating status and fault alarm signals to the PLC.
The S7-300 315-2DP has two communication interfaces: MPI and Profibus-DP. In this control system, the MPI interface is used to connect to the Proface human-machine interface, facilitating operator control of the entire machine. The Profibus-DP interface leads to the Profibus bus, through which the ATV71 frequency converter communicates with the PLC in real time. The Profibus bus can support up to 127 slave stations, increasing the flexibility for system expansion.
Schneider Electric's ATV71 frequency converter is a high-performance flux vector control frequency converter with excellent low-frequency characteristics, which can well meet the low-frequency requirements of roving processes. However, its built-in bus interface is a Modbus bus, which only conforms to the Modbus bus protocol standard, so it cannot be directly connected to the PROFIBUS bus. Therefore, a PROFIBUS-DP communication card needs to be selected for each frequency converter to support communication with Siemens PLCs.
Figure 1: Hardware structure of the control system for a four-motor roving frame
3. Bus communication frame format and state machine
According to the PROFIBUS-DP protocol, data exchange follows a master-slave model, where only the master station can initialize bus communication. The slave station operates similarly to a server, responding to request signals from other slave stations. Multiple masters can operate on the same bus, all capable of reading I/O signals from slave stations, but only one master station can perform write operations on a slave station.
3.1 Communication Protocol Frame Format
Data exchange requires a fixed frame format, which is called PPO (Parameter-Process Data-Object) in the PROFIBUS-DP bus protocol. A PPO consists of two parts: PKW and PZD. The PKW is used to read and write non-periodic data, including parameter settings, configuration, and diagnostics; the PZD is used to read and write periodic data, i.e., I/O signals. In the PROFIBUS-DP bus protocol, there are five types of PPOs, each with different word count requirements for the PKW and PZD.
The ATV71 Profibus communication card requires PPO5 as the input/output data frame format, as shown in the table below:
Table 1: Output Data Frame Format
Table 2: Output Data Frame Format
PKW section: PKE represents the logical address of the parameter to be modified, which can be obtained by querying the inverter's internal communication variable table; R/W/N represents read/write requests or error messages, respectively represented by ASCII codes; PWE is the value of the parameter to be modified.
The PZD section includes: CMDD (Command Word), which sends instructions to the inverter to perform a specified function; LFRD (Reference Speed in RPM), which is the reference speed to be sent to the inverter; and CMI (Internal Command Register), which mainly supplements the commands. ETAD (Status Word), which reflects the current state of the inverter; and RFRD (Actual Motor Speed in RPM). This control system is most concerned with the value of PZD2. The PLC needs feedback from the inverter on the actual motor speed, and then performs speed correction through the program. Other parameters relate to the motor's current, voltage, and other states.
3.2 State Machine
The ATV71 is a flux vector inverter, meaning that before the inverter can send a frequency signal, it must first establish magnetic flux to the motor. Once the flux is established, a frequency signal is sent to the inverter to start it. The flux establishment process must strictly follow the inverter's communication state machine, as follows:
Figure 2: Communication State Machine
According to the state machine diagram above, after the inverter is powered on, its state changes to "enabled but not enabled," and it is in a locked state with a state word ETA of 16#xx40. First, command word 16#0006 is sent, changing the inverter's state to "ready to start," and it is in a waiting state with an ETA of 16#xx21. Then, command word 16#0007 is sent, changing the inverter's state to "on," and it is in a ready state with an ETA of 16#xx23. Finally, command word 16#000F is sent, changing the inverter's state to "operating potential energy," and it is in a running state with an ETA of 16#xx27. At this point, the inverter has pre-established magnetic flux for the motor, and sending a frequency will start the motor. Furthermore, command word 16#0000 can be sent to turn off the established magnetic flux, returning the inverter to the locked state, and the ETA will also change to 16#xx40.
4. Communication Program Flowchart
Based on the state machine, the communication program consists of three main modules: the inverter initialization module, the speed transmission module, and the inverter shutdown module. All three modules are housed in the OB1 main loop module within the PLC. During machine operation, the three states of the inverter are continuously checked in a loop. Start-up and shutdown are controlled by switches on the human-machine interface. The specific program flowchart is as follows:
Figure 3: Program Flowchart
5. Summary
The innovation of this paper lies in proposing a programming method for connecting Siemens S7-300 series PLCs and ATV71 series frequency converters via the Profibus bus. This communication scheme has been successfully applied to the four-motor control system of roving frames in the textile industry, which well meets the stringent requirements for the low-frequency characteristics of frequency converters in the roving process, and greatly improves product quality.
References
[1] Wang Renxiang and Wang Xiaoman, eds. Selection, Application and Maintenance of General-Purpose Frequency Converters [M]. Beijing: Posts & Telecom Press, 2005.6
[2] Qiu Gongwei (ed.). Programmable Logic Controller Network Communication and Applications [M]. Beijing: Tsinghua University Press, 2000.3
[3] Liao Changchu (ed.). S7-300/400 PLC Application Technology [M]. Beijing: China Machine Press, 2005.3
[4] Wu Zhengang, Chen Hu. PLC Human-Machine Interface and Programming [J]. Microcomputer Information, 2005, 8-1: 21-23
About the author:
1. Gao Yong (1982-), male (Han), Hefei, Anhui, Department of Automation, Shanghai Jiao Tong University, Master's degree, research interests: fieldbus communication and intelligent control as well as multi-motor synchronous coordination control.
2. Gong Liang, male (Han), Shandong, Department of Automation, Shanghai Jiao Tong University, Master's degree, research direction: industrial intelligent system control
3. Yang Yupu, male (Han), from Shaanxi, is a professor and doctoral supervisor in the Department of Automation at Shanghai Jiao Tong University. His research interests include neural networks and intelligent control.
