Abstract: To reduce motor energy consumption in industrial settings, a system controlling multiple frequency converters was designed and implemented using an S7-200 PLC as the control master station and employing USS serial communication. The communication control program and MM440 frequency converter settings are easy to implement, and the underlying equipment wiring is simple and easily understood by technical personnel. Experimental results show that the entire system can fulfill control requirements such as segmented speed regulation and stepless speed regulation, operates stably and reliably, and demonstrates strong practicality and high cost-effectiveness.
Keywords: USS communication, MM440 frequency converter, S7-200 PLC, touch screen
0 Introduction
With the increasing development of power electronics technology and automatic control technology, the speed regulation of motors is no longer a concept of a single speed regulation system. The speed regulation control of frequency converters has been developed to the point that it has been widely used in various industrial fields. In modern industrial automation control systems, such as textiles, light industry, building materials, etc., a common speed regulation system is to use PLC to control frequency converters to realize the speed regulation control of multiple motors. This speed regulation system can realize the variable frequency speed regulation of multiple motors based on the network communication between PLC and frequency converter [1].
The control system connects the touchscreen, PLC, and frequency converter via a bus. Variable frequency control of the motor is achieved through project components downloaded to the touchscreen and programs in the PLC, including start/stop, forward/reverse rotation, multi-stage speed regulation, stepless speed regulation, and parameter reading/writing. This paper uses an S7-200 series PLC as the control core, and controls the operating status of the MM440 frequency converter by changing parameters in the PLC through operation of the TP270-6 touchscreen. The variable frequency speed control system designed in this paper mainly focuses on the variable frequency speed control of the motor and the communication between the frequency converter network and the PLC.
1. Composition of the speed control system
1.1 System Equipment Introduction
The entire speed control system consists of: one host computer (installed with WinCCflexible and STEP7-MicroWIN software), one TP270 touch screen, one Siemens 224 PLC, three MM440 frequency converters, and matching motors. The structure of the entire system is shown in Figure 1.
The host computer uses programming software to design the touchscreen control screen and the USS communication and control program in the PLC. The program is downloaded to the touchscreen via RS-232 serial communication and to the PLC via PC/PPI communication. After editing, the host computer disconnects from the system and does not participate in the motor speed control process. The Siemens touchscreen is connected to the PLC's Port1 serial port, exchanging data via MPI serial bus. The S7-200 PLC's Port0 serial port connects to three frequency converters via RS-485 serial bus. The MM440 frequency converters have communication ports 29 (P+) and 30 (N-).
Figure 1. Composition of the speed control system
1.2 USS Communication Protocol
The USS protocol is a serial data communication protocol defined by SIEMENS AG. All SIEMENS drive products support this universal protocol. Compared with Profibus and other protocols, the USS protocol does not require the purchase of communication accessories and is a low-cost, high-performance industrial network configuration connection solution [2].
The USS protocol uses a master-slave architecture, allowing one master station and up to 31 slave stations to be connected on the bus. Slave stations cannot send data first unless the master station requests communication, and direct data transmission between slave stations is also prohibited. The master station is typically a PLC or PC, while the slave stations can be frequency converters or DC speed controllers.
In other serial communication scenarios, such as communication between PCs and smart terminals or embedded system networks, users have to develop their own protocols due to the lack of readily available standard protocols. The USS protocol, due to its simplicity, efficiency, flexibility and ease of implementation, is also widely used in these scenarios.
2. USS Communication Program and TP270 Screen Design
2.1 USS Communication Programming
STEP7-MicroWIN programming software has library instructions that implement the USS communication protocol. It is necessary to add and allocate the corresponding library storage area. It includes 14 subroutines, 3 interrupt routines and 8 instructions. When writing the communication program, the instruction block needs to be called in the main program, as shown in Figure 2 [3].
Figure 2 shows the invocation of USS communication commands.
As shown on the left of Figure 2, the initialization instruction is: Mode 1 in USS_INIT indicates that the Port0 communication port is used for the USS protocol and the protocol is enabled; Baud is the baud rate, which can be set as needed (including 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200); Active indicates the driver activation number. All activated drivers will be automatically polled in the background to control the driver search status and prevent the serial link of the driver from timeout; Done bit indicates that the output is on; Error byte stores the execution result of the instruction [4].
Figure 2 on the right shows the inverter drive control command USS_CTRL. The EN bit is the enable terminal, which needs to be kept on using SM0.0; the RUN bit indicates whether the drive is on or off; the OFF2 bit is used to allow the MicroMaster drive ramp to decrease to a stop; the OFF3 bit is used to command the MicroMaster drive to stop quickly; the F_ACK (fault acknowledgment) bit is used to acknowledge drive faults. When F_ACK changes from 0 to 1, the drive clears the fault; the DIR (direction) bit indicates which direction the drive should move; Drive (drive address) is the address of the MicroMaster drive, to which the USS_CTRL command is sent, with valid addresses ranging from 0 to 31; Type (drive type) selects the type of drive, which is 1 for the 4-series MicroMaster drive; Speed_SP (speed setpoint) is the drive speed, which is a percentage of full speed. The percentage; a negative value of Speed_SP causes the drive to rotate in the opposite direction, ranging from -200.0% to 200.0%; Error is an error byte containing the execution result of the most recent communication request sent to the drive; Status is the original value of the status word returned by the drive; Speed is the drive speed, which is a percentage of full speed, ranging from -200.0% to 200.0%; Run_EN (RUN enable) indicates whether the drive is running (1) or stopped (0); D_Dir indicates the direction of drive rotation; Inhibit indicates the state of the inhibit bit on the drive (0--not inhibited, 1--inhibited), to clear the inhibit bit, the Fault bit must be zero, and the RUN, OFF2 and OFF3 inputs must be disconnected; Fault indicates the state of the fault bit (0--no fault, 1--faulty), to clear the Fault, the fault must be cleared and the F_ACK bit must be turned on[4].
2.2 TP270 Screen Design
Before designing the interface, a comprehensive system analysis is necessary to ensure a good understanding of the design's structure, functions, and control requirements. After completing the hardware connection and software communication debugging of the touchscreen, the control interface design can begin.
The interface of the TP270-6 touch screen is designed using WinCCflexible software. There is one main user interface, which is used to display the control schematic diagram and access the monitoring interface, which can be accessed by administrators and users (password required). There are four data object interfaces, namely the monitoring interface (Figure 3), the continuously variable transmission interface (Figure 4), the parameter reading interface (Figure 5), and the parameter writing interface (Figure 6).
The design of the control screen can be carried out in the following steps [5]:
① Open WinCCflexible software and create a new "project".
② In the "Project" window, click the "Object" option to edit the image. Return to the "Project" window and double-click the "Image" option to complete adding the image. In the Image window's menu bar, select the "Object" option, select the desired graphic or component to insert, and then edit them.
③ Return to the "Project" window and click the "Variables" option to add and edit variables. Double-click the "Variables" option in the "Project" window. Multiple variables need to be created here. Edit the variables in the "Variables" dialog box. Variable name, PLC, type, range, acquisition cycle, and number of components can all be set and modified according to actual needs. Select the button in the "Object" option, and then drag it with the left mouse button in a blank area of the interface to generate a button icon. The button's displayed text can be changed in the "Text" section of the "General" tab. The operation settings for this icon are located at the bottom of the main interface; make the corresponding modifications and selections to ultimately achieve the control requirements.
④ Connect the screen to the variables, and then click the "Compile" and "Download" options in the toolbar in sequence to complete the compilation of the project and download it to the touch screen.
3. Parameter settings for the MM440 frequency converter
Siemens frequency converters offer three panel options for users: the Status Display Panel (SDP) with two LEDs to indicate the converter's operating status; the Basic Operation Panel (BOP) for setting various parameters, with values and units displayed using 5 digits; and the Advanced Operation Panel (AOP) for reading and writing parameter settings, capable of storing up to 10 sets of settings. This article uses the most commonly used BOP for setting the converter's parameters.
To achieve USS communication, the Siemens MM440 frequency converter needs to perform specific parameter settings in three steps. The first step is to set P0010=30 and P0970=1 to restore the frequency converter parameters to the factory settings. The second step is to modify various parameters related to the motor, including the motor nameplate parameters, ramp-up time and ramp-down time. The third step is to determine the communication method and communication address. The parameter settings for this step are shown in Table 1 [6].
Table 1 MM440 Inverter Parameter Settings
Communication between the USS master station (S7-200PLC) and the USS slave station (drive unit, i.e., frequency converter) is asynchronous. The communication operation runs in the background, and sending and receiving data is independent of the control logic. The user program controls the start/stop of the frequency converter and changes the set frequency value by changing the STW and HSW values in the USS message. The USS message is sent to the drive unit via a send command, and the USS message returned by the frequency converter is received via a receive command. Only one send command or receive command can be activated at a time.
1 Conclusion
Energy shortages have become increasingly severe over time, making energy conservation, emission reduction, and environmental protection urgent priorities. As the most common electrical equipment in industrial settings, motors, particularly variable frequency drives (VFDs), have garnered significant attention from technical experts for their energy-saving measures. This paper utilizes the popular Siemens MM440 VFD to achieve USS communication between a single master station and multiple slave stations. This simplifies system wiring, improves motor performance, saves electricity, and increases reliability, providing a superior control solution for various industrial settings such as metallurgy, manufacturing, and textiles.
Author Basic Information
Name: Chen Fei Postal Code: 830000
Institution: School of Electrical Engineering, Xinjiang University; Graduate Student Email: [email protected]; Mobile: 15026035070
Contact Address: 2011 Graduate Students, School of Electrical Engineering, South Campus, Xinjiang University, Yan'an Road, Tianshan District, Urumqi, Xinjiang
