1. Introduction
With the rapid development of modern industry, multi-motor AC control systems have been widely used in industries such as papermaking, chemical engineering, steel, and food processing, and each industry has developed its own unique frequency converter control system. In actual operation, such control systems offer high reliability and strong anti-interference capabilities, but the corresponding maintenance costs increase over time. For example, a certain type of frequency converter uses its unique communication protocol and interface; a communication failure causing the communication interface board to burn out will directly lead to system failure unless a new communication interface of that model is purchased. Similarly, if the frequency converter burns out, a replacement of the same model must be purchased. Furthermore, system upgrades require the complete replacement of all frequency converters. As a result, huge procurement costs and excess spare parts inventory are inevitable.
This paper introduces a multi-motor AC control system based on fieldbus, which combines the most mainstream fieldbus technology and takes into account the convenience of practical applications. The main features of the multi-motor AC control system are described in detail from three aspects: bus introduction, hardware configuration, and software principle.
2. Introduction of Profibus fieldbus
Profibus, or Processfieldbus, is a fieldbus system developed since 1984. After more than a decade of development, production, and application, Profibus has become Europe's leading open fieldbus system. Currently, the PNO (Profibus User Organization) has over 600 members and produces nearly a thousand products. Profibus products hold a market share of over 40% in Europe and are widely used in manufacturing automation, building automation, process automation, power generation and transmission.
Based on ISO 7498 and using the Open Systems Interconnection (OSI) model as its reference, Profibus defines physical transmission characteristics, bus access protocols, and application functions. Profibus-DP , Profibus-PA, and Profibus-FMS constitute the Profibus family. Profibus-DP (Decentralized Periphery) is a high-speed and inexpensive communication connection specifically designed for communication between automated control systems and distributed I/O at the device level. Profibus-DP modules can replace 24V or 4-20mA serial signal transmission. The Direct Data Link Mapping (DDLM) provides a user interface that simplifies access to the data link layer, and transmission can use RS-485 technology or fiber optic media. This article focuses on the DP component.
The hardware of the standard fieldbus PROFIBUS DP consists of three parts: master devices, slave devices, and network. The master device controls data transmission on the bus and sends information and is authorized to access the bus when no external request is made. Slave devices are simpler external devices compared to the master devices and are not authorized to access the bus. The network consists of transmission media and network connectors. The former includes electrical networks constructed with shielded twisted-pair cables, fiber optic networks constructed with plastic or glass fiber cables, or hybrid networks based on OLM conversion between the two media. The latter includes RS485 bus connectors, RS485 bus terminals, RS485 repeaters, and optical link modules (OLMs).
In multi-motor AC control systems, the motor control points and AC frequency converters are characterized by long and dispersed distribution lines. By using fieldbus technology, distributed I/O modules can be installed in the control room and on the field control console, forming a bus network. This allows for the serial transmission of control data in a digital manner, which gives the control system better anti-interference capabilities and provides more accurate reference values to the frequency converters.
3. Hardware Configuration of Multi-Motor AC Control System
Figure 1 (top) is the hardware connection diagram of the multi-motor AC control system. As shown in the figure, the master device of the Profibus fieldbus uses a Siemens programmable logic controller (PLCS5), which communicates with the Profibus bus via an IM308 interface module. As the master device, the PLCS5 is responsible for reading the inverter status words (including digital and analog values) of all distributed I/O modules connected to the bus, and simultaneously transmitting inverter control words (including digital and analog values). The slave devices are distributed input/output modules (ET200). Each ET200 slave can connect up to 32 digital or analog input/output cards (such as DI, DO, AI, AO, etc.), and the address value of the connected I/O card can be confirmed by setting the address DIP of the ET200 slave interface card IM318.
Due to the remote distribution characteristics of the ET200 slave stations, corresponding slave stations are set up in areas where frequency converters are concentrated, and point-to-point connections are made to the associated frequency converters. The specific wiring method is shown in Figure 1 (below). All frequency converters have I/O interfaces, including digital inputs, digital outputs, analog inputs, and analog outputs. Digital inputs include forward start, reverse start, external fault (such as motor temperature rise), jogging, enable, and set frequency start. Digital outputs include frequency converter operating signals, fault signals, operating frequency range signals, and alarm signals. Analog inputs and outputs must first be set in U/I mode to confirm whether it is a 4-20mA signal or a 0-10V signal. Analog inputs are mainly used to set reference values for speed or frequency, while analog outputs are actual current values, actual speed values, actual frequency values, and DC circuit voltage values.
Figure 1 Hardware connection diagram of multi-motor AC control system
The hardware structure of the multi-motor AC control system is relatively simple. Centered on a programmable logic controller (PLC), it connects associated ET200 slave stations sequentially via a Profibus bus, and each slave station connects to a corresponding frequency converter group. The bus uses electrical twisted-pair wiring, making the cabling clear. A short control line connects the ET200 slave stations to the associated frequency converter groups. This data transmission method primarily uses serial digital signals, only employing a portion of analog signals when reaching the frequency converters; therefore, it does not affect the reliability or interference immunity of the transmitted data.
4. Software Design of Multi-Motor AC Control System
4.1 Software Flow of Multi-Motor AC Control System
The software design of a multi-motor AC control system based on fieldbus includes the following points, as shown in Figure 2 (above):
1) Initialization of PLCIM308 interface module: Before starting the fieldbus, the Profibus bus must be configured and the parameters of each bus station must be assigned (transmission rate on the bus, bus standard (usually DP-Standard), and fault mode settings (including diagnostic settings and diagnostic addresses, etc.).
2) Read the status word and actual value of inverter #1: By operating the remote I/O through the I/O terminal of the ET200 slave station connected to the inverter, the status word (digital quantity) and actual value (analog quantity) of the inverter can be read.
3) Determine if the frequency converter is faulty. If it is faulty, alarm and save the data. Otherwise, output analog values to the I/O terminals of the ET200 slave station connected to the No. 1 frequency converter according to the system requirements, including control words and set values.
4) Repeat steps 2 and 3 to perform read and write operations on inverters #2, #3, #4...N in sequence.
4.2 Logic Control Principle of Multi-Motor AC Control System
Logic control refers to the logical interlocking between the frequency converter and other external control signals to enable the start-up and operation of specific equipment. For example, when using frequency conversion control for a feed pump in a paper mill, the necessary conditions for the feed pump to start are: (1) the valve is fully open; (2) the liquid level in the tank is normal; and (3) the sealing water of the feed pump is normal. If any of these three conditions are missing, the equipment cannot be started. In addition to the above three conditions, the continuous operation of the feed pump also requires the condition that the pipeline flow rate and pressure are normal; otherwise, the operation of the feed pump will automatically stop.
The purpose of logic control is to protect personal safety, ensure normal equipment operation and stable production quality. Therefore, logic interlocking is an essential step in multi-motor AC control.
4.3 Closed-loop control principle of frequency converter
Normally, frequency converters operate in open-loop mode. However, for high production requirements, especially in multi-motor AC drive control, frequency converters must operate under closed-loop control to achieve the desired production goals. For example, closed-loop control is particularly important in continuous production lines such as film machines, cold rolling mills, and paper machines.
In Figure 2 (below), the frequency converter operates using closed-loop control. The torque control within the current loop is directly handled internally by the frequency converter and is not shown in this figure. The figure depicts the speed control closed loop. The analog input signal (whether current or voltage, depending on the frequency converter's manual, and requiring proper DIP switch and parameter settings) is compared with the filtered signal from the speed sensor after being limited by the highest and lowest frequencies. This signal then enters PID control (PID parameters can be tuned in the frequency converter settings), and after passing through the slope generator (including RAMPONTIME/RAMPDOWNTIME), it directly enters the current loop to control the motor output. This constitutes the frequency converter's closed-loop control.
Figure 2. Software flow of the multi-motor AC control system
5. Conclusion
By constructing this system, it can be widely applied in small pulp and paper mills, powder processing plants, building ventilation pump control and other fields. Because it is constructed through an open fieldbus Profibus system with IEC standards, it has (1) low-flexibility modification and expansion functions; (2) distributed control, improving system response speed and control accuracy; (3) reducing system unreliability and enhancing maintainability.
