With the rapid development of modern industry, multi motor AC control system has been widely used in papermaking, chemical industry, steel, food and other industrial fields, and formed their own unique inverter control system. In actual operation, such a control system has strong reliability and high anti-interference rate, but the corresponding maintenance costs are getting higher and higher with the passage of time. For example, a certain type of frequency converter adopts its unique communication protocol and communication interface. Once the communication interface board is burned due to communication failure, the system will collapse directly, unless the communication interface of the model is purchased; Similarly, if the frequency converter is burnt out, it is necessary to purchase the same type of frequency converter; In addition, to upgrade the system, all inverters must be replaced. In this way, the phenomenon of huge purchase cost and overstock of spare parts inventory will inevitably exist.
This paper introduces a multi motor AC control system based on fieldbus, which combines the most mainstream fieldbus technology and considers the convenience of practical application. The main characteristics of the multi motor AC control system are described in detail from three aspects of bus introduction, hardware structure and software principle.
2. Introduction of PROFIBUS
PROFIBUS, or process fieldbus, has been developing fieldbus products since 1984. After more than ten years of development, production and application, PROFIBUS has become the leading open fieldbus system in Europe. At present, PNO (PROFIBUS user organization) has more than 600 members and produces nearly 1000 kinds of products. PROFIBUS products occupy more than 40% of the European market and are widely used in processing automation, building automation, process automation, power generation, transmission and distribution, etc.
PROFIBUS is based on iso7498 and takes open system interconnection (OSI) as a reference model. It defines physical transmission characteristics, bus access protocol and application functions. PROFIBUS DP, PROFIBUS PA and PROFIBUS FMS constitute the Profibus family. Among them, PROFIBUS DP (decentralized peripheral) is a kind of high-speed and cheap communication connection, which is specially designed for communication between automatic control system and equipment level distributed I / O. PROFIBUS DP module can replace 24 V or 4-20 mA serial signal transmission. The user interface provided by direct data link imaging (Ddlm) makes the access to data link layer simple and convenient. RS-485 transmission technology or optical fiber media can be used for transmission. This paper introduces the DP part.
The hardware of standard fieldbus PROFIBUS DP consists of master equipment, slave equipment and network. The master device is used to control the data transmission on the bus, send information and be authorized to access the bus when no external request is provided; The slave device is a relatively simple external device relative to the master device, and is not authorized to access the bus; Network network includes transmission medium and network linker. The former includes shielded twisted pair cable to form electrical network, optical fiber network with plastic or glass fiber cable, or hybrid network based on OLM conversion between the two media. The latter includes RS485 bus connector, RS485 bus terminal, RS485 repeater, optical link module OLM, etc.
In the multi motor AC control system, the motor control points and AC frequency converters have the characteristics of long and scattered distribution lines. Using fieldbus technology, the distributed I / O modules can be installed in the control room and the field console to form a bus network, and the serial transmission of control data can be carried out in a digital way. In this way, the control system will have better anti-interference ability, The reference value provided to the inverter is also more accurate.
3. Hardware architecture of multi motor AC control system
Figure 1 (left) is the hardware connection diagram of multi motor AC control system. It can be seen from the figure that the main equipment of PROFIBUS adopts Siemens PLC S5, which communicates with PROFIBUS through im308 interface module. As the main equipment, PLC S5 is responsible for reading the inverter status words (including digital quantity and analog quantity) of all distributed I / O modules suspended on the bus, and transmitting the inverter control words (including digital quantity and analog quantity). The slave device is the distributed input / output module et200. Each et200 slave station can be connected with 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 dip setting the address of the interface card im318 of et200 slave station.
Due to the remote distribution characteristics of et200 slave stations, the corresponding slave stations are set in the places where the frequency converters are concentrated, and the associated frequency converters are connected point-to-point. The specific wiring mode is shown in Figure 1 (right). All inverters have I / O interface, including digital input, digital output, analog input and analog output. The digital input includes forward start, reverse start, external fault (such as motor temperature rise), inching, enable, set frequency start, etc. the digital output includes inverter operation signal, fault signal, operation frequency interval signal, alarm signal, etc; Analog input and output must be set in U / I mode to confirm 4-20mA signal or 0-10V signal. Analog input is mainly the reference value of set speed or frequency, and analog output is the actual value of current, speed, frequency, DC circuit voltage, etc.
The hardware structure of the multi motor AC control system is relatively simple. Taking the programmable controller as the center, the related et200 slave groups are connected through PROFIBUS bus, and the corresponding inverter groups are connected through each slave group. The bus adopts electrical twisted pair circuit, and the wiring is clear; From the et200 slave station to the associated inverter group, it can be connected by a shorter control line. This data transmission mode is mainly serial digital mode, and only when it reaches the frequency converter, it uses part of the analog signal, so it does not affect the reliability and immunity of the data transmission at all.
4. Software design of multi motor AC control system
4.1 software flow of multi motor AC control system
The software design of multi motor AC control system based on Fieldbus includes the following points, as shown in Figure 2 (left)
1) Initialization of PLC im308 interface module: before starting the fieldbus, the Profibus must be configured, and the parameters of each bus station (the transmission rate on the bus, the standard of the bus (generally DP standard), and the fault mode (including diagnosis setting and diagnosis address, etc.) must be allocated page］
2) Read the status word and actual value of 1 frequency converter: read the status word (digital quantity) and actual value (analog quantity) of frequency converter through remote I / O operation through the I / O terminal of et200 slave station connected at the end of frequency converter.
3) Judge whether the frequency converter is in fault, if it is in fault, alarm and archive, otherwise output analog value to the I / O terminal of et200 slave station at the end of 1 frequency converter according to the requirements of the system, including control word and set value.
4) Repeat steps 2 and 3 to read and write 2, 3, 4… N converter in turn.
4.2 logic control principle of multi motor AC control system
Logic control refers to the realization of logic interlock between inverter and other external control signals for the opening and operation of specific equipment. For example, in the paper mill, when the feed pump is controlled by frequency conversion, the necessary conditions for changing the feed pump to open are: (1) the valve is opened in place（ 2) The liquid level of material tank is normal（ 3) The sealing water of feed pump is normal. If any of the three conditions is missing, the device cannot be turned on. In addition to the above three conditions, the condition of normal pipeline flow and pressure should be increased for the continuous operation of the feed pump, otherwise the operation of the feed pump will be stopped automatically.
The purpose of logic control is to protect personal safety, ensure the normal and stable production quality of equipment, so it is an essential step to protect the logic interlock in multi motor AC control.
4.3 closed loop control principle of frequency converter
Usually, the frequency converter runs in open-loop mode, but for higher production requirements, especially in the multi motor AC drive control, the frequency converter must be under closed-loop control to achieve the production goals. For example, film machine, cold rolling machine, paper machine and other continuous production lines, closed-loop control is particularly important.
In Figure 2 (right), the inverter works with closed loop control. The current inner loop torque control is directly completed by the inverter, which is not indicated in this figure. The figure describes the speed control closed-loop, from the analog input signal (according to the instructions of each inverter to determine the current or voltage signal, and need to set the corresponding dip switch and parameters), through the limit of the highest and lowest frequency, that is, compared with the filtered signal fed back from the speed sensor, enter the PID control (PID parameters can be set in the inverter parameters), Then through the slope generator (including ramp on time / ramp down time), it directly enters the current loop to control the output of the motor. In this way, the closed-loop control of the inverter is formed.
5. Concluding remarks
Through the construction of the system, it can be widely used in small pulp and paper mills, powder processing plants, building air pump control and other fields. Because it is constructed by an open PROFIBUS system with IEC standard, it has (1) low flexibility modification and expansion function（ 2) The distributed control is realized to improve the response speed and control precision of the system（ 3) Reduce the unreliability of the system and enhance the maintainable function.
Editor in charge: GT