Several Methods of Data Acquisition in Weighing System

With the progress of technology, the electronic weighing instrument made by weighing sensor has been widely used in all walks of life. The weighing sensor has realized the rapid and accurate weighing of materials, which has become a necessary device in process control. The automation of steelmaking and material management are inseparable from the link of data collection and transmission. The data acquisition of the detection instrument is the necessary condition to ensure the operation of PLC system. In recent years, a large number of new projects and technical transformation have been completed in the steel mill. Weighing system has been widely used in converter, refining, molten iron pretreatment and casting machine. Weighing data is the main basis of production control and material statistics management in each PLC control system. In each PLC control system, the method of data acquisition of symmetrical heavy system is different. This paper describes several methods of weighing data collection applied in the production practice of this steel mill, and analyzes and explains their respective characteristics in detail.

1. Technical characteristics of weighing instrument

1.1 function of weighing instrument

Weighing instrument is mainly used to measure, display and control the quality of materials by electronic technology. Weighing instruments are mostly used in automatic systems such as fast, long-distance measurement, digital display and process control. The weighing range can be from several grams to hundreds of tons, and the weighing accuracy can reach 0.001-0.000 1. In application, it can be used to form hopper scale, crane scale, track scale, belt scale, etc. Compared with mechanical scale and mechanical electrical scale, the electronic scale has the advantages of simple mechanical structure and small maintenance workload, and the outstanding advantage is that it can be combined into a high degree of automation and multiple distribution control systems. In the control system of the steel mill, weighing system plays an important role. For example, the auxiliary raw material feeding system, ferroalloy weighing system, magnesium powder tank and lime tank of the molten iron pretreatment system, refining feeding system, ladle and tundish weighing of casting machine are all indispensable technical parameters in their control system. Therefore, it is very important to collect accurate weighing data.

1.2 technical characteristics of weighing instrument

The weighing instruments used in every process of steelmaking are intelligent weighing instruments, which have the following technical characteristics.

1) It is used in combination with various sensors and transmitters to realize weight measurement, measurement and control, data collection and real-time printing on various fields, and has the functions of gross weight, peeling, net weight, peak value and peak valley processing.

2) Advanced hardware circuit and intelligent software processing are adopted, which have the functions of wathdog circuit, software trap and redundancy, power off protection, digital filtering and strong error correction and anti-interference function.

3) Input signal, current and voltage signals output by various force measuring sensors. The comparison output types are OC gate output and relay output. The optional analog output photoelectric isolation is 4-20mA DC.

4) Rs232/rs485 serial data output interface.

2 data acquisition of PLC

The basic automation system of Bengang Steel Plant adopts the three electric integration design, all the electric control and I & C signals are connected to PLC. In each PLC control system, the pressure transmitter, temperature transmitter, flowmeter, molten steel weighing, temperature measurement and other detection instruments installed on site collect instrument data, and equipment such as switch and limit position collect process data and field equipment operation status. All data are integrated into PLC system, and then the instructions issued by terminal operation station, In PLC, the process loop adjustment, electrical equipment sequence control and transmission equipment control are realized according to the preset program.

The data collected by PLC are transmitted to the HMI of the field operation station in the main control room after processing. According to the process requirements, each operation screen corresponding to the field process is set on HMI to display the status of the field equipment, the actual value of each process parameter and the system control process. The operator can observe the operation status of the equipment in real time through monitoring HMI. The operator can modify the process data according to the actual situation, and handle the alarm event in time. If necessary, the operation of the equipment can be interfered manually. Thus, the accuracy of data acquisition is the necessary condition to ensure the stable and reliable operation of PLC control system.

3.Several methods of data acquisition in weighing system

3.1 collect weighing data by analog input template data acquisition

In the iron water pretreatment station of this steel mill, the weight of magnesium powder tank and lime tank shall not only be displayed on the operation screen for the operator to monitor, but also used in the control program to calculate the amount of spray during the spraying process. When the injection amount reaches the set blow weight, the spray will be stopped automatically. In this control system, the method of analog input data acquisition is used to collect the weight of magnesium powder and lime. The data acquisition method of analog quantity template requires that the weighing instrument has 4-20mA current output signal. The signal is output to the analog input template of PLC, and the signal can be processed in PLC.

The weighing signal measured by the weighing sensor is transmitted to the weighing instrument. The weight measured by the weighing instrument is converted into 4-20mA current signal through d/a conversion. After inputting the signal to a channel of PLC Analog input template, the current signal is converted into the corresponding weight value by PLC program. This data acquisition method is simple, and the connection, data processing and calculation of PLC are the same as the general analog quantity, and PLC technicians can quickly master. But in this way, due to the instability of instrument zero point and signal drift, there are some errors in current signal conversion. The data after processing may have some errors with the displayed value on the weighing instrument.

3.2 collect weighing data with weighing module

This method is widely used in the auxiliary raw material blanking, ferroalloy and refining weighing system of converter in this steel mill. In converter steelmaking, the auxiliary material feeding system can display and record the weight of the current weighing bucket, and it also directly participates in the automatic control of various materials, which is an indispensable technical parameter for the automatic feeding function. It uses Siemens weighing module to collect symmetrical heavy data. Siemens weighing module can be directly applied to Siemens S7 control system, which is a function module of PLC, and is mostly used in S7300PLC or et200 remote station. The measured voltage signal is sent to the weighing module by the weighing sensor on site. After the calculation processing (including AD conversion, filtering, etc.) by the weighing module, it is directly transmitted to the CPU through PLC bus, and then displayed on the operation screen.

This mode is the most advanced system of industrial weighing. It directly enters PLC with the millivolt signal of sensor. Weighing module is a part of PLC, which avoids intermediate links such as transmitter and weighing instrument, so as to enhance the anti-interference ability of the system and reduce the fault point. In addition, Siemens weighing also provides a solution for weighing adjustment. It provides siwatool configuration software for calibration. It can be calibrated by computer serial port. The process is simple and fast. Even if we don't understand the basic knowledge of PLC, it can quickly calibrate the scale. This weighing mode has good application effect, simple and convenient, and it is almost maintenance free after put into use. As long as the sensor is calibrated regularly, the accuracy can be ensured.

3.3 collect weighing data by point-to-point data communication

In the first-class control system and thin slab caster control system upgraded by the steel mill, the weighing of large and medium ladle is an important data in the casting process, which not only provides accurate steel weight value for operators, but also directly participates in the interlocking control of casting. It uses point-to-point data communication mode, and Siemens special point-to-point data communication module. Siemens point-to-point data communication module is a special module of serial data communication, which can realize data exchange with single equipment with standard serial interface. It has CP340 and CP341 series suitable for application in s7300 and et200 remote stations, and cp440 and cp441 series that can be applied in S7-400. Taking CP341 communication module configured in et200m sub station as an example, the paper illustrates the method of collecting data by point-to-point data communication. CP341 communication module is connected through point to point, which is used for high-speed and powerful serial data exchange to reduce the burden of CPU. The module provides three different types of transmission interfaces: RS232C (V.24), 20mA (TTY), rs422/rs485 (x.27). At the same time, the module has ASCII, 3964 (R) and rk512 communication protocol. The user should choose according to the parameters of weighing instrument.

To use cp41 communication module, first, ensure the connection between template and instrument is accurate and reliable, and ensure that the connection of special interface is correct. Secondly, the CP341 template is configured in S7 hardware configuration. The communication parameters of the template are set according to the communication parameters of the connected weighing instrument, including baud rate, data bit, stop bit and calibration bit of communication data, which shall be identical to the setting of weighing instrument. After the configuration is successful, program with the special function block line provided by S7 to read the data of the instrument. The point-to-point data communication method of CP341 template is adopted in thin slab continuous casting control system. The cp421 template used in the first level continuous casting system upgraded this year has good effect in application, and has been recognized and praised by the field operators and instrument maintenance personnel. In the communication of both systems, the function of both sides to respond to communication is to send a request signal to the weighing instrument when PLC needs to read the weighing data. The instrument sends out serial data after receiving the request signal, and the PLC will inform the instrument to receive the data successfully after receiving the data, and process the received data in PLC, Send to HMI operation screen display or PLC program to participate in interlocking and control. Although this method of data transmission is reliable, it is required to be high in hardware configuration, and it is difficult to program. It requires technical personnel with certain professional knowledge to master.

3.4 collect weighing data by PROFIBUS Fieldbus

In the control system of No.7 wide slab continuous casting which was put into operation recently in this steel mill, a new technology PROFIBUS Fieldbus was taken out to collect the weighing data. In this data acquisition mode, the interface equipment with RS485 special communication protocol is connected to PROFIBUS bus by pm-120t converter, which makes the device a slave station on PROFIBUS bus, which enables users to realize the connection communication in a short time. Pm-120 converter is a converter which converts RS485 protocol into PROFIBUS-DP protocol. It realizes data communication between serial port and PROFIBUS-DP. It uses transparent conversion mode to convert serial port input data, and outputs it by PROFIBUS-DP side port of converter in the form of byte stream; At the same time, the data input from the Profibus DP side port of the converter will be output by serial port in the form of byte stream. It is universal, and the user can realize communication by programming on the master station according to the communication protocol of RS485 interface equipment. It has baud rate adaptive function, and does not need special configuration.

The steps of this method are as follows: firstly, the GSD file of pm-120 converter is installed in S7 hardware configuration, so pm120 can be found in the configuration of S7 accessories; Secondly, set the station number of pm120 according to the actual station number (station number 5 in the figure), add 16byte backward in, 16byte out data area for it; Finally, the data communication can be completed by simply reading the communication data of the data area. This data communication method does not have any protocol, and the configuration is simple. It is not only weighing instrument, but also can be used by all devices with RS485 serial interface to realize interconnection with PROFIBUS. Users can realize the connection without knowing the technical details of PROFIBUS, and can realize the connection according to the requirements, and can realize data communication in a short time without complicated programming. In the wide slab casting machine system by the technical personnel and maintenance personnel praise.

4 Conclusion

1) Through the description of four communication modes, we can see the advantages and disadvantages of their communication methods clearly, and make correct choice according to the actual situation in practical application.

2) The input mode of analog quantity is too simple and has some errors; Although the weighing template is very precise, it has certain limitations; The communication mode of point-to-point data and the data acquisition by Profibus fieldbus are suitable for many single devices with standard serial interface.

3) The communication mode of point-to-point data and PROFIBUS Fieldbus provide reference and direction for serial data acquisition in modern industrial control system.

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