Automatic batching system is a very important process in the production process of fine chemical plant, and the quality of batching process plays an important role in the quality of the whole product. Automatic batching control process is a multi-input and multi-output system. All batching production lines are strictly coordinated and controlled, and the material level and flow rate are monitored and adjusted in time and accurately. The system consists of programmable controller and electronic belt scale, forming a two-level computer control network, which is connected with field instruments, control computers, PLC, frequency converters and other devices with high intelligence and fast processing speed through field buses. In the process of automatic batching production, the main materials and auxiliary materials are proportioned according to a certain proportion, and the materials transported by belt conveyor are measured by electronic belt scale. PLC is mainly responsible for the real-time control of conveying equipment and weighing process, and completes system fault detection, display and alarm, and at the same time outputs signals to frequency converter to adjust the speed of belt conveyor.
Composition of batching system software: The conventional batching system software part is a visual computer operating system designed and developed for batching workshop monitoring and automatic batching. It has the characteristics of simple operation, strong reliability, friendly man-machine interface and complete functions, and can be widely used in industries that need computer automatic batching, such as feed, grain, pharmacy, metallurgy and chemical industry. High level of intelligence and informatization: the upper computer has the function of menu library management; Intelligent report software provides a lot of data information for production management, such as ingredient list, raw material consumption list, production quantity list, formula use result record and so on. , and can generate statistics and printing functions such as class report, daily report, monthly report and annual report according to time and formula. At the same time, it provides two user-defined report components, one is to use crystal report for secondary design; The other is to embed data seamlessly into EXCEL reports. In addition, the system can interact with other management systems to meet the requirements of in-depth data analysis. The upper computer stores the cumulative quantity, proportion, start-stop time and other parameters of each operation, which is convenient for query. The software part of the batching system can automatically complete the batching process of the system, and the computer screen shows the batching system (workflow) in real time. The software operation is simple and the picture is realistic. At the same time, it has the following characteristics: the upper computer software sets the operation password and password modification protection of important parameters, the user realizes hierarchical management and can define the personnel authority at will.
The core hardware of batching system adopts imported or domestic high-quality products, and the control instrument adopts high-quality weighing control instrument, which has the characteristics of high precision, high reliability and strong anti-interference ability. The sensor is a high-precision weighing module. The weighing module is easy to install and maintain, which provides a reliable guarantee for the long-term stability of the system. Therefore, our batching system has high precision, high speed, good stability and high degree of automation.
1. Composition of automatic batching system
The automatic batching system consists of five batching lines of electronic belt scales, numbered 1#, 2#, 3#, 4# and 5#, in which 1# ~ 4 # is a group, 1# is the main batching scale, and the other three are auxiliary weighing scales. When there is no need to add auxiliary materials, the 5# electronic scale works independently to transport the main materials. The system has two functions: constant current and proportional control. For constant flow control, the electronic belt scale automatically adjusts the belt speed according to the amount of material on the belt to meet the set flow requirements. The technological process of the main scale (1#) system is analyzed, and the technological process is shown in figure 1.
After the automatic batching system is electrified, the belt drive motor starts to rotate, and the microprocessor controls the motor speed according to the current operation situation. The materials in the hopper fall into the blanking area and are transported to the weighing area by the belt, and the materials on the belt are weighed by the electronic belt scale. The weighing sensor outputs a voltage signal according to the force, which is amplified by the transmitter and outputs a metering level signal proportional to the weight of the material. The signal is sent to the host computer interface, sampled and converted into a flow signal, and the current flow value is displayed on the host computer. At the same time, this flow signal is sent to the PLC interface, compared with the given values of various ingredients set by the upper computer, and then adjusted and calculated, and its control quantity is sent to the frequency converter, thus changing the output value of the frequency converter and then changing the speed of the driving motor. Adjust the given amount to make it equal to the set value to complete the automatic batching process.
Flow is the amount of substance that passes through the endothelial region in a certain period of time. Electronic belt scale weighs instantaneous flow, and the upper computer gives the set flow, which is different from real-time measurement. PID control is the most widely used control method in industrial control, which is used for actual flow control. According to the flow deviation, the control quantity is calculated by proportion, integration and differentiation. The relationship between the input and output (error) of the control quantity in time domain can be expressed as:
In the formula, e(t) represents the error and input of the controller, u(t) is the output of the controller, kp is the proportional coefficient, Ti is the integral time constant, and Td is the differential time constant. Fig. 2 is a structural diagram of PID closed-loop regulation of system flow. During automatic adjustment in the production process, based on the flow measurement of main ingredients, the upper computer sets the total flow and the ratio parameters of main ingredients and auxiliary materials according to the production process requirements, and the rest auxiliary materials are doped according to the formula ratio. Flow metering control is a combination of metering deviation and frequency conversion speed regulation, which has the advantages of simple structure, good stability, reliable operation and convenient adjustment.
When the automatic batching system starts to work, start the batching production line. First, initialize the system program, set the batching ratio through the upper computer or touch screen, and check whether there is any material in the hopper. If there is no material, feed it into the hopper, start the batching production line, weigh it with an electronic belt scale and measure it in real time, and CPU calculates the real-time flow and accumulated flow. If there is a deviation between the set flow and the actual flow, the regulator compares the deviation between the set flow and the actual flow according to the system control requirements, changes the output signal through PID adjustment, and controls the frequency converter to adjust the speed of the conveying motor to realize constant flow control. All the auxiliary materials are mixed and metered at the same time according to the proportion, and added according to the formula process requirements. The main program control flow of the system is shown in the following figure:
In the automatic batching system, the main and auxiliary scales are controlled by programmable controller (PLC) and upper computer. Take the PLC control analysis of four electronic belt weighers 1 # ~ 4 # as an example. Each electronic belt scale has a belt drive motor, two material level sensors, a speed sensor, a weighing sensor and a frequency converter, which constitute the controlled object. The start and stop of the motor is controlled by the switch value, and the digital output signal of PLC is used as the input signal of the control end of the frequency converter to modulate and output high-frequency pulses to the belt drive motor. The material level sensor detects whether there is any material in the hopper, and the speed sensor measures the speed of the motor. The system needs 8 digital input signals, 25 switch input signals and 24 switch output signals, and the total number of I/O points is 57. See table 1 for the number and types of I/O points.
SIMATICS7-/300 is a small modular PLC system. Various independent modules can be widely combined to form systems with different requirements.
According to the I/O point, process requirements, scanning speed and self-diagnosis function of the controlled object of the system, CPU3 15-2DP of Siemens S7-300 series PLC is selected. CPU3 15-2DP is the only CPU template with SINE CL 2-DP interface of Profibus. It has 48KB of memory and 80KB of loading memory. The storage capacity can be extended to 5 12KB with the memory card, and the maximum capacity can be extended to 1024 digital points or 128 analog quantities. According to the statistical I/O points, choose 32-point DC and 16-point SM32 1 digital input module and 32-point SM322 relay output module.
3.2 Selection and function setting of frequency converter
Mitsubishi Corporation provided the technical parameters of FR-A540 series inverter when matching with the company's standard motor. Mitsubishi's standard motor is adopted. The rated power of 1# belt conveyor is 2.2KW, the rated power of 2 ~ 4 # belt conveyor is 0.4KW, the rated voltage is 380V, the rated current is 5A, the rotating speed is 1420r/min, and the speed regulating range is 120 ~ 1200 r/min. Mitsubishi FR-A540 inverter has its own PID adjustment function, and PID control is carried out according to the production process requirements of automatic batching system. Some parameter settings that need to be detected and set are as follows:
(1) pr. 1 = 50 Hz, Pr.2= 5 Hz, and the system Pr. 18= 120 Hz remains unchanged.
② Pr. 19=9999, which is the same as the power supply voltage.
③ Pr.7=2s, and the acceleration time (the factory setting below 7.5K is 5s, 0 ~ 3600s/0 ~ 360s).
Pr.8=2s, deceleration time (5s below 7.5K, 0 ~ 3600s/0 ~ 360s).
④ Pr.9 is determined by the motor rating.
⑤ Pr. 14=0, which is suitable for constant torque load.
⑥ Pr.79=3, external /PU combined operation mode.
⑦ Pr. 183=8, realizing RT switch =REX switch.
⑧ Pr. 128，Pr. 129，Pr. 130，Pr. 13 1，Pr. 132，Pr. 133，Pr .
STEP7 is a programming language used by Siemens S7-300 series PLC. It is a programming language that can run on a general microcomputer and can be programmed in WINDOWS environment. Through STEP7 programming software, it is very convenient to use ladder diagram and statement table for off-line programming, and it can be directly sent to the memory of PLC through switching cable for execution. During debugging, it is also possible to monitor the on-off status of various input/output or state points in the program online, and even modify the values of variables in the program online, which brings great convenience to debugging.
STEP7 divides the user program into different types of blocks. Program blocks are divided into two categories: system blocks and user blocks. User block includes: OB= organization block, FB= function block, FC= function, and DB= data block. The main program can be put into the "organization block" (OB), and the subprogram can be put into the "function block" (FB or FC).
In this system, the main task of PLC is to accept the input of external switch signals (buttons, relay contacts) and digital signals generated by sensors, judge the current system state and output signals to control contactors, relays, solenoid valves and other equipment, so as to complete the corresponding control tasks. In addition, another important task is to accept the control command of industrial computer (upper computer) and carry out automatic batching control.
Automatic batching program * * * has seven "blocks" from OB 1 and FC 1 to FC6. OB 1 is the main program, and the functional modules such as FC 1 to FC6 are called in turn through six "call" statements to achieve the purpose of organizing the whole program. The task assignment of the six functional blocks in the program is as follows:
FCl is responsible for the startup of system operation and the setting of operation mode; FC2 is responsible for stopping the system;
FC3 is responsible for the proportioning control of metering pump and metering pump; FC4 is responsible for handling and controlling faults and accidents;
FC5 is responsible for controlling the frequency converter; FC6 is responsible for the display control of indicator light.
5. Concluding remarks
PLC replaces the traditional mechanical transmission and huge control appliances, and realizes the automatic control of electricity. Through the frequency conversion speed regulation of belt motor, energy can be saved and the batching accuracy can be improved.
The innovation of this paper is that the automatic batching system adopts PLC control scheme, which has the advantages of powerful function, convenience, flexibility, high reliability, low cost and easy maintenance, greatly improves the batching accuracy, is convenient for microcomputer control of metering, realizes networked production management, and has achieved good economic benefits in operation. The economic benefit of this project is 200,000 yuan.