The constant flow control of the automatic batching system adopts PID adjustment, and the flow metering control is a combination of metering deviation and frequency conversion speed regulation. Based on the system process flow, the flow control method and system control process of the batching system are introduced, and the selection of PLC and the hardware settings, parameter settings and software design process in the frequency conversion control of the PLC batching system are described in detail.
The automatic batching system is a very important process in the production process of fine chemical plants. The quality of the batching process plays a decisive role in the quality of the entire product. The automatic batching control process is a multi-input, multi-output system. Each batching conveying production line is strictly coordinated and controlled, and the material level and flow rate are monitored and adjusted in a timely and accurate manner. The system consists of a programmable controller and an electronic belt scale to form a two-level computer control network, which connects field instruments, control computers, PLCs, frequency converters and other highly intelligent and fast processing equipment through field buses. In the automatic batching production process, the main materials and auxiliary materials are mixed in a certain proportion, and the electronic belt scale completes the measurement of the materials transported by the belt conveyor. PLC is mainly responsible for 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 the frequency converter to adjust the speed of the belt conveyor.
The software component of the batching system: The conventional batching system software part is a visual computer operating system designed and developed for monitoring and automating batching in the batching section. It has the characteristics of simple and convenient operation, strong reliability, friendly human-machine interface and complete functions. It can be widely used in feed, grain, pharmaceutical, metallurgy, chemical and other industries that require computer automatic batching. It has a high level of intelligent informatization: the host computer has Formula library management function; the intelligent report software provides a large amount of data information for production management, such as ingredient result list, raw material consumption list, production volume list, formula usage result record, etc. It can produce class reports, daily reports, monthly reports and other reports based on time, formula, etc. Annual report and other statistical and printing functions. At the same time, two user-defined report components are provided, one is to use Crystal Reports for secondary design; the other is to seamlessly embed data into EXCEL reports. In addition, the system can interact with other management systems to meet in-depth data analysis requirements. The host stores the parameters such as the cumulative amount, ratio, and start and end time of each run for each run for easy query. The software part of the batching system can automatically complete the system batching process, and the computer screen displays the batching system (work flow) in real time. The software is simple to operate and the screen is lifelike. It also has the following characteristics: the upper computer software sets the operating password and important parameter password modification protection, and the user Achieve hierarchical management, and you can define the authority of personnel arbitrarily.
The core hardware of the batching system is made of imported or domestic high-quality products, and the control instrument uses high-quality weighing control instrument, which has high precision and high reliability. , strong anti-interference ability. The sensor adopts high-precision weighing module. The weighing module is simple to install and easy to maintain, which provides a reliable guarantee for the long-term stability of the system. Therefore, our batching system has high precision, fast speed and good stability. , high degree of automation.
1. The composition of the automatic batching system
The automatic batching system consists of 5 electronic belt scale batching lines, numbered 1#, 2#, and 3#. , 4#, 5#, among which 1#~4# are a group, 1# is the main material scale, and the other three are auxiliary material scales. When there is no need to add auxiliary materials, the 5# electronic scale works alone to transport the main material. It has two functions: constant flow and proportioning 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 based on the main scale (1#) system process. Analysis, the process flow is shown in Figure 1.
After the automatic batching system is powered on, the belt drive motor starts to rotate, and the microprocessor controls the motor speed according to the current operation. The material in the hopper falls into the blanking area. After being transported by the belt to the weighing area, the material on the belt is weighed by the electronic belt scale. The weighing sensor outputs a voltage signal according to the force exerted on it, which is amplified by the transmitter and outputs a measurement voltage proportional to the weight of the material. Level signal. This signal is sent to the interface of the host computer, and is converted into a flow signal after sampling. The current flow value is displayed on the host computer. At the same time, the flow signal is sent to the PLC interface and communicated with various ingredients set by the host computer. The fixed values ??are compared, and then the adjustment operation is performed, and the control value is sent to the inverter to change the output value of the inverter, thereby changing the speed of the drive motor.
Adjust the given amount to make it equal to the set value to complete the automatic batching process.
Flow rate is the amount of material passing through the belt within a certain period of time. The electronic belt scale measures the instantaneous flow rate, while the host computer measures the set flow rate. There is a deviation between the two in real-time measurement. In the actual flow control, PID regulation, which is the most widely used in industrial control, is used. According to the flow deviation, the control quantity is calculated using proportion, integral, and differential for control. The relationship between the input and output (error) of the control quantity is available in the time domain. The formula is expressed as follows:
In the formula, e(t) represents the error and controller input, u(t) is the controller output, kp is the proportional coefficient, Ti is the integral time constant, and Td is the differential time constant. . Figure 2 is the system flow PID closed-loop adjustment structure diagram. When the production process is automatically adjusted, based on the flow measurement of the main ingredient, the total flow rate and the main and auxiliary material ratio parameters are set through the host computer according to the production process requirements, and the remaining auxiliary materials are doped according to the formula proportion. Flow measurement control is a combination of measurement deviation and frequency conversion speed regulation. It 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, the system program is initialized, the ingredient ratio is set through the host computer or the touch screen, and the hopper is checked for materials. If there is no material, feed the material to the hopper, start the batching production line, weigh and measure in real time by the electronic belt scale, and the CPU calculates the real-time flow rate and cumulative flow rate. If there is a deviation between the set flow rate and the actual flow rate, the regulator compares the deviation between the set value and the actual flow rate according to the system control requirements, and changes the output signal through PID adjustment to control the frequency converter to adjust the speed of the conveying motor, thereby achieving constant flow control. All auxiliary materials are mixed and measured at the same time according to the proportion, and added according to the formula and process requirements. The main program control flow of the system is as shown in the figure:
In the automatic batching system), the main and auxiliary material scales are controlled at two levels by the programmable controller (PLC) and the host computer. Now take the PLC control analysis of four electronic belt scales 1# to 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. They constitute the controlled objects. The start and stop of the motor are controlled by the switching value. The PLC digital output signal is used as the control terminal input signal of the frequency converter. The frequency converter modulates and outputs high-frequency pulses to the belt drive motor. The material level sensor detects whether there is material in the hopper, and the speed sensor measures the rotation speed of the motor. The system requires 8 digital input signals, 25 switching input signals and 24 switching output signals. The total number of I/O points is 57. The number and type of I/O points are shown in Table 1.
The company's SIMATICS7-/300 is a modular small PLC system, and various individual modules can be widely combined to form systems with different requirements.
Based on the number of I/O points of the controlled objects of the system and the process requirements, scanning speed, self-diagnosis function and other considerations, the CPU315-2DP of the S7-300 series PLC of SIEMENS Company was selected. CPU315-2DP is the only CPU template with fieldbus (PROFIBUS) SINECL2-DP interface. It has 48KB of RAM and 80KB of load memory. The load storage capacity can be expanded by a memory card to a maximum of 512KB. It can be expanded to a maximum of 1024 digital points or 128 points. Analog quantity. Select a DC 32-point and a 16-point SM321 digital input module and a 32-point SM322 relay output module based on the counted I/O points.
3.2 Inverter selection and function settings
Mitsubishi provides technical parameters for matching the FR-A540 series inverter with the company's standard motors. Using Mitsubishi's standard motor, 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 rotation speed is 1420r/min, and the speed adjustment range is 120~1200r/min. Mitsubishi FR-A540 inverter has its own PID adjustment function. It performs PID control according to the production process requirements of the automatic batching system. Some parameters that need to be detected and set are as follows:
① Pr.1=50 Hz, Pr.2= 5 Hz, Pr.18=120 Hz of this system remains unchanged.
② Pr.19=9999, the same as the power supply voltage
③ Pr.7=2s, acceleration time (factory setting value below 7.5K: 5s, 0~3600s/0~ 360s)
Pr.8=2s, deceleration time (factory setting value below 7.5K: 5s, 0～3600s/0～360s)
④ Pr.9 is determined by the motor rating Decision
⑤ Pr.14=0, suitable for constant torque load
⑥ Pr.79=3, external/PU combination operation mode
⑦ Pr. 183=8, realize RT switch = REX switch
⑧ Pr.128, Pr.129, Pr.130, Pr.131, Pr.132, Pr.133, Pr.134 are adjusted according to the on-site PID requirements to set.
STEP7 is the programming language used by Siemens' S7-300 series PLC. It is a language that can run on a general-purpose microcomputer and be programmed in the WINDOWS environment. Through the STEP7 programming software, not only can it be very convenient to use ladder diagrams and statement lists for offline programming, and can be directly sent to the PLC's memory for execution through an adapter cable, but also during debugging and running, each element in the program can also be monitored online. The on-off status of input and output or status points, and even online modification of the values ??of variables in the program, also bring great convenience to debugging work.
STEP7 divides the user program into blocks of different types. Program blocks are divided into two major categories: system blocks and user blocks. User blocks include: OB=organization block, FB=function block, FC=function, DB=data block. The main program can be placed in an "organization block" (OB), and the subprogram can be placed in a "function block" (FB or FC).
In this system, the main task of the PLC is to accept the input of external switch signals (buttons, relay contacts) and digital signals generated by sensors, determine the current system status and output signals to control contactors, Relays, solenoid valves and other devices to complete corresponding control tasks. In addition, another important task is to accept the control commands from the industrial computer (host computer) for automatic batching control.
The automatic batching program has 7 "blocks" including OB1 and FC1 to FC6. OB1 is the main program. Through 6 "CALL" calling statements, functional modules such as FC1 to FC6 are called in sequence to achieve the purpose of organizing the entire program. The task distribution of the six function blocks in the program is as follows:
FCl is responsible for starting the system and setting the operating mode; FC2 is responsible for stopping the system;
FC3 is responsible for the metering pump and metering pump proportioning control; FC4 is responsible for fault and accident handling control;
FC5 is responsible for controlling the frequency converter; FC6 is responsible for the display control of the indicator light.
5. Conclusion
PLC has replaced traditional mechanical transmission and huge control appliances, realizing automated electrical control. Through frequency conversion speed regulation of the belt motor, energy can be saved and batching accuracy can be improved.
The innovation point of this article is: the automatic batching system adopts a PLC control scheme, which has the advantages of powerful functions, convenience and flexibility, high reliability, low cost, and easy maintenance. It greatly improves the batching accuracy and facilitates the measurement of microcomputers. It realizes network-based production management through centralized control, and has achieved good economic benefits through production and use. The economic benefit of this project is 200,000 yuan.