Design of a PLC-based liquid temperature control system
Zhang Shuo
Tianjin Jinghai Urban Infrastructure Construction Investment Group Co., Ltd., Tianjin
Email: shuoshuo316@126.com
Submission date: February 28, 2021; Acceptance date: March 30, 2021; Publication date: April 6, 2021
Abstract
With the improvement of modern industrial automation levels, the requirements for temperature control in industrial processes are becoming increasingly stringent, especially in the pharmaceutical process where temperature requirements are particularly strict. To ensure the quality and yield of the pharmaceutical solution, this article designs a temperature control system specifically for the pharmaceutical process. This design takes the temperature control of a pharmaceutical factory as an example, using a PLC as the control core. By comparing the deviation between the set temperature and the actual temperature, the PID controller of the PLC generates control signals to adjust the steam flow entering the medicine tank to achieve temperature control. The upper and lower machine system consists of Kingview software and S7-200 PLC, realizing real-time monitoring of the temperature of the pharmaceutical solution.
Keywords
PLC, liquid temperature control, King of Configuration
With the improvement of modern industrial automation level, the requirements of temperature control in the process of industrial control are higher and higher, especially in the pharmaceutical process. In order to ensure the quality and output of liquid medicine, this paper designs a special temperature control system for the pharmaceutical process. This design takes the temperature control of pharmaceutical factory as an example to design. PLC is selected as the control core. By comparing the deviation between the set temperature and the actual temperature, the PID con-
The controller of PLC generates the control signal to adjust the steam flow into the medicine tank to realize the temperature control. The upper and lower computer system is composed of Kingview software and S7-200 PLC, which realizes the real-time monitoring of liquid temperature
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Open Access
1. Introduction
Temperature is the most common physical quantity in people's daily lives and industrial production processes. With the improvement of modern industrial automation levels, the requirements for temperature control in industrial processes are becoming increasingly stringent. The quality and yield of products are closely related to the accuracy of temperature control during production, especially in the pharmaceutical process where temperature requirements are particularly strict. To ensure the quality and yield of pharmaceutical solutions, designing a temperature control system specifically for the pharmaceutical process is of significant practical importance.
In addition, with the continuous development and improvement of industrial automation levels, people are paying more and more attention to the design and monitoring of control systems. Human-machine interfaces have gradually been researched, designed, and widely applied in industrial production processes. Through human-machine control interfaces, real-time monitoring of various parameters of the control system, alarm functions, data and information processing, etc., can be achieved. This makes the industrial control process more streamlined, operations easier, and more intuitive.
2. PLC controller
PLC has characteristics such as high reliability, ease of use, simple operation, complete control functions, and high precision, making it the most commonly used controller in industrial control processes. Therefore, in industrial control systems, human-machine interaction interface products are generally used to achieve real-time monitoring of the entire control process, with PLC as the core controller, together with computers, forming a complete industrial control system. This article takes the drug tank in a pharmaceutical process of a certain pharmaceutical factory as the controlled object, with the material temperature of the drug tank as the controlled variable for the temperature control system, using PID control, and employing PLC as the controller to complete the design of the temperature control system, achieving automatic control of the drug solution temperature.
PLC can be used for digital logic operations and manipulations. It is a digital operation controller with a microprocessor used for automation control, capable of loading control instructions into memory for storage and execution at any time. The PLC mainly consists of a CPU module, memory, communication interface, I/O module, expansion interface, and power supply. Based on actual control needs, the PLC selected is Siemens S7-200.
3. Temperature Control System
3.1. Temperature Control System Structure
This design takes the temperature control of a pharmaceutical factory as an example. During the preparation of the medicinal solution, it is required to maintain the temperature of the medicine tank at 80^(@)C∼90^(@)C80^{\circ} \mathrm{C} \sim 90^{\circ} \mathrm{C} for 2 hours. A PLC is selected as the control core, which generates control signals through the PID controller of the PLC by comparing the set temperature and the actual temperature deviation, to adjust the opening of the proportional control valve, thereby controlling the flow of high-temperature steam entering the medicine tank, achieving the purpose of controlling the temperature of the medicinal solution. At the same time, a flow meter is used to monitor the water inflow and drug extraction flow, and a liquid level sensor is used to monitor the liquid level in the medicine tank and the storage tank. This way, the system can not only control the temperature but also monitor other physical quantities, making the system safer and more reliable. The system has two modes: automatic control and manual control.
When in automatic control, it is a closed-loop control system. The system first uses a thermocouple to detect the temperature and converts it into a standard range voltage.
However, after the A/D conversion, it is transformed into a digital quantity, compared with the set temperature value, and the PID controller analyzes the error to generate a control signal. Finally, the control signal is converted into a voltage signal through D//AD / A to control the heater voltage.
When in manual control, it is open-loop control. The operator changes the control voltage by entering numbers on the upper computer, thereby controlling the voltage of the heater to achieve control over the temperature of the solution.
In summary, the temperature control is set in both manual and automatic modes. At the same time, data collection is performed on temperature, liquid level, flow rate, etc., and real-time monitoring of the temperature control process is conducted, with historical curves viewable through the upper computer. The control requirements are: the temperature must be maintained within the specified range, the liquid level in the storage tank stops working when it is below the set value of X2 meters, and an alarm is triggered when it exceeds the set value of X1 meters. The temperature control scheme for the drug tank is shown in Figure 1.
Figure 1. Temperature control scheme Figure 1. Temperature control scheme for the medicine jar
3.2. Sensor
In this system, we use thermocouple temperature sensors as the tool for measuring temperature. The principle is as follows: if two homogeneous conductors of different compositions form a loop, the end where the temperature is measured is called the hot end, and the terminal end is called the cold end. The thermocouple loop and its schematic diagram are shown in Figure 2. When there is a temperature difference between the two ends, an electromotive force will be generated in the loop, and the direction and magnitude of this electromotive force are related to the materials of the conductors and the temperatures at the two junctions. This phenomenon is called the "thermoelectric effect." The magnitude of the thermoelectric potential only depends on the materials of the thermocouple conductors and the temperature difference between the two ends, and is independent of the length and diameter of the thermocouple conductors. The thermocouple temperature sensor converts the temperature changes of the liquid medicine into changes in electrical signals, which are sent to the PLC for regulation, thus achieving temperature control. The measurement range of this type of sensor can reach 0^(@)C∼1500^(@)C0^{\circ} \mathrm{C} \sim 1500^{\circ} \mathrm{C} , making it suitable for a wider range of measurement requirements.
Figure 2. Thermocouple circuit and its schematic diagram Figure 2. Thermocouple circuit and its schematic diagram
3.3. PID Control
PID control, short for proportional, integral, and derivative control, is one of the earliest developed control strategies. Due to its simple algorithm, good robustness, and high reliability, it is widely used in industrial process control, and to this day, there are about 90%90 \% control loops with a PID structure.
Proportional control can adjust the system's open-loop gain, improve the system's steady-state accuracy, and reduce the system's inertia. Integral control has an output that is proportional to the integral of the input error signal, which helps eliminate the system's steady-state error. Derivative control can sensitively perceive changes in the input quantity and respond early, increasing the system's damping and improving the system's response speed. The effective combination of proportional, integral, and derivative control can meet different control requirements. The PLC comes with a PID controller, which is a continuous PID controller, and the signal is a continuously varying analog quantity. Set
p(t)p(t) is the given value, y(t)y(t) is the feedback value, the error is e(t)=p(v)-y(t)e(t)=p(v)-y(t) , the output of the PID controller is =P+I+D+=P+I+D+ initial value, and its controller expression is:
In the formula: K_(p)K_{p} is the proportional coefficient, T_(I)T_{I} is the integral time constant, T_(D)T_{D} is the differential time constant, u_(0)u_{0} is the initial value of the output.
The controller design is carried out through the PID instruction wizard of the S7-200. Enter the settings interface by clicking on the "Wizard 1 PID" icon in the instruction tree to complete basic settings such as sampling time. It is important to note that the proportional, integral, and derivative times are adjusted and controlled by the upper computer. The wizard automatically generates PID1_INIT (initialization program), PID1_DATA (data block), PID1_SYM (symbol table), and PID_EXE (interrupt program). During the control process, PID_EXE is called to execute PID calculations.
3.4. Control System Design
According to the system design requirements, complete the design of the liquid temperature control system in the PLC. First, allocate memory addresses, then complete the controller design using the PID instruction wizard provided by the PLC program, and finally complete the overall control system ladder diagram design according to actual requirements. The main program includes 1 subroutine and 1 interrupt program [4]. See Figure 3.
Figure 3. Program flow chart: (a) Main program; (b) Subroutine; (c) Interrupt program
Figure 3. Program flowchart: (a) main program; (b) subroutine; (c) interrupt program
3.5. Human-Computer Interaction Design
Configuration King, specifically the Configuration King development monitoring system software, is a new type of industrial automatic control system. It replaces traditional closed systems with an integrated system composed of standard industrial computer software and hardware platforms. It has advantages such as strong adaptability, good openness, easy expansion, cost-effectiveness, and a short development cycle. Such systems can typically be divided into three hierarchical structures: the control layer, the monitoring layer, and the management layer. The monitoring layer connects to the control layer below and the management layer above, achieving real-time monitoring and control of the field, and plays an important role in the upload and download processes within the automatic control system. Based on the application characteristics of this design, the human-computer interaction interface design of the control system uses Configuration King 6.55.
The upper and lower machine system of this system is composed of KingView software and S7-200 PLC, and the main interface of human-computer interaction is shown in Figure 4. It mainly achieves the following functions and has been effectively used in the actual pharmaceutical process.
1) Start, stop, and manual/automatic switching of the liquid temperature control system;
2) The temperature control process of the liquid medicine shows: the complete control process can be seen through the King of Configuration window interface; 3) Real-time monitoring: Real-time curve of liquid temperature changes; 4) Fault alarm: high/low temperature limit alarm, high/low liquid level limit alarm.
Figure 4. Main interface of control system Figure 4. Main interface of the liquid temperature control system
4. Summary
This article takes the temperature during the pharmaceutical manufacturing process of a certain drug factory as the controlled variable, uses PLC as the controller to complete the design of the temperature control system, and designs a human-machine interface based on KingView to achieve real-time monitoring of the temperature control process. The overall design can achieve temperature control and monitoring, but the design and application environment are relatively simple. Future research will continue to focus on achieving temperature control under more complex working conditions.
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