Application of PLC Control System in Gas Regulation

0 Overview

The gas company's regulating room was put into operation in October 2001. It is a supporting project of our factory's technical transformation project. Its function is to provide and regulate appropriate production gases (oxygen, nitrogen and argon) to meet the needs of production units.

In recent years, with the gradual expansion of our factory's production scale, the gas regulation in production has become more stable and rapid, and the current traditional secondary instruments can no longer meet the existing control needs. Mainly manifested in:

1) The process has a low degree of automation, and information collection and feedback still use traditional secondary instruments, resulting in slow data collection and delayed adjustment, which reduces the stability of system operation.

2) The internal information storage capacity of the instrument is small, and the collected data such as pressure and flow cannot be stored for a long time, which is not convenient for historical query and analysis of future production work.

3) The system reliability is low, and a dedicated person is required to be responsible for the operation of the system, resulting in an increase in labor costs.

In view of the above three points, the current problems are solved by adopting a PLC (programmable controller) control system.

1 Characteristics and composition of PLC control system

PLC has been widely used in the field of modern industrial control. In terms of the control function of PLC, it has the advantages of being rigorous, convenient, easy to program, easy to install, and highly reliable. It has strong versatility and wide adaptability, especially in the field of logic control such as digital input/output. PLC has a wealth of logic control instructions and advanced application instructions. It provides high-quality hardware, a high-level system software platform, and an application software platform that is easy to learn and program. In addition, PLC has its own network system and open I/O and communication interfaces, making it easy to build a network and achieve remote access.

The PLC uses the S7-300 series produced by Siemens. Since the PLC system on site is far away from the host computer in the main control room (about 800 meters), the communication system needs to be equipped with RS-485 repeaters in pairs. In addition, a TP27-10// touch screen is added on site for data display to ensure the stability of system operation. The system composition is shown in Figure 1.

1.1 System structure and hardware configuration

According to the control requirements, the CPU module uses CPU314, the digital input (DI) uses SM321 module, the digital output (DO) uses SM322 module, the analog input (AI) uses SM331 module, the analog output (AO) uses SM332 module and IM365 and other modules. IM365 realizes rack expansion, and the upper monitoring machine uses SIEMENS CP5611 network card to complete the data communication between the computer and PLC. The entire communication network adopts the MPI communication protocol, and the entire gas regulation process can be monitored and operated from the upper computer.

1.2 Functional realization of control system

The compilation of PLC program is directly related to whether the gas supply system can work normally, and the key to program design lies in the programmer's understanding of the process system and the flexible application of program compilation technology. Therefore, in the program design, the characteristics of the gas supply pressure regulation system are first considered, and the program design is refined and divided into multiple program modules to implement modular programming. This can not only easily add or delete program modules, facilitate on-site process adjustments, but also improve different program modules according to the controllability of supporting equipment.

The PLC programming software uses SIEMENS's SIMATIC STEP7 V5.1 software platform to complete hardware configuration, address and station address allocation, and compile the control program for the entire production process. The host computer monitoring software uses the domestic software Kingview, which uses a Chinese interface to facilitate system development and operation. The system runs on the Windows 2000 Chinese platform and can achieve comprehensive monitoring of the production process, form historical records of important parameters, and display them to operators in the form of reports or curves. Through VB language scripts, the historical trends and real-time trends of important parameters can be displayed on the host computer in the main control room, and the manual and automatic switching, operation, high and low limit alarms of pressure regulation, and display and accumulation of flow data can be realized to meet the scheduling needs of high productivity.

1.3 On-site display

The TP27 touch screen is used on site for parameter display and control. The touch screen program is completed by the configuration software. The human-machine interface uses Chinese menus, which are user-friendly, easy to operate, and powerful. It is mainly used for the display and operation of pressure, flow, and valve position on site. It can be used as a basis for on-site operations by operators.

1.4 Industrial Computer Configuration

The industrial computer uses Advantech IPC-610, and the CP5611 card is used to complete the communication between S7-300 PLC and the industrial computer. It mainly completes the following tasks: transmitting on-site monitoring data; operation monitoring; fault recording and troubleshooting prompts; parameter setting; production data management and processing; graphical teaching and offline programming.

2 The system realizes automatic control and monitoring of the gas supply system, mainly including the following functions:

1) Flexible operation mode and powerful system control function:

The system can realize host computer operation, control cabinet touch screen operation and local manual operation;

2) Alarm function:

When the pressure exceeds the process requirements, high and low limit pressure alarms can be realized on-site;

3) Simple and convenient parameter setting:

The pressure setting value, P, I, D and other parameters of the pressure regulating valve can be set in the host computer.

2. System control function

(1) Functions of process control:

1) The system realizes PID automatic regulation and control of the gas supply pressure;

2) Linearize, filter, and convert the collected analog signals into engineering units;

3) Temperature and pressure compensation of flow signal is realized, which improves the measurement accuracy of the instrument.

(2) Logic control

Interlocking logic control realizes on/off control, logic control and user-defined function blocks, etc. The system realizes solenoid valve control and parameter over-limit alarm and other functions.

(3) Human-machine interface

The HMI system includes the main process screen, and multiple screens such as the gas supply pressure, flow, and gas supply pressure adjustment of each system. The screen is intuitive and rich, and has the functions of PID online adjustment and online display of adjustment curves, including real-time trend curves and historical trend curves of process quantity change trends.

(4) Report printing

Draw alarm records and historical records in the form of reports, and adjust the data report. The real-time trend curve and historical trend curve can set the time period at will, and print the online trend and historical trend curve.

3 Software Design

According to the specific situation of the system, the following aspects are important to consider during the design of PLC system software:

(1) Data collection and engineering quantity conversion

(2) PID algorithm

(3) Calculation of flow rate temperature and pressure compensation and accumulation of flow rate

For the logic control in the system, ladder diagram (LADDER) programming is selected, which is intuitive and convenient; for the PID loop control flow temperature and pressure compensation calculation and the cumulative calculation of flow, statement list (STL) programming is adopted, which is compact and flexible.

PID regulation is the most important control program in this system, so the PID algorithm is introduced in detail.

3.1 PID algorithm

STEP7 provides two commonly used PID algorithms: continuous PID (FB41) and discrete PID (FB42). According to actual requirements, FB41 is selected. In the KingView, the drawing function is used to simulate the operation panel of a PID regulator to complete the manual/automatic switching, given value input, manual output value input, PID parameter (proportional coefficient, integral time) input and other functions in PID regulation control.

The output of the PID algorithm is actually the sum of the three parts of proportion (P), integration (I), and differentiation (D):

Mn=MPn+MIn+MDn

MPn = GAIN(SPn- PVn)

MPn = GAINTS/TI(SPn-PVn)+MX

MDn = GAIN TD/ TS(PVn-1-PVn)

Mn: output value at the nth sampling moment.

MPn: Proportional effect at the nth sampling moment, proportional to the deviation.

MIn: The integral effect at the nth sampling moment can eliminate static error and improve control quality.

MDn: The differential action at the nth sampling moment, which is adjusted according to the rate of change of the difference to suppress overshoot. SPn: The set value at the nth sampling moment.

PVn: process value at the nth sampling moment.

MX: Integral effect at the n-1th sampling moment, automatically refreshed after each sampling calculation.

GAIN: loop gain, P parameter.

TI: Integration time constant, i.e. I parameter.

TI: Differential time constant, i.e. D parameter.

TS: sampling time.

From the above formula, we can see that the parameter P (GAIN) is proportional to the effects of P, I, and D. It determines the sensitivity of the PID loop, that is, the speed of regulation. The larger the I parameter, the weaker the integral effect, and the larger the D parameter, the stronger the differential effect. The PID parameters cannot be determined by theoretical calculation alone. The only criterion is the accuracy and stability of the controlled parameter (pressure). Therefore, in actual debugging, the real-time curve of the controlled parameter is referred to, and repeated observation and analysis are performed to achieve the best control effect.

4 Significance of adopting this system

(1) Computerized management enables the system to store a large amount of information, and data collection and feedback are timely and accurate. The system's production data can be stored for a long time, which is conducive to historical query of production data and immediate troubleshooting;

(2) After the system was put into operation, the degree of process automation was improved through computer display and control, which enabled unmanned adjustment operation, reduced operation links, lowered operating costs, and brought the management and control of the system to a new level.

5 Conclusion

Since the system was put into operation in July 2005, it has been operating stably and reliably, successfully realizing all control functions, achieving relatively good control effects, and meeting the expected control indicators.

[References]

1. Yin Huawen, ed. Programmable Controller and Industrial Control Network. Xi'an: Xi'an Map Publishing House, 2001

2. SIEMENS. SIMATIC S7-300 M7-300 Programmable Controller Template Specification Reference Manual, October 2001

3. SIEMENS. STEP7_v5.1 Programming Chinese Manual. 2001

4. Asia Control Company. KingView Manual Version 6.5

5. Liao Changchu, ed. Application Technology of Programmable Controller (3rd Edition). Chongqing: Chongqing University Press, 2000