Design of Grey Water Fuzzy Control System Based on LabVIEW

1 Introduction

At present, hydraulic ash removal is the main method of ash removal in thermal power plants in my country. When the alkaline substances such as active calcium oxide contained in the coal ash come into contact with the ash flushing water, they will dissolve in the water, causing the pH value of the ash water to exceed the standard. The treatment method generally adopts acid neutralization. Due to the serious nonlinearity, time delay and non-parametric model of the neutralization process, it is difficult to achieve ideal results by using conventional control technologies such as PID to precisely control the pH value. For such a nonlinear, strongly coupled, large time lag system that is difficult to establish an accurate mathematical model, fuzzy control can achieve better results [1].

Virtual instruments are a new generation of virtual measurement and control instruments based on personal computers. They use the display function of computer monitors to simulate the control panel of traditional instruments, output test results in a variety of forms, use the powerful software functions of computers to realize the calculation, analysis and processing of signal data, and use I/O interface devices to complete signal acquisition, measurement and conditioning. LabVIEW is an efficient virtual instrument development tool designed for scientists and engineers based on graphical programming. Here, LabVIEW is used as the development platform, and the fuzzy logic toolbox is used to quickly and conveniently design the power plant ash water pH fuzzy control system.

2. Process flow and control principle

    The system adopts industrial computer control mode, and sends the collected ash water pH2 signal and flow signal to the industrial computer, which then calculates the required acid addition amount according to the set pH value control range, and converts it into a 4-20mA adjustment signal and sends it to the signal converter to control the speed of the electromagnetic metering pump, realize the adjustment of the acid addition amount, and thus achieve the purpose of qualified pH2 value of ash field drainage. The process flow and control principle are shown in Figure 1.

Figure 1 System process flow and control principle

3. Software Design

The virtual instrument system is run on the industrial computer to perform fuzzy control and display the instantaneous flow rate and pH value of the gray water. The fuzzy controller is the core of the whole system. The following focuses on the design process of the fuzzy controller based on the LabVIEW platform.

3.1 Software Development Platform LabVIEW and Its Fuzzy Logic Toolbox

LabVIEW is a graphical programming environment developed by National Instruments (NI) for data acquisition, instrument control, data analysis and data expression. It is aimed at test engineers rather than professional programmers. It is very convenient to program, has an intuitive and friendly human-computer interaction interface, and has powerful data visualization analysis and instrument control capabilities.

The LabVIEW Fuzzy Logic for G Toolkit is used to design rule-based fuzzy controllers [2]. Its main application areas are industrial process control and expert systems. It consists of four subVIs:

①Fuzzy Logic Controller Design VI

It is a VI that runs independently in the LabVIEW environment. It consists of three parts: fuzzy membership function editor, fuzzy rule base editor, and input and output performance test. It provides a friendly human-computer interaction interface, and users can intuitively and conveniently design various fuzzy logic controllers that meet different requirements. The fuzzy controller designed by this VI is saved in a data file with the suffix fc format to be called by the control system.

②Load Fuzzy Controller VI

This VI is used as a graphical function module in the block diagram program and connected to the fuzzy controller VI. When the program starts running, it loads the control parameters stored in the data file with the suffix fc into the fuzzy controller VI.

③Fuzzy Controller VI

This VI is the implementer of fuzzy controller in LabVIEW. It is applied in the block diagram program of LabVIEW, reads the parameters of fuzzy controller, and outputs the corresponding results. Each controller has a maximum of four inputs and one output.

④Test Fuzzy Control VI

Mainly used to test the basic performance of fuzzy controller.

3.2 Design of fuzzy controller

The implementation of a typical fuzzy controller requires solving the following problems: (1) fuzzification, that is, the setting of membership functions, including the number, shape, location distribution, degree of overlap, etc. of membership functions; (2) determination of control rules; (3) fuzzy algorithm; (4) defuzzification [3].

The fuzzy controller is designed in a "two-input and one-output" mode. The input variables are the deviation e between the pH2 value of the gray water and the given value of the qualified gray water pH and the rate of change of the deviation ec. The output variable is the adjustment value u of the frequency converter. The corresponding fuzzy languages ​​are E, EC and U. The range of input and output variables is mapped to the interval [-3, 3] through specific mapping rules, belonging to the fuzzy set {negative large, negative medium, negative small, zero, positive small, positive medium, positive large}, and the seven fuzzy subsets are recorded as NB, NM, NS, ZE, PS, PM, PB. The membership function adopts the commonly used triangular function. The fuzzy membership function editor in the fuzzy logic toolbox can easily set each language variable and its membership function.

Summarizing the technical knowledge and practical operation experience of engineering staff, combined with experimental conditions, a language control rule table is compiled, as shown in Table 1. The fuzzy control rules are input using the fuzzy rule base editor, and the weight value of each rule is set to the default value of 1. The entire fuzzy reasoning process uses the commonly used Max-Min method, and the method of defuzzification is the centroid method.

         　　Table 1 Fuzzy language control rules table

3.3 Testing and simulation of fuzzy controller

LabVIEW is a graphical development platform for virtual instruments. It provides a large number of input and output instrument panels, as well as various functions and signal generators. It can easily input and output various data and generate different analog signals. It also has various additional software packages, such as disk management, automatic testing, control and simulation, signal processing, graphics acquisition and processing, numerical analysis tools, etc., which can simulate real systems. You can also use the input and output performance test function of the fuzzy logic controller design VI and the test fuzzy controller VI to intuitively observe whether the output obtained by different deviations and deviation change rates meets the required requirements, verify whether the control rules are correct and reliable, and then modify and improve the fuzzy controller. After the test is completed, save the data in a data file with the suffix fc.

3.4 System Implementation

The designed fuzzy controller is applied to the block diagram program of LabVIEW by loading the fuzzy controller VI and the fuzzy controller VI, and the corresponding instrument control front panel and background block diagram program are designed according to the functions that the system needs to realize. Figure 2 shows the main block diagram program. The designed system has a friendly human-computer interface, which displays the real-time status of pH and flow through curves, and also has the functions of querying historical data, storing a large amount of data, sound and light alarm reminders, printing reports, etc. The control interface can also be published on the network using LabVIEW to realize remote monitoring.

Figure 2 System flowchart

4 Conclusion

The grey water fuzzy control system based on LabVIEW makes full use of the openness and graphical programming of LabVIEW, and brings into play the characteristics of strong robustness and good dynamic response of fuzzy control, and achieves good control effect in practical applications. LabVIEW is an extremely powerful virtual instrument development tool. With it as a platform, powerful and high-performance control systems can be developed conveniently and flexibly. The fuzzy controller designed using the LabVIEW fuzzy logic toolbox does not need to establish a mathematical model of the controlled object, and has a certain adaptability to the time lag, nonlinearity and time-varying nature of the controlled object. Moreover, the design process is very convenient and fast, and can be quickly applied to various industrial process control and automation software developed based on LabVIEW, providing a new way to efficiently develop fuzzy control systems.

         references

         　　[1] Xiao Bingyan. Application of fuzzy control mechanism in treating ash flushing water in Baosteel Power Plant [J]. Baosteel Technology, 2002, 2: 44-46.

         　　[2]Fuzzy Logic for G Toolkit Reference Manual[EB].National Instruments, 1997.

         　　[3] Zhang Jianmin, Wang Tao, Wang Zhongli, et al. Intelligent control principles and applications [M]. Beijing: Metallurgical Industry Press, 2003.