Application of LabVIEW in Dynamic Characteristics Test of Automobile ABS Brake Pipe

1 Introduction

LabVIEW is the abbreviation of Laboratory Virtual Instrument Engineering Workbench. It is a revolutionary graphical programming language, G language, launched by National Instruments in 1986, which opened a new era of virtual instrumentation and control [1].

The goal of LabVIEW is to simplify program development and enable engineers and scientists to make full use of PCs to complete their work quickly and concisely. Since the birth of LabVIEW in 1986, after more than ten years of development, LabVIEW's functions have become increasingly rich and powerful, and can be widely used in various fields such as automatic measurement systems, industrial process automation, real-time monitoring, and laboratory system simulation. Almost all colleges and research institutions in the United States are using LabVIEW. In recent years, LabVIEW has been introduced to China, and more and more domestic research institutions are using LabVIEW to develop virtual instruments [3]. Users can use ordinary computers with economical hardware equipment to build their own instrument control systems. These software-based systems make full use of the computer's super-powerful computing, presentation and connection capabilities to form powerful and flexible instrumentation and control equipment. Users can integrate data acquisition, data analysis, instrument control hardware and existing instruments and equipment to build a virtual instrumentation and control system that fully meets their specific needs. Traditional instruments and equipment are often limited by the functions assigned by the manufacturer, while virtual instruments can be used as many instruments and equipment, such as temperature monitors, voltmeters, trend chart recorders, oscilloscopes and spectrum analyzers [1].

The concept of "virtual instrument" (VI) proposed by LabVIEW enables instruments built on software to be freely combined, and their operation panels are no different from real instruments. On the one hand, it increases the flexibility of the hardware; on the other hand, the program is compiled using block diagrams and is directly compiled into execution code without the involvement of other languages ​​or drive systems. Compared with other "languages", LabVIEW is more flexible and time-saving. It has a variety of dedicated function libraries and development tools for data processing and control.

LabVIEW uses a graphical programming language, G language (Graphical Programming Language), a data flow programming model, a WYSIWYG programming method. It is different from the linear structure of text-based languages. Unlike languages ​​such as C and Qbasic, which are restricted by numerous grammatical rules, it is simple and intuitive, greatly saving program development time [2]. In LabVIEW, the order of program execution is determined by the data flow between blocks, rather than the traditional text language continuous execution method of command line order [2].

LabVIEW contains a rich library of functions and subroutines, such as data signal processing, probability statistics, linear algebra, filtering, windowing, etc., as well as GPIB, VXI, PLC and serial instrument control subroutines. Through the various functions and subroutines it provides, the hardware system can be softened and a test control system that meets user requirements can be designed.

LabVIEW also provides a simple, convenient and intuitive program debugging environment, where users can easily find the location and cause of the error, and can also use probes to view the results at any location. In addition, the platform also provides a debugging method for observing the program execution process, so that users can clearly see the data flow hierarchy and intermediate results. LabVIEW provides a large number of data acquisition subroutines, which range from simple to advanced and can be provided to users.

In short, LabVIEW is powerful, flexible and convenient. It has many similarities with traditional programming languages, such as similar data types, data flow control mechanisms, program debugging tools, etc., but the biggest difference between the two is that traditional programming languages ​​are programmed in text languages, while LabVIEW is programmed in graphical languages ​​(i.e., various icons, graphic symbols, connections, etc.). Programming with LabVIEW does not require much programming experience, and the interface is very intuitive and vivid, with knobs, switches, graphics, etc. that engineers are familiar with. Therefore, LabVIEW is undoubtedly an excellent choice for engineers who do not have rich programming experience. Furthermore, LabVIEW also provides interfaces for traditional programming languages ​​(such as C language). For tasks that are difficult or not good at completing by itself, it can be achieved by using other programming languages, thereby ultimately enhancing the overall function of LabVIEW.

2 Application of LabVIEW

We use LabVIEW to perform dynamic testing on automobile brake pipes. It mainly allows LabVIEW to complete the generation of control signals, that is, to generate step and other control signals; to realize A/D data acquisition; and to perform data processing functions such as file storage and curve printing.

The dynamic test of automobile ABS brake pipe mainly tests the dynamic process of pressure at four pressure measuring points of automobile brake pipe. The pressure measuring points are: valve outlet (hard pipe inlet), long hard pipe outlet (hose inlet), hose outlet (disc brake inlet), drum brake inlet. Two more test points are added: control voltage Ui and output current Ii. The system shown in Figure 1 is designed for the above tasks. [page]

The process of data acquisition and control is: first use LabVIEW to control the data acquisition card Axiom5412-HG to generate a step control signal, that is, let the data acquisition card output a control voltage to the solenoid coil of the normally open electro-hydraulic proportional pressure control valve developed by this laboratory, which is converted into current by the amplifier to control the opening and closing of the solenoid coil, and then control the opening of the proportional valve to be too small to achieve the purpose of controlling the outlet pressure of the valve. Then the voltage signal on each pressure sensor on the brake pipe is read into the computer from the analog input channel of the data acquisition card. And the data obtained at that time is drawn into a curve using LabVIEW, and the file I/O function of LabVIEW software is used to save the data to the hard disk for future dynamic performance analysis of the automobile brake pipe.

As can be seen from Figure 1, the data acquisition system generally consists of the following parts: computer, LabVIEW software, data acquisition card, sensor, dynamic strain gauge, hardware interface and connection cable. The computer provides a specific environment for the operation of LabVIEW software, and becomes the processing center of process communication, data acquisition and analysis with the support of virtual instrument LabVIEW software; the signal is picked up by the sensor, sampled by the data acquisition card under the control of the software through the dynamic strain gauge, and then saved or processed by the computer through A/D conversion. It can be seen from the flow of signals in the system that LabVIEW software occupies a dominant position in the data acquisition system. Software support enables the computer to have the ability to acquire, control, process and output results of signals. The core functions of the system are completed by it, while the hardware provides the material basis for the normal operation of the entire system.

Among them: Data B--valve outlet pressure; Data C--long hard pipe outlet pressure; Data D--hose outlet pressure; Data E--brake inlet pressure.

3 Experimental results

Here, the main research is the dynamic response of the front and rear brake pressures of the brake pipe to the electromagnetic force FMEM, and the rising edge triggering of the electromagnetic force, where the system pressure PVV1 is 15MPa. The signals collected by the dynamic test data are the switch signal that controls the input voltage of the proportional amplifier and the output pressure signal of the pressure reducing valve.

The experiment tests the drum brake system, whose slender brake pipe consists of a brake hard pipe and a brake hose, with an inner diameter of 3.2mm and an outer diameter of 4.8mm. The brake pipe length refers to the total length of the hard pipe and the hose. The length of the test hose is 113mm, and the wall thickness = 0.8mm. The brake pipe length PL = 3.54mm, and the brake pressure step response from 15MPa to 0MPa.

Figure 2 is the experimental curve. From the pressure curve in the figure, it can be seen that the pressure drops when the electromagnetic force is triggered by the rising edge. The pressure at the outlet of the brake pipe lags behind the inlet of the brake, with a pressure lag time of about 5ms, a pressure lag time between the two hoses of about 1ms, and an adjustment time of about 45ms when the pressure in the brake chamber transitions to a steady state. This shows that the hose has little effect on the dynamic performance of the entire system.

4 Conclusion

It can be seen that LabVIEW software fully utilizes the functions of computer hardware and software, with software technology as the core, making the computer the processing center for signal acquisition control, processing and result output, and replacing traditional electronic instruments with computerized soft instruments. As a virtual instrument development platform with good openness, LabVIEW software provides strong support for instrument-oriented programming.

References
1 LabVIEW Version 5.1 Addemdum. National Instruments Corporation Manual, 1999
2 LabVIEW Function and VI Reference Manual[M]National Instruments Corporation, 1998
3 Lin Zhengsheng. Virtual Instrument Technology and Its Application[J]. Electronic Technology Application. 1997, 23(3):24
4 Shen Lansun. Data Acquisition Technology. Hefei: University of Science and Technology of China Press, 1990
5 Ma Mingjian, Zhou Changcheng. Data Acquisition and Processing Technology. Xi'an: Xi'an Jiaotong University Press, 1998