Research on the Application of Virtual Instrument Technology in Engine Testing

1 Introduction

The traditional internal combustion engine bench test has a single function, low test efficiency, and lacks a unified data processing system during the test process. Equipment produced by different manufacturers has different data recording formats, and both software and hardware are incompatible, so software and hardware resources cannot be shared. The product development cycle is long, the equipment upgrade capability and scalability are poor, and there is a lot of repetitive work, resulting in a large waste of resources. Virtual instrument technology was proposed and gradually implemented in the early 1990s. This technology uses a small number of standardized hardware modules and a powerful graphical programming platform to perfectly combine computer technology with instrument technology. It has the characteristics of short development cycle, low cost, less repetitive work, easy application of new algorithms, and easy upgrade. It has been widely used in the field of measurement and control.

The virtual instrument programming language LabWindows/CVI is a 32-bit software development platform for virtual instruments in the field of computer measurement and control developed by NI (National Instruments) of the United States using virtual instrument technology. It takes ANSI C as the core and organically combines the powerful and flexible C language platform with professional measurement and control tools for data acquisition, analysis and expression. Its integrated development platform, interactive programming method, rich function panels and library functions have greatly enhanced the functions of C language, and provided an ideal software development environment for developers familiar with C language to establish detection systems, automatic measurement environments, data acquisition systems, process monitoring systems, etc.

2 Hardware system design

The structure of the system is shown in Figure 1. The whole system is roughly composed of three parts: the first part is the sensor and the primary instrument, whose function is to convert the performance parameters of the engine into corresponding electrical signals through the sensor. In order to make full use of existing resources and save costs, for the measurement of parameters such as torque and fuel consumption, we directly lead the signal line from the output port on the dynamometer and fuel consumption meter to the wiring terminal of the signal conditioning module; the second part is the signal conditioning module and the data acquisition card, whose main function is to sample, amplify, A/D convert the signal, and transmit the collected data to the upper computer in a certain format; the third part is the computer processing system, whose function is to realize data processing, display, storage and chart printing.

The system uses the Lab-PC-1200 data acquisition card of NI, which is a low-priced acquisition card with excellent performance suitable for PC and its compatible machines. It can complete signal acquisition (A/D), analog output of digital signals (D/A) and timing/counting functions; it has 8 analog input channels, two analog output channels, 24 digital I/O interfaces, and 3 16-bit counters. Insert the data acquisition card into the PCI slot of the computer. Before the data acquisition card software driver is used, the parameters should be set. The parameter setting is achieved through the "Measurement & Automation" software provided by NI.

There are many electromagnetic interference sources in the engine test bench environment. The key to whether the test system can work normally and reliably lies in effectively suppressing external noise. In order to improve the test accuracy, while ensuring a good grounding state, the input signal line is shielded, the length of the signal line is shortened as much as possible, and the converter is equipped with photoelectric isolation technology. The test shows that even without software filtering, the experimental requirements can be met.

Figure 1 Hardware structure diagram

3 Design of software system

The software system mainly consists of four parts: parameter setting, data acquisition and storage, experimental result display and printing, and experimental process demonstration. Here we introduce the functions and implementation methods of each part.

3.1 Parameter setting part

As shown in Figure 2. Since the engine bench test generally requires several experiments such as load characteristics and speed characteristics. After entering the test system, you can select the experimental items to be done according to the requirements. In order to protect the safety of experimental equipment and personnel, you can also set the alarm values ​​of speed, oil temperature, cooling water temperature and exhaust temperature according to different engines. As long as the measured experimental data exceeds the set alarm value. It will automatically shut down.

Figure 2 Parameter setting panel

3.2 Data acquisition and storage

After entering the system, the program automatically executes the ConnectToDDESevr function in the code to open the connection with the database so as to exchange data with the database. Before exiting the system, the function DisconnectFromDDEServer must be used to disconnect the connection with the database. By setting the parameters of the data acquisition card, the port number corresponding to each signal is determined. Use the Input Byte From Pot function in the Utility Library (setting library function) to read the data from each port, and store the data in the specified file through the ClientDDEWrite function. As shown in Figure (2), the Chart of the operation panel (upper right corner of Figure 3) displays the collected data in real time and distinguishes them with different colors, so that the experimenter can easily see the changes in each parameter. During the acquisition process, if a certain data measured exceeds the preset alarm value, the system will immediately sound and light alarm. [page]

3.3 Display and print of experimental results

After the test is completed, execute the function ClientDDERead to read the data from the data file into the memory, use the least square method of curve fitting to fit the data, and display the result of the operation on the Graph of the operation panel (lower right corner of Figure 3). There are two ways to print: if you only want to print the characteristic curve of the engine, you can use the implicit call of the EXCEL data table method to open the connection with the database, and then call the function PrintCurve to print the Graph on the operation panel; if you want to make an experimental report, you need to output both the graph and the data, you can use the explicit call of the EXCEL data table method to directly activate the data file in the operating system.

Figure 3 Load characteristics experiment operation panel

3.4 Experimental Process Demonstration

Sometimes, in order to find the cause of abnormal phenomena during the experiment, it is hoped that the experimental process can be reproduced on the computer at any time, just like playing a movie. The system we designed can achieve such a function. During the experiment, the system automatically records the total time used for collection and the collection time corresponding to each set of data while collecting data. This information is written into the database together with the data set. When it is necessary to observe the measurement process again, call out the data set, call the function Timer, and display the data on the virtual instrument at a certain interval (such as 0.5 seconds) according to the order of each set of data collection, and display the curve changes on the Chart.

4 Conclusion

The engine bench test system developed in this paper realizes the automatic data collection and processing functions of the engine performance experiment, and has the following two features:

• The sound and light alarm system can automatically monitor the experimental conditions to ensure the safety of the experimental system and operators;

• The reproducibility of the experimental process is realized on the computer, which provides a guarantee for finding abnormal conditions in the experimental process.

References:
[1] Liu Junhua, ed., Virtual Instrument Programming Language LabWindows/CVI Tutorial, Electronic Industry Press, 2001, 8
[2] NI LabWindows/CVI User Manual, 2001
[3] Yu Bianzhang, ed., Digital Signal Processing, Northwestern Polytechnical University Press, 2002, 8