Computer data acquisition system for stiffness tes

2022-07-23
  • Detail

Computer data acquisition system for automobile leaf spring stiffness test Abstract: This paper introduces a computer data acquisition system used in automobile leaf spring stiffness test, and puts forward a low-cost method to realize high-precision and high-efficiency continuous data acquisition. Using force and displacement sensor, a/d converter, general computer and corresponding office automation software Microsoft Excel, the continuous stiffness test of automobile leaf spring, automatic data acquisition and processing are realized, and the test accuracy and work efficiency are improved

key words: leaf spring stiffness acquisition system

1 introduction

there are many elastic elements in the automobile structure, and the automobile leaf spring is the largest load-bearing elastic element in the automobile suspension system. Therefore, the relationship between load (force) and deformation (linear displacement) - stiffness is a very important assessment index. In practical application, static stiffness is frequently involved. In the production process of automobile leaf spring, the performance of leaf spring must be tested when the material and process change

the stiffness of general objects is non-linear, and the load deformation forms a hysteresis loop. The area surrounded by the loop is the energy consumed by the object from loading to unloading. Using a simple ratio to express the stiffness of an object is the result of linear processing, which is not accurate. However, the stiffness of some objects is close to linear, so the static stiffness is often used to replace and express the complex mechanical mechanism. Static stiffness refers to the deformation of an object under the action of a force. The ratio between the force and the corresponding deformation is the static stiffness of the object, k= △ p/△ F, where △ P is the force change value, unit n; △ f is the displacement change value, in mm. Therefore, in the stiffness test of leaf spring, we adopt the whole process continuous data acquisition method, which can accurately and intuitively reflect the actual situation of leaf spring stiffness

since the stiffness change of the general leaf spring is close to the linear change, we can use the static stiffness to express its elastic characteristics. The stiffness of nonlinear leaf spring can also be simplified into several straight segments

stiffness measurement is mainly to collect force and displacement data. Usually, the traditional static stiffness test method of leaf spring is point by point measurement and reading. The standard hydraulic (or mechanical) pressure testing machine shall be used for loading, the leaf spring shall run to the corresponding detection point, its load shall be indicated by the measuring disk of the testing machine, the displacement value of deformation shall be indicated by the ruler, and the load and displacement value of the point shall be recorded by manual reading, and then the stiffness value of the leaf spring shall be calculated manually. The disadvantage of this method is that the test accuracy is poor, the workload is heavy, the efficiency is low, and the continuous stiffness curve cannot be obtained, which may lead to significant deviation of the test results. Moreover, the full scale of the trimming machine here is large, and the error is large when testing the leaf spring with small load. Therefore, if the method of continuous data acquisition and software analysis is adopted in the leaf spring stiffness test, the problem can be solved conveniently

2 system principle

before the test, install the force and displacement sensors on the standard hydraulic (or mechanical) pressure testing machine and connect them with the computer. The leaf spring shall be clamped in place according to the loading status. Preload three times to eliminate the influence of residual stress and accidental error. Open the special data acquisition software of the computer, set the parameters (switch to the calibration coefficient of the used sensor, etc.), start the loading and unloading test after the self-test program of the instrument is correct, and start the data acquisition

during the test, the two-way signals of the force and displacement sensors are collected by the special data acquisition software through the interface card, amplifier and a/d converter, and the force and displacement curves are displayed on the computer screen. After reaching the set maximum load point, start unloading and repeat the acquisition process

at the same time, the data is stored in the Microsoft Excel workbook in a special data format. After the loading and unloading test, close the application acquisition program, enter the Microsoft EXCEL program, read the saved data into the Microsoft EXCEL program, call the chart function, and draw the complete load deformation curve (stiffness curve) of the loading and unloading of the leaf spring with the X and Y scatter diagram (X: displacement value/f, Y: force value/p). In the chart, the trend line, data fitting and regression analysis are used to obtain the quadratic equation algebraic formula, in which the slope is the stiffness value that the composite polyurethane adhesive used in the field of leaf spring accounts for more than 50% of the total production and sales of composite polyurethane. In the case of multi-level stiffness leaf spring, the stiffness value of this section of leaf spring can be obtained by regression analysis of each straight section. Although some people think that this operation is too cumbersome, Microsoft Excel software is widely used, and the trend line analysis form in the chart function is flexible, so it is completely competent for the analysis of stiffness curve, so it is not necessary to compile the analysis software by yourself

the maximum number of acquisition points in this program can reach 500 points. It can collect the data at any designated detection position during the operation of the leaf spring, and calculate its stiffness value. In case of multi-level stiffness leaf spring, the stiffness value of each section can be calculated by sections

3 system hardware

the system consists of four parts: sensor, data acquisition control device, computer and acquisition and analysis software

sensor: displacement and force sensor

data acquisition control device: acquisition and test core, with the functions of sensor amplifier, a/d converter, data acquisition, transmission, storage, etc

computer: hardware platform for acquisition software. The basic requirements are cpu:486 processor, 64M memory, 2G hard disk and Windows98 operating system. In consideration of cost reduction, desktop office computers already available in most manufacturers are also used as computers and analysis software, and the office automation software provided by the platform is used as data analysis software

acquisition and analysis software: the acquisition software completes the tasks of data acquisition, transmission and storage. The analysis software uses Microsoft EXCEL program, an office automation software included in Windows98, to plot, trend analyze and calculate the stiffness of the collected data

technical data:

input: 1 channel displacement sensor signal, 1 channel force sensor signal

sampling period: 100ms (adjusted by the user)

storage media: all test data can be stored on the HDD of the computer for further processing

data output: read all test data in EXCEL program, and display the test data by diagram or table. This data is saved in a Microsoft Excel workbook and can be easily output by the equipped printer or inserted into other files

display: it can display load, deformation value and stiffness curve during loading

temperature range: use - 25~65 ℃, storage: - 25~85 ℃

power supply: 220V

the core of the system is that the electrical signal of the sensor is sent to the computer through amplifier and a/d conversion, and the computer is used to receive, analyze and process the signal, send control instructions in time, print and store, as shown in Figure 1

Large randomness leads to larger relative error

Figure 1 hardware block diagram

4 system software

according to the requirements of the system and in combination with the computer configuration of the application, C language is selected to write the acquisition, display, mapping, transmission, storage and calibration program. The program makes full use of the operating characteristics of the DOS operation method, which is to make a special small hammer fall freely from a fixed height to impact the sample system of the tested material. The data acquisition adopts the query method, and takes the set time unit (the shortest time slice can be up to 5ms) as the interval. If the time unit is set to 500 ms, the computer will conduct data sampling every 0.1s. Considering that the static stiffness is obtained under slow loading, the standard promoted by China Automotive Industry Corporation is 8 mm/min (loading and unloading speed), or 0~1.25p is used to complete loading and unloading within 30s (P is the maximum static force actually borne by the object during use). Therefore, the computer acquisition frequency should not be too high. We set the time unit as 200 ms. The displacement or pressure unit can also be set as an interval and collected by query. It can also be collected with several pressure, displacement and time as specific points

theoretically, when the leaf spring is not loaded, the displacement changes to zero. Therefore, in order to ensure the validity of the sampling data, the system controls the operation of the sampling program by displacement by default. At the beginning, the system first clears to zero, and then samples the displacement of the leaf spring

the key points of the acquisition system are: calibration of sensors, real-time display of force and displacement curves

for the calibration of the system, five (or more) points are generally used, which are distributed evenly in full scale. Software is used in the design. That is, the standard value is manually input, the system automatically remembers, 5 (or more) points are corresponding one by one, and the quadratic equation is regressed by the least square method. Each sensor can be calibrated once and memorized. The system can store the calibration parameters of multiple sensors at the same time, which not only ensures the accuracy of data acquisition, but also facilitates implementation

in order to display the loading and unloading curves in real time, the system adopts the method of real-time "tracking" in the programming of the software, so that the graphics are clear at a glance

the storage format of collected data is a format that can be automatically recognized by Microsoft EXCEL program, which is very convenient to read. The following is a general description of the system process

the acquisition and control system running under DOS is compiled in C language, and its frame structure completely follows the C language structure style

main()//the main program

{

...

mainu menu();//the main screen menu displays

...

getu dada();//collect data

...

demarcate();//calibrate

//other statements

}

adv (CHA)//a/d conversion

int Cha

{

int adl,adh,l;

……

adl=inp(base+2);

adh=inp(base+3);

……

adh=adh

data[l]=adh*256+adl;

……

}

void reset_ All (void)

{

//all equipment initialization

int graphdriver=detect, graphmode=vga;

initgraph (&graphdriver, &graphmode, "");

...

}

Figure 2 program block diagram

5 system test results

place the tested leaf spring on the tension and pressure tester, load it after the system is set, and the computer will automatically collect test data. The collected data is displayed on the computer screen in real time, and the data is saved in the form of Microsoft Excel workbook. After the test, open Microsoft Excel software to comprehensively analyze the test data. Figure 3 shows the results of the stiffness test of the rear transverse leaf spring of nj6400 car of Nanya Automatic Vehicle Co., Ltd. with this system. Carry out linear regression on the stiffness curve in Microsoft Excel, and the linear regression equation is: p=34.132f+450.5, and the static stiffness of leaf spring is: 34.1 (n/mm)

figure 3

6 conclusion

the stiffness curve of leaf spring provided in this paper is drawn based on the measured test data. The system flow of data acquisition program gives the general structure, and the rest of the readers can improve themselves according to the actual situation

the leaf spring stiffness testing system introduced in this paper has low requirements for the equipment and personnel quality of general existing automobile leaf spring factories, and has strong operability, which is easy to popularize and use. Leaf spring factories generally have leaf spring loading equipment (hydraulic or mechanical pressure testing machine), so in the specific implementation of the new system, as long as sensors and data acquisition and control devices are added and reused

Copyright © 2011 JIN SHI