Analysis of static characteristics of sensors based on virtual instruments

The working characteristics of the sensor are directly reflected by its static and dynamic characteristics. The static characteristics represent the input-output relationship of the sensor when the various values ​​of the measured input quantity are in a stable state. It can more prominently reflect the various indicators of the sensor. The static characteristics of the sensor mainly refer to linearity, hysteresis, repeatability and static error. In the past, the testing and analysis of the static characteristics of the sensor required a variety of instruments to complete together, and the measurement, recording of input and output signals, and calculation of characteristic indicators all needed to be done manually, which was labor-intensive, inefficient and unreliable. The sensor characteristic analysis based on virtual instruments has strong data processing capabilities, high efficiency, good flexibility and consistency, rich display content, convenient printing output, and extremely high cost performance, and has been widely used.

Measurement and calculation of static characteristics of sensors

The computer system constitutes a virtual test instrument to complete the analysis of the static characteristics of the sensor, achieving the purpose of fast, accurate, flexible and reliable

For a sensor characteristic cycle test, when 5 calibration points are taken, the distribution of the measurement points is shown in Table 1.

In order to ensure the reliability of the measurement in the actual test, the experiment should be repeated for at least 3 cycles, 5 calibration points should be taken, and a total of 25 measurement values ​​should be obtained. Generally, the experiment is repeated for 5 cycles, 7 calibration points should be taken, and a total of 61 measurement values ​​are obtained. The linearity, hysteresis, repeatability and static error of the sensor are calculated from these measurement values.

PC-based virtual instruments

Virtual Instrument Hardware

Table 1 Measurement point distribution table

From the analysis, it can be known that if you want to measure the characteristic index of the sensor, you must input the calibration signal to the sensor at each calibration point and measure the standard output signal value of the sensor at the same time. The standard output signal of the sensor is generally 4-20mA or 1-5V, and the current signal can be converted into a voltage signal through the sampling resistor. The traditional measurement method is to use a multimeter to measure and record the output signal value of the sensor each time. Nowadays, using a PC plus an A/D expansion board to measure and record the output signal of the sensor can greatly improve the measurement accuracy and speed. In addition, the PC has extremely strong computing power, which can conveniently and flexibly calculate and analyze the various characteristic indexes of the sensor, and has a very high cost performance.

At present, PCs all have more than three PCI expansion slots, and there are many types of industrial-grade analog signal acquisition boards based on the PCI bus. Most of them can collect signals in the range of -10 to +10V, and the sampling accuracy ranges from 12 bits to 16 bits, and the sampling speed ranges from tens of thousands to millions of times per second. As an industrial product, its working stability and reliability have been recognized, and it has been widely used in the field of industrial monitoring.

In view of the high precision of sensor testing, the test instrument accuracy is generally required to reach 5×10-4. The virtual instrument uses an ordinary PC computer and expands the PCI-816 analog acquisition board based on the PCI bus of Taiwan Advantech. The main functions of the board are: 16 analog signal differential inputs, 16-bit resolution, maximum sampling speed of 100kHz/s, wide signal sampling range and software programmable settings, support for software and hardware triggering, programmable selection of interrupt levels and DMA transmission channels, and 16 digital inputs and outputs.

Virtual instrument software

The hardware of virtual instruments is relatively simpler than the software. The convenience of operation, complete functions, rich display screen, and the ability to print out various analysis reports are all determined by the software design level. The software is compiled in VC++6.0 under Windows environment. The program function structure diagram is shown in Figure 1. It can run under WindowsNT/98/2000/XP operating system and has the characteristics of intuitive screen display, convenient and flexible operation, etc.

Figure 1 Program function structure diagram

After the virtual instrument software is running, data collection or input, modification, calculation, display screen switching and result printing and output can be completed through the drop-down menu or toolbar button. In order to facilitate the input and modification of various data, a highly integrated data input and modification dialog window as shown in Figure 2 is used, in which the sensor name, model and number of cycle tests can be directly input, while the sensor calibration point setting value and the measured value in the test can be input between adding and modifying according to the setting. For example, if the setting value of each calibration point needs to be continuously input, you can select the [Calibration Input or Test Data] check box in the operation method group box (uncheck it for the operation of measurement data) and select the [Continuous Operation] check box at the same time, as shown in Figure 2, and you can enter continuously. In addition, you can set different operations on the current subscript (trip number) data by selecting different buttons from the 4 radio buttons [Set], [Insert], [Append] and [Delete]. In this way, the entire data collection and modification can be completed in one dialog window, which greatly improves the efficiency of data operation. At each step of the operation, the current data of all parameters can be displayed in the program window.

Figure 2 Data input and modification dialog window

The number of calibration points and the number of required measurements are automatically calculated by the program according to the number of calibration points and the number of cycles. The program automatically checks whether the number of calibration points (M) and the number of measurements (N) satisfy equation (1). If not, the calculation menu item and the calculation button in the toolbar are invalid until the two satisfy equation (1). After checking and confirming the correctness of the data, the static indicators of the sensor can be calculated.

Display and Printing of Test Data

For input or measured data and calculation results, the program can display them in different pop-up windows in the form of tables, curves, histograms, etc. It provides storage in the form of independent files and classified databases, and pre-stores different print output report formats. You can select different display and print output methods by selecting different menu items or buttons.

The print output report provides a secondary design function, which allows you to design the print output report format yourself through the drawing and string editing functions in the software.

Application Examples

Virtual instruments have been applied in actual sensor testing. Table 2 shows some of the data measured and calculated for a pressure sensor (range: 1MPa) using 5 calibration points and repeated cyclic tests 3 times (confidence coefficient α=3).

Table 2 Partial data of pressure sensor measurement and calculation

Conclusion

Although this article does not introduce the use of this virtual instrument to measure and analyze the dynamic characteristics of the sensor, the dynamic characteristics of the sensor can be analyzed by referring to the analysis method of the static characteristics. By utilizing the fast A/D conversion function of the virtual instrument, the output can be continuously A/D sampled while the sensor input undergoes a step or continuous change. By analyzing the sampling results, the response characteristics of the sensor to the input that changes over time can be obtained.