Design of test system for gear tester based on vision technology

The gear tester based on visual technology is a new type of gear measurement tool. It is gradually being studied and developed with the substantial improvement of computer data processing capabilities and the continuous development of CCD measuring equipment. At present, the project is still in the research stage in my country. There are many factors that affect the measurement accuracy of this equipment. First of all, image acquisition is the input link of the system, which is a very important component. The quality of image acquisition directly affects the subsequent processing and measurement. A good quality picture can simplify the processing link and improve efficiency and reliability. In the image input stage, the deviation between the lens center and the gear center, the quality of the lens, the accuracy of the image acquisition card, and the light source will affect the quality of the acquired image; in the image processing stage, it must go through image denoising, threshold segmentation, median filtering, boundary extraction and other processes. The selection of image processing algorithm at this stage is crucial. A good algorithm will greatly improve the accuracy of image processing; in the parameter measurement stage, the selection of gauge blocks, the determination of the gear center, the establishment of parameter measurement models, the calculation parameter algorithm, the calculation accuracy, etc. will have a direct impact on the final result. Considering that some errors cannot be avoided, in the following experiment, the influence trend of measurement errors is mainly studied from three aspects: the selection of gauge blocks, the resolution of the acquired image, and the modulus.

From the qualitative analysis, it is not difficult to find that when only one of the calibration value, image resolution, and modulus changes, its impact on accuracy is: the smaller the calibration value, the higher the accuracy; the higher the image resolution, the higher the accuracy; the larger the modulus, the higher the accuracy. For the convenience of verification, here we mainly measure the diameter of the tooth top circle to examine the impact of the above three factors on the measurement error, so as to provide a reference for the measurement of more parameters and tooth shape errors later, and improve the measurement accuracy.

1 Measurement system composition

The measurement system consists of a workbench, a CCD camera, an image acquisition card, and a computer. The workbench completes the position displacement of the gear and places the gauge block. The CCD camera is used to capture the gear image. The image acquisition card converts the captured image into a digital signal and sends it to the computer. The system uses VC++6.0 to pre-process the captured image, including binarization, edge extraction, and sub-pixel edge fitting; secondly, the gear center is determined and calibrated; then the gear-related parameters are measured: module, addendum circle, root circle, common normal length, and involute tooth profile data; finally, error analysis is performed.

In the experiment, a high-precision standard gear with a small modulus (m≤2) was selected as the research object, and the gauge block was placed on the left side of the gear and captured by the CCD together with the gear. The purpose here is to explore the impact of changing only one of the three factors, modulus, resolution, and gauge block, on the measurement error, and provide experimental data and qualitative analysis of the data respectively.

2 Effect of modulus size on measurement error

When collecting images, the height of the gauge block is 20 mm, the number of teeth is 20, the pressure angle is α=20°, and the image resolution is 800×600. When these conditions remain unchanged and the module of the gear is continuously changed, 4 different groups of data are selected from the collected images, and the calculated data are taken as absolute values. The results are shown in Table 1 and Figure 2.

From the above results, it can be seen that when only the modulus is changed, while the calibration value and the number of teeth remain unchanged, the gear image increases with the increase of the modulus, the change of the tooth top circle difference is not obvious, and the tooth top circle error decreases with the increase of the modulus. Therefore, it is concluded that when only the modulus increases among the measurement parameters, under certain conditions, the value of the measurement error decreases. Similarly, the following conclusion can be drawn: when the modulus, calibration value, image resolution, etc. remain unchanged, when only the number of teeth is changed, the difference of the measurement parameters does not change significantly. Since the gear image increases with the increase of the number of teeth, the corresponding measurement error of the tooth top circle gradually decreases.

3 Impact of resolution on measurement error

When the system is measuring, the conditions such as the size of the gauge block, the number of teeth, and the module remain unchanged, only the resolution of the CCD camera is changed, and 5 sets of images are collected under different resolutions. Image processing and parameter measurement are performed in the gear measurement program, and the measurement results are shown in Table 2 and Figure 3.

From the above results, it can be seen that when only the image resolution is changed, the higher the resolution, the smaller the difference in measurement parameters, the smaller the measurement error, and the higher the measurement accuracy; from the measurement data, the resolution of the acquired image has a greater impact on the measurement error.

4 Effect of gauge block size on measurement error

When collecting images, only the size of the gauge block is changed while the image resolution, number of teeth, modulus and other conditions remain unchanged, that is, only the calibration value is changed, and 7 groups of different images are collected. The measurement results are shown in Table 3 and Figure 4.

From the above results, it can be seen that with the increase of the gauge block value, the tooth top circle difference and measurement error become smaller and smaller, and the accuracy becomes higher and higher. The main reason is that the increase of the gauge block value improves the accuracy of the calibration value, thereby improving the accuracy of the measurement parameters.

5 Conclusion

From the above experiment, it can be seen that the selection of resolution and gauge block has a very obvious impact on the measurement accuracy. The higher the resolution, the larger the value of the gauge block, and the higher the measurement accuracy of the gear. The change of the module does not change the measurement accuracy very obviously. It mainly affects the accuracy of the relative error and has little effect on the absolute error. At the same time, a CCD camera with high resolution should be used as much as possible to improve the resolution of the collected image to improve the measurement accuracy. The measurement plan should be arranged according to the specific conditions of the module and the number of teeth.