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Analysis of measurement accuracy of image measuring instrument
Author:Administrator   Published in:2019-07-31 15:23

    A contactless measuring instrument that acquires an optical image of a workpiece by using a camera (or a digital camera), an optical tube, an illumination source, and a mechanical motion system, and processes the image signal and measures the geometrical dimensions of the element. Image measuring instrument, also known as secondary measuring instrument.

    In daily applications, for the performance of image measuring instruments, especially measurement accuracy, there are some statements that cannot be easily explained and not standardized, such as: What is the measurement accuracy? What is the repeatability? What is the measurement error?

    The accuracy of an image measuring instrument is related to the measuring instrument itself, especially the performance of the image capturing device---camera (or digital camera), for example, the camera-specific camera TM-C6597E equipped with the US TEO brand. The measurement accuracy is higher than that of other camera-equipped image measuring instruments. If equipped with the US TEO brand TM-C520HP image acquisition system, due to the dedicated camera, balanced transmission line, and image acquisition card, the three models are designed uniformly, the matching is excellent, and the test accuracy is Higher, it will be 2.6 times better than the accuracy of a normal camera.

    However, everyone ignores the fact that the workpiece itself has an influence on the measurement accuracy. Because of a qualified image measuring instrument after stereotypes, the mounted camera (or digital camera), optical tube, illumination source, etc. are fixed, and different workpieces are measured due to workpiece shape, material properties, photographic properties, and reflection spectra. Differently, the optical image taken is not able to accurately represent the real artifact. Then the image signal generated from the optical image can not truly reflect the actual object. Because the workpiece is different, the measurement accuracy naturally varies greatly.

    In order to verify the accuracy of an image measuring instrument. There are two methods. One is to test the same known attribute and geometric size of the workpiece on two image measuring instruments, and compare the difference between the measured reading and the actual size to determine which measuring instrument has high precision, but There are also workpieces with different properties (such as metal and plastic), and the measurement accuracy of the same measuring instrument is very different.

    Another method is to test the measurement accuracy of the image measuring instrument by using the same standard instrument (quantity block, line scale, and dot mask) that conforms to the national standard. The accuracy of this method is expressed in a special term, namely: Indication error E. The image measuring instrument measures the difference between the measured value and the actual value of the standard, and is defined as the indication error E.

    This test method excludes the workpiece itself and its effect on the test results. It is the only scientific and uncontroversial method of national standards. For the specific inspection method, please refer to the national standard GB/T 24762-2009 "Product Geometrical Specification (GPS) Image Measuring Instrument Acceptance Inspection and Re-examination Inspection" which was officially released by the National Standards Management Committee on September 1, 2010. On June 1, 2016, the Ministry of Industry and Information Technology officially released the JB/T 12639-2016 "Flash Image Measuring Instrument" ministerial standard.

    In addition, the indication error reflects the degree of deviation of the measured value of the measuring instrument, but how is the accuracy of the indication value expressed? In accordance with national standards, with a degree of accuracy of this specification defines the terms, namely: the maximum permissible error MPEV (Maximum Permissible Errors Value) or the allowable error limit (limit of error). Is the maximum allowable range of indication error specified by the image measuring instrument. The allowable error limit has an upper limit and a lower limit, usually a symmetry limit, so the amount value should be represented by a "±" sign. It is an important technical indicator of the image measuring instrument, reflecting the accuracy of the measuring instrument. The maximum allowable error for the general measurement length is:

MPEV = ± (E + L / 200) μm

Where E: indication value; L: length of the tested standard

In summary, the measurement accuracy of the image measuring instrument should be expressed by two indicators, the indication error E and the maximum allowable error MPEV.

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