###### Abstract  
Aiming to address the disadvantages of the low automation degree of the traditional steel measuring tape verification device, as well as the high labor intensity of metrologists, high time costs, and low measurement accuracy, this paper develops a new automatic steel measuring tape calibration device based on machine vision technology. In this study, the key components, such as the tape-winding mechanism, machine vision system, force measuring device, and tape-retracting device, were designed. Three different scribed line identification and location algorithms, processing the longest line, the closest to the starting line, the highest line, etc., were designed to adapt to different types of steel measuring tape. Lastly, indication error and measurement repeatability tests were carried out. The results showed that the standard device can carry out the verification and calibration of steel measuring tape, and the maximum repeatability of the indication error of the test objects was 0.025 mm, 0.038 mm, and 0.030 mm, respectively, and the indication error of each object was within the allowable error limits. The uncertainty evaluation of the indication error of each test object was 0.16 mm, 0.54 mm, 0.78 mm, which was not more than one third of the maximum permissible error value. Hence, the measuring principle, method, and procedure of the calibration device are shown to be scientific and feasible. metrology; steel measuring tape; calibration device; machine vision; pixel calibration; indication error calibration; scribed line recognition algorithms +
Footnote †: journal: Applied Sciences  
high, and thus not suitable for verifying the SMT. At present, most provincial measuring institutions in China still use the traditional method to complete the calibration process of SMT. The ST with high precision is used as the measurement standard and gives the standard value. The calibration process is to place the SMT and ST on a special calibration platform side by side. After identifying the scribed line of the SMT and ST through a reading microscope and recording the indication values of the lines, then the indication error can be calculated. It takes at least an hour for a skilled tester to calibrate an SMT with a range of 100 m. This process is characterized by low automation, low digitalization, low measuring accuracy, and high labor intensity [6, 7]. To sum up, a measurement standard for calibrating SMT with low cost, high measurement accuracy, high efficiency and low labor intensity is urgently needed. Following the development of photoelectric and image-detection vision technology, PLC and computer vision technology were introduced to the steel-tape automatic measurement system [8]. The use of machine vision technology in the automatic verification of steel tape can effectively increase efficiency and accuracy [9, 10, 11]. Grating scale measurement technology is also used in this field; [PERSON] et al. developed a model of the system for measuring the displacement of the tape in a raster formation device to investigate and assess the possible effects of external and internal factors on steel tape parameters [12]. Therefore, aiming to address the disadvantages of the low automation degree of the traditional steel measuring tape verification device as well as the high labor intensity of metrologists, high time costs, and low measurement accuracy, a new automatic steel measuring tape calibration device has been designed based on the research of domestic and foreign scholars. The scope of a single measurement has been broadened based on the use of the visual array image acquisition technology (currently, the upper limit of a single measurement of the common automatic detection device for SMT is 1 m, whereas the upper limit of a single measurement of the object in this paper is 5 m). In addition, the structure and working principle of key components are described in detail. Finally, a prototype has been experimentally produced, and the steel tape measure indication error and repeatability tests carried out.