A novel method of steel tape zero calibration

LI Yi-ming, WANG Zhong, CHEN Xi, LU Rui-jun

(State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China)

0 Introduction

According to the national standard steel tape verification regulation of JJG 4-2015[1], the metrological verification units verify half and integer meter scales of steel tape periodically. Normally, the indication errors of steel tape under test and standard tape can be calibrated by deviation of scales[2]. However, there are some special steel tapes with movable structures on zero position, as shown in Fig.1. Traditional zero scale collimation methods are not applicable to this kind of steel tape due to the lack of zero scale.

Fig.1 Steel tape with movable hook structure

The steel tape with movable hook structure on zero position is a widely used length measuring instrument. Push-type measurement and hook-type measurement are two different operation modes, and the zero position depends on frontal datum plane and rear datum plane of the hook, respectively, as shown in Fig.2. Displacementdis equal to the thickness of the hook (including cock quantity), which compensates for the deviation between the two datum planes.

Fig.2 Principle of hook structure

Heree0is the indication error of measured steel tape of 500 mm, and it is regarded as the zero error in conventional verification of the whole steel tape[3]. On the long sized stage of whole steel tape,e0will affectei,the indication error of subsequent verification, andeican be calculated by

ei=Ni-Li+e0,

(1)

whereNiis indication value, andLiis standard value. Δ will be introduced in the process of zero scale collimation. Δ is caused by the deviation of zero scales which should have been aligned completely.

(3)

1 Analyses of existing methods

To meet the needs of zero scale collimation in steel tape calibration, a variety of methods are researched.

1.1 Zero adjustment system with manual mechanism

Yu C L[4]used a direct method to align zero scale, as shown in Fig.3.

Fig.3 Zero adjustment system with manual mechanism

The zero adjustment system was composed of loading mechanism, zero scale collimation mechanism, manual adjustment mechanism and computer vision system. The relative position between zero scale and standard scale of measured steel tape was shown by the image signal. Then steel tape’s conveying was driven by a manual adjustment mechanism, and zero scale collimation was realized.

This method has a better accuracy compared with optical reading devices like microscopes[5]. However, it has some limitations. Firstly, due to manual adjustment, it is difficult to guarantee the stability. What’s more, the two operation modes of steel tape are not taken into account.

1.2 Zero collimation system with motorized platform

Lei T[6]mentioned an automatic zero collimation system, as shown in Fig.4. After the rough localization, zero scale was in the scope about ±2 mm from the standard datum line. Then the position signal would be sended to computer, and the distance could be calculated. Next conveying motor drove the micro-displacement platform with measured steel tape to move to the standard datum line. At last servo motor was stopped while the zero scale and standard datum line were coincided.

Fig.4 Zero collimation system with real-time motorized platform

Automatic conveying and collimation can save labor. As to a servo system, a very high degree of accuracy is required when driving the steel tape movement. But it is nearly impossible to drive micro-displacement platform to align zero position without any deviation. Besides, this system is not applicable to the steel tape with movable hook structure.

2 New zero calibration system

The zero calibration method proposed in this paper is specifically designed for the steel tape with movable hook structure. This system is aimed to solve the indeterminacy of zero position. Standard datum planes and standard scales are designed to determine zero position and calibrate indication error. A new system based on this principle is put forward, as shown in Fig.5.

Fig.5 Mechanical structure of steel tape zero calibration system

2.1 Main component elements

Zero calibration system is composed of mechanical desktop, location structure of standard datum planes, loading mechanism, high precision dovetail guide rails, one-dimensional displacement mechanism,multi-freedom fine-tuning structure of camera, machine vision system and computer system software. Experimental devices and key parameters are shown in Table 1.

Table 1 Experimental devices and key parameters

2.2 Principle

In this new system based on machine vision, an indirect approach is proposed. The calculation of deviation value is done by camera and computer, which can get much more accuracy results than traditional observational methods. The distance between standard datum planes and standard scales is regarded as a datum quantity. We compared this datum quantity with the distance between zero position and 500 mm scale, it can be found that the indication error is equal to the zero error. Push-type measurement datum and hook-type measurement datum are designed to get the measured value. Then the result can be got indirectly, although zero position is a plane rather than a scale. Traceability operation is designed to confirm the accuracy of datum quantity.

2.3 Calibration operation

Steps of the zero calibration are as follows.

1) Determine the type of measured steel tape and operation modes.

2) Push or catch the hook structure tightly against the positioning blocks and fix the box of tape.

3) Camera collects images of 500 mm scale.

4) Computer processes the images to calculate the pixel equivalent and takes the equivalent as standard component, and then the indication error is calculated.

5) Repeat steps 2 and 3 while changing the operation mode.

2.4 Traceability operation

Standard datum planes are determined by traceability operation. The mechanical structure is composed of positioning blocks, loading mechanism and micrometer handwheel. The positioning blocks are taken as reference of different standard datum planes of push-type and hook-type measurements. Combined with different modes of operation, steps of the traceability are as follows.

1) Determine the position of standard datum planes in push-type measurement. The standard component used as a higher precision reference is knife-type steel scale, which is designed by Tianjin Institute of Metrological Supervision and Testing. The knife-type steel scale has a standard length of 500 mm between standard scale line and standard surface. Micrometer handwheel and setscrew are used to determine and fix the the position of push-type standard datum planes. It is shown in Fig.6.

Fig.6 Mechanical structure of standard datum location device

2) Determine the position of standard datum planes in hook-type measurement. After the standard datum in push-type has been fixed, the 4 mm gauge block[7]is sandwiched between two positioning blocks as a higher precision standard component. We should ensure that the distance between the two datum planes is equal to the distance between the two standard scales on board.

2.5 Float pixel equivalent method

Pixel equivalent is an important parameter to describe the relationship between pixel of digital image and actual size of measured piece in the 2D image measuring system. Also it plays a role of precision standard in machine vision system.

This system adopts a float equivalent method[8]. During the image processing, the two standard scales in the field, whose distance is known exactly, are used as standard to calculate dynamic pixel equivalent. As previously mentioned, distance between the two standard scales on board isl=4 mm. The corresponding pixel numberncan be counted in image processing. The pixel equivalentbcan be calculated by

(4)

In this system, the width of field is around 40 mm, and effective pixel of industrial camera is 2 592×1 944. It is obvious that the calibration process has a high degree of accuracy, and the resolution is 0.02 mm.

2.6 Operation and displace interface

The operation interface is designed as shown in Fig.7. In addition to 500 mm scale, other integer decimeter scales between 0 mm and 500 mm can also be calibrated by this system. The image in the left side is a processed binary image. The indication error can be calculated according to pixel equivalent. Identifying line center and horizontal projection of the pixels in binary image[9]are used in image processing. What’s more, the mode of operation, push-type or hook-type, can be chosen before calibration, so does the type of measured tape.

Fig.7 Operation and displace interface

3 Experiments and analysis

An experiment was designed to examine repeatability of new system. Repeatability tests were carried out at 500 mm scale on the same steel tape. Indication errors were recorded in two different operation modes. The records are shown in Table 2 and Fig.8.

Table 2 Experimental results

Fig.8 Records of experiment

(5)

(7)

Taking uncertainty of measurementUas expanded uncertainty, it is

U=kuA,

(8)

wherekis coverage factor, generallyk=2.

As shown in Table 2 and Fig.8, the system’s calibration repeatability is approximately 0.006 mm, and the uncertainty is less than 0.002 mm, which is sufficient for practical use. Besides, the range of pixel equivalent is very little, which can be neglected. The traditional observational system usually introduces errors caused by the naked-eye and zero scale alignment process, and the repeatability is around 0.1 mm. It is obvious that this novel zero calibration method aiming to solve indeterminacy of zero position is far superior to the traditional methods in not only accuracy but also the repeatability of the result.

4 Conclusion

In this paper, a new zero calibration method of steel tape is put forward to deal with the indeterminacy of zero position caused by movable hook structure. Standard datum planes and standard scales are designed as the reference standard. The float pixel equivalent method is applied to ensure the accuracy of calibration process. Meanwhile, cooperated with 5 m verification platform[10-12], this system shows obvious superiority over traditional methods in both accuracy and stability.

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