阅读说明：本技术 最小二乘法椭圆拟合活塞裙部最大点处尺寸测量方法 (Least square method ellipse fitting piston skirt maximum point size measurement method ) 是由 陶唐飞 杨兴宇 徐佳宇 赵晋民 徐光华 于 2019-08-19 设计创作，主要内容包括：最小二乘法椭圆拟合活塞裙部最大点处尺寸测量方法,先采集活塞竖直放置状态下的俯视图,然后对其进行像素级边缘检测,再进行亚像素级边缘检测,接着用采集到的亚像素边缘点进行基于最小二乘法的椭圆拟合,并计算椭圆参数；然后通过长轴转角参数对拟合的椭圆进行角度矫正,并进行边缘点筛选,对筛选出的边缘点进行椭圆拟合,计算椭圆长轴即为裙部最大点处尺寸；本发明实现活塞裙部最大点处的尺寸测量,具有测量方法简单、尺寸测量精度较高的优点。(The method for measuring the size of the maximum point of the skirt part of the piston by the least square method elliptic fitting comprises the steps of firstly collecting a top view of the piston in a vertical placement state, then carrying out pixel level edge detection on the top view, then carrying out sub-pixel level edge detection, then carrying out the least square method elliptic fitting by using the collected sub-pixel edge points, and calculating elliptic parameters; then, carrying out angle correction on the fitted ellipse through the major axis corner parameters, carrying out edge point screening, carrying out ellipse fitting on the screened edge points, and calculating the major axis of the ellipse, namely the size of the maximum point of the skirt part; the invention realizes the size measurement of the maximum point of the skirt part of the piston and has the advantages of simple measurement method and higher size measurement precision.)

1. The method for measuring the size of the maximum point of the piston skirt by least square ellipse fitting is characterized by comprising the following steps of:

step 1), collecting an overlook image of the piston in a vertical placement state: collecting a piston image under the state that the piston is vertically placed, wherein the maximum size measured in the image is the size of the maximum point of the skirt;

Step 2), performing sub-pixel edge detection on the piston image:

2.1) carrying out pixel level edge detection on the collected piston image to obtain a piston image pixel level edge;

2.2) carrying out sub-pixel level edge detection subdivision on the pixel level edge, extracting sub-pixel level edge points of the piston image, and recording the point set as N;

Step 3), ellipse fitting based on the least square method and ellipse parameter calculation: carrying out ellipse fitting on the acquired piston sub-pixel edge points by using an ellipse fitting method based on a least square method to obtain an ellipse equation and ellipse parameters of a major semi-axis, a minor semi-axis, an ellipse center and a major axis corner;

step 4), carrying out angle correction on the fitted ellipse and screening edge points:

4.1) rotating the ellipse according to the calculated ellipse parameters, and correcting the angle to ensure that the rotation angle of the long axis of the ellipse becomes 0;

4.2) determining end points on two sides of the major axis of the ellipse, and sequentially screening N/8 points in each direction to serve as edge points on the ellipse;

Step 5), carrying out ellipse fitting on the screened edge points and calculating the major axis of the ellipse: and carrying out ellipse fitting based on minimum multiplication on the screened edge points, and calculating to obtain an ellipse equation and major semiaxis parameters, wherein the major axis of the ellipse is the size of the maximum point of the skirt part.

2. a method of measuring a dimension at a maximum point of a skirt of a piston by least squares ellipse fitting according to claim 1, wherein: on the basis of the parameters of the major and half axes of the fitted ellipse obtained in the steps 3) and 5), a size conversion coefficient-calibration coefficient of a world coordinate system and an image coordinate system is obtained through camera calibration, and then the pixel size is converted into an actual physical size.

Technical Field

The invention relates to the technical field of piston skirt measurement, in particular to a method for measuring the size of the maximum point of a piston skirt by least square ellipse fitting.

Background

the piston, as the heart of the engine, plays a crucial role in the overall operational performance of the device. The working condition of the piston is severe, the qualified size of the piston can ensure the accurate matching of the piston and the cylinder, and is beneficial to the formation and maintenance of a dynamic lubricating oil film, so that the air leakage of the engine is reduced, the friction resistance is reduced, the running noise is reduced, the running performance of the engine is improved, and the service life is prolonged; the unqualified size of the piston can cause the deterioration of the lubrication state between the piston and the cylinder, aggravate the abrasion between the piston and the cylinder and seriously affect the working performance of the engine. Therefore, the measurement of the geometric dimension of the piston has great significance to the safe operation of the whole mechanical equipment and the safety guarantee of personnel. Wherein the dimension of the skirt at its maximum point is an extremely important one of a number of dimensions that determine the piston's operational quality.

In the field of piston production machining, the measurement of the dimensions of the skirt at the maximum point has been carried out manually using special tools. In order to measure the size, a set of dial indicator-based piston skirt maximum point size measuring instrument is developed, firstly, the plumb face where the size of the maximum point of the piston skirt is located is determined manually, then, the height of the measuring instrument is adjusted to enable the measuring instrument to meet the size measurement of pistons of different models, and the key point is to accurately find the position where the size of the maximum point of the piston skirt is located and measure the size. The method has the advantages of low measurement precision, measurement results influenced by human factors, low efficiency, improper data record and storage and poor piston product size traceability.

The sub-pixel edge detection algorithm is used for decomposing pixels into smaller units by subdividing the pixels under the condition that the hardware condition of a vision measurement system is not changed, so that the accuracy of edge detection is improved, and the sub-pixel edge detection algorithm is often applied to workpiece dimension measurement.

the least square method was first proposed in 1806 by mary legendre, and by minimizing the square sum of errors and finding the best function matching of data, unknown data can be easily found by the least square method, and the square sum of errors between these found data and actual data is minimized, showing many unique advantages in curve fitting.

The measured piston adopts a vertical posture, the measurement of the size of the maximum point of the piston can be realized by applying methods based on sub-pixel edge detection gold and least square method ellipse fitting, and no document is disclosed for measuring the maximum point of the skirt of the piston by combining the sub-pixel edge detection gold and the least square method ellipse fitting.

disclosure of Invention

In order to overcome the defects of the prior art, the invention aims to provide a least square method ellipse fitting method for measuring the size of the maximum point of the skirt of the piston, so that the size measurement of the maximum point of the skirt of the piston is realized, and the method has the advantages of simple measurement method and higher size measurement precision.

In order to achieve the purpose, the invention adopts the technical scheme that:

the method for measuring the size of the maximum point of the piston skirt by least square ellipse fitting comprises the following steps:

step 1), collecting an overlook image of the piston in a vertical placement state: collecting a piston image under the state that the piston is vertically placed, wherein the maximum size measured in the image is the size of the maximum point of the skirt;

step 2), performing sub-pixel edge detection on the piston image:

2.1) carrying out pixel level edge detection on the collected piston image to obtain a piston image pixel level edge;

2.2) carrying out sub-pixel level edge detection subdivision on the pixel level edge, extracting sub-pixel level edge points of the piston image, and recording the point set as N;

step 3), ellipse fitting based on the least square method and ellipse parameter calculation: carrying out ellipse fitting on the acquired piston sub-pixel edge points by using an ellipse fitting method based on a least square method to obtain an ellipse equation and ellipse parameters of a major semi-axis, a minor semi-axis, an ellipse center and a major axis corner;

step 4), carrying out angle correction on the fitted ellipse and screening edge points:

4.1) rotating the ellipse according to the calculated ellipse parameters, and correcting the angle to ensure that the rotation angle of the long axis of the ellipse becomes 0;

4.2) determining end points on two sides of the major axis of the ellipse, and sequentially screening N/8 points in each direction to serve as edge points on the ellipse;

Step 5), carrying out ellipse fitting on the screened edge points and calculating the major axis of the ellipse: and carrying out ellipse fitting based on minimum multiplication on the screened edge points, and calculating to obtain an ellipse equation and major semiaxis parameters, wherein the major axis of the ellipse is the size of the maximum point of the skirt part.

On the basis of the parameters of the major and half axes of the fitted ellipse obtained in the steps 3) and 5), a size conversion coefficient-calibration coefficient of a world coordinate system and an image coordinate system is obtained through camera calibration, and then the pixel size is converted into an actual physical size.

the invention has the beneficial effects that:

1. the invention can realize the measurement of the size of the maximum point of the skirt part of the piston, and has the characteristics of convenient image acquisition, strong algorithm adaptability and low cost.

2. the invention carries out ellipse fitting by a least square method on the basis of the detected sub-pixel edge points of the piston image, carries out angle correction on the ellipse, screens out partial edge points and carries out ellipse fitting again, thereby improving the ellipse fitting precision of the skirt maximum point and improving the measurement precision of the size of the skirt maximum point.

drawings

FIG. 1 is a flow chart of the method of the present invention.

fig. 2 is a diagram showing the result of ellipse fitting based on the least square method based on the detected sub-pixel edge points in the embodiment.

FIG. 3 is a graph of the results of the least squares ellipse fitting performed on the results of FIG. 2 to screen edge points based on the long axis endpoints.

FIG. 4 is a comparison of the dimensional errors of the measurement results of the maximum skirt point size for different individual pistons based on the least square ellipse fitting algorithm of the full edge point and the partial edge point.

Detailed Description

the invention is described in detail below with reference to the figures and examples.

referring to fig. 1, a least square ellipse fitting method for measuring the size of a piston skirt at a maximum point comprises the following steps:

step 1), collecting an overlook image of the piston in a vertical placement state: the size of the maximum point of the skirt part of the piston is the maximum size in the piston, and the section of the skirt part of the piston is an ellipse, so that an image of the piston in a vertically placed state of the piston is acquired, and the maximum size measured in the image is the size of the maximum point of the skirt part;

step 2), performing sub-pixel edge detection on the piston image:

2.1) carrying out pixel level edge detection on the collected piston image to obtain a piston image pixel level edge;

2.2) carrying out sub-pixel level edge detection subdivision on the pixel level edge, extracting sub-pixel level edge points of the piston image, and recording the point set as N;

Step 3), ellipse fitting based on the least square method and ellipse parameter calculation: carrying out ellipse fitting on the acquired piston sub-pixel edge points by using an ellipse fitting method based on a least square method to obtain an ellipse equation and ellipse parameters of a major semi-axis, a minor semi-axis, an ellipse center and a major axis corner; the result of the ellipse fitting of the present embodiment is shown in fig. 2, and it can be seen from fig. 2 that fitting of the ellipse of the skirt portion is achieved by ellipse fitting based on the least square method, but the measurement error is large, and correction of the fitted ellipse is required.

step 4), carrying out angle correction on the fitted ellipse and screening edge points:

4.1) rotating the ellipse according to the calculated ellipse parameters, and correcting the angle to ensure that the rotation angle of the long axis of the ellipse becomes 0;

4.2) determining end points on two sides of the major axis of the ellipse, and sequentially screening N/8 points in each direction to serve as edge points on the ellipse;

Step 5), carrying out ellipse fitting on the screened edge points and calculating the major axis of the ellipse: and carrying out ellipse fitting based on minimum multiplication on the screened edge points, and calculating to obtain an ellipse equation and major semiaxis parameters, wherein the major axis of the ellipse is the size of the maximum point of the skirt part. The result of the ellipse fitting of this embodiment is shown in fig. 3, and it can be seen from fig. 3 that the least square ellipse fitting effect based on the screened partial edge points is better than the accuracy before screening.

Referring to table 1, table 1 shows that the partial edge point method of the present invention measures the maximum point size of a piston skirt of QC480Q model based on the full edge point method, and it can be seen from table 1 that the partial edge point method of the present invention is more accurate than the full edge point method for piston samples.

TABLE 1 two ellipse fitting methods measure QC480Q piston skirt maximum point size (units: mm)

referring to fig. 4, fig. 4 is a comparison of the dimensional errors of the measurement results of the maximum skirt point size of different individual pistons based on the full edge point method and the partial edge point method of the present invention, and it can be seen that the accuracy of the partial edge point method of the present invention is superior to that based on the full edge point method, and the method has no individual contingency, and can measure different sizes within a certain range.