CN110376207A - A kind of ultra-wide slab surface defect on-line detecting system image-pickup method - Google Patents

Abstract

The invention provides an image acquisition method for an online detection system for super-wide and thick plate surface defects, which belongs to the technical field of machine vision non-destructive detection. In this method, the number of line scan cameras used is determined according to the maximum width of the board, the minimum size of defects to be detected, and the camera resolution, and the number of narrow-band LED light sources is consistent with the number of cameras. After the steel plate enters the camera acquisition area, the laser velocimeter acquires the speed of the steel plate, thus determining the acquisition line rate of the line array camera. Then, the multi-channel PWM signal generator is controlled by the industrial computer to generate the PWM signal with the same pulse frequency and acquisition line rate, and output the PWM signal with the same number of cameras to control the synchronous acquisition of the cameras, and finally realize the image acquired by each camera in length and Widthwise shift stitching yields a complete surface image. The method can realize the rapid adjustment of the brightness of the light source without changing the acquisition rate of the camera, and ensure the uniform brightness of the image background.

Description

An image acquisition method for an online detection system for surface defects of ultra-wide and thick plates

technical field

The invention relates to the technical field of machine vision non-destructive testing, in particular to an image acquisition method for an online detection system for surface defects of ultra-wide thick plates.

Background technique

At present, the online detection system of steel plate surface defects based on machine vision has been widely used in the production lines of major steel plants, which plays a key role in ensuring the surface quality of steel plates. The key to this system is to collect complete steel plate images with uniform background for accurate defect identification. For the detection of ultra-wide boards larger than 5 meters, due to the requirements of detection accuracy, it is necessary to arrange multiple cameras in the width direction to complete the collection of the entire board surface. These multiple cameras need to collect images in the width direction and motion direction. Accurate splicing can facilitate subsequent defect analysis. Since the board is very wide, a strip of LED light source cannot guarantee the uniformity of the entire lighting area, which will cause the background brightness of the collected steel plate image to appear uneven in the width direction. Therefore, multiple strips of LED light sources are required to provide illumination. How to control the brightness changes of these LED light sources in time to ensure that the camera corresponding to the light source captures an image with a uniform background needs to be considered.

Contents of the invention

The technical problem to be solved by the present invention is to provide an image acquisition method for an online detection system for surface defects of ultra-wide and thick plates.

The method realizes image acquisition through the combination of a line array camera, a narrow-band LED light source, a multi-channel PWM signal generator, a laser velocimeter, and a signal enhancement module. The implementation process includes the following steps:

(1) According to the maximum width L of the ultra-wide and thick plate surface, the minimum size α of the defect to be detected and the resolution M of the line camera, determine the number N of line cameras used, the number of narrow-band LED light sources and the number of line cameras to maintain consistent;

(2) After the ultra-wide and thick plate enters the acquisition area of the line camera, the laser tester obtains the speed ν of the ultra-wide and thick plate, and determines the acquisition line rate of the line camera according to the obtained ultra-wide and thick plate speed ν and the minimum size α of the defect to be detected f;

(3) Control the multi-channel PWM signal generator to generate a PWM signal with the same pulse frequency as the acquisition line rate f, and output N pulse signals identical to the number of line array cameras, and each signal is divided into two parts;

(4) By pre-calibrating the position difference between the light bands of the narrow-band LED light source, the images collected by each line array camera are stitched in the length direction using shifting;

(5) The images collected by each line camera are shifted and stitched in the length and width directions to obtain a complete surface image.

Wherein, the number of line-scan cameras in step (1) N=2L/(α×M).

In step (2), the acquisition line rate of the line array camera is f=2ν/α. In order to ensure the detection of defects with a minimum size of α, the acquisition accuracy here needs to reach α/2.

In step (3), part of each signal is sent to a line array camera using an external trigger to realize synchronous acquisition, and the other part is directly used as a power supply for a narrow-band LED light source after being enhanced by a signal enhancement module. The signal used as the power supply of the narrow-band LED light source can ensure that the acquisition rate is unchanged, that is, the acquisition accuracy is unchanged, and the brightness of the light source can be adjusted by controlling the PWM signal generator to change the signal duty cycle, thereby realizing the adjustment of the brightness of the image captured by the camera.

The collection illumination areas of each group of line scan cameras are arranged in a staggered pattern.

Step (4) is specifically: the misalignment distance of the illumination areas of two adjacent line scan cameras is σ, the acquisition accuracy θ in the length direction of the ultra-wide thick plate is α/2, and α is the minimum size of the defect that needs to be detected, then the shifted The number of pixels Δ is:

Δ=σ/θ=2σ/α.

The beneficial effects of above-mentioned technical scheme of the present invention are as follows:

In the above scheme, multiple sets of line array cameras and narrow-band LED light sources are used to obtain the accurate movement speed of the steel plate through the laser velocimeter, and the PWM signal is used to realize the external trigger synchronous acquisition control of multiple cameras to ensure the acquisition positions of multiple cameras. Using the pulse width of the same signal to control the brightness of each line of illumination light source for image acquisition, this method can realize the adjustment of the brightness of the light source for each line of the image acquisition, and at the same time use the same PWM signal hardware method to realize the control of acquisition speed and image brightness , which can realize the brightness adjustment of a single line in the image, so that the light source can quickly respond to the brightness change request.

Description of drawings

Fig. 1 is an overall scheme diagram of the image acquisition method of the online detection system for super-wide and thick plate surface defects of the present invention;

Fig. 2 is the layout diagram of each camera acquisition area in the embodiment of the present invention;

FIG. 3 is a schematic diagram of PWM signals in an embodiment of the present invention.

Detailed ways

In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will describe in detail with reference to the drawings and specific embodiments.

The invention provides an image acquisition method of an online detection system for super-wide and thick plate surface defects.

As shown in Figure 1, this method realizes image acquisition through the combination of a line array camera, a narrow-band LED light source, a multi-channel PWM signal generator, a laser velocimeter, and a signal enhancement module. The implementation process includes the following steps:

(1) According to the maximum width L of the ultra-wide and thick plate, the minimum size α of defects to be detected and the resolution M of the line camera, determine the number N of line cameras used, and the number of narrow-band LED light sources should be consistent with the number of line cameras ;

(2) After the ultra-wide and thick plate enters the acquisition area of the line camera, the laser tester obtains the speed ν of the ultra-wide and thick plate, and determines the acquisition line rate of the line camera according to the obtained ultra-wide and thick plate speed ν and the minimum size α of the defect to be detected f;

(3) Control the multi-channel PWM signal generator to generate a PWM signal with the same pulse frequency as the acquisition line rate f, and output N pulse signals identical to the number of line array cameras, and each signal is divided into two parts;

(4) By pre-calibrating the position difference between the light bands of the narrow-band LED light source, the images collected by each line array camera are stitched in the length direction by shifting;

(5) The images collected by each line camera are shifted and stitched in the length and width directions to obtain a complete surface image.

The following will be described in conjunction with specific embodiments.

According to the maximum width L of the board surface and the minimum size α of the defect to be detected, and the number M of pixels of the line scan camera used, determine the number of cameras to be used in the width direction N=2L/(αM);

At the same time, the number of narrow-band LED light sources is the same as the number of cameras.

According to the obtained steel plate velocity ν and the minimum size α of defects to be detected in the width direction, the collection linear velocity f of the line scan camera is determined, f=2ν/α.

As shown in Figure 3, use the obtained acquisition line rate f to control the multi-channel PWM signal generator to generate N-channel PWM signals with the same pulse frequency as the acquisition line rate. Each signal is divided into two parts, and one part is sent to the external trigger. In this way, the synchronous acquisition of N cameras can be guaranteed, and the detection accuracy can be guaranteed in the width and length directions of the steel plate. The period value t of the signal is:

t=1/f

The other part of the PWM signal is amplified by the signal enhancement circuit and used as the power supply for the narrow-band LED light source matched with the camera. By calculating the average gray value of one or more lines collected by each camera in real time, evaluate the average gray value and set the benchmark The difference in gray scale, send an instruction to adjust the pulse width of the PWM signal, and adjust the pulse width λ value of the PWM signal. It can be considered that there is a nearly linear relationship between the brightness ν of the light source and the change of the pulse width λ.

ν=Kλ+β

K is a positive constant, and β is a constant.

Therefore, in the case of ensuring that the pulse frequency of the PWM signal remains unchanged, that is to say, the acquisition rate of all cameras is the same, by changing the pulse width value of the PWM signal that supplies power to the LED light source, that is, the duty cycle of the PWM signal can be changed. The brightness of the light source realizes the adjustment of the background brightness of the images collected by each camera. The whole adjustment process can be carried out in a self-feedback manner to achieve the purpose of quickly adjusting the brightness of the light source.

In this way, the collection of all cameras and the illumination brightness of each camera are controlled through a set of PWM signals, so as to ensure the splicing accuracy of images and uniform background.

In order to avoid the mutual influence of the lighting sources of each camera, the collection lighting areas of each group of cameras adopt a pattern of interlacing, as shown in Figure 2, which can ensure that the influence of mutual light sources is avoided to a certain extent, because the entire collection process accurately controls the movement according to the speed Accuracy in the direction, by pre-calibrating the position difference between each light band, the images collected by each camera can be shifted and stitched in the length direction. For example, the misalignment distance of the illumination areas of two adjacent cameras is σ, and the steel plate The acquisition accuracy θ in the length direction is α/2, where α is the minimum size of the defect to be detected, and the number of pixels Δ to be shifted is Δ=σ/θ=2σ/α.

In the specific application process, the workflow steps are as follows:

Step 1. The steel plate in motion enters the acquisition area of the camera. (The camera adopts a line array camera to achieve accurate splicing of images on the steel plate movement, and the acquisition area is very narrow, which can avoid the interference of different camera light sources). The laser velocimeter measures The exact movement speed of the current steel plate.

Step 2. According to the design acquisition accuracy and the current steel plate movement speed, calculate the acquisition line rate that the camera needs to adopt, and control the multi-channel PWM signal generator to generate multiple PWM pulse signals with the same frequency and the same number of cameras, and a part of each signal is directly It is sent to the line array camera, and the other part is amplified by the signal power enhancement module to supply power to the LED light source. The camera adopts an external trigger method, and will start synchronous acquisition after receiving the signal.

Step 3. By calculating the average gray value of the single-line or multi-line collected by each camera, compare it with the set benchmark. If the difference is large, adjust the pulse width of the PWM pulse signal of the channel. Without changing the acquisition Adjust the brightness of the light source in time to ensure that the background brightness of each camera is uniform.

Step 4. After the images of each camera are obtained, the images are shifted in the width direction according to the coincident positions obtained by calibration in advance, and after the shifting, the images of the entire surface are obtained by splicing.

The above description is a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, these improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims (7)

1. An image acquisition method of an ultra-wide and thick plate surface defect on-line detection system, characterized in that: the method realizes ultra-wide through the combination of a line array camera and a narrow-band LED light source, a multi-channel PWM signal generator, a laser velocimeter, and a signal enhancement module. The image acquisition of wide and thick plates, the realization process includes the following steps: (1) According to the maximum width L of the ultra-wide and thick plate surface, the minimum size α of the defect to be detected and the resolution M of the line camera, determine the number N of line cameras used, the number of narrow-band LED light sources and the number of line cameras to maintain consistent; (2) After the ultra-wide and thick plate enters the acquisition area of the line camera, the laser tester obtains the velocity ν of the ultra-wide and thick plate, and determines the acquisition line rate of the line-scan camera according to the obtained ultra-wide and thick plate speed ν and the minimum size α of the defect to be detected f; (3) The industrial computer controls the multi-channel PWM signal generator to generate a PWM signal with the same pulse frequency as the acquisition line rate f, and outputs N pulse signals with the same number as the line array camera, and each signal is divided into two parts; (4) By pre-calibrating the position difference between the light bands of the narrow-band LED light source, the images collected by each line array camera are stitched in the length direction by shifting; (5) The images collected by each line camera are shifted and stitched in the length and width directions to obtain a complete surface image. 2. The image acquisition method of the online detection system for super-wide and thick plate surface defects according to claim 1, characterized in that: the number of line cameras in the step (1) is N=2L/(α×M). 3. The image acquisition method of the online detection system for super-wide and thick plate surface defects according to claim 1, characterized in that: in the step (2), the acquisition linear velocity of the line array camera is f=2ν/α. 4. The image acquisition method of the ultra-wide and thick plate surface defect on-line detection system according to claim 1, characterized in that: in the step (3), part of each signal is sent to a line-scan camera that adopts an external trigger mode to realize synchronization The other part is enhanced by the signal enhancement module and directly used as the power supply of the narrow-band LED light source. 5. The image acquisition method of the online detection system for super-wide and thick plate surface defects according to claim 1, characterized in that: the acquisition illumination areas of each group of line array cameras are arranged in an interlaced pattern. 6. The image acquisition method of the online detection system for super wide and thick plate surface defects according to claim 1, characterized in that: the step (4) is specifically: the misalignment distance of the illumination areas of two adjacent line array cameras is σ, which is To ensure that the defect with the minimum size of α can be detected, and the acquisition accuracy θ in the length direction of the ultra-wide and thick plate is α/2, the number of pixels that need to be shifted Δ is: Δ=σ/θ=2σ/α. 7. The image acquisition method of the ultra-wide and thick plate surface defect on-line detection system according to claim 4, characterized in that: the enhanced PWM signal as the power supply of the narrow-band LED light source can ensure that the acquisition rate is constant, that is, the acquisition accuracy is unchanged, By controlling the PWM signal generator to change the signal duty cycle, the brightness of the light source can be adjusted, so as to realize the rapid adjustment of the brightness of the image collected by the camera.