The imaging system is mainly composed of a camera (CCD/CMOS), a lens, and a light source, which is the basis of machine vision detection. The planning intention of the imaging system is to obtain qualified original images, and a good imaging system must ensure stable image quality during the operation of the system. Stable image capture is the basic guarantee of stability of visual inspection.
1. The impact of industrial cameras on imaging stability
In terms of the selection of industrial cameras, machine vision system planners should first consider its sensor type, resolution, and frame rate. Among them, sensors are divided into CCD and CMOS. CMOS image sensors are highly integrated, and the distances between components and circuits are very close. Compared with CMOS cameras, CCD sensor cameras have the characteristics of high sensitivity, low noise, and fast response speed. In terms of stability, CCD cameras are also more resistant to shock and vibration. Generally speaking, CCD sensor cameras are superior to CCD cameras in terms of image quality and stability.
Another important factor that affects the image quality of the camera is the lens of the camera. In addition to the need to select suitable parameters such as focal length, depth of field, and aperture according to the specific operating conditions, an important factor that affects the detection accuracy of the system is the geometric distortion of the image. It is the inherent perspective distortion of optical lenses and it is affected by the manufacturing technology and cannot be eliminated but can only be compensated. Although many industrial cameras use various methods to compensate for the fault of lens distortion, geometric distortion is still in the field of high-precision detection. It will still affect the detection accuracy.
2. The influence of light source on imaging stability
The light source has the effect of expanding the characteristics and defects of the image, weakening the disorder and the scenery, and directly affecting the quality of the input data. Since there is no general lighting equipment, the planning of the light source has always been a difficulty for the machine vision system. Generally, it is not only necessary to select the type of light source for each specific application instance but also to consider the light source device and the irradiation method of the light source according to the specific environment, so as to achieve the best effect.
There are differences in the stability of different types of light sources. Common visible light sources include LED light sources, halogen lamps, fluorescent lamps, and sodium lamps. The biggest defect of visible light is that it cannot continue to output light energy stably. For example, within an hour, the light energy of fluorescent lamps will drop by about 15%, and, with the increase of the use time, the light energy output will continue to decline.
There is no doubt about the impact of the stability of detection software on machine vision. After all, the vision system will use the software on the computer to select targeted algorithms for image filtering, edge detection, edge extraction, and a series of image processing. Different image processing, analysis methods, different detection methods, and calculation formulas will bring different faults, and the algorithm determines the detection accuracy.
The detection environment during the operation of the machine vision system includes temperature, light, power changes, dust, humidity, and electromagnetic disturbances. A good environment is the guarantee for the normal operation of the vision system. External light will affect the total light intensity irradiated on the measured object, increasing the noise of image data output. And the change of power supply voltage will also cause the light source to be unstable, resulting in noise that changes with time.
The temperature change will also affect the function of the camera. The temperature range for normal operation will be marked on the camera when it leaves the factory. Too hot or too cold environments will affect the normal operation of the camera.
The electromagnetic disturbance is an unavoidable disturbance factor in industrial detection sites. It has a particularly severe impact on weak current circuits such as industrial camera circuits and data signal transmission circuits. Qualified machine vision products will undergo strict anti-disturbance tests before leaving the factory. Therefore, the influence of external electromagnetic disturbance on the hardware circuit can be eliminated.