Selection knowledge that visual engineers must know

Visual engineer must know the selection

1.What causes the problem of dropped frames in industrial cameras?

Some machine vision engineers often think that industrial cameras with USB interfaces will cause frame drops. In general, industrial camera frame loss has nothing to do with the transmission interface used by industrial cameras, whether it is USB, 1394, GigE, or CameraLink. Poorly designed drivers or industrial camera hardware are the real causes of dropped frames: The reason for poorly designed industrial cameras to lose frames is actually the blockage of the transmission channel, which cannot be processed in time, so when new images come in, The previous image may be forced to be discarded, or the new image may be forced to be discarded. To solve this problem, designers need to make precise designs for each link of the driver and industrial camera hardware data transmission.

2. How is the resolution of an industrial camera defined?

Resolution is the most basic parameter of the camera. It is determined by the resolution of the chip used by the camera. It is the number of pixels arranged on the target surface of the chip. Generally, the resolution of area scan cameras is expressed by two numbers of horizontal and vertical resolution, such as: 1920 (H) x 1080 (V). The first number indicates the number of pixels in each line, that is, a total of 1920 pixels, and the latter number Represents the number of pixel lines, which is 1080 lines. Now the resolution of the camera usually indicates how many K, such as 1K (1024), 2K (2048), 3K (4096) and so on. When capturing images, the resolution of the camera has a great impact on the image quality. When imaging the same large field of view (range of objects), the higher the resolution, the more obvious the display of details.

3. What does the frame rate and line frequency of an industrial camera mean?

The frame rate / line rate of the camera indicates the frequency at which the camera collects images. Normally area scan cameras use the frame rate. The unit is fps (Frame Per second), such as 30fps. The camera usually uses the line frequency as the unit of KHz. For example, 12KHz means that the camera can collect up to 12000 lines of image data in 1 second. Speed is an important parameter of the camera. In practical applications, it is often necessary to image moving objects. The speed of the camera needs to meet certain requirements in order to clearly and accurately image the object. The frame rate and line rate of the camera are first affected by the frame rate and line rate of the chip. The maximum design speed of the chip is mainly determined by the highest clock that the chip can withstand.

4. What does the noise of industrial cameras mean?

Industrial camera noise refers to unwanted acquisition during imaging. Generally speaking, noise can be divided into two categories: one is statistical fluctuation noise that conforms to Poisson distribution caused by effective signals, also called shot noise (Shot noise), this kind of noise is the same for any camera, it is inevitable, especially a certain calculation formula. (That is: the square of the noise = the mean of the signal). The second type is the signal-independent noise inherent in the camera itself. It is noise caused by the image sensor readout circuit, camera signal processing and amplification circuit, etc. The inherent noise of each camera is different. In addition, for digital cameras, quantization noise is generated when analog conversion is performed on a video signal. The higher the number of quantization bits, the lower the noise.

5. What does the signal-to-noise ratio of an industrial camera mean?

The signal-to-noise ratio of a camera is defined as the ratio of the signal to the noise in the image (the ratio of the average gray value of the effective signal to the root-mean-square of the noise), which represents the quality of the image. The higher the signal-to-noise ratio of the image, the better the image quality.

6. What does dynamic range mean in industrial cameras?

The dynamic range of the camera indicates the range of the light signal detected by the camera. The dynamic range can be defined by two methods. One is the optical dynamic range, which refers to the ratio of the maximum light intensity at saturation to the light intensity equivalent to the noise output. It is determined by the characteristics of the chip. Decide. The other is electronic dynamic range, which refers to the ratio between saturation voltage and noise voltage. For a fixed camera, its dynamic range is a fixed value and does not change with external conditions. In linear response, the dynamic range of the camera is defined as the ratio of the saturation exposure to the equivalent exposure of the noise: dynamic range = full well capacity of the photosensitive cell / equivalent noise signal dynamic range can be expressed in multiples, dB or Bit. If the dynamic range is large, the camera has a stronger adaptability to different light intensities.

7. What does the pixel depth in an industrial camera mean?

The digital signal output by a digital camera, that is, the gray value of a pixel, has a special number of bits, which is called the pixel depth. For black and white cameras, the orientation of this value is usually 8-16bit. The cell depth defines the number of gray levels in which gray levels are illuminated by dark channels. For example, for an 8-bit camera, 0 means full darkness and 255 means full light. A number between 0 and 25 represents a certain brightness indicator. 10bit data has 1024 gray levels and 12bit has 4096 gray levels. For each application we have to carefully consider whether we need very fine gray levels. Increasing from 8-bit to 10-bit or 12-bit can indeed increase the measurement accuracy, but it also reduces the speed of the system and increases the difficulty of system integration (increased cables and larger sizes), so we must also choose carefully.

8. What is the difference between industrial cameras and ordinary digital cameras?

1. The shutter time of industrial cameras is extremely short, which can capture fast-moving objects clearly, while ordinary cameras capture fast-moving objects very blurry;

2. The image sensors of industrial cameras are scanned progressively, while the image sensors of ordinary cameras are interlaced, or even scanned at three lines;

3. The shooting speed of industrial cameras is much higher than ordinary cameras; industrial cameras can take ten to several hundred pictures per second, while ordinary cameras can only take 2-3 pictures;

4. Industrial cameras output naked data, and their spectral range is often wider, which is more suitable for high-quality image processing algorithms, and is generally used in machine vision systems. For a picture taken by a common camera, its spectral range is only suitable for human vision, and after MPEG compression, the image quality is also poor;

9. How to choose a line scan camera?

1. Calculate resolution: Divide the width by the minimum detection accuracy to get the pixels needed for each line.

2. Detection accuracy: The width is divided by the pixels to get the actual detection accuracy.

3. Number of scanning lines: The length of the movement speed per second is divided by the accuracy to obtain the number of scanning lines per second.

An example of selecting a line scan camera based on the above calculation results is as follows:

For example, the width is 1600 mm, the accuracy is 1 mm, and the moving speed is 22000mm / s. Camera: 1600/1 = 1600 pixels with a minimum of 2000 pixels. If you select 2k camera, 1600/2048 = 0.8. Actual accuracy: 22000mm / 0.8mm = 27.5KHz. Camera is a 2048 pixel 28kHz camera

10. What are the characteristics of a line scan camera?

1. Line scan sensors used by line scan cameras usually have only one line of light sensing units (a few color line scan sensors use three lines of light sensing units);

2. Line scan camera only captures one line of images at a time;

3. The line scan camera only outputs one line of images at a time;

4. Compared with traditional area scan cameras, area scan scans collect several lines of images at a time and output them as frames.

11. What is the difference between a line scan camera and an area scan camera?

Linear array CCD industrial cameras are mainly used for image processing in industrial, medical, scientific research and security fields. In the field of machine vision, linear array cameras are a special type of vision machine. Compared with area scan cameras, its sensor has only one row of photosensitive elements, thus enabling high scanning frequency and high resolution. The typical field of application of linear array cameras is to detect continuous materials such as metal, plastic, paper and fiber. The detected object usually moves at a uniform speed, and it is continuously scanned line by line with one or more industrial cameras to achieve uniform detection of its entire surface. The image can be processed line by line, or the area array image composed of multiple lines can be processed. In addition, the linear array industrial camera is very suitable for measurement occasions. Thanks to the high resolution of the sensor, it can accurately measure to the micron.

For area array CCD, the application area is wide, such as the measurement of area, shape, size, position, and even temperature. The advantage of area array CCD is that it can obtain two-dimensional image information and the measurement image is intuitive. The disadvantage is that the total number of pixels is large, and the number of pixels in each row is generally less than the linear array, and the frame rate is limited. Area array CCD has few industrial cameras, more flexible pixel size and high frame size, which is especially suitable for the measurement of one-dimensional dynamic targets.

12. What is a line scan camera?

Linear array industrial cameras, as the name suggests, are "linear". Although it is also a two-dimensional image, it is extremely long, a few K in length, and only a few pixels wide. This camera is generally used in only two cases:

1. The field of view to be measured is a long and thin strip, which is mostly used for the problem of detection on the drum.

2. Requires great field of vision or high precision.

13. What are the general steps for selecting an industrial camera?

Step 1: First need to know the system accuracy requirements and industrial camera resolution;

Step 2: Need to know system speed requirements and industrial camera imaging speed;

The third step: the industrial camera and the frame grabber need to be considered together, because this involves the matching of the two;

Step 4: Compare prices.

14. How to use the accuracy required by the machine vision system to calculate the resolution (pixels) of the camera?

Knowing the actual detection accuracy and inferring how large an industrial camera should be can be calculated by the formula: X-direction system accuracy (X-direction pixel values) = field of view (X-direction) / CCD chip pixels (X-direction); Y Directional system accuracy (pixel values in the Y direction) = field of view (Y direction) / number of CCD chip pixels (Y direction). Of course, the theoretical pixel value is obtained by comprehensive consideration of system accuracy and sub-pixel method;

15. How to deduce which industrial camera to choose based on the actual required detection speed?

System single operation speed = system imaging (including transmission) speed + system detection speed, although the system imaging (including transmission) speed can be theoretically calculated based on the asynchronous trigger function of industrial cameras, shutter speed, etc., the best method is to actually use software test;

16. Which industrial cameras need to match the frame grabber for normal use?

Industrial cameras need to match the frame grabber for normal use, and generally need to match the following:

a. For video signal matching, there are two formats for black and white analog signal cameras, CCIR and RS170 (EIA). Generally, the capture card supports both industrial cameras;

b. Matching resolution, each board only supports cameras in a certain resolution range;

c. Matching of special functions. If the special functions of the camera are used, first determine if the board used supports this function. For example, if multiple cameras are to take pictures at the same time, the capture card must support multiple channels. If the camera is scanned progressively , Then the capture card must support progressive scanning;

d. Match the interface to determine whether the interface between the camera and the board matches. Such as CameraLink, Firewire1394, etc.

17. What role does the image processing unit play in intelligent industrial cameras?

In intelligent industrial cameras, the image processing unit is similar to an image acquisition and processing card. It can store the image data of the image acquisition unit in real time and perform image processing with the support of image processing software.

18. What is the main role of image processing software in smart industrial cameras?

The image processing software mainly completes the image processing function with the support of the hardware environment of the image processing unit. Such as geometric edge extraction, Blob, gray histogram, OCV / OVR, simple localization and search. In the smart camera, the above algorithms are packaged as fixed modules, which can be applied directly by the user without programming.

19. How to set the "Automatic Gain Control" function in industrial cameras?

The industrial camera has a video amplifier that amplifies the signal from the CCD to a level that can be used. The amplification, that is, the gain, is equivalent to a high sensitivity. However, in a bright light environment, the amplifier will overload and distort the video signal. When the switch is ON, the lens aperture is fully opened in low brightness conditions, and the gain is automatically increased to obtain a clear image. When the switch is OFF, natural and low-noise images can be obtained at low brightness.

20. What are the advantages of high-speed industrial cameras compared to general industrial cameras?

1. High-speed real-time uncompressed image recording, real-time display, set speed display;

2. The system adopts the recording method of directly writing data to the hard disk, which solves the problem of short recording time in the traditional memory recording method, and solves the traditional acquisition;

The transmission speed of the system is limited by the bandwidth of the PCI bus;

3. Guarantee 100% frame loss, solving the problems of easy frame loss and lack of power failure protection in traditional memory recording methods;

4. The system works independently, occupies almost no computer resources, and has high reliability;

5. A system can support multiple boards and cameras, and track and record multiple targets at the same time;

6. Support the superposition and fusion of multiple external signals;

7. Support a variety of image formats, with a variety of software and hardware external trigger functions;

8. The software interface is simple and convenient for secondary development and real-time processing.

21. What is the white balance of an industrial camera?

White balance is a technology used in color cameras. White balance is a balance of the three components of red, green, and blue, so that the camera can reflect the actual color of the actual scene. Because the output of the three RGB components of the light-sensitive element is unbalanced under different lighting conditions, the color distortion, bluishness or redness of the image will be poor, so white balance is needed to restore the color of the image. Generally, the camera can be divided into two ways: automatic and manual white balance. In addition, white balance can be achieved by software.

22. How to improve the signal-to-noise ratio of an image?

The Signal to Noise Ratio (SNR) reflects the anti-interference ability of the camera imaging. The reflection is the quality of the picture whether the picture is clean and noise-free. The following techniques can improve the signal-to-noise ratio of the image and make the acquired image clearer and cleaner.

23. What is the difference between an industrial camera's mechanical shutter and an electronic shutter?

Mechanical shutter: use spring or electromagnetic means to control the opening and closing of several blades, or the two-layer curtain "sliding" the imaging image field window with a certain width gap up and down like a stage "pull curtain", let the window Get the "opportunity to see light" for a specified length of time-this makes the usual mechanical shutter concept.

Electronic shutter: Directly operate the CCD / CMOS to control the shutter exposure through a circuit, which is called an electronic shutter. Using the principle that the CCD / CMOS is not powered on and does not work, when the CCD is powered off, the image cannot be generated even though the window is "open". If the electronic time circuit is used when pressing the shutter button, the CCD / CMOS is powered on for "a specified period of time", and the same effect as that of the shutter "open instantly" can be obtained.

Generally speaking, the advantage of a mechanical shutter is that it can work without electricity, but the disadvantage is that the comparison between high and low speed gears will be inaccurate. Electronic shutters are more accurate and perform better than pure mechanical shutters (the shortest exposure time can be shorter, etc.), have higher reliability, and last longer.

24. What is the difference between a digital industrial camera and an analog industrial camera?

Conceptually, these two cameras differ only in output signals. Analog industrial cameras output analog signals, while digital industrial cameras output digital signals. In other words, A / D conversion of analog industrial cameras is performed outside of industrial cameras, and A / D conversion of digital industrial cameras is performed inside industrial cameras.

25. How to maintain an industrial camera?

1. Try to avoid direct exposure of the camera to sunlight, so as not to damage the image sensing device of the camera;

2. Avoid contacting the camera with substances such as oil, steam, water vapor, moisture and dust, and avoid direct contact with water;

3. Do not use harsh cleaning agents or organic solvents to wipe the camera;

4. Don't pull and twist the connecting wire;

5.Unless necessary, do not disassemble the camera at will and try to touch its internal parts. This will easily cause damage to the camera. Under normal circumstances, the dealer does not guarantee the warranty;

6. When storing, the camera should be stored in a clean and dry place.

26. What is a frame grabber?

An image capture card is also called an image card. It transfers the camera's image and video signals to the computer's memory and VGA frame memory in frames, for computer processing, storage, display and transmission. In the machine vision system, the image collected by the image acquisition card is used by the processor to process whether the workpiece is qualified, the movement deviation of the moving object, the location of the defect, and the like.