Knowledge about visual products

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 reasons for dropped frames: The reason for poorly designed industrial cameras to drop frames is actually the blockage of the data channel, which ca n’t be processed in time. 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: What are the input and output interfaces of industrial cameras?

In machine vision inspection technology, the input and output interfaces of industrial cameras include Camera Link, IEEE 1394, USB2.0, Ethernet, and USB3.0;

3: Know the length, width, height, and required measurement accuracy of the measured object, how to choose a CCD camera and an industrial lens, and what do I need to pay attention to when selecting the above devices?

First, choose the right lens. The lens selection should follow these principles:

1). How big is the chip size of the camera?

2). What is the camera interface type, C interface, CS interface or other interfaces;

3). Working distance of the lens;

4). Angle of view of the lens;

5) .Spectral characteristics of the lens;

6). Lens distortion rate;

7). Lens mechanical structure size;

When choosing a CCD camera, you should consider the following aspects:

1). Photosensitive chip type; CCD or CMOS

2). Video features; including point frequency and line frequency.

3) .Signal output interface;

4). Camera working mode: continuous, trigger, control, asynchronous reset, long time integration.

5). Video parameter adjustment and control method: Manual, RS232.

At the same time, when choosing a CCD, you should pay attention to l inch = 16mm instead of 25.4mm.

4: What is the difference between a CCD camera and a CMOS camera?

1.Imaging process

The principle of photoelectric conversion of CCD and CMOS image sensors is the same. The main difference between them is the signal readout process. Since CCD has only one (or a few) output nodes for unified readout, the consistency of its signal output is very good; In a CMOS chip, each pixel has its own signal amplifier, which performs charge-voltage conversion. The consistency of its signal output is poor. However, in order to read the entire image signal, the CCD requires a wide signal bandwidth of the output amplifier. In a CMOS chip, the bandwidth requirement of the amplifier in each pixel is low, which greatly reduces the chip's power consumption. This is the CMOS chip. The main reason for lower power consumption than CCD. Despite the reduced power consumption, the inconsistency of millions of amplifiers brings higher fixed noise, which in turn is an inherent disadvantage of CMOS over CCD.

2.Integrity

From the perspective of the manufacturing process, the circuits and devices in the CCD are integrated in the semiconductor single crystal material manufacturer, and the process is more complicated. Only a few manufacturers in the world can produce CCD wafers, such as DALSA, SONY, Panasonic, etc. The CCD can only output analog electrical signals, which requires subsequent address decoders, analog converters, and image signal processors, and it also needs to provide three sets of power supply synchronous clock control circuits with different voltages. The integration is very low. CMOS is integrated on a monolithic material called metal oxide. This process is the same as the process of producing tens of thousands of semiconductor chips such as computer chips and storage devices. Therefore, the cost of sound field CMOS is much lower than that of CCD. . At the same time, the CMOS chip can integrate the image signal amplifier, signal reading circuit, A / D conversion circuit, image signal processor and controller into a single chip, and only one chip can realize all the basic functions of the camera. High, chip-level camera concepts are born from this. With the continuous development of CMOS imaging technology, more and more companies can provide high-quality CMOS imaging chips, including: Micron, CMOSIS, Cypress, etc.

3.Speed

The CCD uses one-by-one photosensitive output, which can only be output according to the prescribed program, and the speed is slow. CMOS has multiple charge-to-voltage converters and row / column switch controls. The read speed is much faster. At present, most high-speed cameras above 500fps are CMOS cameras. In addition, the address strobe of the CMOS can be sampled randomly to achieve sub-window output, which can achieve higher speed when only outputting sub-window images.

4.Noise

CCD technology has developed earlier and is more mature. It uses PN junction or silicon dioxide (SiO2) isolation layer to isolate noise. The imaging quality has certain advantages over CMOS photoelectric sensors. Due to the high integration of CMOS image sensors, the distance between components and circuits is very close, interference is serious, and noise has a great impact on image quality. In recent years, with the continuous development of CMOS circuit noise reduction technology, it has provided good conditions for the production of high-density and high-quality CMOS image sensors.

5: What are the main parameters of industrial cameras?

Resolution

2. Speed (frame frequency / line frequency)

Noise

Signal-to-noise ratio

5. Dynamic range

6. Pixel Depth

7. Spectral response

8. Optical interface

6: 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.

7: 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.

8: What does the noise of industrial cameras mean?

Industrial camera noise refers to signals that are not expected to be acquired during the imaging process and are outside the actual imaging target. According to the European camera test standard EMVA1288, the noise in the defined camera can be divided into two categories as a whole: one is the statistical fluctuation noise that conforms to the Poisson distribution caused by effective signals, also known as shot noise (shot noise) ), This kind of noise is the same for any camera, it is inevitable, especially the definite 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.

9: What does the signal-to-noise ratio of industrial cameras 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.

10: 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.

11: 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.

12: What interfaces do industrial cameras have?

The interface refers to the excuse between the camera and the lens. Commonly used excuses for the lens are the C port, CS port, and F port.

13: How are industrial cameras classified?

1. Classified by chip structure: CCD camera & CMOS camera

2. According to the sensor structure: area scan camera & line scan camera

3. Classified by output mode: analog camera & digital camera

4. Color Camera & Black & White Camera

14: 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;

15: 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

16: What are the characteristics of a line scan camera?

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

17: Why use a line scan camera in machine vision inspection?

1. Line scan cameras have higher resolution; line scan cameras generally have 1024, 2048, 4096, 8012 pixels per line; while general area scan cameras are only 640, 768, 1280, and area scans larger than 2048 are rare. .

2. The acquisition speed of line scan cameras is faster; the acquisition speed of different models of line scan cameras ranges from 5000 lines to 60,000 lines per second, and users can choose a few frames or every ten lines to form a frame image for processing once. Therefore, very high frame rates can be achieved.

3. Linear array camera can continuously collect and process uninterruptedly; linear array camera can continuously collect linearly moving objects (linear guide rails, paper on drums, fabrics, printed matter, objects on conveyor belts, etc.).

4. Linear array camera has simpler and more reasonable structure. Compared with area scan cameras, line scan cameras do not waste resolution and collect useless data.

18: What is a smart industrial camera?

Smart industrial camera is not a simple camera, but a highly integrated micro-small machine vision system. It integrates image acquisition, processing, and communication functions into a single camera, thereby providing a versatile, modular, highly reliable, and easy-to-implement machine vision solution. Intelligent industrial cameras generally consist of an image acquisition unit, an image processing unit, image processing software, and a network communication device. Due to the application of the latest DSP, FPGA and mass storage technology, its intelligence has been continuously improved, which can meet the needs of various machine vision applications.

19: What are the main parameters of CCD chip and CMOS chip?

The two types of photoelectric sensor chips mainly used in machine vision are CCD chips and CMOS chips. CCD is the abbreviation of ChargeCoupled Device, and CMOS is the Complementary Metal-Oxide-Semiconductor Transistor. abbreviation. Whether CCD or CMOS, their role is to convert optical signals into electrical signals (voltage / current) through the photoelectric effect, and store them to obtain images. The main parameters of CCD chip and CMOS chip are:

Pixel size

Pixel size refers to the actual physical size of each pixel on the chip's pixel array. The usual sizes include 14um, 10um, 9um, 7um, 6.45um, 3.75um, etc. The pixel size reflects the chip's ability to respond to light to a certain extent. The larger the pixel size, the more photons can be received, and the more charge generated in the same lighting conditions and exposure time. For low-light imaging, pixel size is a characterization of chip sensitivity.

Sensitivity

Sensitivity is one of the important parameters of the chip, and it has two physical meanings. One refers to the photoelectric conversion capability of an optical device, which has the same meaning as the response rate. That is, the sensitivity of the chip refers to the output signal voltage (current) per unit exposure within a certain spectral range. The unit can be nanoamperes / lux nA / Lux, volts / watt (V / W), and volts / lux (V / Lux). Volt / lumen (V / lm). The other refers to the radiant power (or illuminance) to the ground that the device can sense, which has the same meaning as the detection rate. Units can be expressed in Watts (W) or Lux.

3. Bad points

Due to the limitation of the manufacturing process, it is almost impossible for a sensor with millions of pixels to have all the pixels in a good condition. The number of bad pixels refers to the bad pixels in the chip (pixels that cannot be effectively imaged or corresponding pixels). Inconsistency is greater than the number of pixels allowed by the parameter. The number of swap points is an important parameter to measure the quality of the chip.

4. Spectral response

Spectral response refers to the chip's response to different wavelengths of light, and is usually given by a spectral response curve.

20: 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.

21: How is a line scan camera defined?

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.

22: What are the general steps for selecting an industrial camera?

The first step is to first know the system accuracy requirements and the resolution of the industrial camera;

The second step is to know the system speed requirements and the imaging speed of the industrial camera;

The third step is to consider the industrial camera and the frame grabber together, because this involves the matching of the two;

The fourth step is the comparison of prices.

23: 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;

24: 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;

25: 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.

26: What is the difference between a USB camera and a 1394 camera?

From the aspect of interface of USB camera and 1394 camera, the factors that affect our choice are the following:

a) Protocol specifications: There are more than 50 types of industry-standard protocols related to 1394 equipment, which include cameras, industrial cameras, and other equipment. Most manufacturers' 1394 industrial cameras follow DCAM industry specifications. The interface of the USB industrial camera is a commercial specification recently developed from commercial PC applications.

b) Power supply mode: 1394 industrial camera operating voltage is 8 to 30VDC, USB industrial camera operating voltage is 5VDC. From the perspective of the power supply range, the 1394 interface meets the DC power supply requirements of individual equipment in the industrial field, such as 12VDC or 24VDC; while the USB interface uses the TTL standard voltage of electronic circuits for power supply and is generally used for internal power supply of the device.

c) Operating system cooperation: 1394 interface industrial camera can keep the original address after system restart, but USB interface industrial camera needs system re-allocation after each startup.

d) Data transmission: 1394 interface has inherent advantages when processing data transmission of multiple industrial cameras. From the development background, the USB interface is a new generation of high-speed data transmission interfaces that inherit the RS232 interface, while the 1394 industrial cameras are designed to replace SCSI and PCI buses.

27: What is the difference between a smart industrial camera and a general industrial camera?

The difference between a smart camera and an industrial camera is, in short: a smart camera is a highly integrated micro-small machine vision system; and an industrial camera is one of the components of a machine vision system.

28: What are the main functions of the image acquisition unit in the intelligent industrial camera?

In smart cameras, the image acquisition unit is equivalent to a CCD / CMOS camera and an image acquisition card in the ordinary sense. It converts the optical image into an analog / digital image and outputs it to the image processing unit.

29: 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.

30: 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.

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