■ Minimum illuminance
The minimum illumination is an important indicator that measures the sensitivity of CCD camera. It indicates that the corresponding ambient illuminance is lowered when the ambient light illuminance is lowered to a certain extent. For example, when the ambient illuminance is lowered to 0.04LX, the magnitude of the video signal output by the CCD camera is reduced to 50% of the maximum amplitude, and the minimum illuminance of the CCD camera is 0.04X (F1.2). When the ambient illumination continues to decrease, the image quality of the video image output of the CCD camera will be difficult to guarantee.
The minimum illumination of the CCD camera is related to the maximum relative pore size of the lens used. While providing the camera's minimum illuminance, the relative pore diameter of the lens used when the test is used.
■ Fixed Image Noise (FPN)
When exposure control is not used, the non - consistency of the transfer gate structure will result in a small change in the gate potential. At the same time, the gate restriction resistance also causes a small change in the gate potential to generate a small potential difference in the start of the photodiode during each integration period. Since the characteristics of the gate structure are fixed, these tiny potentials are called "fixed pattern noise"; when exposed control is used, the initial potential of the photodiode is determined by the base of the reset tube. If the base potential is increased than the previous potential, a certain amount of charge will be introduced. These charges are also transmitted to the CCD register even under zero-sensitivity conditions. This is not a major problem because the DC signal can be easily removed from the output signal. However, the non-consistency of the transfer grid will produce a DC bias, and this offset is different from the pixels and the pixels, thereby improving the fixed image noise.
Fixed image noise can be corrected by a non-uniform correction circuit or a software method.
■ Resolution
The resolution is one of the most important performance parameters of the CCD camera, mainly used to measure the ability of the camera to be in-dimensional dimensionality in the image.
The resolution of the camera can be measured by both spatial frequencies or TV lines. These two measurements are not one thing. If half of the area of a CCD image sensor is shielded, the resolution of the camera is indicated by a spatial frequency, and its resolution is unchanged; if the resolution of the camera is indicated by a TV line, its resolution falls to the original 50%.
The resolution of the CCD camera is affected by a variety of factors, including the spectrum of the light source, the F number of the lens, and the like. At the same time, it is also affected by certain subjective factors. Such as: In general, what kind of target image is "identifiable", its standard is inconsistent. If the camera resolution is a key parameter, it is best to test the camera in the actual operating environment.
Spatial frequency and TV line
■ Scan mode
□ "Standard" 2: 1 Valley Scan
This is the scanning order of the RS-170 "Standard Camera". This scanning method developed by the radio television system provides a more clear image with relatively low (30 Hz). From the top of one frame of images, the camera reads all odd lines in the first half frame time (1.3.5 ... 479). Then start all even numbers (0.2.4 ... ..478) from the frame in the second half frame time. On only one time, only half frame images can be changed, and the flash of the image can be reduced.
Interlaced scans may have trouble for machine vision. Because the adjacent lines are scanned at different times, any moving object may differ in the odd line and the position of the even line, thereby affecting the image quality.
□ Continuous scan
In visual applications, continuous scan cameras are getting more and more popular. A continuous scan camera is continuously scanned from the top to bottom of one frame image (0.1.2 ... .479). Some linear continuous scan cameras have additional circuits that convert continuously acquired data into 2: 1 interlaced data to prepare RS-170 monitors and acquisition card processing.
□ Asynchronous trigger and partial scan
When the CCD camera is in an asynchronous trigger, the camera is not continuously scanned and outputs a continuous signal at a fixed clock. Instead, after receiving a trigger signal, start scanning the new frame signal. This feature is suitable for instant image acquisition of fast moving targets on the production line. Some scans refers to the data read by the CCD camera is less than its full frame data. Since the amount of read data is relatively small, the corresponding read time should be less, thereby increasing the frame speed. This feature is very important for high-speed image acquisition systems. For example, an object only accounts for 1/4 of the upper portion of one frame of images, that is, the continuous scan camera can collect this part of the image only when 1/4 frame. When a partial scan mode is used, the camera is reset using the asynchronous reset function to reset the acquisition of the next frame image in the acquisition of the overhead, and only the 1/4 frame of the original image is still collected. This increases the effective frame rate of the camera to the original four times.
■ Interface
□ Analog
The camera that outputs analog signal is still widely used. This system has many advantages, simple and available, low cost, can directly connect video signals to the TV, which is good enough for most use performance, models available to choose. Generally use 1/4, 1 door, or L / 2 inch color or black and white CCD chip, most of which take PAL or NTSC system, output 768 × 596 or 640 × 480 pixels, the speed of these two types is 25 Frames / seconds and 30 frames / sec, each frame is composed of two fields, that is, the so-called interlaced scan, there is no choice, the CCD chip pixel used is also this range, the signal-to-noise ratio is generally 48dB less than 60 dB. There are many lens interfaces in the C or CS format. Signals typically use standard 75 Ω video coaxial transmission, BNC connector. There is a camera scan, but you must use the corresponding image card to support.
For machine visual applications, you should pay attention to several issues when choosing this camera: 1) Select various parameters to manually set the camera, such as gain, shutter speed, etc. 2) The camera selects high sensitivity, signal-to-noise ratio, the general manual will mark the minimum illumination of 0.1lux, the smaller the better. 3) The larger the horizontal sharpness, such as 600 lines. 4) CCD size is advantageous for subsequent image quality, but considers the performance of the lens when choosing.
When the camera output analog signal is used. You can choose a low-cost image card. Generally, the digitalization rate of 8bit is high enough, and 256 grayscale or 24bit color images can be provided, more than enough applications. The speed of image processing has become quickly, which reduces the speed requirements of the back computer system. The key to this image card is that the image quality is high, because the voltage amplitude of the video signal is 0-1V, if the input portion of the image card is not designed, it will cause various results affect image quality, for images The algorithm is extremely unfavorable; the image card must also be convenient to use the trigger input and control output, and the image acquisition is triggered by the signal emitted by the sensor, thereby controlling the time and position of the image acquisition, the result of the image processing (e.g., PASS or FALL) through the output interface. To control the automation of the entire system to the PLC or other control system, control the machine's movement and running logic, complete the automation of the entire system. The format of the output signal is preferably selected, since the computer itself can only output 5V or 12V signals, and the industrial PLC and sensor use 24V DC, the image card has the corresponding output, no additional conversion Circuit.
Of course, the camera of the analog signal has many shortcomings, the most important thing is that the resolution is not high, the number of frames is not fast enough, and the noise is large. Despite this, due to its low cost advantages and multi-varieties, the current market share is the largest.
□ RS422
RS422 is an electrical specification for data signal transmission. This standard uses twisted pairs to transmit the same signal in different modes, and the figure below shows the relationship between the transmission data rate and the length of the cable. When a signal is high, the other signal must be a low level. In the RS422 specification, the high level is 3V and the low level is 0V. In order to reduce noise, the twisted pair must be applied to all RS422 signals. Its cable has a resistance of 100 Ω and has a 110Ω terminal load. In the machine vision system, all RS422 signals of all input cameras require a 110Ω terminal load. □ CAMERA LINK
Camera Link is a communication interface for visual application digital cameras and image acquisition cards. This interface expands Channel Link technology, providing detailed specifications for visual applications.
Camera Link is designed for machine visual high-end applications, which is the Channel Link technology of the Drive Tablet Display of National Semiconductor, US Second Semiconductor, which is jointly released by several companies specializing in image cards and cameras in 2000, so they started Considering wiring, data format, triggering, camera control, high resolution, and frame rate, the result is a lot of convenience for machine visual applications, such as the transmission rate of data is very high, up to 1Gbits / s, output It is a digital format that provides high resolution, high digitalization rate and various frame frequencies, and the signal-to-noise ratio is also improved. Moreover, according to the requirements of the application, the basic (BASE), MIDIUM, all (FULL), and other support formats can be provided, which can be freely selected according to resolution, speed. The communication between the image card and the camera uses the LVDS (Low Voltage Differential Signaling) format, fast speed and better noise. The image card and the camera are used to use a specialized cable distance from 10 meters. Generally provided is a standard 3m MDR26-PIN wiring.
If your application requires high speed, high resolution, and you have enough funds, consider the camera and image card for selecting the Camera Link interface, it is important that you can choose a lot of camera and image card. The disadvantage of the Camera Link interface is that its interface itself is too mechanical (1.55 × 0.51 inches), although the semiconductor integrated circuit technology can make the imaging system very small, but this interface determines the size of the camcorder May be small. In addition, the transmission distance of this protocol is closer, although the distance can be added to hundreds of meters, but the cost is high.
As shown in the figure below: The standard Camera Link cable provides camera control signal lines, serial communication signal lines, and video cables. Where the camera control signal line is 4 LVDs, they are defined as the camera input and image acquisition card output; serial communication line is 2 LVDs for asynconic serial communication between the camera and the image acquisition card.
□ LVDS (EIA-644)
LVDS (Low Voltage Differential Signaling) is a low swing of differential signal technology, which allows signals to transmit at 400 mbps on a differential PCB line or balanced cable, and its low voltage and low current drive output realize low noise and Low power consumption.
The LVDS interface is a single way. The half-duplex can also be used if necessary, but generally in the case where noise is smaller, the distance is shorter. The differential pair of point-to-point connections consists of a driver, interconnect, and receiver. The low swing drive signal implements high-speed operation and reduces power consumption, and the differential signal provides a low pressure swing of the appropriate noise edge and power consumption. A large reduction in power allows multiple interface drivers and receivers to be integrated on a single integrated circuit. This increases the performance of the PCB board and reduces costs. LVDS requires the use of a terminal resistance (100 ± 20Ω) that matches the medium, the resistance terminates the circulation signal, and it should be placed as much as possible to the receiver input. The LVDS driver can drive twisted pairs at a speed of more than 155.5 Mbps, and the distance is more than 10m, and the picture below shows the relationship between the LVDS transmission data rate and the length of the cable. □ IEEE 1394
The IEEE1394 interface is the serial interface standard developed by Apple, also known as the FireWire interface. The IEEE 1394 interface can provide 100,200,400 mbps transmission rate between the computer and the peripheral device. This interface does not require a PC side as a controller that accesss peripherals, and different peripherals can be directly transmitted between each other.
With the topology of IEEE 1394, the interface can connect to 63 devices without a hub, and can connect a separate subnet to the bridge, which does not need to force computers to control these devices. The IEEE 1394B interface specification enables high-speed communication mode of 800Mbps and 1.6Gbps transmission speed, and transmissions for longer distance data can be achieved. Wireless Mode IEEE 1394 Ultra-high-speed data transmission technology can achieve 400 Mbps wireless communication speed. The transmission distance can reach 12m during accessibility, and the transmission radio wave is 60 GHz microwave.
□ Gigabit Ethernet
The currently used network connection protocol Elheranet has been released more than 20 years ago, and the transmission speed can be selected at 10, 100, 1000 Mbits standard, and Gigabit Ethernet is more than enough for most machine visual purposes, especially for machine visuals, The bandwidth is distributed to the camera, no sharing with other devices. Use a low price RJ45 interface and a network cable to reach a transmission distance of 1 00 m.
Considering the requirements of machine visual requirements, this standard protocol seems to meet all aspects, with sufficient speed and transmission distance, standard hardware software support. It uses the wiring price, and other standards are hard to look back. Moreover, it saves a dedicated image card. In general, the cost of use may be the lowest.
As each camera manufacturer supports this, the Gigabit Ethernet interface can implement the same plug-and-play function as the USB and FireWire, and the continuous high speed transmission of data can be achieved, and the speed of the CPU processing image is sacrificed as USB and FireWire. . At present, some manufacturers have released a camera that supports such an interface, and the future development is worthy of attention.
