来源:MookLab马克拉伯
Machine vision technology is an important application of automation and intelligence in industrial production inspection. The machine vision system is divided into two parts: image acquisition and image processing, and the image grabber is an important component connecting these two plates.
The image acquisition part of the machine vision system is mainly coordinated by the LED light source, the industrial lens and the industrial camera, and the image processing part is realized by the image processing software.
Common concepts for frame grabbers
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1. A/D conversion
The frame grabber can realize the conversion of analog signals to digital signals, which is called A/D conversion, and the corresponding components that realize the conversion are called A/D converters.
2. Number of transmission channels
In order to meet the production needs, frame grabbers have the function of A/D conversion of several cameras at the same time. The number of transmission channels refers to the number of simultaneous conversion of the same image capture card, and the optional transmission channels of the acquisition card developed and produced on the market are single channel, dual channel, and four channel.
3. Sampling frequency
The sampling frequency is the frequency of the frame grabber when collecting image information, which reflects the speed and ability of the frame grabber to process images.
4. Frames and fields
A video signal can be progressively sampled through a series of frames, or it can be sampled by interlacing a sequence of interlaced fields in which half of the data of a frame is sampled at each time sampling interval.
How to choose a capture card
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1. Interface format and data format
The interface system includes digital (Camera Link, LVDS/RS422) and analog (PAL, NTSC, CCIR, RS170/EIA, non-standard analog standard), which must be consistent with the camera selected for the vision system.
2. Digital accuracy
The digital accuracy of the analog capture card mainly includes two aspects:
(1) Pixel jitter
Pixel jitter is caused by an error in the sampling clock of the frame grabber's A/D converter, which produces a small error in the pixel position, resulting in an error in the distance measurement.
(2) Grayscale noise
The process of digitizing a frame grabber involves amplifying the analogue video signal and measuring its brightness (grayscale).
3. The size of the data rate
To calculate whether the data rate of the digital capture card meets the requirements of the system, the following formula can be used:
Data Rate(Grabber)>1.2 * Data Rate(Camera)
Data Rate(Camera)=R * f * d / 8
where Data Rate(Grabber) is the data rate of the frame grabber, Data Rate(Camera) is the data rate of the camera, R is the resolution of the camera, f is the frame rate of the camera, and d is the digital depth (or gray level) of the camera.
4. Memory size, PCI bus transfer rate
The PCI bus supports BUS Master devices to transmit data at a burst rate of 132MB/s. The average sustained data transfer rate is generally 50-90MB/s.
5. Camera control signal and external trigger signal
External Trigger: The process of acquisition is initiated by an external event.
Synchronous trigger: Without changing the synchronization relationship between the camera and the board, the acquisition starts from the next field active signal.
Asynchronous trigger: Change the synchronization relationship between the camera and the board, and the acquisition starts from the first field active signal after the camera is reset.
When the vision system wants to detect objects in motion, the camera and frame grabber must have the function of asynchronous triggering.
6. Reliability of hardware system
The reliability of the hardware is very important in the production system, and the cost of downtime due to equipment failure is far greater than the equipment itself.
(1) Choose a capture card with lower power consumption. A complex card with more devices dissipates more heat than a card with fewer devices.
(2) Overvoltage protection is an important indicator of reliability. Overvoltage protection circuitry is added to the video input and I/O ports to protect the capture card from the high voltage breakdown caused by electromagnetic interference in the industrial environment.
7. Support the functions of the software
Consider whether the software you want to use for your vision system is compatible with the frame grabber. With the continuous development of machine vision technology, the technology of the capture card is constantly strengthened and the functions are constantly updated. Choosing the right capture card for your vision system plays a key role in the proper functioning of a vision system.
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