Cognex Vision Systems – What are the key points to choose a line scan vision system?

1. Selection of line scan industrial cameras

Step 1: Calculate the resolution. The width of the object to be inspected (perpendicular to the direction of motion) is divided by the minimum detection accuracy to obtain the required pixel value for each row.

Step 2: Calculate the actual accuracy. Select the camera based on the pixel values above. The actual detection accuracy is then obtained by dividing the width of the object to be inspected by the number of pixels.

Step 3: Calculate the number of lines scanned per second, which is the line scan rate. The length of the movement speed per second divided by the accuracy gives the number of lines scanned per second.

Based on the above values, you can select a camera.

For example, the width of the object to be inspected is 160mm, the accuracy is 0.1mm, and the movement speed is 2200mm/s, and the camera resolution: 160/0.1=1600 pixels should be about 2000 pixels.

First, select a 2K camera

The actual accuracy is: 160mm/2048 pixels=0.08mm

Line scan rate: 2200mm/0.08mm=27.5KHz

Therefore, the camera should be selected as a 2K camera with a line scan rate of about 28kHz.

Second, the selection of line scan lens

Common line scan camera resolutions are currently 0.5k, 1K, 2K, 4K, 6K, 8K, 12K, 16K, etc., and the pixel size is 5um, 7um, 10um, 14um, etc., so that the size of the chip ranges from 7.168mm (512x14um) to 81.920mm (16Kx5um). Obviously, the C interface is far from meeting the requirements, because the C interface can only be connected to a maximum of 22m chips, that is, 1.3 inches. Therefore, the interface of many cameras is F-type, and different lens mounts correspond to different back focal points (Flange distance), which determines the different working distances of the lenses. Optical magnification (β, Magnification)

The camera resolution and pixel size are determined, and the sensor size (Sensor siz) can be calculated. The chip size divided by the field of view (FOV) equals the optical magnification. β=CCD/FOV

Mount:

There are mainly C, CS, F, M42x1, T2, M72x0.75 and so on.

Flange Distance

The back focus refers to the distance from the camera interface plane to the chip, which is a very important parameter determined by the camera manufacturer according to its own optical path design. Cameras from different manufacturers, even if they have the same interface, may have different back focus.

With optical magnification, interface, and back focus, the working distance and loop length can be calculated. After selecting these, there is another important link, which is to see if the MTF value is good enough? Many machine vision engineers don't know about MTF, and for high-end lenses, MTF must be used to measure optical quality. MTF covers a wealth of information such as contrast, resolution, spatial frequency, chromatic aberration, and expresses the optical quality in great detail at the center and edges of the lens. Please understand for yourself about the MTF form, and I will not repeat it here. The lens not only meets the requirements of the working distance and field of view, but the contrast at the edges is not good enough, and it is necessary to reconsider whether to choose a higher resolution lens.

Cognex Vision Systems – What are the key points to choose a line scan vision system?

Cognex Vision Systems – What are the key points to choose a line scan vision system?

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