Use of line scan cameras

foreword

The most common application of line scan cameras in life is the scanning device on copiers.

principle

Every time a line is collected, it just moves to the next unit length, and then continues to collect the next line. After a period of time, the lines are assembled into a two-dimensional picture.
Two ways:
1. Uniform motion, take a single frame at the same interval.
2. The encoder triggers image acquisition, and triggers single-frame image acquisition at equal intervals.

product application

A typical field of application for line scan cameras is the detection of continuous materials such as metals, plastics, paper and fibers. The detected object usually moves at a constant speed, and one or more cameras are used to continuously scan it line by line to achieve uniform detection of its entire surface. Its image can be processed line by line, or an area image composed of multiple lines. In addition, line scan cameras are very suitable for measurement occasions, thanks to the high resolution of the sensor, it can measure accurately down to the micron.

1. A line scan camera, as the name suggests, is in the shape of a "line". Although it is also a two-dimensional image, it is extremely long, with a length of several K, but a width of only a few pixels. Generally, this kind of camera is only used in two situations: 1. The field of view to be measured is a slender strip, which is mostly used for detection on the roller. Second, a large field of view or high precision is required.

2. In the second case (requiring a large field of view or high precision), it is necessary to use the excitation device to stimulate the camera multiple times, take multiple pictures, and then combine the multiple "strip" images taken into a huge picture. Therefore, if you use a line array camera, you must use a capture card that can support the line array camera.
   Line array cameras are expensive, and in the case of large field of view or high-precision detection, their detection speed is also slow - the image of a general camera is
   400K ~ 1M, and the merged image is as large as several M, and the speed Naturally it is slow.

Advantage

1. The cost of the linear array CCD plus the scanning mechanism and position feedback link is still much lower than that of the area array CCD with the same area and the same resolution;
2. The coordinates of the scanning line are provided by the grating, and the indication accuracy of the high-precision grating ruler can be higher than the manufacturing accuracy of the pixel pitch of the area array CCD. In this sense, the accuracy of the image acquired by the line array CCD in the scanning direction can be high. For area array CCD images;
3. The latest linear CCD sub-pixel splicing technology can optically displace the pixels of two CCD chips by 1/2 pixel in the direction of the array length of the line array, which is equivalent to dividing the pixels of the second CCD All pixels are sequentially inserted into the pixel gap of the first CCD, indirectly "reducing" the pixel size of the linear array CCD, improving the resolution of the CCD, and alleviating the difficulty of reducing the CCD pixel due to the influence of technology and materials Due to the problem of size, in theory, higher resolution and precision than area array CCD can be obtained.

How to choose a line scan camera

1. Calculation resolution: width/minimum detection accuracy = pixels required for each line

2. Selected camera: width/number of pixels = actual detection accuracy

3. Movement speed length/accuracy per second = scanning lines per second

For example, the width is 1800mm, the precision is 1mm, and the movement speed is 25000mm/s

Camera: 1800/1=1800 pixels, at least 2000 pixels, selected as 2k camera 1800/2048=0.9 actual precision 25000mm/0.9mm=27.8KHz should be selected as 2048 pixels 28 KHz camera.

Common parameters

size of the picture

The height (Height) and width (Width) together determine the size of a picture.

trigger mode

Frame Burst Start

Only when the camera receives a frame trigger signal, it will start
exposure and give image data. The camera
outputs images according to the currently set line frequency, and there is no need to set a line
trigger signal.

Line Trigger (Line Start)

The horizontal frequency is determined by the frequency of the external horizontal trigger control signal
. In this mode,
only one line is exposed when a trigger signal is input.

frame+line trigger

The external frame trigger signal and line trigger signal of the camera
are required. The number of lines in a frame
is controlled by the frame height register, and the line frequency
is controlled by the line trigger signal provided externally, and is limited by the internal
setting value. After the camera receives the frame trigger signal
, the line trigger signal will work.

free trigger mode

The trigger signals of this mode are all generated by the camera itself , and the horizontal frequency
parameters can be adjusted according to requirements.

Frequency Converter and Encoder Control

When the line trigger is turned on, when the external encoder trigger signal is connected, the speed of the encoder signal will control the actual line frequency of the camera.
If the captured picture is slightly compressed or stretched, a frequency converter is required to control it.
The input signal of the predivider (PreDivider) first enters the predivider module, and through an integer division,
the purpose of reducing the frequency of the source signal is achieved, and the processed signal is sent to the multiplier module. The pre-divider module can reduce
the periodic jitter of the input signal, and the signal whose frequency exceeds 100kHz must pass through the pre-divider to reduce the frequency, because the multiplier can only
accept signals in the frequency range of 10~100kHz. Periodic jitter in the signal from the shaft encoder is acceptable.
Multiplier (Multiplier) After the signal is processed by the predivider, it will be sent to the multiplier. The multiplier will
multiply the signal frequency by an integer to achieve the purpose of increasing the signal frequency, and then the signal will be sent to the voltage divider module. The adjustment parameter can
be set as rising edge or falling edge. If set to rising edge, each rising edge of the signal coming from the prescaler will
be locked to match the signal generated by the rising edge, and vice versa. Be sure not to increase the frequency of the signal with too large a multiplier, and
avoid triggering the signal frequency beyond the maximum supported line frequency of the camera. Even if a small multiplier is selected, too high a frequency may be
generated during frequency adjustment, which may exceed the maximum line frequency of the camera.
Frequency divider (PostDivide) The frequency divider reduces the frequency of the signal by an integer factor, and uses the generated
signal of the new frequency as the trigger signal of the camera.
The signal processed by the above three modules is the final trigger signal of the camera.
The more commonly used ones are multiplier and predivider. The simple description is: when the image is stretched, the predivider is used, and when the image is compressed, the
multiplier is used.

direction

For reciprocating translation stages, it can be handled with shaft encoder control. After the camera input is connected to two phases, the shaft encoder
can choose two modes: ignore direction count (Ignore Direction) and fixed direction count (Follow Direction)
. Ignoring direction counting will record data in all directions; fixed direction counting will only record data in the forward direction once, and
will not record data in the repeated back and forth.