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Machine vision systems with line scan cameras used for quality control and monitoring of various types of production, sorting, and reading are gradually overtaking the job of human quality control in many industries. And this is happening for several good reasons. One is that a well-set and configured machine vision system with one or more line scan cameras can perform very detailed and precise quality control, and spot any defects on a fast-moving conveyor line or on free falling or rotating objects without the process having to be stopped.
What exactly is a line scan camera?
A line scan camera in most cases has one single line of pixels which are light sensitive and which can capture a very precise image of a continuously moving object r action by capturing a series of consecutive “slices” of the object.
This type of camera is the basis of many machine vision systems for process inspection and quality control in a wide number of industries, sports and other activities which involve fast moving objects.
What are some of the most common uses of line scan cameras
These ingenious cameras are used for providing high quality, detailed images and defect spotting in various production and other processes. Some of the common applications of the line scan cameras include continuous web inspection of paper, textile, metal sheets, and glass while they are on the conveyor line. These cameras are also essential for document scanning, print inspection, as well as for the inspection of fast-moving rotating objects, fruit, food, pharmaceutical and postal sorting, and for inspection of continuously free falling objects and materials such as molten glass, metal, and others.
Line scan cameras have also been traditionally used for sports events, to capture the finish line or other moments which cannot be seen with a normal camera or the naked eye. The photo finish images are often the only way to determine the actual winner of a sprint or other fats sports race, and line scan cameras make this possible by capturing the smallest details up to microns.
How exactly does a machine vision system with a line scan camera work
In order to be efficient, the line scan camera or a series of line scan cameras need to be properly installed and synchronized with the other elements of the machine vision system for inspection and control.
For the separate slices of the object captured by the line scan camera to be lined precisely together, it is essential that the frame rate or the number of single slices captured by the camera is synchronized with the speed of the moving object.
Either linear or rotary motion encoders are used for determining the start point and the end point of a separate frame. The linear motion encoders are used for conveyor lines and other continuous web productions. Rotary motion encoders are installed when the objects being monitored are constantly rotating. The former detects the speed of the conveyor line to determine the number of frame rate of the line scan camera. The latter type of motion encoder detects the point where the object begins rotating and the endpoint when it has completely rotated.
The resulting 2D images are a result of very precisely lined together slices of a whole image, which if the system is synced correctly are very clear and with no distortion or blurring.
When separate moving objects need to be inspected, trigger pulses are used to detect the exact start and finish of the object, so that a full image of the entire object is captured by the line scan camera.
One single frame consists of a specific predetermined number of slices which are captured for a certain time frame.
Another vital element of a machine vision system for inspection is the frame card. It is used for proper synchronization of the line scan camera sensors and the light controller.
All elements of the system must be interconnected with reliable connections and cables in order for the process to go smoothly and for the inspection and control to be effective.
Specialized software is used for precisely lining each slice next to the consecutive one. It creates the full image of the object allowing for very detailed inspection of the surface.
Advantages of line scan cameras
Line scan cameras are relatively inexpensive, and they are very compact with a small footprint. This makes them particularly useful when detailed inspection in a tight space needs to be performed. This includes installing the cameras underneath a conveyor line or in between the rollers.
The line scan cameras along with the other elements of a machine vision system for control are much more precise in spotting defects and other problems on the continuous sheets of paper, fabric, metal, wood and many others than human quality control is. They are also much faster in sorting postal packages, pharmaceuticals, foodstuffs and other fast-moving objects. In other words, they help capture the details or sort products much faster and more efficiently, without the process having to be stopped than a human quality control inspector or another worker can.
A well-synchronized system can automatically increase the image acquisition rate of the line scan camera is the speed increases, and decrease the rate in the cases when the production line slows down.
The different types of line scan cameras
Apart from the traditional line scan cameras which have a single line of pixels, today there are cameras with 2-pixel lines and even TDI line scan cameras which capture a single slice of an image multiple times. This allows for much brighter and even more detailed end results. These cameras are very useful for high precision control and for the inspection of objects in low lighting, as the separate slices are overlaid to create a much more precise and bright image.
There are also monochromatic and color line scan cameras which are implemented for the different needs for the processes being inspected.
Final words
The line scan camera may seem like a relatively simple type of camera, but as you can see it is absolutely irreplaceable when it comes to machine vision systems for the inspection and quality control of continuous processes and fast moving objects.
