Line scan cameras are becoming essential tools for machine vision inspection and control systems in various industries.
The line scan camera integrated into such a system for detecting defects and other detailed inspections helps create very precise images by capturing single line slices of a moving object.
This is especially useful when it comes to ongoing control and inspection of high speed and continues applications including document scanning, web inspection of steel sheets, paper and textiles on conveyor lines, print inspection, glass inspection and for any other task requiring that control is performed in microns.
Line scan cameras are superb for assisting postal sorting, pharmaceutical production as well as for quality control of other continuous free falling objects such as molten glass or metal, or of rotating objects on the conveyor line.
Benefits of using line scan cameras
A line scan camera features a single line of light sensitive pixels. These capture single lines or slices of the object moving in front of the camera. An image is created thanks to the preset frame rate which is set in accordance with the speed of the moving object being monitored.
Since line scan cameras typically have a small footprint, they can be fitted in very compact spaces, including in between the rollers of the conveyor or beneath it.
The 2D images which are built by precisely lining each of the slices of the image are typically very detailed and with no perspective distortion.
The continuous images built by a line scan camera are with high resolutions allowing for detecting defects which no human quality control or other types of cameras can detect.
Also, line scan cameras help increase the productivity of the production process as they do not require that the conveyor line or other ongoing process be stopped for proper quality control.
How a line scan camera works
When a line scan camera is properly installed and synchronized with all other components of a machine vision system for inspection they can provide ongoing images of a high speed moving, rolling or falling objects without any disruption of the production process.
The single pixel line of the camera captures single lines of the object. The number of lines which need to be taken in order for a precise 2D image to be captured is determined by the speed of the moving object. One frame includes a specific predetermined number of slices which have to be taken for a certain time frame.
A linear motion encoder connected to the line scan camera helps capture the speed of the conveyor or the moving objects. When synchronized properly, the images resulting from this process are very detailed with no blurring or distortion.
In case the objects being inspected are separate one from another, the start and end of each passing object are detected by trigger pulses. These estimate the frame rate and allow for the line scan camera to capture slices of the entire object as it is rotating.
A rotary motion encoder is used to determine the frame depending on when an object is rotating or has moved at a specific distance.
Apart from these essential elements of the machine vision system, frame grabber cards must be integrated as well. They enable the correct synchronization of the line scan camera with the sensors and the light controller.
A reliable connection between the camera and the other components of the machine vision inspection system provided by special input lines ensures that the synchronization is consistent at all times during the process.
Specialized software is used to correctly put the slices back together and produce full images of the object being inspected.
This helps increase the acquisition rate of the line scan camera in case the speed of the moving object or conveyor line increases, and decrease the rate of acquisition when the production process slows down.
When setting up a machine vision system featuring line scan cameras, it is crucial that the camera is also synchronized with the fluctuations in lighting which commonly occur during a high-speed production process.
Overall, setting up such an automated system for control and inspection is quite a complicated process and requires perfect synchronization of every single element in the machine vision system in order to provide the expected results and be efficient for quality control and monitoring.
Types of line scan cameras
Although the typical line scan camera operates via a single line of light sensitive pixels, due to the technological developments and the increasing number of applications of these cameras, a number of different types of line scan cameras have been introduced to serve the specific needs of different businesses and services.
Today, there are line scan cameras which have 2-pixel lines instead of just one. These allow for capturing that single slice of the image twice. The result is a much brighter image. Such cameras are used for settings with low or insufficient lighting.
TDI line scan cameras feature even more pixel lines and can capture each slice of the object multiple times. This type of line scan cameras provides very detailed and brighter images of the moving object which is indispensable when high precision is required and when there is low light availability.
Some line scan cameras integrate many or all of the components of a traditional machine vision system within them. This means that they can produce an image directly without the use of a separate frame grabber, memory, processor and software.
Line scan cameras which provide 3D images are also available for some specific production inspection needs.
Manufacturers and service providers can also choose between monochromatic and color line scan cameras for their inspection and control needs.
A line scan camera in itself is a simple tool, but when set-up and synchronized properly within a machine vision system, it can be an absolutely indispensable tool which helps identify any defects, control the quality of the production or help with the proper sorting of falling and moving objects without the production process having to be stopped, and without human intervention