Using Line Scan Cameras or Frame Cameras: Solving the Pixel Puzzle

Using the right tool to handle the job at hand is a mission critical task in the different phases of industrial automation. Here's an example: long rolls of paper and plastic materials need inspection for defects.

A machine vision system will check the quality of round objects moving at high rates of speed. Cameras are integral part of the process. Would you choose frame cameras or line scan cameras? Both can seem adequate.

Differences in Capturing Images

A frame camera, also known as area scan cameras, uses a two-dimensional array of sensors to capture an image. The outputs from the sensors are then scanned and transferred to the frame grabber.

Line scan cameras have a one-dimensional array of sensors. A line scan camera builds an image one horizontal line at a time. Only part of the object needs to be visible to be inspected.

A motion encoder then "triggers the acquisition" of each line of the image as noted in the article Understanding Line Scan Applications available on Vision Online.org. A few thousand lines are built as the object zips past the camera.

4 Challenges with Frame Grab Cameras

Frame grab cameras have weaknesses in this type of process.

• Defective pixels are common in large-format frame cameras and the product's own defects might not get identified.

• Second, the frame camera’s exposure time must be short to prevent blurring due to the material’s motion. A short exposure requires high intensity lighting over the entire imaged area to get an acceptable image.

• Third, effectively aligning the necessary cameras is difficult.

• Last, corrections in pixel responses are necessary to detect low contrast defects and that's time consuming with frame cameras.

Line Scan Camera Advantages

Line scan cameras are useful for producing a flat image of cylindrical objects, imaging very large objects with good resolution, and producing images of objects moving past a fixed point, such as parts on an assembly line. The technology is useful for making sure items like bottles, jars, cans and vials are free of defects.

Consider inspecting a long roll of plastic and the machine vision system camera needs 16,000 pixels to highlight product defects.

In the article on line scan applications, one option described is setting up four large-format frame cameras mounted across the web of material. The disadvantages of the equipment come into play and a 100 percent inspection of the product may not happen.

One line scan camera with a 16,000 pixel capability is a much better solution. One camera covers the material with no defective pixels.

What happens when a defect is noticed? In scenarios with continuous web applications the web is moving too fast to stop. Instead, a “roll map” records and describes the position and type of defects.

Best Uses

Some of the best uses for line scan applications are inspections for paper, textiles, metal, and glass. Line scan technology is required to inspect flat panel displays, solar cells, or printed circuit boards that must have large, defect-free images. Railway and road inspections are other uses.

Here's a scenario to consider using a line scan application. The case study is covered in Machine Vision System Detects Defective Parts for Precision Stampings Supplier.

A company needed to prevent shipments of defective stampings while meeting required production quotas. A line-scan vision inspection system was incorporated into a conveyor system and defective pieces would be removed using a robotic arm.

The manufacturer was making components in the electronics industry and there needed to be 100 percent inspection of all components. The company didn't want to risk the customer receiving one defective component. There were 3,200 pieces per hour for a single production line.

Trays measuring 250x400mm and containing 50 parts each were placed on a conveyor and processed once a sensor confirms that the tray is accurately positioned. Imaging is accomplished in a cycle time of 1.5 seconds, resulting in a throughput rate of 2,400 pieces per hour.

Compared to manual inspection, a machine vision system offers improved accuracy, high consistency, and works non-stop without fatigue. Selecting the right components within a system maximizes its capabilities to deliver top quality results with every use.

Industrial automation holds tremendous benefits for manufacturers in all industries. Visit A3 to stay on top of important trends and developments.