Line scan rates above 1 MHz

The High-Performance Vision Systems research group at the AIT Austrian Institute of Technology in Vienna, Austria, has been working on fast machine vision since the 1990s. However, based on an EoSens Creation camera from SVS-Vistek, the AIT team is now breaking new ground in terms of speed.
Highly automated production processes can be incredibly fast: electronic components, battery foils, and printed products are manufactured at speeds of up to 10 meters per second. Even faster processes can be found, for example, in the manufacture of wires or cables, where production speeds of up to 100 meters per second are not uncommon.

Such speeds are beyond the resolution capabilities of the human eye anyway, but even machine vision systems used for quality inspection of very fast-produced components are often unable to keep up reliably with the specified speed: Although the performance of this technology has steadily increased in recent years, even high-end systems sometimes reach their limits during extremely fast inspection processes.

“Recording speeds of around 1 MHz line scan rate were previously unattainable for industrial applications,” says Dr. Ernst Bodenstorfer, Senior Expert at the High-Performance Vision Systems research group at the AIT Austrian Institute of Technology (AIT) in Vienna, Austria. “We have now broken through this barrier.”

 

Clever tricks to increase speed
The AIT achieves high image capture speeds in a clever way. Line scan cameras, which operate on the basis of line scan sensors, are generally used to inspect long or almost endless objects. This allows images to be captured “slice by slice” and then stitched together before evaluation.

However, AIT is taking a different approach here, explains Bodenstorfer: “With today's CMOS image sensor technology, area sensors are available that allow scanning rates many times faster than the fastest line scan sensors in terms of lines captured per second. Our approach is to extend the FPGA logic of a standard camera with integrated real-time processing. This allows area scans to be precisely synchronized with the movement of the object and the images to be seamlessly merged into a continuous image stream. With this concept, unwanted shading artifacts are corrected within the camera itself.”

According to Bodenstofer, the key innovation in this development is that AIT has enhanced a conventional area scan camera with 1920 x 1080 pixels with special FPGA functionality for image stitching and precise synchronization with ultra-fast LED flash lighting. This trick results in unprecedented scanning speeds that match those of a line scan camera with a 1 MHz line rate.

 

The right camera as a foundation
The camera for the high-speed machine vision system developed by AIT, called xposure:1M, naturally played a key role. Bodenstorfer and his team opted for the Mikrotron EoSens Creation 2.0 XGE from SVS-Vistek, which the German Gilching-based manufacturer initially provided to its customer on loan. “One of the main reasons for choosing this camera was the fast integrated LUX19HS sensor from Luxima, which which operates without frame overhead time and therefore still offers good performance even when reading out a small number of lines,” says Bodenstorfer. “In addition, this camera allows us to integrate our own IP for high-performance, intelligent image preprocessing into the camera's FPGA and to stitch the images together into larger images for optimized processing. We use this unique capability to synchronize image capture with object movement and flash control of the lighting with sub-microsecond precision and to divide the image data into manageable frame sizes.”

In its standard version, the Mikrotron EoSens Creation 2.0 XGE offers a resolution of 1920 x 1080 pixels and a maximum frame rate of 2200 frames per second. For the AIT system, the resolution was reduced to 8 lines with a width of 1024 pixels in order to achieve a remarkable sensor speed of 125,000 frames per second. This now makes it possible to operate the camera similarly to a line scan camera with a line rate of 1 MHz.

Another advantage of the selected camera is its 10GigE Vision SFP+ interface, which can transfer large amounts of recorded data very quickly and over long distances via fiber optic cabling. Depending on the field of application, this feature can be crucial in ensuring the delay-free transfer of image data from the recording position to the processing computer.

Illumination as a decisive criterion
The image capturing speed is one thing — but equally important is the image quality that can be achieved at high scan rates of 1 MHz and above. This is where the illumination comes into play, because outstanding results can only be achieved by combining a high-speed area scan camera with fast pulsed lighting.

 

“A line scan rate of 1 MHz requires a particularly short exposure time of around 1 µs with the highest possible light intensity in order to achieve good image quality,” explains Bodenstorfer. “In contrast to standard illumination for area scan cameras, which illuminate a larger area, our xposure:1M technology only needs to illuminate a few lines. This has the advantage that, on the one hand, very high pulse powers are possible thanks to the overcurrent capability of modern LEDs. On the other hand, a narrow recording area can be illuminated very brightly and homogeneously, which is more difficult to achieve with an area illumination.”

 

With today's high-performance LEDs, light output can be significantly increased by overdriving pulsed lighting with a small duty cycle. This applies to the illumination of the field of view of the area scan camera, as the exposure time must be only a fraction of the image period to avoid motion blur. The light source therefore only needs to be switched on for a short part of each cycle. One possible lighting solution for the xposure:1M system is AIT's fast xposure:flash stroboscopic line light technology with pulse durations of up to 1 μs.

Another key factor for optimal image quality is precisely synchronized triggering of the camera with the feed of the object. “In the current configuration of our system, in which around 10 lines are read out at a time, a synchronization accuracy with deviations of up to approx. +/- 5% is sufficient, which is easily achievable for most industrial applications,” says Bodenstorfer.

 

Competent support
In addition to the technical features of the Mikrotron EoSens Creation 2.0 XGE, SVS-Vistek also impressed the AIT team on another level, according to Bodenstorfer: “The excellent and competent support provided by our contacts at SVS-Vistek during the training phase for HDL (Hardware Description Language) programming of the camera helped us in this project, as did the ongoing support. The customer-specific adaptations that SVS-Vistek implemented exclusively for AIT to enable the camera's required special functions were also particularly valuable to us.”

 

Bodenstorfer and his team have now demonstrated the effectiveness of the concept using various examples. For example, they mounted a banknote on a drum rotating at a speed of 10 m/s. When capturing a region of interest of 8 lines at a speed of 125,000 frames per second, the xposure:1M system achieved an impressive total speed of 1 million lines per second and was able to capture details with a resolution of 10 µm/pixel. Even 200 μm small characters on the banknote could still be read reliably.

 

Based on these results, Bodenstorfer sees numerous areas of application for AIT's xposure:1M technology and cites a few examples: “Among other things, this high-speed system is suitable for the fast optical inspection of wires, cables, or metal strips in the metalworking industry, for checking security printing such as passports or banknotes, and for checking packaging printing, for example, to check Braille code on medication packaging. Examples from electronics manufacturing include the rapid inspection of printed circuit boards or battery foils and electrodes. However, the system can also be used to inspect infrastructure such as rail or road surfaces, or for many other tasks where flat and narrow surfaces of very fast-moving objects need to be scanned at high resolution.” This is particularly true when objects need to be captured “simultaneously” under different lighting geometries using fast time multiplexing, such as with inline photometric stereo systems. For very fast applications that still require high precision from the vision system, AIT's xposure:1M technology is therefore a hot candidate.

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