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Vision & Sensors: Machine Vision Standards

March 8, 2012
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How Systems Can Take Advantage of Camera Link HS Technology



Camera Link HS (CLHS) employs component technology developed for the high volume telecommunication and computing markets and, as such, there are multiple suppliers providing extended component lifetimes, ensuring stable system designs. This article explores how to employ features that are found in the CLHS protocol to deliver robust, leading-edge inspection systems, ensuring stable system designs. This article explores how to employ features that are found in the CLHS protocol to deliver robust, leading-edge inspection systems.

Customers demand reliability and CLHS achieves this. The CLHS compliant device is single bit, error tolerant and data is resent immediately should a transmission error occur. The short delay will not impact system level latencies and inspection decisions can be made correctly and consistently.

Machines need to run 24/7 and, should a device fail, it needs to be repaired quickly. Indicator LED use on CLHS devices have been standardized and enable repair personnel to quickly diagnose the issue and get the vision system back on line in a minimum amount of time.

Some machine vision systems trigger a camera when an object is present and, if objects come too closely spaced, the camera may not be able to capture the second object. CLHS makes it easy for the camera to notify the frame grabber with the Missed Trigger Flag in the video transport header. The system will know the precise time or location of the missed object. The system can than take appropriate action, ensuring reliable inspections.

Some systems move the cameras and, as a result, cables degrade with time, resulting in a rise in error rates. Bit error counters are available so that the maintenance department can schedule cable replacement. If the bit error count gets so high that too many data resend requests are made, the Buffer Overflow flag of the video header is set by the camera and can be passed to the system to inform it that the data is corrupted.

The ability to meet different systems requirements with one machine vision transport technology simplifies implementation and support of different systems. CLHS is designed to support 300 MB/s to virtually unlimited bandwidth for both line-scan and area-array cameras, including multispectral images. Costs are scaled with the capability needed and the data packetization methodology enables simple parallel processing with lower bandwidth frame grabbers. Low cost fibre optic solutions are available that answer the distance requirements of some applications and some cameras and frame grabbers will feature fibre optic interfaces. The scalable architecture and varied transmission media of CLHS enables a single transmission technology to meet all customer needs.

Machine vision systems accept inputs from the system and send control outputs to the system. CLHS makes it possible to optimize the system cabling by supporting up to 32 GPIO being sent/received from the camera head with latencies as low as 100 ns and a maximum latency of 500 ns plus cable delay. This low latency and jitter enables simpler control algorithms and system debug of time-critical applications. The fact cameras can effectively support I/O and the frame grabber card gives system designers the choice of how to connect the system together.

Changing the camera operating parameters frame by frame is a requirement of some of the most demanding systems and CLHS supports this capability with the Trigger Effectivity Byte and the return of it in the Image Acquisition Byte with the video. The Trigger Effectivity Byte is used by the application software to switch the camera’s predefined operating parameters for the next or subsequent exposure. The Acquisition Byte in the video header confirms the condition under which the data was captured and is passed to the application software, allowing it to properly interpret the image. It can be that inputs at the camera control the predefined operating conditions, in which case the camera responds to the inputs and the Acquisition Byte reflects the input conditions for the captured frame. Some of the expected uses for this feature include frame-by-frame Region of Interest selection and frame-by-frame gain selection.

Camera Command Communication used to control gain and other operating modes of the camera has up to 300 MB/sec of bandwidth This would enable streaming 16 bit pixel correction coefficients to a 1 Mp area array in about 7 ms in a fully optimized solution.

CLHS technology is designed to be “future proof” and take advantage of the improvements in the bit rates of serializer/deserializer technology. It is possible to run the CLHS protocol with a 10 GB serializer available today and the CLHS protocol will easily take advantage of the improvements that are coming in the future. The CLHS committee is made up of leading edge camera, frame grabber and cable companies that are pushing the envelope of performance to meet their customer’s needs. V&S





Tech Tips

CLHS makes it possible to optimize the system cabling by supporting up to 32 GPIO being sent/received from the camera head with latencies as low as 100 ns and a maximum latency of 500 ns plus cable delay.

CLHS is designed to support 300 MB/s to virtually unlimited bandwidth for both line-scan and area-array cameras, including multispectral images.

It is possible to run the CLHS protocol with a 10 GB serializer.

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