Visitors to the recent Vision show in Stuttgart would probably have been struck by two key buzzwords: ‘CoaXPress’ (CXP) and ‘USB 3’ with products based around these two bright new shiny standards in abundance. By Dr John Haddon, technical consultant to UK Industrial Vision Association and director, Panther Vision
The world of machine vision standards has moved on rapidly over the last few years, with applications requiring either faster data transfer rates or longer data transmission distances or both than can be
handled by the well-established GigE Vision or CameraLink standards. The need to accommodate these higher end requirements has led to the emergence of CXP, USB 3 and CameraLink HS, another standard based on the original CameraLink methodology.
Perhaps unsurprisingly, there is no definitive ‘best’ standard – the most appropriate standard should be chosen according to the requirements of the application. The new standards fall into two categories: ‘on the wire direct interfaces’ which have proprietary machine vision protocols that require a frame grabber, and ‘host-based standard on the wire interfaces’ which do not. CXP and CameraLink HS are examples of the former and USB3 is an example of the latter. However, all of the new emerging standards are underpinned by the Gen<i>Cam standard.
Gen<i>Cam was originally developed at the same time as the GigE Vision standard in 2006 to give a consistent programming interface for all compliant cameras by providing a method of configuring the camera (resolution, camera control, exposure modes, special features etc) and a standard camera feature naming convention.
In 2008, a transport layer interface, the GenTL standard, was introduced to provide a technology-independent interface for enumerating cameras and acquiring images from them.
It is the GenTL that has allowed Gen<i>Cam to be a common platform that can be used with all of the new emerging standards.
CXP was developed especially for machine vision applications, utilising coaxial cable for data transmission. Originally specified by a consortium of camera and frame grabber vendors, it won the Vision award in November 2009. It was adopted and is now maintained by the Japan Industrial Imaging Association. In April 2011, it was approved as an international standard and although a trickle of CXP products appeared after ratification, it has taken until the end of 2012 for there to be a real choice from a variety of manufacturers. CXP is characterised by its ability to transmit much more pixel data than the current Camera Link and GigE Vision interface standards, and over distances between 40m and 100m (without the need for repeaters) depending on data rates. The downlink (from camera to frame grabber) can transmit image data at up to 6.25 Gbits/sec (Gbps) while the uplink (from frame grabber to camera) can handle command and control data at up to 20Mbits/sec. Greater downlink performance can be achieved by using link aggregation.
The flexibility of coaxial cable makes CXP well suited for applications where a camera is mounted on a moving arm. Coaxial cable is already installed in many analogue systems, which makes it easier to move these to digital with CXP. CXP also helps reduce cable complexity and cost by offering triggering and 13W of power over the same coax cable as well as having low latency and low jitter trigger characteristics.
CXP is finding favour in applications which need higher resolution cameras, running at very fast frame rates, which includes both areascan and linescan cameras. One UKIVA member reports working on a CXP project which requires a camera resolution of 4k x 1k running at 500fps.
While it has taken well over a year for CXP products to come to market, the adoption of the ‘superspeed’ USB 3.0 as a vision data transfer interface is quite different. USB 3.0 has been available in the mass market since 2011. Improved data transfer rates of up to 400Mbyte/s, the fact that no framegrabber is required and, of course, the low cost, led to a wide range of USB 3.0 cameras being available well in advance of the CXP products. USB3 cable length will be dependent on data rate and the quality of the cable itself.
For the fastest cameras transmission distances could be as short as 3-4m but could be over 5m at lower data rates. Given these variables, manufacturers and suppliers may need to define maximum data rates at each length. However, it is important to differentiate between the USB 3.0 standard used on the mass market and the actual Machine Vision Standard, USB3 Vision. USB3 Vision is a Standard on the wire, which is fully Gen<i>Cam compliant and provides power, control and data over a single cable with a lockable connector.
The role of the UKIVA
As more machine vision standards become available, vision technology suppliers and vision systems integrators including UKIVA members will bear a greater responsibility in guiding end-users and OEMs to the optimum solution. We are grateful to UKIVA members Alrad Imaging, Clearview Imaging, Imperx, Matrox Imaging, MultiPix, National Instruments, Panther Vision and Stemmer Imaging, for their contributions to this article. They offer experience with either their own CoaXPress or USB 3.0 products or products from the world’s leading manufacturers so they will be happy to advise potential users on the best route to take.
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