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Three Popular Machine Vision Standards Cheat Sheet by

Three Popular Machine Vision Standards
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Introd­uction

Advanc­ements in machine vision have augmented industrial automation and provided a platform to advance imagin­g-based automatic inspection and analysis applic­ations. This techno­logy, however, comes with its hardware challenges — cameras and their cabling must often fit into cramped spaces while being able to withstand constant strain. This article explains and explores three of the more popular machine vision standards: GigE Vision, USB3 Vision, and CoaXPress. The inherent constr­aints within these standards affect their implem­ent­ation, from the lack of vendor diversity with CoaXPress to the limited cable length afforded by USB 3.0.

GigE Vision Overview

Released by the Automated Imaging Associ­ation (AIA) in May 2006, GigE Vision allows for simple interf­acing between a GigE Vision compliant device and a network card using standard Category 5 (CAT-5) Ethernet cable. This standard builds on the gigabit Ethernet commun­ication protocol with two custom protocols: GigE Vision Control Protocol (GVCP) and GigE Vision Streaming Protocol (GVSP), which are specif­ically geared towards machine vision industrial cameras. The GVCP protocol is mainly utilized for camera config­uration and connection while the GVSP protocol concerns the transfer of image data. Although the specif­ica­tion's name refers to gigabit Ethernet, it can be used for any speed grade, such as fast Ethernet. GigE Vision leverages the applic­ation progra­mming interface (API): Generic Interface for Camera­s(G­eni­Cam). This firmware was developed by the European Machine Vision Associ­ation (EMVA) with the goal of providing a progra­mming interface for all kinds of cameras and devices, despite vendor diversity. GeniCam also supports the USB3 Vision, CoaXPress, Camera Link HS, and Camera Link vision standards

GigE Hardware Consid­era­tions

Cables and connectors in industrial automation applic­ations are often mounted in and around robotic arms and various machinery in constant motion. The locking clips that come with standard RJ45 connectors are inadequate in these scenarios. They can often become dislodged, or unmated, from constant jostling and vibration. Vendors will often provide screw mounted hardware with the connector head in order to hold the mated connec­tion, thereby limiting the costs of plant downtime. However, connector heads from different vendors may not mate properly due to propri­etary designs of the screw interface. This will likely be solved with the release of Version 2.1, where the connector design is specified.
 

USB3 Vision Overview

The USB3 Vision standard is built upon the USB 3.0 (Super­speed USB) specif­ication with custom transport layers defined for the needs of machine vision. The Control Transport Layer and Event Transport Layer transfer asynch­ronous events from the device to the host PC. This differs from the USB 2.0 half-d­uplex polled traffic flow, as the receiver can simult­ane­ously transmit data acknow­led­gements without interr­upting the burst of data. Tradit­ional USB 2.0 bus transa­ction protocols can leave a processor in idle for long periods of time while I/O operations complete, wasting precious bus bandwidth. The USB3 standard draws upon the GeniCam platform to provide a uniform progra­mming interface. The standard also specifies the design of screw-down cable connectors (Micro-B, Standa­rd-A, Standa­rd-B, and Powered-B) to allow for the mating of components across vendors.

USB3 Hardware Consid­era­tions

The increased data rate offered by USB3 Vision comes with the caveat of cable length — while GigE vision cable runs can go as long as 100 meters without signal degrad­ation, USB3 vision cables can only go up to 5 meters. If cable length is not an issue, then USB3 would be a natural altern­ative. While the power that can be sent to peripheral devices from the USB port has improved from 2.5W for USB 2.0 to 4.5 W for USB 3.0, it still may not be enough to power high perfor­mance, high resolution cameras. In this case, the camera is often powered with an external GPIO connector — more cabling to worry about in a dynamic enviro­nment.

CoaXPress (CXP) Overview

Released by the Japan Industrial Imaging Associ­ation (JIIA) in 2013, CoaXPress (CXP) is a high-speed imaging standard for point-­to-­point, serial commun­ica­tions with camera­-to­-host speeds (downlink) as high as 6.25 Gbps and host-t­o-c­amera speeds as high as 20 Mbps. Similar to the other standards (e.g. 10 GigE, USB 3.1/.2) there is a roadmap to 10 Gbps and 12.5 Gbps throug­hputs per cable. This protocol leverages commercial off-th­e-shelf (COTS) coaxial cables with a charac­ter­istic impedance of 75 ohms (e.g. RG11, RG6, RG59). These cables often use either BNC or DIN 1.0/2.3 connec­tors. CXP offers plug-a­nd-play capabi­lities with built-in mechanisms for automatic link setup.

CXP Hardware Consid­era­tions

CXP really only supports point-­to-­point commun­ica­tions where multi-­camera setups are enabled through link aggreg­ation. Multi-­des­tin­ation support will, however, be introduced in the future with CXP v2.0, where a single camera can send data to frame grabbers in multiple PCs. There is not yet much vendor diversity available for CXP chipsets — essent­ially one CXP-en­abled driver­/eq­ualizer manufa­cturer, EcqoLogic. However, MACOM is set to release a chipset that supports CXP v2.0 as soon as it is released

The CXP standard also utilizes IP cores, or FPGAs, for camera or frame grabber develo­pment. There are three providers of frame grabber CXP cores: Easii IC, Sensor to Image, and Kaya Instru­ments. There are also only three manufa­cturers of CXP cores for cameras: Demand Creation, Kaya Instru­ments, and Easii IC. The lack of vendor diversity for key components can make integr­ation of this technology less accessible than with USB3 or GigE vision.

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