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.

Figure 1. USB3 was predicted to be the most popular machine vision interface in 2015. (Source: Vision Systems - Popular Interface Design Poll 2015)

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 Vision Benefits

The GigE Vision standard has the benefit of leveraging the widely utilized Ethernet platform, which allows for high scalability at a relatively low price. The widespread adoption of GigE vision also provides excellent interoperability and vendor independence, while the revisions allow for more flexible configurations aside from point-to-point architecture with multiple camera setups. GigE Vision offers a large passive cable length of 100 meters, allowing for unimpeded upgrades in large installations. Revisions that include technologies such as 3D data support and PTP utilization will allow this platform to stay relevant for years to come. The upgrade to support 10 GigE throughputs is very significant, as customers can still have the advantages of using the dominant interface with integration across multiple industries while attaining ten times the speed.

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 Vision Benefits

Figure 2. Contrary to USB3 or GigE vision standards, CXP typically leverages a frame grabber as opposed to directly interfacing with a PC. (Adimec rugged CMOS low light camera application)

The USB3 Vision standard allows for lower system cost without sacrificing performance. The USB3.0 interface offers throughputs nearly three times higher than the popular GigE Vision standard and its CPU load is significantly lower. At 85 MB/s, the CPU load for USB 3.0 would be approximately 1% while GigE would be about 6% due to the copy processing that GigE requires. The USB 3.0 interface is backwards compatible with USB 2.0, allowing for simple upgrades of a native USB 2.0 vision system. A poll run in 2015 by Vision Systems noted that 80% of respondents felt that USB 3.0 would be the most popular interface for machine vision, likely due to some of its significant benefits. The USB 3.0 interface affords the cost and ease-of-implementation benefits of leveraging a commonly used technology while offering considerably higher data rates than other popular vision standards. The USB 3.0 interface is also widely available on newer laptops, and since there is a general trend of moving away from desktop PCs in favor of laptops and tablets, this platform has some competitive advantages over Ethernet.

Table 1. Parameters of various machine vision standards.

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.