An overview of machine vision interfaces

Benefits of having a 10GigE interface:

• It is built on a transmission data standard for data centers and IT infrastructure. This means that its core technology is constantly being improved and solidified by key players within the Ethernet IT business such as Google, Cisco, IBM and Intel.

• Due to the widespread availability of GigE Vision compliant software and hardware devices, integration across networks is greatly simplified. This helps to achieve better scalability and flexibility for machine vision applications.

• It doesn't require a frame grabber. Most systems use standard network adapter cards supporting GigE/10GigE. These are inexpensive and available at many vendors, helping to lower system costs.

• It can also save on cabling and maintenance costs. GigE cables are easily available at IT hardware stores. These cables, as well as other networking components, are easy to replace and are low maintenance, thus helping to reduce inventory.

• It can operate over long cable lengths. Standard twisted pair copper cables (Cat 6, Cat 6e, Cat 6a and Cat 7) can all be used with 10GigE connections. Cat 6 and Cat 6e can support cable lengths up to 55m whereas Cat 6a and Cat 7 can support cable lengths up to 100m.

• It is based on GenICam which helps to provide consistency for programmers, standardized pixel formats, and better interoperability with other GenICam-based interfaces.

• 10GigE and GigE Vision support multi-video streaming which allows the streaming of two or more parallel video streams using the same interface. This can be especially useful for multi-sensor cameras as well as multi-processor architectures.

• It supports Precision Time Protocol (IEEE 1588), which is an integral part of the GigE Vision 2.0 standard. With the ever-increasing use of multiple camera systems in machine vision applications, precise synchronization of various vision and non-vision components in an application plays an important role to minimize jitter and other non-sync effects. Precision Time Protocol (PTP) provides that.

• It can be implemented in a way that supports network auto-negotiation. This can make 10GigE backwards compatible to 1000BASE-T (1 Gbps) and NBASE-T (2.5 and 5 Gbps) helping users to transition to faster GigE speeds.

Challenges

• As with all GigE Vision interfaces, there can be latency issues related to the IP networking configuration. Low latency and jitter can be critical in high-speed and real-time applications. Network optimization focusing on host computer and resource sharing (between buses, memories, CPUs, operating system, imaging cores, and graphic libraries) can help to minimize latency issues, but cannot eliminate them completely.

• It requires dealing with some network complexity and network bandwidth limitations. For example, multi-camera applications must have IP addresses assigned, as opposed to a true "plug-and-play" model. In addition, the sharing of network bandwidth in multi-camera applications may require complicated packet delay setups, and may still not have enough bandwidth to support high-speed applications.

• New multi-streaming cameras may not be fully supported by third-party software tools. Though multi-streaming is a part of the GigE Vision standard, its use in cameras is still relatively new. If multi-streaming is required, it is important to select a camera vendor who can provide an SDK with sample code for building multi-stream applications so that this capability can be fully leveraged.

• It may require separate cabling for power and/or triggering. Power over Ethernet (PoE) has been widely implemented in traditional 1000BASE-T (1 GigE) cameras providing a one cable option for power and data transfer. However, many cameras do not support triggering over Ethernet, which is typically implemented using GigE Vision Action Commands. Thus, two cables/connections may still needed for many applications, even when PoE is supported. This becomes even more challenging for 10GigE cameras due to their maximum power consumption which may exceed the basic 12.95W limit of the PoE standard. While PoE+ (IEEE 802.3at) with a consumption limit of 25.5W is an option, it is more complex and expensive to implement, and many 10GigE cameras have simply opted for a two-cable approach, with the second connection used for power and I/O.

Typical applications

The fact that GigE is integrated across almost all state-of-the-art IT hardware devices, and the fact that special frame grabber hardware is not required, has made GigE Vision a very popular machine vision interface. This use is particularly widespread in some applications such as:

• Internet of Things (IoT) and Industry 4.0: Interconnectivity of devices is central to Industry 4.0. The rise of digital-industrial technologies makes it possible to gather and analyze data across systems. A few popular application examples are additive manufacturing and production processes; intelligent robot automation systems; and quality control and monitoring.

• Boxed Vision Systems: GigE Vision-based systems are not exactly plug and play because in networked systems, the IP address may still need to be setup before running the system smoothly. However, they do fall under the category of boxed vision systems. They may be used in automotive, semiconductor, food sorting & recycling, industrial quality control stations, medical diagnosis, traffic systems & speed enforcement, and calibration & alignment devices, among others.

• Standalone Vision Systems: These are vision systems with no inter-connectivity to other systems. They are mainly used to inspect smaller parts and overcome the size constraints of smaller manufacturers. Such systems are also often used as secondary inspection units. Typical examples are automotive parts inspection and microscopy.

• Intelligent Vision Systems: These systems can include an embedded system architecture for real-time, AI/decision making based on deep learning algorithms. Typical application examples are intelligent farming, food sorting, automotive parts inspection, etc.

Benefits of an optical fiber physical layer based on a GigE Vision protocol

• The 10GBASE-SR standard-based transceivers can support very long transmission distances (up to 10 km). Very long cable lengths are helpful in industrial and outdoor environments.

• Being an optical fiber, the transmission noise is extremely low and stable over long distances, and there is high immunity to external noise sources including EMI and RFI that might be emitted by equipment in large factory environments.

• The data protocol is GigE Vision and hence the data packet management is based on this well established standard.

• Compared to fixed interfaces (e.g. standard Ethernet connectors), SFP+ systems can be equipped with any suitable type of transceiver (refer to the standards table above).

• Optical fibers are low cost even for very long cable lengths.

• All functions supported by the GigE Vision protocol are supported by 10GigE-based SFP+ models including multi-streaming, PTP, chunk data, and more.

Challenges

• SFP+ transceivers are not backwards compatible to 1 Gbps and NBASE-T speeds and hence cannot provide auto-negotiation with the network.

• Depending on the choice of transceivers, patch panels, and distance extenders, the cost of the total system can be very high. But it is often the case that customers who need such a network architecture, are aware of these costs.

• Fiber cables are fragile and need proper handling.

• Functions such as triggering, encoder control, etc. should be managed with additional cables. The optical fiber is purely for physical transport. Additional operation-focused camera functions need to be run using additional cables connected to the camera.

Typical applications

• Long distance outdoor: Many outdoor applications like railway track inspection, container goods inspection, etc. need very long cable distances. The imaging module is often located far away from the PC and data processing module. Stable data transfer under rough conditions is required and hence the use of optical fibers with SFP+ transceivers is advantageous.

• Long distance indoor: Some industrial applications like paper manufacturing require very large machinery spread across more than 100 meters. The inspection units are often located in critical areas of paper making which have high humidity and temperature. The image processing stations are often located far away near the control rooms.

• Electrostatic environments: Applications like battery inspection and inspection of other electronic devices have high electrostatic environments. Twisted copper gigabit Ethernet cannot be used due to high electrical conduction. This makes optical fiber cables are a good option.

Benefits of the CXP interface

• It offers high throughput. With its latest standard, CXP offers the highest raw data throughput when it comes to machine vision applications. A 4xCXP-12 connection offers 50 Gbps speed with a per lane speed via CXP-12 of 12.5 Gbps.

• It efficiently supports multiple high-speed cameras. CXP's high bandwidth and multi-lane architecture is well-suited to applications with multiple high-speed/high resolution cameras. Up to four cameras can be connected to a 4xCXP-12 frame grabber giving each camera 12.5 Gbps of dedicated bandwidth in a simple, point-to-point configuration.

• It simplifies multi-processing. The CXP 2.0 standard has introduced a multi-destination capability where the data from a single camera can be easily divided or replicated to multiple frame grabbers, which may be located on different PCs.

• It supports long cable lengths (though not as long as GigE Vision). Depending on the standard and bandwidth implemented, the coaxial cable lengths used can range between over 200m (for CXP-1) to about 30m (for CXP-12). This cable length can be further extended by using thicker coaxial cables or extenders based on optical fibers.

• It has low electrostatic noise.

• It supports both triggering and power over a single cable. Power over CXP (PoCXP) can deliver 24V and 13W per lane. The CXP standard also includes a low-speed uplink channel for triggering or device control via the frame grabber.

• It has very low latency and jitter. Depending on the frame grabber, CXP triggering typically has a latency of less than 5 microseconds with jitter of a few nanoseconds. This is an important feature in many high-speed industrial machine vision applications where the latency of a GigE network could create problems.

• Wide range of regulatory approvals. As the CXP standard uses standard coaxial cables at high speeds, it can also be used in highly regulated industries and applications such as medical, life sciences and defense. The transition from analog to digital is easier as the cables used in analog systems have the same architecture.

• Support for the GenICam standard. This has always been a cornerstone of the CXP standard and is important when it comes to data management and compatibility between different devices streaming complex data.

• CXP cables are more cost effective than Camera Link cables. Though CXP cables are typically more expensive than Ethernet cables, they are less than half the cost of the cables needed for the other frame-grabber-based interface, Camera Link. A good CXP cable can range anywhere between $50 – $90 (~42€ – 76€) whereas, a Camera Link cable can cost between $150 – $200 (~128€ - 170€).

Challenges

• Unlike GigE and USB interfaces, the implementation of CXP requires an appropriate plug-in card or frame grabber. This is because CXP is not a part of the standard PC architecture like GigE and USB are. Hence, transferring data using standard Ethernet infrastructure is not possible with the CXP interface.

• In higher resolution cameras with high frame rates, the maximum power consumption can exceed the 13W supported over a single CXP lane. This requires a more complicated PoCXP design including more expensive frame grabbers that can supply power to multiple lanes over a bundled cable. In such cases, PoCXP may not be supported or may be less advantageous.

• In most cases, the total system cost of a CXP-based vision system is higher than GigE and USB systems. This is often not a concern if an application really demands CXP performance.

Typical applications

• Defense & Medical: Ease of integration into existing coaxial cable systems, long cable lengths, high-speed data transmission and the complex data handing functions of CXP such as link sharing are useful in these types of applications. Some typical applications include situational awareness, target positioning systems, border and entry surveillance, surgical live viewing, etc.

• Industrial Machine Vision: Low jitter and latency, real-time triggering at high speeds, robust and flexible cables help CXP to perform well in harsh industrial environments. Typical applications include roll-based inspection of paper, metal and plastic foil, and also semiconductor and PCB inspection.

Benefits of the Camera Link interface

• It seamlessly supports multiple configurations/speeds. Depending on the application requirements, different speeds and components (such as cables) can be selected.

• The digital transmission is free of data loss and takes place over standardized signals.

• There are many variants and geometries available for the connectors. Depending on the application and available space, the connectors can be clipped or screwed. These are well-suited for robust industrial environments.

• It is the most "real-time" of the interfaces. Like CXP, Camera Link features a dedicated, point-to-point signaling channel for camera control/triggering. This triggering allows for a very low delay (less than 75 ns) and guarantees a nearly real-time, deterministic control of the camera.

• Camera Link supports power over the interface (PoCL) and, assuming both camera and frame grabber are so equipped, an external power supply to the camera is not required. The PoCL function is supported over all types of connectors and configurations.

Challenges

• Cable lengths with Camera Link are limited. Base and Medium configurations are limited to 10m for the 85 MHz frequency version while Full configurations are limited to 5m and Deca configurations are limited to 4m.

• CL cables are relatively expensive compared to other interface cables used in machine vision.

• CL cables are less flexible and hence difficult to use in moving vision assemblies.

• High bandwidth output (anything over 2 Gbps) requires two data cables and possibly a separate cable for power and I/O. Most CL frame grabbers have 2 or 4 connectors and thus can support a maximum of 2 high-speed cameras. This makes multi-camera applications both complicated and expensive.

Typical applications

• High speed line scan applications: Being a high-speed interface with very low latency and jitter, Camera Link is popularly used in web applications like print, paper, foil and metal inspection.

• Reliability applications: The high point-to-point reliability of Camera Link operation has also led to its use in critical inspection tasks within electronics and semiconductor industry where network delays or other instabilities cannot be tolerated.

Benefits of the USB3 Vision interface

• USB is the most popular and standardized method to connect to computers and peripheral devices.

• As part of the standard PC architecture, replacement of parts such as buses and cables is easy and cost effective.

• The latest USB3 Vision standard is completely plug-and-play. In a vast majority of cases, there are no compatibility issues between cameras and PC chipsets.

• Due to increasing bandwidth and reliability, USB3 Vision has rapidly replaced Firewire and USB 2.0 interfaces. The USB3 Vision interface is backwards compatible to USB 2.0 allowing simple upgrades from a native USB 2 vision system.

• USB3 Vision is highly compatible and functional in multi-camera setups as long as speeds and resolutions are moderate.

• The basic design of the USB3 Vision interface is based on GenICam which helps to provide consistency for programmers and better interoperability with other GenICam-based interfaces.

• It has lower latency and jitter than a network interface like GigE Vision, though it does not achieve the near real-time performance of Camera Link or CoaXPress.

• Variable image size: USB3 Vision allows for the sending of images in variable sizes by providing the host with information about the image in advance.

• Low CPU load: The use of zero copy (Direct Memory Access — DMA) keeps the necessary CPU load for image retrieval very low.

• USB 3.0 offers 4.5W of power supply to connected devices. This means many basic cameras can work without an additional power supply.

• Its simplicity can reduce system costs while still providing three times faster speed than standard 1000BASE-T GigE Vision.

Challenges

• The passive cable length of USB 3.0 Vision is limited to 5m which gives other interfaces, particularly GigE Vision, a significant advantage in certain types of applications.

• The power supply of 4.5 W is often not enough for high performance, high resolution cameras, thus either these cameras are not available with a USB3 Vision interface or a separate power connection is required.

• As USB 3.0 is first and foremost a consumer interface used with a wide range of PC peripherals, care must be taken in selecting cables, hubs, and other devices for industrial applications. Many consumer-level USB accessories cannot stand up to the requirements of machine vision applications, leading to instability, poor performance, and field failures.

Typical applications

• Plug and play: Known for its table-top, plug and play capabilities, USB3 Vision is a popular interface in microscopy, standalone inspection systems, portable inspection devices, etc.

• Multi-camera environments: Using a hub-based topology, USB3 Vision can be used effectively in many types of moderate-speed multi-camera environments, such as 3D vision systems on robot arms, autonomous vehicles, and multi-camera virtual reality "heads" providing 360-degreee views.