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Feature Articles

Digital Camera Network Varieties Solve Complex Application Needs

by Winn Hardin, Contributing Editor - AIA

 

This is a part II of AIA's look at existing camera network protocols and supporting cabling, what's driving the adoption of various camera network technologies, and how the machine vision industry is responding. Part I of this series looked at the new CameraLink 1.2* and 2.0 specifications, and can be found by following this link.

1394a/1394b  400/800 Mbps 4.5 meters
Ethernet 10/100 Mbps 100 meters
GigE Vision™ 1000 Mbps 100 meters
USB 2.0 480 Mbps 5 meters
Camera Link® 1.2* 5200-6800 Mbps 10 meters

TABLE 1

The length of cable running between the camera and PC or other image processor in a machine vision system is still the least complex part of the vision system, but making the wrong choice among cable types and technologies can cost you more than money.

Slower analog solutions have given way to a variety of digital connectivity options for industrial cameras. Among these digital camera connectivity options, different network protocols view issues like latency, bandwidth, power supply, and channel control – well, differently. Some like Camera Link® are designed for industrial applications and have methods for ensuring data integrity, reducing latency, and providing power. Other networks designed mainly for consumers, can deliver the same robustness, but require some customization and know how on the part of camera and I/O card manufacturers. This article will look at the specifics of USB 2.0, 1394 (a and b), as well as 10/100 Ethernet and the AIA's new GigE Vision™ standard and see what vision equipment manufacturers are saying about trends within the peripheral networking market as they apply to machine vision.

USB, Ethernet, Everywhere
Of the four digital camera connectivity options today (USB, 1394, Ethernet, and GigE Vision) only two of them typically do not require a digital I/O card: USB and Ethernet. And the reason for this isn't because these networks are 'better' – they aren't. It's because of how PC's are built.

Lumenera Corporation (Ottawa, Ontario Canada) makes cameras for a wide selection of industries, and regularly supplies cameras with built-in USB, Ethernet, Camera Link, or FireWire connectivity. Ethernet and USB dominate their commercial camera offerings, however, because ‘‘…of how common an interface they are,’‘ says Lumenera's Greg Bell, Vice President of Business Development. ‘‘Every laptop or host computer has an embedded USB 2.0 port, and it’s the same with Ethernet. We started with USB 2.0 and then followed along to Ethernet because we do more than sell industrial products. We're heavily into security cameras as well.’‘

Lumenera's Bell points out that USB 2.0, with its 480 Mbps theoretical bandwidth, jives well with machine vision applications running around 1 megapixel at 30 fps. A 1.3 megapixel sensor outputting 12 bits per pixel at 30 fps generates 468 Mbps, very close to USB 2.0's 480 Mbps. ‘‘I don’t know if it was intentional, but it's certainly convenient,’‘ Bell concludes.

Ruggedness is a downfall often cited against USB, but Lumenera recently began offering USB cameras with custom, ruggedized connectors that won't easily work loose compared to standard USB connectors. The ruggedness issues continue through to the cable construction, which typically isn't constructed to meet the ruggedness and strength requirements for moving camera applications, such as robot guidance.

Real-Time Challenges
Vision systems with image transmission requirements that do not exceed a maximum cable length of 5 m and bandwidth of 480 Mb/s can benefit from the prevalence of USB ports in PCs, but when it comes to guaranteeing data integrity in real-time operations, it's clear that the application will be limited in the number of cameras or frame rates that can be achieved using USB technology.

The other most prevalent network, Ethernet, with speeds of 10/100 Mbps offers significantly less bandwidth than USB, but the RJ45 jacks are designed with a simple locking mechanism and are less likely to work themselves lose, and the cable run is extended to 100m or more, making this technology extremely useful for some low-bandwidth vision applications and a majority of security applications.

‘‘In a pure machine vision environment latency is an issue [for Ethernet], but there's lots of opportunity out there for non-real-time machine vision applications, even some applications that afford some compression,’‘ explains Lumenera's Bell. ‘‘Compression doesn't work if you're doing pixel-to-pixel analysis, but presence-absence vision systems can live with compression.’‘

This has, in part, been a driving force behind the adoption of 'smart camera' machine vision systems that process the images at the camera head and only supply partial image data or logical response back to the PC host. 

Although some security systems use machine vision technology for access control (retinal/biometric scanners) or automated intruder detection, the majority of security systems are manually monitored CCTV type system. Security applications typically do not require high-resolution sensors, or have concerns about latency between reading data at the sensor and delays processing the sensor data at the host. The main concern is transmitting the images, i.e., cable lengths and costs. Lumenera recently introduced a TCP/IP camera specifically for easy networking of security systems, as well as a power over Ethernet (PoE) capable camera for security systems, which eliminates the need for running a separate power cable. ‘‘Power over Ethernet is becoming a standard RFP request for security applications,’‘ adds Lumenera's Bell.

Power and Bandwidth
Cameras with FireWire connectivity, either 1394a or the newer 1394b, also draw power from their I/O board, similar to USB and PoE. However, utilizing PoE means buying a specialized NIC card that can provide the power, which eliminates the cost savings of using consumer-driven networks.

With a maximum cable length of 4.5m, 1394a offers 400 Mbps, while 1394b uses 9 pins instead of 6 pins and accommodates up to 800 Mbps. Sony, one of the first adopters of 1394 technology for industrial cameras, recently offered a 1394b camera. Sony's product manager, Ilias Levis, explains that the additional bandwidth, and more specifically the additional pins, allowed Sony to integrate additional error checking capabilities into the 1394b camera. ‘‘The 1394b cameras are also better at daisy chaining since you have so much more additional bandwidth,’‘ Levis adds. ‘‘1394b is an excellent interface for industrial applications offering reliable data transfer and minimal CPU latencies, Ethernet and GigE Vision can go 100 meters, but only a small percentage of industrial applications require that.

Discussions of 1394b often generate comparisons to GigE Vision. In 2006, AIA introduced the first GigE Vision standard built on the Gigabit Ethernet communication protocol. At 1000 Mbps, GigE Vision offers similar bandwidth with 1394b and no power, but the cable run extends up to 100m. Proponents of both 1394b and GigE Vision say the other side suffers from technical issues. Many of those issues may be limited to specific skills of the system integrator, but there are two complaints that have merit: Windows® offers poor support for 1394b cards, and GigE Vision is still growing and not as prevalent as 10/100 Ethernet, necessitating the purchase of non-powered Gigabit Ethernet cards.

The GigE infrastructure is likely to follow the same growth curve as 10/100 Ethernet, so preparing an early vision standard based on the communications protocol will help ease compatibility concerns as more cameras become GigE Vision compliant. The GigE Vision standard has four elements:

  1. The GigE Vision Control Protocol (GVCP), which runs on top of Universal Datagram Protocol (UDP) IPv4. It defines how to control and configure compliant devices such as cameras, specifies stream channels and provides mechanisms for cameras to send image and control data to host computers.
  2. The GigE Vision Stream Protocol (GVSP), which defines data types and describes how images are transmitted over GigE.
  3. The GigE Device Discovery Mechanism, which defines how cameras and other compliant devices obtain IP addresses.
  4. An XML description file based on the emerging GenICam™ standard, which provides the equivalent of a computer-readable datasheet to allow access to camera controls and image stream.

Prosilica Inc. (Burnaby, British Columbia, Canada) has been one of the leading proponents of embedded Ethernet connectivity for industrial imaging applications. ‘‘We've been doing GigE Vision now for over a year,’‘ notes Prosilica's CEO, Marty Furse. ‘‘We're finding that GigE Vision is really a very versatile interface, especially in multiple camera situations, but even in high-speed, time-critical, machine vision applications.’‘

According to Furse, it's not just about having more bandwidth than you need to manage conflicts; it's about managing that bandwidth. ‘‘For instance, our GigE Vision cameras have a sophisticated resend mechanism, so if any data is lost it's resent immediately within that frame. So the level of data integrity is much higher than what's available from FireWire, Camera Link, or USB which do not have such a resend mechanism.’‘ GigE Vision will continue to require additional power cabling for the indefinite future, however.

With the wealth of connectivity options available to industrial vision applications today, designers can pick and choose the right combination of bandwidth, length, data integrity, and customization for their application. Data bottlenecks that used to exist between the camera and the image processor have moved again inside the PC. Designers will have to make sure that their image processing boards use PCI Express with up to 16000 Mbps versus PCI's 1280 Mbps if they want to avoid the problems that come with overloaded processors and lost data.


* Be sure to read Part I of this article on the new Camera Link 1.2 specification.

 

 

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