By Zuschant Chugh
Zuschant Chugh, Product Management Toolbox + Accessories for Basler, is responsible for cables, PC cards, peripherals, and accessories product categories.
When using cameras together with PC cards, users can choose between several camera interfaces. The dominant options at this time are USB 3.0, CoaXPress 2.0 and Gigabit Ethernet (GigE). Each of these interfaces has its own advantages and disadvantages, which can justify the respective selection depending on the application area and usage scenario. This article examines these advantages and disadvantages with regard to multi-camera systems. It also identifies the optimal interface for such systems and additionally discusses PC cards as the most important component in a multi-camera system. These PC cards form the interface to the camera used; they are installed in the PC for smooth image acquisition and transmission and thus digitize the analog camera signals.
Different camera interfaces
CoaXPress (CXP) was published in 2008 and has become a new standard in industrial image processing in recent years. If users decide to use the CoaXPress interface, the use of a CXP-12 interface card or a CXP-12 frame grabber is automatically required, creating some differences in terms of price and technical design. The frame grabber is technically more sophisticated because it contains pre-processing and other image processing features. For many applications, even those with several cameras, a less expensive interface card is often sufficient.
The CoaXPress interface is optimal for applications that require high-speed or high-resolution and real-time behavior; CoaXPress 2.0 offers high data rates and low latency, allowing users to take full advantage of modern image sensor capabilities and achieve maximum performance in their applications. Advantages of this interface include the plug-and-play connectivity of CoaXPress components, cable lengths up to 40 meters with very high bandwidth (12.5 Gbps per channel) and a single cable solution for data transmission and power supply. By combining frame grabber technology with the cameras used, very precise camera synchronization is possible even in multi-camera systems (limited to four cameras) with very low latency. However, individual components required for this interface technology, such as the frame grabber, are more cost-intensive; CoaXPress cameras also tend to be larger compared with compact USB 3.0 or GigE cameras.
Launched in early 2013, the USB3 Vision standard defines all necessary elements that make USB 3.0 suitable for industrial image processing applications. Besides connectors and cable characteristics, it describes the communication between a USB 3.0 device and USB 3.0-compliant software. Since USB 3.0 has become a widespread standard in the consumer market, the majority of hardware currently available supports USB 3.0. Advantages of this interface are high bandwidths of greater than 350 Mbps, no additional cost for a separate frame grabber, widely available plug-and-play hardware, and a low CPU load. Most new devices—from PCs to the smallest PC cards—have USB 3.0 embedded connections. However, the standard only supports short cable lengths up to eight meters. Another disadvantage is that multi-camera systems are possible, but the maximum bandwidth is limited by the individual cameras. This makes complex multi-camera systems with more than four cameras difficult to implement.
Gigabit Ethernet (GigE) has been an established standard in industrial image processing since 2006. Measured by the number of installed applications, it is currently the fastest growing interface technology for industrial cameras. GigE owes its popularity to its ability to solve some key problems effectively. Restrictions in cable lengths, which were common with older interfaces, are no problem with GigE, which is advantageous for multi-camera systems.
In addition, the combination of several cameras with one PC is much easier and more cost-effective. As well, this interface technology is widely used in traditional IT, which means there is a wealth of know-how on this technology as well as standard components. GigE already dominates numerous areas of image processing, from production to intelligent traffic systems, in which multi-camera systems are often used.
Advantages of this interface are a simple infrastructure and minimal hardware requirements for multi-camera systems, data transfer rates from 1 Gbps up to 10 Gbps with scalability for future higher requirements, and the lack of a requirement for a frame grabber. Cameras can be operated synchronously and/or in real time without additional cabling via Precision Time Protocol (PTP). Maximum cable lengths of up to 100 meters as well as single cable solutions via Power over Ethernet (PoE) are possible, and that length can be further expanded by connecting additional network components (e.g. switches). A disadvantage is that the CPU load required for image acquisition is typically about 10 to 15%. However, this can be minimized to 3 to 8% CPU load when using drivers and settings optimized for machine vision.
Possible combinations with GigE Vision
Of the three interface technologies presented, the GigE Vision interface is usually the most suitable technology for multi-camera systems because of the simpler infrastructure, maximum cable lengths, and cost efficiency. Using multiple cameras, however, requires PC cards that have the appropriate number of ports and represent the appropriate data transfer rate. Since a conventional PC often has only one GigE port, which is usually occupied for machine control, these cards are indispensable for such a scenario. In addition, machine vision cameras require the full bandwidth of the port to which they are connected. Coupling several cameras via a distributor (usually switches) to the port available on the PC therefore fails because of the port’s bandwidth. The above-mentioned PC cards, which also allow a higher bandwidth at one port, provide a solution.
Accordingly, GigE interface cards with 1 Gbit and one, two, or four ports are usually used as PC cards for machine vision. In very rare cases, it is possible for users to use interface cards that provide even more ports. Thus, a single interface card can reduce the system complexity immensely by eliminating additional network components that can cause system errors. In addition, the PoE function makes it possible to implement a single cable solution, eliminating the need for a "complicated" power supply for the application. Thus, the system is as easy to handle as a USB camera setup and has the additional advantages of the GigE interface.
In addition to the 1 Gbit interface cards, 10 Gbit interface cards are also available on the market, making it possible, for example, to operate 10 1 Gbit cameras at full bandwidth, i.e. at full speed. By connecting a switch, it is even possible to create a camera setup with more than 10 cameras. Furthermore, industrial camera manufacturers are already increasingly using fast interface technologies and will do so even more in the near future. Such cameras with more than 1 Gbit can be used with corresponding 10 Gbit interface cards, allowing the full performance of the cameras to be used. Since there are currently no 5 Gbit interface cards available that meet the machine vision requirements, users in most cases use a 10 Gbit interface card for this scenario, which must be backward compatible. However, this is not a real drawback, since a 10 Gbit interface card with two ports allows, for example, multiple GigE cameras to be used at full performance.
Application scenarios
Different scenarios are possible when using GigE cameras in a multi-camera setup. On the one hand, in an application with one to four cameras, trigger cables can supply power to the camera, which means that a PoE function of the interface cards is no longer necessary. On the other hand, if five to eight cameras are used, the software controls the triggering and thus the PoE function of the interface cards is used, so that only one cable is required to supply power to the camera. Triggering via the software can, of course, also be used with fewer than four cameras. However, since the system complexity in this setup is low, users usually use additional trigger cables and do not need the PoE function.
If a 10GigE interface card with one port and a 10GigE switch are combined, up to 10 1 Gbit cameras can be used. Analogously, this also works with a 10GigE interface card with two ports to connect up to 20 cameras. The following use cases show that these previously described application scenarios do not only work in theory but are actually implemented by users and are therefore common practice.
Use cases
In many camera applications, it is often necessary to use multiple cameras to acquire images, such as in 3D triangulation, sports and motion applications, quality inspection, and assembly line applications.
When capturing a goal shot in a soccer game, for example, it can be crucial to capture multiple images at precisely defined points in time. A powerful video and statistical analysis program specifically designed for professional soccer uses three GigE cameras that capture every inch of the field. Proprietary software then seamlessly combines the three images into a single, high-resolution, real-time view of the field from corner flag to corner flag. The result is an ultra-high-resolution video that covers the entire field and includes a pan and zoom function to give users complete control over the displayed video.
In another sports use case, a camera-based, computerized tennis analysis system helps tennis players of all levels improve their games. The system uses five cameras to digitally record every aspect of the game or training session. The cameras capture and record every movement of the ball and players and automatically classify each shot.
In another application regularly inspecting railroad tracks is crucial to ensure rail traffic safety. Among the many inspection techniques used by modern railroad companies, ultrasonic inspection based on a recorded image of a rail is a particularly good and promising method. The inspection vehicle that carries out the inspection is equipped with four GigE cameras to inspect the rail from the top and side surface and check for defects.
A multi-camera setup is also used to carry out train quality control and check the individual wagons and wheels of a moving train for defects. In this application example, freight trains travel along the track at 50 to 60 kmh, and three GigE cameras inspect for defects: one camera mounted on the rail bed and two more beside the rail. Experts in a control room carefully examine the captured images of the undercarriage and wheels for defects and flaws. A decisive factor in the use of cameras with GigE interface was capability for long cables, which ensure easy connection between the inspection system and the control room.
In an automotive application, software analyzes the images, checks the dimensions of the objects, and ensures that all the necessary components are present. The inspection environment consists of a black room or cabinet in which walls, floor, and ceiling are equipped with dozens (up to 100) GigE cameras. The inspected object, such as a front frame, is brought from the production line into the cabinet. There, the cameras record the object from all sides using the optimally designed lighting in the cabinet.
Since each camera and each segment of the LED illumination performs its task according to a precisely defined cycle, the imaging process takes only a few seconds. A PoE interface reduces the number of cables required and simplifies maintenance and installation. Via GigE transmission, a single software component controls several dozen cameras. Fast electronic shutter speeds, programmable down to a fraction of a second, enable effective imaging applications on a fast-moving production line.
In times of Internet trade, lean production, and increasingly efficient value-added chains, fast and timely delivery of goods is of high and ever-increasing importance. Fast and reliable logistics is an important component of every efficient company in all areas of the value-added process. Automated systems are increasingly helping to make the dispatch and delivery of goods and material provision processes faster and more reliable. Possible cable lengths of up to 100 meters when using the GigE interface are ideally suited for this application scenario. In addition, multi-camera systems can be used with high efficiency in large warehouses and to optimize various processes.
The retail sector also requires reliable camera technology. Camera systems can be used in many different ways, such as in reverse vending machines or cash dispensers. The multi-camera setups also support the recording of customer numbers and behavior. Accordingly, cameras are used in the retail sector to track which customers (women or men) stop where longer, which posters they look at longer, which items they spend time with, and which goods they ignore and simply walk past.
Retailers use the data obtained to optimize the display of goods in the store and ultimately attract more buyers. In addition, there is a growing trend toward self-checkout systems, where customers pay at a vending machine without a cashier. Various cameras handle customer recognition and payment. The GigE interface technology is an ideal match for the demand for a simple infrastructure of multi-camera systems and the long cables required in this application area.
Outlook
GigE is the interface with the greatest technological flexibility in terms of cable length and multi-camera functionality. It will therefore continue to play an important role in many types of applications and will be the first choice for users when multi-camera systems are used. Accordingly, the interface cards used in these systems are of great importance for the overall application. When selecting and using GigE cameras and interface cards, it is important to rely on the GigE Vision standard and comply with it to guarantee the appropriate quality of the systems. It is crucial to rely on tested and approved products to meet the high requirements of machine vision applications.
However, the ultimate "all things to everyone" solution for industrial cameras does not exist. GigE, USB, and CoaXPress will probably collectively dominate the future interface landscape. However, certain interface technologies are more suited for certain applications.
