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The Fieldbus Wars: Who will Machine Vision support?
by Winn Hardin, Contributing Editor - AIA Posted 01/11/2005
Machine vision has already experienced a few revolutionary events in its history. The move from VME-based image processors to PC hosts was one revolutionary event, enabling the machine vision industry to significantly improve system performance without carrying the massive costs of developing commodity-priced, high-performance microprocessors.
Other vision proponents might say the growth of the CCD sensor to provide enough pixels for both high resolution and color was another transition that opened up a new body of applications. The same with the movement to object based programming and human-machine interfaces (HMI) is another benchmark event in machine vision. And few would contest the movement from stand-alone machine vision systems to vision as a system node on the plant network.
Low-cost consumer-based networking protocols and cabling helped to push the benefits of automated inspection systems. Machine vision initially leveraged consumer network protocols such as Ethernet and the universal serial bus (USB), but while these protocols can carry significant bandwidth, and in the case of Ethernet, across long distances, these networks are not designed for control systems. Industrial networks depend on real time communications -- or fieldbus networks -- that use low bandwidth transmissions to maintain the correct sequence of production events while guaranteeing safe operation for laborers.
Today, manufacturing plants have a wide variety of fieldbus networks to choose from, including varieties of Allen-Bradley's DeviceNet, ControlNet and Ethernet/IP; the growing Foundation Fieldbus network; control area networks (CAN); and European standards such as ASi-Bus, InterBus and the dominant Profibus -- to name a few. When considering how to integrate a machine vision system with a fieldbus network, system designers should consider several factors, including the location of the facility, the level of the network with which the vision system will communicate; bandwidth; hardware and network architecture.
According to Rob Hulsebos, fieldbus seminar instructor and software architect for Philips' Center for Industrial Technology (Eindhoven, Netherlands), ‘‘Machine vision is characterized by its large bandwidth demand, requiring a high-speed network like 100 Mbit/s Ethernet, USB or Firewire… What I see here is a conflict between the real-time demands of audio and video, and the real-time demands of control systems. The latter do not want data to be dropped, the former don't mind if data is dropped, as there is always a 'new' picture or more audio coming along.’‘ (Click here to see Mr. Hulsebos' and Grid Connect's fieldbus comparison charts for operational specifications of a variety of fieldbus architectures.)
Hulsebos goes on to say that the Ethernet, Firewire and USB cables are also not designed for movement, such as connecting a mobile camera on a robotic arm, for instance, because the constant flex and movement will degrade non-ruggedized copper cables. Fieldbus industrial cables are typically hardier than consumer cables, and allow a vision system to communicate with distant PLCs or other industrial nodes without having to run a new cable between the vision system and the actuator.
‘‘When you’re dealing with real time data rather than plant or system wide data, like in a closed loop process, a fieldbus is a fast way to get data from one process to another, while reducing the needs and costs of cabling,’‘ concludes PPT Vision’s vice president of marketing, Joe Rogers.
Because vision systems typically output high bandwidth signals, integrating a machine vision system with a fieldbus network -- even varieties such as Modbus/TCP or EtherNet/IP -- require specialized PCI cards located inside the PC host to handle the digital handshakes and transmission protocols of individual industrial networks. In some cases where the cable runs are short, RS-232 is used between the vision PC and the PLC, but that still requires adapting the data into PLC compatible formats inside the vision system.
Although examples like Allen-Bradley's move to a NetLinx architecture that uses a common (Control and Information Protocol) language for the device, system and network levels (see diagram), and commonality of protocols among other fieldbus networks such as Modbus/TCP and Foundation Fieldbus, compatibility issues among different fieldbus networks challenge both the plant operator and the vision system integrator. Common protocols mean that companies can improve communication among different levels of the network and save on hardware by unifying around certain chipset standards among different network level devices and pass that savings on to the user.
However, this trend toward fieldbus compatibility has yet to spill over between fieldbus types, leading to universal compatibility. The result is a fragmented fieldbus industry and product inventory that translates to the individual plant level by forcing plants to depend on and support legacy fieldbus architectures to save on spare parts and maintenance rather than choosing a plant network based solely on the application and performance. This is why location is important to the vision integrator because while DeviceNet and variants may dominate U.S. manufacturing markets, Profibus is the king in Europe. To best serve a vision client, the integrator needs to be familiar with the most common fieldbus architectures and protocols for that geographic area.
Today, approximately 5 to 10 percent of vision customers ask for a vision system to be integrated into a fieldbus network. Because of the low demand, resulting in low product development between vision systems and fieldbus architectures, many vision systems accommodate the high bandwidth needs of the vision system and the real-time needs of the fieldbus network by incorporating separate communication channels: an Ethernet connection for picture archiving and remote viewing, and a PLC compatible PCI board and software to communicate pass/fail or action signals to a PLC. This two pronged approach is likely to increase as smart cameras provide widely distributed inspection and process control systems within manufacturing plants, while facing challenges because of the variety of fieldbus connectors and protocols, limiting the vision systems ability to build a smart camera that fills network requirements in the U.S., Asia and Europe.
Comparison .pdf used with permission from Rob Hulsebos, Philips Center for Industrial Technology, Eindhoven, The Netherlands
Grid Connect field bus comparison chart .pdf used with permission from Grid Connect Inc. (Naperville, Illinois USA)
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