With only about a quarter of the overall installed base of industrial networking using 4-20 mA and other hardwired methods, fieldbus communication systems are becoming the de facto choice for automation. Now the debate is shifting to the question of whether fieldbus-based networks will remain the standard or if it will be supplanted by industrial Ethernet.
Fieldbus vs Ethernet
According to the Industrial Networking Q2 cover story “Industrial Ethernet Cruises Powerfully Along,” an IMS Research report found that fieldbus-based communications systems accounted for 75 percent of new industrial automation network connections in 2011, three times more than those using industrial Ethernet.
However, IMS adds that Ethernet is growing faster and will become the dominant industrial networking technology within 10 to 15 years. Although the IMS study labels all non-Ethernet industrial networks as fieldbus, for our discussion we’ll define fieldbus networks as those used in process applications to connect instruments and analyzers to the main control system, typically a distributed control system (DCS). Leading examples are Foundation H1, HART and Profibus-PA.
Device-level networks will be identified as those that connect discrete devices such as sensors, switches and motor starters to controllers, usually PLCs and PACs. Some of the most popular device-level networks use AS-i, CANopen, DeviceNet, IO-Link, Modbus and Profibus-DP.
While industrial Ethernet networks are certainly likely to grow in popularity in the automation world, not everyone is ready to abandon fieldbus-based communications. The explanation for this goes beyond fieldbus having a large installed base; there are definite situations in which fieldbus is the better choice over Ethernet. We’ll look at real-life examples that demonstrate the value of fieldbus-based communications, and we will consider the future of device, fieldbus and industrial Ethernet networks.
Advantage of Device and Fieldbus Networks Over Ethernet
- More economical
- Straightforward installation requiring less wiring
- Ease of calibration of instruments
Access to more data – measurement as well as diagnostic
- Easier troubleshooting
- Less sensitive to electrical noise
- More physically robust connectors and components
- Easier to transmit power over network
- Spans longer distances without repeaters or switches
Easier and Economical
When it comes to connecting field-level equipment — such as transmitters, control valves, motors, proximity sensors, encoders and similar monitoring and control devices — to control systems, device and fieldbus networks are often the simplest and cheapest way to go.
CMD Corp. located in Appleton, Wisconsin, manufactures converting equipment for blown film (Figure 1). In the Industrial Networking cover story, Paul Johnson, senior electrical engineer at CMD Corp., explains, “Process controllers, temperature controllers, small AC drives and similar equipment typically have built in RS-232/RS485 ports.” He adds that most HMIs and PLCs have the same ports, which enable them to support general-purpose protocols such as Modbus or DF1.
“We choose Modbus to save money. Ethernet support in many cases is an added expense. In some cases, Ethernet doesn’t meet application requirements,” says Johnson. Ethernet often can’t communicate with legacy equipment, and when it does, static noise and high bandwidth issues arise.
Benefits of Ethernet-Based Networks
- Same network for IT automation
- More topology options
- Large variety of network analysis tools
- Easier to expand
- Readily available technical support
- Universal connectivity to controllers, I/O and other components
- Ability to transmit multiple protocols simultaneously
- Superior and rapidly improving price/performance ratio
- Easier integration with multiple wireless networks
This reasoning is sound, according to Carl Henning, the deputy director at PI North America, the association for both Profibus and Profinet. In the cover story he states, “It doesn’t make sense to put an Ethernet connection in a proximity switch or any other simple sensor or actuator.”
Mallard Creek Polymers (MCP) in Charlotte, North Carolina, is a maker of latex products that also uses Modbus. Matt Bothe, engineer at MCP, says the company uses Modbus to communicate with several smart mass flowmeters to obtain flow and totalized measurements.
While Bothe admits device networks are preferable in many cases as a result of their broad installation base, determinism and familiarity, he also adds there are distinct advantages of these networks.
“Device networks generally possess advantages over analog/discrete systems, namely far less wire, ease of calibration, access to more data, ease of troubleshooting, and ease of installation,” explains Bothe.
Determinism and Consistency
Determinism is a key difference between device and fieldbus networks and Ethernet. Supporters claim device and fieldbus networks guarantee a response time, while Ethernet can’t. This differentiator is very essential for certain applications, such as position-dependent operations.
For example, if a dispenser is told to turn on exactly one second before the robot arrives on point, it must be extremely consistent. Sometimes a slight delay, such as 1.32 seconds, is okay as long as it’s consistent. The latency in the robot controller’s I/O configuration file can be adjusted for the 1.32 seconds.
While Ethernet is very fast, it isn’t deterministic; it can’t provide the consistency needed for factory automation when timing is critical. Sometimes the speed of Ethernet can overcome most of the determinism concerns, but not all of them. Speed will determine the packet transfer rate but not how collisions are handled.
The Importance of Distance
Distance is another important factor when considering Ethernet. While wired Ethernet is limited to about 100 meters, device and fieldbus network distances can be much longer.
Brian Radichel is business development manager at Purvis Industries in Dallas, Texas, a manufacturer of material handling and mechanical systems. He talks about his experience at a prior company that creates gypsum board. The number of drives in the system went from 40 to 70 during a process speed increase.
He explains that in a gypsum plant the distances between drive nodes and CPUs can be long. The drives were clustered in many cases, but one application required distances of 200 to 300 feet between each drive. The backbone was Profibus-DP, which easily handled the increased devices and distances with minimal network changes. He concedes the installation was slightly more complicated due to the daisy-chain topology of Profibus, but once the network was set up and addressed, the commissioning went smoothly.
The 100-meter distance limitation of Ethernet-based systems means they have no multi-drop capabilities, which is another factor that impedes their performance. In addition, they’re more susceptible to noise that can degrade overall network performance. Fiber optic cable can be used, but its installation requires a great deal of skill.
Perhaps more importantly, using Ethernet to cover the same distance as a fieldbus network is more expensive. Fiber optic cable or routers are required, especially for a daisy-chain configuration, making Ethernet impractical in terms of both performance and installation costs for outdoor, long-lead-length applications.
Fieldbus in Process Automation
Fieldbus networks are widely used in the process industries where they offer several advantages over Ethernet. One factor to consider is familiarity. As a result of fieldbus networks being used for a long time, engineers have discovered ways to use their advantages to solve problems that Ethernet can’t.
For example, one company had applications that require devices to be connected through slip rings, and 10 MHz/100 MHz Ethernet doesn’t transmit very well through slip ring assemblies. The engineers used their experience with device networks and implemented low-speed DeviceNet running at 56 kHz to avoid the problem.
While Ethernet is trying to establish itself in the process industry, Larry O’Brien, the global marketing manager at Fieldbus Foundation, isn’t concerned. For O’Brien it comes down to energy usage, “Process field devices require power, and Foundation H1 provides digital communications and power over standard twisted-pair wiring,” he says. He concedes that Ethernet does support power over Ethernet (PoE), but conveys the primary use of PoE is for phones, panels, access points and cameras — not field instrumentation.
Many process industries contain hazardous areas where implementing Ethernet at the physical layer is problematic. On the other hand, fieldbus networks can offer a two-wire, twisted-pair field level network that can be installed safely in a hazardous area. In addition, simple screw terminations are used, which are familiar to a device installer.
Diagnostics specific to process automation and control in the field are other key advantages of fieldbus networks. O’Brien explains that Foundation H1 provides sophisticated diagnostic and data-management capabilities, and a block structure that allows end users to implement function blocks in control valves or field devices for the purpose of implementing control in the field. He adds, “There’s evidence that control in the field provides an 80 percent increase in meantime between failures compared to traditional DCS control.”
O’Brien illustrates his point with an example of a chemical plant in the Netherlands. The interface card in the DCS at the facility failed, which meant communication from the DCS to the field devices was interrupted. However, a plant shutdown was avoided because the control-in-the-field functionality of Foundation H1 enabled continued operation via direct communication among the measurement devices and valves.
While proponents point out Ethernet’s ability to support wireless, the same capability is available with WirelessHART. In fact, since hundreds of thousands of process instruments already have HART installed, connecting to them via WirelessHART is simpler than with Ethernet.
Ethernet is moving into automation at the device level. Ethernet-based protocols like EtherNet/IP and Profinet have been widely accepted as viable in robot end-of-arm tooling and PLC communications.
The initial concern with using Ethernet-based networks was fear that the level of determinism wasn’t suitable for communication between the scanner and the device nodes. The determinism debate continues, but it doesn’t appear to be cause for concern. Although respected and considered robust, proven and deterministic, device networks are being replaced by Ethernet.
Ethernet will work its way further down into simpler and simpler devices. Industrial Ethernet networks are faster, have greater bandwidth, unlimited node counts, improved diagnostics, easier upward integration, and can use standard wireless. They also have more topology options and clearly a place in the automation network mix.
The Fieldbus Foundation also sees the value of Ethernet in industrial applications, which is why Foundation HSE is based on high-speed Ethernet. Foundation HSE, running at 100 Mbit/s, is designed for device, subsystem and enterprise integration. It supports the entire range of fieldbus capabilities, including standard function blocks and device descriptions, as well as application-specific, flexible function blocks for advanced process and discrete/hybrid/batch applications.
While fieldbus networks are the standard and often a better choice for the process industries, Ethernet does have its advantages and uses. In fact, many companies are starting to use both network types, depending on the application.
By, Christine Lesher, ControlsPR
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Originally Posted: Sept. 24, 2013