Go With What Works: Are Simpler, Hard-Working Device-Level Networks a Better Choice? 04/30/2013
According According to IMS Research fieldb Research fieldbus–based us–based communications communications systems accounted for f or 75% of new industrial automation network connections in 2011, three times more than those using industrial Ethernet. IMS does, however, say Ethernet is grow g rowing ing faster and and will become the dominant dominant industrial ind ustrial networking networking technology t echnology within 10–15 10–15 years. years. The IMS IMS study labels l abels all non-Ethernet n on-Ethernet industrial networks as fieldbu fieldbus, s, but for this article we'll use u se terminology more common common to industrial indu strial automation. automation. We'll We'l l define fieldbus netwo n etworks rks as as those used in proce p rocess ss 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. SEE ALSO: Ethernet: The Promised Land?
We'll identify device-level networks as networks as those that connect
About the Author
discrete devices such as sensors, switches and motor
Dan Hebert is Hebert is senior
starters to controllers, most typically PLCs and PACs. Some
technical technical editor for
of the most popular are AS-i , CANopen, DeviceNet, IO-Link,
Control,, Control Design Control and Industrial Networking. Networking.
Modbus and Profibus-DP.
Email him at
Despite Despite the Ethernet Eth ernet onslaught, device device and fieldbus fieldb us networks are still widely used in industrial indust rial applications. This article will will
[email protected] or
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explain explain why, why, using u sing real-world real-world applications, these netwo n etworks rks are preferred preferred over over Ethernet in particular applications. We'll also gaze into the future, and speculate on the future fu ture of device, device, fieldbus and industrial indust rial Ethernet networks. networks.
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Simpler and Cheaper
When it comes to connecting field-level equipment — such as
Providing Top Notch
transmitters, control valves, motors, proximity sensors, encoders
Network Support
and similar monitoring and control devices devices — to control systems,
How Machine Builder
device device and fieldbus fieldb us netwo n etworks rks are often the simplest and cheapest
Combines Its Networks
way to go.
"Process "Process controllers, tempera t emperature ture controllers, small ac drives drives and similar equipment typically typ ically have built in RS-232/RS48 RS-232/RS485 5 ports," explains explains Paul Johnson, senior electrical electrical engineer at CMD Corp., Corp. , Appleton, Wis., which manufactures converting converting equipment for blown film (Figure (Figu re 1). "Since most most HMIs HMIs and PLCs have the same ports, they can support supp ort 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, cases, Ethernet doesn't meet appl ication ication requirements. The issues include includ e too high a bandwidth, static noise, and its inability to interface to legacy equipment." Carl Carl Henning, deputy direc d irector tor at PI North America, America, the th e associatio association n for f or both Profibus and Profinet, agrees. "It makes no sense to put an Ethernet connection in a proximity switch or any other simple sensor or actuator," he maintains. Ethernet can get complicated, too. "Ethernet typically calls for external hardware, hardware, architecture design and IT department involvement involvement that th at industrial indu strial control engineers want to avoid avoid if possible," says Bryan Sisler, product manager at AB ABB B Low Voltage Drives Drives.. Keeping Keeping the IT department out of the picture is easier with
Reasons to Use Device and Fieldbus
device device and fieldbus networks. networks. "Modbus can be handled h andled by by
Networks Instead of Ethernet
currently embedded protocols, no additional hardware is
Deterministic
required, and speeds are adequate to handle h andle most system requirements," Sisler says. "CANopen is simple to connect and use, is really open, PC-friendly, and great for low
Cheaper Less wiring required and easier installation Ease of calibration of instruments
node-count systems. DeviceNet provides determinism,
Access to more data — measurement measurement
plug-and-play plug -and-play system components, and a hardy transceiver transceiver
as well as diagnostic
that can survive survive transients that th at Ethernet transceive transceivers rs cannot."
Ease of troubleshooting
These, adds Sisler, are all reasons to avoid the expense, time
Less sensitive to electrical noise
consumption and complications brought on by more sophisticated sophisticated networks, "especially "especially those that might require
More physically rugged connectors and components Easier to transmit power over network
the involvement involvement and training of a new group of
Can span longer distances without
non-engineering folks."
repeaters or switches
Mallard Creek Polymers, Polymers , Charlotte, N.C., a maker of latex products, also uses u ses Modbus. Modbus. "Device "Device networks networks tend to be
Reasons to Use Ethernet-based Networks Faster
preferred preferred in many cases due to their broad install ation ation base b ase,,
Same network handles IT and
determinism and familiarity," says engineer Matt Bothe, who
automation
explains that MCP uses Modbus to communicate with several
More topology options
smart, mass flowmeters in order to extract flow and totalized
Wide range of available network
measurements.
analysis tools Easier to expand
"Device networks generally possess advantages over
Widespread availability of technical
analog/discrete systems, namely far less wire, ease of
support personnel
calibration, access to more data, ease of troubleshooting, and ease ease of installation," Bothe adds.
Universal connectivity to controllers, I/O and other components Can simultaneously transmit multiple protocols
Determinism Determinism and Distance
Superior and rapidly improving price/performance ratio Easier to integrate with multiple wireless networks
One difference between between dev d evice ice and fieldbus fieldbu s networks and Ethernet is determi d eterminism. nism. Proponents argue that device and and fieldbus f ieldbus netwo n etworks rks guarantee a response response time, while Ethernet can't, and this can be vitally important in certain certain applications. app lications. "For position-dependent operations (Figure 2), determi d eterministic nistic behavior behavior is a must," states Nick Hunt, manager of automotive automotive technology and support at robot builder build er AB ABB B Robotics Robotics.. "For example, when a dispenser is told to t o turn on exactly exactly one second before b efore the robot arrives arrives on point, it needs to be be extremely extremely consistent. consistent. We can even even liv l ive e with the fact that one second actually actuall y turns turn s out to be 1.32 seconds. If it's consistent, it easily can be tuned in by including the latency in the robot controller's I/O config file."
Figure 1 For performance perf ormance and and cost efficiencies, efficiencies, t he CMD Global Drawtape Drawt ape System and High-Speed High-Speed Bag Winder makes use of several networks including Ethernet, DeviceNet and Modbus. Source: CMD Corp.
And consistency consistency seems to be the key key. Ethernet, as fast as it is, isn't deterministic, deterministic, so it can't provide provide the consistency needed for factory automation automation when timing t iming is critical. "Speed is part p art of it, b ut speed sp eed really doesn't address the determinism issue, only the packet transfer rate," Hunt notes.
"Determinism "Determinism is addressed addressed by the way collisions collisions are handled, which they're not with Ethernet." Hunt goes on to say the raw speed of Ethernet can overcome most of the determinism concerns, but not all. Another issue issue is distance. distance. Wired Ethernet is limited to about 100 100 m, but dev d evice ice and fieldbus network distances can be much longer. "We worked worked in a gypsum board mill during a process speed speed increase, and were increasing increasing the th e number of drives drives in the system from 40 to 70," relates Brian Radichel, bu siness development development manager at Purvis Industries, Industries , Dallas, Texas, a manufacturer of material handling and mechanical systems, of his experience at a prior company. "The backbone was Profibus-DP, and in a gypsum plant the distances between drive nodes and and CPUs can can be b e long. The drives drives were clustered in many cases, but one application required distances of 200-300 ft between each drive. Profibus easily handled the increased devices devices and distances d istances with with minimal network changes. The installation was slightly slightl y more complicated complicated due to th e daisy-chain daisy-chain topology of Profibus, but b ut once the netwo n etwork rk was set set up and addresse add ressed, d, the commissioning commissioning proceeded proceeded smoothly." SEE ALSO: 1 Gigabit Industrial Ethernet Field Network Delivers Determinism and Accommodates Accomm odates 10/100/1000 Mb TCP/IP T CP/IP Field Device Devices s
Getting Ethernet to cover the same distance with fiberoptic cable or routers can get expensive, particularly in a daisy-chain configuration. "In outdoor, outdoor, long-lead-length long -lead-length applications, the cost of running fiber for Ethernet Eth ernet makes Profibus-DP Profibus-DP or other h igh-speed serial networks attractive attractive for both performance performance and cost of installation," he adds. Device networks have been around for a long time, and engineers have found ways to use their advantages advantages to solve solve problems that Ethernet Eth ernet can't. Johnson had just such a problem with netwo n etworking rking on CMD's CMD's machines. "We have applications that require devices devices to be connected through slip rings," ring s," he explains. "10 MHz/100 MHz Ethernet doesn't transmit very well through slip ring assemblies. We had to use a low-speed DeviceNet running at 56 kHz to avoid the problem." Fieldbus in Process Automation
Fieldbus networks are widely used in the process industries and offer several advantages over Ethernet. Larry O'Brien, global marketing marketing manager at Fieldbus Foundation, Foundation , doesn't see Ethernet encroaching encroaching on process automation automation networks networks at the fieldbus fieldb us lev l evel. el. "Process field d evices evices require power, and Foundation H1 provides digital communications and power over standard twisted-pair wiring," wiring," O'Brien says. "Ethernet does support power over Ethernet (PoE), Ethernet (PoE), but the primary use of PoE is for phones, panels, access points and cameras — not field instrumentation."
Process Process industries also have other requirements requirements that are not met by simply implementing Ethernet at the physica ph ysicall layer l ayer,, such as operation in hazardous areas . "Foundation H1 is a two-wire, twisted-pair field lev l evel el network that can b e installed safely in a hazardous area. area. It uses u ses simple screw terminations, which are comfortable to a device installer." Diagnostics specific to process automation and control in the field are other key advantages of fieldbus networks. networks. "Foundation H1 provides provides sophisticated, diagnostic, data-ma d ata-management nagement capabilities, capabilities, and a block b lock structure that all ows end users to implement function blocks b locks in control valves valves or field devices devices for the purpose p urpose of implementing control in the field," f ield," O'Brien O 'Brien explains. explains. "There is evidence that control in the field has an 80% increase in meantime between failures compared to traditional DCS control. "A good example example of the value of control in the th e field can be found at the Shin-Etsu Shin-Etsu chemical chemical plant in the Netherlands. At this facility f acility,, the th e interface card card in the DCS failed, which meant communication from the DCS to the field devices devices was no longer taking place pl ace.. Instead of a plant shutdown, the th e control-in-the-field control-in-the-field functionality of Foundation Foun dation H1 enabled continued operation operation via direct direct communication among measurement devices and valves." The 100-m distance limitation of Ethernet-based systems is also a factor. With Ethernet, O'Brien says, "distances are significantly significantly shorter; it has no multi-drop mul ti-drop capabilities; and it's more susceptible to noise that can degrade overall network performance. You can use fiberoptic cable with Ethernet, but that's even more challenging to install."
While proponents talk of Ethernet's ability to support sup port wireless, wireless, the th e same capability capability is available available with WirelessHART. In fact, since hundreds of thousands of process instruments already have HART installed, connecting to them via WirelessHA WirelessHART RT is simpler than with Ethernet. Future Device Networks
ABB ABB Robotics' Robotics' Hunt says Ethernet is moving moving into automation at the device device level. level. "Ethernet-based protocols like EtherNet/IP and Profinet have been widely accepted as viable in robot end-of-arm tooling and PLC communications," communications," he h e points out. "The original concern with using Ethernet-based networks was networks was fear that the level level of determinism was was not suitable for communication between between the t he scanner and the th e device device nodes. The determinism debate still rages, b ut doesn't d oesn't appear to be cause for concern. Device networks, though respected and considered as robust, proven and deterministic, are being replaced by Ethernet." Henning seems to agree. "Ethernet will work its way further down into simpler and simpler devices," devices," he opines. "If devices devices are available available with industrial ind ustrial Ethernet connections, connections, use them. Industrial
Ethernets are faster, faster, hav h ave e greater bandwidth, unlimited node n ode counts, improved improved diagnostics, easier upward integration, can use u se standard wireless, wireless, and have have more topology options. We actually spend sp end some time in both our Profibus and Profinet one-day training classes on th e advantages advantages of Profinet over Profibus." SEE ALSO: Industry Ready to Adopt Wireless Ethernet
O'Brien also sees sees the value of Ethernet in industrial indu strial applications; that's why Foundation Found ation HSE, HSE, is based on high-speed high -speed Ethernet. "Running "Runn ing at 100 Mbit/s, Mbit/s, Foundation Foun dation HSE is designed for device, device, subsystem and enterprise integration," integration," he h e explains. explains. "It supports sup ports the entire range of fieldbus fieldbu s capabilities, capabilities, including includ ing standard function blocks bl ocks and device device descriptions, descriptions, as well as appl icationicationspecific, specific, flex fl exible ible function fun ction blocks for advanced process and discrete/hybrid/batch appl ications." ications." Joey Stubbs, North American representative of the EtherCAT Technology Group, Group , adds that, "Actually, there is no reason why a device network can't also be Ethernet-based for industrial applicatio appl ications. ns. Obvi Ob viously ously,, trying to replace a simple serial network on legacy devices wouldn't be attractive if this results in a complex network with many switches or routers, and sudd enly requires IT support. However However,, not n ot all Ethernet Figure 2
fieldbus systems are created equal. EtherCAT, for instance, is
Recent advancements in controllers,
appropriate for high-speed, high-precision high-precision devices, devices, and also
programming programming software sof tware and networking networking
for enabling simple devices to communicate at lower scan
technologies have made the operation of robots much simpler and intuitive. Systems like this paint drum palletizer make use of
rates — even with low-powered embedded controllers with less processing processing power. power. It's difficult to t o find controllers without
EtherNet/IP to increase flexibility, and are
on-board Ethernet ports, which is the only h ardware ardware
able to handle a wide variety of drum sizes
requirement for an EtherCAT master."
and pallet configurations. Source: ABB Robotics and Systems
"EtherCAT is the standard network for our control solution,
Automated
with approximately 99% of our controls and I/O devices having EtherCAT connections," says James Wood, director of
engineering engineering at Adv at Advanced anced Blending Solutions , Menominee, Menominee, Wis., a manufacturer of blending bl ending and extrusion systems. Mike Rasner, Advanced's president and CEO, says the biggest factor favoring EtherCAT was its easy connectivity to CANopen and networks such as DeviceNet, Profibus and EtherNet/IP via OPC. "We use bus couplers to tie different networks to the controller via EtherCAT, and this provides quick and seamless connectivity," Rasner eplains. "In our business, we need to interface to a variety of networks from our main controller, so connectivity is very important."
Similarly, XCS Systems, Systems, a system integrator in Worcestershire, Worcestershire, U.K., specializes in high-speed high -speed process control in th e packaging industry ind ustry.. Using Using EtherNet/IP EtherNet/IP, the th e company recently integrated more than 160 motors and drives into a new can conveyor conveyor line, simplifying equipment design, configuration and commissioning. In the past, XCS XCS Systems relied on a dedicated motion control network for motor and drive synchronization. Today, they rely on a single network infrastructure for discrete and motion control. "The EtherNet/IP EtherNet/IP network does all the interlocking between between machines, improving machine synchronization," says Paul Croad, systems integrator at XCS. "It allows engineers to program any machine from anywhere in in the th e line." Ethernet also can serve the needs of both automation automation and process control control at the same time. Gram Equipment,, Vojens, Denmark, a manufacturer of ice cream equipment, uses EtherNet/IP for discrete, Equipment motion, process and safety control, as well as HMI and enterprise-wide information management. Scott Bivens, director of electrical engineering at R.A Jones & Co., Co. , a packa p ackaging ging machinery OEM that was recently recently bought b ought by Coesia Group Group from Oystar Group, views views the future of industrial netwo n etworking rking by saying, "Everything is going Ethernet. We are migrating to Ethernet on p retty much everything everything because Ethernet is easier to design and commission, and speeds are faster."
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