This trend will have to solve another problem first. Power is a big issue. “Wireless device-level network and sensors have yet to take off due to limitations of battery technology and lack of standardization,” says Jonas Berge, marketing manager at Smar
. “Unlike wireless consumer devices such as mobile phones or PDAs, sensors can’t have their batteries charged or changed every week. Conserving battery power is therefore very important.” Because of the power problem, wireless sensors often transmit less often than wired sensors, and at slower speeds. “As a result, wireless sensors are not suitable for control or functional safety,” says Berge.
Slow response also happens when too much wireless traffic occurs nearby, so the sensor has to wait for the frequency to clear to transmit. No one wants to base a real-time control decision on inputs from a wireless sensor. For the immediate future, it appears that wireless will be limited to monitoring. (See this article’s sidebar for a solution to both problems.FIELDBUS FORGES AHEAD
The October 2005 issue of CONTROL includes an account of a mammoth Foundation fieldbus project in SECCO’s ethylene cracker complex in China. The DeltaV system from Emerson Process Management involves 10,000 fieldbus devices in 10 plants with 48,000 loops and 166,000 I/O tags. All PID control is performed in fieldbus instrumentation, and overall control and supervision comes from a central control room, where workstations run asset management, loop tuning and maintenance diagnostic software. The installation demonstrates the control capabilities of fieldbus, its ability to handle giant systems, and shows that it actually works.
Meanwhile, standards organizations are working hard to make fieldbus easier to use. “Of greatest interest are Electronic Device Description Language (EDDL) extensions,” says John Yingst, fieldbus systems product manager at Honeywell. “This enhancement will provide a common means for device vendors to specify and interact with complex data such as valve signatures.” EDDL has the support of the Fieldbus Foundation, HART, OPC and Profibus organizations, and EDDL is expected to make it easier for vendors and end users to implement all kinds of fieldbus instrumentation.
Fieldbus systems have been way ahead of the current trend toward remote diagnostics. “On-line diagnostics available with Profibus include a status byte incorporated in the transmission of process variable data,” explains Fabian Monino, senior applications engineer at Siemens Energy & Automation
. “This permits a control system to evaluate data and perform alarming if necessary.”
Fieldbus also can be used in hazardous areas, but it is not considered a “safety system,” says Yingst. “Significant changes would be required to adapt the FF protocol to the requirements of a safety standard. There is an ongoing effort to define a workable Safety Instrumented Systems (SIS) specification.” Berge agrees. “There are plenty of safety buses for the machinery/factory automation world, but nothing for process,” he notes. “FF-SIS is under development and already has TÜV concept approval.”
In addition, Pilz
has successfully integrated its PSS safety network with EtherNet/IP, ControlNet, Profibus and Interbus, and it developed a safe fieldbus technology of its own, called SafetyBus p. Lee Burk, training and consulting manager at Pilz, says the Ethernet protocol can be used, but Pilz believes a separate fieldbus should be implemented for communication of safety related signals.
"Supported by the Fieldbus Foundation, HART, OPC and Profibus organizations, EDDL is expected to make it easier to implement all kinds of fieldbus instrumentation."
One problem with fieldbus in hazardous areas is the power requirement. “At the introduction stage of fieldbus technology, cabinet-mounted barriers and power supplies were the standard solution,” explains Bernd Schuessler, business development manager at Pepperl+Fuchs
. “This solution barely supplied enough power for three or four instruments.” Users complained, so standards organizations and manufacturers developed the FISCO (Fieldbus Intrinsically Safe Concept) and FNICO (Fieldbus Nonincendive Concept), which can power up to eight devices on a segment.
P+F also developed the High-Power Trunk concept, which allows up to 16 instruments in a segment. “This allows end users to get the maximum number of devices on a segment, while also being able to achieve maximum cable lengths,” says Schuessler.
Foundation fieldbus and Profibus-PA are the two most successful fieldbuses in process control, while Profibus-DP and DeviceNet appear to be the most successful in discrete automation.
“We're seeing a very strong trend towards the use of Ethernet as a fieldbus,” says Graham Harris, president, Beckhoff Automation
. “This is reflected in current I/O sales and in the emergence of technical organizations such as the EtherCat Technology Group
. Over time, we'll see that the most successful Ethernet fieldbus technologies will be truly real-time, deterministic, offer a variety of wiring topologies, and push minimum cycle times below the millisecond mark. In addition, the cost of implementing these fieldbuses will continue to go down when standard cabling is used and off-the-shelf Ethernet cards continue to replace unnecessarily expensive fieldbus cards.”
Sergei Furduy, a control engineer at Concept Systems
(), a system integrator in Albany, Ore., sees the same trend unfolding. “We still use DeviceNet in some applications, but more often we see DeviceNet being replaced by Ethernet,” he says. “One reason for this is that Ethernet interfaces typically cost less to install than DeviceNet.”