“Not all field-based intelligence will revolve around PID loops and not all intelligence will utilize the same communications mechanisms,” notes Johnson. “Foundation fieldbus is just one of the many technology enablers behind this trend toward distributed intelligence. Others include industrial Ethernet, OPC, Profibus, HART, Modbus and, increasingly, the emerging wireless communications standards. Each has strengths and weaknesses for different distributed intelligence applications, with the trade off often being cost vs. security. For example, wires are secure but they have a very high cost and, in some implementations, very low bandwidth."
Every month, we publish new products that feature fieldbus, internet or web connections. As an example, last month (August 2005) we ran an item about Red Lion
’s new Master Controller, which lets you connect 32 PID loops as a single node on Ethernet, view and control loops via a web server and a virtual HMI, or link to the device via normal interfaces, GSM wireless devices and a cellular phone.
None of this is particularly expensive, either. Advantech
’s XScale communication controller has a web server, FTP server and TelNet server, plus RS232, RS485, 10/100BaseT Ethernet and Modbus interfaces, and sells for $595. Both Wago and Beckhoff offer fieldbus- and Ethernet-compatible controllers starting in the $150-$405 range (See Figure 3).
FIGURE 3: INEXPENSIVE CONTROL
Distributed fieldbus I/O and controls, such as Wago’s PFC (programmable fieldbus controller), installed in a pharmaceutical plant, start at about $405. Source: Wago
This means that you can put monitoring and control all the way down to the field devices, virtually eliminating much of the expensive, traditional field wiring. But how far out can you go?Far Out, Tovarisch
Transneft operates the world’s largest oil pipeline, encompassing more than 30,000 miles of oil pipe that transports 4.2 billion tons of oil per year across Russia, from Siberia to the Baltic (See Figure 4 below).
The Trans-Russian oil pipeline has 350 pumping stations and 850 holding tanks supplying 35 refineries. Iconics
Genesis 32 software runs the entire system, including 600,000 tags. It is claimed to be the world’s largest PC-based SCADA and dispatch system, although Citect
claims one in the same size range in Australia.
As an example of how far out you can go with distributed control, the system has 2,500 PLCs and 500 networked PCs spread out over those 30,000 miles, all linked by 20 satellite data links, microwave and land telephone lines using standard TCP/IP protocol. The 1,500 operator screens have an average response time of 3-5 sec.
Pipelines, offshore platforms and water/wastewater facilities are the most obvious examples of process applications that have widely dispersed control and monitoring equipment, but many other plants are also going distributed, says Todd Stauffer, manager of product marketing at Siemens Energy & Automation
. “We are seeing more customers migrate from the centralized architecture of the classical DCS, where all controllers, I/O and field wiring terminations are in a central location, to where the DCS equipment is distributed throughout the facility in remote locations that are chosen to maximize maintainability and minimize installation cost,” he says.
Modern wireless communications technology makes such distribution possible, because it eliminates field wiring altogether. “Myriad new technologies are being developed to increase distributed communications,” says Raether. “GPS and GPRS have been around for years, and there are more and more devices that can talk to and listen through these networks. The latest to join the list are WiFi networks such as 802.11b, a, g, and even n. Even more popular are longer-distance networks such as WiMax and xMax. There are also more and more devices that include the ability to communicate through these networks, and they can be installed anywhere electromagnetic waves can reach.”
Plugging devices into the Internet or a virtual private network over the Internet makes it possible to reach anywhere on the globe that the Internet goes.
Some plants use a combination of communications. Wates GmbH in Meschede, Germany, is a consulting engineering company that specializes in water and wastewater; it designed a control system for the Bruck Sewage Works in Bruck, Germany. “We used the full range of access options, including dedicated lines, dial-up and GSM,” says Henry Sanders, managing director of Wates.
“Making communications technology part of an embedded process control system is fairly easy to accomplish given the processing power available in such devices,” adds Raether. “One example of such a device is the recently-released module which S.E.A. Datentechnik GmbH
in Germany released. This module enables the NI CompactRIO embedded control system to communicate via GPRS, GPS, and radio clock frequencies.”FIGURE 4: AS DISTRIBUTED AS POSSIBLE