Distributed intelligence stands alone

Perhaps they envisioned, 30 years ago, the ability to truly distribute a system as low and as far out as possible. Today, microprocessor technology makes it possible to put monitoring, diagnostics and control logic all the way down to the sensor level. “As sensors go digital--and the transmitters that house them contain microprocessors--information, diagnostics and computational abilities become available at the device level,” says Bruce Jensen, manager of systems marketing at Yokogawa.

"Continuing the trend that began more than a decade ago, microprocessor-based intelligence is becoming increasingly more distributed throughout refineries, petrochemical plants, power plants, pulp and paper mills and other continuous process plants,” says Alex Johnson, system architect, Invensys Process Systems. “This has had, and will continue to have, a major impact on the way these process plants are engineered, operated and maintained. Intelligence will clearly keep moving into the field.”

“With today’s semiconductor capabilities, extreme miniaturization, and exponentially increasing processing power, the ability to embed intelligence or decision-making capabilities into smaller and smaller devices is not only an option, but a requirement,” adds Gricha Raether, industrial control and distributed I/O product manager at National Instruments. “There are currently hundreds of smart sensors on the market that feature built-in microprocessors. These sensors acquire the signal, convert it to a digital signal, then transmit it to a controller or central monitoring system through a fieldbus or industrial protocol.”

“In the case of fieldbus, distributed intelligence is really the name of the game,” says John Yingst, Experion product manager at Honeywell. “We presently have fieldbus devices that can do calibration, diagnostics, control, and then alarm when there is a process control or a device problem. Device warnings range from telling us they will soon need maintenance, to loss of an air supply, to overheating, and all the way to letting us know of complete sensor or actuator failure. Control algorithms complete with alarming running in fieldbus devices is commonly know in the fieldbus world as ‘control on the wire.’ Control functions include PID, totalization, signal characterization, signal splitting, input selection, and general-purpose math.”

Terry Krouth, vice president of PlantWeb Technology at Emerson Process Management, adds that fieldbus is designed to be inherently redundant, and to operate independent of a host. "With a Backup Link Active Scheduler (BLAS) in one of the devices on each segment, a fieldbus system can operate without connections to the main system," he explains. A Link Active Scheduler (LAS) controls all the communications in a segment, ensuring deterministic response within the segment and the system. "If the main LAS fails, the designated BLAS takes over."

Krouth agrees with Yingst that fieldbus is perfectly capable of running process loops. "In many cases, PID control in the field can operate without needing a host of any kind."

Smart devices put paid to all the traditional methods we’ve been using to configure control systems. Traditionally, we’ve wired field I/O to a termination rack, connected it to signal conditioners, fed it to multiplexers, and transmitted the collected data via a “home run” network to a central or local control system, which logged all the data, stored it in a process historian or database, ran it through assorted software processing routines, and put it up on an HMI’s display for operators to see.

FIGURE 2: DAQ AT THE SENSOR

This Beckhoff Bus Terminal at Shanghai Drainage, Shanghai, China, acquires eight channels of data in the field and sends it to the control system via DeviceNet, eliminating all the usual I/O cabling, terminations and enclosures. Source: Shanghai Drainage

With today’s smart sensors and I/O devices, much of that is unnecessary (See Figure 2). Individual devices can now connect via a fieldbus or Ethernet-based network, and eliminate much of that wiring, terminations, enclosures and boxes.

It’s even possible to bypass everything from the field wiring to the central HMI/SCADA system. Today’s sensors and control devices sometimes have their own web servers embedded, so they can jump on the Internet or a plant Intranet, and make their data and control settings available to any legitimate user with a standard Web browser.

 “Distributed intelligent control systems are giving engineers a new choice to control their projects,” says Terry Lenz, senior product support engineer at Wago. “It’s time to rethink how the large local/remote rack PLCs control I/O.”

Lenz points out that the traditional method for PLCs (and other control systems) was to connect I/O to remote racks and send the data to a central PLC via a proprietary network. “When distributed fieldbus networks were introduced, this let smaller nodes of I/O be placed closer to the devices or process, but it still involves scan time to gather the information to the main PLC, process the data, and reply to the I/O. Decentralized intelligent devices are like remote PLCs controlling the I/O locally. Although they are connected to the network they operate independent from the network scan times. Decentralized control reduces scanner load because it only passes data the main controller needs.”