Today's programmable logic controllers (PLCs) answer to many descriptions: smaller, smarter, faster, more flexible, expandable, scalable, lower cost, highly reliable.
Units ranging from simple to sophisticated address virtually any control need a manufacturer might have. Increasingly they are also being called on to serve as data-gathering devices, while at the same time, suppliers are expanding communications capabilities and improving migration paths.
As their capabilities increase, their availability also grows. Lower price points and compact sizes make it possible to install PLCs on machines and lines where it was previously impractical. In many cases, PLCs are replacing standard controls like relays. "PLC actuation means less point-to-point wiring," explains Tim Roberts, product manager at Schneider Electric (Palatine, IL).
Functionality is substantially broader, too, especially on lower-end models. For example, the CompactLogix platform from Rockwell Automation (Milwaukee, WI) now includes integrated motion control, a feature previously available only on the higher-level ControlLogix platform. Indeed, high-end PLCs like the ControlLogix have become so sophisticated, ARC and some vendors describe them as "programmable automation controllers" (PACs) rather than PLCs.
Many top-end PLCs are multidisciplinary. Rockwell combines control, sequential, process, motion, drive, and safety in its flagship controller, the ControlLogix. "One platform gives users a single common development environment," explains Mike Miclot, director of Rockwell's Commercial Programs. The single platform also improves asset management, since spares and training are the same for each functionality.
In addition, integrated safety control permits warm shutdowns while still protecting the operator. Eliminating the cold shutdowns required by all-mechanical, electromechanical, and standalone safety controllers reduces downtime and improves productivity.
In 2004, GE Fanuc introduced the PAC RX3i controller, based on Intel's Celeron and M-class processors. It supports multiple programming languages and possesses 10 MB of memory, allowing documentation, diagnostics, and other information to reside in the controller. An RX3i Universal Analog Input module (configurable on a per channel basis for voltage, current, RTD, Thermocouple, strain gage, or resistive) delivers the functionality of four modules in one, reducing input/output (I/O) module space requirements, decreasing the number of spare parts and inventory, and simplifying wiring and configuration.
A newer model, the PAC RX7i, is even more powerful, boasting 65 MB of user memory and a high-speed 1.8 GHz processor. It's designed for applications like paint booths, where hundreds of recipes need to be stored in the controller. "Certain applications want to store as much process information as possible at the controller level rather than relying on the network server to have all the data," explains Bill Black, controllers product manager at GE Fanuc Automation Americas, Inc. (Charlottesville, VA).
Despite the increased capabilities, some features haven't changed: PLCs continue to rely primarily on ladder logic programming, which is familiar to plant personnel. And PLCs remain one of the most reliable components on the factory floor. In fact, today's units are more reliable than ever due to the replacement of hardware with software and improved quality in the PC board supply chain. Failure rates of less than 0.5% are not uncommon, meaning that time before failure numbers can reach into the tens of thousands of hours.
With PLCs gaining functionality expanding, the worldwide market will grow 5.9% per year, rising from $7 billion in 2004 to $9 billion in 2009, according to a report from ARC Advisory Group titled "Globalization Drives PLC Market Growth."
"Besides automation, applications for PLCs are also driven by crucial factors such as energy saving, condition monitoring, safety, collaborative manufacturing, and real-time optimization strategies," writes Himanshu Shah, a senior analyst with ARC and principal author of the study.
Beyond Control
Their continuing evolution has driven PLCs beyond traditional control functions and into data gathering. "Users are figuring out the genesis of data is on the factory floor," Miclot explains.
"Manufacturers are looking for statistical information such as yield, total quantity produced, and reject count," notes Richard Hutton, automation marketing product manager at Schneider Electric. "That information is already present in the PLC and now needs to be accessed at multiple levels up to the enterprise." The goal is for data to flow seamlessly from the lowest sensor to the Web dashboard on the chief executive officer's desk if need be. As a result, vendors are focusing on facilitating the passage of data from the PLC up to the manufacturing-execution and enterprise levels.
To that end, systems are being designed with intelligent modules. For example, the FactoryCast human-machine interface (HMI) module from Schneider Electric can capture information and organize it in a format that can be accessed from anywhere through a Web-based browser. This is accomplished by integrating a ruggedized processor or coprocessor into the PLC rack and equipping it with intelligent software to obtain and configure data from the PLC. Data can reside in the HMI or be sent to a standardized database via a standardized network like Ethernet.
Input/outputs (I/Os) are multiplying, adding intelligence, and, increasingly, are being distributed closer to the application to be controlled and to reduce point-to-point wiring. Some sensors being added on the manufacturing floor link directly to the PLC, which captures the data and transmits it to a plant-wide network for analysis. In fact, some newer sensors like the GHSE/GHSER 750 Series LVDT position sensors from Macro Sensors (Pennsauken, NJ) are powered by the PLC. Constructed of stainless steel, these rugged, hermetically sealed, three-quarter-inch sensors feature radially mounted connectors to shorten installed length and enable usage in restricted spaces. Applications include gauging systems, statistical process control data collection, and electronic dial indicators.
PLCs can provide data for diagnostics as well as for production. In fact, some PLCs like the PAC RX3i from GE Fanuc incorporate multi-channel configurable alarms that eliminate the need to write a ladder logic program to provide a warning if parameters are exceeded.
To make use of the data stored in the PLC, communication with the network takes on new importance. As a result, there is a trend toward Ethernet-enabled devices. In fact, a survey by GE Fanuc found that 85% of its customers selected Ethernet as their first choice. "The openness of Ethernet is very appealing," GE Fanuc's Bill Black says. Not only is it easy to connect, but it eliminates the need for the expensive cabling and connectors required by other buses. "All it takes is an RJ45 connector and cable," Black explains.
To address the preference for Ethernet, GE Fanuc introduced its VersaMax Micro 64 PLC in 2005 with an optional plug-in Ethernet board. This design makes it possible to buy an Ethernet-enabled PLC for less than $1,000 -- about $550 for the PLC plus $200-$300 for the Ethernet board. The VersaMax Micro 64 supports 48k of user ladder logic programming and 32k of data registers. The memory allows the system to solve complex applications requiring storage of multiple programs and large volumes of data and enables faster set-up/changeover or system monitoring. The data also can be written to an internal Flash Memory Module to eliminate the need for a battery.
Schneider Electric's Telemecanique Twido nano controller, the first nano PLC with embedded Ethernet capability, features high-density analog I/O expansion modules for measurement and regulation. New wireless capabilities, based on a Bluetooth gateway, simplify diagnostics and troubleshooting and permit remote monitoring and control with wire-free connections of up to 30 feet. Updated software simplifies controller programming by allowing users to modify program instructions in instruction list or ladder logic while the controller is in run mode. Programming is done via a laptop with TwidoSoft programming software or with a pocket PC equipped with a new TwidoAdjust tool.
Another PLC using Ethernet as the standard interface (along with PROFIBUS Mater and RS232) is the IndraControl L40 controller from Bosch Rexroth Corp. (Hoffman Estates, IL). The unit also can be equipped with expansion modules to add other communications interfaces. Additional features include an integrated display for extensive diagnostic and status messages and an easily accessible CompactFlash module for program and data storage. A built-in power supply minimizes installation space.
To connect legacy Modbus serial devices to Ethernet networks, Schneider Electric offers a FactoryCast gateway. The new TSX ETG1000 supports all Telemecanique and Modicon PLCs and devices and eliminates the need for serial communication interfaces on third-party devices, as well as expensive PC gateways. The FactoryCast gateway also enables e-mail messages and alarm notifications from standalone applications. An embedded Web server provides simple Web browser access to machine maintenance information, enabling OEMs to offer remote diagnostics and repair services for their machines.
Increased reliance on software makes today's PLCs more configurable and easier to program and install. Many of today's systems rely on point-and-click functionality to eliminate the need for preprogramming. "You simply name a tag once and it is used pervasively at all levels," Miclot says.
Unity Pro software from Schneider offers a library of function blocks so instructions don't have to be created from scratch. Unity Pro also uses open-interface XML to simplify integration. "New, more open communications improve the connection to legacy products and competitor products," says Roberts of Schneider Electric.
Virtually all PLC vendors offer a family of controllers to address a range of needs. "A lot of customers start out with the low end," Black says. "Once they get comfortable, they move to high-end units," he adds.
In the past, this migration would require a manufacturer to remove existing PLCs and start over, a lengthy, expensive proposition since hardware accounts for about 85% of the cost. To remedy this and also provide scalability, many of today's PLC families rely on a common architecture. As a result, the user experience is the same irrespective of the network, drive, amplifier, or power range used. For example, GE Fanuc ensures an easy transition to its PAC RX3i and PAC RX7i by designing the units to accept I/O and logic for earlier models. Most application code can be converted too.
New PLCs from Rockwell also incorporate migration elements so customers with older units can augment their systems without starting over. "We design from a clean sheet of paper, but don't make our customers do that," Miclot notes.
Tomorrow's PLC
Functionality will continue to expand, particularly for low-end controllers. Units will be increasingly integrated so applications like discrete, process control, motion, operator interface, and Web-based technology reside on the same platform. There also will be a lot of innovation in software development. "Software allows more configuration," Roberts says. "It you can configure the unit and eliminate a program that has to be written, the PLC can be applied more quickly," he explains.
Future PLCs also will be more robust to withstand harsh conditions like washdown with high-pressure water/steam. Schneider, for example, plans to introduce an IP67-rated PLC this year. The IP67-rated housing eliminates the need for special shielding, permits the PLC to be positioned closer to I/Os, and reduces point-to-point wiring.
As the level of data residing in the PLC grows, so too does interest in improving battery backup systems to prevent data loss during power outages or shutdown periods. Finally, use of Ethernet communication will continue to grow and, eventually, will yield to wireless communication.