Programmable Logic Controllers Transformed

Programmable logic controllers (PLCs), says Himanshu Shah, senior analyst at ARC Advisory Group (Pittsburgh, PA), “are continually improving to meet rising challenges in functionality, communication, size, software, implementation, and diagnostics. Demands for open standards, multi-control disciplines, modular architecture, and comprehensive automation solutions software are leading to an expanding role for PLCs, especially for high-end PLC applications. Interoperability is the driving force in creating the multi-disciplined controller. Consequently, a new role of the high-end PLC will shift toward the PAC—programmable automation controller—for plant and factory automation.”

“INTEL INSIDE”

PLC vendors are riding the curve in consumer electronics—smaller size, faster CPUs, more memory, higher screen resolutions, more capabilities. Some vendors are also riding the Intel curve, says Bill Black, controllers product manager for GE Fanuc Automation Americas, Inc. (www.gefanuc.com; Charlottesville, VA). Back in 2004, GE Fanuc introduced its PACSystems RX3i, which used Intel Celeron processors. A year and a half later, the latest PACSystems, the RX7i, use Intel’s 1.8-GHz Pentium M processors (with 65 MB of user memory). These processors run fast and generate less heat than is the norm. “I can get more horsepower in that controller and don’t have to fight the battle of getting heat out of the system,” says Black.

GE Fanuc also uses AMD 586 processors and, at its low-end controller line, Hitachi processors. Using the latter, GE Fanuc’s VersaMax Micro 64 PLC is about 9-in. wide and 2-in. square, and supports up to 64 I/O points (expandable to 176 I/O). It supports four 100-KHz high-speed counters (or one counter for precise motion positioning) and four 65-KHz pulse train/PWM outputs for high-speed motion applications. The unit has lots of communications options: serial (SNP, SNP Master, Serial Read/Write, RTU Slave and RTU Master, USB) and Ethernet (SRTP and Modbus TCP). This controller also supports 48 KB of user ladder logic programming and 32 KB of data registers. All this for about $550, plus about another $300 for the Ethernet board.

Data communications, of course, is a key capability, especially in the multi-network environment of industrial automation. The CPU 319-3 PN/DP, the latest and most powerful CPU from Siemens Energy and Automation, Inc. (www.siemens.com; Alpharetta, GA) for the Simatic-S7-300 controller family, comes with Profibus DP/MPI, Profibus DP, and Profinet ports right on the CPU. “This takes out the structural costs and complexity from adding third-party interface modules,” explains Raj Batra, vice president of Siemens Automotive Center of Competence. It also makes for a fast controller where data-intensive tasks and closed-loop control exist.

But even small PLCs require communications capabilities. “Smaller controller” doesn’t mean it has a slow scan time, explains Lee Lane, Logix marketing manager for Rockwell Automation (www.rockwellautomation.com and www.ab.com; Mayfield Heights, OH). “It means that it has less communications capabilities. Instead of a full spectrum of integrated communications, plus seamless bridging and routing, the smaller controller may be locked into one or two types of communications platforms.” The Rockwell Automation MicroLogix 1100, for instance, a small controller aimed at applications with up to about 100 I/O points, comes with embedded EtherNet/IP. It also lists for $550, has a text LCD display for basic controller and I/O status information, and allows users to modify programs without interrupting operations.

Other technological mergings are occurring. Instead of just plugging a block of motion control in a rack, Rockwell Automation has merged motion control with the PLC. This merger eliminates “handshaking” and other complexities that arise from implementing asynchronous production. The Rockwell Automation CompactLogix 1768-L43, for example, can control up to four axes of motion; support a variety of networks, including DeviceNet, ControlNet, and EtherNet/IP; and support removable CompactFlash memory that can be used for program and firmware storage (the controller comes with 2 MB of memory). The controller supports the 38 embedded motion instructions and preloaded motion parameters currently found in ControlLogix controllers. Naturally, CompactLogix uses Rockwell Automation RSLogix 5000 programming software, but note that Rockwell has built motion instructions into its ladder logic.

SAFETY FIRST

The four primary control disciplines have been drive, integrated motion, integrated process, and sequential control, explains Kevin Colloton, GuardLogix product manager for Rockwell Automation. Now there’s another one, he says: safety. Why is a “safety controller” important? More and more, the plant floor is being populated by safety-related components and equipment: light curtains, door switches, pressure sensors, motion detectors to monitor the movement of equipment and people, and so on. But instead of deploying two sets of control systems—one for machine control, the other for safety-related control—users are installing one box that can perform both standard machine control and safety control across a single safety-rated network. For Safety Integrity Level (SIL) 3 compliance within that single box, dual-processor operation is required. This way, if a processor goes down, the other can take over in an orderly fashion and perform an orderly shutdown.

The NE1A Safety Network Controller from Omron Electronics LLC (www.omron.com; Schaumburg, IL) also creates a safety/control network so that both safety and standard control devices can reside on an existing DeviceNet network. “The main advantage of this product on a larger network,” says Joe Rubino, Omron’s marketing manager, PLC Group & Automotive Planning, “is decentralized control that can greatly increase processing speed and reduce network communication time. By connecting time-critical devices, such as light curtains, directly to the controller, the remote input time plus communication time can be eliminated, virtually halving the response time. Also, multiple processors allow for faster execution time of programs, which also increases the reaction speed. If that’s not enough, remote device response times can be increased or decreased to free up bandwidth for more time-critical devices.”

From Rockwell, SIL 3 compliance comes in GuardLogix, introduced in February 2005 in limited quantities. Today, there are two GuardLogix processors—L61S and the L62S. The main difference between the two is memory capacity (2 MB and 4 MB, respectively), which determines how big an application each can control. Both come with EtherNet/IP. While GuardLogix controllers have two processors, from a control programming perspective, they look like they have a single processor. Programming GuardLogix, again, is through RSLogix 5000.

Generally, says Colloton, GuardLogix are roughly 150% the cost of a standard controller. “That’s not over the top; we’re not talking three or four times the cost.” Because GuardLogix handles both machine and safety control, its extra cost is quickly recouped by the savings in reduced programming and installation time, the elimination of duplicated controls systems, and increased machine production efficiency. One of Rockwell’s customers reports that it was able to install a new stamping line in five days rather than 14—a 65% reduction. 

NOW ON THE BIG SCREEN

Some new PLCs are mostly screen—with integrated control logic. For example, the NSJ series from Omron are fully deterministic controllers with integrated display and integrated DeviceNet capability. List price is from $3,200 to $4,575, depending on display screen size, resolution, and control features. Their sizes range from 7.7 in. x 5.7 in. (320 x 240 resolution) to 12.4 in. x 9.5 in. (800 x 600 resolution)—and no more than 3.1-in. deep. The NSJ include built-in Ethernet, USB, and serial programming ports and printer ports. They can support up to 32,000 points of DeviceNet I/O, and can hold 60K-steps program memory and 128K-words data memory. The controllers include CompactFlash memory slots for storing screen/controller data, read/write recipes, downloaded applications, and FTP transfers.

The NSJ, explains Rubino, lets users distribute industrial control, downsize control cabinet space, and eliminate those costs spent on wiring separate human/machine interface and logic controllers. While other control products provide these same capabilities in a single package, they do so by sharing the controller’s central processor. The NSJ has separate processors, so compute-intensive control tasks don’t affect the display, and vice versa.

DO YOU SPEAK OOP?

Then there are programming “challenges.” PLC users are looking for key things like reuse of software, modularity, and scalability, says Mike Richards, GE Fanuc’s programming software product manager. In object-oriented programming (OOP), user-defined function blocks are the main program building blocks. These blocks give users, continues Richards, the ability to create an object or a thing that models something in the process, such as a motor, a conveyor, or even a whole machine. This lets users move away from octal programming and from managing memory locations in the controller. Instead, they can use the entity’s name in the control program and in screen displays. For instance, if an I/O point is called “StartButton1” on the machine, it can be named “StartButton1” in the control program. Such programming code, adds Colloton, is pretty much autodocumenting.

These features are incorporated in the latest version of GE Fanuc’s Proficy Machine Edition software for programming, configuring, and diagnosing all GE Fanuc controllers. Proficy now features IEC 61131-3 Function Block Diagram (FBD) programming and symbolic IO variable configuration support for GE Fanuc PACSystems, a monitor mode to provide a look and do not touch interface to running applications, and Asian-language capabilities.

For hybrid control applications— applications with both discrete and process control—users can mix programming languages to best suit each portion of their application. For example, the control program can use relay ladder diagrams for the discrete control portions of the application, and use FBD for the process portions. In fact, users can mix ladder diagram, FBD, structured text, and C blocks for each portion of their applications. That’s a lot to pack in these PACs, but that is what makes them the next generation PLCs.