Electronic Design

PACs Become Task Masters

Faster speeds and greater memory make it easier to integrate many functions in one system using programmable automation controllers.

As global pressures squeeze manufacturers, many feel the urgency to optimize equipment in their plants. Existing controls are being tossed out and replaced with modern programmable automation controllers (PACs) that deliver more processing capabilities and more network connectivity.

PACs have been gaining acceptance for years, giving engineers a single platform to address a broad range of needs. These platforms include software, which eradicates the need for a range of tools to do programming. PACs also leverage technologies developed for the PC world, assuring industrial users they’ll see steady advances in performance and compatibility with front office equipment, among other benefits.

The versatility of PACs has prompted many plant managers to replace their programmable logic controllers (PLCs). However, this changeover isn’t expected to mean the end of the line for PLCs. Product developers and plant managers are clearly focusing on PACs, but PLCs are also adopting faster processors and adding networking capabilities. They’re now seeing use in jobs that don’t change often and don’t require lots of computing power.

“There are still a lot of applications that don’t need the flexibility of a PAC. A PLC is ideal for customers who can use ladder logic for simple discrete control,” says Bill Black, controllers product manager at GE Fanuc.

In some instances, PLCs have become one of the many modules that fit into PAC architectures. For example, Mitsubishi Electric Automation’s iQ automation architecture blends a PLC, motion control, computer numerical control (CNC), and robotic control in a single platform (Fig. 1). Many of the modules that work in this architecture are small, which is helping developers create complex systems without taking up a lot of space.

“From a physical standpoint, it has a very small footprint. The CPU measures only 4 by 1 inches and it’s 4 inches deep,” explains Scott Rohlfs, director of product marketing for Mitsubishi.

By shrinking size while increasing performance, equipment and system designers can continue to combine operations that were once the domain of dedicated controllers. As these systems boost performance and handle more jobs, the control modules also manage more complex tasks.

Five-axis control was an advanced feature a few years ago, but now it’s becoming fairly routine for PACs to manage more motion. Rockwell Automation’s recently introduced Allen- Bradley CompactLogix L45 PAC expands motion capability to handle eight-axis applications (Fig. 2).

THE ROLE OF BACKPLANES
The controllers are only part of the equation. Backplanes play a critical role, linking the modules so they act as a single system. These backplanes also employ ruggedized PC architectures for industrial environments. They offer enough bandwidth to make motion control just another element in a closely coupled system.

“Motion controllers and PACs are being tightly integrated using high-speed backplanes that assure the best of both worlds. You don’t have to do a lot of handshaking, which is good because motion is very compute-intensive,” says Paul Derstine, motion products manager at GE Fanuc (Fig. 3).

Communication between the cards in these backplanes makes it possible to run complex motion tasks while other operations are running. The basic PCI architecture used by many systems routinely moves data at 27 MHz, which is fast enough for many operations, says Derstine.

Users who want more speed can move to the ruggedized PXI and PXI Express. They give users a ruggedized bus that’s compatible with PCI. PXI Express has bandwidth up to 2 Gbytes/s per slot along with timing and synchronization capabilities. National Instruments heavily endorses PXI Express, rolling out a number of products based on the architecture.

These releases include the CompactRIO and CompactFieldPoint families, which extensively employ FPGAs. These products add even greater versatility, allowing engineers to alter hardware when operations change. This hardware programmability can help plant managers configure systems that meet their specific needs without the additional cost of customized equipment.

FPGAs were once avoided because of the difficulties in programming and reconfiguring them. But the advent of graphical programming techniques has helped eliminate those problems. Thus, plant managers can more easily exploit the flexibility provided by programmable logic devices.

“Users don’t have to learn the abstract commands usually needed to program an FPGA. They can use LabVIEW and control their real-time systems,” says Arun Veeramani, product manager at National Instruments.

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Another factor helping boost the PAC’s task load is the large volume of memory that now fits on a single card. The Allen- Bradley ControlLogix L65 from Rockwell was upgraded to 32 Mbytes. This helps in information-intensive applications like batch processing, where added capacity provides more capability for recipe management.

Easily accessible memory can bring a number of benefits for plant managers. Some aren’t dramatic technical capabilities, but they’re important nonetheless. PACs can store documentation on each piece of equipment so operators can quickly access instructions for tasks they aren’t familiar with.

Similarly, maintenance technicians can quickly access information they need. Storing these files directly on the system assures managers that personnel are using up-to-date documentation, reducing problems when misplacing books or disks.

Though PACs handle many complex tasks, important facets of control still run on other controllers. Most PACs currently don’t have human-machine interface (HMI) capabilities, which typically run on dedicated processors. Many contain proprietary interfaces, though the industry seems to be migrating to commercial operating systems such as Windows CE.

That’s likely to change, bringing tighter integration and eliminating the physical space required by separate controllers while reducing the memory space required for a dedicated operating system. “As we go to dual-core and multicore processors, it’s possible that a processor can handle the HMI and conventional PAC tasks,” says Derstine.

NETWORKED WORLD
Another key benefit of modern controllers is their connectivity. The drive to eliminate duplication of effort has made Ethernet the standard throughout most corporations. Extending Ethernet to the factory floor makes it simpler to link equipment in the plant to front office computing. That means inventory data and orders can use the same data files, wiping away the problems that may occur when data must be re-entered for different architectures.

Industrial managers, as a result, can reap many benefits. They no longer have to deal with a multitude of networks, and the compatibility with TCP/IP makes it possible to tap into any piece of equipment regardless of where it’s located throughout the global enterprise. “With PACs, you can bring up a browser and see what’s going on from your home or a remote office. You can’t do that with a PLC,” says NI’s Veeramani.

This freedom creates further challenges for the engineers who design industrial controllers. Ethernet connectivity is a must, but several technologies still use a number of field buses, including DeviceNet, Profibus, AS-Interface, and Interbus. Moreover, realtime Ethernet alternatives require some minor tweaking in addition to conventional Ethernet installation efforts.

Product developers are doing everything they can to let controllers communicate with many of these networks. “One requirement for a PAC is openness, being able to talk to any bus, any industrial network,” says Veeramani. And, companies continue to extend their offerings in this area.

Opto 22’s SNAP I/O Systems now communicate via Allen Bradley/Rockwell Automation’s Ethernet/IP (Fig. 4). Functions like high-speed counting and latching, quadrature inputs, pulsing, thermocouple linearization, and proportional-integral-derivative (PID) loop control can be distributed to the I/O level so the central controller needn’t address these time-consuming tasks. The Opto 22 PAC can serve as a slave or adapter, sharing data with Ethernet/IP-enabled devices.

Nearly all PACs require this type of connectivity. The Bosch Rexroth IndraControl L10 is a low-end board that’s designed to be compact, but it still includes Ethernet connectivity and talks to many field buses (Fig. 5). “With our PLCs, you can talk to any network. It’s relatively transparent to the user which network they’re using,” explains Ted Thayer, PLC product manager at Bosch Rexroth.

Controllers are also taking advantage of increases in Ethernet bandwidth. CC-Link leverages the Gigabit Ethernet standard that pushes bandwidth up by a factor of 10. Mitsubishi is a primary proponent, and others are expected to employ the latest commercial version of Ethernet in industrial environments. Until that happens, plant managers are using real-time versions of Ethernet to gain determinism. Engineers exploit the enhanced capabilities to add more nodes to their networks and manage more tasks.

“Users are improving their systems by designing larger network segments, simplifying maintenance and programming on the total architecture. They can also implement multifunctional networks by running different applications like I/O control, video, and HTTP on the same wire,” says Scott Tenorio, Logix product manager at Rockwell

TAGS: Robotics
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