Narrowing the PC, PLC Gap

Two Camps Continue to Compete for Business, but Draw Closer to the Same Architecture

By Rich Merritt

Once again, as part of our anniversary retrospective of content we created during the past 15 years, we offer up, as it originally appeared in February 2001, a discussion of the role of PC-based control in discrete manufacturing and its impact on applications that once were the sole domain of PLCs. It was becoming clearer that PCs might not take over the industrial world after all, but the landscape was undergoing permanent change. Note the number of companies that no longer exist as standalone entitities.

The subject comes up every year, and for good reason. Choosing whether to use a PC or a PLC as the heart of a machine or system controller can make or break an OEM. Software and technology change regularly — sometimes daily — so OEMs try to keep up as best they can. Someday a killer app will come along to tilt the scales in one direction or another, they fear, and nobody wants to miss it.

Over the past year or so, we've seen three major trends developing that will impact the PC vs. PLC decision for OEMs: communications, web-enabling, and hybrid PLCs, some of which are becoming PCs.

PLCs Becoming PCs
Aside from the technical arguments, marketing issues may have an impact on the decision making. Semicore Equipment makes vacuum deposition equipment (Figure 1), which it sells to sophisticated, high-tech customers such as Nortel and Lucent. "We used PLCs for 17 years," says Trey Haight, automation engineer. "We switched over to pure PCs for a few years, and now we've returned to the new PLC hybrids."

Semicore president Matt Hughes says vacuum deposition systems have been around for 40 years and the players in the industry are well-known. "What sets us apart from our competitors is that we are using the latest automation equipment, and this is important to our customers," he says. "In fact, we recently sold a system to Nortel, and they came back and asked us if we could install our controls on two of their older machines."

The need to support Ethernet and web communications functions is driving many PLC vendors to add PC-like features to stay competitive. "Information requirements from the plant floor are pushing a PC mindset into the controls arena," says Diane Trentini, manager of enterprise solutions at Optimation Technology, a system integrator.

As a result, some PLCs are actually becoming PCs. This began when PLC manufacturers offered coprocessors and plug-in modules to compete with PCs, but now the functions are being built in. "Most PLC manufacturers are introducing products with more low-cost, PC-like connectivity capabilities and open features integrated into the CPU, such as an Ethernet port," says Jim Allison, PC controls product manager at AutomationDirect. "PLC systems are becoming network-aware."

"Some PLC companies are offering slot PCs and soft PLCs to address customer demand for high-level PC features," adds Mark Smith, business development manager, CTC Parker Automation.

Patrick Rousselot, product director at control system developer AlterSys, says, "PLC manufacturers try to maintain their reliability advantage [over PCs] by integrating new technologies directly into their hardware, and the result is a PC in a PLC box."

AutomationDirect was one of the first to try this with its WinPLC product, a low-end PLC with the Windows CE operating system, Think & Do software, and HMI capability. Introduced in April 1999, it is a hybrid of PLC and PC technology.

At the upper end are the Rockwell Automation line of Logix PLC/PC hybrid controllers, WinAC soft PLCs from Siemens E&A, ONC from Omron Electronics, MachineLogic from CTC Parker Automation, and similar systems that blend PLC-like hardware with PC processors and software.

David Gee, vice president of Steeplechase Software, says this is a continuing trend. "PLC makers are aware they will never be able to keep up with the PC industry, so they are moving to adopt PC technologies," he says. "This trend will continue until PLCs are PCs running specialized applications software. Hybrid PC/PLCs are just a stepping stone."

This sentiment is shared by many on the PC side, who agree that hybrids are just the next generation in the PLC evolution to the PC architecture.

Omron Electronics has a different perspective. They believe the hybrid platform is gaining acceptance because of its functionality. "We believe the hybrid product is more than a niche product, and we plan to extend the line," says Dave Quebbemann, Omron's industrial automation marketing manager. "Because they combine the best of both platforms, hybrid controllers will gain acceptance in the industrial market."

So does CTC. "We feel that PCs should become more like PLCs," Smith says, "so that the safety, familiarity and lower cost found in PLCs can be delivered on a standardized platform for open machine control."

Real PLCs Fight Back
PLCs have been around since 1968 and have a tenacious hold on the discrete control, automation and motion control business. PLC manufacturers have been fighting back against every advance in PC-based control and continue to be competitive. For OEMs that use PLCs, this keeps them loyal. "When people have invested in a technology and it works, they don't rush to change," Trentini says. "The costs, development, reliability and support are known and part of the OEM's business model. The business model must be changed for an OEM to consider a major upgrade such as e-business."

Dave Rapini, application engineer at Rockwell Automation, agrees. "Most of us implement the type of system we already are familiar with, whether it's 20-year-old technology or last year's technical innovation. It's easy to choose what we know best. But the essential question today is whether this will be best for our particular application."

It is very difficult for a PC system to compete with low-end PLCs for simple machine controls. The WinPLC from AutomationDirect is one of the lowest-cost hybrid controllers available, and it still costs several hundred dollars plus software. Compare that to several PLCs available for $100 or less at list price, complete with I/O.

From a pure cost standpoint, a subject near and dear to the heart of every OEM, it has always been very difficult for PCs to compete. Even if PC hardware was available at less than $100, there is still the issue of paying for a Windows license and obtaining HMI, SCADA or control software and OPC drivers from third parties. Specialty x86 boards are available dirt cheap, but they can't always run modern software.

Software licensing is a killer for OEMs. "For PC applications, each PC must have a licensed version of software," Quebbemann says. "There is no operating system license for PLCs."

At the higher end of control, where more networking and interfaces to software are required, PLCs are trying to keep up, but it's getting expensive. New PLCs have Ethernet connectivity, embedded web servers, and OPC drivers.

Rapini points out that PLCs are tough to beat for motion control applications that combine sequential control with complex computation requirements and the interpolation of position data with velocity. "The most recent advance in PLC technology hardware is a backplane-embedded network that allows multiple processors, networks and I/O to be mixed without restriction in a single chassis," Rapini says. "In Allen-Bradley ControlLogix, motion instructions and coarse trajectory calculations take place on the central processor. An axis card closes the fine trajectory loops for the two axes connected to it using a digital signal processor [DSP]. As axes are added, more processing load is placed on the processor and the tasks can be split among multiple processors."

One intriguing element emerges from the research that went into this article. Few PLC companies chose to defend their bread-and-butter PLCs. Instead, they all wanted to talk about their hybrids and PC-based systems. This may be the best indicator of all that the popularity of conventional PLCs is beginning to wane.

Best for Real-Time Control?
PLC proponents have long claimed that PC-based systems running a Windows operating system (OS) are not a good solution for real-time operations because a PC is not deterministic, i.e., you cannot guarantee a response time to interrupts and tasks. A PLC, on the other hand, has a guaranteed scan time, ensuring its I/O and interrupts are serviced on a regular basis.

Recent developments in Windows and other operating systems may be close to making this a non-issue. Even worse for PLC manufacturers, some people think that PCs are now more responsive for real-time control applications than PLCs. Isn't that heresy?

This stems from the PLC's penchant for processing relay ladder logic sequentially. Historically, a PLC started at the top of a list of logic. It refreshed its I/O, and then ran through the entire sequence from top to bottom. This is called a scan time, and it is fixed. Many PLCs still run this way; others may use different languages such as Function Block or Flowchart to avoid running all the logic.

In a PC, scan time is not a limiting function. A PC's non-ladder logic control programs are often based on interrupt service and changes in I/O, so they don't have to run through all the logic every scan cycle. Instead, they deal only with those portions of the software affected by the change in I/O. They can accomplish more in the same processing time because they aren't evaluating all the logic all the time.

PCs also can be equipped with a real-time operating system (RTOS) to provide the necessary determinism. One of the latest is Windows CE 3.0.

"Tests we have done demonstrate that the Windows CE 3.0 OS is even more predictive and more real-time than any PLC because it guarantees a cycle time and provides a more precise writing and reading cycle, which is very important in discrete control," says AlterSys's Rousselot.

If response speed becomes a problem, PC-based controls can simply upgrade processors, while PLC users have to upgrade to an entirely different model. "If you can't reach a 1 ms cycle time with a 33 MHz 386 embedded processor, just use a faster Pentium III processor from the same supplier," Rousselot says. "This is not a big investment. Sometimes bigger processors cost less than the smaller ones."

A similar upgrade issue involving PLC capabilities faced Shigekazu Gyohten, manager of software development at Youtec, an OEM of semiconductor assembly equipment. "We have been using Omron PLCs and a PC with SCADA software to control our equipment, but I was asked by a customer to add some new functions," Gyohten says. "The memory in the existing PLC was not enough, and I didn't have time to add another PLC to the system."

Gyohten decided he could move some of the PLC functions to a soft-PLC system in the existing PC. "I used DataWorX, GraphWorX, and ControlWorX from Iconics and connected the PC to the Omron PLC directly using an OPC Server." This clever arrangement (Figure 2) let Gyohten add all the features the customer wanted without modifying the existing system control hardware.

OEMs have faced similar problems of improving response time and adding performance for years. For PC-based OEMs, very fast PC processors and specialized real-time operating systems such as QNX, RTX and others provide blazing response speeds. PLC manufacturers have responded by increasing their processor speeds and capabilities, so it's a never-ending battle. The difference these days is that a machine has to be very, very fast to stymie a processor on either side of the aisle.

The more complex the control system, the more PCs prevail. "PC solutions are used where motion, vision and high-speed data acquisition all need to be seamlessly integrated with an HMI into a machine control application," says Don Holley, industrial automation marketing manager, National Instruments. "A PC makes an ideal platform for applications that have to integrate diverse measurement and control equipment."

Or for applications that build things. Alpine Engineering Products manufactures Autoset automated equipment to make building trusses, which they sell to truss fabrication companies all over the country. Their control system is a Siemens WinAC PC-based hybrid controller. "The system ties the database for CAD drawings to the manufacturing scheduling and the machine instructions for the automated stop positioning assigned to each job order," explains David McAdoo, director of engineering services.

This is necessary because Alpine's customers are all CAD-based. When the design for a structure is finalized, all the specs are transmitted over a T1 line to the manufacturing plant and distributed to automated jigs (Figure 3) and saws. The Alpine system has two WinACs and nine automated tables, each with four motor drives, all connected via Profibus. McAdoo explains this was all once handled by PLCs and mechanical stops, but the systems were so complex that setup took 45 minutes. Today, he says, operators can call up the proper configurations in 12 seconds.

PLCs prevail when reliability issues emerge. Although PCs have certainly improved, they are definitely at a dis-advantage here.

Bay Associates, Virginia Beach, Va., builds aviation fueling systems for the military. Not only do its systems have to be completely reliable, most of them operate standalone and unattended. Ray Boseman, Bay's president, says they use Series 90 PLCs from GE Fanuc Automation. "With the PLCs, we're able to maximize fuel-dispensing rates and, more importantly, protect operators and equipment from dangerous surge conditions."

Although personnel safety is most important, Boseman notes that fuel spill liabilities can cost as much as $8 per pound for soil remediation. "If the fuel cell of a B2 Stealth bomber ruptured because of too much pressure from the control system, the cleanup cost would be astronomical," he says.

For maximum safety, Bay has built a number of hot-backup redundancy systems using Series 90-30 PLCs, the Genius bus, and HBR-30 backup software from GE Fanuc partner Trimation. The redundant systems use a parallel CPU arrangement — one the primary and the other operating as the backup.

Ethereal Influences
The use of Ethernet hardware and TCP/IP for I/O networks has the potential to significantly change the economics of control systems. "However, this technology will remain a fringe curiosity until one or two standards emerge as the clear winner in the protocol arguments," says Steeplechase's Gee. At present, there are more than a dozen competing and mutually incompatible standards.

Ethernet will bring about less-expensive communications because it will no longer be necessary to use proprietary communications networks for PCs or PLCs. Instead, devices such as communications cards, connectors, cables and so on will be available off the shelf at reasonable prices. It also will be easier to communicate with other vendor systems.

"The use of Ethernet as a fieldbus makes it easier than ever for users and OEMs to mix and match components when developing a system," explains Rainer Wischinski, director of control products marketing at Intellution. "These new connectivity options are also helping the industry migrate to a truly distributed architecture."

Emphasis on E-Commerce
Unless you've been living under a rock for the past year, you're aware of the e-madness that has struck our industry. Even an OEM of motion controls or vision systems has to be aware that every device is becoming an Internet appliance. Cheap communications chips make it possible to web-enable everything.

But why would you want to? Roy Kok, vice president of operating system developer VenturCom, thinks he knows why. "A PLC does not need to operate faster, but a PLC does need to offer status information via web-browser interfaces, send emails, update inventories, drive local graphical user interfaces, and support the needs of the future," Kok says. "The driving forces are moving us toward multifunctional equipment, and communications is accelerating the trend."

Here's another — far more important — reason for OEMs: remote diagnostics and maintenance. "We think that web-enabling is an up-and-coming standard for diagnostic tools," says Diane Corey, programmer at CamSoft. CTC's Smith agrees. "There is an emerging interest in remote diagnostics, so an OEM can solve a machine problem at a distant location," he says.

Actually, it makes a lot of sense. With the entire world running e-muck, an OEM could build web-enabling hardware and software into its products to provide an easy way to perform online diagnostics and maintenance via a standard browser, and perform system upgrades via FTP downloads.

If all your products are web-enabled, then doing a worldwide upgrade could be as simple as sending a single email to a few hundred or few thousand customers.

Whether an OEM builds on a PC or PLC base, it needs to make sure the platform will be able to go with the Ethernet flow, and then continue onward to the web. PC platforms have the capability already, but PLC vendors are catching up fast. "PLC vendors are realizing that their products do not work in a vacuum," Gee says. "They're adding features such as the ability to serve up HTML documents."

Omron agrees with the premise. "Communication via the web is already embedded in Omron PLCs through Ethernet TCP/IP," Quebbemann says. "Some of our customers have shown a high interest in the ability to share data from factory-floor devices with enterprise computers locally and around the world using the web."

Omron isn't the only one, Optimation Technology's Trentini says. "We see our PLC vendors adding web-server and Ethernet cards to their product set, along with supporting software," she says. This is not the easiest solution for PLC vendors, because they cannot always take advantage of open hardware and software like PCs can. Instead, as Intellution's Wischinski points out, "It is a challenging process for PLC vendors, because they must put web-enabled functionality on top of existing proprietary solutions."

Making motion controls and vision systems into Internet appliances is not a deal-killer for OEMs yet, but the e-snowball is definitely rolling and gathering momentum, albeit hindered by all the communications standards floating around.

Still Converging
We've all seen Windows CE-based products from companies such as Indusoft and Iconics running on Palm and HP handheld processors, and we know those CPU chips cost less than $10. Soon, someone will make a Windows CE-based embeddable board available for less than $20 in OEM quantities. CE is already available for less than $10 to OEMs, so what's left is for the CE software vendors to make their software licenses available for less than $20, too. When the cost of the ingredients for a powerful embedded processor — hardware, operating system and application software development system — becomes less than $100 to an OEM, then the fun will start.

Meanwhile, the two camps are converging. "PCs are affecting PLCs and vice-versa," says Mike Jabbour, product manager of PC-based products, Siemens E&A. "The PLC camp is driving PC-based systems toward determinism and diskless media, and the PC camp is driving PLC systems to open standards, integration, and ease of use. The core technology paths will intersect in the near future, and it will favor the PC platform."

In fact, don't be at all surprised to see the ultimate convergence: a PLC with PC software. Insiders tell us that one or two PLCs based on Windows CE, inexpensive chips and soft-PLC HMI software may become available next year.

That means we'll have to review this situation again. Right now, it's still a tossup.