Personal Computers Lose the Pretty Face

PC-based control gains acceptance with new and more practical form factors

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It could be an episode of "Fear Factor": A control systems engineer is forced to perform control of a critical process using a desktop Windows PC!

Rational or not, many end users and system integrators fear using PCs for real-time process control. "We have not implemented PC-based control, and I am not aware of any client applications. Everyone I speak to expresses many reservations regarding PC-based control," says David Kennedy, PE, a control systems engineer with Fresno, Calif., system integrator Ginosko (www.ginosko.com).

Many engineers and technicians believe PCs are unreliable, and many others don't see a need for PCs in process control. "PCs have too much overhead, which results in bugs in the Windows software that cause crashes, hang-ups, and re-boots," says Paul Parker, a plant engineer with KSL Services at Los Alamos National Labs in Los Alamos, N.M. "Why stress out over this, when PLCs cost less than PCs? What is the advantage?" he asks.

When we visited this topic two years ago, we showed concrete reasons for using PCs instead of PLCs in process control. These reasons are shown in Table I, and perusal of the table shows how PCs can be a cost-effective alternative when there is a need for more than just simple discrete and analog control.

Even if one or more of these reasons apply to your application, it still would not make sense to use a PC unless it proved to be a reliable, inexpensive, and accepted alternative to a PLC or DCS.

Industry acceptance is coming along slowly but surely. PC-based control is a mainstay of discrete parts manufacturing, most notably in the auto industry. According to the Venture Development (www.vdc-corp.com), a technology market research group, PC control also has a significant and fast-growing presence in process control.

Table I: Top Ten Reasons for Using a PC Instead of a PLC

1

Networking to higher-level platforms

2

Advanced control algorithms

3

Extensive database manipulation

4

HMI functionality in one platform

5

Integrated custom control routines

6

Complex process simulation

7

Very fast CPU processing

8

Memory requirements exceed PLC specs

9

Interfaces thorough multiple protocols

10

Wireless access

 

"Distributed and remote I/O for use with PC-based control systems in industrial process industry applications is forecast to increase from $143 million in 2001 to $254 million in 2005. This gives PC-based control about 10.6% of the total control market in 2001 and a projected 14.5% of the total in 2005," according to Jim Taylor, a group manager with Venture Development.

Industry acceptance is growing, but what about cost effectiveness? This is perhaps the area where PCs have made their greatest inroads, and this trend is sure to accelerate.

Office-grade PCs decline in price on a seemingly daily basis, and industrial grade PCs now follow the same trend. "Several years ago the cost ratio of an industrial PC to an office-grade was about 4:1, but now the ratio has declined to around 2:1 or less," says Ed Boutilier, president and CEO of Stealth Computer (www.stealthcomputer.com).

Industry acceptance is on the rise and costs are declining, so the only hurdle left for PCs in process control is reliability. Unfortunately for PC-based control vendors and their supporters in the user community, this is a high hurdle, largely due to poor past implementations of PC-based control.

Windows Won't Work

It is hard to escape history, and PCs in process control have a rather unpleasant past. Many users have fallen to the temptation of using low-cost desktop Windows-based PCs for critical process control, often with disastrous results. "The whole PC-based control scenario is riddled with amateurish implementations," observes George Turnbull, managing director of Open Automation and Control in Essex, England (www.oacg.co.uk).

One of the main problem areas has been the Windows operating system. Desktop Windows is primarily designed for non-real time and non-critical applications. Earlier versions of Windows (prior to NT) were especially plagued by unexplained glitches, the blue screen of death, and frequent re-boots.

As detailed in our May 2002 cover story, this problem has largely been alleviated by the use of real-time PC-based operating systems. In addition to the various embedded versions of Windows, such as CE and embedded XP, real-time PC operating systems are also available in Linux. Firms such as VxWorks (www.windriver.com), QNX (www.qnx.com), and LynxOS (www.lynuxworks.com) also offer rock-solid real-time operating systems.

Table II: Industrial Grade PCs vs. Office PCs

Features to consider

Industrial Grade

Office Grade

Can it withstand extended temperatures?

Yes

No

Can it withstand shock & vibration?

Yes

No

Is it dust proof, or does it have filtering?

Yes

No

Is it splash proof?

Yes

No

Does it have RFI/EMI shielding?

Yes

No

Does it have power surge protection?

Optional

No

What type of chassis construction?

Cold rolled steel, rigid

Flimsy, often plastic

Does it meet industry classifications?

NEMA/IP, MIL, etc.

None

What type of security does it provide?

Locking doors & drive bays

None

How much lifecycle technical support?

3-5 years

6-12 months

Does it provide Intrinsic Safety?

Optional

No

Mean Time Between Failure Data

Sometimes provided

Not provided

Mean Time to Repair

5-15 minutes

1-2 hours plus

Mounting & enclosures

Multiple Options

desktop or tower style

Is it fault tolerant?

Optional

No

 

How reliable are these systems? The control engine for Westinghouse's Ovation DCS (www.westinghousepc.com) is the VxWorks operating system running on dual redundant PCs. Ovation DCSs are used to control power and water/wastewater plants worldwide with some systems controlling over 10,000 I/O. The VxWorks operating system also is used to control the NASA Mars rover. The QNX operating system is used to control robot hands in surgical applications.

Desktop Windows may not be ready for real-time process control, but other PC-based operating systems are. This leaves one last remaining hurdle for PC-based process control, and that is the reliability of the PC itself.

Gritty Bricks

A white box PC is not designed to run 24/7 in a harsh environment. Unfortunately for proponents of PC-based control, the first image most people have of a PC is the ubiquitous white box. Another problem is the moniker "soft PLC" to describe PC-based control. Users don't want soft, they want hard real-time. Vendors are trying to change perceptions, and some have chosen the lowly and humble but always reliable and rugged brick as a suitable icon.

"The small form factor, about the size of a brick, combined with DIN rail mounting allows the integrator to place the PC-based processor in areas not easily serviced by desktop or laptop PCs," says Wayne McGee, vice president of business development with SBS Technologies (www.sbs.com).

"We currently only make PCs that plug into our PLC backplanes, but we are investigating standalone or brick-style I/O footprints," says Jim Allison, the PC control product manager with AutomationDirect (www.automationdirect.com).

Bricks are good, and so is grit. "Our embedded CE-based controller is a PC with PLC grit," says Tom LeBay, director of marketing for Online Development (www.oldi.com). It is obvious vendors of hardened and embedded PCs are trying to change user perceptions about the ruggedness of PCs.

The changes go beyond mere terminology. These PCs often have totally different form factors than a desktop PC. In addition to the aforementioned brick, other form factors include cards that plug into PLC backplanes, rack-mounted computers, and single-board computers.

Table II summarizes some of the key differences between a desktop PC and an industrially hardened PC. Most industrial PCs won't meet all of these criteria, but all satisfy some of these specifications. Application needs drive users to select PCs with proper specifications, and we will next look at some process control applications to see where these rugged PCs are used.

 

Figure 1: Thick as a Brick

With a brick-like form factor, some new industrial PCs not only lack a resemblance to their desktop cousins but are tougher and smaller.

In the interest of full disclosure, I must say that I have a soft spot for soft PLCs. The first control system that I designed used PC-based control. Back in 1988 I was charged with a complete re-design of an obsolete relay, timer, and analog instrument control system. My employer was an OEM skid builder, and the control system needed extensive process control, a sophisticated HMI, and advanced control.

Our choices were simple. We could use a PLC for process control, an industrial PC with visualization software for the HMI, and a custom single-board computer for advanced control. Or we could use an industrial PC for everything.

We decided on a PC with a real-time OS operating under DOS. We found an off-the-shelf software package that combined real-time function block control, an HMI, and a C++ toolkit to handle our advanced control algorithm.

We used an STD bus PC, an industrially hardened monitor, and Opto 22 (www.opto22.com) I/O. The system worked beautifully, was very reliable, and was by far the lowest-cost alternative. The only problem was the customer's perception of reliability.

We overcame these reservations by offering a "lifetime" warranty on the PC, the monitor, and the I/O. In the four-year period from 1988 to 1991 (when I left the company), we had no hardware failures and thus no claims under the warranty.

Programming Standards a Boost

The IEC 61131 standard defines five different programming languages: ladder diagram, structured text, instruction list, function block diagram, and sequential function chart. Many software vendors market Windows-based software packages that use these five languages. Applications are programmed on a PC and then downloaded to the desired target platform or control engine, in many cases a rugged PC.

Open Automation and Control in England uses ISaGRAF IEC 61131 programming software from ICS Triplex ISaGRAF (www.isagraf.com) for many process control applications. The company use various PC targets including a CE-based controller from Online Development.

After configuration on a Windows PC, the ISaGRAF software can be downloaded to virtually any target control engine. Windows (NT/2000/XP/NT Embedded/XP Embedded), Windows CE (3 and .Net), QNX, Linux (RTAI), and Venturcom RTX (www.pharlap.com) are directly supported. A customer can download configured applications to any of these operating systems without custom programming.

Other platforms and operating systems are supported through a source code toolkit. "We provide a toolkit with example source code that can be used to port the control engine, as well as options like alarm, event, trend, web, etc., to any operating system or any platform," says Jim Desrosiers, global sales manager for ICS Triplex ISaGRAF. "Our customers can stick these engines anywhere they want to use them including a PC, a drive, or network equipment."

Open Automation and Control finds the ISaGRAF/Windows CE platform to be a better fit for process control than a PLC. "The openness of IEC 61131 and PC-based control engines lead to better solutions than PLCs. PLC I/O is not normally sufficient for serious process applications," says Turnbull.

PC-based control engines can be used with I/O from any number of vendors. Multiple communication ports allow the simultaneous use of different types of I/O, also from different vendors.

"Good algorithms and a regular execution time are essential in process control, and PLC vendors are not normally good in these areas. The IEC 61131 function block diagram language works well for many process applications," adds Turnbull. The simultaneous and interactive use of any of the five IEC 61131 languages is a key attribute of PC-based control with IEC 61131.

Open Automation and Control considered desktop PCs and industrial PCs for its applications, but both options were rejected. "Generally, a PC is not rugged enough. It could be mounted in a suitable enclosure, but this is costly and large. The TECLA CE-based controller is very compact and rugged in all aspects," concludes Turnbull.

HyRadix (www.hyradix.com) makes gas processing equipment that produces a hydrogen-rich gas from natural gas or LPG. This gas is used in fuel cells and in industrial hydrogen applications. The Des Plaines, Ill., company uses PC Worx programming software from Phoenix Contact (www.phoenixcon.com/automation).

PC Worx is an IEC 61131 programming software package designed for use with PC-based logic controllers as well as with Phoenix's Interbus controllers. HyRadix uses an embedded CE platform containing an HMI as its target control engine.

Control of analog loops in the HyRadix system is via a combination of conventional PID loops as well as other complex control algorithms. The system includes state control and process fault handling logic, and there are also significant requirements for data logging on prototype and field test units. Field test units have a requirement for remote access to enable configuration from anywhere in the world.

Based on these requirements, the best solution was PC-based control. "A Windows CE-based system with an integral HMI (HMI screens developed in Microsoft embedded Visual Basic) became a more attractive solution than a conventional PLC," says Janus Kubinski, a senior controls engineer with HyRadix. "The integration of a PLC controller with an HMI panel addresses our size and cost requirements, and the Windows CE environment allows for limited but adequate data acquisition and interconnectivity."

PCs Keep the Water Running

Automated Control Systems (www.automation-software.com), Vancouver, Wash., uses shoebox industrial PCs to control five of the largest hydroelectric facilities in the U.S. The company claims these power plants represent about 11% of the total U.S. hydroelectric capacity. ACS is installing PC-based control in seven more plants representing another 6% of hydroelectric capacity.

ACS uses more than 250 SoftPLC controllers (www.softplc.com) in these applications. Each is a Pentium-based 133-266 MHz system with 32 MB Ram and 20-32 MB of solid state flash disk. Some of the SoftPLCs have additional communication ports to allow integration of power meters, governors, and excitation systems. The systems use over 30,000 points of MTL I/O (www.mtl-inst.com) along with 96 SCADA nodes from Intellution (www.gefanucautomation.com).

"PC-based control lets us use best-of-breed components such as I/O that meets strict IEEE surge-withstand criteria," says Daniel Perrier, PE, ACS president. "We also added functionality to the relay ladder logic in the form of a Modbus Master Communication program. Even when PLCs offer Modbus communications, the nature of the proprietary hardware makes adding additional communication ports expensive or impossible."

Additional requirements included keeping the SoftPLC clock synchronized to a satellite clock for certain top-of-the-hour reporting requirements. This extension to the SoftPLC instruction set was done in Java. "The SoftPLC gives us rock-solid real-time performance and the ability to have an almost limitless programming ability in terms of functionality and flexibility," adds Perrier.

Another utility using PC-based control is water and wastewater treatment in Rock Hill, S.C. The city uses an AutomationDirect (www.automationdirect.com) system consisting of a PLC backplane, a WinPLC industrial PC that plugs into the backplane, and local and remote PLC I/O.

The wastewater system consists of 18 racks of I/O (15 with WinPLC processors), one main server running Entivity's Think and Do software, and seven PC workstations. The system has 1,600 digital inputs, 500 digital outputs, and 300 analog inputs and outputs.

The water treatment plant consists of 21 racks of I/O (18 with WinPLC processors), one main server, and three PC workstations. This system has 1,056 digital inputs, 500 digital outputs, and 352 analog inputs and outputs.

PC-based control offered two key features for these applications: flow chart programming and distributed control. "Our water plant filter tables have 30-40 pages of flow chart control, which in ladder logic would convert to several thousand lines of code. Troubleshooting is straightforward with the flow charts but would be very unwieldy with ladder logic," says Jonathan White, maintenance superintendent with Rock Hill.

"Most of our processes are spread over plant sites that can be up to 40 acres. In the water and wastewater business it makes sense to use distributed process control that is monitored by a master or server-based HMI system as opposed to a central controller," adds White.

If your application requires functionality, as outlined in Table I, then PC-based control might be a good fit. The operating systems are robust, and the newer form factors are not only rugged but are also easier to integrate into industrial control systems than desktop PCs.

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