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.
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.