By Martin Piszczalski
Programmable logic controllers (PLCs) could fast become historical relics. Personal computers (PCs) doing direct, real-time control are meeting or exceeding the robustness and reliability of PLCs. More decisively, however, these PCs are thumping PLCs in terms of cost and flexibility. The automotive industry is wholeheartedly embracing direct PC control. Major users include the Spring Hill, Tennessee, Saturn plant and Chrysler's Kokomo Indiana, transmission facility.
All manufacturers should be getting production experience with this technology now. Cost savings of 70% or more are too compelling to ignore. In addition, low-cost network computers (NCs) may be an even more enticing substitute for today's PLC-dominated plant floor.
Direct PC Control.
The big news is that PCs can do a whole lot more than just control. They can also serve as the operator interface, program-development console and supervisory-control station.
Their key function, nevertheless, is real-time control. Enabling a PC to do this is a real-time, operating-system kernel. This kernel always gives highest-priority to real-time, control functions. As a result, non-controls functions such as servicing the hard disc or responding to a mouse click can never slow down the processing of industrial I/O, the traditional responsibility of PLCs.
Direct PC control vendors using such a real-time kernel include Taylor Instruments (Edmonton, Alberta) and Steeplechase Software (Ann Arbor, MI). The latter uses the iRMX kernel running under Windows NT. In contrast, traditional PLCs use entirely proprietary operating systems that cannot also run off-the-shelf PC software (such as for supervisory control) in the same unit. Unlike PLCs, one direct PC control computer can connect to industrial I/O networks from a variety of PLC vendors, such as from Allen-Bradley or Modicon. In contrast, each brand of PLC typically connects only to that maker's proprietary network.
Advantages of Direct PC Control. Direct PC control compared to traditional PLCs is
- more flexible
- more seamlessly integrable
- downright faster.
Compared one-to-one, a PC is a far cheaper unit than a PLC (except possibly for low-cost micro-PLCs). However, the real cost advantage is in the ancillary equipment and integration services when a PC is made the heart of controls.
A striking example is General Motors' Pittsburgh stamping plant. A turnkey, PLC-based system there would have cost $15 million. Instead, GM opted for one based on Steeplechase Software's direct PC control The savings: $10 million.
Separate boxes and networks are not needed for functions now done outside a PLC, namely:
- supervisory control and data acquisition (SCADA), and
- man-machine interfaces.
Data management also becomes far less convoluted since data is not distributed across different platforms and incompatible formats. Dramatic cost-savings also occur because direct PC control instantly opens up the possibility to buy industrial I/O devices from any maker. Also, all the major, PLC-centric networks can simultaneously hang off the same PC.
Over the next two years, expect to see PC solutions run away from PLCs in terms of cost and performance. The incredible pressure of the huge PC industry to slash costs, accelerate performance and add functionality will leave special-purpose, hardware boxed in the dust.
Furthermore, direct PC control installations are fast to configure, easy to modify and quick to diagnose. Thanks to flow-chart programming (vs. the far-less-intuitive ladder-logic of PLCs), even operators can modify the PC-based software.
Likewise, direct PC control allows for the possibility of one seamless architecture stretching from the office right down to the shop floor. For instance, Saturn has 1,800 Windows NT boxes in its business offices and manufacturing—all managed, maintained and networked together. An Excel spreadsheet operating anywhere in the corporation can analyze data stored in a plant-floor PC, for instance. Try doing that on PLC-based data!
Furthermore, PCs are faster than PLCs. They will become even more so as symmetric multiprocessors kick in and further accelerate their performance.
Today's Pentium Pro chips in PCs are at least 20 times faster than any PLC-based processor. With power to spare, PCs can do many additional functions while simultaneously performing real-time control. For instance, generating reports on the same unit is no problem. This can be maintenance or end-of-shift reports. Using easy-to-use PC software such as Excel or Microsoft Word, an operator can analyze up-to-date production or machine-performance data.
For instance, Saturn does hundreds of tests on each assembled vehicle. In the past it took one day for that data to trickle back to the plant workers whose processes, in effect, produced those test results. Now Saturn feeds back that data in one second, enabling fast correction of processes about to drift out of control.
Argument Against Direct PC Control. PC technology is faulted for hardware deficiencies. Making them inappropriate for real-time control, critics claim are PCs'
- CRT screens
- power suppliers, and
- hard discs.
To overcome these early weaknesses direct PC control vendors have responded with
- liquid crystal displays (LCD)
- uninterruptible power supplies (UPS), and
- diskless, real-time kernels.
Furthermore, industrially hardened PCs can withstand the harshest plant-floor environments. Nevertheless, PLCs have been in plants for decades. Software for direct PC control will not match overnight the vast libraries of control programs developed for PLCs. Probably the biggest factor inhibiting the more widespread adoption of this technology is cultural resistance. For instance, nearly every plant has staff deeply experienced with a particular brand of PLC and its associated hardware and software. Much of that hard-earned, PLC-brand-specific knowledge goes out the window when installing direct PC control. Furthermore, some plants have purchasing policies that mandate a particular PLC brand for all new production equipment. Textron Automotive's Farmington, MA, plant, for example, must first change this long-established, purchasing practice before it can even evaluate a direct PC control alternative.
Furthermore, there is a strong fear factor rooted in PCs from a bygone era. Consequently, direct PC control will most slowly penetrate controls applications where failure is most catastrophic, such as loss of life. Nuclear power plants and some continuous-process facilities are not expected to be early adapters.
NT Everywhere or Network Computers (NCs) in the Works? Clearly, many auto-industry firms are moving toward a Microsoft-dominated architecture, from top to bottom. For instance, the fastest growing market for the German enterprise resource planning (ERP) vendor, SAP America, is for its NT-based product. Having one NT architecture span everything from corporate financials down to real-time industrial control is an enticing image to companies constantly struggling with incompatible systems. Even more provocative is deploying on the shop-floor the stripped-down, network computers (NC) championed by Sun and Oracle. Adding industrial I/O interfaces to these inexpensive boxes could offer the best of the PC world without the PC clutter. Coupled with a low-cost, plant-wide server, expect to see hardware costs for total plant I.T./controls really plummet if NCs take hold on the plant floor.
Recommendations: Start Exploring Direct PC Control Now. Every manufacturer planning a new production line should at least be getting a quote for direct PC control. Larger firms should already have some production lines using this technology. Educating and training controls staff now will help smooth staff acceptance for this radical change. Furthermore, management should consider melding more closely its information systems and controls staff. These two worlds will increasingly share more territory. Indeed, Electronic Data Systems Corp., not known as a powerhouse on the plant floor, is now active in real-time, shop-floor control thanks to this technology. Tomorrow's agile manufacturer will certainly have machine tools, sensors, actuators, business systems, and indeed, the whole supply chain operating like one seamless whole.