Looking into the Future, PACs for Industrial Control

The "80-20" Rule During the following three decades since their introduction, PLCs have evolved to incorporate analog I/O, communication over networks, and new programming standards such as IEC 61131-3. However, engineers create 80 percent of industrial applications with digital I/O, a few analog I/O points, and simple programming techniques. Experts from ARC, Venture Development Corporation (VDC), and the online PLC training source PLCS.net estimate that: ï 77 percent of PLCs are used in small applications (less than 128 I/O) ï 72 percent of PLC I/O is digital ï 80 percent of PLC application challenges are solved with a set of 20 ladder-logic instructions

Since 80 percent of industrial applications are solved with traditional tools, there is strong demand for simple low-cost PLCs. This has spurred the growth of low cost micro PLCs with digital I/O that use ladder logic. It has also created a discontinuity in controller technology, where 80 percent of applications require simple, low cost controllers and 20 percent relentlessly push the capabilities of the control systems. These 20 percent applications are built by engineers who require higher loop rates, advanced control algorithms, more analog capabilities, and better integration with the enterprise network.

In the 80s and 90s, these "20 percenters" evaluated PCs for industrial control. The PC provided the software capabilities to perform advanced tasks, offered a graphical rich programming and user environment, and utilized COTS components allowing control engineers to employ technologies developed for other applications. They now could use Intel floating point processors; high speed I/O busses, such as PCI and Ethernet; non-volatile data storage; and graphical development software tools. The PC also provided unparalleled flexibility, highly productive software, and advanced low-cost hardware.

However, PC-based control was not perfect so while many engineers used the PC when incorporating advanced functionality, like analog control and simulation, database connectivity, web based functionality, and communication with third party devices, the PLC was still king for control. The main problem with PC-based control was that standard PCs are not designed for rugged environments.

The PC suffered from three main problems: 1. Stability: Often, the PC's general-purpose operating system was not stable enough for control. PC controlled installations were forced to handle system crashes and unplanned rebooting. 2. Reliability: With rotating magnetic hard drives and non-industrially hardened components, such as power supplies, PCs were more prone to failure. 3. Unfamiliar Programming Environment: Plant operators needed the capability to override a system for maintenance or troubleshooting. Using ladder logic, they knew how to manually force a coil and patch code to quickly override a system. But with PC systems, operators needed to learn new tools.

Although some engineers use special industrial computers with hardened hardware and special operating systems, most engineers avoided PCs for control because of problems with PC reliability. In addition, the devices used within a PC for different automation tasks, such as I/O, communications, or motion, may have different development environments.

So the "twenty percenters" either lived without functionality not easily accomplished with a PLC or cobbled together a system that included a PLC for the control portion of the code and a PC for the more advanced functionality. This is the reason many factory floors today have PLCs used in conjunction with PCs for data logging, connecting to bar code scanners, inserting information into databases, and publishing data to the Web. The big problem with this type of setup is that these systems are often difficult to construct, troubleshoot and maintain. The system engineer often is left with the unenviable task of incorporating hardware and software from multiple vendors, which poses a problem since the equipment is not designed to work together.