More and more PLCs are springing up in motion control, and in places where you normally wouldn’t expect to find them. These include car washes, vending machines, restaurants, elevators, and even private residences.
What’s fueling this trend is the advent of “nanoPLCs,” low-cost programmable logic controllers with 14 I/O points or less. Taking advantage of sensor-actuator networks, nanoPLCs are challenging even “microPLCs” (20 to 60 I/O points), the dominant controller in small OEM machines used in printing, bagging, packaging, and other such applications.
Whether or not a PLC is the best solution for a particular job depends on several factors. The alternative, an industrial PC running motion-control software, has come a long way in the past few years and is making the decision ever more difficult.
Industrial PCs offer an open architecture, capitalizing on open hardware and software and a high level of connectivity. With almost endless options, they can implement a wide range of intelligent, distributed control functions.
On the down side, PCs tend to slow when handling a large number of I/Os. The reason: PC operating systems are oriented more toward file handling and user interfaces, and do not respond well to the frequent and random interrupts typical of PLC-based systems. Newer OSs like Windows NT and OS/2 were supposed to take care of this shortcoming, but what they actually do is pass the problem on to the programmer. PCs also tend to be single-tasking, often failing in real-time applications. And when they fail, they usually do it in an unpredictable way, making diagnosis and recovery a hit-or-miss process.
Further complicating matters is the fact that PLC makers are increasingly embedding PC modules within their products. The resulting hybrid systems offer a PC-like interface and development environment with PLC I/O. PC makers, on the other hand, are actively wooing PLC users integrating software that emulates programmable logic. A third development blurring the line between PCs and PLCs is the emergence of “soft logic,” PC software that runs on PLCs as well as PCs equipped with third-party I/O such as Opto 22’s.
One thing is clear, though. In low-end applications, industrial PCs cannot compete with PLCs, at least on a cost basis. A nanoPLC, for example, can be purchased for about $14 an I/O point. In medium to high-end applications, however, the price advantage swings to PCs. In this range, PLCs can cost from $4,000 and $5,000, while a 486-based industrial PC can be had for less than $2,000.
