PLCs Maximize Machine, Motion Control

Control engineers now have at their disposal a wide variety of logic devices to program processes and machines. Computers and computer technology provide almost unlimited control possibilities no matter what the application. PLCs remain a strong part of the mix—a recent survey finds more than half expect to increase PLC spending in the next 12 months. More than 40% expect to spend as least much as they did in the past 12 months.

Early machine and motion control were rooted in basic logic, the same basic logic that computers still leverage today. Depending on the machine and its function, mid-20th-century logic could be mechanical, pneumatic, or electrical, or a hybrid of any of the three. Years past, control engineers used basic relay-ladder logic to map out the sequence and build control systems for manufacturing machines and lines of all types. This type of ladder logic used many components (relays, timers, coils, switches, pilot lights, pushbuttons, etc.) and an enclosure, adding protection (and wiring). Even though the process to create this type of control system was straightforward, demands for faster, cheaper, more accurate, and more complex machinery clearly demanded something more sophisticated. Enter the programmable logic controller or PLC.

PLCs are now, and have been for some time, the control device of choice for a wide variety of control applications. According to the most recent Control Engineering/Reed Research online poll (using feedback from more than 300 respondents, who specify, recommend, or buy PLCs), a majority use PLCs for machine-control applications. Process control applications are a close second, followed by motion control, batch control, and diagnostic applications. Most applications are in-plant (54%), with the reminder OEM (25%) or both (17%).

Spending up, size down

The 2005 survey found that 51% of respondents expected to increase their spending on PLCs, while 8% expected to spend less. Remaining respondents (42%) expected spending would remain the same over the next year. These figures set a solid spending trend; in the 2002 survey only 29% of respondents expected an increase in spending, 64% expected spending to remain the same, and 7% expected a decrease. The 2005 survey saw a 2-5% increase in nano PLCs (<15 I/O connections), micro PLCs (12 to 128 I/O), and medium PLCs (128-512 I/O). Large PLCs (>512 I/O) and PC-based control each fell 4%.

The slight “decline” in PC-based control among PLC users may be attributed to a melding of product categories. Use of PC-based silicon in a PLC form factor, (often called a programmable automation controller or PAC), tends to blur the line between PCs and PLCs. Some manufacturers even refer to their PACs as PLCs or “controllers.” That terminology also blurs the line between PLCs and products traditionally known as “loop controllers,” covered in a separate July 2005 article. Software-based PLCs and embedded control use stayed nearly the same in this 2005 PLC survey.

Mike Miclot, division product manager at Rockwell Automation, sees identical trends from his vantage point as a supplier—smaller PLCs are gaining ground over larger and PC-based configuration. Miclot says three reasons push the move to smaller PLCs. First, newer, smaller controllers have more power and can replace their larger counterparts. Next, the controls community has embraced a distributed architecture that uses networks of smaller PLCs (performance and autonomy are the key factors here). The third reason, Miclot says, is lower overall cost.

Is there a limit to how small these devices can be? If PLCs do get any smaller than the current “nano” configuration, it likely will be because the electronic components that manufacturers put inside continue to be miniaturized. “But even as these devices become smaller and more powerful, there will be a point where they will become impractical within the factory environment—just too complex or difficult to install or work on,” Miclot warns. Pushing these limits of shrinkage, Divelbiss Corp. (see online link for more information) even touts a silicon-based PLC.

One rung at a time

The control engineering profession has its enduring traditions. There is neither an engineer nor a tech unfamiliar with one of its oldest and longest-lasting PLC programming languages—the venerable ladder diagram. Firmly rooted in the days of relay logic, the ladder diagram remains the overwhelming top pick among the respondents in the 2005 survey (see accompanying diagram). Little has changed in this portion of the 2005 survey from 2004’s, and even before that.

The top three programming languages remained ladder diagram, function block, and sequential function chart, in that order. Changes at the bottom of the list saw both C programming and instruction list move up in the standings.

According to Joe Rubino, PLC product manager for Omron Corp., ladder diagrams may hold this top slot for some time to come. “The requirement for ladder diagram programming is directly driven through supporting the equipment on the manufacturing floor. The domestic market of PLC vendors established deep-seated roots with ladder diagram programming by migrating the legacy equipment control panels, which used relays to perform the control logic, to programmable logic controllers. Ladder diagram programming offered the easiest migration path, mimicking the physical wiring of the legacy relay panels. This greatly reduced the learning curve for using the PLC. Existing personnel, who previously supported relay panels, are able to develop and support the PLC’s electronic representations of the physical panel wiring that they were accustom to,” says Rubino.

Does Omron see other programming languages as eventually unseating the ladder diagram? “No,” Rubino retorts, “Instead of function blocks or sequential function charts overtaking ladder diagrams in the future, we see them complementing the way to write PLC control code. The key is in offering the programmer flexible ways of creating control code.”

No failure to communicate

Networking trends varied only slightly from the previous survey. Most respondents reported that most oftheir PLCs were standalone (32.9%). A slightly smaller number claimed that they networked with personal computers (26.5%). Respondents also reported that the ability to network with other PLCs (22.8%) and distributed control systems (17.6%) were necessary to the functioning of their control network. Overall, respondents reported that the greater majority of their PLCs worked on some sort of a network, the margin being slightly greater than two to one.

When asked which features they would expect to specify in future PLC purchases, respondents chose the same attributes for the top three positions, in slightly different order than the earlier survey (see accompanying chart). Based on the responses, it is clear that the ability to network programmable logic controllers (and do so easily) is on the mind of the control engineer. Ability to use universal programming software for multiple targets/ platforms claimed the number one position. Number two among the features deemed very important for future purchase was the ability to network PLC I/O connections with a PC. Third most-wanted feature was the availability of PLCs with more remote I/O subsystems.

When Mike Shulim, chief technical officer for DST Controls, a San Francisco Bay area systems integrator, was asked which features “got his vote” as the most important advances in PLC development, he cited PLCs with onboard PC processors, nano/micro PLCs, and PLCs with more remote I/O subsystems, in that order. Even though Shulim’s picks do not mirror survey results, it is easy to see that working control engineers like PLCs that are small, fast, and smart. Doing a job for less money (PLCs with remote I/O subsystems save on wiring-related costs) can’t hurt either.