A diversity of applications and technical changes keep PLCs a viable contender in the control arena.
By Jon Giardina
Omron Electronics Inc.
Edited by Miles Budimir
After more than a decade of articles touting the benefits of PC-based control over programmable controllers and predictions of the PLC's demise, this much is clear: both PCs and PLCs have their place in contemporary control systems. In many cases, they will work as a team.
In lower-end process industries such as food plants where the PLC is strongly entrenched, there is little interest in switching to PC-based control. The reason is the PLC's suitability. Sequential logic control provided by PLCs is best suited for round-the-clock operations common in those industries. In particular, PLCs offer repeatable, reliable control conducive to high-speed, repetitive processes. Dedicated performance and a high level of security are also benefits of a PLC. The packaging side of a food plant lends itself well to sequential logic control which makes PLCs the best choice for conveying and sorting applications.
Five years ago, the prediction was that PCs were going to take traditional PLC applications by storm. The reason they didn't is that the PLC has continued to be a good, low-cost control option. For instance, PLCs with highspeed counter inputs, loop controls, analog points, and basic math calculations are available in the $200 to $400 range with 100 or more I/O and with options for expansion. On the other hand, even buying an inexpensive PC requires loading software, setting it up, and making sure it's robust enough to not crash.
However, PC-based control has its merits. Specifically, it has greater number-crunching capability, more communication options, and is geared more toward data logging and trending, plant monitoring, report generation, and documentation. The key reasons to switch to PC-based control include networking the control system to higher-level applications such as ERP, handling complex mathematical applications such as vision inspection systems, and connecting to bar-code scanners, in-motion weigh scales, and other devices. In short, most users believe that when data must be transferred from the controller into a database for analysis, the transfer goes more smoothly when the data is already on a PC-based system.
The numbers tell a tale of the PLC's firm entrenchment in control applications. For instance, hardware sales figures for 2001 show $80 million spent for open control (PCbased control), with $1.1 billion being spent on PLCs. Even as the trend is for both figures to grow by 2004, PLCs still have a sizable advantage over PCs.
Engineers these days are calling for networking capabilities. PLCs are now low-cost dedicated controllers. They offer more processing power and have added networking capabilities with other PLCs through serial connections. Micro PLCs have an Ethernet connection with optional DeviceNet or Profibus modules. So the total system is not just a stand-alone controller, but one that can network to bigger PLCs. Also, increased processing speeds let PLCs do more. Because of these changes, the PLC remains a viable control option.
One trend is towards so-called hybrid PLCs. These more common hybrid applications are challenging the capabilities of each control approach. Their number is growing because fewer applications are pure discrete or pure process installations. One example is a PLC on a board with sequence logic-controller functions. It plugs into a PC ISA slot and has the same programming and instruction set as stand-alone PLCs. So if you already have a PC on the factory floor, you don't need to buy a PLC in a box, just the board that plugs into the PC. It has a separate power supply so that if the PC crashes, the PLC keeps running. The board also has its own I/O connectors.
Another type of PLC is an industrially hardened gateway device. It comes with a number of ports and can be loaded with a soft
PLC. It has Ethernet, DeviceNet, and serial communications and serves as a bridge between protocols on the factory floor between legacy systems and current systems. For instance, data from serial devices such as bar code scanners can be connected to the gateway and through the soft PLC program.
Generally, defining PLCs in terms of I/O is the best way to go. Micro PLCs, for instance, have up to a maximum of 256 I/O. Some have Ethernet connections, good resolution, analog capabilities, high-speed counter inputs, and broad instruction sets.
Nano PLCs are application specific with limited options for expansion. Examples include highly defined tasks such as commercial lighting and vending machines. They have a narrow instruction set and small programs, typically only 100 lines or so. At its simplest, it's an inexpensive replacement for relays. You can add on a very limited number of I/O later on. Typically there is no analog capability, no computation ability, and the programming is very basic.
Nano PLCs let vending machines be networked back to a center to monitor what needs refilling. This is what designers want: a $100 controller with an Ethernet connection, proving that connectivity is more valuable than other functions.
One front on which PLCs are seeing more action is elevator control. Mid-America Elevator of Indianapolis has found a way to address the attrition rate of PC-based OEM elevator-control equipment. Instead of using PCbased equipment, they create an overlay, a type of retrofit, using a PLC. Overlay installations run the gamut from two-stop hydraulic elevators to multistoried high-rise elevators.
With PC-based control, it can take 10 to 12 weeks for an OEM to wire up the controls for an elevator car. With the PLC overlay, the same job takes about 2 weeks, and costs are about half. "A PC control might have 20 to 25 interface boards that are $2,000 to $5,000 a piece," says Mike Siler, vice president of engineering for Mid-America Elevator. "We can do the same thing with a small PLC."