The PLC is dead! Introducing the PAC!
Once written off by some as a dinosaur, the programmable logic controller is morphing into a more powerful programmable automation controller.
By Bob Trebilcock Editor at Large -- Modern Materials Handling, 2/1/2003
Software controls or programmable logic controllers (PLCs)? Which is best when it comes to controlling materials handling equipment on the factory and distribution center floor? That discussion has been raging for several years now.
For a time, PC-based software controls appeared to be gaining the upper hand. That's because PC-based software controls allowed users to control multiple functions, like motion control, logic, and process control on a single platform, rather than with multiple PLCs. What's more, PC-based software controls could be programmed in common languages, like Windows, and integrated with plant floor and enterprise-level ERP systems with standard, rather than proprietary, interfaces.
In Europe, there are those who say the battle is over, with software controls the victor, especially in high-volume order fulfillment centers and complex manufacturing environments.
"The PLC is very good at controlling conveyors," argues Juergen Conrad, president and CEO of viastore (616-656-8876), a European provider of automated storage and retrieval systems (AS/RS) that favors software controls. "But if you have an AS/RS, robot, or other materials handling systems with multi-axis movement, you really need a PC."
Not so fast, counter the PLC manufacturers. To counter the trend, they are introducing a new generation of PLC controlling devices with many of the capabilities that once gave the nod to PC controls, blurring the lines between PLCs and industrial computers.
"There's no question that part of the market believed that PLCs are dinosaurs and that everyone will run their factories and distribution centers from a desktop," says John Nesi, strategic marketing manager for Rockwell Automation (440-646-5000). "That just hasn't happened."
One reason PLCs haven't gone the way of T-Rex is their reputation for reliability. "The PLC offers users a degree of reliability for materials handling applications that just isn't available from PC controls today," argues Rob Dussault, marketing manager, motion control, Schneider Electric (847-397-2610).
Another is that PLC manufacturers have responded to the market's demands with a new breed of device that marries the reliability of a PLC with the open programming languages used by PCs. That means controlling devices utilizing commercial computer chips and Windows CE
"You're seeing the convergence of PC technology with the PLC," says Mike Berryman, product manager for Advantech Automation Corp. (513-742-8895). "That creates a more integrated approach to the systems on the factory or warehouse floor."
Given the number of PLCs already in the field, those are big pluses to users who want more sophisticated control without ripping out their legacy systems to install PC controls.
"By definition, a PLC is an industrial box with instant on and high reliability," says Nesi of Rockwell Automation. "It's built with languages suited to industrial applications. That diminishes the risk to the user, especially if that user is someone doing high-speed sortation with high-speed actuation."
PLC to PAC
Craig Resnick, director of hybrid manufacturing research at ARC Advisory Group (781-471-1000), calls this new generation of controlling device a PAC, which stands for programmable automation controller.
"The label PLC simply understates the capability of current automation systems," contends Resnick.
What differentiates a PAC from a PLC? For starters, a PAC is capable of true multi-tasking. One device can control motion, drive, logic, and process control on one platform rather than several. That means the system is not being developed in a multitude of environments.
"Our low-level PAC can now run three simultaneous tasks with the same device," says Carl Larsson, general manager, product sales, ABB Inc. (262-785-8301). "At the higher level, one PAC will run up to 16 tasks."
Under the hood, you'll find the same off-the-shelf chips managing the applications that you find in commercial PC technology. That reduces the cost of maintenance and upgrades.
What's more, PACs use de-facto standards like OPC and XML for interoperability. That allows users to move more of the decision-making to the machine level, and to exchange data as part of a network of multi-vendor systems. It's that combination of commercial chip technology and open software that is blurring the line between PC-based and traditional control systems.
Finally, PACs bring a new level of computing and controlling power to smaller devices than traditional PLCs. "You can now get full control capability plus the ability to send e-mails from a device that's the size of two cigarette packs," says Bill Lydon, product manager of electronic products for Wago Corp. (800-346-7245)."
The flexibility of these new PACs is particularly useful in distribution centers where speed is important. "There's more of a demand on materials handling applications for high speed," says Dussault of Schneider Electric. "That includes the speed of throughput through the work cells or the warehouse, and it includes the speed of bringing a system back on line if it goes down."
The expanded power and memory in today's PACs provides the "hot standby" functionality that's required by more and more distribution centers. That allows the system to retain the knowledge of where things were at the time the system went down.
Finally, PACs facilitate modularity in materials handling systems—the creation of individual cells of automation that can be easily added to a larger network. "What we're seeing in the field is the demand for PLCs with just a few input/output points that can be connected to a larger network and integrate to a higher-level PLC," says Dussault. "The advantage is that you have infinitely modular design capability. Individual stations can be maintained and delivered at a very economical level."
Information driven
The multi-functionality of a PAC enables more than just the movement of materials through a factory or warehouse. It also facilitates the exchange of production and order fulfillment information throughout factory, warehouse, and distribution center operations or to the enterprise resource planning (ERP) applications.
Traditionally, there were three layers of communication in these systems: controllers at the machine level that communicated with inputs and outputs; a second layer that collected information from and between controllers; and finally the Ethernet network where enterprise applications are located.
There was, however, lag time, or disconnect, as information passed between these various layers, especially if that information was batched from one system to the next. That disconnect between the shop floor and the ERP running the business was significant, especially as the demand for real-time information increased. "A consultant I know in the ERP business has a saying: what we typically get isn't the truth about our systems, but a version of the truth that's updated in the middle of the night," says Bill Lydon of Wago Corp. "That's why it's important for these devices to capture data in real time."
With new PACs tied directly to the Ethernet, those three architectural layers are collapsing into a single communications network. Remove the layers, translators and proprietary networks and data flows freely.
"The controller is now a true extension of the enterprise computing system," Lydon says. "The closer you can capture real-time data to the source of the data, the more accurate that data is going to be."
What's more, these de-vices utilize common computer and communication languages. "The next generation of PLCs can use XML language for communication," says Glenn Graney, manager of commercial marketing for GE Fanuc Automation. (800-433-2682). "That means you can publish the data from your PAC representing the process, and anyone on a PC can use a browser screen to see that data directly from the PAC in real time, like a current stock price on the Internet."
What's more, Graney adds, the data is better. "Every time you combine or move data, you potentially change the data like the old game of telephone."
Predictive maintenance
The ability to collect accurate real-time data can also be used for predictive maintenance, rather than preventative maintenance. That means addressing an issue because something is needed, rather than doing it just because it's scheduled to be done.
"Smarter controllers can do more effective gathering, filtering, and measuring at lower levels of the operational food chain," says Rockwell's Nesi. "As the PAC senses things going wrong in the process, you can make predictive decisions and then link that back to scheduling software for maintenance people."
The expanded informational qualities also allow the PAC to be used for quality control during the production or order fulfillment process, rather than waiting to do a quality control check at the end of the line.
"A PAC can capture data about quality at each work station as the tasks are being performed," says Graney of GE Fanuc Automation. "When you're making a widget, you're immediately checking to see that it's made correctly at the work station, rather than finding out at the tenth station that an error occurred at the second station."
Going forward, suppliers and users will continue to find new uses for PACs in complex materials handling systems, displacing traditional PLCs and taking on even more of the attributes of PC controls and industrial computers.
"The PLC still has a long, long life ahead of it," says Resnick of ARC Advisory.
Click on the icon to read more about new developments in automation control.
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