Controls basics
Programmable logic controls, PC-based controls and embedded controls are the building blocks that make your automated materials handling systems run.
By Bob Trebilcock, Editor at Large -- Modern Materials Handling, 4/1/2007
Anyone who’s ever walked through a highly automated distribution center or factory can’t help but wonder how the system knows what to do next. For instance, how does the system decide where to put away a pallet in an automated storage and retrieval system, when to turn on a conveyor, when to divert a carton to a specific shipping lane, or how to palletize cartons in a specific pattern?
Making that happen with speed and accuracy falls to automation control devices that sit between the machines they control and a business application software system like an enterprise resource planning (ERP) system, warehouse management system (WMS) or warehouse control system (WCS).
Think of them as the building blocks and the brains behind automated systems.
Three types of controls predominate on the warehouse and factory floor:
- Programmable logic control (PLC): A PLC is a dedicated programmable processor that uses the information it receives from a business application system to control automated machinery in the right sequence to perform operations.
- PC-based control: Also known as soft controls, PC-based controls manage the same processes as a PLC, but with an industrialized PC. These controls typically have more memory and processing power than a PLC.
Embedded control: An emerging technology, embedded controls are a compromise between a PLC and a PC-based control. Housed in a form factor that resembles a PLC, an embedded control is loaded with a kernel of an operating system, like a PC, and the application software required for a specific task.
While each of the three technologies has its own unique characteristics, the line between these different control devices is blurring. That is especially the case with PLCs and PC-based controls.
While there is still a performance gap, PLCs are processing transactions nearly as fast as PCs. And while PLCs were designed specifically to withstand the rigors of a factory or warehouse setting, industrialized PCs are more reliable than ever. Finally, while still developing, embedded controls combine the best of both worlds into one design, say some experts.
For that reason, the decision to use one over the other is often driven as much by the functionality of the device as by the philosophy of the system’s designers or by what existing systems use. Still, differences exist between the three.From PLC to PAC
PLCs have been on the job in industrial settings for more than 40 years. They are the brains of an automated system—housed in a non-descript industrialized package. No one will invite a PLC to the prom, but what happens inside the box is pretty remarkable.
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| Programmable logic control. PLCs are the brains of an automated system, using information received from a business application to run motors and monitor the status of a system. |
Using information received from a business application, a PLC: turns on and off the motors that run machinery like conveyors and sortation systems; monitors the status of the system and conditions from the amount of vibration to the temperature of equipment that could affect performance or require maintenance; and collects important data like when a specific cart passed a photo eye.
As the name suggests, a PLC is a programmable device that includes a processor and some memory but is without moving parts that can fail. For instance, a PLC doesn’t have a hard drive or fan.
When PLCs were first introduced in the 1960s, they replaced electromechanical devices. “To make an automated system work, people used big relay panels with complicated wiring schemes between the relays,” says Bob Nelson, marketing manager for controller and I/O products at Siemens Energy & Automation. “If something went wrong or your processes changed, they weren’t very user-friendly to work with.”
Because a PLC was programmable, it eliminated those big relay panels. But early PLCs were still limited in their function, and it often took more than one PLC to get the job done. One PLC, for instance, might control motion while another controlled positioning.
“You had controllers for materials handling, controllers for production and controllers for packaging,” says Lee Lane, marketing manager for Rockwell Automation. “That meant you needed maintenance personnel and spare parts for all of those applications.”
Today’s PLC can handle the vast majority of those applications in one unit. They are also continuing to evolve into more powerful devices known as programmable automation controls, or PACs. They operate at faster speeds than a PLC, can be programmed in more languages with more functionality, and have some memory. They can also communicate and share more information across a network than a PLC.
“A PC is still faster and has more memory than a PAC,” says Bill Black, product manager of controllers for GE Fanuc Automation. “But the gap between what a PC and a PAC can do is getting smaller, and a PAC costs a lot less money than an industrialized PC.”
Taking the PC to the shop floor
There was a time in the 1990s when PC-based control systems, or soft controls, were expected to be the nail in the coffin for PLCs.
That didn’t happen. By some estimates, PLCs account for more than 90% of the control systems at work in industrialized settings today. The biggest disadvantage—and the biggest hurdle—to PC-based control systems is fear of the blue screen of death any PC user dreads.
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| PC-based controls. PC-based controls perform the same function as a PLC, but with more speed and more memory for high-speed applications with high amounts of data. |
Still, there are applications where PCs shine.
“The advantage of a PC is that it is very fast and has a large amount of memory on board, which means you can have a database as part of the control system,” says Bjoern Falke, lead product marketing specialist for Phoenix Contact.“And since industrial PCs have expansion slots, you can support several motion cards to control multiple applications in one unit that can be integrated to a network or field device.”
A PC-based control system includes a screen and some way to easily interact with an operator, like a keyboard or touch screen, known as a human machine interface (HMI).
Raw computing power means that PC-based controls are ideal for applications that require: very high speeds, such as a conveyor operating at 600 feet per minute or faster; a high degree of complexity, like an automated storage and retrieval system (AS/RS) storing a variety of products; and high amounts of data, such as a high-speed sortation system receiving bar scan reads.
“You are now getting PLCs with the capability to do all those functions,” says Falke. “But at a higher price than a traditional PC.”However, given that PLCs can do much of what PC-based controls did in the past, the decision to use a PC often comes down to one of two considerations:
Personal preference: Some systems designers simply prefer working with PCs in those applications. Others prefer PLCs.
Whether other applications in the facility are already using PCs or require an HMI. “With a PC, you can have control, a screen and the HMI all in one device,” says Black.
Embedded controls emerge
In the future, end users may not have to choose between a PLC and a PC-based control system, thanks to embedded controls.
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| Embedded control. An embedded control combines the power of a PC, including a software operating system, with the compact industrialization form of a PLC. |
Just what is an embedded controller? It’s an industrialized device that looks like a PLC. And like a PLC, it has no moving parts. Yet like a PC, an embedded controller includes a computer processor, a software operating system and the kernel of a software application that performs specific functions.
“An embedded controller combines the best of the PLC world with the best of the PC world,” says Falke.
An embedded controller, for instance, can combine an HMI, like the color touch-screen operator panel you would expect in a PC-based control system, with the functions of a PLC in a compact form. An embedded controller has the robust network connectivity capabilities of a PC.
There’s one other advantage to embedded controllers. End users who have developed proprietary software solutions to control their processes can put that software on an embedded controller to get the advantages of a PC in a rugged PLC form on the factory floor.“What we’re really starting to see with embedded controls is the merger of PC-based controls and PLCs,” says Nelson of Siemens Energy & Automation. “Over the next several years, the line between PLCs, PC-based controls and embedded controls is going to disappear and they will start to become one and the same.”
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Materials handling automation controls manufacturers
These manufacturers carry Programmable logic controls, PC-based controls, and Embedded Controls. |
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Company
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Telephone
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| ABB Group | 203-750-2200 |
| Allen-Bradley (Rockwell Automation) | 414-382-2000 |
| GE Fanuc | 800-433-8622 |
| Mitsubishi Electric | 973-291-4600 |
| Phoenix Contact | 800-322-3225 |
| Schneider Electric/Square D | 888-778-2733 |
| Siemens Energy & Automation | 800-964-4114 |
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