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It's a challenge every design engineer faces. For some it is merely a nuisance, but for others it can be a sleep-depriving exercise in frustration. Choosing the right hardware platform for industrial and control applications is not an easy task.
Industrial applications need to incorporate integrated hardware and software; offer a short time to market; and be highly reliable, powerful and flexible, low cost, and easy to maintain and upgrade.
For the last decade, a passionate debate has raged over which platform provides the best solution for industrial control and automation. The debate has mainly centered around two solutions: programmable logic controllers (PLC) and PC-based control. Each platform has its own strengths and limitations, and in some simple cases the answer is clear.
However, when applications become complex and demand such features as high-speed control, motion, special analog I/O, machine vision, and more, the line between the various platforms for industrial control becomes blurry and challenging to navigate.
The PLC
For nearly three decades, the PLC has been the automation solution of choice for industrial control engineers. Familiar and reliable, the PLC has evolved to incorporate analog I/O, network communication, and new programming standards. Experts from Automation Research Corporation (ARC) and Venture Development Corporation (VDC) estimate:
80% of industrial applications can be solved with digital I/O and a few analog I/O points, and programming requiring simple logic
77% of PLCs are used in small applications (fewer than 128 I/O channels)
72% of PLC I/O is digital
Because engineers meet so many industrial application challenges with traditional PLC solutions, there is a strong demand for simple low-cost PLCs.
This demand has spurred the growth of the low-cost PLC market, but it has also created a discontinuity in controller technology. Most PLCs are designed to provide solutions for straightforward control applications.
However, the PLC has not traditionally delivered viable solutions for applications that require higher loop rates, advanced control algorithms, more analog capabilities, and better integration with an enterprise network.
The PC
To address these PLC hardware limitations, many engineers have evaluated using PCs for industrial control. The PC provides software capabilities for advanced tasks, offers a graphical, rich development and user environment, and uses commercial off-the-shelf (COTS) components. The PC also delivers unparalleled flexibility, highly productive software, and advanced low-cost hardware.
However, PCs are not ideal for control applications. Although they are still used when incorporating advanced functionality such as analog control and simulation, database connectivity, Web-based functionality, and communication with third-party devices, the PCs fail in one key industrial aspect: standard PCs are not designed for rugged environments.
This lack of rugged design presents three challenges for PCs in industrial settings. The first and most limiting of these is stability. In the industrial setting, where hourly downtime is often measured in thousands or even millions of dollars lost per hour, the PC's general-purpose operating system has experienced system crashes and unplanned rebooting that can make it a risky choice for control.
The second issue is closely related to the first: reliability. With the majority of computer hardware sources serving the consumer market, many standard PC components cannot withstand the often harsh conditions in an industrial setting.
Heat and dust wreak havoc on rotating magnetic hard drives and power supplies, and humidity and dirty power can lead to shortened life spans and reliability issues with other core PC components.
Finally, the PC has presented automation engineers with a complex and unfamiliar programming environment. Plant operators need the ability to override a system for maintenance or troubleshooting.
With PLCs, they can manually force coils to desired states and quickly patch the affected code to override a system. But with PC systems, operators are required to learn new, more advanced tools.
The PAC
The industrial PC sector has expanded and improved in recent years, but the PC is still a challenging platform for industrial automation applications. Although some engineers use these newer systems with more rugged hardware and special operating systems, they face the task of piecing together a system that has the reliability of a PLC and the added functionality the PC provides.
These hybrid systems, while offering a solution to the design problem, add multiple-vendor hardware and software integration issues as well as the difficulties that come from the inevitable maintenance and system upgrades.
Figure 1: Incorporating the benefits of PCs and PLCs, PACs can often be as powerful, and I/O rich as an industrial PC and as rugged and reliable as a PLC.
With no clear PC or PLC solution, engineers who have implemented such hybrid systems have worked closely with control vendors to help develop a new class of industrial controllers.
The resulting new controllers, designed to address the needs of architecturally complex applications, combine the best PLC features with the best PC features.
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