Tuesday, March 1, 2011

PC based Control Challenges Compare to PLC

The PC provided the software capabilities to offer a graphical rich programming, perform advanced tasks, and user environment, and utilized COTS components allowing control engineers to take advantage of technologies developed for other applications. These technologies include high speed I/O busses, floating point processors; such as PCI and Ethernet; non-volatile data storage; and graphical development software tools. The PC also provided highly productive software, unparalleled flexibility, and advanced low-cost hardware.

However, PCs were still not ideal for control applications. Although the PC was used by many engineers when incorporating advanced functionality, such as database connectivity, analog control and simulation, web based functionality, and communication with third party devices, the PLC still ruled the control realm. The main problem with PC-based control was that standard PCs were not designed for rugged environments.

The PC presented three main challenges:
1. Reliability: With non-industrially hardened components and rotating magnetic hard drives, PCs were more prone to failure.
2. Unfamiliar Programming Environment: Plant operators require the ability to override a system for troubleshooting or maintenance. They can manually force a coil to a desired state, and quickly patch the affected code to quickly override a system using ladder logic. However, PC systems require operators learn new, more advanced tools.
3. Stability: The PC’s general purpose operating system was not stable enough for control. PC controlled installations were forced to handle unplanned rebooting and system crashes.

Although some engineers use special industrial computers with special operating systems and rugged hardware, most engineers avoided PCs for control because of problems with PC reliability. In addition, the devices used within a PC for different automation tasks, such as communications, I/O, or motion, may have different development environments.

So the "twenty percenters" either lived without functionality not accomplished easily with a PLC or cobbled together a system that included a PLC for the control portion of the code and a PC for the more advanced functionality. This is the reason why so many factory floors today have PLCs used in conjunction with PCs for connecting to bar code scanners data logging, , inserting information into databases, and publishing data to the Web.

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