PCs point the way

PLC or a PC? A turn to PC-based control is in the wind.

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By Dan Hebert, PE, Technical Editor

Dan HebertGiven enough time and resources, a controls engineer can do just about anything. This includes shoehorning a PLC into an application suited for a PC, or vice-versa. However, in the real world of financial and time constraints, it pays to use the control platform that fits the application. Across various OEM applications there are very specific factors that drive the PC versus PLC decision.

PLCs have long been the control platform of choice for industrial OEM machinery. These workhorses have performed well for years and are ubiquitous across the OEM landscape. Nevertheless, new factors are coming into play that make PCs a compelling choice for many monitoring and control tasks. Applying a PLC to a task suited for a PC can result in a complex, expensive, and high-maintenance control system.

Cost Plus
PLCs are often considered the low-cost option, but machine control and data-handling requirements often make a PC the economical choice. “None of the PLC manufacturers we contacted could provide the motion control [with linear interpolation], recipe storage, and OI requirements within our budget,” says Tim Burnier, vice president of engineering, Research Automation, Aurora, Ill.
While Research Automation manufacturers various types of machinery, this example focuses on their seal trimmer. The seal trimmer has seven axes of motion with about 50 I/O. Two encoders (in addition to seven associated with the motion) are used to monitor tool holder location. Recipe storage requires capacity for 1,000 part numbers, and each number has about 75 variables associated with it. The operator interface has more than 50 screens customized to each application and customer.

PLCs were not only expensive for this application, they also needed multiple platforms. “And we were concerned about the robustness of the drivers used for communication among these platforms,” notes Burnier. The PC solution allowed control, data handling, and operator interface to be executed on one platform.

Research Automation chose an integrated operator interface and motion control PC-based solution. “Eason Technology provided, installed, and configured an operator interface (OI) with a Galil seven-axis PC/104 motion control system. The OI software included the drivers required to share variables with the motion controller,” says Burnier.

High Speed Demands a PC 
That's not a misprint. PLCs have always been thought of as the first option for real-time control of high-speed applications, but this is a misconception. “There is a perception that PLCs are hard real-time controllers with scan time deviations of less than 100 microsec., but this isn't the case,” says Nat Frampton, president of Real Time Development, Pearl River, La. “In today's real-time operating system discussions, the PLC is classified as a soft real-time system.” Real Time Development provides system integration services to OEMs.
Very-high-speed applications require dedicated control boards that plug into the PC bus. PLC scan times are neither fast enough nor deterministic enough to handle such tasks. “PCs provide the speed required by applications such as quality rejects of discrete parts, high-speed sortation, and many motion control applications,” says John Nesi, strategic marketing manager, Rockwell Automation.

I/O scan times are one of the key speed requirements for a control system, but processing of I/O data is also important. “A PLC could perform the machine sequencing [without problem], but could not come close to the speeds required to interface to high-speed, on-line sensors,” reports Michael Iaquinta, vice president of tobacco operations for Automation and Control Technology, Columbus, Ohio. ACT makes custom precision measurement systems for the tobacco and metal industries. "The PC also allows us to add I/O without impacting processor speeds."

Another important speed requirement is data handling and manipulation. This is an area where PCs excel. “PLCs simply do not have the high-speed arithmetic and data handling capability required for our application,” says John Wargelin, director of operations for Pile Dynamics, Cleveland. "A PLC could control the pile driver, but for the high-speed, data-intensive tasks, we needed a PC." The company's pile driver analyzer monitors movement, acceleration, and force of pile drivers.

Pile Dynamics worked with Applied Data Systems to implement a PC-based solution running under Windows CE. “Specialists like Pile Dynamics can encode their vast know-how into a rugged device that can be used by a construction worker at the site,” adds Larry Ricci, business development manager for Applied Data. "For end users, it is like buying a world-class Ph.D. consulting engineer in a box."

One Platform for a Total Solution
Many OEM control systems must perform three functions: machine control, operator interface, and communications to other computing systems. PLC-based systems often employ a PLC for control, a graphics terminal for operator interface, and a PC or other protocol translation device for communications. A PC can perform all three of these functions in many machine control applications.

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