PCs versus PLCs for process control

Soapbox: Don’t settle for faults inherent in PLCs and PCs. Take the best of each for a more effective hybrid system for process control, according to a software and hardware provider to the process industries.


PLCs have been an old-time favorite for automation and control. The top-down programs and input-output type of ladder logic offered reliable control of process. However, with the ever-growing need for production information and quality control, the inadequacies of the PLCs were exposed.

Turbomation engineers software and hardware integrated systems; hybrid PC and PLC control systems are more popular than ever, the company says. Courtesy: Turbomation

Enter the PC as a tool for automation and control. The PC, with its user friendly operating systems, looked more like a candidate to displace the PLCs. However, the reliability of the operating systems left a lot to be desired. If working on a word processing document and the PC freezes, it is simple to hit the reset button. This, however, is not an acceptable solution when a PC-controlled machine is reaching dangerous operating conditions. Hence the PC is not an answer for process control.

The PLC is inadequate when complex mathematics is required. PLCs do not handle image analysis well either. Even process control like temperature and pressure are not well handled by PLCs where high levels of accuracy are needed. This kind of temperature and pressure control is better left to intelligent field devices like dedicated temperature or pressure controllers. Recently PLCs have been benefitting from special process control modules that may be added to PLCs. One thing that PLCs do very well is discrete control like opening and closing valves. PLCs have been fairly efficient in positioning for robotics.

In short, the question is how to integrate the reliability of the PLC’s discrete control, the precise control of intelligent field devices, and the processing power of the PC. The obvious answer is a hybrid system with the PC as the supervisory device with delegated controls to the PLC and other field devices.

All calibration, data analysis, and data history are handled by the PC. The human machine interface is provided by the PC. Actual process control is carried out by the PLC and the field devices best suited for the application. With the advent of interdevice communication capabilities, hybrid control systems are more popular than ever.

This way the best capabilities of all the devices are safely leveraged for the best results. This model also lends itself to cloud-based remote control or data acquisition.

- Jacob George is application engineering manager at Turbomation, a company engaged in engineering and automation consultation for process control. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.