The loop, the whole loop, and nothing but the loop

Sometimes we need to think about an individual control loop. Other times we need to consider the larger control system. You should be able to do either when appropriate.

04/09/2013


As control engineers, we often get tunnel vision. We get very focused on a single task such as creating our control module configuration and lose sight of the bigger picture – creating a control system. While working on my bachelor’s degree, I worked as an instrument designer for big A&E firms that built power generation stations. My job was to specify instrumentation and control elements for these projects, design control panels, create loop sheets for the hookup of the electronic instruments, and installation details for the sensing side – or in the case of pneumatic instruments – the pneumatic tubing. At that point in my career, a loop was from sensor to controller and from controller to control element. I had nothing to do with specifying the controller itself, which at that time was typically a standalone device mounted on the control panel. I also completed all drawings to control the operation of electrical prime movers, which were wired back to the switches on the control panel.

Jump forward in time a few years to the introduction of the DCS. Instead of following my usual practice of calling the Bailey or Foxboro guy and just buying a standalone PID controller, I was now expected to program the control function as well as direct the designer on the instrument and control element design. This really did open up a world of possibilities for the control design strategy that would have been impractical before for creating highly interactive and complex controls. Unfortunately, for many of my contemporaries who hadn’t come up through the path I had, it also focused them on what was happening in the space between the input and output terminals with little or no regard to what was happening outside the box.

During that period, I moved from the design engineering firms to work for a DCS supplier. In working with some of their project teams, I would often inquire into the design of specific control modules, which they referred to as control loops. My inquiries would usually take the form of, “What kind of transmitter are you using or what kind of control element, valve, pump, or fan are you operating?”

To my horror, the usual answer was, “I don’t know.” To make matters worse, that was often followed with, “It doesn’t really matter, this will control anything.”

Now, on the surface, this is a reasonable answer since you can control almost anything in a typical process with a PID function and some appropriate interlocks or permissives. The problems arise when you can’t answer simple loop questions like:

“Is the loop direct or reverse acting?”
“Does the valve fail open or fail closed?”or,
“Is the measurement linear?”

Without understanding the whole loop, you can easily misapply the control functions within the control module. Other more esoteric aspects of the loop that might need understanding could include the need to start a very large fan against a closed damper to prevent overloading the motor. If the loop designer doesn’t know this, then the controls may be designed to force the operator to startup the process incorrectly. In many ways, it is now more important than ever that the person designing the controls understand the whole loop to ensure that he or she doesn’t prevent the operators from doing their job.

This post was written by Bruce Brandt, PE. Bruce is the DeltaV technology leader at MAVERICK Technologies, a leading system integrator providing industrial automation, operational support, and control systems engineering services in the manufacturing and process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, and business process optimization. The company provides a full range of automation and controls services – ranging from PID controller tuning and HMI programming to serving as a main automation contractor. Additionally MAVERICK offers industrial and technical staffing services, placing on-site automation, instrumentation and controls engineers.



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 Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.
Electric motor power measurement and analysis: Understand the basics to drive greater efficiency; Selecting the right control chart; Linear position sensors gain acceptance
Protecting standby generators for mission critical facilities; Selecting energy-efficient transformers; Integrating power monitoring systems; Mitigating harmonics in electrical systems

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.