Gain scheduling and process linearity

Can we really have a linear process nonlinearity? Sure!


Dear Control Engineering: I was reading the article on gain scheduling and was trying to relate that to process linearity, or a lack of linearity. How do those all interact?

The example that Vance VanDoren used in his article is a spherical tank, and how the amount of liquid required to change the level by one inch changes a great deal if you compare the top or bottom of the tank with the middle. Manipulating the level is subject to the non-linearity that is intrinsic to that kind of tank. That’s why you need something like gain scheduling.

Some months ago we considered the notion of process linearity and what that means. There are elements of a process that tend to be more linear than others. Some of those elements are unavoidable and part of the nature of how chemicals interact, or they’re determined by the characteristics of the equipment, like the spherical tank. Some of those nonlinear elements have linearizable nonlinearities. That is not a contradiction in terms. In the case of the spherical tank, you can predict exactly how the fill rates behave at each end and your gain scheduling scheme can take that into consideration. If you can characterize it properly, it ceases to be a nonlinearity.

Others nonlinearities are due to bad design, like a poorly sized pump or valve that is always working at one end of its adjustability range or another saturated element. They are harder to control because they are more erratic in every respect. They can also destabilize other parts of the process. Those things can usually be fixed if you want to make the effort, but it’s more a hardware issue in most cases.

Peter Welander,

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.