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 Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Make Big Data and Industrial Internet of Things work for you, 2017 Engineers' Choice Finalists, Avoid control design pitfalls, Managing IIoT processes
Engineering Leaders Under 40; System integration improving packaging operation; Process sensing; PID velocity; Cybersecurity and functional safety
Mobile HMI; PID tuning tips; Mechatronics; Intelligent project management; Cybersecurity in Russia; Engineering education; Road to IANA
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance

(copy 5)

click me