Applying gain scheduling

Exploring a process scenario that may be a good place to apply a gain scheduling strategy.


Dear Control Engineering: I enjoyed your article about gain scheduling. I’m curious as to your thoughts about another variable gain scenario. Suppose a PID loop heats a measured but unregulated flowing stream. The changing flow rate would seem to change the response dynamics of the heating loop, requiring different tuning parameters. Is this a suitable case for implementing a gain schedule based on flowrate similar to your tank level application?

Thanks, Dale Cresap.

Vance VanDoren’s Back to Basics article drew a number of comments including this one. Here is Vance’s response:

Yes, I think it is. The time constant of your heating process probably lengthens as the flowrate goes up since a larger portion of the heat added by your PID loop is carried downstream, thereby increasing the time required to reach the setpoint. And since more heat is required to achieve the same temperature change, your process gain probably drops as the flowrate goes up. A gain schedule in your controller should help you combat those effects by forcing the controller to be more aggressive at higher flowrates.

I talked to Vance about his suggestion to clarify a couple points. First, we’re assuming that the temperature sensor feeds its signal to the PID controller, which modulates the heater. Vance thinks that setting the gain schedule based on flow can probably be done more easily from empirical data based on observation rather than a formula, although either could be possible.

I would add another thought, and ask which measuring device should be operative sensor. If the temperature is the wider variable and flow is less subject to change, then the discussion above is probably the way to go. On the other hand, if temperature is relatively stable and the flow can change more drastically, maybe it should be flowmeter? Of course that suggest a completely different control strategy, which we’ll leave for another time.

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.
Choosing controllers: PLCs, PACs, IPCs, DCS? What's best for your application?; Wireless trends; Design, integration; Manufacturing Day; Product Exclusive
Variable speed drives: Smooth, efficient, electrically quite motion control; Process control upgrades; Mobile intelligence; Product finalists: Vote now; Product Exclusives
Machine design tips: Pneumatic or electric; Software upgrades; Ethernet advantages; Additive manufacturing; Engineering Leaders; Product exclusives: PLC, HMI, IO
This article collection contains the 5 most referenced articles on improving the use of PID.
Learn how Industry 4.0 adds supply chain efficiency, optimizes pricing, improves quality, and more.

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

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