Self tuning has its limits

Regarding Self-Tuning Control (July 2007 by Vance Van Doren). A linear control loop will not develop a limit cycle — its cycle will either decay or expand, depending on whether loop gain is below or above unity, respectively. Limit cycling requires a nonlinear function somewhere in the loop to limit the amplitude of the oscillation, and the waveform is typically non-sinusoidal.

05/01/2008


Regarding Self-Tuning Control (July 2007 by Vance Van Doren). A linear control loop will not develop a limit cycle — its cycle will either decay or expand, depending on whether loop gain is below or above unity, respectively. Limit cycling requires a nonlinear function somewhere in the loop to limit the amplitude of the oscillation, and the waveform is typically non-sinusoidal. Changing the tuning of a controller in a limit-cycling loop usually does not eliminate the cycling, but only affects its amplitude and period, rendering self-tuning ineffective. The ineffectiveness of tuning efforts, along with a non-sinusoidal waveform, should direct the user to look for and eliminate the nonlinear element, such as valve deadband.

Greg Shinskey, shinskey@hughes.net

I could quibble over your assertion that self-tuning is ineffective because other phenomena can cause limit cycling, but I believe for different reasons that self-tuners are hard to implement successfully. In general, though, I agree with your observations. Limit cycling can only occur if a limiter of some sort (such as a valve’s maximum and minimum positions) to keep an unstable oscillation constant. Still, the information available from a limit cycle waveform can be used to tune a loop, as Ziegler and Nichols showed in 1942.

Vance VanDoren, controleng@msn.com

Greg responds: I agree that a limit cycle caused by an on-off controller, or by excessive proportional gain, can be used to tune a controller, and some auto-tuners operate this way. But it would not make sense for a self-tuner to wait until the valve cycled between its limits before retuning. And for a velocity-limited valve, the period of that large limit cycle could be much longer than that of a small-amplitude cycle. However, some limit cycles do not drive the controller output to its limits. A very common limit cycle is caused by valve deadband in a PI level-control loop. The manipulated flow cycles in a clipped sine wave, well within the limits of the valve, with amplitude and period varying with the P and I settings. No combination of those settings will eliminate the cycle. A self-tuner will fail in this loop because there is no stable state.

TalkBack is the online comment section of www.controleng.com . Add your opinion to this discussion or any article online by scrolling to the bottom and clicking the button. Comments may appear in print, as space allows.





Engineers' Choice Awards
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers.
System Integrator Giants
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
How to Maximize Factory Automation Efficiency with Low Cost Machine Vision
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Wireless Reliability in Harsh Environments
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
Human Factors and the Impact on Plant Safety
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
June 2018
Discrete and process sensor fundamentals, autotuning controls, system integrator roundtable
May 2018
Salary and Career Survey, IT and OT convergence, robotic standards and safety, secure circuit protection
April 2018
Cybersecurity best practices, artificial intelligence, robotic additive manufacturing, embedded systems, IIoT integration, energy efficiency
Edge Computing
This article collection contains several articles on how today's technologies heap benefits onto an edge-computing architecture such as faster computing, better networking, more memory, smarter analytics, cloud-based intelligence, and lower costs.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
PLCs
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. Featured articles in this digital report compare PLCs and programmable automation controllers (PACs), industrial PCs, and robotic controllers.
SIDB

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

June 2018
Machine learning, produced water benefits, progressive cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
February 2018
Focus on power systems, process safety, electrical and power systems, edge computing in the oil & gas industry
John O. Ayuk, PE, CFSE, PMP, CAP
Automation Engineer; Wood Group
Doug Baker
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Engineers' Choice Awards
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers.
System Integrator Giants
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
How to Maximize Factory Automation Efficiency with Low Cost Machine Vision
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Wireless Reliability in Harsh Environments
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
Human Factors and the Impact on Plant Safety
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
June 2018
Discrete and process sensor fundamentals, autotuning controls, system integrator roundtable
May 2018
Salary and Career Survey, IT and OT convergence, robotic standards and safety, secure circuit protection
April 2018
Cybersecurity best practices, artificial intelligence, robotic additive manufacturing, embedded systems, IIoT integration, energy efficiency
Edge Computing
This article collection contains several articles on how today's technologies heap benefits onto an edge-computing architecture such as faster computing, better networking, more memory, smarter analytics, cloud-based intelligence, and lower costs.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
PLCs
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. Featured articles in this digital report compare PLCs and programmable automation controllers (PACs), industrial PCs, and robotic controllers.
SIDB

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

June 2018
Machine learning, produced water benefits, progressive cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
February 2018
Focus on power systems, process safety, electrical and power systems, edge computing in the oil & gas industry
John O. Ayuk, PE, CFSE, PMP, CAP
Automation Engineer; Wood Group
Doug Baker
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Engineers' Choice Awards
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers.
System Integrator Giants
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
How to Maximize Factory Automation Efficiency with Low Cost Machine Vision
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Wireless Reliability in Harsh Environments
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
Human Factors and the Impact on Plant Safety
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
June 2018
Discrete and process sensor fundamentals, autotuning controls, system integrator roundtable
May 2018
Salary and Career Survey, IT and OT convergence, robotic standards and safety, secure circuit protection
April 2018
Cybersecurity best practices, artificial intelligence, robotic additive manufacturing, embedded systems, IIoT integration, energy efficiency
Edge Computing
This article collection contains several articles on how today's technologies heap benefits onto an edge-computing architecture such as faster computing, better networking, more memory, smarter analytics, cloud-based intelligence, and lower costs.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
PLCs
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. Featured articles in this digital report compare PLCs and programmable automation controllers (PACs), industrial PCs, and robotic controllers.
SIDB

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

June 2018
Machine learning, produced water benefits, progressive cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
February 2018
Focus on power systems, process safety, electrical and power systems, edge computing in the oil & gas industry
John O. Ayuk, PE, CFSE, PMP, CAP
Automation Engineer; Wood Group
Doug Baker
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me