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.
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, firstname.lastname@example.org
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, email@example.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.
- Events & Awards
- Magazine Archives
- Oil & Gas Engineering
- Salary Survey
- Digital Reports
Annual Salary Survey
Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.
There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.
But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.
Read more: 2015 Salary Survey