Surprisingly Nostalgic

» A B O U T T H I S I S S U E

What is it about classical controlthat attracts us? Is it the maththat we find so intriguing? Is it

the sense that, despite decades of“modern control” research, classicalcontrol tools and insights remain—arguably—the most effective weaponin the control practitioner’s arsenal? Oris it a touch of nostalgia for the simplersingle-input, single-output (SISO) andanalog days, when embedded code didnot yet exist?

In the February 2005 special section,“Innovations in Undergraduate ControlEducation, Part II,” IEEE Control Sys-tems Magazine published two articlesthat discussed links between Bode androot locus analysis. The insights ofthose articles—by Associate EditorsKent Lundberg and Takis Tsiotras—suggested to me that classical control isa remarkably subtle subject that cancontinue to fascinate and surprise us.What other surprises might be hiding?

The articles in this special section,Classical Control Revisited, cametogether gradually and unexpectedlyfrom diverse sources. The germ was atalk that Dan Davison gave at the Uni-versity of Michigan in 2004 on the num-ber of controller degrees of freedom.The basic setting was classical, but theresults were—again—surprising.

Around that time I received a sub-mission from Bill Messner, Mark Bedil-lion, Lu Xia, and Duane Karns on anonclassical variation of classical leadand lag filters as well as a manuscriptfrom Wieslaw ‘Wiesiek’ Krajewski andUmberto Viaro on new insights into rootlocus. It slowly dawned on me that Ihad the components of a special sectionthat could take a fresh look at classicalcontrol issues and tools. The project

8 IEEE CONTROL SYSTEMS MAGAZINE » FEBRUARY 2007

Contributors

Jie Chen, after descending from a climb-ing trip above 6,000 meters in Sichuanprovince in Northwestern China.

Daniel Davison, his wife May-Jane, andtheir daughter Miranda.

Wieslaw Krajewskiand Karol.

Mark Bedillion.

From left:DavidTrumper,Kent Lundberg,and HaynesMiller.

FEBRUARY 2007 « IEEE CONTROL SYSTEMS MAGAZINE 9

became a reality when Dan agreed towrite up his ideas on controller degreesof freedom, and he, together with JieChen (the Bode integral guru), consent-ed to serving as guest editors.

In the meantime, Kent Lundberg,David Trumper, and Haynes Miller vol-unteered to tackle one of the thorniestpoints in Laplace transform theory,namely, the “zero-plus-versus-zero-minus” schism, known infamously as“troubles at the origin.” Informal discus-sion on this point had continued spo-radically since Jessy Grizzle’s bookreview appeared in the June 2004 issueof IEEE Control Systems Magazine. It wasclear that the special section would pro-vide an excellent venue for getting tothe bottom of this issue.

Finally, an article by JaganathChandrasekhar and myself addressesthe question of what can be done if aclassical servo loop has an unknownbias in the sensor measurements as wellas an unknown offset in the actuator. Itturns out that integral control is onlypart of the story.

The leadoff article byDan and Jie providesadditional background onthe special section andmotivates the content ofeach article. To comple-ment these feature arti-cles, we bring you ahistory column by ChrisBissell on the Moniacanalog computer. Whilemany of us know thatmoney can flow likewater, the Moniac exploitedthe converse fact, namely, that water canflow like money. This issue also includesreviews of three books with a direct rela-tion to classical control. For the LighterSide, Dan Simon contributes his secondcrossword puzzle, which reflects thetheme of the special section. For Dan’sfirst contribution, see the October 2006issue but don’t peek at the solutions.

Now, a few words about what’s instore for the rest of 2007 and beyond. Inaddition to more articles on classical con-trol, we’ll soon begin a series of articles

on modeling. These arti-cles are major invitedworks that reflect theimportance of modelingto our community as wellas our unique insightsinto the process and toolsof modeling for analysis,simulation, and control.We also have several spe-cial issues in preparationon topics ranging fromcomplex networks to sys-

tem identification to iner-tially stabilized platforms. Lookingfarther ahead—as far as 2009 and 2010—our Kalman filter anniversary specialissue plans are moving forward and—dare I say it?—gaining momentum.

Finally, let me remind you that IEEEControl Systems Magazine publishes let-ters from readers on any aspect of themagazine. Please share your views. Oh,yes, I almost forgot to mention thatwe’re always seeking short, technology-related articles on any aspect of control.I hope to hear from you.