» A B O U T T H I S I S S U E

8 IEEE CONTROL SYSTEMS MAGAZINE » DECEMBER 2008

Imagine that you need to assemble astructure, such as the one depictedon the cover of this magazine. The

pieces would need to be joined togeth-er, one at a time, until the structure iscomplete. To do this, you would choosea starting point and then add one pieceat a time. You might find out, however,that, as you assemble the structure,some kind of external support is need-ed since the intermediate structures arenot as rigid as the completed structure.You might wonder, then, whether thereis a way to assemble the structure sothat all of the intermediate structuresare inherently rigid. The reverse prob-lem is also of interest: How do you take

the structure apart so that it doesn’t col-lapse in the process?

These assembly and disassemblyproblems are a metaphor for the aug-mentation and reduction of forma-tions, which Brian Anderson,Changbin Yu, Baris Fidan, and JulienHendrickx discuss in this issue ofIEEE Control Systems Magazine (CSM).In particular, they consider the prob-lem of adding nodes to a formation sothat, after each modification, theresulting formation maintains rigidi-ty. Rigidity means that the formationcannot be continuously deformedwhile satisfying the formation’sinternode distance constraints.

A formation may be minimallyrigid, or it may have redundancy in the

sense that rigidity persists even whensome nodes are removed. Therefore,during reduction, the removal of a sin-gle node may or may not destroy rigid-ity. A more extreme form of reductionis to decompose the formation into twoseparate multi-node formations. Theproblem then is to determine how toperform this decomposition so thateach of the resulting formations isrigid. Remarkably, there are powerfultools and methods—some classical andsome recent—for all of these problems.The subject is fascinating, and many ofthe results are surprising.

In this issue, we also continue thespecial section on modeling systemswith friction, the first installment ofwhich appeared in the October 2008

Brick by Brick

Digital Object Identifier 10.1109/MCS.2008.930159

Farid Al-Bender and Marina Van denBranden with their children Idries, Noor,and Hanna.Baris Fidan during a visit to Brussels. Josh Bishop

Brian Anderson and Changbin (Brad) Yu at Tianmen Cave inZhangjiajie, China before the 2007 CCC.

Avraham Harnoy (left) and Bernard Friedland.

A self-portrait of FaridAl-Bender.

Contributors

DECEMBER 2008 « IEEE CONTROL SYSTEMS MAGAZINE 9

issue. The first article in this follow-onsection, by Farid Al-Bender and JanSwevers, presents a large-scale approachto friction modeling in which small-scaleasperities combine to replicate the effectsof friction on a macroscopic level. Next,the article by Avraham Harnoy, BernardFriedland, and Simon Cohn discusses atechnique for measuring the effects offriction to validate a postulated frictionmodel. In a different direction, the articleby Y. Guo, Zhihua Qu, YehudaBraiman, Zhenyu Zhang, and JacobBarhen considers physics-based mod-els for friction that mimic molecularinteraction. Finally, the article by KarlAstrom and Carlos Canudas-de-Witdiscusses properties of the Lund-Grenoble (LuGre) friction model.

In “Applications of Control,” wehave an article on friction modeling forpart placement, which is needed forassessing manufacturing accuracy. Thisdepartment also includes an applica-tion of the minimum-variancesmoother to the control of machines forcoal-mine automation.

For “People in Control” we speakwith two journal editors, Jie Chen

and Brett Ninness, whoare editors of new andestablished controljournals, respectively.We also speak withTariq Samad, who iswell known to CSMreaders as a past editor-in-chief of this maga-zine and will soonbegin a term as presi-dent of IEEE ControlSystems Society (CSS).

But that is not all. For “HistoricalPerspectives,” we have an autobio-graphical essay by Jurgen Acker-mann, who relates his experiences inthe control field and shares somevaluable advice. We also have fourbook reviews and two conferencereports, and we end with a longoverdue pun.

Since this is the December issue, I’llclose with a few end-of-the-yearthoughts. First, I see the coming yearas auspicious on many levels, from themany new projects that CSS is embark-ing on to exciting developments in thediverse areas in which CSS members

are active. CSM is work-ing on numerous specialprojects, ranging fromautomotive safety tohybrid systems to Kalmanfilter applications. Thescope of control technolo-gy continues to expandinto new and challengingfields. Finally, the CDC—the CSS flagship confer-ence—will be held soon in

Cancun, Mexico. This is theideal venue for meeting others in thefield, exchanging ideas, and learningabout recent developments. During theconference, I hope to hear about yourideas for either long or short articlesfor future issues. If you cannot attend,then e-mail is always welcome, but myfirst choice is to hear your ideas in per-son. Finally, please be aware that the2009 CDC to be held in Shanghai,China, necessitates an earlier-than-usual submission deadline, so be sureto check the conference Web siteASAP. Happy New Year!

Dennis S. Bernstein

Chloe, Garry, and Jak Einicke.

Garry Einicke

Julien Hendrickx visiting Niagara Falls.

Laine Mears and his daughters Marina(left) and Maddie May.

Yi Guo with her mother Zhenfu Liu (left)and her son Raymond (center).