(("is presently" can mean "soon" or "now" -it is best to avoid this ambiguity; and the present tense indicates the present/current condition, so this is redundant if "now" is meant
'''Variable structure control''',or '''(VSC)''', is a form of [[Classification of discontinuities|discontinuous]] [[nonlinear control]]. The method alters the [[dynamic system|dynamics]] of a [[nonlinear system]] by application of a high-frequency ''switching control''. The [[state space (controls)|state]]-[[feedback]] control law is ''not'' a [[continuous function]] of time; it ''switches'' from one smooth condition to another. So the ''structure'' of the control law ''varies'' based on the position of the state trajectory; the method switches from one smooth control law to another and possibly very fast speeds (e.g., for a [[countably infinite]] number of times in a finite time interval). Variable structure control (VSC) and associated sliding mode behaviour was first investigated in early 1950s in the Soviet Union by Emelyanov and several coresearchers.<ref name="E1967">{{Cite book|title= Variable Structure Control Systems|editor1-last=Emelyanov|editor1-first=S.V.|publisher=Nauka|___location=Moscow|year=1967|postscript= <!--None-->}}</ref>
Presently, theThe main mode of VSC operation is [[sliding mode control]] (SMC). The strengths of SMC include:
* Low sensitivity to [[plant (control theory)|plant]] parameter uncertainty
* Greatly reduced-order modeling of plant dynamics
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* [[Chatter]]ing due to implementation imperfections
* Over-focus on ''matched'' uncertainties (i.e., uncertainties that enter into the control channel)
However, the evolution of VSC is presently an active area of research.<ref name="EFF06">{{Cite book|title=Advances in Variable Structure and Sliding Mode Control|editor1-last=Edwards|editor1-first=Cristopher|editor2-last=Fossas Colet|editor2-first=Enric|editor3-last=Fridman|editor3-first=Leonid|publisher=Springer-Verlag|___location=Berlin|year=2006|series=Lecture Notes in Control and Information Sciences|volume=vol 334|isbn=978-3-540-32800-1|postscript=<!--None-->}}</ref>
