title: Variable structure control
text: # Variable structure control
  
  Variable structure control (VSC) is a form of discontinuous nonlinear control. The method alters the dynamics of a nonlinear system by application of a high-frequency switching control. The 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). VSC and associated sliding mode behaviour was first investigated in early 1950s in the Soviet Union by Emelyanov and several coresearchers.
  
  The main mode of VSC operation is sliding mode control (SMC). The strengths of SMC include:
  
  * Low sensitivity to plant parameter uncertainty
  * Greatly reduced-order modeling of plant dynamics
  * Finite-time convergence (due to discontinuous control law)
  
  The weaknesses of SMC include:
  
  * Chattering due to implementation imperfections
  * Over-focus on matched uncertainties (i.e., uncertainties that enter into the control channel)
  
  However, the evolution of VSC is an active area of research.
  
  