Variable structure control

Variable structure control, or VSC, is 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).

Presently, 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 presently an active area of research.

See also

• Variable structure system
• Sliding mode control
• Hybrid system
• Nonlinear control
• Robust control
• Optimal control
• H-bridge — A topology that combines four switches forming the four legs of an "H". Can be used to drive a motor (or other electrical device) forward or backward when only a single supply is available. Often used in actuator in sliding-mode controlled systems.
• Switching amplifier — Uses switching-mode control to drive continuous outputs
• Delta-sigma modulation — Another (feedback) method of encoding a continuous range of values in a signal that rapidly switches between two states (i.e., a kind of specialized sliding-mode control)
• Pulse density modulation — A generalized form of delta-sigma modulation.
• Pulse-width modulation — Another modulation scheme that produces continuous motion through discontinuous switching.

Further reading

• Khalil, H.K. (2002). Nonlinear Systems (3^rd edition ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-067389-7. {{cite book}}: |edition= has extra text (help)
• Utkin, V.I. (1992). Sliding Modes in Control and Optimization. Springer-Verlag. ISBN 978-0387535166. {{cite book}}: External link in |author= (help)
• Zinober, A.S.I., ed. (1990). Deterministic control of uncertain systems. London: Peter Peregrinus Press. ISBN 978-0863411700. {{cite book}}: Unknown parameter |editorlink= ignored (|editor-link= suggested) (help)
• Zinober, Alan S.I., ed. (1994). Variable Structure and Lyapunov Control. London: Springer-Verlag. doi:10.1007/BFb0033675. ISBN 978-3-540-19869-7.