Strict-feedback form


In control theory, dynamical systems are in strict-feedback form when they can be expressed as
where
Here, strict feedback refers to the fact that the nonlinear functions and in the equation only depend on states that are fed back to that subsystem. That is, the system has a kind of lower triangular form.

Stabilization

Systems in strict-feedback form can be stabilized by recursive application of backstepping. That is,
  1. It is given that the system
  2. ::
  3. :is already stabilized to the origin by some control where. That is, choice of to stabilize this system must occur using some other method. It is also assumed that a Lyapunov function for this stable subsystem is known.
  4. A control is designed so that the system
  5. ::
  6. :is stabilized so that follows the desired control. The control design is based on the augmented Lyapunov function candidate
  7. ::
  8. :The control can be picked to bound away from zero.
  9. A control is designed so that the system
  10. ::
  11. :is stabilized so that follows the desired control. The control design is based on the augmented Lyapunov function candidate
  12. ::
  13. :The control can be picked to bound away from zero.
  14. This process continues until the actual is known, and
  15. * The real control stabilizes to fictitious control.
  16. * The fictitious control stabilizes to fictitious control.
  17. * The fictitious control stabilizes to fictitious control.
  18. *...
  19. * The fictitious control stabilizes to fictitious control.
  20. * The fictitious control stabilizes to fictitious control.
  21. * The fictitious control stabilizes to the origin.
This process is known as backstepping because it starts with the requirements on some internal subsystem for stability and progressively steps back out of the system, maintaining stability at each step. Because
then the resulting system has an equilibrium at the origin that is globally asymptotically stable.