Strict-feedback form

In control theory, dynamical systems are in strict-feedback form when they can be expressed as
where

with,
are scalars,
is a scalar input to the system,
vanish at the origin,
are nonzero over the domain of interest.

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,

It is given that the system
::
: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.
A control is designed so that the system
::
:is stabilized so that follows the desired control. The control design is based on the augmented Lyapunov function candidate
::
:The control can be picked to bound away from zero.
A control is designed so that the system
::
:is stabilized so that follows the desired control. The control design is based on the augmented Lyapunov function candidate
::
:The control can be picked to bound away from zero.
This process continues until the actual is known, and
* The real control stabilizes to fictitious control.
* The fictitious control stabilizes to fictitious control.
* The fictitious control stabilizes to fictitious control.
*...
* The fictitious control stabilizes to fictitious control.
* The fictitious control stabilizes to fictitious control.
* 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

vanish at the origin for,
are nonzero for,
the given control has,

then the resulting system has an equilibrium at the origin that is globally asymptotically stable.