Applications of Robust Control to Nonlinear Systems

Chapter 6: Direct Approach to Nonlinear H? Control

The example problem of Chapter 5, Sec. II, is solved directly in this chapter using a nonlinear extension to the method of Chang.56 The generalized plant of Eq. (5.8) is repeated here, along with appropriate dimensions:58

(6.1)
(6.2, 6.3, 6.4)
(6.5, 6.6)
(6.7, 6.8, 6.9, 6.10)

I. Riccati Equation Solution Initialization

The solutions for the Riccati matrices X and Y will be derived next. First, the following intermediate matrices are calculated:58

(6.11)
(6.12)
(6.13)
(6.14)
(6.15)
(6.16)

II. Hamiltonian Matrix H ? Solution

Next, when solving for the Hamiltonian matrix H ? additional intermediate matrices are generated:58

(6.17)
(6.18)
(6.19)
(6.20)

The first term in the H ? equation is

(6.21)

Similarly, the second term in the H ? equation is

(6.22)

or

(6.23)

Subtracting Eq. (6.23) from Eq. (6.21) gives the Hamiltonian matrix H ? of Eq. (6.25):

(6.24)
(6.25)

III. Solution to H ? Riccati Equation

The corresponding Riccati equation is

(6.26)

or

(6.27)

Equation (6.27) is equivalent to the following four equations:

(6.28)
(6.29)
(6.30)
(6.31)

Subtracting Eq. (6.30) from (6.29) gives X 12 X 21 = 0, or X 12 = X 21. Equations (6.28 6.31) simplify to become

(6.32)
(6.33)
(6.34)

Solving for X 12 in Eq. (6.34) gives

(6.35)

or

(6.36)

To generate the solution for the Riccati matrix X, first solve for X 11 = f( X 12) using Eq. (6.32), then solve for X 22

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