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Crossover frequencies for sector bound



wc = getSectorCrossover(H,Q) returns the frequencies at which the following matrix M(ω) is singular:


When a frequency-domain sector plot exists, these frequencies are the frequencies at which the relative sector index (R-index) for H and Q equals 1. See About Sector Bounds and Sector Indices for details.


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Find the crossover frequencies for the dynamic system G(s)=(s+2)/(s+1) and the sector defined by:


for various values of a and b.

In U/Y space, this sector is the shaded region of the following diagram (for a, b > 0).

The Q matrix for this sector is given by:


getSectorCrossover finds the frequencies at which H(s)HQH(s) is singular, for H(s)=[G(s);I]. For instance, find these frequencies for the sector defined by Q with a = 0.1 and b = 10.

G = tf([1 2],[1 1]); 
H = [G;1];

a = 0.1;  
b = 10; 
Q = [1 -(a+b)/2 ; -(a+b)/2 a*b];

w = getSectorCrossover(H,Q)
w =

  0x1 empty double column vector

The empty result means that there are no such frequencies.

Now find the frequencies at which HHQH is singular for a narrower sector, with a = 0.5 and b = 1.5.

a2 = 0.5;  
b2 = 1.5; 
Q2 = [1 -(a2+b2)/2 ; -(a2+b2)/2 a2*b2];

w2 = getSectorCrossover(H,Q2)
w2 = 1.7321

Here the resulting frequency is where the R-index for H and Q2 is equal to 1, as shown in the sector plot.


Thus, when a sector plot exists for a system H and sector Q, getSectorCrossover finds the frequencies at which the R-index is 1.

Input Arguments

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Model to analyze against sector bounds, specified as a dynamic system model such as a tf, ss, or genss model. H can be continuous or discrete. If H is a generalized model with tunable or uncertain blocks, getSectorCrossover analyzes the current, nominal value of H.

To get the frequencies at which the I/O trajectories (u,y) of a linear system G lie in a particular sector, use H = [G;I], where I = eyes(nu), and nu is the number of inputs of G.

Sector geometry, specified as:

  • A matrix, for constant sector geometry. Q is a symmetric square matrix that is ny on a side, where ny is the number of outputs of H.

  • An LTI model, for frequency-dependent sector geometry. Q satisfies Q(s)’ = Q(–s). In other words, Q(s) evaluates to a Hermitian matrix at each frequency.

The matrix Q must be indefinite to describe a well-defined conic sector. An indefinite matrix has both positive and negative eigenvalues.

For more information, see About Sector Bounds and Sector Indices.

Output Arguments

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Sector crossover frequencies, returned as a vector. The frequencies are expressed in rad/TimeUnit, relative to the TimeUnit property of H. If the trajectories of H never cross the boundary, wc = [].

Version History

Introduced in R2016a