Defines the Zames-Falb IQCs for a integrator encapsulated in a deadzone nonlinearity with slope in [0, kdzn], see Figure.

The encapsulation defines the operator

$$ \begin{align*}&\dot{z}=\varphi(v-z),\quad z(0)=0\\&w=\varphi(v-z),\end{align*}$$

where \(\varphi\) denotes the deadzone nonlinearity. The IQCs holds for any encapsulated nonlinearity \(\varphi\) with slope in [0, kdzn]. However, the IQCs are particularly interesting for nonlinearities of deadzone type, i.e., when the slope is zero for small values of the input to the nonlinearity.


Figure 1: Illustration of the encapsulation operator.





The IQCs are of the form

$$\left\langle w,(h_{0}-H)(v-\frac{1}{k_{dzn}}w)\right\rangle + \left\langle w, Fw \right\rangle \geq 0,$$

where \(F=(H(s)-H(0))/s\), and


Here \(a\) is any nonzeros real number and \(N \geq 0\).

The parameters \(h_{0}\) and \(x_{k}\) are subject to the constraint

$$ ||h||_{1}=\int_{-\infty}^{\infty}|h(t)|dt \leq h_{0}$$


$$ h(t)=\sum_{k=0}^{N}\frac{\textrm{sign}(a)x_{k}}{k!}t^{k}e^{-at}\theta(\textrm{sign}(a)t),$$

where \(\theta(t)\) is the unit step function, i.e., \(\theta(t) = 1, t \geq 0\) and \(\theta(t) = 0\) otherwise. We refer to the

manual regarding the implementation of the constraint.



v      Input to the encapsulated nonlinearity.

a      Pole location. Default a=1.

N      Defines number of terms in expansion for H. Default N=0.

kdzn Slope bound for the encapsulated nonlinearity. Default kdz=1.


w     Output from the encapsulated nonlinearity.

h_0  Parameter h0 (optional).

H      Multiplier H (optional).

F      Multiplier F = (H(s) − H(0))/s (optional).

x      The parameters defining H, x=[x0,..,xN] (optional).

Example 1

In order to obtain more than one pole in H(s) call the iqc_slope command twice as



The constraints (i) and (ii) will be conservative in general except for the case when a1 and a2 have different signs. See the manual for a more thorough discussion.

See also

iqc_sector, iqc_monotonic, iqc_slope_oddiqc_polytope_stvp


U. Jönsson and A. Megretski. The Zames Falb IQC for critically stable systems. To appear in the proceedings of the 1998 American Control Conference.

U Jönsson and A. Megretski. The Zames Falb IQC for critically stable systems. Technical Report LIDS-P-2405, Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, 1997.