# CDS 212, Homework 1, Fall 2010

1. REDIRECT HW draft
 J. Doyle Issued: 28 Sep 2010 CDS 112, Fall 2010 Due: 7 Oct 2010

• DFT, Chapterss 1 and 2
• Dullerud and Paganini, Ch 3

### Problems

1. DFT 2.1, page 28
Suppose that <amsmath>u(t)</amsmath> is a continuous signal whose derivative <amsmath>\dot u(t)</amsmath> is also continuous. Which of the following quantities qualifies as a norm for $u$:
1. <amsmath>\textstyle \sup_t |\dot u(t)|</amsmath>
2. <amsmath>\textstyle |u(0)| + \sup_t |\dot u(t)|</amsmath>
3. <amsmath>\textstyle \max \{ \sup_t |u(t)|,\, \sup_t |\dot u(t)| \}</amsmath>
4. <amsmath>\textstyle \sup_t |u(t)| + \sup_t |\dot u(t)|</amsmath>

Make sure to give a thorough answer (not just yes or no).

2. DFT 2.4, page 29]
Let $D$ be a pure time delay of $\tau$ seconds with transfer function \begin{displaymath} \widehat D(s) = e^{-s \tau}. \end{displaymath} A norm $\|\cdot\|$ on transfer functions is {\em time-delay invariant} if for every bounded transfer function $\widehat G$ and every $\tau > 0$ we have \begin{displaymath} \| \widehat D \widehat G \| = \| \widehat G \| \end{displaymath} Determine if the 2-norm and $\infty$-norm are time-delay invariant.
3. [DFT 2.5, page 30]
Compute the 1-norm of the impluse response corresponding to the transfer function \begin{displaymath} \fract{1}{\tau s + 1} \qquad \tau > 0. </li> <li> DFT 2.7, page 30] <br> Derive the $\infty$-norm to $\infty$-norm system gain for a stable, proper plant $\widehat G$. (Hint: write $\widehat G = c + \widehat G_1$ where $c$ is a constant and $\widehat G_1$ is strictly proper.) </li> <li> [DFT 2.8, page 30] <br> Let $\widehat G$ be a stable, proper plant (but not necessarily strictly proper). # Show that the $\infty$-norm of the output $y$ given an input $u(t) = \sin(\omega t)$ is $|\widehat G(jw)|$. # Show that the 2-norm to 2-norm system gain for $\widehat G$ is $\| \widehat G \|_\infty$ (just as in the strictly proper case). </li> <li>[DFT 2.11, page 30] <br> Consider a system with transfer function \begin{displaymath} \widehat G(s) = \fract{s+2}{4s + 1} \end{displaymath} and input $u$ and output $y$. Compute \begin{displaymath} \| G \|_1 = \sup_{\|u\|_\infty = 1} \| y \|_\infty \end{displaymath} and find an input which achieves the supremum.
4. [DFT 2.12, page 30]
For a linear system with input $u$ and output $y$, prove that \begin{displaymath} \sup_{\|u\| \leq 1} \| y \| = \sup_{\|u\| = 1} \| y \| \end{displaymath} where $\|\cdot\|$ is any norm on signals.
5. Consider a second order mechanical system with transfer function \begin{displaymath} \widehat G(s) = \fract{1}{s^2 + 2 \omega_n \zeta s + \omega_n^2} \end{displaymath} ($\omega_n$ is the natural frequence of the system and $\zeta$ is the damping ratio). Setting $\omega_n = 1$, write a short MATLAB program to generate a plot of the $\infty$-norm as a function of the damping ratio $\zeta > 0$.