# CDS 212, Homework 1, Fall 2010

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- REDIRECT HW draft

J. Doyle | Issued: 28 Sep 2010 |

CDS 112, Fall 2010 | Due: 7 Oct 2010 |

### Reading

- DFT, Chapters 1 and 2
- Dullerud and Paganini, Ch 3

### Problems

- 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 <amsmath>u</amsmath>:- <amsmath>\textstyle \sup_t |\dot u(t)|</amsmath>
- <amsmath>\textstyle |u(0)| + \sup_t |\dot u(t)|</amsmath>
- <amsmath>\textstyle \max \{ \sup_t |u(t)|,\, \sup_t |\dot u(t)| \}</amsmath>
- <amsmath>\textstyle \sup_t |u(t)| + \sup_t |\dot u(t)|</amsmath>

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

- DFT 2.4, page 29]

Let <amsmath>D</amsmath> be a pure time delay of <amsmath>\tau</amsmath> seconds with transfer function <amsmath> \widehat D(s) = e^{-s \tau} </amsmath>. A norm <amsmath>\|\cdot\|</amsmath> on transfer functions is*time-delay invariant*if for every bounded transfer function <amsmath>\widehat G</amsmath> and every <amsmath>\tau > 0</amsmath> we have<amsmath> \| \widehat D \widehat G \| = \| \widehat G \|

</amsmath>Determine if the 2-norm and <amsmath>\infty</amsmath>-norm are time-delay invariant.

- [DFT 2.5, page 30]

Compute the 1-norm of the impluse response corresponding to the transfer function <amsmath> \frac{1}{\tau s + 1}, \quad \tau > 0 </amsmath>. - DFT 2.7, page 30]

Derive the <amsmath>\infty</amsmath>-norm to <amsmath>\infty</amsmath>-norm system gain for a stable, proper plant <amsmath>\widehat G</amsmath>. (Hint: write <amsmath>\widehat G = c + \widehat G_1</amsmath> where <amsmath>c</amsmath> is a constant and <amsmath>\widehat G_1</amsmath> is strictly proper.) - [DFT 2.8, page 30]

Let <amsmath>\widehat G</amsmath> be a stable, proper plant (but not necessarily strictly proper).- Show that the <amsmath>\infty</amsmath>-norm of the output <amsmath>y</amsmath> given an input <amsmath>u(t) = \sin(\omega t)</amsmath> is <amsmath>|\widehat G(jw)|</amsmath>.
- Show that the 2-norm to 2-norm system gain for <amsmath>\widehat G</amsmath> is <amsmath>\| \widehat G \|_\infty</amsmath> (just as in the strictly proper case).

- [DFT 2.11, page 30]

Consider a system with transfer function <amsmath>\widehat G(s) = \frac{s+2}{4s + 1}</amsmath> and input <amsmath>u</amsmath> and output <amsmath>y</amsmath>. Compute<amsmath> \| G \|_1 = \sup_{\|u\|_\infty = 1} \| y \|_\infty

</amsmath>and find an input which achieves the supremum.

- [DFT 2.12, page 30]

For a linear system with input <amsmath>u</amsmath> and output <amsmath>y</amsmath>, prove that<amsmath> \sup_{\|u\| \leq 1} \| y \| = \sup_{\|u\| = 1} \| y \|

</amsmath>where <amsmath>\|\cdot\|</amsmath> is any norm on signals.

- Consider a second order mechanical system with transfer function
<amsmath> \widehat G(s) = \frac{1}{s^2 + 2 \omega_n \zeta s + \omega_n^2}

</amsmath>(<amsmath>\omega_n</amsmath> is the natural frequence of the system and <amsmath>\zeta</amsmath> is the damping ratio). Setting <amsmath>\omega_n = 1</amsmath>, write a short MATLAB program to generate a plot of the <amsmath>\infty</amsmath>-norm as a function of the damping ratio <amsmath>\zeta > 0</amsmath>.