## Control Theory – is it an exciting topic? Impressions from the Summer School on Control of Evolution Equations @ Wuppertal

From September 30 to October 4, 2018 the Summer School on Control of Evolution Equations took place in Wuppertal and I appreciated it very much to be one of its participants. The program was divided into two interesting topics. Both lecture sessions shared an open and very enjoyable atmosphere due to the enthusiasm of the lecturers, Hans Zwart (Twente) and Marius Tucsnak (Bordeaux). My participation in the summer school lead to this blog article about control theory.

Participants of the Summer School on Control of Evolution Equations @ Wuppertal

After this short introduction, some of you might have noticed that I have not answered the question in the title – a summer school with great lectures does not necessarily mean that the topic itself  is interesting or even exciting. Maybe you even want to add the following questions:

What are the main issues and goals of control theory?
How is it related to the CRC on wave phenomena?

As usual, the best way to motivate the concepts of control theory and try to answer all these questions is to use an example, which I encountered in the lectures of Marius Tucsnak and Hans Zwart. For the sake of simplicity and evidence, I do only select the single example of a one-dimensional vibrating string. Nevertheless, this summer school taught me that there are many other examples having interesting connections to various parts within our CRC. If you are further interested in the example, it can be found in chapters 6.2 and 11.2 of the book Observation and Control for Operator Semigroups by M. Tucsnak and G. Weiss (Birkhäuser 2009) as well as chapter 9.2 of the book Linear Port-Hamiltonian Systems on Infinite-dimensional Spaces by B. Jacob and H. Zwart (Birkhäuser 2012). Note furthermore that control theory is a wide subject, so I will only motivate the three concepts of observability, controllability and stabilizability.

Consider a vibrating elastic string (of your favorite length, say 1) that is fixed at both ends, which models for instance the string of a violin. First, there are no external forces acting on it (no gravity, damping, etc.). We call the vibrating string our system and the amplitude of its oscillations behaves according to a one-dimensional wave equation. Imagine now, that we do not know anything about the state of our system, i.e., its initial and current shape is not visible. Consequently, our system has become a black box and we cannot deduce anything about it.
So, we assume that we can measure an output of our system, say the slope at its left end (i.e., the spatial derivative at this point). One natural question concerns (exact) observability: can we deduce the initial (and thus the current) state of our system by means of our observations? The answer is yes, but we must observe our system on time scales larger than 2 which is also called the minimal observation time.

However, observing is not sufficient for most applications – acting (or controlling) is better. Thus, we follow up the path of control theory and get to the aspect of controllability. The goal of (exact) controllability is to move our system to a given state within a given time. The port over which we can manipulate the system is called input. That definition sounds quite abstract, so how is controllability related to our string? Let us unloose our string at the left end, so that it can move freely here. We now manipulate our system by means of a force that acts on the loose left end of the string and is caused by a damper or an engine. In this case, our system is exactly controllable for every time larger than 2 and this means the following. Suppose, we have a given initial shape $$w_{\text{start}}$$, an initial velocity $$\partial_t w_{\text{start}}$$ and a certain desired final shape $$w_{\text{final}}$$ as well as a final velocity $$\partial_t w_{\text{final}}$$ that our system should attain at a time $$T$$ larger than 2. Then we can find a time-dependent force that acts on the loose left end of our string during the time $$[0,T]$$ and drives its state from $$(w_{\text{start}},\partial_t w_{\text{start}})$$ to the desired one $$(w_{\text{final}},\partial_t w_{\text{final}})$$ at time $$T$$.

A vibrating elastic string, fixed at its right end and free at its left. In our model, we influence our string via a force acting on the left end.

Our system with a damping force acting on the left end of our string is also a suitable instance of the concept of stabilization. Can we design a damper in such a way that the oscillations of our string decay in a uniform way, say exponentially, on large time intervals? Let us assume that we can observe also the vertical velocity of our string at the left end. Now we construct a damper that induces a force being negatively proportional to the string’s velocity at its left end.  In this way we have designed a closed loop system, i.e., we have constructed a control (our input force) based on the knowledge of a given output (the observable velocity at the left end). Returning to our goal of stabilization: the oscillations of our closed loop system decay indeed in a uniform exponential way. Of course, there are many other ways of designing controls based on certain outputs and one can also ask for cost-optimal controls and the numerical approximation of controls.

During summer school, we considered many other examples of control theory for wave equations such as coupled strings, vibrating plates and other higher dimensional wave equations on various domains. Most fascinating for me is the interplay of abstract functional analysis with complex analysis having in mind real applications such as the design of machines by engineers – think of the manufacture of integrated circuits for computer chips requiring machines that are moving fast with extreme precision. In this way we have returned to the question from the title and I hope, I could convince you that control theory is indeed an exciting topic and worth to deal with.

## Conferences and Summer Schools

The CRC recently held a summer school on wave phenomena in Karlsruhe bringing 53 participants from 12 countries around the world to Karlsruhe to focus on their shared interest in and faszination of wave phenomena. I personally recently took part in a conference in Italy (IMSE) leading some friends and family to ask me, what a conference or summer school amongst mathematicians is like. Since you might be thinking about a future in academic research yourself, while you read this, I want to take some time to tell you a little bit about what my experiences with these two formats have been and what they offer to you as a student or young researcher.

## Harmonic Analysis at its Boundaries

In June 2016 we, Christine Grathwohl, Fabian Hornung and Luca Hornung, had the opportunity to participate in the summer school “Harmonic Analysis at its Boundaries” in Nantes. The aim of this event was to present recent advances in abstract and applied harmonic analysis in depth through three minicourses and to give an overview about connected topics through accompanying talks.

The origins of the harmonic analysis go back to the end of the 18th century. One of the founders of this theory was the French mathematician Joseph Fourier, who developed it during his attempts to solve differential equations. Nowadays applications are found in various fields of science and technology, reaching from acoustics and optics over signal processing till cryptography.

Consider for example a music song that you want to record as an MP3 file in such a way that you need as low memory capacity as possible. All the information on frequencies that are not in the frequency range that can be heard by us humans is not needed. It is impossible to delete this information directly from the signal caused by the air pressure in the microphone. Instead of this, one computes the Fourier transform of a signal that divides the information on the different frequencies. Now the very high and very low frequencies can be cut off easily and a retransform gives the desired result.

In the following, we want to give an insight into the topics of the minicourses mentioned above. A little cautionary reminder: this short report requires knowledge of some advanced mathematics.

## CRC summer school on Wave Phenomena: analysis and numerics

From 12th to 15th of September 2016 the CRC organized its first summer school on wave phenomena. It was directed to Ph.D. and advanced master students with a solid background in analysis and/or numerical analysis of partial differential equations. In the morning from Monday to Thursday two lecture series were held by  Herbert Koch (Bonn, Germany) and Alexander Ostermann (Innsbruck, Austria).

## Grand opening of CRC 1173

Time flies – our CRC is now 15 months old, and a lot has happened since its beginning. In fact, there were so many activities that up to now we did not find time to celebrate the official beginning of our CRC. This was made good for by a two-day event on September 15-16, 2016. On the evening of the first day many PIs, doctoral students, summer-school participants and friends came to celebrate the official opening in the atrium of Department of Mathematics with a common dinner.

KIT‘s vice-president for research Prof. Oliver Kraft had accepted our invitation to join us for the grand opening.  After having worked his way through thick traffic on the A8-motorway he honored our CRC through his motivating and appreciative speech. Our CRC is very grateful for the ongoing support from him and the entire presidential committee of KIT. Continue Reading →