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Stephan Trenn

Stephan Trenn

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Category: Submitted

Posted on 2021-05-172021-05-17

Nonlinear DAE paper accepted

Our paper

Chen, Yahao; Trenn, Stephan; Respondek, Witold

Normal forms and internal regularization of nonlinear differential-algebraic control systems Journal Article

In: International Journal of Robust and Nonlinear Control, vol. 2021, no. 31, pp. 6562-6584, 2021, (open access).

Abstract | Links | BibTeX

@article{ChenTren21d,
title = {Normal forms and internal regularization of nonlinear differential-algebraic control systems},
author = {Yahao Chen and Stephan Trenn and Witold Respondek},
url = {https://stephantrenn.net/wp-content/uploads/2021/06/ChenTren21d.pdf, Paper},
doi = {10.1002/rnc.5623},
year = {2021},
date = {2021-04-13},
urldate = {2021-04-13},
journal = {International Journal of Robust and Nonlinear Control},
volume = {2021},
number = {31},
pages = {6562-6584},
abstract = {In this paper, we propose two normal forms for nonlinear differential-algebraic control systems (DACSs) under external feedback equivalence, using a notion called maximal controlled invariant submanifold. The two normal forms simplify the system structures and facilitate understanding the various roles of variables for nonlinear DACSs. Moreover, we study when a given nonlinear DACS is internally regularizable, i.e., when there exists a state feedback transforming the DACS into a differential-algebraic equation (DAE) with internal regularity, the latter notion is closely related to the existence and uniqueness of solutions of DAEs. We also revise a commonly used method in DAE solution theory, called the geometric reduction method. We apply this method to DACSs and formulate it as an algorithm, which is used to construct maximal controlled invariant submanifolds and to find internal regularization feedbacks. Two examples of mechanical systems are used to illustrate the proposed normal forms and to show how to internally regularize DACSs.},
note = {open access},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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In this paper, we propose two normal forms for nonlinear differential-algebraic control systems (DACSs) under external feedback equivalence, using a notion called maximal controlled invariant submanifold. The two normal forms simplify the system structures and facilitate understanding the various roles of variables for nonlinear DACSs. Moreover, we study when a given nonlinear DACS is internally regularizable, i.e., when there exists a state feedback transforming the DACS into a differential-algebraic equation (DAE) with internal regularity, the latter notion is closely related to the existence and uniqueness of solutions of DAEs. We also revise a commonly used method in DAE solution theory, called the geometric reduction method. We apply this method to DACSs and formulate it as an algorithm, which is used to construct maximal controlled invariant submanifolds and to find internal regularization feedbacks. Two examples of mechanical systems are used to illustrate the proposed normal forms and to show how to internally regularize DACSs.

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  • Paper
  • doi:10.1002/rnc.5623

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was accepted for publication.

Furthermore, Yahao and I have finished a follow-up paper discussing the generalization of the consistency-projector to the nonlinear case: 

Chen, Yahao; Trenn, Stephan

Impulse-free jump solutions of nonlinear differential-algebraic equations Journal Article

In: Nonlinear Analysis: Hybrid Systems, vol. 46, no. 101238, pp. 1-17, 2022, (open access).

Abstract | Links | BibTeX

@article{ChenTren22a,
title = {Impulse-free jump solutions of nonlinear differential-algebraic equations},
author = {Yahao Chen and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2024/02/ChenTren22a.pdf, Paper},
doi = {10.1016/j.nahs.2022.101238},
year = {2022},
date = {2022-11-01},
urldate = {2022-11-01},
journal = {Nonlinear Analysis: Hybrid Systems},
volume = {46},
number = {101238},
pages = {1-17},
abstract = {In this paper, we propose a novel notion called impulse-free jump solution for nonlinear differential-algebraic equations (DAEs) of the form E(x)x' = F(x) with inconsistent initial values. The term “impulse-free” means that there are no Dirac impulses caused by jumps from inconsistent initial values, i.e., the directions of jumps stay in ker E(x). We find that the existence and uniqueness of impulse-free jumps are closely related to the notion of geometric index-1 and the involutivity of the distribution defined by ker E(x). Moreover, a singular perturbed system approximation is proposed for nonlinear DAEs; we show that solutions of the perturbed system approximate both impulse-free jump solutions and C1-solutions of nonlinear DAEs. Finally, we show by some examples that our results of impulse-free jumps are useful for the problems like consistent initializations of nonlinear DAEs and transient behavior simulations of electric circuits.},
note = {open access},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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In this paper, we propose a novel notion called impulse-free jump solution for nonlinear differential-algebraic equations (DAEs) of the form E(x)x' = F(x) with inconsistent initial values. The term “impulse-free” means that there are no Dirac impulses caused by jumps from inconsistent initial values, i.e., the directions of jumps stay in ker E(x). We find that the existence and uniqueness of impulse-free jumps are closely related to the notion of geometric index-1 and the involutivity of the distribution defined by ker E(x). Moreover, a singular perturbed system approximation is proposed for nonlinear DAEs; we show that solutions of the perturbed system approximate both impulse-free jump solutions and C1-solutions of nonlinear DAEs. Finally, we show by some examples that our results of impulse-free jumps are useful for the problems like consistent initializations of nonlinear DAEs and transient behavior simulations of electric circuits.

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  • Paper
  • doi:10.1016/j.nahs.2022.101238

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Posted on 2020-01-222021-05-10

Two extended abstracts submitted to MTNS

In responds to invitations from Rafal Goebel and Tim Hughes/Malcom Smith I have submitted the following two extended abstracts to the MTNS 2020:

Iervolino, Raffaele; Vasca, Francesco; Trenn, Stephan

Discontinuous Lyapunov functions for discontinous piecewise-affine systems Miscellaneous

Extended Abstract, 2020, (accepted for cancelled MTNS 20/21).

Abstract | Links | BibTeX

@misc{IervTren20m,
title = {Discontinuous Lyapunov functions for discontinous piecewise-affine systems},
author = {Raffaele Iervolino and Francesco Vasca and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2020/01/Preprint-ITV200122.pdf, Extended Abstract},
year = {2020},
date = {2020-01-22},
urldate = {2020-01-22},
abstract = {Asymptotic stability of continuous-time piecewise affine systems defined over a polyhedral partition of the state space, with possible discontinuous vector field on the boundaries, is considered. We first introduce the feasible Filippov solution concept by characterizing single-mode Caratheodory, sliding mode and forward Zeno behaviors. Then, a global asymptotic stability result through a (possibly discontinuous) piecewise Lyapunov function is presented. The sufficient conditions are based on pointwise classifications of the trajectories which allow the identification of crossing, unreachable and Caratheodory boundaries. It is highlighted that the sign and jump conditions of the stability theorem can be expressed in terms of linear matrix inequalities by particularizing to piecewise quadratic Lyapunov functions and using the cone-copositivity approach. },
howpublished = {Extended Abstract},
note = {accepted for cancelled MTNS 20/21},
keywords = {},
pubstate = {published},
tppubtype = {misc}
}

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Asymptotic stability of continuous-time piecewise affine systems defined over a polyhedral partition of the state space, with possible discontinuous vector field on the boundaries, is considered. We first introduce the feasible Filippov solution concept by characterizing single-mode Caratheodory, sliding mode and forward Zeno behaviors. Then, a global asymptotic stability result through a (possibly discontinuous) piecewise Lyapunov function is presented. The sufficient conditions are based on pointwise classifications of the trajectories which allow the identification of crossing, unreachable and Caratheodory boundaries. It is highlighted that the sign and jump conditions of the stability theorem can be expressed in terms of linear matrix inequalities by particularizing to piecewise quadratic Lyapunov functions and using the cone-copositivity approach.

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  • Extended Abstract

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Trenn, Stephan

The Laplace transform and inconsistent initial values Miscellaneous

Extended Abstract, 2020, (accepted for cancelled MTNS 20/21, presented at MTNS 2022).

Abstract | Links | BibTeX

@misc{Tren20m,
title = {The Laplace transform and inconsistent initial values},
author = {Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2020/01/Preprint-Tre200122.pdf, Extended Abstract},
year = {2020},
date = {2020-01-22},
urldate = {2020-01-22},
abstract = {Switches in electrical circuits may lead to Dirac impulses in the solution; a real word example utilizing this effect is the spark plug. Treating these Dirac impulses in a mathematically rigorous way is surprisingly challenging. This is in particular true for arguments made in the frequency domain in connection with the Laplace transform. A survey will be given on how inconsistent initials values have been treated in the past and how these approaches can be justified in view of the now available solution theory based on piecewise-smooth distributions.},
howpublished = {Extended Abstract},
note = {accepted for cancelled MTNS 20/21, presented at MTNS 2022},
keywords = {},
pubstate = {published},
tppubtype = {misc}
}

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Switches in electrical circuits may lead to Dirac impulses in the solution; a real word example utilizing this effect is the spark plug. Treating these Dirac impulses in a mathematically rigorous way is surprisingly challenging. This is in particular true for arguments made in the frequency domain in connection with the Laplace transform. A survey will be given on how inconsistent initials values have been treated in the past and how these approaches can be justified in view of the now available solution theory based on piecewise-smooth distributions.

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  • Extended Abstract

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[Update 04-02-2020] My postdoc Yahao Chen was able to finish a nice result about the index of nonlinear DAEs just on time to submit it also to the MTNS (as a full paper):

Chen, Yahao; Trenn, Stephan

On geometric and differentiation index of nonlinear differential-algebraic equations Proceedings Article

In: IFAC-PapersOnLine (Proceedings of the MTNS 2020/21), pp. 186-191, IFAC Elsevier, 2021, (open access).

Abstract | Links | BibTeX

@inproceedings{ChenTren21b,
title = {On geometric and differentiation index of nonlinear differential-algebraic equations},
author = {Yahao Chen and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2022/03/ChenTren21b.pdf, Paper},
doi = {10.1016/j.ifacol.2021.06.075},
year = {2021},
date = {2021-04-06},
urldate = {2021-04-06},
booktitle = {IFAC-PapersOnLine (Proceedings of the MTNS 2020/21)},
volume = {54},
number = {9},
pages = {186-191},
publisher = {Elsevier},
organization = {IFAC},
abstract = {We discuss two notions of index, i.e., the geometric index and the differentiation index for nonlinear differential-algebraic equations (DAEs). First, we analyze solutions of nonlinear DAEs by revising a geometric reduction method (see e.g. Rabier and Rheinboldt (2002),Riaza (2008)). Then we show that although both of the geometric index and the differentiation index serve as a measure of difficulties for solving DAEs, they are actually related to the existence and uniqueness of solutions in a different manner. It is claimed in (Campbell and Gear, 1995) that the two indices coincide when sufficient smoothness and assumptions are satisfied, we elaborate this claim and show that the two indices indeed coincide if and only if a condition of uniqueness of solutions is satisfied (under certain constant rank assumptions). Finally, an example of a pendulum system is used to illustrate our results on the two indices.},
note = {open access},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

Close

We discuss two notions of index, i.e., the geometric index and the differentiation index for nonlinear differential-algebraic equations (DAEs). First, we analyze solutions of nonlinear DAEs by revising a geometric reduction method (see e.g. Rabier and Rheinboldt (2002),Riaza (2008)). Then we show that although both of the geometric index and the differentiation index serve as a measure of difficulties for solving DAEs, they are actually related to the existence and uniqueness of solutions in a different manner. It is claimed in (Campbell and Gear, 1995) that the two indices coincide when sufficient smoothness and assumptions are satisfied, we elaborate this claim and show that the two indices indeed coincide if and only if a condition of uniqueness of solutions is satisfied (under certain constant rank assumptions). Finally, an example of a pendulum system is used to illustrate our results on the two indices.

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  • Paper
  • doi:10.1016/j.ifacol.2021.06.075

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[Update March 2021] Unfortunately the MTNS was cancelled, which means that the extended abstracts will not be published on the conference webpage (but I will keep them online under Miscellaneous). The full paper coauthored with Yahao Chen will be published in the proceedings.

Posted on 2019-12-062019-12-06

Paper on PDEs coupled to swDAEs submitted

Finally we have published our key theoretical paper four our DFG-project “Coupling hyperbolic PDEs with switched DAEs: Analysis, numerics and application to blood flow models”:

Borsche, Raul; Kocoglu, Damla; Trenn, Stephan

A distributional solution framework for linear hyperbolic PDEs coupled to switched DAEs Journal Article

In: Mathematics of Control, Signals, and Systems (MCSS), vol. 32, pp. 455-487, 2020, (Open Access).

Abstract | Links | BibTeX

@article{BorsKoco20,
title = {A distributional solution framework for linear hyperbolic PDEs coupled to switched DAEs},
author = {Raul Borsche and Damla Kocoglu and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2020/11/23-MCSS2020.pdf, Paper},
doi = {10.1007/s00498-020-00267-7},
year = {2020},
date = {2020-11-18},
urldate = {2020-11-18},
journal = {Mathematics of Control, Signals, and Systems (MCSS)},
volume = {32},
pages = {455-487},
abstract = {A distributional solution framework is developed for systems consisting of linear hyperbolic partial differential equations (PDEs) and switched differential-algebraic equations (DAEs) which are coupled via boundary conditions. The unique solvability is then characterize in terms of a switched delay DAE. The theory is illustrated with an example of electric power lines modeled by the telegraph equations which are coupled via a switching transformer where simulations confirm the predicted impulsive solutions.},
note = {Open Access},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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A distributional solution framework is developed for systems consisting of linear hyperbolic partial differential equations (PDEs) and switched differential-algebraic equations (DAEs) which are coupled via boundary conditions. The unique solvability is then characterize in terms of a switched delay DAE. The theory is illustrated with an example of electric power lines modeled by the telegraph equations which are coupled via a switching transformer where simulations confirm the predicted impulsive solutions.

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  • Paper
  • doi:10.1007/s00498-020-00267-7

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In this paper, we provide a novel distributional solution framework to handle jumps and Dirac impulses on the boundaries of the domain of a hyperbolic PDE. It also includes a nice example of electrical power lines (modeled via the telegraph equation) with a switching transformer (modeled by a switched DAE). The simulations nicely show how a Dirac impulse (induced by a switch) is moving through time and space as predicted by the theory.

Posted on 2019-12-062019-12-06

Two submission to IFAC WC ’20

Together with my PhD-students we have prepared the following two submission to the IFAC World Congress 2020 to be held in Berlin next year:

Wijnbergen, Paul; Jeeninga, Mark; Trenn, Stephan

On stabilizability of switched differential algebraic equations Proceedings Article

In: IFAC-PapersOnLine 53-2, pp. 4304-4309, 2020, (Proc. IFAC World Congress 2020, Berlin, Germany. Open access.).

Abstract | Links | BibTeX

@inproceedings{WijnJeen20,
title = {On stabilizability of switched differential algebraic equations},
author = {Paul Wijnbergen and Mark Jeeninga and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2021/06/WijnJeen20.pdf, Paper},
doi = {10.1016/j.ifacol.2020.12.2580},
year = {2020},
date = {2020-07-06},
booktitle = {IFAC-PapersOnLine 53-2},
pages = {4304-4309},
abstract = {This paper considers stabilizability of switched differential algebraic equations (DAEs). We first introduce the notion of interval stabilizability and show that under a certain uniformity assumption, stabilizability can be concluded from interval stabilizability. A geometric approach is taken to find necessary and sufficient conditions for interval stabilizability. This geometric approach can also be utilized to derive a novel characterization of controllability.},
note = {Proc. IFAC World Congress 2020, Berlin, Germany. Open access.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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This paper considers stabilizability of switched differential algebraic equations (DAEs). We first introduce the notion of interval stabilizability and show that under a certain uniformity assumption, stabilizability can be concluded from interval stabilizability. A geometric approach is taken to find necessary and sufficient conditions for interval stabilizability. This geometric approach can also be utilized to derive a novel characterization of controllability.

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  • Paper
  • doi:10.1016/j.ifacol.2020.12.2580

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Hossain, Sumon; Trenn, Stephan

A time-varying Gramian based model reduction approach for Linear Switched Systems Proceedings Article

In: IFAC PapersOnline 53-2, pp. 5629-5634, 2020, (Proc. IFAC World Congress 2020, Berlin, Germany. Open access.).

Abstract | Links | BibTeX

@inproceedings{HossTren20a,
title = {A time-varying Gramian based model reduction approach for Linear Switched Systems},
author = {Sumon Hossain and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2021/06/HossTren20a.pdf, Paper (open access)},
doi = {10.1016/j.ifacol.2020.12.1580},
year = {2020},
date = {2020-07-05},
urldate = {2020-07-05},
booktitle = {IFAC PapersOnline 53-2},
pages = {5629-5634},
abstract = {We propose a model reduction approach for switched linear system based on a balanced truncation reduction method for linear time-varying systems. The key idea is to approximate the piecewise-constant coefficient matrices with continuous time-varying coefficients and then apply available balance truncation methods for (continuous) time-varying systems. The proposed method is illustrated with a low dimensional academic example.},
note = {Proc. IFAC World Congress 2020, Berlin, Germany. Open access.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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We propose a model reduction approach for switched linear system based on a balanced truncation reduction method for linear time-varying systems. The key idea is to approximate the piecewise-constant coefficient matrices with continuous time-varying coefficients and then apply available balance truncation methods for (continuous) time-varying systems. The proposed method is illustrated with a low dimensional academic example.

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  • Paper (open access)
  • doi:10.1016/j.ifacol.2020.12.1580

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Posted on 2019-10-10

Two submission two ECC’20

We have submitted the following two papers to the ECC’20 taking place in St. Petersburg, Russia:

Wijnbergen, Paul; Trenn, Stephan

Impulse controllability of switched differential-algebraic equations Proceedings Article

In: Proc. European Control Conference (ECC 2020), pp. 1561-1566, Saint Petersburg, Russia, 2020.

Abstract | Links | BibTeX

@inproceedings{WijnTren20,
title = {Impulse controllability of switched differential-algebraic equations},
author = {Paul Wijnbergen and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2020/02/Preprint-WT200204.pdf, Preprint},
doi = {10.23919/ECC51009.2020.9143713},
year = {2020},
date = {2020-05-15},
booktitle = {Proc. European Control Conference (ECC 2020)},
pages = {1561-1566},
address = {Saint Petersburg, Russia},
abstract = {This paper addresses impulse controllability of switched DAEs on a finite interval. First we present a forward approach where we define certain subspaces forward in time. These subpsaces are then used to provide a sufficient condition for impulse controllability. In order to obtain a full characterization we present afterwards a backward approach, where a sequence of subspaces is defined backwards in time. With the help of the last element of this backward sequence, we are able to fully characterize impulse controllability. All results are geometric results and thus independent of a coordinate system.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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This paper addresses impulse controllability of switched DAEs on a finite interval. First we present a forward approach where we define certain subspaces forward in time. These subpsaces are then used to provide a sufficient condition for impulse controllability. In order to obtain a full characterization we present afterwards a backward approach, where a sequence of subspaces is defined backwards in time. With the help of the last element of this backward sequence, we are able to fully characterize impulse controllability. All results are geometric results and thus independent of a coordinate system.

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  • Preprint
  • doi:10.23919/ECC51009.2020.9143713

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Lee, Jin Gyu; Berger, Thomas; Trenn, Stephan; Shim, Hyungbo

Utility of edge-wise funnel coupling for asymptotically solving distributed consensus optimization Proceedings Article

In: Proc. European Control Conference (ECC 2020), pp. 911-916, Saint Petersburg, Russia, 2020.

Abstract | Links | BibTeX

@inproceedings{LeeBerg20,
title = {Utility of edge-wise funnel coupling for asymptotically solving distributed consensus optimization},
author = {Jin Gyu Lee and Thomas Berger and Stephan Trenn and Hyungbo Shim},
url = {https://stephantrenn.net/wp-content/uploads/2020/02/Preprint-LBTS200204.pdf, Preprint},
doi = {10.23919/ECC51009.2020.9143983},
year = {2020},
date = {2020-05-14},
booktitle = {Proc. European Control Conference (ECC 2020)},
pages = {911-916},
address = {Saint Petersburg, Russia},
abstract = {A new approach to distributed consensus optimization is studied in this paper. The cost function to be minimized is a sum of local cost functions which are not necessarily convex as long as their sum is convex. This benefit is obtained from a recent observation that, with a large gain in the diffusive coupling, heterogeneous multi-agent systems behave like a single dynamical system whose vector field is simply the average of all agents' vector fields. However, design of the large coupling gain requires global information such as network structure and individual agent dynamics. In this paper, we employ a nonlinear time-varying coupling of diffusive type, which we call `edge-wise funnel coupling.' This idea is borrowed from adaptive control, which enables decentralized design of distributed optimizers without knowledge of global information. Remarkably, without a common internal model, each agent achieves asymptotic consensus to the optimal solution of the global cost. We illustrate this result by a network that asymptotically finds the least-squares solution of a linear equation in a distributed manner.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

Close

A new approach to distributed consensus optimization is studied in this paper. The cost function to be minimized is a sum of local cost functions which are not necessarily convex as long as their sum is convex. This benefit is obtained from a recent observation that, with a large gain in the diffusive coupling, heterogeneous multi-agent systems behave like a single dynamical system whose vector field is simply the average of all agents' vector fields. However, design of the large coupling gain requires global information such as network structure and individual agent dynamics. In this paper, we employ a nonlinear time-varying coupling of diffusive type, which we call `edge-wise funnel coupling.' This idea is borrowed from adaptive control, which enables decentralized design of distributed optimizers without knowledge of global information. Remarkably, without a common internal model, each agent achieves asymptotic consensus to the optimal solution of the global cost. We illustrate this result by a network that asymptotically finds the least-squares solution of a linear equation in a distributed manner.

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  • Preprint
  • doi:10.23919/ECC51009.2020.9143983

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Posted on 2019-07-19

Journal paper on funnel synchronization submitted

In cooperation with colleagues from Seoul National University we have submitted the following paper

Lee, Jin Gyu; Trenn, Stephan; Shim, Hyungbo

Synchronization with prescribed transient behavior: Heterogeneous multi-agent systems under funnel coupling Journal Article

In: Automatica, vol. 141, no. 110276, pp. 13, 2022, (open access).

Abstract | Links | BibTeX

@article{LeeTren22,
title = {Synchronization with prescribed transient behavior: Heterogeneous multi-agent systems under funnel coupling},
author = {Jin Gyu Lee and Stephan Trenn and Hyungbo Shim},
url = {https://stephantrenn.net/wp-content/uploads/2022/08/LeeTren22.pdf, Paper
https://arxiv.org/abs/2012.14580, Extended ArXiv-version},
doi = {10.1016/j.automatica.2022.110276},
year = {2022},
date = {2022-07-01},
urldate = {2022-07-01},
journal = {Automatica},
volume = {141},
number = {110276},
pages = {13},
abstract = {In this paper, we introduce a nonlinear time-varying coupling law, which can be designed in a fully decentralized manner and achieves approximate synchronization with arbitrary precision, under only mild assumptions on the individual vector fields and the underlying (undirected) graph structure. The proposed coupling law is motivated by the so-called funnel control method studied in adaptive control under the observation that arbitrary precision synchronization can be achieved for heterogeneous multi-agent systems by a high-gain coupling; consequently we call our novel synchronization method ‘(node-wise) funnel coupling.’ By adjusting the conventional proof technique in the funnel control study, we are even able to obtain asymptotic synchronization with the same funnel coupling law. Moreover, the emergent collective behavior that arises for a heterogeneous multi-agent system when enforcing arbitrary precision synchronization by the proposed funnel coupling law, is analyzed in this paper. In particular, we introduce a single scalar dynamics called ‘emergent dynamics’ which describes the emergent synchronized behavior of the multi-agent system under funnel coupling. Characterization of the emergent dynamics is important because, for instance, one can design the emergent dynamics first such that the solution trajectory behaves as desired, and then, provide a design guideline to each agent so that the constructed vector fields yield the desired emergent dynamics. We illustrate this idea via the example of a distributed median solver based on funnel coupling.},
note = {open access},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

In this paper, we introduce a nonlinear time-varying coupling law, which can be designed in a fully decentralized manner and achieves approximate synchronization with arbitrary precision, under only mild assumptions on the individual vector fields and the underlying (undirected) graph structure. The proposed coupling law is motivated by the so-called funnel control method studied in adaptive control under the observation that arbitrary precision synchronization can be achieved for heterogeneous multi-agent systems by a high-gain coupling; consequently we call our novel synchronization method ‘(node-wise) funnel coupling.’ By adjusting the conventional proof technique in the funnel control study, we are even able to obtain asymptotic synchronization with the same funnel coupling law. Moreover, the emergent collective behavior that arises for a heterogeneous multi-agent system when enforcing arbitrary precision synchronization by the proposed funnel coupling law, is analyzed in this paper. In particular, we introduce a single scalar dynamics called ‘emergent dynamics’ which describes the emergent synchronized behavior of the multi-agent system under funnel coupling. Characterization of the emergent dynamics is important because, for instance, one can design the emergent dynamics first such that the solution trajectory behaves as desired, and then, provide a design guideline to each agent so that the constructed vector fields yield the desired emergent dynamics. We illustrate this idea via the example of a distributed median solver based on funnel coupling.

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  • Paper
  • Extended ArXiv-version
  • doi:10.1016/j.automatica.2022.110276

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Already in our 2015 CDC paper

Shim, Hyungbo; Trenn, Stephan

A preliminary result on synchronization of heterogeneous agents via funnel control Proceedings Article

In: Proc. 54th IEEE Conf. Decis. Control, Osaka, Japan, pp. 2229–2234, 2015.

Abstract | Links | BibTeX

@inproceedings{ShimTren15,
title = {A preliminary result on synchronization of heterogeneous agents via funnel control},
author = {Hyungbo Shim and Stephan Trenn},
url = {http://stephantrenn.net/wp-content/uploads/2017/09/Preprint-ST150902.pdf, Preprint},
doi = {10.1109/CDC.2015.7402538},
year = {2015},
date = {2015-12-01},
booktitle = {Proc. 54th IEEE Conf. Decis. Control, Osaka, Japan},
pages = {2229--2234},
abstract = {We propose a new approach to achieve practical synchronization for heterogeneous agents. Our approach is based on the observation that a sufficiently large (but constant) gain for diffusive coupling leads to practical synchronization. In the classical setup of high-gain adaptive control, the funnel controller gained popularity in the last decade, because it is very simple and only structural knowledge of the underlying dynamical system is needed. We illustrate with simulations that “funnel synchronization” may be a promising approach to achieve practical synchronization of heterogeneous agents without the need to know the individual dynamics and the algebraic connectivity of the network (i.e., the second smallest eigenvalue of the Laplacian matrix). For a special case we provide a proof, but the proof for the general case is ongoing research.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

Close

We propose a new approach to achieve practical synchronization for heterogeneous agents. Our approach is based on the observation that a sufficiently large (but constant) gain for diffusive coupling leads to practical synchronization. In the classical setup of high-gain adaptive control, the funnel controller gained popularity in the last decade, because it is very simple and only structural knowledge of the underlying dynamical system is needed. We illustrate with simulations that “funnel synchronization” may be a promising approach to achieve practical synchronization of heterogeneous agents without the need to know the individual dynamics and the algebraic connectivity of the network (i.e., the second smallest eigenvalue of the Laplacian matrix). For a special case we provide a proof, but the proof for the general case is ongoing research.

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  • Preprint
  • doi:10.1109/CDC.2015.7402538

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we proposed the idea of funnel synchronization and the simulations looked very promising, but at that time we were not able to prove the synchronization property and also were not able to characterize the emergent behavior. Jin Gyu Lee was able to find the right proof techniques which is now the corner stone of our new submission.

Posted on 2019-04-18

Paper on stability of switched singular systems in discrete time submitted

We have submitted the manuscript

Anh, Pham Ky; Linh, Pham Thi; Thuan, Do Duc; Trenn, Stephan

Stability analysis for switched discrete-time linear singular systems Journal Article

In: Automatica, vol. 119, no. 109100, 2020.

Abstract | Links | BibTeX

@article{AnhLinh20,
title = {Stability analysis for switched discrete-time linear singular systems},
author = {Pham Ky Anh and Pham Thi Linh and Do Duc Thuan and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2020/02/Preprint-ALTT200515.pdf, Preprint},
doi = {10.1016/j.automatica.2020.109100},
year = {2020},
date = {2020-09-01},
urldate = {2020-09-01},
journal = {Automatica},
volume = {119},
number = {109100},
abstract = {The stability of arbitrarily switched discrete-time linear singular (SDLS) systems is studied. Our analysis builds on the recently introduced one-step-map for SDLS systems of index-1. We first provide a sufficient stability conditions in terms of Lyapunov functions. Furthermore, we generalize the notion of joint spectral radius of a finite set of matrix pairs, which allows us to fully characterize exponential stability.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

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The stability of arbitrarily switched discrete-time linear singular (SDLS) systems is studied. Our analysis builds on the recently introduced one-step-map for SDLS systems of index-1. We first provide a sufficient stability conditions in terms of Lyapunov functions. Furthermore, we generalize the notion of joint spectral radius of a finite set of matrix pairs, which allows us to fully characterize exponential stability.

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  • doi:10.1016/j.automatica.2020.109100

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for publication.

My three Vietnames co-authors and I have analyzed the stability of switched linear singular system in discrete-time. The results build on our recently introduced notion of a one-step-map for index-1 switched singular systems:

Anh, Pham Ky; Linh, Pham Thi; Thuan, Do Duc; Trenn, Stephan

The one-step-map for switched singular systems in discrete-time Proceedings Article

In: Proc. 58th IEEE Conf. Decision Control (CDC) 2019, pp. 605-610, Nice, France, 2019.

Abstract | Links | BibTeX

@inproceedings{AnhLinh19,
title = {The one-step-map for switched singular systems in discrete-time},
author = {Pham Ky Anh and Pham Thi Linh and Do Duc Thuan and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2019/03/Preprint-ALTT190910.pdf, Preprint},
doi = {10.1109/CDC40024.2019.9030154},
year = {2019},
date = {2019-12-11},
urldate = {2019-12-11},
booktitle = {Proc. 58th IEEE Conf. Decision Control (CDC) 2019},
pages = {605-610},
address = {Nice, France},
abstract = {We study switched singular systems in discrete time and first highlight that in contrast to continuous time regularity of the corresponding matrix pairs is not sufficient to ensure a solution behavior which is causal with respect to the switching signal. With a suitable index-1 assumption for the whole switched system, we are able to define a one-step- map which can be used to provide explicit solution formulas for general switching signals.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

Close

We study switched singular systems in discrete time and first highlight that in contrast to continuous time regularity of the corresponding matrix pairs is not sufficient to ensure a solution behavior which is causal with respect to the switching signal. With a suitable index-1 assumption for the whole switched system, we are able to define a one-step- map which can be used to provide explicit solution formulas for general switching signals.

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  • doi:10.1109/CDC40024.2019.9030154

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Posted on 2019-03-222019-06-17

Four CDC papers submitted

We have submitted the following four papers to the CDC 2019 in Nice, France.

Lee, Jin Gyu; Berger, Thomas; Trenn, Stephan; Shim, Hyungbo

Utility of edge-wise funnel coupling for asymptotically solving distributed consensus optimization Proceedings Article

In: Proc. European Control Conference (ECC 2020), pp. 911-916, Saint Petersburg, Russia, 2020.

Abstract | Links | BibTeX

@inproceedings{LeeBerg20,
title = {Utility of edge-wise funnel coupling for asymptotically solving distributed consensus optimization},
author = {Jin Gyu Lee and Thomas Berger and Stephan Trenn and Hyungbo Shim},
url = {https://stephantrenn.net/wp-content/uploads/2020/02/Preprint-LBTS200204.pdf, Preprint},
doi = {10.23919/ECC51009.2020.9143983},
year = {2020},
date = {2020-05-14},
booktitle = {Proc. European Control Conference (ECC 2020)},
pages = {911-916},
address = {Saint Petersburg, Russia},
abstract = {A new approach to distributed consensus optimization is studied in this paper. The cost function to be minimized is a sum of local cost functions which are not necessarily convex as long as their sum is convex. This benefit is obtained from a recent observation that, with a large gain in the diffusive coupling, heterogeneous multi-agent systems behave like a single dynamical system whose vector field is simply the average of all agents' vector fields. However, design of the large coupling gain requires global information such as network structure and individual agent dynamics. In this paper, we employ a nonlinear time-varying coupling of diffusive type, which we call `edge-wise funnel coupling.' This idea is borrowed from adaptive control, which enables decentralized design of distributed optimizers without knowledge of global information. Remarkably, without a common internal model, each agent achieves asymptotic consensus to the optimal solution of the global cost. We illustrate this result by a network that asymptotically finds the least-squares solution of a linear equation in a distributed manner.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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A new approach to distributed consensus optimization is studied in this paper. The cost function to be minimized is a sum of local cost functions which are not necessarily convex as long as their sum is convex. This benefit is obtained from a recent observation that, with a large gain in the diffusive coupling, heterogeneous multi-agent systems behave like a single dynamical system whose vector field is simply the average of all agents' vector fields. However, design of the large coupling gain requires global information such as network structure and individual agent dynamics. In this paper, we employ a nonlinear time-varying coupling of diffusive type, which we call `edge-wise funnel coupling.' This idea is borrowed from adaptive control, which enables decentralized design of distributed optimizers without knowledge of global information. Remarkably, without a common internal model, each agent achieves asymptotic consensus to the optimal solution of the global cost. We illustrate this result by a network that asymptotically finds the least-squares solution of a linear equation in a distributed manner.

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  • Preprint
  • doi:10.23919/ECC51009.2020.9143983

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Lee, Jin Gyu; Trenn, Stephan

Asymptotic tracking via funnel control Proceedings Article

In: Proc. 58th IEEE Conf. Decision Control (CDC) 2019, pp. 4228-4233, Nice, France, 2019.

Abstract | Links | BibTeX

@inproceedings{LeeTren19,
title = {Asymptotic tracking via funnel control},
author = {Jin Gyu Lee and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2019/03/Preprint-LT190910.pdf, Preprint},
doi = {10.1109/CDC40024.2019.9030274},
year = {2019},
date = {2019-12-13},
booktitle = {Proc. 58th IEEE Conf. Decision Control (CDC) 2019},
pages = {4228-4233},
address = {Nice, France},
abstract = {Funnel control is a powerful and simple method to solve the output tracking problem without the need of a good system model, without identification and without knowledge how the reference signal is produced, but transient behavior as well as arbitrary good accuracy can be guaranteed. Until recently, it was believed that the price to pay for these very nice properties is that only practical tracking and not asymptotic tracking can be achieved. Surprisingly, this is not true! We will prove that funnel control – without any further assumptions – can achieve asymptotic tracking.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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Funnel control is a powerful and simple method to solve the output tracking problem without the need of a good system model, without identification and without knowledge how the reference signal is produced, but transient behavior as well as arbitrary good accuracy can be guaranteed. Until recently, it was believed that the price to pay for these very nice properties is that only practical tracking and not asymptotic tracking can be achieved. Surprisingly, this is not true! We will prove that funnel control – without any further assumptions – can achieve asymptotic tracking.

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  • Preprint
  • doi:10.1109/CDC40024.2019.9030274

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Trenn, Stephan; Unger, Benjamin

Delay regularity of differential-algebraic equations Proceedings Article

In: Proc. 58th IEEE Conf. Decision Control (CDC) 2019, pp. 989-994, Nice, France, 2019.

Abstract | Links | BibTeX

@inproceedings{TrenUnge19,
title = {Delay regularity of differential-algebraic equations},
author = {Stephan Trenn and Benjamin Unger},
url = {https://stephantrenn.net/wp-content/uploads/2019/03/Preprint-TU190910.pdf, Preprint},
doi = {10.1109/CDC40024.2019.9030146},
year = {2019},
date = {2019-12-12},
booktitle = {Proc. 58th IEEE Conf. Decision Control (CDC) 2019},
pages = {989-994},
address = {Nice, France},
abstract = {We study linear time-invariant delay differential-algebraic equations (DDAEs). Such equations can arise if a feedback controller is applied to a descriptor system and the controller requires some time to measure the state and to compute the feedback resulting in the time-delay. We present an existence and uniqueness result for DDAEs within the space of piecewise-smooth distributions and an algorithm to determine whether a DDAE is delay-regular.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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We study linear time-invariant delay differential-algebraic equations (DDAEs). Such equations can arise if a feedback controller is applied to a descriptor system and the controller requires some time to measure the state and to compute the feedback resulting in the time-delay. We present an existence and uniqueness result for DDAEs within the space of piecewise-smooth distributions and an algorithm to determine whether a DDAE is delay-regular.

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  • Preprint
  • doi:10.1109/CDC40024.2019.9030146

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Anh, Pham Ky; Linh, Pham Thi; Thuan, Do Duc; Trenn, Stephan

The one-step-map for switched singular systems in discrete-time Proceedings Article

In: Proc. 58th IEEE Conf. Decision Control (CDC) 2019, pp. 605-610, Nice, France, 2019.

Abstract | Links | BibTeX

@inproceedings{AnhLinh19,
title = {The one-step-map for switched singular systems in discrete-time},
author = {Pham Ky Anh and Pham Thi Linh and Do Duc Thuan and Stephan Trenn},
url = {https://stephantrenn.net/wp-content/uploads/2019/03/Preprint-ALTT190910.pdf, Preprint},
doi = {10.1109/CDC40024.2019.9030154},
year = {2019},
date = {2019-12-11},
urldate = {2019-12-11},
booktitle = {Proc. 58th IEEE Conf. Decision Control (CDC) 2019},
pages = {605-610},
address = {Nice, France},
abstract = {We study switched singular systems in discrete time and first highlight that in contrast to continuous time regularity of the corresponding matrix pairs is not sufficient to ensure a solution behavior which is causal with respect to the switching signal. With a suitable index-1 assumption for the whole switched system, we are able to define a one-step- map which can be used to provide explicit solution formulas for general switching signals.},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

Close

We study switched singular systems in discrete time and first highlight that in contrast to continuous time regularity of the corresponding matrix pairs is not sufficient to ensure a solution behavior which is causal with respect to the switching signal. With a suitable index-1 assumption for the whole switched system, we are able to define a one-step- map which can be used to provide explicit solution formulas for general switching signals.

Close

  • Preprint
  • doi:10.1109/CDC40024.2019.9030154

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Posted on 2019-03-162019-03-16

Paper on stability of discontinuous PWA submitted

Finally, we have finished our paper

Iervolino, Raffaele; Trenn, Stephan; Vasca, Francesco

Asymptotic stability of piecewise affine systems with Filippov solutions via discontinuous piecewise Lyapunov functions Journal Article

In: IEEE Transactions on Automatic Control, vol. 66, no. 4, pp. 1513-1528, 2021.

Abstract | Links | BibTeX

@article{IervTren21,
title = {Asymptotic stability of piecewise affine systems with Filippov solutions via discontinuous piecewise Lyapunov functions},
author = {Raffaele Iervolino and Stephan Trenn and Francesco Vasca},
url = {https://stephantrenn.net/wp-content/uploads/2020/02/Preprint-ITV200204.pdf, Preprint},
doi = {10.1109/TAC.2020.2996597},
year = {2021},
date = {2021-04-01},
urldate = {2021-04-01},
journal = {IEEE Transactions on Automatic Control},
volume = {66},
number = {4},
pages = {1513-1528},
abstract = {Asymptotic stability of continuous-time piecewise affine systems defined over a polyhedral partition of the state space, with possible discontinuous vector field on the boundaries, is considered. In the first part of the paper the feasible Filippov solution concept is introduced by characterizing single-mode Caratheodory, sliding mode and forward Zeno behaviors. Then, a global asymptotic stability result through a (possibly discontinuous) piecewise Lyapunov function is presented. The sufficient conditions are based on pointwise classifications of the trajectories which allow the identification of crossing, unreachable and Caratheodory boundaries. It is shown that the sign and jump conditions of the stability theorem can be expressed in terms of linear matrix inequalities by particularizing to piecewise quadratic Lyapunov functions and using the cone-copositivity approach. Several examples illustrate the theoretical arguments and the effectiveness of the stability result.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

Close

Asymptotic stability of continuous-time piecewise affine systems defined over a polyhedral partition of the state space, with possible discontinuous vector field on the boundaries, is considered. In the first part of the paper the feasible Filippov solution concept is introduced by characterizing single-mode Caratheodory, sliding mode and forward Zeno behaviors. Then, a global asymptotic stability result through a (possibly discontinuous) piecewise Lyapunov function is presented. The sufficient conditions are based on pointwise classifications of the trajectories which allow the identification of crossing, unreachable and Caratheodory boundaries. It is shown that the sign and jump conditions of the stability theorem can be expressed in terms of linear matrix inequalities by particularizing to piecewise quadratic Lyapunov functions and using the cone-copositivity approach. Several examples illustrate the theoretical arguments and the effectiveness of the stability result.

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  • doi:10.1109/TAC.2020.2996597

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and submitted it for publication.

We have started to work on this topic almost four years ago when I visited Benevento. Inspired by the promising approach of using the cone-copositivity approach to find Lyapunov functions for piecewise-affine (PWA) systems in an automatic way, we were trying to extend this idea in two main directions: 1) We wanted to allow for discontinuous Lyapunov functions and 2) we wanted to cover also sliding and Zeno solutions. We obtained first results by focusing on the first point (i.e. we only considered classical solutions) and presented these ideas at the CDC 2017 in Melbourne, Australia. It turned out that the technicalities involved in dealing with general Filippov solutions were quite tricky and we still haven’t resolved all of them. In particular, we had to make two technical assumptions for PWA systems which we believe are always satisfied, because we could not construct counter examples which violates these assumptions, but we were also not able to prove them so far. Furthermore, classifying boundaries as crossing, non-reachable and sliding turned out to be harder as expected as well, in particular, due to the presence of Zeno-behavior. Nevertheless we have a very strong (i.e. not very conservative) Lyapunov stability theorem which is formulated in terms of pointwise-conditions. In order to use the cone-copositive approach it is necessary to make some uniformity assumption on the solution behavior along the boundaries, but these assumptions do not exclude Zeno and sliding behavior and allows discontinuities of the Lyapunov function on crossing boundaries.

We are now looking forward to receive constructive feedback from our peers to further improve on the manuscript for the final published version.

Posted on 2017-12-202019-09-11

Preprint on switched-induced instability in power grids

We have submitted the manuscript

Gross, Tjorben B.; Trenn, Stephan; Wirsen, Andreas

Switch induced instabilities for stable power system DAE models Proceedings Article

In: IFAC-PapersOnLine, pp. 127-132, 2018, (Proc. IFAC Conf. Analysis Design Hybrid Systems (ADHS 2018)).

Abstract | Links | BibTeX

@inproceedings{GrosTren18,
title = {Switch induced instabilities for stable power system DAE models},
author = {Tjorben B. Gross and Stephan Trenn and Andreas Wirsen},
url = {https://stephantrenn.net/wp-content/uploads/2018/04/Preprint-GTW180413.pdf, Preprint},
doi = {10.1016/j.ifacol.2018.08.022},
year = {2018},
date = {2018-07-11},
booktitle = {IFAC-PapersOnLine},
journal = {IFAC-PapersOnLine},
volume = {51},
number = {16},
pages = {127-132},
abstract = {It is well known that for switched systems the overall dynamics can be unstable despite stability of all individual modes. We show that this phenoma can indeed occur for a linearized DAE model of power grids. By making certain topological assumptions on the power grid, we can ensure stability under arbitrary switching.},
note = {Proc. IFAC Conf. Analysis Design Hybrid Systems (ADHS 2018)},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}

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It is well known that for switched systems the overall dynamics can be unstable despite stability of all individual modes. We show that this phenoma can indeed occur for a linearized DAE model of power grids. By making certain topological assumptions on the power grid, we can ensure stability under arbitrary switching.

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  • doi:10.1016/j.ifacol.2018.08.022

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for publication. The results of this manuscript are based on Tjorben’s PhD-thesis.

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