# Prof. Dr.-Ing. Thomas Hemker

Wirtschaftsprüfer, Diplom-Mathematiker

## Kontakt

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Telefon: 0228 / 303 95657
Büro: Raum 9110

Sprechstunde: nach Vereinbarung

## Lehre

### Lehrveranstaltungen im gehobenen Verwaltungsinformatikdienst

• M 7 Projektmanagement und Softwareengineering (Teil 2)
• M 11 Grundlagen des E-Governments
• M 12 ERP- und BI Systeme

## Projekte

• ERP-Systeme in der öffentlichen Verwaltung - ein Teilpropjekt des eGov-Campus

## Vita

Seit Juli 2017 lehre ich als Professor im Studiengang Verwaltungsinformatik und vertrete Themen der angewandten Informatik. Neben meiner Lehrtätigkeit berate ich in Projekten der Bundesbehörden und stehe auch als Ansprechpartner zu Fachthemen der Informatik und numerischen Mathematik zur Verfügung. Mehr zu mir gibt es auch bei LinkedIn

### Beruflicher Werdegang

 seit Juli 2017 Professor für Informatik am Fachbereich Finanzen der HS Bund, im Studiengang Verwaltungsinformatik seit 2016 Beratung im Bereich Digitalisierung, Simulation und Optimierung, öffentlich bestellter Wirtschaftsprüfer 2010-2016 KPMG AG Wirtschaftsprüfungsgesellschaft, Berlin, Bereich Consulting als Prokurist / Manager 2004-2010 Wissenschaftlicher Mitarbeiter, später Post-Doc und Lehrbeauftragter am Fachbereich Informatik der TU Darmstadt

### Ausbildung

 2015 Prüfung als Wirtschaftsprüfer bei der Wirtschaftsprüferkammer, Berlin 2013 Certified Information Systems Auditor (CISA) 2011 Certified Document Imaging Architect (CDIA+) 2008 Promotion zum Doktor-Ingenieur am Fachbereich Informatik der TU Darmstadt 2004-2007 DFG-Stipendiat im Graduiertenkolleg "Modellierung, Simulation und Optimierung von Ingenieuranwendungen" an der TU Darmstadt 1998-2003 Studium zum Diplom-Mathematiker mit Nebenfach Betriebswirtschaftlehre an der WWU Münster und der UC Madrid

## Bücher

1. Robert Müller-Török Thomas Hemker. Entwicklung und Einführung ERP-basierter Lehre an den HÖD. In Gleichwertigkeit der Lebensverhältnisse: Möglichkeiten und Wege der Gestaltung für die öffentliche Verwaltung. 2020, 263-276. BibTeX

@inproceedings{ThomasHemker2020,
author = "Thomas Hemker, Robert Müller-Török",
booktitle = {{Gleichwertigkeit der Lebensverh\"altnisse: M\"oglichkeiten und Wege der Gestaltung f\"ur die \"offentliche Verwaltung}},
title = "Entwicklung und Einführung ERP-basierter Lehre an den HÖD",
year = 2020,
pages = "263-276",
publisher = "{Nomos Verlagsgesellschaft mbH \& Co. KG}",
series = {{Schriften des Praxis- und Forschungsnetzwerks der Hochschulen f\"ur den \"offentlichen Dienst}}
}

2. Prof. Dr.-Ing. Berend Denkena (ed.). Analysis of Industrial Robot Structure and Milling Process Interaction for Path Manipulation. Springer, 2011. BibTeX

@inbook{2011:Bauer_etal_ProcessMaschineInteractions,
title = "Analysis of Industrial Robot Structure and Milling Process Interaction for Path Manipulation",
publisher = "Springer",
year = 2011,
editor = "Prof. Dr.-Ing. Berend Denkena",
author = "J. Bauer and M. Friedmann and T. Hemker and M. Pischan and C. Reinl and E. Abele and O. von Stryk",
booktitle = "Process Machine Interactions"
}

3. N Ando, S Balakirsky, T Hemker, M Reggiani and O Stryk (eds.). Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2010). Lecture Notes in Artificial Intelligence series, volume 6472, Springer, 2010. URL, DOI BibTeX

@book{2010:SIMPAR-Proc,
title = "Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2010)",
publisher = "Springer",
year = 2010,
editor = "N. Ando and S. Balakirsky and T. Hemker and M. Reggiani and O. von Stryk",
author = "N. Ando and S. Balakirsky and T. Hemker and M. Reggiani and O. von Stryk",
volume = 6472,
series = "Lecture Notes in Artificial Intelligence",
abstract = "Proceedings of the Second International Conference on Simulation, Modeling and Programming for Autonomous Robots, SIMPAR 2010, Darmstadt, Germany, November 15-18, 2010.",
doi = "10.1007/978-3-642-17319-6",
url = "http://dx.doi.org/10.1007/978-3-642-17319-6"
}

4. T Hemker. Derivative Free Surrogate Optimization for Mixed-Integer Nonlinear Black Box Problems in Engineering. Reihe 10: Informatik/Kommunikation series, number 797, VDI Verlag, 2008. URL BibTeX

@book{2008:HemkerDiss,
title = "Derivative Free Surrogate Optimization for Mixed-Integer Nonlinear Black Box Problems in Engineering",
publisher = "VDI Verlag",
year = 2008,
author = "T. Hemker",
number = 797,
series = "Reihe 10: Informatik/Kommunikation",
month = "December 19",
howpublished = "http://www.vdi-nachrichten.com/onlineshops/buchshop/literaturshop/langanzeige.asp?vr_id=7941",
pdf_short = "hemker.pdf",
}


## Conferences

1. Thomas Hemker, Robert Müller-Török and Alexander Prosser. Interactive ELearning with ERP Systems: Advancing Some Refutable Hypotheses on Interactivity in ELearning. In Proceedings of the Central and Eastern European EDem and EGov Days. 2022, 130–134. URL, DOI BibTeX

@inproceedings{10.1145/3551504.3551562,
author = {Hemker, Thomas and M\"{u}ller-T\"{o}r\"{o}k, Robert and Prosser, Alexander},
booktitle = "Proceedings of the Central and Eastern European EDem and EGov Days",
title = "Interactive ELearning with ERP Systems: Advancing Some Refutable Hypotheses on Interactivity in ELearning",
year = 2022,
address = "New York, NY, USA",
pages = "130–134",
publisher = "Association for Computing Machinery",
series = "CEEeGov '22",
abstract = "COVID-19 accelerated the drive towards distance learning, typically supported by web-based eLearning resources. There are also studies reviewing the transition to such as teaching style, whether in conjunction with traditional classroom teaching or as a supplement. This paper will focus on ERP-based teaching within a larger program to establish tertiary distance education in eGovernment in Germany (“eGov Campus”). It attempts to structure the topic and to advance some hypotheses that will be empirically tested against real-world data and experience from introducing this style of teaching in the eGov Campus.",
doi = "10.1145/3551504.3551562",
isbn = 9781450397667,
keywords = "eGov Campus, ERP systems, eGovernment",
location = "Budapest, Hungary",
numpages = 5,
url = "https://doi.org/10.1145/3551504.3551562"
}

2. T Hemker and C Werner. DIRECT using local search on surrogates. Pacific Journal of Optimization 7(3):443-466, September 2011. URL BibTeX

@article{hemker2011,
author = "T. Hemker and C. Werner",
title = "DIRECT using local search on surrogates",
journal = "Pacific Journal of Optimization",
year = 2011,
volume = 7,
pages = "443-466",
number = 3,
month = "September",
abstract = "The solution of noisy nonlinear optimization problems with nonlinear constraints and derivative information is becoming increasingly important, as many practical applications can be described by this type of problem in e.g., engineering applications. Existing local optimization methods show good convergence properties. However, they often depend on sufficiently good starting points and/or the approximation of gradients. In turn, global derivative free methods, which need no starting values to be initialized, require many evaluations of the objective function, particularly in the vicinity of the solution. A derivative free optimization algorithm is developed that combines advantages of both local and global methods. The DIRECT algorithm, which is often used for problems where no prior knowledge is available as kind of a brute force start, is extended by an inner loop using a surrogate based optimization method. The local search on the surrogate function determines better candidates for sampling than the hypercube center points chosen by DIRECT, especially if constraints are arising. This inner loop needs no additional evaluation of the original problem. Standard test problems and a computational more expensive test problem are chosen to show the performance of the new algorithm.",
file = "2011_hemker.pdf:2011_hemker.pdf:PDF",
url = "http://www.ybook.co.jp/online2/oppjo/vol7/p443.html"
}

3. J D Griffin, K R Fowler, G A Gray, T Hemker and M D Parno. Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search. Pacific Journal of Optimization 7(3):425-443, September 2011. URL BibTeX

@article{griffin2011,
author = "J. D. Griffin and K. R. Fowler and G. A. Gray and T. Hemker and M. D. Parno",
title = "Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search",
journal = "Pacific Journal of Optimization",
year = 2011,
volume = 7,
pages = "425-443",
number = 3,
month = "September",
abstract = "In this paper, we describe the technical details of HOPSPACK (Hybrid Optimization Parallel Search Package), a new software platform which facilitates combining multiple optimization routines into a single, tightly-coupled, hybrid algorithm that supports parallel function evaluations. The framework is designed such that existing optimization source code can be easily incorporated with minimal code modification. By maintaining the integrity of each individual solver, the strengths and code sophistication of the original optimization package are retained and exploited.",
file = "2010-griffin_etal.pdf:2010-griffin_etal.pdf:PDF",
url = "http://www.ybook.co.jp/online2/oppjo/vol7/p425.html"
}

4. M Parno, K Fowler and T Hemker. Applicability of Surrogates to Improve Efficiency of Particle Swarm Optimization for Simulation-based Problems. Engineering Optimization, pages online, 2011. DOI BibTeX

@article{parno2011,
author = "M. Parno and K. Fowler and T. Hemker",
title = "Applicability of Surrogates to Improve Efficiency of Particle Swarm Optimization for Simulation-based Problems",
journal = "Engineering Optimization",
year = 2011,
pages = "online",
abstract = "Particle swarm optimization (PSO) is a population-based, heuristic technique based on social behaviour that performs well on a variety of problems including those with non-convex, non-smooth objective functions with multiple minima. However, the method can be computationally expensive in that a large number of function calls is required. This is a drawback when evaluations depend on an off-the-shelf simulation program, which is often the case in engineering applications. An algorithm is proposed which incorporates surrogates as a stand-in for the expensive objective function, within the PSO framework. Numerical results are presented on standard benchmarking problems and a simulation-based hydrology application to show that this hybrid can improve efficiency. A comparison is made between the application of a global PSO and a standard PSO to the same formulations with surrogates. Finally, data profiles, probability of success, and a measure of the signal-to-noise ratio of the the objective function are used to assess the use of a surrogate.",
doi = "10.1080/0305215X.2011.598521"
}

5. T Hemker, De H Gersem, S Koch and O Stryk. Bessere Synchrotron-Magnete durch simulationsbasierte Optimierung. In M Schaefer (ed.). Computational Engineering. Forschung series, number 2/2011, TU Darmstadt, 2011, pages 58-62. URL BibTeX

@incollection{2011:TUD-Forschung,
author = "T. Hemker and H. De Gersem and S. Koch and O. von Stryk",
title = "Bessere Synchrotron-Magnete durch simulationsbasierte Optimierung",
booktitle = "Computational Engineering",
year = 2011,
editor = "M. Schaefer",
number = "2/2011",
series = "forschung",
pages = "58-62",
abstract = "Während die numerische Optimierung von kontinuierlichen Parametern supraleitender Magnete basierend auf elektromagnetischen Feldsimulationen trotz hoher Rechenzeitanforderungen schon fast alltäglich ist, stellt die für ein optimales Design notwendige, simultane Berücksichtigung auch diskreter Parameter und der damit verbundene, hohe zusätzliche Rechenaufwand enorme Schwierigkeiten dar. Eine neue, methodische Vorgehensweise zu deren Überwindung wird vorgestellt und zur Entwicklung von besonders leistungsfähigen, supraleitenden Magneten für das neue internationale Beschleunigerzentrum FAIR erfolgreich angewendet.",
file = "2011-Forschen_02_2011_S58-62.pdf:2011-Forschen_02_2011_S58-62.pdf:PDF",
}

6. Prof. Dr.-Ing. Berend Denkena (ed.). Analysis of Industrial Robot Structure and Milling Process Interaction for Path Manipulation. pages (accepted for publication), Springer, 2011. BibTeX

@inbook{2011:Bauer_etal_ProcessMaschineInteractions,
pages = "(accepted for publication)",
title = "Analysis of Industrial Robot Structure and Milling Process Interaction for Path Manipulation",
publisher = "Springer",
year = 2011,
editor = "Prof. Dr.-Ing. Berend Denkena",
author = "J. Bauer and M. Friedmann and T. Hemker and M. Pischan and C. Reinl and E. Abele and O. von Stryk",
booktitle = "Process Machine Interactions"
}

7. E Abele, J Bauer, T Hemker, R Laurischkat, H Meier, S Reese and O Stryk. Comparison and Validation of Implementations of a Flexible Joint Multibody Dynamics System Model for an Industrial Robot. CIRP Journal of Manufacturing Science and Technology 4(1):38-43, 2011. DOI BibTeX

@article{2011:Abele_etal_JMST,
author = "E. Abele and J. Bauer and T. Hemker and R. Laurischkat and H. Meier and S. Reese and O. von Stryk",
title = "Comparison and Validation of Implementations of a Flexible Joint Multibody Dynamics System Model for an Industrial Robot",
journal = "CIRP Journal of Manufacturing Science and Technology",
year = 2011,
volume = 4,
pages = "38-43",
number = 1,
abstract = "In this paper, different implementations of elastic joint models of industrial robots are described and compared established in ADAMS and SimMechanics. The models are intended to be used for path prediction under process force load due to Roboforming and High Speed Cutting, respectively. The computational results have been compared and showed good agreement. In experiments of robot forming and robot milling the measured and simulated path deviations according to the process force are compared. The experiments are descriped and the results are discussed within the paper as a basis of a next step model based compensation of the path deviation.",
doi = "10.1016/j.cirpj.2011.01.006"
}

8. E Abele, J Bauer, M Pischan, O Stryk, M Friedmann and T Hemker. Prediction of the Tool Displacement for Robot Milling Applications using Co-Simulation of an Industrial Robot and a Removal Process. In CIRP 2nd International Conference Process Machine Interactions. Juni 2010. BibTeX

@inproceedings{2010:AbeleBauerPrischanStrykFriedmannHemker,
author = "E. Abele and J. Bauer and M. Pischan and O. v. Stryk and M. Friedmann and T. Hemker",
booktitle = "CIRP 2nd International Conference Process Machine Interactions",
title = "Prediction of the Tool Displacement for Robot Milling Applications using Co-Simulation of an Industrial Robot and a Removal Process",
year = 2010,
month = "jun",
organization = "CIRP",
abstract = "Industrial robots are used in a great variety of applications for handling, welding and milling operations. They represent a cost-saving and flexible alternative for machining applications. A reduced pose and path accuracy, especially under process force load due to the high mechanical compliance, restrict the use of industrial robots for further machining applications. In this paper a method is presented to predict the resulting path deviation of the robot under process force. A process force simulation based on a material removal calculation is proposed. The presented material removal simulation allows the calculation of the chip geometry even in complex cutter work piece engagement conditions. The removal simulation is validated by means of conducted milling tests. By coupling the removal simulation with a robot model the path deviation can be predicted.",
file = "2010-PMI_preprint.pdf:2010-PMI_preprint.pdf:PDF",
keywords = "Milling, Industrial Robot, Machine Process Interaction, Material removal simulation"
}

9. K Radkhah, T Hemker and O Stryk. Self-calibration for industrial robots with rotational joints. Int. J. Mechatronics and Manufacturing Systems 3(3/4):187-209, 2010. BibTeX

@article{2010:IJMMS-Radkhah-etal,
author = "K. Radkhah and T. Hemker and O. von Stryk",
title = "Self-calibration for industrial robots with rotational joints",
journal = "Int. J. Mechatronics and Manufacturing Systems",
year = 2010,
volume = 3,
pages = "187-209",
number = "3/4",
file = "2010_ijmms_rdkhh_preprint.pdf:2010_ijmms_rdkhh_preprint.pdf:PDF"
}

10. M Friedmann, T Hemker, S Kohlbrecher, K Petersen, S Petters, K Radkhah, M Risler, D Scholz, D Thomas and O Stryk. Darmstadt Dribblers - Team Description for Humanoid KidSize League of RoboCup 2010. Technische Universität Darmstadt, 2010. BibTeX

@techreport{2010:dd_tdp,
author = "M. Friedmann and T. Hemker and S. Kohlbrecher and K. Petersen and S. Petters and K. Radkhah and M. Risler and D. Scholz and D. Thomas and O. von Stryk",
title = "Darmstadt Dribblers - Team Description for Humanoid KidSize League of RoboCup 2010",
year = 2010,
abstract = "This paper describes the hardware and software design of the kidsize humanoid robot systems of the Darmstadt Dribblers in 2010. The robots are used as a vehicle for research in control of locomotion and behavior of autonomous humanoid robots and robot teams with many degrees of freedom and many actuated joints. The Humanoid League of RoboCup provides an ideal testbed for such aspects of dynamics in motion and autonomous behavior as the problem of generating and maintaining statically or dynamically stable bipedal locomotion is predominant for all types of vision guided motions during a soccer game. A modular software architecture as well as further technologies have been developed for ecient and eective implementation and test of modules for sensing, planning, behavior, and actions of humanoid robots.",
file = "2010-tdp-hum.pdf:2010-tdp-hum.pdf:PDF"
}

11. N Ando, S Balakirsky, T Hemker, M Reggiani and O Stryk (eds.). Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2010) 6472. Springer, 2010. URL, DOI BibTeX

@proceedings{2010:SIMPAR-Proc,
title = "Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2010)",
year = 2010,
editor = "N. Ando and S. Balakirsky and T. Hemker and M. Reggiani and O. von Stryk",
volume = 6472,
series = "Lecture Notes in Artificial Intelligence",
publisher = "Springer",
abstract = "Proceedings of the Second International Conference on Simulation, Modeling and Programming for Autonomous Robots, SIMPAR 2010, Darmstadt, Germany, November 15-18, 2010.",
author = "N. Ando and S. Balakirsky and T. Hemker and M. Reggiani and O. von Stryk",
doi = "10.1007/978-3-642-17319-6",
url = "http://dx.doi.org/10.1007/978-3-642-17319-6"
}

12. K Radkhah, T Hemker, M Friedmann and O Stryk. Towards the deployment of industrial robots as measurement instruments - An extended forward kinematic model incorporating geometric and nongeometric effects. In Proc. 2009 IEEE/ASME Conference on Advanced Intelligent Mechatronics (AIM). 2009, 124-129. URL BibTeX

@inproceedings{radkhah:2009,
author = "K. Radkhah and T. Hemker and M. Friedmann and O. von Stryk",
booktitle = "Proc. 2009 IEEE/ASME Conference on Advanced Intelligent Mechatronics (AIM)",
title = "Towards the deployment of industrial robots as measurement instruments - An extended forward kinematic model incorporating geometric and nongeometric effects",
year = 2009,
month = "Jul. 14 - Jul. 17",
pages = "124-129",
abstract = "In the area of mounting and spot-welding of body-in-white, absolutely accurate robots are installed as measuring instruments, replacing expensive coordinate and other external measuring machines. Measurement technologies based on industrial robots play an increasingly important role. Such applications require highly accurate robots. Prior to deployment of highly accurate robot, however, it needs to be ensured that the implemented robot model fits the real model. Robot calibration can offer a significant opportunity to improve the positioning accuracy and to cut production costs. Existing calibration approaches fail to capture geometric and elastic effects occurring in the robot forward kinematics. Therefore, in this work an extended forward kinematic model incorporating both geometric and elastic effects has been developed in which the positioning accuracy of a manipulator, with or without an accurate internal robot model in the robot controller, is improved.",
}

13. M D Parno, K R Fowler and T Hemker. Framework for Particle Swarm Optimization with Surrogate Functions. Number TUD-CS-2009-0139, Technische Universität Darmstadt, Department of Computer Science, 2009. BibTeX

@techreport{Parno2009,
author = "M. D. Parno and K. R. Fowler and T. Hemker",
title = "Framework for Particle Swarm Optimization with Surrogate Functions",
institution = {Technische Universit\"at Darmstadt, Department of Computer Science},
year = 2009,
number = "TUD-CS-2009-0139",
abstract = "Particle swarm optimization (PSO) is a population-based, heuristic minimization technique that is based on social behavior. The method has been shown to perform well on a variety of problems including those with nonconvex, nonsmooth objective functions with multiple local minima. However, the method can be computationally expensive in that a large number of function calls is required to advance the swarm at each optimization iteration. This is a significant drawback when function evaluations depend on output from an off-the-shelf simulation program, which is often the case in engineering applications. To this end, we propose an algorithm which incorporates surrogate functions, which serve as a stand-in for the expensive objective function, within the PSO framework. We present numerical results to show that this hybrid approach can improve algorithmic efficiency.",
file = "Parno2009.pdf:Parno2009.pdf:PDF"
}

14. T Hemker, H Sakamoto, M Stelzer and O Stryk. Efficient walking speed optimization of a humanoid robot. International Journal of Robotics Research 28(2):303 - 314, 2009. URL, DOI BibTeX

@article{2008_hemker_ijr,
author = "T. Hemker and H. Sakamoto and M. Stelzer and O. von Stryk",
title = "Efficient walking speed optimization of a humanoid robot",
journal = "International Journal of Robotics Research",
year = 2009,
volume = 28,
pages = "303 - 314",
number = 2,
month = "Feb.",
abstract = "The development of optimized motions of humanoid robots that guarantee a fast and also stable walking is an important task especially in the context of autonomous soccer playing robots in RoboCup. We present a walking motion optimization approach for the humanoid robot prototype HR18 which is equipped with a low dimensional parameterized walking trajectory generator, joint motor controller and an internal stabilization. The robot is included as hardware-in-the-loop to define a low dimensional black-box optimization problem. In contrast to previously performed walking optimization approaches we apply a sequential surrogate optimization approach using stochastic approximation of the underlying objective function and sequential quadratic programming to search for a fast and stable walking motion. This is done under the conditions that only a small number of physical walking experiments should have to be carried out during the online optimization process. For the identified walking motion for the considered 55 cm tall humanoid robot we measured a forward walking speed of more than 30 cm/sec. With a modified version of the robot even more than 40 cm/sec could be achieved in permanent operation.",
doi = "10.1177/0278364908095171",
file = "2009_hemker_ijr.pdf:2009_hemker_ijr.pdf:PDF",
url = "http://dx.doi.org/10.1177/0278364908095171"
}

15. T Hemker, K R Fowler, M W Farthing and O Stryk. A mixed-integer simulation-based optimization approach with surrogate functions in water resources management. Optimization and Engineering 9(4):341-360, Dezember 2008. DOI BibTeX

@article{2007:OptEng-HemkerFowlervonStryk,
author = "T. Hemker and K.R. Fowler and M.W. Farthing and O. von Stryk",
title = "A mixed-integer simulation-based optimization approach with surrogate functions in water resources management",
journal = "Optimization and Engineering",
year = 2008,
volume = 9,
pages = "341-360",
number = 4,
month = "December",
abstract = "Efficient and powerful methods are needed to overcome the inherent difficulties in the numerical solution of many simulation-based engineering design problems. Typically, expensive simulation codes are included as black-box function generators; therefore, gradient information that is required by mathematical optimization methods is entirely unavailable. Furthermore, the simulation code may contain iterative or heuristic methods, low-order approximations of tabular data, or other numerical methods which contribute noise to the objective function. This further rules out the application of Newton-type or other gradient-based methods that use traditional finite difference approximations. In addition, if the optimization formulation includes integer variables the complexity grows even further. In this paper we consider three different modeling approaches for a mixed-integer nonlinear optimization problem taken from a set of water resources benchmarking problems. Within this context, we compare the performance of a genetic algorithm, the implicit filtering algorithm, and a branch-and-bound approach that uses sequential surrogate functions. We show that the surrogate approach can greatly improve computational efficiency while locating a comparable, sometimes better, design point than the other approaches.",
doi = "10.1007/s11081-008-9048-0",
file = "2008_hemker_opte.pdf:2008_hemker_opte.pdf:PDF"
}

16. K Radkhah, T Hemker and O Stryk. A Novel Self-Calibration Method for Industrial Robots Incorporating Geometric and Nongeometric Effects. In Proc. IEEE International Conference on Mechatronics and Automation (ICMA). 2008, 864-869. URL BibTeX

@inproceedings{radkhah:2008,
author = "K. Radkhah and T. Hemker and O. von Stryk",
booktitle = "Proc. IEEE International Conference on Mechatronics and Automation (ICMA)",
title = "A Novel Self-Calibration Method for Industrial Robots Incorporating Geometric and Nongeometric Effects",
year = 2008,
month = "Aug. 05 - Aug. 08",
pages = "864-869",
abstract = "We propose a novel problem formulation for the solution of the nonlinear least squares appearing in the static calibration of the parameters of an industrial robot with rotational joints. A further contribution of the presented work is the use of an extended forward kinematic model incorporating both geometric and nongeometric parameters. These novelties facilitate the use of industrial robots as measurement systems. Both the automobile and the general industry are interested in the use of highly accurate robots that can be deployed as measurement instruments, e.g. for noticing incorrectly welded points early in the production process. Existing approaches are disadvantageous because of (1) the use of additional external measurement systems, and (2) the difficult, time-consuming, and cost-intensive determination of the robot base. Instead of absolute measurement data, we make use of relative measurement data by means of a camera attached to the robot flange. In tests based on data generated from simulation of a real experimental setup, we obtain absolute accuracies after calibration better than $\pm$ \unit[100]{$\mu$m}. This work lays the foundations for a cost-minimal and effective realization of a robot as a measurement instrument.",
url = "http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4798870&isnumber=4798708"
}

17. T Hemker. Derivative Free Surrogate Optimization for Mixed-Integer Nonlinear Black Box Problems in Engineering. Technische Universität Darmstadt, 2008. URL BibTeX

@phdthesis{2008:HemkerDiss,
author = "T. Hemker",
title = "Derivative Free Surrogate Optimization for Mixed-Integer Nonlinear Black Box Problems in Engineering",
year = 2008,
month = "December 19",
howpublished = "http://www.vdi-nachrichten.com/onlineshops/buchshop/literaturshop/langanzeige.asp?vr_id=7941",
number = 797,
pdf_short = "hemker.pdf",
publisher = "VDI Verlag",
series = "Reihe 10: Informatik/Kommunikation",
}

18. T Hemker, De H Gersem, O Stryk and T Weiland. Mixed-Integer Nonlinear Design Optimization of a Superconductive Magnet. IEEE Transactions on Magnetics 44(6):1110-1113, Juni 2007. BibTeX

@article{hemker_et_al_2008,
author = "T. Hemker and H. De Gersem and O. von Stryk and T. Weiland",
title = "Mixed-Integer Nonlinear Design Optimization of a Superconductive Magnet",
journal = "IEEE Transactions on Magnetics",
year = 2007,
volume = 44,
pages = "1110-1113",
number = 6,
month = "June",
abstract = "The numerical optimization of continuous parameters in electrotechnical design using electromagnetic field simulation is already standard. When integer-valued variables are involved, the complexity of the optimization problem rises drastically. In this paper, we describe a new sequential surrogate optimization approach for simulation-based mixed-integer nonlinear programming problems. We apply the method for the optimization of combined integer- and real-valued geometrical parameters of the coils of a superconductive magnet.",
file = "2007_hemker_ieee_tm.pdf:2007_hemker_ieee_tm.pdf:PDF"
}

19. T Hemker, O Stryk, De H Gersem and T Weiland. Simulation-based design improvement of a superconductive magnet by mixed-integer nonlinear surrogate optimization. In 16th Intl. Conf. on the Computation of Electromagnetic Fields - Compumag 2007. 2007, 449-450. BibTeX

@inproceedings{2007:Compumag-Hemkeretal,
author = "T. Hemker and O. von Stryk and H. De Gersem and T. Weiland",
title = "Simulation-based design improvement of a superconductive magnet by mixed-integer nonlinear surrogate optimization",
booktitle = "16th Intl. Conf. on the Computation of Electromagnetic Fields - Compumag 2007",
year = 2007,
pages = "449-450",
month = "June 24-28",
abstract = "The numerical optimization of continuous parameters in electrotechnical design using electromagnetic field simulation is already standard. In this paper, we describe a new sequential surrogate optimization approach for simulation-based mixed-integer nonlinear programming problems. We apply the method for the optimization of combined integer- and real-valued geometrical parameters of the coils of a superconductive magnet.",
file = "2007_hemker_compumag.pdf:2007_hemker_compumag.pdf:PDF"
}

20. T Hemker, H Sakamoto, M Stelzer and O Stryk. Hardware-in-the-loop optimization of the walking speed of a humanoid robot. In CLAWAR 2006: 9th International Conference on Climbing and Walking Robots. 2006, 614-623. BibTeX

@inproceedings{2006:CLAWARHemkeretal,
author = "T. Hemker and H. Sakamoto and M. Stelzer and O. von Stryk",
title = "Hardware-in-the-loop optimization of the walking speed of a humanoid robot",
booktitle = "CLAWAR 2006: 9th International Conference on Climbing and Walking Robots",
year = 2006,
pages = "614-623",
month = "September 11-14",
abstract = "The development of optimized motions of humanoid robots that guarantee a fast and also stable walking is an important task especially in the context of autonomous soccer playing robots in RoboCup. We present a walking motion optimization approach for the humanoid robot prototype HR18 which is equipped with a low dimensional parameterized walking trajectory generator, joint motor controller and an internal stabilization. The robot is included as hardware-in-the-loop to define a low dimensional black-box optimization problem. In contrast to previously performed walking optimization approaches we apply a sequential surrogate optimization approach using stochastic approximation of the underlying objective function and sequential quadratic programming to search for a fast and stable walking motion. This is done under the conditions that only a small number of physical walking experiments should have to be carried out during the online optimization process. For the identified walking motion for the considered $55$ cm tall humanoid robot we measured a forward walking speed of more than $30$ cm/sec. With a modified version of the robot even more than $40$ cm/sec could be achieved in permanent operation.",
file = "2006-Hemker_etal_CLAWAR_pre.pdf:2006-Hemker_etal_CLAWAR_pre.pdf:PDF"
}

21. T Hemker, M Glocker, De H Gersem, O Stryk and T Weiland. Mixed-integer simulation-based optimization for a superconductive magnet design. In Sixth International Conference on Computational Elektromagnetics, Proceedings, 4-6 April 2006 in Aachen. 2006, 125-127. BibTeX

@inproceedings{2006_hemker_CEM,
author = "T. Hemker and M. Glocker and H. De Gersem and O. von Stryk and T. Weiland",
title = "Mixed-integer simulation-based optimization for a superconductive magnet design",
booktitle = "Sixth International Conference on Computational Elektromagnetics, Proceedings, 4-6 April 2006 in Aachen",
year = 2006,
pages = "125-127",
month = "4-6 April",
organization = "VDE",
abstract = "The optimization of continuous parameters in electrotechnical designs using electromagnetic field simulation is already standard. Typically, the simulation tools only carry out evaluations of the objective function and do not provide gradient information. If in addition to continuous design parameters also integer design parameters have to be optimized, only computational expensive random search methods like genetic algorithms are well known. In this paper, we present a new sequential modeling approach to solve mixed-integer simulation-based optimiza-tion problems for an electrotechnical design problem for superconductive magnets. Each step of this approach uses stochastic modeling techniques to predict the simulation output by a surrogate function. The surrogate function treats the integer variables as real-valued ones. New promising parameter con-figurations are predicted by a “branch-and-bound” method, which solves the purely continuous subproblems by classical optimization methods for continuous and differentiable functions. The additional information of these simulation runs improves the quality of the surrogate function step by step. The proposed approach is applied to optimize the distribution of coil blocks and coil windings of a superconduc-tive magnet such that a maximal homogeneity of the magnetic field in the aperture is achieved.",
file = "2006-Hemker_etal_CEM_pre.pdf:2006-Hemker_etal_CEM_pre.pdf:PDF"
}

22. T Hemker, K R Fowler and O Stryk. Derivative-Free Optimization Methods for Handling Fixed Costs in Optimal Groundwater Remediation Design. In P J Binning, P Engesgaard, H Dahle, G Pinder and W G Gray (eds.). Proc. of the CMWR XVI - Computational Methods in Water Resources. 2006, electronic resource. BibTeX

@inproceedings{2006_hemker_CMWR,
author = "T. Hemker and K.R. Fowler and O. von Stryk",
booktitle = "Proc. of the CMWR XVI - Computational Methods in Water Resources",
title = "Derivative-Free Optimization Methods for Handling Fixed Costs in Optimal Groundwater Remediation Design",
year = 2006,
editor = "P.J. Binning and P. Engesgaard and H. Dahle and G. Pinder and W.G. Gray",
month = "19-22 June",
pages = "electronic resource",
abstract = "Groundwater remediation design problems are routine in water resource management. The starting point for such a design problem is to formulate an objective function that represents a measure of the manager´s goal. For example, in plume migration control, we need to determine the cost to design a well field to alter the direction of groundwater flow and thereby control the destination of a contaminant. Constraints must be specified to ensure that the plume is captured, the physical domain is protected, and the wells operate under realistic conditions. Optimization algorithms must work in conjunction with groundwater flow and possibly contaminant transport simulators to determine the minimal cost well design subject to the constraints, but typically these numerical simulation codes have been developed for many years and have usually not been designed to meet the specific needs of optimization methods as, e.g., providing gradient information. Decision variables can be real-valued, in the case of pumping rates and well locations, or integer valued in the case of the number of wells in the design. In this work we focus on formulations that include a fixed installation cost as well as an operating cost, resulting in a simulation-based nonlinear mixed-integer optimization problem. The motivation is that our preliminary studies have shown that convergence to an unsatisfactory, local minimum with many wells operating at low pumping rates is common when the fixed cost is ignored. The challenge in the fixed cost formulation is the integer variable for the number of wells in the design. Removing a well from the design space leads to a large decrease in cost meaning optimizers must be equipped to either handle a mixed-integer or approximate mixed integer, black-box problem and discontinuities in the objective function. Moreover since evaluation of the objective function requires numerical results from a simulation, derivative information is unavailable. Gradient based optimization methods are not appropriate for these applications, hence methods that rely only on function values are more appealing. We compare three methods for handling the installation cost on a hydraulic capture benchmarking problem proposed in the literature. All the approaches described below do not use the gradient of the objective function, only function values for minimization. In one approach, we use penalty coefficients proposed in the literature for the installation term to transform the discontinuous problem into a continuous one. In another approach, we bypass including the number of wells as a decision variable by defining an inactive-well threshold. In the course of the optimization, if a well rate falls in this threshold, the well is removed from the design space, leading to large discontinuities in the objective function. For the two above formulations, we use the implicit filtering algorithm, a method which uses a sequence of finite difference gradients, for minimization. In the third approach, we use sequential stochastic modeling to build surrogate functions to approximate the original objective function. With this procedure the use of a branch and bound technique becomes possible to solve the mixed integer problem in contrast to methods working directly on the simulation results, which impedes relaxation of integer variables. We present promising preliminary numerical results on the benchmarking problem and point the way towards improvement and future work.",
file = "2006-Hemker_etal_CMWR_pre.pdf:2006-Hemker_etal_CMWR_pre.pdf:PDF"
}


## Trelse

1. Robert Hemker Thomas und Müller-Törk. (Zu) Wenig Digital-Kompetenzen in der Ausbildung für die öffentliche Verwaltung. zur Veröffentlichung angenommen, Hochschule des Bundes, Technische Berichte zur angewandten Informatik, 2022. BibTeX

@article{,
author = "Hemker, Thomas und Müller-Törk, Robert",
journal = "zur Veröffentlichung angenommen, Hochschule des Bundes, Technische Berichte zur angewandten Informatik",
title = "(Zu) Wenig Digital-Kompetenzen in der Ausbildung für die öffentliche Verwaltung",
year = 2022
}

2. U Lohmann, S Schmittwilken and T Hemker. Virtuelle Labore in der IT Didaktik. Innovative Verwaltung 12, 2018. BibTeX

@article{lohmann2018,
author = "U. Lohmann and S. Schmittwilken and T. Hemker",
title = "Virtuelle {L}abore in der {IT D}idaktik",
journal = "Innovative Verwaltung",
year = 2018,
volume = 12,
issn = "1618-9876"
}

3. T Hemker, De H Gersem, S Koch and O Stryk. Bessere Synchrotron-Magnete durch simulationsbasierte Optimierung. In M Schaefer (ed.). Computational Engineering. Forschung series, number 2/2011, TU Darmstadt, 2011, pages 58-62. URL BibTeX

@incollection{2011:TUD-Forschung,
author = "T. Hemker and H. De Gersem and S. Koch and O. von Stryk",
title = "{Bessere Synchrotron-Magnete durch simulationsbasierte Optimierung}",
booktitle = "Computational Engineering",
year = 2011,
editor = "M. Schaefer",
number = "2/2011",
series = "Forschung",
pages = "58-62",
abstract = "Während die numerische Optimierung von kontinuierlichen Parametern supraleitender Magnete basierend auf elektromagnetischen Feldsimulationen trotz hoher Rechenzeitanforderungen schon fast alltäglich ist, stellt die für ein optimales Design notwendige, simultane Berücksichtigung auch diskreter Parameter und der damit verbundene, hohe zusätzliche Rechenaufwand enorme Schwierigkeiten dar. Eine neue, methodische Vorgehensweise zu deren Überwindung wird vorgestellt und zur Entwicklung von besonders leistungsfähigen, supraleitenden Magneten für das neue internationale Beschleunigerzentrum FAIR erfolgreich angewendet.",
file = "2011-Forschen_02_2011_S58-62.pdf:2011-Forschen_02_2011_S58-62.pdf:PDF",
}

4. J.~D. Griffin, K.~R. Fowler, G.~A. Gray, T. Hemker and M ~D Parno. Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search. Sandia Report SAND2010-3023J, Sandia National Laboratories, Mai 2010. BibTeX

@techreport{JoshuaD.Griffin2010,
author = "J.~D. Griffin, and K.~R. Fowler, and G.~A. Gray, and T. Hemker, and M.~D. Parno",
title = "Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search",
institution = "Sandia National Laboratories",
year = 2010,
type = "Sandia Report",
number = "SAND2010-3023J",
address = "Albuquerque, New Mexico 87185 and Livermore, California 94550",
month = "May",
owner = "thomas",
timestamp = "2013.08.18"
}

5. M Friedmann, T Hemker, S Kohlbrecher, K Petersen, S Petters, K Radkhah, M Risler, D Scholz, D Thomas and O Stryk. Darmstadt Dribblers - Team Description for Humanoid KidSize League of RoboCup 2010. Technische Universität Darmstadt, 2010. BibTeX

@techreport{2010:dd_tdp,
author = "M. Friedmann and T. Hemker and S. Kohlbrecher and K. Petersen and S. Petters and K. Radkhah and M. Risler and D. Scholz and D. Thomas and O. von Stryk",
title = "Darmstadt Dribblers - Team Description for Humanoid KidSize League of RoboCup 2010",
year = 2010,
abstract = "This paper describes the hardware and software design of the kidsize humanoid robot systems of the Darmstadt Dribblers in 2010. The robots are used as a vehicle for research in control of locomotion and behavior of autonomous humanoid robots and robot teams with many degrees of freedom and many actuated joints. The Humanoid League of RoboCup provides an ideal testbed for such aspects of dynamics in motion and autonomous behavior as the problem of generating and maintaining statically or dynamically stable bipedal locomotion is predominant for all types of vision guided motions during a soccer game. A modular software architecture as well as further technologies have been developed for ecient and eective implementation and test of modules for sensing, planning, behavior, and actions of humanoid robots.",
file = "2010-tdp-hum.pdf:2010-tdp-hum.pdf:PDF"
}

6. M D Parno, K R Fowler and T Hemker. Framework for Particle Swarm Optimization with Surrogate Functions. Number TUD-CS-2009-0139, Technische Universität Darmstadt, Department of Computer Science, 2009. BibTeX

@techreport{Parno2009,
author = "M. D. Parno and K. R. Fowler and T. Hemker",
title = "Framework for Particle Swarm Optimization with Surrogate Functions",
institution = {Technische Universit\"at Darmstadt, Department of Computer Science},
year = 2009,
number = "TUD-CS-2009-0139",
abstract = "Particle swarm optimization (PSO) is a population-based, heuristic minimization technique that is based on social behavior. The method has been shown to perform well on a variety of problems including those with nonconvex, nonsmooth objective functions with multiple local minima. However, the method can be computationally expensive in that a large number of function calls is required to advance the swarm at each optimization iteration. This is a significant drawback when function evaluations depend on output from an off-the-shelf simulation program, which is often the case in engineering applications. To this end, we propose an algorithm which incorporates surrogate functions, which serve as a stand-in for the expensive objective function, within the PSO framework. We present numerical results to show that this hybrid approach can improve algorithmic efficiency.",
file = "Parno2009.pdf:Parno2009.pdf:PDF"
}


## Articles

1. Robert Hemker Thomas und Müller-Törk. Digital Competences in Public Sector Education as a Prerequisite for e-Government. Pro Publico Bono – Public Administration accepted for publication, 2023. BibTeX

@article{,
author = "Hemker, Thomas und Müller-Törk, Robert",
journal = "Pro Publico Bono – Public Administration",
title = "Digital Competences in Public Sector Education as a Prerequisite for e-Government",
year = 2023,
volume = "accepted for publication",
ranking = "rank5"
}

2. Thomas Hemker, Robert Müller-Török and Alexander Prosser. Interactive ELearning with ERP Systems: Advancing Some Refutable Hypotheses on Interactivity in ELearning. In Proceedings of the Central and Eastern European EDem and EGov Days. 2022, 130–134. URL, DOI BibTeX

@inproceedings{Hemker2022,
author = {Hemker, Thomas and M\"{u}ller-T\"{o}r\"{o}k, Robert and Prosser, Alexander},
booktitle = "Proceedings of the Central and Eastern European EDem and EGov Days",
title = "Interactive ELearning with ERP Systems: Advancing Some Refutable Hypotheses on Interactivity in ELearning",
year = 2022,
address = "New York, NY, USA",
pages = "130–134",
publisher = "Association for Computing Machinery",
series = "CEEeGov '22",
abstract = "COVID-19 accelerated the drive towards distance learning, typically supported by web-based eLearning resources. There are also studies reviewing the transition to such as teaching style, whether in conjunction with traditional classroom teaching or as a supplement. This paper will focus on ERP-based teaching within a larger program to establish tertiary distance education in eGovernment in Germany (“eGov Campus”). It attempts to structure the topic and to advance some hypotheses that will be empirically tested against real-world data and experience from introducing this style of teaching in the eGov Campus.",
doi = "10.1145/3551504.3551562",
isbn = 9781450397667,
keywords = "eGov Campus, ERP systems, eGovernment",
location = "Budapest, Hungary",
numpages = 5,
url = "https://doi.org/10.1145/3551504.3551562"
}

3. T Hemker and C Werner. DIRECT using local search on surrogates. Pacific Journal of Optimization 7(3):443-466, September 2011. URL BibTeX

@article{hemker2011,
author = "T. Hemker and C. Werner",
title = "DIRECT using local search on surrogates",
journal = "Pacific Journal of Optimization",
year = 2011,
volume = 7,
pages = "443-466",
number = 3,
month = "September",
abstract = "The solution of noisy nonlinear optimization problems with nonlinear constraints and derivative information is becoming increasingly important, as many practical applications can be described by this type of problem in e.g., engineering applications. Existing local optimization methods show good convergence properties. However, they often depend on sufficiently good starting points and/or the approximation of gradients. In turn, global derivative free methods, which need no starting values to be initialized, require many evaluations of the objective function, particularly in the vicinity of the solution. A derivative free optimization algorithm is developed that combines advantages of both local and global methods. The DIRECT algorithm, which is often used for problems where no prior knowledge is available as kind of a brute force start, is extended by an inner loop using a surrogate based optimization method. The local search on the surrogate function determines better candidates for sampling than the hypercube center points chosen by DIRECT, especially if constraints are arising. This inner loop needs no additional evaluation of the original problem. Standard test problems and a computational more expensive test problem are chosen to show the performance of the new algorithm.",
file = "2011_hemker.pdf:2011_hemker.pdf:PDF",
url = "http://www.ybook.co.jp/online2/oppjo/vol7/p443.html"
}

4. J D Griffin, K R Fowler, G A Gray, T Hemker and M D Parno. Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search. Pacific Journal of Optimization 7(3):425-443, September 2011. URL BibTeX

@article{griffin2011,
author = "J. D. Griffin and K. R. Fowler and G. A. Gray and T. Hemker and M. D. Parno",
title = "Derivative-Free Optimization Via Evolutionary Algorithms Guiding Local Search",
journal = "Pacific Journal of Optimization",
year = 2011,
volume = 7,
pages = "425-443",
number = 3,
month = "September",
abstract = "In this paper, we describe the technical details of HOPSPACK (Hybrid Optimization Parallel Search Package), a new software platform which facilitates combining multiple optimization routines into a single, tightly-coupled, hybrid algorithm that supports parallel function evaluations. The framework is designed such that existing optimization source code can be easily incorporated with minimal code modification. By maintaining the integrity of each individual solver, the strengths and code sophistication of the original optimization package are retained and exploited.",
file = "2010-griffin_etal.pdf:2010-griffin_etal.pdf:PDF",
url = "http://www.ybook.co.jp/online2/oppjo/vol7/p425.html"
}

5. M Parno, K Fowler and T Hemker. Applicability of Surrogates to Improve Efficiency of Particle Swarm Optimization for Simulation-based Problems. Engineering Optimization, pages online, 2011. DOI BibTeX

@article{parno2011,
author = "M. Parno and K. Fowler and T. Hemker",
title = "Applicability of Surrogates to Improve Efficiency of Particle Swarm Optimization for Simulation-based Problems",
journal = "Engineering Optimization",
year = 2011,
pages = "online",
abstract = "Particle swarm optimization (PSO) is a population-based, heuristic technique based on social behaviour that performs well on a variety of problems including those with non-convex, non-smooth objective functions with multiple minima. However, the method can be computationally expensive in that a large number of function calls is required. This is a drawback when evaluations depend on an off-the-shelf simulation program, which is often the case in engineering applications. An algorithm is proposed which incorporates surrogates as a stand-in for the expensive objective function, within the PSO framework. Numerical results are presented on standard benchmarking problems and a simulation-based hydrology application to show that this hybrid can improve efficiency. A comparison is made between the application of a global PSO and a standard PSO to the same formulations with surrogates. Finally, data profiles, probability of success, and a measure of the signal-to-noise ratio of the the objective function are used to assess the use of a surrogate.",
doi = "10.1080/0305215X.2011.598521"
}

6. E Abele, J Bauer, T Hemker, R Laurischkat, H Meier, S Reese and O Stryk. Comparison and Validation of Implementations of a Flexible Joint Multibody Dynamics System Model for an Industrial Robot. CIRP Journal of Manufacturing Science and Technology 4(1):38-43, 2011. DOI BibTeX

@article{2011:Abele_etal_JMST,
author = "E. Abele and J. Bauer and T. Hemker and R. Laurischkat and H. Meier and S. Reese and O. von Stryk",
title = "Comparison and Validation of Implementations of a Flexible Joint Multibody Dynamics System Model for an Industrial Robot",
journal = "CIRP Journal of Manufacturing Science and Technology",
year = 2011,
volume = 4,
pages = "38-43",
number = 1,
abstract = "In this paper, different implementations of elastic joint models of industrial robots are described and compared established in ADAMS and SimMechanics. The models are intended to be used for path prediction under process force load due to Roboforming and High Speed Cutting, respectively. The computational results have been compared and showed good agreement. In experiments of robot forming and robot milling the measured and simulated path deviations according to the process force are compared. The experiments are descriped and the results are discussed within the paper as a basis of a next step model based compensation of the path deviation.",
doi = "10.1016/j.cirpj.2011.01.006"
}

7. E Abele, J Bauer, M Pischan, O Stryk, M Friedmann and T Hemker. Prediction of the Tool Displacement for Robot Milling Applications using Co-Simulation of an Industrial Robot and a Removal Process. In CIRP 2nd International Conference Process Machine Interactions. Juni 2010. BibTeX

@inproceedings{2010:AbeleBauerPrischanStrykFriedmannHemker,
author = "E. Abele and J. Bauer and M. Pischan and O. v. Stryk and M. Friedmann and T. Hemker",
booktitle = "CIRP 2nd International Conference Process Machine Interactions",
title = "Prediction of the Tool Displacement for Robot Milling Applications using Co-Simulation of an Industrial Robot and a Removal Process",
year = 2010,
month = "Jun",
organization = "CIRP",
abstract = "Industrial robots are used in a great variety of applications for handling, welding and milling operations. They represent a cost-saving and flexible alternative for machining applications. A reduced pose and path accuracy, especially under process force load due to the high mechanical compliance, restrict the use of industrial robots for further machining applications. In this paper a method is presented to predict the resulting path deviation of the robot under process force. A process force simulation based on a material removal calculation is proposed. The presented material removal simulation allows the calculation of the chip geometry even in complex cutter work piece engagement conditions. The removal simulation is validated by means of conducted milling tests. By coupling the removal simulation with a robot model the path deviation can be predicted.",
file = "2010-PMI_preprint.pdf:2010-PMI_preprint.pdf:PDF",
keywords = "Milling, Industrial Robot, Machine Process Interaction, Material removal simulation"
}

8. K Radkhah, T Hemker and O Stryk. Self-calibration for industrial robots with rotational joints. Int. J. Mechatronics and Manufacturing Systems 3(3/4):187-209, 2010. BibTeX

@article{2010:IJMMS-Radkhah-etal,
author = "K. Radkhah and T. Hemker and O. von Stryk",
title = "Self-calibration for industrial robots with rotational joints",
journal = "Int. J. Mechatronics and Manufacturing Systems",
year = 2010,
volume = 3,
pages = "187-209",
number = "3/4",
file = "2010_ijmms_rdkhh_preprint.pdf:2010_ijmms_rdkhh_preprint.pdf:PDF"
}

9. T Hemker, H Sakamoto, M Stelzer and O Stryk. Efficient walking speed optimization of a humanoid robot. International Journal of Robotics Research 28(2):303 - 314, 2009. URL, DOI BibTeX

@article{2008_hemker_ijr,
author = "T. Hemker and H. Sakamoto and M. Stelzer and O. von Stryk",
title = "Efficient walking speed optimization of a humanoid robot",
journal = "International Journal of Robotics Research",
year = 2009,
volume = 28,
pages = "303 - 314",
number = 2,
month = "Feb.",
abstract = "The development of optimized motions of humanoid robots that guarantee a fast and also stable walking is an important task especially in the context of autonomous soccer playing robots in RoboCup. We present a walking motion optimization approach for the humanoid robot prototype HR18 which is equipped with a low dimensional parameterized walking trajectory generator, joint motor controller and an internal stabilization. The robot is included as hardware-in-the-loop to define a low dimensional black-box optimization problem. In contrast to previously performed walking optimization approaches we apply a sequential surrogate optimization approach using stochastic approximation of the underlying objective function and sequential quadratic programming to search for a fast and stable walking motion. This is done under the conditions that only a small number of physical walking experiments should have to be carried out during the online optimization process. For the identified walking motion for the considered 55 cm tall humanoid robot we measured a forward walking speed of more than 30 cm/sec. With a modified version of the robot even more than 40 cm/sec could be achieved in permanent operation.",
doi = "10.1177/0278364908095171",
file = "2009_hemker_ijr.pdf:2009_hemker_ijr.pdf:PDF",
url = "http://dx.doi.org/10.1177/0278364908095171"
}

10. K Radkhah, T Hemker, M Friedmann and O Stryk. Towards the deployment of industrial robots as measurement instruments - An extended forward kinematic model incorporating geometric and nongeometric effects. In Proc. 2009 IEEE/ASME Conference on Advanced Intelligent Mechatronics (AIM). 2009, 124-129. URL BibTeX

@inproceedings{radkhah:2009,
author = "K. Radkhah and T. Hemker and M. Friedmann and O. von Stryk",
booktitle = "Proc. 2009 IEEE/ASME Conference on Advanced Intelligent Mechatronics (AIM)",
title = "Towards the deployment of industrial robots as measurement instruments - An extended forward kinematic model incorporating geometric and nongeometric effects",
year = 2009,
month = "Jul. 14 - Jul. 17",
pages = "124-129",
abstract = "In the area of mounting and spot-welding of body-in-white, absolutely accurate robots are installed as measuring instruments, replacing expensive coordinate and other external measuring machines. Measurement technologies based on industrial robots play an increasingly important role. Such applications require highly accurate robots. Prior to deployment of highly accurate robot, however, it needs to be ensured that the implemented robot model fits the real model. Robot calibration can offer a significant opportunity to improve the positioning accuracy and to cut production costs. Existing calibration approaches fail to capture geometric and elastic effects occurring in the robot forward kinematics. Therefore, in this work an extended forward kinematic model incorporating both geometric and elastic effects has been developed in which the positioning accuracy of a manipulator, with or without an accurate internal robot model in the robot controller, is improved.",
}

11. T Hemker, K R Fowler, M W Farthing and O Stryk. A mixed-integer simulation-based optimization approach with surrogate functions in water resources management. Optimization and Engineering 9(4):341-360, Dezember 2008. DOI BibTeX

@article{2007:OptEng-HemkerFowlervonStryk,
author = "T. Hemker and K.R. Fowler and M.W. Farthing and O. von Stryk",
title = "A mixed-integer simulation-based optimization approach with surrogate functions in water resources management",
journal = "Optimization and Engineering",
year = 2008,
volume = 9,
pages = "341-360",
number = 4,
month = "December",
abstract = "Efficient and powerful methods are needed to overcome the inherent difficulties in the numerical solution of many simulation-based engineering design problems. Typically, expensive simulation codes are included as black-box function generators; therefore, gradient information that is required by mathematical optimization methods is entirely unavailable. Furthermore, the simulation code may contain iterative or heuristic methods, low-order approximations of tabular data, or other numerical methods which contribute noise to the objective function. This further rules out the application of Newton-type or other gradient-based methods that use traditional finite difference approximations. In addition, if the optimization formulation includes integer variables the complexity grows even further. In this paper we consider three different modeling approaches for a mixed-integer nonlinear optimization problem taken from a set of water resources benchmarking problems. Within this context, we compare the performance of a genetic algorithm, the implicit filtering algorithm, and a branch-and-bound approach that uses sequential surrogate functions. We show that the surrogate approach can greatly improve computational efficiency while locating a comparable, sometimes better, design point than the other approaches.",
doi = "10.1007/s11081-008-9048-0",
file = "2008_hemker_opte.pdf:2008_hemker_opte.pdf:PDF"
}

12. K Radkhah, T Hemker and O Stryk. A Novel Self-Calibration Method for Industrial Robots Incorporating Geometric and Nongeometric Effects. In Proc. IEEE International Conference on Mechatronics and Automation (ICMA). 2008, 864-869. URL BibTeX

@inproceedings{radkhah:2008,
author = "K. Radkhah and T. Hemker and O. von Stryk",
booktitle = "Proc. IEEE International Conference on Mechatronics and Automation (ICMA)",
title = "A Novel Self-Calibration Method for Industrial Robots Incorporating Geometric and Nongeometric Effects",
year = 2008,
month = "Aug. 05 - Aug. 08",
pages = "864-869",
abstract = "We propose a novel problem formulation for the solution of the nonlinear least squares appearing in the static calibration of the parameters of an industrial robot with rotational joints. A further contribution of the presented work is the use of an extended forward kinematic model incorporating both geometric and nongeometric parameters. These novelties facilitate the use of industrial robots as measurement systems. Both the automobile and the general industry are interested in the use of highly accurate robots that can be deployed as measurement instruments, e.g. for noticing incorrectly welded points early in the production process. Existing approaches are disadvantageous because of (1) the use of additional external measurement systems, and (2) the difficult, time-consuming, and cost-intensive determination of the robot base. Instead of absolute measurement data, we make use of relative measurement data by means of a camera attached to the robot flange. In tests based on data generated from simulation of a real experimental setup, we obtain absolute accuracies after calibration better than $\pm$ \unit[100]{$\mu$m}. This work lays the foundations for a cost-minimal and effective realization of a robot as a measurement instrument.",
url = "http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4798870&isnumber=4798708"
}

13. T Hemker, De H Gersem, O Stryk and T Weiland. Mixed-Integer Nonlinear Design Optimization of a Superconductive Magnet. IEEE Transactions on Magnetics 44(6):1110-1113, Juni 2007. BibTeX

@article{hemker_et_al_2008,
author = "T. Hemker and H. De Gersem and O. von Stryk and T. Weiland",
title = "Mixed-Integer Nonlinear Design Optimization of a Superconductive Magnet",
journal = "IEEE Transactions on Magnetics",
year = 2007,
volume = 44,
pages = "1110-1113",
number = 6,
month = "June",
abstract = "The numerical optimization of continuous parameters in electrotechnical design using electromagnetic field simulation is already standard. When integer-valued variables are involved, the complexity of the optimization problem rises drastically. In this paper, we describe a new sequential surrogate optimization approach for simulation-based mixed-integer nonlinear programming problems. We apply the method for the optimization of combined integer- and real-valued geometrical parameters of the coils of a superconductive magnet.",
file = "2007_hemker_ieee_tm.pdf:2007_hemker_ieee_tm.pdf:PDF"
}

14. T Hemker, O Stryk, De H Gersem and T Weiland. Simulation-based design improvement of a superconductive magnet by mixed-integer nonlinear surrogate optimization. In 16th Intl. Conf. on the Computation of Electromagnetic Fields - Compumag 2007. 2007, 449-450. BibTeX

@inproceedings{2007:Compumag-Hemkeretal,
author = "T. Hemker and O. von Stryk and H. De Gersem and T. Weiland",
title = "Simulation-based design improvement of a superconductive magnet by mixed-integer nonlinear surrogate optimization",
booktitle = "16th Intl. Conf. on the Computation of Electromagnetic Fields - Compumag 2007",
year = 2007,
pages = "449-450",
month = "June 24-28",
abstract = "The numerical optimization of continuous parameters in electrotechnical design using electromagnetic field simulation is already standard. In this paper, we describe a new sequential surrogate optimization approach for simulation-based mixed-integer nonlinear programming problems. We apply the method for the optimization of combined integer- and real-valued geometrical parameters of the coils of a superconductive magnet.",
file = "2007_hemker_compumag.pdf:2007_hemker_compumag.pdf:PDF"
}

15. T Hemker, M Glocker, De H Gersem, O Stryk and T Weiland. Mixed-integer simulation-based optimization for a superconductive magnet design. In Sixth International Conference on Computational Elektromagnetics, Proceedings, 4-6 April 2006 in Aachen. 2006, 125-127. BibTeX

@inproceedings{2006_hemker_CEM,
author = "T. Hemker and M. Glocker and H. De Gersem and O. von Stryk and T. Weiland",
booktitle = "Sixth International Conference on Computational Elektromagnetics, Proceedings, 4-6 April 2006 in Aachen",
title = "Mixed-integer simulation-based optimization for a superconductive magnet design",
year = 2006,
month = "4-6 April",
organization = "VDE",
pages = "125-127",
abstract = "The optimization of continuous parameters in electrotechnical designs using electromagnetic field simulation is already standard. Typically, the simulation tools only carry out evaluations of the objective function and do not provide gradient information. If in addition to continuous design parameters also integer design parameters have to be optimized, only computational expensive random search methods like genetic algorithms are well known. In this paper, we present a new sequential modeling approach to solve mixed-integer simulation-based optimiza-tion problems for an electrotechnical design problem for superconductive magnets. Each step of this approach uses stochastic modeling techniques to predict the simulation output by a surrogate function. The surrogate function treats the integer variables as real-valued ones. New promising parameter con-figurations are predicted by a “branch-and-bound” method, which solves the purely continuous subproblems by classical optimization methods for continuous and differentiable functions. The additional information of these simulation runs improves the quality of the surrogate function step by step. The proposed approach is applied to optimize the distribution of coil blocks and coil windings of a superconduc-tive magnet such that a maximal homogeneity of the magnetic field in the aperture is achieved.",
file = "2006-Hemker_etal_CEM_pre.pdf:2006-Hemker_etal_CEM_pre.pdf:PDF",
ranking = "rank5"
}

16. T Hemker, K R Fowler and O Stryk. Derivative-Free Optimization Methods for Handling Fixed Costs in Optimal Groundwater Remediation Design. In P J Binning, P Engesgaard, H Dahle, G Pinder and W G Gray (eds.). Proc. of the CMWR XVI - Computational Methods in Water Resources. 2006, electronic resource. BibTeX

@inproceedings{2006_hemker_CMWR,
author = "T. Hemker and K.R. Fowler and O. von Stryk",
booktitle = "Proc. of the CMWR XVI - Computational Methods in Water Resources",
title = "Derivative-Free Optimization Methods for Handling Fixed Costs in Optimal Groundwater Remediation Design",
year = 2006,
editor = "P.J. Binning and P. Engesgaard and H. Dahle and G. Pinder and W.G. Gray",
month = "19-22 June",
pages = "electronic resource",
abstract = "Groundwater remediation design problems are routine in water resource management. The starting point for such a design problem is to formulate an objective function that represents a measure of the manager´s goal. For example, in plume migration control, we need to determine the cost to design a well field to alter the direction of groundwater flow and thereby control the destination of a contaminant. Constraints must be specified to ensure that the plume is captured, the physical domain is protected, and the wells operate under realistic conditions. Optimization algorithms must work in conjunction with groundwater flow and possibly contaminant transport simulators to determine the minimal cost well design subject to the constraints, but typically these numerical simulation codes have been developed for many years and have usually not been designed to meet the specific needs of optimization methods as, e.g., providing gradient information. Decision variables can be real-valued, in the case of pumping rates and well locations, or integer valued in the case of the number of wells in the design. In this work we focus on formulations that include a fixed installation cost as well as an operating cost, resulting in a simulation-based nonlinear mixed-integer optimization problem. The motivation is that our preliminary studies have shown that convergence to an unsatisfactory, local minimum with many wells operating at low pumping rates is common when the fixed cost is ignored. The challenge in the fixed cost formulation is the integer variable for the number of wells in the design. Removing a well from the design space leads to a large decrease in cost meaning optimizers must be equipped to either handle a mixed-integer or approximate mixed integer, black-box problem and discontinuities in the objective function. Moreover since evaluation of the objective function requires numerical results from a simulation, derivative information is unavailable. Gradient based optimization methods are not appropriate for these applications, hence methods that rely only on function values are more appealing. We compare three methods for handling the installation cost on a hydraulic capture benchmarking problem proposed in the literature. All the approaches described below do not use the gradient of the objective function, only function values for minimization. In one approach, we use penalty coefficients proposed in the literature for the installation term to transform the discontinuous problem into a continuous one. In another approach, we bypass including the number of wells as a decision variable by defining an inactive-well threshold. In the course of the optimization, if a well rate falls in this threshold, the well is removed from the design space, leading to large discontinuities in the objective function. For the two above formulations, we use the implicit filtering algorithm, a method which uses a sequence of finite difference gradients, for minimization. In the third approach, we use sequential stochastic modeling to build surrogate functions to approximate the original objective function. With this procedure the use of a branch and bound technique becomes possible to solve the mixed integer problem in contrast to methods working directly on the simulation results, which impedes relaxation of integer variables. We present promising preliminary numerical results on the benchmarking problem and point the way towards improvement and future work.",
file = "2006-Hemker_etal_CMWR_pre.pdf:2006-Hemker_etal_CMWR_pre.pdf:PDF"
}

17. T Hemker, H Sakamoto, M Stelzer and O Stryk. Hardware-in-the-loop optimization of the walking speed of a humanoid robot. In CLAWAR 2006: 9th International Conference on Climbing and Walking Robots. 2006, 614-623. BibTeX

@inproceedings{2006:CLAWARHemkeretal,
author = "T. Hemker and H. Sakamoto and M. Stelzer and O. von Stryk",
title = "Hardware-in-the-loop optimization of the walking speed of a humanoid robot",
booktitle = "CLAWAR 2006: 9th International Conference on Climbing and Walking Robots",
year = 2006,
pages = "614-623",
abstract = "The development of optimized motions of humanoid robots that guarantee a fast and also stable walking is an important task especially in the context of autonomous soccer playing robots in RoboCup. We present a walking motion optimization approach for the humanoid robot prototype HR18 which is equipped with a low dimensional parameterized walking trajectory generator, joint motor controller and an internal stabilization. The robot is included as hardware-in-the-loop to define a low dimensional black-box optimization problem. In contrast to previously performed walking optimization approaches we apply a sequential surrogate optimization approach using stochastic approximation of the underlying objective function and sequential quadratic programming to search for a fast and stable walking motion. This is done under the conditions that only a small number of physical walking experiments should have to be carried out during the online optimization process. For the identified walking motion for the considered $55$ cm tall humanoid robot we measured a forward walking speed of more than $30$ cm/sec. With a modified version of the robot even more than $40$ cm/sec could be achieved in permanent operation.",