Browsing by Author "Walter, Tobias"

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Open Access
Active exoskeleton reduces erector spinae muscle activity during lifting
(2023) Walter, Tobias; Stutzig, Norman; Siebert, Tobias
Musculoskeletal disorders (MSD) are a widespread problem, often regarding the lumbar region. Exoskeletons designed to support the lower back could be used in physically demanding professions with the intention of reducing the strain on the musculoskeletal system, e.g., by lowering task-related muscle activation. The present study aims to investigate the effect of an active exoskeleton on back muscle activity when lifting weights. Within the framework of the study, 14 subjects were asked to lift a 15 kg box with and without an active exoskeleton which allows the adjustment of different levels of support, while the activity of their M. erector spinae (MES) was measured using surface electromyography. Additionally, the subjects were asked about their overall rating of perceived exertion (RPE) during lifting under various conditions. Using the exoskeleton with the maximum level of support, the muscle activity was significantly lower than without exoskeleton. A significant correlation was found between the exoskeleton’s support level and the reduction of MES activity. The higher the support level, the lower the observed muscle activity. Furthermore, when lifting with the maximum level of support, RPE was found to be significantly lower than without exoskeleton too. A reduction in the MES activity indicates actual support for the movement task and might indicate lower compression forces in the lumbar region. It is concluded that the active exoskeleton supports people noticeably when lifting heavy weights. Exoskeletons seem to be a powerful tool for reducing load during physically demanding jobs and thus, their use might be helpful in lowering the risk of MSD.
Open Access
Evaluation and application of estimated gaze depth in Virtual Reality
(2022) Walter, Tobias
Eye Tracking Kameras werden zum Standard in neuen Virtual Reality Brillen. Während Evaluation und Bewertung von zweidimensionalen Eye Tracking Daten schon Einsatz in Forschung und Designprozessen finden, ist der Einsatz von dreidimensionaler Blicktiefe weitgehend unerforscht. Üblicherweise wird zur Schätzung der Blicktiefe der Blickstrahl mit einer zweidimensionalen Ebene, z.~B. dem Bilschirm, geschnitten. Allerdings setzt dieser Ansatz voraus, dass Abstände der Szene bekannt sind und keine Verdeckung auftritt. In dieser Arbeit wird die Blicktiefe durch das Schneiden der Blickgeraden beider Augen geschätzt. Dies ermöglicht die Verwendung von semi- transparenten Objekten, mit denen ein Benutzer interagieren kann. Einblicke in die Blicktiefe können wertvolle Einsichten in Benutzerverhalten liefern, in verdeckungsreichen Szenen die Frage klären, welches Ziel fokussiert wird und neue Interaktionstechniken ermöglichen. Ziel dieser Arbeit ist es, Blicktiefenschätzung zu evaluieren und neue Verwendungsmöglichkeiten zu erfrorschen. Um eine zuverlässige Schätzung zu erhalten, werden zwei Kalibrierungsprozeduren entwickelt, die auf aktuellen Methoden aufbauen und Modalitäten verglichen, die Einfluss auf die Kalibrierung haben könnten. Die Implementierung wurde in einer Pilotstudie (n=10) verglichen. Die Ergebnisse zeigen, dass Interaktion gut in Distanzen bis zu 1.2 Metern funktioniert, während Objekte, die nur 30 cm vom Benutzer entfernt waren, teilweise als unangenehm empfunden wurden. Außerdem legen die Ergebnisse nahe, dass ein sich bewegendes Kalibrierungsziel zu einer besseren Allzweckkalibrierung führt. Eine sorgfältige Kalibrierung des Raumes, in dem Interaktion verwendet wird, kann daher die Blicktiefenschätzung und Interaktion verbessern.
Open Access
Local divisors in formal languages
(2016) Walter, Tobias; Diekert, Volker (Prof. Dr.)
Regular languages are exactly the class of recognizable subsets of the free monoid. In particular, the syntactic monoid of a regular language is finite. This is the starting point of algebraic language theory. In this thesis, the algebraic connection between regular languages and monoids is studied using a certain monoid construction - local divisors. Using the local divisor construction, we give a Rees decomposition of a monoid into smaller parts - the monoid is a Rees extension of a submonoid and a local divisor. Iterating this concept gives an iterated Rees decomposition of a monoid into groups appearing in the monoid. This decomposition is similar to the synthesis theorem of Rhodes and Allen. In particular, the Rees decomposition shows that closure of a variety V of finite monoids under Rees extensions is the variety H̅ induced by the groups H contained in V. Due to the connection between H̅ and local divisors, we turn our attention to a language description of H̅. The language description is a continuation of classical work of Schützenberger. He studied prefix codes of bounded synchronization delay and used those codes to give a language description of H̅ in the case that the variety H of groups contains only abelian groups. We use the local divisor approach to generalize Schützenberger's language description of H̅ for all varieties H of finite groups. The main ingredient of this generalization is the concept of group-controlled stars. The group-controlled star is an operation on prefix codes of bounded synchronization delay which generalizes the usual Kleene star. The language class SDH(A∞) is the smallest class which contains all finite languages and is closed under union, concatenation product and group-controlled stars for groups in H. We show that SDH(A∞) is the language class corresponding to H̅. As a by-product of the proof we give another language characterization of H̅: the localizable closure LocH(A∞) of H. In the last part of this thesis, we deal with Church-Rosser congruential languages (CRCL). A language is Church-Rosser congruential if it is a finite union of congruence classes modulo a finite, confluent and length-reducing semi-Thue system. This yields a linear time algorithm for the membership problem of a fixed language in CRCL. A natural question, which was open for over 25 years, is whether all regular languages are in CRCL. We give an affirmative answer to this question by proving a stronger statement: for every regular language L and for every weight, there exists a finite, confluent and weight-reducing semi-Thue system S such that A*/S is finite and recognizes L. Lifting the result from the special case of length-reducing to weight-reducing allows the use of local divisors. Next, we focus on Parikh-reducing Church-Rosser systems for regular languages. Instead of constructing a semi-Thue system for a fixed weight, a Parikh-reducing Church-Rosser system is weight-reducing for every weight. We construct such systems for all languages in A̅b̅, that is, for all languages such that the groups in the syntactic monoid are abelian. Additionally, small changes in the proof of this result also yield that for all languages L over a two letter alphabet there exists a Parikh-reducing Church-Rosser system S of finite index such that L is recognized by A*/S. Lastly, we deal with the size of the monoid A*/S for the constructed systems S. We show that in the group case this size has an exponential lower bound and a triple exponential upper bound. The key observation is that one can restrict the alphabet used in the inductive construction. Using the same observation, one can lower the upper bound in the general monoid case from a non-primitive function without this optimization to a quadruple exponential upper bound.
Open Access
Über die Billaudsche Vermutung
(2011) Walter, Tobias
Diese Arbeit thematisiert die Billaudsche Vermutung. Diese handelt von einer Induktivität von Fixpunktwörtern, Wörtern, die Fixpunkte nicht-trivialer Morphismen sind. Billaud stellte 1993 in der Newsgroup comp.theory seine Vermutung auf. Seitdem konnte wenig zur Vermutung veröffentlicht werden. In dieser Arbeit wird zunächst die Vermutung auf kleiner Alphabetgröße untersucht. Dabei wird die Vermutung auf drei Buchstaben bewiesen und Teilresultate der Vermutung auf vier Buchstaben erzielt. Der zweite Teil der Arbeit beschäftigt sich mit einer schwächeren Vermutung und beweist diese teilweise.