Papers by Christian Daniel
Simulative Determination of Ideal Fluid Properties for an Automatic Ball Balancer Under Different Run-up Scenarios
Vibration Engineering for a Sustainable Future, 2021
Automatic ball balancers (ABB) can be applied to rotating machinery in order to reduce the effect... more Automatic ball balancers (ABB) can be applied to rotating machinery in order to reduce the effective unbalance excitation and resulting deflections of the system. The passive working mechanism omits active components such as sensors, control units and actuators. In an ABB several balls are enclosed in a cylindrical cavity filled with a viscous fluid and can orbit on a circular track positioned centrically to the axis of rotation of the equipped rotor. Inherent to the functional principle, the rotating speed has to exceed the first critical frequency of the system for the balls to be positioned at beneficial orbit positions and counterbalancing the primary unbalance.
Vibration Analysis of an Electric Wheel Hub Motor at Stationary Operating Points
Mechanisms and Machine Science, 2018
In the context of environmentally friendliness and challenging pollution limits the electrificati... more In the context of environmentally friendliness and challenging pollution limits the electrification of passenger cars becomes more and more important. In this contribution an innovative electrical drive for automobiles is presented and its vibration behavior is analyzed experimentally. In special, the vibrations of an electric wheel hub motor are studied in detail. For this purpose, a laser scanning vibrometer is used. To be able to measure the vibrations at the running engine a derotator is needed additionally to the laser scanning vibrometer. The electric wheel hub motor is investigated on a test bench at different stationary operating points which differ in the rotational speed as well as in the torque that is applied by an electric brake. Analyzing the vibroacoustic behavior of this special electric machine is of utmost importance as its sound radiation is directed straight to the passers-by of the car. The sound radiation of conventional cars drives is normally shielded and attenuated by the vehicle body and for this reason less critical. Moreover, the application of damping materials is more difficult if the engine is placed within a wheel. In the paper at hand different prototype stages of the electric wheel hub motor are presented. The working principle of this special engine is also explained. For the numerical simulations a holistic simulation workflow has been developed which takes into account the electromagnetic field as the most important vibration excitation as well as the structural vibrations coupled with an air volume around the engine to calculate the sound pressure. First, the electromagnetic forces are calculated which are then applied to excite the structural vibrations of the engine. Finally, the calculated surface velocity is used to excite the surrounding air volume under free field conditions to determine the radiated sound pressure level. In all three steps of the holistic methodology, the finite element method (FEM) is used for the numerical simulations. Beside identifying weak points of the engine prototype as well as obtaining a general understanding of such an electrical machine, the experimental data are used to validate a numerical model of the electric wheel hub motor. With the help of both the validated model and the gained experimental experiences the design of the wheel hub motor is improved. However, this contribution focusses on the experimental analysis of the structural vibrations of the running wheel hub motor.
Medical engineering & physics, Feb 1, 2018
This paper presents the development of a compact torsion spring for use as an elastic element in ... more This paper presents the development of a compact torsion spring for use as an elastic element in a lightweight series elastic actuator for an active orthosis. This orthosis is going to be utilised as an assistive device for motorically impaired stroke-patients. In the design a two-step optimisation strategy was implemented to meet all requirements for the torsion spring. The first step was to identify a promising topology for the element. In the second step, the shape was optimised based on a finite element model using two different optimisation methods in order to minimise the von Mises equivalent stresses. Four promising variants of the identified topology were extracted from these calculations, one of which was then chosen as the final design. A prototype was manufactured by a laser cutting process, which is a new procedure in the context of elastic elements for series elastic actuators. The calculation results were validated successfully by measurement of the spring properties o...
Machine Learning, 2016
Tasks that require many sequential decisions or complex solutions are hard to solve using convent... more Tasks that require many sequential decisions or complex solutions are hard to solve using conventional reinforcement learning algorithms. Based on the semi Markov decision process setting (SMDP) and the option framework, we propose a model which aims to alleviate these concerns. Instead of learning a single monolithic policy, the agent learns a set of simpler sub-policies as well as the initiation and termination probabilities for each of those sub-policies. While existing option learning algorithms frequently require manual specification of components such as the sub-policies, we present an algorithm which infers all relevant components of the option framework from data. Furthermore, the proposed approach is based on parametric option representations and works well in combination with current policy search methods, which are particularly well suited for continuous real-world tasks. We present results on SMDPs with discrete as well as continuous state-action spaces. The results show that the presented algorithm can combine simple sub-policies to solve complex tasks and can improve learning performance on simpler tasks.
2015 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), 2015
Reinforcement learning of motor skills is an important challenge in order to endow robots with th... more Reinforcement learning of motor skills is an important challenge in order to endow robots with the ability to learn a wide range of skills and solve complex tasks. However, comparing reinforcement learning against human programming is not straightforward. In this paper, we create a motor learning framework consisting of state-of-the-art components in motor skill learning and compare it to a manually designed program on the task of robot tetherball. We use dynamical motor primitives for representing the robot's trajectories and relative entropy policy search to train the motor framework and improve its behavior by trial and error. These algorithmic components allow for high-quality skill learning while the experimental setup enables an accurate evaluation of our framework as robot players can compete against each other. In the complex game of robot tetherball, we show that our learning approach outperforms and wins a match against a high quality hand-crafted system.
Journal of Laser Applications, 2016
Chipping of hardened steel is often performed by the use of polycrystalline cubic boron nitride (... more Chipping of hardened steel is often performed by the use of polycrystalline cubic boron nitride (PCBN). Moreover, PCBN tools are also suitable for many machining tasks with a lower degree of specialization, due to their excellent mechanical properties. Despite the significantly higher lifetime and the associated process reliability plus the reduction of nonproductive times, tools from PCBN cannot yet prevail over competing cutting materials. The reason for this can be seen in high tool costs, which especially arise from an expensive, mechanical final preparation process that is subject to a high tool wear. Laser material processing with ultrashort pulsed laser sources can be regarded as an alternative to the conventional preparation of PCBN cutting tools. However, the multitude of laser process parameters in combination with different material properties by a variety of available PCBN-types requires an each individual setup and review of the process. This represents a barrier for introducing laser processes into industrial applications. Therefore, the objective of this research is to establish an efficient method for a global parameter determination which is performed by varying the parameters power, repetition rate, scan speed, and track-distance of the laser process. Investigations are carried out by use of picosecond pulsed laser radiation on a PCBN grade with a high cubic boron nitride content and a titanium-based binder to gain a basic understanding of the ablation behavior of PCBN. The results of the experimental investigations show that processing of PCBN cutting tools by laser ablation is feasible. Due to the achieved volume ablation rates of Q A > 18 mm 3 /min and an overall minimal surface quality of S a ¼ 1.52 lm, this process shows potential as an alternative to the conventional preparation method. Therefore, also the processing of enhanced tool geometries in PCBN by use of laser technology becomes possible.
2015 IEEE International Conference on Robotics and Automation (ICRA), 2015
Most manipulation tasks can be decomposed into a sequence of phases, where the robot's actions ha... more Most manipulation tasks can be decomposed into a sequence of phases, where the robot's actions have different effects in each phase. The robot can perform actions to transition between phases and, thus, alter the effects of its actions, e.g. grasp an object in order to then lift it. The robot can thus reach a phase that affords the desired manipulation. In this paper, we present an approach for exploiting the phase structure of tasks in order to learn manipulation skills more efficiently. Starting with human demonstrations, the robot learns a probabilistic model of the phases and the phase transitions. The robot then employs model-based reinforcement learning to create a library of motor primitives for transitioning between phases. The learned motor primitives generalize to new situations and tasks. Given this library, the robot uses a value function approach to learn a high-level policy for sequencing the motor primitives. The proposed method was successfully evaluated on a real robot performing a bimanual grasping task.
The paper deals with the numerical simulation of stability problems of the bearings of a turbocha... more The paper deals with the numerical simulation of stability problems of the bearings of a turbocharger model. Therefore a full-dynamical run-up of the turbocharger shaft up to operating speed is simulated. The usage of multi-body-systems (MBS) for the numerical simulation of complex dynamically loaded sys-tems is well established. The formulation of the acting nonlinear force law, given by the Reynolds' differential equation, can be achieved by usage of a user defined force element. The full floating ring bearings implicate special problems of coupling of inner and outer lubricating film. The communication drill-holes in the full float-ing ring ensure the oil supply of the inner lubricating film and lead to a coupling of pressure. The integration of these boundary conditions is discussed by usage of finite differences as well as finite elements for the solution of Reynolds' differential equation.
Simulation der Kolbendynamik unter Berücksichtigung der EHD-Kopplung
This paper deals with the calculation of a full-dynamical run-up of a turbocharger shaft up to op... more This paper deals with the calculation of a full-dynamical run-up of a turbocharger shaft up to operating speed to investigate the stability of the bearing. Due to the full floating ring bearings special boundary conditions result, which lead to a coupling of outer and inner lubrication film. Furthermore the rotor is able to take a skew position in such a way, that for the solution of Reynolds equation the simplistic assumption of a parallel gap isn't longer valid. Therefore the modeling is done in a multi-body-system(MBS), whereat the finite element method is used for the calculation of hydrodynamic pressures in every timestep. Concluding a comparison is done between measured data and numerical simulation. Thereby the development of the instable region is discussed by the influence of shaft inclination as well as the communication drill-holes.
Validierung der Hochlaufsimulation für automotive Abgasturbolader
Abgasturbolader sind ein probates Mittel die Effizienz von Verbrennungsmotoren zu steigern und si... more Abgasturbolader sind ein probates Mittel die Effizienz von Verbrennungsmotoren zu steigern und sind aus heutigen Fahrzeugkonzepten nicht mehr wegzudenken. Zur Verkürzung des Entwicklungsprozesses kommen für die notwendige Optimierung der Systemparameter statt kostenintensiver Prüfstandsversuche vermehrt Simulationen des Hochlaufverhaltens von Turbolader-Rotoren zum Einsatz. Von besonderer Bedeutung ist die Lagerung des Rotors, welche vorwiegend gleitgelagert ausgeführt wird. Bedingt durch den daraus entstehenden Zielkonflikt zwischen maximal nutzbarem Drehzahlbereich und der gleitlagerbedingten Stabilitätsgrenzdrehzahl wird auf eine Schwimmbuchsenlagerung mit semi- oder full-floating-ring zurückgegriffen. Das daraus resultierende extrem nichtlineare Rotorsystem erfordert ein aufwändiges Simulationsmodell zur Vorhersage des transienten Schwingungsverhaltens. Im Beitrag werden Simulationsergebnisse mit Prüfstandsversuchen vergleichen. Das Simulationsmodell wurde innerhalb des Mehrkörp...
Journal of the Ceramic Society of Japan, 2015
Although the research on laser ablation advances steadily, mainly on ultra-short pulses (in the o... more Although the research on laser ablation advances steadily, mainly on ultra-short pulses (in the order of pico-and femtoseconds), research on the low-cost and low-power-consuming nanosecond regime (>100 ns) is relatively scarce. This process is still difficult to predict, due to the many simultaneous and interacting physical processes that take place in a relatively short time. This study provides an experimental analysis for a ytterbium pulsed fibre-laser on Al 2 O 3 , by evaluating surface morphology and fracture strength for two sets of parameters. A very well-defined difference in removal rate and resulting surface topographies was observed, suggesting a threshold point between distinct ablation mechanisms. Laser machining also caused a clear increase on the Weibull modulus, while reducing the characteristic stress.
Movement Primitives (MP) are a well-established approach for representing modular and re-usable r... more Movement Primitives (MP) are a well-established approach for representing modular and re-usable robot movement generators. Many state-of-the-art robot learning successes are based MPs, due to their compact representation of the inherently continuous and high dimensional robot movements. A major goal in robot learning is to combine multiple MPs as building blocks in a modular control architecture to solve complex tasks. To this effect, a MP representation has to allow for blending between motions, adapting to altered task variables, and co-activating multiple MPs in parallel. We present a probabilistic formulation of the MP concept that maintains a distribution over trajectories. Our probabilistic approach allows for the derivation of new operations which are essential for implementing all aforementioned properties in one framework. In order to use such a trajectory distribution for robot movement control, we analytically derive a stochastic feedback controller which reproduces the given trajectory distribution. We evaluate and compare our approach to existing methods on several simulated as well as real robot scenarios.
Gleitlager sind ein wichtiger Bestandteil heutiger Maschinenkonstruktionen. Durch die steigenden ... more Gleitlager sind ein wichtiger Bestandteil heutiger Maschinenkonstruktionen. Durch die steigenden Anforde-
Iterative SLE Solvers over a CPU-GPU Platform
2010 IEEE 12th International Conference on High Performance Computing and Communications (HPCC), 2010
AbstractGPUs (Graphics Processing Units) have become one of the main co-processors that contribu... more AbstractGPUs (Graphics Processing Units) have become one of the main co-processors that contributed to desktops towards high performance computing. Together with multi-core CPUs, a powerful heterogeneous execution platform is built for massive calculations. To ...
Autonomous reinforcement learning with hierarchical REPS
The 2013 International Joint Conference on Neural Networks (IJCNN), 2013
ABSTRACT Future intelligent robots will need to interact with uncertain and changing environments... more ABSTRACT Future intelligent robots will need to interact with uncertain and changing environments. One key aspect to allow robotic agents to adapt to such situations is to enable them to learn multiple solution strategies to one problem, such that the agent can remain flexible and employ alternative solutions even if the preferred solution is no longer viable. We propose a unifying framework that allows the use of hierarchical policies and which can, thus, learn multiple solutions at once. We build our method on the basis of relative entropy policy search, an information theoretic policy search approach to reinforcement learning, and evaluate our method on a real robot system.
Neural Computation, 2010
Most conventional policy gradient reinforcement learning (PGRL) algorithms neglect (or do not exp... more Most conventional policy gradient reinforcement learning (PGRL) algorithms neglect (or do not explicitly make use of) a term in the average reward gradient with respect to the policy parameter. That term involves the derivative of the stationary state distribution that corresponds to the sensitivity of its distribution to changes in the policy parameter. Although the bias introduced by this omission can be reduced by setting the forgetting rate γ for the value functions close to 1, these algorithms do not permit γ to be set exactly at γ = 1. In this article, we propose a method for estimating the log stationary state distribution derivative (LSD) as a useful form of the derivative of the stationary state distribution through backward Markov chain formulation and a temporal difference learning framework. A new policy gradient (PG) framework with an LSD is also proposed, in which the average reward gradient can be estimated by setting γ = 0, so it becomes unnecessary to learn the valu...
Biointerphases, 2007
Understanding the formation of supported lipid bilayers via vesicle fusion-A case that exemplifie... more Understanding the formation of supported lipid bilayers via vesicle fusion-A case that exemplifies the need for the complementary method approach (Review) Biointerphases 11, 020801 (2016);
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Papers by Christian Daniel