arXiv subjects

Christian Ott

Publications and source records attributed to Christian Ott.

At least 19 recordsLinked to original sources

arXiv2026

The Nonsmooth Impact Direction (NSID) of Robotic Systems

Collisions of rigid-link robots and rigid environments are often modeled as instantaneous events. Under this idealization, the impact forces become impulsive and the system velocities nonsmooth. In this work, we systematically analyze pre- and post-impact velocities focusing on what we refer to as the nonsmooth impact direction (NSID). We show that it is a characteristic direction of a robotic impact and largely independent of contact properties. The results are directly applicable to large classes of backdrivable robotic systems with rigid links. We address particularities of systems with nonelastic and flexible joints, unconstrained as well as constrained systems. Further, we show that the approach direction w.r.t the NSID sets the direction of the impulsive force in frictional, inelastic impacts. The comprehensive theoretical analysis of this work supported by an experimental validation may serve as a foundation for future planning and control algorithms for various robotic impact applications. These can include humanoid locomotion on a slippery surface or repetitive hammering.

cs.RO↗

arXiv2026

Adaptive Control of Motor-Position-Controlled Flexible Joint Robots with Uncertain Joint Stiffness

Model-based control of flexible joint robots with position-controlled actuators relies on accurate knowledge of the joint compliance. In practice, precise stiffness models are often unavailable as the properties of physical elastic elements vary with operating conditions and slowly change over time due to wear and aging. To improve model-based control of these systems, we propose an adaptive control approach in this work, which updates an estimate of the uncertain, nonlinear torque-deflection relation of each joint. As opposed to classical adaptive control approaches for non-elastic robots, we rely on an implicit control law and a control-input-dependent regressor matrix to account for the uncertain joint stiffness. We analyze robustness of the approach against errors induced by the motor position controller. Experimental results on a flexible joint with nonlinear stiffness characteristics demonstrate the effectiveness of the proposed approach.

cs.RO↗

arXiv2026

Efficiently Learning Robust Torque-based Locomotion Through Reinforcement with Model-Based Supervision

We propose a control framework that integrates model-based bipedal locomotion with residual reinforcement learning (RL) to achieve robust and adaptive walking in the presence of real-world uncertainties. Our approach leverages a model-based controller, comprising a Divergent Component of Motion (DCM) trajectory planner and a whole-body controller, as a reliable base policy. To address the uncertainties of inaccurate dynamics modeling and sensor noise, we introduce a residual policy trained through RL with domain randomization. Crucially, we employ a model-based oracle policy, which has privileged access to ground-truth dynamics during training, to supervise the residual policy via a novel supervised loss. This supervision enables the policy to efficiently learn corrective behaviors that compensate for unmodeled effects without extensive reward shaping. Our method demonstrates improved robustness and generalization across a range of randomized conditions, offering a scalable solution for sim-to-real transfer in bipedal locomotion.

cs.RO↗

arXiv2025

Humanoid Robot Running Through Random Stepping Stones and Jumping Over Obstacles: Step Adaptation Using Spring-Mass Trajectories

This study proposes a step adaptation framework for running through spring-mass trajectories and deadbeat control gain libraries. It includes four main parts: (1) Automatic spring-mass trajectory library generation; (2) Deadbeat control gain library generation through an actively controlled template model that resembles the whole-body dynamics well; (3) Trajectory selection policy development for step adaptation; (4) Mapping spring-mass trajectories to a humanoid model through a whole-body control (WBC) framework also accounting for closed-kinematic chain systems, self collisions, and reactive limb swinging. We show the inclusiveness and the robustness of the proposed framework through various challenging and agile behaviors such as running through randomly generated stepping stones, jumping over random obstacles, performing slalom motions, changing the running direction suddenly with a random leg, and rejecting significant disturbances and uncertainties through the MuJoCo physics simulator. We also perform additional simulations under a comprehensive set of uncertainties and noise to better justify the proposed method's robustness to real-world challenges, including signal noise, imprecision, modeling errors, and delays. All the aforementioned behaviors are performed with a single library and the same set of WBC control parameters without additional tuning. The spring-mass and the deadbeat control gain library are automatically computed in 4.5 seconds in total for 315 different trajectories.

cs.RO↗

arXiv2025

Partial Feedback Linearization Control of a Cable-Suspended Multirotor Platform for Stabilization of an Attached Load

In this work, we present a novel control approach based on partial feedback linearization (PFL) for the stabilization of a suspended aerial platform with an attached load. Such systems are envisioned for various applications in construction sites involving cranes, such as the holding and transportation of heavy objects. Our proposed control approach considers the underactuation of the whole system while utilizing its coupled dynamics for stabilization. We demonstrate using numerical stability analysis that these coupled terms are crucial for the stabilization of the complete system. We also carried out robustness analysis of the proposed approach in the presence of external wind disturbances, sensor noise, and uncertainties in system dynamics. As our envisioned target application involves cranes in outdoor construction sites, our control approaches rely on only onboard sensors, thus making it suitable for such applications. We carried out extensive simulation studies and experimental tests to validate our proposed control approach.

cs.RO↗

arXiv2025

Dark-fringe interferometer with dynamic phase control for M\"ossbauer science

Interference is a powerful tool for measuring and control. In M\"ossbauer science, interference effects are essential to most applications, due to the coherent scattering nature. However, M\"ossbauer interferometry remains challenging, due to stability requirements imposed by the short x-ray wavelength. Here, we put forward a ``dark fringe'' interferometer with vanishing transmission in the empty state, thereby facilitating sensitive measurements. The relative interferometer phase can dynamically be tuned by displacing a M\"ossbauer target. We experimentally demonstrate the tuning capabilities of this interferometer by controlling the transmitted x-ray intensity on nanosecond time scales. Then, we demonstrate sensitive measurements by observing the propagation of impulsively launched sound waves in the target over $\sim 10\,\mu s$. The interferometer concept opens avenues towards polarization-sensitive phase measurements, the generation of coherent multi-pulse sequences for controlling nuclear dynamics, and the implementation of feedback loops to adaptively optimize the interferometer, thereby fueling the further development of nuclear quantum optics.

quant-ph↗

arXiv2025

Experimental Comparison of Whole-Body Control Formulations for Humanoid Robots in Task Acceleration and Task Force Spaces

This paper studies the experimental comparison of two different whole-body control formulations for humanoid robots: inverse dynamics whole-body control (ID-WBC) and passivity-based whole-body control (PB-WBC). The two controllers fundamentally differ from each other as the first is formulated in task acceleration space and the latter is in task force space with passivity considerations. Even though both control methods predict stability under ideal conditions in closed-loop dynamics, their robustness against joint friction, sensor noise, unmodeled external disturbances, and non-perfect contact conditions is not evident. Therefore, we analyze and experimentally compare the two controllers on a humanoid robot platform through swing foot position and orientation control, squatting with and without unmodeled additional weights, and jumping. We also relate the observed performance and characteristic differences with the controller formulations and highlight each controller's advantages and disadvantages.

cs.RO↗

arXiv2025

Learning Swing-up Maneuvers for a Suspended Aerial Manipulation Platform in a Hierarchical Control Framework

In this work, we present a novel approach to augment a model-based control method with a reinforcement learning (RL) agent and demonstrate a swing-up maneuver with a suspended aerial manipulation platform. These platforms are targeted towards a wide range of applications on construction sites involving cranes, with swing-up maneuvers allowing it to perch at a given location, inaccessible with purely the thrust force of the platform. Our proposed approach is based on a hierarchical control framework, which allows different tasks to be executed according to their assigned priorities. An RL agent is then subsequently utilized to adjust the reference set-point of the lower-priority tasks to perform the swing-up maneuver, which is confined in the nullspace of the higher-priority tasks, such as maintaining a specific orientation and position of the end-effector. Our approach is validated using extensive numerical simulation studies.

cs.RO↗

arXiv2025

Coincidence measurement of two-photon double ionization of argon through an autoionizing resonance

We present coincidence measurements of two-photon double-ionization (TPDI) of argon driven by femtosecond pulses tunable around 26.5 eV photon energy, which are obtained from a high-harmonic generation source. The measured photoelectron spectra are interpreted with regard to three TPDI mechanisms. Theoretical predictions are obtained by an approximate model for direct TPDI and atomic structure calculations, which are implemented into a Monte Carlo simulation. The prevailing mechanism involves the excitation and prompt photoionization of an autoionizing resonance in neutral argon. We provide evidence for pronounced electron-electron interaction in this ultrafast ionization process. Furthermore, we show that the dominant TPDI mechanism can be altered by slight tuning of the photon energy. The present work paves the way for scrutinizing and controlling non-linear photoionization in the extreme ultraviolet using table-top sources.

physics.atom-ph↗

arXiv2025

Redirection and Reshaping of Intense Extreme-Ultraviolet Radiation

The goal to control short-wavelength radiation for the investigation and manipulation of ultrafast dynamics in quantum systems coevolves with the growing availability of extreme-ultraviolet (XUV) and x-ray sources from high-harmonic generation and free-electron lasers. Here, we present an XUV spatio-spectral phase modulator based on an intense XUV laser beam propagating through an optically thick resonant target, introducing dispersion profile variations around the resonance both perpendicular to and along the laser propagation direction. The resulting dipole radiation gets spectrally reshaped and becomes more divergent as compared to the original beam in the far field. As an experimental demonstration, the intense-XUV-induced double-peak off-axis structure in the far-field spectrum obtained at the Free-Electron Laser in Hamburg (FLASH) shows indications of the underlying XUV-driven Rabi dynamics and resonant pulse propagation effects. The presented work highlights a ubiquitous phenomenon occurring when an intense laser beam passes through a resonant medium.

physics.atom-ph↗

arXiv2025

Phase evolution of strong-field ionization

We investigate the time-dependent evolution of the dipole phase shift induced by strong-field ionization (SFI) using attosecond transient absorption spectroscopy (ATAS) for time-delays where the pump-probe pulses overlap. We study measured and calculated time-dependent ATA spectra of the ionic 4d-5p transition in xenon, and present the time-dependent line shape parameters in the complex plane. We attribute the complex, attosecond-scale dynamics to the contribution of three distinct processes: accumulation of ionization, transient population, and reversible population of excited states arising from polarization of the ground state.

physics.atom-ph↗

arXiv2025

Realtime Limb Trajectory Optimization for Humanoid Running Through Centroidal Angular Momentum Dynamics

One of the essential aspects of humanoid robot running is determining the limb-swinging trajectories. During the flight phases, where the ground reaction forces are not available for regulation, the limb swinging trajectories are significant for the stability of the next stance phase. Due to the conservation of angular momentum, improper leg and arm swinging results in highly tilted and unsustainable body configurations at the next stance phase landing. In such cases, the robotic system fails to maintain locomotion independent of the stability of the center of mass trajectories. This problem is more apparent for fast and high flight time trajectories. This paper proposes a real-time nonlinear limb trajectory optimization problem for humanoid running. The optimization problem is tested on two different humanoid robot models, and the generated trajectories are verified using a running algorithm for both robots in a simulation environment.

cs.RO↗

arXiv2025

An Efficient Numerical Function Optimization Framework for Constrained Nonlinear Robotic Problems

This paper presents a numerical function optimization framework designed for constrained optimization problems in robotics. The tool is designed with real-time considerations and is suitable for online trajectory and control input optimization problems. The proposed framework does not require any analytical representation of the problem and works with constrained block-box optimization functions. The method combines first-order gradient-based line search algorithms with constraint prioritization through nullspace projections onto constraint Jacobian space. The tool is implemented in C++ and provided online for community use, along with some numerical and robotic example implementations presented in this paper.

cs.RO↗

arXiv2024

Enhancing Model-Based Step Adaptation for Push Recovery through Reinforcement Learning of Step Timing and Region

This paper introduces a new approach to enhance the robustness of humanoid walking under strong perturbations, such as substantial pushes. Effective recovery from external disturbances requires bipedal robots to dynamically adjust their stepping strategies, including footstep positions and timing. Unlike most advanced walking controllers that restrict footstep locations to a predefined convex region, substantially limiting recoverable disturbances, our method leverages reinforcement learning to dynamically adjust the permissible footstep region, expanding it to a larger, effectively non-convex area and allowing cross-over stepping, which is crucial for counteracting large lateral pushes. Additionally, our method adapts footstep timing in real time to further extend the range of recoverable disturbances. Based on these adjustments, feasible footstep positions and DCM trajectory are planned by solving a QP. Finally, we employ a DCM controller and an inverse dynamics whole-body control framework to ensure the robot effectively follows the trajectory.

cs.RO↗

arXiv2024

Coherent all X-ray four wave mixing at core shell resonances

Nonlinear wave mixing in the X-ray range can provide valuable insights into the structural and electron dynamics of atomic and molecular systems on ultrafast time scales, with state- and site-selectivity and atomic resolution. This promising experimental toolbox was so far limited by requiring at least one near-visible laser, thus preventing core-shell two-dimensional X-ray spectroscopy. In this work, we demonstrate the generation of background-free all-X-ray four-wave mixing (XFWM) signals from a dilute gaseous sample (Ne). The measured and simulated two-dimensional spectral maps ($\omega_{\text{in}},\omega_{\text{out}}$) show multiple contributions involving the coherent response from core electrons. Notably, two-color resonant XFWM signals, essential for generalized multi-color schemes that allow to locally probe the electronic excitation of matter, are observed in neutral Ne. Moreover, stimulated Ne$^+$ emission in each of the propagating X-ray pulses leads to an increase of the temporal coherence in a narrow-bandwidth, which results in the coherent mixing of three X-ray lasers. Preliminary X-ray excitation experiments making use of multi-color time-delayed X-ray pulses demonstrate temporal resolution capability and show a time dependency consistent with a signal dominated by resonant XFWM processes. This first all-X-ray four-wave-mixing approach represents a major breakthrough towards multidimensional X-ray correlation spectroscopy and the general application of nonlinear all-X-ray wave-mixing.

physics.optics↗

arXiv2024

Attosecond soft X-ray pulses generated by chirp-dispersed manipulation in an XFEL reveal nonlinear core-electron dynamics in neon

Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of the electron-bunch chirp-dispersion and working with the full 4.5 MHz repetition rate at the European XFEL in Germany. With a high-fidelity single-shot temporal characterisation, we detect X-ray pulses with durations of down to 200 attoseconds and peak powers reaching into the terawatt regime at ~1 keV photon energy. As a direct application, we present simultaneous measurements of nonlinear X-ray-matter interaction via time-resolved electron spectroscopy. Using the derived temporal pulse information and restricting the durations to a regime where individual X-ray pulses are shorter than the single-core-hole life time in neon atoms, we reveal an otherwise hidden peak-intensity dependence in the nonlinear dynamics of double-core-hole formation. Our results open the field of attosecond science to the investigation of electronic processes not only in the ground state but also in systems driven far off their equilibrium. They shed light on highly transient intermediate steps in complex electronic dynamics and thus promise to help build the conceptual bridge between fundamental physical processes and chemical photo-reactions.

physics.optics↗

arXiv2024

Transmission spectroscopy of CF$_4$ molecules in intense x-ray fields

The nonlinear interaction of x-rays with matter is at the heart of understanding and controlling ultrafast molecular dynamics from an atom-specific viewpoint, providing new scientific and analytical opportunities to explore the structure and dynamics of small quantum systems. At increasingly high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific electronic states and emerging short-lived transient intermediates is of particular relevance for our understanding of fundamental multi-photon absorption processes. In this work, intense x-ray free-electron laser (XFEL) pulses at the European XFEL (EuXFEL) are combined with a gas cell and grating spectrometer for a high-intensity transmission spectroscopy study of multiphoton-induced ultrafast molecular fragmentation dynamics in CF$_4$. This approach unlocks the direct intra-pulse observation of transient fragments, including neutral atoms, by their characteristic absorption lines in the transmitted broad-band x-ray spectrum. The dynamics with and without initially producing fluorine K-shell holes are studied by tuning the central photon energy. The absorption spectra are measured at different FEL intensities to observe nonlinear effects. Transient isolated fluorine atoms and ions are spectroscopically recorded within the ultrashort pulse duration of few tens of femtoseconds. An isosbestic point that signifies the correlated transition between intact neutral CF$_4$ molecules and charged atomic fragments is observed near the fluorine K-edge. The dissociation dynamics and the multiphoton absorption-induced dynamics encoded in the spectra are theoretically interpreted. Overall, this study demonstrates the potential of high-intensity x-ray transmission spectroscopy to study ultrafast molecular dynamics with sensitivity to specific intermediate species and their electronic structure.

physics.atom-ph↗

arXiv2024

Shared Autonomy via Variable Impedance Control and Virtual Potential Fields for Encoding Human Demonstration

This article introduces a framework for complex human-robot collaboration tasks, such as the co-manufacturing of furniture. For these tasks, it is essential to encode tasks from human demonstration and reproduce these skills in a compliant and safe manner. Therefore, two key components are addressed in this work: motion generation and shared autonomy. We propose a motion generator based on a time-invariant potential field, capable of encoding wrench profiles, complex and closed-loop trajectories, and additionally incorporates obstacle avoidance. Additionally, the paper addresses shared autonomy (SA) which enables synergetic collaboration between human operators and robots by dynamically allocating authority. Variable impedance control (VIC) and force control are employed, where impedance and wrench are adapted based on the human-robot autonomy factor derived from interaction forces. System passivity is ensured by an energy-tank based task passivation strategy. The framework's efficacy is validated through simulations and an experimental study employing a Franka Emika Research 3 robot. More information can be found on the project website https://shailjadav.github.io/SALADS/

cs.RO↗